Cupric ion binding by coal humic acids at pH's 1–3

Abstract

The extent and mechanism of cupric ion binding by humic acid (HA) at low pH's where the material was insoluble in aqueous media was investigated. It was shown that significant amounts of Cu(II) were bound by HA even at pH's as low as 1.35. Two major types of HA—Cu2+ binding were observed. Binding which was accompanied by exchange of H+ was more prevalent at high pH's and [Cu2+]. Nonexchange bonding occurred at all pH's, but assumed greater importance at low pH's. Since the pattern of Cu(II) uptake could not be interpreted in terms of basic ion exchange or surface adsorption theory, a new model was proposed which qualitatively explained the dependence of Cu(II) on pH and [Cu2+]. The model assumed that the intermolecular bonding in flocculated HA was determined by the relative amounts of H+ and Cu2+ available for forming bridging bonds between the HA molecules. The model suggested that at high concentrations of Cu(II) and/or high pH's, the floc structure was such that the formation of copper bridge bonds was maximized.