Peracetic acid-based UVA photo-Fenton reaction: Dominant role of high-valent iron species toward efficient selective degradation of emerging micropollutants

Chloride-enhanced degradation of micropollutants in natural water by the iron/biochar/peroxymonosulfate system: Role of iron(IV) and radicals

The increased occurrence and concentration of micropollutants in water supplies raise public health concerns. Advanced oxidation of micropollutants in real water sources remains challenging due to scavenging reactions involving background anions and natural organic matter. For the first time, this paper demonstrates that chloride (Cl-) accelerates the activation of peroxymonosulfate (PMS) by iron-biochar (Fe/BC) composites. Under the tested conditions, this novel system completely degraded bisphenol A (BPA), a representative micropollutant, within 1.0 min. Micropollutant degradation was investigated at different Cl- contents, PMS levels, Fe/BC doses, and solution pH. The primary reactive species involved with BPA degradation were iron(IV) (Fe(IV)), sulfate radical (SO4•-), hydroxyl radical (•OH), and reactive chlorine species (Cl•, ClO•, Cl2•-). The steady-state concentrations of these reactive species were evaluated to determine their relationships to the Cl- and PMS contents. Fe(IV) was confirmed as the dominant reactive species, with Fe(IV) concentrations increasing with Cl- content and salinity to enhance the overall BPA degradation. Importantly, BPA degradation by the Fe/BC/PMS/Cl- system was not greatly affected by background anions or natural organic matter (NOM) present in real water sources, and the system was successfully applied for five sequential cycles of BPA treatment.

High-valent ferryl intermediates generation, reactivity and kinetic characterization with contaminants of emerging concern via a facile photo-Fenton competition kinetic methodology

High-valent ferryl (FeIV) intermediates are important reactive species in biological oxidation and Fe-catalyzed advanced oxidation processes (Fe-AOPs). Notwithstanding notable progress has been made on FeIV identification, the second-order reaction rate constants of FeIV with contaminants of emerging concern (CECs) were rarely reported in literature, severely hindering understanding its reactivity and kinetics toward various CECs. To this end, we discovered a novel system, i.e., photo-Fenton reaction of peracetic acid with Fe3+-nitrilotriacetate complex, which enabled stable generation of FeIV with steady-state concentrations at ∼10-8-10-7 M at neutral pH, as evidenced by electron spin resonance (ESR) trapping detection, quenching experiments and probe testing. Notably, a facile competition kinetic methodology was developed by using methyl phenyl sulfoxide (PMSO) as a probe, which enabled to characterize the reactivity and kinetics of FeIV with 12 target CECs (e.g., phenolic compounds, endocrine disruptor, herbicide, and pharmaceuticals). The measured second-order rate constants were in a range of 3.99 × 103-4.75 × 105 M-1 s-1, which were correlated to the ionization potential of the target CECs, owing to electrophilic attack by FeIV. This achievement can fill a critical gap in uncovering the reactivity and kinetics of FeIV toward CECs for promising environmental application.

New Insights into Natural Polyphenol-Enhanced Fe(III)/Peracetic Acid System under Acidic pH Conditions: The Overlooked Role of Coexisting Hydrogen Peroxide

Natural polyphenols have been extensively utilized as reducing agents to enhance contaminant degradation in the Fe(III)/peracetic acid (PAA) system. However, the roles of coexisting hydrogen peroxide (H2O2) remain insufficiently explored. This study, using protocatechuic acid (PCA) as a representative natural polyphenol, demonstrated that contaminant removal within the PCA/Fe(III)/PAA system under acidic pH conditions exhibited two kinetic stages: an initial rapid stage driven by PAA, followed by a slower stage driven by H2O2. The presence of H2O2 facilitated the complete degradation (100%) of contaminants even at low concentrations (<1.0 μM). Interestingly, these two stages contributed differently to various contaminants' degradation. Mechanistic investigations revealed that Fe(IV) was the major reactive species (RSs) for contaminant degradation during the PAA stage, while •OH dominated during the H2O2 stage. In brief, H2O2 enriched the generation pathways and types of RSs. Notably, besides PCA itself, the reaction intermediates (i.e., phenoxy radicals) formed during the reaction between PCA and RSs also played a key role in reducing Fe(III), which explained why the PCA/Fe(III)/PAA system was able to maintain sufficient Fe(II) to further interact with H2O2. Overall, this study highlighted the synergistic role of coexisting H2O2 and provided valuable insights for optimizing various contaminants' degradation in actual waters using PAA-based Fenton-like systems.

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.

Enhancing microfiltration membrane performance by sodium percarbonate-based oxidation for hydraulic fracturing wastewater treatment

Low pressure membrane takes a great role in hydraulic fracturing wastewater (HFW), while membrane fouling is a critical issue for the stable operation of microfiltration (MF). This study focused on fouling mitigation by sodium percarbonate (SPC) oxidation, activated by ultraviolet (UV) and ferrous ion (Fe(II)). The higher the concentration of oxidizer, the better the anti-fouling performance of MF membrane. Unlike severe MF fouling without oxidation (17.26 L/(m2·h)), UV/SPC and Fe(II)/SPC under optimized dosage improved the final flux to 740 and 1553 L/(m2·h), respectively, and the latter generated Fe(III) which acted as a coagulant. Fe(II)/SPC oxidation enabled a shift in fouling mechanism from complete blocking to cake filtration, while UV/SPC oxidation changed it to standard blockage. UV/SPC oxidation was stronger than Fe(II)/SPC oxidation in removing UV254 and fluorescent organics for higher oxidizing capacity, but the opposite was noted for DOC removal. The deposited foulants on membrane surface after oxidation decreased by at least 88% compared to untreated HFW. Correlation analysis showed that UV254, DOC and organic fraction were key parameters responsible for membrane fouling (correlation coefficient＞0.80), oxidizing capacity and turbidity after oxidation were also important parameters. These results provide new insights for fouling control during the HFW treatment.

Highly Efficient Degradation of Emerging Contaminants with Sodium Bicarbonate-Enhanced Mn(II)/Peracetic Acid Process: Formation and Contribution of Mn(V)

Organic ligands have been extensively used to enhance the catalytic performance of manganese ion (Mn(II)) for peracetic acid (PAA). In this study, sodium bicarbonate (NaHCO3), an economical and eco-friendly inorganic ligand, was introduced to enhance the degradation of emerging contaminants (ECs) in the Mn(II)/PAA process. NaHCO3 could significantly improve the oxidizing ability of the Mn(II)/PAA process over the initial pH range of 3.0-11.0. Mn(V) was identified as the primary reactive species for degrading naproxen in the NaHCO3/Mn(II)/PAA process. HCO3- could complex with Mn(II) to generate Mn(II)-HCO3-, which has a lower redox potential to enhance the catalytic activity of Mn(II). Mn(II)-HCO3- reacted with PAA to produce Mn(III)-HCO3- and CH3C(O)O•. Mn(V)-HCO3- was generated via two-electron transfer between Mn(III)-HCO3- and PAA. Although organic radicals were detected in the NaHCO3/Mn(II)/PAA process, naproxen was mainly degraded by Mn(V)-HCO3- via one-electron transfer along with the formation of MnO2. Notably, the coexisting hydrogen peroxide was vital in the reduction of MnO2 to Mn(II/III), thereby enhancing the continuous generation of Mn(V)-HCO3-. NaHCO3/Mn(II)/PAA process exhibited exceptional oxidation performance in actual water samples. This study proposed a strategy utilizing an eco-friendly inorganic ligand to address the inherent drawbacks of organic ligand-enhanced Mn(II)/PAA processes and highlighted its potential applications in the removal of ECs.

Enhanced degradation of emerging contaminants by Far-UVC photolysis of peracetic acid: Synergistic effect and mechanisms

Krypton chloride (KrCl*) excimer lamps (222 nm) are used as a promising irradiation source to drive ultraviolet-based advanced oxidation processes (UV-AOPs) in water treatment. In this study, the UV222/peracetic acid (PAA) process is implemented as a novel UV-AOPs for the degradation of emerging contaminants (ECs) in water. The results demonstrate that UV222/PAA process exhibits excellent degradation performance for carbamazepine (CBZ), with a removal rate of 90.8 % within 45 min. Notably, the degradation of CBZ in the UV222/PAA process (90.8 %) was significantly higher than that in the UV254/PAA process (15.1 %) at the same UV dose. The UV222/PAA process exhibits superior electrical energy per order (EE/O) performance while reducing resource consumption associated with the high-energy UV254/PAA process. Quenching experiments and electron paramagnetic resonance (EPR) detection confirm that HO• play a dominant role in the reaction. The contributions of direct photolysis, HO•, and other active species (RO• and 1O2) are estimated to be 5 %, 88 %, and 7 %, respectively. In addition, the effects of Cl-, HCO3-, and humic acid (HA) on the degradation of CBZ are evaluated. The presence of relatively low concentrations of Cl-, HCO3-, and HA can inhibit CBZ degradation. The UV222/PAA oxidation process could also effectively degrade several other ECs (i.e., iohexol, sulfamethoxazole, acetochlor, ibuprofen), indicating the potential application of this process in pollutant removal. These findings will propel the development of the UV222/PAA process and provide valuable insights for its application in water treatment.

Protocatechuic acid enhanced the selective degradation of sulfonamide antibiotics in Fe(III)/peracetic acid process under actually neutral pH conditions

The practical application of the Fe-catalyzed peracetic acid (PAA) processes is seriously restricted due to the need for narrow pH working range and poor anti-interference capacity. This study demonstrates that protocatechuic acid (PCA), a natural and eco-environmental phenolic acid, significantly enhanced the removal of sulfonamide antibiotics in Fe(III)/PAA process under actually neutral pH conditions (6.0-8.0) by complexing Fe(III). With sulfamethoxazole (SMX) as the model contaminant, the pseudo-first-order rate constant of SMX elimination in PCA/Fe(III)/PAA process was 63.5 times higher than that in Fe(III)/PAA process at pH 7.0, surpassing most of the previously reported strategies-enhanced Fe-catalyzed PAA processes (i.e., picolinic acid and hydroxylamine etc.). Excluding the primary contribution of reactive species commonly found in Fe-catalyzed PAA processes (e.g., •OH, R-O•, Fe(IV)/Fe(V) and 1O2) to SMX removal, the Fe(III)-peroxy complex intermediate (CH3C(O)OO-Fe(III)-PCA) was proposed as the primary reactive species in PCA/Fe(III)/PAA process. DFT theoretical calculations indicate that CH3C(O)OO-Fe(III)-PCA exhibited stronger oxidation potential than CH3C(O)OO-Fe(III), thereby enhancing SMX removal. Four potential removal pathways of SMX were proposed and the toxicity of reaction solution decreased with the removal of SMX. Furthermore, PCA/Fe(III)/PAA process exhibited strong anti-interference capacity to common natural anions (HCO3-, Cl-and NO3-) and humic acid. More importantly, the PCA/Fe(III)/PAA process demonstrated high efficiency for SMX elimination in actual samples, even at a trace Fe(III) dosage (i.e., 5 μM). Overall, this study provided a highly-efficient and eco-environmental strategy to remove sulfonamide antibiotics in Fe(III)/PAA process under actually neutral pH conditions and to strengthen its anti-interference capacity, underscoring its potential application in water treatment.

Infancy of peracetic acid activation by iron, a new Fenton-based process: A review

The exacerbated global water scarcity and stricter water directives are leading to an increment in the recycled water use, requiring the development of new cost-effective advanced water treatments to provide safe water to the population. In this sense, peracetic acid (PAA, CH3C(O)OOH) is an environmentally friendly disinfectant with the potential to challenge the dominance of chlorine in large wastewater treatment plants in the near future. PAA can be used as an alternative oxidant to H2O2 to carry out the Fenton reaction, and it has recently been proven as more effective than H2O2 towards emerging pollutants degradation at circumneutral pH values and in the presence of anions. PAA activation by homogeneous and heterogeneous iron-based materials generates - besides HO• and FeO2+ - more selective CH3C(O)O• and CH3C(O)OO• radicals, slightly scavenged by typical HO• quenchers (e.g., bicarbonates), which extends PAA use to complex water matrices. This is reflected in an exponential progress of iron-PAA publications during the last few years. Although some reviews of PAA general properties and uses in water treatment were recently published, there is no account on the research and environmental applications of PAA activation by Fe-based materials, in spite of its gratifying progress. In view of these statements, here we provide a holistic review of the types of iron-based PAA activation systems and analyse the diverse iron compounds employed to date (e.g., ferrous and ferric salts, ferrate(VI), spinel ferrites), the use of external ferric reducing/chelating agents (e.g., picolinic acid, l-cysteine, boron) and of UV-visible irradiation systems, analysing the mechanisms involved in each case. Comparison of PAA activation by iron vs. other transition metals (particularly cobalt) is also discussed. This work aims at providing a thorough understanding of the Fe/PAA-based processes, facilitating useful insights into its advantages and limitations, overlooked issues, and prospects, leading to its popularisation and know-how increment.