Comprehensive experimental design for preparation of zirconium-modified coal gangue–based molecular sieves and investigation of their sulfate ion adsorption performance

As a major producer and consumer of coal, China faces significant environmental pressures owing to the accumulation of coal gangue and contamination from high-sulfate mine water. Coal gangue, rich in silicon and aluminum, is an ideal raw material for synthesizing molecular sieves. Among these, NaA molecular sieves exhibit structural stability, high specific surface area, and strong ion exchange capacity; however, their negatively charged lattice limits their anion adsorption capacity. Zirconium hydroxide (Zr(OH)₄) has a strong affinity for SO₄²⁻ but tends to agglomerate during practical applications, diminishing effective adsorption sites. To address these limitations, this study employs a zirconium modification strategy to construct Zr(OH)₄/ZrA composite adsorbents, aiming to simultaneously valorize coal gangue and efficiently remove SO₄²⁻ from mine water.

Coal gangue was ground and calcined at 800 ℃ for 2 h to produce metakaolin, which was then used to synthesize NaA molecular sieve via hydrothermal reaction. Subsequently, the Zr(OH)₄/ZrA adsorbent was synthesized through a multistep modification process: first, NH₄A molecular sieve was obtained; subsequently, a secondary ion exchange with ZrOCl₂ solution at 70 ℃ for 4 h formed the ZrA molecular sieve carrier; finally, zirconium hydroxide was loaded using the impregnation–precipitation method. Specifically, ZrA molecular sieves were impregnated with 0.1 mol/L ZrOCl₂ solution at 60 ℃ for 12 h, followed by a second impregnation with ammonia solution (1∶1 volume ratio) for an additional 12 h. The resulting material was vacuum filtered, washed, and dried to yield the Zr(OH)₄/ZrA adsorbent. Characterization techniques, including X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and Brunauer–Emmett–Teller specific surface area analysis, were employed to systematically investigate the crystal structure, surface functional groups, microstructure, and pore characteristics of the sample. Static adsorption experiments investigated the influence of initial SO₄²⁻ concentration, adsorption time, reaction temperature, and solution pH on adsorption performance. Adsorption processes were analyzed using pseudo-first-order and pseudo-second-order kinetic models alongside Langmuir and the Freundlich isotherm models.

Findings indicate that Zr(OH)₄ was successfully loaded onto the surface of NaA molecular sieves without compromising their crystalline structure. The modified Zr(OH)₄/ZrA adsorbent exhibits a rough surface with a well-developed mesoporous structure, achieving a specific surface area of 441.36 m²/g, thereby providing ample active sites for SO₄²⁻ adsorption. Adsorption performance increased with initial SO₄²⁻ concentration, reaching a maximum adsorption capacity of 205 mg/g at an initial concentration of 10 g/L. Adsorption was rapid during the first 120 min and gradually approached equilibrium thereafter. Elevated temperature and high pH were found to inhibit adsorption efficiency. Kinetics analysis indicates that adsorption follows pseudo-second-order kinetics, while isotherm fitting predominantly aligns with the Langmuir model, confirming monolayer chemisorption.

The Zr(OH)₄/ZrA adsorbent exhibits excellent structural stability and high SO₄²⁻ affinity, achieving outstanding SO₄²⁻ removal and demonstrating regeneration potential. This study presents a viable approach for valorizing coal gangue and offers an effective, economical method for treating high-sulfate mine water. The findings hold significance for advancing the green transformation of the coal industry and supporting China’s “dual carbon” goals.