Performance and mechanisms of thermally treated bentonite for enhanced phosphate removal from wastewater

Optimization of clays as adsorbent for low concentration phosphorus removal from wastewater has received increasing attention in recent years. This study explored the feasibility of using bentonite as an adsorbent for phosphate (P) removal from synthetic wastewater, by assessing the performance of thermally treated bentonite for P removal and elucidating the mechanisms of P adsorption. Natural bentonite (B25) was thermally treated at 100-1000 °C (B100-B1000) for 2 h. Physical and chemical properties were measured by the SEM, XRD, pore size distribution, EDX, and cation exchange capacity (CEC) methods. Thermal treatment increased P sorption capacity of bentonite and that B800 had a higher P sorption capacity (6.94 mg/g) than B25 (0.237 mg/g) and B400 (0.483 mg/g) using the Langmuir isotherm equation. Study of sorption kinetics indicated that B800 rapidly removed 94% of P from a 10 mg P/L solution and the pseudo-second-order equation fitted the data well. The Ca²⁺ release capacity of B800 (1.31 mg/g) was significantly higher than that of B25 (0.29 mg/g) and B400 (0.40 mg/g) (p < 0.05). The initial pH level had a smaller impact on P removal efficiency for B800 than that of B25 and B400. Ca-P was the main fraction of P adsorbed onto B800, and Ca₁₀-P was the main species (41.4%). The main factors affecting the phosphorous adsorption capacity of B800 were changed crystal structure, strong calcium release capacity, and improved stability in different pH solutions. The results demonstrated that thermally treated bentonite (B800) has the potential to be an efficient adsorbent for removal of low-concentration phosphorus from wastewater.