低分子量有机酸对可变电荷土壤铝活化动力学的影响

从动力学角度研究了几种低分子量有机酸对2种酸性土壤中铝的活化和活化铝在土壤固/液相之间分配的影响。结果表明:对于络合能力弱的醋酸和乳酸,主要通过质子作用活化铝,且活化作用明显小于盐酸。而络合能力较强的苹果酸、草酸和柠檬酸,主要通过络合作用促进铝的释放,且这种作用随有机酸根阴离子络合能力的增强而增加。在氧化铁含量较高的砖红壤中,苹果酸、草酸和柠檬酸通过专性吸附增加土壤表面负电荷,从而增加土壤交换态铝;但在氧化铁含量较低的红壤中,草酸和柠檬酸主要通过形成可溶性铝络合物降低交换态铝。活化铝在土壤固/液相间的分配主要决定于溶液中有机阴离子与土壤固相表面对铝离子的竞争。醋酸和乳酸活化的铝主要以交换态铝存在;而草酸和柠檬酸活化的铝主要以有机酸-铝络合物存在于溶液中,特别是在氧化铁低的红壤中,这将促进铝在土壤-水体中的迁移。

Effect of low-molecular-weight organic acids on dynamics of mobilization of Al in variable charge soils

Effects of low-molecular-weight (LMW) organic acids on mobilization of Al and distribution of mobilized Al between soil solid and solution phase were studied in two acid soils from the angle of dynamics. Results show that LMW organic acids weak in chelating ability, such as lactic acid and acetic acid, promoted Al mobilization mainly with the aid of protons. They were less effective than HCl in promoting Al mobilization. While LMW organic acids strong in chelating ability, such as malic acid, oxalic acid and citric acid, promoted Al release mainly through chelation with organic anions. This effect was more effective for organic acids stronger in chelating ability with Al. In latosol high in content of iron oxides, malic acid, oxalic acid and citric acid increased the content of exchangeable Al mainly through specific adsorption of organic anions. While they decreased the content of exchangeable Al in red soil low in content of iron oxides mainly through formation of soluble Al-organic complexes. Distribution ratio of mobilized Al between soil solution and soil solid depended on competition between organic anions in soil solution and negative charge sites on soil surface for Al ions. Thus, most Al mobilized by lactic acid and acetic acid existed as exchangeable Al, while Al mobilized by oxalic acid and citric acid was mainly distributed as soluble Al-organic complexes in soil solution, which was especially significant in red soil low in content of free iron oxides, and would promote Al mitigation from soil to water body.