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

Abstract

Anion exchange between ubiquitous low-molecular-weight organic acids (LMWOA), such as oxalate, and surface-retained inorganic anions, such as F − , SO 4 2− 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. H 2 O ! 1.32 × 10 −3 mol l −1 F − ! D.I. H 2 O ! 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.

Editorial for the special issue on “Understanding soil interfacial reactions for sustainable soil management and climatic change mitigation” (ISMOM 2019)

“…The study by Evans and Jacobs (2021) investigates the complexity of tertiary anion exchange reactions in soils from different horizons of alpine tundra soils in order to assess their impact on aluminium biogeochemistry. A combination of batch and column experimental approaches was used to follow oxalate/hydroxide/fluoride exchange along with aluminium solubilization.…”