Formation Of Iron Complexes In Soil Organic Matter And Their Influence On Mobility And Bioavailability Of Antimony

Antimony (Sb) is a metalloid belonging to group 15 of the periodic table. Chemical similarities between arsenic (As) and Sb produce concerns about potential health effects of Sb and enrichment in the Environment. Sb is found in the environment as an oxyanionic species, antimonate (Sb(OH)6-). As a result of its net negative charge, antimonate was not initially predicted to have strong interactions with natural organic matter. It has been suggested that oxyanionic species could bind the negatively charged organic matter via a ternary complexation mechanism, in which cationic metals mediate the strong association between organic matter functional groups and oxyanions. The structure of these complexes remains poorly characterized. XANES spectroscopy was performed on organic soils amended with increasing iron levels in order to elucidate the structure of organically complexed iron. Humic acid complexes of iron were also synthesized and examined using XANES and Mossbauer spectroscopy. Two distinct iron sites were found in organic materials. A monomeric iron site and an oligomeric site consisting of small clusters of iron at sub-oxide levels. Phosphate exchangeable Sb was predicted to represent the majority of soil bound Sb. However, phosphate extractable Sb from soils is lower than anticipated. The affect to phosphate on Sb retention in organic soils was examined. Phosphate addition significantly reduced Sb retention in organic soils treated with Fe. The influence of organically complexed Fe on the mobility of Sb was assessed. Increasing Fe amendments resulted in an increase in Sb retention in organic soils. Further examination of the bioavailability of Sb to maize seedlings as a function of organically complexed Fe was examined using a greenhouse study. An unexpected increase in plant tissue Sb was observed as organically complexed Fe increased, which was not predicted by extractions commonly used to assess bioavailable Sb. Extraction of soils with organic acids common to the maize rhizosphere suggested that organic acid exudation can readily mobilize Sb bound by organic iron complexes. Overall, iron complexes in soil organic materials were found to have significant implications on mobility and bioavailability of Sb. Additionally, methods used to assess bioavailable Sb underestimate Sb mobility in organic soils