The treatment of dye wastewater remains a formidable challenge within the realm of industrial wastewater management. The existing methods for treating dye wastewater include adsorption, electrochemical oxidation, photocatalysis, biodegradation, and membrane separation. Among these, membrane separation is particularly advantageous due to its energy efficiency, high efficacy, and ease of process control. Separation membranes can be categorized into two primary types, i.e., organic and inorganic membranes. Inorganic membranes have attracted significant attention due to their high mechanical strength, controllable size distribution, resistance to high temperature and pressure, and chemical stablility. Despite these benefits, the advancement of conventional inorganic ceramic membranes is hindered by their complex preparation processes and high energy demands. This limitation underscores a urgent need for the development of simple and cost-effective inorganic membranes. A promising innovation in this field is a geopolymer-zeolite composite membrane, which is synthesized through in-situ hydrothermal conversion following the formation of reactive silica-aluminum materials under alkali or acid excitation. Coal gangue (CG), as a solid waste from coal mining primarily composed of SiO₂ and Al₂O₃, can be used as a raw material for the production of geopolymer-zeolite composite membrane. In this paper, A cost-effective geopolymer-zeolite composite membrane was prepared by a polymerization-hydrothermal method with CG as the main raw material for the separation of RhB in water. The objective was to propose a novel utilization strategy for CG and to lay a foundation for the development of membrane separation technology.

Coal gangue (CG) (Zhunger, Inner Mongolia, China) was calcinated at 800 ℃ for 2 h, resulting in the formation of calcined coal gangue (CCG). The CCG was then uniformly mixed with other additives in a mass ratio of CCG:NaOH:Na₂SiO₃: H₂O (100∶10∶40∶45). This mixture was poured into polytetrafluoroethylene moulds with the diameters of 50 mm and thicknesses of 5 mm. The samples were cured at 70 ℃ for 24 h to produce coal gangue-geopolymer membrane (CCG-GM). Afterwards, the CCG-GM was hydrothermally treated in a 100 mL 1 mol/L NaOH solution at 140 ℃ for 12 h, resulting in a coal gangue-geopolymer-zeolite composite membrane (CCG-GZCM).

The prepared CCG-GZCM was incorporated into a custom-built terminal filtration system, and its performance was evaluated via measuring the pure water flux, membrane flux, and RhB removal rate under varying conditions (i.e., system pressure, initial RhB solution concentrations, and pH values of the RhB solution). In addition, the circular utilization of CCG-GZCM was also examined to assess its reusability. The selectivity of CCG-GZCM for the removal contaminants (i.e., RhB, methylene blue (MLB), methyl violet (MV), methyl blue (MB), methyl orange (MO), and eosin Y (EY)) was determined.

The XRD patterns reveals that CG primarily consists of quartz and kaolinite. Upon calcination at 800 ℃ for 2 h, kaolinite in CG transforms into metakaolinite, thus froming CCG. For alkali activation, a broad diffraction peak of CCG initially appears at 15°-25°, and then shifts to 20°-40°, indicating the conversion of CCG into a geopolymer, thus forming CCG-GM. Subsequently, CCG-GZCM, with a main phase composition of NaP1 zeolite, is obtained via in-situ hydrothermal transformation of CCG-GM. The FTIR spectra of CCG-GZCM confirm the presence of NaP1 zeolite absorption bands. The XPS spectra further identify Na, Si, O, and Al as the predominant elements on the surfaces of both CCG-GM and CCG-GZCM. Note that CCG-GZCM exhibits a larger specific surface area and pore volume, compared to CCG-GM, albeit with a smaller average pore size. The TG-DSC spectra indicate that the weight loss of CCG-GZCM primarily occurs at 30-253 ℃, indicating its good thermal stability. In addition, the compressive strength of CCG-GM also increases from 18.84 MPa to 47.01 MPa (CCG-GZCM) after hydrothermal transformation.

CCG-GZCM exhibits varying removal efficiencies for different dye types. At an initial concentration of 10 mg/L and a pH value of 7, the pure water flux and membrane flux to RhB solution of CCG-GM and CCG-GZCM increase linearly with the increase of system pressure, while the RhB removal rate decreaseslinearly. At a system pressure of -0.08 MPa and a pH value of 7, the RhB removal rate by CCG-GZCM gradually decreases with the increase of initial concentration within 60 min. Conversely, at the same system pressure and an initial RhB concentration of 10 mg/L, the RhB removal rate by CCG-GZCM gradually increases with increasing pH value of the RhB solution within 60 min, and the maximum is 99.11%. However, when the system pressure maintains at -0.08 MPa, at an initial RhB concentration of 10 mg/L and an initial pH value of 11, the RhB removal rate by CCG-GZCM decreases with an increasing number of cycles, reducing to 94.25% after 6 cycles. In addition, CCG-GZCM demonstrates different removal rates within 30 min for different dye types. It achieves a high removal rate of exceeding 99% for cationic dyes including RhB, MLB, and MV. In contrast, the removal rates for anionic dyes, including MB, MO, and EY, are 70.00%, 25.82%, and 42.69%, respectively.

The pore structure of geopolymer membrane was modified and converted it into CCG-GZCM with larger specific surface area, higher pore volume and smaller average pore size by a hydrothermal method. The CCG-GZCM was characterized by an amorphous geopolymer matrix with NaP1 zeolite and a minor presence of NaA zeolite on the surface. At a system pressure of-0.08 MPa, an initial RhB concentration of 10 mg/L, and a pH value of 11, the CCG-GZCM achieved a remarkable RhB removal rate of 99.11% for 60 min. In addition, the CCG-GZCM also demonstrated a high circular utilization efficiency for RhB removal, maintaining a removal rate of exceeding 99% for cationic dyes for 30 min. The CCG-GZCM could be used as a promising approach for the effective utilization of coal gangue, having a significant potential for application in water pollutant treatment.