membranes Article Separation and Recycling of Concentrated Heavy Metal Wastewater by Tube Membrane Distillation Integrated with Crystallization Xiang-Yang Lou 1,2,3 , Zheng Xu 1,4 , An-Ping Bai 5 , Montserrat Resina-Gallego 2 and Zhong-Guang Ji 1,4, * 1 National Engineering Lab. of Biohydrometallurgy, GRINM Technology Group Co., Ltd., Beijing 101407, China; Xiangyang.Lou@uab.cat (X.-Y.L.); xzh63@126.com (Z.X.) 2 GTS Research Group, Department of Chemistry, Faculty of Science, Universitat Autònoma de Barcelona, 08290 Bellaterra, Spain; montserrat.resina@uab.cat 3 General Research Institute for Nonferrous Metals, Beijing 100088, China 4 GRINM Resources and Environmental Tech. Co., Ltd., Beijing 101407, China 5 Beijing Vocational College of Labor and Social Security, Beijing 102200, China; baianping1992@163.com * Correspondence: jizhongguang@grinm.com; Tel.: +86-010-6066-2768  Received: 7 December 2019; Accepted: 17 January 2020; Published: 20 January 2020  Abstract: Tube membrane distillation (MD) integrated with a crystallization method is used in this study for the concurrent productions of pure water and salt crystals from concentrated single and mixed system solutions. The effects of concentrated Zn2+ and Ni2+ on performance in terms of membrane flux, permeate conductivity, crystal recovery rates, and crystal grades are investigated. Preferred crystallization and co-crystallization determinations were performed for mixed solutions. The results revealed that membrane fluxes remained at 2.61 kg·m−2 ·h−1 and showed a sharp decline until the saturation increased to 1.38. Water yield conductivity was below 10 µs·cm−1 . High concentrated zinc and nickel did not have a particular effect on the rejection of the membrane process. For the mixed solutions, membrane flux showed a sharp decrease due to the high saturation, while the conductivity of permeate remained below 10 µs·cm−1 during the whole process. Co-crystallization has been proven to be a better method due to the existence of the SO4 2− common-ion effect. Membrane fouling studies have suggested that the membrane has excellent resistance to fouling from highly concentrated solutions. The MD integrated with crystallization proves to be a promising technology for treating highly concentrated heavy metal solutions. Keywords: wastewater treatment; membrane distillation; crystallization; heavy metals; concentrated solutions; membrane fouling 1. Introduction The increase of heavy metal wastewater has not only strained already limited freshwater but also caused the waste of heavy metal resources all over the world [1]. At the same time, electroplating, electrolysis, and other industrial processes are generating highly concentrated zinc and nickel wastewater every year [2]. These solutions are characterized by their high acidity, strong corrosion capacity, and elevated metal concentrations. Nowadays, the industry uses precipitation as a low-cost and straightforward method to treat this wastewater, but unfortunately, it produces a large amount of high-risk sludge in the meantime [3,4]. The traditional evaporation crystallization method takes lots of energy and needs reactors with excellent resistance to corrosion [5]. Besides, the most desirable goal of substantial metal wastewater treatment is to achieve the recovery of valuable Membranes 2020, 10, 19; doi:10.3390/membranes10010019 www.mdpi.com/journal/membranes Membranes 2020, 10, 19 2 of 13 metals and water. Therefore, it is essential to develop new technologies to deal with this kind of wastewater [6]. Over the past thirty years, membrane-based separation processes have emerged as the most promising techniques in the wastewater treatment field, owing to the significant evolution of membrane technologies [7–9]. Among membrane technologies, membrane distillation (MD) is a thermally driven desalination process in which a porous hydrophobic membrane is employed as a physical barrier between the hot feed, where the volatile compounds are evaporated, and the distillate side, where the diffused vapors are condensed [10]. The driving force of MD is the difference of vapor pressure on both sides of the membrane. Therefore, it is unnecessary to heat the feed solution to boiling temperature. In the MD process, solar, wind, and other clean energies or waste heat can be used [11,12]. Besides, due to its mild operating conditions and excellent sealing properties, MD is suitable for the treatment of toxic and harmful concentrated heavy metal or radioactive wastewater. On the other hand, crystallization is a very mature technology that has been widely used in the industrial sector [13]. A complete set of the theoretical and technical system is formed by the application of cooling crystallization to the recovery of solute in a saturated solution. Membrane distillation integrated with crystallization is a combination of these two technologies. Its principle is to remove the volatile solvent (generally water) by the MD process, concentrate the solution to reach saturation, then allow cooling crystallization to do the solid–liquid separation to get crystals [14,15]. The treatment of heavy metal solutions by MD has been reported. Yuan et al. [16] utilized the hollow fiber membrane in MD to deal with multiple heavy metal dilute solutions and found that copper, nickel, and zinc ions have no significant effects on the stability of flux and the quality of water yield. There are also some reports on the research of concentrated solutions by MD integrated with crystallization technology [17–22]. For instance, Edwie et al. [19] used three different hollow fiber membranes to treat saturated NaCl solution and found that a smaller membrane pore size and a more compact membrane surface are beneficial for the stability and continuity of the MD integrated with crystallization process. Chan et al. [22] utilized a flat sheet membrane to treat NaSO4 and NaCl single element solutions, respectively, and found that temperature and saturation levels are critical operating parameters in the MD integrated with crystallization process. On the other hand, the treatment of mixed heavy metal solutions by means of MD integrated with crystallization technology has rarely been reported. Besides, when treating concentrated solutions, a rapid flux decline is usually observed due to crystal deposition and scale formation on the membrane surface, which reduces membrane permeability. It is found that hollow fiber and flat membranes are generally used in the existing MD integrated with crystallization processes. The filament diameter of the hollow fiber membrane is small, so it is easy to crystallize at the fiber inlet when treating highly concentrated solutions. Moreover, the flow pattern of the flat membrane is difficult to control and it is easy to form crystallization at the membrane surface. All these limitations restrict the development of MD integrated with crystallization technology; in order to overcome them, tube membranes were used in this work. This study, therefore, aims to investigate the efficiency of MD integrated with crystallization technology in treating concentrated heavy metal wastewater. Concentrated single zinc and nickel system solutions and mixed system solutions were used to get pure crystals and distilled water. As the foremost concern, membrane fouling was investigated by using SEM and EDS. 2. Materials and Methods 2.1. Materials The reagents used in this study were ZnSO4 ·7H2 O and NiSO4 ·6H2 O. All are analytical grade and purchased from Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China. As it is shown in Table 1 [23], these reagents have different solubilities at 10 ◦ C (cooling crystallization temperature) and 65 ◦ C (heat side temperature of MD). According to this condition, it is possible to cool crystallization to obtain solid salt before MD is blocked by membrane fouling. Membranes 2020, 10, 19 3 of 13 Table 1. Solubility and corresponding mass fraction of ZnSO4 and NiSO4 at different temperatures [22]. Material Temperature/◦ C Solubility Mass Fraction/% 10 47.20 32.07 ZnSO4 65 74.33 42.63 10 32.00 24.24 NiSO4 65 57.83 36.64 For the mixed system, if one reagent reaches its saturation point at 65 ◦ C, while the other is not saturated at 10 ◦ C, it would thus be possible to separate one from the other. Otherwise, co-crystallization will be a better option. Without considering the common-ion effect, three different ratios of ZnSO4 and NiSO4 were selected, and three different methods were chosen to treat the mixed solutions in this study accordingly, as listed in Table 2. Table 2. Three different methods to treat mixed solutions. Ratios of Zinc Sulfate and Nickle Sulfate Method 3:1 Preferential crystallization of Zinc sulfate 0.5:1 Preferential crystallization of Nickle sulfate 1.5:1 Mixed co-crystallization PTFE tube membranes were kindly supplied by Huzhou Sano Environmental Technology, China. The membrane module is a self-made air gap type component. 2.2. Membrane Characterizations The diameter and thickness were measured using a Vernier caliper. The measurement of the advancing contact angle was performed using a dynamic contact angle meter (DCAT21, Data Physics, Filderstadt, Germany). The membrane nominal and maximum pore sizes were conducted with a capillary flow parameter (Proulx 1000, Porometer NV, Nazareth, Belgium). The mechanical properties of the tube membrane were characterized in terms of strain and tensile stress at break using an electronic fabric strength tester (YG065CQS/pc, Changzhou No.1 Textile Equipment Co., Ltd., Changzhou, China). Table 3 shows the parameters of the membrane material and module. Table 3. Parameters of membrane material and module. Membrane and Module Properties Diameter/mm 22 Membrane thickness/mm 0.300 Nominal pore size/µm 0.083 Maximum pore size/µm 0.211 Contact angle/deg 115.7 Tensile stress at break/MPa 54 Strain at break/% 240 Effective length/m 0.308 Effective membrane area/(m2 ) 0.213 2.3. MD Integrated with Crystallization Experiments MD integrated with crystallization experiments were carried out by a laboratory-scale setup. The experimental apparatus diagram of air gap membrane distillation is shown in Figure 1. The membrane modules were self-made, and there were tube glasses, tube membranes, and tube condensers from the outside to the inside. During the MD process, hot feed flows between the membrane and the glass, and cold water flows inside the condense tube. There was not a certain width of the gap in this case. A 1000 mL beaker flask was used as a cooling mold. module driven by a magnetic force pump and recirculates on the module’s hot side. The same procedure is followed for the chilled water system. The vapor passing through the membrane is condensed and recovered on the cold side. As the experiment goes on, when the feed solution is supersaturated, the concentrated solution is pumped out to the mold and cooled to crystallize. Finally, crystallization Membranes 2020, 10, 19 products are obtained by solid-liquid separation. The remaining crystalline 4 of 13 solution returns to the MD unit and continues to concentrate. Figure 1. Figure 1. Experimental Experimental apparatus apparatus diagram of air diagram of air gap gap membrane membrane distillation distillation system. system. (1) (1) Thermostatic Thermostatic hot bath; (2) magnetic pump; (3) liquid flowmeter; (4) thermometer; (5) product water hot bath; (2) magnetic pump; (3) liquid flowmeter; (4) thermometer; (5) product water collector; (6) collector; membrane (6) membranemodule; (7) (7) module; thermostatic cold thermostatic bath. cold bath. Accordingfeed Prepared to previous solution experiments, is heated by athe optimized average thermostatic hot bath,operating then flows conditions through theare membrane as follows: temperature module of the driven by hot side, 65 °C; a magnetic temperature force pump andofrecirculates the cold side, on 20 the°C; velocityhot module’s of the hotThe side. side,same 0.16 m·s ; velocity −1 procedure of the cold is followed for side, 0.26 m·s the chilled −1 water; temperature system. The of the vaporcooling crystallization, passing through the10 °C; time of membrane is cooling crystallization, condensed and recovered 8 h. on the cold side. As the experiment goes on, when the feed solution is supersaturated, the concentrated solution is pumped out to the mold and cooled to crystallize. Finally, 2.4. MD Integrated crystallization with Crystallization products are obtained Characterizations by solid-liquid separation. The remaining crystalline solution returns to the MD unit and continues The obtained membrane flux is one to concentrate. of the most critical characterizations in the MD process [24,25]. It can be calculated using Equationoptimized According to previous experiments, the (1), whereaverage V∗ρ isoperating permeated conditions mass, Aareis as thefollows: active temperature of the hot side, 65 ◦ C; temperature of the cold side, 20 ◦ C; velocity of the hot side, 0.16 m·s−1 ; membrane area, and t is the operating time. In this study, a 100 mL measuring cylinder was used to velocity measureofthe thevolume cold side, of 0.26 m·s−1 ; Rejection permeate. temperature rateofisthe cooling also crystallization, an essential 10 ◦ C; time characterization of of cooling operation crystallization, 8 h. stability in the MD process [26–28]. The rejection rate was indicated by permeate conductivity in this study, which was measured using a conductivity meter (S470 Seven Excellence, Mettler Toledo, 2.4. MD Integrated with Crystallization Characterizations Switzerland). The lower the permeate conductivity, the higher the reject rate. The obtained membrane flux is one of the most critical characterizations in the MD process [24,25]. 𝐽 𝑣∗𝜌 / 𝐴∗𝑡 (1) It can be calculated using Equation (1), where V∗ρ is permeated mass, A is the active membrane area, and tThe crystallization is the product operating time. In thisis study, characterized a 100 mLbymeasuring a recovery rate and cylinder wasgrade, used towhich measurecan the be calculated volume using Equations of permeate. Rejection(2)rate and (3), an is also respectively, where m is the essential characterization of weight of stability operation the target crystal in the MD product,[26–28]. process M is theThe total weight rate rejection of thewastarget product, indicated by and p is theconductivity permeate total weightinofthis crystal product. study, which was measured using a conductivity meter (S470 Seven Excellence, Mettler Toledo, Switzerland). The lower η = m/M∗100% (2) the permeate conductivity, the higher the reject rate. 𝛾 𝑚/𝑝 ∗ 100% (3) J = (v ∗ ρ) / (A ∗ t) (1) The crystals obtained from the single system were observed by stereomicroscope (XZT-CT, Shanghaicrystallization The product Optics, Shanghai, is characterized China). Obtained mix by crystals a recoveryandrate and grade,were membranes which can be calculated determined using a using Equations (2) and (3), respectively, where m is the weight of the target crystal product, M is the total weight of the target product, and p is the total weight of crystal product. η = mM ∗ 100% (2) γ = (m/p) ∗ 100% (3) Membranes 2020, 10, 19 5 of 13 Membranes 2020, 10, x FOR PEER REVIEW 5 of 13 field The emission scanning crystals electron obtained microscope from the (FE-SEM, single system were SUPRA55, Carl observed by Zeiss AG, Oberkochen, stereomicroscope (XZT-CT, Germany) and an Shanghai, Shanghai Optics, energy-dispersive X-ray spectroscopy China). Obtained (EDS, mix crystals and Oxford INCA membranes x-act, were High Wycombe, determined using a UK). field emission scanning electron microscope (FE-SEM, SUPRA55, Carl Zeiss AG, Oberkochen, Germany) and an energy-dispersive X-ray spectroscopy (EDS, Oxford INCA x-act, High Wycombe, UK). 3. Results and Discussion 3. Results and Discussion 3.1. Single System Crystallization by MD Integrated with Crystallization 3.1. Single System Crystallization by MD Integrated with Crystallization To investigate the feasibility of the MD integrated with crystallization process and to compare To investigate the effect of zinc ionsthe feasibility with of the20MD nickel ions, wt integrated with crystallization % ZnSO4 solution and 20 wt %process and to compare NiSO4 solution were usedthe effect of zinc ions with nickel ions, 20 wt % ZnSO solution and 20 wt separately. The correspondences of saturation with mass score of ZnSO4 and NiSO4 at 65 °C are 4 % NiSO 4 solution were used separately. shown The correspondences in Supplementary Tables of S1 saturation and S2.with Themass score ofin variations ZnSO 4 and NiSO membrane flux 65 ◦water 4 atand C are shown yield in Supplementary conductivity Tables S1ofand as functions S2. The variations saturation in membrane are represented flux and in Figure 2. Aswater can yield conductivity be seen, when the as functions of saturation are represented in Figure 2. As can be seen, when saturation was lower than 1 (42.64 wt % for ZnSO4 and 36.64 wt % for NiSO4), the fluxes of both the saturation was lower than 1 (42.64 wtkept % for −2 ·h−1 . solutions at ZnSO 4 and−236.64 2.61 kg·m wt % for ·h−1. When theNiSO 4 ), the fluxes saturation of both was higher solutions than 1 and kept lower at than 2.61 kg·m 1.38 (50.63 When wt % forthe ZnSO saturation 4 andwas44.39higher wt %thanfor 1NiSO and 4lower ), the than fluxes1.38 (50.63 wt showed % fordecline. a slow ZnSO4 and 44.39when Finally, wt %thefor NiSO saturation 4 ), the fluxes showed a slow decline. Finally, when the saturation was higher was higher than 1.38, the fluxes decreased sharply. During the whole MD operation, the than 1.38, the fluxes decreased water yieldsharply. During conductivity of the bothwhole MD remained solutions operation,below the water yield−1conductivity 10 μs·cm . Moreover, the of both solutions rejection rates remained below 10 µs·cm −1 . Moreover, the rejection rates of heavy metal salt reached 99.99%. of heavy metal salt reached 99.99%. Figure Figure 2. 2. Evolution Evolutionofofflux flux and and water water yield yield conductivity conductivity during during the the membrane membrane distillation distillation (MD) (MD) operation. operation. (a) ZnSO44solution; (a)ZnSO solution;(b) (b)NiSO NiSO4 4solution. solution. Besides, the Besides, the experiments experiments involving ZnSO44 and andNiSO NiSO44solutions solutionsin inair airgap gapmembrane membranedistillation distillation were consistent. were consistent. Highly Highlyconcentrated concentratedzinc zincandandnickel nickelions ionsdiddidnot nothave haveaadifferent different addition addition effect effect on on the performance of MD, except that membrane fluxes were slightly lower the performance of MD, except that membrane fluxes were slightly lower than that of diluted than that of diluted solutions. Therefore,Therefore, solutions. for the zinc forandthe nickel mixed zinc and solution, nickel mixed it is possible solution, it is to selectively possible separate separate to selectively the valuable the metals according valuable to their solubility metals according differences to their solubility at different differences at temperatures. different temperatures. After cooling cooling crystallization crystallization at at 10 ◦ C, both the zinc and nickel sulfate crystalline products were 10 °C, After both the zinc and nickel sulfate crystalline products were obtained. According obtained. Accordingto tocrystallization crystallizationchemistry, chemistry,when when thethe cooling cooling temperature temperature of of the the zinc zinc sulfate sulfate solution is below 39 ◦ C, and for the nickel sulfate solution, below 31.5 ◦ C, the crystalline products solution is below 39 °C, and for the nickel sulfate solution, below 31.5 °C, the crystalline products will will be be ZnSO ZnSO 4·7H24O ·7H 2 Oand [29] [29]NiSO and NiSO 4·7H2O4 ·7H 2 Orespectively. [30], [30], respectively. In theInview the view of theofexistence the existence of carbonate of carbonate and and hydroxide hydroxide ions ions (from (from the the air and air and water) water) in in thethe solution, solution, thepossible the possibleformation formationofofcarbonate carbonate and and hydroxide precipitations is discussed in the supplemental information. The results indicate that itit is hydroxide precipitations is discussed in the supplemental information. The results indicate that is impossible to impossible to form form carbonate carbonate and and hydroxide hydroxide precipitations precipitations under under the the condition condition of of the the MD MD integrated integrated with crystallization with crystallization experiments. experiments. Obtained Obtainedsolid solidproducts products werewere observed observed by by stereomicroscope stereomicroscope and and compared with commercial products, compared with commercial products, respectively, respectively, as shown in Figure 3. The crystal products of The crystal products of zinc zinc sulfate obtained sulfate obtained fromfrom the the MD MD integrated integratedwith withcrystallization crystallizationprocess processwere werecolorless colorlessand andneedle-like, needle-like, belonging to the orthorhombic system. The average size was 4 mm. Furthermore, its shape was more Membranes 2020, 10, 19 6 of 13 Membranes2020, Membranes 2020,10, 10,xxFOR FORPEER PEERREVIEW REVIEW 66 of of 13 13 belonging to the orthorhombic system. The average size was 4 mm. Furthermore, its shape was more regularregular regular than than than the the observed the observed observed commercial commercial commercial products. products. products. OnOn On thethe the other other other hand,the hand, hand, theobtained the obtained nickel obtained nickel nickel sulfate sulfate sulfate crystals crystals were were green, green, transparent, transparent, and and belonged belonged to to the the orthorhombic orthorhombic system. system. Their Their average average crystals were green, transparent, and belonged to the orthorhombic system. Their average size was size size was22 was 2mm, mm, mm, and and shapes and shapes were shapes were quite were quite different quite different from different from commercial from commercial NiSO commercial NiSO ·6H NiSO444·6H O, ·6H222O, which O, which was which was used was used to used to prepare prepare the to prepare the the initial solution. initial solution. initial solution. Figure 3. Figure 3. Morphology 3. Morphology of of crystalline ofcrystalline products crystallineproducts and productsand commercial andcommercial products. commercialproducts. (a)(a) products.(a) ZnSO ZnSO ·7H 44·7H O crystals 22O crystals by by Figure Morphology ZnSO 4 ·7H2 O crystals MD MDMD integrated integrated with with crystallization; crystallization; crystallization;(b)(b) commercial (b)commercial commercialZnSOZnSO ZnSO44·7H 4 ·7H ·7H2O; 2 O; (c) NiSO ·7H O crystals (c) NiSO4·7H2O crystals by MD 4 2 by MD by integrated with 2 O; (c) NiSO4 ·7H2 O crystals by MD integrated integrated with with crystallization; crystallization; (d) (d) commercial commercial NiSO NiSO integrated with crystallization; (d) commercial NiSO4 ·6H2 O. ·6H 44·6H O. 22O. 3.2. 3.2.Preferential CrystallizationofofZinc Preferential Crystallization ZincSulfate Sulfateby byMD MDIntegrated Integratedwith withCrystallization Crystallization In this In this experiment, experiment, 21.43 experiment, 21.43 wtwt % wt %%ZnSO ZnSO444 and ZnSO and7.14 and 7.14wt 7.14 wt% wt %NiSO % NiSO NiSO mixedsolution 444mixed mixed solutionwas solution wasused. was used. MD used. MD integratedwith integrated integrated withcrystallization with crystallization crystallization experiments experiments experiments werewere performed performed were performed to separate separate to separate to and obtain and obtain and obtain zinc sulfate zinc sulfate zinc sulfate crystal crystalproducts products crystal products selectively. selectively. selectively. Asseen As can As be canbe can be seen from seen fromFigure Figure from Figure 4, 4,membrane membrane 4, membrane flux has flux has anhas flux anapparent apparent an apparent decreasing decreasing trend decreasing trendthe with trend with with the increase increase the increase of of saturation. saturation. of saturation. Additionally, Additionally, when the when the saturation Additionally, when thewassaturation higher than saturation was was higher 1.15, than 1.15, membrane higher than 1.15, flux membrane decreased flux rapidly.decreased At the rapidly. end of the At the process, end whenof the the process, saturation when was the 1.28, saturation the flux was was 0.381.28, kg·m −2 ·h the −1 . flux membrane flux decreased rapidly. At the end of process, when the saturation was 1.28, the flux wasmembrane The was 0.38kg·m 0.38 kg·m−2−2·h ·h −1. The flux −1 drop . The membrane was probably membrane fluxdrop flux drop causedwasby was probably causedby the combined probably caused bythethecombined action combined action of solutionaction vapor ofpressure of solution solution vaporconcentration drop, vapor pressure drop, pressure drop, concentration polarization, concentration polarization, temperature temperature polarization, polarization, temperature polarization, andpolarization, membrane foulingand membrane and membrane fouling [31–33]. During fouling [31–33]. the During the whole MD process, the conductivity of permeate remained −1below 10 μs·cm −1. The [31–33]. During the whole MD process, the conductivity of permeate remained below 10 μs·cmrate whole MD process, the conductivity of permeate remained below 10 µs·cm . The rejection −1 . Theof rejection heavy metal rejection ratesalt rate of heavy of heavy reached metal salt reached 99.99%. metal salt reached 99.99%. 99.99%. Figure Figure4. Figure Evolution 4.Evolution 4. of Evolutionof flux offlux and fluxand water andwater yield wateryield conductivity conductivityduring yieldconductivity duringpreferential during preferentialcrystallization preferential crystallizationof crystallization ofZnSO of ZnSO4.4.4. ZnSO After cooling After cooling crystallization crystallization atat 10 10 °C, °C, aa mixture mixture of of zinc zinc sulfate sulfate and and nickel nickel sulfate sulfate crystalline crystalline products were obtained. Figure 5 shows that the recovery rate and grade of products were obtained. Figure 5 shows that the recovery rate and grade of ZnSO4·7H2OZnSO 4 ·7H 2 O crystals crystals Membranes 2020, 10, 19 7 of 13 Membranes After2020, Membranes 10, 10, xx FOR cooling 2020, PEER PEER REVIEW crystallization FOR REVIEW at 10 ◦ C, a mixture of zinc sulfate and nickel sulfate crystalline 77 of of 13 13 products were obtained. Figure 5 shows that the recovery rate and grade of ZnSO4 ·7H2 O crystals varied at variedat varied different at different saturation differentsaturation values saturationvalues during valuesduring duringthe the theMDMD integrated MDintegrated with integratedwith crystallization withcrystallization process. crystallizationprocess. As process.As can As can can bebe be seen, seen, with seen,with the withthe increase theincrease of increaseofof saturation, saturation, saturation, the thethe recovery recovery recovery rate raterate increased increased increased linearly. linearly. linearly. WhenWhen When the supersaturation the supersaturation the supersaturation was was 1.18, 1.18, was the 1.18, the the membrane membrane flux wasflux membrane flux was 1.5 was −2 1.5 kg·m 1.5 kg·m kg·m ·h−1 −2·h−1 ·h−1,, the , the−2recovery recovery the rate of zincrate recovery rate of zinc zinc sulfate of reached sulfate 35%. reached sulfate reached 35%. 35%. Nevertheless, Nevertheless, Nevertheless, the the the grade grade of crystals wasof grade of crystals 70%was crystals about andabout was about changed70% and 70%little andatchanged changed little little different at at different different saturations. saturations. saturations. Figure Figure 5. Figure 5.5. Recovery Recovery rate Recovery rate and rate and grade grade of of crystals crystals produced produced by produced by MD by MD integrated MD integrated with integrated with crystallization with crystallization process crystallization process process at at different different saturation saturation values. values. at different saturation values. After After drying, drying, the the obtained obtained crystal crystal products products were were observed under observed under SEM under SEM and SEM and measured and measured measured byby EDS. by EDS. EDS. The results are shown The results are shown in shown in Figure in Figure 6. Figure 6.6. The composition of the main elements of crystals was 29.52% O, The composition of the main elements of crystals was 29.52% O, 33.80% 33.80% S, 29.44% Zn, and 7.23% Ni. Both Both the the results results obtained from ICP-OES and EDS 33.80% S, 29.44% Zn, and 7.23% Ni. Both the results obtained from ICP-OES and EDS analysis suggestanalysis suggest that that this this process process does does not not have have aa zinc zinc sulfate sulfate preferential crystallization due preferential crystallization due to to the the common-ion common-ion effect effect of 2− of SO SO4442−... 2− (a) (a) (b) (b) Figure Figure 6. 6. Scanning Figure 6. Scanning electron Scanning electron microscope microscope (SEM) (SEM) and and energy-dispersive energy-dispersive X-ray energy-dispersive X-ray spectroscopy spectroscopy (EDS) spectroscopy (EDS) (EDS) diagrams diagrams of diagrams of ZnSO444 crystal ZnSO ofZnSO crystal products. crystal products. (a) products. (a) SEM; SEM; (b) (b) EDS. EDS. 3.3. 3.3. Preferential Crystallization of Nickel Sulfate by MD Integrated with Crystallization 3.3. Preferential Preferential Crystallization Crystallization ofof Nickel Nickel Sulfate Sulfate by by MD MD Integrated Integrated with with Crystallization Crystallization The The mass fractions of 10.34 wt % ZnSO and 20.69 wt % NiSO mixed solution were used to The mass mass fractions fractions ofof 10.34 10.34 wtwt %% ZnSO ZnSO444 and and 20.69 20.69 wtwt %% NiSO NiSO444 mixed mixed solution solution were were used used toto selectively selectively separate and obtain nickel sulfate crystal products. As can be seen from Figures 7 and 8, selectively separate separate andand obtain obtain nickel nickel sulfate sulfate crystal crystal products. products. As As can can be be seen seen from from Figures Figures 77 and and 8, 8, the the preferential separation of NiSO showed the the same tendency tendency as thethe preferred separation separation of ZnSOZnSO . the preferential preferential separation separation ofof NiSO NiSO444 showed showed the same same tendency as as the preferred preferred separation of of ZnSO444.. The The conductivity conductivity of of permeate permeate remained remained below below 10 10 μs·cm μs·cm−1 during −1 during the the MD MD process, process, but but the the membrane membrane flux flux showed a decline when the saturation increased. The recovery rate rose linearly with the showed a decline when the saturation increased. The recovery rate rose linearly with the increase increase of of saturation. saturation. When When the the saturation saturation waswas 1.18, 1.18, the the membrane membrane fluxflux was was 1.5 1.5 kg·m kg·m−2·h·h−1,, and −2 −1 and the the recovery recovery Membranes 2020, 10, 19 8 of 13 The conductivity of permeate remained below 10 µs·cm−1 during the MD process, but the membrane flux showed Membranes Membranes 2020,a10, 2020, decline 10, xxFOR whenREVIEW FORPEER PEER the saturation increased. The recovery rate rose linearly with the increase REVIEW 88 of of 13 13 of saturation. When the saturation was 1.18, the membrane flux was 1.5 kg·m−2 ·h−1 , and the recovery rate was rate was 35% was35%35%byby by primary primary MD primary MD MD integrated integrated withwith integrated crystallization crystallization with operating. operating. crystallization However, However, operating. the theofgrade the grade However, crystals grade of of crystals achieved achieved 72% and 72% and changed changed little at little at different different saturations. saturations. crystals achieved 72% and changed little at different saturations. Figure Figure 7. Figure 7. Evolution Evolution of of flux flux and and water water yield yield conductivity conductivity during conductivity during the the preferential the preferential crystallization crystallization of crystallization of NiSO NiSO NiSO44 4 experiment. experiment. Figure Figure 8. Figure 8. Recovery Recovery rate rate and and grade grade of of crystals crystals produced produced by by the by the MD integrated with MD integrated crystallization with crystallization crystallization process process at at different differentsaturations. saturations. The obtained The obtained crystal obtainedcrystal products crystal products products were were also alsoobserved observed underunder SEM SEMand and measured measured measured by EDS. The byEDS. The results results are shown in are shown inFigure in Figure9. Figure 9.9.The The The main main main element element element composition composition composition of of crystals of crystals crystals was waswas 12.57% 12.57%12.57%O, O, 26.07% O, 26.07% 26.07% S, S, 6.52% S, 6.52% 6.52% Zn, Zn, Zn, andand and 54.85% 54.85% 54.85% Ni. Ni. results Ni. The The The results results showed showed showed that that preferential that preferential preferential crystallization crystallization crystallization of of NiSOof NiSO NiSO 44 by by MD4 byintegrated MD MD integrated integrated with with with crystallization crystallization crystallization was was feasible was feasible feasible while whilewhile therethere there was was was still still still 7.23% 7.23%7.23% Ni Ni in Nithe in in the the ZnSOZnSO ZnSO ·7H42·7H 44·7H 2O 2 O and O and and NiSO NiSO NiSO ·7H422·7H 44·7H O 2O O salt salt salt products. products. products. Membranes 2020, 10, 19 9 of 13 Membranes Membranes2020, 2020,10, 10,x xFOR FORPEER PEERREVIEW REVIEW 9 9ofof1313 (a) (a) (b) (b) Figure Figure9.9. 9.SEM SEMand andEDS EDSdiagrams diagramsofof ofNiSO4 crystal products. (a) SEM; (b) EDS. NiSO 4 crystal products. (a) SEM; (b) EDS. Figure SEM and EDS diagrams NiSO4 crystal products. (a) SEM; (b) EDS. 3.4. 3.4.Co-Crystallization 3.4. Co-Crystallizationofof Co-Crystallization ofZinc Zincand Zinc andNickel and NickelSulfate Nickel Sulfateby Sulfate byMD by MDIntegrated MD Integratedwith Integrated withCrystallization with Crystallization Crystallization InIn Inthese theseexperiments, these experiments,a aaZnSO experiments, ZnSO ZnSO 4 andand 4 4and NiSO 4 mixed NiSO NiSO 4 mixed 4 mixed solution was solution solution was used was totoinvestigate used used to investigate investigate the theco- the co- crystallization. co-crystallization.The mass crystallization. The mass fractions The mass of ZnSO fractions fractions of ZnSO and of4 ZnSO 4 NiSO 4 and and NiSO 4 were 20.00 NiSO 4 were wt 4 were 20.00 % and wt %20.0013.33 wt wt %wtand and 13.33 %, respectively. 13.33 wt %, %, respectively. The Thevariations respectively. variations ininmembrane The variations membrane influx and andwater membrane flux fluxyield water and conductivity yieldwater asasfunctions yield conductivity conductivity asof functions offiltration functions oftime filtration timeare filtration are illustrated time are in Figure illustrated 10. in illustrated in Figure 10. Figure 10. Figure Figure Figure10.10. Evolution ofofmembrane 10.Evolution Evolution membrane membrane distillation fluxflux distillation distillation andand flux water yieldyield andwater water conductivity yieldconductivityof the mixed conductivity ofofthe thesolution. mixed mixed solution. solution. It can be seen from the figure, as the experiment goes on, the membrane flux reduces continuously. Six hours ItIt can later,seen the membrane flux decreased rapidly, and goesthe conductivity of permeate increased. can bebe seen from fromthe thefigure, figure,asas the the experiment experiment −2 goes−1on, on,the the membrane membrane flux fluxreduces reduces Furthermore, continuously. after Six 12 hours h of operation, later, the the membrane flux was flux 0.38 decreased kg·m ·h rapidly, . andHowever, the during the conductivity of whole permeate MD continuously. Six hours later, the membrane flux decreased rapidly, −1 and the conductivity of permeate process, the increased. conductivity of permeate remained below 20 µs·cm . The rejection rate of heavy metal increased.Furthermore, Furthermore,after after1212hhofofoperation, operation,the theflux fluxwas was0.38 0.38kg·m −2·h−1. However, kg·m−2 ·h−1. However,during duringthe the salt whole still MD reached process,99.99%. the After natural conductivity ofdrying, permeate the crystals remained were belowdissolved 20 μs·cmin distilled −1 . The water and rejection rate then of whole MD process, the conductivity of permeate remained below 20 μs·cm . The rejection rate of −1 analyzed heavy utilizing an ICP-OES. Among the natural products, the mass fraction of ZnSO4 ·7H2 O was 64.96%, heavymetalmetalsaltsaltstill stillreached reached99.99%. 99.99%.After After naturaldrying, drying,thethecrystals crystalswere weredissolved dissolvedinindistilled distilled and the water mass fraction of NiSO4 ·7H 2O was 35.04%. waterand andthen thenanalyzed analyzedutilizing utilizingananICP-OES. ICP-OES.AmongAmongthe theproducts, products,the themass massfraction fractionofofZnSO ZnSO 4·7H2O 4·7H2O was 64.96%, wasMembrane and 64.96%, and the mass fraction of NiSO 4·7H2O was 35.04%. the mass fraction of NiSO4·7H2O was 35.04%. 3.5. Fouling 3.5. 3.5.Membrane Fouling As mentioned Membrane Foulingbefore, during the MD integrated with crystallization process, a decrease in the membrane flux was observed. After washing the system with tap and MILIQ water, membrane flux As Asmentioned returned to its initial mentioned before, valueduring before, and the theMD MDintegrated permeated during integratedwith conductivity crystallization again with process, remained below crystallization a adecrease 10 µs·cm process, decrease ininthe −1 . However, the membrane membrane flux was observed. After washing the system with tap and MILIQ water, membraneflux flux was observed. After washing the system with tap and MILIQ water, membrane flux returned returnedtotoits itsinitial initialvalue valueand andpermeated permeatedconductivity conductivityagain againremained remainedbelow below1010μs·cm −1. However, μs·cm−1 . However, Membranes 2020, 10, 19 10 of 13 as an essential indicator of a comprehensive and in-depth evaluation of the membrane process [34–36], membrane surface morphology and surface deposits were investigated in this study. According to the differences in the treated liquids, tube membranes were labeled, as shown in Table 4. The working time of each membrane was calculated. The outer surface, inner surface, and cross-section of the Membranes membrane 2020, 10, x observed were FOR PEER REVIEW by SEM, and the composition of the potential sediments was analyzed 10 of by 13 EDS. In particular, to investigate how deeply the salts penetrate the pores of the membranes, EDS liner as an essential analysis was usedindicator insteadofofapoint comprehensive analysis. Inand orderin-depth evaluation to get neat of thesamples, cross-section membrane process were membranes [34– 36], membrane pre-treated withsurface morphologycan liquid nitrogen.As andbesurface seen from deposits Figurewere investigated 11, the membranes inused this study. in this According study have to the differences in the treated liquids, tube membranes were labeled, a density network structure. A small number of deposits can be seen on the outer surfaces as shown in Table 4. The of membrane working time of each membrane was calculated. The outer surface, inner surface, 1 and membrane 3. This is mainly because of the highly concentrated solutions directly contacted with and cross-section of thethe membrane outer surfaces. were observed However, by SEM, the network and the structure of composition the membraneofsurfaces the potential can stillsediments was be seen clearly. analyzed by EDS. In particular, to investigate how deeply the salts penetrate The deposition phenomenon on the membrane surfaces was not significant. Furthermore, there were the pores of the membranes, EDS liner analysis was used instead of point analysis. In order to no deposits found on the outer surfaces of membranes 0 and 1, nor the inner surfaces or cross-sectionsget neat cross-section samples, of all the membranes membranes.were pre-treated The results of EDS with linerliquid nitrogen.As analysis indicatedcanthatbetheseen from existed deposits Figure only11, the on membranes used in this study have a density network structure. A small the membrane outer surfaces and did not penetrate deep into the membranes. Element and content number of deposits can be seen on the analysis outer of the surfaces deposits of membrane on the outer membrane 1 and surface membrane 3. This is shown is mainly in Table 5. The because chemicalof the highly formula of concentrated solutions directly contacted with the outer surfaces. However, the PTFE membrane is (C2 F4 )n . Apart from the C and F elements, the amounts of sediment elementsthe network structure of the suchmembrane as Zn, Ni, S,surfaces and O werecan still be seen relatively smallclearly. on theThe deposition surface phenomenon of the membrane. on the All this provedmembrane that the surfaces membrane has excellent resistance to the fouling of highly concentrated zinc and nickel solutions. of was not significant. Furthermore, there were no deposits found on the outer surfaces membranes 0 and 1, nor the inner surfaces or cross-sections of all the membranes. The results of EDS liner analysis indicated that the Tabledeposits existedofonly 4. Information on themembranes. the studied membrane outer surfaces and did not penetrate deep into the membranes. Element and content analysis of the deposits on the outer membrane Number surface is shown in Table 0 5. The chemical 1 formula of the2PTFE membrane is (C 3 2F4)n. Apart from Composition the C and Fofelements, the themembrane amounts of sediment elements Nickel such assulfate Zn, Ni, S, andandO were New Zinc sulfate Zinc nickelrelatively sulfate treated solutions small on the surface of the membrane. All this proved that the membrane has excellent resistance to Work time 0h 515 h 22 h 95 h the fouling of highly concentrated zinc and nickel solutions. Figure Figure 11. 11. Morphologies Morphologiesof of the the membrane membrane outer outer surface, surface, inner inner surface, surface, and and cross-section. cross-section. (0) (0) new new membrane; membrane; (1) (1) zinc zinc sulfate sulfate system; system; (2) (2) nickel nickel sulfate sulfate system; system; (3) (3) mixed mixed system. system. Table 4. Information of the studied membranes. Number 0 1 2 3 Composition of the New Zinc Nickel Zinc and treated solutions membrane sulfate sulfate nickel sulfate Work time 0h 515 h 22 h 95 h Membranes 2020, 10, 19 11 of 13 Table 5. Elements and contents of deposits on the membrane outer surface. Membrane C O F S Zn Ni Number/Element Content % 0 23.33 - 76.67 - - 0 1 24.39 - 73.63 0.32 3.86 - 2 19.99 - 80.01 - - - 3 24.99 6.15 56.58 - 11.2 1.08 4. Conclusions The MD integrated with crystallization process has been confirmed to permit pure water and crystal products to be obtained from highly concentrated zinc and nickel mixed solutions. For the single element system, membrane fluxes remained at 2.61 kg·m−2 ·h−1 and showed a sharp decline until the saturation increased to 1.38. Water yield conductivity was maintained below 10 µs·cm−1 . Highly concentrated zinc and nickel ions did not have extra influence on the membrane. For the mixed solutions, membrane flux showed a sharp decrease as an increase in saturation was observed, while the conductivity of permeate remained under 10 µs·cm−1 during the whole process. In this study, co-crystallization proves to be a better method due to the existence of the SO4 2− common-ion effect. Furthermore, membrane fouling studies suggest that the membrane has excellent resistance to the fouling of highly concentrated solutions. The MD integrated with crystallization process has been demonstrated to be a promising technology in treating highly concentrated heavy metal solutions. However, nowadays, this technology is not used in the industry. Currently, one of the barriers to the implementation of MD integrated with crystallization is the price of the commercial membranes. It can be predicted that with the progress of membrane technology and materials science, MD integrated with crystallization technology will be widely used in the industry. Supplementary Materials: The following are available online at http://www.mdpi.com/2077-0375/10/1/19/s1, Table S1: Correspondence of saturation with the mass fraction of ZnSO4 solution at 65 ◦ C. Table S2: Correspondence of saturation with the mass fraction of NiSO4 solution at 65 ◦ C. Author Contributions: X.-Y.L. carried out the experiments and wrote the draw manuscript text. Z.X. and Z.-G.J. provided the idea and helped with the result discussion and revision of the manuscript. A.-P.B. helped revise the manuscript. M.R.-G. helped improve the English writing. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by Beijing Municipal Natural Science (2182085) and the National Key Research and Development Program of China (2018YFC1900304). Acknowledgments: X.-Y.L. acknowledges the China scholarship council for the fellowship (201708110179). Conflicts of Interest: The authors declare no conflict of interest. References 1. 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