Humic ion-binding model VI: An improved description of the interactions of protons and metal ions with humic substances

Humic Ion-Binding Model VI, a discrete site/electrostatic model of the interactions of protons and metals with fulvic and humic acids, is applied to 19 sets of published data for proton binding, and 110 sets for metal binding. Proton binding is described with a site density, two median intrinsic equilibrium constants, two parameters defining the spread of equilibrium constants around the medians, and an electrostatic constant. Intrinsic equilibrium constants for metal binding are defined by two median constants, log K(MA) and log K(MB), which refer to carboxyl and weaker-acid sites respectively, together with a parameter, Delta LK(1), defining the spreads of values around the medians. A further parameter, Delta LK(2), takes account of small numbers of strong binding sites. By considering results from many data sets, a universal average value of Delta LK(1) is obtained, and a correlation established between log K(MB) and log K(MA). In addition, a relation between Delta LK(2) and the equilibrium constant for metal-NH(3) complexation is tentatively suggested. As a result, metal-binding data can be fitted by the adjustment of a single parameter, log K(MA). Values of log K(MA) are derived for 22 metal species. Model VI accounts for competition and ionic strength effects, and for proton-metal exchange.