Dissolution of lead smelter dust compounds by humic acid

The ability of humic acids to complex metal ions is well established; however, the chemistry of lead dissolution and structure of the organo-metallic compound is not well understood. To better understand these problems, experiments were undertaken reacting the common smelter by-products PbSO₄, PbCO₃, and PbO with humic acid as a function of time, pH, and humic acid concentration. Examinations were made of the structure and mechanisms of complexing through the use of model humic acid functional group compounds and infra-red spectroscopy. Low pH conditions and increased humic acid concentrations favor PbO and PbCO₃ complexing. Lead sulfate complexation was favored by increased pH and humic acid concentrations. At low pH values the particulate lead compounds adsorbed humic acid inhibiting lead dissolution. Active sites of complexation found through infra-red analysis were at carboxyl, phenol, phenyl, and aliphatic groups, along with formation of metalocyclohexanes and pentanes. Model functional group analysis proved inconclusive for characterization of active complexing sites.