| Abstract: | Allophanic soils are known to accumulate organic matter, but the underlying mechanism is not well understood. Here we have investigated the sorption of humic acid (HA) by an allophanic clay in the presence of varied concentrations of either CaCl2 or NaCl as background electrolytes. Both the HA and the clay were separated from New Zealand soils. Much more HA was sorbed in CaCl2 than in NaCl of the same ionic strength. Apparently Ca2+ ions were more effective than Na+ ions in screening the negative charge on HA. In CaCl2 the HA molecule might also assume a more compact configuration than in NaCl. In the presence of CaCl2 sorption increased, reached a maximum, and then declined as the concentration of HA in solution was increased. This behaviour was not observed in NaCl where sorption showed a gradual and steady increase with HA concentration. We propose that ligand exchange occurs between the surface hydroxyl groups of allophane and the carboxylate groups of HA. As a result, the allophane–HA complex acquires negative charges, requiring the co‐sorption of extraneous cations (Ca2+ or Na+) for charge balance. The Ca2+ co‐sorbed can attract more HA to the complex possibly by a cation‐bridging mechanism, giving rise to a maximum in sorption. The decline in sorption beyond the maximum may be ascribed to a decrease in the concentration of free Ca2+ ions through binding to HA molecules in solution. The increase in supernatant pH may be attributed to a ligand exchange reaction between the surface hydroxyls of allophane and the carboxylate groups of HA, and proton binding to the allophane–HA complex. |
