Tight Ultrafiltration Ceramic Membrane for Separation of Dyes and

Article pubs.acs.org/IECR

Tight Ultrafiltration Ceramic Membrane for Separation of Dyes and Mixed Salts (both NaCl/Na2SO4) in Textile Wastewater Treatment Xiao Ma, Pengli Chen, Ming Zhou, Zhaoxiang Zhong, Feng Zhang, and Weihong Xing* State Key Laboratory of Materials-Oriented Chemical Engineering, National Engineering Research Center for Special Separation Membrane, Nanjing Tech University, Nanjing 210009, Jiangsu, China ABSTRACT: Commercial nanofiltration (NF) membranes have been used to separate dyes and salts in industry; however, NF membrane’s high rejection to divalent salts (i.e., Na2SO4) leads to a reduction of salt recovery. In this study, a tight ultrafiltration (t-UF) ceramic membrane (MWCO 8800 Da) is proposed to fractionate dyes and mixed salts (NaCl/Na2SO4) for textile wastewater treatment. Performance of the t-UF ceramic membrane and DK polymeric membrane (from GE) has been compared regarding to permeability, retention of reactive dyes, and permeation of salts. The t-UF ceramic membrane presents better permeability, competitive rejection of dye molecules (>98%), and reduced rejection of NaCl ( 98.9% for anion dyes including Direct Red 80, Direct Red 23, Congo Red, and Reactive Blue 2 with the desalination efficiency up to 98% Received: Revised: Accepted: Published: 7070

April 7, 2017 May 11, 2017 May 19, 2017 May 19, 2017 DOI: 10.1021/acs.iecr.7b01440 Ind. Eng. Chem. Res. 2017, 56, 7070−7079

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Industrial & Engineering Chemistry Research Table 1. Information on the Dye Molecules

in Lin et al.’s work.18 In contrast to the polymeric ones, ceramic membranes generally perform with better thermal, chemical, and mechanical stability and improved permeability,19,20 and some UF ceramic membranes have been selected in textile wastewater treatment. Barredo-Damas et al. studied the performance of ceramic membranes with MWCO of 30, 50, and 150 kDa for dye effluent treatment, and the membranes presented a maximum decoloring rate between 82% and 98%.21 Alventosa-de Lara et al. selected ultrafiltration ceramic membrane (MWCO 150 kDa) to separate dye Reactive Black 5 and NaCl in solution leading to dye rejection of 45.6% at TMP of 1 bar.22 Desalination of dye solution (Erichrome black T) containing NaCl/Na2SO4 was investigated by Chen et al. using a ceramic nanofiltration membrane of MWCO 900 Da and yielding the retention of dye > 99%, NaCl < 10%, and Na2SO4 < 20% at an operating pressure of 3 bar.23 With the development of membrane preparation technology, ceramic membranes can be fabricated having an active layer of tight structure (mean pore size as low as 1−5 nm), which can lead to their applications in chemical processes including food, pharmaceutical, and wastewater treatment.24−27 In this work, a tight ultrafiltration (t-UF) ceramic membrane consisting of a TiO2/ZrO2 skin layer with a mean pore size of 1.16 nm on porous Al2O3 support was studied. The feasibility of the t-UF membrane for separation of dye and mixed salts (NaCl/Na2SO4) has been investigated with the aim of recovering dye and mineral sources. First, the t-UF ceramic membrane has been compared to DK membrane (from GE) in filtration experiments using monocomponent of Reactive Blue KN-R, Reactive Black 5, Reactive Red H-E7B, NaCl, and Na2SO4 in water solution. Further on, the performance of the ceramic membrane has been intensively studied for the dye and NaCl/Na2SO4 (mass ratio 1:1) fraction by adjusting the pressure, temperature, pH, and salt concentration and changing the valence of the salt ions and charge of dyes in the membrane process. The effect of operative parameters and the interaction between the solutes and the ceramic membrane will be discussed in the following sections.

(RB5, from Zhejiang Runtu Co. Ltd.), and Reactive Red H-E7B (RR HE7B, from Everlight Chemical Industrial Co.) and a cation dye Cation Yellow X-2RL (CY X-2RL, from Shijiazhuang Wancai Chemical Industry Co.) were used in the study. Chemical and structural information on the dye molecules are listed in Table 1. Also, the zeta potential of dye solutions has been measured with pH from 2 to 11 as presented in Figure 1, giving information on the nature of dye charges at

Figure 1. Zeta potential of the dye solution.

corresponding pH (also listed in Table 1). NaCl and CaCl2 were provided by Xilong Chemical Co. Ltd., and KCl, MgCl2, AlCl3, and Na2SO4 were provided by Sinopharm Chemical Reagent Co. Ltd. all of analytical purity grade. All chemicals were used without any further purification. Solutions were prepared with pure water with conductivity less than 15 μS/cm. The t-UF ceramic membranes were supplied by Jiangsu Jiuwu High-tech Co. Ltd. The multichannel tubular membrane is of length of 0.5 m and has 19 channels with an inner diameter of 4.0 mm. The effective filtration area of the membrane is 0.1 m2. The upmost active layer and porous support are composed of TiO2/ZrO2 and α-Al2O3, respectively. The commercial polymeric NF membranes in spiral-wound module (DK 1812) were provided by GE Osmonics, having an effective membrane area of 0.38 m2.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Membranes. Three anion dyes including Reactive Brilliant Blue KN-R (RB KNR, from Shanghai Eighth Dyestuff Chemical Plant), Reactive Black 5 7071

DOI: 10.1021/acs.iecr.7b01440 Ind. Eng. Chem. Res. 2017, 56, 7070−7079

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Industrial & Engineering Chemistry Research

Figure 2. (a) Cross-section and surface SEM images of t-UF ceramic membrane. (b) MWCO cut off curve and pore size distribution (inset) of the membrane.

2.2. Membrane Characterization. The morphology of the t-UF ceramic membrane was scanned with SEM (S-4800, Hitachi, Japan), and the cross-section and surface images are displayed in Figure 2a. Pure water permeability of the t-UF membrane (32.4 L·m−2·h−1·bar−1) has been measured at 25 °C with TMP from 1 to 3 bar and CFV = 3 m/s and the DK membrane (5.1 L·m−2·h−1·bar−1) with TMP 4−12 bar and flow rate 6.5 L/min. The pore size distribution of the ceramic membrane was characterized using polyethylene glycols (PEG) of various molecular weights of 2000, 4000, 6000, and 10 000 Da. The solute transport method28,29 is to estimate the pore size distribution of the membrane using the PEG solution 0.6 g/L at TMP 0.2 bar. The concentration of PEG was measured with a Total Organic Carbon analyzer (TOC, Multi 3100, Analytik Jena, Germany). The relationship between the Stokes radius and the molecular weight (MW) can be expressed as the following equation18 molecular radius = 0.01673 × MW 0.5

were returned to the feed tank. The duration of each test was 60 min with a constant CFV of 3 m/s at 25 °C. In all experiments, permeate flux was continuously monitored and solutions were collected from both feed and permeate streams for analysis. The solution pH was adjusted with hydrochloric acid and sodium hydroxide. After each filtration experiment the used membrane was rinsed with pure water and then cleaned with sodium hydroxide solution at pH 10−11. To follow up, hydrogen peroxide of concentration 0.5% (v/v) was used to remove the organic pollutants on the membrane surface. Eventually the membrane was rinsed with deionized water until the pH turned neutral, and pure water flux of the membranes was tested again. It is noted that pure water flux of the ceramic membrane must recover to 95% of the initial flux before any new filtration tests would be carried out. In the processes, a low operating pressure and high cross-flow rate were practiced. 2.4. Analytical Methods. The concentration of dyes in solution was determined with a UV/vis spectrophotometer from Thermo Scientific AQ 7000. The concentration of monosalt NaCl contained in dye solutions was determined with a conductivity meter (Rex DDSJ-308A, Shanghai Rex Instrument Factory). The concentration of mixed salts NaCl and Na2SO4 contained in dye solutions was analyzed by ion chromatography (Dionex ICS-2000 System, USA). Solution pH was measured with a pH meter (ZDJ-4A, Shanghai Precision Scientific Instrument Co. Ltd.). The zeta potential and particle size of dyes in solution was measured using a Zetasizer Nano ZS apparatus (Malvern Instruments Ltd., UK). The flux, J, was calculated by the following equation

(1)

The pore size distribution of the nanoporous ceramic membrane can be obtained through the probability density function30,31 dRT(rp) drp

⎡ (ln r − ln μ )2 ⎤ p 1 p ⎥ exp⎢ − = 2 ⎥⎦ ⎢ rp ln σp 2π 2(ln σp) ⎣

(2)

where μp is the mean effective pore radius which is determined at the PEG rejection coefficient of R = 50% and σp is the geometric standard deviation which is defined as the ratio of rp at R = 84.13% over that at R = 50%. Accordingly, the MWCO of the t-UF ceramic membrane arriving at R= 90% pointed at a PEG of molecular weight 8800 Da, and accordingly the mean effective pore radius of the membrane was 1.16 nm as given in Figure 2b. 2.3. Membrane Unit. The performance of the t-UF ceramic membrane was evaluated in a lab-scale cross-flow permeation setup whose configuration can be found in the previous work of our group.32 Feed solution was pumped from a tank (20 L) into the membrane cell with a centrifugal pump. The transmembrane pressure (TMP) and cross-flow velocity (CFV) were regulated by valves. Full circulation mode was used for all experiments, where both the retentate and the permeate

J=

V A·t

(3) 2

where V (L) is the volume of permeated water, A (m ) is the effect area of the membrane, and t (h) is the permeation duration. The observed rejection (R) of solutes was calculated as a percentage according to the following equation ⎛ Cp ⎞ R(%) = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

(4)

where Cp (g/L) and Cf (g/L) are the concentrations of solutes in permeate and retentate, respectively. 7072


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Figure 3. Comparison of t-UF ceramic membrane and NF polymeric membrane: (a) permeation flux and (b) dye rejection (RB KNR, RB5, and RR HE7B) and salt rejection (NaCl and Na2SO4) (conditions: TMP = 2 bar and CFV = 3 m/s for t-UF membrane; TMP = 10 bar and flow rate = 6.5 L/min for DK membrane; T = 25 °C, C(dye) = 0.5 g/L, C(NaCl) = 1.0 g/L, C(Na2SO4) = 1.0 g/L).

Figure 4. Flux (a) and rejection (b) of the t-UF membrane at different transmembrane pressure using dye and NaCl/Na2SO4 aqueous mixture (conditions: C(dye, RR HE7B) = 0.5 g/L, C(salts) = 1.5 g/L (m(NaCl):m(Na2SO4) = 1:1), CFV = 3 m/s, and T = 25 °C).

UF ceramic membrane has an upmost layer of tightly structured TiO2/ZrO2 on porous support α-Al2O3; the polymeric NF membrane is composed of polyamide (DK from GE). The permeation flux of the monocomponent solution (containing RB KNR, RB5, RR HE7B, NaCl, or Na2SO4) through both membranes under transmembrane pressure at 2 and 10 bar, respectively, are displayed in Figure 3a. The permeability of t-UF membrane is 21.8 L·m−2·h−1·bar−1 and DK membrane 5.3 L·m−2·h−1·bar−1 using the dye solution of RR HE7B. The t-UF ceramic membrane has 4 times higher permeability than that of DK membrane due to the larger pore size. The DK membrane with a negative-charged surface presented a less fouling tendency (i.e., stable flux within 1 h as in Figure 3) owing to electrostatic repulsion to the anion dyes, while the ceramic surface is normally slighter negative charged. Rejection to the individual species has been analyzed by both membranes in monocomponent solution as given in Figure 3b. In general, ceramic t-UF membrane retained dyes of RB KNR, RB5, and RR HE7B with competitive percentage (above 97%) in comparison to DK membrane (above 99%). On the contrary, the t-UF membrane had much lower rejection to NaCl (5%) and Na2SO4 (10%) than that of DK membrane (NaCl 70% and Na2SO4 99%). In other words, the ceramic membrane provided

The ionic strength (I) of the aqueous phase empirically is calculated as in eq 5 I=

1 2

∑ CiZi2 i=1

(5)

where Zi is the valence of ion and Ci is the concentration of ion i.

3. RESULTS AND DISCUSSION Inorganic salts NaCl and Na2SO4 are common and usually simultaneously present in dyeing wastewater; correspondingly, the aqueous mixture “dye/dual-salts” is practically more interesting to be analyzed than the dye/monosalt mixture. First, the performance of the ceramic and polymeric membranes has been evaluated and compared in solutions containing merely a single component of RB KNR, RB5, RR HE7B, NaCl, or Na2SO4. To follow up, a mixture solution containing negative-charged dye with both inorganic salts NaCl/Na2SO4 (mass ratio 1:1) was studied in the t-UF membrane process to evaluate the operative conditions and relevant effect on separation efficiency. 3.1. Comparison of Ceramic t-UF Membrane and Polymeric NF Membrane. As presented in section 2.2, the t7073


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Industrial & Engineering Chemistry Research better passage, allowing the inorganic salts to penetrate while maintaining a good rejection to dyes. It is a very important property of the t-UF membrane being capable of desalinating textile wastewater, especially for dye effluents containing Na2SO4. Detailed studies on the operative parameters in t-UF membrane separation will be discussed in the following section. 3.2. Operation Conditions in the Dye/Salt Separation Process. 3.2.1. Effect of Transmembrane Pressure. Permeation flux of pure water, dye RR HE7B solution, and dye/dualsalts solution through the t-UF membrane with increasing TMP from 1 to 3 bar is depicted in Figure 4a. Flux increases linearly with operating pressure. However, greater flux drop from pure water to dye solution occurs at higher pressure (reduction 28.0% at 1 bar and 43.1% at 3 bar) due to stronger filtration resistance at higher pressure with the presence of dyes. The resistance could be strengthened by concentration polarization at higher TMP as described in the work of Alventosa-de Lara.22 On the other hand, the difference for fluxes of dye solution and dye/dual-salts solution is negligible due to a very small change of osmosis pressure with the presence of salts because the t-UF membrane has low rejection to salt ions. The t-UF membrane rejection to RR HE7B dye (without salt effect) remains stable to 97% from 1 to 3 bar as displayed in Figure 4b. Yet with the presence of NaCl and Na2SO4 in dye solution it is evident that rejection of RR HE7B decreased, for instance, by 5% at 1 bar. It could be attributed to the screening effect caused by the existence of salt ions on membrane surface which weakens the electrostatic repulsion between dye molecules and the membrane.22,33,34 We find that the membrane rejection to chloride ions increases from −13.9% to 9.5% and sulfate ions from 12.8% to 30.5% with increasing TMP from 1 to 3 bar. “Dilute effect” is one of the reasons that the solvent (water) transports faster than the solute (salt) owing to electric and steric exclusion.35,36 In the meantime, increased concentration polarization at higher operating conditions could also reduce the salt rejection by the ceramic membrane. Moreover, the Donnan effect plays a significant role in separation of various charged species through the porous channels of the membrane.37 The notable phenomenon of negative rejection of NaCl is consistent with many other works including Bowen et al.,38 who developed an approach predicting negative rejection of Cl−. In a summary, the transport mechanism of small inorganic ions is mainly convective (pressure dependent) because they can enter the pores in the membrane, while the transport of large dye molecules with high charge density is mainly diffusive (pressure independent).37 3.2.2. Effect of Working Temperatures. Dyeing wastewater is normally processed at 40−60 °C, which may be a challenging condition for most polymer membranes in long-term running. Flux and dye/salt rejection of the t-UF ceramic membrane has been studied from 25 to 80 °C in Figure 5. It is apparent that permeation flux goes up along with increased temperatures: the flux at 80 °C (131.1 L·m−2·h−1) is about 2.3 times higher than that at 25 °C (56.6 L·m−2·h−1). The resistance of total mass transfer is reduced at higher temperature since the viscosity decreases and the diffusion coefficient increases.39 An insignificant thermal effect was witnessed on dye rejection (all above 97%) from room temperature to 80 °C in Figure 5. In Jian’s study a similar result was reported that dye rejection was reduced only by 1% from 20 to 70 °C.40 High dye rejection indicates that the structural property of the active layer of the ceramic membrane is thermally stable up to 80 °C. In the

Figure 5. Flux and dye/salt rejection of t-UF ceramic membrane at different temperatures in dye and NaCl/Na2SO4 aqueous mixture (conditions: TMP = 2 bar, C(dye, RR HE7B) = 0.5 g/L, C(salts) = 1.0 g/L (m(NaCl):m(Na2SO4) = 1:1), and CFV = 3 m/s).

meantime, salt rejection reduces slightly with enhanced mass transfer at higher temperature. Freger et al. discovered that salt solutes distribute more evenly between the solution and the membrane phases at high temperature resulting in lower rejection.33 3.2.3. Effect of Solution pH. Solution pH affects the electrochemical properties of both solutes and the membrane surface and therefore can effectively alter the separation efficiency in the membrane process.36 It can be noted that the permeation flux of dye solutions containing NaCl, Na2SO4, and NaCl/Na2SO4 reduced by 9%, 5%, and 8%, respectively, on increasing pH from 4.06 to 11.15 (Figure 6a). Kim et al.41 and Zabkova et al.42 suggested that solute particles could precipitate onto the membrane surface more easily under higher pH. Besides, it is interesting to find that the permeation flux of the dye solution with both NaCl/Na2SO4 is higher than that with monosalt component NaCl. The effect has been studied in our previous work,23 insisting that salt ion environment has a significant influence on dye aggregation in aqueous solution and thus affects the cake layer structure and solution flux. The ceramic membranes are positively charged when the pH is below the isoelectric point (IEP) and negatively charged when the pH is above the IEP. In the study, the IEP of the ceramic t-UF membrane with TiO2/ZrO2 active layer is ca. pH 6.25,36 In general, rejection of dye (containing both salts) by the membrane increases from 96.6% to 98.1% at varying pH of 4− 11 as shown in Figure 6b because stronger repulsion is formed between dye molecules (negative) and ceramic membrane (negative) under more alkali condition. It has been consistently proved by the measurement on the zeta potential of dye solution as given in Figure 6d: the absolute value of the dye particle charge density increases with higher pH; the charge density also increases with the appearance of Na2SO4. Intensified charge density (i.e., zeta potential) of dye particles tends to form stronger electrostatic repulsion to the membrane surface and leads to higher rejection of dye with increased pH. Retention of salts fluctuating with pH is presented in Figure 6c as sulfate ions rejection (in the case of monosalt) first goes up from 33.7% to 55.3% and then declines to 42.4% when changing the pH from 4 to 11; rejection of chloride ions fluctuates marginally over the pH range having an average rate of 20.3%. There are similar trends, respectively, for the changes of SO42− and Cl− retention over the same pH range in the case of using mixed salts in dye solution. Rejection of SO42− in mixture is higher than that of monosalt due to variant ionic 7074


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Figure 6. Permeation flux (a), dye rejection (b), and salt rejection (c) of t-UF ceramic membrane at different pH using dye/monosalt and dye/dualsalts solution mixtures and (d) zeta potential of the dye solution (conditions: C(dye, RR HE7B) = 0.5 g/L, C(NaCl) = 1.0 g/L, C(Na2SO4) = 1.0 g/L, C(salts) = 1.0 g/L (m(NaCl):m(Na2SO4) = 1:1), TMP = 2 bar, and T = 25 °C).

changing total concentration from 0 to 15 g/L. It can be explained by the ion environment’s effect on dispersing dye aggregation in aqueous solution that the mixture salts could make the cake layer even looser on the membrane surface.23 It reveals that the salt concentration in dye solution has an effective impact on the membrane performance on dye/salts fraction. With the presence of salts, dye rejection by the membrane reduces dramatically from 99.32% to 55.5% with the increased initial concentration of NaCl/Na2SO4 from 0 to 15 g/L and the dye rejection reduces from 99.32% to 69.7% with NaCl from 0 to 15 g/L (Figure 7b). The results are in accordance with shielding effect when the counterions (i.e., Na+) are adsorbed on the membrane surface. It neutralizes the negative groups of membrane and allows more dye molecules permeable with less Donnan exclusion.44 Meanwhile, the salt ions make a contribution to a more homogeneous dispersion of dye particles in water and may help to minimize cake layer blockage on the membrane surface as schematically depicted in Figure 7c. In general, explanations of the salt effect on dye rejection can include a weakened Donnan effect by salt ions45 and uniform disperses of dye particles with increased salt content.46−48 In conclusion, the existence of a larger quantity of salt can weaken the electrostatic interaction between the membrane and the dye particles, and therefore, the steric hindrance would be the dominant mechanism over electric exclusion. The dye particles are less impeded when penetrating the pore channels in tight ultrafiltration membrane with the increased concentration of coexisting salt.

strength in solution containing either mixed salts or pure salt. The ionic strength for NaCl/Na2SO4 solution is calculated as 0.019 mol/L, while it is 0.018 mol/L for pure Na2SO4 at the same total concentration (eq 5). It implies that there is more concentration of ions present in the former case with higher ionization, which properly enforces concentration polarization and thus lead to higher sulfate rejection. Rejection of SO42− is outstandingly more effective than Cl− in the range of pH 6−8 and becomes close to that of Cl− in caustically acid and basic conditions. In caustic acid condition, the membrane surface is positively charged and the counterions (i.e., SO42−) pass through the porous membrane driven by electrostatic attraction. In caustic alkali condition, the membrane surface is negatively charged. However, in order to maintain charge balance in bulk solution the co-ions (i.e., SO42−) are forced to pass through the porous channels which also leads to low sulfate retention.37,43 In conclusion, both caustic pH conditions have led to a decrease of membrane retention to the sulfate ions. 3.3. Salt Concentration Influencing the Dye Rejection. The impact of salt concentration on the membrane separation process has been investigated as changing the concentration of mixed salts from 0 to 15 g/L (NaCl/Na2SO4 mass ratio 1:1). As can be seen in Figure 7a, permeation flux apparently declines with increased initial concentration of either mixed salts or monosalt. There is a greater drop of flux when experimenting with the dye solution containing both NaCl/Na2SO4 (flux decline 18.0%) than that of only NaCl (flux decline 7.8%) with 7075


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Figure 7. Salt concentration effect on t-UF membrane separation of dye and salts: (a) permeation flux, (b) dye and salts rejections, and (c) schematic diagram of dye−salt interactions with increased salt concentration (conditions: TMP = 2 bar, CFV = 2 m/s, C(dye, RR HE7B) = 0.5 g/L, T = 25 °C, C(total salts) = 0−15 g/L, pH = 6.5 ± 0.3).

Figure 8. Charges of inorganic salts influence t-UF membrane separation of dye (RR HE7B) and salts including (a) cations effect, (b) anions effect, (c) zeta potential of the mixture solution, and (d) particle size of feed dye solution with AlCl3 (test conditions: TMP = 2 bar, CFV = 2 m/s, C(salt) = 2.5 g/L, C(dye) = 1.5 g/L and T = 25 °C).

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two types of dye molecules RB KNR (MW 626.54 g/mol) and CY X-2RL (MW 526.93 g/mol) have been studied which are of opposite charges but close molecular weight. The molecular weights were selected smaller than MWCO of ceramic t-UF membrane to analyze electric exclusion and avoid size exclusion as the main separation mechanism. Permeability and fraction of dye/salt by the ceramic membrane using dyes RB KNR (negative ly charged) and CY X-2RL (positively charged) are compared in Figure 9. The flux of both pure dye solutions

In parallel, retention of salt anions in the dye/salt mixture solution has been analyzed, whose results are given in Figure 7b. The ceramic membrane has performed a reduced rejection to the inorganic anions with increased concentration of salt of either NaCl or NaCl/Na2SO4 in dye solution. Rejection of chloride ions drops from 32.1% to 4.5% in dye and NaCl mixture and from 16.6% to −8.0% in dye and NaCl/Na2SO4 mixture; rejection of sulfate ions drops from 53.4% to 4.3% in the second case. Similarly, enhanced permeation of NaCl/ Na2SO4 (1−10 g/L, mass ratio 1:1) through the NF270 membrane has been reported by Hilal et al.49 Reduced salt rejection, which is improved free permeation, with increased feed salinity can be caused by accelerating diffusion transport of salt ions through the membranes at a larger concentration gradient between permeate and feed. In addition, Cl− rejection becomes a negative value (typically with the existence of SO42−) with increased salinity of dye solution. A higher concentration of Cl− in the feed can contribute to an increase in the ionic or Donnan equilibrium of Cl− ions in the membrane.50−53 Furthermore, the sulfate ions being more effectively retained by the ceramic membrane than chloride ions can be explained with the different ionic radius,54,55 charge density, and diffusion coefficients56 of the inorganic ions. 3.4. Charge of the Salt Ions Influencing Membrane Retention of the Dyes. Permeability and separation performance of ceramic t-UF membrane in the dye solution containing various inorganic cations including Na+, K+, Mg2+, Ca2+, and Al3+ are compared and summarized in Figure 7a. The figures demonstrate that the flux declines following the order of contained salts KCl > NaCl > MgCl2 > CaCl2 > AlCl3 ,and dye rejection declines following the order AlCl3 > KCl > NaCl > MgCl2 > CaCl2. With the perspective of cation effect, the flux reduces with the appearance of monovalent, divalent, to trivalent cations. The dye rejection goes down with a similar cation effect yet with the exception of containing AlCl3. The accumulated cations on the membrane surface could reduce membrane electrostatic repulsion, and electromigration of the dye particles is less hindered as transporting within the active layer. Simultaneously, the zeta potential of dye/salt solution decreases greatly with the presence of divalent cations as shown in Figure 8c, which also weakens the electrostatic repulsion between dyes and membrane. The cation effect on salt rejection follows the sequence KCl > NaCl > MgCl2 > CaCl2. The outcome is in agreement with Yu’s study on the separation performance of a positively charged nanofiltration membrane for different types of salts.57 Regarding the anion effect (Cl− and SO42−), it has an insignificant effect on membrane permeability and dye rejection as displayed in Figure 7b, while sulfate rejection is higher than that of chloride due to different ionic radius, ionic strength, and valence states. The noticeable flux decline of dye/AlCl3 solution through the t-UF membrane could be caused by reaction occurring between dye molecules and AlCl3. A macromolecular complex has been formed possibly leading to serious membrane fouling as can been seen in the particle size distribution of dye/AlCl3 feed solution in Figure 8d. The change of the dye/AlCl3 permeation solution after 12 h is also displayed in the same figure. It is clear that precipitation of macromolecular complex takes place after standing for some time, which should be the main reason for the exceptionally high dye rejection in this situation. 3.5. Charge of the Dyes Influencing Salt Permeation through the Membrane. To study the impact of dye charges,

Figure 9. Charges of dye molecules influence membrane separation of dye and NaCl salt (Conditions: TMP = 2 bar, CFV = 3 m/s, C(dye, RB KNR) = 0.5 g/L, C(dye, CY X‑2RL) = 0.5 g/L, and C(salt, NaCl) = 1.0 g/L).

(without salt) varies little; however, the flux of the negative dye solution is reduced when containing NaCl, while the positive dye is reduced barely. Another interesting finding is that the dye rejection has been dramatically suppressed with the presence of NaCl salt uniquely in the case of dye CY X-2RL (positive charge) as in Figure 9. First, retention of positively charged dye is much less than that of the negative one due to electrostatic attraction. Retention of anion dye (RB KNR) remains stable above 97% either containing the salt or not. The rejection by tight UF ceramic membrane is found higher than that of negative regenerated cellulose UF membranes (reactive blue KNR rejection 72%) prepared by Chen et al.58 In contrast, rejection to the cation dye (CY X-2RL) decreased sharply from 24.4% to 3.4% with the appearance of NaCl as seen in Figure 9. Since the CY X2RL dye particles tend to attach and accumulate on the ceramic membrane by charge attraction it induces aggregation and impedes dye permeation. Xu et al. modified the NF membrane surface turning negative to positive, and the membrane rejection to dye crystal violet (positive) increased from 20% to 94.3%.59 When salt ions emerge in dye CY X-2RL solution it contributes to a more homogeneous suspension of the dye particles,46,60 which assists the dye particles to pass through porous membrane and accordingly decreases the dye rejection as found in Figure 9.

4. CONCLUSION In this work, a tight ultrafiltration (t-UF) ceramic membrane has been proposed as an alternative to a nanofiltration membrane for desalination of textile wastewater and for recovery of dyes and minerals. The t-UF membrane has improved the fraction of dye and salts, in particular, with the existence of Na2SO4 whose separation is difficult for polymeric NF membrane thanks to a larger pore size and reduced Donnan effect in the ceramic membrane. The ceramic membrane has 7077


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presented competitive rejection to reactive anion dyes including Blue KN-R, Black 5, and Red H-E7B (all >98%) and 4-fold permeability (43.5 L·m−2·h−1·bar−1) in comparison to the DK membrane. The salt rejection in aqueous dye mixture by the ceramic membrane was as low as −2.12% for NaCl and 14.38% for Na2SO4. When the salt concentration was over 5 g/L in dye solution, it has even enhanced the separation of dyes/salts by the membrane. Separation of Reactive Red H-E7B dye and NaCl/Na2SO4 has been investigated in the t-UF membrane process by regulating the conditions of pressure, temperature, pH, salts, and dyes. Increased concentration of salts and increased valence of salt ions could suppress both dye rejection in the mixture owing to a zeta potential change of dye particles and the shielding effect on the membrane surface. The results point out that the t-UF membrane is more compatible to work with negatively charged dyes instead of positively charged dyes taking account of electrostatic interactions on the ceramic surface. Other parameters such as temperature and pH had less impact on separation efficiency. In summary, it implies that the t-UF ceramic membrane has potential application in treatment to dyes effluent of high salinity; in the meantime, the t-UF membrane process is capable of recovering the dye and salt sources separately from the discharged wastewater.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhaoxiang Zhong: 0000-0002-0574-0185 Weihong Xing: 0000-0002-3967-485X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Sincere thanks for the financial support from the National Natural Science Foundation of China (No. 21490580) and the Prospective Joint Research Project of Jiangsu Province (BY2014005-06).

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REFERENCES

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