Ions binding to solid organic matter was investigated in this study. A simple surface complexation model, the diffuse double-layer model, was used to describe Pb(II), Cd(II), Zn(II), and Ni(II) binding to a lignocellulosic substrate extracted from wheat bran. The lignocellulosic substrate was represented by two acid sites: a low-pH ("carboxylic") site and a high-pH ("phenolic") site, the phenolic-type sites contributing significantly to the binding behavior, even at relatively low pH. By using the previously determined concentrations of sites and acidity constants, the surface complexation model was applied to ph, Cd, Zn, and Ni binding as a function of pH in a 0.1 M NaNO3 medium. The model fits were good over a wide range of pH (2 < pH < 10). The parameters deduced from these single-metal data were then used to predict metals binding under different conditions. The model accounted well for the change in solid concentration ([solid] = 0.2-1 g/L), metal concentration ([M] = 2 x 10(-5) to 1.4 x 10(-3) M), and ionic strength ([NaNO3] = 0.01-0.1 hi). It also allowed reasonably good prediction of the competition between up to five metals for the available sites.