Humic acid adsorption onto bacterial and mineral surfaces was measured as a function of pH and humic:surface ratio. The bacterial species used was the aerobic gram-positive species Bacillus subtilis, and Al mineral surface sire interactions were isolated by using corundum (Al2O3). Adsorption of humic acid onto both surfaces is strongly pH-dependent, increasing with decreasing pH over the pH range 2.5-11.5, Humic acid adsorption also increases with increasing concentration of the sorbing surface. Because adsorption of humic acid onto the bacterial surface is strongest under low-pH conditions when both the humic acid (with three proton-active functional group types: L1, L2, and L3) and the bacterial surface are uncharged, the interaction is dominantly hydrophobic. We model the adsorption with a site-specific surface complexation approach based on the reaction: (1) X-H(L3)(0) + R-COOH0 double left right arrow R-COOH-H(L3)-X-0, where X and R represent the humic acid molecule and bacteria, respectively, to which the L3 and carboxyl functional groups are attached. Conversely, humic acid adsorption onto the mineral surface is strongest when the humic acid is negatively charged and the mineral surface is positively charged, with the adsorption decreasing as the concentration of positively charged mineral surface sites decreases with increasing pH. This type of adsorption is dominantly electrostatic, and can successfully be modeled also with a site-specific approach using the reaction: (2) X-(L1)(-1) + > Al(OH2)(+) double left right arrow Al(OH2)-(L1)-X-0, where > Al represents a crystallographically bound surface Al atom, We test the surface complexation approach by comparing observed humic acid adsorption in systems containing both bacteria and mineral surfaces to quantitative estimates of adsorption in these systems. The estimates are conducted independent of the experiments, based on the results from the simplified systems. The model predictions of humic acid adsorption in mixed humic-bacteria-mineral systems are reasonably accurate in describing the observed adsorption behavior in these mixed systems, indicating that the surface complexation approach can be successfully used to quantify the distribution of humic acid in realistic subsurface environments. (C) 1999 Elsevier Science B.V. All rights reserved.