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Article

Potential of Textile Wastewater Decolorization Using Cation Exchange Membrane Electrolysis Coupled with Magnesium Salt Precipitation (CEM-MSP)

by
Yujing Zhao
1,
Nuo Cheng
1,
Ruihan Jiang
1,
Jian Jiao
1,
Chen Chen
2,
Jiahao Liang
1,
Longfeng Hu
1,
Hesong Wang
1 and
Jinlong Wang
1,*
1
State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, 73 Huanghe Road, Nangang District, Harbin 150090, China
2
Hainan Litree Purifying Technology Co., Ltd., Haikou 571126, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(12), 1785; https://doi.org/10.3390/w17121785 (registering DOI)
Submission received: 19 May 2025 / Revised: 9 June 2025 / Accepted: 12 June 2025 / Published: 14 June 2025
(This article belongs to the Special Issue Electrochemical Approaches to Sustainable Water Treatment: Tackling Pollution and Advancing Resource Recovery)
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Abstract

:
To overcome the low efficiency, high cost and less environmentally friendly limitations in existing textile wastewater disposal technology, an innovative approach of cation exchange membrane electrolysis coupled with magnesium salt precipitation (CEM-MSP) was implemented. This method simultaneously achieved the high-efficiency adsorption decolorization of dyes and the recovery of lye. The results indicated that cation exchange membrane electrolysis with MgSO4 added to the anode chamber (CEM-EA) exhibited excellent decolorization performance on DB86 dye and achieved low residual Mg2+ concentration. Furthermore, the adsorption mechanism of Mg(OH)2 on DB86 was systematically investigated. The adsorption process fitted with the first-order kinetic, where the adsorption of DB86 by Mg(OH)2 was dominated by electrostatic attraction. Detailed comparison of the four systems demonstrated that CEM-EA was superior to the single magnesium addition method (85.24%) or the stand-alone membrane electrolysis method (10.36%), with 99% decolorization efficiency. In comparison to the cation exchange membrane electrolysis with MgSO4 added to the cathode chamber (CEM-EC), the CEM-EA could diminish the Mg2+ concentration in the effluent to facilitate the lye recovery while guaranteeing the decolorization efficiency. In addition, the DB86 adsorption behavior during the formation of Mg(OH)2 in the cathode chamber was investigated. The Mg(OH)2 particles were relatively dense copper-blue agglomerates with a thin lamellar layer on the surface. Notably, only slight mass contamination was observed on the cation exchange membrane (CEM) surface after multiple cycles. Minor CEM contamination illustrated the stable treatment efficiency of the CEM-EA after several cycles. This study constructed a novel approach integrating membrane electrolysis with magnesium salt precipitation, delivering valuable technical solutions for textile wastewater disposal.

1. Introduction

The textile industry is a long-established and global industry that has expanded rapidly in recent years. Statistically, the global textile market size reached USD 1837.27 billion in 2023 and is expected to rise at a 7.4% CAGR by 2030 [1,2]. Nevertheless, the volume of wastewater discharged from the textile industry is enormous. The total discharge of textile wastewater accounts for approximately 17–20% of the entire industrial wastewater emissions [3]. Moreover, the regional agglomeration of the textile industry further enlarges and concentrates the pollution level, significantly contributing to the environmental load [4]. The textile wastewater mainly originates from the washing, bleaching, mercerizing and dyeing processes, containing a variety of organics such as pulp, auxiliaries and surfactants [5]. Notably, it also contains high concentrations of dyes including azo, anthraquinone and phthalocyanine dyes, resulting in a high chroma of effluents [6]. Raw textile effluents generally present chroma levels in the range of 200–1000 Pt-Co units, even up to 2000 Pt-Co units [7]. Most dyes are severely toxic, among which common azo dyes are degraded by intestinal bacteria to form toxic amino acids with potential carcinogenicity [8]. Therefore, the discharge of high chroma textile wastewater into the aquatic environment not only impacts aquatic organisms, but may also be hazardous to human beings through the food chain and skin contact [9,10]. Consequently, it is urgent to develop efficient textile wastewater disposal technology to ensure the safety of the aquatic environment and promote the green transformation of the textile industry.
The conventional disposal method for textile wastewater is pre-treatment combined with biological processing [11]. Biological processing utilizes the metabolism of microorganisms to degrade dyes, involving aerobic techniques, anaerobic techniques and their combinations. Microorganisms decompose the chromogenic groups in dyes under anaerobic conditions, with intermediate metabolites further mineralized under aerobic conditions, thereby achieving low chromaticity and biological toxicity of effluents [12,13]. Meanwhile, the biological process as the core unit features long hydraulic retention times (HRTs), bulky structures and unstable treatment performance [14]. Emerging technologies include separation by modified ultrafiltration membranes and advanced oxidation based on Fenten methods [15]. Some modified membranes have demonstrated high rejection rates for dyes but no selectivity for inorganic salts while maintaining outstanding water permeability [16,17]. Advanced oxidation technology allows faster and more efficient decolorization of dyes through in situ generation of hydroxyl radicals [18,19]. While membrane separation has high operating costs and extensive energy consumption due to frequent cleaning membranes, the advanced oxidation actually yields more oxidation by-products with high biological toxicity [20,21]. In conclusion, these approaches each have their own limitations and hazards. Further investigation is required to address how to treat textile wastewater efficiently, greenly and in an energy-saving and cost-effective manner.
Recently, magnesium salt precipitation has gained increasing attention due to its simple operation and low cost [22]. Mg(OH)2 derived from the alkalization of magnesium salts enables the adsorption of colloidal particles in solution owing to its higher surface free energy and larger adsorption surface area. For instance, it has been utilized for removal of sulfate as gypsum in mine water, heavy metals in industrial wastewater and dyes in textile wastewater [23,24,25]. Rasilingwani et al. prepared MgO-bentonite clay nanocomposite particles for the treatment of real dyeing wastewater and the removal rate of Congo red dye was 80%, which was further verified as a monolayer adsorption process [26]. Shen et al. proposed a crosslinking-induced precipitation disposal technology, which demonstrated 99% decolorization efficiency for RB5 dye by dosing MgSO4 as coagulants after 10 min of reaction [27]. However, studies have revealed that the pH of magnesium salt precipitation is approximately maintained at 12 [28,29]. Hence, additional alkalizers are required for maintaining excellent decolorization efficiency, which significantly increases wastewater salinity and maintenance costs. In summary, a more environmentally friendly pH adjustment method is urgently needed to overcome the shortage of alkalizers in the decolorization of textile wastewater.
Cation exchange membrane (CEM) electrolysis employs a selectively permeable membrane to separate the cathode and anode chambers [30]. With the joint effects of the electric field and CEM, anions and cations migrate across the membrane and are split on the two sides of the CEM. During CEM electrolysis, the hydrogen evolution reaction (HER) occurs at the cathode and the generated OH− results in an alkaline pH environment in the cathode chamber [31]. Increasingly, studies have adopted the stable alkaline environment of the cathode chamber to recover metal resources in the form of solids precipitation [32,33]. Wang et al. designed a novel two-step membrane electrolysis process coupling the AEM and CEM to treat nanofiltration concentrate, and recovered metal ions through the formation of CaCO3 and Mg(OH)2 in the cathode chamber of the AEM [34]. Zaslavschi et al. induced scale precipitation through an alkaline environment in the cathode chamber to remove the hardness of brackish water [35]. Rögener et al. also utilized membrane electrolysis to recover nickel and iron from a stainless steel pickling solution and investigated the optimal current density conditions [36]. Consequently, the alkaline environment derived from membrane electrolysis could compensate for the insufficiency of alkalizers and offer favorable conditions for the precipitation of magnesium salts.
Therefore, an innovative method utilizing CEM electrolysis coupled with magnesium salt precipitation (CEM-MSP) was proposed. The stable alkaline environment formed in the cathode chamber through membrane electrolysis enables MgSO4 to form Mg(OH)2 precipitates, which adsorb dyes and achieve the decolorization of textile wastewater. This study focused on the adsorption kinetics of the DB86 by Mg(OH)2 to discover the type of adsorption. And then four different experimental systems involving non-membrane electrolysis with MgSO4 addition, stand-alone membrane electrolysis and membrane electrolysis with MgSO4 addition were set up to investigate and compare the dye decolorization performance on alkaline textile wastewater. Additionally, the DB86 adsorption behavior during the formation of Mg(OH)2 was investigated. The fouling of the CEM after multiple cycle operations was also evaluated. The results shed light on the adsorption mechanism of Mg(OH)2 on DB86, providing a novel technical approach and theoretical guidance for the treatment of textile wastewater.

2. Materials and Methods

2.1. Experimental Materials

Nafion 117 (DuPont) was used as the ion exchange membrane (IEM) in this experiment, which was soaked in deionized water and stored at 4 °C after pretreatment. In this study, magnesium sulphate (MgSO4) was used as coagulant and magnesium hydroxide (Mg(OH)2) was employed to determine the adsorption capacity. Sodium chloride (NaCl) and sodium hydroxide (NaOH) were obtained as auxiliary reagents, which were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). The modeling dye used in this experiment was C.I. Direct Blue 86 (DB86) that was obtained from Shanghai Shenhong Pigment Co., Ltd. (Shanghai, China). Its main structure is copper phthalocyanine as a blue powder, whose chemical formula is C32H14CuN8Na2O6S2. The concentration of DB86 in the simulated dye wastewater was 100 mg/L. An amount of 2M NaCl was added to the feed water to simulate the salt used for color fixation in the dyeing process and the pH was adjusted to 11.0 using NaOH.

2.2. Experimental Procedures and Methods

As depicted in Figure 1, this experiment utilized an ion exchange membrane electrolysis reactor as the main device. The electrolysis reactor consisted of a plexiglass square tube with cathode and anode chambers separated by an ion exchange membrane. The ion exchange membrane had an effective contact area of 5 cm × 5 cm with the solution and was sandwiched between the cathode and anode chambers by silica gasket. The reactor had rectangular chambers with a total working volume of 160 mL. Platinum-coated titanium electrodes were chosen as the cathode and anode, with an effective area of 10 cm2, which were placed vertically in the cathode chamber and completely immersed in the solution.
In the membrane electrolysis reactor, 150 mL of simulated dye wastewater was added to the cathode chamber and the equal volume of 50 mM NaCl solution was added to the anode chamber. As shown in Figure S1, four distinct experimental systems were designed according to the MgSO4 dosing method and to whether membrane electrolysis was performed. Two systems were employed to separately investigate the decolorization efficiency of membrane electrolysis and magnesium salt application: (i) the cation exchange membrane electrolysis without additional chemicals (CEME) and (ii) a non-electrolysis system with the addition of magnesium sulphate to cathode chamber (MDC). Two systems incorporated membrane electrolysis and magnesium sulphate dosing, and further explored the dosing methods: (iii) cation exchange membrane electrolysis with magnesium sulphate added to the cathode chamber (CEM-EC) and (iv) cation exchange membrane electrolysis with magnesium sulphate added to the anode chamber (CEM-EA). Table 1 displays the experimental operating conditions for four different systems, involving temperature, electrolysis time, current density and magnesium sulphate dosages. Moreover, the magnetic stirrer was operated at a speed of 320 rpm to ensure homogeneous mass transfer. At regular intervals, 3 mL of dye wastewater was collected from the cathode chamber and filtered through 0.45 µm membranes to determine the absorbance of samples. The dye concentration was calculated based on the standard curve of DB86 and the dye decolorization efficiency was determined according to Equation (1).
D E % = ( A b s 0 − A b s t ) / A b s 0
where Abs0 represents absorbance at the maximum absorption wavelength of simulated dye wastewater; Abst denotes absorbance at the maximum absorption wavelength of the sample obtained in the cathode chamber at time t.

2.3. Instrumental Analysis

The absorbance of the dye effluent at the maximum absorption wavelength was determined using a UV–Vis spectrophotometer (SPECORD 200, Jena, Jena, Germany). Stereomicroscope (SM) equipped with a camera (Olympus C-7070, Olympus, Tokyo, Japan) and scanning electron microscopy (Sigma 500, Zeiss, Oberkochen, Germany) were employed to obtain the morphology characterization of the IEM surface and hydroxide magnesium precipitates. The IEM was dried under natural conditions and Mg(OH)2 precipitate was dried in an oven at 65 °C for 24 h, which was adhered to the slide with copper conductive adhesive. Elements in samples were analyzed using an X-ray spectrometer (X-Max N, Oxford Instruments, Oxfordshire, UK).

2.4. Adsorption Kinetics Model Fitting

The feed water used in the adsorption kinetics experiment was 150 mL of simulated dye wastewater, with a GB86 concentration of 100 mg/L. Certain quantity of Mg(OH)2 was added to the reactor and the adsorption experiments were performed in a shaking chamber at 298 K, 180 rpm.
The study of adsorption kinetics exerts significant significance in exploring the mechanism of the adsorption process. Multiple adsorption kinetic models are available to describe the adsorption process, the most widespread being primary and secondary kinetic models. The primary kinetic model is usually used to describe diffusion-dominated types of adsorption and is expressed as in Equation (2). The secondary kinetic model is frequently used to describe the type of chemically dominated adsorption and is expressed as in Equation (3).
l n ⁡ q e − q t = l n q e − k 1 t
t q t = 1 k 2 q e 2 + 1 q t t
where qe is the adsorption capacity (mg/g) at adsorption equilibrium and qt is the adsorption capacity at time t. k1 and k2 are primary and secondary kinetic adsorption rate constants, respectively.

3. Results and Discussion

3.1. Dye DB86 Adsorption Mechanism

3.1.1. Kinetic Analysis of Dye DB86 Adsorption by Mg(OH)2

Magnesium salt, a well-performed adsorption agent, has been frequently applied to decolorize dyes containing sulfonic acid groups [22]. A kinetic fitting of the adsorption process was carried out to explore the kinetic adsorption mechanism of the dye DB86 by Mg(OH)2. Figure 2a presents the adsorption capacity of Mg(OH)2 for the dye DB86 over time and the fitting results of the first-order and second-order kinetics. Adsorption of dyes by Mg(OH)2 could reach the saturation state swiftly followed by adsorption equilibrium within 5 min, indicating that Mg(OH)2 offers a promising adsorption performance for DB86 and is capable of rapidly removing the dye from wastewater by adsorption. Table 2 shows the parameters associated with the kinetic fitting. Based on the R2 results of the kinetic fitting, the adsorption of DB86 by Mg(OH)2 was more consistent with the first-order kinetic model, which explains that the adsorption of Mg(OH)2 on the dye DB86 is dominated by physical interaction [37]. Positively charged Mg(OH)2 combined with the negatively charged sulphonic acid groups generated by the DB86 dissociation through electrostatic attraction. Consequently, the adsorption process is reversible and unstable. This interpretation can be further elaborated by integrating the adsorption principle and the variation trend of DB86 decolorization efficiency in Section 3.1.2 and Section 3.2.2.

3.1.2. DB86 Adsorption Principle by Mg(OH)2

Depending on the molecular structure of DB86 and the adsorption characteristics of Mg(OH)2, the adsorption and desorption processes of DB86 were elaborated. In the dissociation process, as shown in Figure 2b, the DB86 is soluble in water while dye molecules dissociate in aqueous solution. The copper phthalocyanine structure containing two sulfonic acid groups hydrolyzes in water, dissolving into negatively charged dye molecules. The isoelectric point (IEP) of Mg(OH)2 is typically between pH 10–12 [38,39]. When the pH of the system stays below the IEP, the Mg(OH)2 colloid is positively charged; above the IEP, it becomes negatively charged. In the adsorption process, Mg2+ combines with OH− and initially hydrolyzes to positively charged Mg(OH)2 when magnesium salt is added to alkaline dye wastewater. Mg(OH)2 adsorbs negatively charged dye molecules to form a complex bound by hydrogen bonds as shown in Figure 2c, which settles under gravity thereby separating the dye molecules from the system. As depicted in Figure 2d, a desorption process occurs between the dye and Mg(OH)2 when the pH of this system elevates to a certain level. In this experiment, a charge reversal of Mg(OH)2 (from +ζ to −ζ) was observed at approximately pH 11.5. The positively charged Mg(OH)2 formed in the previous stage adsorbs OH− and forms a negatively charged Mg(OH)2 precipitate which desorbs from the dye molecules. Consequently, it is necessary to strictly control the pH range of the solution when removing dyes using the magnesium salt addition method to avoid desorption.

3.2. Dye Decolorization Performance

3.2.1. Dye Decolorization Performance of Systems CEME and MDC

The cation exchange membrane electrolysis (CEME) system was applied to evaluate the effect of the electrolysis process on the dye decolorization performance. Without external Mg2+ addition, current densities from 2.5 to 20 mA/cm2 were selected to assess the decolorization performance of the CEME system for dye DB86. As shown in Figure 3a, the dyes exhibited minimal decolorization efficiency in the cathode chamber. Decolorization efficiency progressively increased with electrolysis time at all current densities, with higher current density achieving a better dye removal rate. Current density primarily regulated solution pH, thereby controlling decolorization efficiency [40]. At 20 mA/cm2, the decolorization efficiency reached only 10.36% after 30 min electrolysis, demonstrating the limited efficiency of stand-alone membrane electrolysis without Mg2+ addition. The decolorization under electrolysis conditions may originate from either (i) electric field-induced molecular reduction in the cathode chamber, or (ii) electrolytically driven pH changes modifying dye chromophores [41,42].
The addition of magnesium salt to alkaline dye wastewater could induce the precipitation and adsorption process, but its DB86 adsorption performance remains unclear. Therefore, the Mg(OH)2 precipitation from MgSO4 alkalization and its subsequent dye adsorption was examined, while evaluating dosage-dependent effects on the decolorization efficiency. As indicated in Figure 3b, the decolorization trend of dye DB86 remained consistent at all MgSO4 levels. In the initial 5 min, dissolved Mg2+ reacted with OH− to form Mg(OH)2 precipitates to adsorb dyes, enhancing the decolorization efficiency. After 5 min, the reaction reached equilibrium, maintaining a constant decolorization efficiency. Notably, after 30 min of reaction time, the decolorization rates of various systems with 5–40 mM MgSO4 dosage were comparable, ranging from 70% to 80%. Previous studies have demonstrated that the adsorption decolorization efficiency depends on pH, with optimal performance occurring at pH 12 [29]. Consequently, the MDC system achieved a limited decolorization rate (max 85%) due to insufficient alkalizing agents in the solution for complete Mg(OH)2 formation. To address this difficulty, the membrane electrolysis system was employed to stably generate alkaline conditions necessary for efficient Mg(OH)2 formation [43].

3.2.2. Dye Decolorization Performance of System CEM-EC

In the membrane electrolysis system, OH⁻ may be generated and accumulated in the cathode chamber through electrochemical reactions and IEM, which established a stable alkaline environment in the cathode chamber [33]. Consequently, the CEM electrolysis system coupled with Mg(OH)2 precipitation could compensate for insufficient alkalizing agents and enhance dye decolorization efficiency. Here, we examined the decolorization performance of dye DB86 treated by CEM electrolysis with MgSO4 added to the cathode chamber (CEM-EC). As described in Equation (4), chemical reaction kinetics revealed a MgSO4 dissolution rate, Mg2+ concentration and OH− concentration as critical control factors on the decolorization performance.
r = k [ M g 2 + ] x [ O H − ] y
where r is the chemical reaction rate, [A] represents the activity of reactant A, k denotes the chemical reaction rate constant, x and y are reaction orders.
Accordingly, the experimental conditions with various gradients of current density and MgSO4 dosage were designed. As illustrated in Figure 4, the increasing MgSO4 dosages significantly enhanced both the decolorization rate and the final efficiency, demonstrating a clear dose-dependent performance. Remarkably, the system achieved beyond 99% dye removal efficiency at MgSO4 concentrations of 3–5 mM and current densities ranging from 2.5 to 20 mA/cm2, indicating exceptional treatment stability. The duration required to achieve 99% decolorization efficiency differed across experimental conditions. Temporally, the decolorization process exhibited three distinct phases. Phase I: Rapid MgSO4 dissolution released Mg2+, which combined with OH− to form dye-adsorbing Mg(OH)2 precipitates, thus swiftly improving the decolorization efficiency. Phase II: Sustainable OH− generation through membrane electrolysis elevated pH in the cathode chamber, enabling continuous Mg(OH)2 precipitation and stable decolorization efficiency. Phase III: Excess OH⁻ induced negative surface charging of Mg(OH)2 precipitates, causing electrostatic repulsion and subsequent desorption of anionic dyes, ultimately reducing decolorization efficiency. At a low MgSO4 dosage (1 mM), increased current density accelerated both the Phase I decolorization rate and the Phase III efficiency decline. Higher current densities accelerated OH− generation, promoting rapid Mg(OH)2 formation but also inducing faster surface charge development, which triggered premature dye desorption [44]. Accordingly, it is inferred that the force of Mg(OH)2 with dye molecules is electrostatic attraction, and the adsorption process is ionic adsorption, which is compatible with previous kinetic analysis results. At a specific Mg2+ dosage, increasing the current density enhanced the decolorization rate. The increased generation of OH− at higher current densities facilitated the rapid formation of Mg(OH)2, thereby enabling significantly faster dye removal.

3.2.3. Dye Decolorization Performance of System CEM-EA

In fact, the CEM-EC system could not guarantee that the added MgSO4 was completely converted to Mg(OH)2 precipitation, so the residual Mg2+ in the effluent from the cathode chamber was relatively high. A higher concentration of Mg2+ in the effluent from the cathode chamber would affect the reuse of lye and influence the uniformity of the dyeing process [45]. Thus, the CEM-EA system was constructed, in which membrane electrolysis was performed accompanied by MgSO4 addition into the anode chamber. In this case, Mg2+ passed through the CEM and entered the cathode chamber to precipitate and adsorb the dyes. After the decolorization process, the residual level of Mg2+ in the effluent from the cathode chamber remained relatively low to ensure normal lye reuse. Similar to the CEM-EC system, various current densities and MgSO4 dosages were set to explore the effects of the dye decolorization performance. As presented in Figure 5, the dye decolorization process consisted of three obvious phases of adsorption, stabilization and desorption at 5 mM MgSO4 dosage. However, with the increase in the MgSO4 dosage, the desorption process was weakened and the decolorization efficiency showed a tendency to become steady. Meanwhile, the efficiency and rate of dye decolorization improved with the increase in the MgSO4 dosage and current density, which might be associated with the elevated transmembrane rate of Mg2+. Comparatively, Mg2+ has to across the CEM to the cathode chamber driven by electric field in order to form Mg(OH)2 and thus adsorb the dyes. Hence, under the same MgSO4 dosage and current density conditions, the decolorization efficiency peak of the CEM-EA system reached its peak with a time delay compared with the CEM-EC system.

3.2.4. Comparison of Decolorization Performance of Four Different Systems

Figure 6 compares the effects of the four systems on decolorizing the dye DB86. In the dimension of electrolysis time, the decolorization efficiency of the MDC and CEM-EC systems showed the consistent trend when MgSO4 was added to the cathode chamber dye wastewater (Figure 6a). The decolorization efficiency rose rapidly, reached a stable level and then leveled off. When MgSO4 was added to the anode chamber, the decolorization efficiency increased with a lag because Mg2+ had to cross the membrane to the cathode chamber for the reaction. In terms of maximum decolorization efficiency, the dye decolorization performance of the four different systems varied considerably (Figure 6b). In the MDC system, OH− from alkaline dye wastewater combined with Mg2+ from dissolved MgSO4 to form Mg(OH)2, which adsorbed the dissociated dye DB86 thereby achieving decolorization. However, the maximum decolorization efficiency of the dye could only reach 85.24% due to the insufficient alkalinity of the wastewater. The decolorization efficiency of the CEME system was relatively low at 10.36%. The pH of the cathode chamber solution increased to 12.2 after electrolysis, so the dye decolorization could be attributed to the change in solution pH. In the CEM-EC, the membrane electrolysis provided sufficient alkalinity for the system, and the Mg2+ rapidly combined with OH− in the cathode chamber to form Mg(OH)2 precipitates adsorbing the dye, so that the dye decolorization efficiency could reach 100%. Nevertheless, due to the excessive addition of MgSO4, a large amount of Mg2+ remained in the effluent of the cathode chamber, affecting the reuse of the lye. In the CEM-EA, the dissolved Mg2+ from the anode chamber passed through the CEM into the cathode chamber, and Mg(OH)2 formed in the alkaline environment adsorbed the dye DB86 to achieve decolorization. Compared with the CEM-EC system, the CEM-EA can effectively reduce the Mg2+ in the effluent water, simultaneously guaranteeing the decolorization efficiency.

3.3. Precipitates Analysis and Cation Exchange Membranes Fouling

3.3.1. Morphology Characterizations of Mg(OH)2 Precipitates

The removal of dyes in the CEM-EA system primarily depended on the adsorption by Mg(OH)2 precipitates formed within the cathode chamber. To verify the process of adsorption co-precipitation of dyes by Mg(OH)2, the morphology characterizations of Mg(OH)2 precipitates formed in the cathode chamber under the optimal experimental condition were examined. As depicted in Figure 7a, optical microscope observations revealed that the precipitates displayed an intense copper blue color, confirming the adsorption of DB86 by Mg(OH)2. Morphologically, the formed Mg(OH)2 particles were relatively dense agglomerates, whose surface presented a thin lamellar layer (Figure 7b). The primary particles of Mg(OH)2 exhibited a particle size of approximately 1 μm and aggregated to form agglomerates with sizes in the range of 5–10 μm. Lamellar-shaped Mg(OH)2 crystals had a larger specific surface area and better adsorption performance than agglomerated Mg(OH)2 particles [46]. The adsorption performance of monolayer-shaped aggregated Mg(OH)2 crystals formed by membrane electrolysis was better [47]. The precipitate was analyzed by EDS as shown in Figure 7d–f, where highly visible Mg elements were observed, but no intense presence of Cu element was observed. This could be attributed to the centered position of the Cu atom in the DB86, while DB86 was located between the lamellar structure of Mg(OH)2 crystals when adsorbed.

3.3.2. Morphology Characterizations of the Fouling Cation Exchange Membrane

In order to investigate the contamination of the CEM after several runs of CEM-EA, the surface morphology of the membrane was observed and analyzed through SEM. As seen in Figure 7c, the surface of the initial CEM is very smooth and free of depressions and protrusions. After 90 cycles of CEM-EA treatment for dye wastewater, only slight lumpy fouling was observed on the surface of the CEM (Figure 7g). Overall, the CEM was lightly contaminated, demonstrating the stable treatment efficiency of the CEM-EA system after multiple cycles. Furthermore, it was observed during the experiment that the CEM after 90 cycles in the CEM-EA system had a faster dye decolorization rate than new membranes. This may be due to the increased pore size of the CEM after multiple electrolysis cycles, which is more favorable for the passage of Mg2+ through the pores [48]. To further elaborate the formation mechanism of membrane fouling, the contaminated CEMs were characterized by EDS (Figure 7h,i). There were magnesium elements on the fouling CEM, whereas no copper elements were detected, so it was presumed that the main contamination was inorganic scaling [49]. When Mg2+ passed through the CEM into the cathode chamber, scaling occurred and adhered to the membrane surface under an alkaline environment. Scale crystals were small in size and could be removed from the surface of the CEM by soaking and cleaning with water.

3.4. A Prospective Technology for Simultaneous Dye Degradation, Magnesium Salt Recovery and Lye Reuse

The CEM-EA system mentioned above has attained efficient decolorization of the dye DB86 while ensuring a relatively lower concentration of Mg2+ in the effluent from the cathode chamber. To further achieve the simultaneous dye degradation, magnesium salt recovery and lye reuse from textile wastewater, a prospective technology was proposed, whose operation mechanism is illustrated in Figure 8. MgSO4 is added in the anode chamber and the Mg2+ enters the cathode chamber through the CEM. An alkaline environment is developed in the cathode chamber due to water electrolysis, allowing Mg2+ to precipitate as Mg(OH)2. Mg(OH)2 and the dye DB86 combine by electrostatic attraction and subsequently settle by gravity in the cathode chamber. The Mg(OH)2 precipitates adsorbing dye molecules are separated from the decolorized dye wastewater by membrane filtration. Dye wastewater after decolorization can be reused for lye reuse. The oxygen evolution reaction (OER) in the anode chamber creates an acidic environment. The filtered Mg(OH)2 precipitates are injected into the anode chamber in an acidic environment and dissolved to obtain Mg2+. Mg2+ migrates to the cathode chamber driven by electric field and again proceeds to precipitation and adsorption. The dye is dissolved into the anode chamber and degraded by chemical oxidation. In electrochemical reactions using NaCl as the electrolyte, the common reactive oxygen species (ROS) include •OH, Cl• and Cl2−• [50]. The degradation of dye DB86 occurred in the presence of various types of free radicals.

4. Conclusions

In this study, a novel technique of cation exchange membrane electrolysis coupled with magnesium salt precipitation (CEM-MSP) was introduced to achieve the high-efficiency adsorption decolorization of dyes. The conclusions are presented below:
(1)
The adsorption of the dye DB86 by Mg (OH)2 matches the first-order kinetic model, as a process dominated by electrostatic interactions. The adsorption equilibrium is promptly reached within 5 min, demonstrating the feasibility of DB86 removal by Mg(OH)2 precipitation adsorption. During membrane electrolysis, the Mg(OH)2 colloid is positively and then negatively charged due to the pH variation in the cathode chamber. Hence, the adsorption process of DB86 by Mg(OH)2 might be followed by the desorption process.
(2)
Four different experimental systems were designed to investigate and compare the dye decolorization performance on alkaline textile wastewater. Without external Mg2+ addition, the max decolorization rate of CEME only reached 10.36%. The MDC achieved limited decolorization efficiency (max 85.24%) due to insufficient alkalizing agents in the single magnesium addition system. Remarkably, the CEM-EC achieved beyond 99% dye removal at 5 mM MgSO4, indicating exceptional treatment performance. Meanwhile, increasing MgSO4 dosage significantly enhanced decolorization efficiency, demonstrating a clear dose-dependent performance. Compared with CEM-EC, the CEM-EA enabled a reduction in the Mg2+ in the effluent water while guaranteeing the decolorization efficiency. The decolorization efficiency peak of CEM-EA reached its peak with a time delay due to the requirement for the transmembrane transport of Mg2+.
(3)
The morphology characterizations of Mg(OH)2 precipitates formed in the cathode chamber were observed to prove the adsorption co-precipitation process of dyes. The formed Mg(OH)2 particles were dense agglomerates of copper blue color, whose surface presented a thin lamellar layer facilitating dye adsorption. Additionally, only slight lumpy fouling was observed on the surface of the CEM after several electrolysis cycles. Analysis of contaminated CEM illustrated the stable decolorization efficiency of the CEM-EA system.
Future research should focus on the following aspects: (1) long-term decolorization stability (over 50 cycles); (2) anode reaction mechanisms using electron paramagnetic resonance (EPR) techniques to identify ROS types and dye oxidation pathways; (3) adsorption process simulation via molecular dynamics; and (4) scalability assessment through comparative analysis of energy consumption (kWh/m3) and operational costs against conventional methods.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17121785/s1, Figure S1. Schematic diagram of four experimental systems (a) CEME, (b) MDC, (c) CEM-EC and (d) CEM-EA.

Author Contributions

Conceptualization, Y.Z.; methodology, N.C.; formal analysis, J.J. and J.L.; investigation, N.C. and C.C.; resources, R.J.; data curation, Y.Z., J.J., J.L., L.H. and H.W.; writing—original draft preparation, Y.Z.; writing—review and editing, N.C., R.J., J.J., C.C., J.L., L.H., H.W. and J.W.; visualization, L.H. and H.W.; supervision, J.W.; project administration, J.W.; funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was jointly supported by the Postdoctoral Science Foundation of China (No. 2022M710975), the Heilongjiang Province Natural Science Foundation (TD2023E003), the Hei Long Jiang Postdoctoral Foundation (Grant No. LBH-Z22141), the specific research fund of The Innovation Platform for Academicians of Hainan Province (SQ2022PTKY0028), the Key-Area Research and Development Program of Guangdong Province (2022B0101090004) and the Fundamental Research Funds for the Central Universities.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Chen Chen was employed by the company Hainan Litree Purifying Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

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Water 17 01785 g001
Figure 1. Experimental setup of CEM electrolysis coupled with magnesium salt coagulation and precipitation.
Figure 1. Experimental setup of CEM electrolysis coupled with magnesium salt coagulation and precipitation.
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Figure 2. Adsorption mechanism of dye DB86 by Mg(OH)2. (a) The adsorption capacity and the kinetics fitting results, (b–d) DB86 adsorption principle by Mg(OH)2.
Figure 2. Adsorption mechanism of dye DB86 by Mg(OH)2. (a) The adsorption capacity and the kinetics fitting results, (b–d) DB86 adsorption principle by Mg(OH)2.
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Figure 3. Dye decolorization performance of systems (a) CEME and (b) MDC (current density within Figure (a) = 0, 2.5, 5, 10, 20 mA/cm2, MgSO4 dosages within Figure (b) = 0, 1, 2, 5, 10, 20, 30 mM).
Figure 3. Dye decolorization performance of systems (a) CEME and (b) MDC (current density within Figure (a) = 0, 2.5, 5, 10, 20 mA/cm2, MgSO4 dosages within Figure (b) = 0, 1, 2, 5, 10, 20, 30 mM).
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Figure 4. Dye decolorization performance of system CEM-EC (MgSO4 dosages = 1, 2, 3, 4, 5 mM, current density within Figure (a–d) = 2.5, 5, 10, 20 mA/cm2).
Figure 4. Dye decolorization performance of system CEM-EC (MgSO4 dosages = 1, 2, 3, 4, 5 mM, current density within Figure (a–d) = 2.5, 5, 10, 20 mA/cm2).
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Figure 5. Dye decolorization performance of system CEM-EA (current density = 2.5, 5, 10, 20 mA/cm2, MgSO4 dosages within Figure (a–d) = 5, 10, 20, 40 mM).
Figure 5. Dye decolorization performance of system CEM-EA (current density = 2.5, 5, 10, 20 mA/cm2, MgSO4 dosages within Figure (a–d) = 5, 10, 20, 40 mM).
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Figure 6. Comparison of decolorization performance of four different systems. (a) Decolorization rate over time, (b) final decolorization rate.
Figure 6. Comparison of decolorization performance of four different systems. (a) Decolorization rate over time, (b) final decolorization rate.
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Figure 7. Morphology characterizations of Mg(OH)2 precipitates and the fouling CEM. (a) SM, (b) SEM, (d–f) EDS images of Mg(OH)2 precipitates formed in the cathode chamber. (c) SEM image of initial CEM. (g–i) EDS images of CEM after multiple electrolysis cycles.
Figure 7. Morphology characterizations of Mg(OH)2 precipitates and the fouling CEM. (a) SM, (b) SEM, (d–f) EDS images of Mg(OH)2 precipitates formed in the cathode chamber. (c) SEM image of initial CEM. (g–i) EDS images of CEM after multiple electrolysis cycles.
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Figure 8. Experimental procedures for the prospective technology.
Figure 8. Experimental procedures for the prospective technology.
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Table 1. Experimental operating conditions for four different systems.
Table 1. Experimental operating conditions for four different systems.
SystemsTemperature
(K)		Electrolysis Time
(min)		Current Densities
(mA/cm2)		MgSO4 Dosages
(mM)
CEME298302.5, 5, 10, 20-
MDC30-1, 2, 5, 10, 20, 40
CEM-EC302.5, 5, 10, 201, 2, 3, 4, 5
CEM-EA602.5, 5, 10, 205, 10, 20, 40
Table 2. Parameters of the adsorption kinetics fitting for DB86 dye by Mg(OH)2.
Table 2. Parameters of the adsorption kinetics fitting for DB86 dye by Mg(OH)2.
First-Order Kinetic FittingSecond-Order Kinetic Fitting
Qe (mg/g)k1R2Qe (mg/g)k2k2qeR2
308.72.35520.9896308.77.9000.29810.9865
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MDPI and ACS Style

Zhao, Y.; Cheng, N.; Jiang, R.; Jiao, J.; Chen, C.; Liang, J.; Hu, L.; Wang, H.; Wang, J. Potential of Textile Wastewater Decolorization Using Cation Exchange Membrane Electrolysis Coupled with Magnesium Salt Precipitation (CEM-MSP). Water 2025, 17, 1785. https://doi.org/10.3390/w17121785

AMA Style

Zhao Y, Cheng N, Jiang R, Jiao J, Chen C, Liang J, Hu L, Wang H, Wang J. Potential of Textile Wastewater Decolorization Using Cation Exchange Membrane Electrolysis Coupled with Magnesium Salt Precipitation (CEM-MSP). Water. 2025; 17(12):1785. https://doi.org/10.3390/w17121785

Chicago/Turabian Style

Zhao, Yujing, Nuo Cheng, Ruihan Jiang, Jian Jiao, Chen Chen, Jiahao Liang, Longfeng Hu, Hesong Wang, and Jinlong Wang. 2025. "Potential of Textile Wastewater Decolorization Using Cation Exchange Membrane Electrolysis Coupled with Magnesium Salt Precipitation (CEM-MSP)" Water 17, no. 12: 1785. https://doi.org/10.3390/w17121785

APA Style

Zhao, Y., Cheng, N., Jiang, R., Jiao, J., Chen, C., Liang, J., Hu, L., Wang, H., & Wang, J. (2025). Potential of Textile Wastewater Decolorization Using Cation Exchange Membrane Electrolysis Coupled with Magnesium Salt Precipitation (CEM-MSP). Water, 17(12), 1785. https://doi.org/10.3390/w17121785

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