Study on the Adsorption Mechanism and Response Surface Optimization of Fluorine and Phosphorus by the Red Clay in Guizhou

The presence of fluorine and phosphorus in phosphogypsum leachate directly affects the local ecological environment and groundwater quality. This paper examined the adsorption characteristics of fluorine and phosphorus by red clay near a phosphogypsum landfill in Guizhou Province, China, under different pH values, initial concentrations, soil dosages, and particle sizes of the soil. Response surface methodology (RSM) was employed to optimize the three factors (pH, initial concentration, and soil dosage) selected by Box-Behnken design that have a great influence on single-factor adsorption. The optimum condition for the adsorption of fluorine by red clay was identified as pH = 3, initial concentration of 91.08 mg/L, and soil dosage of 5.31 g, while that for the adsorption of phosphorus by red clay was pH = 4.17, initial concentration of 99.49 mg/L, and soil dosage of 5.82 g. The Langmuir equation, the quasi-secondary kinetic model, and the Elovich model were used for fitting, which demonstrated good fitting. Finally, comparing the characterization results of scanning electron microscopy (SEM-EDX), X-ray diffraction (XRD), and infrared spectroscopy (FTIR), the adsorption process of red clay on fluorine and phosphorus was accompanied by electrostatic adsorption in addition to chemical adsorption and ligand exchange, which was the result of multiple adsorption effects. The findings of this study will provide a reference for the prevention, control, and management of fluoro-phosphorus pollution in phosphogypsum dumps.