Enhanced arsenic sorption by hydrated iron (III) oxide-coated materials--mechanism and performances.

Mechanism and performances of arsenic(III) [As(III)] and arsenic(V) [As(V)] sorption onto hydrated iron(III) oxide (HFO)-coated materials were investigated at neutral pH where arsenic occurs in both molecular and ionic forms. Arsenic sorption by HFO-coated materials was proven to be a multistage process consisting of both macropore and intraparticle diffusion. Higher mass-transfer velocities were obtained for As(III), which is attributed to the beneficial features of HFO. Equilibrium studies revealed the spontaneous and favorable nature of the arsenic sorption process. The maximum sorption capacity and the Gibbs free energy values indicated that HFO-coated materials exhibit more affinity towards As(III). The Langmuir and Freundlich isotherm models revealed both the chemical and physical nature of the sorption process, while the Dubinin-Radushkevich model indicated that physical sorption is a more dominant process with HFO-coated materials.