Effects of Humic Acid on the Solubility Product Constants of some Environmentally Significant Calcium Compounds

Warren C. Fiskus and Thomas J. Manning

Abstract: The effect that humic acid (HA) has on the solubility product constant (K_sp) of calcite (CaCO₃) and gypsum (CaSO₄-2H₂0) was determined. A flame atomic absorbance spectrometer was used to measure the aqueous phase calcium concentration in the solutions. The solubilities were measured as functions of humic acid aconcentration and pH. Calcite solubility was determined to be a function of [H+] and humic acid was determined to have a minimum effect on solubility. Gypsum solubility was unaffected by humic acid under normal encironmental conditions but decreased at basic pH(9) and high humic acid concentration (10 ppm).