Non-catalytic and catalytic degradation of effluent dissolved organic matter under UVA-and UVC-irradiation tracked by advanced spectroscopic tools

Behavior of organic components and the migration of heavy metals during sludge dewatering by different advanced oxidation processes via optical spectroscopy and molecular fingerprint analysis

A comparative study of the different advanced oxidation processes (Fe(II)-Oxone, Fe(II)-H2O2, and Fe(II)-NaClO) was carried out herein to analyze the characteristics of organic components and the migration of heavy metals in waste activated sludge. With the Fe(II)-Oxone and Fe(II)-H2O2 treatments, sludge dewaterability was significantly improved, however, sludge dewaterability was deteriorated by the Fe(II)-NaClO treatment. The enhanced sludge dewaterability by the Fe(II)-Oxone and Fe(II)-H2O2 treatments was strongly correlated with the shifted organic components, particularly proteins, in soluble extracellular polymeric substances (S-EPS), while the deteriorated sludge dewaterability by the Fe(II)-NaClO treatment was strongly correlated with the over release of organic components from bound EPS (B-EPS) to S-EPS. For both the Fe(II)-Oxone and Fe(II)-H2O2 treatments, the radicals preferentially attacked humic acid-like organic components over the protein-like organic components in S-EPS, while for the Fe(II)-NaClO treatment, interestingly, the radicals preferentially attacked the protein-like organic components in both S-EPS and B-EPS. The hydrophilic functional groups like phenolic OH and CO of polysaccharides may be more preferentially migrated to S-EPS of sludge by the Fe(II)-NaClO treatment compared to the other two treatments. With the Fe(II)-Oxone and Fe(II)-H2O2 treatments, the proportion of aliphatic compounds as well as the much oxygenated organic components with a low desaturation and a low molecular weight increased. While with the Fe(II)-NaClO treatment, the proportion of low oxygenated organic components with a high desaturation and a high molecular weight increased. The concentration of total organic carbon, particularly the concentration of proteins, may be the key factor determining the shift of Zn and Cu from sludge solid to liquid phase, along with the high oxidation extent of organic components and close binding to CHOS and CHON compounds as indicated by density functional theory (DFT) calculation. This study systematically revealed the simultaneous sludge dewatering and migration of heavy metals when the role of organic components was factored into herein.

Interactions between white and black carbon in water: A case study of concurrent aging of microplastics and biochar

Microplastics and biochar, as particulate matter that is prevalent in the water environment, will inevitably encounter and interact with each other during environmental aging. The potential interaction of microplastics and biochar, and the associated impact on their environmental behavior remains largely unknown. In this study, we exposed microplastics and biochar concurrently to ultraviolet light to mimic the aging process, investigated the release and fluorescence characteristics of dissolved organic matter (DOM) in water, and analyzed the effects of co-existing microplastics and biochar on their sorption of organic contaminants. We demonstrate that early-stage interactions of microplastics and biochar could entangle to promote the release of DOM from biochar, while their long-term interactions after light irradiation resulted in the sorption of hydrophobic and small molecules of microbial byproduct-like DOM. Simultaneously, early-stage interactions of microplastics and biochar showed a promotion for sorption of organic contaminants with an increase of 5.3-17.7%. After aging, however, long-term interactions between microplastics and biochar made it no longer promote the sorption of organic contaminants due to the influence of heterogeneous aggregation. Our results provide new insights into the time-dependent interactions between microplastics and biochar and highlight the need to incorporate their interactions into future environmental risk assessments for microplastics in the water environment.

Interfacial reactions of catalytic ozone membranes resulting in the release and degradation of irreversible foulants

An efficient in-situ self-cleaning catalytic ceramic-membrane tailored with MnO₂-Co₃O₄ nanoparticles (Mn-Co-CM) was fabricated. Density functional theory calculations result substantiated that molecular ozone could be effectively adsorbed by oxygen vacancies (OV) on the Mn-Co-CM surface and then direct activated into a surface-bound atomic oxygen (*O_ad) and a peroxide (*O_{2, ad}), ultimately producing ^·OH. Mn-Co-CM coupling with ozone efficiently removed foulants from the permeate and the membrane surface simultaneously and leading to in-situ formation of ^·OH that changed the nature of the irreversible foulants and ultimately resulted in the rapid release and degradation of humic acid-like substances causing irreversible fouling. However, the commercial CM with ozone mainly removed cake layer fouling including protein-like and fulvic acid-like substances, followed by the slow release and degradation of irreversible foulant, resulting in many humic acid-like substances remain on the membrane surface as irreversible fouling. Based on these, the flux growth rate of Mn-Co-CM was 3.5 times higher than that of CM with ozone. This study provides new insights into the mechanism of in-situ membrane fouling mitigation, when using an efficient catalytic ceramic-membrane. This will facilitate the development of membrane antifouling strategies.

New insights into the transformation of effluent organic matter during Fe(II)-assisted advanced oxidation processes: Parallel factor analysis coupled with self-organizing maps

The negative effects of effluent organic matter (EfOM) on receiving aquatic environments and advanced treatment facilities pose significant concerns. However, the effective removal of EfOM is challenging due to its chemically complex nature and its refractory characteristics. In this study, two Fe(II)-assisted oxidation processes including UV/Fe(II)/H₂O₂ and UV/Fe(II)/persulfate (UV/Fe(II)/PS) were investigated to promote EfOM reduction. Fe(II) was essential for promoting EfOM degradation. The mineralization rate of EfOM increased from 7 to 29% with 2 mM Fe(II) addition in the UV/H₂O₂ process and to 23% with 0.8 mM Fe(II) addition in the UV/PS process. A preliminary experiment was conducted to obtain the optimal molar ratio of oxidant to Fe(II) for practical applications based on different indicators. The form of Fe(III) prevalent at different pH values strongly affected Fe(II)/Fe(III) cycling, thus determining the progress of EfOM degradation. A machine learning approach consisting of parallel factor analysis coupled with self-organizing maps (PARAFAC-SOM) was employed with fluorescence spectra to visualize the degradation behavior of EfOM in the different reaction systems. Four components (i.e., two humic-like substances, one fulvic acid, and one tryptophan-like substance) were eventually identified, and their reductions reached more than 62% during the Fe(II)-assisted oxidation processes. The degradation orders for each component in the different oxidation processes were initially evaluated by SOM analysis with F_max percentage data. The degradation behavior of EfOM in the UV/Fe(II)/H₂O₂ and UV/Fe(II)/PS systems exhibited different trends based on the best matching unit map and component planes. The humic-like component was more refractory than the other three components in both oxidation processes. The microbial humic-like and high-molecular-weight fulvic acid substances showed higher reactivity with SO·₄^- than with ·OH, while the tryptophan-like substance was more reactive in the UV/Fe(II)/H₂O₂ system than in the UV/Fe(II)/PS system. The outcomes of this study provide new insights into the degradation behavior of EfOM, promoting the development of advanced wastewater treatments.

Variations in NOM during floc aging: Effect of typical Al-based coagulants and different particle sizes

Most studies on the interaction between coagulation and NOM (natural organic matter) currently focus on pollutant removal and coagulant species distribution, while studies on floc aging are lacking. Investigation onto the effects of floc aging could guide further processes that utilize flocs, such as densadeg sludge recirculation, floc predeposition for ultrafiltration, sludge condensation, and other traditional sludge reflux processes. In this study, flocs generated by Al₁₃ and AlCl₃ in microparticle- and nanoparticle-containing water were investigated, and the effect of floc aging on NOM was quantified based on several organic matter characterization techniques. Flocs absorb and release organics during aging. The flocs generated from micro-SiO₂ have a significant absorbing effect for LWM-N (low-molecular-weight neutral substances) and protein-like substances, while the absorption of NOM by flocs generated from nano-SiO₂ is insignificant. HS (humic substances) with high aromaticity are released during floc aging. From the molecular perspective, the molecules released during floc aging are those with higher double bond equivalents and higher aromaticity, while the absorbed molecules are those with lower double bond equivalents and lower aromaticity. 2D-COS (two-dimensional correlation spectroscopy) demonstrated that the flocs generated by Al₁₃ and AlCl₃ had the same organic release patterns but different intensities, while the flocs generated in the micro-SiO₂ and nano-SiO₂ systems had different organics release patterns. Abundant aluminum hydrolysates with low polymerization and amorphous Al(OH)₃ would be produced from AlCl₃ during the coagulation process and then undergo hydroxyl‑bridging reaction and crystallization during floc aging, thus releasing more HS with high aromaticity into the supernatant; in comparison, prehydrolyzed Al₁₃ produces a more stable floc and releases less HS during aging. The flocs produced by nano-SiO₂ and Al-based coagulants release higher aromaticity HS into the water than those produced by micro-SiO₂, which may be related to the formation of more highly polymerized degree hydrolysates and nanocrystalline Al(OH)₃ in the nano-SiO₂ system. The flocs generated in water with micro-SiO₂ may contain a large amount of Al-OH and have a loose structure, thus further absorbing NOM, such as protein-like substances and LWM-N. In contrast, the flocs generated from nano-SiO₂ possess abundant adsorbed water and a denser structure; thus, organic matter cannot be absorbed stably.

Photo-persulfate oxidation and mineralization of benzoic acid: Kinetics and optimization under UVC irradiation

The strong oxidant, persulfate (PS, S₂O₈^2-), was applied to treat the synthetic wastewater of benzoic acid (BA) under UV irradiation. UVC light initiated a chain reaction that derived the sulfate radical (SO₄•^-) and hydroxyl radical (HO•) from S₂O₈^2- ion. The experiment parameters, including light irradiation (UVA and UVC), pH, dose ratio ([PS]₀/[BA]₀), initial concentration ([BA]₀, mg/L), was optimized based on degradation efficiency and total organic carbon (TOC) removal of BA, which reached up to 100% and 96%, respectively, under pH 3.0. The best dose ratio was close to equivalent stoichiometry (and [PS]₀/[BA]₀ = 15) for the treatment of 100 mg-BA/L, suggesting that UV/S₂O₈^2- was able to completely convert BA to carbon dioxide and water. The scavenging test showed that SO₄•^- contributed to about 60% of degradation rate, which the HO• predominated the mineralization rate, i.e., TOC removal. A consecutive kinetic model was proposed to clarify the reaction sequence and rate-determining factor of photo-persulfate oxidation for benzoic acid.

Organic removal from coal-to-chemical brine by a multistage system of adsorption-regeneration and electrochemically driven UV/chlorine processes

Treatment of coal-to-gas brine (CGB) is a great challenge since it contains elevated inorganic salts and a high level of toxic and bio-accumulative organics. In this study, CGB treatment was conducted by adsorptionregeneration and electrochemically driven UV/chlorine (E-UV/Cl₂) processes. LS-109D macroporous resin was optimal adsorbent primarily due to unique pore structure, which preferably adsorbed the aromatic fluorescent components with quenching Cl∙ effect and low molecular weight acids recalcitrant to ∙OH. The E-UV/Cl₂ process outperformed the UV photolysis process and electrochemical advanced oxidation processes (EAOPs) for oxidation of organic compounds due to its full utilization of Cl^- in CGB to produce highly active oxidation agents. Thanks to the synergy between process units in organic matter removal, dissolved organic carbon (DOC) of CGB was reduced from 163.41 mg/L to 26.58 mg/L by the multistage system. Furthermore, the CGB with characteristics of high fluorescence and molecular weight (MW) distribution was converted to effluent with low fluorescence and MW distribution. The exhausted LS-109D was regenerated by ultrasound-assisted hot water elution at 363 K. After pretreated by ozonation, the eluate can be easily treated by biological process. The study suggests that the multistage system can provide an effective treatment option for removing organics from CGB.

Mineralization, characteristics variation, and removal mechanism of algal extracellular organic matter during vacuum ultraviolet/ozone process

Extracellular organic matter (EOM) produced by algal blooms in source water is detrimental to drinking water treatment processes and supplied water quality. Ozonation has been used to treat algal EOM, but it could not mineralize EOM effectively. In this study, mineralization and characteristics variation of EOM by vacuum ultraviolet/ozone (VUV/O₃) and its sub-processes were comprehensively investigated. Results showed that EOM removal in different processes followed the order of VUV/O₃ > UV/O₃ > O₃ > VUV > UV. For VUV/O₃ process, removal efficiencies of dissolved organic carbon (DOC), UV₂₅₄, protein, and polysaccharide at 50 min were 75.6%, 80.8%, 80.1%, and 78.0%, respectively, and fluorescence components received very high removal rates (≥92.8%, at 10 min). The yield of trichloromethane dropped from 102.0 to 30.1 μg/L after treating for 50 min by VUV/O₃. Besides, effects of O₃ dosage, initial pH, and water matrices on EOM removal in VUV/O₃ process were investigated. Moreover, fluorescent molecular probe experiments confirmed that hydroxyl radical and superoxide radical were the main reactive oxygen species (ROS) in VUV/O₃ process, and the transformation of ROS was proposed. The mechanism of EOM removal by VUV/O₃ included VUV photolysis, direct O₃ oxidation, and ROS oxidation. Furthermore, the removal of EOM in filtered water by VUV/O₃ was satisfactory. All results indicated that VUV/O₃ process had great application potential in treating EOM-rich filtered water.

Application of UV radiation for in-situ Cr(VI) reduction from contaminated soil with electrokinetic remediation

Restoring hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) from contaminated soil is a cost-effective alternative for attenuating Cr(VI) toxicity to the ecosystem. A new electrokinetic remediation (EKR) system with UV light was explored to overcome an energy barrier to catalyze Cr(VI) reduction from the surface soil near the anodic reservoir. Natural organic matters and minerals from the contaminated soil acted as electron donors and catalysts for Cr(VI) photo-reduction and no additional chemical reagent. There was almost no residual Cr(VI) in anolyte after UV/EKR compared with the conventional EKR. The reduction improved the efficiency of EKR in the soil near the anodic reservoir by dropped the Cr(VI) negative mass flux caused by electroosmosis advection and concentration diffusion. The pathways of Cr(VI) photo-reduction are possibly dominated by ligand-to-metal charge transfer, i.e., photocatalytic cyclic reduction by Fe(III)/Fe(II) complexes on the surface of the minerals and in soil pore fluid and the photo-induced decomposition of chromate ester. It is concluded that UV/EKR is a clean, efficient, and low-cost method for remediation of Cr(VI)-contaminated soil.

Advances in the characterization and monitoring of natural organic matter using spectroscopic approaches

Natural organic matter (NOM) is ubiquitous in environment and plays a fundamental role in the geochemical cycling of elements. It is involved in a wide range of environmental processes and can significantly affect the environmental fates of exogenous contaminants. Understanding the properties and environmental behaviors of NOM is critical to advance water treatment technologies and environmental remediation strategies. NOM is composed of characteristic light-absorbing/emitting functional groups, which are the "identification card" of NOM and susceptive to ambient physiochemical changes. These groups and their variations can be captured through optical sensing. Therefore, spectroscopic techniques are elegant tools to track the sources, features, and environmental behaviors of NOM. In this work, the most recent advances in molecular spectroscopic techniques, including UV-Vis, fluorescence, infrared, and Raman spectroscopy, for the characterization, measurement, and monitoring of NOM are reviewed, and the state-of-the-art innovations are highlighted. Furthermore, the limitations of current spectroscopic approaches for the exploration of NOM-related environmental processesand how these weaknesses/drawbacks can be addressed are explored. Finally, suggestions and directions are proposed to advance the development of spectroscopic methods in analyzing and elucidating the properties and behaviors of NOM in natural and engineered environments.

Monitoring dissolved organic matter in wastewater and drinking water treatments using spectroscopic analysis and ultra-high resolution mass spectrometry

Dissolved organic matter (DOM) plays a critical role in determining the quality of wastewater and the safety of drinking water. This is the first review to compare two types of popular DOM monitoring techniques, including absorption spectroscopy and fluorescence excitation-emission matrices (EEMs) coupled with parallel factor analysis (PARAFAC) vs. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), for the applications in wastewater and drinking water treatments. The optical techniques provide a series of indices for tracking the quantity and quality of chromophoric and fluorescent DOM, while FT-ICR-MS is capable of identifying thousands of DOM compounds in wastewater and drinking water at the molecule level. Both types of monitoring techniques are increasingly used in studying DOM in wastewater and drinking water treatments. They provide valuable insights into the variability of DOM composition in wastewater and drinking water. The complexity and diversity of DOM highlight the challenges for effective water treatments. Different effects of various treatment processes on DOM are also assessed, which indicates that the information on DOM composition and its removal is key to optimize the treatment processes. Considering notable progress in advanced treatment processes and novel materials for removing DOM, it is important to continuously utilize these powerful monitoring tools for assessing the responses of different DOM constituents to a series of treatment processes, which can achieve an effective removal of DOM and the quality of treated water.

Leachability of endocrine disrupting chemicals (EDCs) in municipal sewage sludge: Effects of EDCs interaction with dissolved organic matter

In this study, experiments were performed to assess the significance of dissolved organic matter (DOM) on the leachability of four common EDCs, i.e., bisphenol A (BPA), 17α-ethinylestradiol (EE2), progesterone (PGT) and testosterone (TST), in municipal sewage sludge (MSS) under landfill conditions. The DOM was derived from two sources: MSS (MDOM), and natural soil represented by organic matter obtained from the Suwannee River (NDOM). Fluorescence excitation-emission matrix quenching combined with parallel factor analysis was adopted to characterize the interaction properties between the EDCs and DOM. The accumulative leachability of the target EDCs ranged from 0.09% (PGT) to 3.8% (TST). In particular, the leaching of BPA, EE2 and TST followed S-shaped curves, while PGT exhibited continuous leaching potential in untreated MSS. With the introduction of DOM, (i) the leachability of BPA and EE2 increased to 13.4% and 61.6%, respectively, whereas those of PGT and TST declined by 61.3% and 45.8%, respectively, and (ii) BPA, EE2 and PGT no longer reached leaching equilibrium but the S-shaped leaching property of TST persisted. The differential effects of MDOM and NDOM at identical concentrations on the EDCs leachability increased with curing time. BPA, EE2 and PGT quenched the MDOM fluorophores attributed to aromatic protein-like components. The fluorescence quenching of NDOM by BPA, EE2 and PGT was centered on soluble microbial by-product-like and humic-like substances. Compared with PGT, EE2 and BPA had greater capability for binding with DOM components largely via hydrophobic interactions, whereas PGT preferentially interacted with the DOM hydrophilic functionalities through specific interactions. TST had no binding capability but displayed potentials competing for sorption sites with DOM moieties. Our findings suggested that the management of MSS increased the risk of environmental contamination by EDCs for a long duration and that DOM was a useful indicator to predict the migration and transport properties of EDCs.

Fluorescence Signatures of Dissolved Organic Matter Leached from Microplastics: Polymers and Additives

Despite the numerous studies that have investigated the occurrence and fate of plastic particles in the environment, only a limited effort has been devoted toward exploring the characteristics of dissolved organic matter (DOM) leached from microplastics. In this study, using excitation emission matrix-parallel factor analysis (EEM-PARAFAC), we explored the fluorescence signatures of plastic-derived DOM from commonly used plastic materials, which included two polymers (polyvinyl chloride (PVC) and polystyrene (PS)), two additives (diethylhexyl phthalate (DEHP) and bisphenol A (BPA)), and two commercial plastics. The exposure of the selected plastics to UV light facilitated the leaching of DOM measured in terms of dissolved organic carbon and fluorescence intensity. Four fluorescent components were identified, which included three protein/phenol-like components (C1, C3, and C4) and one humic-like component (C2). The C1 and C4 components were highly correlated with the amounts of DOM leached from DEHP and BPA, respectively, under both leaching conditions, while both C2 and C4 presented good correlations with the DOM leached from polymers under UV light. The C4 may serve as a good fluorescence proxy for DOM leached from BPA or BPA-containing plastics. This study highlights the overlooked issue of plastic-derived DOM leaching into the aquatic environment through optical characterization.

TiO2 microspheres with cross-linked cyclodextrin coating exhibit improved stability and sustained photocatalytic degradation of bisphenol A in secondary effluent

Photocatalytic water treatment has significant potential to disinfect and degrade recalcitrant organic pollutants while minimizing the need to add chemicals, but current approaches have poor energy efficiency due, in part, to inefficient utilization of photo-generated reactive oxygen species (ROS). Organic coatings such as cyclodextrin (CD) can adsorb target contaminants and bring them close to the photocatalyst surface to enhance ROS utilization efficiency, but the coatings themselves are susceptible to ROS attack. Here, we report an ROS-resistant fluorinated CD polymer (CDP) that can both adsorb contaminants and resist degradation by ROS, yielding a more efficient material for "trap and zap" water treatment. We produced the CDP through condensation polymerization of β-cyclodextrin and tetrafluoroterephthalonitrile, resulting in a cross-linked, covalently bound CD film that is much more stable than prior approaches involving physi-sorption. We optimized the coating thickness on TiO₂ microspheres to improve the efficiency of contaminant degradation, and found that increasing the CDP content enhanced BPA adsorption but also occluded photocatalytic sites and hindered photocatalytic degradation. The optimum content of CDP was 5% by weight, and this optimal CDP-TiO₂ composition had a BPA adsorption capacity of 36.9 ± 1.0 mg g^-1 compared with 24.1 ± 1.1 mg g^-1 for CD-coated TiO₂ (CD-TiO₂) and 21.9 ± 1.5 mg g^-1 for bare TiO₂. CDP-TiO₂ exhibited minimal photoactivity loss after 1000 h of repeated use in DI water under UVA irradiation (365 nm, 3.83 × 10^-6 E L^-1s^-1), and no release of organic carbon from the coating was detected. Photocatalytic treatment using CDP-TiO₂ only showed a small decrease in BPA removal efficiency in secondary effluent after four 3-h cycles, from 80.2% to 71.7%. In contrast, CD-TiO₂ and P25 removed only 29.8% and 6.2% of BPA after 4 cycles, respectively. Altogether, the CDP-TiO₂ microspheres represent promising materials for potential use in photocatalytic water treatment.

Investigation of hydrolysis acidification process during anaerobic treatment of coal gasification wastewater (CGW): Evolution of dissolved organic matter and biotoxicity

Coal gasification wastewater (CGW) contains several types of aromatic pollutants, which impart high biotoxicity and reduce the quality of anaerobic treatment. Two types of hydrolysis acidification processes, namely microaerobic hybrid reactor (HA-1) and upflow anaerobic sludge blanket reactor (HA-2), were developed for pre-treatment before the anaerobic treatment. The changes in the dissolved organic matter and biotoxicity were investigated to comprehensively understand the degradation process. The results showed that HA-2 coupled with an anaerobic reactor achieved a 12.3% and 13.4% higher removal efficiency for chemical oxygen demand and total phenols, respectively, compared with the coupled process with HA-1. Furthermore, HA-2 could transform macromolecules into small molecules more efficiently and produce fewer intermediates. The coupled process with HA-2 preferentially removed complex aromatic substances with absorption wavelengths of 285 and 254 nm, according to the sequential orders interpreted from two-dimensional correlation spectroscopy. In addition, the results of fluorescence excitation-emission-matrix with regional integration analysis revealed that the contents of typical cyclic compounds in CGW, such as phenolic, heterocyclic, and polycyclic aromatic compounds were remarkably reduced by HA-2. In addition, HA-2 reduced the toxic unit value of CGW by 67.5% and increased the resazurin dehydrogenase activity of the sludge by 37.5% during CGW treatment, thus improving the biotoxicity removal and biodegradability. However, the coupled process with HA-2 did not significantly affect the "indirect estrogenic activity" of CGW. A Pearson correlation analysis indicated that spectral indicators, such as UV₂₅₄ and Φ_T,n, presented a high positive correlation with the reduction of acute toxicity and organics.

Hydrogen production from natural organic matter via cascading oxic-anoxic photocatalytic processes: An energy recovering water purification technology

Photocatalysis provides a "green" strategy to produce the clean energy of H₂. However, the realization of efficient H₂ production is usually accomplished by the consumption of electron donors, which are costly energy carriers themselves. Here, we attempted to utilize the naturally abundant humic acid (HA), a representative natural organic matter (NOM), as the source of electron donor in a cascading oxic-anoxic photocatalytic system. Results showed that degradation of HA and remarkable H₂ yield (1660.9 μmol g^-1 h^-1 at optimal condition) were obtained successively, whereas the anoxic photocatalytic treatment of pristine HA did not improve H₂ yield but substantially eliminated the H₂ production and HA degradation efficiency. These phenomena suggested the preoxidation process played a vital role in counteracting the detrimental effect of HA on photocatalytic H₂ production. Electrochemical measurement indicated that the preoxidized HA harbored more redox-active moieties than the untreated HA and thus leading to a higher photo-induced charge carrier separation efficiency. A variety of advanced spectroscopic analyses revealed that the photocatalytic oxic pre-treatment resulted in breakdown of chemically inert, electron mediating and chromophoric aromatic macrostructure of HA to form smaller sized oxygenated organic intermediates. These intermediates were more nucleophilic than the pristine HA and acted as sacrificial reagent in the subsequent anoxic process for boosting H₂ production. This study showcases an energy recovering water remediation process and paves the way for the design of novel photocatalytic technologies for environmental application.

Photocatalytic degradation of organic micropollutants: Inhibition mechanisms by different fractions of natural organic matter

Natural organic matter (NOM) can inhibit the photocatalytic degradation of organic micropollutants (OMPs) through inner filter effect, reactive oxygen species (ROS) scavenging, and competitive adsorption. However, previous studies have focused solely on the bulk properties of NOM and our understanding of the inhibition mechanism by NOM fractions during photocatalytic degradation of OMP is still fragmentary. In this study, five well-characterized different NOM samples (i.e., secondary treated wastewater, river water, and three standard NOM surrogates) were used to elucidate the inhibition mechanisms during photocatalytic degradation of carbamazepine (a model OMP) using TiO₂ and its composites with carbon nanotubes (CNT-TiO₂) under UVC and solar-light irradiation. The results indicated that terrestrially derived NOM with high aromaticity, a low oxygen/carbon atom ratio, and large molecular weight is the major fraction that participates in ROS scavenging, competitive adsorption, and inner filter effect. Furthermore, the modeling analysis suggested that inner filter effect due to NOM and ROS scavenging was the most influential inhibitory mechanism. In the case of secondary treated wastewater, the presence of high concentrations of inorganic species (e.g., PO₄^3-, Cl^-, and NO₃^-) together with NOM significantly reduced the photocatalytic degradation of carbamazepine. Overall, the methods and the results of this study provide a comprehensive understanding of the effects of NOM fractions on photocatalysis and highlight the need to further consider the interplay between NOM and background inorganic constituents in photocatalytic degradation of OMP.

Formation of assimilable organic carbon (AOC) during drinking water disinfection: A microbiological prospect of disinfection byproducts

Disinfection processes might alter the chemical structure of biological recalcitrant natural organic matter (NOM) in source water to form assimilable organic carbon (AOC), which can be readily utilized by microbes for growth. However, AOC has not been classified as disinfection byproducts (DBPs) before and little is known about the chemical and structural nature of AOC. This study, for the first time, considers the disinfection-induced AOC as DBPs from a microbiological perspective. The AOC formation by three types of disinfection processes, i.e., chlorination, UVC irradiation (254 nm) and photocatalysis represented by TiO₂-UVA in drinking water containing two reference NOM materials of Suwannee River and Nordic Reservoir (SRNOM and NRNOM, respectively) were comparatively benchmarked using Pseudomonas aeruginosa as inoculum. Results showed that chlorination caused a substantial increase in AOC content, whereas TiO₂-UVA led to a moderate increase in AOC content and UVC rendered the AOC content unchanged, independent of the types of NOM. Molecular weight indicated by spectral slope ratio and fluorescence fingerprint were found to not provide critical information about the AOC formation potential. FTIR and FT-ICR-MS results indicated that the AOC formation by chlorination was attributed to the oxidation and chlorine substitution on aromatic molecules to form molecules with carboxylic- and alcohol- functionalities, as well as chlorinated aromatics. These molecules could be metabolized and assimilated by Pseudomonas species by a catechol pathway. The results obtained in this study can provide valuable insight regarding the selection of proper technologies for disinfection to prevent microbial growth/regrowth in the distributing system and is intended to encourage more thinking and research on AOC as a new prospect of DBPs during disinfection of drinking water.

Using EEM-PARAFAC to probe NF membrane fouling potential of stabilized landfill leachate pretreated by various options

Pretreatment processes substantially modify the organic composition of landfill leachate, which affect the fouling behavior in the post-treatment of membrane filtration. In this study, the changes in the chemical composition of stabilized landfill leachate upon various pretreatments, which encompassed coagulation/flocculation (C/F), ion exchange resins (MIEX), granular activated carbon (GAC) adsorption, and their combinations, were tracked via excitation emission matrix - parallel factor analysis (EEM-PARAFAC), and the membrane fouling potentials were assessed in the subsequent processes of nanofiltration (NF). Fluorescence components, fulvic-like (C1), protein-like (C2), and humic-like (C3), were identified and validated using EEM-PARAFAC. MIEX and C/F pretreatments were not effective to remove C1 and C2, which were associated with relatively small sized and hydrophilic molecules. GAC adsorption did not show any preference with the removal towards different components. These differences in the chemical heterogeneity among the variously pretreated leachates led to the discrepancies in membrane fluxes at a similar leachate concentration. The result also signified the importance of probing the chemical composition of pretreated leachate for the optimization of the post membrane filtration. The sum of C2 and C3 in the pretreated leachate showed a good correlation with reversible membrane fouling resistance (r = 0.93; p < 0.05), while C1 was highly correlated with irreversible membrane resistance (r = 0.872; P < 0.05). These findings provided a new insight into the applicability of fluorescence spectroscopy for tracking the changes in the membrane fouling potential of stabilized landfill leachate after various pretreatments.

A comparison of dissolved organic matter transformation in low pressure ultraviolet (LPUV) and ultraviolet light-emitting diode (UV-LED)/chlorine processes

This study compared the degradation of dissolved organic matter (DOM) by UV/chlorine advanced oxidation processes (AOPs) with emerging ultraviolet light-emitting diode (UV-LED, 275 nm) and traditional low pressure UV (LPUV, 254 nm) as UV sources. Excitation emission matrix-parallel factor (EEM-PARAFAC) analysis and two-dimensional (2D) correlation gel permeation chromatograph were applied to explore the evolutions of DOM during oxidation processes. The degradation behaviors of DOM indicated by UV absorbance at 254 nm (UV₂₅₄), dissolved organic carbon (DOC), and fluorophores fitted the pseudo-first-order kinetics well. The removal efficiency of DOM was similar under UV-LED and LPUV irradiation alone. However, UV-LED exhibited much higher degradation rates (increased by 29-160%) than LPUV regardless of the tracking variables during UV/chlorine processes. For three PARAFAC components, humic-like fluorescences were preferentially degraded by UV/chlorine oxidation compared with protein-like fluorescence potentially due to the differences of electronic moieties and molecular weight (MW). The decline in UV₂₅₄, DOC, and fluorophores increased with increasing chlorine dosage; linear correlations between those indicators were observed during the two AOPs. Moreover, UV-LED/chlorine could achieve greater extents of MW change. Our study demonstrated that UV-LED could be a superior alternative for the future selection of UV source in the UV/chlorine process.

Visible light-activated degradation of natural organic matter (NOM) using zinc-bismuth oxides-graphitic carbon nitride (ZBO-CN) photocatalyst: Mechanistic insights from EEM-PARAFAC

In this study, the complex degradation behavior of natural organic matter (NOM) was explored using photocatalytic oxidation systems with a novel catalyst based on a hybrid composite of zinc-bismuth oxides and g-C₃N₄ (ZBO-CN). The photooxidation system demonstrated the effective removal of NOM under low-intensity visible light irradiation, presenting removal rates of 53-74% and 65-88% on the basis of dissolved organic carbon (DOC) and the UV absorption coefficient (UV₂₅₄), respectively, at 1.5 g/L of the catalyst. The NOM removal showed an increasing trend with a higher ZBO-CN dose. Comparative experiments with the hole and OH radical scavengers revealed that the direct oxidation occurring on the catalyst's surface might be the governing photocatalytic mechanism. Fluorescence excitation emission matrix-parallel factor analysis (EEM-PARAFAC) revealed the individual removal behavior of the different constituents in bulk NOM. Different tendencies towards preferential adsorption and subsequent oxidative removal were found among dissimilar fluorescent components within a bulk terrestrial NOM, following the order of terrestrial humic-like (C1) > humic-like (C2) > microbial humic-like (C3) components. The result suggests the dominant operation of π-π and/or hydrophobic interactions between the NOM and the catalyst. The discriminative removal behavior was more pronounced in visible light versus UV-activated systems, probably due to the incapability of visible light to excite è - h⁺ pairs of ZnO and the triplet state of NOM. The high photoactivity and structural stability of ZBO-CN under visible light implies its potential for an effective, low-cost and energy-saving treatment technology to selectively remove large sized humic-like substances from water.

Effect of UV wavelength on humic acid degradation and disinfection by-product formation during the UV/chlorine process

The efficiency of the ultraviolet (UV)/chlorine process strongly depends on UV wavelength because chlorine photolysis and its subsequent radical formation are highly wavelength-dependent. This study compared the degradation of humic acid (HA) during the UV/chlorine process by low pressure mercury lamp (LPUV, 254 nm) and ultraviolet light-emitting diode (UV-LED, 275 and 310 nm). The results indicated that HA degradation followed the pseudo-first-order kinetics, and the fluence-based degradation rate constants (k_obs) were significantly affected by UV wavelength and solution pH. HA degradation decreased greatly with increasing solution pH during the UV/chlorine process at 254 nm, while the opposite trend was observed at 275 and 310 nm. In the meantime, k_obs decreased in the order of 275 nm > 254 nm > 310 nm at pH > 7.0. The changes of chlorine molar absorption coefficients at different UV wavelengths resulted in the variation of chlorine photodecay rates (k_{obs, chlorine}), and the synergistic effects of k_{obs, chlorine} and chlorine quantum yields (Φ_chlorine) affected HA reduction. The formation of disinfection by-products (DBPs) during the UV/chlorine process was also evaluated. A significant suppression on DBP formation and DBP-associated calculated theoretical cytotoxicity were observed at 275 nm high UV fluence and alkaline pHs. These findings in this study demonstrate that UV wavelength at 275 nm is more suitable for HA degradation by the UV/chlorine advanced oxidation process in practical applications.

Investigation on degradation behavior of dissolved effluent organic matter, organic micro-pollutants and bio-toxicity reduction from secondary effluent treated by ozonation

The environmental risk of secondary effluent has caused increasing attention in recent years, the negative effect of dissolved effluent organic matters (dEfOM) and organic micro-pollutants (OMPs) was a hot research point. In this research, the degradation behavior of dEfOM and fourteen OMPs in the ozonation was revealed using spectroscopic and chromatographic tools. Ozonation was effective for reducing UV₂₅₄, but had limited effect in dissolved organic carbon reduction. The dEfOM with shorter absorption wavelength was preferentially removed in the ozonation (230 nm  >  240 nm > 254 nm) and high molecular weight humics was largely reduced by the ozonation. Soluble microbial by-products were more reactive with ozone than humic acid as reflected by the fluorescence. Degradation behavior of the OMPs was identified based on their elimination kinetics and molecular structures and a simplified classification method was proposed. The group I OMPs (logk_O3>5) showed high removal efficiency with 1 mg/L of ozone, while the removal of group II OMPs (1< logk_O3<5) was largely dependent on the ozone dose. The CC bond, deprotonated amidogen, phenolic, aniline and anisole groups in these OMPs structures were the main reaction sites with ozone. The group III OMPs without active groups in the molecules showed slight removal in the ozonation. Moreover, genotoxicity and estrogenic activity were simultaneously analyzed for further evaluation on the risk of the effluent. The genotoxicity and estrogenic activity of the secondary effluent were 73.46 μg 4-NQO/L and 519.86 ng E2/L, respectively and an ozone dose of 10 mg/L could reduce the bio-toxicity to the detection limit.

Tracking the reactivity of ozonation towards effluent organic matters from WWTP using two-dimensional correlation spectra

The characteristics of effluent organic matter (EfOM) from a wastewater treatment plant (WWTP) during ozonation were investigated using excitation and emission matrix (EEM) spectra, Fourier transform infrared spectroscopy (FT-IR) and high-performance size exclusion chromatography (HPSEC) at different ozone dosages. The selectivity of ozonation towards different constituents and functional groups was analysed using two-dimensional correlation spectra (2D-COS) probed by FT-IR, synchronous fluorescence spectra and HPSEC. The results indicated that ozonation can destroy aromatic structures of EfOM and change its molecular weight distribution (MWD). According to 2D-COS analysis, microbial humic-like substances were preferentially removed, and then the protein-like fractions. Terrestrial humic-like components exhibited inactivity towards ozonation compared with the above two fractions. Protein-like substances with small molecular weight were preferentially reacted during ozonation based on 2D-COS probed by HPSEC. In addition, the selectivity of ozone towards different functional groups of EfOM exhibited the following sequence: phenolic and alcoholic CO groups > aromatic structures containing CC double bonds > aliphatic CH. X-ray photoelectron spectroscopy (XPS) further elucidated the preferential reaction of aromatic structures in EfOM during ozonation.

Spectroscopic insights into photochemical transformation of effluent organic matter from biological wastewater treatment plants

The photodegradation of discharged effluent organic matter (EfOM) changes its composition and shifts its impacts on pollutant migration and transformation in receiving waters. However, to date, EfOM photodegradation processes are not well understood due to the complexity and heterogeneity of EfOM. Herein, the spectroscopic analysis including ultra violet-visible (UV-Vis), fluorescence and FTIR spectroscopies coupled with two-dimensional correlation analysis (2D-COS) were used to draw a comprehensive view of EfOM photodegradation and involving mechanisms. Results revealed that the photolability of each component in EfOM followed the order: tannin-like > humic-like > protein-like > carbohydrate-like and aliphatic compounds. The photolability of different components of EfOM were found to be related to the photolability of their functional groups. Specifically, the aromatic, carboxylic, phenolic and quinonoid groups associated with humic or tannin-like compounds were more prone to be photodegraded than amides in proteins or C-OH and C-O-C in carbohydrates. Furthermore, the humic-like components, dominating the light absorption of EfOM, were found to be degraded by direct photolysis. Nevertheless, the photodegradation of tannin-like and protein-like components were mainly due to the indirect photodegradation by ROS and ³OM^⁎. Furthermore, results indicated that UV light, rather than visible light, was responsible for the photodegradation of EfOM. The spectroscopic techniques integrated with 2D-COS analysis could serve as a powerful tool with which to clarify complex EfOM photodegradation process as well as to improve our understanding of the fate of discharged EfOM and related environmental processes.

Unraveling complex removal behavior of landfill leachate upon the treatments of Fenton oxidation and MIEX® via two-dimensional correlation size exclusion chromatography (2D-CoSEC)

The complex removal behavior of stabilized landfill leachate was explored for the treatments of Fenton oxidation (FnO) and magnetic ion exchange (MIEX^®) resin using two-dimensional correlation size exclusion chromatography (2D-CoSEC) and fluorescence excitation emission matrix-parallel factor analysis (EEM-PARAFAC). The overall removal rates of the bulk parameters (∼45% for dissolved organic carbon and ∼78% for UV absorbance) were similar between the two treatment options, while distinct differences were found with respect to different molecular sizes and chemical composition. The resin treatment eliminated humic substances (HS) and low molecular weight acid (LMWA) fractions to a greater extent than other fractions (i.e., HS: 62% and LMWA: 99%), while low molecular weight neutral (LMWN) and biopolymers (BP) fractions were more effectively treated by the FnO with the removal rates of 56% and 92%, respectively. The 2D-CoSEC further revealed that the sequential or preferential changes of different size fractions with increasing the resin or H₂O₂ were opposite between the two treatment options in the order of HS → LMWA → LMWN → BP for MIEX^®. Due to their complementary roles in treating leachate, the combined processes removed a wider ranges of different molecular sizes compared to the single operation.

Effect of UV irradiation on iodinated trihalomethane formation during post-chloramination

Ultraviolet (UV) irradiation has been widely used in drinking water treatment processes, but its influence on the formation of disinfection by-products (DBPs), especially the emerging iodinated trihalomethanes (I-THMs) during post-chloramination remains unclear. This study evaluated the impact of low pressure (LP) UV treatment on the formation of I-THMs during post-chloramination through two pathways. The first pathway is through the transition of DOM structure and composition during UV-chloramination, resulting significant increase of I-THM formation with increasing UV dosage in different dissolved organic matter (DOM)-containing water (49.7%-90.5% at 1160 mJ/cm²). With the application of excitation emission matrix-parallel factor analysis (EEM-PARAFAC), we found that I-THM formation in UV-chloraminated water correlated well with two ratios of three PARAFAC humic-like components (C3/C2 and C1/C2, R² = 0.958-1.000), suggesting that the ratios of fluorescent components can be used as reliable indicators for I-THM formation. Moreover, the shift in these fluorescent components is crucial for I-THM formation during UV-chloramination. Another pathway for UV irradiation to affect I-THM formation during post-chloramination is through the transformation of iodine species. Large amounts of reactive iodine species (HOI/I₂ and I₃^-) can be generated directly in the mixed iodine system by UV light, leading to the enhancement of iodine utilization factor (IUF) (up to 0.040) after post-chloramination. These results suggest that UV application to DOM-containing water may induce changes in organic precursors and iodine species so as to enhance I-THM formation during post-chloramination.

Dynamics of dissolved organic matter in a wastewater effluent-impacted Japanese urban stream: characteristics, occurrence and photoreactivity of fluorescent components

We report the results of using the excitation-emission matrix (EEM) method combined with parallel factor analysis (PARAFAC) to investigate the characteristics and occurrence of dissolved organic matter (DOM) in an urban stream impacted by effluent from a wastewater treatment plant (WWTP). The PARAFAC model divides the bulk EEM spectra into six individual fluorescent components with three humic-like components (C1-C3), two protein-like components (C4 and C5) and a wastewater-derived component (C6). In general, intensities of fluorescent components are abundant in WWTP effluent impacted samples, thus showing that such an effluent is a major source of DOM in urban rivers, but C5 is considered to have autochthonous sources within the stream. In areas where the effluent is released, the fluorescent intensity from components (except C5) gradually decreases as these components are transported downstream. However, concentrations of dissolved organic carbon remain almost constant downstream of the release area. These results would be attributed to degradation and/or modification of fluorophore. Photolysis experiments confirmed that fluorescent intensities can decrease with increase of irradiation times. C6 particularly showed a rapid photodegradation, remaining only 24.1% after 48 h photolysis. These findings would be important when assessing DOM source and water quality in aquatic environments by EEM-PARAFAC.

Applying ultraviolet/persulfate (UV/PS) pre-oxidation for controlling ultrafiltration membrane fouling by natural organic matter (NOM) in surface water

Membrane fouling is a recognized obstacle for the application of ultrafiltration (UF) for drinking water treatment. In this study, ultraviolet/persulfate (UV/PS) oxidation was employed as a pretreatment to control membrane fouling caused by natural organic matter (NOM) in surface water. The effects of UV/PS pretreatment on amounts and characteristics of NOM were investigated in terms of dissolved organic carbon, fluorescent spectrum, molecular weight distribution and hydrophobicity. UF membrane fouling during filtration of raw and pre-oxidized water was compared with transmembrane pressure development, and the fouled membranes were further characterized using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The results indicate that NOM was considerably degraded and partially mineralized (∼58%) by UV/PS pretreatment at a PS dose not exceeding 0.6 mM and a UV irradiation time within 120 min, which was attributed to the generation of sulfate and hydroxyl radicals. The fluorescent compounds in NOM were almost completely degraded (>98%) by the UV/PS pretreatment at a PS dose of 0.4 mM, except for tyrosine-like proteins (∼80%). Moreover, UV/PS pretreatment decreased the ratio of macromolecular compounds and increased the hydrophilic fractions, resulting in reduced NOM adhesion to the membrane. Hence, irreversible fouling by NOM was significantly retarded (∼75%) by the UV/PS pretreatment due to reduction in NOM, and more importantly by preferential degradation of fluorescent, macromolecular and hydrophobic compounds. Fouling control performance was considerably improved at increased PS doses and extended UV irradiation time.

Biosorption performance evaluation of heavy metal onto aerobic granular sludge-derived biochar in the presence of effluent organic matter via batch and fluorescence approaches

In present study, the biosorption process of Cu(II) onto aerobic granular sludge-derived biochar was evaluated in the absence and presence of effluent organic matter (EfOM) by using batch and fluorescence approaches. It was found that EfOM gave rise to enhancement of Cu(II) removal efficiency onto biochar, and the sorption data were better fitted with pseudo-second order model and Freundlich equation, in despite of the absence and presence of EfOM. According to excitation-emission matrix (EEM), EfOM was mainly comprised by humic-like substances and fulvic-like substances and their intensities were reduced in the addition of biochar and Cu(II) from batch biosorption process. Synchronous fluorescence spectra coupled to two-dimensional correlation spectroscopy (2D-COS) further implied that a successive fluorescence quenching was observed in various EfOM fractions with the increasing Cu(II) concentration. Moreover, fulvic-like fraction was more susceptibility than other fractions for fluorescence quenching of EfOM.

Evolution of molecular weight and fluorescence of effluent organic matter (EfOM) during oxidation processes revealed by advanced spectrographic and chromatographic tools

Effluent organic matter (EfOM) is an emerging concern to receiving aquatic environment due to its refractory property. The degradation of EfOM in ozonation and other two advanced oxidation processes (AOPs), UV/H₂O₂ and UV/persulfate (PS), was investigated in this study. Fluorescence spectra coupled with parallel factor analysis (PARAFAC) and two-dimensional correlation gel permeation chromatography (2D-GPC) were used to track the evolution of EfOM during each oxidation process. Results showed that the degradation of EfOM indicated by dissolved organic carbon (DOC), UV₂₅₄ and fluorescence components, fitted well with pseudo-first-order kinetic model during the oxidation processes. Ozonation showed higher degradation efficiency than AOPs, while UV/PS was more effective than UV/H₂O₂ with equimolar oxidants dosage. Ozone and SO·₄^- were more reactive with terrestrial humic-like substances, while hydroxyl radical preferentially reacted with protein-like substances. Organic molecules with higher molecular weight (MW) were susceptible to ozone or radicals. Ozonation could transform higher MW (MW of 3510 and 575) organic matters into lower MW organic matters (MW of 294), while reductions of all the organics were observed in both AOPs. Due to the higher reaction rates between ozone and EfOM, ozonation maybe serve as a pre-treatment for AOPs to reduce the radical and energy consumption and improve mineralization of EfOM by AOPs. The decline in DOC, UV₂₅₄, fluorescence and reduction in oxidants increased with the increase of oxidants dosage, and linear correlations among them were found during the ozonation and AOPs.