Phenol removal from water by polyamide and AgCl mineralized thin

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Phenol removal from water by polyamide and AgCl mineralized thin film composite forward osmosis membranes Yangbo Huang, Pinar Cay Durgun, Tianmiao Lai, Ping Yu, and Mary Laura Lind Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00205 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Industrial & Engineering Chemistry Research

Phenol removal from water by polyamide and AgCl mineralized thin film composite forward osmosis membranes Yangbo Huang1,2, Pinar Cay-Durgun2,3, Tianmiao Lai2, Ping Yu1, Mary Laura Lind2,3* 1

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, People’s Republic of China 2

School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, United States 3

Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment, Arizona State University, Tempe, Arizona 85287, United States *corresponding author: [email protected]

Abstract: This study systematically investigated phenol removal from water and phenol fouling of membranes in forward osmosis (FO) using thin film composite (TFC) polyamide and silver chloride (AgCl) mineralized thin film composite (MTFC) membranes. The influence of operating parameters (membrane orientation, phenol concentration, draw solution concentration, pH, and ionic strength) on phenol rejection and phenol adsorption to membrane were investigated to elucidate the phenol transport behaviors in FO process. Overall, phenol rejection improved with increased draw solution concentration or feed solution pH. At a feed solution pH of 11, TFC membranes exhibited their highest phenol rejection of 97.0% and MTFC membranes exhibited their highest phenol rejection of 98.8%. Phenol adsorption on the membrane surface may be related to the solute hydrophobic character, electrostatic interaction, and reverse salt diffusion. Six-hour fouling experiments show that the phenol fouling of FO membranes is reversible and easily cleaned by physical flushing. Additionally, the MTFC membranes have an increased flux and rejection for phenol in FO than the TFC membranes. Keywords: forward osmosis, phenol removal, adsorption, thin film composite membrane 1. Introduction Industrial chemical production is accompanied by many waste streams that may have negative impacts on the environment and human health from their toxicity, carcinogenicity, and teratogenicity. Phenol is one of the most common contaminants present in the effluents of petrochemical production, coal refineries, paper production, plastics and resin industries.1-3 Phenol and its derivatives are highly toxic and difficult to degrade. They affect organisms even at very low concentrations and result in serious ecological problems when improperly discharged.4 According to the World Health Organization, the concentration limit for phenolic compounds in potable and mineral water is 1 µg/L.5 Hence, removal of phenol and its derivatives from wastewater is critical prior to wastewater discharge.

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Research has focused on removal of phenol and its derivatives from wastewater by improvement of current treatment methods and by development of new technologies.6, 7 Conventional methods to eradicate phenol are categorized into two groups: separation technologies (e.g. distillation, extraction, adsorption, and membrane-based separation) and destruction technologies (e.g. chemical oxidation, electrochemical oxidation, photocatalytic oxidation, and biochemical abatement).8 However, challenges remain for practical applications of both types of technology including poor performances, high financial investment, and secondary pollution. For example, adsorption and extraction of phenol may result in secondary pollution from the disposal of the spent adsorbent and extractant.8 Additionally, biodegradation of phenol removal often has low efficiency because of phenol’s high toxicity to microbial species.9 Membranebased separation methods such as reverse osmosis (RO) and nanofiltration (NF) are widely used for wastewater treatment. Unfortunately, despite the excellent performance of RO process in desalination, RO and NF processes often show low retention for many organic micro-pollutants and suffer from severe fouling problems.10, 11 In recent years, forward osmosis (FO) membrane processes have shown promising performances in wastewater treatment, seawater desalination, and power generation applications.11-14 In FO processes, water transports from a low-concentration feed solution (FS) through the FO membrane to dilute a highconcentration draw solution (DS); solutes in feed solution are effectively rejected by the FO membranes. The osmotic pressure gradient between the feed and draw solution is the driving force in FO processes, thus no applied external pressure is required. Currently, two major obstacles face the commercialization of FO processes. First, current FO membranes do not achieve the desired performance with both high water flux and low reverse salt flux; second, FO processes lack an effective draw solution that can be easily regenerated.15, 16 Nevertheless, compared to the pressure driven processes like RO and NF, FO shows the advantages of high water recovery, low fouling tendency, and ease of operation. Although reversible fouling is commonly observed on FO membrane surfaces, the membranes can be easily regenerated after physical cleaning.17, 18 Recent research reported that FO processes are able to reject small molecules, such as boron, aniline, nitrobenzene or other trace organic micropollutants.18-21 Thus, FO is a promising alternative method to treat phenol wastewater. In addition, liquid fertilizer has been studied as a draw solution in FO for a variety of wastewater treatments; this would enable direct use of the diluted draw solution for irrigation, thus mitigating energy and cost issues.22, 23 Similarly, fertilizer may be a good choice as draw solute for FO removal of phenol in practical applications. Other compounds such as NaCl, MgSO4, and glucose are potential draw solutes for FO because the diluted draw solution can be easily regenerated by NF or RO. This study explores the removal efficiency of phenol from wastewater during FO processes using thin film composite (TFC) polyamide membranes and silver chloride (AgCl) mineralized TFC (MTFC) membranes. The rejection behavior mainly depends on the physicochemical properties and interaction of feed solution, draw solution, and FO membranes. Our previous study demonstrated that AgCl mineralized TFC membranes had higher solute rejection and better fouling resistance compared to un-modified TFC membranes.24 Based on these results, we hypothesized that the MTFC membrane would achieve better removal efficiency than TFC membranes in phenol wastewater treatment. In this work, we also investigated the impact of operating conditions on phenol rejection to find conditions to achieve better removal efficiency of phenol. This study also investigated the adsorption of phenol on the membrane surface and the water flux recovery after physical cleaning. This study provides a better understanding of


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phenol rejection behavior and gives practical insight on critical parameters for an efficient FO process for phenol removal. 2. Materials and experimental methods 2.1 Chemicals Polysulfone beads (PSf, Mw=7500, Acros Organics), 1-Methyl-2-pyrrolidinone (NMP, 99%, Fisher Scientific) and polyvinylpyrrolidone (PVP, Fisher Scientific) were used to make the membrane substrates. 1,3-Phenylenediamine (MPD, 99%, Sigma-Aldrich), trimesoyl chloride (TMC, 98.0%, Sigma-Aldrich) and Isopar-G (100%, Fisher Scientific) were used for interfacial polymerization. Silver nitrate (AgNO3, 99%) and sodium chloride (NaCl, 99.5%) purchased from Sigma-Aldrich were used for surface mineralization to prepare MTFC membranes. Sodium chloride was used as draw solution and phenol (99.5%, Sigma-Aldrich) was used as feed solution for FO tests to simulate wastewater. 2.2 Preparation of TFC and MTFC membranes PSf substrates were prepared according to our previous study.25 A polyamide active layer was interfacially polymerized on the surface of the PSf substrate via the following procedure based on our previous work.26 First, the PSf substrate was cut to a suitable size and taped onto a glass plate. Then the glass plate was dipped in 200 mL 2 wt% MPD aqueous solution for 120 s. The excess MPD droplets were removed with a fingerprint ink rubber roller. Next, the glass plate was immersed into a vertical membrane holder containing 0.15 wt% TMC in Isopar-G solution for 120 s. Then the glass plate was taken out and placed in the fume hood for 15 min to evaporate the solvent. The synthesized membrane was rinsed with deionized (DI) water and stored in DI water until testing. Alternate soaking process (ASP) was utilized for surface mineralization.24, 27 Specifically, the TFC membrane was first soaked in 0.1 M AgNO3 solution for 1 min, and then washed by DI water for 1 min to remove the residual ions. Next, the membrane was immersed in 0.1 M NaCl solution for 1 min and then rinsed again in DI water for 1 min. The two steps described above were considered as a single ASP cycle. In this study, we prepared the MTFC membrane with 4 ASP cycles since it was previously reported that membranes with 4 ASP coating cycles had optimal FO desalination performance.20 2.3 Membrane characterizations The surface morphologies of clean and fouled membranes were examined by an 1910 field emission scanning electron microscope (FESEM, Amray). Surface hydrophilicity of the membranes was determined using a deionized water sessile drop contact angle instrument (Easy Drop DSA-20, Kruss). 2.4 Forward osmosis performance evaluation To investigate phenol removal efficiency, FO tests were conducted with a bench-scale cross-flow FO testing cell with details described previously.24 The membrane cell has two identical channels on either side of the membrane. The dimensions of both channels were 2.45 cm in radius and 0.21 cm in depth, which provided a membrane effective area (Am) of 18.86 cm2. The flow rates of feed solution (500 mL phenol aqueous solution) and draw solution (500 mL NaCl aqueous solution) were both set at 5 gallons per hour. We conducted the FO tests in both AL-DS mode (active layer facing draw solution) and AL-FS



mode (active layer facing feed solution). This enabled assessment of the effects of feed concentration and membrane orientation on phenol removal efficiency. Only AL-FS mode was performed for experiments. Preparation of phenol containing solutions and FO experiments were conducted in the fume hood because of the high toxicity of phenol. Each of the experimental performance tests included three steps.21, 28 First, the FO system was rinsed thoroughly with deionized water. Second, a phenol solution (100-500 mg/L) was cycled through the feed-side of the FO system for 1 h to mitigate the influence of mass adsorption onto the system (pipes, flow meter, and pump) during subsequent data collection. Third, the FO system was operated for 1 h using a new membrane to collect data. During the FO test, the DS is diluted. However, in our tests using a draw solution volume of 500 mL with an initial concentration of 1 mol/L NaCl, enabled the concentration variation of both the DS and FS to stay within 5% of the initial concentration. Phenol concentration in solution was determined by UV-Vis spectroscopy (UV 1700, Shimadze, Japan) at the absorbance wavelength of 270 nm. The water flux (Jw), phenol solute flux (Js) and phenol rejection (R) were calculated by the following equations:

Jw =

∆mdraw Am ∆t

(1)

Js =

CdVd Am ∆t

(2)

R = (1 −

CdVd C f Vp

) ×100%

(3)

where ∆ mdraw (g) is the weight increment of the DS during the testing time ∆t (h), Cd (mol/L) is the phenol concentration in DS at the end of tests. Cf (mol/L) is the initial phenol concentration in FS, Vd (L) is the final volume of DS and Vp (L) is the volume increment in DS. The amount of phenol adsorbed (Ma, g/m2) on the membrane and adsorption percentage (Ya, %) were calculated by Eq. (4) and (5):

Ma =

Ya =

C f V f − (CdVd + CrVr ) Am

C f V f − (CdVd + CrVr ) C fVf

(4)

× 100% (5)

where Vf (L) is the initial volume of the FS, Vr (L) is the final volume of FS and Cr (mol/L) is the final concentration of FS. Error bars in all figures correspond to standard deviation. Every data point represents the average of at least four replicates. A single membrane sample was tested only once to avoid the influence of phenol fouling or adsorption on the membrane surface.


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2.5 FO fouling and cleaning procedure All fouling tests were operated in AL-FS mode for 6 h with 1000 mg/L phenol solution (pH=7) as feed solution and 1 M NaCl solution as draw solution. Prior to the tests, a control experiment was conducted to correct for the flux decline caused by the concentration change of feed and draw solution during the testing interval. Membrane physical cleaning was performed immediately after the fouling tests. The fouled membrane was rinsed in the FO system using DI water for 1 h. The flow rate was set at 5 gallons per hour. Then the cleaned membrane was evaluated under the same fouling conditions. 3 Results and discussion 3.1 Effect of phenol concentration and membrane orientation To investigate the influence of phenol concentration and membrane orientation on phenol removal efficiency, experiments were carried out using 1 M NaCl solution as DS and FS of phenol with concentrations ranging from 0 to 500 mg/L, pH=7. The experiments were conducted in both AL-DS and AL-FS mode. Fig. 1 and Fig. 2 show the results of water flux and phenol rejection. Fig. 1 shows that the water flux of TFC and MTFC membranes decline as the concentration of phenol increases from 0 to 500 mg/L in both AL-DS and AL-FS mode. The increased osmotic pressure in the feed solution was much lower than the transmembrane osmotic pressure difference provided by 1 M NaCl, therefore the flux reduction primarily resulted from increased fouling with increased FS phenol concentration. Previous researchers reported, for general FO processes, the water flux was higher in ALDS mode than in AL-FS mode,19, 29, 30 which is consistent with our results. This difference in fluxes, based on membrane active layer orientation, is a result of the different types of internal concentration polarization (ICP) in the support layer of TFC membranes. For example, concentrative ICP occurs in ALDS mode and dilutive ICP occurs in AL-FS mode. Dilutive ICP leads to a larger decrease in chemical driving force, therefore membranes in AL-FS mode have lower water flux.31 Fig. 1 also shows that the water flux in AL-DS mode decreases more rapidly than that of AL-FS mode as the phenol concentration increases. This is primarily the result of an accumulation of phenol within the support layer in AL-DS mode, which accelerates the decrease of water flux.



Figure 1. Effect of phenol concentration on water flux in (a) AL-DS mode and (b) AL-FS mode. Experimental conditions：pH=7; and draw solution=1 M NaCl.

Figure 2. Effect of phenol concentration on phenol rejection in (a) AL-DS mode and (b) AL-FS mode. Experimental conditions: pH=7; and draw solution=1 M NaCl. Fig. 2 shows that the phenol rejection of TFC and MTFC membranes are nearly constant in both AL-DS and AL-FS modes, regardless of the phenol concentration. Similar results were found by Fan19 and Liu21 for removal of boron and cobalt ions using FO process. Fig. 2 also indicates that overall phenol rejection was much lower in AL-DS mode. This may be attributed to the two different ICP types. The concentrative ICP that occurs in AL-DS mode leads to a high phenol concentration within the support layer and this further enhances phenol transport across the membrane resulting in lower phenol rejection. In addition, the rejection of phenol in both modes is satisfactory. T. Xiao et al. reported that at the same water flux, TFC membranes showed higher phenol rejection in FO tests than in RO tests.32 This indicates that the FO process is an alternative method for phenol wastewater treatment. While investigating the influence of membrane orientation helps us understand the phenol transport mechanism in FO process, AL-FS modes should be applied as the most practical application for


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wastewater treatment. Operating FO in AL-DS mode often leads to irreversible fouling in the substrate, which significantly reduces the lifetime of FO membranes.17, 20 Additionally, Fig. 2 shows that the FO separation performance for phenol of MTFC membranes is better than TFC membranes. Table 1 reports the sessile drop contact angles for water of the membranes. The MTFC membrane has a smaller contact angle (46±2.5°) than that of TFC membrane (71±1.7°). This indicates that the MTFC has a more hydrophilic surface than the TFC membrane, which may result in a higher water flux. Additionally, the MTFC membranes have a higher NaCl rejection (66.1 %) than the TFC membranes (62.2 %), which is consistent with the results of phenol rejection. Table 1 Properties of TFC and MTFC membranes. Membrane

Thicknessa (µm)

Contact angle (°)

Zeta potentialb at pH 6 (mV)

NaCl rejectionc (%)

TFC

98±3

71

-27.0

62.2

MTFC

97±4

46

-37.5

66.1

a

Membrane thickness measured with a digital micrometer. Zeta potential reported from previous work.24 c NaCl rejection was measured with brackish water (20 mM NaCl) at pressure of 2.5 bar in reverse osmosis. b

3.2 Effect of draw solution concentration To investigate the effect of the draw solution concentration on phenol rejection, we increased the sodium chloride concentration in the draw solution from 0.5 to 2 M while we kept the phenol concentration at 500 mg/L and pH=7. The initial transmembrane pressure difference increased from about 24.4 atm to 90.6 atm. We calculated the osmotic pressure according to M. Hamdan’s report33. During these experiments, the membrane was kept in AL-FS orientation.



Figure 3. The effect of draw solution concentration on water flux and phenol rejection. Experimental conditions: AL-FS mode; pH=7; and feed solution=500 mg/L phenol. Fig. 3 shows that the water flux drastically increased as the NaCl concentration increased from 0.5 to 2 M. For example, in MTFC membranes the water flux increased from an average of about 5 LMH (at a draw solution concentration of 0.5 M NaCl) to an average of 12 LMH (at a draw solution concentration of 2 M NaCl). These improvements result from the increased osmotic driving force. Fig. 3 clearly shows that the increased draw solution concentration of NaCl correlated with a significant improvement of phenol rejection. Liu and Kim observed a similar phenomenon that an increased osmotic driving force resulted in improved cobalt and boron rejection rates.21, 34 The increased phenol rejection is mainly the result of the drastic increase of water flux caused by the increased osmotic driving force, which is much more significant than the increment of phenol solute flux. We observed an increase in the phenol rejection from 81.3±1.1% (at FS concentration 0.5 M NaCl) to 92.1±2.0% (at FS concentration 2 M NaCl) for TFC membranes and an increase in phenol rejection from 83.1±1.4% (FS concentration 0.5 M NaCl) to 92.4±1.1% (FS concentration of 2 M NaCl) for MTFC membranes. A. Bódalo et al.10 tested the phenol rejection of three commercial RO membranes in RO mode. They observed 40% phenol rejection; and by adjusting the testing conditions, they achieved a maximum rejection of 80%. Apparently, in RO mode, the phenol rejection of RO membranes is much lower than our mineralized MTFC membranes.10 Therefore, increasing the draw solution concentration is a simple, general method to improve phenol removal efficiency in FO processes, which is not applicable in NF or RO processes.


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3.3 Effect of feed solution pH The pH is an important parameter in the treatment of phenol and an important factor influencing membrane phenol rejection. We adjusted the pH of the feed solutions from 3.0, 5.0, 7.0, 9.0 and 11.0 by adding either 0.1 M hydrochloric acid or sodium hydroxide to the solution. For the FO tests, the FS contained 500 mg/L phenol and the DS contained 1 M NaCl solution. The experiments were conducted in AL-FS mode.

Figure 4. The effect of pH on phenol removal. Experimental conditions: AL-FS mode; feed solution=500 mg/L phenol; draw solution=1 M NaCl. The error is about ±0.8~ 2.2% for phenol rejection, and ± 0.5~2.0% for solute flux, error bars not shown for clarity. As shown in Fig. 4, the phenol rejection increases drastically with increasing feed solution pH, especially at pHs above 10. The results show that the FS pH has a major influence on phenol retention. Previous works showed that for some polyamide membranes used in RO the pollutant retention increases with the pH of the FS,34, 35 this can be explained by the existence of ionizable groups in the membrane structure or the dissociation of solutes in the feed water. Phenol is weakly acidic and at high pH, phenol starts to ionize: C6H5OH+ H2O ⇋ C6H5O- + H3O+ pKa=9.96



Phenol has a pKa value of 9.96 at the standard temperature and pressure. The neutral molecule (C6H5OH) dominates in the FS when the pH is below the pKa, while the phenolate anions (C6H5O-) dominate when the pH is above the pKa. In our experiments, when the feed water pH increased from 9 to 11, the phenol rejection increased from 86.1±2.1% to 97.0±1.2% for TFC membranes and 87.0±1.8% to 98.8±0.6% for MTFC membranes. The higher phenol rejection in alkaline solution is mainly due to the electrostatic repulsion between the phenolate anions and the membrane surface. The surface of a typical polyamide membrane is negatively charged at higher pH as carboxylic functional groups in polyamide structure deprotonate.5, 36 Therefore, increasing the pH of the feed solution can effectively change phenol chemistry that favors phenol retention in FO application. 3.4 Effect of feed solution ionic strength The feed solution contained 500 mg/L phenol and 10-3~10-1 M KCl while the draw solution was 1 M NaCl. The pH of the feed solution was adjusted to 11.0 and membranes operated in AL-FS mode.

Figure 5. The effect of ionic strength on water flux and phenol rejection. Experimental conditions: AL-FS mode; feed solution=500 mg/L phenol with 10-3~10-1 M KCl; draw solution=1 M NaCl; and pH=11. Fig. 5 shows that the increase of KCl in the feed solution results in a decrease of both water flux and phenol rejection. There is no doubt that the decline of water flux resulted mainly from the decreased osmotic pressure difference. According to the reports by Elimelech’s group,37, 38 the electric double layer


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around charged phenolate anions was compressed at higher ionic strength, which weakened the electrostatic repulsion between phenol and the membrane surface. This resulted in lower phenol rejection. In addition, the reduced phenol rejection may also result from the charge neutralization on the membrane surface. The increased K+ in feed solution partially neutralized the negative charges of polyamide membrane, which reduced the electrostatic repulsion of the membrane and phenolate anion and facilitated the phenol passage.5 3.5 Phenol adsorption Organic pollutants adsorbed to the membrane surfaces impede the membranes’ separation performance.20 Here, the removal efficiency of phenol molecules in FO process has contributions from both membrane rejection and phenol adsorption to the membrane. The adsorption percentage (Ya, %) of phenol to the membrane was calculated by equation (5), and the adsorption percentage under different operating parameters (phenol concentration, draw solution concentration, pH, and ionic strength) was discussed. All the data was collected from the experiments in section 3.1 to 3.4. Fig. 6 shows that the adsorption of phenol on the membrane surface was enhanced as the increase of initial phenol concentration in the feed solution. But the overall adsorption percentage was relatively low and below 7%. Some researchers reported that hydrophobic interactions between the solutes and membrane may be responsible for this surface phenomenon.5, 39, 40 Because the polyamide membranes synthesized in this work are hydrophilic, the hydrophobic interaction between phenol and the membrane is weak and leads to limited fouling. It can also be seen that the adsorption percentage of MTFC membranes is lower than that of TFC membranes. This is because the MTFC has a more hydrophilic surface than the TFC membrane. Thus, the MTFC membranes are less prone to phenol fouling than TFC membranes.



Figure 6. The effect of phenol concentration on phenol adsorption to the membrane surface. Experimental conditions: AL-FS mode; pH=7; and draw solution=1 M NaCl. Fig. 7 shows that increasing the draw solution concentration reduced phenol adsorption of both TFC and MTFC membranes. We concluded that the increment of transmembrane solute concentration difference led to an increase of reverse salt flux. As the reverse diffusion of salt molecules from the opposite direction is similar to a backwash process, the increased reverse salt flux may retard phenol attachment to the membrane surface.


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Figure 7. The effect of draw solution concentration on phenol adsorption. Experimental conditions: ALFS mode; pH=7; and feed solution=500 mg/L phenol. Fig. 8 shows a significant decrease of phenol adsorption percentage as the pH increases from 9 to 11. As we discussed in Section 3.3, phenolate anions (C6H5O-) dominates at pH above the pKa. The electrostatic repulsion between the phenolate anions and the membrane surface leads to phenol molecular diffusion away from the membrane surface, resulting in less phenol adsorption.



Figure 8. The effect of pH on phenol adsorption. Experimental conditions: AL-FS mode; feed solution=500 mg/L phenol; draw solution=1 M NaCl. As depicted in Fig. 9, the phenol adsorption increased as the KCl concentration in the FS increased from 10-3 mol/L to 0.1 mol/L. According to the discussion in Section 3.4, the electric double layer around charged phenolate anions was compressed at higher ionic strength, which resulted in an increase in the intermolecular adhesion of phenol to the membrane surface.37, 38 In addition, the increased phenol adsorption may result from the presence of increased K+ in the FS which partially neutralized the negative charges of the polyamide membrane; this ultimately weakened the electrostatic repulsion of membrane and phenolate anions. In summary, the operating parameters (phenol concentration, draw solution concentration, pH, and ionic strength) greatly influence phenol adsorption. The adsorption behavior appears to be related to solute hydrophobic character, electrostatic interaction, and reverse salt diffusion. However, the adsorption of phenol to the membranes was relatively low in our testing parameters. The detrimental effects of phenol adsorption on membrane performance was limited, especially when the FS pH is above 9.96.


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Figure 9. The effect of ionic strength on phenol adsorption. Experimental conditions: feed solution=500 mg/L phenol with 10-3~10-1 M KCl; draw solution=1 M NaCl; and pH=11. 3.6 Fouling and recovery In this section, we conducted six-hour fouling experiments to evaluate the phenol antifouling properties of TFC and MTFC membranes. All fouling tests were operated in AL-FS mode with 1000 mg/L phenol solution (pH=7) as feed solution and 1 M NaCl solution as draw solution. Fig. 10 shows the fouling behavior of TFC and MTFC membranes. The water flux of both membrane types declined about 30% after the 6-hour test. After physical cleaning with DI water, the water flux recovered to 85% of its initial flux for TFC membranes and 90% for MTFC membranes. The high flux recovery indicates that the phenol fouling in FO processes is reversible and is easily cleaned by a physical flush. Similar results reported by Mi et al. showed that alginate fouling in FO is almost completely reversible as the fouling layer is loosely attached to the membrane surface because there is little hydraulic pressure.41 MTFC membranes exhibit lower water decline ratios and higher flux recovery rates than the TFC membranes. The SEM images (Fig. 10 (b) and (c)) also show that the fouling of the TFC membrane is more severe. The results indicate that MTFC membranes are more antifouling to phenol than TFC membranes. We hypothesize this is primarily a result of the enhanced negative charge and hydrophilicity (shown in Table 1) of the membrane surface after AgCl mineralization.



Figure 10. Fouling behavior of TFC and MTFC membranes. (a) water flux before physical cleaning and after physical cleaning indicated by the dash line; (b) SEM image of fouled MTFC membrane; (c) SEM image of fouled TFC membrane. Experimental conditions: initial feed solution=1000 mg/L phenol; draw solution=1 M NaCl; and pH=7. 4. Conclusion This study systematically investigated phenol removal from water by FO processes using TFC and MTFC membranes. We investigated the influences of membrane orientation, phenol concentration, draw solution concentration, pH, and ionic strength on phenol rejection and phenol adsorption. Results showed that the FO process is a potential alternative method for phenol wastewater treatment. (1) Phenol rejection remained constant regardless of the initial phenol concentration in the feed solution. (2) Membrane orientation had a strong influence on phenol rejection. Although the AL-DS mode resulted in higher water flux, it was also accompanied by lower phenol rejection and irreversible fouling in the polysulfone support layer. (3) Increasing the draw solution concentration is a simple, general method to improve phenol removal efficiency in FO processes. On the contrary, higher ionic strength in the feed solution has a negative effect on both phenol rejection and water flux because of charge neutralization and charge screening effects. (4) Phenol rejection was significantly improved by adjusting the pH of feed solution above 9.96 (the pKa of phenol). Phenol is weakly acidic and the phenolate anion dominates in the feed solution when pH is above the pKa. So the electrostatic repulsion between the phenolate anion and the membrane surface results in increased rejection. Therefore, increasing the pH of the feed solution can effectively change phenol chemistry that favors phenol retention in FO applications. (5) The operating parameters greatly influenced phenol adsorption. The adsorption behavior may be related to solute hydrophobic character, electrostatic interaction, and reverse salt diffusion.


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(6) The long-term fouling experiments showed that the phenol fouling in FO processes is reversible and can be easily cleaned by a physical flush. It is worth noting that the MTFC membranes have increased antifouling property to phenol compared to TFC membranes. This may be mainly the result of the enhanced negative charge and hydrophilicity of the membrane surface after AgCl mineralization. In all, the relatively high rejection, low fouling tendency, and fouling reversibility properties of FO processes make them a potentially better method for phenol wastewater treatment than NF/RO processes.

Acknowledgment The authors gratefully acknowledge the China Scholarship Council. We gratefully acknowledge Margarita Judith from Dr. Jerry Lin’s lab at Arizona State University for the phenol concentration determination. Pinar Cay-Durgun is supported by the National Science Foundation through a CAREER award CBET-1254215 and the Nanotechnology Enabled Water Treatment (NEWT) Engineering Research Center (ERC-1449500).

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Phenol removal from water by polyamide and AgCl mineralized thin

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