Comparative Analysis of Photocatalytic and Electrochemical Degradation of 4-Ethylphenol in Saline Conditions

Modification of Sugar Cane Bagasse with CTAB and ZnO for Methyl Orange and Methylene Blue Removal

Sugar cane bagasse (SB) was modified with cetyltrimethylammonium bromide (CTAB), followed by impregnation with zinc oxide (ZnO) to create a synergistic adsorption and photocatalytic system for methyl orange (MO) and methylene blue (MB) removal. The presence of CTAB and ZnO was confirmed by X-ray diffraction, Fourier transform infrared, and energy dispersive X-ray (for Zn and O). Modification of SB with CTAB (CSB) generated more positive sites on the surface of SB, which enhanced MO removal compared with that of pristine SB. ZnO impregnation induces a decrease in MO removal due to the ZnO presence on the CSB surface, which might reduce the positive sites on the CSB. In addition, the positive sites on CSB can interact with Zn2+ and O2- to form ZnO and lead to a decrease in MO removal. In contrast, the presence of ZnO facilitated good removal of MB compared to CSB, indicating that the photocatalytic process plays a greater role in removing MB. However, the addition of H2O2 can improve MO and MB removal under irradiation due to the formation of external •OH. The photocatalytic performance of MO and MB was also observed to be favored under acidic and alkaline conditions, respectively.

Theoretical study on the potential environmental and ecological risk of 4-ethylphenol induced by hydroxyl radical

This study closely examines the environmental fate of 4-ethylphenol (4-EP), a significant byproduct of biomass combustion. We employed quantum chemical calculations to investigate the reaction mechanism, kinetics, and ecotoxicity of 4-EP initiated by OH radicals in various environments (aqueous, atmospheric liquid, atmospheric and inhomogeneous phases). Our findings highlight that solvent effects contribute to a higher OH-addition reaction branching ratio (Γadd) of 0.68 for 4-EP in an aqueous solution, compared to 0.26 in the gas-phase environment and 0.22 in the inhomogeneous environment at 298 K. We determined the rate constants for the liquid-phase, gas-phase, and nonhomogeneous phase to be 1.14 × 109 s-1 M-1, 3.09 × 109 s-1 M-1, and 6.19 × 1014 s-1 M-1, respectively. Notably, the adsorption of mineral particles considerably enhances the reaction rate of 4-EP with OH radicals. 4-ethylbenzene-1,2-diol, 4-hydroxycyclohexa-3,5-diene-1,2-dione, 1-ethyl-6-methyl-6H-benzo(c)chromene-4,9-diol, 5-ethyl-6'-(1-hydroxyethyl)-(1,1'-biphenyl)-2,3,3'-triol and 2-ethyl-4,6,9-trimethyl-6H-benzo(c) chromene are major products in both gas-phase and liquid-phase reactions, and (2Z, 4Z)-4-ethyl-6-oxohexa-2,4-dienoic acid is also one of the major products in gas-phase reactions. Toxicological predictions indicate that the ecotoxicity of 4-ethyl-6-methyl-6H-benzo(c)chromene-1,9-diol, 2-ethyl-6-methyl-6H-benzo(c)chromene-3,9-diol, and 2-ethyl-4,6,9-trimethyl-6H-benzo(c) chromene surpassed that of 4-EP. However, the toxicity of the reaction products is reduced in the presence of NOx. This investigation provides an exhaustive theoretical foundation for comprehending the degradation behavior of 4-EP and underscores the need to consider various environmental factors in assessing the potential risk of biomass combustion by products.

Removal of organic pollutants in shale gas fracturing flowback and produced water: A review

Shale gas has been developed as an alternative to conventional energy worldwide, resulting in a large amount of shale gas fracturing flowback and produced water (FPW). Previous studies focus on total dissolved solids reduction using membrane desalination. However, there is a lack of efficient and stable techniques to remove organic pollutants, resulting in severe membrane fouling in downstream processes. This review focuses on the concentration and chemical composition of organic matter in shale gas FPW in China, as well as the hazards of organic pollutants. Organic removal techniques, including advanced oxidation processes, coagulation, sorption, microbial degradation, and membrane treatment are systematically reviewed. In particular, the influences of high salt on each technique are highlighted. Finally, different treatment techniques are evaluated in terms of energy consumption, cost, and organic removal efficiency. It is concluded that integrated coagulation-sorption-Fenton-membrane filtration represents a promising treatment process for FPW. This review provides valuable information for the feasible design, practical operation, and optimization of FPW treatment.

Defective TiO2 Nanotube Arrays for Efficient Photoelectrochemical Degradation of Organic Pollutants

Oxygen vacancies (OVs) are one of the most critical factors that enhance the electrical and catalytic characteristics of metal oxide-based photoelectrodes. In this work, a simple procedure was applied to prepare reduced TiO₂ nanotube arrays (NTAs) (TiO_2-x) via a one-step reduction method using NaBH₄. A series of characterization techniques were used to study the structural, optical, and electronic properties of TiO_2-x NTAs. X-ray photoelectron spectroscopy confirmed the presence of defects in TiO_2-x NTAs. Photoacoustic measurements were used to estimate the electron-trap density in the NTAs. Photoelectrochemical studies show that the photocurrent density of TiO_2-x NTAs was nearly 3 times higher than that of pristine TiO₂. It was found that increasing OVs in TiO₂ affects the surface recombination centers, enhances electrical conductivity, and improves charge transport. For the first time, a TiO_2-x photoanode was used in the photoelectrochemical (PEC) degradation of a textile dye (basic blue 41, B41) and ibuprofen (IBF) pharmaceutical using in situ generated reactive chlorine species (RCS). Liquid chromatography coupled with mass spectrometry was used to study the mechanisms for the degradation of B41 and IBF. Phytotoxicity tests of B41 and IBF solutions were performed using Lepidium sativum L. to evaluate the potential acute toxicity before and after the PEC treatment. The present work provides efficient PEC degradation of the B41 dye and IBF in the presence of RCS without generating harmful products.

Preparation and characterization of PMT-TiO2-NTs@NiO-C/Sn-Sb composite electrodes by a two-step pulsed electrodeposition method for the degradation of crystalline violet

To overcome the limitations imposed by Sn-Sb electrodes, the titanium foam (PMT)-TiO₂-NTs@NiO-C/Sn-Sb composite electrodes with cubic crystal structure are synthesized by introducing NiO@C nanosheet arrays interlayer on the TiO₂-NTs/PMT matrix through hydrothermal and carbonization process. Then a two-step pulsed electrodeposition method is used to prepare the Sn-Sb coating. Benefiting from the advantages of stacked 2D layer-sheet structure, the obtained electrodes exhibit enhanced stability and conductivity. Synergy of inner and outer layers fabricated by different pulse times strongly influence the electrochemical catalytic properties of the PMT-TiO₂-NTs@NiO-C/Sn-Sb (Sn-Sb) electrode. Hence, the Sn-Sb (b0.5 h + w1 h) electrode is the optimal electrode to degrade the Crystalline Violet (CV). Next, the effect of the four experimental parameters (initial CV concentration, current density, pH value and supporting electrolyte concentration) on the degradation of CV by the electrode are investigated. The degradation of the CV is more sensitive to alkaline pH, and the rapid decolorization of CV when the pH is 10. Moreover, the possible electrocatalytic degradation pathway of CV is performed using HPLC-MS. Results from the tests show that the PMT-TiO₂-NTs/NiO@C/Sn-Sb (b0.5 h + w1 h) electrode is an interesting alternative material in industrial wastewater applications.

Potential of low-cost TiO2-PVC composite in photoelectrocatalytic degradation of reactive orange 16 under visible light

In recent years, previously reported studies revealed a high efficiency of pollutant degradation by coupling photocatalysis and electrochemical processes (PECs) using titanium dioxide (TiO₂) photoelectrode rather than using photocatalysis or electrocatalysis alone. However, some of the TiO₂ photoelectrodes that have been reported were not cost-effective. This is due to the use of expensive chemicals and certain expensive equipment in the fabrication process, other than involving complicated preparation steps. Therefore, this study is aimed at investigating the PEC performance and stability of low-cost TiO₂-polyvinyl chloride (TiO₂-PVC) composite photoelectrode for Reactive Orange 16 (RO16) degradation. The materials characterisation using the ATR-FTIR, XRD and UV-Vis DRS proved that TiO₂ and TiO₂-PVC were successfully synthesised. The micrograph obtained for the surface characterisation using the FESEM showed that the smooth surface of freshly prepared photoelectrodes turned slightly rough with tiny pits formation after five continuous PEC processes. Nevertheless, the photoelectrode retained its original shape in good condition for further PEC processes. By PEC process, the fabricated photoelectrode showed 99.4% and 51.1% of colour and total organic carbon (TOC) removal, respectively, at optimised PEC parameters (1.0 mol L^-1 NaCl concentration, 10 V applied voltage, 120 min degradation time and initial pH 2). Moreover, the fabricated photoelectrode demonstrated sufficient reusability potential (~ 96.3%) after five cycles of PEC processes. In summary, a low-cost and stable composite photoelectrode with high efficiency in RO16 degradation was successfully fabricated and could be potentially applied for other emerging pollutants degradation via the PEC degradation technique.

Photocatalytic nanocomposite membranes for environmental remediation

We report the design and one-pot synthesis of Ag-doped BiVO₄embedded in reduced graphene oxide (BiVO₄:Ag/rGO) nanocomposites via a hydrothermal processing route. The binary heterojunction photocatalysts exhibited high efficiency for visible light degradation of model dyes and were correspondingly used for the preparation of photocatalytic membranes using polyvinylidene fluoride (PVDF) or polyethylene glycol (PEG)-modified polyimide (PI), respectively. The surface and cross-section images combined with elemental mapping illustrated the effective distribution of the nanocomposites within the polymeric membranes. Photocatalytic degradation efficiencies of 61% and 70% were achieved after 5 h of visible light irradiation using BiVO₄:Ag/rGO@PVDF and BiVO₄:Ag/rGO@PI (PEG-modified) systems, respectively. The beneficial photocatalytic performance of the BiVO₄:Ag/rGO@PI (PEG-modified) membrane is explained by the higher hydrophilicity due to the PEG modification of the PI membrane. This work may provide a rational and effective strategy to fabricate highly efficient photocatalytic nanocomposite membranes with well-contacted interfaces for environmental purification.

Effect of electrolyte composition on electrochemical oxidation: Active sulfate formation, benzotriazole degradation, and chlorinated by-products distribution

Electrochemical oxidation is an effective technique for treating persistent organic pollutants, which are hardly removed in conventional wastewater treatment plants. Sulfate and chloride salts commonly used and present in natural wastewater influence the electrochemical degradation process. In this study, the effect of electrolyte composition on the active sulfate species (SO₄^●⁻ and S₂O₈²⁻) formation, benzotriazole degradation-a model organic compound, and chlorinated by-products distribution have been investigated while using a boron-doped diamond (BDD) anode. Different Na₂SO₄:NaNO₃ and Na₂SO₄:NaCl ratios with constant conductivity of 10 mS/cm were used in the experiments and applied anode potential was kept constant at 4.3 V vs. Ag/AgCl. The electrogenerated SO₄^●⁻ and S₂O₈²⁻ formation were faster in 10:1 and 2:1 Na₂SO₄:NaNO₃ ratios than in the 1:0 ratio. The ^●OH-mediated SO₄^●⁻ production has prevailed in 10:1 and 2:1 ratios. However, ^●OH-mediated SO₄^●⁻ production has hindered the 1:0 ratio due to excess chemisorption of SO₄²⁻ on the BDD anode. Similarly, the faster benzotriazole degradation, mineralization, and lowest energy consumption were achieved in the 10:1 Na₂SO₄:NaNO₃ and Na₂SO₄:NaCl ratio. Besides, chlorinated organic by-product concentration (AOX) was lower in the 10:1 Na₂SO₄:NaCl ratio but increased with the increasing chloride ratio in the electrolyte. LC-MS analysis shows that several chlorinated organic transformation products were produced in 0:1 to 2:1 ratio, which was not found in the 10:1 Na₂SO₄:NaCl ratio. A comparatively higher amount of ClO₄⁻ was formed in the 10:1 ratio than in 2:1 to 0:1 ratio. This ClO₄⁻ formation train evidence the effective ^●OH generation in a sulfate-enriched condition because the ClO₄⁻ formation is positively correlated to ^●OH concentration. Overall results show that sulfate-enriched electrolyte compositions are beneficial for electrochemical oxidation of biorecalcitrant organic pollutants.

Minimizing toxic chlorinated byproducts during electrochemical oxidation of Ni-EDTA: Importance of active chlorine-triggered Fe(II) transition to Fe(IV)

The formation of chlorinated byproducts represents a significant threat to the quality of the effluent treated using electrochemical advanced oxidation processes (EAOPs), thus spurring investigation into alleviating their production. This study presents a new strategy to minimize the release of chlorinated intermediates during the electrochemical oxidation of Ni-EDTA by establishing a dual mixed metal oxide (MMO)/Fe anode system. The results indicate that the dual-anode system achieved a substantially higher rate (0.141 min^-1) of Ni-EDTA destruction and accordingly allowed a more pronounced removal of aqueous Ni (from 39.85 to 0.63 mg L^-1) after alkaline precipitation, compared with its single MMO anode (0.017 min^-1 of Ni-EDTA removal, with 14.38 mg L^-1 Ni remaining) and single Fe anode (insignificant Ni-EDTA removal, with 38.37 mg L^-1 Ni remaining) counterparts. Compared to reactive chlorine species (RCS) produced from the single MMO anode system, Fe(IV) was in situ generated from the dual-anode system and was predominantly responsible for the attenuation of chlorinated byproducts and thus the decrease in the acute toxicity of the treated solution (evaluated using luminescent bacteria). The Fe(IV)-dominated dual-anode system also exhibited superior performance in removing multiple pollutants (including organic ligands, Ni, and phosphite) in the real electroless plating effluent. The findings suggest that the strategy for Fe(II) transition to Fe(IV) by active chlorine paves a new avenue for yielding less chlorinated products with lower toxicity when EAOPs are used to treat chloride-containing organic wastewater.

Iron Phosphide Precatalyst for Electrocatalytic Degradation of Rhodamine B Dye and Removal of Escherichia coli from Simulated Wastewater

Electrocatalysis using low-cost materials is a promising, economical strategy for remediation of water contaminated with organic chemicals and microorganisms. Here, we report the use of iron phosphide (Fe2P) precatalyst for electrocatalytic water oxidation; degradation of a representative aromatic hydrocarbon, the dye rhodamine B (RhB); and inactivation of Escherichia coli (E. coli) bacteria. It was found that during anodic oxidation, the Fe2P phase was converted to iron phosphate phase (Fe2P-iron phosphate). This is the first report that Fe2P precatalyst can efficiently catalyze electrooxidation of an organic molecule and inactivate microorganisms in aqueous media. Using a thin film of Fe2P precatalyst, we achieved 98% RhB degradation efficiency and 100% E. coli inactivation under an applied bias of 2.0 V vs. reversible hydrogen electrode in the presence of in situ generated reactive chlorine species. Recycling test revealed that Fe2P precatalyst exhibits excellent activity and reproducibility during degradation of RhB. High-performance liquid chromatography with UV-Vis detection further confirmed the electrocatalytic (EC) degradation of the dye. Finally, in tests using Lepidium sativum L., EC-treated RhB solutions showed significantly diminished phytotoxicity when compared to untreated RhB. These findings suggest that Fe2P-iron phosphate electrocatalyst could be an effective water remediation agent.

Effect of Potential and Chlorides on Photoelectrochemical Removal of Diethyl Phthalate from Water

Removal of persistent pollutants from water by photoelectrocatalysis has emerged as a promising powerful process. Applied potential plays a key role in the photocatalytic activity of the semi-conductor as well as the possible presence of chloride ions in the solution. This work aims to investigate these effects on the photoelectrocatalytic oxidation of diethyl phthalate (DEP) by using TiO2 nanotubular anodes under solar light irradiation. PEC tests were performed at constant potentials under different concentration of NaCl. The process is able to remove DEP following a pseudo-first order kinetics: values of kapp of 1.25 × 10−3 min−1 and 1.56 × 10−4 min−1 have been obtained at applied potentials of 1.8 and 0.2 V, respectively. Results showed that, depending on the applied potential, the presence of chloride ions in the solution affects the degradation rate resulting in a negative effect: the presence of 500 mM of Cl− reduces the value of kapp by 50 and 80% at 0.2 and 1.8 V respectively.

Enhancement of the photocatalytic synchronous removal of Cr(vi) and RhB over RP-modified flower-like SnS2

Although photocatalysis is frequently employed to remove various pollutants in water, it still suffers from low efficiency due to the rapid recombination of photogenerated electrons and holes. In this study, a red phosphorus/tin disulfide (RP/SnS₂) composite photocatalyst is fabricated by loading nano-sized RP on flower-like SnS₂ films with a facile hydrothermal method. It is noteworthy that the 2D heterojunction formed between SnS₂ and RP provided channels for the rapid transfer of photon-generated carriers and their effective separation. Furthermore, the separated electrons can react with absorbed O₂ for the generation of superoxide radicals (˙O₂ ^-), thereby impacting the photocatalytic degradation oxidation reaction. The application of photocatalytic synchronous removal of Cr(vi) and RhB over RP/SnS₂ was implemented first. Compared with pristine SnS₂, the photocatalytic degradation activity of Cr(vi) and RhB over the RP/SnS₂ composite was significantly enhanced and the kinetic rate constant reached 8.2, which is 10.8 times that of pristine SnS₂. Moreover, the hybrid photocatalysts exhibited prominent reusability and stability. Therefore, a photocatalytic degradation mechanism and pathway of carriers are proposed in the study. Furthermore, it is considered that the present study is a promising method in the treatment of wastewater by photocatalysis.