Degradation Of Trace Organic Contaminants Research Articles

Dichloramine Hydrolysis in Membrane Desalination Permeate: Mechanistic Insights and Implications for Oxidative Capacity in Potable Reuse Applications.

Dichloramine (NHCl2) naturally exists in reverse osmosis (RO) permeate due to its application as an antifouling chemical in membrane-based potable reuse treatment. This study investigated mechanisms of background NHCl2 hydrolysis associated with the generation of oxidative radical species in RO permeate, established a kinetic model to predict the oxidative capacity, and examined its removal efficiency on trace organic contaminants in potable reuse. Results showed that NHCl2 hydrolysis generated transient peroxynitrite (ONOO-) and subsequently dissociated into hydroxyl radical (HO•). The maximal HO• exposure was observed at an RO permeate pH of 8.4, higher than that from typical ultraviolet (UV)-based advanced oxidation processes. The HO• exposure during NHCl2 hydrolysis also peaked at a NH2Cl-to-NHCl2 molar ratio of 1:1. The oxidative capacity rapidly degraded 1,4-dioxane, carbamazepine, atenolol, and sulfamethoxazole in RO permeate. Furthermore, background elevated carbonate in fresh RO permeate can convert HO• to carbonate radical (CO3•-). Aeration of the RO permeate removed total carbonate, significantly increased HO• exposure, and enhanced the degradation kinetics of trace organic contaminants. The kinetic model of NHCl2 hydrolysis predicted well the degradation of contaminants in RO permeate. This study provides new mechanistic insights into NHCl2 hydrolysis that contributes to the oxidative degradation of trace organic contaminants in potable reuse systems.

Assessing the Chemical-Free Oxidation of Trace Organic Chemicals by VUV/UV as an Alternative to Conventional UV/H2O2.

Low-pressure mercury lamps with high-purity quartz can emit both vacuum-UV (VUV, 185 nm) and UV (254 nm) and are commercially available and promising for eliminating recalcitrant organic pollutants. The feasibility of VUV/UV as a chemical-free oxidation process was verified and quantitatively assessed by the concept of H2O2 equivalence (EQH2O2), at which UV/H2O2 showed the same performance as VUV/UV for the degradation of trace organic contaminants (TOrCs). Although VUV showed superior H2O activation and oxidation performance, its performance highly varied as a function of light path length (Lp) in water, while that of UV/H2O2 proportionally decreased with decreasing H2O2 dose regardless of Lp. On increasing Lp from 1.0 to 3.0 cm, the EQH2O2 of VUV/UV decreased from 0.81 to 0.22 mM H2O2. Chloride and nitrate hardly influenced UV/H2O2, but they dramatically inhibited VUV/UV. The competitive absorbance of VUV by chloride and nitrate was verified as the main reason. The inhibitory effect was partially compensated by •OH formation from the propagation reactions of chloride or nitrate VUV photolysis, which was verified by kinetic modeling in Kintecus. In water with an Lp of 2.0 cm, the EQH2O2 of VUV/UV decreased from 0.43 to 0.17 mM (60.8% decrease) on increasing the chloride concentration from 0 to 15 mM and to 0.20 mM (53.5% decrease) at 4 mM nitrate. The results of this study provide a comprehensive understanding of VUV/UV oxidation in comparison to UV/H2O2, which underscores the suitability and efficiency of chemical-free oxidation with VUV/UV.

Quantitative structure-activity relationships for the reaction kinetics of trace organic contaminants with one-electron oxidants.

Understanding the reactivity between trace organic contaminants (TrOCs) and radicals involved in advanced oxidation processes (AOPs) is necessary for a good process design, but the experimentally determined rate constants (k values) are not sufficient for numerous artificial TrOCs. Thus, the development of quantitative structure-activity relationships (QSARs) for predicting k values may be an effective way to address this limitation. In this work, we developed QSARs for the reactions of TrOCs with AOP-related one-electron oxidants. Specifically, 15 QSARs using Hammett constants and 8 cross-correlations were developed based on the k values of over 400 reactions between TrOCs (most contain electron-rich moieties, such as phenol, aniline, and alkoxy benzene) and 5 one-electron oxidants (SO4˙-, Br˙, Br2˙-, Cl2˙-, and CO3˙-). Overall, the developed QSARs show a good predictive performance with 94% (237/251, for Hammett constant-based QSARs) and 80% (218/274, for cross-correlations) of the k values predicted within a factor of 3. All the Hammett constant-based QSARs show negative slope values and all cross-correlations show positive relationships, suggesting all 5 one-electron oxidants mainly share similar electrophilic mechanisms with the TrOCs highlighted in this work. Previous QSAR studies on the k values of one-electron oxidants were compared and integrated into their model analysis. Furthermore, k values predicted herein from the QSARs were used to evaluate the degradation of TrOCs during UV/persulfate and UV/chlorine treatment in multiple wastewater matrices, which were demonstrated to be useful. Finally, remarks on the use of the developed QSARs were presented.

Removal of Emerging Contaminants by Degradation during Filtration: A Review of Experimental Procedures and Modeling

Here, we review the efficient removal of organic micropollutants from water by degradation during filtration using specialized bacteria and enzymes. In both approaches, the filter provides essential binding sites where efficient degradation can occur. A model is presented that enables the simulation and prediction of the kinetics of filtration for a given pollutant concentration, flow rate, and filter dimensions and can facilitate the design of experiments and capacity estimates; it predicts the establishment of a steady state, during which the emerging concentrations of the pollutants remain constant. One method to remove cyanotoxins produced by Microcystis cyanobacteria, which pose a threat at concentrations above 1.0 µg L−1, is to use an activated granular carbon filter with a biofilm; this method resulted in the complete removal of the filtered toxins (5 µg L−1) during a long experiment (225 d). This system was analyzed using a model which predicted complete toxin removal when applied at a 10-fold-higher concentration. Enzymes are also used in filtration processes for the degradation of trace organic contaminants, mostly through the use of membrane bioreactors, where the enzyme is continuously introduced or maintained in the bioreactor, or it is immobilized on the membrane.

Elucidating the performance of UV-based photochemical processes for the removal of trace organic contaminants: Degradation and toxicity evaluation

In this study, the performance of standalone ultraviolet (UV) photolysis and UV-based advanced oxidation processes (AOPs), namely, UV/hydrogen peroxide, UV/chlorine, UV/persulphate, and UV/permonosulphate, were investigated for the degradation of 31 trace organic contaminants (TrOCs). Under the tested conditions, standalone UV photolysis did not achieve effective removal of TrOCs. To improve the degradation efficiency of UV photolysis, four different oxidants were added individually to the test solution. The effect of these oxidants in the absence of UV irradiation was also explored and only chlorine showed promising degradation of some contaminants. During the chlorination of 31 investigated TrOCs, only six demonstrated greater than 50% degradation. The combined UV-based AOPs demonstrated much improved degradation (ranging from 65 to 100%) depending on TrOC-structure and oxidant concentration. The UV/hydrogen peroxide process showed similar degradation of TrOCs, irrespective of the functional groups (i.e., electron withdrawing groups, EWGs and electron donating groups, EDGs) present in their structures. Conversely, the UV/sulphate and UV/chlorine based processes achieved better degradation of the TrOCs with EDGs in their structures. TrOCs degradation improved up to 40% when oxidants concentrations were increased from 0.1 to 1 mM, and further increasing the concentration to 2 mM did not improve degradation. Toxicity evaluation using bioluminescence test (BLT assay) demonstrated that except for UV/hydrogen peroxide, all UV-based AOPs increased the toxicity of the treated effluent, indicating generation of toxic by-products. This study elucidates the performance of four different UV-based AOPs for the removal of commonly detected diverse TrOCs for the first time.

Colloid-bound radicals formed in NOM-enhanced Fe(III)/peroxymonosulfate process accelerate the degradation of trace organic contaminants in water

The omnipresence of natural organic matter (NOM) in water bodies traditionally hinders the degradation of trace organic contaminants (TrOCs) in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs). This study elucidates the positive role of NOM in enhancing the degradation of TrOCs through the Fe(III)/PMS process. During this process, NOM reduces Fe(III), yielding semiquinone-like radical (NOM•) and concurrently forming NOM-Fe(III) colloids. In addition to the Fe(II)-mediated activation pathway, Fe(III) sites on NOM-Fe(III) colloids effectively transfer electrons from NOM• or some redox-active moieties to PMS, resulting in the generation of long-lived colloid-bound SO4•−, which can readily undergo hydrolysis to produce HO•. The stabilization of SO4•− and HO• by NOM-Fe(III) colloids, combined with their moderate adsorption of TrOCs, results in surface-confined reactions that significantly enhance TrOC removal, despite the presence of concurrent quenching reactions between radicals and NOM. Further, the significant positive correlation between the phenolic contents of eight NOM types and TrOC degradation kinetics suggests phenolic moieties as the primary electron source for PMS activation. By in-situ utilizing NOM in raw water, a PMS-amended iron coagulation process with 0.2 mM Fe(III) and PMS effectively removes 90–100 % of six coexisting TrOCs. This study unveils the previously unrecognized role of colloid-bound radicals in decontamination processes, offering valuable insights into harnessing NOM's influence in advanced oxidation water treatment processes.

Reactivity of dissolved effluent organic matter (EfOM) with hydroxyl radical as a function of its isolated fractions during ozonation of municipal secondary effluent

Dissolved effluent organic matter (EfOM) plays an important role in ozonation decomposition and hydroxyl radical (·OH) scavenging in ozonation for the degradation of trace organic contaminants (TrOCs) in municipal secondary effluents. The properties of EfOM have been considered a major concern in this process because EfOM fractions act differently as initiator, promoter or inhibitor of ozone decomposition and ·OH scavenging, which further impacts the degradation effectiveness of TrOCs. This study isolated EfOM from three wastewater treatment plants into six fractions (HoA, HoB, HoN, HiA, HiB and HiN) to understand the reaction kinetics as a function of EfOM and its isolated fractions. Their reaction rate constants with ·OH (kEfOM-·OH), and their roles as the initiator, promoter and inhibitor in the ·OH chain reactions in ozone decomposition were further quantified. The results showed that kEfOM-·OH of hydrophilic fractions (HiF) (accounting for 17%) reached up to 10 times as high as that for hydrophobic fractions (HoF) (accounting for 83%) (7.92 × 108 M−1 s−1 versus 0.78 × 108 M−1 s−1), suggesting the dominating role of HiF in ·OH scavenging. Hydrophilic base (HiB) was the most important fraction dominating the ozone decomposition and reaction with ·OH due to its highest rate constants of initiation and promotion. This study quantifies the kinetics and contribution of EfOM fractions in ·OH scavenging, which will guide the optimization implementation of ozonation and other ·OH-mediated AOPs toward wastewater effluents.

The multiple roles of phenols in the degradation of aniline contaminants by sulfate radicals: A combined study of DFT calculations and experiments.

Recent research revealed inhibition or enhancement of dissolved organic matter (DOM) to the degradation of trace organic contaminants (TrOC) in natural and engineered water systems. Phenols containing acetyl, carboxyl, formyl, hydroxy, and methoxy groups were selected as the model DOM to quantitatively study their roles in the degradation of simple anilines, sulfonamide antibiotics, phenylurea pesticides by sulfate radicals (SO4•-). Experimental results found that p-methoxyphenol inhibited aniline and sulfamethoxazole degradation by thermally activated peroxydisulfate (TAP), while p-acetylphenol slightly promoted aniline degradation. Quantum chemical calculations were applied to study the microscopic mechanism and kinetics of phenols affecting the degradation of aniline pollutants (AN) in three ways: competitively reacting with SO4•-, repairing aniline cationic radicals (AN•+) and phenylaminyl radicals (AN(-H)•), and generating phenoxy radicals to degrade anilines. Generally, the degradation of sulfonamides and phenylureas prefer to be inhibited by hydroxy- and methoxy-phenols with low oxidation potential (Eox), due to their diffusion-limiting reaction with SO4•- and rapid back-reduction AN•+ with the calculated rate constants of (0.02 - 6.38)×109 M-1 s-1. Phenols repairing AN(-H)• through H abstraction reaction is speculated to possibly dominate the joint degradation of phenols and anilines by TAP, which has a poor correlation with Eox. This study provides mechanistic insight into the chemical behavior of complex and heterogeneous DOM in complex aqueous environments.

Multiple Roles of Dissolved Organic Matter in Advanced Oxidation Processes.

Advanced oxidation processes (AOPs) can degrade a wide range of trace organic contaminants (TrOCs) to improve the quality of potable water or discharged wastewater effluents. Their effectiveness is impacted, however, by the dissolved organic matter (DOM) that is ubiquitous in all water sources. During the application of an AOP, DOM can scavenge radicals and/or block light penetration, therefore impacting their effectiveness toward contaminant transformation. The multiple ways in which different types or sources of DOM can impact oxidative water purification processes are critically reviewed. DOM can inhibit the degradation of TrOCs, but it can also enhance the formation and reactivity of useful radicals for contaminants elimination and alter the transformation pathways of contaminants. An in-depth analysis highlights the inhibitory effect of DOM on the degradation efficiency of TrOCs based on DOM's structure and optical properties and its reactivity toward oxidants as well as the synergistic contribution of DOM to the transformation of TrOCs from the analysis of DOM's redox properties and DOM's transient intermediates. AOPs can alter DOM structure properties as well as and influence types, mechanisms, and extent of oxidation byproducts formation. Research needs are proposed to advance practical understanding of how DOM can be exploited to improve oxidative water purification.

Selective and rapid degradation of organic contaminants by Mn(V) generated in the Mn(II)-nitrilotriacetic acid/periodate process

The rapid generation of Mn(V) as the dominant reactive species for the selective and efficient degradation of trace organic contaminants (TrOCs) remains still challenging. In this work, aqueous Mn(II) complexed with nitrilotriacetic acid (Mn(II)-NTA complex) was employed for the first time to activate periodate (PI) (Mn(II)-NTA/PI process) to degrade TrOCs. Experiments showed that TrOCs could be degraded in the Mn(II)-NTA/PI process within 30 s. Mn(V), existing as Mn(V)-NTA complex, was identified as the active oxidant in this process by using methyl phenyl sulfoxide as a probe, scavenging experiments, and mass spectrometry analysis. Three-dimensional UV–vis spectra indicated that Mn(III)-NTA complex was firstly generated through oxidation of Mn(II)-NTA complex by PI and then oxidized to Mn(V)-NTA complex via two-electron transfer. The generation of Mn(V)-NTA complex was dependent on NTA concentration, PI concentration, and solution pH, whereas the reactivity of Mn(V)-NTA complex toward TrOCs strongly relied on the functional groups of TrOCs. A high PI utilization efficiency could be achieved in this process. Moreover, due to the catalyst-like role of Mn(II)-NTA complex in activating PI, Mn(V)-NTA complex could be continuously generated and thus the sustainable and efficient degradation of TrOCs occurred in this process even when the concentration of Mn(II)-NTA complex was low. This work advances the understanding of Mn(V) generation from Mn(II) commonly present in aquatic environments.

Reinvestigation of the oxidation of organic contaminants by Fe(VI): Kinetics and effects of water matrix constituents

Since previous studies mostly ignored the contributions of Fe(IV) and Fe(V) during the determination of reaction rate constants of ferrate (Fe(VI)) with trace organic contaminants (TrOCs), the intrinsic oxidation ability of Fe(VI) was overestimated. For the first time, this study systemically evaluated the reactivity of Fe(VI) towards four kinds of TrOCs by blocking Fe(IV)/Fe(V) over the TrOCs degradation, and evaluated the effects of coexisting water matrix constituents. Results revealed that Fe(VI) exhibited superior reactivity towards phenolic compounds. Different from other tested TrOCs, phenolic compounds were mainly degraded by Fe(VI) rather than Fe(IV)/Fe(V). Taking bisphenol A (BPA) as the target TrOC, we found that the coexisting constituents can not only affect the reactivity of different ferrate species (i.e., Fe(IV), Fe(V), and Fe(VI)), but also alter the concentrations of ferrates. HPO42- inhibited the reaction between Fe(VI) and H2O2, while Ca2+, Mg2+, and NH4+ promoted the generation of Fe(IV)/Fe(V) from Fe(VI). Besides, humic acid could increase the contribution of Fe(IV)/Fe(V) to the oxidation of BPA. These findings were validated in real water samples. Taken together, this study provides a new perspective regarding the intrinsic oxidation reactivity of Fe(VI), thereby urging reconsideration of the proper strategies for utilization of high-valent Fe species in practices.

Role of Antioxidant Moieties in the Quenching of a Purine Radical by Dissolved Organic Matter.

Dissolved organic matter (DOM) has been known to inhibit the degradation of trace organic contaminants (TrOCs) in advanced oxidation processes but quantitative understanding is lacking. Adenine (ADN) was selected as a model TrOC due to the wide occurrence of purine groups in TrOCs and the well-documented transient spectra of its intermediate radicals. ADN degradation in the presence of DOM during UV/peroxydisulfate treatment was quantified using steady-state photochemical experiments, time-resolved spectroscopy, and kinetic modeling. The inhibitory effects of DOM were found to include competing for photons, scavenging SO4•- and HO•, and also converting intermediate ADN radicals (ADN(-H)•) back into ADN. Half of the ADN(-H)• were reduced back to ADN in the presence of about 0.2 mgC L-1 of DOM. The quenching rate constants of ADN(-H)• by the 10 tested DOM isolates were in the range of (0.39-1.18) × 107 MC-1 s-1. They showed a positive linear relationship with the total antioxidant capacity of DOM. The laser flash photolysis results of the low-molecular-weight analogues of redox-active moieties further supported the dominant role of antioxidant moieties in DOM in the quenching of ADN(-H)•. The diverse roles of DOM should be considered in predicting the abatement of TrOCs in advanced oxidation processes.

Redox-Active Moieties in Dissolved Organic Matter Accelerate the Degradation of Nitroimidazoles in SO4•--Based Oxidation.

The presence of dissolved organic matter (DOM) is known to inhibit the degradation of trace organic contaminants (TrOCs) in SO4•--based advanced oxidation processes (AOPs) due to filtering of the photochemically active light and radical scavenging effects. This study revealed an unexpected contribution for DOM in the degradation of nitroimidazoles (NZs) in the UV/persulfate AOP. The apparent second-order rate constants of NZs with SO4•- increased by 2.05 to 4.77 times in the presence of different DOMs. The increments were linearly related to the total electron capacity of DOM. Quinone and polyphenol moieties were found to play a dominant role. The reactive species generated from SO4•-'s oxidation of DOM, including semiquinone radical (SQ•-) and superoxide (O2•-), were found to react with NZs via Michael addition and O2•- addition. The second-order rate constants of tinidazole with SQ•- is determined to be (5.69 ± 0.59) × 106 M-1 s-1 by laser flash photolysis. Reactive species potentially generated from DOM may be considered in designing processes for the abatement of different types of TrOCs.

Role of TEMPO in Enhancing Permanganate Oxidation toward Organic Contaminants.

Permanganate (Mn(VII)) has been widely applied as an oxidant in water treatment plants. However, compared with ozone, Fenton, and other advanced oxidation processes, the reaction rates of some trace organic contaminants (TrOCs) with Mn(VII) are relatively low. Therefore, further studies on the strategies for enhancing the oxidation of organic contaminants by Mn(VII) are valuable. In this work, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), as an electron shuttle, enhanced Mn(VII) oxidation toward various TrOCs (i.e., bisphenol A (BPA), phenol, estrone, sulfisoxazole, etc.). TEMPO sped up the oxidative kinetics of BPA by Mn(VII) greatly, and this enhancement was observed at a wide pH range of 4.0-11.0. The exact mechanism of TEMPO in Mn(VII) oxidation was described briefly as follows: (i) TEMPO was oxidized by Mn(VII) to its oxoammonium cation (TEMPO+) by electron transfer, which was the reactive species responsible for the accelerated degradation of TrOCs and (ii) TEMPO+ could decompose TrOCs rapidly with itself back to TEMPO or TEMPOH (TEMPO hydroxylamine). To further illustrate the interaction between TEMPO and target TrOCs, we explored the transformation pathways of BPA in Mn(VII)/TEMPO oxidation. Compared to Mn(VII) alone, adding TEMPO into the Mn(VII) solution significantly suppressed BPA's self-coupling and promoted hydroxylation, ring-opening, and decarboxylation. Moreover, the Mn(VII)/TEMPO system was promising for the abatement of TrOCs in real waters for humic acid, and ubiquitous cations/anions had no adverse or even beneficial impact on the Mn(VII)/TEMPO system.

Impact of Inorganic Ions and Organic Matter on the Removal of Trace Organic Contaminants by Combined Direct Contact Membrane Distillation-UV Photolysis.

This study investigated the degradation of five trace organic contaminants (TrOCs) by integrated direct contact membrane distillation (DCMD) and UV photolysis. Specifically, the influence of inorganic ions including halide, nitrate, and carbonate on the performance of the DCMD–UV process was evaluated. TrOC degradation improved in the presence of different concentrations (1–100 mM) of fluoride ion and chloride ion (1 mM). With a few exceptions, a major negative impact of iodide ion was observed on the removal of the investigated TrOCs. Of particular interest, nitrate ion significantly improved TrOC degradation, while bicarbonate ion exerted variable influence—from promoting to inhibiting impact—on TrOC degradation. The performance of DCMD–UV photolysis was also studied for TrOC degradation in the presence of natural organic matter, humic acid. Results indicated that at a concentration of 1 mg/L, humic acid improved the degradation of the phenolic contaminants (bisphenol A and oxybenzone) while it inhibited the degradation of the non-phenolic contaminants (sulfamethoxazole, carbamazepine, and diclofenac). Overall, our study reports the varying impact of different inorganic and organic ions present in natural water on the degradation of TrOCs by integrated DCMD–UV photolysis: the nature and extent of the impact of the ions depend on the type of TrOCs and the concentration of the interfering ions.

A critical review of advanced oxidation processes for emerging trace organic contaminant degradation: Mechanisms, factors, degradation products, and effluent toxicity

The persistent nature and low biodegradability of a large number of trace organic contaminants (TrOCs) reduce effectiveness of their removal by conventional wastewater treatments. In this context, advanced oxidation processes (AOPs), such as photolysis, photocatalysis, ozonation, Fenton process, anodic oxidation, sonolysis and wet air oxidation, have been studied extensively for the effective degradation of the wide range of TrOCs. All AOPs produce reactive oxygen species (HO2●, O2●−), especially hydroxyl radicals that unselectively attack contaminants and oxidise them. Factors affecting the degradation rates of TrOCs by different AOPs include the concentration and nature of the TrOCs, bulk wastewater characteristics, dose of chemicals or catalysts used, and other reaction parameters. This review critically analyses the overview of already established AOPs, the effect of the structure of TrOCs based on different functional groups such as electron donating groups (EDGs) and electron withdrawing groups (EWGs) on their degradation by each AOP. The overall degradation rates based on data collected from a comprehensive literature review show that ozonation achieves effective degradation for a broad range of TrOCs, but it can lead to the production of toxic degradation by-products. By comparison, photocatalysis shows moderate to high degradation rate for TrOCs. Photolysis and Fenton processes show TrOC-specific suitability. This review also demonstrates that optimum doses of chemicals/catalysts are required for each AOP. This is because excessive concentrations of catalysts or other chemicals (e.g., H2O2: iron dose in Fenton process) may result in low TrOC degradation. Degradation of individual TrOCs can result in a number of degradation by-product that varies in nature. Different AOP has different reaction mechanisms that also affect the number and nature of by-product formation during TrOC degradation.

Combining enzymatic membrane bioreactor and ultraviolet photolysis for enhanced removal of trace organic contaminants: Degradation efficiency and by-products formation

Coupling of membrane distillation with bioreactors containing enzymes such as ‘laccase’ forms an enzymatic membrane bioreactor resulting in complete retention of both trace organic contaminants and enzyme, facilitating simultaneous trace organic contaminants degradation. Integration of enzymatic membrane bioreactor and ultraviolet photolysis may result in further improved degradation of trace organic contaminants in membrane-concentrate. We studied the degradation as well as by-products formation of five selected trace organic contaminants, namely, sulfamethoxazole, diclofenac, bisphenol A, oxybenzone, and carbamazepine by ‘membrane distillation — ultraviolet photolysis’ system and ‘enzymatic membrane bioreactor — ultraviolet photolysis’ system. In the former, the membrane effectively retained the trace organic contaminants and then ultraviolet photolysis of membrane-concentrate resulted in trace organic contaminant degradation in the following order: diclofenac (88 %) > sulfamethoxazole (71 %) > oxybenzone (35 %) > bisphenol A (33 %) > carbamazepine (27 %). By contrast, the enzymatic membrane bioreactor — ultraviolet photolysis system resulted in 100 % degradation of diclofenac, sulfamethoxazole, and bisphenol A and around 70 % degradation of oxybenzone and carbamazepine. This system also resulted in more than 50 % reduction in number of degradation products with 60–70 % lower abundance. Our results indicate that laccase degradation led to products that were more amenable to the post-treatment by ultraviolet photolysis. Overall, it can be concluded that for enzymatic membrane bioreactor — ultraviolet photolysis system, enzymatic pre-treatment not only helped in better degradation of the parent trace organic contaminants but also led to the formation of fewer by-products with lower abundance (i.e., more complete degradation).

Overlooked Role of Fe(IV) and Fe(V) in Organic Contaminant Oxidation by Fe(VI).

Fe(VI) has received increasing attention since it can decompose a wide range of trace organic contaminants (TrOCs) in water treatment. However, the role of short-lived Fe(IV) and Fe(V) in TrOC decomposition by Fe(VI) has been overlooked. Using methyl phenyl sulfoxide (PMSO), carbamazepine, and caffeine as probe TrOCs, we observed that the apparent second-order rate constants (kapp) between TrOCs and Fe(VI) determined with the initial kinetics data were strongly dependent on the initial molar ratios of TrOCs to Fe(VI). Furthermore, the kapp value increases gradually as the reaction proceeds. Several lines of evidence suggested that these phenomena were ascribed to the accumulation of Fe(IV) and Fe(V) arising from Fe(VI) decay. Kinetic models were built and employed to simulate the kinetics of Fe(VI) self-decay and the kinetics of PMSO degradation by Fe(VI). The modeling results revealed that PMSO was mainly degraded by Fe(IV) and Fe(V) rather than by Fe(VI) per se and Fe(V) played a dominant role, which was also supported by the density functional theory calculation results. Given that Fe(IV) and Fe(V) have much greater oxidizing reactivity than Fe(VI), this work urges the development of Fe(V)/Fe(IV)-based oxidation technology for efficient degradation of TrOCs.

A critical review on advanced oxidation processes for the removal of trace organic contaminants: A voyage from individual to integrated processes

Advanced oxidation processes (AOPs), such as photolysis, photocatalysis, ozonation, Fenton process, anodic oxidation, sonolysis, and wet air oxidation, have been investigated extensively for the removal of a wide range of trace organic contaminants (TrOCs). A standalone AOP may not achieve complete removal of a broad group of TrOCs. When combined, AOPs produce more hydroxyl radicals, thus performing better degradation of the TrOCs. A number of studies have reported significant improvement in TrOC degradation efficiency by using a combination of AOPs. This review briefly discusses the individual AOPs and their limitations towards the degradation of TrOCs containing different functional groups. It also classifies integrated AOPs and comprehensively explains their effectiveness for the degradation of a wide range of TrOCs. Integrated AOPs are categorized as UV irradiation based AOPs, ozonation/Fenton process-based AOPs, and electrochemical AOPs. Under appropriate conditions, combined AOPs not only initiate degradation but may also lead to complete mineralization. Various factors can affect the efficiency of integrated processes including water chemistry, the molecular structure of TrCOs, and ions co-occurring in water. For example, the presence of organic ions (e.g., humic acid and fulvic acid) and inorganic ions (e.g., halide, carbonate, and nitrate ions) in water can have a significant impact. In general, these ions either convert to high redox potential radicals upon collision with other reactive species and increase the reaction rates, or may act as radical scavengers and decrease the process efficiency.

Removal of trace organic contaminants by enzymatic membrane bioreactors: Role of membrane retention and biodegradation

Performance of an enzymatic membrane bioreactor (EMBR) equipped with either an ultrafiltration (UF) or a nanofiltration (NF) membrane was explored for the degradation of a set of 29 chemically diverse trace organic contaminants (TrOCs). The NF membrane provided effective retention (90-99%) of TrOCs within the NF-EMBR. On the other hand, partial retention of charged and significantly hydrophobic (log >3) TrOCs was achieved by the UF membrane via charge repulsion and adsorption on the enzyme gel-layer formed on the membrane surface during UF-EMBR operation. Laccase achieved TrOC-specific degradation in both EMBRs. The extent of TrOC degradation was significantly (5 to 65%) better by NF-EMBR as compared to that achieved by UF-EMBR. Addition of a redox-mediator (violuric acid) at concentrations ranging from 10-100 μM improved the degradation of non-phenolic TrOCs, but degradation efficiency reached a plateau when its concentration was increased beyond 25 μM. Although the permeate flux of the UF/NF membranes dropped with time due to membrane fouling caused by enzyme gel-layer and/or concentration polarization, membrane flushing with water was effective in recovering the flux by up to 95%.