Article pubs.acs.org/est
Molecular and Spectroscopic Characterization of Water Extractable Organic Matter from Thermally Altered Soils Reveal Insight into Disinfection Byproduct Precursors Kaelin M. Cawley,*,†,‡ Amanda K. Hohner,† David C. Podgorski,§ William T. Cooper,∥ Julie A. Korak,† and Fernando L. Rosario-Ortiz*,† †
Department of Civil, Environmental and Architectural Engineering, University of Colorado, 4001 Discovery Drive, Boulder, Colorado 80309-0607, United States ‡ National Ecological Observatory Network (NEON), 1685 38th Street, Suite 100, Boulder, Colorado 80301, United States § National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States ∥ Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States S Supporting Information *
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/mgC) 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.
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INTRODUCTION
water quality changes depend on many variables including topography, hydrology, weather events, and wildfire specific characteristics, such as temperature and duration, which may be confounding.16 Particularly important water quality parameters for the drinking water treatment19,20 and aquatic ecology21,22 are the concentration and character of dissolved organic matter (DOM) in a water body, which can be linked to soil WEOM content and character.23 DOM quantity and quality in source water are monitored by drinking water utilities because DOM reacts with disinfectants, such as chlorine, to form disinfection byproducts (DBPs), which are regulated by the US Environmental Protection Agency (EPA).24,25 Changes in DOM quality following a wildfire have been reported to increase DBP formation for some classes of DBPs, with nitrogen
While soil heating and solid characterization have been valuable for understanding C and N character and sequestration in soils,1−4 the characteristics, including molecular size and composition, of water extractable organic matter (WEOM) from thermally altered soils has not been well characterized. Wildfires are a modern source of natural soil heating that has received attention lately due to an increase in their frequency and duration in recent decades that is expected to continue.5−7 This increase can be attributed to climate change, extreme droughts, land disturbances, and increased fuel loads.5,7−9 Wildfires are known to severely impact the ecology of forests and grasslands by altering C and N cycling,2,10 macroinvertebrate populations,11 and vegetation cover.12,13 There are also concerns about how wildfires may alter water quality in streams and reservoirs that provide water for potable water systems.14,15 Following wildfires, alterations to water quality include increased nutrient concentrations, sediment loads, and metals concentrations, which may have negative impacts on drinking water treatment processes.16−18 However, the exact © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
October 9, 2016 November 22, 2016 December 7, 2016 December 7, 2016 DOI: 10.1021/acs.est.6b05126 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
(LCT), which was produced by running City of Boulder tap water through a large granular activated carbon bed to remove organic compounds and chlorine, while retaining the inorganic composition (pH = 7.1, alkalinity = 40.2 mgCaCO3 L−1, conductivity = 100.6 μS/cm, DOC = 0.2 mgC L−1). The leaching mixture was comprised of 100 g soil in 2 L LCT. Following 24 h of leaching (2 min of stirring followed by stagnant conditions for the remainder of the leaching) at room temperate in the dark, samples were filtered through a Whatman GF/F (0.7 μm) filter and refrigerated at 4 °C until further analysis. Concentration and Character of WEOM. DOC was measured with a UV-persulfate oxidation method (5310C, GE). UV254 was measured using a UV−vis spectrophotometer (Cary 100, Agilent Technologies) with a 1 cm path length quartz cuvette. Molecular size distribution in solution was determined using size exclusion chromatography (SEC) with a highpressure liquid chromatograph (HPLC) equipped with a protein-pak column (Waters) and UV detector (set to 280 nm) and calibrated using polystyrenesulfonate standards (PSS, 210, 1000, 4300, 6800, and 17000 Da). A 5 mM sodium sulfate and phosphate buffer solution (pH 6.8) was used as the mobile phase at a flow rate of 0.7 mL/min. Values are reported as weight-averaged molecular weight estimates relative to the PSS standards. Total iron (Fe) concentration of filtered leachates was determined at the CU Boulder Laboratory for Environmental and Geological Studies (LEGS) using an inductively coupled optical emission spectrometer method. Fluorescence excitation emission matrices (EEMs) were collected using a Horiba FluoroMax-4 spectrofluorometer over a range of excitation 240−450 nm (10 nm increment) and emission 300−600 nm (2 nm increment) and corrected for instrument optics, inner-filter effects, Raman normalized, and blank subtracted (see additional details in the Supporting Information).33,38−40 The fluorescence index (FI) is defined as the ratio of the emission intensity at 470 nm to that at 520 nm excited at 370 nm and has been related to dissolved organic matter source with higher FI values (e.g., 1.8) indicating more microbial DOM and lower FI values (e.g., 1.2) indicating more terrestrial DOM.41,42 A six component parallel factor analysis (PARAFAC) model (Matlab software, DOMFluor toolbox) was created using EEMs of surface water collected from the CLP River during 2013, the year following the High Park Fire, EEMs of leached sediments collected along the banks of the CLP River, and the leachates of laboratory burn simulations presented here (n = 135). The model resulted in noisy, low intensity residuals (10%. FT-ICR MS Methods. Leachates were acidified to pH ∼2 and solid phase extracted with PPL Bond Elut (Agilent) resin cartridges, which were eluted in methanol.46 Each extract was diluted in methanol to a final concentration of 200 μgC/mL and analyzed in triplicate by direct infusion negative-ion electrospray ionization ((−) ESI) coupled to a custom-built Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with a 9.4 T 220 mm room-temperature bore superconducting magnet (Oxford Instruments, Abingdon, U.K.).47−51 Multiple (50) time-domain acquisitions were coadded, Hanning-apodized, zero-filled once, and then Fourier transformed with a modular ICR data acquisition station (MIDAS).52 Signals were only considered if greater than 6σ root-mean-square (RMS) baseline noise. Each mass spectrum was internally calibrated with homologous series of compounds that differ in elemental composition by integer multiples of CH2 common in the majority of the spectra.53 The average mass error of all assigned formulas was 100−200 ppb after internal calibration with an average resolving power (m/ Δm50%) > 725,000 at m/z 500. Variations in total formula assigned were generally less than 5% for the triplicate analyses, while variations in relative abundances of compound classes were less than 1−2% (Table S2). Calibration of mass spectra, molecular formula assignments and van Krevelen diagrams were completed with PetroOrg© software.54 The constraints for molecular formula assignment were C, 2−100; H, 4−200; O, 1−25; N, 0−4; S, 0−1; and P, 0−1. Each molecular formula was assigned a modified aromaticity index (AImod) value, a measure of the degree of unsaturation calculated from the number of C, H, O, N, S, and P atoms in a molecular formula,55 and grouped into compound classes: black carbon = AImod ≥ 0.67, N = 0; black nitrogen = AImod ≥ 0.67, N > 0; polyphenol = AImod 0.5−0.67; unsaturated, low/high oxygen = AImod < 0.5, H/C < 1.5, O/C < 0.5 (low oxygen)/O/C 0.5−1.0 (high oxygen); aliphatic = H/C ≤ 1.5, O/C < 1.0, N = 0; and peptide = H/C ≤ 1.5, O/C < 1.0, N > 0.56,57.
Table 1. Percent Mass Loss, Dissolved Organic Carbon (DOC) Concentration, Fe Concentration, Fluorescence Index (FI), and Weight-Averaged Molecular Weight (Mw) Estimates for Low Carbon Tap Water (LCT, No Soil Added) and Unheated and Heated Soilsa sample LCT HGunheated HG-225 HG-350 HG-500 PBRunheated PBR-225 PBR-350 PBR-500
% mass loss
1.6 (±0.2) 4.7 (±0.2) 6.8 (±0.4)
2.8 (±0.9) 7.3 (±0.3) 9.2 (±0.3)
DOC (mgC L−1)
Fe (ppm)
FI
weight-averaged MW (Da)
