The metastability of monohydrocalcite (CaCO3 center dot H2O: MHC) suggests high reactivity to dissolved trace elements. Using kinetic and isotherm sorption experiments with different reaction times (24 h, 48 h), As(V) sorption on MHC was examined to elucidate As(V) uptake by MHC. Although the MHC was transformed to aragonite with time, the MHC in higher As(V) loading conditions was able to persist longer than in lower loading conditions. Actually, As(V) uptake was negligible for samples in which the MHC remained. However, remarkable uptake of As(V) was observed for samples in which a complete transformation of MHC to aragonite occurred. Results of kinetic study confirmed that the timing of the MHC transformation coincided perfectly with that of As(V) removal from the solution. XAFS measurements showed that the local structure of As after the MHC transformation was almost identical to that of As in-the As(V) coprecipitated aragonite. Sorption behavior of As(V) during the transformation is explainable theoretically by the substitution of As(V) into the aragonite structure. The distribution coefficient and (apparent) maximum sorption capacity of As(V) sorption on MHC after 48 h at low-to-moderate As(V) concentrations are 500 L/kg and 25 mu mol/g, respectively, which are much higher than those of simple adsorption of As(V) on calcite. (C) 2015 Elsevier B.V. All rights reserved.