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Interactions of allophane with humic acid and cations
Author(s) -
Yuan G.,
Theng B. K. G.,
Parfitt R. L.,
Percival H. J.
Publication year - 2000
Publication title -
european journal of soil science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.244
H-Index - 111
eISSN - 1365-2389
pISSN - 1351-0754
DOI - 10.1046/j.1365-2389.2000.00295.x
Subject(s) - allophane , sorption , chemistry , humic acid , ionic strength , carboxylate , inorganic chemistry , ion exchange , electrolyte , ligand (biochemistry) , soil water , molecule , ion , clay minerals , aqueous solution , adsorption , mineralogy , organic chemistry , geology , soil science , fertilizer , biochemistry , receptor , electrode
Summary 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 CaCl 2 or NaCl as background electrolytes. Both the HA and the clay were separated from New Zealand soils. Much more HA was sorbed in CaCl 2 than in NaCl of the same ionic strength. Apparently Ca 2+ ions were more effective than Na + ions in screening the negative charge on HA. In CaCl 2 the HA molecule might also assume a more compact configuration than in NaCl. In the presence of CaCl 2 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 (Ca 2+ or Na + ) for charge balance. The Ca 2+ 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 Ca 2+ 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.