Thermodynamic constraints on limestone-based arsenic removal from water

Limestone-based material is effective for reducing arsenic concentrations below the current US limit of 0.010 mg/L for drinking water, typically resulting in final concentrations of about 0.004–0.006 mg/L (4–6 parts per billion). However, in laboratory and field testing, reductions to the 1 ppb range are difficult to achieve with limestone unless other material such as iron is added. Scanning electron microscopy and energy-dispersive X-ray analysis show the formation of calcium arsenate on limestone after arsenic removal. The arsenic removal mechanism with untreated limestone appears to be the formation of a low-solubility precipitate of calcium arsenate hydroxide, in the form of Ca 5 (AsO 4 ) 3 OH (arsenate apatite). Likely reactions and thermodynamic data show strongly negative Gibbs free energy changes and indicate a theoretical removal limit of about 4 ppb for arsenic using native limestone. Impurities in limestone, such as iron, also could enhance the removal process. The solubility product of arsenate apatite is approximately 10 −40 . This low solubility appears to account for the stability of the waste product after arsenic removal with limestone.