Valorization of high-ash coal slime: lanthanum-modified porous ceramsite for efficient phosphate removal and...

High-ash coal slime (HACS) is an unavoidable byproduct of coal washing and processing, poses significant environmental challenges during conventional disposal. In this study, HACS was employed as the primary raw material, with lanthanum hydroxide serving as a modifier, to synthesize a porous ceramsite for efficient phosphorus removal from aqueous solutions. The preparation conditions were optimized using response surface methodology, and the optimal parameters were determined to be a HACS: bentonite: CaCO₃ ratio of 7:2:1, preheating at 500 °C for 20 min, and sintering at 1100 °C for 20 min. Adsorption experiments demonstrated that in a 10 mg P/L phosphate solution, the modified ceramsite achieved a maximum phosphorus removal rate of 98.45 %, with the adsorption process following the pseudo-second-order kinetic model and Langmuir isotherm, indicating chemisorption and monolayer coverage. Even in the presence of competing ions, the ceramsite maintained high phosphorus removal efficiency, and after five adsorption-desorption cycles, it retained a removal rate above 80 %. Characterization revealed the porous structure and confirmed phosphate binding through the formation of La-O-P coordination structures on the ceramsite surface. Mechanistic insights from density functional theory calculations and molecular dynamics simulations elucidated the atomic-scale interactions: (1) H₂PO₄^- coordinates strongly with La³⁺ sites via O atoms, forming stable inner-sphere La-O-P complexes. (2) H₂PO₄^- exhibits superior interfacial kinetics (higher diffusion coefficient) and adsorption affinity compared to HPO₄^2-, as evidenced by mass density profiles and radial distribution function analysis.