Mechanism of arsenic sorption onto laterite iron concretions

Abstract

We investigated arsenic sorption mechanisms onto laterite iron concretions (LIC) using electrophoretic mobility (EM) measurements, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and surface complexation modeling. Competitive sorption experiments were also conducted in the presence of phosphate and sulfate, as these compounds may interfere with arsenic sorption in waters from sulfide mining waste and ore stock piles. Electrophoretic mobility measurement results indicate that arsenic sorption lowered the point of zero charge of the LIC at pH 3–10. This observation implies both As (III) and As (V) form inner-sphere complexes on the LIC. The FTIR studies suggest that both As (III) and As (V) also form inner-sphere complexes with LIC. Arsenic sorption onto LIC was modeled by surface complexation modeling using the generalized composite approach. The model assumes that all mineral phases contributing to sorption are represented by one type of surface group. Adsorption edge experimental data was used to verify model performance. Arsenic (III) sorption onto LIC, which is markedly reduced by increasing ionic strength, was modeled using the triple-layer model, while As (V) sorption onto LIC was modeled using both the diffuse and the triple-layer models. The presence of phosphate reduces the amount of both As (V) and As (III) sorbed, while sulfate only reduces adsorption of As (III).