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Effects of cations on the formation of ultrafiltration membrane fouling layers when filtering fulvic acid
Desalination 352 (2014) 174–180
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Effects of cations on the formation of ultrafiltration membrane fouling layers when filtering fulvic acid Congcong Tang a,b, Zhangwei He a,b, Fangbo Zhao a,b,⁎, Xiaoyang Liang a, Zhanshuang Li a a b
Key Laboratory of Superlight Materials and Surface Technology, MOE, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China State Key Laboratory of Urban Water Resources and Environment, Harbin Institute of Technology, Harbin 150090, China
H I G H L I G H T S • • • •
Membrane fouling caused by FAs with existence of cations has been investigated. Monovalent cations (K+ & Na+) caused membrane pore blocking by forming a gel layer. Divalent cations reacted with FAs and formed a cake layer with porous structures. Different valence states of ions could influence the NOM in different ways.
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Article history: Received 7 June 2014 Received in revised form 19 August 2014 Accepted 22 August 2014 Keywords: Fulvic acid Ultrafiltration membrane fouling Divalent cations Monovalent cations Cake layer Gel layer
a b s t r a c t Fulvic acid (FA) is the main water-soluble component in humic substances which usually cause membrane fouling in the drinking water treatment process. This study investigates the ultrafiltration membrane fouling characteristics and mechanisms caused by FAs in the presence of various salt ions (Ca2+, Mg2+, K+ and Na+). A series of experiments had been carried out to characterize the decline of permeate flux, pollutant rejection, FA aggregates, and the morphology of the membrane's fouling layer. Compared with pure FA solution, FAmonovalent solution containing cations (K+ and Na+) caused much lower permeate flux and smaller amount of fouling layer, which was difficult to remove by flushing with water. The presence of divalent cations (Ca2+ and Mg2+), caused the permeate flux to noticeably increase and membrane fouling can be easily removed. Laser particle size analysis showed that the FA formed larger aggregates in the presence of Ca2+ or Mg2+ cations. Based on an atomic force microscope and scanning electron microscope analysis, results showed that monovalent cations made the colloids form a gel layer, which led to pore blocking and permeate flux decrease. And divalent cations reacted with FA and formed a cake layer of larger particles creating porous structures on the membrane surface. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Natural organic matter (NOM) causes odor, chromaticity, biological instability and corrosion in water distribution networks. It is a major precursor of halogenated by-products and creates disinfection byproducts (DBPs) which contain trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs) and mutagens (MX). Most types of DBPs increase the mutagenicity of drinking water [1,2]. In addition, NOM can form complexes with pesticides and heavy metals, which increase the biological accumulation and persistence of those substances [3,4].
⁎ Corresponding author at: Key Laboratory of Super Light Materials and Surface Technology, MOE, College of Materials Science and Chemical Engineering, Harbin Engineering University, 145 Nantong Street, Harbin 150001, China. Tel.: + 86 15114587583. E-mail address: [email protected] (F. Zhao).
http://dx.doi.org/10.1016/j.desal.2014.08.020 0011-9164/© 2014 Elsevier B.V. All rights reserved.
It is well known that, humic substance (HS) is a kind of NOM widely existing in nature. Earlier research has shown that HS is the main component of NOM and is the major organic constituent of soil (humus), peat, coal, dystrophic lakes and oceanwater [5–7]. Humic substances can be divided into three main groups: humic acid (HA), fulvic acid (FA) and humin. Humin is insoluble in dilute alkalis and acids [8]. HAs and FAs are the major organic constituents in water sources. Humic and fulvic acids can be extracted as colloid from soil and other solid phase sources in a strongly basic solution of sodium hydroxide or potassium hydroxide. A typical difference between humic and fulvic acids is that HAs are precipitated from water solution by adjusting the pH to 1 with hydrochloric acid, on the contrary, FAs are still present in the solution [9]. The atomic numbers of the elements C, H, O and N, and the corresponding relative percentage contents of each element are 45–60%, 25–45%, 4–7% and 1.5–4%, respectively [10]. Rice et al. have reported that the H/C ratio can characterize the aromaticity and fat content of
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humus molecules [10]. The C/N ratio of HS is related to its humification degree. When C/N ratio is higher, the humification degree is lower [11]. FA has an aliphatic and aromatic molecule, with three functional groups, carboxyl (COOH−), hydroxyl (OH−) and aldehyde (C_O). Compared with HA, FA has a higher humification degree, and contains more oxygen functional groups and aliphatic structures. A previous study has shown that the number of functional groups from the above list contributes to physical and chemical properties of FA [12]. Thus, FA is the main fraction of NOM contained in the water body because of its higher solubility and stability [13,14]. Over time, each component of NOMs moves toward stability. That is, substances such as polysaccharide and protein can be broken down and used, while substances like lignins are stable after the humification over a long period of time [15]. Compared with other kinds of NOM, the influence of FA in an aqueous solution is more durable, stable and harder to treat [14]. Membrane separation technology has become increasingly important as a removal technology for FAs because of its advantages such as high separation efficiency, a small physical footprint and convenient operation and control. Many FAs in aqueous solutions have sizes close to 1 nm, so they can be removed effectively by reverse osmosis (RO) and nanofiltration (NF) technologies. However, RO and NF membranes are high pressure membrane technologies which can lead to high energy consumption [16–18]. Ultrafiltration (UF) can partly remove some FAs, even though the molecular weight of the FA is often lower than the molecular weight cut-off of the UF membrane. Many researchers have reported that combined processes including membrane filtration technology showed high efficiency in removing dissolved components of NOMs (i.e. a mixture of FAs and HAs) [19–21]. Compared with NF or RO technology, UF technology has a great development potential that will support low carbon emissions and sustainable development objectives. In the filtration process of FAs with a UF membrane, membrane fouling is influenced by the surrounding environment, especially ions. This kind of membrane fouling is both inconvenient and significant. Both Kloster et al. [22] and Baalousha et al. [23] have reported the divalent ion effects on the form and solvency of the HA and FA molecules. In addition, Roger et al. have reported the monovalent ion and divalent ion effects on the aggregation and the central charge of the HA and FA in an aqueous solution [24]. Moreover, there are reports that the interaction between some organic compounds and humic substances is influenced by calcium ions (Ca2+) present on the membrane [25]. However, there is little information or consensus about the effect of salt ions [26,27]. This is an area where additional information is necessary to gain a complete and detailed understanding of how various salt ions affect organic fouling mechanisms. The molecular weight and hydrophily/hydrophobicity of NOMs are additional factors that influence membrane fouling [28,29]. Humic substance has a complex and diverse structure because of its different sources. Thus, the membrane fouling mechanism or effect of each factor is not well understood. That is, the organic fouling, which is believed to be caused by NOMs from the source waters, is not well explained [27]. In order to investigate the relationship between organic fouling and the complex or unknown speciation in NOMs, Zularisam et al. have reviewed limited details about the behavior of NOMs [26]. With more stable and consistent structure [14], FA in aqueous solutions is more easily identified as target pollutants. In this work, we have conducted a preliminary evaluation of FA fouling on UF membranes when influenced by various kinds of ions using macro phenomena (the observation of permeate flux, back-flushing and FA rejection) and the microanalysis (the characterization on membrane fouling). This study gives theoretical support to interactions between FA and various salt ions and lays a primary investigation for the change of FA and the formation for its membrane fouling in real situations with mixed salt ions.
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2. Materials and methods 2.1. Chemicals Fulvic acid (FA) was provided by Mudanjiang Fengda Chemical Corporation (China) and dissolved in deionized water at a concentration of 10 mg/L. The molecular formula of FA was C14H12O8 and its molecular weight was 308. To study the effects of different salts, mixed FA/inorganic salts were prepared with analytical grade NaCl, KCl, MgCl2 and CaCl2. The salt concentrations in these mixed-solute solutions were fixed at 0.01 mM, 0.1 mM, 1 mM and 10 mM while the FA concentration was kept at 10 mg/L. The pH value of each sample was 6 ± 0.2. 2.2. Membrane module Filtration/backwash experiments were carried out with a polyvinyl chloride (PVC) UF membrane produced by Shanghai SINAP Membrane Tech Co., Ltd., Shanghai, China. The molecular weight cut-off (MWCO) of the UF membrane in this study is 150 kDa and the thickness of the membrane is 0.1 mm. The UF membrane was fixed in a filter tank in the center of the membrane module. The membrane area is 25.07 cm2. 2.3. Filtration experiments Filtration experiments were carried out using a cross-flow filtration unit. The lab-scale apparatus consists of a plate UF membrane module. The feed vessel (∼350 mL), which contained the feed solutions of each set, was connected by piping to a pump to obtain the desired operating pressure (0.01 MPa) for filtration. Clean water membrane flux at this pressure was found to be 150 ± 5 L/m2·h. Permeate was collected in a beaker placed on an electronic balance connected to a computer for automatic data acquisition. The smaller vessel (∼150 mL) pressurized with air, contained deionized water which was used in the backwashing step. The feed solution for the filtration process was pumped to the membrane module from a feed tank. The liquid flow was tangent to the membrane surface and permeate was gathered in a beaker (3). After filtration, concentrated solution was collected in a smaller vessel (4) (∼150 mL). All samples of the permeate solutions were taken after a 30-min filtration run. Every set of data was collected three times to guarantee the integrity and repeatability of the experiment. Back-flushing and chemical cleaning were used to clean membrane from reversible and irreversible fouling. For the back-flushing process, the deionized water was pumped with air into the membrane module to clean the membrane. Chemical cleaning (NaOH/HCl cleaning) involved three steps: first, 1% NaOH cleaning, second, water cleaning and finally, 37% HCl cleaning. All experiments were carried out at room temperature of 20 ± 2 °C. Fig. 1 shows the whole ultrafiltration system. 2.4. Characterization methods Concentrations of FAs in both feed and permeate solutions were determined by their absorbency on 311 nm using an ultraviolet–visible spectrophotometer. A calibration curve of FA solutions has been prepared. Thus the absorbance could be determined directly and a working curve could be also drawn. The response observed was linear over a limited range. The grain size distribution of the FA solution is determined by a laser particle size analyzer (LPS). The brand of LPS is MASTERSIZER 2000 (UK) and the method of sample preparation is the same as the method of solution preparation. After filtration of each sample, membrane surface is characterized by an atomic force microscope (AFM) and a scanning electron microscopy energy dispersive spectrometer (SEM–EDS). The type of AFM is BioScope (Germany), working at a tapping mode. And
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Fig. 1. The whole ultrafiltration system. *Note: 1. Feed tank (V ~ 350 mL); 2. Backwash tank (V ~ 150 mL); 3. Permeate; 4. Backwash water drain; 5. Plate membrane module.
each sample (small piece of membrane) has been pasted on the sample holder and its surface topography has been scanned. The type of SEM– EDS is JSM-6480A (Japan). Each sample has been pasted on the stage using conducting resin and treated by vacuum drying and spray-gold (for 12 min). SEM–EDS is used to study the microstructure and elements of membrane contaminant. 3. Results and discussion 3.1. The effect on membrane flux of adding cations to the FA solution Filtration tests were performed with FA solutions prepared as described in the previous section. Membrane flux as a function of time for various salt ion concentrations is shown in Fig. 2. The influences of K+ and Ca2 + in different concentrations were investigated. Both
(a)
calcium and potassium influenced the rate of flux decline. In Fig. 2(a), as the potassium concentration was increased from 0 to 10 mM, the observed flux decline became more and more severe. Meanwhile, as shown in Fig. 2(b), the trend of flux decline was reversed in the presence of calcium. When the calcium concentration is increased from 0 to 10 mM, the observed flux was gradually increased. This behavior for the UF membrane is in contrast with previous findings with HAs which exhibit a wider range of particle size distribution, and where a dramatic increase in flux has been observed only at the highest end of calcium concentration (more than 4 mM) in Katsoufidou's study [30]. This can be explained by the different properties between HAs and FAs. Compared with HA, the FA has a higher humification degree and a more uniform molecular size. Also, FA has greater complexing ability with metal ions [31]. In order to determine the property of monovalent cations and divalent cations, the flux variations of 4 kinds of FA/electrolyte (K+, Na+, Ca2+ and Mg2+) solutions have been investigated. As shown in Fig. 3, the trends of the flux variations were consistent with the variations of ion valence in solutions. The addition of monovalent cations (K+ and Na+) could decrease the FA flux while the addition of divalent cations (Ca2+ and Mg2+) could increase the FA flux. The impact of salt ions on FA flux variations was as follows: Mg2+ b Ca2+ and K+ b Na+. This phenomenon shows that the mechanism of divalent cations or monovalent cations affected on FAs follows a single rule, respectively. The influence between divalent cations and monovalent cations on FA is different. FA aggregates are larger and may form a cake layer on the membrane surface with a relatively porous structure in the presence of divalent cations. Meanwhile, FA aggregates are much smaller in the presence of monovalent cations and may form a cohesive gel layer on the membrane surface.
(b)
Fig. 2. Filtration time vs. the flux ratio of the mixed-solute (FA and electrolyte) solution, for various salt concentrations. *Note: added inorganic electrolytes: (a) KCl; (b) CaCl2.
Fig. 3. Filtration time vs. the flux ratio of the pure FA solution and mixed-solute solutions, for various kinds of electrolytes (10 mM).
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3.2. Effects on membrane fouling when cations are added to the FA solution Previous research has indicated that, metal acting on HS containing FA can dramatically alter the properties of both the cations and HS by increasing the dissolution kinetics of the HS [32], inducing the aggregation of the HS or the mobilizing of either HS or metal previously adsorbed to mineral particles [22,33,34]. Back-flushing experiments have been performed to show the different types of membrane fouling affected by 4 kinds of salts. The clean water fluxes were shown after filtration, back-flushing and NaOH/HCl cleaning in Fig. 4. As it can be seen, the clean water flux recovery of FA/divalent salt solutions after back-flushing was much higher than that of the FA solution. The clean water flux recovery of FA/monovalent salt solutions after back-flushing was lower than that of FA solution. In addition, the clean water flux recovery of all kinds of solutions was nearly the same after NaOH/HCl cleaning. These results indicates that the membrane fouling affected by divalent cations and FA together is a little easier to wash off than that affected by FA only. Meanwhile, the membrane fouling created by monovalent cations and FA together is more difficult to wash off than that affected by FA only. Moreover, the membrane fouling of all solutions in these experiments can be washed away easily by NaOH/HCl solutions. That is to say, the addition of monovalent cations can lead to more irreversible fouling of FA. Thus, the membrane fouling of FA influenced by monovalent cations is more serious than that affected by other kinds of salt ions. FA rejections by the membrane in different concentrations and kinds of salts are shown in Fig. 5. The filtration time at which the retention was measured was identical. The apparent rejection of solutes (R) is C defined as: R ¼ 1− C pf , where Cp is the permeate concentration and Cf is the solute concentration in the feed solution. As shown in Fig. 5, the original rejection of FA was about 20% in a pure FA solution. In general, both monovalent cations and multivalent cations can increase the rejection of FA to a different extent. As expected, multivalent cations affected the rejection of FA obviously and reached a maximum value of about 86% in high Ca2+ concentration (10 mM). In general, the rejection of FA affected by monovalent cations is smaller, which is close to about 30%. FA may diffuse through the membrane pores and then with higher FA concentration in the permeate as permeate flux is reduced. According to the solute rejection calculation, rejections of the FA in mixsolute solutions can be reduced in parallel to reduction in permeate flux. In the presence of monovalent cations, the amount of FA in the permeated solution (1-R) is almost doubled compared to that in the presence of multivalent cations. Lower permeate flux (with Na+ and K+), may partially lead to the higher concentration of FA in the permeated solution, which eventually elevate the FA passage as calculated from the concentrations of FA in the feed and permeate (not the absolute mass passage of FA). So the variation of mass transfer of FA molecules by diffusion is one of the possible reasons leading to the variation of FA rejection.
Fig. 4. The mean water flux ratio vs. various kinds of pure FA solution and mix-solute solutions.
Fig. 5. Rejection of fulvic acid by ultrafiltration membrane during the filtration cycles vs. the electrolyte concentration in the various mixed-solute solutions.
From macro perspectives, the rejection, flux variation and flux recovery rate of fulvic acid in different salt solutions have been observed. And the effects of monovalent ions and multivalent ions were different and have certain regularity. So the microstructure characterization of the membrane fouling in different situations is necessary.
3.3. Characterization of the effects of membrane fouling by different cations The size distribution of FA with different kinds of salt ions added obtained by laser particle size analyzers is shown in Fig. 6. The concentration of every kind of salt ions was 10 mM in each mix-solute solution. It is observed that FA particles are uniform and below the nominal MWCO of the membrane. Based on the order of magnitude of its size distribution, FA molecules dissolve in water in the form of colloids [35]. However, as salt ions were added in the feed solutions the aggregation tendency and the particle size ranges of FAs were increased. The destabilization of the FA molecular colloid could be observed in the presence of salt ions. Indeed, as shown in Fig. 6, the influence of salt ions on FA solutions is based on the following order: Ca2 + N Mg2 + N K+ N Na+, which was compliant with the Hofmeister series [36]. The influence of multivalent cations on the aggregation tendency of FA was much stronger than that of monovalent cation. The addition of multivalent cations (Ca2+ and Mg2+) made the particle size ranges of fulvic acid aggregates reach to over 10 μm while the addition of monovalent cations (K+ and Na+) made it reach to only about 0.1 μm. With the aggregation of FA particles, the individual FA molecules reduced (almost disappeared in the presence of multivalent cations). This behavior corresponds to a previous study showing that the aggregation of humic substances is less affected by monovalent cations [24]. But the fouling tendency in mix-solute solution (FA + Na+ or FA + K+) is more serious than that in a pure FA solution. In the presence of monovalent cations, the FA aggregates are much smaller (compared with the
Fig. 6. The grain size distribution of the commercial fulvic acid obtained by LPS.
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aggregates in the presence of multivalent cations) and may form a cohesive gel layer on the membrane surface. So the membrane flux of FAs decreased (shown in Fig. 3). In addition, in the pure FA solution, the original rejection of FA was low (shown in Fig. 5), which indicated that the molecular size of FA was much smaller than the pore size of the membrane used in this experiment. Thus, most of the FA molecules transport through the membrane and dissolve in water. With the addition of monovalent cations, the distribution of FA particles is uneven, destabilized and its particle sizes increase slightly, which narrows the gap between FA particles and membrane pores. Thus the FA particles affected by monovalent cations has a larger impact on membrane pores, i.e., is more likely to lead to pore blocking. As the influence of multivalent cations on the aggregation tendency of FA is much stronger, a majority of particle sizes for FA aggregates are larger than the MWCO of
the membrane. Therefore, FA aggregates can be rejected by the membrane efficiently and primarily build up at the membrane surface with a relatively porous structure. In general, monovalent cations may decrease the permeate flux, leading to greater pore blocking or enhancing the mass transfer of FA by diffusion. The two factors may both influence the variation of FA rejection. Multivalent cations may increase the FA rejection visibly by increasing the aggregation tendency of FA. AFM analyses were performed to compare morphological changes between fouled membranes affected by different salt ions. Fig. 7 displays 3-D views of the membrane in each solution and shows its surface roughness respectively. From the image, the clean membrane, the membrane fouled with pure FA solution and the membrane fouled with FA/salts solutions are sequentially shown. The average roughness parameters calculated for each membrane were 63.651, 77.742,
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 7. AFM images of clean and fouled membranes. *Note: (a) clean membrane surface; (b) fouled membrane affected by pure FA (FA); (c) added NaCl; (d) added KCl; (e) added MgCl2; (f) added CaCl2. The X and Y dimensions are both 20 μm (5 μm/division), while the Z-scale is 2000, 2000, 3000, 3000, 4000 and 4000 nm, respectively.
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166.78, 168.46, 316.95 and 377.71 nm, respectively from Fig. 7(a) to (f). The fouled membrane surfaces appeared rougher than the clean membrane surfaces. Furthermore, the roughness variation of fouled membrane surface was affected by the salt ions based on the degree of FA aggregation affected by salt ions, i.e. Ca2+ N Mg2 + N K+ N Na+. This fact indicates that salt ions could increase the amount of membrane fouling layer on the micro level. In addition, the 3-D structures of fouled membrane affected by multivalent cations (Ca2 + and Mg2 +) and monovalent cations (K+ and Na+) were different. Though the roughness of the fouled membrane
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affected by multivalent cations (Ca2+ and Mg2+) is greater and the 3-D views exhibit a surface coverage with a “higher topography”, the structure of its pollution layer is more porous in the fouling layer. Moreover, the structure of the fouled membrane affected by monovalent cations (K+ and Na+) is compact and there is almost no gap, indicating that the membrane flux decline affected by the monovalent cations is more serious. SEM images provided additional detailed information on structural features of the UF membrane. Fig. 8 reveals the metal elements of the membrane pollutant. From SEM–EDS, the metal elements contained in
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(b)
(c)
(d)
Fig. 8. SEM–EDS images of deposits on fouled membranes after the filtration experiments without a backwash.
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membrane contamination are sequentially shown. The membrane contamination contains multivalent cations (Ca2+ and Mg2+) and has no monovalent cations (K+ and Na+). There are different possible explanations for this phenomenon. Compared with the pure FA solution, FA molecules were induced to slightly more aggregation in the presence of monovalent ions. The addition of monovalent cations (K+ and Na+) may have increased the ionic strength of the FA solution, which compressed the electric double layer and induced the FA molecules to aggregate. Previous researchers have also reported a similar phenomenon showing that monovalent cations can lead to the aggregation of other macro molecule matter such as fullerene (C60) [37]. The membrane fouling affected by multivalent cations (Ca2 + and Mg2+) is much greater and thicker. Elemental calcium and magnesium have been found in the fouled layer respectively. The magnesium content is slightly more than the calcium content in the fouled layer. This phenomenon is analogous to the scale formation phenomenon by the hardness of ions (Ca2 + and Mg2+). In other words, divalent ions can be combined with macro molecule matter to form deposits easily. 4. Conclusions In this work, an experimental laboratory scale apparatus using a PVC ultrafiltration membrane was used to study the influence of ultrafiltration membrane fouling affected by four kinds of inorganic salts. Two important parameters are the kinds and concentrations of electrolytes (Na+, K+, Mg2 + and Ca2 +). These two factors promote the intraregional and molecular forces between FA molecules and accelerate their aggregation, thereby leading to flux decline, fouling reversibility and FA rejection. But the mechanisms and experimental observations of fouling between monovalent cations (Na+ and K+) and divalent cations (Mg2+ and Ca2+) were different. From macro and micro perspective, a more complete and detailed explanation of the pollution mechanism has been developed which matches the experimental observations. The aggregation of FA was less affected by monovalent cations than that by multivalent cations. Monovalent cations (Na+ and K+) could influence fulvic acid to form a dense fouling layer. Meanwhile, the multivalent cations (Mg2 + and Ca2 +) could be combined with fulvic acid molecules to form deposits with porous structures. Monovalent cations mainly affected the membrane flux decline while multivalent cations primarily improved the rejection of FA. In this study, it is suggested that different valence states of ions could influence the NOM in different ways. Acknowledgments This work was supported by the National Natural Science Foundation of China (51108112), the Natural Science Foundation of Heilongjiang Province (E201252), the Fundamental Research Funding of Harbin Engineering University (HEUFT06029), and the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (ESK201004). Also, some of the modeling work was carried out with the help of Dr. Jaehong Kim in Georgia Institute of Technology, USA. References [1] B. Bolto, D. Dixon, R. Eldridge, Removal of natural organic matter by ion exchange, Water Res. 36 (2002) 5057–5065. [2] S. Vreysen, A. Maes, Adsorption mechanism of humic and fulvic acid onto Mg/Al layered double hydroxides, Appl. Clay Sci. 38 (2008) 237–249. [3] L. Zhao, F. Luo, J.M. Wasikiewicz, Adsorption of humic acid from aqueous solution onto irradiation-crosslinked carboxymethyl chitosan, Bioresour. Technol. 99 (2008) 1911–1917. [4] J.N. Wang, Y. Zhou, A.M. Li, L. Xu, Adsorption of humic acid by bi-functional resin JN10 and the effect of alkali-earth metal ions on the adsorption, J. Hazard. Mater. 176 (2010) 1018–1026.
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Effects of cations on the formation of ultrafiltration membrane fouling layers when filtering fulvic acid Congcong Tang a,b, Zhangwei He a,b, Fangbo Zhao a,b,⁎, Xiaoyang Liang a, Zhanshuang Li a a b
Key Laboratory of Superlight Materials and Surface Technology, MOE, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China State Key Laboratory of Urban Water Resources and Environment, Harbin Institute of Technology, Harbin 150090, China
H I G H L I G H T S • • • •
Membrane fouling caused by FAs with existence of cations has been investigated. Monovalent cations (K+ & Na+) caused membrane pore blocking by forming a gel layer. Divalent cations reacted with FAs and formed a cake layer with porous structures. Different valence states of ions could influence the NOM in different ways.
a r t i c l e
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Article history: Received 7 June 2014 Received in revised form 19 August 2014 Accepted 22 August 2014 Keywords: Fulvic acid Ultrafiltration membrane fouling Divalent cations Monovalent cations Cake layer Gel layer
a b s t r a c t Fulvic acid (FA) is the main water-soluble component in humic substances which usually cause membrane fouling in the drinking water treatment process. This study investigates the ultrafiltration membrane fouling characteristics and mechanisms caused by FAs in the presence of various salt ions (Ca2+, Mg2+, K+ and Na+). A series of experiments had been carried out to characterize the decline of permeate flux, pollutant rejection, FA aggregates, and the morphology of the membrane's fouling layer. Compared with pure FA solution, FAmonovalent solution containing cations (K+ and Na+) caused much lower permeate flux and smaller amount of fouling layer, which was difficult to remove by flushing with water. The presence of divalent cations (Ca2+ and Mg2+), caused the permeate flux to noticeably increase and membrane fouling can be easily removed. Laser particle size analysis showed that the FA formed larger aggregates in the presence of Ca2+ or Mg2+ cations. Based on an atomic force microscope and scanning electron microscope analysis, results showed that monovalent cations made the colloids form a gel layer, which led to pore blocking and permeate flux decrease. And divalent cations reacted with FA and formed a cake layer of larger particles creating porous structures on the membrane surface. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Natural organic matter (NOM) causes odor, chromaticity, biological instability and corrosion in water distribution networks. It is a major precursor of halogenated by-products and creates disinfection byproducts (DBPs) which contain trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs) and mutagens (MX). Most types of DBPs increase the mutagenicity of drinking water [1,2]. In addition, NOM can form complexes with pesticides and heavy metals, which increase the biological accumulation and persistence of those substances [3,4].
⁎ Corresponding author at: Key Laboratory of Super Light Materials and Surface Technology, MOE, College of Materials Science and Chemical Engineering, Harbin Engineering University, 145 Nantong Street, Harbin 150001, China. Tel.: + 86 15114587583. E-mail address: [email protected] (F. Zhao).
http://dx.doi.org/10.1016/j.desal.2014.08.020 0011-9164/© 2014 Elsevier B.V. All rights reserved.
It is well known that, humic substance (HS) is a kind of NOM widely existing in nature. Earlier research has shown that HS is the main component of NOM and is the major organic constituent of soil (humus), peat, coal, dystrophic lakes and oceanwater [5–7]. Humic substances can be divided into three main groups: humic acid (HA), fulvic acid (FA) and humin. Humin is insoluble in dilute alkalis and acids [8]. HAs and FAs are the major organic constituents in water sources. Humic and fulvic acids can be extracted as colloid from soil and other solid phase sources in a strongly basic solution of sodium hydroxide or potassium hydroxide. A typical difference between humic and fulvic acids is that HAs are precipitated from water solution by adjusting the pH to 1 with hydrochloric acid, on the contrary, FAs are still present in the solution [9]. The atomic numbers of the elements C, H, O and N, and the corresponding relative percentage contents of each element are 45–60%, 25–45%, 4–7% and 1.5–4%, respectively [10]. Rice et al. have reported that the H/C ratio can characterize the aromaticity and fat content of
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humus molecules [10]. The C/N ratio of HS is related to its humification degree. When C/N ratio is higher, the humification degree is lower [11]. FA has an aliphatic and aromatic molecule, with three functional groups, carboxyl (COOH−), hydroxyl (OH−) and aldehyde (C_O). Compared with HA, FA has a higher humification degree, and contains more oxygen functional groups and aliphatic structures. A previous study has shown that the number of functional groups from the above list contributes to physical and chemical properties of FA [12]. Thus, FA is the main fraction of NOM contained in the water body because of its higher solubility and stability [13,14]. Over time, each component of NOMs moves toward stability. That is, substances such as polysaccharide and protein can be broken down and used, while substances like lignins are stable after the humification over a long period of time [15]. Compared with other kinds of NOM, the influence of FA in an aqueous solution is more durable, stable and harder to treat [14]. Membrane separation technology has become increasingly important as a removal technology for FAs because of its advantages such as high separation efficiency, a small physical footprint and convenient operation and control. Many FAs in aqueous solutions have sizes close to 1 nm, so they can be removed effectively by reverse osmosis (RO) and nanofiltration (NF) technologies. However, RO and NF membranes are high pressure membrane technologies which can lead to high energy consumption [16–18]. Ultrafiltration (UF) can partly remove some FAs, even though the molecular weight of the FA is often lower than the molecular weight cut-off of the UF membrane. Many researchers have reported that combined processes including membrane filtration technology showed high efficiency in removing dissolved components of NOMs (i.e. a mixture of FAs and HAs) [19–21]. Compared with NF or RO technology, UF technology has a great development potential that will support low carbon emissions and sustainable development objectives. In the filtration process of FAs with a UF membrane, membrane fouling is influenced by the surrounding environment, especially ions. This kind of membrane fouling is both inconvenient and significant. Both Kloster et al. [22] and Baalousha et al. [23] have reported the divalent ion effects on the form and solvency of the HA and FA molecules. In addition, Roger et al. have reported the monovalent ion and divalent ion effects on the aggregation and the central charge of the HA and FA in an aqueous solution [24]. Moreover, there are reports that the interaction between some organic compounds and humic substances is influenced by calcium ions (Ca2+) present on the membrane [25]. However, there is little information or consensus about the effect of salt ions [26,27]. This is an area where additional information is necessary to gain a complete and detailed understanding of how various salt ions affect organic fouling mechanisms. The molecular weight and hydrophily/hydrophobicity of NOMs are additional factors that influence membrane fouling [28,29]. Humic substance has a complex and diverse structure because of its different sources. Thus, the membrane fouling mechanism or effect of each factor is not well understood. That is, the organic fouling, which is believed to be caused by NOMs from the source waters, is not well explained [27]. In order to investigate the relationship between organic fouling and the complex or unknown speciation in NOMs, Zularisam et al. have reviewed limited details about the behavior of NOMs [26]. With more stable and consistent structure [14], FA in aqueous solutions is more easily identified as target pollutants. In this work, we have conducted a preliminary evaluation of FA fouling on UF membranes when influenced by various kinds of ions using macro phenomena (the observation of permeate flux, back-flushing and FA rejection) and the microanalysis (the characterization on membrane fouling). This study gives theoretical support to interactions between FA and various salt ions and lays a primary investigation for the change of FA and the formation for its membrane fouling in real situations with mixed salt ions.
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2. Materials and methods 2.1. Chemicals Fulvic acid (FA) was provided by Mudanjiang Fengda Chemical Corporation (China) and dissolved in deionized water at a concentration of 10 mg/L. The molecular formula of FA was C14H12O8 and its molecular weight was 308. To study the effects of different salts, mixed FA/inorganic salts were prepared with analytical grade NaCl, KCl, MgCl2 and CaCl2. The salt concentrations in these mixed-solute solutions were fixed at 0.01 mM, 0.1 mM, 1 mM and 10 mM while the FA concentration was kept at 10 mg/L. The pH value of each sample was 6 ± 0.2. 2.2. Membrane module Filtration/backwash experiments were carried out with a polyvinyl chloride (PVC) UF membrane produced by Shanghai SINAP Membrane Tech Co., Ltd., Shanghai, China. The molecular weight cut-off (MWCO) of the UF membrane in this study is 150 kDa and the thickness of the membrane is 0.1 mm. The UF membrane was fixed in a filter tank in the center of the membrane module. The membrane area is 25.07 cm2. 2.3. Filtration experiments Filtration experiments were carried out using a cross-flow filtration unit. The lab-scale apparatus consists of a plate UF membrane module. The feed vessel (∼350 mL), which contained the feed solutions of each set, was connected by piping to a pump to obtain the desired operating pressure (0.01 MPa) for filtration. Clean water membrane flux at this pressure was found to be 150 ± 5 L/m2·h. Permeate was collected in a beaker placed on an electronic balance connected to a computer for automatic data acquisition. The smaller vessel (∼150 mL) pressurized with air, contained deionized water which was used in the backwashing step. The feed solution for the filtration process was pumped to the membrane module from a feed tank. The liquid flow was tangent to the membrane surface and permeate was gathered in a beaker (3). After filtration, concentrated solution was collected in a smaller vessel (4) (∼150 mL). All samples of the permeate solutions were taken after a 30-min filtration run. Every set of data was collected three times to guarantee the integrity and repeatability of the experiment. Back-flushing and chemical cleaning were used to clean membrane from reversible and irreversible fouling. For the back-flushing process, the deionized water was pumped with air into the membrane module to clean the membrane. Chemical cleaning (NaOH/HCl cleaning) involved three steps: first, 1% NaOH cleaning, second, water cleaning and finally, 37% HCl cleaning. All experiments were carried out at room temperature of 20 ± 2 °C. Fig. 1 shows the whole ultrafiltration system. 2.4. Characterization methods Concentrations of FAs in both feed and permeate solutions were determined by their absorbency on 311 nm using an ultraviolet–visible spectrophotometer. A calibration curve of FA solutions has been prepared. Thus the absorbance could be determined directly and a working curve could be also drawn. The response observed was linear over a limited range. The grain size distribution of the FA solution is determined by a laser particle size analyzer (LPS). The brand of LPS is MASTERSIZER 2000 (UK) and the method of sample preparation is the same as the method of solution preparation. After filtration of each sample, membrane surface is characterized by an atomic force microscope (AFM) and a scanning electron microscopy energy dispersive spectrometer (SEM–EDS). The type of AFM is BioScope (Germany), working at a tapping mode. And
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Fig. 1. The whole ultrafiltration system. *Note: 1. Feed tank (V ~ 350 mL); 2. Backwash tank (V ~ 150 mL); 3. Permeate; 4. Backwash water drain; 5. Plate membrane module.
each sample (small piece of membrane) has been pasted on the sample holder and its surface topography has been scanned. The type of SEM– EDS is JSM-6480A (Japan). Each sample has been pasted on the stage using conducting resin and treated by vacuum drying and spray-gold (for 12 min). SEM–EDS is used to study the microstructure and elements of membrane contaminant. 3. Results and discussion 3.1. The effect on membrane flux of adding cations to the FA solution Filtration tests were performed with FA solutions prepared as described in the previous section. Membrane flux as a function of time for various salt ion concentrations is shown in Fig. 2. The influences of K+ and Ca2 + in different concentrations were investigated. Both
(a)
calcium and potassium influenced the rate of flux decline. In Fig. 2(a), as the potassium concentration was increased from 0 to 10 mM, the observed flux decline became more and more severe. Meanwhile, as shown in Fig. 2(b), the trend of flux decline was reversed in the presence of calcium. When the calcium concentration is increased from 0 to 10 mM, the observed flux was gradually increased. This behavior for the UF membrane is in contrast with previous findings with HAs which exhibit a wider range of particle size distribution, and where a dramatic increase in flux has been observed only at the highest end of calcium concentration (more than 4 mM) in Katsoufidou's study [30]. This can be explained by the different properties between HAs and FAs. Compared with HA, the FA has a higher humification degree and a more uniform molecular size. Also, FA has greater complexing ability with metal ions [31]. In order to determine the property of monovalent cations and divalent cations, the flux variations of 4 kinds of FA/electrolyte (K+, Na+, Ca2+ and Mg2+) solutions have been investigated. As shown in Fig. 3, the trends of the flux variations were consistent with the variations of ion valence in solutions. The addition of monovalent cations (K+ and Na+) could decrease the FA flux while the addition of divalent cations (Ca2+ and Mg2+) could increase the FA flux. The impact of salt ions on FA flux variations was as follows: Mg2+ b Ca2+ and K+ b Na+. This phenomenon shows that the mechanism of divalent cations or monovalent cations affected on FAs follows a single rule, respectively. The influence between divalent cations and monovalent cations on FA is different. FA aggregates are larger and may form a cake layer on the membrane surface with a relatively porous structure in the presence of divalent cations. Meanwhile, FA aggregates are much smaller in the presence of monovalent cations and may form a cohesive gel layer on the membrane surface.
(b)
Fig. 2. Filtration time vs. the flux ratio of the mixed-solute (FA and electrolyte) solution, for various salt concentrations. *Note: added inorganic electrolytes: (a) KCl; (b) CaCl2.
Fig. 3. Filtration time vs. the flux ratio of the pure FA solution and mixed-solute solutions, for various kinds of electrolytes (10 mM).
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3.2. Effects on membrane fouling when cations are added to the FA solution Previous research has indicated that, metal acting on HS containing FA can dramatically alter the properties of both the cations and HS by increasing the dissolution kinetics of the HS [32], inducing the aggregation of the HS or the mobilizing of either HS or metal previously adsorbed to mineral particles [22,33,34]. Back-flushing experiments have been performed to show the different types of membrane fouling affected by 4 kinds of salts. The clean water fluxes were shown after filtration, back-flushing and NaOH/HCl cleaning in Fig. 4. As it can be seen, the clean water flux recovery of FA/divalent salt solutions after back-flushing was much higher than that of the FA solution. The clean water flux recovery of FA/monovalent salt solutions after back-flushing was lower than that of FA solution. In addition, the clean water flux recovery of all kinds of solutions was nearly the same after NaOH/HCl cleaning. These results indicates that the membrane fouling affected by divalent cations and FA together is a little easier to wash off than that affected by FA only. Meanwhile, the membrane fouling created by monovalent cations and FA together is more difficult to wash off than that affected by FA only. Moreover, the membrane fouling of all solutions in these experiments can be washed away easily by NaOH/HCl solutions. That is to say, the addition of monovalent cations can lead to more irreversible fouling of FA. Thus, the membrane fouling of FA influenced by monovalent cations is more serious than that affected by other kinds of salt ions. FA rejections by the membrane in different concentrations and kinds of salts are shown in Fig. 5. The filtration time at which the retention was measured was identical. The apparent rejection of solutes (R) is C defined as: R ¼ 1− C pf , where Cp is the permeate concentration and Cf is the solute concentration in the feed solution. As shown in Fig. 5, the original rejection of FA was about 20% in a pure FA solution. In general, both monovalent cations and multivalent cations can increase the rejection of FA to a different extent. As expected, multivalent cations affected the rejection of FA obviously and reached a maximum value of about 86% in high Ca2+ concentration (10 mM). In general, the rejection of FA affected by monovalent cations is smaller, which is close to about 30%. FA may diffuse through the membrane pores and then with higher FA concentration in the permeate as permeate flux is reduced. According to the solute rejection calculation, rejections of the FA in mixsolute solutions can be reduced in parallel to reduction in permeate flux. In the presence of monovalent cations, the amount of FA in the permeated solution (1-R) is almost doubled compared to that in the presence of multivalent cations. Lower permeate flux (with Na+ and K+), may partially lead to the higher concentration of FA in the permeated solution, which eventually elevate the FA passage as calculated from the concentrations of FA in the feed and permeate (not the absolute mass passage of FA). So the variation of mass transfer of FA molecules by diffusion is one of the possible reasons leading to the variation of FA rejection.
Fig. 4. The mean water flux ratio vs. various kinds of pure FA solution and mix-solute solutions.
Fig. 5. Rejection of fulvic acid by ultrafiltration membrane during the filtration cycles vs. the electrolyte concentration in the various mixed-solute solutions.
From macro perspectives, the rejection, flux variation and flux recovery rate of fulvic acid in different salt solutions have been observed. And the effects of monovalent ions and multivalent ions were different and have certain regularity. So the microstructure characterization of the membrane fouling in different situations is necessary.
3.3. Characterization of the effects of membrane fouling by different cations The size distribution of FA with different kinds of salt ions added obtained by laser particle size analyzers is shown in Fig. 6. The concentration of every kind of salt ions was 10 mM in each mix-solute solution. It is observed that FA particles are uniform and below the nominal MWCO of the membrane. Based on the order of magnitude of its size distribution, FA molecules dissolve in water in the form of colloids [35]. However, as salt ions were added in the feed solutions the aggregation tendency and the particle size ranges of FAs were increased. The destabilization of the FA molecular colloid could be observed in the presence of salt ions. Indeed, as shown in Fig. 6, the influence of salt ions on FA solutions is based on the following order: Ca2 + N Mg2 + N K+ N Na+, which was compliant with the Hofmeister series [36]. The influence of multivalent cations on the aggregation tendency of FA was much stronger than that of monovalent cation. The addition of multivalent cations (Ca2+ and Mg2+) made the particle size ranges of fulvic acid aggregates reach to over 10 μm while the addition of monovalent cations (K+ and Na+) made it reach to only about 0.1 μm. With the aggregation of FA particles, the individual FA molecules reduced (almost disappeared in the presence of multivalent cations). This behavior corresponds to a previous study showing that the aggregation of humic substances is less affected by monovalent cations [24]. But the fouling tendency in mix-solute solution (FA + Na+ or FA + K+) is more serious than that in a pure FA solution. In the presence of monovalent cations, the FA aggregates are much smaller (compared with the
Fig. 6. The grain size distribution of the commercial fulvic acid obtained by LPS.
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aggregates in the presence of multivalent cations) and may form a cohesive gel layer on the membrane surface. So the membrane flux of FAs decreased (shown in Fig. 3). In addition, in the pure FA solution, the original rejection of FA was low (shown in Fig. 5), which indicated that the molecular size of FA was much smaller than the pore size of the membrane used in this experiment. Thus, most of the FA molecules transport through the membrane and dissolve in water. With the addition of monovalent cations, the distribution of FA particles is uneven, destabilized and its particle sizes increase slightly, which narrows the gap between FA particles and membrane pores. Thus the FA particles affected by monovalent cations has a larger impact on membrane pores, i.e., is more likely to lead to pore blocking. As the influence of multivalent cations on the aggregation tendency of FA is much stronger, a majority of particle sizes for FA aggregates are larger than the MWCO of
the membrane. Therefore, FA aggregates can be rejected by the membrane efficiently and primarily build up at the membrane surface with a relatively porous structure. In general, monovalent cations may decrease the permeate flux, leading to greater pore blocking or enhancing the mass transfer of FA by diffusion. The two factors may both influence the variation of FA rejection. Multivalent cations may increase the FA rejection visibly by increasing the aggregation tendency of FA. AFM analyses were performed to compare morphological changes between fouled membranes affected by different salt ions. Fig. 7 displays 3-D views of the membrane in each solution and shows its surface roughness respectively. From the image, the clean membrane, the membrane fouled with pure FA solution and the membrane fouled with FA/salts solutions are sequentially shown. The average roughness parameters calculated for each membrane were 63.651, 77.742,
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 7. AFM images of clean and fouled membranes. *Note: (a) clean membrane surface; (b) fouled membrane affected by pure FA (FA); (c) added NaCl; (d) added KCl; (e) added MgCl2; (f) added CaCl2. The X and Y dimensions are both 20 μm (5 μm/division), while the Z-scale is 2000, 2000, 3000, 3000, 4000 and 4000 nm, respectively.
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166.78, 168.46, 316.95 and 377.71 nm, respectively from Fig. 7(a) to (f). The fouled membrane surfaces appeared rougher than the clean membrane surfaces. Furthermore, the roughness variation of fouled membrane surface was affected by the salt ions based on the degree of FA aggregation affected by salt ions, i.e. Ca2+ N Mg2 + N K+ N Na+. This fact indicates that salt ions could increase the amount of membrane fouling layer on the micro level. In addition, the 3-D structures of fouled membrane affected by multivalent cations (Ca2 + and Mg2 +) and monovalent cations (K+ and Na+) were different. Though the roughness of the fouled membrane
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affected by multivalent cations (Ca2+ and Mg2+) is greater and the 3-D views exhibit a surface coverage with a “higher topography”, the structure of its pollution layer is more porous in the fouling layer. Moreover, the structure of the fouled membrane affected by monovalent cations (K+ and Na+) is compact and there is almost no gap, indicating that the membrane flux decline affected by the monovalent cations is more serious. SEM images provided additional detailed information on structural features of the UF membrane. Fig. 8 reveals the metal elements of the membrane pollutant. From SEM–EDS, the metal elements contained in
(a)
(b)
(c)
(d)
Fig. 8. SEM–EDS images of deposits on fouled membranes after the filtration experiments without a backwash.
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membrane contamination are sequentially shown. The membrane contamination contains multivalent cations (Ca2+ and Mg2+) and has no monovalent cations (K+ and Na+). There are different possible explanations for this phenomenon. Compared with the pure FA solution, FA molecules were induced to slightly more aggregation in the presence of monovalent ions. The addition of monovalent cations (K+ and Na+) may have increased the ionic strength of the FA solution, which compressed the electric double layer and induced the FA molecules to aggregate. Previous researchers have also reported a similar phenomenon showing that monovalent cations can lead to the aggregation of other macro molecule matter such as fullerene (C60) [37]. The membrane fouling affected by multivalent cations (Ca2 + and Mg2+) is much greater and thicker. Elemental calcium and magnesium have been found in the fouled layer respectively. The magnesium content is slightly more than the calcium content in the fouled layer. This phenomenon is analogous to the scale formation phenomenon by the hardness of ions (Ca2 + and Mg2+). In other words, divalent ions can be combined with macro molecule matter to form deposits easily. 4. Conclusions In this work, an experimental laboratory scale apparatus using a PVC ultrafiltration membrane was used to study the influence of ultrafiltration membrane fouling affected by four kinds of inorganic salts. Two important parameters are the kinds and concentrations of electrolytes (Na+, K+, Mg2 + and Ca2 +). These two factors promote the intraregional and molecular forces between FA molecules and accelerate their aggregation, thereby leading to flux decline, fouling reversibility and FA rejection. But the mechanisms and experimental observations of fouling between monovalent cations (Na+ and K+) and divalent cations (Mg2+ and Ca2+) were different. From macro and micro perspective, a more complete and detailed explanation of the pollution mechanism has been developed which matches the experimental observations. The aggregation of FA was less affected by monovalent cations than that by multivalent cations. Monovalent cations (Na+ and K+) could influence fulvic acid to form a dense fouling layer. Meanwhile, the multivalent cations (Mg2 + and Ca2 +) could be combined with fulvic acid molecules to form deposits with porous structures. Monovalent cations mainly affected the membrane flux decline while multivalent cations primarily improved the rejection of FA. In this study, it is suggested that different valence states of ions could influence the NOM in different ways. Acknowledgments This work was supported by the National Natural Science Foundation of China (51108112), the Natural Science Foundation of Heilongjiang Province (E201252), the Fundamental Research Funding of Harbin Engineering University (HEUFT06029), and the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (ESK201004). Also, some of the modeling work was carried out with the help of Dr. Jaehong Kim in Georgia Institute of Technology, USA. References [1] B. Bolto, D. Dixon, R. Eldridge, Removal of natural organic matter by ion exchange, Water Res. 36 (2002) 5057–5065. [2] S. Vreysen, A. Maes, Adsorption mechanism of humic and fulvic acid onto Mg/Al layered double hydroxides, Appl. Clay Sci. 38 (2008) 237–249. [3] L. Zhao, F. Luo, J.M. Wasikiewicz, Adsorption of humic acid from aqueous solution onto irradiation-crosslinked carboxymethyl chitosan, Bioresour. Technol. 99 (2008) 1911–1917. [4] J.N. Wang, Y. Zhou, A.M. Li, L. Xu, Adsorption of humic acid by bi-functional resin JN10 and the effect of alkali-earth metal ions on the adsorption, J. Hazard. Mater. 176 (2010) 1018–1026.
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