High-salinity wastewater is usually considered for wastewater with total dissolved solids up to 10,000 ppm. To date, large volumes of high-salinity wastewater have been discharged from various industrial sections, such as the coal mining industry, petroleum and petrochemical industry, pulp and paper industry, power generation industry and fertilizer industry, among others. The composition of high-salinity wastewater is usually very complex and the wastewater contains various organic components, monovalent and multivalent inorganic salts such as Cl−
, among others. The inappropriate discharge of high-salinity wastewater will result in deleterious consequences such as the destruction of ecosystems, environmental contamination and salinization of soil [1
]. The high-salinity wastewater treatment system directly affects the sustainability of modern industrial societies.
Considering the coal mining industry, the Chinese government encourages the development of modern coal chemical technologies based on coal gasification technologies and other advanced chemical technologies to produce oil, syngas, olefins and ethylene glycol owing to the special energy resource structure of “rich coal, less gas, lack of oil” in China [4
]. Notably, coal mining is an intensive water consumption process. It is estimated that 0.5 tons of water are consumed to produce 1–2 tons of coal. However, the coal resources and water resources in China are in reverse distribution. Approximately 70% of coal mines are located in water-scarce areas and about 40% experience severe water shortage problems [5
]. Conversely, coal mining produces approximately 3–6 billion tons of wastewater per year. The coal mining industry faces a dilemma: the discharge of large volumes of wastewater and the severe water scarcity problem. The strict regulations on wastewater disposal and lack of water availability are driving the implementation of a zero liquid discharge (ZLD) system [6
]. Figure 1
indicates a typical wastewater treating procedure for the ZLD process, including biological treatment systems such as the traditional aerobic activated sludge process, aerobic granular sludge and the sequencing batch reaction (SBR) system, ultrafiltration system (i.e., UF, to remove the organic matters), nanofiltration system (i.e., NF, to separate the mono- and divalent salts) system, reverse osmosis system (i.e., RO, to concentrate the brine) system, membrane concentrating system such as electrodialysis (ED) and disc-tube reverse osmosis (DTRO) as well as the evaporation system. In recent years, the ZLD concept is developing rapidly since the legislation of the “Water Pollution Prevention and Control Action Plan” (alternatively known as the “Water Ten Plan”) in 2015. Theoretically, the implementation of a ZLD system can augment the water supply, eliminate the disposal of waste streams and valorize the valuable by-products. However, in reality, the ZLD process faces two crucial challenges during practical operation. On one hand, the ZLD system has a high capital cost and intensive energy input. All the soluble salts are finally required by thermal evaporation to generate the solid salts. It is a huge energy-consuming process even though emerging thermal evaporation technologies such as mechanical vapor recompression (MVR) and multiple-effect evaporation are developing rapidly. On the other hand, the evaporated and crystalized salts are usually mixtures of monovalent and divalent salts as well as organic matters that are difficult to reuse. Table 1
shows the estimated waste salt discharging capacities from the coal chemical industry in China. It can be seen that large amounts of waste salts are generated every year, which will become a hazardous threat to the environment. If the evaporated salts are not appropriately treated, the ZLD will become meaningless from the original aspiration of resource recycling. Therefore, how to valorize the mixed salts from the ZLD process has become the bottleneck for the ZLD process.
Currently, the chlor-alkali industry is the largest salt-consuming industry. The principle of chlor-alkali is a membrane electrolysis process that converts NaCl into chlorine, hydrogen and sodium hydroxide in the presence of a direct electric current field. Unfortunately, there are stringent requirements for the salts in the chlor-alkali industry, but the recycled salts from the ZLD process cannot meet these requirements. In contrast, bipolar membrane electrodialysis (BMED) [9
], which has less strict requirements for the feed compared with membrane electrolysis, is a perfect solution for the valorization of recycled salts into acids and bases in the ZLD process.
BMED is an advanced electro-membrane process that combines the water splitting in the bipolar membrane and directional migration of ionic components in the conventional electrodialysis process [11
]. The saline wastewater inputted to the BMED system can be effectively converted to acids and bases, achieving a closed loop industrial production. As a consequence, there is no need to evaporate the saline effluents and there will be no disposal of waste salts. Numerous studies have proved the feasibility of BMED for the production of acids and bases from industrial saline wastewater [13
]. However, the saline wastewater usually contains both monovalent and divalent anions, but the conventional BMED can only convert the mixed salts into mixed acids such as HCl and H2
, and bases. Owing to the low values of mixed acids, the technological sustainability and viability of BMED is restricted during the valorization of salts into acids and bases in the ZLD process. To address this problem, selectrodialysis (SED), a novel electrodialysis assembly system, was firstly proposed by Zhang et al. [17
] in 2012. The authors assembled monovalent selective ion exchange membranes into the conventional ED stack to achieve the separation of sulphate from a NaCl/Na2
mixture. In a subsequent study [18
], the authors further developed an integrated SED-struvite reactor to improve phosphate recovery efficiency using the high perm-selectivity of MSAEM for Cl−
ions against Hx
ions. Similarly, Xu et al. [19
] investigated SED for the selective removal of arsenic and monovalent ions from a brackish water reverse osmosis concentrate. Cohena and co-authors [20
] used SED for reducing chloride and sodium while preserving most of the hardness ions in the groundwater for irrigation. Zhang et al. [21
] performed SED for the separation of divalent ions from a seawater concentrate to enhance the purity of coarse salt. After that, Reig et al. [22
] ex situ combined SED with the BMED process to achieve monovalent–divalent ions separation and acids and bases production. However, the experiments were performed in two steps. In the first step, SED was used for the separation of Cl−
. In the second step, BMED was used for the conversion of NaCl and Na2
into their corresponding acids (HCl and H2
) and base (NaOH). Recently, Qiu et al. [23
] also ex situ integrated SED with BMED for the production of lithium hydroxide from salt lakes. This combination was also performed in two stages. Li+
ions were separated from Li+
mixtures by SED in the first stage, then Li+
ions were converted to LiOH by BMED in the second stage. Aspired from the advantages of the monovalent selective ion exchange membrane and BMED, in our previous study [24
], we in situ integrated SED with the mono-selective cation-exchange membrane and BMED for selectively regenerating monovalent ions and producing acids/bases in a single route. In this way, the pretreatment procedure to remove the multivalent cations, which cause membrane fouling, can be eliminated.
Apart from the concentrated brine from the RO plant, the wastewater from industrial societies such as the coal mining industry is rich in monovalent and divalent anions, and the valorization of these mixed salts is indispensable to the sustainability of the ZLD process. Herein, we in situ integrated BMED with MSAEM in a novel electrodialysis stack (SBMED). The MSAEM was placed between the bipolar membrane and the conventional anion-exchange membrane. By taking advantage of the perm-selectivity of MSAE for Cl− ions against the SO42− ions, it is possible to valorize the mixed salts into high-purity monoprotic and diprotic acids in one step. Therefore, the main objectives of this study are: (i) to test the feasibility of the in situ combination of SED and BMED for the production of monoprotic and diprotic acids; and (ii) to test the effects of the current density and mixed salt components on the purities of monoprotic and diprotic acids.