Reactions of compost-derived humic substances with lead, copper, cadmium, and zinc

Thermodynamic stability constants of the formation of complexes from the reactions of humic substances with various metals are usually used as parameters to judge the reactivities of both humic substances and metals. However, in calculating the thermodynamic stability constants, complicated processes for the acquisition of activities of components in reactions are absolutely inevitable. In this study, we investigated the average conditional concentration quotients of the complexes formed from the reaction of metals with humic substances and the relations of these quotients to thermodynamic stability constants. The characterized humic substances including HA (MW > 1000), FA (MW > 1000), and FA (MW < 1000) extracted from a swine compost were prepared to react with Pb, Cu, Cd, and Zn at 25 °C and at pH 4.00 and 6.50. Reactions of HA (MW > 1000), FA (MW > 1000), and FA (MW < 1000) with the four metals were carried out at 1:0.1, 1:0.5, 1:1, 1:5, and 1:10 ligand:metal stoichiometry. The concentrations of the free ions of Pb, Cu, Cd, and Zn in the reaction systems of metal-HA suspensions and metal-FA solutions were measured by anodic stripping voltammetry (ASV). The sequence of the average conditional concentration quotients of the formed complexes from the reaction of humic substances with metals was FA (MW < 1000) > FA (MW > 1000) > HA (MW > 1000), showing the relative reactivities of the fractions of swine compost-derived humic substances. The sequence of reacting metals with humic substances was Pb > Cu > Cd > Zn, which is in good agreement with the sequence reported by judging the thermodynamic stability constants. The average conditional concentration quotients of the formed complexes from the reaction of humic substances with metals were thus useful parameters that can be directly related to thermodynamic stability constants and other parameters.