Advanced Reduction Processes Research Articles

Mechanism of 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide degradation in UV light-emitting diodes lamp driven sodium sulfite activation process

DOPO (9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) is emerging organic phosphorus flame retardant, which has gained greatly attention because of its adverse health effects on aquatic organisms. In this study, UV light-emitting diodes lamp driven sodium sulfite activation (UV254/Sulfite) process was employed as an advanced reduction process to remove DOPO, and its mechanisms were revealed. The pseudo first order rate constant of DOPO (kobs-DOPO) in UV254/Sulfite process initially decreased from 6.95 × 10−5 s−1 to 5.11 × 10−5 s−1, and subsequently increased to 1.61 × 10−4 s−1 as the sulfite concentration increased from 0 to 10 mM, indicating that the negative effect of HSO3- on DOPO degradation was the dominate factor in UV254/Sulfite process at low sulfite concentration (< 2 mM), while the yields of hydrated electron (eaq-) increased as the sulfite concentration gradually reaching to a high value (10 mM), resulting in enhancing DOPO degradation significantly. In addition, the coexisted water matrix in deionized water and natural surface water weakened DOPO degradation in UV254/Sulfite process, and this inhibition strengthened as the concentrations of water matrix raised. The DOPO degradation pathway indicated that eaq- attacked phosphaphenanthrene structure of DOPO and contributed to its detoxification which was confirmed by Quantitative Structure-Activity Relationship (QSAR) assessment.

Degradation of metoprolol by UV/sulfite as an advanced oxidation or reduction process: The significant role of oxygen

The degradation of metoprolol (MTP) by the UV/sulfite with oxygen as an advanced reduction process (ARP) and that without oxygen as an advanced oxidation process (AOP) was comparatively studied herein. The degradation of MTP by both processes followed the first-order rate law with comparable reaction rate constants of 1.50×10−3sec−1 and 1.20×10−3sec−1, respectively. Scavenging experiments demonstrated that both eaq− and H• played a crucial role in MTP degradation by the UV/sulfite as an ARP, while SO4•− was the dominant oxidant in the UV/sulfite AOP. The degradation kinetics of MTP by the UV/sulfite as an ARP and AOP shared a similar pH dependence with a minimum rate obtained around pH 8. The results could be well explained by the pH impacts on the MTP speciation and sulfite species. Totally six transformation products (TPs) were identified from MTP degradation by the UV/sulfite ARP, and two additional ones were detected in the UV/sulfite AOP. The benzene ring and ether groups of MTP were proposed as the major reactive sites for both processes based on molecular orbital calculations by density functional theory (DFT). The similar degradation products of MTP by the UV/sulfite process as an ARP and AOP indicated that eaq−/H• and SO4•− might share similar reaction mechanisms, primarily including hydroxylation, dealkylation, and H abstraction. The toxicity of MTP solution treated by the UV/sulfite AOP was calculated to be higher than that in the ARP by the Ecological Structure Activity Relationships (ECOSAR) software, due to the accumulation of TPs with higher toxicity.

Quantifying Hydrated Electron Transformation Kinetics in UV-Advanced Reduction Processes Using the Re-,UV Method.

Ultraviolet advanced reduction processes (UV-ARP) have garnered significant attention recently for the degradation of several hard to treat contaminants, including recalcitrant per- and polyfluoroalkyl substances (PFAS). The rate of contaminant degradation in UV-ARP is directly related to the available hydrated electron concentration ([eaq-]). However, reports of [eaq-] and other parameters typically used to characterize photochemical systems are not widely reported in the UV-ARP literature. Deploying monochloroacetate as a probe compound, we developed a method (Re-,UV) to quantify the time-based hydrated electron concentration ([eaq]t) available for contaminant degradation relative to inputted UV fluence. Measured [eaq]t was then used to understand the impact of eaq- rate of formation and scavenging capacity on the degradation of two contaminants─nitrate and perfluorooctane sulfonate (PFOS)─in four source waters with varying background water quality. The results show that the long-term treatability of PFOS by UV-ARP is not significantly impacted by the initial eaq- scavenging conditions but rather is influenced by the presence of eaq- scavengers like dissolved organic carbon and bicarbonate. Lastly, using [eaq]t, degradation of nitrate and PFOS was modeled in the source waters. We demonstrate that the Re-,UV method provides an effective tool to assess UV-ARP treatment performance in a variety of source waters.

Organic photoredox catalysts for wastewater remediation: Beyond the established advanced oxidation processes

Contaminants of emerging concern (CECs) constitute a wide class of organic molecules, which are found dissolved in wastewaters. Chemical properties of CECs are as wide as their chemical structures; nevertheless, most of the CECs can be removed from wastewaters using Advanced Oxidation Processes (AOPs). However, there are examples of CECs that have proven recalcitrant to AOPs. In such cases, Advanced Reduction Processes (ARPs) include those techniques able to achieve degradation of contaminants by producing highly reactive reducing radicals.Organic photocatalysts have demonstrated to be an efficient alternative to other types of photocatalysts in AOPs. Moreover, several organic photocatalysts, according to their redox properties, could also act as reductants under appropriate scenarios, a process that has been much less explored than oxidation. As an example, Riboflavin (RF) is a natural water-soluble molecule, well-known as vitamin B2. The photophysical and redox properties of RF have been extensively applied for the oxidative photodegradation of contaminants in water. More recently, the potential of RF has been expanded to the photoreduction of more recalcitrant pollutants. For instance, benzotriazoles UV-stabilizers (BUVSs) constitute a vast family of recalcitrant contaminants, which have been successfully degraded in the presence of a commercially available derivative of RF (2′,3′,4′,5′-tetraacetylated riboflavin, RFTA) and visible light, assisted by an amine, which acts as electron donor to the excited states of RFTA, generating the RFTA.−. This species ultimately produces the reduction of the selected BUVSs. Photophysical experiments demonstrated the key role of the radical anion RFTA.−, and allowed determining the rate constants that were correlated to the observed photodegradations.

Selective and Efficient Reduction of Nitrate to Gaseous Nitrogen from Drinking Water Source by UV/Oxalic Acid/Ferric Iron Systems: Effectiveness and Mechanisms

Nitrate (NO3−) reduction in water has been receiving increasing attention in water treatment due to its carcinogenic and endocrine-disrupting properties. This study employs a novel advanced reduction process, the UV/oxalic acid/ferric iron systems (UV/C2O42−/Fe3+ systems), in reducing NO3− due to its high reduction efficiency, excellent selectivity, and low treatment cost. The UV/C2O42−/Fe3+ process reduced NO3− with pseudo-first-order reaction rate constants of 0.0150 ± 0.0013 min−1, minimizing 91.4% of 60 mg/L NO3− and reaching 84.2% of selectivity for gaseous nitrogen after 180 min at pHini. 7.0 and 0.5 mg/L dissolved oxygen (DO). Carbon dioxide radical anion (CO2•−) played a predominant role in reducing NO3−. Gaseous nitrogen and NH4+, as well as CO2, were the main nitrogen- and carbon-containing products, respectively, and reduction pathways were proposed accordingly. A suitable level of oxalic acids (3 mM) and NO3− (60 mg/L) was recommended; increasing initial iron concentrations and UV intensity increased NO3− reduction. Instead, increasing the solution pH decreased the reduction, and 0.5–8.0 mg/L DO negligibly affected the process. Moreover, UV/C2O42−/Fe3+ systems were not retarded by 0.1–10 mM SO42− or Cl− or 0.1–1.0 mM HCO3− but were prohibited by 10 mM HCO3− and 30 mg-C/L humic acids. There was a lower reduction of NO3− in simulated groundwater (72.8%) than deionized water after 180 min at pHini. 7.0 and 0.5 mg/L DO, which meets the drinking water standard (&lt;10 mg/L N-NO3−). Therefore, UV/C2O42−/Fe3+ systems are promising approaches to selectively and efficiently reduce NO3− in drinking water.

Reductive Dehalogenation Pathways and Inhibition Mechanisms of 2,5-Dichloronitrobenzene in Uv/Sulfite Advanced Reduction Process

Reductive Dehalogenation Pathways and Inhibition Mechanisms of 2,5-Dichloronitrobenzene in Uv/Sulfite Advanced Reduction Process

Determination of Optimum Conditions for the Degradation of 2,4,6-Trinitrotoluene (Tnt) by Advanced Reduction Processes

Determination of Optimum Conditions for the Degradation of 2,4,6-Trinitrotoluene (Tnt) by Advanced Reduction Processes

Reactivities of hydrated electrons with organic compounds in aqueous-phase advanced reduction processes

Novel linear free energy relationships were determined between experimentally measured rate constants of solvated electrons and theoretically calculated one electron reduction potential for the elucidation of reductive reaction mechanisms.

Ni2P/biocarbon composite derived from an unusual phosphorus-rich precursor as a superior catalyst for 4-nitrophenol reduction

Conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) through advanced reduction process is important to lower the hazardous risk of 4-NP, which requires the development of highly efficient catalyst. Herein, we proposed a novel strategy to fabricate nickel phosphide/carbon composite for 4-NP reduction in the presence of NaBH4. An unusual precursor of adenosine 5′-triphosphate (ATP) was applied to provide the essential phosphorus and carbon elements for nickel phosphide and biocarbon, respectively. ATP-Ni complex was firstly prepared and then transformed to nickel phosphide/biocarbon composite by an anoxic pyrolysis treatment. Material characterizations confirmed both large particles of hundreds of nanometers and small particles of ∼2.4 nm existing for Ni2P, which were dispersed on N/P-doped biocarbon with thin layer sheet-like structure. When applied for 4-NP reduction, Ni2P/BC exhibited fast reaction kinetic with a pseudo-first-order reaction rate constant of 0.019 s−1 (normalized rate constant 253 s−1 g−1) and good reusability in 10 consecutive reaction runs (95.8% conversion efficiency). It is suggested that such superior activity may originate from the high affinity of Ni2P/BC towards reactants and the good electron transfer ability of Ni2P/BC. This study not only enriches the synthesis strategy of transition metal phosphide, but also provides a superior catalyst for advanced reduction process.

Degradation of Perfluorooctanoic Acid with Hydrated Electron by a Heterogeneous Catalytic System.

Hydrated electron (eaq-)-induced reduction protocols have bright prospects for the decomposition of recalcitrant organic pollutants. However, traditional eaq- production involves homogeneous sulfite photolysis, which has a pH-dependent reaction activity and might have potential secondary pollution risks. In this study, a heterogeneous UV/diamond catalytic system was proposed to decompose of a typical persistent organic pollutant, perfluorooctanoic acid (PFOA). In contrast to the rate constant of the advanced reduction process (ARP) of a UV/SO32-, the kobs of PFOA decomposition in the UV/diamond system showed only minor pH dependence, ranging from 0.01823 ± 0.0014 min-1 to 0.02208 ± 0.0013 min-1 (pH 2 to pH 11). As suggested by the electron affinity (EA) and electron configuration of the diamond catalyst, the diamond catalyst yields facile energetic photogenerated electron emission into water without a high energy barrier after photoexcitation, thus inducing eaq- production. The impact of radical scavengers, electron spin resonance (ESR), and transient absorption (TA) measurements verified the formation of eaq- in the UV/diamond system. The investigation of diamond for ejection of energetic photoelectrons into a water matrix represents a new paradigm for ARPs and would facilitate future applications of heterogeneous catalytic processes for efficient recalcitrant pollutant removal by eaq-.

Efficient reductive and oxidative decomposition of haloacetic acids by the vacuum-ultraviolet/sulfite system

Haloacetic acids (HAAs), as a representative category of halogenated disinfection byproducts, are widely detected in disinfected water. In this work, the vacuum ultraviolet (VUV)/sulfite process under N2 saturated conditions was proposed to eliminate a series of HAAs (i.e., monochloroacetic acid (MCAA), difluoroacetic acid (DFAA), trifluoroacetic acid (TFAA), dichloroacetic acid (DCAA), etc.). The in situ generated hydrated electron (eaq−) demonstrated to be the main species to fulfill the initial degradation and dechlorination of MCAA, while hydroxyl radicals (˙OH) were in charge of the mineralization of MCAA. This means that the VUV/sulfite system is a combination of advanced reduction and oxidation processes (ARPs and AOPs). A significant enhancement of MCAA removal was observed with increasing pH values from 6.0 to 10.0, and surprisingly, kobs correlated well with the proportion of SO32- as the pH changed. This can be explained by the production of eaq− from VUV irradiation of SO32− rather than HSO3− and also due to eaq− being more stable under alkaline conditions. Increasing the sulfite dosage also elevated the degradation of MCAA. However, the addition of certain anions (i.e., chloride (Cl−), bicarbonate (HCO3−), and nitrate (NO3−)) and dissolved organic matter (DOM) inhibited the removal of MCAA to varying degrees. The VUV/sulfite system was effective toward various types of halogenated disinfection byproducts, supporting its broad applicability. Nevertheless, even in real waters, the VUV/sulfite system was also promising for the simultaneous abatement of HAAs and other oxyanions.

Effectiveness of UV/SO32- advanced reduction process for degradation and mineralization of trichlorfon pesticide in water: identification of intermediates and toxicity assessment.

This study aimed to investigate the degradability, mineralization, proposed decomposition pathway, intermediate products, and toxicity of effluent from trichlorfon (TCF) degradation in water by UV/sulfite-advanced reduction process (UV/S-ARP). This study was experimentally performed in a photochemical reactor as a batch operation. The source of light was a UV lamp. Sulfite ion was used as the reducing agent. After the treatment, the residual concentration of TCF was measured by liquid chromatography equipped with tandem mass spectrometry (LC-MS/MS). UV/S-ARP had the highest performance at an initial pH of 7, a sulfite ion concentration of 120mg/L, a contact time of 60min, and a TCF concentration of 10mg/L. Under such conditions, the degradation efficiency of TCF was 96.0%, and the amount of mineralization based on the removal of TOC and COD was 74.6% and 79.5%, respectively. The results of the degradation mechanism showed that eaq- and SO3•- have played the greatest role in dechlorination and transformation of TCF. Based on the identified intermediates, more complex compounds are transformed into compounds with simpler structures by UV/S-ARP. Evaluating the toxicity of TCF by-products via ECOSAR bioassay showed that as-generated intermediates do not have acute and chronic adverse effects on fish. The results of our study indicated that the advanced reduction process could be an effective process for the purification of TCF-contaminated water.

NO3− photolysis-induced advanced reduction process removes NO3− and 2, 4, 6-tribromophenol

Reductive processes are an important type of pollutant removal technology, particularly for organic halogens. NO3− is an anion and pollutant that is commonly present in wastewater. In this study, a novel advanced reduction process (ARP) induced by NO3− photolysis was developed to remove 2,4,6-tribromophenol (TBP) and NO3−. The UV/NO3−/formate acid (FA) process achieved NO3− removal and improved the debromination of TBP (initial TBP concentration = 0.1 mM) (up to 97.8%), however, their coexistence adversely affected the reductive removal of each component. Acidic conditions (pH 3 in this study) benefited the removal of NO3− and the debromination of TBP. Cl− promoted NO3− removal in UV/NO3−/FA, however, it decreased the debromination effect of TBP by 27.8%. Humic acid, a typical dissolved organic matter, suppressed NO3− removal, TBP degradation and debromination under all experimental conditions. Methyl viologen significantly inhibited the performance of ARP, and this verified the role of CO2•- in this ARP. Insufficient reduction and over-reduction of NO3− were observed under different conditions and a greater amount of NH4+ was formed under the influence of TBP. The data also indicated that as much as 80% of the removed NO3− was converted to NO2−, and this is noteworthy. Due to the reductive radicals generated from the oxidation of FA, both oxidative and reductive products of TBP were detected in the effluent. The results of this study provide a potential technology for the reductive removal of organic halogens from NO3−-rich wastewater.

Reductive degradation of perfluorooctanoic acid in complex water matrices by using the UV/sulfite process

Hydrated electrons (e-aq,E= -2.9 V) generated by advanced reduction processes (ARPs) have been proved to be a promising approach to eliminate various per- and polyfluoroalkyl substances (PFASs) in water. In this study, the decomposition of perfluorooctanoic acid (PFOA) in a complex water matrix by e-aq generated from the UV/sulfite process was investigated. The effect of pH (9–12) and co-existing compounds (chloride, nitrate, phosphate, carbonate and humic acid) on PFOA degradation efficiency was studied. In addition, the intermediates and possible degradation pathways were analyzed by ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS). The results showed that the concentration of PFOA was below the detection limit (10 μg/L) after 1 h (conditions: C0 10 mg/L, initial pH = 10, sulfite 10 mM) while 89% defluorination was achieved after 24 h. Using a higher initial pH (pH = 12) greatly enhanced the PFOA degradation as 100% degradation and 98% defluorination were achieved after 24 h. The presence of carbonate (> 5 mM), nitrate (> 2 mM) and humic acid (> 25 mg/L) showed a significant negative effect on PFOA degradation via a UV blocking effect or quenching of hydrated electrons while the presence of chloride and phosphate had a smaller effect on PFOA degradation. Even at extremely high concentrations of chloride (1.709 M, pH = 11.25), the defluorination ratio reached 97% after 24 h of reaction time. During the process, short-chain perfluorinated carboxylic acids (PFCAs, C < 7) and hydrogen substituted compounds were detected, which implies that chain-shortening and H/F change reactions had occurred. Moreover, this confirmed the generation of sulfonated and unsaturated intermediates during the process, which disclosed valuable new mechanistic insights into PFOA degradation.

Insights into the hydrated electron generation from UV/aniline: Mechanism and quantum efficiency

The insight into the neglected reduction process accompanied by UV-based oxidation process may provide new ideas for the development of advanced oxidation and reduction technologies. In this study, aniline was comprehensively investigated as an unexpected indicator of hydrated electron (eaq−) under UV irradiation. Monochloroacetic acid (MCAA) was selected as the probe of eaq− and the balance of chloride ions indicated the reduction of MCAA. Further, laser flash photolysis experiments demonstrated the generation of eaq− in the UV/aniline process and the half-life period of formed eaq− was demonstrated to be 0.13 μs. The photolysis of aniline along with the decay of the excited state of aniline was responsible for the eaq− generation. Besides, the hydrogen atom (H) generated from the photolysis can subsequently reacted with OH− to generate eaq−. The photolysis pathways of aniline were proposed by the results of GC-MS. Aniline was abstracted of H in solution to the formation of aniline radical (PhNH) or form aminophenol in three different isomers (orto-, meta- and para-aminophenol). Moreover, UV/aniline showed a higher reducing capacity of MCAA compared with other organic electron donors and sustained a highly reducing ability in a wide pH. And the calculation results of quantum efficiency (Φ) showed that excessive aniline was not conducive to the elevation of Φ. This study introduced a novel pathway of eaq− generation during the photolysis of aniline and provided a new perspective for eaq−-based advanced reduction processes.

Advanced reduction process to achieve efficient degradation of pyridine

Pyridine and its derivatives are widely consumed and detected in the environment persistently, which can cause potential adverse impacts on environment and human health. Considering the fact that pyridine could absorb UV light at 254 nm to generate excited one, which could react with reductive radicals, promoting its structural changes, we proposed that one typical efficient advanced reduction process (ARP) which combines UV irradiation with sulfite could be used to eliminate pyridine quickly. Sulfite/UV process showed a higher pyridine removal rate with a pseudo-first-order reaction rate constant of 0.1439 min−1, which was 3 times of that in UV irradiation and 1.3 times in UV/H2O2 process. This was primarily due to reductive radicals (eaq–, H• and SO3•–) produced by UV irradiation. The removal rate of pyridine was highest in slightly alkaline environment. And the presence of oxygen, as well as certain concentration of humid acid just showed slight inhibition, indicating the possibility of application in practical environment. A positive impact was observed with increasing sulfite dosage, but it was gradually inhabited when the dosage was over 5 mM. The present study may provide an alternative efficient technology for the degradation of pyridine ring-containing substances.

Advanced reduction of bromate by UV/TiO2-Bi process without external sacrificial agents: Mechanism and applications

Recently, the heterogeneous advanced reduction processes (ARPs) for bromate (BrO3-) degradation have attracted considerable attention due to its stable performance and easy combination with UV disinfection. In this study, a UV/TiO2-Bi advanced reduction process (ARP) was established for BrO3- removal without adding external sacrificial agents. In the UV/TiO2-Bi ARP, Bi nanoparticles (NPs) doped on the TiO2 acted as hole trapping centers, while TiO2 NPs were used as reduction centers for the reduction of BrO3-, which enhanced the charge carrier separation. The UV/TiO2-Bi-1% ARP exhibited the highest BrO3- removal rate without the addition of sacrificial agents in the reaction solution compared to UV/TiO2 ARP and the UV/TiO2-Bi ARP at other Bi/Ti ratios. The result of 72% removal of 200 µg/L BrO3- at 60 min by 0.2 g/L TiO2-Bi-1% in tap water at pH 6.8 and 7.71 mg/L dissolved oxygen (DO) demonstrates the promising potential of the UV/TiO2-Bi ARP for the removal of BrO3- in practical applications. Compared to the 99.5% removal in deionized (DI) water at 60 min, lower BrO3- reduction in tap water was likely attributed to the consumption of photo-generated electrons by NO3– and the adsorption of natural organic matter on the surface of TiO2-Bi-1% nanocomposites. The study provides not only a novel BrO3- removal system with good photo-reduction performance, but also some guidance for the influence of constituents in raw water on the treatment of BrO3-.

Nitrate Reduction by the Ultraviolet-Sulfite Advanced Reduction Process

Advanced reduction processes (ARPs) form highly reactive reducing radicals and aqueous electrons, which can reduce oxidized contaminants, by activating reducing agents. Experiments were conducted to investigate the kinetics of nitrate degradation by the sulfite/ultraviolet (UV) ARP, which has been proven to be the most effective ARP. Experimental variables were pH, UV (253.7 nm) intensity, and sulfite concentration. Rate of nitrate reduction increases with increasing pH, UV intensity, and initial sulfite concentration. The efficiency of photochemical degradation of nitrate (quantum yield based on overall reaction rate) was unaffected by UV intensity, but decreased with pH and initial sulfite concentration. Nitrite was identified as a major intermediate product of nitrate mineralization by the UV/sulfite ARP, which was simultaneously degraded along with nitrate.

Structure-Specific Aerobic Defluorination of Short-Chain Fluorinated Carboxylic Acids by Activated Sludge Communities

Per- and polyfluoroalkyl substances (PFAS) are a large group of manmade chemicals that impose emerging environmental concerns. Among them, short-chain per- and polyfluorinated carboxylic acids represent an important subgroup used as building blocks of biologically active chemicals and functional materials. Some are also considered PFAS alternatives, and some could be byproducts of the physicochemical treatment of PFAS. However, little is known about the environmental fate of short-chain fluorinated carboxylic acids (FCAs) and their defluorination/transformation by microorganisms. To fill the knowledge gap, we investigated the structure-reactivity relationships in the aerobic defluorination of C3-C5 FCAs by activated sludge communities. Four structures exhibited greater than 20% defluorination, with 3,3,3-trifluoropropionic acid being almost completely defluorinated. We further analyzed the defluorination/transformation pathways and inferred the structures susceptible to aerobic microbial defluorination. We also demonstrated that the defluorination was via cometabolism. The findings advance the fundamental understanding of aerobic microbial defluorination and help assess the environmental fate of PFAS. Since some short-chain PFAS, such as 3,3,3-trifluoropropionic acid, are the incomplete defluorination byproducts of advanced reduction processes, their defluorination by activated sludge communities sheds light on the development of cost-effective chemical-biological PFAS treatment train systems.

UV/sulphite as alternative for UV/H2O2 for micropollutant degradation in drinking water

Abstract The UV/SO32– process can be applied to convert organic micropollutants (OMP). The reaction is induced by UV photolysis of sulphite (preferably using a medium pressure (MP) UV lamp). Although in general it is assumed that the UV/SO32− process is an advanced reduction process, our results show degradation of OMP under both anoxic and oxic conditions. Several recalcitrant OMP even show higher degradation rates under oxic conditions than under anoxic conditions. This conversion is affected by the presence of dissolved organic carbon (DOC), HCO3− and NO3−, and thus was more effective in drinking water than in Milli-Q water. The reaction rate constants of the oxic UV/SO32− process in general are lower than those of a UV/H2O2 process, but in cases where the use of H2O2 is not preferred, the UV/SO32− process may be an interesting alternative.