Arsenic(III) carbonate complexing

A recent hypothesis that As(III) forms strong complexes with (bi)carbonate is tested by measuring the solubility of As2O3 in concentrated (up to 0.72 m) bicarbonate solutions at near-neutral pH and 25.0 ± 1.8 °C. A small, but statistically significant solubility enhancement is observed in NaHCO3-containing solutions compared to NaCl solutions of essentially the same ionic strength. The effect is well-explained by formation of a single complex: As(OH)30+HCO3-=As(OH)2CO3-+H2O, Kc = 0.22 ± 0.04 (corrected to infinite dilution). This complex's small stability constant value is consistent with a published, quantum-based computational prediction. The measurements also yield a value for the solubility of arsenolite that agrees reasonably with recently reported values: 1/2As2O3(arsenolite)+3/2H2O=As(OH)30, Ks = 0.178 ± 0.004 (also corrected to infinite dilution). The new data show that As(III) carbonate complexes will be negligible at carbonate concentrations found in most natural waters, but could be significant, though minor, in extremely carbonate-rich waters, such as found in interior drainage basins.