- Email: [email protected]
Comparison of ALD coated nanofiltration membranes to unmodified commercial membranes in mine wastewater treatment
Accepted Manuscript Comparison of ALD coated nanofiltration membranes to unmodified commercial membranes in mine wastewater treatment Piia Juholin, Marja-Leena Kä äriäinen, Markus Riihimäki, Rafal Sliz, Junkal Landaburu Aguirre, Minna Pirilä, Tapio Fabritius, David Cameron, Riitta L. Keiski PII: DOI: Reference:
S1383-5866(17)31732-X http://dx.doi.org/10.1016/j.seppur.2017.09.005 SEPPUR 14014
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
31 May 2017 1 September 2017 1 September 2017
Please cite this article as: P. Juholin, M-L. Kä äriäinen, M. Riihimäki, R. Sliz, J. Landaburu Aguirre, M. Pirilä, T. Fabritius, D. Cameron, R.L. Keiski, Comparison of ALD coated nanofiltration membranes to unmodified commercial membranes in mine wastewater treatment, Separation and Purification Technology (2017), doi: http:// dx.doi.org/10.1016/j.seppur.2017.09.005
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Comparison of ALD coated nanofiltration membranes to unmodified commercial membranes in mine wastewater treatment Piia Juholina, Marja-Leena Kääriäinenb, Markus Riihimäkia, Rafal Slizd, Junkal Landaburu Aguirreac, Minna Piriläa, Tapio Fabritiusd, David Camerone, Riitta L. Keiskia* a
Environmental and Chemical Engineering, Faculty of Technology, P.O. Box 4300, FI-90014 University of Oulu, Finland b
Department of Chemistry and Biochemistry and Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, USA c
IMDEA Agua, Calle Punto Net 4, 2A Planta, 28805 Alcala de Henares Madrid, Spain
d
Optoelectronics and Measurement Techniques, Faculty of Information and Electrical Engineering, FI-90014 University of Oulu, Finland e
CEPLANT, Masaryk University, Kotlářská 267/2, 611 37 Brno, Czech Republic *Corresponding author, e-mail: [email protected], Tel: +358 40 7263018
Abstract In this study, commercial nanofiltration (NF270 and NF90, Dow Filmtec) and zinc oxide (ZnO) coated NF270 nanofiltration membranes prepared by atomic layer deposition (ALD) were applied to the purification of mine wastewaters. The characteristics of the coated nanofiltration membranes were evaluated with contact angle, roughness and zeta-potential analyses. Membrane wearing was also studied from microscopic pictures. The waters used were purified process water and mildly contaminated wastewaters from mine sites. The target compounds were sulphate, manganese, nitrate and chloride, the removal efficiencies of which were studied. In addition, membrane fouling was under interest. It was observed that the flux recovery ratio (FRR) after the experiment was 73-85% and 83-93% with the original and the coated membranes, respectively. The relative flux was slightly increased with the coated membrane when testing the membrane efficiency with three different wastewaters. The ZnO layer on the active membrane surface reduced the reversible fouling of the NF270 membrane. Ion removal efficiencies of the coated membranes were in the same level with the uncoated membrane, i.e. removal efficiencies were high. Keywords Anti-fouling, atomic layer deposition, hybrid process, membrane coating, mine wastewater, nanofiltration, sulphate removal, zinc oxide
Abbreviations: ALD, atomic layer deposition; AMD, acid mine drainage; FRR, flux recovery ratio; NF, nanofiltration; WW, wastewater
Nomenclature A effective membrane area (m2) CP concentration of the contaminant in permeate (mg/dm3) CR concentration of the contaminant in retentate (mg/dm3) J permeate flux (dm3/m2 h) Jrel relative flux J0 initial permeate flux of the clean membrane with deionized water (dm3/m2 h) Jwater,1 deionized water flux before experiment (dm3/m2 h) Jwater,2 deionized water flux after experiment (dm3/m2 h) Jww average permeate flux during filtration of wastewater (dm3/m2 h) p pressure (Pa) R rejection coefficient (%) t time of permeation (h) V permeate volume (dm3)
1. Introduction Mining of ores has enabled the development of the existing modern society and production of most of everyday items. Mining industry has been a key also for the technological development. However, mining industry has affected negatively the environment globally due to the acid mine drainage (AMD), soil erosion and heavy metal releases and tailings contamination. [1] As an example, in Finland the mining industry has had a boom during the past few years due to the increased demand for metals and industrial minerals. Consequently, six new mines have started their operation in Finland between the years 2007-2013. [2]; [3] Due to the increased number of mine sites, it has been found that mining operations have caused growing problems of polluted waters and thus new more efficient water purification technologies are needed. At mine sites, water volumes are very high and water is mildly contaminated. Typical contaminants in mine waters are sulphate, heavy metals and nitrogen compounds. The concentrations of organic compounds in mine waters are usually relatively low. Sulphide-containing minerals, e.g. pyrite (FeS2) or pyrrhotite (Fe1-xS) are typical mineral types. During mining activities sulphides encounter oxidative conditions due to the atmospheric oxygen and moisture which is yielding acid mine drainage (AMD) which means elevated concentrations of sulphate and increased acidity in mine waters. Low pH makes heavy metals more soluble in water and thus can increase the dissolving of heavy metals into waters. AMD discharges to the environment can cause environmental problems in ground waters, streams and rivers causing increased water salinity, water eutrophication, and heavy metal bioaccumulation. [4] Despite its harmfulness, sulphate releases are not heavily restricted in legislation. As an example, in the Finnish mining sector the sulphate release limits have been set case-by-case depending on the surrounding water bodies and their sensitivity to wastewater contaminants.
The formation of wastewaters cannot be fully inhibited in the mining industry and therefore new water purification technologies are required to avoid contaminants such as sulphate being released into the surrounding environment. Precipitation, adsorption, and biological treatment are conventional separation techniques for sulphate removal [5]; [6] However, their ability for sulphate separation can be limited at low concentrations. The use of membranes has been in a minor role in the mining sector. The main reasons might have been the membrane fouling, relatively low permeate fluxes for large water volumes which has led to high operating and investment costs. In addition, the concession of environmental permits that made it possible to lead waters having some contaminants to the environment is one of the reasons. These have prevented the use of membranes in mining industry. However, membrane technologies such as nanofiltration and reverse osmosis are applicable techniques for mine wastewater treatment. With membranes, e.g. nanofiltration, it is possible to obtain economically high quality, even potable, water [7] that certainly meets tightening environmental permits. The main drawback in membrane technologies is the membrane fouling which decreases membrane performance and efficiency. Possible foulants in mining industry are sulphate containing salts such as gypsum (CaSO4·2H2O) that cannot always be removed completely from membrane and piping surfaces due to low solubility. [8] Membrane fouling mitigation is one of the factors affecting the economy of membrane separation processes. Membrane fouling can be diminished by several methods such as the development of new membrane materials e.g. by membrane coating that shows a better fouling resistance and can even degrade foulants from the membrane surface. Membrane surface modification can result in reduced membrane fouling and thus improved membrane functionality, selectivity, or in creating a new separation function [9] Surface modification can change the membrane surface charge, hydrophilicity, roughness and permeability, and increase the rejection of removable compounds. [10]; [11]; [12] One attractive technique for membrane surface modification is atomic layer deposition (ALD) [13] ALD is a surface controlled gas phase chemical vapor deposition process where thin films can be deposited by one molecular or atomic layer at a time. ALD can be used to deposit continuous, pinhole-free films with large area uniformity and conformality even on complex surfaces and deformities [14]; [15]. The thickness of the films can be controlled by a number of reaction cycles, thereby enabling the precise growth of ultra-thin layers. Currently ALD is the best technique to provide conformality in high aspect structures [15]. ALD coated membranes as well as other membrane modification techniques have been investigated in several studies [12]; [13]; [16]. A variety of inorganic compounds has been used as a (membrane) coating material including titanium dioxide (TiO2 ), alumina (Al2O3), silver, silica (SiO2) and ZnO [17]; [18]. TiO2 and Al2O3 are the most widely studied compounds used in the modification of polymeric membranes [17]. Nikkola et al. [17] deposited Al2O3 on a polyamide thin film composite reverse osmosis membrane. The investigators found that the deposition of Al2O3 for 10 or 50 ALDcycles resulted in an increased antifouling effect while the salt rejection was slightly decreased.
3
In this study, an ALD coating method was applied to coat polymeric membranes with ZnO, which has advantageous and versatile properties. Zinc oxide is a non-toxic and biocompatible material suitable for environmental applications [19]. In addition, it is a (photo)catalyst having antibacterial properties and therefore may increase the antifouling effect on membranes [20]; [21]; [22]. ALDdeposited ZnO thin films have been found effective in killing bacteria without photocatalytic activation [23] and based on several studies zinc ions (Zn2+) seem to be the cause for the antibacterial effect [23]; [24]; [25]; [26]. Despite the advantageous and versatile properties of ZnO it has been studied less than other oxides in membrane modifications. [27]; [28] In this study the aim was to investigate how mine wastewater purification can be improved with a nm-scale thin film ZnO coated nanofiltration membranes compared to commercial nanofiltration membranes NF90 and NF270 (Dow, USA). Two commercially available membranes were selected to better compare the changes of the modified membrane and to see if characteristics of a tighter membrane can be achieved. Removal efficiencies of typical mine water polluting ions including sulphate, manganese, nitrate and chloride were studied as well as changes in water conductivity which indicates the total ion concentration of water. In fact, plenty of efforts have been made in testing modified membranes with synthetic wastewaters according to the literature. However, studies where modified membranes are tested for real wastewater purification are still scarce and their performance has not been widely investigated. Wastewater from the mining industry exposes membranes to reversible and irreversible fouling. Therefore, the removal of sulphate as well as manganese which can be membrane foulants were investigated. To observe functionality of the coated membrane, characteristics of coated and uncoated nanofiltration membranes were analyzed and compared to each other using zeta-potential, surface charge, surface profilometry, contact angle and microscopic imaging.
2. Experimental 2.1. Membranes and coating materials Nanofiltration membranes NF90 and NF270 from DowFilmtec were used in the experiments. The properties of the used membranes are presented in Table 1. These membranes (NF270 and NF90) were used as received from the manufacturer. In addition, these commercial membranes were coated by the atomic layer deposition (ALD) technique. The untreated NF90, NF270, and the ALD coated NF270 membranes were used for filtration of mine wastewater and also some membrane characteristics were measured and their results were compared to verify the influence of the coated layer on the membrane characteristics. In these preliminary tests the NF270 membranes were coated with 10 and 20 nm ZnO coating layers and for the filtration experiments the coating layer of 20 nm was selected.
4
Table 1. Characteristics of the commercial membranes used. NF90 Dow Membrane manufacturer Polyamine thin-film composite Membrane material Max. operating pressure pH range Max. temperature
41 bar 3-10 45 °C
NF270 Dow Polypiperazine thin-film composite 41 bar 3-10 45 °C
2.2. Membrane coating process Heat treatment of commercial NF membranes Nanofiltration membranes were heat-treated to remove the excess water adsorbed in the membrane prior to the ALD process. The optimal dewatering temperature was studied to avoid membrane damage at high temperatures. The heat treatment temperatures tested were 80 °C, 120 °C, 150 °C, and 180 °C. Temperature was kept constant during the heat treatment and its duration was 4 hours. The membrane surface morphology was then studied with a Field Emission Scanning Electron Microscope (FESEM) and compared to the original untreated membrane materials. ALD process Nanofiltration membranes NF90 and NF270 were modified with zinc oxide deposited by ALD. Zinc oxide was grown onto membranes by ALD using diethylzinc (Zn(C 2H5)2) (DEZ) and deionized water as precursors. Reaction temperatures were 60 °C and 120 °C for NF90 and NF270, respectively. Nitrogen with 99.999% purity was used as the carrier and purging gas (INMATEC IMT-PN 1150, INMATEC GaseTechnologie GmbH & Co., KG). The deposition was carried out in a TSF-500 ALD 3D-reactor (Beneq Oy) at 5x102 – 1x103 Pa pressure. The precursors were kept at 20 °C during the deposition. The growth rate was checked on co-deposited samples of polished silicon substrates with a spectroscopic ellipsometer M-2000FI from J.A.Woollam Co., Inc. The used ALD-cycle was: 30 sec (TiCl4-pulse) – 30 sec (N2 purge) – 30 sec (H2O-pulse) – 30 sec (N2purge) The NF90 and NF270 membranes were both modified using two different thicknesses (10 and 20 nm) of ZnO. 10 nm refers to 100 and 20 nm to 200 deposition cycles, respectively. Based on the thicknesses on co-deposited silicon samples the growth rate was 2Å/cycle at both process temperatures. 2.3. Membrane characterization Zeta-potential analysis The zeta potential analysis for the membrane materials was done with a Becmann Coulter DELSA Nano C particle analyzer using a Flat Surface Cell unit. The sample size was 35 × 15 mm. The Quartz Glass sheet was used as the control material. Standard Solid Particles (Otsuka Electronics) 5
in 10 mM NaCl solution were used in surface zeta potential control measurements and in membrane sample measurements. The cell constant was measured using a 10 mM NaCl solution. Microscopic figures A Mitutoyo Quick Vision Ace light microscope was used for visual characterization and imaging of the topography of membrane materials. The microscope has a calibrated dimension measurement option in 3D. Membranes were used in the same size that had been used in the experiment and the analysis was nondestructive so that the microscopy analysis was done before and after the tests for the same sample. FESEM analysis A Field Emission Scanning Electron Microscope (FESEM) Zeiss Ultra Plus was used to analyse the membrane surface changes after the membrane heat treatment, which was performed prior to the ALD coating. The membrane surface was sputtered with platinum. Contact angle measurements To measure the wettability of the surface the contact angle measurement method was applied. Specifically, the height-width method was used to extract the contact angle from recorded videos. In this method, a contour line enclosed by a rectangle is regarded as being the segment of a circle. As a result, the contact angle can be calculated from the height-width relationship of the enclosing rectangle. Importantly, to avoid gravitational flattening, the drop size was limited to be 1.5×10-9 m3. Finally, the surface contact angles of droplets at three different locations on the same sample were averaged out and the standard deviations were calculated. Surface roughness measurements The roughness measurements were performed using a Bruker ContourGT optical profilometer in the VSI mode. A vertical scanning interferometry method, with a broadband (white) light source, was applied during the measurements. The size of the inspected area was 0.3 mm2 (0.634 × 0.475 mm). Morphology images for each sample were taken in three different locations, and root square (Rq) roughnesses were extracted. Consequently, the obtained values of each sample were averaged and standard deviations calculated. 2.5. Membrane filtration experiments with real wastewaters 2.5.1. Equipment The nanofiltration experiments were performed at bench-scale with a cross-flow membrane cell. The solution was pumped into the cell with a Wanner Hydra Cell diaphragm pump model G-03. Temperature was measured from the feed tank and it was kept constant within 21 ±1 °C with a thermal bath. Fig. 1 shows the flow diagram of the nanofiltration system. The system was operated in a total recycle mode where the retentate and the permeate streams were recycled back to the feed tank. The feed volume was 5 dm3 and the flow rate was 1.6 dm3/min. The superficial velocity on the membrane surface was approximately 0.3 m/s. The transmembrane pressure was 8 × 105 Pa (8 bar). Samples were taken from the feed before the experiments and during the experiments, permeate and
6
retentate samples were collected initially every 5 minutes and later every 10 minutes when the fluxes were stabilized.
Figure 1. Cross-flow equipment diagram. The system consisted of a Sepa CF membrane cell (1), a feed tank (2), a Wanner Hydra-Cell diaphragm pump (3), a retentate valve (4), and pressure gauges before and after the membrane cell (5). Temperature of feed water was measured from the feed tank (6). 2.5.2 Wastewaters Four different sulphate containing mine wastewaters were used in the experiments. The water characteristics are presented in Table 2. The wastewaters were mildly contaminated mine waters from different process steps. In addition, the waters are also originated from two mine sites. Wastewater 1 is process water, which is treated further at an overland flow area. The other waters are from an overland flow area and mildly contaminated mine waters, which have to be post-treated before leading them to the surrounding water bodies. The wastewaters were prefiltered with a Millipore Polygard 0.45 µm cartridge filter to remove colloidal particles from the waters prior to the nanofiltration experiments. Colloids were removed to avoid membrane damage and fouling. The wastewater analyses were done after the prefiltration. Table 2. pH, conductivity and average concentrations of sulphate, manganese, in the feed water. Parameter Wastewater 1 Wastewater 2 Wastewater 3 pH 8.3 9.1 5.7 Conductivity 11.8 6.3 4.0 [mS/cm]
chloride and nitrate Wastewater 4 6.5 11.1
7
SO42- [g/ dm3] Mn [mg/ dm3] Cl- [mg/dm3] NO3- [mg/dm3]
9.6 1.1 60.7 25.7
3.3 0.2 40.0 3.6
2.3 0.08 15.7 39.5
5.6 1.0 14.6 2.2
The sulphate and manganese concentrations were analysed using inductively coupled plasma optical emission spectrometry (ICP-OES). Nitrate and chloride concentrations were measured by ion-selective electrodes manufactured by Sentek. 2.5.4. Compaction and cleaning of the membranes Before the experiment, both the commercial and modified membranes were prepared for the filtration experiment by compacting the membrane. Pressure was increased slowly step by step up to 15 × 105 Pa. The commercial original membranes were kept pressurized for 4 hours. However, to avoid the mechanical degradation of the membrane coating layer, the coated membranes were kept pressurized for shorter times (1.5 h). The effect of shorter pressurization time was estimated to be sufficient since even shorter pressurization times have been used according to the literature [29]. The commercial membranes were cleaned with 1.5% citric acid solution for 30 minutes after each experiment. After acidic cleaning, the membranes were flushed with deionized water approximately for 30 minutes. Since ZnO coated membranes suffer from wearing in high pH conditions as described later these membranes were cleaned by flushing with deionized water for 30 minutes, which was noticed to be enough to remove the foulant. 2.7.2. Rejection coefficient and water permeate flux To study the removal efficiency of sulphur, chloride and manganese, as well as to observe changes in conductivity, the rejection coefficient was calculated in order to study the membrane performance. The rejection coefficient (R) for the compounds was calculated as follows (1) where CP and CR are concentrations of feed and permeate solutions of the compound i. The permeate flux (J) was calculated as follows, (2) where V is the volume of the permeate sample, A is the active membrane area and t is the time needed for collecting the permeate sample. The relative flux (Jrel) was calculated using the following Equation (3), (3) where J is the average permeate flux during filtration of wastewaters and J0 is the initial permeate flux of a clean membrane. The average permeate flux with the real wastewater was used since the flux decrease during the experiments was not observed. 8
To analyse the fouling tendency of membranes, the flux recovery ratio (FRR) was calculated. The permeate flux of deionized water was measured before the experiments and after the filtration experiment when water has been recirculated in the filtration system for 10 minutes. (4) where Jwater,1 and Jwater,2 are deionized water fluxes before and after the experiment, respectively.
3. Results and discussion 3.1. Membrane characterization FESEM FESEM was used to analyse the changes on the membrane surfaces before and after the heat treatment of membranes. In Figures 2 a-d the thermally treated and untreated membranes are presented. The membrane surface had some changes that occurred at high temperatures including breakages and wrinklings of the active surface layer. The membrane surface had also spots where the membrane was compacted. To avoid changes in the membrane performance the membranes used in the actual filtration experiments were heat treated prior to the ALD process below these temperatures that were causing the effects. The maximum dewatering temperatures in which visible changes on the membrane structure occurred were at 80 °C and 160 °C for NF90 and NF270, respectively. In general, the lower the temperature the less damaged the membrane was. NF90 suffered more than NF270 from the heat treatment and therefore the temperature which was used to remove moisture from the ALD coating process was lower (80 °C) whereas NF270 was treated at 120°C.
a)
b)
9
c)
d)
)
Figure 2. FESEM images of the uncoated membrane surface before and after heating in an oven for 4 hours. a) NF90 membrane before the heat treatment, b) NF90 after the treatment at 120 °C, c) NF270 before the heat treatment d) NF270 after the treatment at 160 °C.
Zeta-potential Thec)zeta-potential values of NF270 and NF90 are in accordance with the zeta potential results found from the literature being -28 mV and -15 mV for NF270 and NF90 at pH 7, respectively [30]; [31]; [32]. The zeta-potential of the uncoated NF270 membrane changed from -28 mV to -13 mV after coating with 20 nm of ZnO at pH 7. Since the zeta-potential value has increased closer to the isoelectric point (IEP), the charge of the membrane is smaller and thus interactions between the nanofiltration membrane and ions are weaker. In addition, membranes can be less prone to fouling when the membrane attracts less effectively the oppositely charged ions. Membrane roughness and contact angle measurements It was observed that the ZnO coating layer increased membrane hydrophobicity compared to the original membrane material. The contact angle was tested for the uncoated membranes and for the membranes coated with 10 nm and 20 nm coating layer thicknesses. For both membranes the hydrophobicity was higher with the 20 nm coating layer (200 ALD cycles) (Figure 3). However, it is interesting to note that even though the number of deposition cycles were the same in both NF90 and NF270, the overall hydrophobic character of these membranes was different. This is most probably because the membrane itself affected the overall hydrophobicity. NF270 is less hydrophobic than NF90 and showed a lower hydrophobic character with the coating layer. Hydrophobicity usually reduces the values of the permeate flux. In general, hydrophilic membranes are favored in wastewater purification applications due to their better anti-fouling properties. Fouling compounds in wastewaters are often hydrophobic and hydrophilic membranes do not attract them. In addition, in highly hydrophilic membranes a layer of water molecules can be formed on the membrane surface and hydrophobic foulants cannot deposit on the membrane surface. [33]; [34]; [35]; [36] 10
120
Contact angle [°]
100 80
60 40 20 0 0
10
20
Coating thickness [nm] NF270
NF90
Figure 3. Contact angle of NF270 and NF90 membranes as a function of the coating layer thickness. The coating layer also affected the roughness of the membranes (Figure 4). The roughness of the NF90 membrane increased slightly from 1050 nm to 1180 nm measured from the uncoated and the membrane having a coating layer of 20 nm. With NF270, the increase in roughness was more evident since the roughness increased from 737 nm to 1189 nm. An increased membrane roughness can affect the colloidal fouling. However, in this study, the coating layer has a minor role in changes of roughness since the coating layer thickness is small (10-20 nm) compared to the roughness of the membrane surface. Rough surfaces alter the membrane behavior to be more sensitive for the deposition of particles. [33]; [37]
1600
Roughness Rq [nm]
1400 1200 1000 800 600 400 200 0 0
10
20
Coating thickness [nm] NF270
NF90
Figure 4. Roughness of the uncoated and coated NF270 and NF90 membranes.
11
3.2. Wastewater filtration experiments Water permeability Permeate fluxes did not decrease considerably during the filtration experiments and therefore it can be concluded that considerable fouling was not observed. In most of the experiments, NF270 had the highest permeate flux during the filtration experiment, being 31.4-49.7 dm3/m2 h. The permeate flux of the ZnO modified membrane had variable permeate flux values in the range from 26.0 to 47.2 dm3/m2 h. The permeate flux of the NF90 membrane was between 18.4-46.6 md3/m2 h during the filtration experiments. With wastewater 1 and 3 the permeate flux was in the same range with NF270. However, with wastewater 2 and 4 the permeate flux was noticeably lower. That might be due to the characteristics of the feed waters. The relative fluxes of NF270, coated NF270 and NF90 membranes with four wastewaters are presented in Figure 5. The relative flux compares the clean water flux and the flux during the filtration experiments. It can be observed that the relative flux was slightly increased with the coated membranes when compared to the original NF270 membrane, however, that can be in the range of variation. As a comparison, when filtrating wastewaters 1 and 2 with the NF90 membrane relative fluxes got higher values than those with the modified and original NF270 membranes. The ZnO coating layer increased permeability compared to the unmodified membrane. That can be due to the coating layer, which has increased the membrane roughness (Figure 4). Increased membrane roughness is often related to the increased membrane fouling when foulants can deposit on an uneven surface. However, it has been found in some studies that the increased roughness leads to increased permeability. Increased roughness can be regarded as an enlargement of the active membrane area when liquid has a larger contact area on the membrane surface. [38] 0.9 0.8
Relative flux (Jrel]
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
WW1
WW2 NF270
NF270 20 nm
WW3
WW4
NF90
Figure 5. Relative fluxes with NF270, coated NF270 and NF90 membranes. The permeate flux was measured before and after the filtration experiment at pressures 5-15 × 105 Pa to detect reversible membrane fouling. It was observed that the permeate flux decreased 12
relatively more in the case of uncoated membrane compared to the ZnO coated membranes with all four real wastewaters examined (Figure 6). With each wastewater filtered, the FRR was higher when the coated membranes were applied. Therefore, this study shows that reversible fouling, deposition of colloidal particles and concentration polarization [39] were reduced when coated membranes were applied.
100 90 80
FRR [%]
70 60 50 40 30 20 10 0
WW1
WW2 NF270
NF270 20 nm
WW3
WW4
NF90
Figure 6. Flux recovery ratio (FRR) with coated and uncoated membranes and four wastewaters.
It has to be noticed that the permeate flux with deionized water was lower with the coated membranes compared to the uncoated ones. The lower permeate flux can be due to a resistance caused by the coating layer. The coating layer having thickness of 20 nm alters the characteristics of a membrane which pore sizes are 0.31 and 0.38 nm for NF90 and NF270, respectively. In addition, increased hydrophobicity of the membrane has probably decreased the permeate flux due to its more hydrophobic character. An increasing membrane hydrophobic character increases the fouling tendency of membranes. That is due to the hydrophobic character of inorganic fouling compounds, which are attracted by hydrophobic surfaces. Despite the increased hydrophobicity of the modified membrane surfaces compared to unmodified membranes, fouling was noticed to decrease. Therefore we can even conclude that hydrophobicity is not the most important phenomenon in this study in causing membrane fouling. However, membrane’s zeta potential values also have an important role on membrane fouling phenomena. Zeta-potential is related to membrane’s surface charge. However, besides membrane’s surface characteristics, zeta potential takes into account also the chemistry and thermodynamics of the solution and the flow characteristics. [40] Therefore, having similar solution conditions the surface charge can be estimated. In this study, by measuring zeta potential by streaming potential, the surface charge of a coated membrane was lower compared to that of the uncoated membrane.
13
That can result in lower fouling tendency since the surface is less electrostatically attractive to oppositely charged ions, which can foul the membrane. The permeate flux after membrane cleaning (deionized water flushing with the coated membranes and nitric acid cleaning with an uncoated membrane) was restored to the same level with both original and coated membranes and therefore a clear difference was not seen in the irreversible fouling. Rejection coefficients Rejection of salts from the real wastewaters with ZnO coated NF270 membrane was compared to the behavior of uncoated NF270 and NF90 membranes. The rejection coefficients for sulphate, nitrogen compounds, zinc, chloride and manganese are shown in Figure 7. It was observed that the concentration of manganese decreased during the experiments in the feed/retentate stream. It can be assumed that manganese was precipitated on membrane and on the surface of piping, pumps etc. during the filtration experiment. In fact, manganese is known for its tendency to precipitate and foul surfaces of process equipment [41]. All the membranes removed sulphate efficiently having rejection coefficient >91%. The removal efficiency was the highest with NF90 due to its small pore size. When the uncoated NF270 membrane is compared to the coated membrane, a clear change cannot be found in the removal efficiency. Thus, it can be said that the coating layer did not have a clear effect on the sulphate removal. In the removal of nitrate ions, negative rejection was observed. Negative rejection of nitrate ions increased when the coated membranes were applied compared to the uncoated ones. As it is known about the negative rejection, a compound concentrates in the permeate stream. Better permeating compounds such as monovalent ions tend to go through the membrane more easily while some other more easily removable compounds, such as divalent ions, are more retained to keep electroneutrality in the vicinity of a membrane. [42] Chloride, which is a monovalent ion, had the best removal efficiency with the NF90 membrane, the value being approximately 87.0-91.1%. With NF270 and the coated NF270 membranes, the removal efficiency was 18.8-57.2%. Relatively low removal efficiency of Cl- with the NF270 membranes can be due to the larger pore size compared to the NF90 membrane. In this study monovalent nitrate ions had a tendency to permeate the membrane easily due to the presence of multivalent ions such as sulphate. The negative rejection of NO3- was observed with wastewaters 1 and 3, which had the highest NO3- concentrations. Also the removal of chloride ions decreased with the coated membrane although a negative rejection was not observed. For the coated membranes, this phenomenon might have been enhanced due to the lower membrane zeta-potential values when negatively charged compounds were less repulsed by the membrane since the zetapotential of NF270 membrane changed from -28 to -13 mV after coating. Negative rejection was not observed with the NF90 membrane. That might be due to a smaller pore size of the membrane when it rejects NO3- ions more effectively. 14
In fact, negative rejection and selective separation of nanofiltration membranes can be exploited in applications where the selective separation of ionic compounds is needed [43]. Coated nanofiltration membranes can be used as a technique in NO3 - separation from sulphate containing wastewaters. The permeated NO3- can then be processed further and reused. Conductivity of electrolyte solution refers to ionic content in a solution. Conductivity of permeate samples remained at the same level with the NF270 coated and the original membrane. Thus, the coating layer of the NF270 membrane did not change the rejection efficiency of ions significantly. NF90 had the highest ion rejection efficiency due to the smaller pore size compared to the NF270 membrane. In this study, the effect of increased hydrophobicity was not observed clearly on the rejection coefficient nor the membrane fouling. In fact, the effect of electrostatic repulsion has an inevitable effect when charged membranes, such as nanofiltration membranes, are used and the effect of hydrophobicity is not always that obvious. [42] Therefore, hydrophobicity did not affect the membrane performance very clearly. An increase in the hydrophobicity can affect the removal efficiency due to the attraction of foulants to the membrane surface. [44] If foulants attach on the membrane surface, they can alter the electrostatic interactions of the membrane and thus change the rejection efficiency. However, since fouling was decreased with the modified membranes, the effect of hydrophilicity was estimated to be smaller compared to other characteristics analysed. Besides the studies performed at 8 × 105 Pa the sulphate removal was studied also at transmembrane pressure of 15 × 105 Pa. The sulphate removal was slightly improved at higher pressures with all the membranes (original NF270 and NF90 and ZnO modified NF270), however, an increase in the sulphate removal was observed to be between 0.0 - 3.4%. Also the conductivity of permeate samples decreased from 0.1 to 9.4% compared to the conductivity values at the pressure value of 8 × 105 Pa which indicates an increased ion removal efficiency. The removal efficiency was improved at higher pressure as it is known to happen with nanofiltration membranes. Variation in the values of the rejection coefficients between the wastewaters was also noticed. As an example, the negative rejection was observed with two wastewaters. In addition, conductivity was not similar throughout the wastewaters. Differences can be explained with different chemical compositions of wastewaters. Several variables such as pH, ionic strength, permeating compounds etc. affect the removal efficiency.
15
Rejection efficiency and change in conductivity [%]
Wastewater 1 100 80 60 40 20 0 -20 -40 -60 -80
SO4
Mn
NF270
NO3
NF270 20 nm
Cl
Conductivity
Cl-
Conductivity
NF90
Rejection efficiency and change in conductivity [%]
Wastewater 2 100 80 60 40 20 0 SO4 2-
Mn NF270
NO3 NF270 coated
NF90
16
Rejection efficiency and change in conductivity [%]
Wastewater 3 100 80
60 40 20 0 -20
SO4 2-
Mn
NO3
Cl-
Conductivity
Cl-
Conductivity
-40 -60 -80 NF270
NF270 coated
NF90
Rejection efficiency and change in conductivity [%]
Wastewater 4 100 80 60 40 20
0 SO4 2-
Mn NF270
NO3 NF270 coated
NF90
Figure 7. Rejection efficiencies of NF270, modified NF270 and NF90 membranes for four different real mine wastewaters. Membrane wearing stability It was observed that the membrane surface suffered from wearing during the filtration experiments as can be seen from Figures 8 a and b. From the microscopic images, it can be noticed that the ZnO coating layer was cracked and flakes were detached (c.a. 1/5 from the area) from the membrane surface. Dimension of the microscopic pictures was 1.257 × 0.938 mm.
17
Figure 8. Microscopic pictures of the NF270 membrane surface a) before and b) after a filtration experiment. The concentration of zinc in the water was analyzed from some samples to see if the ZnO coating suffered from wearing off the membrane surface. Zinc concentration increased during the experiments especially in the permeate stream. These results indicated that wearing could not been avoided during the experiments. By using prefiltration with a 0.45 µm cartridge filter, mechanical abrasion of particulates in water was minimized. Table 3. Zn concentration in feed, retentate and permeate samples from the experiments with the modified membranes. Wastewater1 Wastewater 2 Wastewater 3 Wastewater 4 3 Feed [mg/dm ] N/A <0.01 0.01 0.085 5 Permeate 8 × 10 Pa 0.049 0.5 0.56 3 [mg/dm ] Retentate 8 × 105 Pa 0.013 0.082 0.13 3 [mg/dm ]
Increased zinc concentration in the permeate stream might be due to properties such as inflexibility of the inorganic layer on a flexible polymeric membrane surface. As can be noticed from the microscopic figures and the increased concentration of zinc in the water samples, wearing of the coating layer is apparent. This may be due to the fact that ZnO dissolves to some extent in aqueous solutions [19]; [23]. In this study, relatively high shear forces existed during the experiments since the experiments were conducted in a cross-flow system with a flow rate of 1.6 dm3/min. Chemical characteristics of wastewaters can also promote the wearing phenomena. At low pH values, the zinc oxide layer becomes more soluble. If the zinc concentrations are compared to the pH values of the real wastewaters a relationship can be seen. pH of wastewater 2 was 9.12 and in that case the zinc concentrations had the lowest values. pH values of wastewater 3 and 4 were 5.7 and 6.49, respectively, and with these wastewaters the zinc concentrations were similar being a bit higher compared to wastewater 2. To reduce the coating depletion, operating conditions should be taken into account. Another option is to mix the ZnO nanoparticles in the membrane matrix allowing it to be more firmly bound to the membrane material. Conclusions The ZnO coated commercial membranes were used for the filtration of real mine wastewaters and their characteristics were compared to the original uncoated membrane. The relative flux was slightly increased with the coated membrane with the three wastewaters used, however, the increment can be in the range of variation. The ZnO layer on the membrane’s active surface reduced the membrane reversible fouling. The flux recovery ratio was measured after 10 minutes 18
flushing with deionized water. The FRR of the NF270 membrane varied with four real wastewaters from 73 to 85%. In the same conditions, the FRR values of the coated NF270 membrane were between 83 - 93%. However, a change in the irreversible fouling was not observed when coated and uncoated membranes were compared. The ZnO coating did not significantly improve the removal efficiency of the compounds studied. The coated nanofiltration membranes retained the ability to separate multivalent ions more readily compared to the monovalent ions as the original uncoated membranes do. Membrane characteristics such as membrane roughness, contact angle and zeta-potential values changed when the coating layer was added. This study proved that membrane characteristics can be improved with a ZnO coating layer. Especially membrane fouling was reduced. However, some challenges of ALD coated membranes were observed. The ZnO coating layer had a tendency to wear off from the membrane surface. Wearing of coating did not affect the permeability of membrane but increased the Zn concentration of the permeate stream and therefore the coated membranes have a limited life time. If the coated membranes will be used in a larger scale, durability of the ZnO coating has to be improved. More research is needed to improve the characteristics of membrane material to have a long-lasting coating layer, high rejection efficiency and good permeability. Acknowledgements The authors want to acknowledge Sulka “Sulphur Compounds in Mining Operations-Environmental Impact Assessment, Measurement and Emission Abatement” (A32164, 524/2012) –project funded by European Regional Development Fund as well as Tekes - the Finnish Funding Agency for Innovation funded project HYMEPRO “Hybrid membrane processes for water treatment” (845/31/2011) for financing the study. The Finnish Cultural Foundation, Ahti Pekkalan säätiö, KAUTE foundation, Tauno Tönningin säätiö, and Maa- ja vesitekniikan tuki ry are also acknowledged for financial support. Kirsi Ahtinen is acknowledged for laboratory assistance as well as Tarja Seppänen for coating the membranes. This research has been performed under the ADMA “Advanced materials” doctoral programme of the University of Oulu. References [1] Hilson, G. & Nayee, V. 2002, "Environmental management system implementation in the mining industry: a key to achieving cleaner production", International Journal of Mineral Processing, 64(1): 19-41. doi:10.1016/S0301-7516(01)00071-0 [2] Wessman, H., Salmi, O., Kohl, J., Kinnunen, P., Saarivuori, E. & Mroueh, U. 2014, "Water and society: mutual challenges for eco-efficient and socially acceptable mining in Finland", Journal of Cleaner Production, 84: 289-298. doi:10.1016/j.jclepro.2014.04.026
19
[3] Söderholm, K., Söderholm, P., Helenius, H., Pettersson, M., Viklund, R., Masloboev, V., Mingaleva, T. & Petrov, V. 2015, "Environmental regulation and competitiveness in the mining industry: Permitting processes with special focus on Finland, Sweden and Russia", Resources Policy, 43: 130-142. doi:10.1016/j.resourpol.2014.11.008 [4] Simate G.S. & Ndlovu S. 2014, “Acid mine drainage: challenges and opportunities”, Journal of Environmental Chemical Engineering 2: 1785-1803. doi:10.1016/j.jece.2014.07.021 [5] Almasri, D., Mahmoud, K.A. & Abdel-Wahab, A. 2015, "Two-stage sulfate removal from reject brine in inland desalination with zero-liquid discharge", Desalination 362(0): 52-58. doi:10.1016/j.desal.2015.02.008 [6] Hong, S., Cannon, F.S., Hou, P., Byrne, T. & Nieto-Delgado, C. 2014, "Sulfate removal from acid mine drainage using polypyrrole-grafted granular activated carbon", Carbon, 73(0): 51-60. doi:10.1016/j.carbon.2014.02.036 [7] Bertrand, S., Lemaître I. & Wittmann, E. 1997, “Performance of a nanofiltration plant on hard and highly sulphated water during two years of operation”, Desalination 113(2–3): 277-281. doi:10.1016/S0011-9164(97)00141-0 [8] Silva, A.M., Lima, R.M.F. & Leão, V.A. 2012, "Mine water treatment with limestone for sulfate removal", Journal of hazardous materials, 221–222(0): 45-55. doi:10.1016/j.jhazmat.2012.03.066 [9] Ulbricht, M. 2006, "Advanced functional polymer membranes", Polymer, 47(7): 2217-2262. doi:10.1016/j.polymer.2006.01.084 [10] Tang, C.Y., Kwon, Y. & Leckie, J.O. 2009, "Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: II. Membrane physiochemical properties and their dependence on polyamide and coating layers", Desalination, 242(1–3): 168-182. doi:10.1016/j.desal.2008.04.004 [11] Peyki, A., Rahimpour, A. & Jahanshahi, M. 2015, "Preparation and characterization of thin film composite reverse osmosis membranes incorporated with hydrophilic SiO 2 nanoparticles", Desalination, 368(0): 152-158. doi:10.1016/j.desal.2014.05.025 [12] Xu, Q., Yang, J., Dai, J., Yang, Y., Chen, X. & Wang, Y. 2013, "Hydrophilization of porous polypropylene membranes by atomic layer deposition of TiO 2 for simultaneously improved permeability and selectivity", Journal of Membrane Science, 448(0): 215-222. doi:10.1016/j.memsci.2013.08.018 [13] Shenvi, S.S., Isloor, A.M. & Ismail, A.F. 2015, "A review on RO membrane technology: Developments and challenges", Desalination, 368:10-26. doi:10.1016/j.desal.2014.12.042
20
[14] Maydannik, P.S., Kääriäinen, T.O., Lahtinen, K., Cameron, D. C., Söderlund, M., Soininen, P., Johansson, P., Kuusipalo, J., Moro, L. & Zeng, X. 2014, “Roll-to-roll ALD process for flexible electronics encapsulation applications”, J. Vac. Sci Technol. A, 32: 051603. doi:10.1116/1.4893428 [15] George, S.M. 2010 “Atomic layer deposition: an overview” Chemical Reviews, 110: 111–131. doi: 10.1021/cr900056b [16] Shafi H.Z., Khan Z., Yang R. & Gleason K.K. 2015, “Surface modification of reverse osmosis membranes with zwitterionic coating for improved resistance to fouling”, Desalination, 362(0): 93103. doi:10.1016/j.desal.2015.02.009 [17] Nikkola, J., Sievänen, J., Raulio, M., Wei, J., Vuorinen, J. & Tang, C.Y. 2014, "Surface modification of thin film composite polyamide membrane using atomic layer deposition method", Journal of Membrane Science, 450(0): 174-180. doi:10.1016/j.memsci.2013.09.005 [18] Moghimifar, V., Raisi, A. & Aroujalian, A. 2014, "Surface modification of polyethersulfone ultrafiltration membranes by corona plasma-assisted coating TiO2 nanoparticles", Journal of Membrane Science, 461: 69-80. doi:10.1016/j.memsci.2014.02.012 [19] Rudakova, A.V., Oparicheva, U.G., Grishina, A.E., Murashkina, A.A., Emeline, A.V. & Bahnemann, D.W. 2016, "Photoinduced hydrophilic conversion of hydrated ZnO surfaces", Journal of colloid and interface science, 466: 452-460. doi:10.1016/j.jcis.2015.08.015 [20] Li, M., Li, G., Jiang, J., Zhang, Z., Dai, X. & Mai, K. 2015, "Ultraviolet Resistance and Antimicrobial Properties of ZnO in the Polypropylene Materials: A Review", Journal of Materials Science & Technology, 31(4): 331-339. doi:10.1016/j.jmst.2014.11.022 [21] Srivastava M., Basu B.B.J. & Rajam K.S. 2011, “Improving the Hydrophobicity of ZnO by PTFE Incorporation”, Journal of Nanotechnology, 2011, Article ID 392754. doi:10.1155/2011/392754 [22] Shinde, V.R., Lokhande, C.D., Mane, R.S. & Han, S. 2005, “Hydrophobic and textured ZnO films deposited by chemical bath deposition: annealing effect”, Applied Surface Science, 245(1–4): 407-413. doi:10.1016/j.apsusc.2004.10.036 [23] Kääriäinen M.-L., Weiss C.K., Ritz S., Pütz S., Cameron D.C., Mailänder V. & Landfester K. 2013, “Zinc release from atomic layer deposited zinc oxide thin films and its antibacterial effect on Escherichia coli” Applied Surface Science, 287: 375– 380. doi:10.1016/j.apsusc.2013.09.162 [24] Pilakasiri, K., Molee, P., Sringernyuang, D., Dangjun, N., Channasanon, S. & Tan-odekaew, S. 2011, ”Efficacy of chitin-PAA-GTMAC gel in promoting wound healing: animal study” J. Mater. Sci. Mater. Med., 22: 2497. doi:10.1007/s10856-011-4420-6
21
[25] Sudheesh Kumar, P.T., Lakshmanan, V.-K., Anilkumar, T.V., Ramya, C., Reshmi, P., Unnikrishnan, A.G., Nair, S.V. & Jayakumar, R. 2012, ”Flexible and Microporous Chitosan Hydrogel/Nano ZnO Composite Bandages for Wound Dressing: In Vitro and In Vivo Evaluation”, ACS Appl. Mater. Interfaces 4(5): 2618–2629. doi: 10.1021/am300292v [26] Sudheesh Kumar P.T., Lakshmanan V.-K., Raj M., Biswas R., Hiroshi T., Nair S.V. & Jayakumar R. 2013, ”Evaluation of Wound Healing Potential of β-Chitin Hydrogel/Nano Zinc Oxide Composite Bandage” Pharm. Res., 30: 523-537. doi:10.1007/s11095-012-0898-y [27] Hairom, N.H.H., Mohammad, A.W. & Kadhum, A.A.H. 2014, "Effect of various zinc oxide nanoparticles in membrane photocatalytic reactor for Congo red dye treatment", Separation and Purification Technology, 137(0): 74-81. doi:10.1016/j.seppur.2014.09.027 [28] Rabiee, H., Vatanpour, V., Farahani, M.H.D.A. & Zarrabi, H. 2015, "Improvement in flux and antifouling properties of PVC ultrafiltration membranes by incorporation of zinc oxide (ZnO) nanoparticles", Separation and Purification Technology, 156(2): 299-310. doi:10.1016/j.seppur.2015.10.015 [29] Gomes, S., Cavaco, S.A., Quina, M.J. & Gando-Ferreira, L.M. 2010, "Nanofiltration process for separating Cr(III) from acid solutions: Experimental and modelling analysis", Desalination, 254(1–3): 80-89. doi:10.1016/j.desal.2009.12.010 [30] Hilal, N., Kochkodan, V., Al Abdulgader, H. & Johnson, D. 2015, "A combined ion exchange– nanofiltration process for water desalination: II. Membrane selection", Desalination, 363,(0): 51-57. doi:10.1016/j.desal.2014.11.017 [31] Rizwan T. & Bhattacharjee S. 2007, “Initial Deposition of Colloidal Particles on a Rough Nanofiltration Membrane”, The Canadian Journal of Chemical Engineering, 85: 570-579. doi: 10.1002/cjce.5450850502 [32] Nghiem L.D., Schäfer A.I., Elimelech M. 2005, “Pharmaceutical Retention Mechanisms by Nanofiltration Membranes”, Environ. Sci. Technol. 39: 7698-7705. doi: 10.1021/es0507665 [33] Akbari, A., Aliyarizadeh, E., Mojallali Rostami, S.M. & Homayoonfal, M. 2016, "Novel sulfonated polyamide thin-film composite nanofiltration membranes with improved water flux and anti-fouling properties", Desalination 377: 11-22. doi:10.1016/j.desal.2015.08.025 [34] Vatanpour, V., Shockravi, A., Zarrabi, H., Nikjavan, Z. & Javadi, A. 2015, "Fabrication and characterization of anti-fouling and anti-bacterial Ag-loaded graphene oxide/polyethersulfone mixed matrix membrane", Journal of Industrial and Engineering Chemistry, 30: 342-352. doi:10.1016/j.jiec.2015.06.004
22
[35] Jhaveri, J.H. & Murthy, Z.V.P. 2016, "A comprehensive review on anti-fouling nanocomposite membranes for pressure driven membrane separation processes", Desalination, 379: 137-154. doi:10.1016/j.desal.2015.11.009 [36] Mänttäri, M., Pekuri, T. & Nyström, M. 2004, "NF270, a new membrane having promising characteristics and being suitable for treatment of dilute effluents from the paper industry" Journal of Membrane Science, 242(1–2): 107-116. doi:10.1016/j.memsci.2003.08.032 [37] Vrijenhoek, E.M., Hong, S. & Elimelech, M. 2001, "Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes", Journal of Membrane Science, 188(1): 115-128. doi:10.1016/S0376-7388(01)00376-3 [38] Hirose, M., Ito, H. & Kamiyama, Y. 1996, "Effect of skin layer surface structures on the flux behaviour of RO membranes", Journal of Membrane Science, 121(2): 209-215. doi:10.1016/S0376-7388(96)00181-0 [39] Schäfer, A.I., Andritsos, N., Karabelas, A.J., Hoek, E.M.V., Schneider, R. & Nyström, M. 2004, “Fouling in Nanofiltration, in: Nanofiltration – Principles and Applications”, Schäfer A.I., Waite T.D., Fane A.G. (Eds). Elsevier, Chapter 20, 169-239. [40] Childress, A.E. & Elimelech, M. 1996, "Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes", Journal of Membrane Science, 119(2): 253-268. doi:10.1016/0376-7388(96)00127-5 [41] Xu, J., Chen, G., Huang, X., Li, G., Liu, J., Yang, N. & Gao, S. 2009, "Iron and manganese removal by using manganese ore constructed wetlands in the reclamation of steel wastewater", Journal of hazardous materials, 169(1–3): 309-317. doi:10.1016/j.jhazmat.2009.03.074 [42] Mänttäri, M. & Nyström, M. 2006, "Negative retention of organic compounds in nanofiltration", Desalination, 199(1–3): 41-42. doi:10.1016/j.desal.2006.03.015 [43] Pérez-González A., Ibáñez R., Gómez P., Urtiaga AM, Ortiz I & Irabien JA (2015) Nanofiltration separation of polyvalent and monovalent anions in desalination brines. Journal of Membrane Science 473: 16-27. doi:10.1016/j.memsci.2014.08.045 [44] Rana, D. & Matsuura, T. 2010. Surface modifications for antifouling membranes. Chemical Reviews 110: 2448–2471. doi: 10.1021/cr800208y
23
Highlights Atomic layer deposition was applied for coating of NF membranes with ZnO. Membranes were applied for purification of real mine wastewaters. Rejection of SO4, NO3, Cl and Mn as well as membrane fouling tendency were studied.
24
S1383-5866(17)31732-X http://dx.doi.org/10.1016/j.seppur.2017.09.005 SEPPUR 14014
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
31 May 2017 1 September 2017 1 September 2017
Please cite this article as: P. Juholin, M-L. Kä äriäinen, M. Riihimäki, R. Sliz, J. Landaburu Aguirre, M. Pirilä, T. Fabritius, D. Cameron, R.L. Keiski, Comparison of ALD coated nanofiltration membranes to unmodified commercial membranes in mine wastewater treatment, Separation and Purification Technology (2017), doi: http:// dx.doi.org/10.1016/j.seppur.2017.09.005
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Comparison of ALD coated nanofiltration membranes to unmodified commercial membranes in mine wastewater treatment Piia Juholina, Marja-Leena Kääriäinenb, Markus Riihimäkia, Rafal Slizd, Junkal Landaburu Aguirreac, Minna Piriläa, Tapio Fabritiusd, David Camerone, Riitta L. Keiskia* a
Environmental and Chemical Engineering, Faculty of Technology, P.O. Box 4300, FI-90014 University of Oulu, Finland b
Department of Chemistry and Biochemistry and Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, USA c
IMDEA Agua, Calle Punto Net 4, 2A Planta, 28805 Alcala de Henares Madrid, Spain
d
Optoelectronics and Measurement Techniques, Faculty of Information and Electrical Engineering, FI-90014 University of Oulu, Finland e
CEPLANT, Masaryk University, Kotlářská 267/2, 611 37 Brno, Czech Republic *Corresponding author, e-mail: [email protected], Tel: +358 40 7263018
Abstract In this study, commercial nanofiltration (NF270 and NF90, Dow Filmtec) and zinc oxide (ZnO) coated NF270 nanofiltration membranes prepared by atomic layer deposition (ALD) were applied to the purification of mine wastewaters. The characteristics of the coated nanofiltration membranes were evaluated with contact angle, roughness and zeta-potential analyses. Membrane wearing was also studied from microscopic pictures. The waters used were purified process water and mildly contaminated wastewaters from mine sites. The target compounds were sulphate, manganese, nitrate and chloride, the removal efficiencies of which were studied. In addition, membrane fouling was under interest. It was observed that the flux recovery ratio (FRR) after the experiment was 73-85% and 83-93% with the original and the coated membranes, respectively. The relative flux was slightly increased with the coated membrane when testing the membrane efficiency with three different wastewaters. The ZnO layer on the active membrane surface reduced the reversible fouling of the NF270 membrane. Ion removal efficiencies of the coated membranes were in the same level with the uncoated membrane, i.e. removal efficiencies were high. Keywords Anti-fouling, atomic layer deposition, hybrid process, membrane coating, mine wastewater, nanofiltration, sulphate removal, zinc oxide
Abbreviations: ALD, atomic layer deposition; AMD, acid mine drainage; FRR, flux recovery ratio; NF, nanofiltration; WW, wastewater
Nomenclature A effective membrane area (m2) CP concentration of the contaminant in permeate (mg/dm3) CR concentration of the contaminant in retentate (mg/dm3) J permeate flux (dm3/m2 h) Jrel relative flux J0 initial permeate flux of the clean membrane with deionized water (dm3/m2 h) Jwater,1 deionized water flux before experiment (dm3/m2 h) Jwater,2 deionized water flux after experiment (dm3/m2 h) Jww average permeate flux during filtration of wastewater (dm3/m2 h) p pressure (Pa) R rejection coefficient (%) t time of permeation (h) V permeate volume (dm3)
1. Introduction Mining of ores has enabled the development of the existing modern society and production of most of everyday items. Mining industry has been a key also for the technological development. However, mining industry has affected negatively the environment globally due to the acid mine drainage (AMD), soil erosion and heavy metal releases and tailings contamination. [1] As an example, in Finland the mining industry has had a boom during the past few years due to the increased demand for metals and industrial minerals. Consequently, six new mines have started their operation in Finland between the years 2007-2013. [2]; [3] Due to the increased number of mine sites, it has been found that mining operations have caused growing problems of polluted waters and thus new more efficient water purification technologies are needed. At mine sites, water volumes are very high and water is mildly contaminated. Typical contaminants in mine waters are sulphate, heavy metals and nitrogen compounds. The concentrations of organic compounds in mine waters are usually relatively low. Sulphide-containing minerals, e.g. pyrite (FeS2) or pyrrhotite (Fe1-xS) are typical mineral types. During mining activities sulphides encounter oxidative conditions due to the atmospheric oxygen and moisture which is yielding acid mine drainage (AMD) which means elevated concentrations of sulphate and increased acidity in mine waters. Low pH makes heavy metals more soluble in water and thus can increase the dissolving of heavy metals into waters. AMD discharges to the environment can cause environmental problems in ground waters, streams and rivers causing increased water salinity, water eutrophication, and heavy metal bioaccumulation. [4] Despite its harmfulness, sulphate releases are not heavily restricted in legislation. As an example, in the Finnish mining sector the sulphate release limits have been set case-by-case depending on the surrounding water bodies and their sensitivity to wastewater contaminants.
The formation of wastewaters cannot be fully inhibited in the mining industry and therefore new water purification technologies are required to avoid contaminants such as sulphate being released into the surrounding environment. Precipitation, adsorption, and biological treatment are conventional separation techniques for sulphate removal [5]; [6] However, their ability for sulphate separation can be limited at low concentrations. The use of membranes has been in a minor role in the mining sector. The main reasons might have been the membrane fouling, relatively low permeate fluxes for large water volumes which has led to high operating and investment costs. In addition, the concession of environmental permits that made it possible to lead waters having some contaminants to the environment is one of the reasons. These have prevented the use of membranes in mining industry. However, membrane technologies such as nanofiltration and reverse osmosis are applicable techniques for mine wastewater treatment. With membranes, e.g. nanofiltration, it is possible to obtain economically high quality, even potable, water [7] that certainly meets tightening environmental permits. The main drawback in membrane technologies is the membrane fouling which decreases membrane performance and efficiency. Possible foulants in mining industry are sulphate containing salts such as gypsum (CaSO4·2H2O) that cannot always be removed completely from membrane and piping surfaces due to low solubility. [8] Membrane fouling mitigation is one of the factors affecting the economy of membrane separation processes. Membrane fouling can be diminished by several methods such as the development of new membrane materials e.g. by membrane coating that shows a better fouling resistance and can even degrade foulants from the membrane surface. Membrane surface modification can result in reduced membrane fouling and thus improved membrane functionality, selectivity, or in creating a new separation function [9] Surface modification can change the membrane surface charge, hydrophilicity, roughness and permeability, and increase the rejection of removable compounds. [10]; [11]; [12] One attractive technique for membrane surface modification is atomic layer deposition (ALD) [13] ALD is a surface controlled gas phase chemical vapor deposition process where thin films can be deposited by one molecular or atomic layer at a time. ALD can be used to deposit continuous, pinhole-free films with large area uniformity and conformality even on complex surfaces and deformities [14]; [15]. The thickness of the films can be controlled by a number of reaction cycles, thereby enabling the precise growth of ultra-thin layers. Currently ALD is the best technique to provide conformality in high aspect structures [15]. ALD coated membranes as well as other membrane modification techniques have been investigated in several studies [12]; [13]; [16]. A variety of inorganic compounds has been used as a (membrane) coating material including titanium dioxide (TiO2 ), alumina (Al2O3), silver, silica (SiO2) and ZnO [17]; [18]. TiO2 and Al2O3 are the most widely studied compounds used in the modification of polymeric membranes [17]. Nikkola et al. [17] deposited Al2O3 on a polyamide thin film composite reverse osmosis membrane. The investigators found that the deposition of Al2O3 for 10 or 50 ALDcycles resulted in an increased antifouling effect while the salt rejection was slightly decreased.
3
In this study, an ALD coating method was applied to coat polymeric membranes with ZnO, which has advantageous and versatile properties. Zinc oxide is a non-toxic and biocompatible material suitable for environmental applications [19]. In addition, it is a (photo)catalyst having antibacterial properties and therefore may increase the antifouling effect on membranes [20]; [21]; [22]. ALDdeposited ZnO thin films have been found effective in killing bacteria without photocatalytic activation [23] and based on several studies zinc ions (Zn2+) seem to be the cause for the antibacterial effect [23]; [24]; [25]; [26]. Despite the advantageous and versatile properties of ZnO it has been studied less than other oxides in membrane modifications. [27]; [28] In this study the aim was to investigate how mine wastewater purification can be improved with a nm-scale thin film ZnO coated nanofiltration membranes compared to commercial nanofiltration membranes NF90 and NF270 (Dow, USA). Two commercially available membranes were selected to better compare the changes of the modified membrane and to see if characteristics of a tighter membrane can be achieved. Removal efficiencies of typical mine water polluting ions including sulphate, manganese, nitrate and chloride were studied as well as changes in water conductivity which indicates the total ion concentration of water. In fact, plenty of efforts have been made in testing modified membranes with synthetic wastewaters according to the literature. However, studies where modified membranes are tested for real wastewater purification are still scarce and their performance has not been widely investigated. Wastewater from the mining industry exposes membranes to reversible and irreversible fouling. Therefore, the removal of sulphate as well as manganese which can be membrane foulants were investigated. To observe functionality of the coated membrane, characteristics of coated and uncoated nanofiltration membranes were analyzed and compared to each other using zeta-potential, surface charge, surface profilometry, contact angle and microscopic imaging.
2. Experimental 2.1. Membranes and coating materials Nanofiltration membranes NF90 and NF270 from DowFilmtec were used in the experiments. The properties of the used membranes are presented in Table 1. These membranes (NF270 and NF90) were used as received from the manufacturer. In addition, these commercial membranes were coated by the atomic layer deposition (ALD) technique. The untreated NF90, NF270, and the ALD coated NF270 membranes were used for filtration of mine wastewater and also some membrane characteristics were measured and their results were compared to verify the influence of the coated layer on the membrane characteristics. In these preliminary tests the NF270 membranes were coated with 10 and 20 nm ZnO coating layers and for the filtration experiments the coating layer of 20 nm was selected.
4
Table 1. Characteristics of the commercial membranes used. NF90 Dow Membrane manufacturer Polyamine thin-film composite Membrane material Max. operating pressure pH range Max. temperature
41 bar 3-10 45 °C
NF270 Dow Polypiperazine thin-film composite 41 bar 3-10 45 °C
2.2. Membrane coating process Heat treatment of commercial NF membranes Nanofiltration membranes were heat-treated to remove the excess water adsorbed in the membrane prior to the ALD process. The optimal dewatering temperature was studied to avoid membrane damage at high temperatures. The heat treatment temperatures tested were 80 °C, 120 °C, 150 °C, and 180 °C. Temperature was kept constant during the heat treatment and its duration was 4 hours. The membrane surface morphology was then studied with a Field Emission Scanning Electron Microscope (FESEM) and compared to the original untreated membrane materials. ALD process Nanofiltration membranes NF90 and NF270 were modified with zinc oxide deposited by ALD. Zinc oxide was grown onto membranes by ALD using diethylzinc (Zn(C 2H5)2) (DEZ) and deionized water as precursors. Reaction temperatures were 60 °C and 120 °C for NF90 and NF270, respectively. Nitrogen with 99.999% purity was used as the carrier and purging gas (INMATEC IMT-PN 1150, INMATEC GaseTechnologie GmbH & Co., KG). The deposition was carried out in a TSF-500 ALD 3D-reactor (Beneq Oy) at 5x102 – 1x103 Pa pressure. The precursors were kept at 20 °C during the deposition. The growth rate was checked on co-deposited samples of polished silicon substrates with a spectroscopic ellipsometer M-2000FI from J.A.Woollam Co., Inc. The used ALD-cycle was: 30 sec (TiCl4-pulse) – 30 sec (N2 purge) – 30 sec (H2O-pulse) – 30 sec (N2purge) The NF90 and NF270 membranes were both modified using two different thicknesses (10 and 20 nm) of ZnO. 10 nm refers to 100 and 20 nm to 200 deposition cycles, respectively. Based on the thicknesses on co-deposited silicon samples the growth rate was 2Å/cycle at both process temperatures. 2.3. Membrane characterization Zeta-potential analysis The zeta potential analysis for the membrane materials was done with a Becmann Coulter DELSA Nano C particle analyzer using a Flat Surface Cell unit. The sample size was 35 × 15 mm. The Quartz Glass sheet was used as the control material. Standard Solid Particles (Otsuka Electronics) 5
in 10 mM NaCl solution were used in surface zeta potential control measurements and in membrane sample measurements. The cell constant was measured using a 10 mM NaCl solution. Microscopic figures A Mitutoyo Quick Vision Ace light microscope was used for visual characterization and imaging of the topography of membrane materials. The microscope has a calibrated dimension measurement option in 3D. Membranes were used in the same size that had been used in the experiment and the analysis was nondestructive so that the microscopy analysis was done before and after the tests for the same sample. FESEM analysis A Field Emission Scanning Electron Microscope (FESEM) Zeiss Ultra Plus was used to analyse the membrane surface changes after the membrane heat treatment, which was performed prior to the ALD coating. The membrane surface was sputtered with platinum. Contact angle measurements To measure the wettability of the surface the contact angle measurement method was applied. Specifically, the height-width method was used to extract the contact angle from recorded videos. In this method, a contour line enclosed by a rectangle is regarded as being the segment of a circle. As a result, the contact angle can be calculated from the height-width relationship of the enclosing rectangle. Importantly, to avoid gravitational flattening, the drop size was limited to be 1.5×10-9 m3. Finally, the surface contact angles of droplets at three different locations on the same sample were averaged out and the standard deviations were calculated. Surface roughness measurements The roughness measurements were performed using a Bruker ContourGT optical profilometer in the VSI mode. A vertical scanning interferometry method, with a broadband (white) light source, was applied during the measurements. The size of the inspected area was 0.3 mm2 (0.634 × 0.475 mm). Morphology images for each sample were taken in three different locations, and root square (Rq) roughnesses were extracted. Consequently, the obtained values of each sample were averaged and standard deviations calculated. 2.5. Membrane filtration experiments with real wastewaters 2.5.1. Equipment The nanofiltration experiments were performed at bench-scale with a cross-flow membrane cell. The solution was pumped into the cell with a Wanner Hydra Cell diaphragm pump model G-03. Temperature was measured from the feed tank and it was kept constant within 21 ±1 °C with a thermal bath. Fig. 1 shows the flow diagram of the nanofiltration system. The system was operated in a total recycle mode where the retentate and the permeate streams were recycled back to the feed tank. The feed volume was 5 dm3 and the flow rate was 1.6 dm3/min. The superficial velocity on the membrane surface was approximately 0.3 m/s. The transmembrane pressure was 8 × 105 Pa (8 bar). Samples were taken from the feed before the experiments and during the experiments, permeate and
6
retentate samples were collected initially every 5 minutes and later every 10 minutes when the fluxes were stabilized.
Figure 1. Cross-flow equipment diagram. The system consisted of a Sepa CF membrane cell (1), a feed tank (2), a Wanner Hydra-Cell diaphragm pump (3), a retentate valve (4), and pressure gauges before and after the membrane cell (5). Temperature of feed water was measured from the feed tank (6). 2.5.2 Wastewaters Four different sulphate containing mine wastewaters were used in the experiments. The water characteristics are presented in Table 2. The wastewaters were mildly contaminated mine waters from different process steps. In addition, the waters are also originated from two mine sites. Wastewater 1 is process water, which is treated further at an overland flow area. The other waters are from an overland flow area and mildly contaminated mine waters, which have to be post-treated before leading them to the surrounding water bodies. The wastewaters were prefiltered with a Millipore Polygard 0.45 µm cartridge filter to remove colloidal particles from the waters prior to the nanofiltration experiments. Colloids were removed to avoid membrane damage and fouling. The wastewater analyses were done after the prefiltration. Table 2. pH, conductivity and average concentrations of sulphate, manganese, in the feed water. Parameter Wastewater 1 Wastewater 2 Wastewater 3 pH 8.3 9.1 5.7 Conductivity 11.8 6.3 4.0 [mS/cm]
chloride and nitrate Wastewater 4 6.5 11.1
7
SO42- [g/ dm3] Mn [mg/ dm3] Cl- [mg/dm3] NO3- [mg/dm3]
9.6 1.1 60.7 25.7
3.3 0.2 40.0 3.6
2.3 0.08 15.7 39.5
5.6 1.0 14.6 2.2
The sulphate and manganese concentrations were analysed using inductively coupled plasma optical emission spectrometry (ICP-OES). Nitrate and chloride concentrations were measured by ion-selective electrodes manufactured by Sentek. 2.5.4. Compaction and cleaning of the membranes Before the experiment, both the commercial and modified membranes were prepared for the filtration experiment by compacting the membrane. Pressure was increased slowly step by step up to 15 × 105 Pa. The commercial original membranes were kept pressurized for 4 hours. However, to avoid the mechanical degradation of the membrane coating layer, the coated membranes were kept pressurized for shorter times (1.5 h). The effect of shorter pressurization time was estimated to be sufficient since even shorter pressurization times have been used according to the literature [29]. The commercial membranes were cleaned with 1.5% citric acid solution for 30 minutes after each experiment. After acidic cleaning, the membranes were flushed with deionized water approximately for 30 minutes. Since ZnO coated membranes suffer from wearing in high pH conditions as described later these membranes were cleaned by flushing with deionized water for 30 minutes, which was noticed to be enough to remove the foulant. 2.7.2. Rejection coefficient and water permeate flux To study the removal efficiency of sulphur, chloride and manganese, as well as to observe changes in conductivity, the rejection coefficient was calculated in order to study the membrane performance. The rejection coefficient (R) for the compounds was calculated as follows (1) where CP and CR are concentrations of feed and permeate solutions of the compound i. The permeate flux (J) was calculated as follows, (2) where V is the volume of the permeate sample, A is the active membrane area and t is the time needed for collecting the permeate sample. The relative flux (Jrel) was calculated using the following Equation (3), (3) where J is the average permeate flux during filtration of wastewaters and J0 is the initial permeate flux of a clean membrane. The average permeate flux with the real wastewater was used since the flux decrease during the experiments was not observed. 8
To analyse the fouling tendency of membranes, the flux recovery ratio (FRR) was calculated. The permeate flux of deionized water was measured before the experiments and after the filtration experiment when water has been recirculated in the filtration system for 10 minutes. (4) where Jwater,1 and Jwater,2 are deionized water fluxes before and after the experiment, respectively.
3. Results and discussion 3.1. Membrane characterization FESEM FESEM was used to analyse the changes on the membrane surfaces before and after the heat treatment of membranes. In Figures 2 a-d the thermally treated and untreated membranes are presented. The membrane surface had some changes that occurred at high temperatures including breakages and wrinklings of the active surface layer. The membrane surface had also spots where the membrane was compacted. To avoid changes in the membrane performance the membranes used in the actual filtration experiments were heat treated prior to the ALD process below these temperatures that were causing the effects. The maximum dewatering temperatures in which visible changes on the membrane structure occurred were at 80 °C and 160 °C for NF90 and NF270, respectively. In general, the lower the temperature the less damaged the membrane was. NF90 suffered more than NF270 from the heat treatment and therefore the temperature which was used to remove moisture from the ALD coating process was lower (80 °C) whereas NF270 was treated at 120°C.
a)
b)
9
c)
d)
)
Figure 2. FESEM images of the uncoated membrane surface before and after heating in an oven for 4 hours. a) NF90 membrane before the heat treatment, b) NF90 after the treatment at 120 °C, c) NF270 before the heat treatment d) NF270 after the treatment at 160 °C.
Zeta-potential Thec)zeta-potential values of NF270 and NF90 are in accordance with the zeta potential results found from the literature being -28 mV and -15 mV for NF270 and NF90 at pH 7, respectively [30]; [31]; [32]. The zeta-potential of the uncoated NF270 membrane changed from -28 mV to -13 mV after coating with 20 nm of ZnO at pH 7. Since the zeta-potential value has increased closer to the isoelectric point (IEP), the charge of the membrane is smaller and thus interactions between the nanofiltration membrane and ions are weaker. In addition, membranes can be less prone to fouling when the membrane attracts less effectively the oppositely charged ions. Membrane roughness and contact angle measurements It was observed that the ZnO coating layer increased membrane hydrophobicity compared to the original membrane material. The contact angle was tested for the uncoated membranes and for the membranes coated with 10 nm and 20 nm coating layer thicknesses. For both membranes the hydrophobicity was higher with the 20 nm coating layer (200 ALD cycles) (Figure 3). However, it is interesting to note that even though the number of deposition cycles were the same in both NF90 and NF270, the overall hydrophobic character of these membranes was different. This is most probably because the membrane itself affected the overall hydrophobicity. NF270 is less hydrophobic than NF90 and showed a lower hydrophobic character with the coating layer. Hydrophobicity usually reduces the values of the permeate flux. In general, hydrophilic membranes are favored in wastewater purification applications due to their better anti-fouling properties. Fouling compounds in wastewaters are often hydrophobic and hydrophilic membranes do not attract them. In addition, in highly hydrophilic membranes a layer of water molecules can be formed on the membrane surface and hydrophobic foulants cannot deposit on the membrane surface. [33]; [34]; [35]; [36] 10
120
Contact angle [°]
100 80
60 40 20 0 0
10
20
Coating thickness [nm] NF270
NF90
Figure 3. Contact angle of NF270 and NF90 membranes as a function of the coating layer thickness. The coating layer also affected the roughness of the membranes (Figure 4). The roughness of the NF90 membrane increased slightly from 1050 nm to 1180 nm measured from the uncoated and the membrane having a coating layer of 20 nm. With NF270, the increase in roughness was more evident since the roughness increased from 737 nm to 1189 nm. An increased membrane roughness can affect the colloidal fouling. However, in this study, the coating layer has a minor role in changes of roughness since the coating layer thickness is small (10-20 nm) compared to the roughness of the membrane surface. Rough surfaces alter the membrane behavior to be more sensitive for the deposition of particles. [33]; [37]
1600
Roughness Rq [nm]
1400 1200 1000 800 600 400 200 0 0
10
20
Coating thickness [nm] NF270
NF90
Figure 4. Roughness of the uncoated and coated NF270 and NF90 membranes.
11
3.2. Wastewater filtration experiments Water permeability Permeate fluxes did not decrease considerably during the filtration experiments and therefore it can be concluded that considerable fouling was not observed. In most of the experiments, NF270 had the highest permeate flux during the filtration experiment, being 31.4-49.7 dm3/m2 h. The permeate flux of the ZnO modified membrane had variable permeate flux values in the range from 26.0 to 47.2 dm3/m2 h. The permeate flux of the NF90 membrane was between 18.4-46.6 md3/m2 h during the filtration experiments. With wastewater 1 and 3 the permeate flux was in the same range with NF270. However, with wastewater 2 and 4 the permeate flux was noticeably lower. That might be due to the characteristics of the feed waters. The relative fluxes of NF270, coated NF270 and NF90 membranes with four wastewaters are presented in Figure 5. The relative flux compares the clean water flux and the flux during the filtration experiments. It can be observed that the relative flux was slightly increased with the coated membranes when compared to the original NF270 membrane, however, that can be in the range of variation. As a comparison, when filtrating wastewaters 1 and 2 with the NF90 membrane relative fluxes got higher values than those with the modified and original NF270 membranes. The ZnO coating layer increased permeability compared to the unmodified membrane. That can be due to the coating layer, which has increased the membrane roughness (Figure 4). Increased membrane roughness is often related to the increased membrane fouling when foulants can deposit on an uneven surface. However, it has been found in some studies that the increased roughness leads to increased permeability. Increased roughness can be regarded as an enlargement of the active membrane area when liquid has a larger contact area on the membrane surface. [38] 0.9 0.8
Relative flux (Jrel]
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
WW1
WW2 NF270
NF270 20 nm
WW3
WW4
NF90
Figure 5. Relative fluxes with NF270, coated NF270 and NF90 membranes. The permeate flux was measured before and after the filtration experiment at pressures 5-15 × 105 Pa to detect reversible membrane fouling. It was observed that the permeate flux decreased 12
relatively more in the case of uncoated membrane compared to the ZnO coated membranes with all four real wastewaters examined (Figure 6). With each wastewater filtered, the FRR was higher when the coated membranes were applied. Therefore, this study shows that reversible fouling, deposition of colloidal particles and concentration polarization [39] were reduced when coated membranes were applied.
100 90 80
FRR [%]
70 60 50 40 30 20 10 0
WW1
WW2 NF270
NF270 20 nm
WW3
WW4
NF90
Figure 6. Flux recovery ratio (FRR) with coated and uncoated membranes and four wastewaters.
It has to be noticed that the permeate flux with deionized water was lower with the coated membranes compared to the uncoated ones. The lower permeate flux can be due to a resistance caused by the coating layer. The coating layer having thickness of 20 nm alters the characteristics of a membrane which pore sizes are 0.31 and 0.38 nm for NF90 and NF270, respectively. In addition, increased hydrophobicity of the membrane has probably decreased the permeate flux due to its more hydrophobic character. An increasing membrane hydrophobic character increases the fouling tendency of membranes. That is due to the hydrophobic character of inorganic fouling compounds, which are attracted by hydrophobic surfaces. Despite the increased hydrophobicity of the modified membrane surfaces compared to unmodified membranes, fouling was noticed to decrease. Therefore we can even conclude that hydrophobicity is not the most important phenomenon in this study in causing membrane fouling. However, membrane’s zeta potential values also have an important role on membrane fouling phenomena. Zeta-potential is related to membrane’s surface charge. However, besides membrane’s surface characteristics, zeta potential takes into account also the chemistry and thermodynamics of the solution and the flow characteristics. [40] Therefore, having similar solution conditions the surface charge can be estimated. In this study, by measuring zeta potential by streaming potential, the surface charge of a coated membrane was lower compared to that of the uncoated membrane.
13
That can result in lower fouling tendency since the surface is less electrostatically attractive to oppositely charged ions, which can foul the membrane. The permeate flux after membrane cleaning (deionized water flushing with the coated membranes and nitric acid cleaning with an uncoated membrane) was restored to the same level with both original and coated membranes and therefore a clear difference was not seen in the irreversible fouling. Rejection coefficients Rejection of salts from the real wastewaters with ZnO coated NF270 membrane was compared to the behavior of uncoated NF270 and NF90 membranes. The rejection coefficients for sulphate, nitrogen compounds, zinc, chloride and manganese are shown in Figure 7. It was observed that the concentration of manganese decreased during the experiments in the feed/retentate stream. It can be assumed that manganese was precipitated on membrane and on the surface of piping, pumps etc. during the filtration experiment. In fact, manganese is known for its tendency to precipitate and foul surfaces of process equipment [41]. All the membranes removed sulphate efficiently having rejection coefficient >91%. The removal efficiency was the highest with NF90 due to its small pore size. When the uncoated NF270 membrane is compared to the coated membrane, a clear change cannot be found in the removal efficiency. Thus, it can be said that the coating layer did not have a clear effect on the sulphate removal. In the removal of nitrate ions, negative rejection was observed. Negative rejection of nitrate ions increased when the coated membranes were applied compared to the uncoated ones. As it is known about the negative rejection, a compound concentrates in the permeate stream. Better permeating compounds such as monovalent ions tend to go through the membrane more easily while some other more easily removable compounds, such as divalent ions, are more retained to keep electroneutrality in the vicinity of a membrane. [42] Chloride, which is a monovalent ion, had the best removal efficiency with the NF90 membrane, the value being approximately 87.0-91.1%. With NF270 and the coated NF270 membranes, the removal efficiency was 18.8-57.2%. Relatively low removal efficiency of Cl- with the NF270 membranes can be due to the larger pore size compared to the NF90 membrane. In this study monovalent nitrate ions had a tendency to permeate the membrane easily due to the presence of multivalent ions such as sulphate. The negative rejection of NO3- was observed with wastewaters 1 and 3, which had the highest NO3- concentrations. Also the removal of chloride ions decreased with the coated membrane although a negative rejection was not observed. For the coated membranes, this phenomenon might have been enhanced due to the lower membrane zeta-potential values when negatively charged compounds were less repulsed by the membrane since the zetapotential of NF270 membrane changed from -28 to -13 mV after coating. Negative rejection was not observed with the NF90 membrane. That might be due to a smaller pore size of the membrane when it rejects NO3- ions more effectively. 14
In fact, negative rejection and selective separation of nanofiltration membranes can be exploited in applications where the selective separation of ionic compounds is needed [43]. Coated nanofiltration membranes can be used as a technique in NO3 - separation from sulphate containing wastewaters. The permeated NO3- can then be processed further and reused. Conductivity of electrolyte solution refers to ionic content in a solution. Conductivity of permeate samples remained at the same level with the NF270 coated and the original membrane. Thus, the coating layer of the NF270 membrane did not change the rejection efficiency of ions significantly. NF90 had the highest ion rejection efficiency due to the smaller pore size compared to the NF270 membrane. In this study, the effect of increased hydrophobicity was not observed clearly on the rejection coefficient nor the membrane fouling. In fact, the effect of electrostatic repulsion has an inevitable effect when charged membranes, such as nanofiltration membranes, are used and the effect of hydrophobicity is not always that obvious. [42] Therefore, hydrophobicity did not affect the membrane performance very clearly. An increase in the hydrophobicity can affect the removal efficiency due to the attraction of foulants to the membrane surface. [44] If foulants attach on the membrane surface, they can alter the electrostatic interactions of the membrane and thus change the rejection efficiency. However, since fouling was decreased with the modified membranes, the effect of hydrophilicity was estimated to be smaller compared to other characteristics analysed. Besides the studies performed at 8 × 105 Pa the sulphate removal was studied also at transmembrane pressure of 15 × 105 Pa. The sulphate removal was slightly improved at higher pressures with all the membranes (original NF270 and NF90 and ZnO modified NF270), however, an increase in the sulphate removal was observed to be between 0.0 - 3.4%. Also the conductivity of permeate samples decreased from 0.1 to 9.4% compared to the conductivity values at the pressure value of 8 × 105 Pa which indicates an increased ion removal efficiency. The removal efficiency was improved at higher pressure as it is known to happen with nanofiltration membranes. Variation in the values of the rejection coefficients between the wastewaters was also noticed. As an example, the negative rejection was observed with two wastewaters. In addition, conductivity was not similar throughout the wastewaters. Differences can be explained with different chemical compositions of wastewaters. Several variables such as pH, ionic strength, permeating compounds etc. affect the removal efficiency.
15
Rejection efficiency and change in conductivity [%]
Wastewater 1 100 80 60 40 20 0 -20 -40 -60 -80
SO4
Mn
NF270
NO3
NF270 20 nm
Cl
Conductivity
Cl-
Conductivity
NF90
Rejection efficiency and change in conductivity [%]
Wastewater 2 100 80 60 40 20 0 SO4 2-
Mn NF270
NO3 NF270 coated
NF90
16
Rejection efficiency and change in conductivity [%]
Wastewater 3 100 80
60 40 20 0 -20
SO4 2-
Mn
NO3
Cl-
Conductivity
Cl-
Conductivity
-40 -60 -80 NF270
NF270 coated
NF90
Rejection efficiency and change in conductivity [%]
Wastewater 4 100 80 60 40 20
0 SO4 2-
Mn NF270
NO3 NF270 coated
NF90
Figure 7. Rejection efficiencies of NF270, modified NF270 and NF90 membranes for four different real mine wastewaters. Membrane wearing stability It was observed that the membrane surface suffered from wearing during the filtration experiments as can be seen from Figures 8 a and b. From the microscopic images, it can be noticed that the ZnO coating layer was cracked and flakes were detached (c.a. 1/5 from the area) from the membrane surface. Dimension of the microscopic pictures was 1.257 × 0.938 mm.
17
Figure 8. Microscopic pictures of the NF270 membrane surface a) before and b) after a filtration experiment. The concentration of zinc in the water was analyzed from some samples to see if the ZnO coating suffered from wearing off the membrane surface. Zinc concentration increased during the experiments especially in the permeate stream. These results indicated that wearing could not been avoided during the experiments. By using prefiltration with a 0.45 µm cartridge filter, mechanical abrasion of particulates in water was minimized. Table 3. Zn concentration in feed, retentate and permeate samples from the experiments with the modified membranes. Wastewater1 Wastewater 2 Wastewater 3 Wastewater 4 3 Feed [mg/dm ] N/A <0.01 0.01 0.085 5 Permeate 8 × 10 Pa 0.049 0.5 0.56 3 [mg/dm ] Retentate 8 × 105 Pa 0.013 0.082 0.13 3 [mg/dm ]
Increased zinc concentration in the permeate stream might be due to properties such as inflexibility of the inorganic layer on a flexible polymeric membrane surface. As can be noticed from the microscopic figures and the increased concentration of zinc in the water samples, wearing of the coating layer is apparent. This may be due to the fact that ZnO dissolves to some extent in aqueous solutions [19]; [23]. In this study, relatively high shear forces existed during the experiments since the experiments were conducted in a cross-flow system with a flow rate of 1.6 dm3/min. Chemical characteristics of wastewaters can also promote the wearing phenomena. At low pH values, the zinc oxide layer becomes more soluble. If the zinc concentrations are compared to the pH values of the real wastewaters a relationship can be seen. pH of wastewater 2 was 9.12 and in that case the zinc concentrations had the lowest values. pH values of wastewater 3 and 4 were 5.7 and 6.49, respectively, and with these wastewaters the zinc concentrations were similar being a bit higher compared to wastewater 2. To reduce the coating depletion, operating conditions should be taken into account. Another option is to mix the ZnO nanoparticles in the membrane matrix allowing it to be more firmly bound to the membrane material. Conclusions The ZnO coated commercial membranes were used for the filtration of real mine wastewaters and their characteristics were compared to the original uncoated membrane. The relative flux was slightly increased with the coated membrane with the three wastewaters used, however, the increment can be in the range of variation. The ZnO layer on the membrane’s active surface reduced the membrane reversible fouling. The flux recovery ratio was measured after 10 minutes 18
flushing with deionized water. The FRR of the NF270 membrane varied with four real wastewaters from 73 to 85%. In the same conditions, the FRR values of the coated NF270 membrane were between 83 - 93%. However, a change in the irreversible fouling was not observed when coated and uncoated membranes were compared. The ZnO coating did not significantly improve the removal efficiency of the compounds studied. The coated nanofiltration membranes retained the ability to separate multivalent ions more readily compared to the monovalent ions as the original uncoated membranes do. Membrane characteristics such as membrane roughness, contact angle and zeta-potential values changed when the coating layer was added. This study proved that membrane characteristics can be improved with a ZnO coating layer. Especially membrane fouling was reduced. However, some challenges of ALD coated membranes were observed. The ZnO coating layer had a tendency to wear off from the membrane surface. Wearing of coating did not affect the permeability of membrane but increased the Zn concentration of the permeate stream and therefore the coated membranes have a limited life time. If the coated membranes will be used in a larger scale, durability of the ZnO coating has to be improved. More research is needed to improve the characteristics of membrane material to have a long-lasting coating layer, high rejection efficiency and good permeability. Acknowledgements The authors want to acknowledge Sulka “Sulphur Compounds in Mining Operations-Environmental Impact Assessment, Measurement and Emission Abatement” (A32164, 524/2012) –project funded by European Regional Development Fund as well as Tekes - the Finnish Funding Agency for Innovation funded project HYMEPRO “Hybrid membrane processes for water treatment” (845/31/2011) for financing the study. The Finnish Cultural Foundation, Ahti Pekkalan säätiö, KAUTE foundation, Tauno Tönningin säätiö, and Maa- ja vesitekniikan tuki ry are also acknowledged for financial support. Kirsi Ahtinen is acknowledged for laboratory assistance as well as Tarja Seppänen for coating the membranes. This research has been performed under the ADMA “Advanced materials” doctoral programme of the University of Oulu. References [1] Hilson, G. & Nayee, V. 2002, "Environmental management system implementation in the mining industry: a key to achieving cleaner production", International Journal of Mineral Processing, 64(1): 19-41. doi:10.1016/S0301-7516(01)00071-0 [2] Wessman, H., Salmi, O., Kohl, J., Kinnunen, P., Saarivuori, E. & Mroueh, U. 2014, "Water and society: mutual challenges for eco-efficient and socially acceptable mining in Finland", Journal of Cleaner Production, 84: 289-298. doi:10.1016/j.jclepro.2014.04.026
19
[3] Söderholm, K., Söderholm, P., Helenius, H., Pettersson, M., Viklund, R., Masloboev, V., Mingaleva, T. & Petrov, V. 2015, "Environmental regulation and competitiveness in the mining industry: Permitting processes with special focus on Finland, Sweden and Russia", Resources Policy, 43: 130-142. doi:10.1016/j.resourpol.2014.11.008 [4] Simate G.S. & Ndlovu S. 2014, “Acid mine drainage: challenges and opportunities”, Journal of Environmental Chemical Engineering 2: 1785-1803. doi:10.1016/j.jece.2014.07.021 [5] Almasri, D., Mahmoud, K.A. & Abdel-Wahab, A. 2015, "Two-stage sulfate removal from reject brine in inland desalination with zero-liquid discharge", Desalination 362(0): 52-58. doi:10.1016/j.desal.2015.02.008 [6] Hong, S., Cannon, F.S., Hou, P., Byrne, T. & Nieto-Delgado, C. 2014, "Sulfate removal from acid mine drainage using polypyrrole-grafted granular activated carbon", Carbon, 73(0): 51-60. doi:10.1016/j.carbon.2014.02.036 [7] Bertrand, S., Lemaître I. & Wittmann, E. 1997, “Performance of a nanofiltration plant on hard and highly sulphated water during two years of operation”, Desalination 113(2–3): 277-281. doi:10.1016/S0011-9164(97)00141-0 [8] Silva, A.M., Lima, R.M.F. & Leão, V.A. 2012, "Mine water treatment with limestone for sulfate removal", Journal of hazardous materials, 221–222(0): 45-55. doi:10.1016/j.jhazmat.2012.03.066 [9] Ulbricht, M. 2006, "Advanced functional polymer membranes", Polymer, 47(7): 2217-2262. doi:10.1016/j.polymer.2006.01.084 [10] Tang, C.Y., Kwon, Y. & Leckie, J.O. 2009, "Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: II. Membrane physiochemical properties and their dependence on polyamide and coating layers", Desalination, 242(1–3): 168-182. doi:10.1016/j.desal.2008.04.004 [11] Peyki, A., Rahimpour, A. & Jahanshahi, M. 2015, "Preparation and characterization of thin film composite reverse osmosis membranes incorporated with hydrophilic SiO 2 nanoparticles", Desalination, 368(0): 152-158. doi:10.1016/j.desal.2014.05.025 [12] Xu, Q., Yang, J., Dai, J., Yang, Y., Chen, X. & Wang, Y. 2013, "Hydrophilization of porous polypropylene membranes by atomic layer deposition of TiO 2 for simultaneously improved permeability and selectivity", Journal of Membrane Science, 448(0): 215-222. doi:10.1016/j.memsci.2013.08.018 [13] Shenvi, S.S., Isloor, A.M. & Ismail, A.F. 2015, "A review on RO membrane technology: Developments and challenges", Desalination, 368:10-26. doi:10.1016/j.desal.2014.12.042
20
[14] Maydannik, P.S., Kääriäinen, T.O., Lahtinen, K., Cameron, D. C., Söderlund, M., Soininen, P., Johansson, P., Kuusipalo, J., Moro, L. & Zeng, X. 2014, “Roll-to-roll ALD process for flexible electronics encapsulation applications”, J. Vac. Sci Technol. A, 32: 051603. doi:10.1116/1.4893428 [15] George, S.M. 2010 “Atomic layer deposition: an overview” Chemical Reviews, 110: 111–131. doi: 10.1021/cr900056b [16] Shafi H.Z., Khan Z., Yang R. & Gleason K.K. 2015, “Surface modification of reverse osmosis membranes with zwitterionic coating for improved resistance to fouling”, Desalination, 362(0): 93103. doi:10.1016/j.desal.2015.02.009 [17] Nikkola, J., Sievänen, J., Raulio, M., Wei, J., Vuorinen, J. & Tang, C.Y. 2014, "Surface modification of thin film composite polyamide membrane using atomic layer deposition method", Journal of Membrane Science, 450(0): 174-180. doi:10.1016/j.memsci.2013.09.005 [18] Moghimifar, V., Raisi, A. & Aroujalian, A. 2014, "Surface modification of polyethersulfone ultrafiltration membranes by corona plasma-assisted coating TiO2 nanoparticles", Journal of Membrane Science, 461: 69-80. doi:10.1016/j.memsci.2014.02.012 [19] Rudakova, A.V., Oparicheva, U.G., Grishina, A.E., Murashkina, A.A., Emeline, A.V. & Bahnemann, D.W. 2016, "Photoinduced hydrophilic conversion of hydrated ZnO surfaces", Journal of colloid and interface science, 466: 452-460. doi:10.1016/j.jcis.2015.08.015 [20] Li, M., Li, G., Jiang, J., Zhang, Z., Dai, X. & Mai, K. 2015, "Ultraviolet Resistance and Antimicrobial Properties of ZnO in the Polypropylene Materials: A Review", Journal of Materials Science & Technology, 31(4): 331-339. doi:10.1016/j.jmst.2014.11.022 [21] Srivastava M., Basu B.B.J. & Rajam K.S. 2011, “Improving the Hydrophobicity of ZnO by PTFE Incorporation”, Journal of Nanotechnology, 2011, Article ID 392754. doi:10.1155/2011/392754 [22] Shinde, V.R., Lokhande, C.D., Mane, R.S. & Han, S. 2005, “Hydrophobic and textured ZnO films deposited by chemical bath deposition: annealing effect”, Applied Surface Science, 245(1–4): 407-413. doi:10.1016/j.apsusc.2004.10.036 [23] Kääriäinen M.-L., Weiss C.K., Ritz S., Pütz S., Cameron D.C., Mailänder V. & Landfester K. 2013, “Zinc release from atomic layer deposited zinc oxide thin films and its antibacterial effect on Escherichia coli” Applied Surface Science, 287: 375– 380. doi:10.1016/j.apsusc.2013.09.162 [24] Pilakasiri, K., Molee, P., Sringernyuang, D., Dangjun, N., Channasanon, S. & Tan-odekaew, S. 2011, ”Efficacy of chitin-PAA-GTMAC gel in promoting wound healing: animal study” J. Mater. Sci. Mater. Med., 22: 2497. doi:10.1007/s10856-011-4420-6
21
[25] Sudheesh Kumar, P.T., Lakshmanan, V.-K., Anilkumar, T.V., Ramya, C., Reshmi, P., Unnikrishnan, A.G., Nair, S.V. & Jayakumar, R. 2012, ”Flexible and Microporous Chitosan Hydrogel/Nano ZnO Composite Bandages for Wound Dressing: In Vitro and In Vivo Evaluation”, ACS Appl. Mater. Interfaces 4(5): 2618–2629. doi: 10.1021/am300292v [26] Sudheesh Kumar P.T., Lakshmanan V.-K., Raj M., Biswas R., Hiroshi T., Nair S.V. & Jayakumar R. 2013, ”Evaluation of Wound Healing Potential of β-Chitin Hydrogel/Nano Zinc Oxide Composite Bandage” Pharm. Res., 30: 523-537. doi:10.1007/s11095-012-0898-y [27] Hairom, N.H.H., Mohammad, A.W. & Kadhum, A.A.H. 2014, "Effect of various zinc oxide nanoparticles in membrane photocatalytic reactor for Congo red dye treatment", Separation and Purification Technology, 137(0): 74-81. doi:10.1016/j.seppur.2014.09.027 [28] Rabiee, H., Vatanpour, V., Farahani, M.H.D.A. & Zarrabi, H. 2015, "Improvement in flux and antifouling properties of PVC ultrafiltration membranes by incorporation of zinc oxide (ZnO) nanoparticles", Separation and Purification Technology, 156(2): 299-310. doi:10.1016/j.seppur.2015.10.015 [29] Gomes, S., Cavaco, S.A., Quina, M.J. & Gando-Ferreira, L.M. 2010, "Nanofiltration process for separating Cr(III) from acid solutions: Experimental and modelling analysis", Desalination, 254(1–3): 80-89. doi:10.1016/j.desal.2009.12.010 [30] Hilal, N., Kochkodan, V., Al Abdulgader, H. & Johnson, D. 2015, "A combined ion exchange– nanofiltration process for water desalination: II. Membrane selection", Desalination, 363,(0): 51-57. doi:10.1016/j.desal.2014.11.017 [31] Rizwan T. & Bhattacharjee S. 2007, “Initial Deposition of Colloidal Particles on a Rough Nanofiltration Membrane”, The Canadian Journal of Chemical Engineering, 85: 570-579. doi: 10.1002/cjce.5450850502 [32] Nghiem L.D., Schäfer A.I., Elimelech M. 2005, “Pharmaceutical Retention Mechanisms by Nanofiltration Membranes”, Environ. Sci. Technol. 39: 7698-7705. doi: 10.1021/es0507665 [33] Akbari, A., Aliyarizadeh, E., Mojallali Rostami, S.M. & Homayoonfal, M. 2016, "Novel sulfonated polyamide thin-film composite nanofiltration membranes with improved water flux and anti-fouling properties", Desalination 377: 11-22. doi:10.1016/j.desal.2015.08.025 [34] Vatanpour, V., Shockravi, A., Zarrabi, H., Nikjavan, Z. & Javadi, A. 2015, "Fabrication and characterization of anti-fouling and anti-bacterial Ag-loaded graphene oxide/polyethersulfone mixed matrix membrane", Journal of Industrial and Engineering Chemistry, 30: 342-352. doi:10.1016/j.jiec.2015.06.004
22
[35] Jhaveri, J.H. & Murthy, Z.V.P. 2016, "A comprehensive review on anti-fouling nanocomposite membranes for pressure driven membrane separation processes", Desalination, 379: 137-154. doi:10.1016/j.desal.2015.11.009 [36] Mänttäri, M., Pekuri, T. & Nyström, M. 2004, "NF270, a new membrane having promising characteristics and being suitable for treatment of dilute effluents from the paper industry" Journal of Membrane Science, 242(1–2): 107-116. doi:10.1016/j.memsci.2003.08.032 [37] Vrijenhoek, E.M., Hong, S. & Elimelech, M. 2001, "Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes", Journal of Membrane Science, 188(1): 115-128. doi:10.1016/S0376-7388(01)00376-3 [38] Hirose, M., Ito, H. & Kamiyama, Y. 1996, "Effect of skin layer surface structures on the flux behaviour of RO membranes", Journal of Membrane Science, 121(2): 209-215. doi:10.1016/S0376-7388(96)00181-0 [39] Schäfer, A.I., Andritsos, N., Karabelas, A.J., Hoek, E.M.V., Schneider, R. & Nyström, M. 2004, “Fouling in Nanofiltration, in: Nanofiltration – Principles and Applications”, Schäfer A.I., Waite T.D., Fane A.G. (Eds). Elsevier, Chapter 20, 169-239. [40] Childress, A.E. & Elimelech, M. 1996, "Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes", Journal of Membrane Science, 119(2): 253-268. doi:10.1016/0376-7388(96)00127-5 [41] Xu, J., Chen, G., Huang, X., Li, G., Liu, J., Yang, N. & Gao, S. 2009, "Iron and manganese removal by using manganese ore constructed wetlands in the reclamation of steel wastewater", Journal of hazardous materials, 169(1–3): 309-317. doi:10.1016/j.jhazmat.2009.03.074 [42] Mänttäri, M. & Nyström, M. 2006, "Negative retention of organic compounds in nanofiltration", Desalination, 199(1–3): 41-42. doi:10.1016/j.desal.2006.03.015 [43] Pérez-González A., Ibáñez R., Gómez P., Urtiaga AM, Ortiz I & Irabien JA (2015) Nanofiltration separation of polyvalent and monovalent anions in desalination brines. Journal of Membrane Science 473: 16-27. doi:10.1016/j.memsci.2014.08.045 [44] Rana, D. & Matsuura, T. 2010. Surface modifications for antifouling membranes. Chemical Reviews 110: 2448–2471. doi: 10.1021/cr800208y
23
Highlights Atomic layer deposition was applied for coating of NF membranes with ZnO. Membranes were applied for purification of real mine wastewaters. Rejection of SO4, NO3, Cl and Mn as well as membrane fouling tendency were studied.
24