Manganese-mediated immobilization of arsenic by calcifying macro-algae, Chara braunii

The restoration capability of charophyte Chara braunii was studied in arsenic-polluted water in the context of biogenic calcium and manganese depositions on the plant. In addition to calcite encrustation, formation of craterlike shape deposits of manganese oxides (MnO_x) with diameters of 5-10 μm was detected on the cell walls of the plants grown in Mn-rich media. Relative proportions of arsenic taken up by the plant biomass to those incorporated into the calcium and manganese biominerals were determined using a modified sequential chemical extraction method. The mean total arsenic recovery from water reached its highest value at 375 mg kg^-1 in treatment with HCO₃^- and high concentrations of Ca and Mn (40 and 2 mg L^-1, respectively). The percentage of arsenic associated with the manganese deposits in the plants exposed to 0.5 mg L^-1 As(III) increased from 16.3% to 51.7% of the total arsenic accumulation at low and high Mn levels (<0.05 and 2 mg L^-1, respectively), that accounted for the highest Mn-bound arsenic contribution. Surface oxidation of As(III) by MnO_x and subsequent precipitation-adsorption of the formed As(V) onto the evolving structure of MnO_x could be a plausible mechanism for arsenic removal. The presence, and in some cases dominance of arsenic bound to the biogenic Ca and Mn deposits on the studied aquatic plant may contribute to preservation of arsenic in sediments in a less bioavailable form upon its senescence and decomposition.