Enhanced adsorption of inorganic arsenic by Mg-calcite under circumneutral conditions

Calcite is an important reservoir for arsenic (As) and strongly affects its mobility in various geological environments. However, the method by which bulk As is taken up by calcite needs to be better understood. To broaden our understanding, Mg-containing calcite (Mg-calcite), which is a ubiquitous form of calcite in nature, was investigated to determine its As adsorption capacity. Laboratory experiments were conducted under aerobic and anaerobic conditions using synthetic pure calcite (Ca10Mg0) and Mg-calcite (Ca9Mg1, Ca8Mg2). As speciation (determined using chromatography techniques, such as ion chromatography-hydride generation-atomic fluorescence spectrometry (IC-HG-AFS)) and microscopic characterization (field-emission scanning electron microscopy coupled with energy-dispersive spectroscopy (FE-SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) analysis, and X-ray photoelectron spectroscopy (XPS)) were coupled to investigate the adsorption capacity and potential mechanisms of As adsorption by Mg-calcite. The results showed that Mg-calcite exhibited an As adsorption capacity that was several times higher than pure calcite, especially for As(III). Based on microscopic characterizations, the substitution of CO3/OH groups with As-O groups and the formation of complexes on the Mg-calcite surface are the dominant mechanisms of arsenate (As(V)) adsorption. The doping of Mg into calcite results in a lattice contraction effect that provides additional space for the substitution of larger As oxyanions for carbonate ions. In parallel, as evidenced by the potentiometric automatic titration results, the presence of Mg in calcite led to an increased density of surface positive charges, which promotes greater adsorption of negatively charged As. In addition to the substitution of CO3 groups with As-O groups, the enhanced adsorption of As(III) by Mg-calcite was also attributed to the larger amount of H-bonding yielded by the addition of Mg. To the best of our knowledge, this is the first experimental investigation to quantify the adsorption of As by Mg-calcite, and it provides new insights into the fate and transport of inorganic As by carbonates in aqueous environments.