Synergistic degradation of PNP via coupling H2O2 with persulfate catalyzed by nano zero valent iron

Roles of silica coating on nanosized zero-valent iron in sequential reduction-oxidation process in a system containing persulfate

A sequential reduction-oxidation process using silica-coated nanosized zero-valent iron (nZVI) particles (nZVI@SiO2) and persulfate for mineralizing recalcitrant compounds was developed, and the effects of the process on nitrobenzene were evaluated. This sequential process significantly enhanced contaminant mineralization, which could not be effectively achieved by reduction or oxidation alone. The nZVI@SiO2 rapidly reduced nitrobenzene to aniline, then the aniline concentration gradually decreased after persulfate had been added and initiated sequential oxidative degradation. The SiO2 coating on the nZVI@SiO2 limited outward mass transfer of reaction products and increased the efficiency with which nitrobenzene was converted into aniline. Slow release of Fe(II) caused by the coating caused persulfate activation and subsequent aniline oxidation to be more sustained and efficient than without the coating. The final nitrobenzene-aniline mineralization efficiency was higher for the nZVI@SiO2/persulfate system than the nZVI/persulfate system. The SiO2 coating of the nZVI@SiO2 particles was an excellent protective layer, protecting the particles from undesirable consumption through reactions with groundwater components. nZVI@SiO2 particle transformations during the sequential process were investigated, and the operating conditions were optimized to maximize the recalcitrant compound removal efficiency. The results indicated that nZVI@SiO2 and persulfate could be used to mineralize organic contaminants in groundwater through sequential reduction-oxidation.

Electrocatalytic degradation of p-nitrophenol on metal-free cathode: Superoxide radical (O2•-) production via molecular oxygen activation

Although metal-free electrodes in molecular oxygen-activated Fenton-like wastewater treatment technologies have been developed, the reactive oxygen species (ROS) generation mechanisms are still not sufficiently clear. As a typical example of refractory phenolic wastewater, p-nitrophenol (PNP) has been widely studied. This study demonstrated the critical role of superoxide radicals (O2•-) in PNP degradation by metal-free electrodes through electron spin resonance (ESR), ROS quenching, and density functional theory (DFT) tests. The most superior metal-free electrode exhibited a mass activity of approximately 133.5 h-1 gcatalyst-1. Experimental and theoretical studies revealed the mechanism of O2•- generation via oxygen activation, including one- and three-electron transfer pathways, and found that O2•- mainly attacked the nitro group of PNP to degrade and transform the pollutant. This study enhances the mechanistic understanding of metal-free materials in the electrochemical degradation of refractory pollutants.

Degradation mechanism of Bisphenol S via hydrogen peroxide/persulfate activated by sulfidated nanoscale zero valent iron

Fenton-like oxidation processes are widely used to degrade recalcitrant organic pollutants, but are limited by narrow application pH and low reaction efficiency. This study investigated the synchronous activation of H2O2 and persulfate (PDS) by sulfidated zero valent iron (S-nZVI) in ambient conditions for Fenton-like oxidation of bisphenol S (BPS), an estrogenic endocrine-disrupting chemical. The activation of S-nZVI induced H2O2 or PDS could be greatly enhanced with the assistance of PDS and H2O2, respectively, even across a wide range of pH value (3-11). The first-order rate constant of S-nZVI/H2O2/PDS, S-nZVI/PDS and S-nZVI/H2O2 systems was found to be 0.2766 min-1, 0.0436 min-1, and 0.0113 min-1, respectively. A significant synergy between H2O2 and PDS was achieved when the PDS-H2O2 molar ratio was above 1:1, and where sulfidation promoted iron corrosion and decreased solution pH were observed in the S-nZVI/H2O2/PDS system. Radical scavenging experiments and electron paramagnetic resonance (EPR) investigations suggest that both SO4•- and •OH were generated and that •OH played a crucial role in BPS removal. Furthermore, four BPS degradation intermediates were detected and three degradation pathways were proposed in line with the HPLC-Q-TOF-MS analysis. This study demonstrated that compared to the traditional Fenton-like system, the S-nZVI/H2O2/PDS system could be a more efficient, advanced oxidation technology capable of being used across a broad pH range for emerging pollutants' degradation.

The performance and mechanism of iron-mediated chemical oxidation: Advances in hydrogen peroxide, persulfate and percarbonate oxidation

Many studies have successfully built iron-mediated materials to activate or catalyze Fenton-like reactions, with applications in water and wastewater treatment being investigated. However, the developed materials are rarely compared with each other regarding their performance of organic contaminant removal. In this review, the recent advances of Fenton-like processes in homogeneous and heterogeneous ways are summarized, especially the performance and mechanism of activators including ferrous iron, zero valent iron, iron oxides, iron-loaded carbon, zeolite, and metal organic framework materials. Also, this work mainly compares three O-O bond containing oxidants including hydrogen dioxide, persulfate, and percarbonate, which are environmental-friendly oxidants and feasible for in-situ chemical oxidation. The influence of reaction conditions, catalyst properties and benefits are analyzed and compared. In addition, the challenges and strategies of these oxidants in applications and the major mechanisms of the oxidation process have been discussed. This work can help understand the mechanistic insights of variable Fenton-like reactions, the role of emerging iron-based materials, and provide guidance for choosing appropriate technologies when facing real-world water and wastewater applications.

Degradation of ciprofloxacin by persulfate activated with pyrite: mechanism, acidification and tailwater reuse

Residues of ciprofloxacin (CIP) in the environment pose a threat to human health and ecosystems. This study investigated the degradation of CIP by persulfate (PS) activated with pyrite (FeS2). Results showed that when [CIP] = 30 μM, [FeS2] = 2.0 g L-1, and [PS] = 1 mM, the CIP removal rate could reach 94.4% after 60 min, and CIP mineralization rate reached 34.9%. The main free radicals that degrade CIP were SO4˙- and HO˙, with contributions of 34.4% and 35.7%, respectively. Additionally, compared to the control (ultrapure water), CIP in both tap water and river water was not degraded. However, acidification could eliminate the inhibition of CIP degradation in tap water and river water. Furthermore, acidic tailwater from CIP degradation could be utilized to adjust the pH of untreated CIP, which could greatly promote the degradation of CIP and further reduce disposal costs. The reaction solution was not significantly biotoxic and three degradation pathways of CIP were investigated. Based on the above results and the characterization of FeS2, the mechanism of CIP degradation in the FeS2/PS system was that FeS2 activated PS to generate Fe(iii) and SO4˙-. The sulfide in FeS2 reduced Fe(iii) to Fe(ii), thus achieving an Fe(iii)/Fe(ii) cycle for CIP degradation.

Degradation of methyl orange by pyrite activated persulfate oxidation: mechanism, pathway and influences of water substrates

Degradation mechanism of methyl orange (MO), a typical azo dye, with pyrite (FeS₂) activated persulfate (PS) was explored. The results showed that when the initial concentration of MO was 0.1 mM, FeS₂ was 1.6 g/L and PS was 1.0 mM, the removal rate of MO could reach 92.9% in 150 min, and the removal rate of total organic carbon could reach 14.1%. In addition, both pH ≤ 2 and pH ≥ 10 could have an inhibitory effect in the FeS₂/PS system. Furthermore, Cl^- and low concentrations of HCO^-₃ had little effect on the degradation of MO with FeS₂/PS. However, H₂PO^-₄ and high concentrations of HCO^-₃ could inhibit the degradation of MO in the system. Besides, MO in river water and tap water were not degraded in FeS₂/PS system, but acidification (pH = 4) would greatly promote the degradation. In addition, the removal rate of MO with FeS₂/PS could still reach about 90% after five cycles of FeS₂. Furthermore, the intermediates and possible degradation pathways were speculated by LC-MS, and the degradation mechanism of MO by FeS₂/PS was that the cycle of Fe(III)/Fe(II) could continuously activate persulfate to produce SO₄^•-. The results could provide technical support for azo dye degradation in the FeS₂/PS system.

Recent advances in graphitic carbon nitride as a catalyst for heterogeneous Fenton-like reactions

Graphitic carbon nitride (g-C₃N₄), an appealing metal-free polymer, has featured in extensive research in heterogeneous Fenton-like reactions owing to its advantages of stable chemical and thermal properties, ease of structural regulation and unique redox ability. However, there are still some gaps in the understanding of the mechanism and fate of g-C₃N₄ and its derivatives in heterogeneous Fenton reaction degradation of contaminants. This paper gives systematic emphasis to the development and progress of g-C₃N₄ and its composites as catalysts in heterogeneous Fenton-like reactions. The main synthesis strategies of g-C₃N₄ composites are discussed, including calcination, hydrothermal method and self-assembly method. Then, the key catalytic properties of g-C₃N₄ in Fenton-like applications, including anchoring nanoparticles, increasing specific surface area and exposed active surface sites, as well as regulating charge transfer reactions, are highlighted. Special emphasis is placed on its multifunctional role in heterogeneous Fenton-like reactions and the mechanisms involved in the activation of hydrogen peroxide, persulfates, and photocatalytic activation of persulfate. Lastly, the existing challenges and possible development direction of g-C₃N₄-coupling Fenton reactions are proposed. It is believed that this paper will bring useful information for the development of graphitic carbon nitride in both laboratory studies and practical applications.

Fe3O4-Zeolite Hybrid Material as Hetero-Fenton Catalyst for Enhanced Degradation of Aqueous Ofloxacin Solution

A hetero-Fenton catalyst comprising of Fe3O4 nanoparticles loaded on zeolite (FeZ) has been synthesized using a facile co-precipitation method. The catalyst was characterized using various characterization methods and then, subsequently, was used to degrade ofloxacin (OFL, 20 mg·L−1), an antibiotic, via a heterogeneous Fenton process in the presence of an oxidizing agent. The effects of different parameters such as Fe3O4 loading on zeolite, catalyst loading, initial solution pH, initial OFL concentration, different oxidants, H2O2 dosage, reaction temperature, and inorganic salts were studied to determine the performance of the FeZ catalyst towards Fenton degradation of OFL under different conditions. Experimental results revealed that as much as 88% OFL and 51.2% total organic carbon (TOC) could be removed in 120 min using the FeZ catalyst. Moreover, the FeZ composite catalyst showed good stability for Fenton degradation of OFL even after five cycles, indicating that the FeZ catalyst could be a good candidate for wastewater remediation.