Removal Performance and Mechanism for Treating Phosphorus in Agricultural Wastewater by Three Adsorbents

Abstract

To screen the optimal absorbents for P removal from agricultural wastewater, the P adsorption capacity of bentonite, red soil, and slag was studied using synthetic wastewater. Combing the properties of three adsorbents measured by SEM, XDS, and BET methods, the isothermal adsorption, adsorption kinetics, and Ca2+ release capacity were analyzed to elucidate the mechanisms of P adsorption. The results showed that the P adsorption capacity of slag was higher than that of bentonite and red soil, and the Langmuir isotherm model was able to better fit the adsorption data (R2>0.96). The P theoretical saturation sorption capacity of slag was higher (16.87 mg·g-1) than that of bentonite (1.21 mg·g-1) and red soil (0.92 mg·g-1) (P<0.05). The results for adsorption kinetics indicated that slag rapidly removed 95.6% of P from 10 mg·L-1 solution, and the Elovich equation fit the data well (R2=0.812). The adsorption kinetics of P on bentonite and red soil were better described by the pseudo-second-order kinetic equation (R2=0.982 and 0.959, respectively). The Ca2+ release capacity of slag (10.46 mg·g-1) was significantly higher compared to bentonite (0.31 mg·g-1) and red soil (0.03 mg·g-1) (P<0.05). The P adsorption capacity of red soil was 0.26 mg·g-1 when the pH value was 3, and it decreased as the pH values increased. At the initial pH of 7.0, the P adsorption capacity of bentonite was about 0.01 mg·g-1, lower than 0.04 mg·g-1 at pH 3, and 0.05 mg·g-1 at pH 11. The initial pH value had little effect on the P adsorption capacity of slag. The P-loaded bentonite, red soil, and slag were effectively regenerated by using CaCl2 solution, and bentonite was easier to reuse compared to red soil and slag. The key factors affecting the P adsorption capacity of the three adsorbents were physical and chemical properties, such as crystal structure and the content of metal ions, Ca2+ release capacity, and initial pH. These findings demonstrated that slag was a better choice for P removal compared to bentonite and red soil and could be used as an effective P adsorbent for agricultural wastewater treatment.