Oxalate‐fluoride anion exchange in alpine tundra soil: Impact on aluminium transport

Abstract

AbstractAnion 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 × 10−3 mol l−1 F−, with OH− release decreasing in the order O/A1 ~ A2> > Bw. A similar OH− release pattern was observed in the O/A1 horizon for the 7.81 × 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 × 10−4 mol l−1 and 7.81 × 10−4 mol l−1 oxalate treatment. Individual soil horizons leached with 7.81 × 10−4 mol l−1 oxalate exhibited increased F− leaching in the order O/A1 > A2 ~ Bw, with pulsed F− movement being observed in the O/A1 horizon. Soil columns eluted with 1.32 × 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 × 10−3 mol l−1 F− → D.I. H2O → 7.81 × 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 Oxalate and F− compete for similar OH− exchange sites, increasing soil pH. Adsorbed F− can be displaced by oxalate anions. Al solubilization increases with F− adsorption in alpine tundra soil. Aluminium fluoride speciation changes rapidly within the soil wetting front. Anion exchange reactions should be considered in assessing soil ANC.