Environmental behavior of arsenic(III) and (V) in soils

This paper presents an evaluation of the environmental behavior of arsenic species in soils (F. F. Dias, Master Thesis, University of Delaware, 1997). The results obtained were used to determine adsorption constants that were incorporated in mathematical models using forward and backward stepwise linear regression to correlate data. The amount of adsorption was significantly different depending on soil properties, such as organic matter, iron oxide content, and surface area. Arsenic speciation on the soil surface was deduced from desorption data, with As(V) being more strongly retained in the soil. As(III) was oxidized on the soil surface and desorbed as As(V); an important factor since As(V) is less toxic. In order to develop an adequate adsorption model, Langmuir and Freundlich isotherms were obtained for each soil without pH alteration. Results indicated that the maximum amount of As(V) adsorbed was greater than the amount of As(III) adsorbed. Adsorption edges for As(III) and As(V), with pH varying from 3 to 10, were obtained at concentrations that ranged from 0.1 to 200 mg L-1. The soils studied exhibited an L-type Langmuir isotherm. Maximum As(III) adsorption occurred around pH 6 to 9, while maximum As(V) adsorption occurred in the 4 to 5 pH range. Experiments to determine arsenic kinetics were carried out and showed that adsorption and desorption equilibrium was reached within 48 hours for both species.