Molecular and Spectroscopic Characterization of Water Extractable Organic Matter from Thermally Altered Soils Reveal Insight into Disinfection Byproduct Precursors

Abstract

To characterize the effects of thermal-alteration on water extractable organic matter (WEOM), soil samples were heated in a laboratory at 225, 350, and 500 °C. Next, heated and unheated soils were leached, filtered, and analyzed for dissolved organic carbon (DOC) concentration, optical properties, molecular size distribution, molecular composition, and disinfection byproduct (DBP) formation following the addition of chlorine. The soils heated to 225 °C leached the greatest DOC and had the highest C- and N-DBP precursor reactivity per unit carbon compared to the unheated material or soils heated to 350 or 500 °C. The molecular weight of the soluble compounds decreased with increasing heating temperature. Compared to the unheated soil leachates, all DBP yields were higher for the leachates of soils heated to 225 °C. However, only haloacetonitrile yields (μg/mg) were higher for leachates of the soils heated to 350 °C, whereas trihalomethane, haloacetic acid and chloropicrin yields were lower compared to unheated soil leachates. Soluble N-containing compounds comprised a high number of molecular formulas for leachates of heated soils, which may explain the higher yield of haloacetonitriles for heated soil leachates. Overall, heating soils altered the quantity, quality, and reactivity of the WEOM pool. These results may be useful for inferring how thermal alteration of soil by wildfire can affect water quality.

Laboratory simulation of postfire effects on conventional drinking water treatment and disinfection byproduct formation

“…Previous work by Cawley et al () showed a consistent increase in HAN4 yields of heated soils (225 °C) relative to unheated soils (Cawley et al, ). However, litter materials were not included, which could explain the difference with the results of this study, as litter leachates (no soil) showed higher variability in organic content than soil leachates (Figure ).…”

“…Water-soluble chloropicrin precursors may form at temperatures close to 225 C, whereas HAN4 precursors could form at different temperatures or under different conditions in the natural environment. Previous work by Cawley et al (2016) showed a consistent increase in HAN4 yields of heated soils (225 C) relative to unheated soils . However, litter materials were not included, which could explain the difference with the results of this study, as litter leachates (no soil) showed higher variability in organic content than soil leachates (Figure 1).…”

“…However, litter materials were not included, which could explain the difference with the results of this study, as litter leachates (no soil) showed higher variability in organic content than soil leachates (Figure ). Furthermore, Cawley et al () suggest a shift upon heating in soil composition toward more N‐containing compounds, indicated by high‐resolution mass spectroscopy. Alternatively, for this study, heated litter, the primary source of DOM in the mixed leachates, may not contribute substantial HAN4 precursors relative to unheated litter, implying that heating alters HAN4 precursors in soil and litter layers differently.…”

“…The soil and litter samples were placed in ceramic evaporating dishes or aluminum pans in a preheated (225 °C) muffle furnace (Thermo Scientific Lindberg/Blue M) for 2 h under oxic conditions. A preliminary study was used to establish the heating temperature and duration (Cawley et al, ). Briefly, materials were heated at temperatures of 225, 350, and 500 °C and characterized for water‐soluble organic carbon.…”

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

“…After a wildfire event, elevated levels of dissolved organic matter (DOM), generally measured as DOC, in downstream of nearby water sources has been associated to heavy loads of ash and debris by several previous studies. 18,26,32 Recent studies have linked water extractable organic matter (WEOM) from burned soil 17,33 and ash 34 to the formation of disinfection byproducts (DBPs). 35,36 DBPs are of particular concern due to their effect on public health, which includes risks of bladder cancer and disruption in cell growth.…”