Treatment of Aniline Wastewater by Membrane Distillation and Crystallization

Abstract

Aniline is a highly toxic organic pollutant with “carcinogenic, teratogenic and mutagenesis” characteristics. In the present paper, a membrane distillation and crystallization (MDCr) process was proposed to achieve zero liquid discharge (ZLD) of aniline wastewater. Hydrophobic polyvinylidene fluoride (PVDF) membranes were used in the membrane distillation (MD) process. The effects of the feed solution temperature and flow rate on the MD performance were investigated. The results showed that the flux of the MD process was up to 20 L·m⁻²·h⁻¹ and the salt rejection was above 99% under the feeding condition of 60 °C and 500 mL/min. The effect of Fenton oxidation pretreatment on the removal rate of aniline in aniline wastewater was also investigated, and the possibility of realizing the ZLD of aniline wastewater in the MDCr process was verified.

1. Introduction

Aniline is a kind of highly toxic, refractory organic pollutant with “carcinogenic, teratogen and mutagenic” properties, and is among the 129 priority environmental pollutants in the United States and is included in the pollution blacklist published by the European Community [1,2]. Furthermore, about 30,000 t aniline is discharged into the environment every year worldwide [3,4,5,6]. As of 2019, the domestic aniline production capacity is more than 3.6 million t/a. Aniline wastewater is characterized by a high content of aniline substances (mass concentrations can reach thousands of mg/L [5]), a high salt content, obvious changes in the chemical oxygen demand (COD) content (ranging from 200 mg/L to 2000 mg/L [7]), etc. Aniline wastewater is mainly derived from the production and use process and widely exists in the printing and dyeing, pharmaceutical, and papermaking industries [8,9]. The treatment of aniline wastewater traditionally involves chemical, physical, biological, and electrochemical methods [10]. Among them, the biological method is the most widely used, through which aniline can be made completely biodegradable to CO₂ and N₂/NOx [11]. However, chemical, biological, and electrochemical methods cannot realize the recovery of aniline and discharge a large number of chemical reagents, which not only increases the operating cost but also causes secondary pollution. For example, biological methods are temperature-dependent and are restrained by the toxicity of pollution, especially because they are very sensitive to the concentration of salt in wastewater. The electrochemical method is clean and easy to handle, but it produces more carbon dioxide, which is called greenhouse gas. Aniline wastewater can also be treated with biological enzymes such as Laccases [12], but the interaction mechanism of biological enzymes with different types of wastewater is still unclear and it has relatively high costs. Although aniline can be recovered by physical methods, they are not suitable for industrial applications because it is difficult to separate the extractant and aniline [13]. Therefore, an efficient aniline wastewater treatment process is urgently needed; it is necessary to develop a new process for the treatment of aniline wastewater.

As a new desalination technology, membrane distillation (MD) is especially suitable for treating high-salt solutions [14,15]. In the MD process, a hydrophobic porous membrane acts as the physical barrier between the hot feed and the permeate flow, and the vapor pressure gradient drives the volatile compounds to be transported from the feed side to the permeate side [16]. Since the need for transmembrane pressure is eliminated in MD, this operation is not sensitive to feed concentration [17]. In addition, MD operates at a low temperature below the boiling point of the feed solution, generating water vapor and utilizing renewable heat energy [18]. There are four different modes of MD: Direct Contact Membrane Distillation (DCMD) [19,20]; Air Gap Membrane Distillation (AGMD) [21,22]; Sweeping Gas Membrane Distillation (SGMD) [23,24]; and Vacuum Membrane Distillation (VMD) [25,26]. Compared with the other MD configurations, VMD permits higher partial pressure gradients, and, hence, higher permeate flux can be achieved [27,28]. In addition, it is better than DCMD in terms of energy consumption/permeate flow ratios and thermal evaporation efficiency [29]. One of the most important benefits provided by VMD is that of the possibility of harnessing available renewable energy sources such as solar energy [30,31]. In this study, the VMD configuration was also adopted to carry out membrane distillation and crystallization (MDCr) experiments [32]. In addition, in the treatment of specific wastewater, the unique characteristics of MD allow only volatile components to pass through the pores, resulting in the simultaneous generation of water and recyclable materials [33]. It is also noteworthy that the process can concentrate brine into a supersaturated state for further MDCr processes [34]. Before the MDCr process, the salinity of feed brine can be concentrated to be close to the saturation state by MD. Thus, MDCr can obtain clean water and salt crystals in one step, and, with some additional and simple treatments, can even meet drinking water standards. The nucleation and growth of salt crystals can be controlled by adjusting the feed temperature and flow rate [35]. In addition, the MDCr process can start with a low-level heat source, such as waste heat and solar energy from a factory or power plant [36,37]. Therefore, it is feasible to integrate the MDCr process into the zero liquid discharge (ZLD) system [38]. In sewage treatment, the concept of ZLD is mentioned more and more frequently; ZLD is a concept where most industrial raw materials, such as water and salts, are purified, reused, and separated after multiple stage cycles as much as possible to ensure that no waste liquid is discharged from a factory [39,40].

In this work, the VMD process was used to investigate the treatment feasibility of aniline wastewater. The temperature and flow rate during VMD operation was optimized, and the contamination conditions under different conditions were investigated. The Fenton oxidation process was used to remove aniline from wastewater, and the optimal concentration of H₂O₂ and FeSO₄ was determined. The pretreatment technology of aniline wastewater was also studied for a better treatment effect. Then, a long-time MDCr experiment was carried out with aniline wastewater, and regular cube salt crystal was obtained, showing the feasibility of membrane distillation crystallization to treat aniline wastewater.

2. Experiment

2.1. Material

Commercial hydrophobic flat sheet PVDF (GVHP 04700, 0.22 μm, Millipore, Burlington, MA, USA) membranes were applied in this work. Ferrous sulfate (FeSO₄) and hydrogen peroxide (H₂O₂) are the materials for the Fenton reaction. Aniline wastewater came from the production of anti-aging agents by the Nanjing Research Institute of Chemical Industry. Both FeSO₄ and H₂O₂ came from the China National Pharmaceutical Group, with analytical purity and an H₂O₂ concentration of 30%. Ionized water was produced by a self-made reverse osmosis system. All chemicals were used without further purification.

2.2. Characterization

Inorganic anions and cations were analyzed using an ion chromatograph spectrometer (Thermo ICS 2000, USA) and an inductively coupled plasma spectrometer (ICP, Perkin Elmer Optima 7000DV, USA), respectively. Before each ICP and ICS test, a microwave digestion device (Analytik TOP wave, Germany) was employed to digest the organic components that could cause possible interference. For organic components, COD values of the feed and permeate solution were measured using a spectrophotometer (Hach DR 3900, USA). The values of conductivity and total dissolved solids were both measured by a conductivity meter (Mettler Toledo FE 38, China). Specific organic components (including esters, alcohols, benzenes, and others) were tested by a triple-quadrupole mass analyzer (Bruker Scion-MS-4306GC, Germany). An atomic force microscope (AFM) (Bruke Icon, Germany) operated in tapping mode was used to evaluate the surface topography of the membranes in a scan size of 10 × 10 μm.

Since the original aniline wastewater had undergone many complex treatment procedures in the industry, it was important to determine its composition and properties. The basic information on the aniline wastewater is shown in

Table 1. Basic information on aniline wastewater.

Number	Project	Units	Numerical Value
1	pH	-	12.85–12.96
2	Conductivity	ms·cm−1	162.75
3	Na+	g·L−1	52.86
4	Cl-	g·L−1	66.30
5	NO3-	g·L−1	0.88
6	Turbidity	NTU	2.98
7	TDS	mg·L−1	≈125
8	TOC	mg·L−1	4728
9	COD	mg·L−1	5600

. The untreated aniline wastewater was alkaline, the cations were mainly sodium ions, with a content of about 52.86 g·L⁻¹, the anions were mainly chloride ions, with a content of about 66.30 g·L⁻¹, and trace nitrate ions were also present in the aniline wastewater. In addition, the conductivity value was about 162.75 ms·cm⁻¹, and the COD and TOC (total dissolved solids) values were about 1400 mg·L⁻¹ and 4728 mg·L⁻¹, respectively. Based on this information, it was speculated that the aniline wastewater mainly contained NaCl. In addition, the specific organic components are listed in Table S1 and Figures S1 and S2. Seven kinds of main organics were detected in the aniline wastewater, among which aniline was the main pollutant.

In order to evaluate the effects of aniline wastewater on the membranes before and after MD operation, the membrane morphology and surface element distribution were measured using a field emission scanning electron microscope–energy dispersive spectrometer (FESEM-EDS, Hitachi S4800, Japan). The average pore size and pore size distribution of the membranes were measured by a membrane pore size distribution apparatus (PSDA-20, Gaoqian function Co., Nanjing, China) based on the gas–liquid exclusion method.

The porosity of the PVDF membranes (ε) was measured by the gravimetric method and calculated by Equation (1):

$$\epsilon \left(\%\right)=\frac{\frac{{\mathrm{W}}_{\mathrm{w}}-{\mathrm{W}}_{\mathrm{d}}}{{\mathsf{\rho }}_{\mathrm{i}}}}{\frac{{\mathrm{W}}_{\mathrm{w}}-{\mathrm{W}}_{\mathrm{d}}}{{\mathsf{\rho }}_{\mathrm{i}}}+\frac{{\mathrm{W}}_{\mathrm{d}}}{{\mathsf{\rho }}_{\mathrm{M}}}}\times 100\%$$

(1)

where W_w is the weight of the wet membrane, W_d is the weight of the dry membrane, ${\rho }_i$ is the density of IPA, and ${\rho }_M$ is the density of PVDF. Mechanical strength tests were performed with a tensile strength testing instrument (Model SH-20, Wenzhou Shandu Instrument Co., China). Then, tensile strength (σ) and elongation at break (δ) were calculated by Equations (2) and (3), respectively:

$$\mathsf{\sigma }=\frac{\mathrm{F}}{\mathrm{A}}\times 100\%$$

(2)

$$\mathsf{\delta }=\frac{\mathrm{L}-{\mathrm{L}}_0}{{\mathrm{L}}_0}\times 100\%$$

(3)

where A is the cross-sectional area of the membrane sample, L and F are the final length and tensile stress when the sample was broken, and L₀ is the initial length of the tested sample. Hydrophobicity is one of the most significant parameters of the membranes used in MD and is usually evaluated by contact angle (CA) measurements. In the present work, CA was measured by a commercial contact angle instrument (Dataphysics OCA 25, Germany).

2.3. Fenton Oxidation Pretreatment Process

The Fenton oxidation process is one of the processes of the catalytic oxidation of organic substances in wastewater by hydroxyl radicals generated by the reaction of ferrous ions (Fe²⁺) and H₂O₂. Compared with other methods, the Fenton oxidation process has the advantages of economy, convenient use, non-toxic by-product production, and no need for any complex or expensive instruments [41,42]. Ferrous ions catalyze hydrogen peroxide to produce hydroxyl radicals as follows:

Fe²⁺ + H₂O₂ → Fe³⁺ +·OH + OH⁻

During the Fenton reaction, iron ions and hydroxyl ions are also formed due to the reaction of oxygen free radicals and ferrous ions.

Fe³⁺ + H₂O₂ → Fe²⁺ + H⁺ +HO₂

OH + Fe²⁺ → OH⁻ + Fe³⁺

OH + H₂O₂ → H₂O + HO₂

OH +·OH → H₂O₂

The removal rate of aniline wastewater can be calculated by Equation (4) as follows:

$$\mathrm{Aniline} \mathrm{removal} \mathrm{rate}=\frac{{\mathrm{C}}_0-{\mathrm{L}}_0}{{\mathrm{C}}_0}\times 100\%$$

(4)

where C₁ and C₀ are the concentration detected by an ultraviolet spectrophotometer (Perkin Elmer Lambda 950, USA) of original aniline wastewater and aniline wastewater after pretreatment [43,44].

2.4. Vacuum Membrane Distillation and Crystallization

The membrane used in this experiment was a Millipore GVHP 0.22 μm hydrophobic membrane, and its specific parameters are shown in

Table 2. Basic parameters of the PVDF membrane.

PVDF Characteristics	Value
Contact angle (°)	128
Porosity (%)	65.12
Tensile ratio (%)	69.05
Tensile strength (MPa)	4.52
Pore size (nm)	334
Thickness (mm)	0.122

.

The performance of aniline wastewater treatment was tested by using the experimental MDCr setup, as shown in Figure 1. The effective membrane area was 11.34 cm², and the permeate vacuum degree of permeate side was kept around 95 kPa. In addition, in order to promote crystallization, stirring was performed in the crystallizer. A total volume of 1 L aniline wastewater was circulated by a peristaltic pump. On the permeate side, the cooling bath temperature was kept at 5 °C. For each test, the permeate and conductivity were recorded. The permeate flux (J) and concentration factor of the feed solution were calculated by Equations (5) and (6) [45].

$$\mathrm{J}=\frac{m}{A\times \mathrm{t}}$$

(5)

$$\mathrm{Concentration} \mathrm{factor}=\frac{{\mathrm{Q}}_0}{{\mathrm{Q}}_0-{\mathrm{Q}}_{\mathrm{p}}}$$

(6)

where $m$ is the mass of the permeable liquid (kg), A is the effective area of the flat membrane (m²), t is the time interval (h) of operation, and ${\mathrm{Q}}_0$ and ${\mathrm{Q}}_{\mathrm{p}}$ are the initial quantity of the feed and the cumulative quantity of permeated water, respectively.

A conductivity meter was used to measure the conductivity of the product water, and the salt rejection can be calculated by Equation (7) as follows:

$$\mathrm{Salt} \mathrm{rejection}=\frac{{\mathrm{C}}_{\mathrm{f}}-{\mathrm{C}}_{\mathrm{p}}}{{\mathrm{C}}_{\mathrm{f}}}\times 100\%$$

(7)

where ${\mathrm{C}}_{\mathrm{f}}$ and ${\mathrm{C}}_{\mathrm{p}}$ are the conductivity of the feed solution and the permeate solution, respectively.

3. Results and Discussion

3.1. Optimization of Operating Parameters for VMD Treatment of Aniline Wastewater

Under the conditions of different feed temperatures of 50 °C, 60 °C, and 70 °C, feed flow of 500 mL/L, and absolute pressure of 5 kPa at the permeate side, the experiment of treating aniline wastewater with VMD for 15 h was carried out. Figure 2a shows the MD flux variation with different feed temperatures. Among them, when the temperature was 50 °C, 60 °C, or 70 °C, the permeation flux of membrane distillation was 16.7, 21.8, and 36.9 kg·m⁻²·h⁻¹, respectively. With the gradual increase in feed temperature, the saturated vapor pressure of the volatile components gradually increased at the corresponding temperature, which increased the mass transfer impetus and permeation flux of the process [46,47]. At 50 °C and 60 °C, the permeation flux of VMD decreases slightly but is relatively stable within 15 h, however, when the feed temperature was 70 °C, the initial flux was relatively higher. After 8 h of operation, the flux began to decay, and after 14 h of operation, the flux dropped from 36.9 kg·m⁻²·h⁻¹ at the beginning to about 14.5, a 50% drop in flux. This probably occurred because the more volatile substances clogged the pores along with the water steam when going through the membrane pore channel at higher temperatures; pollutant deposition also occurred on the membrane surface and so the pollution layer was thickened [48,49].

An experiment was carried out to evaluate the influence of different feed flow rates of 400, 500, and 600 mL/min under the conditions of a feed temperature of 60 °C and absolute pressure of 5 kPa at the permeate side. The results are shown in Figure 2b. The initial permeation flux was 18.8, 21.3, and 22.3 kg·m⁻²·h⁻¹, corresponding to different flow rates of 400, 500, and 600 mL/min, respectively. With the increase in the feed flow rate, the permeation flux of MD gradually increased. The high flow rate is conducive to the turbulent state, which enables better mixing of the feed solution and decreases the thermal boundary layer and temperature polarization [50,51]. However, the increasing trend is not obvious relative to different temperatures because the temperature is the main influence factor of membrane distillation flux [52,53].

In addition to studying the flux changes under different MD operating conditions, we also investigated the fouling of the membrane surface after the vacuum membrane distillation treatment of aniline wastewater, as shown in Figure 3. As seen in the SEM images, with the increase in feed temperature, the fouling phenomenon on the membrane surface became more serious; the color changes can be seen in the pictures in the upper left corners of the SEM images. Correspondingly, it can be observed from SEM that more and more pollutants accumulated on the membrane surface, resulting in fewer and fewer visible pores. This was attributed to the complex organic composition of aniline wastewater. More and more foulants accumulated on the membrane’s surface or volatilized into the pores, which led to a rapid decay of membrane distillation flux after a few hours at 70 °C [54]. However, the influence of the flow rate on membrane surface fouling presented a completely opposite trend. It can be seen from the SEM images and photos of the membrane surface that the membrane surface fouling was more serious when the flow rate was 400 mL/min, while the fouling was not serious when the flow rate was 600 mL/min. This is because an increase in the flow rate will increase the Reynolds number, and a high flow rate is conducive to obtaining a turbulent flow state. A turbulent state can better relieve the deposition of fouling on the membrane surface [55,56].

As is shown in

Table 3. AFM images of the original PVDF membrane and the PVDF membrane after operation in different conditions.

Operating Condition	Mean Roughness (Ra), nm	AFM Images
Original Membrane	207
60 °C/400 mL/min	102
60 °C/500 mL/min	170
60 °C/600 mL/min	174
500 mL/min/50 °C	90.8
500 mL/min/60 °C	164
500 mL/min/70 °C	28

, when the temperature was 60 °C and the flow rate was 400, 500, and 600 mL/min, the average surface roughness of the membrane decreased by 50.7%, 17.9%, and 15.9%, respectively. This is because when the flow velocity increases, the concentration of pollutants in the feed liquid accumulates on the surface of the membrane during a long membrane distillation; the deposition of organic matter also resulted in a decrease in surface hydrophobicity [57], which also explains the lower average surface roughness at the flow rate of 400 mL/min. This result is consistent with the SEM results. Under the flow rate condition of 500 mL/min and temperatures of 50, 60, and 70 °C, the average surface roughness of the membrane decreased by 56.1%, 20.7%, and 86.5%, respectively. This is because when the temperature is low, the diffusion coefficient of the organic matter in the wastewater is low, and the membrane surface pollution deposition is fast, so the average surface roughness is low. When the temperature was too high, the aniline wastewater was concentrated quickly, the pollutant concentration in the wastewater increased, and a large number of pollutants were deposited on the membrane surface, which made the membrane surface smooth, decreased the hydrophobicity, and finally, wetted the membrane [58,59]. Therefore, using an appropriate temperature was very important for the MD process.

3.2. Pretreatment of Aniline Wastewater by Fenton Reaction

Since the organic foulants on the membrane surface cannot be effectively treated during the membrane cleaning process, it was planned to use the Fenton oxidation method to pretreat the aniline wastewater to remove the aniline content in the aniline wastewater. Figure 4, Figure 5, Figure 6 and Figure 7 show the change of COD, aniline removal rate, and liquid surface tension under different H₂O₂ and FeSO₄ concentrations, respectively.

Firstly, when the concentration of H₂O₂ was 0.1, 0.3, 0.5, 1, 2, 3, 4, 5, 6, and 7 mL/L (FeSO₄ was 1000 mg/L), the COD, aniline removal rate, and liquid surface tension in the wastewater were measured; the results are shown in Figure 4a, Figure 5a, Figure 6a. The COD value shows a trend of first decreasing and then increasing with the increase in H₂O₂ concentration. When the H₂O₂ concentration was 3 mL/L, the COD reached its lowest value. Due to the increase in the amount of H₂O₂ added to the Fenton reaction, the output of ·OH increased, and the removal rate of the organic foulants also increased. However, when excessive H₂O₂ was added, it inhibited the generation of ·OH, and H₂O₂ itself constantly decomposed into oxygen and water, oxidizing Fe²⁺ into Fe³⁺ with low catalytic ability, increasing the amount of hydrogen peroxide, and reducing the production efficiency of the ·OH radical [60,61,62,63,64]. The change of COD also partly explains why the removal rate of aniline increases first and then decreases.

Especially, at the concentration of 3 mL/L H₂O₂ and 1000 mg/mL FeSO₄, the liquid surface tension of the aniline wastewater increased, and the liquid surface tension increased, albeit by a small amount. This is a good performance for MD. Because ·OH has strong oxidation, it will oxidize and decompose aniline, thus increasing the liquid surface tension of wastewater. According to Laplace’s equation, an increase in liquid surface tension will reduce the occurrence of membrane wetting during membrane distillation.

When wetting, the liquid enters the membrane hole, as seen in the Laplace Equation (8):

$$\Delta \mathrm{P}=\frac{2{\mathsf{\gamma }}_1\cos \mathsf{\theta }}{\mathrm{r}}$$

(8)

where $\Delta \mathrm{P}$ is the pressure difference on the membrane surface, ${\mathsf{\gamma }}_1$ is the liquid surface tension, r is the pore size of the membrane, and $\mathsf{\theta }$ is the contact angle of the membrane surface. When $\mathsf{\theta }>90°$, $\cos \mathsf{\theta }<0$, $\Delta \mathrm{P}>0$, and only when a certain pressure is applied, will the liquid penetrate into the membrane pores. From the equation, it can be seen that wettability depends on three factors: membrane pore size, liquid surface tension, and membrane material surface energy. The smaller the liquid surface tension, the easier it is to cause membrane wetting.

Secondly, the results from a FeSO₄ concentration of 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, and 1200 mg/L and an H₂O₂ concentration of 3 mL/L were also measured and are shown in Figure 4b, Figure 5b, Figure 6b. From Figure 4b, it can be seen that the COD removal rate increases with the increase in FeSO₄ concentration. When the concentration is 1000 mg/L, the COD removal effect is the best. Afterward, the COD removal rate decreases with an increase in FeSO₄ concentration. That is because when the mass concentration of Fe²⁺ is low, the amount of Fe²⁺ added increases, the catalytic effect is enhanced, the amount of ·OH free radicals generated increases, and the COD removal rate increases. When the mass concentration of Fe²⁺ is too high, it will reduce a portion of H₂O₂, causing the decomposition rate of H₂O₂ to be too fast and producing a large amount of ·OH in a short period of time. Thus, some ·OH cannot react with the organic pollutants in time, and free radicals will have already formed mutual reactions and been consumed, resulting in a decrease in the utilization rate of ·OH [65,66].

Figure 7 shows that the initial permeation flux of treating original aniline wastewater is about 18 kg·m⁻²·h⁻¹ and the initial permeation flux after pretreatment is about 22 kg·m⁻²·h⁻¹; the wastewater after pretreatment shows a more stable and lasting permeation flux in long-term operation.

3.3. Membrane Distillation and Crystallization Process

The performance of the hydrophobic MDCr process on Fenton oxidation-pretreated aniline wastewater will be discussed in the following section. The hydrophobic PVDF membrane was adopted during the process, and the experimental operation parameters of the MDCr process are shown in

Table 4. The operation parameters of membrane distillation and crystallization.

Processing Capacity/L	Feed Flow Rate /mL∙min−1	Feed Temperature /°C	Vacuum Pressure /MPa
2	500	60	0.0095

.

It can be seen from Figure 8a that during the 30 h operation process, the overall flux remains stable and the salt retention rate of the membrane remains relatively stable; both are above 99.99%. In addition, after 30 h, the concentration factor reached 1.5, which means that during this process, the original aniline wastewater had been concentrated to 1.5 times the original concentration. It can be seen from the SEM images (Figure 8b,c) of the membrane after the experiment that there was no obvious membrane pore blockage or large-scale pollutant deposition on the membrane surface, and that it still maintained a porous state.

The organic components in the original solution, concentrated crystallization mother solution, and crystallization salt were characterized, as shown in

Table 5. Analysis of the organic composition of wastewater and crystal salt.

TOC (mg/L)
Original Solution	Crystalline Mother Solution	Crystalline Salt (10 ppm)
4728	2471.1	2.99

. It can be seen that the TOC value of the concentrated crystallization mother solution decreased compared to the original solution, and it has not been ruled out that the organic substances with lower boiling points can be removed due to their permeation through the membrane in a vacuum environment.

Crystalline salt precipitated during the membrane distillation crystallization process and was obtained through filtration and drying. In order to understand the residual organic compounds on the surface of the crystal salt, the crystal salt was reintegrated into deionized water (with a concentration of 10 ppm), and the TOC value of the crystal salt was measured to be 2.99 mg/L.

An EDX analysis was then performed on the crystalline salt, as shown in Figure 8b and

Table 6. Surface element analysis of the crystal salt.

Element	Mass Ratio %	Atomic Ratio %
O	4.54	7.95
Na	38.66	47.14
Cl	56.80	44.91

. Figure 8b shows the surface morphology of the crystalline salt, a cubic shape with clear edges and a complete structure [32]. The salt surface analysis by EDX is shown in Table 5. The main elements are Na and Cl, and their atomic ratios are close to 1:1, which means the membrane distillation and crystallization process can be used to achieve ZLD of the aniline wastewater in this case. In order to further confirm this conclusion, we also conducted XRD characterization, as shown in Figure 9. The XRD spectrum of the PVDF membrane after the MDCr process showed obvious characteristic peaks of NaCl with peaks of 2θ = 27.36°, 31.73°, 45.44°, and 56.47°. This further confirmed that the crystals we obtained were NaCl. However, due to the complex composition of the wastewater, there were still some peaks of unknown organic and inorganic substances.

Nevertheless, when membrane distillation was used to treat aniline wastewater, although the Fenton oxidation process was used to pretreat the wastewater, the final membrane surface fouling was still very serious, and further modification of the membrane surface is needed to improve the antifouling performance. Moreover, the aniline wastewater obtained from the actual factory has complex components, so better pretreatment methods should be tried to further improve the long-term stability of membrane distillation.

4. Conclusions

In this paper, MDCr was applied to the treatment of aniline wastewater, providing a feasible solution for the realization of ZLD. When the feed temperature is 60 °C and the feed flow rate is 500 mL/min, the flux of the MD process is 21.3 kg·m⁻²·h⁻¹; under this condition, the effect of MD is the best and the influence of foulants on the performance of MD can be avoided to the greatest extent. After that, the concentration of H₂O₂ and FeSO₄ in Fenton oxidation was optimized, and when the optimal concentration of H₂O₂, 3 mL/L, and FeSO₄, 1000 mg/L, was discovered, the aniline removal rate was found to reach 91%. Finally, the salt was extracted and analyzed. The purity of the NaCl was up to 94 wt.%. Overall, in this case, the MDCr process can be used to achieve ZLD of the aniline wastewater, showing the possibility and great potential of the large-scale treatment of aniline wastewater.