REDUCTION IN TOXIC ALUMINUM TO PLANTS BY SULFATE COMPLEXATION

Excess SO42- in solution has been demonstrated to reduce aluminum (Al) toxicity in several studies, but the mechanism of SO42- induced reduction of Al toxicity was not clearly understood. In this study, Size Exclusion Chromatography (SEC; Sephadex G-25, superfine resin) was employed to investigate the interaction between Al and SO42- in unhydrolyzed Al solutions at pH 3.5. A dual peak elution of Al was observed when Al solution with SO4/Al molar ratio greater-than-or-equal-to 2.0 was injected into the SEC column. The first Al peak, which was eluted substantially earlier than the bed volume of the column, where the monomeric Al species are expected, was due to co-elution of Al with SO42- as evident from the position of this peak corresponding to the elution volume for peak elution Of SO42-. The elution fraction at the first Al peak contained SO42- and Al at a SO4/Al molar ratio = 2.7. The co-elution of Al and SO42- WOUld appear to be due to outer sphere SO42-complexation which has been referred to as "SO42- -complexed Al" in this paper. The Al peak for co-elution of Al and SO42- increased, while the peak at the bed volume decreased with an increase in S04/Al molar ratio of the injected sample. A nutrient solution experiment was conducted to evaluate the differential toxicity of SO42- -complexed Al (eluted by the SEC column), and monomeric Al in the absence of SO42- [as Al(NO3)3] in the range of 0 to 40-mu-M total applied Al concentrations. The SO42- complexed Al was less phytotoxic to soybean (Glycine max L.) root growth than the monomeric Al. The shielding effect of SO42- in the outer sphere of Al would appear to be responsible for the reduction in Al toxicity.