Enhanced degradation of organic contaminants by Fe(III)/peroxymonosulfate process with l-cysteine

Activation of Metabisulfite by Dissolved Fe(III) at Environmentally Relevant Concentrations for Organic Contaminants Degradation

Currently, iron-catalyzed low-valent sulfur species processes are regarded as potentially valuable advanced oxidation processes (AOPs) in wastewater treatment. As a commonly used low-valent sulfur species in the food industry, metabisulfite (MBS) can undergo decomposition to bisulfite when dissolved in water. Therefore, the combination of MBS with dissolved Fe(III) at environmentally relevant concentrations is proposed in this study to accelerate organic contaminants degradation while simultaneously minimizing the production of iron sludge. The results show that the Fe(III)/MBS process could degrade various organic contaminants, including acid orange 7 (AO7), and the removal efficiency of AO7 obeyed the pseudo-first-order kinetic. The rate constant values exhibited variations depending on the initial concentrations of Fe(III) and MBS, pH values, as well as the reaction temperature. Moreover, the contribution of HO• and SO4•- to AO7 degradation was estimated as 51.59% and 46.45%, respectively. Furthermore, Cl- showed a minimal effect while HCO3- and humic acid resulted in a significant inhibitory effect on AO7 degradation. The satisfactory degradation of AO7 was achieved in three real water bodies. Ultimately, the results of gas chromatography-mass spectrometry and the theoretical calculations greatly facilitate the proposal of AO7 degradation pathways, including N=N cleavage, hydroxylation, and hydrogen abstraction. The findings of this study indicate that the Fe(III)/MBS process may be a promising AOP for further application in organic contaminants degradation during wastewater treatment.

New insights into the Fe(III)-activated peroxyacetic acid: Oxidation properties and mechanism

Iron-activated peroxyacetic acid (PAA) represents an innovative advanced oxidation process (AOP). However, the efficiency of PAA activation by Fe(III) is often underestimated due to the widespread assumption that Fe(III) exhibits much lower ability than Fe(II) to activate PAA. Herein, the oxidative degradation of Rhodamine B (RhB) by Fe(III)-activated PAA process was investigated, and some new insights into the performance and mechanism of the Fe(III)/PAA system were presented. Although the reaction rate of Fe(III) with PAA was slightly slower than that of Fe(II), Fe(III) was still able to activate PAA effectively, and the degradation efficiency of RhB was comparable to that of the Fe(II)/PAA system after 30 min of reaction. Notably, the Fe(III)/PAA system demonstrated superior oxidation capacity compared to conventional oxidant systems, including Fe(III)/H2O2, Fe(III)/PDS, Fe(III)/PMS. The degradation efficiency varied significantly across different water substrates. While Cl- exhibited a certain inhibitory effect on the degradation of RhB, H2PO4- exerted a pronounced inhibitory influence, whereas NO3-, SO42- and HCO3- had negligible effects. The increase of humic acid (HA) showed a facilitating effect in the initial stage, followed by an inhibitory effect. Furthermore, mechanistic studies indicated that H2O2 in PAA solution was not effectively activated. The degradation of RhB primarily occurred through a non-radical pathway generated by PAA activation, with the contribution of reactive species (RS) in the order of FeIVO2+ > •OH > R-O• (CH3COO• and CH3COOO•). RhB degradation was achieved not only by attacking the chromophore of RhB molecules, but also the effective destruction of the stable structures such as benzene rings. This study enhances the understanding of Fe(III)-activated PAA and broadens its potential for developing and applying PAA-based AOPs.

Phenolic acid-containing compounds enhanced Fe3+/peroxides processes for efficient removal of sulfamethoxazole in surface waters

Sulfamethoxazole (SMX) in surface waters has raised widespread concerns due to its potential environmental and biological hazards. In this study, the performance, mechanism, and environmental application of phenolic acid-containing compounds (PACCs) enhanced Fe3+/peroxides processes for SMX degradation were investigated. PACCs with two Ar-OH groups exhibited the lowest toxicity and the best enhancement performance (65%-66% of PDS, 47%-58% of PMS and 61%-63% of H2O2), which were attributed to the excellent chelating and reducing ability towards Fe3+. Free radicals played the predominant role in PDS (37% of SO4-·, 34% of ·OH), PMS (37% of SO4-·, 35% of ·OH) and H2O2 (61% of ·OH) oxidation processes. FeIVO2+ play a non-negligible role in PDS and PMS processes (ŋ[PMSO2] = 52%-80% and ŋ[PMSO2] = 59%-72%). PDS and PMS processes were suitable for a pH range of 3.0-9.0, while the H2O2 process was 3.0-10.0. PDS and PMS processes exhibited stronger resistance to the common anions in surface waters. PMS process exhibited higher adaptability to surface waters quality (92%-98%). This study provides a novel approach for enhancing the degradation of SMX in natural surface waters.

Sulfur-doped α-Fe2O3 as an efficient and recycled peroxydisulfate activator toward organic pollutant degradation: performance and mechanism

Sulfur (S)-doped α-Fe2O3 has been regarded as an excellent catalyst for eliminating organic pollutants in the photo-Fenton-like reaction. Yet, the synthetic complexity and extremely low activity in the dark Fenton-like reaction still need to be solved. In this study, magnetic α-Fe2O3 with sulfide was successfully fabricated via hydrothermal and calcination processes, for the first time, where thiourea acted as both S source and reducing agent, and then, it was applied for activating peroxydisulfate (PDS) to degrade organic contaminants. Important influencing factors were systemically investigated, and the results showed that this catalyst activating PDS was highly effective in the removal of organic pollutants in dark- and photo-Fenton-like reactions. In addition, the catalyst possessed good stability and recyclable ability. The structure of catalyst was analyzed by several characterizations, such as XRD and XPS. The results revealed that sulfide had an important effect on the structure and performance of α-Fe2O3. The detected mechanism indicated that the main reactive oxygen species were altered after switching from darkness to LED illumination. This work offered a promising method to rationally design for S/α-Fe2O3 in the environmental remediation.

The synergistic degradation of pollutants in water by photocatalysis and PMS activation

In recent years, the synergistic degradation of water pollutants through advanced oxidation technology has emerged as a prominent research area due to its integration of various advanced oxidation technologies. The combined utilization of peroxymonosulfate (PMS) activation technology and photocatalysis demonstrates mild and nontoxic characteristics, enabling the degradation of water pollutants across a wide pH range. Moreover, this approach reduces the efficiency of electron hole recombination, broadens the catalyst's light response range, facilitates electron transfer of PMS, and ultimately improves its photocatalytic performance. The paper reviews the current research status of photocatalytic technology and PMS activation technology, respectively, while highlighting the advancements achieved through the integration of photocatalytic synergetic PMS activation technology for water pollutant degradation. Furthermore, this review delves into the mechanisms involving both free radicals and nonradicals in the reaction process and presents a promising prospect for future development in water treatment technology. PRACTITIONER POINTS: Degradation of water pollutants by photocatalysis and PMS synergistic action has emerged. Synergism can enhance the generation of free radicals. This technology can provide theoretical support for actual wastewater treatment.

Structure of a novel Co-based heterogeneous catalyst via Mn3(PO4)2 as a carrier to efficiently activate peroxymonosulfate for improving degradation of sulfonamides

Effective degradation of sulfonamides (SAs) in water is of global importance for decreasing its pathogenicity and bioaccumulation. In this study, Mn₃(PO₄)₂ was used as a carrier to fabricate a novel and high-efficient catalyst with Co₃O₄ anchored (Co₃O₄@Mn₃(PO₄)₂) for the activation of peroxymonosulfate (PMS) to degrade SAs. Surprisingly, the catalyst exhibited superior performance, and nearly 100% of SAs (10 mg L^-1) including sulfamethazine (SMZ), sulfadimethoxine (SDM), sulfamethoxazole (SMX), and sulfisoxazole (SIZ) was degraded by Co₃O₄@Mn₃(PO₄)₂-activated PMS within 10 min. A series of characterization of the Co₃O₄@Mn₃(PO₄)₂ composite were conducted and the main operational parameters of SMZ degradation were investigated. SO₄^•-, •OH, and ¹O₂ were determined to be the dominating reactive oxygen species (ROS) responsible for the degradation of SMZ. Co₃O₄@Mn₃(PO₄)₂ also exhibited excellent stability and the removal rate of SMZ still maintained over 99% even in the fifth cycle. The plausible pathways and mechanisms of SMZ degradation in the system of Co₃O₄@Mn₃(PO₄)₂/PMS were deduced on the basis of the analyses of LCMS/MS and XPS. This is the first report on high-efficient heterogeneous activating PMS by mooring Co₃O₄ on Mn₃(PO₄)₂ to degrade SAs, which provides us with a strategy to structure novel bimetallic catalysts for PMS activation.

Peroxymonosulfate Activation by Facile Fabrication of α-MnO2 for Rhodamine B Degradation: Reaction Kinetics and Mechanism

The persulfate-based advanced oxidation process has been an effective method for refractory organic pollutants' degradation in aqueous phase. Herein, α-MnO₂ with nanowire morphology was facially fabricated via a one-step hydrothermal method and successfully activated peroxymonosulfate (PMS) for Rhodamine B (RhB) degradation. Influencing factors, including the hydrothermal parameter, PMS concentration, α-MnO₂ dosage, RhB concentration, initial pH, and anions, were systematically investigated. The corresponding reaction kinetics were further fitted by the pseudo-first-order kinetic. The RhB degradation mechanism via α-MnO₂ activating PMS was proposed according to a series of quenching experiments and the UV-vis scanning spectrum. Results showed that α-MnO₂ could effectively activate PMS to degrade RhB and has good repeatability. The catalytic RhB degradation reaction was accelerated by increasing the catalyst dosage and the PMS concentration. The effective RhB degradation performance can be attributed to the high content of surface hydroxyl groups and the greater reducibility of α-MnO₂, and the contribution of different ROS (reactive oxygen species) was ¹O₂ > O2·- > SO4·- > ·OH.

Efficient Fe(III)/Fe(II) cycling mediated by L-cysteine functionalized zero-valent iron for enhancing Cr(VI) removal

Herein, L-cysteine (Cys) was modified on zero-valent iron (C-ZVI^bm) by using a mechanical ball-milling method to improve the surface functionality and the Cr(VI) removal efficiency. Characterization results indicated that Cys was modified on the surface of ZVI by the specific adsorption of Cys on the oxide shell to form a -COO-Fe complex. The Cr(VI) removal efficiency of C-ZVI^bm (99.6%) was much higher than that of ZVI^bm (7.3%) in 30 min. The attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analysis inferred that Cr(VI) was more likely to be adsorbed on the surface of C-ZVI^bm to form bidentate binuclear inner-sphere complexes. The adsorption process was well-matched to the Freundlich isotherm and the pseudo-second-order kinetic model. Electrochemical analysis and electron paramagnetic resonance (ESR) spectroscopy revealed that Cys on the C-ZVI^bm lowered the redox potential of Fe(III)/Fe(II), and favored the surface Fe(III)/Fe(II) cycling mediated by the electrons from Fe⁰ core. These electron transfer processes were beneficial to the surface reduction of Cr(VI) to Cr(III). Our findings provide new understandings into the surface modification of ZVI with a low-molecular weight amino acid to promote in-situ Fe(III)/Fe(II) cycling, and have great potential for the construction of efficient systems for Cr(VI) removal.

Photosensitive Dye as an Ideal Peroxymonosulfate Activator for Efficient Self-Degradation: A Novel Idea of Using Waste to Treat Waste

Commonly used peroxydisulfate (PS) or peroxymonosulfate (PMS) activation methods have been limited in their practical application due to certain drawbacks, such as high cost, high energy consumption and secondary pollution. In this study, a catalyst-free alizarin green (AG) self-activating PMS catalytic system was constructed based on photosensitization properties of dye, which ultimately achieved efficient degradation of the dye activator, also the target pollutant. Here, 52.5% of the 100 mL mixture of 10 mg/L AG decomposed within 60 min with 1 mM PMS under visible-light irradiation, thereby showing a strong pH adaptation. Mechanism of AG self-activating PMS was revealed that the photo-excited AG can effectively transfer photo-induced electrons to PMS for its activation, which generates reactive oxidizing species dominated by singlet oxygen (¹O₂), and supplemented by hydroxyl radical (•OH), superoxide radical (O₂^•-) and sulfate radical (SO₄^•-) to realize the efficient self-degradation of the dye pollutants. Moreover, such self-catalytic system operated well under natural sunlight irradiation, indicating the great application potential in the actual wastewater treatment. Herein, photosensitive dye acted as an ideal PMS activator realizing its efficient self-degradation, which provides a novel idea of "using waste to treat waste" for developing wastewater treatment process in a high-efficiency and low-consumption way.

Highly Efficient Copper Doping LaFeO3 Perovskite for Bisphenol A Removal by Activating Peroxymonosulfate

A series of copper doping LaFeO3 perovskite (LaCuxFe1−xO3, LCFO, x = 0.1, 0.4, 0.5, 0.6, 0.9) are successfully synthesized by the sol-gel method under mild conditions. In this study, it is applied for the activation of peroxymonosulfate (PMS) for bisphenol A (BPA) removal. More than 92.6% of BPA was degraded within 30 min at 0.7 g/L of LCFO and 10.0 mM of PMS over a wide pH range with limited leaching of copper and iron ions. The physical–chemical properties of the catalysts were demonstrated by using X-ray diffraction (XRD), N2 adsorption–desorption isotherms, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Furthermore, the effects of catalyst dosage, PMS concentration, initial pH value, and inorganic anions on the LCFO/PMS system were fully investigated. Quenching experiments were performed to verify the formation of reactive oxidant species, which showed that the radical reaction and mechanisms play a great role in the catalytic degradation of BPA. The perovskite LCFO is considered a stable, easy to synthesize, and efficient catalyst for the activation of PMS for wastewater treatment.

Application of BiOX Photocatalyst to Activate Peroxydisulfate Ion-Investigation of a Combined Process for the Removal of Organic Pollutants from Water

The persulfate-based advanced oxidation processes employing heterogeneous photocatalysts to generate sulfate radicals (SO4•−) from peroxydisulfate ion (PDS, S2O82−) have been extensively investigated to remove organic pollutants. In this work, BiOX (X = Cl, Br, and I) photocatalysts were investigated to activate PDS and enhance the transformation rate of various organic substances under UV (398 nm) and Vis (400–700 nm) radiation. For BiOCl and BiOBr, in addition to excitability, the light-induced oxygen vacancies are decisive in the activity. Although without organic substances, the BiOI efficiency highly exceeds that of BiOBr and BiOCl for PDS activation (for BiOI, 15–20%, while for BiOBr and BiOCl, only 3–4% of the PDS transformed); each BiOX catalyst showed enhanced activity for 1,4-hydroquinone (HQ) transformation due to the semiquinone radical-initiated PDS activation. For sulfamethoxypyridazine (SMP), the transformation is driven by direct charge transfer, and the effect of PDS was less manifested. BiOI proved efficient for transforming various organic substances even under Vis radiation. The efficiency was enhanced by PDS addition (HQ is wholly transformed within 20 min, and SMP conversion increased from 40% to 90%) without damaging the catalyst; its activity did change over three consecutive cycles. Results related to the well-adsorbed trimethoprim (TRIM) and application of biologically treated domestic wastewater as a matrix highlighted the limiting factors of the method and visible light active photocatalyst, BiOI.

Degradation of petroleum pollutants in oil-based drilling cuttings using an Fe2+-based Fenton-like advanced oxidation processes

Oil-based drilling cuttings (OBDC) contain a large amount of total petroleum hydrocarbon (TPH) pollutants, which are hazardous to the environment. In this study, Fe²⁺-activating hydrogen peroxide (Fe²⁺/H₂O₂), peroxymonosulfate (Fe²⁺/PMS), and peroxydisulfate (Fe²⁺/PDS) advanced oxidation processes (AOPs) were used to treat OBDC due to the difference in the degradation capacity of TPH caused by the type of free radical generated and effective activation conditions observed for the different oxidants studied. The results showed that the oxidant concentration, Fe²⁺ dosage, and reaction time in the three AOPs were greatly positively correlated with the TPH removal rate in a certain range. The initial pH value had a significant effect on the Fe²⁺/H₂O₂ process, and its TPH removal rate was negatively correlated in the pH range from 3 to 11. However, the Fe²⁺/PMS and Fe²⁺/PDS processes only displayed lower TPH removal rates under neutral conditions and tolerated a wider range of pH conditions. The optimal TPH removal rates observed for the Fe²⁺/H₂O₂, Fe²⁺/PMS, and Fe²⁺/PDS processes were 45.04%, 42.75%, and 44.95%, respectively. Fourier transform infrared spectrometer and gas chromatography-mass spectrometer analysis showed that the alkanes in OBDC could be effectively removed using the three processes studied, and their degradation ability toward straight-chain alkanes was in the order of Fe²⁺/PMS > Fe²⁺/PDS > Fe²⁺/H₂O₂, among which Fe²⁺/PMS exhibited the optimal removal effect for aromatic hydrocarbons. Scanning electron microscope, energy dispersive spectroscopy, and X-ray diffraction results showed no significant changes in the elemental and mineral composition of OBDC before and after treatment. Therefore, this study provided a theoretical reference for the effective degradation of TPH pollutants in OBDC.

Constant oxidation of atrazine in Fe(III)/PDS system by enhancing Fe(III)/Fe(II) cycle with quinones: Reaction mechanism, degradation pathway and DFT calculation

Quinones are potential pollutants and redox active compounds widely distributed in environmental media. In this study, methyl-p-benzoquinone (MBQ) was introduced into Fe(III)/peroxydisulfate system (Fe(III)/PDS) to expedite the conversion of Fe(III) to Fe(II) and the degradation of atrazine (ATZ), ultimately establishing an environmentally friendly system of "treating pollution with pollution". MBQ/Fe(III)/PDS system showed superior performance to traditional Fe(II)/PDS system in pH range of 2-7. Sulfate radical (SO₄^•-) and hydroxyl radical (^•OH) were confirmed to exist in MBQ/Fe(III)/PDS system according to alcohol quenching experiments and ESR tests. Meanwhile, stable 80% of η[PMSO₂] (i.e., the molar ratio of PMSO₂ generation to PMSO consumption) was achieved and manifested that highly reactive substance Fe(IV) also participated in MBQ/Fe(III)/PDS system. The spontaneous transformation of MBQ and methyl-hydroquinone (MHQ) drove Fe(III)/Fe(II) cycle, during which MHQ induced Fe(III) reduction and Fe(II) regeneration. Transformation pathways of ATZ were proposed based on HPLC-MS detection and DFT calculation and ATZ degradation could be initiated by lateral chain oxidation and dechlorination-hydroxylation. The acute toxicity, bioaccumulation factor, developmental toxicity and mutagenicity of ATZ and its degradation intermediates were evaluated by Toxicity Estimation Software Tool, and the luminescent bacteria test was conducted to investigate the acute toxicity variation of the reaction solution. Cl^- obviously inhibited ATZ degradation and three main by-products generation, while humic acid (HA) had little effect on them probably due to the established balance between inhibition (some components in HA competed to consume reactive species) and acceleration (quinone units in HA also facilitated Fe(III)/Fe(II) cycle).

Enhanced Activation of Peroxymonosulfate for Tetracycline Degradation Using CoNi-Based Electrodeposited Films

Synthesizing efficient heterogeneous catalysts with multiple active sites able to activate peroxymonosulfate (PMS) for the degradation of persistent organic pollutants continues to be a challenge for societies worldwide. In response, cost-effective, eco-friendly oxidized Ni-rich and Co-rich CoNi micro-nanostructured films were fabricated following a two-step process based on simple electrodeposition using green deep eutectic solvent as an electrochemical media and thermal annealing. The CoNi-based catalysts demonstrated exceptional efficiency in the heterogeneous catalyzed activation of PMS for tetracycline degradation and mineralization. The effects of the catalysts' chemical nature and morphology, the pH, the concentration of PMS, irradiation with visible light, and the duration of contact with the catalysts on the degradation and mineralization of tetracycline were also studied. In dark conditions, oxidized Co-rich CoNi degraded more than 99% of tetracyclines in only 30 min and mineralized more than 99% of them in only 60 min. Moreover, the degradation kinetics doubled from 0.173 min^-1 in dark conditions to 0.388 min^-1 under visible light irradiation. In addition, the material demonstrated excellent reusability and can be easily recovered with simple heat treatment. Given those findings, our work provides new strategies for constructing high-efficiency and cost-effective PMS catalysts and elucidating the effects of operational parameters and primary reactive species formed by the catalyst-PMS system on water treatment technologies.

Pyrite Cinder as an Effective Fenton-like Catalyst for the Degradation of Reactive Azo Dye: Effects of Process Parameters and Complete Effluent Characterization

This research investigates the potential use of pyrite cinder (PC) as an efficient Fenton-like catalyst for the removal of the reactive azo dye Reactive Red 120 (RR120) from aqueous solutions. The characterization of its PC structure and composition confirmed its great potential to act as catalytic iron source in a heterogeneous Fenton system. Dye removal optimization was performed in terms of PC dosage (0.4–8 g/L), H2O2 concentration (2–25 mM), pH value (2–4.6), initial dye concentration (50–200 mg/L), and mixing time. The highest decolorization efficiency (92%) was achieved after a reaction time of 480 min under following conditions: RR120 = 50 mg/L, PC = 4 g/L, H2O2 = 10 mM, and pH = 3. After decolorization, an extensive analysis of the generated effluent was performed regarding metal leaching, mineralization, toxicity, and degradation product formation. The metal leaching indicated the necessity for a pH increase in order to remove the settled metal hydroxides. The mineralization efficiency was satisfactory, reaching 85% and 62% of the COD and TOC removal, respectively. The respirometry measurements and bioluminescence tests indicated the detoxification of the treated solution. The absorption spectra and GC/MS analysis confirmed the changes in the molecular structure in the form of the destruction of the azo bond, with a simpler aromatic and aliphatic intermediates formation. This study provides an effective method for removing azo dye in polluted water by employing waste tailings as alternative Fenton-like catalysts, while also using waste tailings as the secondary resource.

Acid-washed zero-valent aluminum as a highly efficient persulfate activator for degradation of phenacetin

Phenacetin (PNT) is one of the most frequently detected nonsteroidal anti-inflammatory drugs in the water ecosystems, which poses a potential risk to environmental aquatic organisms. Acid-washed zero-valent aluminum (ZVAl) as a highly efficient activator for persulfate (PS) process was investigated to degrade PNT from the aqueous solution. The results indicated that acid-washed pretreatment for ZVAl could efficiently increase the degradation efficiency of PNT in the PS treatment. The degradation efficiency of PNT (50 μM) was up to 90% in 4 hours with the addition of 0.2 g/L acid-washed ZVAl and 8 mM PS at pH 6.8 and 25 °C. The PNT degradation followed pseudo-first order kinetics in the present system. High activator dosage, PS concentration, and reaction temperature could enhance the PNT degradation. The presence of inorganic anions (i.e., NO₃^-, HCO₃^-) and humic acid (HA) showed inhibitory effects on the PNT degradation. The reuse results illustrated the acid-washed ZVAl material would have continuous and efficient activation performance for PS to degrade the PNT. Radical scavenger experiments and electron paramagnetic resonance indicated that both SO₄^•- and •OH were major reactive species during the PNT degradation. The possible degradation pathways of PNT mainly included the break of C-N and C-O bonds and further oxidation.

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.

Spontaneous FeIII/FeII redox cycling in single-atom catalysts: Conjugation effect and electron delocalization

The mechanism of spontaneous Fe^III/Fe^II redox cycling in iron-centered single-atom catalysts (I-SACs) is often overlooked. Consequently, pathways for continuous SO₄ ^·-/HO⋅ generation during peroxymonosulfate (PMS) activation by I-SACs remain unclear. Herein, the evolution of the iron center and ligand in I-SACs was comprehensively investigated. I-SACs could be considered as a coordination complex created by iron and a heteroatom N-doped carbonaceous ligand. The ligand-field theory could well explain the electronic behavior of the complex, whereby electrons delocalized by the conjugation effect of the ligand were confirmed to be responsible for the Fe^III/Fe^II redox cycle. The possible pyridinic ligand in I-SACs was demonstrably weaker than the pyrrolic ligand in Fe^III reduction due to its shielding effect on delocalized π orbitals by local lone-pair electrons. The results of this study significantly advance our understanding of the mechanism of spontaneous Fe^III/Fe^II redox cycling and radical generation pathways in the I-SACs/PMS process.

UV and Zero-Valent Iron (ZVI) Activated Continuous Flow Persulfate Oxidation of Municipal Wastewater

Currently, sulfate-radical-based advanced oxidation processes are promising candidates to become viable post-treatment processes for wastewater purification. In this work, a continuous flow UV light/persulfate (PS)/zero-valent iron (ZVI) system has been applied for wastewater treatment for the first time. The influence of certain photo-Fenton-like process parameters, such as space time, PS concentration, and PS to ZVI molar ratio, on the removal of total organic carbon (TOC), was examined using the Box–Behnken design. First, synthetic municipal wastewater was used for the experiments, and the polynomial regression model was constructed utilizing the real data by using the response surface methodology (RSM). The adequacy of the RSM model was assessed by analysis of variance, which showed that the model was reliable and could be applied to improve the process parameters for TOC removal. Moreover, both synthetic and real municipal wastewater were spiked with carbamazepine (CBZ), which is commonly prescribed as an antiepileptic drug, to investigate its fate in the UV/PS/ZVI system. With a space time of 60 min, PS concentration of 60 mM, and PS to ZVI molar ratio of 15, it was possible to remove 71% of TOC and completely remove CBZ from the synthetic municipal wastewater, whereas a 60% TOC removal and complete removal of CBZ were achieved at a space time of 50 min, PS concentration of 50 mM, and PS/ZVI molar ratio of 15 for the real municipal wastewater. This difference in TOC removal could possibly be linked to the complex matrix of the real wastewater and the presence of radical scavenging agents.

Integrated Adsorption-Photocatalytic Decontamination of Oxytetracycline from Wastewater Using S-Doped TiO2/WS2/Calcium Alginate Beads

Integrated wastewater treatment processes are needed due to the inefficient removal of emerging pharmaceuticals by single methods. Herein, an adsorbent-photocatalyst integrated material was fabricated by coupling calcium alginate with sulfur-doped TiO2/tungsten disulfide (S-TiO2/WS2/alginate beads) for the removal of oxytetracycline (OTC) from aqueous solution by an integrated adsorption-photocatalysis process. The semiconductor S-TiO2/WS2 hybrid photocatalyst was synthesized with a hydrothermal method, while the integrated adsorbent-photocatalyst S-TiO2/WS2/alginate beads were synthesized by blending S-TiO2/WS2 with sodium alginate using calcium chloride as a precipitating agent. The physicochemical characteristics of S-TiO2/WS2/alginate beads were analyzed using X-ray diffraction , scanning electron microscopy, elemental mapping, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy. The integrated adsorption-photocatalysis process showed enhanced removal from 92.5 to 72%, with a rise in the OTC concentration from 10 to 75 mg/L respectively. The results demonstrated that the adsorption of OTC onto S-TiO2/WS2/alginate beads followed the Elovich kinetic model and Redlich–Peterson isotherm models. The formations of H-bonds, cation bridge bonding, and n-π electron donor-acceptor forces were involved in the adsorption of OCT onto S-TiO2/WS2/alginate beads. In the integrated adsorption-photocatalysis, surface-adsorbed OTC molecules were readily decomposed by the photogenerated active radical species (h⁺, O2•−, and HO•). The persulfate addition to the OTC solution further increased the photocatalysis efficacy due to the formation of additional oxidizing species (SO4•⁻, SO4⁻). Moreover, S-TiO2/WS2/alginate beads showed favorable efficiency and sustainability in OTC removal, approaching 78.6% after five cycles. This integrated adsorption-photocatalysis process offered significant insight into improving efficiency and reusability in water treatment.

Peroxymonosulfate Activation by BaTiO3 Piezocatalyst

Peroxymonosulfate (PMS) plays an important role in the advanced oxidation process for environmental remediation. In this study, barium titanate (BTO) piezocatalyst was selected for the activation of PMS driven by ultrasonic power. The degradation of Rhodamine B (RhB) by BTO single component, PMS single component, and BTO/PMS double components were investigated. The results indicated that PMS can be efficiently activated by BTO under an ultrasound with an RhB degradation rate of 98% within 20 min. The ultrasound not only promoted the activation of the PMS itself, but the surface charge carriers of BTO induced by the ultrasound also contributed to the activation of PMS. ·O2−, ·OH, and ·SO4− radicals were found to be the main active species that participated in the reaction. In order to verify the reaction’s environmental applicability, amoxicillin (AMX) as a typical environmental pollutant was studied. BTO/PMS displayed 80% removal efficiency of AMX, and the products generated were less toxic as demonstrated by eco-toxicity comparison. This work provides a promising strategy to improve the utilization of ultrasonic energy and apply it to the field of environmental pollutants treatment.

Enhanced Fe(III)/Fe(II) Redox Cycle for Persulfate Activation by Reducing Sulfur Species

The activation of persulfate (PS) by Fe(III) for the removal of environmental organic pollutants was severely limited by the low reduction rate from Fe(III) to Fe(II). In present study, we reported that reducing sulfur species (i.e., SO32−, HSO3−, S2−, and HS−) under low concentration could significantly accelerate the Fe(III)/Fe(II) cycle in the Fe(III)/PS system. Under the condition of 1.0 mM Fe(III) and 4.0 mM PS, the removal performance of Fe(III)/PS system was poor, and only 21.6% of BPA was removed within 40 min. However, the degradation efficiency of BPA increased to 66.0%, 65.5%, 72.9% and 82.7% with the addition of 1.0 mM SO32−, HSO3−, S2−, and HS−, respectively. The degradation efficiency of BPA was highly dependent on solution pH and the concentration of reducing sulfur species. When the reductant was excessive, the removal efficiency would be significantly inhibited due to the elimination of reactive species. This study provided some valuable insights for the treatment of organic wastewater containing these inorganic reducing ions.

Core-Shell Hierarchical Fe/Cu Bimetallic Fenton Catalyst with Improved Adsorption and Catalytic Performance for Congo Red Degradation

The preparation of heterogeneous Fenton catalysts with both adsorption and catalytic properties has become an effective strategy for the treatment of refractory organic wastewater. In this work, 4A-Fe@Cu bimetallic Fenton catalysts with a three-dimensional core-shell structure were prepared by a simple, template-free, and surfactant-free methodology and used in the adsorption and degradation of Congo red (CR). The results showed that the open three-dimensional network structure and the positive charge of the surface of the 4A-Fe@Cu catalyst could endow a high adsorption capacity for CR, reaching 432.9 mg/g. The good adsorption property of 4A-Fe@Cu for CR not only did not inactivate the catalytic site on 4A-Fe@Cu but also could promote the contact between CR and the active sites on the catalyst surface and accelerate the degradation process. The 4A-Fe@Cu bimetallic catalyst exhibited higher catalytic activity than monometallic 4A@Cu and/or 4A-Fe catalysts due to low work function value. The effects of different pH, H2O2 dosages, and catalyst dosages on the catalytic performance of 4A-Fe@Cu were explored. In the conditions of 7.2 mM H2O2, 2 g/L 4A-Fe@Cu, and 1 g/L CR solution, the degradation ratio of CR by 4A-Fe@Cu could reach 99.2% at pH 8. This strategy provided guidance to the design of high-performance Fenton-like catalysts with both adsorption and catalysis properties for dye wastewater treatment.

Photo-Induced Holes Initiating Peroxymonosulfate Oxidation for Carbamazepine Degradation via Singlet Oxygen

Peroxymonosulfate (PMS) has been intensively used to enhance the photocatalytic activity of catalysts, which is adopted as an electron acceptor to inhibit the recombination of electrons and holes. However, the effect of holes generated by visible light (VL) on PMS activation is always overlooked. Herein, the VL/Bi2WO6/PMS process was constructed for the efficient removal of organics, in which the degradation rate of carbamazepine (CBZ) increased by over 33.0 times by the introduction of PMS into Bi2WO6 under visible light. The radical quenching and determination experiments confirmed that the photogenerated holes could firstly oxidize PMS to form SO5•− and react with HSO5− to produce 1O2, then inducing the formation of other reactive species to greatly enhance the performance of pollutant removal by the VL/Bi2WO6/PMS process. Density functional theory (DFT) predicted that sites with high Fukui index (f0) on CBZ were more susceptible to being attacked, resulting in hydroxylation, ring closure, and C=C bond cleavage of CBZ. Toxicity estimation indicated that photocatalysis degradation products from CBZ were less toxic compared to the parent compound. This study provides a potential avenue for improving photocatalytic efficiency and widening the application of photocatalytic technology in wastewater purification.

Electro-Chemical Degradation of Norfloxacin Using a PbO2-NF Anode Prepared by the Electrodeposition of PbO2 onto the Substrate of Nickel Foam

A novel three-dimensional network nickel foam/PbO2 combination electrode (PbO2-NF) with high electrochemical degradation efficiency to norfloxacin was successfully fabricated through the electrodeposition of PbO2 on the substrate of nickel foam. The characterization of an PbO2-NF electrode, including surface morphology, elemental components, electrochemical performance, and stability was performed. In electrochemical oxidation tests, the removal efficiency of norfloxacin (initial concentration for 50 mg/L) on PbO2-NF reached 88.64% within 60 min of electrolysis, whereas that of pure nickel foam was only 30%. In the presence of PbO2-NF, the optimum current density, solution pH, electrode spacing for norfloxacin degradation were 30 mA/cm2, 11, and 3 cm, respectively. The electric energy consumption for 80% norfloxacin was approximately 5 Wh/L. Therefore, these results provide a new anode to improve the removal of norfloxacin in the wastewater with high efficiency and low energy consumption.

Wastewater Purification and All-Solid Z-Scheme Heterojunction ZnO-C/MnO2 Preparation: Properties and Mechanism

Unlike many studies on the preparation of Z-scheme heterojunctions by doping precious metals, in this paper we first prepared a core-shell material obtained by C doping in ZnO and then composite with MnO2 to form a heterojunction; that is, a low-cost and highly catalytic ternary composite catalyst was prepared by a simple hydrothermal reaction. Meanwhile, a large amount of experimental data have enabled the heterostructure type as well as the mechanism of photocatalytic performance to be fully demonstrated. It is proven that C as an intermediate medium achieves electron transport while making up the deficiency of ZnO, and constitutes an all-solid state Z-scheme heterojunction, which enables the rapid transfer of photogenerated electron pairs and visible light irradiation to the stream to improve the photocatalytic performance of the composite photocatalyst. In terms of examination of degradation performance, this catalyst showed a high photodegradation rate of tetracycline hydrochloride (TC) of 92.6% within 60 min, and the surface ZnO-C/MnO2 catalysts also showed good degradation effect on practical petrochemical wastewater in CODcr degradation experiments.

Study on the catalytic degradation of Acid Orange 7 and the potential mechanism by ferrous-percarbonate

Factors affecting the degradation of Acid Orange 7 (AO7) were evaluated and optimized when ferrous was used to catalyze percarbonate in the present study. The optimized conditions included the initial pH values ranging from 3 to 11 for AO7 solution, the initial level of AO7, sodium percarbonate (SPC), and Fe²⁺ . Some ions and natural organic materials, which commonly exist in natural water, were also tested to evaluate their potential impacts on the degradation of AO7. The degradation efficiency of AO7 was up to 95% under the optimized test conditions, where the ferrous/percarbonate/AO7 molar ratio was 15/10/1 in the 0.285 mmol/l AO7 aqueous solution. The presence of Cl^- , SO₄ ^2- , NO₃ ^- , Na⁺ , and Mg²⁺ did not affect the removal of AO7. The addition of HCO₃ ^- significantly inhibited its removal, even when the concentration of HCO₃ ^- was low to 0.6 mmol/l. A slight inhibition effect was observed when the added concentration of humic acid ranged from 0.5 to 5 mg/l, whereas the residue of AO7 was significantly enhanced when the level of humic acid was continually increased from 50 to 100 mg/l. Hydroxyl radicals (•OH) were the main reactive intermediates controlling the oxidation of AO7 in the present Fe²⁺ /SPC system. The produced intermediates through the degradation of AO7 were identified to include 2-coumaranone, 2-naphthol, phthalic acid, phthalimide, N-methylnaphthylamine, and 2-methylphenol. The proposed degradation pathways are consistent with the radical formation and the identified intermediates. PRACTITIONER POINTS: The ferrous/percarbonate system can remove 95% of AO7 under the optimized conditions. AO7 removal was inhibited by adding HCO₃ ^- and humic acid, but not affected by Cl^- , SO₄ ^2- , NO₃ ^- , Na⁺ , and Mg²⁺ . Hydroxylation, ring opening, and mineralization driven by the generated hydroxyl radicals were derived as the major processes for degrading AO7.

Promising Low-Cost Adsorbent from Waste Green Tea Leaves for Phenol Removal in Aqueous Solution

Phenol is the most common organic pollutant in many industrial wastewaters that may pose a health risk to humans due to its widespread application as industrial ingredients and additives. In this study, waste green tea leaves (WGTLs) were modified through chemical activation/carbonization and used as an adsorbent in the presence of ultrasound (cavitation) to eliminate phenol in the aqueous solution. Different treatments, such as cavitation, adsorption, and sono-adsorption were investigated to remove the phenol. The scanning electron microscope (SEM) morphology of the adsorbent revealed that the structure of WGTLs was porous before phenol was adsorbed. A Fourier Transform Infrared (FTIR) analysis showed an open chain of carboxylic acids after the sono-adsorption process. The results revealed that the sono-adsorption process is more efficient with enhanced removal percentages than individual processes. A maximum phenol removal of 92% was obtained using the sono-adsorption process under an optimal set of operating parameters, such as pH 3.5, 25 mg L⁻¹ phenol concentration, 800 mg L⁻¹ adsorbent dosage, 60 min time interval, 30 ± 2 °C temperature, and 80 W cavitation power. Removal of chemical oxygen demand (COD) and total organic carbon (TOC) reached 85% and 53%. The Freundlich isotherm model with a larger correlation coefficient (R², 0.972) was better fitted for nonlinear regression than the Langmuir model, and the sono-adsorption process confirmed the pseudo-second-order reaction kinetics. The findings indicated that WGTLs in the presence of a cavitation effect prove to be a promising candidate for reducing phenol from the aqueous environment.