A Critical Evaluation of NH4-Oxalate Extraction for Assessment of Fe and As in Soils.
Richard H. Loeppert and Bhajan K. Biswas. Texas A&M University, Soil and Crop Sciences Department, College Station, TX 77843
Chemical extraction is a useful methodology for addressing arsenic (As) retention, release potential and speciation in soils, as well as for assessment and management of As-contaminated sites. A commonly used extractant for assessment of Fe and As is pH 3.0 HN4+-oxalate (in the dark). In spite of its widespread use, questions remain regarding the factors impacting results by this method. The objectives of this study were to evaluate reaction kinetics and the influences of Fe oxide mineralogy, pH, light, and the presence of CaCO3 on AsIII and AsV desorption and speciation during desorption. Extractions of AsIII and AsV and dissolution of Fe from ferrihydrite with pH 3.0 HN4+-oxalate (2 hrs, in the dark) approached quantitative levels. In the dark, there was no appreciable transformation of AsIII to AsV or vice versa. Desorption of AsV was limited (< 6 %) from synthetic goethite across the entire experimental pH range of 2-9 in the dark. Under similar conditions AsIII desorption approached quantitative levels at pH 2-3 and decreased to negligible values at pH ≥ 6. Of 472 Bangladesh rice-paddy soils, HN4+-oxalate extractable Fe represented a median of 28% of the Fe-oxide Fe and 62% of total soil As, which indicates the importance of poorly ordered Fe oxide versus well crystalline Fe oxide in retention of As by the Bangladesh soils. The comparative kinetic behavior of different soils indicated some interesting trends that are related to Fe oxide mineralogy and reactivity.