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Low-molecular-weight organic acids occur widely in soils. Results in pure mineral systems and podzols suggest hat LMW organic acids can promote the dissolution of Al from kaolinite, Al oxides and soils, but limited information is vailable concerning the role of these organic acids on Al mobilization in variable charge soils as yet. This paper deals with he effect of LMW organic acids on Al mobilization and mobilized Al distributed between the solution phase and exchange ble sites in two acidic variable charge soils. The results indicated that organic acids accelerated Al mobilization hrough proton- and ligand- promoted reactions. The ability of different organic acids to mobilize Al followed the order: citric cid > oxalic acid > malonic acid > malic acid > tartaric acid > salicylic acid > lactic acid > maleic acid. This order was in
eneral agreement with the magnitude of the stability constants of Al–organic acid complexes. The ratio of soluble Al to xchangeable Al also increased as the stability constants increased. These results showed that the organic acids with strong l-complexation capacity were most effective in Al mobilization, whereas the weak organic acids promoted the retention of obilized Al by the soil exchangeable sites. Increase in both organic acid concentration and solution pH promoted Al mobization and also increased the ratio of soluble Al to exchangeable Al due to the increase in the concentration of the effective rganic ligands, especially in the strong organic acid systems. These findings may have their practical significance for estabshing more effective amelioration procedures for variable charge soils with increased acidity and higher mobility of Al.

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应助: 5 (幼儿园)
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Previous work on adsorption of oxalate at aluminosilicate surfaces suggests that maximum adsorption occursthrough a bidentate attachment of the organic ligand, at nearneutral pH. Rates of ligand-promoted dissolution areexpected to be greatest at this pH as well. We tested this model by measuring oxalate adsorption on the surfaceof andesine (An47), in solutions of pH 3-5 and totaloxalate concentrations of 0-8 mM. Contrary to expectation,the greatest adsorption density of 24 μmol m-2 totaloxalate was observed at pH 3 and 8 mM total oxalate.Adsorption is dependent upon the activities of both oxalate(C2O42-) and bioxalate (HC2O4-) in solution and can bemodeled with either a two-term Langmuir or a two-termFreundlich isotherm. A Freundlich dsorption model providedthe best fit to rate data because it was not constrainedto a finite number of adsorption sites, as was the Langmuirmodel. The two-term ligand adsorption model wasincorporated into a rate model: Rtot ) kH+[Hads + ]L + kHOx-
[HOxads - ] + kOx2-[Oxads 2- ] where Rtot is the net dissolutionrate of the feldspar, [iads] is the concentration of species iadsorbed to the surface, and ki is the rate constant forrelease of the surface complex. The model was fit to datafor oxalate-promoted dissolution of andesine, resulting inestimates for the rate constants of kHOx- ) 1.16 × 10-12, kOx2-) 1.05 × 10-12, and kH+ ) 9.61 × 10-13 mol of feldspar(μmol of i)-1 s-1.

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2楼2016-07-14 15:32:58

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