Treatment of Wastewater Solutions from Anodizing Industry by Membrane Distillation and Membrane Crystallization

applied sciences

Article Treatment of Wastewater Solutions from Anodizing Industry by Membrane Distillation and Membrane Crystallization

Aamer Ali, Josephine Hvid Jacobsen, Henriette Casper Jensen, Morten Lykkegaard Christensen and Cejna Anna Quist-Jensen *

Center for Membrane Technology, Department of Chemistry and Bioscience, Aalborg University, Fredrik Bajers Vej 7H, 9220 Aalborg East, Denmark; [email protected] (A.A.); [email protected] (J.H.J.); [email protected] (H.C.J.); [email protected] (M.L.C.) * Correspondence: [email protected]; Tel.: +45-9940-3612

Received: 18 December 2018; Accepted: 10 January 2019; Published: 15 January 2019

Abstract: The treatment of wastewater containing various metal ions is a challenging issue in the anodizing industry. The current study investigates the application of membrane distillation/crystallization (MD/MCr) for the simultaneous recovery of freshwater and sodium sulfate from wastewater originating from a Danish anodizing industry. MD/MCr experiments were performed on supernatant from wastewater obtained after centrifugation. The effect of various feed temperatures and cross-flow velocities on flux and crystal characteristics was investigated. The crystal growth in the feed tank was monitored through the use of an online PaticleView microscope. The crystals’ morphology and form were determined by using scanning electron microscope (SEM) and X-ray powder diffraction (XRD), respectively, while inductively coupled plasma (ICP) was applied to determine the purity of the obtained crystals. The weight and dimensions of the MD/MCr unit that were required to treat the specific amount of wastewater were evaluated as a function of the feed inlet temperature. It was demonstrated that the application of MCr allows extracting high-purity sodium sulfate crystals and more than 80% freshwater from the wastewater.

Keywords: sodium sulphate; mineral recovery; membrane crystallization; membrane distillation; anodizing industry

1. Introduction Anodizing is an electrochemical process applied at the metallic surfaces to enhance their decorative, durable, appearance enhancement, and corrosion resistance properties. Anodizing treatment can be broadly divided into two main steps: (i) surface cleaning and preparation (degreasing, pickling etc.) and (ii) metallic coating and surface finishing (electroplating, anodizing, and immersion). The process generates various wastewater streams, which are generally mixed into a single stream before disposal [1]. Due to the presence of several metals into the final stream, the anodizing industry is moving toward zero liquid discharge to meet stringent environmental regulations [2]. Depending upon the composition of effluent and final requirements, various technologies are applied in practice for the treatment of wastewater from the anodizing industry. However, each of the applied technologies addresses a very specific problem within the overall wastewater treatment line. For instance, combined chemical precipitation, coagulation, and flocculation followed by filtration is used to partly remove the dissolved metals from wastewater. However, this approach is time consuming, uses excessive toxic chemicals, generally requires a polishing step to meet the discharge requirements [3], and has large footprints due to the involvement of multiple unit operation.

Appl. Sci. 2019, 9, 287; doi:10.3390/app9020287 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 287 2 of 15

Reverse osmosis (RO) or ion exchange are applied to remove dissolved salts and thus to produce water with low conductivity. However, RO requires extensive pretreatment, and generates a concentrated stream that needs proper disposal, while ion exchange technology requires the regeneration of the resins. Zero liquid discharge units, being applied in the anodizing industry today, are a complex combination of several unit operations such as chemical–physical purification, mechanical separation, membrane bases purification, and evaporation [4]. Various chemicals are used to adjust pH levels at different stages during these processes. The energy consumption of such methods is high, and footprints are large. Furthermore, this approach does not consider the recovery of valuable components from the wastewater streams. Membrane distillation/crystallization (MD/MCr) has emerged as an attractive process to concentrate the solutions to their saturation level [5]. MD is a temperature-driven process in which the vapors of volatile components pass through a hydrophobic membrane, due to a vapor pressure difference across the membrane. Due to the hydrophobic nature of the membrane, it has theoretically a 100% rejection of non-volatile substances present in wastewater streams. The process is operated at lower temperatures (~40–60 ◦C[6]) compared to the conventional distillation process; accordingly, cheap low-grade heat can be used to heat the feed solution [7]. Compared to conventional thermal desalination processes, the process has smaller footprints, and is lightweight [8,9]. The process can simultaneously produce freshwater and crystals of the dissolved salts, thus approaching zero liquid discharge without any additional post-treatment [10]. The technical potential of MD for various applications, such as desalination, environmental/waste clean-up, water reuse, and in the food and medical industry has been well-demonstrated in the literature [11–13]. MCr has been particularly investigated for the simultaneous recovery of salts and freshwater from various types of brine solution [14–16]. Ji et al. [17] applied MCr to recover high-purity NaCl from the reverse osmosis concentrate of real seawater. In another study [10], MCr was applied to recover NaCl crystals from microfiltered produced water. Li et al. [18] integrated a reverse osmosis unit with MCr to recover Na2SO4 from a single salt solution. Curcio et al. [19] coupled MCr with nanofiltration to recover Na2SO4 from its single salt aqueous solution. In another study, sodium sulfate was precipitated from an industrial solution by applying MCr standalone, as well as in integration with nanofiltration [20]. In this study, MD/MCr has been investigated for the treatment of real wastewater originating from the anodizing industry. The particular novel aspect of the study is the application of MCr for the simultaneous recovery of dissolved salts and freshwater from wastewater from the anodizing industry. The study investigates the effect of operating temperature and hydrodynamics upon the freshwater production and quality of the recovered crystals. The ultimate objective is to produce clean water, minimize the waste stream, and recover the minerals contained in the waste streams.

2. Materials and Methods

2.1. Wastewater Properties The feed solution used in the experiments was real wastewater from an anodizing industry where aluminum parts are anodized. The company neutralizes their wastewater stream to pH 7 before disposal. Since the wastewater was highly concentrated and neutralization was performed, a large fraction of suspended solids was present in the solution. Therefore, centrifugation was carried out before MD/MCr treatment. The dry matter content of the wastewater was estimated by drying 20 mL of the wastewater in an oven at 105 ◦C for 24 hours. Afterwards, the dry matter was incinerated at 550 ◦C for two hours to determine the organic/inorganic fraction. The weight loss was declared as the organic matter. The characteristics of the wastewater before centrifugation have been provided in Table1. The composition of the supernatant from the centrifugation was analyzed by using inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermo Scientiﬁc iCap 6300) and ion Appl. Sci. 2019, 9, 287 3 of 15 chromatography (IC). Cations were measured by ICP-OES operated in an axial view mode. The spectrometer was calibrated against matrix matched multi element standards. IC (Metrohm IC system) analyzed the anions with components 820, 819, 818, 771, and 833. The composition of the supernatant has been given in Table2, which shows that the dry matter content in supernatant is 48% less than that in wastewater.

Table 1. Characteristics of wastewater solution before centrifugation.

pH 7.36 Redox potential [mV] 102 Conductivity [mS/cm] 37.61 Dry matter [g/L] 64.4 ± 0.4 Inorganic matter [%] 88.7 Organic matter [%] 11.3

Table 2. Composition of supernatant after centrifugation of the wastewater.

Supernatant [mg/L] Dry matter [g/L] 43.3 Inorganic matter [%] 97.9 Organic matter [%] 2.1 Conductivity [ms/cm] 39 Mn2+ 3.648 Fe2+ 6 Mg2+ 7.813 P3− 90 Na+ 2302 2− SO4 18990

2.2. Membrane Distillation and Crystallization Setup MD/MCr experiments were carried out by using polypropylene (PP) hollow fiber membranes (Membrana Accurel®® PP S6/2) assembled into a transparent plastic module. A detailed description of the membrane fibers and modules can be found in Table3. MD and MCr setup was operated in direct contact configuration mode with feed in the lumen side of fibers (in counter current with permeate on the shell side). The feed side consisted of a feed tank with a stirrer. The feed solution was pumped with a peristaltic pump (Masterflex L/S) from the feed tank through a heater followed by the membrane module and back into the feed tank. Permeate side pumps distillate water from the permeate tank through a cooler and into the membrane module and back into the permeate tank. The temperature is measured at the membrane inlet and outlet at the feed and permeate sides. A schematic diagram of the MD/MCr setup has been presented in Figure1. The experiments were conducted both using distillate water and the supernatant from the centrifuged wastewater described in Table2 as the feed solutions. Feed temperatures at ~38 ◦C, ~47 ◦C, and ~56 ◦C with a flow rate of 21.6 L/h (corresponding to feed and permeate velocities of 1.2 m/s and 0.8 m/s, respectively) and permeate temperature of ~15 ◦C were used in the experiments. Feed and permeate flow rates of 14.8 L/h, 21.6 L/h, and 30.7 L/h (corresponding to feed velocities of 0.8 m/s, 1.2 m/s, and 1.7 m/s, respectively) were tested while fixing the feed and permeate temperature at ~47 ◦C and ~15 ◦C, respectively. The experimental time varied from 40 to 100 hours for each test, depending upon the operating conditions applied; therefore, all of the experiments have been conducted only once. The feed solution was concentrated until precipitation, where the crystallization occurred in the feed tank. The crystallization process was continuously monitored by using a ParticleView V19 microscope (PVM) inserted into the feed tank. When crystals start appearing in the feed tank, a sample was taken from the feed tank to analyze the crystals through an optical microscope. The crystals were recovered by filtering off the solution after the completion of the Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 15 appearing in the feed tank, a sample was taken from the feed tank to analyze the crystals through an opticalAppl. Sci. microscope.2019, 9, 287 The crystals were recovered by filtering off the solution after the completion4 of of 15 the experiments for X-ray diffraction (XRD), ICP-OES, and scanning electron microscopy (SEM) analysexperimentsis. Further for X-ray details diffraction about these (XRD), procedures ICP-OES, have and been scanning provided electron in Section microscopy 2.5. (SEM) analysis. Further details about these procedures have been provided in Section 2.5. Table 3. Description of membrane and membrane module used in membrane distillationTable 3. Description/crystallization of membrane (MD/MCr and) experiments membrane module. used in membrane distillation/crystallization (MD/MCr) experiments. Material. Polypropylene Material Type HollowPolypropylene fiber TypeNo. of fibers Hollow19 fiber No.Length of fibers of fibers (cm) 4219 LengthInner of fibers fiber (cm) diameter (mm) 1.842 Inner fiber diameter (mm) 1.8 Outer fiber diameter (mm) 2.7 Outer fiber diameter (mm) 2.7 MembraneMembrane thickness thickness (mm) (mm) 0.450.45 AverageAverage pore size pore (µm) size (μm) 0.20.2 PorosityPorosity (%) (%) 7373 2 SurfaceSurface area (cm area) (cm2) 45.145.1

Figure 1. MembraneMembrane distillation and crystallization setup used in the experiments. 2.3. Fouling/Scaling Analysis 2.3. Fouling/Scaling Analysis In order to assess the presence of fouling/scaling at the membrane surface, pure water flux In order to assess the presence of fouling/scaling at the membrane surface, pure water flux through the membrane before and after MCr test was measured. Subsequently, the used membrane through the membrane before and after MCr test was measured. Subsequently, the used membrane was cleaned with citric acid solution of pH 4 for 30 minutes. Flux measurements were performed was cleaned with citric acid solution of pH 4 for 30 minutes. Flux measurements were performed before and after cleaning, and were compared with the flux obtained with the virgin membrane. before and after cleaning, and were compared with the flux obtained with the virgin membrane. 2.4. Footprint, Weight, and Waste Reduction Analysis 2.4. Footprint, Weight, and Waste Reduction Analysis In order to evaluate the specific dimensions and weight of the MD/MCr unit, footprint and In order to evaluate the specific dimensions and weight of the MD/MCr unit, footprint and weight analysis was performed. These parameters were also evaluated in terms of the corresponding weight analysis was performed. These parameters were also evaluated in terms of the corresponding process intensity metrics, including productivity to size (P/S) and productivity to weight (P/W) ratios process intensity metrics, including productivity to size (P/S) and productivity to weight (P/W) ratios provided in the literature [9], which are defined mathematically as following: provided in the literature [9], which are defined mathematically as following: Productivity kg Productivity kgh P/S =  h  (1) PS/  Size(m3) (1) Sizem3  kg Productivity h = Productivity kg P/W  h  (2) PW/  Weight(kg) (2) Weight kg The conversion of wastewater into valuable products (freshwater and salt crystals) was monitored accordingThe conversion to the following of wastewater correlation: into valuable products (freshwater and salt crystals) was monitored according to the following correlation: Total waste(kg) Waste Intensity = (3) Mass of product(kg) Appl. Sci. 2019, 9, 287 5 of 15

For the calculation of P/S and P/W metrics, each hollow ﬁber membrane module was assumed to have a 0.201 m diameter, one-meters length, 34 m2 of area, and 16 kg of weight according to the information provided in the literature [8]. For the calculation of the size of the MD/MCr unit, 50% space density between the modules was assumed. The calculations were based upon a wastewater volume of 10 m3 per week.

2.5. Crystal Characterization The formed crystals were analyzed by an in-line microscope (Mettler Toledo, ParticleView V19) to follow the crystallization process during the experiments. Moreover, a one-mL solution containing crystals were withdrawn from the feed tank and characterized by optical microscope (SWEISS, Axioskop, West Germany) with a camera (Infinity X) at a magnification of 10×. At the end of the experiments, the solution containing crystals was filtered through a filter paper with a pore size of 20–25 µm, and dried at 45 ◦C in an oven overnight. Analysis of the dried crystals was carried out by XRD, ICP-OES, and SEM. XRD (Empyrean, PANalytical) analysis was performed to identify the phase of the crystal that was precipitated. The crystalline samples were finely ground before measuring. The program measured at 2θ from 10–110◦ degrees with a beam source of Cu Kα 1.54 Å, and the step size was 0.013◦ degrees. ICP-OES was carried out to determine the composition of the precipitated salts including impurities, and furthermore, SEM (EVO60, Zeiss) was used to identify the crystal morphology. SEM was operated at 10 kV with magnifications of 215, 401, 961, and 2030.

3. Results and Discussion

3.1. Membrane Characterization with Pure Water The flux for clean water as a function of time was determined under different thermal and hydrodynamic conditions. The experiments with different feed temperatures showed clear difference in the flux (Figure2a). The flux increased with temperature due to increases in the corresponding driving force i.e., the vapor pressure difference between the feed and permeate sides. The average fluxes for 38 ◦C, 47 ◦C, and 56 ◦C at a flow rate of 21.6 L/h were measured to be 1.29 L/m2·h, 2.36 L/m2·h, and 3.86 L/m2·h, respectively. The flux was also measured at a feed temperature of 56 ◦C, and flow rates of 14.8 L/h and 21.6 L/h were used to compare the effect of flow rates on water flux (Figure2b). The average flux at 14.8 L/h was measured to be 3 L/m 2·h, which is significantly lower than for the flux at 21.6 L/h. A high flow rate decreases the temperature polarization and temperature drop along the membrane module, which contributes to the observed corresponding high Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 15 flux, as explained in the literature [21].

(a) (b)

FigureFigure 2. 2.Flux Flux as as a a function function ofof timetime forfor clean water. ( (aa)) A Att different different feed feed temperatures temperatures,, all allwith with flow ﬂow raterate of of 21.6 21.6 L/h, L/h, ((bb)) atat differentdifferent flow ﬂow rates rates for for a afeed feed temperatures temperatures of of56.9 56.9 ± 0.1± °C0.1 and◦C 56.4 and ± 56.4 0.2 ±°C0.2 for ◦C for14.8 14.8 L/h L/h and and 21.6 21.6 L/h, L/h, respectively. respectively.

3.2. MD/MCr Tests with the Wastewater The flux for the MCr tests, which were performed for the centrifuged wastewater at different feed temperatures, is shown in Figure 3. Similar to the pure water flux, the flux increased with temperature. The mean flux was measured to be 0.95 ± 0.2 L/m2.h, 1.50 ± 0.2 L/m2.h, and 2.43 ± 0.2 L/m2.h at feed inlet temperatures of 38 °C, 47 °C, and 56 °C, respectively. Even at high recovery factors (>60%), the flux decline was more evident only at a low feed inlet temperature (37 °C). This observation is consistent with the literature [10], and can be associated with the relatively high viscosity of the used solution at low temperature, which increases the concentration polarization [22], and consequently, possible salt accumulation at the membrane surface. The conductivity of the permeate (Figure 3b) increased slightly at the beginning of the experiments, but remained constant during the rest of the time. For all of the conducted experiments, the final permeate conductivity remained below 250 µS/cm (corresponding to a minimum rejection toward conductivity of 99.4%), which indicates no or only a partly wetting of the membrane. No clear tendency of permeate conductivity with respect to the driving force (i.e., temperature) was observed during the experiments. In general, the permeate quality is of good standard. Furthermore, this is confirmed by analyzing the composition of the permeate stream (Table 4), which indicates that for all of the conducted experiments, only small amounts of impurities passed through the membrane, which was possibly due to the wetting of some of the membrane pores.

(a) (b)

Figure 3. (a) Flux and (b) conductivity of permeate as a function of water recovery factor at different feed temperatures, and a flow rate of 21.6 L/h.

Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 15

(a) (b)

Figure 2. Flux as a function of time for clean water. (a) At different feed temperatures, all with flow Appl. Sci.rate2019 of 21.6, 9, 287 L/h, (b) at different flow rates for a feed temperatures of 56.9 ± 0.1 °C and 56.4 ± 0.2 °C for6 of 15 14.8 L/h and 21.6 L/h, respectively.

3.2. MD/MCr MD/MCr Tests Tests withwith the Wastewater The fluxflux forfor thetheMCr MCr tests, tests, which which were were performed performed for for the the centrifuged centrifuged wastewater wastewater at different at different feed feedtemperatures, temperatures is shown, is shown in Figure in 3Figure. Similar 3. to Similar the pure to water the pure flux, water the flux flux increased, the flux with increased temperature. with 2 2 2 temperature.The mean flux The was mean measured flux was to bemeasured 0.95 ± 0.2 to L/mbe 0.95·h, ± 1.50 0.2 ±L/m0.22.h L/m, 1.50· h,± 0.2 and L/m 2.432.h±, and0.2 L/m2.43 ±·h 0. at2 ◦ ◦ ◦ Lfeed/m2. inleth at feed temperatures inlet temperatures of 38 C, of 47 38C, °C and, 47 56°C, andC, respectively. 56 °C, respectively. Even at Even high recoveryat high recovery factors (>60%),factors ◦ (>60%),the flux the decline flux was decline more was evident more only evident at a low only feed at inleta low temperature feed inlet (37 temperatureC). This observation(37 °C). This is obsconsistentervation with is consistent the literature with [10 the], and literature can be associated[10], and with can be the associated relatively high with viscositythe relatively of the usedhigh viscositysolution atof lowthe us temperature,ed solution at which low temperature increases the, which concentration increases polarization the concentration [22], and polarization consequently, [22], andpossible consequently, salt accumulation possible atsalt the accumulation membrane surface. at the The membrane conductivity surface. of theThe permeate conductivity (Figure of 3 theb) permeateincreased ( slightlyFigure 3 atb) theincreased beginning slightly of the at experiments,the beginning but of remainedthe experiments, constant but during remained the rest constant of the duringtime. For the all rest of the of conductedthe time. For experiments, all of the theconducted final permeate experiments conductivity, the final remained permeate below cond 250uctivityµS/cm remained(corresponding below to250 a minimumµS/cm (corresponding rejection toward to a conductivityminimum rejection of 99.4%), toward which conductivity indicates noof 99. or4 only%), whicha partly indicates wetting ofno the or membrane. only a partly No wetting clear tendency of the membrane.of permeate No conductivity clear tendency with respect of permeate to the conductivitydriving force (i.e., with temperature) respect to was the observed driving during force (i.e.the, experiments. temperature) In general,was observed the permeate during quality the experiments.is of good standard. In general, Furthermore, the permeate this quality is confirmed is of good by standard. analyzing Furthermore, the composition this ofis confirmed the permeate by analyzingstream (Table the4 composition), which indicates of the that permeate for all of stream the conducted (Table 4), experiments, which indicates only that small for amounts all of the of conductedimpurities experiments, passed through only the sm membrane,all amounts whichof impurities was possibly passed through due to the the wetting membrane, of some which of was the possiblymembrane due pores. to the wetting of some of the membrane pores.

(a) (b)

Figure 3. ((a)) Flux Flux and and ( (b)) conductivity of of permeate permeate as as a a function function of of water water recovery recovery factor factor at at different different feedfeed temperatures temperatures,, and a flow ﬂow rate of 21.6 L/h.

Table 4. Maximum concentration of ions in the permeate at the end of experiments. The initial

concentration was below the detection limit.

Concentration [mg/L] Mn2+ 0.0043 Mg2+ 0.076 P3− 0.0367 Na+ 16.6

MD and MCr tests have also been carried out on the wastewater under various hydrodynamic conditions (Figure4a). The maximum flux was observed at the highest flow rate. It is well acknowledged that a high feed velocity decreases the temperature polarization and temperature drop along the module in MD [23,24], which results in a higher flux, as is also observed for the studies on pure water flux. However, there was no significant difference in the flux measured at feed flow rates of 14.8 L/h and 21.6 L/h. The feed inlet temperature for the experiment performed at 14.7 L/h (47.7 ± 0.3 ◦C) was slightly higher than for that at 21.6 L/h (47.2 ± 0.3 ◦C). This slight difference in both the temperatures has possibly narrowed down the difference between the observed flux at these Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 15

Table 4. Maximum concentration of ions in the permeate at the end of experiments. The initial concentration was below the detection limit.

Concentration [mg/L] Mn2+ 0.0043 Mg2+ 0.076 P3− 0.0367 Na+ 16.6 MD and MCr tests have also been carried out on the wastewater under various hydrodynamic conditions (Figure 4a). The maximum flux was observed at the highest flow rate. It is well acknowledged that a high feed velocity decreases the temperature polarization and temperature drop along the module in MD [23,24], which results in a higher flux, as is also observed for the studies on pure water flux. However, there was no significant difference in the flux measured at feed flow rates ofAppl. 14.8 Sci. L/h2019 and, 9, 287 21.6 L/h. The feed inlet temperature for the experiment performed at 14.7 L/h (47.77 of 15± 0.3 °C) was slightly higher than for that at 21.6 L/h (47.2 ± 0.3 °C). This slight difference in both the temperatures has possibly narrowed down the difference between the observed flux at these two flowtwo flowrates. rates. As expected, As expected, the flux the flux decreases decreases slightly slightly with with the the recovery recovery factor, factor, due due to to decrease decrease in in the the waterwater activity activity and p potentialotential scaling. TheThe reasonreason forfor thethe disruptiondisruption in in the the flux flux for for the the higher higher flow flow rate rate is isthat that this this experiment experiment was was conducted conducted overnight overnight withoutwithout automaticautomatic recordingrecording of the mass changes changes of of thethe permeate. permeate. The The dependence dependence of of conductivity conductivity of of the the permeate permeate does does not not sh showow any any clear clear trend trend with with respectrespect to to the the flow flow rate rate,, and and remains remains below below 240 240 µS/cmµS/cm (corresponding (corresponding to to a a minimum minimum rejection rejection toward toward conductivityconductivity of of 99.4%) 99.4%) in in all all of of the the cases. cases.

5 FR = 14.8 L/h FR = 21.6 L/h FR = 30.7 L/h

0 0 10 20 30 40 50 60 70 80 90 100 Water recovery factor [%] (a) (b)

FigureFigure 4. 4. ((aa)) Flux Flux and and ( (bb)) c conductivityonductivity of of permeate permeate as as a a function function of of recovery recovery factor factor for for the the experiment experiment conductedconducted at at 14. 14.88 L/h, L/h, 21.6 21.6 L/h L/h,, and and 30.7 30.7 L/h L/h and and feed feed temperature temperaturess of 47.7 of 47.7 ± 0.3± °C,0.3 47.2◦C, 47.2± 0.3± °C0.3, and◦C, 47.1and ± 47.1 0.2 ±°C,0.2 respectively.◦C, respectively.

ForFor all all of of the the conducted conducted experiments, experiments, a a water water recovery recovery factor factor between between 8 84–92%4–92% was was achieved. achieved. ThisThis highlights highlights the the possibility possibility of of minimiz minimizinging the the waste waste stream stream with with at at least least a a factor factor of of six six;; simultaneouslysimultaneously,, this has beenbeen convertedconverted intointo aapure pure water water stream. stream. Moreover, Moreover, precipitation precipitation occurred occurred in inall all of theof conductedthe conducted experiments, experiments potentially, potentially producing producing another anothervaluable valuableresource from resource the wastewater. from the wastewater.The nature and The characteristics nature and characteristics of the crystals of arethe evaluatedcrystals are in evaluated the sections in below.the sections below.

3.3.3.3. Crystallization Crystallization Study Study 3.3.1. Crystal Morphology 3.3.1. Crystal Morphology The in-line PVM microscope showed two different structures precipitated in the feed tank. The in-line PVM microscope showed two different structures precipitated in the feed tank. When When 80–90% of the feed water was removed, needle-shaped crystals started to precipitate, followed by 80–90% of the feed water was removed, needle-shaped crystals started to precipitate, followed by a a precipitation of prismatic shaped crystals (Figure5a,b, respectively). Both types of crystals were precipitation of prismatic shaped crystals (Figure 5a,b, respectively). Both types of crystals were observed under all of the thermal and hydrodynamic conditions that were investigated, negating any observed under all of the thermal and hydrodynamic conditions that were investigated, negating any correlation between the operating conditions and the type of crystals obtained. However, in all of correlation between the operating conditions and the type of crystals obtained. However, in all of the the cases, needle-shaped crystals appeared before the prismatic crystals, perhaps due to the greater induction time of the latter compared to the former. The crystallization occurred fast when the precipitation point was reached; the needle-shaped structures grew especially faster. This could indicate that two different polymorph structures were precipitated. The same precipitation trend was observed in all of the experiments. Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 15 cases, needle-shaped crystals appeared before the prismatic crystals, perhaps due to the greater induction time of the latter compared to the former. The crystallization occurred fast when the precipitation point was reached; the needle-shaped structures grew especially faster. This could indicateAppl. Sci. 2019that, 9two, 287 different polymorph structures were precipitated. The same precipitation trend8 was of 15 observed in all of the experiments.

(a)

(b)

FigureFigure 5. 5. PicturesPictures from anan in-linein-line PVM PVM 19 19 microscope microscope from from experiment experiment conducted conducted at 56at◦ 56C and°C and 21.6 21.6 L/h. L/h.(a) Needle-shaped (a) Needle-shaped crystalline crystalline structures, structures, (b) needle (b) needle and and prismatic-shaped prismatic-shaped crystalline crystalline structures. structures.

TheThe obtained obtained crystals crystals were were also also analyzed analyzed by by using an optical microscope. Images Images from from the the opticaloptical microscope microscope showed showed that that the the size size of of the the prismatic prismatic crystalline crystalline structures structures increased increased with with the the temperature (Figure6). The experiment at the lowest feed temperature precipitated the smallest prismatic crystals (~0.01 mm), while 56 ◦C precipitated the largest prismatic crystals (~0.08 mm). The size of the needle-shaped crystals did not change significantly with temperature (length of Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 15 Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 15 temperature (Figure 6). The experiment at the lowest feed temperature precipitated the smallest Appl.temperatureprismatic Sci. 2019 crystals, 9 , ( 287Figure (~0.01 6). mm), The experiment while 56 °C atprecipitated the lowest the feed largest temperature prismatic precipitated crystals (~0.08 the mm smallest).9 ofThe 15 prismaticsize of the crystals needle- (~0.01shaped mm), crystals while did 56 not °C change precipitated significant the largestly with prismatic temperature crystals (length (~0.08 of mm~0.1). mm). The sizeMoreover, of the needle the prismatic-shaped crystals crystals increased did not change in size significantwith a decreasely with in temperature the flow rate (length (Figure of 7), ~0.1 but mm).there ~0.1Moreover,was mm).no clear Moreover,the tendency prismatic the for crystals prismatic the needle increased crystals-shaped increasedin size crystals. with in a sizeThe decrease withexperiment a in decrease thes flow carried in rate the out ( flowFigure at rate47 7°),C (Figure but and there 14.87), ◦ butwasL/h there producedno clear was tendency nolarge clearr prismatic tendencyfor the needle crystals for the-shaped needle-shapedand needle crystals.-shaped The crystals. experimentcrystals The than experimentss carried the other out carried experiments. at 47° outC and at 47 14.8TheC andL/hneedle produced 14.8-shaped L/h produced large crystalsr prismatic largerin the prismaticexperiment crystals crystalsand at 47needle ° andC and- needle-shapedshaped 30.7 L/h crystals were crystals alsothan larger the than other than the otherexperiments. those experiments. in the other The ◦ Theneedleexperiments needle-shaped-shaped, and crystals only crystals ina few the in smallexperiment the experiment prismatic at 47 crystals at °C 47 andC were30.7 and L/h 30.7formed. were L/h also wereThe largerexperiment also larger than those thanat 47 thosein°C th ande inother 21.6 the ◦ otherexperimentsL/h showed experiments,, and both only andprismatic a only few a small and few small needleprismatic prismatic-structured crystals crystals were crystals, wereformed. but formed. theThe needleexperiment The experiment-shaped at 47 crystalsat °C 47 andC were21.6 and 21.6L/hsmaller showedL/h compared showed both both to prismatic the prismatic other and experiments and needle needle-structured-structured when the crystals,flow crystals, rate but was but the changed. the needle needle-shaped -shaped crystals crystals were smaller compared to the other experimentsexperiments whenwhen thethe flowflow raterate waswas changed.changed.

(a) (b) (c) (a) (b) (c) Figure 6. Optical microscope images of the crystals precipitated in experiments carried out at a flow Figurerate of 6.21.6 OpticalO pticalL/h. M microscopemicroscopagnificatione images 10×. (a ) of 38 the °C crystals, (b) 47 ° precipitatedC, (c) 56 °C. in experiments carried out at a ﬂowflow rate of 21.6 L/h.L/h. M Magniﬁcationagnification 10 10××. (.(a)a 38) 38 °C◦C,, (b ()b 47) 47 °C◦,C, (c ()c 56) 56 °C◦.C.

(a) (b) (c) (a) (b) (c) FigureFigure 7.7. Optical microscopemicroscope images ofof the precipitated crystalscrystals inin anan experimentexperiment carriedcarried outout atat aa feedfeed temperatureFiguretemperature 7. Optical ofof 4747 microscop◦ °CC andand flowflowe imagesrates rates of of ( athe(a)) 14.8 14.8precipitated L/h, L/h, ((bb)) 21.6 21.6crystals L/h, L/h, in andand an ((experimentcc)) 30.730.7 L/h.L/h. carriedMagnification Magnification out at a in feed allall oftemperatureof thethe casescases waswas of 47 11× ×°.C. and flow rates of (a) 14.8 L/h, (b) 21.6 L/h, and (c) 30.7 L/h. Magnification in all of the cases was 1×. TheThe imagesimages fromfrom SEMSEM showed showed a a sharp sharp prismatic prismatic structure structure of of the the crystalline crystalline phases. phases. Compared Compared to theto the picturesThe pictures images from from from the the SEM optical optical showed microscope microscope a sharp taken prismatictaken under under structure wet wet conditions, conditions, of the crystalline the the needle-shapedneedle phases.-shaped Compared crystalscrystals decreasedtodecreased the pictures inin amount.amount. from the The optical SEM picturespicturesmicroscope atat differentdifferent taken under magnificationsmagnifications wet conditions, cancan bebe the seenseen needle inin FigureFigure-shaped8 8.. The The crystals same same tendencydecreasedtendency ofof in thethe amount. crystal The sizesize SEM waswas pictures observed at fromf romdifferent thethe SEMSEM magnifications images,images, i.e,i.e, anancan increasing be seen in sizesize Figure of the 8. The prismatic same crystalstcrystalsendency withwith of the temperature,temperature, crystal size whereaswhereaswas observed nono clearclear from tendencytendency the SEM waswa imagess observedobserved, i.e, an forfor increasing thethe changechange size inin of flowflow therate. prismaticrate. crystals with temperature, whereas no clear tendency was observed for the change in flow rate. Appl. Sci. 2019, 9, 287 10 of 15 Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 15

FigureFigure 8. 8.SEM SEM pictures pictures of the crystals of the atcrystals different at magniﬁcations different magnifications for the membrane for distillation/ the membrane crystallizationdistillation/crystallization (MD/MCr) experiments(MD/MCr) experiments conducted at conducted different operatingat different conductions. operating conductions.

TheThe crystallinecrystalline phasesphases fromfrom the the previous previous described described MD/MCr MD/MCr experimentsexperiments werewere analyzedanalyzed withwith XRDXRD and and ICP ICP to to determine determine the the purity purity and and composition composition of of the the crystals. crystals.

3.3.2.3.3.2. IdentiﬁcationIdentificationand andPurity Purity ofof the the Crystals Crystals

TheThe XRD XRD spectra spectra showed showed that thethat crystalline the crystalline phase that phase precipitated that precipitated was Na2SO was4. Equal Na2SO crystalline4. Equal peakscrystalline were also peaks found were in also the literature found in [the19, 25literature] and in the[19, XRD25] and database in the by XRD analyzing database powdered by analyzing pure Napowdered2SO4 (Figure pure9 ).Na2SO4 (Figure 9). The crystalline phases that precipitated in the experiments were found to be Na2SO4, which can precipitate in the anhydrous form and as the hydrated forms, Na2SO4·10H2O (mirabilite) and Na2SO4·7H2O (heptahydrate) [25]. Na2SO4 anhydrous can precipitate in different polymorphs where ﬁve different phases have been identiﬁed (phase I, II, III, IV, and V) [26]. At room temperature, phase V is stable, while phase III is metastable. Phase I and II are high-temperature polymorphs (>270 ◦C and >225 ◦C) [27]. Phase IV is considered to be metastable, and the phase transition has not yet been well established [26]. The structure of Na2SO4 (III) and Na2SO4 (V) were found to be needle-shaped and prismatic/bipyramidal crystals by the evaporation of solution at 60◦C[26], which corresponds to the structures found in this study. The needle-shaped and prismatic crystals obtained in the MD and Appl. Sci. 2019, 9, 287 11 of 15

MCr experiments were consistent with the Na2SO4 structures found in the literature [25], where the needles were said to be phase III, and the prisms were said to be phase V. The prismatic structure of Na2SO4 that was obtained in these experiments was described in the literature [28], and is said to be phase V. In previous studies on the membrane crystallization of sodium sulfate crystallization, other structures such as orthorhombic structures [20,29] or rectangular/circular crystals [18] were observed, which is slightly different from this study. This can be attributed to the differences in feed composition or operative conditions, which also highlights the potential of controlling polymorph structures by membrane crystallizers through changing operational parameters. Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 15

FigureFigure 9. XRD 9. XRD spectra spectra of theof the crystalline crystalline phases phases precipitatedprecipitated in in the the MD MD and and MCr MCr experiments. experiments. XRD XRDof of pure Na2SO4 was also made (blue curve). pure Na2SO4 was also made (blue curve).

The crystalline phases that precipitated in the experiments were found to be Na2SO4, which can The purity of the Na2SO4 crystals was analyzed by ICP (Table5). The largest percentage of cations precipitate in the anhydrous form and as the hydrated forms, Na2SO4·10H2O (mirabilite) and in the crystalline structures was Na, which was expected due to the XRD results. The data showed Na2SO4·7H2O (heptahydrate) [25]. Na2SO4 anhydrous can precipitate in different polymorphs where that the crystalline structures contained small amounts of Fe, Mg, Mn, and P, which could indicate five different phases have been identified (phase I, II, III, IV, and V) [26]. At room temperature, phase that residueV is stable wastewater, while phase remained III is metastable. with crystals Phase afterI and theirII are separationhigh-temperature from thepolymorphs mother liquid,(>270 °C or the possibleand co-crystallization>225 °C) [27]. Phase of IV other is considered minerals. to Although be metastable Al was, and not the detected phase transition in the supernatant; has not yet been this was perhapswell dueestablished to its quantity [26]. The being structure below of Na the2SO detection4 (III) and limit Na2SO of4 the(V) we instrument,re found to which be needle was-shaped eventually incorporatedand prismatic/bipyramidal into crystal lattice. crystals by the evaporation of solution at 60°C [26], which corresponds to the structures found in this study. The needle-shaped and prismatic crystals obtained in the MD and MCr experimentsTable were 5. Contentconsistent [%] with of cations the Na in2SO crystalline4 structures phases. found Not in the detected literature (nd). [25], where the needles were said to be phase III, and the prisms were said to be phase V. The prismatic structure of Na2SO4 that Experimentwas obtained in these Al experiments Fe was described Mg in Mn the literature Na [28], and P is said to be phase V. In47 previous◦C-21.6 studies L/h on thend membrane nd crystallization 0.00042 of 0.0021 sodium sul 28.47fate crystallization nd , other structures such56 ◦C-21.6 as orthorhombic L/h 0.065 structures [20 nd,29] or 0.0233rectangular/circular 0.0032 crystals 32.84 [18] 0.0053 were observed, ◦ which is slightly38 C-21.6 different L/h from0.012 this study. 0.00005 This 0.016can be attributed 0.0023 to 26.21 the difference 0.0048 s in feed ◦ composition47 orC-30.7 operative L/h conditions,0.0032 which 0.0008 also highlights 0.015 the 0.0026 potential 30.53of controlling 0.0028 polymorph ◦ structures by47 membraneC-14.8 L/h crystallizers0.146 through 0.0027 changing 0.045 operational 0.0059 parameters. 23.32 nd The purity of the Na2SO4 crystals was analyzed by ICP (Table 5). The largest percentage of 3.4. Fouling/Scalingcations in the crystalline Analysis structures was Na, which was expected due to the XRD results. The data showed that the crystalline structures contained small amounts of Fe, Mg, Mn, and P, which could Theindicate normalized that residue flux wastewater (flux divided remained with thewith corresponding crystals after their driving separation force tofrom flatten the mother the effect liquid of, small temperatureor the possible variations) co-crystallization obtained by of usingother miner virginals. and Although used (after Al was and not before detected cleaning) in the supernatant membranes; has beenthis shown wasin perhaps Figure due10. Theto its mean quantity values being of below normalized the detection flux before limit andof the after instrument cleaning, which was calculated was to beeventually 0.00036 LMH/Pa incorporated and into 0.00052 crystal LMH/Pa, lattice. where the latter is similar to the flux measured for the virgin membrane. These observations clearly indicate the buildup of scaling at the membrane surface during MCr tests. However,Table 5. Content the recovery [%] of cations of fluxin crystalline after cleaning phases. Not indicates detected (nd) that. the scaling built-up during the test was reversible.Experiment Al Fe Mg Mn Na P 47 °C-21.6 L/h nd nd 0.00042 0.0021 28.47 nd 56 °C-21.6 L/h 0.065 nd 0.0233 0.0032 32.84 0.0053 38 °C-21.6 L/h 0.012 0.00005 0.016 0.0023 26.21 0.0048 47 °C-30.7 L/h 0.0032 0.0008 0.015 0.0026 30.53 0.0028 47 °C-14.8 L/h 0.146 0.0027 0.045 0.0059 23.32 nd

3.4. Fouling/Scaling Analysis The normalized flux (flux divided with the corresponding driving force to flatten the effect of small temperature variations) obtained by using virgin and used (after and before cleaning) membranes has been shown in Figure 10. The mean values of normalized flux before and after Appl. Sci. 2019,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 12 of 15 cleaning was calculated to be 0.00036 LMH/Pa and 0.00052 LMH/Pa, where the latter is similar to the flux measured for the virgin membrane. These observations clearly indicate the buildup of scaling at Appl.the membrane Sci. 2019, 9, 287 surface during MCr tests. However, the recovery of flux after cleaning indicates12 that of 15 the scaling built-up during the test was reversible.

0.001 Virgin membrane Before cleaning After cleaning

0.0008

0.0006

0.0004

0.0002

0 0 10 20 30 40 50 60 70 80 90 100 Time [min] FFigureigure 10. NormalizedNormalized flux ﬂux for virgin and used (before and after cleaning) membrane.

3.5. Weight, Size Size,, and Waste Reduction Analysis The f footprintsootprints and weight of the system as a function of the feed inlet temperaturetemperature have been shown in in Figure Figure 1111a.a. The The figure figure shows shows th thatat the the footprints footprints of of the the MD/MCr MD/MCr unit unit reduce reduce by almost by almost 2.3 ◦ ◦ times2.3 times by increasing by increasing the feed the feed inlet inlet temperature temperature from from 38 °C 38 to C56 to °C. 56 Similarly,C. Similarly, the weight the weight of the of unit the ◦ dropsunit drops down down by more by morethan 2.5 than times 2.5 timesby increasing by increasing the feed the inlet feed temperature inlet temperature from 38 from °C to 38 56C °C. to ◦ These56 C. observations These observations are due areto increased due to increased flux at a high flux- atfeed ahigh-feed inlet temperature inlet temperature (Figure 3a). (Figure High 3fluxa). decreasesHigh flux decreasesthe membrane the membrane area that areais required that is required to treat toa particular treat a particular volume volume of the wastewater of the wastewater;; thus, thethus, associated the associated weight weight and footprint and footprint of the ofmembrane the membrane unit reduce unit reduce.. Figure Figure 11b reveals 11b reveals the same the trend same, wheretrend, wherethe P/W the and P/W P/S and ratios P/S ratios increase increase with with the thefeed feed inlet inlet temperature temperature due due to to the the corresponding corresponding increase of productivity, which is defineddefined asas thethe productproduct ofof fluxflux andand thethe correspondingcorresponding membranemembrane area of a given MD/MCrMD/MCr unit unit with with the the feed feed temperature. temperature. As As shown shown in in Figure Figure 1111c,c, the the waste waste intensity intensity approaches zero at the final final freshwater recovery factor obtained in this study (approximately 90%), 90%), indicating that almost all of the the wastewater wastewater has has been been converted converted into into freshwater freshwater and and some some salt salt crystals, crystals, thus minimizing the wastewater volumevolume toto bebe disposed.disposed.

(a) (b) Figure 11. Cont. Appl.Appl. Sci. 2019,, 99,, 287x FOR PEER REVIEW 1313 of 1515

(c)

Figure 11.11. BasedBased onon aa plantplant withwith capacitycapacity ofof 270270 kg/h,kg/h, weightweight module = 16 kg, areaarea module = 3434 m2.. ((aa)) FootprintFootprint andand weight,weight, ((bb)) P/SP/S and and P/W, P/W, ( (cc)) w wasteaste intensity. intensity.

4. Conclusions High-purityHigh-purity crystals and freshwater were simultaneously recovered from wastewater from the anodizing industry by by applying applying MD/ MD/MCr.MCr. The The applied applied membrane membrane showed showed stable stable flux flux under under all of all the of thethermal thermal and and hydrodynamic hydrodynamic conditions conditions that that were were investigated. investigated. In all In of all the of thecases, cases, more more than than 80% 80% of ofthe the freshwater freshwater from from the the wastewater wastewater was was recovered, recovered, thus thus significantly significantly reducing reducing the the volume of thethe wastewaterwastewater to bebe disposed.disposed. The high-qualityhigh-quality water that was produced can be used in thethe anodizinganodizing processprocess to to avoid avoid the the failure failure of the of the bath bat chemistry,h chemistry which, which might might force the force disposal the disposal of expensive of expensive solution. Atsolution. the same At time, the same sodium time, sulfate sodium crystals sulfate were crystals recovered, were recovered which can, which find applications can find applications within the anodizingwithin the process anodizing or elsewhere, process or thus elsewhere adding, value thus toadding the process. value Itto wasthe observedprocess. It that was two observed polymorphs that (intwo needle polymorphs and prism (in shapes) needle of and sodium prism sulfate shapes) were of sodiumprecipitated. sulfate The were relative precipitated proportion. The and relative size of theproportion recovered and crystalline size of phases the recov dependsered upon crystalline the applied phases operating depends conditions. upon the Overall, applied the MD/MCroperating unitconditions was able. Overall, to convert the MD/MCr approximately unit was 90% able of to the convert wastewater approximately into products 90% of (freshwater the wastewater and into salt crystals).products The(freshwater footprints and and salt weight crystals). of theThe MCr footprints unit that and were weight required of the for MCr the unit wastewater that were treatment required significantlyfor the wastewater decrease treatment with the significantly temperature decrease of the with feed the solution. temperature Thus, of MD/MCr the feed issolution. a promising Thus, innovativeMD/MCr is solution a promising to approach innovativ zeroe solution liquid discharge to approach in the zero anodizing liquid industry,discharge while in the at anodizing the same timeindustry recovering, while at high-quality the same time freshwater recovering and high salt crystals.-quality freshwater and salt crystals.

Author Contributions: conceptualization, A.A. and C.A.Q.-J.; methodology, A.A. and J.H.J.; validation, J.H.J., Author Contributions: Conceptualization, A.A. and C.A.Q.-J.; methodology, A.A. and J.H.J.; validation, J.H.J., H.C.J.H.C.J. and C.A.Q.-J.;C.A.Q.-J.; Formal analysis, H.C.J.H.C.J. and C.A.Q.-J.;C.A.Q.-J.; investigation, A.A. and J.H.J.;J.H.J.; writing—originalwriting—original draft preparation,preparation, A.A.A.A. andand J.H.J.;J.H.J.; writing—reviewwriting—review and editing,editing, A.A.,A.A., M.L.C.M.L.C. andand C.A.Q.-J.;C.A.Q.-J.; visualization,visualization, A.A.A.A. andand C.A.Q.-J.;C.A.Q.-J.; supervision,supervision, M.L.C.M.L.C. andand C.A.Q.-J.;C.A.Q.-J.; projectproject administration,administration, C.A.Q.-J.C.A.Q.-J.

Funding: This research received nono externalexternal funding.funding

Acknowledgments:Acknowledgments: TheThe Danish Danish Anodizing Anodizing Industry Industry (DAI) (DAI) is greatly is acknowledgegreatly acknowledge for providing for the providing wastewater. the Conﬂictswastewater. of Interest: The authors declare no conﬂict of interest.

Conflicts of Interest: The authors declare no conflict of interest References

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