Quantification and Redox Dynamics of Phenols and Quinones in Organic Matter from Northern Peatlands

Northern peatlands store significant amounts of carbon as organic matter in water-saturated, anoxic peat. Redox transformations of electron-donating phenols and electron-accepting quinones in peat organic matter are increasingly recognized as key processes that govern carbon cycling in peat soil, including the release of the greenhouse gases carbon dioxide and methane from these systems. First, phenolic dissolved organic matter (DOM) in peat is considered to inhibit the activity of extracellular hydrolytic enzymes, thus decreasing peat decomposition rates. Increasing oxygenation events as a result of changing climatic conditions may lead to enzymatic oxidation of these phenols and thus loss of their inhibitory pressure, leading to accelerated decomposition of peat organic matter. Second, quinone moieties in peat organic matter in water-saturated, anoxic peat are considered to serve as terminal electron acceptors for anaerobic respiration. Respiration to quinones under formation of the corresponding hydroquinones is considered to suppress methane emissions from peatlands, provided that the electron-accepting quinones are regularly regenerated through oxidation with O2. Despite the importance of phenols and quinones to peatland carbon cycling, little information was available on the concentration ranges of these moieties in peatlands as well as on their redox transformations in oxidative and reductive environments. This lack of information reflected analytical challenges associated with the quantification of these moieties in organic matter. The goal of my dissertation research, summarized herein, was to overcome existing knowledge gaps by (i) developing analytical approaches to quantify the redox properties of peat organic matter, (ii) providing a systematic characterization of the redox properties of peat organic matter and (iii) elucidating changes in the redox properties of peat organic matter during key reduction and oxidation processes that occur in peatlands. The research involved both field and laboratory studies. In the first part my PhD research, I developed and validated a flow-injection analysis (FIA) system coupled to electrochemical detection to accurately determine the redox properties of dilute DOM under defined EH and pH conditions. This novel system has significantly higher sensitivity to oxidizable and reducible moieties than previously employed analytical techniques. Furthermore, the system allows for automated sample analysis which was not previously possible. The development of this method was instrumental to the characterization of the redox properties of peat DOM in the next part of my PhD research. In the second part of my PhD work, I used the FIA system to quantify the number of electron-donating and electron-accepting moieties in peat DOM collected from pristine ombrotrophic bogs in Värmland, Sweden. DOM sampled from anoxic peat pore water in these bogs had high phenol contents as compared to a diverse set of model DOM isolates from different aquatic and terrestrial environments. Analysis of DOM collected from oxic pools located in the same bogs rather than anoxic pore water revealed that the pool DOM had lower phenol contents, suggesting oxidative processes in the pools that resulted in a loss of phenolic moieties. Consistently, incubating peat DOM with phenol oxidase under oxic conditions resulted in irreversible oxidation of phenols. The finding of oxidative removal of phenols in peat DOM supports that oxygenation events remove phenols in peat organic matter and thus possibly also their inhibitory role in peat organic matter decomposition. The FIA analysis further demonstrated, for the first time, the presence of electron-accepting quinones in peat DOM from both, peat pore water and pool water. Electron transfer to peat DOM over a cycle of electrochemical reduction and subsequent O2 reoxidation was fully reversible, supporting the hypothesized role of quinone moieties in DOM as regenerable terminal electron acceptors. In the third part of my PhD research, I switched focus from DOM to particulate organic matter (POM) in bogs. More specifically, we studied the redox state and reactivity of POM in anoxic peat with dissolved O2 directly in the field. Using push-pull tests with dissolved O2 as reactant, we demonstrated rapid and complete consumption of dissolved oxygen injected in anoxic peat as a result of abiotic electron transfer from reduced moieties in POM to O2. We thereby demonstrated that POM was in a highly reduced state and that the reduced moieties in POM rapidly transfer electrons to oxygen, consistent with the hypothesized role of peat POM as a regenerable terminal electron acceptor in temporarily anoxic parts of peats. Finally, we highlighted the potential of using consecutive oxygen push-pull tests to quantify the total reducing capacity of POM directly in the field. In the last part of my PhD research, we demonstrated the applicability of the developed FIA system to monitor changes in the redox state of DOM also in the context of chemical water treatment. To this end, treated DOM samples and DOM isolates with increasing specific doses of chlorine and ozone and subsequently quantified the resulting decrease in electron-donating phenols in the DOM samples. The results from this work highlight the potential of the FIA system to be employed in routine analysis during chemical water treatment to monitor changes in the redox state of DOM. Taken together, my PhD research summarized in this thesis advances analytical methods to quantify phenols and quinones in organic matter in environmental and engineered systems and contributes to a more holistic understanding of the role of these moieties in the carbon cycling of peatlands

Application of UV absorbance and electron-donating capacity as surrogates for micropollutant abatement during full-scale ozonation of secondary-treated wastewater

Ozonation of secondary-treated wastewater for the abatement of micropollutants requires a reliable control of ozone doses. Changes in the UV absorbance of dissolved organic matter (DOM) during ozonation allow to estimate micropollutant abatement on-line and were therefore identified as feed-back control parameter. In this study, the suitability of the electron-donating capacity (EDC) as an additional surrogate parameter which is independent of optical DOM properties was evaluated during full-scale ozonation. For this purpose, a recently developed EDC analyzer was enhanced to enable continuous on-line EDC and UV absorbance measurements. During a multi-week monitoring campaign at the wastewater treatment plant of Zurich, Switzerland, specific ozone doses were varied from 0.13 to 0.91 mgO3⋅mgDOC−1 and selected micropollutants with different ozone reactivities were analyzed by LC-MS in conjunction with bromate analysis by IC-MS. In agreement with previous laboratory studies, the relative residual UV absorbance and EDC both decreased exponentially as a function of the specific ozone dose and, in comparison to the residual UV absorbance, residual EDC values showed a more pronounced decrease at low specific ozone doses ≤0.34 mgO3⋅mgDOC−1. Logistic regression models allowed to estimate relative residual micropollutant concentrations in the ozonation effluent using either the residual UV absorbance or EDC as explanatory variable. Averaging those models along the explanatory variables allowed to estimate target values in relative residual UV absorbances and EDC for specific micropollutant abatement targets. In addition, both parameters allowed to identify conditions with elevated conversions of bromide to bromate. Taken together, these findings show that the integration of relative residual EDC values as a second control parameter can improve existing absorbance-based ozonation control systems to meet micropollutant abatement targets, particularly for treatment systems where low ozone doses are applied.ISSN:0043-1354ISSN:1879-244

Electron transfer properties of peat organic matter: from to redox processes in peatlands

ISSN:1029-7006ISSN:1607-796

Electron-Donating Phenolic and Electron-Accepting Quinone Moieties in Peat Dissolved Organic Matter: Quantities and Redox Transformations in the Context of Peat Biogeochemistry

ISSN:0013-936XISSN:1520-585

Oxidant-reactive carbonous moieties in dissolved organic matter: Selective quantification by oxidative titration using chlorine dioxide and ozone

The application of oxidants for disinfection or micropollutant abatement during drinking water and wastewater treatment is accompanied by oxidation of matrix components such as dissolved organic matter (DOM). To improve predictions of the efficiency of oxidation processes and the formation of oxidation products, methods to determine concentrations of oxidant-reactive phenolic, olefinic or amine-type DOM moieties are critical. Here, a novel selective oxidative titration approach is presented, which is based on reaction kinetics of oxidation reactions towards certain DOM moieties. Phenolic moieties were determined by oxidative titration with ClO2 and O3 for five DOM isolates and two secondary wastewater effluent samples. The determined concentrations of phenolic moieties correlated with the electron-donating capacity (EDC) and the formation of inorganic ClO2-byproducts (HOCl, ClO2−, ClO3−). ClO2-byproduct yields from phenol and DOM isolates and changes due to the application of molecular tagging for phenols revealed a better understanding of oxidant-reactive structures within DOM. Overall, oxidative titrations with ClO2 and O3 provide a novel and promising tool to quantify oxidant-reactive moieties in complex mixtures such as DOM and can be expanded to other matrices or oxidants.ISSN:0043-1354ISSN:1879-244

Redox Properties of Pyrogenic Dissolved Organic Matter (pyDOM) from Biomass-Derived Chars

Chars are ubiquitous in the environment and release significant amounts of redox-active pyrogenic dissolved organic matter (pyDOM). Yet, the redox properties of pyDOM remain poorly characterized. This work provides a systematic assessment of the quantity and redox properties of pyDOM released at circumneutral pH from a total of 14 chars pyrolyzed from wood and grass feedstocks from 200 to 700 degrees C. The amount of released pyDOM decreased with increasing pyrolysis temperature of chars, reflecting the increasing degree of condensation and decreasing char polarity. Using flow-injection analysis coupled to electrochemical detection, we demonstrated that electron-donating capacities (EDCpyDom; up to 6.5 mmol(e-)center dot g(c)(-1)) were higher than electron-accepting capacities (EAC(pyDom); up to 1.2 mmol(e-)center dot g(c)(-1)) for all pyDOM specimens. The optical properties and low metal contents of the pyDOM implicate phenols and quinones as the major redox-active moieties. Oxidation of a selected pyDOM by the oxidative enzyme laccase resulted in a 1.57 mmol(e-).g(c)(-1) decrease in EDCpyDom and a 0.25 mmol(e-.)g(c)(-1) increase in EAC(pyDom), demonstrating a largely irreversible oxidation of presumably phenolic moieties. Non-mediated electrochemical reduction of the same pyDOM resulted in a 0.17 mmol(e-).g(c)(-1) increase in EDCpyDom and a 0.24 mmol(e-).g(c)(-1) decrease in EDCpyDom consistent with the largely reversible reduction of quinone moieties. Our results imply that pyDOM is an important dissolved redox-active phase in the environment and requires consideration in assessing and modeling biogeochemical redox processes and pollutant redox transformations, particularly in char-rich environments.ISSN:0013-936XISSN:1520-585

Two analytical approaches quantifying the electron donating capacities of dissolved organic matter to monitor its oxidation during chlorination and ozonation

Electron-donating activated aromatic moieties, including phenols, in dissolved organic matter (DOM) partially control its reactivity with the chemical oxidants ozone and chlorine. This comparative study introduces two sensitive analytical systems to directly and selectively quantify the electron-donating capacity (EDC) of DOM, which corresponds to the number of electrons transferred from activated aromatic moieties, including phenols, to the added chemical oxidant 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonate) radical cation (i.e., ABTS•+). The first system separates DOM by size exclusion chromatography (SEC) followed by a post-column reaction with ABTS•+ and a spectrophotometric quantification of the reduction of ABTS•+ by DOM. The second system employs flow-injection analysis (FIA) coupled to electrochemical detection to quantify ABTS•+ reduction by DOM. Both systems have very low limits of quantification, allowing determination of EDC values of dilute DOM samples with <1 mg carbon per liter. When applied to ozonated and chlorinated model DOM isolates and real water samples, the two analytical systems showed that EDC values of the treated DOM decrease with increasing specific oxidant doses. The EDC decreases detected by the two systems were in overall good agreement except for one sample containing DOM with a very low EDC. The combination of EDC with UV-absorbance measurements gives further insights into the chemical reaction pathways of DOM with chemical oxidants such as ozone or chlorine. We propose the use of EDC in water treatment facilities as a readily measurable parameter to determine the content of electron-donating aromatic moieties in DOM and thereby its reactivity with added chemical oxidants

Quantification of the electron donating capacity and UV absorbance of dissolved organic matter during ozonation of secondary wastewater effluent by an assay and an automated analyzer

Ozonation of secondary wastewater treatment plant effluent for the abatement of organic micropollutants requires an accurate process control, which can be based on monitoring ozone-induced changes in dissolved organic matter (DOM). This study presents a novel automated analytical system for monitoring changes in the electron donating capacity (EDC) and UV absorbance of DOM during ozonation. In a first step, a quantitative photometric EDC assay was developed based on electron-transfer reactions from phenolic moieties in DOM to an added chemical oxidant, the radical cation of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS·+). The assay is highly sensitive (limit of quantification ∼0.5 mgDOC·L−1) and EDC values of model DOM isolates determined by this assay were in good agreement with values determined previously by mediated electrochemical oxidation (slope = 1.01 ± 0.07, R2 = 0.98). In a second step, the photometric EDC measurement method was transferred onto an automated fluidic system coupled to a photometer (EDC analyzer). The EDC analyzer was then used to monitor changes in EDC and UV absorbance of secondary wastewater effluent treated with ozone. While both parameters exhibited a dose-dependent decrease, a more pronounced decrease in EDC as compared to UV absorbance was observed at specific ozone doses up to 0.4 mgO3·gDOC−1. The concentration of 17α-ethinylestradiol, a phenolic micropollutant with a high ozone reactivity, decreased proportionally to the EDC decrease. In contrast, abatement of less ozone-reactive micropollutants and bromate formation started only after a pronounced initial decrease in EDC. The on-line EDC analyzer presented herein will enable a comprehensive assessment of the combination of EDC and UV absorbance as control parameters for full-scale ozonation.ISSN:0043-1354ISSN:1879-244

Quantification of Phenolic Antioxidant Moieties in Dissolved Organic Matter by Flow-Injection Analysis with Electrochemical Detection

Phenolic moieties in dissolved organic matter (DOM) play important roles as antioxidants in oxidation processes in natural and engineered systems. This work presents an automated and highly sensitive flow injection analysis (FIA) system coupled to both spectrophotometric and electrochemical detection to quantify electron-donating phenolic moieties in DOM by determining the number of electrons that these moieties transfer to an added chemical oxidant, the radical cation of 2,2′-azino-bis­(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^•+). The FIA system was successfully validated using Trolox as a redox standard. Highest method sensitivity was attained when combining the FIA with chronoamperometric detection, resulting in limits of quantification of picomolar amounts of Trolox and nanogram amounts of DOM (corresponding to solutions with <1 mg carbon per liter). The analysis of DOM isolates showed a strong linear correlation between the number of electrons donated and their titrated phenol contents, supporting oxidation of phenols by ABTS^•+. The broad application spectrum of the FIA system to dilute natural DOM samples was illustrated by analyzing water samples collected from northern peatlands and by monitoring the oxidation of phenols in one peat sample upon incubation with a phenol oxidase. The superior analytical capability of the FIA system allows quantifying phenols and monitoring phenol dynamics in dilute DOM samples