Adsorption of arsenite onto a soil irrigated by sewage

The present work investigates the adsorption of As (III) on a soil irrigated by sewage, as a function of reaction time, initial As (III) concentrations, pH, phosphate, and humic acid. The results show that As (III) adsorption onto the soil takes place in two processes: a fast one that takes place in less than 1h and a slow one that takes place in about 6h, and the rate data are best described by the pseudo second-order kinetic model at pH7.5. As (III) adsorption can be better described by the Freundlich isotherm model, although Langmuir, Temkin and Dubinin–Radushkevich isotherm models can also give a good fit for the adsorption of As (III) onto the soil. The maximum adsorption amount of As (III) derived from Langmuir isotherm for the soil is 0.368mg/g. The adsorption of As (III) decreases slightly at final pH1.21–4.3 due to the decrease of net positive charge for the soil and the apparent species transformation of As, then increases rapidly up to pH about 7 and decreases again as pH continues to increase. The presence of phosphate or humic acid prevents the adsorption of As (III) onto the soil, and the inhibition efficiencies of phosphate for As (III) adsorption onto the soil with the initial phosphate/As (III) molar ratio ≥1 are higher than that with the initial phosphate/As (III) molar ratio <1.