Oxidation Of Dissolved Organic Carbon Research Articles

Aquatic processing enhances the loss of aged carbon from drained and burned peatlands.

Water-logged peatlands store tremendous amounts of soil carbon (C) globally, accumulating C over millennia. As peatlands become disturbed by human activity, these long-term C stores are getting destabilized and ultimately released as greenhouse gases that may exacerbate climate change. Oxidation of the dissolved organic carbon (DOC) mobilized from disturbed soils to streams and canals may be one avenue for the transfer of previously stored, millennia-aged C to the atmosphere. However, it remains unknown whether aged peat-derived DOC undergoes oxidation to carbon dioxide (CO2) following disturbance. Here, we use a new approach to measure the radiocarbon content of CO2 produced from the oxidation of DOC in canals overlying peatland soils that have undergone widespread disturbance in Indonesia. This work shows for the first time that aged DOC mobilized from drained and burned peatland soils is susceptible to oxidation by both microbial respiration and photomineralization over aquatic travel times for DOC. The bulk radiocarbon age of CO2 produced during canal oxidation ranged from modern to ~1300 years before present. These ages for CO2 were most strongly influenced by canal water depth, which was proportional to the water table level where DOC is mobilized from disturbed soils to canals. Canal microbes preferentially respired older or younger organic C pools to CO2, and this may have been facilitated by the use of a small particulate organic C pool over the dissolved pool. Given that high densities of canals are generally associated with lower water tables and higher fire risk, our findings suggest that peatland areas with high canal density may be a hotspot for the loss of aged C on the landscape. Taken together, the results of this study show how and why aquatic processing of organic C on the landscape can enhance the transfer of long-term peat C stores to the atmosphere following disturbance.

Controls on the Respiration of Ancient Carbon Draining From Permafrost Soils Into Sunlit Arctic Surface Waters

AbstractThe thawing of ancient organic carbon stored in arctic permafrost soils, and its oxidation to carbon dioxide (CO2, a greenhouse gas), is predicted to amplify global warming. However, the extent to which organic carbon in thawing permafrost soils will be released as CO2 is uncertain. A critical unknown is the extent to which dissolved organic carbon (DOC) from thawing permafrost soils is respired to CO2 by microbes upon export of freshly thawed DOC to both dark bottom waters and sunlit surface waters. In this study, we quantified the radiocarbon age and 13C composition of CO2 produced by microbial respiration of DOC that was leached from permafrost soils and either kept in the dark or exposed to ultraviolet and visible wavelengths of light. We show that permafrost DOC most labile to microbial respiration was as old or older (ages 4,000–11,000 a BP) and more 13C‐depleted than the bulk DOC in both dark and light‐exposed treatments, likely indicating respiration of old, 13C‐depleted lignin and lipid fractions of the permafrost DOC pool. Light exposure either increased, decreased, or had no effect on the magnitude of microbial respiration of old permafrost DOC relative to respiration in the dark, depending on both the extent of DOC oxidation during exposure to light and the wavelength of light. Together, these findings suggest that photochemical changes affecting the lability of permafrost DOC during sunlight exposure are an important control on the magnitude of microbial respiration of permafrost DOC in arctic surface waters.

Groundwater‐surface water interaction, dissolved organic carbon oxidation and dissolution in carbonate aquifers

AbstractThe high primary porosity and permeability of eogenetic karst aquifers permit water recharged through secondary dissolution features to be temporarily stored in aquifer matrix porosity. The recharged water contains elevated dissolved organic carbon (DOC) concentrations that, when oxidized, enhance limestone dissolution and impact carbon cycling. We evaluate the relationship between DOC oxidation and limestone dissolution using observations at a stream sink‐rise system and reversing spring in the Floridan aquifer, north‐central Florida, USA, where subsurface residence times of recharged water are days and months, respectively. We estimate water chemical compositions during surface water‐groundwater interactions at these two systems with mixing models of surface water and groundwater compositions and compare them with measured DOC, dissolved inorganic carbon (DIC), Ca2+ and dissolved organic nitrogen (DON) concentrations. Differences between measured and modelled concentrations represent net changes that can be attributed to calcite dissolution and redox reactions, including DOC oxidation. DOC losses and Ca2+ gains exhibit significant (p < 0.01) inverse linear correlations at both the reversing spring (slope = −0.9, r2 = 0.99) and the sink‐rise system (slope = −0.4, r2 = 0.72). DOC oxidation in both systems was associated with decreases in the molar C:N ratio (DOC:DON). Significant (p < 0.01) positive linear correlations between increases in Ca2+ and DIC concentrations after correcting for DIC derived from calcite dissolution occurred at both the reversing spring (slope = 1.3, r2 = 0.99) and the sink‐rise system (slope = 1.61, r2 = 0.75). Greater deviations from the expected slope of −1 or +1 at the sink‐rise system than at the reversing spring indicate DOC oxidation contributes less dissolution at the sink‐rise system than at the reversing spring, likely from shorter storage in the subsurface. A portion of the deviation from expected slope values can be explained by the dissolution of Mg‐rich carbonate or dolomite rather than pure calcite dissolution. Despite this, slope values reflect kinetic effects controlling incomplete consumption of carbonic acid during dissolution reactions.

The hidden role of dissolved organic carbon in the biogeochemical cycle of carbon in modern redox-stratified lakes

Abstract. The dissolved organic carbon (DOC) reservoir plays a critical role in the C cycle of marine and freshwater environments because of its size and implication in many biogeochemical reactions. Although it is poorly constrained, its importance in ancient Earth's C cycles is also commonly invoked. Yet DOC is rarely quantified and characterized in modern stratified analogues. In this study, we investigated the DOC reservoirs of four redox-stratified alkaline crater lakes in Mexico. We analyzed the concentrations and isotopic compositions of DOC throughout the four water columns and compared them with existing data on dissolved inorganic and particulate organic C reservoirs (DIC and POC). The four lakes have high DOC concentrations with great variability between and within the lakes (averaging 2 ± 4 mM; 1 SD, n=28; i.e., from ∼ 15 to 160 times the amount of POC). The δ13CDOC signatures also span a broad range of values from −29.3 ‰ to −8.7 ‰ (with as much as 12.5 ‰ variation within a single lake). The prominent DOC peaks (up to 21 mM), together with their associated isotopic variability, are interpreted as reflecting oxygenic and/or anoxygenic primary productivity through the release of excess fixed carbon in three of the lakes (Alberca de los Espinos, La Preciosa, and Atexcac). By contrast, the variability of [DOC] and δ13CDOC in the case of Lake Alchichica is mainly explained by the partial degradation of organic matter and the accumulation of DOC in anoxic waters. The DOC records detailed metabolic functions such as active DIC-uptake and DIC-concentrating mechanisms, which cannot be inferred from DIC and POC analyses alone but which are critical to the understanding of carbon fluxes from the environment to the biomass. Extrapolating our results to the geological record, we suggest that anaerobic oxidation of DOC may have caused the very negative C isotope excursions in the Neoproterozoic. It is, however, unlikely that a large oceanic DOC reservoir could overweigh the entire oceanic DIC reservoir. This study demonstrates how the analysis of DOC in modern systems deepens our understanding of the C cycle in stratified environments and helps to set boundary conditions for the Earth's past oceans.

Anoxic granular activated sludge process for simultaneous removal of hazardous perchlorate and nitrate

An airtight, anoxic bubble-column sequencing batch reactor (SBR) was developed for the rapid cultivation of perchlorate (ClO4-) and nitrate (NO3-) reducing granular sludge (GS) in this study. Feast/famine conditions and shear force selection pressures in tandem with a short settling time (2-min) as a hydraulic section pressure resulted in the accelerated formation of anoxic granular activated sludge (AxGS). ClO4- and NO3- were efficiently (>99.9%) reduced over long-term (>500-d) steady-state operation. Specific NO3- reduction, ClO4- reduction, chloride production, and non-purgeable dissolved organic carbon (DOC) oxidation rates of 5.77 ± 0.54 mg NO3--N/g VSS·h, 8.13 ± 0.74 mg ClO4-/g VSS·h, 2.40 ± 0.40 mg Cl-/g VSS·h, and 16.0 ± 0.06 mg DOC/g VSS·h were recorded within the reactor under steady-state conditions, respectively. The AxGS biomass cultivated in this study exhibited faster specific ClO4- reduction, NO3- reduction, and DOC oxidation rates than flocculated biomass cultivated under similar conditions and AxGS biomass operated in an up-flow anaerobic sludge blank (UASB) bioreactor receiving the same influent loading. EPS peptide identification revealed a suite of extracellular catabolic enzymes. Dechloromonas species were present in high abundance throughout the entirety of this study. This is one of the initial studies on anoxic granulation to simultaneously treat hazardous chemicals and adds to the science of the granular activated sludge process.

A refined recycled carbonatite model for the δ13C negative anomaly in the upper Doushantuo Formation

The upper Doushantuo Formation (terminated at 551 Ma) in the Yangtze platform keeps a large carbonate δ13C (carbon isotope composition) negative anomaly, which has been proposed to be related to the Shuram Excursion (SE). However it is still controversial whether such anomaly records the carbon isotope variation of seawater caused by oxidizing dissolved organic carbon (DOC) or the input of a local exotic carbon reservoir. Here, we conducted further chemical and mineralogical studies of samples from the 13C-depleted strata of the Doushantuo Formation at Jiulongwan. The 13C-depleted strata are featured by flat REY patterns and constant δ13Ccarb (δ13C of carbonate) but variable δ13Corg (δ13C of organic carbon), which disagree with the prediction of DOC oxidation model. Euhedral quartz grains separated from these strata show bluish-violet catholuminescence indicating a high temperature origin. Carbonate inclusions are found in some of these quartz grains. Above chemical and mineralogical features of the 13C-depleted strata can be explained by the previously suggested recycled carbonatite model. However, zircons separated from the 13C-depleted carbonates record not only a young age of 567.17 ± 3.57 M, but also two old age clusters of 640 ∼ 960 Ma and 1760 ∼ 2160 Ma. The young age of 567.17 ± 3.57 Ma may record the carbonatite magmatic activity. The age spectrums of old zircons imply that the recycled carbonatite is more likely to be transported and deposited in the intra-shelf area with other clastics through weathering.

New method for simultaneous determination of dissolved organic carbon and its stable carbon isotope ratio in liquid samples: environmental applications

ABSTRACT This study reports the development of an all-in-one elemental analyser isotope ratio mass spectrometry (EA-IRMS) system modified for simultaneous analysis of dissolved organic carbon (DOC) concentration and its stable carbon isotope footprint (δ 13CDOC) in aqueous samples. The method involves a quantitative oxidation of DOC in a 200 µL liquid sample to CO2, after sample acidification and stripping by nitrogen. The detection limit of the method for DOC quantification was 0.2 mg C/L with an analytical precision of 12 %. Uncertainty of stable isotope determinations was 2 % at 0.2 mg DOC/L, while decreasing to 0.3 % at 20 mg DOC/L. Quantitative oxidation of DOC in aqueous samples was validated by using ring test water samples and Deep Sea reference seawater. The method performances of isotope analysis were evaluated by analysing different isotopic standard solutions. The applicability of the method was tested through the analysis of different environmental types of water, showing that δ 13CDOC ranged from – 23.30 to –31.85 ‰, allowing to characterize samples of different environmental origin. The developed method offers several advantages including rapidity, use of small sample volumes and minimal sample pre-treatment, making it a valuable tool for routine DOC concentration measurements paired with isotopic characterization.

Barite in the Ediacaran Doushantuo Formation and its implications for marine carbon cycling during the largest negative carbon isotope excursion in Earth’s history

The Ediacaran Period (∼635–539 Ma) witnessed the largest negative inorganic carbon isotope (δ13Ccarb) excursion in Earth’s history (i.e., the Shuram Excursion), which is characterized by decoupling from the organic carbon isotope (δ13Corg) record. The cause(s) of this event remains highly debated. Here, we report a major (∼8–9-Myr-long) episode of strong barium (Ba) accumulation during the Shuram Excursion in the form of barite, as recorded in the Ediacaran Doushantuo Formation (∼635–551 Ma) of South China. The inner-shelf Zhangcunping section exhibits minimal Ba enrichment, while the slope Siduping section shows maximal Ba enrichment with the intrashelf basinal Jiulongwan section in the middle. The Siduping section contains ∼5 μm-diameter, ellipsoidal barite crystals of marine origin; and the Jiulongwan section contains large (>50 μm), euhedral barite crystals and cements that are partly replaced by pyrite, pointing to a diagenetic origin with barite formation within the sulfate-methane transition zone. The barite δ34S is ∼10‰ higher than δ34S of carbonate-associated sulfate at Jiulongwan in contrast to similar values of these two components at Siduping, suggesting a limited influence of methane oxidation, if any, on the formation of Shuram Excursion at Jiulongwan. The Ce/Ce* exhibits a lateral gradient among study sections which is reverse to the Ba enrichment, supporting the hypothesis that local surface-water productivity controlled dissolved oxygen levels in the Ediacaran surface ocean. Based on these findings, we attribute the highest Ba enrichment at Siduping to oceanic upwelling which enhanced local marine productivity. Furthermore, we propose that episodic oceanic upwelling in the Ediacaran shelf regions likely transported phosphorus and dissolved organic carbon (DOC) to shallow waters, increasing their productivity and facilitating the oxidation of DOC, contributing to the largest negative carbon isotope excursion in Earth’s history.

Chloride Interference during Analysis of Dissolved Organic Carbon Using Wet Chemical Oxidation Methods

Analysis of dissolved organic carbon (DOC) is widely used to quantify the sum of organic carbon species in water. Analyzers based on wet chemical oxidation (WCO) usually have a relatively low detection limit (±1 μg of C/L) and are, therefore, well suited for analyses of DOC in environmental samples (e.g., surface water, groundwater, or seawater). However, these matrices might contain relatively high concentrations of chloride that can affect the DOC oxidation efficiency. We validated the DOC analysis using a combined persulfate and ultraviolet oxidation method in various prepared samples (humic and fulvic acids and selected organic acids) with different background concentrations of chloride. The results show that chloride is an effective radical scavenger (at concentrations of >355 mg of Cl–/L) that reduces the oxidation efficiency of DOC analyses based on WCO. The oxidation efficiency correlates well with the proton dissociation constant of the selected organic acids. Our findings imply that the DOC analyses by WCO methods should include an assessment of the chloride concentrations in the samples to check for potential interference. The results are relevant for DOC analysis in samples containing high background levels of chloride (e.g., brackish water) and for samples from DOC fractionation methods to quantify humic substances.

Fire reduces riverine DOC concentration draining a watershed and alters post-fire DOC recovery patterns

The loading of dissolved organic carbon (DOC) from soils to inland waters and ultimate transport to the ocean is a critical flux pathway in the terrestrial biosphere carbon cycle. Fires can significantly affect this flux through biogeochemical processes related to oxidation and mobilization of DOC in the soil. Therefore, in order to accurately estimate and model terrestrial carbon storage and export to the marine environment, we need to better understand the effects of fire on DOC flux. In this analysis, we compiled available observational data sets from seven watersheds across the conterminous United States generally spanning the years 1999–2019. We used these data sets to examine the effects of fire on riverine DOC concentration draining a watershed to study both the immediate impacts and the post-fire recovery patterns. Our results suggest that these fires result in an immediate decrease in riverine DOC concentration draining the watershed by 26 ± 15%, and the time required for DOC concentrations to recover to pre-fire levels was estimated to be, on average, approximately 9 months. During recovery, DOC concentration was 24 ± 11% lower than the long-term average for the watershed. In addition, the larger the proportion of the watershed that burned, the greater the concentration decrease and the longer the time period for post-fire recovery.

Leaching of organic carbon from grassland soils under anaerobiosis

The projected increase of extreme precipitation and freeze-thawing events may lead to frequent occurrence of anaerobiosis in upland soils, which has significant impacts on biogeochemical processes affecting soil carbon loss. However, compared to mineralization, the impacts of anaerobiosis (potentially accompanied by fermentation) on soil organic carbon (SOC) leaching is limited. Here we conducted microcosm and intact soil column incubation experiments to examine processes influencing SOC leaching from four typical Chinese grassland soils under simulated anaerobiosis. Compared to aerobiosis, non-fermenting anaerobiosis increased the pH, dissolved phenol concentrations and aromaticity of soil leachates. In contrast, fermenting anaerobiosis induced acetate accumulation, lowered pH, stimulated phenol oxidative activity and generally decreased aromaticity in soil leachates in both microcosm and soil column experiments relative to aerobiosis. Both anaerobiosis potentially induced a strong release of dissolved organic carbon (DOC) accompanied by iron and nitrate reduction, especially with fermentation. However, DOC in soil leachates decreased in alpine subsoils under fermentation relative to aerobiosis. This interesting phenomenon was mainly attributed to (i) minimal iron reduction and dissolution in the alpine subsoils and (ii) enhanced DOC oxidation by elevated phenol oxidative activity in the fermentation relative to aerobiosis treatments. These results collectively indicate that anaerobiosis may increase SOC leaching and its magnitude is dependent on the extent of iron reduction and pH variations. Fermentation-enhanced release of ferrous iron and acetate may have an even stronger influence on the downstream biogeochemistry. Hence, temporary anaerobiosis warrants better recognition and investigation in the Mongolian (relative to Qinghai-Tibetan) grasslands that show high soil iron reduction potentials and are predicted to experience increased extreme precipitation in the future.

A paired carbonate–organic δ13C approach to understanding the Cambrian Drumian carbon isotope excursion (DICE)

The middle Cambrian (Cambrian Series 3) Drumian carbon isotope excursion (DICE) represents a pronounced negative carbon isotopic excursion, and has been proposed as a key chemostratigraphic marker for identifying the base of Cambrian Drumian Stage and global correlation. However, the precise interbasinal correlation of the DICE to the South China remains unsolved and the driving mechanism of the DICE is to be further understood. In this study, we report new paired δ13Ccarb and δ13Corg data from the Wangcun section in South China. Both δ13Ccarb and δ13Corg profiles capture a large negative excursion (DICE) near the FAD of P. atavus. We suggest that the parallel δ13Ccarb and δ13Corg negative excursions during the DICE may have resulted from the shoaling of anoxic deep waters associated with the transgressive event. Shoreward and upward incursion of deep anoxic waters into oxygenated shallower water column during transgression could lead to the enhanced oxidation of dissolved organic carbon (DOC) and introduce additional 13C-depleted carbon to the surface water, resulting in the parallel δ13Ccarb and δ13Corg negative excursions. More importantly, we use a box model to quantify the flux of DOC oxidation needed to produce a carbon isotope excursion of the magnitude and duration of the DICE we observed. The model results indicate that oxidation of DOC during the DICE may require massive amounts of oxidants (i.e., sulfate and oxygen), probably causing further expansion of anoxia in the shallow marine environment. We suggest that shoaling of anoxic water during the DICE could have delayed the recovery of metazoan reef systems from the early–middle Cambrian mass extinctions.

A pulse of oxygen increase in the early Mesoproterozoic ocean at ca. 1.57–1.56 Ga

The relationship between oxygen and evolution of early eukaryotes including algae and primitive animals in geological history has been debated, partly due to the varying estimates of oxygen levels in the mid-Proterozoic (ca. 1.8–0.8 Ga) ocean and atmosphere. The upper part of the Gaoyuzhuang Formation (ca. 1.60–1.54 Ga) in North China hosts decimeter-scale multicellular eukaryotic fossils and is documented with a decrease in cerium anomaly indicative of ocean oxygenation. However, the atmospheric oxygen level across this interval and its subsequent oxidation state require further investigation using additional redox proxies. Here we report I/(Ca+Mg) ratios, carbonate/organic carbon isotopes (δ13Ccarb and δ13Corg), and phosphorous (P) contents across the ca. 1.57–1.56 Ga fossil-bearing interval in the North China Platform. High I/(Ca+Mg) ratios (≥2.6 μmol/mol; up to 3.8 μmol/mol) from shallow-water carbonates of the Gaoyuzhuang Formation suggest an episode of significant oxygen increase up to ≥4% PAL (present atmospheric level). The I/(Ca+Mg) ratios return back to ≤0.5 μmol/mol shortly after the peak values without evidence for increasing water depth or diagenetic alteration, implying a short-lived oxidation event. The increase of I/(Ca+Mg) ratios is associated with a −3.5‰ negative δ13Ccarb and δ13Corg anomaly and an increase in P/Al ratios that are best explained by oxidation of dissolved organic carbon (DOC) in the ocean. Oxygen consumption through oxidation of DOC may have quickly lowered marine and atmospheric O2 levels to the early mid-Proterozoic (1.8–1.4 Ga) background oxygen concentration of ≤0.1–1% PAL. Short-lived oxidation events in an overall anoxic mid-Proterozoic ocean and atmosphere best explain the existing geochemical data and evolutionary stasis of eukaryotes during the “Boring Billion”.

The Controls of Iron and Oxygen on Hydroxyl Radical (•OH) Production in Soils

Hydroxyl radical (•OH) is produced in soils from oxidation of reduced iron (Fe(II)) by dissolved oxygen (O2) and can oxidize dissolved organic carbon (DOC) to carbon dioxide (CO2). Understanding the role of •OH on CO2 production in soils requires knowing whether Fe(II) production or O2 supply to soils limits •OH production. To test the relative importance of Fe(II) production versus O2 supply, we measured changes in Fe(II) and O2 and in situ •OH production during simulated precipitation events and during common, waterlogged conditions in mesocosms from two landscape ages and the two dominant vegetation types of the Arctic. The balance of Fe(II) production and consumption controlled •OH production during precipitation events that supplied O2 to the soils. During static, waterlogged conditions, •OH production was controlled by O2 supply because Fe(II) production was higher than its consumption (oxidation) by O2. An average precipitation event (4 mm) resulted in 200 µmol •OH m−2 per day produced compared to 60 µmol •OH m−2 per day produced during waterlogged conditions. These findings suggest that the oxidation of DOC to CO2 by •OH in arctic soils, a process potentially as important as microbial respiration of DOC in arctic surface waters, will depend on the patterns and amounts of rainfall that oxygenate the soil.

The role of iron and reactive oxygen species in the production of CO2 in arctic soil waters

Hydroxyl radical (OH) is a highly reactive oxidant of dissolved organic carbon (DOC) in the environment. OH production in the dark was observed through iron and DOC mediated Fenton reactions in natural environments. Specifically, when dissolved oxygen (O2) was added to low oxygen and anoxic soil waters in arctic Alaska, OH was produced in proportion to the concentrations of reduced iron (Fe(II)) and DOC. Here we demonstrate that Fe(II) was the main electron donor to O2 to produce OH. In addition to quantifying OH production, hydrogen peroxide (H2O2) was detected in soil waters as a likely intermediate in OH production from oxidation of Fe(II). For the first time in natural systems we detected carbon dioxide (CO2) production from OH oxidation of DOC. More than half of the arctic soil waters tested showed production of CO2 under conditions conducive for production of OH. Findings from this study strongly suggest that DOC is the main sink for OH, and that OH can oxidize DOC to yield CO2. Thus, this iron-mediated, dark chemical oxidation of DOC may be an important component of the arctic carbon cycle.

Photochemical alteration of organic carbon draining permafrost soils shifts microbial metabolic pathways and stimulates respiration

In sunlit waters, photochemical alteration of dissolved organic carbon (DOC) impacts the microbial respiration of DOC to CO2. This coupled photochemical and biological degradation of DOC is especially critical for carbon budgets in the Arctic, where thawing permafrost soils increase opportunities for DOC oxidation to CO2 in surface waters, thereby reinforcing global warming. Here we show how and why sunlight exposure impacts microbial respiration of DOC draining permafrost soils. Sunlight significantly increases or decreases microbial respiration of DOC depending on whether photo-alteration produces or removes molecules that native microbial communities used prior to light exposure. Using high-resolution chemical and microbial approaches, we show that rates of DOC processing by microbes are likely governed by a combination of the abundance and lability of DOC exported from land to water and produced by photochemical processes, and the capacity and timescale that microbial communities have to adapt to metabolize photo-altered DOC.

Organic matter cycling across the sulfate-methane transition zone of the Santa Barbara Basin, California Borderland

Consumption of sulfate (SO42−) in the sulfate-methane transition zone (SMTZ) has often been considered to be due solely to anaerobic oxidation of methane (AOM). However, recent studies show SO42− fluxes into the SMTZ that exceed methane (CH4) fluxes, thereby challenging this conceptual model. Co-occurrence of organoclastic SO42− reduction (oSR) with AOM in the SMTZ has been hypothesized to be the cause for this flux imbalance, but conclusive evidence is lacking. To address this knowledge gap, we investigated organic matter cycling in the SMTZ of the organic-rich sediments of the Santa Barbara Basin, California Borderland, and examined the occurrence of oSR within this zone using bulk solute profiles and Δ14C and δ13C values of selected carbon pools. We also tested the hypothesis that the SMTZ acts as an oxidation front not just for CH4, but also for dissolved organic carbon (DOC) that is produced below the SMTZ and migrates upward. Mass balance calculations for the SMTZ based on reaction stoichiometry and Δ14C and δ13C values of associated carbon fluxes indicate that ∼35–45% of total SO42− reduction in the SMTZ occurs via oSR, with the remainder attributable to AOM. The δ13C value of net DOC production is distinct from that of the fraction of bulk POC undergoing degradation, suggesting that pore-water DOC represents a compositionally unique slice of the metabolizable POC pool. DOC diffusing upward at 450cm is virtually free of 14C and contain low levels of short-chain organic acids. Radiocarbon mass balance shows that >30% of this pre-aged, and presumably refractory, DOC is removed from the pore waters within or immediately below the SMTZ. Although the SMTZ does not appear to be a major net DOC oxidation front, these results show that DOC dynamics provide unique insights into organic matter processing in these subsurface sediments.

An effective method of UV-oxidation of dissolved organic carbon in natural waters for radiocarbon analysis by accelerator mass spectrometry

Radiocarbon (14C) measurement of dissolved organic carbon (DOC) is a very powerful tool to study the sources, transformation and cycling of carbon in the ocean. The technique, however, remains great challenges for complete and successful oxidation of sufficient DOC with low blanks for high precision carbon isotopic ratio analysis, largely due to the overwhelming proportion of salts and low DOC concentrations in the ocean. In this paper, we report an effective UV-Oxidation method for oxidizing DOC in natural waters for radiocarbon analysis by accelerator mass spectrometry (AMS). The UV-oxidation system and method show 95%±4% oxidation efficiency and high reproducibility for DOC in both river and seawater samples. The blanks associated with the method was also low (about 3 µg C) that is critical for 14C analysis. As a great advantage of the method, multiple water samples can be oxidized at the same time so it reduces the sample processing time substantially compared with other UV-oxidation method currently being used in other laboratories. We have used the system and method for 14C studies of DOC in rivers, estuaries, and oceanic environments and have received promise results.

Kinetic analysis and biodegradability of the Fenton mineralization of bisphenol A

AbstractBACKGROUNDThis work reports the kinetic and biodegradability analysis of bisphenol A (BPA) Fenton oxidation. An experimental study was carried out within the variables H2O2/Fe (II)/BPA in the range 10/3/1 to 50/3/1 and at temperatures of 25–70°C. Kinetics of the environmental parameters dissolved organic carbon (DOC) and chemical oxygen demand (COD) were analysed.RESULTSThe concentration of H2O2 and the temperature have a strong influence on the kinetics of Fenton mineralization of BPA. Working at 70°C, the measured DOC values matched that calculated using the measured organic acids, which demonstrates that the oxidation reactions proceeded at a sufficient rate to form organic acids. Moreover, as BPA solutions are not easily biodegradable, the biodegradability of the solution during Fenton oxidation was studied by means of respirometry assays, evaluating the short‐term biochemical oxygen demand (BODst)/COD ratio.CONCLUSIONSThe kinetic models proposed are capable of predicting the rate of H2O2 consumption and the oxidation of DOC and COD as a function of the temperature. Under the process conditions tested the treated effluents increased their biodegradability, facilitating compatibility with further biological treatment. © 2014 Society of Chemical Industry

First assessment of water and carbon cycles in two tropical coastal rivers of south‐west India: an isotopic approach

The contribution of tropical coastal rivers to the global carbon budget remains unmeasured, despite their high water dynamics, i.e. higher run-off with their basin characteristic of warm temperature. Two rivers draining the western part of the Western Ghats, the Swarna (length 80 km) and Nethravati (147 km) Rivers, were studied for water and carbon cycles. The stable isotope ratios of oxygen (δ(18) O values), hydrogen (δ(2) H values) and carbon (δ(13) C values) were used to understand the water circulation, the weathering processes and the carbon biogeochemical cycle. The river water samples were collected during the dry post-monsoonal season (November 2011). The δ(18) O and δ(2) H values of river water suggested that the monsoonal vapour source and its high recycling have a dominant role because of the orographical and tropical conditions. The absence of calcareous rocks has led to dissolved inorganic carbon (DIC) mainly originating from atmospheric/soil CO2 , via rock-weathering processes, and the low soil organic matter combined with high run-off intensity has led to low riverine dissolved organic carbon (DOC) contents. The δ(13) C values increase from upstream to downstream and decrease with increasing pCO2 . There is a positive relationship between the δ(13) CDIC values and the DOC concentrations in these two rivers that is contrary to that in most of the studied rivers of the world. The higher evapotranspiration supported by tropical conditions suggests that there are higher vapour recycling process in the Swarna and Nethravati basins as studied from the water δ(18) O and δ(2) H values. The basin characteristics of higher rainfall/run-off accompanied by warm temperature suggest that the δ(13) C value of riverine DIC is mainly controlled by the weathering of source rocks (silicates) with variation along the river course by CO2 degassing from the river water to the atmosphere and is less dominated by the oxidation of DOC.