Development of new tubular ceramic microfiltration membranes by employing activated carbon in the structure of membranes for treatment of oily wastewater

Journal of Cleaner Production 244 (2020) 118720

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Development of new tubular ceramic microfiltration membranes by employing activated carbon in the structure of membranes for treatment of oily wastewater Behrouz Jafari, Mohsen Abbasi*, Seyed Abdollatif Hashemifard Sustainable Membrane Technology Research Group, Department of Chemical Engineering, Faculty of Petroleum, Gas and Petrochemical Engineering, Persian Gulf University, P.O. Box 75169-13798, Bushehr, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 February 2019 Received in revised form 2 October 2019 Accepted 3 October 2019 Available online 7 October 2019

In this research, the application of activated carbon and natural zeolite as absorbents and hydrophilic agents in the structure of a low-cost and high performance tubular ceramic microfiltration membrane for oily wastewater treatment was investigated. In this respect, Mullite, Mullite-Zeolite, and Mullite-ZeoliteActivated Carbon membranes were fabricated and characterized as ceramic MF membranes by using kaolin clay, natural zeolite and activated carbon powder. Performance of these membranes were assessed by comparing the quantity of permeation flux (PF) and total organic carbon (TOC) rejection during oily wastewater treatment where harsh conditions such as high oil concentration in various concentration of saline feed (0e200 g/L) were considered. Fouling and cleaning experiments were performed on oily waste water using a laboratory scale cross-flow test unit when NaOH was selected as a cleaning agent. Experimental results showed that the presence of activated carbon and natural zeolite in the structure of the membrane can enhance the permeation flux and total organic carbon rejection. Also, the permeation flux was highly dependent on the salt content of the feed solution. Actually, the flux improved sharply with the increase of salt content from 0 to 50 g/L, and then reduced slightly with further increase in salt concentration. In addition, total organic carbon rejection of up to 99.99% for Mullite-Zeolite-Activated carbon membrane was observed. The results of cleaning membrane process illustrated that the NaOH agent was able to clean the fouled MF membrane effectively. The exponential triple smoothing model was also used to predict the permeation flux in long periods. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Prof. S Alwi Keywords: Ceramic membrane Oily wastewater treatment Kaolin Active carbon (AC) Natural zeolite

1. Introduction The increasing rate of population growth, rapid urbanization, and industrialization are all potential threats to human life along with other living species such as plants and animals, specifically owing to the fact that they can lead to pollution of water resources (Firdous et al., 2018). Indeed, discharging a large number of compounds such as oils, solvents, antibiotics, fungicides, and herbicides has caused serious environmental problems in recent years (Zhuo et al., 2018). For instance, oily wastewater arises from various sources such as oil/gas recovery, metal finishing, mining (Huang et al., 2018), transportation and discharge from several industrial works in machining (Changmai et al., 2019), automotive shops, oil-

* Corresponding author. E-mail address: [email protected] (M. Abbasi). https://doi.org/10.1016/j.jclepro.2019.118720 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

drilling and refineries (Rasouli et al., 2017a). The chemical and physical properties associated with oil fluid can influence the environmental impact ability (Changmai et al., 2019). Actually, oily wastewater can endanger human health, growth of plants and animals, atmospheric pollution and natural landscapes (Huang et al., 2018). It is necessary to reduce the toxicity of pollutants (e.g., phenols, petroleum hydrocarbons, and polyaromatic hydrocarbons) before they are released into the environment (Huang et al., 2018). The treated water can be used in washing applications for example, and can largely reduce water consumption (Weber et al., 2003). In this regard, there are several methods such as floatation, sedimentation, adsorption, ultracentrifugation, coagulation, flocculation, electric, thermal methods and membrane separation (Arzani et al., 2016; Changmai et al., 2018; Hilala et al., 2004; Yue et al., 2018). In addition, bio-degradation and enzyme-degradation methods have drawn much attention because of their environmental friendliness

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(Zhuo et al. 2011, 2018). Among these methods, membrane separation seems more suitable because of simplicity of operation, ability to treat large volumes of effluent in less time and small physical footprint (Zhang et al., 2018). Based on porosity size and separation mechanism, the membrane process can be divided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) (Abbasi et al., 2010a). Several factors are influential in the treatment of oily wastewater by the membrane process such as: surface charges, electrostatic interactions, size exclusion (i.e., sieving) and selective wettability (Shi et al., 2013). The size exclusion effect in fact allow the passage of pressurized water through the membrane, while the oil droplets that are larger than the membrane pores are blocked. The selective wettability effect assures that the oil droplets are not got wet and permeated into the membrane due to their hydrophobicity, considering the selective wetting properties of membrane towards water and oil (Huang et al., 2018). The well-known polymeric membranes are regarded as effective membrane types because of their low cost and easy functionality (Kaur et al., 2016). However, nowadays, a great deal of attention has been devoted to ceramic membranes regarding their excellent properties in terms of mechanical strength, thermal stability, capability of enduring extremely harsh environments in addition to their long lifetime and simplicity of cleaning (Ivanets et al., 2016). Several studies have been carried out on the preparation of porous ceramic membranes using materials such as Al2O3, TiO2, SiO2, kaolin, pozzolan, quartz, and ZrO2 (Achiou et al., 2017; Khadijah Hubadillaha et al., 2018). Among the mentioned substances, kaolin has attracted a particular interest because of its unique physical properties along with its very low price (Hubadillah et al., 2017). Moreover, kaolin exhibits hydrophilic behavior (Mgbemena et al., 2013), which pave the way for oily wastewater treatment (Emani et al., 2014). Activated carbon (AC) and natural zeolite are the most common compounds which are used as adsorption agents in oily wastewater treatment. Indeed, employing these materials in the membrane structure can enhance the permeation flux (PF) and total organic carbon (TOC) rejection of the membranes. To explain, high porosity, large surface area and hydrophilicity effect of these materials (i.e., AC and natural zeolite) can lead to the reduction of the interaction between the organic contaminant (oil) and the membrane surface. There are several reports dealing with the adsorption-MF hybrid process using AC and natural zeolite in the literature (Juang et al., 2004; Wang et al., 2013; Yang et al., 2011), whereas incorporation of these materials in the structure of the ceramic membranes has rarely been reported in the literature. Other forms of carbon materials such as carbon nanotubes (CNT) (Yan et al., 2019) and graphene oxide (Cao et al., 2017) have also been used to separate oil from water, but these materials are expensive, and consequently make the treatment process uneconomical. For the first time, Rasouli et al. (2017b), studied the adsorptionMF hybrid process using natural zeolite as an absorption agent in the membrane structure. Kim et al. (2010) prepared alumina-CNT membrane using electrochemical process for purification of water. Their result showed that the incorporation of CNTs could increase the water permeation flux and rate of fouling due to the hydrophilic nature of the CNTs. They attributed the result to the fact that the hydrophobic nature of pore walls increases the slippage of water molecules during transport, and the permeation flux enhances. Yang and Tsai (2008) developed a carbon fiber/carbon/alumina tubular ceramic membrane for treating wastewater. In their study, two steps were used to form the carbon layer on the surface of the alumina tubular Al2O3 substrate: (1) the outside of each tubular

substrate was wrapped with two layers of Polyvinylidene chloride (PVDC) film, and (2) the PVDC-wrapped substrate was put in a quartz tube and carbonized by an electric furnace with an inert atmosphere of nitrogen. Each carbon coated tubular Al2O3 substrate, with two ends plugged by silicone stoppers, was spin-dipped a ferric sulfate solution. Finally, the carbon fibers grew through the surface of the membrane using a CVD process. Moreover, alumina-CNT composite membrane through a pow
der metallurgical method at different temperature (1200e1500 C) was fabricated by Shahzad et al. (2018). In their study, for better dispersion of carbon nanotubes in the alumina matrix, Arabic gum and sodium dodecyl sulfate were used. The results indicated that
the porosity of the membranes at 1200 C was 65% and by increasing the sintering temperature, porosity was decreased. In another work, the hybrid Al2O3/AC membrane was fabricated and the results were compared with the pure alumina membrane. The Al2O3/AC membrane exhibited a very high removal efficiency and retention (more than 99%) for oily wastewater treatment. Up to now, most studies have been done on alumina/carbon ceramic membranes. However, ceramic membranes of alumina need a high
temperature for sintering (up to 1500 C) to achieve good mechanical strength. The fabrication process will be prolonged and extremely expensive (Kayvani Fard et al., 2018). In this novel study, based on previous experiences on fabrication of low-cost and high-performance ceramic membranes for oily wastewater treatment processes, cheap kaolin, natural zeolite, and activated carbon powder have been employed in the membrane structure to improve the PF and TOC rejection. In fact, three types of home-made, cost-effective and high-performance Mullite, MulliteZeolite, and Mullite-Zeolite-AC ceramic MF membranes have been fabricated. As a result, compared to other ceramic membranes, simultaneous use of natural zeolite and activated carbon in the structure of the membrane can boost the performance of oily wastewater treatment effectively. 2. Materials and methods In this investigation, the kaolin clay powder with the particle size (<25 mm) was supplied from Zaminkav company, Iran with purchased price of 0.16 USD per Kg. Natural zeolite with the 500mesh average particle size (<25 mm) and pore size of 26 nm (according to BET analysis) was prepared from Semnan mine, Iran. The price of kaolin and zeolite powder purchased from Iran’s mines is less than 0.1 USD per Kg. Chemical analysis of kaolin and natural zeolite are listed in Table 1. Low-cost industrial activated carbon (Xinyou Aquarium Product Factory) with 170 mesh particle size (<90 mm) and a pore size of 4 nm, was purchased with the price of 2.08 USD per Kg. Starch (Merck KGaA) used as a binder. Tritton X100 (C14H22O(C2H4O)n > 99% purity) and NaCl supplied from Merck Company, Germany. Distilled water with a conductivity of less than 5 ms cm1 was used in experiments. 2.1. Preparation of ceramic membrane The hybrid ceramic/carbon microfiltration membrane was fabricated by extrusion and sintering procedures. Firstly, Kaolin, kaolin/zeolite and kaolin/zeolite/activated carbon with a weight percentage of 100, 90:10 and 80:10:10, were blended using an electrical mixer with high speed for 15 min. Following that, 5 wt % of starch as a binder and 32 wt % of distilled water were added to the solid mixture. After that, these mixtures were extruded and tubular membranes with an inner diameter of 10 mm, the outer diameter of 14 mm and 25 cm length were formed. The membrane was dried for 48 h at room temperature, and then sintered in an electrical furnace (under nitrogen). The temperature gradient from

B. Jafari et al. / Journal of Cleaner Production 244 (2020) 118720

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Table 1 Chemical analysis of kaolin and natural zeolite that used in the preparation of MF ceramic membranes. Kaolin

Natural Zeolite

Component

Percent (%)

Component

Percent (%)

SiO2 Al2O3 CaO Fe2O3 K2O Na2O L.O.I Total

55.25 27.05 0.44 0.91 0.25 0.12 10.98 100

SiO2 AL2O3 Na2O K2O CaO Fe2O3 L.O.I Total

68.5 11 3.8 4.4 0.6 0.2e0.9 10e12 100







room temperature to 550 C was 5 C/min. Then, the temperature
was kept fixed at 550 C for 1 h; subsequently, the temperature
raised from 550 C to 975
C, with the same rate, and kept fixed for

1 h as well. Finally, the temperature raised from 975 C to 1150 C by
a rate of 3 C/min, and remained fixed at this temperature for 3 h. Variation of the temperature gradient in the sintering procedures is shown in Fig. 1. 2.2. Oily wastewater preparation Synthetic oily wastewater was used to measure the performance of MF ceramic membranes. Oil-in-water emulsions (synthetic oily wastewater) with 500, 1000 and 1500 mg/L oil were prepared from Ghachsaran, Iran, and distilled water. Tritton X-100 (0.01 wt %) was used as an emulsifier. A blender mixed the mixture with a high shear rate (20,000 rpm) using laboratory homogenizer (Wise Mix Homogenizer HG 15, Korea) for 30 min. To prepare emulsions with 500, 1000 and 1500 mg/L oil concentrations, all the mixtures were diluted in 9 L distilled water. Chemical-physical properties of Gachsaran crude oil for preparation of synthetic oily wastewater are shown in Table 2. 2.3. Experimental setup

PF ¼

M A:t

(1)

Cp TOC Rejection ð%Þ ¼ 1  Cf

!  100

(2)

where M (Kg) is the collected permeation mass during the time interval, A (m2) is the effective surface area, Cf and Cp are the concentration of oil in the feed and permeation flow. The flux recovery (FR) was calculated as:

FRð%Þ ¼

PFwc  PFww  100 PFwi  PFww

(3)

where “PFwi” refers to initial distilled water flux of the fresh membrane. “PFww” is distilled water flux after fouling by oily wastewater. “PFwc” is distilled water flux after cleaning with chemical agent. It should be noted that all experiments for cleaning were carried out at TMP ¼ 1 bar and CFV ¼ 1.25 m/s as the best operating conditions based on the results of Salahi et al., (2010) and Garmsiri et al. (2017). 2.4. Scanning electron microscopy (SEM)

The schematic diagram of the microfiltration setup is shown in Fig. 2. It is essentially made up of the circulation pump, feed tank of 10 L, flowmeter, rotameter, pressure indicator, and membrane module. The setup was operated in a cross-flow mode. The membrane surface area in contact with the feed was equal to 44 cm2. The set up was rather simple: however, it was designed in such a way that all important operating parameters in the MF process such as operating pressure and cross-flow velocity could be tuned and controlled. Membranes performance during oily wastewater was evaluated by obtaining PF, TOC rejection and flux recovery (FR) of each membrane. The PF and TOC rejection (%) were calculated as the following:

The surface and cross-sectional microstructure of the MF membrane was observed through scanning electron microscopy (SEM). The dried fractured surfaces of the membrane were gold sputter coated and observed using a TESCAN Vega 3 model (Czech Republic) microscope with the accelerating voltage 20 kV. 2.5. X-ray diffraction In order to characterize the structure of fabricated membranes (Mullite, Mullite-Zeolite, and Mullite-Zeolite-Carbon), the evolution of phases in sintered supports was followed by using XRD (PW1,800 X-ray diffractometer, Philips, Netherlands) of samples

Fig. 1. Thermal treatment applied for MF membrane.

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Table 2 Chemical physical properties of Gachsaran crude oil. Specification

Unit

Sp. Gr API Gravity Sulfur Content H2S Total Nitrogen Water Content Salt Content Kin Vis Kin Vis Kin Vis

at 15.6 C e Wt. % Wt. mg/L Wt. % Vol. % PTB
at 10 C cST
at 20 C cST
at 40 C cST




Amount

Specification

Unit

Amount

0.8750 30.21 1.62 ˂1 0.21 0.05 8.00 28.27 16.82 8.41

R.V.P Asphaltene Content Wax Ash Content Acidity Nickel Vanadium Iron Lead Sodium

psi Wt. % Wt. % Wt. % mgKOH g1 mg/L mg/L mg/L mg/L mg/L

4.83 3.60 6.06 0.025 0.05 29.00 105.00 2.60 ˂1 10.00

Fig. 2. Schematic diagram of microfiltration setup.




sintered at 1150 C in the 2q range of 10e80
with the scan rate of 0.02
/s.

Porosityð%Þ ¼

2.6. Shrinkage

v¼

To determine the radial and longitudinal shrinkages during the sintering process, the outer diameter and length of the membranes
were measured at 550, 750, 950 and 1150 C by a slide caliper (precision 0.01 mm). The shrinkage coefficient noted SD and SL were determined directly using the following relations (Kouras et al., 2017):

W2, W1 and rw are dry and wet masses and the water density at the temperature of the experiment. v is cavity volume and Vt is the volume of the membrane, which was calculated through pðRout  Rin Þ2 :L equation. Rout and Rin are external and internal radius, and L is the membrane length (Rasouli et al., 2017a).

SD ¼ ððD0  DÞ = D0 Þ  100

(4)

2.8. Membrane average pore size measurement

SL ¼ ððL0  LÞÞ = L0 Þ  100

(5)

The membrane average pore radius (rm) was calculated according to the Guerout-Elford-Ferry equation (Zhang et al., 2015):

where D0 and L0 are the tube diameter and length of the membrane before sintering and D and L are the tube diameter and length after sintering.

2.7. Porosity The membrane porosity was measured as follows:

r

  v  100 Vt

  W1  W2

rw

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ð2:91:75εÞ8mlJ

m¼

(6)

(7)

(8)

εTMP

ε is the membrane porosity (%), m is the water viscosity (8.9  104 Pa s) at the operating temperature, L is the membrane thickness (m), J is the pure water flux under the specific applied TransMembrane Pressure (TMP). Pure water flux for each TMP in the range of (100e400 KPa) was measured for Mullite, Mullite-Zeolite and Mullite-Zeolite-AC membranes.

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Then, rm was obtained from the slope (S) of the linear line from TMP versus pure J applying the least square method as follows (Rasouli et al., 2017a):

J¼

r 2m εTMP ð2:9  1:75εÞ8mL

(9)

r2m ε ð2:9  1:75εÞ8mL

(10)

S¼

2.9. Dynamic light scattering analyzer (DLS) The size of oil emulsion droplets in the feed was measured by a dynamic light scattering analyzer device (Cilas, Nano DS, France). The emulsion sample (1 mL) was analyzed immediately after preparation. The device measures particles ranging in the size from 0.3 to 10,000 nm. 2.10. Modeling Analyzing and predicting time-oriented data are important problems encountering many fields including industry, engineering, and environmental sciences, (Montgomery et al., 2008; Tsay, 2002). In this section, the PF of membranes was shown in a long time using exponential smoothing method. In this method, the constant process was considered, where the time series data was expected to be around a constant level with random fluctuations, which are usually characterized by uncorrelated errors with mean 0 and constant variance s2f. In fact, the constant process displays a very special case in a more general set of models often used in modeling time series data as a function of time. The general class of time series models can be represented as the following:

yt ¼ f ðt; bÞ þ εt

(11)

where b is the vector of unknown parameters and εt is the uncorrelated error. A special case of the model (10) is the exponential triple smoothing model. The exponential triple smoothing can also be seen as the linear combination of the current observation and the smoothed observation at the previous time unit. As the latter contains the data from all previous observations, the smoothed observation at time t is in fact the linear combination of the current observation and the discounted sum of all previous observations. In addition, exponential smoothing techniques are considered as a visual aid to identify patterns based on time series data. Based on equation (11), at time t, observation at next time unit, t þ 1, are predictable. The exponential triple smoothing model is represented as the following.

yt ¼ b0 þ b1 t þ

b2 2!

t 2 þ εt

(12)

To learn more about the exponential smoothing techniques, refer to Montgomery et al. (2008). 3. Results and discussion

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Fig. 4(a) and (b) that the surface morphology for both samples are rough and similar to each other regarding the fact that both of them are comprised of identical chemical compounds with nearly similar compositions. However, the membrane surface of Mullite-Zeolite is more porous than pristine Mullite membrane, which is confirmed by a study conducted by Rasouli et al. (2017b), reporting a similar result for the incorporation of natural zeolite in Mullite membrane. Figs. 3(c) and 4 (c) represent the surface and cross-section of the Mullite-Zeolite-AC hybrid membrane with AC particles dispersed over the structure of the membrane. Accordingly, the AC particle size is larger than the particles of kaolin and natural zeolite. As aforementioned, the size of kaolin and natural zeolite is less than 20 mm, while powder activated carbon used in the structure of the membranes is lower than 100 mm. It should be noted that since AC is inherently porous, it plays a significant role in enhancing the performance of membranes in terms of the PF. 3.2. X-ray diffraction Wide-angle X-ray diffraction patterns of Mullite, MulliteZeolite, and Mullite-Zeolite-AC, after sintering, are provided in Fig. 5. It can be observed that kaolin and zeolite are transformed to mullite, quartz and cristobalite phases in sintering temperature of
1150 C. Comparable to previous works, the major phase is mullite whereas the minor phases are quartz and cristobalite (Rasouli et al., 2017a). X-ray diffraction of graphite located at 2q ¼ 27, corresponding to the d002 spacing of graphite (Xu et al., 2014). Comparison between XRD pattern of Mullite-Zeolite and MulliteZeolite-AC reveals that with the incorporation of AC, the position of peaks remains unchanged and no new peak is observed. As a result, it can be deduced that no substantial chemical reaction occurs between Mullite-Zeolite and carbon (Kayvani Fard et al., 2018). Fig. 5 also illustrates pattern of Mullite-Zeolite-AC after operations and fouling by oily wastewater. As a result, the intensity and position of the peaks remain unchanged, indicating that structure of the membrane does not change during operations and natural zeolite and AC particles do not leach out from the membranes. 3.3. Filtration test Cross-flow filtration test of distilled water is performed on the Mullite, Mullite-Zeolite and Mullite-Zeolite-AC membrane. Results of distilled water flux vs. the pressure is displayed in Fig. 6. For all the three membrane types, the water fluxes are measured after 15 min of filtration as the transmembrane pressure alters from 1 to 4 bar. It is obviously seen that the flux raises with an increase in the applied pressure, which is due to an enhancement in the driving force as a consequence of increase in the transmembrane pressure (Suresh et al., 2017). Fig. 6 also demonstrates that with incorporation of 10 wt % of natural zeolite into Mullite membrane structure, both the roughness of surface and the porosity improve, and water PF increase. Compared with the Mullite and Mullite-Zeolite membrane, water PF of the Mullite-Zeolite-AC membrane is considerably increased by AC addition. This can be ascribed to the hydrophilic nature of the membrane surface, and the influence of particle on the porosity of composite membranes by thermodynamic instability and kinetic hindrance during fabrication process.

3.1. Morphology 3.4. Dynamic light scattering analyzer (DLS) In this research, SEM was used to observe the morphology of the
Mullite, Mullite-Zeolite, and Mullite-Zeolite-AC sintered at 1150 C. Fig. 3(aec) and 4 (a-c) show the inner surface and cross-sections of the Mullite, Mullite-Zeolite, and Mullite-Zeolite-AC. To compare Mullite with Mullite-Zeolite, it can be seen from

As it is shown in Fig. 7, the mean size of the oil droplet in the synthetic oily wastewater (1000 mg/L oil concentration) is about 300 nm. Under this condition, the emulsion is sufficiently stable for the MF filtration process and no phase separation after 12 h occurs.

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Fig. 3. SEM micrograph of surface ceramic membranes. (a) Mullite, (b) Mullite-Zeolite, (c) Mullite-Zeolite-AC.

3.5. Porosity & shrinkage The porosity of Mullite, Mullite-Zeolite and Mullite-Zeolite-AC membranes is measured according to equations (5) and (6). Over thermal treatment, organic additives are burnt out and formed the porosity in the ceramic body. The open porosity is observed for
tubular ceramic membrane at 1150 C (according to Fig. 1) to be about 34, 41 and 67% for Mullite, Mullite-Zeolite, and MulliteZeolite-AC. In fact, the porosity of the Mullite-Zeolite membrane is boosted by almost 7% relative to Mullite membrane. Comparing the SEM images reveal that by adding the natural zeolite, the void fraction increases. The result also shows that by the addition of only 10 wt % of AC to Mullite-Zeolite membrane, the porosity of the membrane increase from 41% to 67%, which can be derived from the high porosity of AC. Besides, the rough carbon particles increase the free spaces between mullite and zeolite particles (due to less flexibility), which lead to improvement of porosity. Fig. 8 demonstrates the porosity of membranes sintered at
various temperatures (550, 750, 950 and 1150 C). To draw a more meaningful comparison, the samples are placed in the electrical furnace for 1 h at all the four temperature levels separately. It is clearly observant that as the sintering temperature increases, the related porosity declines. By way of illustration, the sintered Mullite
membrane at 550 C has a porosity of 56%, while those sintered at
750, 950 and 1150 C have the porosity of 52, 46 and 37%. The falling trend versus the sintering temperature is similarly seen for the other types of membrane (i.e., Mullite-Zeolite and Mullite-ZeoliteAC) Obviously, the descending trend of porosity towards sintering temperature can be the result of greater shrinkage of the tubes at higher temperatures (Zhou et al., 2011).

Fig. 9(a) and (b) depict the longitudinal and radial shrinkage of the membranes sintered at different temperatures. It is seen that there are two temperatures at which the diameter and length change dramatically.
The first one occurs at the temperature of 550 C where the radial and longitudinal shrinkage of Mullite membane are approximately 1.6 and 1%. The decrease in length and diameter of the tubular membrane in comparison with uncured tubular can be related to the formation of metakaolin from kaolinite phase under an endothermic reaction (1) (Bouzerara et al., 2009; Rasouli et al., 2017b):

2AL2 Si2 O5 ðOHÞ4 / 2AL2 SiO7 þ 4H2 O

(13)

Afterward, although the diameter does not change significantly

at 750 C, it diminishes noticeably at 950 and 1150 C. This comes from the transformation of kaolin to Mullite phase, which is accompanied by release of free silica according to reaction (2) (Bouzerara et al., 2009; Rasouli et al., 2017b):

3ðAL2 O3 :2SiO2 Þ / 3AL2 O3 :2SiO2 þ 4SiO2


(14)

When the sample is sintered at 1150 C, the radial and longitudinal shrinkage of Mullite membrane are approximately 9.9 and 4.7%.
Since the membranes are kept at the temperature of 1150 C for 1 h, it can be concluded that the reaction of the transformation of kaolin to mullite is presumably carried out completely at this temperature. The maximum shrinkage is found to be at this temperature.

B. Jafari et al. / Journal of Cleaner Production 244 (2020) 118720

Fig. 4. SEM micrograph of cross-section ceramic membranes. (a) Mullite, (b) Mullite-Zeolite, (c) Mullite-Zeolite-AC.

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Fig. 5. X-ray diffraction patterns of AC powder and ceramic membrane.

Fig. 6. Pure water flux variation for different membranes as a function of transmembrane pressures.

3.6. Mean pore size calculation Fig. 10 compares the three types of membrane in terms of porosity and pore size. The mean pore diameter of Mullite, MulliteZeolite and Mullite-Zeolite-AC membranes are 0.354, 0.386 and 0.322 mm. As a result, adding 10 wt % of natural zeolite lead to increase in the mean pore diameter slightly owing to embedded zeolite particles between the kaolin clay in the membrane structure. Nevertheless, in Mullite-Zeolite-AC membrane, the mean pore diameter is reduced, which arises from significant increase in its porosity (see Fig. 8). To clarify, in accordance with equation (10), despite the fact that the gradient of pure water flux versus pressure (i.e., slope of the linear line) of the Mullite-Zeolite-AC is steeper than Mullite and

Mullite-Zeolite membranes, the porosity enhancement as a consequence of activated carbon addition exerts a greater effect on the mean pore size in comparison to the effect of the steeper slope of the linear line, leading to the reduction of mean pore diameter. 3.7. Performance of membranes for treatment of oily wastewater Performance of Mullite, Mullite-Zeolite and Mullite-Zeolite-AC membranes at different concentrations of oil in water emulsion (e.g., 500, 1000 and 1500 mg/L oil concentration) are assessed. Fig. 11 displays the evaluation of fluxes for oily wastewater treatment by membranes at a pressure of 1 bar. This experiment is continuously performed for 60 min. Clearly, at the start of the filtration process, the PF of the membrane is boosted by almost 1.7

B. Jafari et al. / Journal of Cleaner Production 244 (2020) 118720

Fig. 7. Droplet size distribution of oil in the emulsion.

times with the incorporation of 10 wt% of natural zeolite likewise,

9

by the addition of only 10 wt % of natural zeolite and 10 wt % AC, the PF enhances around 2.9 times relative to pure Mullite. It can also be seen that the flux is continuously reduced during filtration times for all the membrane types. This reduction of flux is caused by the adsorption and deposition of oil droplets on the inner surface of membrane to form a cake or a gel layer (Abbasi et al., 2010b, 2011) which adds an excessive hydrodynamic resistance (Achiou et al., 2017). It is obvious that by addition of natural zeolite and AC, the PF lowers more severely with the passage of time compared to that of pure Mullite membrane. However, eventually after 60 min of filtration, the PF of Mullite-Zeolite and Mullite-Zeolite-AC are higher than the pure Mullite membrane. From the obtained results, it can generally be concluded that incorporation of AC and natural zeolite exerts two major effects on the performance of the membrane. In the first place, regarding the high surface area, porosity, and hydrophilic nature of the mentioned additives, the PF goes up. In the second place, since the presence of additives can contribute to the adsorption process,




Fig. 8. Porosity of membranes versus sintering temperature (all samples were kept at 550, 750, 950 and 1150 C for 1 h).

Fig. 9. a) longitudinal shrinkage and b) radial shrinkage of membranes versus sintering temperature.

10

Fig. 10. Porosity ( Zeolite-AC.

B. Jafari et al. / Journal of Cleaner Production 244 (2020) 118720

) and mean pore size ( ) for Mullite, Mullite-Zeolite and Mullite-

these materials are saturated after adsorption, and consequently the PF drops more rapidly. It must be noted that the adsorption of oil droplets in membrane pores leads to membrane fouling, as well as adverse effects over the characteristics of membrane and pure water flux (PWF) (Abbasi et al., 2012c). Usually, in membrane filtration processes, it is aimed to retain the pollutants/solutes in the feed solution and reject these solutes as concentrate or brine, rather than adsorbing these solutes on the membrane surface. Recovery of the fouled membrane is essentially required when the adsorbent membranes are utilized. This is why studying the chemical cleaning of ceramic membranes is of vital importance. In this regard, alkalis such as NaOH, KOH or their mixtures can be typically named as low-cost chemical cleaning agents which have efficient neutralization effect on acidic organics (Salahi et al., 2010; Garmsiri et al., 2017). Fig. 12 shows flux recovery of Mullite, Mullite-Zeolite and

Mullite-Zeolite-AC membranes at different concentrations of oil in water emulsion (500, 1000 and 1500 mg/L) during chemical cleaning by using 1 wt% NaOH as the cleaning agent. The flux recovery after 20 min cleaning for Mullite, Mullite-Zeolite and Mullite-Zeolite-AC (500 mg/L oil in water emulsion) are 37.20, 40.12 and 45.20%. Almost similar results are observed for flux recovery of the fouled membranes during treatment of 1000 and 1500 mg/L oil in water emulsion. The flux recovery of Mullite and Mullite-Zeolite membranes are close to each other while that of the MulliteZeolite-AC membrane is slightly greater. This can most likely be due to physical and chemical interactions in membrane cleaning. The first interaction is the chemical reaction between the cleaning agents and the fouled in the fouling layer that results in weakness of fouling layer on the membrane surface. The reaction may be hydrolysis, dissolution or dispersion. The second interaction is the mass transfer of the cleaning agents from the bulk solution to the fouling layer, and vice versa the fouled materials from the fouling layer back to the bulk solution. These interactions cause the removal of the deposited materials from the membrane surface (Salahi et al., 2010; Garmsiri et al., 2017). The TOC rejection is given in Fig. 13. It can be seen that TOC rejection for Mullite membrane only reaches 76% for wastewater containing 1000 mg/L oil, while for the Mullite-Zeolite membrane it reaches 85%. It is also clear that by adding 10 wt % of AC, the TOC rejection can be enhanced to 98%. In fact, the incorporation of AC with high micro/meso-porous structure most presumably contributes to a high degree of oil adsorption and improvement of oil removal by the membranes. Besides, it should be noted that for all membranes, TOC rejection is weakly decreased with increasing oil concentration. To explain, increasing the concentration of oil in the feed ends in an increase in the concentration of oil on the inner surface of the membrane, and subsequently the permeate flux and TOC rejection falls.

Fig. 11. Variation of PF with time during oily wastewater treatment at pressure of 1 bar. (a) 500 mg/L, (b) 1000 mg/L and (c) 1500 mg/L.

B. Jafari et al. / Journal of Cleaner Production 244 (2020) 118720

11

Fig. 12. Flux recovery percent by using NaOH as chemical cleaning agent for the fouled membranes during treatment of oily wastewater with concentration equal to: (a) 500 mg/L, (b) 1000 mg/L and (c) 1500 mg/L.

Fig. 13. The effect of feed concentration on TOC rejection of membranes at pressure 1 bar, (a) 500 mg/L, (b) 1000 mg/L, (c) 1500 mg/L.

3.7.1. Effects of salty oily wastewater on performance of the membranes Sodium chloride (NaCl) is the most abundantly present salt in crude oil that leads to salty oily wastewater discharge into the environment from crude oil desalination unit. The effects of salt concentration on the performance of the membrane, specifically in terms of PF, have been reported by many investigators (Abbasi et al., 2010a; Elzo et al., 1998; Hua et al., 2007, Kayvani Fard et al., 2018; Zhao et al., 2005), which can mainly be associated with alternation of ion concentration in the solution. In fact, the presence of salt

in oily wastewater causes weakening of surfactant hydration, which consequently makes the emulsions a little bit unstable (Chen et al., 2016). Figs. 14 and 15 show variations of PF and FR at various salt concentrations (50e200 g/L NaCl and 1500 mg/L oil in water). It is clear that the PF of the membranes increases and FR slightly decreases with the increase of salt concentration up to 50 g/L. This is because of the fact that high ion concentrations decrease the double layer thickness around the emulsion droplets. The electrostatic barrier to coalescence reduces and PF improves (Abbasi et al., 2010a; Hua et al., 2007). Nevertheless, with an increase of more

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B. Jafari et al. / Journal of Cleaner Production 244 (2020) 118720

Fig. 14. Effects of salt concentration on the performance of the (a) Mullite, (b) Mullite-Zeolite, (c) Mullite-Zeolite-AC ceramic membranes for the synthetic oily wastewater treatment (pressure 1 bar and oil concentration of 1500 mg/L).

than 50 g/L of NaCl, PF starts to decrease and flux recovery increases for all the membrane types. This can be attributed to the salt concentration polarization on the membrane surface. It seems that the addition of salt have both positive and negative influences on

permeate flux. On the positive side, it is capable of producing bigger oil droplets (increase PF). On the negative side, it induces salt crystal formation (decrease PF), and there is an optimum salt concentration.

Fig. 15. The effect of salt concentration on flux recovery of membranes by using NaOH as chemical cleaning agent during treatment of oily wastewater with 1000 ppm oil concentration and salt concentration equal to: (a) 50 g/L, (b) 100 g/L, (c) 150 g/L, (d) 200 g/L (TMP ¼ 1 bar, CFV ¼ 1.25 m/s, oil concentration ¼ 1500 mg/L).

B. Jafari et al. / Journal of Cleaner Production 244 (2020) 118720

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Fig. 16. The effect of salt concentration on TOC rejection of membranes, (a) 50 g/L, (b) 100 g/L, (c) 150 g/L, (d) 200 g/L (pressure 1 bar and oily wastewater of 1500 mg/L).

Analysis of the TOC rejection towards salt concentration (see Fig. 16) shows that as salt concentration alters from 50 to 200 g/L, the TOC rejection improves. For example, the TOC rejection of the pristine Mullite membrane boosts from 73.00% to 83.12% when the concentration of salt increases from 0 to 200 g/L, while that of Mullite-Zeolite-AC is around 99.99% (i.e., the oil droplets are separated up toalmost 99.99%). In fact, the presence of AC enhances the separation of even tiny oil droplets through an adsorption process when the salts destabilizes the emulsion at high concentrations (Kayvani Fard et al., 2018).

presence of 50 g/L of salt, the PF of the Mullite and the MulliteZeolite-AC membrane remains in a comparatively acceptable range after 150 min of operation. These membranes can be regarded as good candidates for the application of oil desalters discharge treatment. For Mullite-Zeolite membrane, predictions shows that after 120 min of operation, the membrane porosity is completely closed. The results also demonstrates that with increasing of salt concentration to 200 g/L, the PF of membranes reaches zero after 100 min of operation. 4. Conclusion

3.8. Modeling The results of PF prediction of oily waste water for the three types of membrane are reported in Table 3. Accordingly with the passage of time and increase in the concentration of oil, the PF diminishes. The predicted results for 500 mg/L wastewater illustrate that after 150 min of operation, the PF still remains at a relatively high level, which reveals the excellent performance of these membranes. Likewise, predication results related to saline oily wastewater are reported in Table 4, where the concentration of oil in water is considered to be 1500 mg/L. According to Table 4, in the

In this study, three types of low-cost tubular porous ceramic MF membranes were successfully prepared by extrusion and used for synthetic oily wastewater treatment. In this respect, Mullite, Mullite-Zeolite, and Mullite-Zeolite-AC membranes were fabricated by using cheap raw materials such as kaolin clay, natural zeolite, and industrial active carbon powder, and their performance were measured experimentally and compared with each other. The results showed that the incorporation of AC and natural zeolite in the structure of ceramic membranes could enhance the porosity, PF and TOC rejection, which could be attributed to the hydrophilic

Table 3 The predicted PF results for Mullite, Mullite-Zeolite and Mullite-Zeolite-AC membranes at various concentration of oil (500, 1000 and 1500 mg/L). Time (min)

2

Mullite

Prediction of Permeate (kg/m .h)

Mullite-Zeolite

Prediction of Permeate (kg/m2.h)

Mullite-Zeolite-AC

Prediction of Permeate (kg/m2.h)

Concentration of oil (mg/L)

70

80

90

100

125

150

500 1000 1500 500 1000 1500 500 1000 1500

153.2 96.5 95.8 174.6 142.5 103.4 208.8 164.2 122.5

148.2 94.3 89.6 157.8 131.1 93.1 170.1 148.5 108.9

143.2 92.1 83.4 141.0 119.7 82.9 143.7 132.7 95.4

138.2 84.9 77.2 124.2 96.9 72.6 117.2 117.0 81.9

125.8 84.3 61.7 132.6 82.2 46.9 97.8 77.7 48.1

113.0 78.8 46.2 108.6 40.2 21.2 47.4 38.3 14.3

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B. Jafari et al. / Journal of Cleaner Production 244 (2020) 118720

Table 4 The predicted PF results for Mullite, Mullite-Zeolite and Mullite-Zeolite-AC membranes at various concentration of salt (50, 100, 150 and 200 g/L) and 1500 mg/L oil in water. Time (min)

2

Mullite

Prediction of Permeate (kg/m .h)

Mullite-Zeolite

Prediction of Permeate (kg/m2.h)

Mullite-Zeolite-AC

Prediction of Permeate (kg/m2.h)

Concentration of Salt (g/L)

70

80

90

100

125

150

50 100 150 200 50 100 150 200 50 100 150 200

98.1 62.3 32.3 16.02 137.4 77.0 33.9 19.5 183.6 117.7 74.9 54.9

91.3 53.7 20.6 6.0 119.1 64.5 15.9 9.0 161.9 96.7 51.8 31.8

84.4 45.1 8.9 0.0 100.8 52.1 0.0 0.0 140.2 75.8 28.8 8.8

77.5 36.5 0.0 0 .0 82.5 39.6 0.0 0 .0 118.5 54.9 5.7 0.0

60.4 15.0 0.0 0.0 36.9 8.5 0.0 0.0 64.3 2.6 0.0 0 .0

43.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 10.0 0 .0 0.0 0.0

nature of these materials and the increase in the micro and mesopore voids in the structure of the membranes. The result of pure water flux showed that the PF increased linearly with the applied pressure difference (from 1 to 4 bar) and the permeability was about 1352 for Mullite, 1766 and 3848 (Kg/m2.h.bar) for MulliteZeolite and Mullite-Zeolite-AC. In addition, the effect of sintering temperature (i.e., 550, 750,
950 and 1150 C) on the tubular membrane shrinkage and porosity were studied. The results indicated that with increasing temperature, the porosity, linear radial, and longitudinal shrinkage decreased due to the phase transformation from kaolin to metakaolin and mullite (according to the results of XRD) as well as release of free silica. Moreover, the oil removal efficiency increased from 78.5 to 98.5, 76.1 to 98.2 and 73.2e97.4% by incorporation of natural zeolite and AC in the structure of Mullite membrane for wastewater containing 500, 1000 and 1500 mg/L oil. The result of flux recovery disclosed that NaOH as the relatively cost-effective chemical cleaning agent was capable of recovering the membrane up to 65.82% and this result could further be enhanced by using backwash and binary and ternary strong chemical cleaning agents in first and second cleaning steps. The result also revealed that at low salt concentrations, PF of the membranes increased, but it decreased under high salt concentration. Moreover, the TOC rejection of membranes improved as the salt concentration increased. For example, treatment by Mullite-Zeolite-AC membrane could achieve 99.99% of TOC rejection for a feed concentration of 1500 mg/L. Finally, an exponential triple smoothing model was also used to predict the PF of membranes at long operation times. The predicted values of the model showed that the PF of the membrane was relatively good after 150 min operation for treatment of oily wastewater. These results suggested that using ceramic membranes made by natural zeolite and active carbon could be an interesting alternative to traditional methods for treatment of oily wastewater regarding their low consumption of chemicals, costeffectiveness, and high efficiency. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References Abbasi, M., Mirfendereski, M., Nikbakht, M., Golshenas, M., Mohammadi, T., 2010a. Performance study of mullite and mulliteealumina ceramic MF membranes for oily wastewaters treatment. Desalination 259, 169e178. Abbasi, M., Salahi, A., Mirfendereski, M., Mohammadi, T., Pak, A., 2010b.

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