Dissolved Organic Carbon Research Articles

Simultaneously reducing methane emissions and arsenic mobility by birnessite in flooded paddy soil: Overlooked key role of organic polymerisation

Flooding of paddy fields enhances methane (CH4) emissions and arsenic (As) mobilisation, which are crucial issues for agricultural greenhouse gas emissions and food safety. Birnessite (δ-MnO2) is a common natural oxidant and scavenger for heavy metals. In this study, birnessite was applied to As-contaminated paddy soil. The capacity for simultaneously alleviating CH4 emissions and As mobility was explored. Soil microcosm incubation results indicated that birnessite addition simultaneously reduced CH4 emissions by 47 %–54 % and As release by 38 %–85 %. The addition of birnessite decreased the dissolved organic carbon (DOC) contents and altered its chemical properties. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) results showed that birnessite reduced the labile fractions of proteins, carbohydrates, lignins, tannins, and unsaturated hydrocarbons, however, increased the abundance of condensed aromatic structures, suggesting the polymerisation of dissolved organic matter (DOM) by birnessite. The degradation of labile fractions and the polymerisation of DOM resulted in an inventory of recalcitrant DOM, which is difficult for microbes to metabolise, thus inhibiting methanogenesis. In contrast, birnessite addition increased CH4 oxidation, as the particulate methane monooxygenase (pmoA) gene abundance increased by 30 %. The enhanced polymerisation of DOM by birnessite also increased As complexation with organics, leading to the transfer of As to the organic bound phase. In addition, the decrease in ferrous ion [Fe(II)] concentrations with birnessite indicated that the reductive dissolution of Fe oxides was suppressed, which limited the release of arsenite [As(III)] under reducing conditions. Furthermore, birnessite decreased As methylation and shaped the soil microbial community structure by enriching the metal-reducing bacterium Bacillus. Overall, our results provide a promising method to suppress greenhouse gas emissions and the risk of As contamination in paddy soils, although further studies are needed to verify its efficacy and effectiveness under field conditions.

The Mechanism of Arsenic Release in Contaminated Paddy Soil with Added Biochar: The Role of Dissolved Organic Matter, Fe, and Bacteria.

The addition of biochar inevitably modifies the acidity (pH), redox potential (Eh), and dissolved organic matter (DOM) level in the soil. These alterations also have coupled effects on the cycling of iron (Fe) and the composition of bacterial communities, thereby impacting the speciation and availability of arsenic (As) in the soil. This study explored the potential mechanisms through which biochar affects As in paddy soil during flooded cultivation with different pyrolysis temperature biochars (300 °C, 400 °C, and 500 °C) added. The results revealed that the TAs concentration increased in the initial 15 days of soil cultivation with SBC300 or SBC400 addition because increasing the concentration of DOM induced the mobility of As though the formation of As-DOM complexes. Meanwhile, biochar addition elevated the pH, decreased the Eh, and promoted the transformation of specific adsorbed As (A-As) and amorphous iron oxide-bound As (Amo-Fe-As) to supernatant As through enhancing the reductive dissolution of Fe(oxy)(hydr)oxides. Moreover, the biochar altered the relative abundance of As (V)-reducing bacteria (such as Firmicutes) and As (III)-oxidizing bacteria (such as Chloroflex), thereby affecting As speciation. However, these mechanistic effects varied depending on the pyrolysis temperature of the biochar. The microbial composition of SBC300 and SBC400 were similar, with both containing larger populations of Enterobacteriaceae (AsRB) and pseudomonas (FeRB) compared to CK and SBC500. It was proposed that lower pyrolysis temperatures (300 °C and 400 °C) are more favorable for the dissolution of Fe(oxy)(hydr)oxides and the reduction of As (V). However, the biochar from the higher pyrolysis temperature (500 °C) showed environmental impacts akin to the control group (CK). This study demonstrated potential mechanisms of biochar's effect on As and the role of pyrolysis temperature.

The effects of dissolved organic carbon and model compounds (DOC analogues) on diffusive water flux, oxygen consumption, nitrogenous waste excretion rates and gill transepithelial potential in Pacific sanddab (Citharichthys sordidus) at two salinities.

Many flatfish species are partially euryhaline, such as the Pacific sanddab which spawn and feed in highly dynamic estuaries ranging from seawater to near freshwater. With the rapid increase in saltwater invasion of freshwater habitats, it is very likely that in these estuaries, flatfish will be exposed to increasing levels of dissolved organic carbon (DOC) of freshwater origin at a range of salinities. As salinity fluctuations often coincide with changes in DOC concentration, two natural freshwater DOCs [Luther Marsh (LM, allochthonous) and Lake Ontario (LO, autochthonous) were investigated at salinities of 30 and 7.5 ppt. Optical characterization of the two natural DOC sources indicate salinity-dependent differences in their physicochemistry. LO and LM DOCs, as well as three model compounds [tannic acid (TA), sodium dodecyl sulfate (SDS) and bovine serum albumin (BSA)] representing key chemical moieties of DOC, were used to evaluate physiological effects on sanddabs. In the absence of added DOC, an acute decrease in salinity resulted in an increase in diffusive water flux (a proxy for transcellular water permeability), ammonia excretion and a change in TEP from positive (inside) to negative (inside). The effects of DOC (10mg C L-1) were salinity and source-dependent, with generally more pronounced effects at 30 than 7.5 ppt, and greater potency of LM relative to LO. Both LM DOC and SDS increased diffusive water flux at 30 ppt but only SDS had an effect at 7.5 ppt. TA decreased ammonia excretion at 7.5 ppt. LO DOC decreased urea-N excretion at both salinities whereas the stimulatory effect of BSA occurred only at 30 ppt. Likewise, the effects of LM DOC and BSA to reduce TEP were present at 30 ppt but not 7.5 ppt. None of the treatments affected oxygen consumption rates. Our results demonstrate that DOCs and salinity interact to alter key physiological processes in marine flatfish, reflecting changes in both gill function and the physicochemistry of DOCs between 30 and 7.5 ppt.

Rainfall Impacts Dissolved Organic Matter and Cation Export From Permafrost Catchments and a Glacial River During Late Summer in Northeast Greenland

ABSTRACTOngoing and amplified climate change in the Arctic is leading to glacier retreat and to the exposure of an ever‐larger portion of non‐glaciated permafrost‐dominated landscapes. Warming will also cause more precipitation to fall as rain, further enhancing the thaw of previously frozen ground. Yet, the impact of those perturbations on the geochemistry of Arctic rivers remains a subject of debate. Here, we determined the geochemical composition of waters from various contrasting non‐glacial permafrost catchments and investigated their impact on a glacially dominated river, the Zackenberg River (Northeast Greenland), during late summer (August 2019). We also studied the effect of rainfall on the geochemistry of the Zackenberg River, its non‐glacial tributaries, and a nearby independent non‐glacial headwater stream Grænse. We analyzed water properties, quantified and characterized dissolved organic matter (DOM) using absorbance and fluorescence spectroscopy and radiocarbon isotopes, and set this alongside analyses of the major cations (Ca, Mg, Na, and K), dissolved silicon (Si), and germanium/silicon ratios (Ge/Si). The glacier‐fed Zackenberg River contained low concentrations of major cations, dissolved Si and dissolved organic carbon (DOC), and a Ge/Si ratio typical of bulk rock. Glacial DOM was enriched in protein‐like fluorescent DOM and displayed relatively depleted radiocarbon values (i.e., old DOM). Non‐glacial streams (i.e., tributaries and Grænse) had higher concentrations of major cations and DOC and DOM enriched in aromatic compounds. They showed a wide range of values for radiocarbon, Si and Ge/Si ratios associated with variable contributions of surface runoff relative to deep active layer leaching. Before the rain event, Zackenberg tributaries did not contribute notably to the solute export of the Zackenberg River, and supra‐permafrost ground waters governed the supply of solutes in Zackenberg tributaries and Grænse stream. After the rain event, surface runoff modified the composition of Grænse stream, and non‐glacial tributaries strongly increased their contribution to the Zackenberg River solute export. Our results show that summer rainfall events provide an additional source of DOM and Si‐rich waters from permafrost‐underlain catchments to the discharge of glacially dominated rivers. This suggests that the magnitude and composition of solute exports from Arctic rivers are modulated by permafrost thaw and summer rain events. This event‐driven solute supply will likely impact the carbon cycle in rivers, estuaries, and oceans and should be included into future predictions of carbon balance in these vulnerable Arctic systems.

Characteristics of organic matter driven by Eichhornia crassipes during co-contamination with per(poly)fluoroalkyl substances (PFASs) and microplastics (MPs)

Co-contamination with MPs and PFASs has been recorded, particularly in surface-water environments. Floating macrophyte microcosms are an important part of the surface water ecosystem, and dissolved organic matter (DOM) driven by floating macrophytes (FMDDOM) is critical for maintaining material circulation. However, knowledge gaps remain regarding the impact of MPs and PFASs co-pollution on FMDDOM. An greenhouse simulation experiment was conducted in this study to investigate the effects of four PFASs, perfluorooctanoic acid (PFOA), perfluoro-octane-sulfonic acid (PFOS), perfluoro-2-methyl-3-oxahexanoic acid (Gen X), and potassium 9-chlorohexadecafluoro-3-oxanonane-1-sulfonate (F-53B), on FMDDOM sourced from Eichhornia crassipes (E. crassipes), a typical floating macrophyte, in the presence and absence of polystyrene (PS) MPs. Four PFASs increased FMDDOM release from E. crassipes, leading to a 32.52–77.49 % increase in dissolved organic carbon (DOC) levels. PS MPs further increased this, with results ranging from −21.28 % to 26.49 %. Based on the parallel factor analysis (PARAFAC), FMDDOM was classified into three types of fluorescent components: tryptophan-like, humic-like, and tyrosine-like compounds. Contaminants of MPs and PFASs modified the relative abundance of these three components. Protein secondary structure analysis showed that fluorocarbon bonds tended to accumulate on the α-helix of proteins in FMDDOM. The relative abundance of fluorescent and chromophorous FMDDOMs varied from 0.648 ± 0.044 to 0.964 ± 0.173, indicating that the photochemical structures of the FMDDOM were modified. FMDDOM exhibits decreased humification and increased aromaticity when contaminated with MPs and PFASs, which may be detrimental to the geochemical cycling of carbon. This study offers a theoretical basis for assessing the combined ecological risks of MPs and PFASs in floating macrophyte ecosystems.

Effect of straw decomposition on hexavalent chromium removal by straw: Significant roles of surface potential and dissolved organic matter

Mobility and bioavailability of hexavalent chromium (Cr(VI)) in agricultural soils are affected by interactions between Cr(VI) and returned crop straws. However, the effect of straw decomposition on Cr(VI) removal and underlying mechanisms remain unclear. In this study, Cr(VI) removal by pristine and decomposed rice/rape straws was investigated by batch experiments and a series of spectroscopies. The results showed that straw decomposition inhibited Cr(VI) removal, regardless of straw types. However, the potential mechanisms of the inhibition were distinct for the two straws. For the rice straw, a lower zeta potential after decomposition suppressed Cr(VI) sorption and subsequent reduction. In addition, less Cr(VI) was reduced by the decomposed rice straw-derived dissolved organic matter (DOM) than the pristine one. In contrast, for the rape straw, due to the increased zeta potential after decomposition, the decreased Cr(VI) removal was mainly ascribed to less Cr(VI) reduction by the rape straw-derived DOM. These results emphasized the significant roles of straw surface potential and DOM in Cr(VI) removal, depending on straw types and decomposition, which facilitate the fundamental understanding of Cr(VI) removal by straws and are helpful for predicting the environmental risk of Cr and rational straw return in Cr(VI)-contaminated fields.

Molecular-level insight into the effect of fertilization regimes on the chemodiversity of dissolved organic matter in tropical cropland

Fertilization is a critical agronomic measure for croplands in tropical regions, owing to their low fertility. However, the effects of fertilization on the quantity and chemodiversity of latosolic dissolved organic matter (DOM) in tropical regions remain largely unknown. Therefore, in this study, the variations in latosol DOM concentrations and chemodiversity induced by inorganic fertilization and the co-application of inorganic fertilization with straw return, sheep manure, biochar, and vermicompost fertilizers at a molecular level were systematically investigated using multispectral techniques and ultrahigh-resolution mass spectrometry. In line with our expectations, the results showed that combined inorganic-organic fertilization improved soil quality by increasing soil organic carbon content compared to that under inorganic fertilization. However, as the most active and bioavailable organic carbon pool, dissolved organic carbonconcentrations between the fertilization treatments were not significantly different (p = 0.07). However, the dissolved organic carbon concentrations under combined inorganic-organic fertilization treatment (NPK plus straw return, 263.45 ± 37.51 mg/kg) were lower than those under inorganic fertilization treatment (282.10 ± 18.57 mg/kg). Spectral analysis showed that the DOM in the combined inorganic-organic fertilization treatments had a higher degree of humification and lower autogenetic contributions. Furthermore, Fourier transform ion cyclotron resonance mass spectrometry analysis indicated that the combined inorganic-organic fertilization increased the chemodiversity of latosolic DOM and promoted the production of large, oxidized, and stable molecules, including lignin, aromatic, and tannin compounds, which potentially benefits soil carbon sequestration in tropical regions. This study could provide a theoretical basis for elucidating on the potentially relevant ecological functions and environmental effects of DOM under fertilization regimes.

Effects of Different Straw Return Modes on Soil Carbon, Nitrogen, and Greenhouse Gas Emissions in the Semiarid Maize Field.

Returning straw to the field is a crucial practice for enhancing soil quality and increasing efficient use of secondary crop products. However, maize straw has a higher carbon-to-nitrogen ratio compared to other crops. This can result in crop nitrogen loss when the straw is returned to the field. Therefore, it is crucial to explore how different methods of straw return affect maize (Zea mays L.) farmland. In this study, a field experiment was performed with three treatments (I, no straw returned, CK; II, direct straw return, SR; and III, straw returned in deep furrows, ISR) to explore the effects of the different straw return modes on soil carbon and nitrogen content and greenhouse gas emissions. The results indicated that the SR and ISR treatments increased the dissolved organic carbon (DOC) content in the topsoil (0-15 cm). Additionally, the ISR treatment boosted the contents of total nitrogen (TN), nitrate nitrogen (NO3--N), ammonium nitrogen (NH4+-N), dissolved organic nitrogen (DON), and DOC in the subsurface soil (15-30 cm) compared with CK. When it comes to greenhouse gas emissions, the ISR treatment led to an increase in CO2 emissions. However, SR and ISR reduced N2O emissions, with ISR showing a more pronounced reduction. The ISR treatment significantly increased leaf and grain biomass compared to CK and SR. The correlation analyses showed that the yield was positively correlated with soil DOC, and soil greenhouse gas emission was correlated with soil NO3--N. The ISR technology has great potential in sequestering soil organic matter, improving soil fertility, and realizing sustainable agricultural development.

Plant Root Secretion Alleviates Carbamate-Induced Molecular Alterations of Dissolved Organic Matter.

Studying the interaction between pesticide contamination in the plant system and the dissolved organic matter (DOM) composition is important to understand the impact of pesticides and plants on the ecological function of DOM. The present study investigated the effects of DOM on the bioaccumulation and biotransformation of carbamates in plants, carbamate exposure on DOM composition, and plant root secretion on the interaction between DOM and carbamates. The concentrations of carbamates and their metabolites in living cabbage plants were continuously tracked through an in vivo analytical method. The presence of DOM was found to reduce the highest bioconcentrations and shorten the time it took to reach the highest bioaccumulated amounts of isoprocarb and carbofuran in plants, while it showed no significant effect on the uptake behavior of carbaryl. DOM profiling results indicated that carbamate exposure substantially decreased the number and molecular diversity of DOM. Notably, plant root secretion alleviated carbamate-induced DOM molecular alterations by inducing a higher turnover rate of DOM compared to that in the uncontaminated group, highlighting the role of plants in mitigating the effects of exogenous pesticide exposure on DOM composition and maintaining DOM molecular homeostasis.

Distinction of facilitating effects between dissolved organic matter derived from corn straw and fulvic acid on transport of titanium dioxide nanoparticles in saturated sand: Dependent on the solution chemistry

The application of organic materials into agricultural soil introduces dissolved organic matter (DOM) into environment. The DOM derived from organic materials would significantly affect the environmental behaviors of nanoparticles. In this study, DOM extracted from corn straw (CSDOM) was characterized and its effects on the transport of titanium dioxides nanoparticles (TiO2 NPs) in porous media were evaluated and compared with those of fulvic acid (FA). The characterization of DOMs indicated marked differences in molecular properties between CSDOM and FA. Weight-average molecular weight (Mw) of CSDOM was 3139 g/mol, higher than 1185 g/mol of FA, whereas CSDOM had a lower aromaticity than FA. Three distinct scenarios of nanoparticle mobility affected by CSDOM and FA were found. Under unfavorable conditions for nanoparticles mobility (pH 4.0 with/without electrolytes, pH 7.0 with electrolytes, and pH 10.0 with CaCl2), both DOMs facilitated TiO2 NPs mobility, and FA with higher aromaticity exerted stronger enhancing effects than CSDOM. However, the promoting effects of DOM on TiO2 NPs mobility were negligible under favorable conditions (pH 10.0 without electrolytes). Under the remaining conditions (pH 7.0 without electrolytes, and pH 10.0 with NaCl), DOM could facilitate the mobility of TiO2 NPs, while there was no difference in the facilitating effects between CSDOM and FA. It could be concluded that distinction of enhancing effects between CSDOM and FA on TiO2 NPs mobility was environmental chemistry-dependent. Furthermore, aromaticity rather than Mw would be a reasonable property determining the enhancing effects of DOM derived from distinct sources on nanoparticle mobility under unfavorable conditions.

Analysis of dissolved organic matter characteristics in pharmaceutical wastewater via spectroscopy combined with Fourier-transform ion cyclotron resonance mass spectrometry

Studying the changes in organic matter and characteristic pollutants during the treatment of penicillin-containing pharmaceutical wastewater, which can be reflected by changes in dissolved organic matter (DOM), is crucial for improving the effectiveness of wastewater treatment units and systems. Herein, water quality indicators, spectroscopic methods, and Fourier-transform ion cyclotron resonance mass spectrometry were utilized to characterize the general molecular compositions and specific molecular changes in DOM during the treatment of typical penicillin-containing pharmaceutical wastewater, including in each of the influent, physicochemical treatment, biological treatment, oxidation treatment, and effluent stages. The influent exhibited a high organic matter content (concentration of dissolved organic carbon >10,000 mg·L−1), its DOM mainly contained protein- and lignin-like substances composed of CHON and CHONS molecules, and the relative intensity (RI) of penicillin was extremely high (RI = 0.220). Compared with the influent, the abundance of CHON and CHONS molecules detected after physicochemical treatment decreased by 70.3 % and 62.5 %, respectively, and the RI of penicillin decreased by 85.5 %. Biological treatment caused substantial changes in DOM components through oxidation, dealkylation, and denitrification reactions, accounting for 36.8 %, 28.9 %, and 14.8 % of the total identified reactions, respectively. Additionally, lignin-like substances were generated in large quantities, the overall humification level significantly increased, and the RI value increased for the penicillin intermediate, 6-aminopenicillanic acid (6-APA). Oxidation treatment effectively removed phosphorus-containing substances and some lignin-like substances produced by biological treatment; however, it was not effective in removing characteristic pollutants such as 6-APA. Such characteristic substances continued to be present in the effluent, and the DOM mainly contained protein- and humus-like substances, accounting for 30.8 % and 47.3 %, respectively. The study findings reveal the changes in organic matter and characteristic pollutants during the treatment of penicillin-containing wastewater from the perspective of the general molecular composition and specific molecular changes in DOM, providing support for further exploration of wastewater treatment mechanisms and improvements in treatment unit efficiency.

The effect of mosses on the relocation of SOC and total N due to soil erosion and percolation in a disturbed temperate forest

Forests cover one-third of the global land and are important components of carbon and nitrogen cycling. Anthropogenic disturbances, such as forest road systems or skid trails for timber harvesting, can dramatically change the nutrient cycling in these ecosystems. Skid trails increase soil erosion and thus the displacement of soil organic carbon (SOC) and total nitrogen (Nt). Additionally, runoff transports high amounts of dissolved organic carbon (DOC), which can have a negative impact on aquatic ecosystems. One of the most important countermeasures against soil erosion is the quick recolonization of vegetation. To date, the extent to which natural vegetation succession influences the relocation of SOC and Nt, and in particular the role of mosses in this context, has not been well investigated. This study investigates the influence of natural vegetation succession and in particular of mosses on the displacement process of SOC and Nt as well as DOC caused by soil erosion. To this end, we combine the results of a field study using in-situ rainfall simulations with small-scale runoff plots in skid trails of the Schönbuch Nature Park in southwestern Germany with the results of ex-situ rainfall simulation experiments with infiltration boxes containing the substrate from the respective skid trails. The eroded sediments of skid trails were on average enriched in SOC by 16% and in Nt by 35% compared to the original soil, which lead to a decrease of the C/N ratio in sediments. As vegetation succession progressed, the displacement of SOC and Nt was reduced, confirmed by a negative correlation between the enrichment ratios of SOC (ERSOC), Nt and total vegetation cover. However, mosses tended to reduce ERSOC more than vascular plants. Additionally, mosses significantly decreased DOC concentration in surface runoff compared to bare soils, while no difference in DOC concentration in percolated water could be observed. Future research should explore the role of mosses in the storage of SOC and Nt in the soil and their impact on soil stability. Thus, utilizing mosses could potentially minimize environmental impacts from soil disturbances in forests.

Erosion of cultivated kelp facilitates dissolved organic carbon release

Growing trend of interests for contributions of cultivation of kelp to carbon sequestration have been driven globally. Saccharina japonica is an important cultivated seaweed, with erosion phenomenon usually occurs at the distal part of the frond in S. japonica throughout the growth cycle. However, the dynamics of dissolved organic carbon (DOC) release induced by erosion of S. japonica are not well understood. This study revealed that erosion induced a substantial increase in DOC release, with a 14% increase under low light (LL) conditions and a 54% increase under high light (HL) conditions. A 10 cm of long slit cut into the distal part of S. japonica increased the rate of DOC release by 56% under LL conditions, and by 13% under HL conditions. Additionally, the epibiotic microorganisms facilitate the release of DOC, and the effects were even more pronounced in erosive S. japonica. Conversely, the proximal part of S. japonica exhibited a higher photosynthetic carbon fixation capacity, with a carbon-to-nitrogen (C/N) ratio approximately 1.76 times higher than that in distal part. During the growth of S. japonica, excess photosynthetic products were often transported from the proximal part into distal part, further facilitating DOC release. In summary, DOC released induced by erosion of S. japonica could make contributions to oceanic carbon sequestration.

Nitrogen fate in riparian zones: Insights from experiments and analysis of sediment porosity and surface water-groundwater exchange

Riparian zones play a vital role in the river ecosystem. Solutes in vertical riparian zones are transported being by alternating hydraulic gradients between river water and groundwater, due to natural or human activities. This study investigates the impacts of porous sediments and alternating rate of surface water-groundwater on nitrogen removal in the riparian zone through experiments based on the field sampled. The experimental results, combined with dimensionless numbers (Péclet and Damköhler) and Partial Least Squares-Path Modeling, analyze the nitrogen fate responding to hydrodynamics changes. The results show that increased sediment porosity contributes to the ammonium removal, particularly when the oxygen content of river water is low, with the removal rate up to 72.57%. High ammonium content and dissolved organic carbon (DOC) in rural rivers lead to a constant low-oxygen condition (4 mg/L) during surface water-groundwater alternation, and promote denitrification. This threatens groundwater with ammonium pollution and causes accumulation at the top of vertical riparian zones during upwelling, potentially causing secondary river pollution. However, increasing the alternating rate hinders the nitrate denitrification and drastically changes in the redox environment of the riparian zone, despite contributing to ammonium removal. Rapid oxygen consumption during aerobic metabolism and nitrification in groundwater-surface water exchange created favorable conditions for denitrification. Floodplains sediment porosity is unfavorable for nitrification. This study improves understanding of coupled hydrologic and solute processes in vertical riparian zones, informing strategies for optimizing nitrogen attenuation and riparian zone construction.

Photolysis of p-phenylenediamine rubber antioxidants in aqueous environment: Kinetics, pathways and their photo-induced toxicity

The widespread use of rubber antioxidants, especially p-phenylenediamines (PPDs), has raised increasing concerns about their risk assessment. However, there is a notable lack of research on their transformation products (TPs). Photolysis, influenced by active components, plays a significant role in the environmental fates of PPDs. This study investigated four emerging PPDs (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD), N, N′-diphenyl-p-phenylenediamine (DPPD), N-isopropyl-N′-phenyl-p-phenylenediamine (IPPD), and N-cyclohexyl-N′-phenyl-p-phenylenediamine (CPPD)) through a combination of experiments (photolysis kinetics, quenching experiments, acute toxicity test to Vibrio Fischeri (V. fischeri) and identification of photolytic products) and theoretical calculations. The results revealed different pathways for indirect photolysis mediated by the hydroxyl radicals (•OH) and singlet oxygen (1O2) of DPPD and IPPD under simulated sunlight irradiation. The effects of dissolved organic matter (DOM) and fulvic acid (FA) on the rates of photolysis of PPDs highlighted the complex interactions among the molecular structure, light absorption properties, and environmental variables. Quenching for reactive oxygen species (ROS) reduced photo-induced toxicity, whereas the addition of DOM and FA increased it, suggesting the crucial role of ROS in the formation of more toxic photolytic products. The study of photolysis pathways and the evaluation of the health risks provide a comprehensive understanding of the environmental effects of these pollutants.

Sorption retards remediation of clayey sulfuric soils with straw‐derived dissolved organic matter

AbstractWhen sulfidic soils become drained, oxidation of pyrite can cause acidification and formation of iron (Fe) oxyhydroxy sulfate phases such as jarosite. Remediation via re‐establishment of reducing conditions requires submergence and addition of biodegradable organic carbon (OC) to stimulate activity of reducing bacteria. Addition of straw‐derived dissolved organic carbon (DOC) has been shown to induce rapid microbial reduction in sandy sulfuric (pH <4) soils. In clayey sulfuric soil, DOC may be less efficient because of limited availability for microbes due to its sorption to reactive minerals. We tested the possible effect of sorption on the remediative potential of straw‐derived DOC using a set of incubation and sorption experiments, and used solid‐state 13C‐NMR spectroscopy for the chemical characterization of OC. The tested materials were a clayey, jarosite‐containing sulfuric soil (pH 3), and artificial model soils composed of synthesized jarosite either mixed with quartz powder or quartz powder + clay minerals. The results showed that addition of DOC from wheat straw induces reduction conditions varying with soil sorptivity. For the model soils, DOC sorption was little, and DOC additions of 0.8 mg OC g−1 were sufficient to achieve permanently reducing conditions and an increase in pH to >6.0. In the natural sulfuric soil, much higher DOC additions were needed (1.8 mg OC g−1) to facilitate continuous reducing conditions, but pH increased only to values no higher than 5.0–5.5. The natural soil revealed strong sorption of added DOC. Sorption preferentially reduced the proportion of proteins, while the proportion of lignin components, which can hardly be used by microorganisms under reducing conditions, remained relatively high in solution. Thus, high DOC additions were required to overcome the sorption‐induced limitations in OC availability. The results suggest that wheat straw‐derived DOC is a promising approach also for remediation of clayey sulfuric soils; however, OC additions need to be adjusted to compensate for possible sorption.

Beyond Hydrology: Exploring the Factors Influencing the Seasonal Variation of the Molecular Composition of Riverine Dissolved Organic Matter

AbstractThe export of dissolved organic matter (DOM) from rivers is essential for linking terrestrial and marine carbon reservoirs in the global carbon cycle. However, there is limited knowledge regarding how the molecular composition of riverine DOM changes under different hydrological conditions, especially during extreme rainfall events. Moreover, the factors beyond hydrology that impact DOM composition have not been well defined. To address these gaps, samples were collected from a human‐impacted medium‐sized subtropical monsoonal river across various hydrological conditions throughout a complete hydrological cycle. Utilizing high‐resolution mass spectrometry, it was discovered that the solid‐phase extractable DOM (SPE‐DOM) during the high‐flow (1 < runoff (Q): annual mean runoff (Qm) < 3) and extreme‐rain (Q:Qm > 3) periods exhibited a higher number of molecular formulae, lower H/C, higher O/C, and a higher proportion of carboxylic‐rich alicyclic molecules compared to the low‐flow period (LFP) (Q:Qm < 1). These alterations were attributed to input from more diverse sources, particularly a greater input from soil organic matter with higher oxidation degrees. Additionally, the P‐containing formulae were more enriched during the extreme‐rain period, likely from agricultural lands and sediment release. Conversely, the fraction of S‐containing formulae was significantly higher during the LFP, possibly due to the amplified influence of anthropogenic input. Furthermore, the DOM aromaticity did not fluctuate with runoff but was significantly associated with temperature. In summary, the study indicated that the composition of DOM varied significantly under different hydrological conditions, with temperature and anthropogenic activities identified as crucial factors influencing riverine DOM export.

Seasonal changes of dissolved CO2 and its linkage with optical characteristics of DOM in groundwater in agricultural areas

Groundwater contains significant quantities of dissolved CO2, which are crucial contributors to the global carbon cycle. Dissolved organic matter (DOM) functions as a principal carbon and energy source for microbial communities within groundwater ecosystems, yet the relationship between dissolved CO2 and optical characteristics of DOM remains ambiguous. Here, we delineate the variations in seasonal and vertical distribution of dissolved CO2 concentrations and DOM optical characteristics, and assess their interconnections in the groundwater of an agricultural region. The results showed that the average concentration of dissolved CO2 in groundwater was 11,300 ± 5,788 ppm, with the highest levels observed in summer and the lowest in winter. Additionally, shallow wells exhibited higher concentrations of dissolved CO2 compared to deep wells. The mass balance calculation suggested that 40,092 t of CO2 are released annually due to the degassing of supersaturated CO2 from groundwater extraction. Three-dimensional fluorescence with parallel factor analysis (EEM-PARAFAC) identified five components of DOM in groundwater: Components C1, C2, C3 and C4 are humic-like substances, and C5 is a tryptophan-like substance associated with protein structures. The analysis of compositional features and fluorescence indices revealed that DOM is predominantly derived from microbial sources, with mixed contributions from terrestrial sources in groundwater. Principal component analysis (PCA) showed that first two axes accounted for 74.7 % of the variance in the seasonal variation of DOM optical characteristics and dissolved CO2 concentration, which reveals that DOM decomposition contributes to CO2 saturation in groundwater. However, a larger proportion of CO2 may be derived from leaching in the vadose zone during the rainy season or irrigation periods in shallow aquifer. This study enhanced the understanding of carbon dynamics within agricultural groundwater.

Influences of wildfire on the soil dissolved organic matter characteristics and its electron-donating capacity

Global increases in the intensity and frequency of wildfires are driving major changes in soil organic matter (SOM) characteristics, including soil dissolved organic matter (DOM). As the most crucial component of SOM, soil DOM plays a pivotal role in the carbon cycle and regulates the environmental fate of contaminants through its versatile reactivities, including electron-donating capacity (EDC). However, it is still being determined how wildfire influences key characteristics of soil DOM and subsequent effects on EDC in forest soils. Thus, we conducted our study to fill this gap with the forest soils of Jinyun Mountain Nature Reserve of China, which experienced an unprecedented wildfire event in 2022. The results from optical characterization, high-performance size-exclusion chromatography (HPSEC), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) showed decreasing molecular weight but elevating nitrogen-containing molecular formulas of soil DOM in the burned soils. This could be attributed to the Maillard reaction and microbial re-colonies. Additionally, wildfires increased the condensed aromatics and lignin components in soil DOM. In the burned soils, we observed increasing EDC of soil DOM, which accounts for an increase in lignin-derived phenolic components. Overall, the findings of this study demonstrate that eco-disturbances, such as wildfires, induce alterations in the properties of DOM, leading to variations in its reactivity and potentially influencing the fate of environmental pollutants beyond carbon dynamics alone. Thus, incorporating the dynamic properties of soil DOM, particularly in the context of climate change, can enhance the assessment of risks associated with contaminants in soil and water, providing valuable insights.

Mechanism and molecular level insight of refractory dissolved organic matter in landfill leachate treated by electroflocculation coupled with ozone

Landfill leachate usually contains a substantial content of refractory dissolved organic matter (RDOM), which mainly consists of humic acid and fulvic acid. RDOM easily blocks the membrane pores resulting in membrane fouling when treated using membrane technologies, and it costs abundant chemical regents using advanced oxidation processes. To achieve the economically efficient treatment of RDOM in landfill leachate, this study established the electroflocculation coupled with ozone (EFCO) system. EFCO achieved a higher removal for organic matter than single electroflocculation and ozonation, where RDOM was the main component of organic matter in landfill leachate. The remaining RDOM was decomposed into lots of micromolecule weight and biodegradable proteins and lipids. Ozone improved the removal of dissolved organic matter (DOM) mainly by changing the functional groups on its surface and improving the mechanical strength of flocs to enhance the complexation between flocs and DOM. Cl2− and ClO produced in the EFCO system were the main active chlorine species, which enhanced the mineralization of RDOM. The study provides an efficient method for the pretreatment of RDOM in landfill leachate.