The interaction of a humic acid with heavy metals

Abstract

The solubility of a coal humic acid and the sorption of heavy metals (Cu^(II), Zn^(II), Co^(II), and Cd^(II)) in the absence and presence of the humic acid were determined as a function of pH and concentration of background electrolyte.

The solubility of the humic acid at low electrolyte concentration increases in a 2-step process with increase in pH. About 80% dissolves in the pH region 3–8·5, and the remainder in the region pH >8·5. The sorption of metals occurs at pH values significantly lower than those associated with the formation of insoluble metal hydroxides, with a maximum occurring in the pH region 5·5–7·5, and involves the solid state fraction of the humic acid. At the higher electrolyte concentration, the solubility of the humic acid is again a 2-step process but the increase in acid solution occurs over a relatively narrow pH range (5–6). At high pH, the presence of the humic acid significantly reduces the precipitation of the metals at both low and high salt concentration, probably due to the formation of soluble metal–humate species. There is evidence to suggest that the major functional group of the humic acid with which the metal cations interact is the carboxyl group.