Origin, Reactivity, and Bioavailability of Mercury in Wildfire Ash

Abstract

Wildfires are expected to become more frequent and intensive at the global scale due to climate change. Many studies have focused on the loss of mercury (Hg) from burned forests; however, little is known about the origins, concentration, reactivity, and bioavailability of Hg in residual ash materials in postfire landscapes. We examine Hg levels and reactivity in black ash (BA, low burn intensity) and white ash (WA, high burn intensity) generated from two recent northern California wildfires and document that all ash samples contained measurable, but highly variable, Hg levels ranging from 4 to 125 ng/g dry wt. (n = 28). Stable Hg isotopic compositions measured in select ash samples suggest that most Hg in wildfire ash is derived from vegetation. Ash samples had a highly variable fraction of Hg in recalcitrant forms (0−75%), and this recalcitrant Hg pool appears to be associated with the black carbon fraction in ash. Both BA and WA were found to strongly sequester aqueous inorganic Hg but not gaseous elemental Hg under controlled conditions. During anoxic ash incubation with natural surface water, we find that Hg in most ash samples had a minimal release and low methylation potential. Thus, the formation of wildfire ash can sequester Hg into relatively nonbioavailable forms, attenuating the potentially adverse effects of Hg erosion and transport to aquatic environments along with eroded wildfire ash. See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ■ REFERENCES (1) Giglio, L.; Randerson, J. T.; Van der Werf, G. R.; Kasibhatla, P. S.; Collatz, G. J.; Morton, D. C.; DeFries, R. S. Assessing variability and long-term trends in burned area by merging multiple satellite fire products. Biogeosciences 2010, 7, 1171−1186. (2) Williams, A. A. J.; Karoly, D.; Tapper, N. The sensitivity of Australian fire danger to climate change.

Difference in characteristics and nutrient retention between biochars produced in nitrogen‐flow and air‐limitation atmospheres

“…The nutrient (K, Ca, Mg, and P) content in BCs derived from NF and AL pyrolysis are shown in Supplemental Figure S3. The observation was consistent with our expectation of increasing nutrients content in BCs with higher pyrolysis temperature (Ku et al., 2018; Zheng et al., 2013) because K, Ca, Mg, and P are predominantly in the form of the inorganic minerals [e.g., K 2 Mg(PO 3 ) 4 , CaMg(CO 3 ) 2 , and K 2 Ca(SO 4 ) 2 ] (Liu et al., 2017; Zheng et al., 2013) and enriched in BCs during pyrolysis, whereas a high pyrolysis temperature enables the C, H, and O oxidized/gasified to form volatile matters, CO 2 , CO, and water vapor (Kozlov, Svishchev, Donskoy, & Shamansky, 2015; Ortiz, Torres, Zalazar, Zhang, & Mazza, 2020), which decreases the retention of C, H, and O in BCs after pyrolysis (Figure 1d; Supplemental Figure S4). Hence, K, Ca, Mg, and P contents in BCs indicate an increasing temperature trend.…”

Isotopic Analyses Fingerprint Sources of Polycyclic Aromatic Compound-Bearing Dust in Athabasca Oil Sands Region Snowpack

“…Ash samples were collected using a stainless steel spoon from random locations within a 1 m 2 area, 25 and represent a mix of white and black ash. 26 The sampling spoon was rinsed with Milli-Q water between sampling sites. The concentrations of PAHs at these sites (Figure 1) and other ground ash samples are reported by Kohl et al 25 Another potential source was also initially considered: haul road dust collected from the surfaces and roofs of transport vehicles in the surface mining region.…”

Big wildfires mobilize mercury. What are the risks to surface water?

“…But the weather can scramble those expectations. When heavy rainfall follows a high-severity fire, sediment and ash are washed into streams as the system is "flushed out," explains Martin Tsz-Ki Tsui, a biogeochemist at the University of North Carolina Greensboro who analyzed and led the interpretation of some of the Cache Creek samples (5). Those samples, collected from streams and analyzed shortly after a big storm, showed a huge increase in mercury fluxes-up to 1,000 times higher than normal, Tsui says.…”

“…The findings, Tsui says, suggest that the organic matter in black carbon creates chemical reactions with mercury that make it nonreactive (5). Thus, although mercury-laden sediments may increase in streams after fires, leading to higher mercury levels in streams, it doesn't look like methylmercury levels are likely to increase significantly, Tsui says.…”