Recalcitrant Organic Pollutants Research Articles

Bacterial enzymatic degradation of recalcitrant organic pollutants: catabolic pathways and genetic regulations.

Contamination of soil and natural water bodies driven by increased organic pollutants remains a universal concern. Naturally, organic pollutants contain carcinogenic and toxic properties threatening all known life forms. The conventional physical and chemical methods employed to remove these organic pollutants ironically produce toxic and non-ecofriendly end-products. Whereas microbial-based degradation of organic pollutants provides an edge, they are usually cost-effective and take an eco-friendly approach towards remediation. Bacterial species, including Pseudomonas, Comamonas, Burkholderia, and Xanthomonas, have the unique genetic makeup to metabolically degrade toxic pollutants, conferring their survival in toxic environments. Several catabolic genes, such as alkB, xylE, catA, and nahAc, that encode enzymes and allow bacteria to degrade organic pollutants have been identified, characterized, and even engineered for better efficacy. Aerobic and anaerobic processes are followed by bacteria to metabolize aliphatic saturated and unsaturated hydrocarbons such as alkanes, cycloalkanes, aldehydes, and ethers. Bacteria use a variety of degrading pathways, including catechol, protocatechuate, gentisate, benzoate, and biphenyl, to remove aromatic organic contaminants such as polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and pesticides from the environment. A better understanding of the principle, mechanisms, and genetics would be beneficial for improving the metabolic efficacy of bacteria to such ends. With a focus on comprehending the mechanisms involved in various catabolic pathways and the genetics of the biotransformation of these xenobiotic compounds, the present review offers insight into the various sources and types of known organic pollutants and their toxic effects on health and the environment.

Heterogeneous Fenton-like Catalyzation of Nanoscale Schwertmannite for Sulfamethoxazole Degradation

Sulfamethoxazole (SMX) contamination in large quantities of wastewater can cause potential environmental problems. Due to difficulty in degrading SMX by natural processes, it is necessary to develop a novel technology to solve this problem. Advanced oxidation processes (AOPs) have been identified as methods with a high potential to treat recalcitrant organic pollutants. The nanoscale schwertmannite (nano-SWT) was prepared with an indoor-temperature synthesis method facilitated by polyvinylpyrrolidone (PVP). In this study, we performed a reaction of the nano-SWT materials with Fenton-like catalysts for SMX degradation in hydrogen peroxide (H2O2) media. The findings showed that the nano-SWT prepared by addition of 0.1 g·L−1 PVP (nano-SWT-n, n = 0.1) could degrade 92.5% of the SMX within 90 min at indoor temperature, which was due to the nano-SWT providing abundant reaction sites at the solid/solution interfaces. Additionally, SMX could be highly mineralized with 75% TOC removal and H2O2 was efficiently utilized during the nano-SWT/H2O2 process. In addition, after six cycles of Fenton-like degradation, the nano-SWT remained stable and reusable as a Fenton-like catalyst for SMX degradation. The nano-SWT performed well as a catalyst for SMX degradation. Additionally, this work provides a feasible environmental purification approach for the efficient degradation of SMX through the use of nanoscale schwertmannite as a catalyst in heterogeneous Fenton-like systems.

Functionalized chitosan and sodium alginate for the effective removal of recalcitrant organic pollutants

In this work, the study of an oxidative-coupling reaction by immobilization of laccase or horseradish peroxidase (HRP) onto chitosan and sodium alginate was reported. The oxidative-coupling reaction of three recalcitrant organic pollutants (ROPs) such as chlorophenol compounds including 2,4-dichlorophenol (DCP), 2,4,6- trichlorophenol (TCP), pentachlorophenol (PCP) was studied. The results showed that the systems with immobilized laccase or horseradish peroxidase had broader range of optimum pH and temperature when compared to that of free laccase and horseradish peroxidase. The removal efficiencies of DCP, TCP and PCP within 6 h were found to be 77 %, 90 % and 83 %, respectively. The rate constants of the first order reactions for laccase were arranged as 0.30 h−1 (TCP) > 0.13 h−1 (DCP) > 0.11 h−1 (PCP) and the rate constants for HRP were arranged as 0.42 h−1 (TCP) > 0.32 h−1 (PCP) > 0.25 h−1 (DCP). The removal rate of TCP was found to be the highest among all and the removal efficiency of ROPs by HRP was always better than that of laccase. The major products of the reaction were identified by LC-MS and confirmed as humic-like polymers.

CoFe2O4/MWCNTs as peroxymonosulfate activator for sulfadiazine degradation in wastewater: Performance, mechanisms, degradation pathway, and products toxicity assessment

Bimetal oxide-based compounds have potential in the degradation of antibiotics in wastewater. In this study, cobalt-ferrite-coated multi-walled carbon nanotubes (CoFe2O4/MWCNTs) have been synthesized by solvothermal method and characterized. Then, the degradation of sulfadiazine (SDZ) by CoFe2O4/MWCNTs activated peroxymonosulfate (PMS) system was determined, mainly including the effects of CoFe2O4/MWCNTs dosage, PMS concentration, solution pH, initial SDZ concentration, and co-existing anions on the degradation of SDZ. Based on reactive oxygen species identification and XPS analysis, the possible reaction mechanisms were investigated. In addition, the potential degradation pathways of SDZ were discussed. The results showed that CoFe2O4/MWCNTs exhibited superior catalytic activity and stability toward SDZ removal, and 97.5% of SDZ was degraded within 30 min under the condition of CoFe2O4/MWCNTs dosage 0.3 g/L, PMS concentration 0.5 mM and solution pH 7. The possible mechanism of PMS activation by CoFe2O4/MWCNTs involved both free-radical and non-radical pathways, and the generated reactive species (SO4•−, •OH, O2•− and 1O2) were responsible for the degradation of SDZ. In addition, four potential degradation pathways were proposed and 15 intermediates were identified. In view of the ecotoxicity assessment, it was found that the toxicity of the degradation intermediates was lower than that of SDZ. Overall, this study established an effective strategy for PMS activation to remove recalcitrant organic pollutants in wastewater.

Oxidative conversion of polycyclic aromatic hydrocarbon phenanthrene by potassium ferrate

Targeting the more and more serious pollution issue caused by recalcitrant bio-toxic organic pollutants in slightly polluted water sources in recent years, the oxidative conversion of polycyclic aromatic hydrocarbon phenanthrene in slightly polluted water by potassium ferrate derived from calcium hypochlorite was studied in this paper. The results showed that the potassium ferrate prepared exhibited obvious spectral absorption in the visible light region, with the maximum absorption value of 0.13 at 510 nm. In addition, the product showed good stability, and the preferable pH range for stability was 9∼10.5, which was wider than the traditional pH range of 9.4∼9.7. This wider pH range increases the application range. The preferable oxidation reaction time with potassium ferrate was preliminarily determined to be 10 minutes. Under the condition of the potassium ferrite content of 8 mg/L, the oxidative conversion rate of phenanthrene reached 97.88%, and the degradation effect for phenanthrene was obvious. With the extension of reaction time, the degradation effect would be further enhanced.

Performance of olive mill wastewater treatment using hybrid system combining sand filtration and vertical flow constructed wetlands

Olive mill wastewater (OMWW) is a main source of noxious and recalcitrant organic pollutants of significant concern in environmental engineering. For this purpose, the current study assessed the performance of system combining sand filtration and vertical flow constructed wetlands (VF-CWs) in reducing the organic load of OMWW. Sand filter system wed in this study is composed of 6 plastic columns of 0.50 m in diameter and 120 L of capacity. The irrigation of sand filter columns was assured by 50 % diluted OMWW at flow rate of 5 L/day. VF-CWs system consists of a 1 m3 tank filled with soil and gravel in the bottom and planted of Typha latifolia, Phragmites australis and Arundo donax aquatic plants. The irrigation of VF-CWs system was assured with treated OMWW with a flow rate of 20 L twice in the week. Obtained results showed that sand filtration achieved a high removal rate of 76.42, 75.12, 79.7 and 78.10 %, respectively, for total suspended solid (TSS), dissolved COD (chemical oxygen demand), total COD and phenolic compounds. The usage of the combination of sand filter and VF-CWS improved significantly the elimination efficiency of total COD up to 91.44 %, dissolved COD up to 77.09 % and phenolic compounds up to 79.47 %, but it did not report any performance in the removal of TSS. Despite the high performance of hybrid system (sand filter+VF-CWs) in reducing the organic pollution, final effluent concentrations remained below the permitted limits for discharge into a municipal sewer system.

Activation of periodate with pinewood biochar-CuO composite for the removal of recalcitrant organic pollutants – Mechanisms and degradation products

The degradation kinetics and mechanisms of various types of recalcitrant organic pollutants (ROPs) by a novel biochar-CuO (B-CuO)/periodate (PI) system have been deeply investigated in the present study. More precisely, the study aimed to explore the significant effects of pH on the formation of various oxidative species using this treatment system. According to the results achieved, iodate (IO3∙) and periodate radicals (IO4∙), ∙OH and O(3P) were mostly generated at pH ≈ 7, resulting in the efficient removal of methylene blue (MB) (90% after 30 min, kobs = 0.09 min−1), while the system favoured the formation of O(3P) under highly alkaline pH values (pH = 11), leading to lower degradation kinetics of the pollutant (51% after 30 min, kobs = 0.028 min−1). Additionally, no oxidative species were found at highly acidic pH values (pH = 3), and no significant MB removal was observed (21% after 30 min, kobs = 0.0081 min−1). Elevating the temperature (from 25 °C to 80 °C) and bubbling with nitrogen or oxygen gas also did not have any significant effects on the degradation of ROPs. Additionally, the system maintained its efficiency in various water matrices, including real wastewater, confirming the applicability of the biochar-CuO/PI system for real water treatment processes. The system also exhibited a high degradation efficiency for other types of ROPs, such as ciprofloxacin. Experiments using ultra-high performance liquid chromatography in combination with quadrupole time-of-flight mass spectrometry were performed to identify the degradation products and pathways of MB, revealing the contribution of two different degradation pathways and the formation of five degradation products in the B-CuO/PI system. The ECOSAR program was employed to evaluate the predicted toxicity of the degradation products. The results showed that the system could efficiently detoxify MB.

Solar photoelectro-Fenton: A very effective and cost-efficient electrochemical advanced oxidation process for the removal of organic pollutants from synthetic and real wastewaters

Recalcitrant and toxic organic pollutants from wastewaters are scarcely removed in conventional wastewater treatment plants. To preserve the water quality, organics need to be removed by developing powerful oxidation technologies. Our laboratory proposed in 2007 a potent electrochemical advanced oxidation process (EAOP) for wastewater remediation, so-called solar photoelectro-Fenton (SPEF). This review summarizes the advances of this emerging technology up to 2022, making evident its effectiveness and cost-efficiency for the destruction of usual organic pollutants. The simultaneous action of generated hydroxyl radicals and the photolysis by sunlight explains the high oxidation power of SPEF respect to other EAOPs. The review is initiated by describing the fundamentals of the process to remark the role of the produced oxidants and the benefits of using solar irradiation in its performance. The photoelectrochemical systems used (bench tank reactor and solar pre-pilot flow plant) and the assessment of the operating variables are discussed. The characteristics of the most common homogeneous SPEF for the degradation and mineralization of several synthetic solutions of industrial chemicals, herbicides, pharmaceuticals, and synthetic organic dyes, as well as of some real wastewaters, are further described. The influence of the photoelectrochemical cell, electrodes, solution pH, electrolyte composition, Fe2+ and pollutant concentration, and current density is analyzed. The performance of a homogeneous SPEF-like process with active chlorine and heterogeneous SPEF processes with solid catalysts such as Fe3O4 and sodium vermiculite is also discussed. Finally, the advances of homogeneous SPEF combined with other techniques like solar photocatalysis, solar photoelectrocatalysis, anaerobic digestion, and nanofiltration are reported.

Single-Atom MnN5 Catalytic Sites Enable Efficient Peroxymonosulfate Activation by Forming Highly Reactive Mn(IV)-Oxo Species.

Four-nitrogen-coordinated transitional metal (MN4) configurations in single-atom catalysts (SACs) are broadly recognized as the most efficient active sites in peroxymonosulfate (PMS)-based advanced oxidation processes. However, SACs with a coordination number higher than four are rarely explored, which represents a fundamental missed opportunity for coordination chemistry to boost PMS activation and degradation of recalcitrant organic pollutants. We experimentally and theoretically demonstrate here that five-nitrogen-coordinated Mn (MnN5) sites more effectively activate PMS than MnN4 sites, by facilitating the cleavage of the O-O bond into high-valent Mn(IV)-oxo species with nearly 100% selectivity. The high activity of MnN5 was discerned to be due to the formation of higher-spin-state N5Mn(IV)═O species, which enable efficient two-electron transfer from organics to Mn sites through a lower-energy-barrier pathway. Overall, this work demonstrates the importance of high coordination numbers in SACs for efficient PMS activation and informs the design of next-generation environmental catalysts.

Challenges and Future Roadmaps in Heterogeneous Electro-Fenton Process for Wastewater Treatment.

The efficiency of heterogeneous electro-Fenton technology on the degradation of recalcitrant organic pollutants in wastewater is glaringly obvious. This green technology can be effectively harnessed for addressing ever-increasing water-related challenges. Due to its outstanding performance, eco-friendliness, easy automation, and operability over a wide range of pH, it has garnered significant attention from different wastewater treatment research communities. This review paper briefly discusses the principal mechanism of the electro-Fenton process, the crucial properties of a highly efficient heterogeneous catalyst, the heterogeneous electro-Fenton system enabled with Fe-functionalized cathodic materials, and its essential operating parameters. Moreover, the authors comprehensively explored the major challenges that prevent the commercialization of the electro-Fenton process and propose future research pathways to countervail those disconcerting challenges. Synthesizing heterogeneous catalysts by application of advanced materials for maximizing their reusability and stability, the full realization of H2O2 activation mechanism, conduction of life-cycle assessment to explore environmental footprints and potential adverse effects of side-products, scale-up from lab-scale to industrial scale, and better reactor design, fabrication of electrodes with state-of-the-art technologies, using the electro-Fenton process for treatment of biological contaminants, application of different effective cells in the electro-Fenton process, hybridization of the electro-Fenton with other wastewater treatments technologies and full-scale analysis of economic costs are key recommendations which deserve considerable scholarly attention. Finally, it concludes that by implementing all the abovementioned gaps, the commercialization of electro-Fenton technology would be a realistic goal.

Facile synthesis of N vacancy g-C3N4 using Mg-induced defect on the amine groups for enhanced photocatalytic •OH generation

Photocatalysis offers opportunities to degrade recalcitrant organic pollutants without adding treatment chemicals. Nitrogen (N) vacancy is an effective point-defect engineering strategy to mitigate electron-hole recombination and facilitate hydroxyl radical (•OH) production via superoxide radical (O2•-) generation during photocatalytic application of graphitic carbon nitride (g-C3N4). Here, we report a novel strategy for fabrication of N-vacancy-rich g-C3N4 (NvrCN) via post-solvothermal treatment of Mg-doped g-C3N4. The addition of the Mg precursor during the polycondensation of urea created abundant amine sites in the g-C3N4 framework, which facilitates formation of N vacancies during post-solvothermal treatment. Elemental analysis and electron paramagnetic resonance spectra confirmed a higher abundance of N vacancies in the resultant NvrCN. Further optical and electronic analyses revealed the beneficial role of N vacancies in light-harvesting capacity, electron-hole separation, and charge transfer. N vacancies also provide specific reaction centers for O2 molecules, promoting oxygen reduction reaction (ORR). Therefore, •OH generation increased via enhanced formation of H2O2 under visible light irradiation, and NvrCN photocatalytically degraded oxytetracycline 4-fold faster with degradation rate constant of 1.85 × 10−2 min−1 (light intensity = 1.03 mW/cm2, catalyst concentration = 0.6 g/L, oxytetracycline concentration = 20 mg/L) than pristine g-C3N4. Overall, this study provides a facile method for synthesizing N-vacancy-rich g-C3N4 and elucidates the role of the defect structure in enhancing the photocatalytic activity of g-C3N4.

Enhancement of TOC removal efficiency of sulfamethoxazole using catalysts in the radiation treatment: Effects of band structure and electrical properties of radiocatalysts

Photocatalysts in AOP have been used for the decomposition of recalcitrant organic pollutants. However, several technical issues associated with energy band gap of catalysts still limit the practical application of photocatalysts with sunlight and UV. Some catalysts with large energy band gap have been successfully used in the radiocatalytic systems using gamma radiation and electron beam. However, there has been little systematic studies for band structure and electrical properties of an appropriate radiocatalyst in the radiation treatment. Kinetic studies were conducted to evaluate the properties (conduction band, valence band, energy band gap, electron mobility, and stability) of catalysts suitable for the radiocatalytic system using high energy sources and the radiocatalytic oxidation performance for sulfamethoxazole (SMX). The lower the CB potential and/or the higher the VB potential was, the higher the oxidation performance of a catalyst was. Unlike the photocatalytic process (activating the catalysts with energy band gaps lower than 6.2 eV), the radiation system using a gamma ray could activate the catalyst with a comparatively large energy band gap (<1,240 keV). TOC removal efficiency for SMX (30 mg/L) with Al2O3 (2 g/L) as a radiocatalyst was enhanced up to 83.9 % at an absorbed dose of 5 kGy, compared to the control (radiation treatment without catalysts) showing 6.6 % TOC removal efficiency at the same condition. The stabilities of metal oxide catalysts were much higher (low self-degradation of 0.0006–0.01 % at 50 kGy) than those (0.07–0.44 % at 50 kGy) of metal sulfide catalysts. Since S2− is less electronegative than O2−, the VB holes of metal sulfide catalyst are more prone to self-degradation. The radiation technology has a wide range of selection for catalysts, and can be an effective alternative-treatment system for the mineralization of recalcitrant organic chemicals.

Degradation of Organic Methyl Orange (MO) Dye Using a Photocatalyzed Non-Ferrous Fenton Reaction

Removal of recalcitrant organic pollutants by degradation or mineralization from industrial waste streams is continuously being explored to find viable options to apply on the commercial scale. Herein, we propose a titanium nanotube array (based on a non-ferrous Fenton system) for the successful degradation of a model contaminant azo dye, methyl orange, under simulated solar illumination. Titanium nanotube arrays were synthesized by anodizing a titanium film in an electrolyte medium containing water and ethylene glycol. Characterization by SEM, XRD, and profilometry confirmed uniformly distributed tubular arrays with 100 nm width and 400 nm length. The non-ferrous Fenton performance of the titanium nanotube array in a minimal concentration of H2O2 showed remarkable degradation kinetics, with a 99.7% reduction in methyl orange dye concentration after a 60 min reaction time when illuminated with simulated solar light (100 mW cm-2, AM 1.5G). The pseudo-first-order rate constant was 0.407 µmol-1 min-1, adhering to the Langmuir-Hinshelwood model. Reaction product analyses by TOC and LC/MS/MS confirmed that the methyl orange was partially fragmented, while the rest was mineralized. The facile withdrawal and regeneration observed in the film-based titanium nanotube array photocatalyst highlight its potential to treat real industrial wastewater streams with a <5% performance drop over 20 reaction cycles.

Simultaneous removal of As(III) and organics in Fenton fluidized bed: The favorable co-crystallization of As(V) and Fe(III)

This study has demonstrated that the Fenton fluidized bed (FFB) technology could successfully immobilize As ions on the iron-crystallized (IC) carriers, with simultaneous crystallization of ferric and mineralization of organic pollutants. It was found that rapid fluidized immobilization of aqueous As(V) on the IC carriers was through surface favorable bidentate complexion, while the adsorptive removal of As(III) was rather slow. Adding Fe3+ caused competitive complexion and inhibited the As(V) immobilization. However, effective oxidative crystallization of Fe(II) and As(III) in FFB would take place. With the simultaneous generation of As(V) and Fe(III) by rapid •OH oxidation, two different crystalline precursors would be formed on the IC carriers, leading to efficient co-crystallization of As(V)/Fe(III). Recursive Fe(III) crystallization in the FFB continuously provided essential surface hydroxyl groups for capturing As(V), inducing layer-by-layer co-crystallized immobilization As(V)/Fe(III). The long-term continuous-flow treatment of a practical As(III)-containing industrial organic wastewater exhibited effective/stable co-crystallizations of As(V)/Fe(III), as well as COD removal. The FFB technology is expected to be a good option in treating complex industrial wastewater containing heavy metal and recalcitrant organic pollutants.

Green Synthesis of Silver Oxide Microparticles Using Green Tea Leaves Extract for an Efficient Removal of Malachite Green from Water: Synergistic Effect of Persulfate

The removal of water pollutants by photocatalysis is a promising technique, mainly due to its environmentally friendly and sustainable nature. In this study, the degradation of a recalcitrant organic pollutant, malachite green (MG), was investigated in water by a microstructured silver oxide photocatalyst. The silver oxide (Ag2O) microparticles (MPs) were synthesized by a low-cost, green method, mediated by green tea leaves extract. The surface, morphological and optical properties of the synthesized Ag2O MPs were determined by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) spectroscopy, and ultraviolet-visible (UV-visible) spectrophotometry. The synthesized Ag2O MPs showed good photoactivity, represented by 83% degradation of malachite green (MG) ([C]0 = 0.4 mM, Ag2O loading = 0.1 g L−1) at neutral pH, in 3 h. Persulfate ions (PS) showed a strong synergistic effect on the efficiency of solar/Ag2O photocatalysis, represented by complete MG removal in 15 min, in the presence of 1.6 mM PS. The results revealed that solar/Ag2O, particularly solar/Ag2O/PS photocatalysis is a promising method for the elimination of toxic organic pollutants, such as malachite green, from the water environment.

Biostimulation of Petroleum-Contaminated Soil Using Organic and Inorganic Amendments.

The most common approaches for the in-situ bioremediation of contaminated sites worldwide are bioaugmentation and biostimulation. Biostimulation has often proved more effective for chronically contaminated sites. This study examined the effectiveness of optimized water hyacinth compost in comparison with other organic and inorganic amendments for the remediation of crude oil-polluted soils. Water hyacinth was found to be rich in nutrients necessary to stimulate microbial growth and activity. An organic geochemical analysis revealed that all amendments in this study increased total petroleum hydrocarbon (TPH) biodegradation by ≥75% within 56 days, with the greatest biodegradation (93%) occurring in sterilized soil inoculated with optimized water hyacinth compost. This was followed by polluted soil amended with a combination of spent mushroom and water hyacinth composts (SMC + WH), which recorded a TPH biodegradation of 89%. Soil amendment using the inorganic fertilizer NPK (20:10:10) resulted in 86% TPH biodegradation. On the other hand, control samples (natural attenuation) recorded only 4% degradation. A molecular analysis of residual polycyclic aromatic hydrocarbons (PAHs) showed that the 16 PAHs designated by the US EPA as priority pollutants were either completely or highly degraded in the combined treatment (SMC + WH), indicating the potential of this amendment for the environmental remediation of soils contaminated with recalcitrant organic pollutants.

Formation of Dichlorine Monoxide for Organic Pollutant Degradation by Free Chlorine in High Chloride-Containing Water

Several ppm levels of free chlorine in high chloride-containing water of no less than 0.20 M show remarkably high degradation rate constants toward carbamazepine (kCBZ′). Dichlorine monoxide (Cl2O) was identified to be responsible for the high kCBZ′. Cl2O degrades aromatic pollutants connected to more electron-donating functional groups faster when comparing kCl2O to 1,4-dimethoxybenzene, bisphenol A, benzoic acid, and CBZ. However, its formation at low free chlorine levels in high chloride-containing water cannot be explained by our current knowledge. We proposed an alternative Cl2O formation mechanism, in which chloride transforms hypochlorous acid/hypochlorite (HOCl/ClO–) to dichlorine (Cl2) and polychloride monoanions (Cln–), which then react with HOCl/ClO– to form Cl2O. Decreasing pH from 8.60 to 4.33 increases Cl2 concentrations and thus generates about 600× higher Cl2O concentrations. Weak acid anions, for example, phosphate and bicarbonate, are strong nucleophiles assisting the Cl2O hydrolysis, but their presence at the testing conditions does not much affect kCBZ′. The proposed alternative Cl2O formation mechanism reveals the roles of chloride in shifting the equilibrium toward the Cl2O formation, allowing the simple chlorine addition strategy to degrade recalcitrant organic pollutants in high chloride-containing wastewater. It may also change our understanding of the speciation of different free chlorine species and their impacts on the DBP formation in high chloride-containing wastewater.

Solar-Light-Responsive Nanomaterials for the Photoelectrocatalytic Degradation of Stubborn Pollutants

Due to the ever increasing demand for cleaner water, a remarkable focus has been on the use of nanomaterials in wastewater treatment application. Photoelectrocatalytic (PEC) degradation, an advanced oxidation process which combines light and electrical energy, has been identified as a suitable technique capable of achieving total mineralisation of recalcitrant organic pollutants in wastewater. PEC degradation is non-selective, environmentally friendly and possesses great efficiency. The efficiency of PEC degradation has been enhanced by fabricating the photoanodes on a nanoscale with distinct morphologies. These nanostructured photoanodes have been extensively used for the removal of pharmaceuticals, dyes and phenolic water from wastewater. In many cases, total degradation of the pollutants is achieved within 2 h with significant TOC removal. This review presents an overview of the remarkable success that has been recorded with the use of nanostructured photoanodes in PEC degradation in the presence of visible light. The techniques that are commonly employed to improve the solar light responsiveness of these photoanodes are well discussed. Additionally, the use of nanostructured photoanodes consisting of heterojunction in PEC degradation is also presented. It is our aim that this review will help researchers to make informed decisions regarding the use of nanomaterials in PEC water treatment.

Prospective Life Cycle Assessment for the Electrochemical Oxidation Wastewater Treatment Process: From Laboratory to Industrial Scale.

Electrochemical oxidation (EO) is a promising technology for water purification, but indirect environmental burdens may arise in association with consumption of materials and energy during electrode preparation and process operation. This study evaluated the life cycle environmental impacts of emerging EO technology from laboratory scale to industrial scale using prospective life cycle assessment (LCA) on a quantitative basis. Environmental impacts of EO technology were assessed at laboratory scale by comparing three representative anode materials (SnO2, PbO2, and boron-doped diamond) and other two typical processes (adsorption and Fenton method), which verified the competitiveness of the EO process and identified the key factors to environmental hotspots. Thereafter, LCA of scale-up EO was performed to offer guidance for practical application, and the life cycle inventory was compiled upon thermodynamic and kinetic simulations, empirical calculation rules, and similar technical information. Results demonstrated EO to be effective for destructing recalcitrant organic pollutants, but visible direct benefits might be outweighed by increased indirect environmental burdens associated with the preparation of anode materials, use of electrolytes, and energy consumption during the operation stage at both laboratory scale and larger scale. This necessitated attention to overall life cycle profiles by taking into account reactor design, anode materials, electrolyte and flow pattern, and decentralized location with a large share of renewable power station and rigorous contamination control strategies for wastewater treatment plants.

Byproduct formation in heterogeneous catalytic ozonation processes

Heterogeneous catalytic ozonation (HCO) is a promising advanced oxidation process (AOP) that can effectively degrade recalcitrant organic pollutants; but formation of harmful byproducts should be carefully evaluated.