Selenium adsorption by New Zealand soils

Abstract

Samples of 66 representative New Zealand surface soils were equilibrated with 0.01 M Na2SeO3 solution to measure their relative adsorption of Se as selenite ions. Nineteen other soil properties were determined and the observed Se adsorption was related to them. Amounts of Se adsorbed ranged 39-3356 mg Se/kg soil and averaged 1621 mg Se/kg soil. This represented removal of 1-85% of added Se (average 41%). Among soils from noncalcareous sedimentary and metamorphic rocks, adsorption increased as weathering increased, from brown-gray earths through to northern yellow-brown earths, except that podzols adsorbed the least Se. Compared with soils from sedimentary and metamorphic parent material, yellow-brown pumice soils developed on rhyolitic volcanic ash adsorbed medium amounts of Se, and yellow-brown loams, red and brown loams, and brown granular clays developed or rhyolitic, andesitic and basaltic materials adsorbed greater amounts. Amounts of Se adsorbed were closely related to specific surface area, organic C, free forms of SiO2, Al2O3, and Fe2O3, and allophane content. Adsorption increased as the ratio between NaOH-extractable SiO2 and Al2O3 approached that of allophane composition. Both amorphous and crystalline forms of Fe2O3 and SiO2 were related to Se adsorption. Multiple regression on dithionite-Fe2O3, Tamm''s-Fe2O3, organic C, Tamm''s-SiO2, and the M SiO2/M Al2O3 ratio and SiO2 content from hot NaOH extraction accounted for 91.8% of variance in Se adsorption.