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

Humic Ion-Binding Model VI, a discrete site/electrostatic model of the inte ractions of protons and metals with fulvic and humic acids, is applied to 1 9 sets of published data for proton binding, and 110 sets for metal binding . Proton binding is described with a site density, two median intrinsic equ ilibrium constants, two parameters defining the spread of equilibrium const ants around the medians, and an electrostatic constant. Intrinsic equilibri um constants for metal binding are defined by two median constants, log K-M A and log K-MB, which refer to carboxyl and weaker-acid sites respectively, together with a parameter, Delta LK1, defining the spreads of values aroun d the medians. A further parameter, Delta LK2, takes account of small numbe rs of strong binding sites. By considering results from many data sets, a u niversal average value of Delta LK1 is obtained, and a correlation establis hed between log K-MB and log K-MA. In addition, a relation between Delta LK 2 and the equilibrium constant for metal-NH3 complexation is tentatively su ggested. 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.