Oxalate-fluorideanion exchange in alpine tundra soil: Impact on aluminium transport

摘要

Anion exchange between ubiquitous low-molecular-weight organic acids (LMWOA), such as oxalate, and surface-retained inorganic anions, such as F-, SO42- and OH-, can play a critical role in Al bioavailability and transport in acidic alpine tundra soil. A series of batch equilibration and composite soil column leaching studies were conducted to examine anion exchange reactions between F-, oxalate and OH-, and the impact of increased soil solution F(-)concentration on Al transport. The addition of either F(-)or oxalate to soil suspensions resulted in significantly higher solution pH, suggesting F(-)and oxalate exchange with surface -OH groups. Distinct stages of OH(-)release were observed for all soil horizons leached with 1.32 x 10(-3) mol l(-1)F(-), with OH(-)release decreasing in the order O/A1 similar to A2 > > Bw. A similar OH(-)release pattern was observed in the O/A1 horizon for the 7.81 x 10(-4)mol l(-1) oxalate treatment, suggesting that both F(-)and oxalate may be competing for common exchange sites in the O/A1 horizon. Fluoride displacement by oxalate, in batch equilibration experiments, was observed for both the 2.84 x 10(-4) mol l(-1) and 7.81 x 10(-4) mol l(-1) oxalate treatment. Individual soil horizons leached with 7.81 x 10(-4) mol l(-1) oxalate exhibited increased F(-)leaching in the order O/A1 > A2 similar to Bw, with pulsed F(-)movement being observed in the O/A1 horizon. Soil columns eluted with 1.32 x 10(-3) mol l(-1) F(-)exhibited increased Al solubilization and transport, with Al leaching increasing in the order Bw > A2 > O/A1. Sequential leaching of soil columns in the order D. I. H2O -> 1.32 x 10(-3) mol l(-1) F- -> D.I. H2O -> 7.81 x 10(-4) mol l(-1) oxalate indicated that previously adsorbed F(-)was displaced by oxalate. Displacement of F(-)by oxalate increased in the order O/A1 > A2 > Bw. Hydroxyl displacement by both F(-)and oxalate may be an important source of acid neutralization in a soil wetting front, affecting Al speciation and transport. Highlights