Article Cite This: Environ. Sci. Technol. 2019, 53, 6273−6281
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Fast Photomineralization of Dissolved Organic Matter in Acid Mine Drainage Impacted Waters Chenyi Yuan,†,⊥ Rachel L. Sleighter,‡ Linda K. Weavers,§ Patrick G. Hatcher,‡ and Yu-Ping Chin*,∥,# †
Environmental Science Graduate Program, The Ohio State University, Columbus, Ohio 43210, United States Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia 23529, United States § Department of Civil, Environmental, and Geodetic Engineering, The Ohio State University, Columbus, Ohio 43210, United States ∥ School of Earth Sciences, The Ohio State University, Columbus, Ohio 43210, United States Downloaded via UNIV OF SOUTHERN INDIANA on July 27, 2019 at 12:59:59 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Acid mine drainage (AMD) formed from pyrite (iron disulfide) weathering contributes to ecosystem degradation in impacted waters. Solar irradiation has been shown to be an important factor in the biogeochemical cycling of iron in AMDimpacted waters, but its impact on dissolved organic matter (DOM) is unknown. With a typical AMD-impacted water (pH 2.7−3) collected from the Perry State Forest watershed in Ohio, we observed highly efficient (>80%) photochemical mineralization of DOM within hours in a solar simulator resembling twice summer sunlight at 40°N. We confirmed that the mineralization was initially induced by •OH formed from FeOH2+ photodissociation and was inhibited 2-fold by dissolved oxygen removal, suggesting the importance of both the photochemical reaction and oxygen involvement. Size exclusion chromatography and Fourier transform ion cyclotron resonance mass spectrometry elucidated that any remaining organic matter was comprised of smaller and highly aliphatic compounds. The quantitative and qualitative changes in DOM are likely to constitute an important component in regional carbon cycling and nutrient release and to influence downstream aquatic ecosystems in AMD-affected watersheds.
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cycling15−17 and hydroxyl radical (•OH) production18,19 from the photolysis of ferric complexes. Although AMD-impacted waters are highly photoreactive, the phototransformation of DOM in these waters, to our knowledge, has not been previously studied, and we hypothesize that the extent of •OH generation in these special aquatic environments plays an oversized role with respect to the fate of DOM. In this article, we quantified DOM photomineralization (i.e., complete conversion to carbon dioxide or monoxide) kinetics, evaluated primary reactive species, and characterized its molecular transformation products in AMD-impacted waters sampled from the Perry State Forest watershed in southeastern Ohio. Besides irradiation inside a solar simulator, we also investigated similar DOM changes between naturally shaded water samples and naturally sunlit water samples.
INTRODUCTION Acid mine drainage (AMD) impairs more than 10 000 miles of streams (Table S1) in over half of the states in the U.S., especially in the Rocky Mountains and Appalachian regions.1−3 AMD is produced from the chemical and biological weathering of sulfide minerals (mainly pyrite, FeS2) in abandoned coal/ mineral mining areas and is rich in heavy metals (such as Fe and Mn) and sulfuric acid.4 With continuing worldwide exploitation of natural resources and ineffective mitigation operations,3 AMD remains a persistent concern to the local and regional aquatic ecosystems. Dissolved organic matter (DOM), ubiquitous in aquatic systems, is a heterogeneous mix of organic molecules derived from biological precursors. It serves important ecological roles as a part of the carbon cycle,5,6 as a substrate for heterotrophic microorganisms,7−9 and as a photoreactive component that can screen cell-damaging UV light and generate reactive intermediates that participate in a number of biogeochemical and environmental processes.10−12 DOM is also present in AMD-impacted waters at low concentrations ( 0.67) areas are marked, and AImod is the modified aromaticity index (Text S1).
initial concentration of 6 mg-C L−1 SRFA) and 4 mg-C L−1 in SP (initial concentration of 5.4 mg-C L−1 in SP) water (Figure 1a). Therefore, electron acceptors other than Fe(III) must also participate in the secondary mineralization of DOM following the photomineralization of DOM mediated by Fe (III) reduction. Dissolved oxygen may be the most important electron acceptor besides Fe(III). In surface waters at equilibrium with the atmosphere (∼300 μM aq O2), we estimate that O2 can mineralize 300 μM or 3.6 mg-C L−1 of DOM based upon redox stoichiometry (eq S19, Table S8). In a study that examined the photodegradation of 2,4-dichlorophenoxy acetic acid (pH 2.8, Fe = 1.0 mM), the presence of O2 enhanced its mineralization by a factor of 4.59 We observed an enhancement by about a factor of 2 for mineralized DOC in air-saturated SP water relative to argon-saturated SP water, and the enhancement was quantitatively attributed to the change in Fe(III) and oxygen levels (Figures 1c and Figure S10). We observed that even in the oxic experiments, the oxygen concentration in the sealed phototubes dropped dramatically (as much as 80%) as a result of photolysis in SP water (Figure S10a). While in the field, where oxygen is continuously replenished by exchange with the atmosphere, a higher enhancement in DOM mineralization is possible compared to what was determined in sealed phototubes. Reaction involving O2, •OH, and DOM could produce organoperoxyl radicals (eqs S9 and S13, Table S8), which may be the first step in the oxygen-dependent secondary transformation of DOM.59 These organoperoxyl radicals may be involved in the reoxidation of Fe(II) back to Fe(III) or in the generation of H2O2 to produce more •OH via reaction with Fe(II) (eqs S15−S18, Table S8).59 Characterizing DOM Phototransformation Products. While most of the target DOM used in this study was mineralized to inorganic carbon, a recalcitrant fraction remained after photolysis and is likely similar in composition to DOM in EL. We determined compositional differences between DOM samples extracted from the sunlit EL and shaded SP waters by FTICR-MS and SEC. DOM was isolated by SPE to remove the undesired high salt content60 and to increase its concentration for the assays. While SPE is a
impacted waters, its photoproduct, SO4•−, is likely a minor contributor to DOM transformation. Light Absorption by DOM and Its Direct Photolysis. Absorption of sunlight by DOM itself can, directly or through generated reactive species, transform DOM51,52 to smaller and/or labile organic compounds53,54 and mineralize it to inorganic carbon.55−57 Although DOM is one of the most important light attenuators in natural waters, the fraction of light absorbed by DOM in AMD-impacted waters is relatively insignificant due to its low abundance relative to lightabsorbing Fe species (Figures 2 and Figure S5). Reported quantum yields of DOM photomineralization typically decrease exponentially with increasing wavelengths, and the averaged quantum yields in the UV portion of sunlight are on the order of 10−4−10−5 for fresh water35,58 and seawater.55 Through integration of the product of absorbed light and quantum yield over 290−400 nm, the rate of direct photomineralization of DOM would be in the range of 4 × 10−4−8 × 10−4 mg-C L−1 h−1 in EL solution spiked with 6 mgC L−1 SRFA under our experimental conditions. This rate is at least 3 orders of magnitude lower than our observed mineralization value (2.4 mg-C L−1 h−1, Figure 1a) in the photic zone, and we surmise that the direct photolysis of DOM would be of minimum importance in AMD waters, and the observed DOM photomineralization was initiated by indirect photolysis. Secondary Reactions Initiated by Photochemical Processes. The aforementioned major light absorption process that leads to reaction with DOM, i.e., light absorption by FeOH2+, requires Fe(III) to be an electron acceptor for the mineralization/oxidation of DOM. If we assume that the oxidation states of carbon in DOM is approximately zero8 and Fe(III) is the only electron acceptor, approximately 0.25 mol of CO2 is produced for every mole of Fe(III) reduced based on redox stoichiometry (eq S20, Table S8). Thus, we anticipate that for the SRFA-spiked EL sample and native SP waters, a maximum of 0.9 and 1.9 mg-C L−1 of DOM would be mineralized, respectively, based upon the initial Fe(III) levels present in each sample. In contrast, we observed photomineralization of 5 mg-C L−1 in EL water (spiked with an 6278
DOI: 10.1021/acs.est.9b00202 Environ. Sci. Technol. 2019, 53, 6273−6281
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Environmental Science & Technology
exceedingly high levels of •OH, accelerated by dissolved oxygen, and was highly dependent on FeOH2+ speciation and other water substituents. Season, weather, water depth, and hydrology of the impacted lake/stream will also greatly impact the actual mineralization rate. Beyond the AMD-impacted waters used in this study, similar DOM phototransformation might be expected in other highly acidic Fe-rich waters, such as those impacted by natural airborne acidic fumigation in extreme environments67 or the general acid rock drainage and waters released from FeS2-containing sediments.68 We also demonstrated that any unmineralized DOM was transformed to smaller and highly aliphatic compounds, whose effect on AMD-impacted watersheds (especially downstream ecosystems) necessitates further research.
necessary sample preparation step, we recognize that it only partially recovers the DOM in our samples (∼50%) and, as such, limits our ability to completely characterize the refractory material.20 SEC of extracted DOM (Figure 4a) clearly shows the shift in the size of the major light-absorbing (λ = 224 nm) molecules from about 1000 Da in SP to the lower size limit of our column (about 100 Da) in EL. Consistently, FTICR-MS showed a difference in both the number of peaks (a 60% decrease between SP and EL DOM, respectively) and numberaveraged m/z for the DOM in EL (467 Da) relative to SP (523 Da) (Tables S6 and S7). Although both SEC and FTICR-MS may detect different DOM pools within their own limitations, the significant and consistent results from the two independent analytical techniques corroborate our observations regarding DOM size. Thus, the resident DOM in sunlit EL consists of smaller compounds and undetectable moieties, likely as a result of extensive photolysis in the lake compared to the shaded SP sample. Compared to SP, EL had a higher H/C ratio and lower double bond equivalents (DBE) (Table S7) and possesses an unusually highly aliphatic content as qualitatively visualized in van Krevelen diagrams (Figures 4b and S6). Aromatic and other light absorbing components were largely transformed to aliphatic moieties upon irradiation. The photolability of aromatic compounds (including its susceptibility by •OH) has been widely recognized by either MS-based techniques or more simple optical techniques (e.g., specific UV absorbance, spectral slope, etc.) in both whole waters and solutions of DOM isolates upon solar irradiation.8,61−64 Low molecular weight acids have also been shown to be formed from reactions between DOM and •OH.8,11 The O/C values decreased from SP to EL (Figure 4b and Table S7), indicating oxygen-based functional groups were degraded upon solar irradiation. In order to assess whether compositional differences between SPDOM and ELDOM are caused by photolysis, we irradiated SP whole waters for 3 or 6 h and found that remaining SPDOM has a similar MS spectrum as ELDOM (Tables S6 and S7, Figures S7 and S8); i.e., aromatic, oxygenrich compounds underwent degradation, and aliphatic compounds remained. Unlike ELDOM, the irradiated SP sample also revealed a high abundance of newly formed highintensity S-containing peaks (Table S6, Figures S7 and S8), suggesting the incorporation of S in DOM molecules. Possible mechanisms include esterification by sulfuric acid65 and SO4•− addition to double bonds66 to form organosulfates. Combining the product molecular signature and the photochemically induced reactive species, we proposed that in sunlit AMD-impacted waters, aromatic organic matters react with photogenerated •OH and subsequently other reactive oxygen species, resulting in OH addition, oxidative radical reaction, ring opening, bond cleavage, and decarboxylation, as depicted by previous studies from other aquatic systems.51,61 In this process, a large amount of inorganic carbon and nutrients, such as N and P, could be released, which might impact the ecosystem.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b00202.
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Materials, experimental procedures, FTICR-MS spectra, and other supplementary data (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Linda K. Weavers: 0000-0002-7956-7901 Yu-Ping Chin: 0000-0003-1427-9156 Present Addresses ⊥
Oak Ridge Institute for Science and Education (ORISE), hosted at National Exposure Research Laboratory, U.S. Environmental Protection Agency, Athens, GA, 30605 # Department of Civil and Environmental Engineering, University of Delaware, Newark, DE, 19716 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding was provided by the John C. Geupel endowment in the Department of Civil, Environmental, and Geodetic Engineering at the Ohio State University. We thank Perry State Forest for providing access to AMD waters. We thank Cody Chandler, Carissa Hipsher, and Jeff Hudson for helping with AMD water sampling; Sue Welch for IC measurement; and Franklin (Sandy) Jones for freeze-drying DOM samples. We also thank Kimberly Parker for discussion and the four anonymous reviewers for their suggestions to improve the manuscript.
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REFERENCES
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IMPLICATIONS FOR FUTURE WORK We discovered near complete mineralization of DOM on a time scale of hours under sunlight in the photic zone of the studied AMD-impacted waters in our phototubes. Our estimates indicate a maximum of 1 and 0.5% DOM could be mineralized in a sunny day in midsummer for a lake with a depth of 10 m and with water properties resembling EL and SP, respectively. This process was initiated by the formation of 6279
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