Characterization of phytotoxic aluminum in soil solutions from phosphogypsum amended soils

Abstract

Phosphogypsum (PG), an industrial by-product from phosphoric acid plants, is being used as an ameliorant for acid soil infertility. Phosphogypsum is primarily CaSO4 and contains F among several impurities. An increase in SO4 2− and F− ligands in soil solutions following amendment with PG is important for the alleviation of Al toxicity. In soil solutions containing abundant SO4 2− and F−, a measure of Al that is not complexed with either of the ligands represents phytotoxic Al. Alleviation of Al toxicity by SO4 2− in some instances is due to SO4 2−-induced precipitation and/or sorption of Al. Some studies have demonstrated a decrease in Al toxicity by SO4 2− even though precipitation of Al was not evident.In such cases, formation of the Al-SO4 + ion-pair (predicted by speciation models) has been attributed as the reason for alleviation of Al toxicity. Recent evidence has indicated that the proportion of Al complexed with SO4 2− was much greater than that predicted by using speciation models, which suggests that alleviation is not fully attributable to the formation of AlSO4 + species. The existing colorimetric Al assay techniques have failed to discriminate Al complexed with SO4 2− and have therefore, proved to be unsuitable for determination of phytotoxic Al in solutions containing SO4 2−. Fractionation of Al complexed with SO4 2− by size exclusion chromatography (SEC) has permitted a more precise characterization of Al complexed with SO4 2− which has been demonstrated as less phytotoxic than the uncomplexed Al. Therefore, size exclusion chromatography is a promising technique for characterization of phytotoxic Al in solutions in the presence of SO4 2−. In solutions containing F−, alleviation of Al toxicity is due to formation of Al-F complexes which are less phytotoxic. The 8-hydroxyquinoline method with a 15 second reaction time excludes Al-F complexes and, therefore is a measure of phytotoxic Al in solutions containing F−.