Adsorption characteristics of As(V) on silty clay

Adsorption of metal cations has been extensively studied. However, the researches on adsorption of anions are still lacking. Adsorption of As(V) on a silty clay is investigated and compared with that of a common cation, Pb(II). The effects of solid-solution ratio, reaction time, concentration, pH and temperature on the adsorption are studied. The adsorption kinetics, isotherms and thermodynamics model sand meso/micro tests are used to investigate the adsorption mechanism involved. The hydraulic conductivities of the contaminated soil are studied. The test results show that the adsorption capacity of As(V) is much lower than that of Pb(II). The adsorption of As(V) increases linearly with the increasing concentration. The adsorption of Pb(II) increases with pH, whereas that of As(V) decreases slightly under alkaline condition. The adsorptions of As(V) and Pb(II) can both be described by the pseudo-second-order kinetic model, and are both dominated by chemical adsorption. Both intraparticle and film diffusions are involved in the adsorption of Pb(II), whereas the intraparticle diffusion is the rate-limiting factor for the adsorption of As(V). The Langmuir model gives poor fit for the adsorption of As(V), which can be rated as moderately difficult. The adsorption of Pb(II) is endothermic and favored by high temperature, whereas that of As(V) is exothermic and decreases with temperature. The meso/micro analysis shows that the adsorptions of As(V) and Pb(II) happen mainly out of the crystal structure. The soil pores become larger after the adsorption of Pb(II) due to particle aggregation, whereas the soil particles become more scattered after that of As(V). Some functional groups participate in the adsorption. As heavy metal content increases, the hydraulic conductivity of Pb(II)-contaminated soil increases whereas that of As(V) decreases. The alkaline condition increases the conductivity. The obtained adsorption mechanisms and parameters are important for the researches on arsenic pollution.