Adsorption behaviors of As(Ⅲ) on bentonite, diatomite, and hematite: implications for scientific remediation of arsenite contamination in soil

Abstract

Comprehensive understanding of As(III) sorption on natural minerals in contaminated soils is important for scientific decision making in remediation. In this study, the characteristics of As(III) adsorption on three minerals with different crystal structures and chemical compositions (bentonite; diatomite; and hematite) were investigated. The adsorption kinetics and thermodynamics were established. Surface complexation modeling was performed using X-ray diffraction spectroscopy, fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results showed that the pH value had a more significant effect on As(III) adsorption on hematite than on bentonite and diatomite, and As(III) was efficiently adsorbed by hematite at pH 7.0. The pseudo-first-order model provided an excellent fit to the As(III) adsorption on bentonite and hematite; the diffusion of ions or groups played an important role in the adsorption of As(III) on bentonite and hematite. The adsorption of As(III) on diatomite could be fitted with pseudo-first-order and pseudo-second order kinetic equations, as their regression coefficients were equal (R²=0.999). It was inferred that the adsorption of As(III) on diatomite occurred through solution diffusion and surface chemisorption. The As(III) adsorption on bentonite and diatomite was mainly physical and multilayer adsorption, whereas the As(III) adsorption on hematite was mainly chemical and monolayer absorption. The As(III) adsorption on hematite was divided into two stages: fast and slow. At first, the inner monodentate complex (such as ☰Fe-OAs₃H^-) formed at a high rate, and with the increase in the coverage of As(III) on the surface of iron oxide, the monodentate complex was slowly converted to the bidentate complex. These results verify the possibility of using bentonite, diatomite, and hematite as alternative materials for the remediation of As(III)-contaminated soils, and also indicate that bentonite and diatomite are suitable for the remediation of low As(III)-contaminated soils, while hematite is suitable for the decontamination of high As(III) polluted soil. Selecting suitable remediation materials according to arsenic contamination level is the key to soil scientific remediation.