Tuesday, November 6, 2007
186-6
Arsenate Sorption on Fe(III) in Soil Organic Matter.
Yu Ting Liu, Soil Science, North Carolina State University, Department of Soil Science, Raleigh, NC 27695-7619, Dean Hesterberg, PO Box 7619, North Carolina State University, North Carolina State University, Department of Soil Science, Raleigh, NC 27695-7619, and Kimberly Hutchison, North Carolina State University, Department of Soil Science, Raleigh, NC 27606.
Soil organic matter potentially serves as an important sorbent for anions like phosphate and arsenate because of the formation of ternary complexes with organically-bound Fe(III) and Al(III). Our objective was to determine the molecular-scale mechanisms of arsenate binding in natural organic matter (OM ) as a function of the concentration of bound Fe(III). Aqueous arsenate was reacted at pH 5.5 with Fe(III)-laden peat at a concentration of 1200 mmol As/kg peat. The peat contained Fe(III) pre-sorbed in the range of 600 to 5400 mmol Fe(III)/kg. X-ray absorption spectroscopy (XAS) at both the Fe and As K-edges was employed to determine the local coordination structures of arsenate-Fe(III)-peat complexes in relation to Fe(III)-hydroxy polynuclear complexes. Both EXAFS and XANES results confirmed bonding between arsenate and peat-bound Fe(III). EXAFS fitting results showed an As-Fe interatomic distance around 2.96 – 3.04 Å. The Fe K-edge XANES spectrum for the peat sample with 600 mmol Fe/kg was similar to an Fe(III)-acetylacetone standard, and the sample with 5400 mmol Fe/kg was similar to that of hematite, indicating the formation of Fe(III) clusters at higher Fe(III) concentration. At the As K-edge, the white line intensity decreased with increasing Fe(III) concentration, indicating a decrease in the probability of an anomalous charge transition of an As 1s electron into a hybridized 4p valence orbital. The combination of XANES and EXAFS results suggested that arsenate sorbs on Fe(III) clusters in a bidentate-binuclear configuration at greater than 1800 mmol Fe/kg peat, and in a bidentate mononuclear configuration at lower Fe(III) concentrations. Our research also showed that the sorption capacity of organic matter for arsenate increases with increasing Fe(III) content, which is important for understanding retention and release of arsenate (and perhaps phosphate) in organic-matter rich soils like those found in the Coastal Plain of North Carolina.