Influence of Organic Ligands on the Redox Properties of Fe(II) as Determined by Mediated Electrochemical Oxidation.

Fe(II) has been extensively studied due to its importance as a reductant in biogeochemical processes and contaminant attenuation. Previous studies have shown that ligands can alter aqueous Fe(II) redox reactivity but their data interpretation is constrained by the use of probe compounds. Here, we employed mediated electrochemical oxidation (MEO) as an approach to directly quantify the extent of Fe(II) oxidation in the absence and presence of three model organic ligands (citrate, nitrilotriacetic acid, and ferrozine) across a range of potentials (EH) and pH, thereby manipulating oxidation over a broad range of fixed thermodynamic conditions. Fe(III)-stabilizing ligands enhanced Fe(II) reactivity in thermodynamically unfavorable regions (i.e., low pH and EH) while an Fe(II) stabilizing ligand (ferrozine) prevented oxidation across all thermodynamic regions. We experimentally derived apparent standard redox potentials, EHϕ, for these and other (oxalate, oxalate2, NTA2, EDTA, and OH2) Fe-ligand redox couples via oxidative current integration. Preferential stabilization of Fe(III) over Fe(II) decreased EHϕ values, and a Nernstian correlation between EHϕ and log(KFe(III)/KFe(II)) exists across a wide range of potentials and stability constants. We used this correlation to estimate log(KFe(III)/KFe(II)) for a natural organic matter isolate, demonstrating that MEO can be used to measure iron stability constant ratios for unknown ligands.

Fast Photomineralization of Dissolved Organic Matter in Acid Mine Drainage Impacted Waters.

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 AMD-impacted 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.

High Pressure Size Exclusion Chromatography (HPSEC) Determination of Dissolved Organic Matter Molecular Weight Revisited: Accounting for Changes in Stationary Phases, Analytical Standards, and Isolation Methods.

We reassessed the molecular weight of dissolved organic matter (DOM) determined by high pressure size exclusion chromatography (HPSEC) using measurements made with different columns and various generations of polystyrenesulfonate (PSS) molecular weight standards. Molecular weight measurements made with a newer generation HPSEC column and PSS standards from more recent lots are roughly 200 to 400 Da lower than initial measurements made in the early 1990s. These updated numbers match DOM molecular weights measured by colligative methods and fall within a range of values calculated from hydroxyl radical kinetics. These changes suggest improved accuracy of HPSEC molecular weight measurements that we attribute to improved accuracy of PSS standards and changes in the column packing. We also isolated DOM from wetlands in the Prairie Pothole Region (PPR) using XAD-8, a cation exchange resin, and PPL, a styrene-divinylbenzene media, and observed little difference in molecular weight and specific UV absorbance at 280 nm (SUVA280) between the two solid phase extraction resins, suggesting they capture similar DOM moieties. PPR DOM also showed lower SUVA280 at similar weights compared to DOM isolates from a global range of environments, which we attribute to oxidized sulfur in PPR DOM that would increase molecular weight without affecting SUVA280.

Abundant carbon substrates drive extremely high sulfate reduction rates and methane fluxes in Prairie Pothole Wetlands.

Inland waters are increasingly recognized as critical sites of methane emissions to the atmosphere, but the biogeochemical reactions driving such fluxes are less well understood. The Prairie Pothole Region (PPR) of North America is one of the largest wetland complexes in the world, containing millions of small, shallow wetlands. The sediment pore waters of PPR wetlands contain some of the highest concentrations of dissolved organic carbon (DOC) and sulfur species ever recorded in terrestrial aquatic environments. Using a suite of geochemical and microbiological analyses, we measured the impact of sedimentary carbon and sulfur transformations in these wetlands on methane fluxes to the atmosphere. This research represents the first study of coupled geochemistry and microbiology within the PPR and demonstrates how the conversion of abundant labile DOC pools into methane results in some of the highest fluxes of this greenhouse gas to the atmosphere ever reported. Abundant DOC and sulfate additionally supported some of the highest sulfate reduction rates ever measured in terrestrial aquatic environments, which we infer to account for a large fraction of carbon mineralization in this system. Methane accumulations in zones of active sulfate reduction may be due to either the transport of free methane gas from deeper locations or the co-occurrence of methanogenesis and sulfate reduction. If both respiratory processes are concurrent, any competitive inhibition of methanogenesis by sulfate-reducing bacteria may be lessened by the presence of large labile DOC pools that yield noncompetitive substrates such as methanol. Our results reveal some of the underlying mechanisms that make PPR wetlands biogeochemical hotspots, which ultimately leads to their critical, but poorly recognized role in regional greenhouse gas emissions.

Isoproturon Reappearance after Photosensitized Degradation in the Presence of Triplet Ketones or Fulvic Acids.

Isoproturon (IPU) is a phenylurea herbicide used to control broad-leaf grasses on grain fields. Photosensitized transformation induced by excited triplet states of dissolved organic matter (3DOM*) has been identified as an important degradation pathway for IPU in sunlit waters, but the reappearance of IPU in the absence of light is observed after the initial photolysis. In this study, we elucidate the kinetics of this photodegradation and dark-reappearance cycling of IPU in the presence of DOM proxies (aromatic ketones and reference fulvic acids). Using mass spectrometry and nuclear magnetic resonance spectroscopic techniques, a semi-stable intermediate (IPUint) was found to be responsible for IPU reversion and was identified as a hydroperoxyl derivative of IPU. IPUint is photogenerated from incorporation of diatomic oxygen to IPU and is subjected to thermolysis whose rate depends on temperature, pH, the presence of DOM, and inorganic ions. These results are important to understand the overall aquatic fate of IPU and structurally similar compounds under diurnal conditions.

Influence of Temperature, Relative Humidity, and Soil Properties on the Soil-Air Partitioning of Semivolatile Pesticides: Laboratory Measurements and Predictive Models.

Soil-air partition coefficient (Ksoil-air) values are often employed to investigate the fate of organic contaminants in soils; however, these values have not been measured for many compounds of interest, including semivolatile current-use pesticides. Moreover, predictive equations for estimating Ksoil-air values for pesticides (other than the organochlorine pesticides) have not been robustly developed, due to a lack of measured data. In this work, a solid-phase fugacity meter was used to measure the Ksoil-air values of 22 semivolatile current- and historic-use pesticides and their degradation products. Ksoil-air values were determined for two soils (semiarid and volcanic) under a range of environmentally relevant temperature (10-30 °C) and relative humidity (30-100%) conditions, such that 943 Ksoil-air measurements were made. Measured values were used to derive a predictive equation for pesticide Ksoil-air values based on temperature, relative humidity, soil organic carbon content, and pesticide-specific octanol-air partition coefficients. Pesticide volatilization losses from soil, calculated with the newly derived Ksoil-air predictive equation and a previously described pesticide volatilization model, were compared to previous results and showed that the choice of Ksoil-air predictive equation mainly affected the more-volatile pesticides and that the way in which relative humidity was accounted for was the most critical difference.

Triplet photochemistry of effluent and natural organic matter in whole water and isolates from effluent-receiving rivers.

Effluent organic matter (EfOM), contained in treated municipal wastewater, differs in composition from naturally occurring dissolved organic matter (DOM). The presence of EfOM may thus alter the photochemical production of reactive intermediates in rivers that receive measurable contributions of treated municipal wastewater. Quantum yield coefficients for excited triplet-state OM (3OM*) and apparent quantum yields for singlet oxygen (1O2) were measured for both whole water samples and OM isolated by solid phase extraction from whole water samples collected upstream and downstream of municipal wastewater treatment plant discharges in three rivers receiving differing effluent contributions: Hockanum R., CT (22% (v/v) effluent flow), E. Fork Little Miami R., OH (11%), and Pomperaug R., CT (6%). While only small differences in production of these reactive intermediates were observed between upstream and downstream whole water samples collected from the same river, yields of 3OM* and 1O2 varied by 30-50% between the rivers. Apparent quantum yields of 1O2 followed similar trends to those of 3OM*, consistent with 3OM* as a precursor to 1O2 formation. Higher 3OM* reactivity was observed for whole water samples than for OM isolates of the same water, suggesting differential recoveries of photoreactive moieties by solid phase extraction. 3OM* and 1O2 yields increased with increasing E2/E3 ratio (A254 nm divided by A365 nm) and decreased with increasing electron donating capacities of the samples, thus exhibiting trends also observed for reference humic and fulvic acid isolates. Mixing experiments with EfOM and DOM isolates showed evidence of quenching of triplet DOM by EfOM when measured yields were compared to theoretical yields. Together, the results suggest that effluent contributions of up to 25% (v/v) to river systems have a negligible influence on photochemical production of 3OM* and 1O2 apparently because of quenching of triplet DOM by EfOM. Furthermore, the results highlight the importance of whole water studies for quantifying in situ photoreactivity, particularly for 3OM*.

Partitioning of polybrominated diphenyl ethers to dissolved organic matter isolated from Arctic surface waters.

Polybrominated diphenyl ethers (PBDEs) are a class of brominated flame retardant that is distally transported to the Arctic. Little is known about the fate of PBDEs in Arctic surface waters, especially in the presence of dissolved organic matter (DOM). DOM has been shown to interact with hydrophobic organic contaminants and can alter their mobility, bioavailability, and degradation in the environment. In this study, the partitioning of six PBDE congeners between Arctic DOM (isolated via solid phase extraction) and water was measured using the aqueous solubility enhancement method. Measured dissolved organic carbon (DOC)-water partition coefficient (KDOC) values were nearly an order of magnitude lower than previously reported values for the same PBDE congeners in soil or commercial organic matter, ranging from 10(3.97) to 10(5.16) L kg(-1) of organic carbon. Measured results compared favorably with values calculated using polyparameter linear free energy models for Suwannee River fulvic acid. Log KDOC values increased with increasing PBDE hydrophobicity. Slightly lower than expected values were observed for the highest brominated congeners, which we attribute to steric hindrance. This study is the first to comprehensively measure KDOC values for a range of PBDE congeners with DOM isolated from Arctic surface waters.

Transformation of natural and synthetic estrogens by maize seedlings.

In agricultural fields, crop plants may transform or degrade hormonally active compounds in manure used as fertilizer and thereby affect the overall endocrine-disrupting activity of agricultural runoff. This study examined the transformation of two natural steroid estrogens [17ß-estradiol (17ß-E2) and estrone (E1)] and two synthetic estrogen mimics [zeranol (α-ZAL) and zearalanone (ZAN)] by maize seedlings. Growing whole maize seedlings in hydroponic solutions of target estrogens resulted in both oxidative (i.e., 17ß-E2 to E1 and α-ZAL to ZAN) and reductive (i.e., E1 to 17ß-E2 and ZAN to α-ZAL) transformations. Although all four estrogens accumulated in maize roots as both parents and products, the shoots contained only 17ß-E2 and α-ZAL, regardless of whether they were the parent or the product. Crude plant enzyme extracts led to substantial reductive transformations but created only trace amounts of oxidation products. In contrast, only oxidative transformations occurred in solutions exposed to plant-associated microbes. Thus, the combined effects of plant enzymes and plant-associated microbes account for the reversible transformations observed with whole plants. These effects are expected to generally decrease the overall estrogenicity of runoff from manure-fertilized fields.

An improved screening tool for predicting volatilization of pesticides applied to soils.

Pesticide volatilization and vapor drift can have adverse effects on nontarget, sensitive ecosystems and human health. Four approaches for pesticide volatilization screening based on Fick's Law were investigated. In each approach, vapor pressures or environmentally relevant partition coefficients were used to describe pesticide behavior in an agricultural field system and to predict 24-h cumulative percentage volatilization (CPV(24h)) losses. The multiphase partitioning approach based on soil-air (K(soil-air)) and water-air (K(water-air)) partition coefficients was found to most accurately model literature-reported pesticide volatilization losses from soils. Results for this approach are displayed on chemical space diagrams for sets of hypothetical K(soil-air) and K(water-air) combinations under different temperature, relative humidity, and soil organic carbon conditions. The CPV(24h) increased with increasing temperature and relative humidity and with decreasing soil organic carbon content. Pesticides and the conditions under which the greatest volatilization losses exist were easily identified using this visual screening technique.

Chlorpyrifos fate in the Arctic: Importance of analyte structure in interactions with Arctic dissolved organic matter.

The insecticide and current use pesticide chlorpyrifos (CLP) is transported via global distillation to the Arctic where it may pose a threat to this ecosystem. CLP is readily detected in Arctic environmental compartments, but current research has not studied its partitioning between water and dissolved organic matter (DOM) nor the role of photochemistry in CLP's fate in aquatic systems. Here, the partition coefficients of CLP were quantified with various types of DOM isolated from the Arctic and an International Humic Substances Society (IHSS) reference material Suwannee River natural organic matter (SRNOM). While CLP readily partitions to DOM, CLP exhibits a significantly higher binding constant with Arctic lacustrine DOM relative to fluvial DOM or SRNOM. The experimental partitioning coefficients (KDOC) were compared to a calculated value estimated using poly parameter linear free energy relationship (pp-LFER) and was found to be in good agreement with SRNOM, but none of the Arctic DOMs. We found that Arctic KDOC values decrease with increasing SUVA254, but no correlations were observed for the other DOM compositional parameters. DOM also mediates the photodegradation of CLP, with stark differences in photo-kinetics using Arctic DOM isolated over time and space. This work highlights the chemo-diversity of Arctic DOM relative to IHSS reference materials and highlights the need for in-depth characterization of DOM that transcends the current paradigm based upon terrestrial and microbial precursors.

Potential for abiotic reduction of pesticides in Prairie pothole porewaters.

Prairie pothole lakes (PPLs) are critical hydrological and ecological components of central North America and represent one of the largest inland wetland systems on Earth. These lakes are located within an agricultural region, and many of them are subject to nonpoint-source pesticide pollution. Limited attention, however, has been paid to understanding the impact of PPL water chemistry on the fate and persistence of pesticides. In this study, the abiotic reductive transformation of seven dinitroaniline pesticides was investigated in PPL sediment porewaters containing naturally abundant levels of reduced sulfur species (i.e., bisulfide (HS(-)) and polysulfides (S(n)(2-))) and dissolved organic matter (DOM). Target dinitroanilines underwent rapid degradation in PPL porewaters and were transformed into corresponding amine products. While the largest fraction of the transformation could be attributed to reduced sulfur species, experimental evidence suggested that other reactive entities in PPL porewaters, such as DOM and mineral phases, might also affect the reaction rates of dinitroanilines. Results from this study highlight the importance of reductive transformation as an abiotic natural attenuation pathway for pesticides entering the PPL sedimentary environment.

Activation of persulfate by humic substances: Stoichiometry and changes in the optical properties of the humic substances.

Persulfate activation through electron transfer from humic substances (HS) was investigated. Persulfate consumption in the presence of standard HS and HS model compounds linearly correlated with the phenol contents of the HS. Redox-active carbonyl groups such as aromatic ketones and quinone also contributed to persulfate consumption by donating electrons while being reduced. Phenols activated persulfate through direct electron transfer from the phenolate forms but reduced ketones activated persulfate through reactions between their organic radicals and persulfate. Persulfate was activated more by terrestrially derived aquatic HS containing large numbers of phenol groups than by other species, and this caused more benzene oxidation to occur in the presence of terrestrially derived aquatic HS than in the presence of other species. Larger amounts of sulfate radicals were scavenged by soil-derived HS than other types of HS because soil-derived HS were composed of larger molecules than other types of HS. The fluorescence regional integration volume for HS reacted with persulfate linearly correlated with persulfate consumption. Decreases in the fluorescence regional integration value could be used to predict persulfate activation through electron transfer from HS to persulfate if the electron-donating capacity cannot be determined. Persulfate activation by HS is expected to be stoichiometrically more advantageous than conventional persulfate-Fe2+ processes when treating an aquifer containing large amounts of electron-rich HS.

Spatial distribution and biogeochemistry of redox active species in arctic sedimentary porewaters and seeps.

Redox active species in Arctic lacustrine sediments play an important, regulatory role in the carbon cycle, yet there is little information on their spatial distribution, abundance, and oxidation states. Here, we use voltammetric microelectrodes to quantify the in situ concentrations of redox-active species at high vertical resolution (mm to cm) in the benthic porewaters of an oligotrophic Arctic lake (Toolik Lake, AK, USA). Mn(II), Fe(II), O2, and Fe(III)-organic complexes were detected as the major redox-active species in these porewaters, indicating both Fe(II) oxidation and reductive dissolution of Fe(III) and Mn(IV) minerals. We observed significant spatial heterogeneity in their abundance and distribution as a function of both location within the lake and depth. Microbiological analyses and solid phase Fe(III) measurements were performed in one of the Toolik Lake cores to determine the relationship between biogeochemical redox gradients and microbial communities. Our data reveal iron cycling involving both oxidizing (FeOB) and reducing (FeRB) bacteria. Additionally, we profiled a large microbial iron mat in a tundra seep adjacent to an Arctic stream (Oksrukuyik Creek) where we observed Fe(II) and soluble Fe(III) in a highly reducing environment. The variable distribution of redox-active substances at all the sites yields insights into the nature and distribution of the important terminal electron acceptors in both lacustrine and tundra environments capable of exerting significant influences on the carbon cycle.

Pesticide processing potential in prairie pothole porewaters.

Prairie pothole lakes (PPLs) are located within the extensively farmed Great Plains region of North America, and many are negatively impacted by nonpoint source pesticide pollution. To date, the environmental fate of pesticides in these lakes remains largely unknown. In this study, two PPLs in the Cottonwood Lake area of North Dakota were sampled, and transformations of four chloroacetanilide pesticides in sediment porewaters were examined. The reduced sulfur species in the porewaters, such as bisulfide (HS(-)) and polysulfides (S(n)(2-)), readily transformed the target pesticides into sulfur-substituted products. Although HS(-) and S(n)(2-) played a dominant role, other reactive constituents in PPL porewaters also contributed to the transformation. Results from this study revealed that abiotic reactions with reduced sulfur species could represent an important removal pathway for pesticides entering PPLs.

Direct and indirect photolysis of polycyclic aromatic hydrocarbons in nitrate-rich surface waters.

The photolysis of three polycyclic aromatic hydrocarbons (PAHs)-pyrene, phenanthrene, and naphthalene-were studied in waters taken from creosote-contaminated sites in Gary (IN, USA) and Wilmington (NC, USA). Direct photolysis of all PAHs was observed under simulated solar radiation, with pyrene degrading at a faster rate than either phenanthrene or naphthalene. Phenanthrene degradation, when compared to its direct photolysis rate, increased in Gary water but decreased in Wilmington water. Analysis of the waters for dissolved organic carbon (DOC) and nitrate revealed higher levels of DOC in the Wilmington sample (9.29 mg/L) compared with the Gary sample (6.73 mg/L), as well as significantly less nitrate (0.046 mM vs 0.205 mM for the Gary sample). The slightly lower rate of phenanthrene degradation observed for the Wilmington sample, corrected for light attenuation effects, is statistically the same as that in the direct photolysis experiments. Therefore, we attribute the lower rate of degradation in the presence of Wilmington water to light screening by DOC, but we believe the faster reaction rate observed for the Gary water results from hydroxyl radical (OH*) chemistry generated by nitrate photolysis. Indeed, degradation of the target compound increased when nitrate (at 0.2 and 0.4 mM) was added to the Wilmington sample, further corroborating this conclusion. Overall photoreaction rates decreased for the lower-molecular-weight PAHs, because the fastest naphthalene photolytic rate was roughly two orders of magnitude slower than that of pyrene.

Photochemical acetochlor degradation induced by hydroxyl radical in Fe-amended wetland waters: Impact of pH and dissolved organic matter.

Iron (Fe) plays a critical role in the formation of hydroxyl radical (OH) which may participate in the indirect photodegradation of aquatic contaminants. While Fe photochemistry has been extensively studied, the efficacy of iron amendments for contaminant attenuation in sunlit natural waters has not been well researched. We studied the efficacy of this approach by monitoring OH induced acetochlor (AC) degradation and determining OH production rates with terephthalate (TPA) as a probe. Surface wetland waters as well as model fulvic acid (FA) solutions were amended with Fe(III) salt at different concentrations at pH values of 2.7, 5, and 7.6. We observed no significant enhancement in the AC degradation rate at circumneutral pH. At pH 5, AC degradation increased by more than 50% with an Fe addition up to an [Fe]T ≈ 6 µM and plateaued at high [Fe]T. At the highly acidic pH of acid mine drainage (AMD) waters, AC degradation was enhanced by two-orders-of magnitude with increasing [Fe]T and no plateau was observed under the conditions tested ([Fe]T ≤ 500 µM). While the Fe induced relative difference in OH production rates determined using TPA was useful in elucidating the reaction mechanism for different dissolved organic matter types at different pH values, the absolute value of OH production rates over-predicted the transformation of AC suggesting the existence of unknown side reactions and/or alternative reactive intermediates.

Reciprocal influences of dissolved organic matter and nanosized zero-valent iron in aqueous media.

We investigated concurrent effects between nano-sized zero-valent iron (NZVI) and dissolved organic matter (DOM). Specific UV absorbance of DOM revealed that aromatic/hydrophobic moieties of DOM were bounded to NZVI surfaces. The DOM fluorescence emission peak shifted toward lower wavelength after NZVI exposure, which indicated removal of aromatic DOM fractions. This blue shift of the emission peak also attributes to the reduction of electron acceptors through NZVI-DOM charge transfer complexes. High molecular weight (103-104 Da) DOM fractions, which are suspected to be both aromatic and hydrophobic, were removed. X-ray absorption spectroscopy (XAS) elucidated that Fe(0) content in the 30-d aged NZVI in the presence of DOM (61.6%) was substantially higher than that in the absence of DOM (25.0%). Corrosion and oxidation of NZVI were mitigated due to interruption of electron transfer by surface bounded DOM and stabilization of Fe(II) by Fe-DOM complexes. The XAS also revealed that the evolution of the iron (oxyhydr)oxide shell of NZVI was significantly altered by complexed aromatic DOM.

Quantifying the electron donating capacities of sulfide and dissolved organic matter in sediment pore waters of wetlands.

Electron donating capacity (EDC) values were determined for a set of pore water samples collected from the sediments of four separate wetlands in the Cottonwood Lakes Study Area in Jamestown, ND by mediated electrochemical analysis, reaction with substituted nitro(so)benzenes, and calculation based on measured organic carbon and sulfide concentrations. The samples were taken from four hydrologically connected and increasingly sulfidic wetlands within the study site. Parallel trends in EDC values related to hydrologic conditions and to in situ reduced sulfur content were observed by all three methodologies. In particular, it was found that sulfide and dissolved organic matter (DOM) are the primary and secondary reductants, respectively, in these systems. The efficacy of these reductants in transforming organic contaminants, however, is largely driven by native pore water reduced sulfur content. Manipulation of the systems demonstrate that while DOM is capable of reducing highly oxidized contaminants or reactive intermediates, this likely only occurs once the reducing capacity of the sulfide is exhausted. Sulfide therefore was the dominant electron donor in the pore water samples.