Metal Reactivity in Laboratory Burned Wood from a Watershed Affected by Wildfires

We investigated interfacial processes affecting metal mobility by wood ash under laboratory-controlled conditions using aqueous chemistry, microscopy, and spectroscopy. The Valles Caldera National Preserve in New Mexico experiences catastrophic wildfires of devastating effects. Wood samples of Ponderosa Pine, Colorado Blue Spruce, and Quaking Aspen collected from this site were exposed to temperatures of 60, 350, and 550 °C. The 350 °C Pine ash had the highest content of Cu (4997 ± 262 mg kg^–1), Cr (543 ± 124 mg kg^–1), and labile dissolved organic carbon (DOC, 11.3 ± 0.28 mg L^–1). Sorption experiments were conducted by reacting 350 °C Pine, Spruce, and Aspen ashes separately with 10 μM Cu­(II) and Cr­(VI) solutions. Up to a 94% decrease in Cu­(II) concentration was observed in solution while Cr­(VI) concentration showed a limited decrease (up to 13%) after 180 min of reaction. X-ray photoelectron spectroscopy (XPS) analyses detected increased association of Cu­(II) on the near surface region of the reacted 350 °C Pine ash from the sorption experiments compared to the unreacted ash. The results suggest that dissolution and sorption processes should be considered to better understand the potential effects of metals transported by wood ash on water quality that have important implications for postfire recovery and response strategies.