Differences in the immobilization of arsenite and arsenate by calcite

The sorption and coprecipitation experiments of arsenic (As) with calcite coupled with determinations of the chemical state of As both in the reaction fluid and in calcite were conducted to investigate the influence of the As oxidation state on its immobilization into calcite. The oxidation states of As in calcite and water were determined via As K-edge XANES and HPLC-ICP-MS analysis, respectively. The results of the sorption experiments at pH 8.2 show that only As(V) is distributed to calcite regardless of the As oxidation state in the solution. In coprecipitation experiments, As(V) is preferentially incorporated into calcite over a wide range of pH (7-12). On the other hand, the incorporation of As(III) into calcite is not observed at circumneutral pH. This difference between As(III) and As(V) is attributed to the fact that their dissolved species are neutral vs. negatively charged, respectively, at circumneutral pH (arsenite as H3AsO3; arsenate as H2AsO4- or HAsO42-). As the pH increases (> 9), up to 33% of As(III)/As-total ratio is partitioned into calcite or a precursor of calcite (metastable vaterite formed during the early stage of precipitation). The higher interaction of As with calcite at an alkaline pH compared with circumneutral pH is due to the negative charge of As(III) at alkaline pH. However, the As(III)/As-total ratio decreases as time progresses and only As(V) can be found finally in calcite. The ratio of distribution coefficients of As(III) and As(V) into calcite (K-As(V)/K-As(III)) at pH similar to 7 is larger than 2.1 x 10(3), suggesting that the oxidation state of As is a significant issue in considering the interaction between As and calcite in groundwater. Moreover, low K-As(III) shows that the sequestration of As via coprecipitation with calcite is not an important chemical process under reducing conditions, such as in the groundwaters in Bangladesh and other As-contaminated areas where As(III) is the dominant dissolved species of As. In the system spiked only with As(III), the XANES analysis detected As(V) in calcite, where the partitioning of As into calcite is very limited as stated above. Therefore, HPLC-ICP-MS and cyclic voltammetric measurements were performed to clarify the As(V)-incorporation mechanism in the As(III) system. As a result, the oxidation of As(III) to As(V) and the stabilization of As(V) in a solution in the presence of the Ca2+ ion are observed. These phenomena result from the formation of calcium arsenate complexes in the aqueous phase. This complexation is an important process for the coprecipitation of As with calcite. Hence, As is incorporated into calcite as As(V) even from the As(III) solution at circumneutral pH, wherein As(III) exists as neutral arsenite with little interaction with calcite. (C) 2012 Elsevier Ltd. All rights reserved.