title: Monohydrocalcite: a promising remediation material for hazardous anions.
abstract: The formation conditions, solubility and stability of monohydrocalcite (MHC, CaCO3·H2O), as well as sorption behaviors of toxic anions on MHC, are reviewed to evaluate MHC as a remediation material for hazardous oxyanions. MHC is a rare mineral in geological settings that occurs in recent sediments in saline lakes. Water temperature does not seem to be an important factor for MHC formation. The pH of lake water is usually higher than 8 and the Mg/Ca ratio exceeds 4. MHC synthesis experiments as a function of time indicate that MHC is formed from amorphous calcium carbonate and transforms to calcite and/or aragonite. Most studies show that MHC forms from solutions containing Mg, which inhibits the formation of stable calcium carbonates. The solubility of MHC is higher than those of calcite, aragonite and vaterite, but lower than those of ikaite and amorphous calcium carbonate at ambient temperature. The solubility of MHC decreases with temperature. MHC is unstable and readily transforms to calcite or aragonite. The transformation consists of the dissolution of MHC and the subsequent formation of stable phases from the solution. The rate-limiting steps of the transformation of MHC are the nucleation and growth of stable crystalline phases. Natural occurrences indicate that certain additives, particularly PO4and Mg, stabilize MHC. Laboratory studies confirm that a small amount of PO4in solution (>30 μM) can significantly inhibit the transformation of MHC. MHC has a higher sorption capacity for PO4than calcite and aragonite. The modes of PO4uptake are adsorption on the MHC surface at moderate phosphate concentrations and precipitation of secondary calcium phosphate minerals at higher concentrations. Arsenate is most likely removed from the solution during the transformation of MHC. The proposed sorption mechanism of arsenate is coprecipitation during crystallization of aragonite. The arsenic sorption capacity by MHC is significantly higher than simple adsorption on calcite.
authors.affiliation: Institute of Nature and Environmental Technology, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa, 920-1192, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Fukushi
authors.affiliation: Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Munemoto
authors.affiliation: Department of Earth and Planetary Sciences, Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa, 920-1192, Japan.
authors.forename: Minoru
authors.initials: M
authors.lastname: Sakai
authors.affiliation: Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa, 920-1192, Japan.
authors.forename: Shintaro
authors.initials: S
authors.lastname: Yagi
keywords: arsenate; formation; monohydrocalcite; phosphate; solubility; sorption; stability