Sulfur Speciation Analyses of Heated Soils using X-ray Absorption Near Edge Structure Spectroscopy

The determination of sulfur speciation changes in soils that result from wildfires is important for understanding the mobility of soil mercury (Hg), which is most strongly bound to reduced sulfur ligands. The transport of leached sulfate and Hg particulates to water bodies is enhanced after a fire as a result of increased susceptibility of soils to erosion, and could increase the production of toxic methyl mercury. X-ray absorption near-edge structure spectroscopy was used to measure the changes in sulfur speciation of soils collected from near Vallecito Reservoir (Durango, CO), an area affected by the 2002 Missionary Ridge wildfire. A soil sample from a burned area in this region was heated at 225°C for 120 min. Separate aliquots of another soil from an unburned area were heated in a muffle furnace for 120 min at 25, 150, 175, and 225°C, and at 225°C for 15, 30, and 345 min. In general, the fraction of sulfate increased by 71 to 209%, and the sulfoxide, reduced aliphatic sulfur, sulfonate, and sulfone fractions decreased by 80 to 98%, 55 to 91%, 40 to 49%, and 23 to 39%, respectively. The fraction of aromatic sulfur remained relatively unchanged. When changes in only the organic sulfur content were considered, the reduced aromatic sulfur content increased by 35 to 86%, while the sulfonate and sulfone content remained relatively the same at 21 to 25% and 5 to 6%, respectively. Sulfoxide and reduced aliphatic sulfur content decreased by 71 to 96% and 30 to 79%, respectively. The fractions of sulfite and sulfonium content were generally < 2% of the total sulfur. The apparent oxidation of reduced aliphatic sulfur due to heating suggests that mercury originally bound to these functional groups is likely released during a wildfire. Some mercury may persist, however, bound to reduced aromatic or aliphatic sulfur that remain in the soil or to weaker chemical functional groups in soil organic matter or minerals. Mercury(II) may therefore be more readily mobilized during precipitation events and, in addition to the increase in oxidized sulfur that provides a leachable source of sulfate, may possibly contribute to enhanced methylation in nearby surface waters.