Application of molecular-scale spectroscopic studies to probe oxyanion reactivity in soils
Oxyanions such as sulfate, selenate, boric acid, and phosphate are important to soil chemists for both environmental and agronomic reasons. Therefore, a clear understanding of the reactions that these oxyanions undergo with mineral phases is crucial to predicting their availability, toxicity, and mobility in natural systems. A variety of molecular scale techniques, such as Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR), Extended X-ray Absorption Fine Structure (EXAFS), and X-ray Absorption Near Edge Structure (XANES) spectroscopy were employed to probe the bonding mechanisms of oxyanions on common soil components and in natural systems. The experimental systems chosen for study were: sulfate and selenate adsorption mechanisms on goethite, hematite, and amorphous iron hydroxide; boric acid adsorption on amorphous iron hydroxide; effects of sulfate on lead adsorption and desorption on goethite; and effects of alum addition on phosphate speciation in poultry litter. It was found that sulfate, selenate, and boric acid all form a mixture of outer-sphere and inner-sphere surface complexes on iron oxides, with solution pH, ionic strength, and the iron oxide chosen affecting the amount of inner-sphere vs. outer-sphere complexation.
The above studies were all conducted with model components and a single oxyanion in suspensions with sodium chloride as an electrolyte. In natural systems, many other ions such as metals, other oxyanions, organic ligands are present that may interact with oxyanions at the mineral/water interface. The effects that sulfate has on lead adsorption and desorption on goethite was studied to better understand how oxyanion chemistry may affect metal reactivity in natural systems. The overall findings were that ternary complex formation between lead and sulfate can significantly change the amount, rate, and stability of lead adsorption onto an iron oxide surface. It is also possible to utilize molecular-scale approaches to directly speciate oxyanions in soils. One such example is the study of how speciation of phosphate is affected by the addition of aluminum sulfate (alum) using XANES spectroscopy. It was learned that alum addition resulted in precipitation of aluminum hydroxides followed by adsorption of phosphates to these phases.
0481: Soil sciences