Arsenite sorption and co-precipitation with calcite

TL;DR

This study investigates how arsenite (As(III)) interacts with calcite, revealing the sorption mechanism, capacity limits, and structural effects, providing insights into arsenic removal processes in industrial applications.

Contribution

It offers a detailed molecular-level understanding of arsenite sorption and co-precipitation with calcite, including sorption capacity, surface complexation, and lattice modifications.

Findings

S-shaped sorption isotherm modeled by surface precipitation

Surface sites limit arsenite adsorption at low concentrations

Arsenic incorporation causes lattice parameter expansion

Abstract

Sorption of As(III) by calcite was investigated as a function of As(III) concentration, time and pH. The sorption isotherm, i.e. the log As(III) vs. log [As(OH)3 degrees / Assat] plot is S-shaped and has been modelled on an extended version of the surface precipitation model. At low concentrations, As(OH)3 degrees is adsorbed by complexation to surface Ca surface sites, as previously described by the X-ray standing wave technique. The inflexion point of the isotherm, where As(OH)3 degrees is limited by the amount of surface sites (ST), yields 6 sites nm-2 in good agreement with crystallographic data. Beyond this value, the amount of sorbed arsenic increases linearly with solution concentration, up to the saturation of arsenic with respect to the precipitation of CaHAsO3(s). The solid solutions formed in this concentration range were examined by X-ray and neutron diffraction. The doped calcite lattice parameters increase with arsenic content while c/a ratio remains constant. Our results made on bulk calcite on the atomic displacement of As atoms along [0001] direction extend those published by Cheng et al., (1999) on calcite surface. This study provides a molecular-level explanation for why As(III) is trapped by calcite in industrial treatments.