Dissolved Organic Matter Fractions Research Articles

Enhanced simultaneous phosphate recovery and organics removal by a nucleation crystallization pelleting process

Naturally co-occurring phosphate (P) and dissolved organic matter (DOM) in raised livestock wastewater have caused increasing public concerns. This study fabricated a novel nucleation crystallization pelleting process (NCP) showing orientated P recovery and HA (as a major fraction of DOM) removal from wastewater. Results showed that after 120 h of continuous operation, the total removal efficiency of P and HA stabilized at 97.5 % and 92 %, respectively, by using a three-stage NCP process. The NCP results further demonstrated that the main recovery mechanism of P involves surface-induced precipitation of P in the form of hydroxyapatite (HAP) on the seed surface by modulating the supersaturation of HAP in the system. The FTIR, SEM-EDS, and XPS analysis combined with DFT calculations revealed the molecular-level mechanisms for removing HA, which were mainly controlled by Calcium ions bridging, H-bonding, and π-π stacking. More importantly, the attached HAP on the seed surface would enhance the mutual interaction with HA, further enhancing the removal efficiency of HA. This contribution not only opens a high potential avenue for further research on the simultaneous capture of P and organic matter but also sheds light on the development of the practical application of the NCP remediation strategies.

Recalcitrant dissolved organic matter in lakes: a critical but neglected carbon sink

Lakes are hotspots for the biogeochemical processing of carbon, including dissolved organic matter (DOM). A less degradable fraction of DOM, preserved for a long time, can be categorized as recalcitrant DOM (RDOM). Lake RDOM is an important but neglected carbon sink, and its characteristics and transformation processes remain largely unknown.

Role of low-proportion, hydrophobic dissolved organic matter components in inhibiting methylmercury uptake by phytoplankton

Uptake of methylmercury (MeHg), a potent neurotoxin, by phytoplankton is a major concern due to its role as the primary pathway for MeHg entry into aquatic food webs, thereby posing a significant risk to human health. While it is widely believed that the MeHg uptake by plankton is negatively correlated with the concentrations of dissolved organic matter (DOM) in the water, ongoing debates continue regarding the specific components of DOM that exerts the dominant influence on this process. In this study, we employed a widely-used resin fractionation approach to separate and classify DOM derived from algae (AOM) and natural rivers (NOM) into distinct components: strongly hydrophobic, weakly hydrophobic, and hydrophilic fractions. We conduct a comparative analysis of different DOM components using a combination of spectroscopy and mass spectrometry techniques, aiming to identify their impact on MeHg uptake by Microcystis elabens, a prevalent alga in freshwater environments. We found that the hydrophobic components had exhibited more pronounced spectral characteristics associated with the protein structures while protein-like compounds between hydrophobic and hydrophilic components displayed significant variations in both distributions and the values of m/z (mass-to-charge ratio) of the molecules. Regardless of DOM sources, the low-proportion hydrophobic components usually dominated inhibition of MeHg uptake by Microcystis elabens. Results inferred from the correlation analysis suggest that the uptake of MeHg by the phytoplankton was most strongly and negatively correlated with the presence of protein-like components. Our findings underscore the importance of considering the diverse impacts of different DOM fractions on inhibition of phytoplankton MeHg uptake. This information should be considered in future assessments and modeling endeavors aimed at understanding and predicting risks associated with aquatic Hg contamination.

Response of a source water quality through a heavy precipitation event: Nutrients, dissolved organic matter and their DBPs formation

To better understand the influence of heavy precipitation event on water supply security, the change of nutrients, dissolved organic matter (DOM) and their corresponding disinfection by-products (DBPs) generation in river were investigated through a heavy precipitation event. The results show that short-term sustained heavy precipitation causes additional load on the river. Significant temporal variations and increases in dissolved oxygen (DO), turbidity, dissolved organic carbon (DOC) and molecular weight of raw water were observed during the heavy precipitation, and dissolved organic nitrogen (DON) showed a remarkably decline with the continuation of precipitation. The molecular weight and aromaticity of DOM increased dramatically with the continuous superposition of precipitation and flow, and the proportion of hydrophobic fraction increased during the study period. Four different fluorescent components were identified, including humic-like (C1, C2 and C3), and protein-like fluorophore C4. Heavy precipitation events affected the mobilization of DOM, while the generation of DBPs increased by 4%–47% during the chlorination, and the elevated river level further increased the DBPs formation. In particular, the HOA fraction was the main contributor of DBPs (up to 70%). The risk of haloacetic acids (HAAs) generation caused by heavy rainfall was concentrated in the river runoff producing and regression periods for both hydrophobic and hydrophilic fractions of DOM. Humification index (HIX) was positively correlated with the yield level of TCM, DCAA, TCAA, and DCAN during chlorination. Chlorination disinfection of raw water during river runoff producing and regression period is likely to increase the haloacetonitriles formation potential (HANsFP), thus posing a vital threat to water supply safety, while chloramination disinfection may significantly relieve the threat. This study provides a theoretical basis for the direct/indirect link between short-term precipitation events and their impact on water quality.

Appling fluorescence spectroscopy with absolute principal component coefficient to explore dynamic migration of DOM fractions from an urbanized river during torrential rainfall

This research delves into the dynamic interplay between urbanization and the characteristics of Dissolved Organic Matter (DOM) in the Anyang River, particularly under the stress of torrential rain. The motivation stems from a critical need to decipher how urban landscapes influence water quality, focusing on the intricate transformations and movements of DOM. Employing advanced fluorescence spectroscopy techniques like Excitation-Emission Matrices (EEM) and Parallel Factor Analysis (PARAFAC), the study meticulously differentiates DOM compositions in urban and agricultural settings. It unveils a pronounced distinction, with urban streams showing elevated proteinaceous DOM from wastewater, contrasting with the humic substances prevalent in agricultural runoff. The analysis also captures how intense rainfall events catalyze significant shifts in DOM profiles, thereby emphasizing the need for tailored water quality management strategies in urbanized catchments. This comprehensive approach not only bridges gaps in understanding the urban impact on riverine ecosystems but also sets a foundation for future research and policy development in the face of escalating environmental changes.

Dissolved organic matter influences the indigenous bacterial community and polycyclic aromatic hydrocarbons biodegradation in soils

In polycyclic aromatic hydrocarbon (PAH) contaminated soils, bioremediation is superior to other strategies owing to its low cost and environmental friendliness. However, dissolved organic matter (DOM) and indigenous bacterial communities can affect the efficiency of PAH-degrading bacteria (PDB). This study found that exogenous PDB (C1) including the genera Acinetobacter, Stenotrophomonas, and Comamonas, decreased the bacterial diversity of Alfisol, Ultisol, Inceptisol, and Mollisol, and DOM enhanced the diffusion of PDB and the bioavailability of PAH. In addition, bacteria preferred to ingest low molecular weight DOM fractions, and the abundances of lipid-like and protein-like substances decreased by 0.12–3.03 % and 1.73–4.60 %. The DOM fractions had a more marked influence on the indigenous bacteria than the exogenous PDB, and PDB dominated the PAH biodegradation process in the soils. More COO functional groups promoted the utilization of higher molecular weight-related homologue fractions by bacteria, and lower molecular weight fractions carrying more CH2 functional groups declined during biodegradation. This study investigated the variations in bacterial communities during biodegradation and revealed the effects of DOM fractions on biodegradation in PAH-contaminated soils at the molecular level. These results will promote the development of bioremediation strategies for organics-contaminated soil and provide guidance for prediction models of soil biodegradation kinetics.

Suppression of mercury methylation in soil and methylmercury accumulation in rice by dissolved organic matter derived from sulfur-rich rape straw

Straw amendment significantly enhances mercury (Hg) methylation and subsequent methylmercury (MeHg) bioaccumulation in Hg-contaminated paddy fields by releasing dissolved organic matter (DOM). This study comprehensively investigates the regulatory mechanisms of DOM and its different molecular weights derived from sulfur-rich rape straw (RaDOM) and composted rape straw (CRaDOM) applied in the rice-filling stage on soil MeHg production and subsequent bioaccumulation in rice grains. The results indicated that the amendment of RaDOM and CRaDOM significantly reduced soil MeHg content by 42.40–62.42%. This reduction can be attributed to several factors, including the suppression of Hg-methylating bacteria in soil, the supply of sulfate from RaDOM and CRaDOM, and the increase in the humification, molecular weight, and humic-like fractions of soil DOM. Additionally, adding RaDOM increased the MeHg bioaccumulation factor in roots by 27.55% while inhibiting MeHg transportation by 12.24% and ultimately reducing MeHg content in grains by 21.24% compared to the control group. Similarly, CRaDOM enhanced MeHg accumulation by 25.19%, suppressed MeHg transportation by 39.65%, and reduced MeHg levels in the grains by 27.94%. The assimilation of sulfate derived from RaDOM and CRaDOM into glutathione may be responsible for the increased retention of MeHg in the roots. Over the three days, there was a significant decrease in soil MeHg content as the molecular weight of RaDOM increased; conversely, altering the molecular weight of CRaDOM demonstrated an inverse trend. However, this pattern was not observed after 12 days. Applying sulfur-rich rape DOM can help mitigate MeHg accumulation in paddy fields by regulating the quality of soil DOM, sulfur cycling, and Hg-methylating bacteria.

Investigating the impacts of solid phase extraction on dissolved organic matter optical signatures and the pairing with high‐resolution mass spectrometry data across a freshwater stream network

AbstractAdvancing our understanding of dissolved organic matter (DOM) chemistry in aquatic systems necessitates the integration of data streams from multiple analytical platforms. Some measurements require pretreatment with solid phase extraction (SPE), while others are performed directly on whole water samples. Evidence has suggested that SPE will be biased against select DOM fractions, leading to concerns over the ability to establish data linkages across platforms with variable needs for SPE pretreatment, such as those from optical measurements and those that provide high‐resolution molecular information. Here, we directly addressed this concern by assessing the impact of SPE on DOM optical properties through excitation–emission matrices with parallel factor analysis (PARAFAC) for 47 samples across a stream network within a single watershed reflective of variable DOM sources. PARAFAC data was further paired with molecular information obtained by Fourier transform ion cyclotron resonance mass spectrometry (FTICR‐MS). A comparison of PARAFAC models first revealed no systematic qualitative differences in major components between whole water DOM and DOM isolated by SPE (SPE‐DOM); however, quantitative biases against select components were observed. Further linkages with FTICR‐MS data revealed that the molecular fingerprint associated with each PARAFAC component was consistent between the whole water DOM and SPE‐DOM. Our results suggest that bulk scale linkages across these analytical platforms could be inferred irrespective of the observed quantitative biases resulting from SPE for samples within this example watershed. This work represents a key step toward the systematic evaluation of linkages between optical and high‐resolution mass spectrometry datasets in freshwater lotic environments.

Carbon Fate, Iron Dissolution, and Molecular Characterization of Dissolved Organic Matter in Thawed Yedoma Permafrost under Varying Redox Conditions.

Permafrost soils store ∼50% of terrestrial C, with Yedoma permafrost containing ∼25% of the total C. Permafrost is undergoing degradation due to thawing, with potentially hazardous effects on landscape stability and water resources. Complicating ongoing efforts to project the ultimate fate of deep permafrost C is the poorly constrained role of the redox environment, Fe-minerals, and its redox-active phases, which may modulate organic C-abundance, composition, and reactivity through complexation and catalytic processes. We characterized C fate, Fe fractions, and dissolved organic matter (DOM) isolates from permafrost-thaw under varying redox conditions. Under anoxic incubation conditions, 33% of the initial C was lost as gaseous species within 21 days, while under oxic conditions, 58% of C was lost. Under anoxic incubation, 42% of the total initial C was preserved in a dissolved fraction. Lignin-like compounds dominated permafrost-thaw, followed by lipid- and protein-like compounds. However, under anoxic incubation conditions, there was accumulation of lipid-like compounds and reduction in the nominal oxidation state of C over time, regardless of the compound classes. DOM dynamics may be affected by microbial activity and abiotic processes mediated by Fe-minerals related to selective DOM fractionation and/or its oxidation. Chemodiversity DOM signatures could serve as valuable proxies to track redox conditions with permafrost-thaw.

Applying synchronous fluorescence with Gaussian band fitting and MW-2DCOS to assess removals of DOM in acrylic fiber wastewater treatment process

Acrylic fiber wastewater (AFW) with hypertoxicity and recalcitrant are treated by advanced oxidation biological techniques. Some water quality parameters as online indicators to monitor pollutant removals, fail to trace removals of organic pollutants. Hence a practicable tool is indispensable for insight into removals of organic pollutants. Synchronous fluorescence spectroscopy (SFS) combined with Gaussian band fitting and two-dimensional correlation spectroscopy (2D-COS) was developed to evaluate removals of dissolved organic matter (DOM) fractions in an AFW treatment plant. Five fluorescence components were identified from DOM by SFS with principal component analysis and Gaussian band fitting, i.e., low-molecular-weight (LMWF1 and LMWF2), microbial humic-like (MHLF) and high-molecular-weight substances (HMWF1 and HMWF2). LMWF2 and HMWF2 were the representative components (52.96%) in the AFW, whose removal efficiencies (94.05% and 73.11%) were much higher than those of LMWF1 (40.16%) and HMWF1 (17.45%). Nitriles and amides occurred in LMWF2, whereas the aromatics in four DOM fractions. Based on 2D-COS, the changing order of DOM fractions was MHLF→ HMWF1→ nitriles→ LMWF2→ HMWF2→ amides→ LMWF1→ aromatics. The nitriles and HMWF1 were continuously removed in treatment process. LMWF2 was degraded by anaerobic and nitrifying bacteria, while the degradation of amides and HMWF2 by anaerobic and aerobic microorganisms. Furthermore, the nitriles could reduce microbial activities, which was mainly removed with the degradation of LMWF2. SFS combined with Gaussian band fitting and 2D-COS could be an applicable approach to monitor variations of DOM fractions and evaluate its removals in the AFW. These findings could be conducive to developing an online equipment in WWTP.

Adsorption-desorption of Atrazine with 9 Agricultural Soils in China.

In this work, the characteristics and mechanisms for atrazine adsorption-desorption with 9 types of soils were investigated with batch equilibrium studies, elemental analyses, infrared spectroscopy, and UV‒visible spectroscopy. The atrazine sorption data for the 9 soils showed better fits with the Freundlich model than the Langmuir model, except with Red earth in Jiangxi (REJ) The results showed that the adsorption capacity was positively correlated with the organic matter (OM) content and negatively correlated with cation-exchange capacity (CEC) and pH. UV‒visible spectroscopy showed that dissolved organic matter (DOM) in the soil enhanced atrazine adsorption, but the adsorption on different DOM fractions was quite different. In addition, the infrared spectra revealed differences in the functional groups of soils and these functional groups may drive the adsorption process via hydrogen bonding and coordination with the -NH2 groups in atrazine.

Controls of Mineral Solubility on Adsorption-Induced Molecular Fractionation of Dissolved Organic Matter Revealed by 21 T FT-ICR MS.

Mineral adsorption-induced molecular fractionation of dissolved organic matter (DOM) affects the composition of both DOM and OM adsorbed and thus stabilized by minerals. However, it remains unclear what mineral properties control the magnitude of DOM fractionation. Using a combined technique approach that leverages the molecular composition identified by ultrahigh resolution 21 T Fourier transform ion cyclotron resonance mass spectrometry and adsorption isotherms, we catalogue the compositional differences that occur at the molecular level that results in fractionation due to adsorption of Suwannee River fulvic acid on aluminum (Al) and iron (Fe) oxides and a phyllosilicate (allophane) species of contrasting properties. The minerals of high solubility (i.e., amorphous Al oxide, boehmite, and allophane) exhibited much stronger DOM fractionation capabilities than the minerals of low solubility (i.e., gibbsite and Fe oxides). Specifically, the former released Al3+ to solution (0.05-0.35 mM) that formed complexes with OM and likely reduced the surface hydrophobicity of the mineral-OM assemblage, thus increasing the preference for adsorbing polar DOM molecules. The impacts of mineral solubility are exacerbated by the fact that interactions with DOM also enhance metal release from minerals. For sparsely soluble minerals, the mineral surface hydrophobicity, instead of solubility, appeared to be the primary control of their DOM fractionation power. Other chemical properties seemed less directly relevant than surface hydrophobicity and solubility in fractionating DOM.

Interactions of manganese oxides with natural dissolved organic matter: Implications for soil organic carbon cycling

Metal (oxyhydr)oxides are key factors for the transformation and stabilization of organic carbon (OC) in soil. While sorptive interactions between Al and Fe (oxyhydr)oxides and dissolved organic matter (DOM) are well studied, the role of Mn(IV) oxides (manganates) in DOM sorption, fractionation, and oxidation has not been extensively investigated. Therefore, we examined sorptive interactions between manganates (δ-MnO2, birnessite, cryptomelane; 5 g L−1) and different DOM types (beech litter and Oa/Oe material under pine; ∼100 mg L−1 dissolved organic carbon, DOC) at pH 4 and 7 in different background electrolytes (BGE; no salt, 0.01 M NaCl or CaCl2) for up to 32 h. Changes of DOM solutions and mineral phases were assessed by solid, liquid, and gas analyses, and statistically evaluated for the effects of manganate, DOM type, pH, and BGE on DOC sorption, fractionation, and oxidative transformation. Manganates sorbed 0.2–8.7 mg OC g−1. Per unit mass, δ-MnO2 was least effective in DOC retention due to its high DOC oxidation capacity. Manganates sorbed more DOC at acidic pH and more aromatic pine than more aliphatic beech DOC, with Ca2+ generally facilitating DOC sorption. Up to 56 % of added DOC (average: 25 %) was oxidized to CO2, which is comparable to the extent of DOC respiration by soil microorganisms. DOC oxidation by δ-MnO2 and cryptomelane exceeded that of birnessite, which had the lowest specific surface area of all manganates. However, we found no difference in the efficiency of manganates to oxidize DOC, implying a similar redox activity of phyllo- and tectomanganates. More beech than pine DOM was oxidized to CO2 and an acidic pH facilitated DOC decomposition. While the presence of Na-BGE increased DOC oxidation to CO2 relative to no-salt treatments, Ca-BGE had the opposite effect, as Ca2+ apparently impeded the electron transfer from sorbed OC to structural Mn(III/IV). Contact of DOM with manganates also produced high concentrations of dissolved low-molecular-weight organic acids (mainly formate, acetate, and oxalate), accounting for up to 19 % of the initial DOC concentration. In addition, reduced specific ultraviolet absorbance of DOM solutions at 280 nm indicated preferential sorption of aromatic moieties, especially in Ca-BGE. However, in the absence of Ca2+ and at neutral pH, manganates increased the aromaticity of beech DOM, most likely due to polymerization reactions. No mineral transformations occurred after reaction of manganates with DOM, despite reductive manganate dissolution. Our results imply that soil manganates accumulate more OC in acidic soils and in presence of more aromatic DOM. However, manganates oxidatively destabilize DOC by generating CO2 and low-molecular-weight organic compounds, which is presumably more relevant in acidic soils with low concentrations of polyvalent cations and for more aliphatic DOM. The produced low-molecular-weight organic compounds may promote microbial activity and foster mineral weathering. Our results suggest a pivotal role of manganates in soil OC cycling, including the fate and bioavailability of nutrient elements associated with DOM, and in supporting ligand-promoted mineral weathering and soil formation.

Development and application of the UVC LED-based spectrophotometer for high frequency online monitoring of nitrate concentration in municipal wastewater

Developing a fast, sensitive and stable sensor for online monitoring nitrate concentration is an urgent need for the optimization of denitrification processes in wastewater treatment plants (WWTPs). In this study, the miniaturized spectrophotometric nitrate sensor was developed using UVC light-emitting diodes (LEDs) coupled with AlGaN or SiC-based photodiodes. The use of novel SiC photodiodes could effectively eliminate UVB and UVA light in the 230 nm LED emission spectra. Replacing 235 nm UVC-LED with 230 nm UVC-LED could effectively increase the detection sensitivity but not the linear range that follows the Lambert-Beer's law. The influences of dissolved organic matter (DOM) on nitrate analysis could be corrected by a unified coefficient α across the NO3--N concentration ranging in 0–50 mg/L. Across the investigated WWTP samples, the correction coefficient α was mainly determined by the fractions of humic substances with apparent molecular weight (MW) of 1.5–9.0 kDa, while the degradation of the large MW proteinaceous biopolymer resulted in dramatic variations of correction coefficient α along the WWTP A/A/O process. Microfiltration membranes with pore sizes ≤ 30 µm could sufficiently eliminate the influences of particles in effluents on photometric nitrate analysis. The ultrafiltration and enhanced coagulation treatment could effectively remove the biopolymer fractions of DOM in wastewater, and thus alleviate its influences on sensitivity and accuracy of nitrate analysis. The spectrophotometric nitrate sensors were successfully applied for on-line monitoring NO3--N concentrations in the influent and effluent of denitrifying biofilter in a municipal WWTP, providing high-frequency feedback signals to precise control of feeding external carbon source.

Unravelling molecular fractionation of dissolved organic matter on ferrihydrite-phosphate complexes

Association of dissolved organic matter (DOM) with iron (Fe) oxyhydroxides leading to significant molecular fractionation, plays a significant role in biogeochemical cycling of carbon in soils. However, the molecular effects of coexisting phosphate, a widely distributed oxyanion in agricultural soils that has a strong affinity with Fe oxyhydroxides on such a process remain poorly understood. Here, using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) with a superiority in deciphering DOM characteristics at the molecular level, this study reveals the DOM molecular fractionation characteristics on ferrihydrite-phosphate (Fh-P) complexes. Results show that the adsorption of DOM on Fh-P complexes can be significantly inhibited by ∼50% compared to that on ferrihydrite. Optical spectroscopy and FT-ICR-MS further indicate that phosphate can greatly change the molecular fractionation of DOM on Fh-P complexes. Phosphate preferentially inhibits the highly aromatic, hydrophobic, and high molecular weight compounds (> 400 Da) of DOM on Fh-P complexes, such as condensed hydrocarbon and tannin; more CHONS compounds preferentially adsorb on the Fh-P complexes than thoseon ferrihydrite. Phosphate inhibits the adsorption of DOM with the characteristics of high oxygen atoms number, aromaticity index (AI), and double bond equivalents (DBE) on ferrihydrite. Our study demonstrates that the inhibitory effects of phosphate on the adsorption of the DOM can significantly influence the fractionation and adsorption of DOM by ferrihydrite. The findings of this work deepen our understanding of biogeochemical cycling of DOM in soil and aquifer environments.

MIEX resin and enhanced coagulation treatment of high-bromide natural water: Chlorine reactivity and DBP precursors removal

Both organic and inorganic constituents play a major role in the formation and speciation of disinfection by-products (DBPs) under chlorination. In this study, the removal of organic and inorganic precursors to DBPs by ion exchange and coagulation was examined, and the relationships between characteristics of dissolved organic matter (DOM), chlorine reactivity, and the formation of DBPs in high-bromide natural water were investigated. Formation potential tests were conducted for trihalomethanes (THMs) and haloacetronitriles (HANs) before and after the different treatments (coagulation, magnetic ion exchange (MIEX) resin, and combined MIEX/coagulation). At a low resin dose, MIEX was more effective than coagulation for DOM removal. The advantages of the MIEX resin were more prominent at a high resin dose because substantial amounts of DOM and bromide were simultaneously removed. The maximum bromide uptake on the MIEX resin was 0.02 mg-Br/mL. A 4-component PARAFAC model was developed using a dataset of 148 fluorescence excitation-emission matrices (EEMs) generated from pre-chlorinated and post-chlorinated samples. The maximum fluorescence intensities of humic-like components were found to be strongly correlated to THMs formation (R2 = 0.79–0.89). The MIEX treatment substantially lowered the formation of THMs (19–51%) and HANs (70–95%) due to its ability to remove humic-like DOM and bromide. The fluorescence loss for humic-like DOM was found to increase over a chlorination time of 3 days, suggesting that these compounds had stronger reactivity to chlorine compared to protein-like DOM. The chlorine reactivity of protein-like DOM increased after treatments as the residual chlorine was able to react with the less reactive fractions of DOM after chlorine-reactive compounds were removed.

Effect of the molecular weight of DOM on the indirect photodegradation of fluoroquinolone antibiotics

Dissolved organic matter (DOM) is ubiquitous and widespread in natural water and influences the transformation and removal of antibiotics. Nevertheless, the influence of DOM molecular weight (MW) on the indirect photodegradation of antibiotics has rarely been reported. This study attempted to explore the influence of the molecular weight of DOM on the indirect photodegradation of two fluoroquinolone antibiotics (FQs), ofloxacin (OFL) and norfloxacin (NOR), by using UV–vis absorption and fluorescence spectroscopy. The results showed that indirect photodegradation was considered the main photodegradation pathway of FQs in DOM fractions. Triplet-state excited organic matter (3DOM*) and singlet oxygen (1O2) were the main reactive intermediates (RIs) that affected the indirect photodegradation of FQs. The indirect photodegradation rate of FQs was significantly promoted in DOM fractions, especially in the low molecular weight DOM fractions (L-MW DOM, MW < 10 kDa). The results of excitation-emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC) showed that terrestrial humic-like substances had a higher humification degree and fluorophore content in L- MW DOM fractions, which could produce more 3DOM* and 1O2 to promote the indirect photodegradation of FQs. This study provided new insight into the effects of DOM at the molecular weight level on the indirect photodegradation of antibiotics in natural water.

A multi-analytical approach to investigate DOM dynamics and alum dose control in enhanced coagulation process using wide-ranging surface waters

In this study, enhanced coagulation (EnC) − as a recommended water treatment process for disinfection by-product control − was investigated using jar testing, multi-analytical techniques, and a wide range of surface water sources. In addition to commonly used dissolved organic matter (DOM) concentration indices, DOM characteristics using UV–Vis spectroscopy, fluorescence EEM (fEEM) data, high-performance size exclusion chromatography (HPSEC), as well as turbidity, zeta potential and chlorophyll-a were studied. The results showed similar removal trends for the DOC, A254 and fluorescent DOM (fDOMs) signals with increasing alum dosing. Absorbance at ∼230 nm and fEEM data near Peak C in raw waters showed the highest correlations with enhanced dose. A decrease in weight-average apparent molecular weight following EnC, and a higher coagulability of DOM fractions with higher molecular weight, was noticed. The optimum zeta potential window was found to be −13 to +4 mV for EnC. Charge neutralization is suggested as the dominant mechanism for the EnC process. In addition to providing comprehensive information on the DOM dynamics during the EnC process, this study helps assess the potential of different water quality analysis techniques for EnC process monitoring and alum dose control, particularly using a field-deployable fDOM probe, offering potential for online, real-time applications.

Electrochemical oxidation treatment of reverse osmosis concentrated landfill leachate: Effect of operation parameters and evolution of dissolved organic matter

Electrochemical oxidation (EO) is a viable option for the advanced treatment of reverse osmosis concentrated landfill leachate (ROCLL). This work characterized dissolved organic matter (DOM) in ROCLL with size exclusion chromatography (SEC) and then investigated the impact of operating parameters on the removal of DOM fractions with response surface methodology (RSM). The SEC chromatograms revealed that the DOM fractions in ROCLL comprised of biopolymers (BP, > 18 kDa), humic substances (HS, 0.45–18 kDa) and low molecular weight substances (LMWS, < 0.45 kDa). For the EO configuration with different anodes, the BDD film anode showed the highest removal efficiency for DOM in terms of COD, DOC, UVA254 and fluorescence indices, followed by Ti, graphite, Ti/RuO2-IrO2 and Ti/SnO2-Sb2O5 anodes. The RSM experiments suggested that pH was the most critical factor to enhance the removal of all DOM fractions, and acidification pretreatment could remove approximately 50% BP and 30% HS fractions due to the pH-induced precipitation. The DOC removal kinetics of DOM fractions could be described by pseudo-zero kinetic model with rates in the order of HS > LMWS > BP, which were controlled by charge transfer process. Differently, the elimination of chromophores and fluorophores followed pseudo-first kinetics, which was limited by mass transport process. Via nonnegativity matrix factorization (NMF) deconvolution of the SEC-OCD chromatograms, BP-C1, HS-C2 and LMWS-C1 subfractions required to be removed through further treatment after EO process. This work demonstrates the potential for SEC analysis coupled with mathematical approaches to give informative insights into the evolution of DOM and provide optimization strategies for the EO treatment of complex ROCLL.

Characterizing dissolved organic matter and bacterial community interactions in a river network under anthropogenic landcover

Anthropogenic landcover could rise nutrient concentrations and impact the characteristics and bioavailability of dissolved organic matter (DOM) in a river network. Exploring the interactions between DOM and microbials might be conducive to revealing biogeochemistry behaviors of organic matter. In this study, synchronous fluorescence spectra (SFS) with Gaussian band fitting and two-dimensional correlation spectroscopy (2D-COS) were employed to identify DOM fractions and reveal their interactions with bacterial communities. DOM was extracted from a river network under eco-agricultural rural (RUR), eco-residential urban (URB), eco-economical town (TOW), and eco-industrial park (IND) regions in Jiashan Plain of eastern China. The overlapping peaks observed in the SFS were successfully separated into four fractions using Gaussian band fitting, i.e., tyrosine-like fluorescence (TYLF), tryptophan-like fluorescence (TRLF), microbial humic-like fluorescence (MHLF), and fulvic-like fluorescence (FLF) materials. Across all four regions, TRLF (44.79% ± 7.74%) and TYLF (48.09% ± 8.85%) were the dominant components. Based on 2D-COS, variations of TYLF and TRLF were extremely larger than those of FLF in RUR-TOW. However, in URB-IND, the former exhibited lower variations compared to the latter. These suggested that FLF be likely derived continuously from lignin and other residue of terrestrial plant origin along the river network, and TYLF and TRLF be originated discontinuously from domestic wastewater in RUR-TOW. By high-throughput sequenced OTUs, the number of organisms in RUR-TOW could be higher than those in URB-IND, while genes associated with carbohydrate metabolism were lower in former than those in the latter. According to co-occurrence networks, microbes could promote the production of TYLF and TRLF in RUR-TOW. In contrast, microbial communities in URB-IND might contribute to decompose FLF. The obtained results could not only reveal interactions between DOM fractions and bacterial communities in the river network, but this methodology may be applied to other water bodies from different landscapes.