ARSENATE-73 UPTAKE BY COMPONENTS OF SEVERAL ACIDIC SOILS AND ITS IMPLICATIONS FOR PHOSPHATE RETENTION

The arsenate (and phosphate) sorbing properties of the various components in the clay fractions of several acidic soils were assessed by autoradiography using radioactive arsenate-73. Arsenate uptake from solution was recognized by localization of Ag grains upon soil particles dispersed on electron microscope grids. The composition and identity of these particles was then established by electron microprobe analysis. In most of the soils examined, arsenate was retained mainly by iron oxides, and to a much lesser extent, by titanium oxides. Both minerals were active either as micron-size pellets or as very fine particles, the latter occurring in surface deposits upon clay minerals and other particles or in microaggregates composed wholly of finely divided material. Gibbsite occurred predominantly in finely divided form and, on a unit mass basis, was much less effective in taking up arsenate than iron or titanium oxides. In 1 soil where it was present in large amounts (50-65%), it accounted for 20-25% of total arsenate uptake. When free of surface deposits of finely divided oxides, other components of the clay fractions examined were comparatively inert towards arsenate. These components included kaolinite, illite, vermiculite, muscovite, potassium-felspar, ilmenite and quartz.