Arsenate uptake by calcite: Macroscopic and spectroscopic characterization of adsorption and incorporation mechanisms

Batch uptake experiments and X-ray element mapping and spectroscopic techniques were used to investigate As(V) (arsenate) uptake mechanisms by calcite, including adsorption and coprecipitation. Batch sorption experiments in calcite-equilibrated suspensions (pH 8.3; PCO2 = 10(-3.5) atm) reveal rapid initial sorption to calcite, with sorption rate gradually decreasing with time as available sorption sites decrease. An As(V)-calcite sorption isotherm determined after 24 h equilibration exhibits Langmuir-like behavior up to As concentrations of 300 mu M. Maximum distribution coefficient values (K-d), derived from a best fit to a Langmuir model, are similar to 190 L kg(-1). Calcite single crystals grown in the presence of As(V) show well-developed rhombobedral morphology with characteristic growth hillocks on (1 0(1) over bar $4) surfaces at low As(V) concentrations (<= 5 mu M), but habit modification is evident at As(V) concentrations >= 30 mu M in the form of macrostep development preferentially on the - vicinal surfaces of growth hillocks. Micro-X-ray fluorescence element mapping of (1014) surfaces shows preferential incorporation of As in the - vicinal faces relative to + vicinals. EXAFS fit results for both adsorption and coprecipitation samples confirm that As occurs in the 5+ oxidation state in tetrahedral coordination with oxygen, i.e., as arsenate. For adsorption samples, As(V) forms inner-sphere surface complexes via corner-sharing with Ca octahedra. As(V) coprecipitated with calcite substitutes in carbonate sites but with As off-centered, as indicated by two Ca shells, and with likely disruption of local structure. The results indicate that As(V) interacts strongly with the calcite surface, similar to often-cited analog phosphate, and uptake can occur via both adsorption and coprecipitation reactions. Therefore, calcite may be effective for partial removal of dissolved arsenate from aquatic and soil systems. (C) 2007 Elsevier Ltd. All rights reserved.