HR: 0830h
AN: B21B-0710    [PDF]
TI: Effects of Organic Chelation on the Behavior of Aluminum Relative to Gallium During Pedogenesis
AU: * Herz, M M
EM: mmh28@cornell.edu
AF: Cornell University, Department of Earth and Atmospheric Sciences, Ithaca, NY 14853 United States
AU: Derry, L A
EM: lad9@cornell.edu
AF: Cornell University, Department of Earth and Atmospheric Sciences, Ithaca, NY 14853 United States
AB: This study of gallium and aluminum behavior in the soil weathering environment provides a first step toward using Ga/Al as a tracer of aluminum dynamics during pedogenesis. Previously, the interpretation of aluminum behavior in soils was confounded by the monoisotopic nature of aluminum and its subsequent lack of a comparative tracer. Like aluminum, gallium is a strongly hydrolyzing group III element. It is included in the same mineral systems as aluminum and the two are believed to have similar chemistries in most natural environments. Aluminum is a highly toxic metal whose removal from mineral matrices is enhanced by acid deposition and chelation by organic ligands. As much as 80% of dissolved aluminum in upper soil horizons can be complexed by organic ligands, most of which are secreted by plant and soil microorganisms to detoxify their surroundings. However, gallium makes comparatively unstable complexes with organic chelators, and is not expected to be carried into solution or leached from the soil profile by them to the same extent as aluminum. The Hawaiian Islands provide a unique opportunity to study the evolution of aluminum and gallium concentrations during soil development along gradients where age or climate varies, but all other soil forming factors are held constant. Ga/Al in older and more intensely weathered soils can be as high as 2.58 (mg/g), whereas the basaltic parent material is almost 10 times lower (0.30 mg/g). The factor driving soil Ga/Al ratios away from those found in parent material may be the strong control that organic chelation exerts over aluminum mobility. The enhancement of aluminum dissolution by organic chelation can be inferred from the decrease in Ga/Al ratios of exchangeable cations along a gradient of increasing rainfall; where wetter, more organic rich sites have Ga/Al ratios 10 to 100 times lower than drier sites where organic chelation is a less important factor in driving aluminum dynamics. Along this same gradient, the bulk soil shows an increase in Ga/Al despite an overall loss of both metals, indicating a relative enrichment of gallium in the secondary minerals. The results of preliminary laboratory syntheses of allophane minerals show that there is no significant fractionation of aluminum and gallium during secondary mineral formation in the absence of organic chelators.
DE: 1045 Low-temperature geochemistry
DE: 1065 Trace elements (3670)
DE: 1886 Weathering (1625)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting