Effect of pH on the adsorption of arsenic(V) and antimony(V) by the black Checksoil in three systems: Performance and mechanism

Arsenic (As) and antimony (Sb) are listed as the priority pollutants by the U.S. Environmental Protection Agency (EPA) and the European Union (EU) due to their toxicity and potential carcinogenicity. It is necessary to investigate their adsorption over soil as such a behavior affects their mobility and bioavailability. In this study, the effect of pH on the adsorption of As(V) and Sb(V) by the black soil was investigated with three systems: the Single system, Binary system, and Sequence system. The operating pH was set at 4.0, 7.0 and 10.0. Based on the Langmuir isothermal and the pseudo-second-order kinetic models, the adsorption for As(V) was always better than Sb(V) in the whole pH range; the best adsorption performance for the two sorbates was achieved at pH of 4.0, followed by 7.0 and 10.0 in the three systems. The reasons could be that the atomic radius of arsenic is smaller than that of antimony, and the positively charged functional groups carried by the inorganic colloids in the soil contributed to binding with the negatively charged As(V)/Sb(V). A lower pH promoted the inorganic colloids to carry more positive charges. Compared to Single system, the maximum adsorption capacity (q(m)) and the initial adsorption rates (k(2)q(e,cal)(2)) of As(V) and Sb(V) in Binary system decreased obviously, suggesting competitive adsorption occurred when As(V) and Sb(V) coexisted. The findings of this work improve the understanding of As(V)/Sb(V) adsorption behavior in soil under different situations and would facilitate a comprehensive evaluation on the risk assessment of arsenic and antimony.