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As using iron-based materials and biomass to prepare effective catalyst has emerged as a hit trend in persulfate-based advanced oxidation processes (PS-AOPs), the problems associated with excessive chemical consumption, costly expenses, and complex synthetic processes have become more urgent to be addressed. Therefore, by using red mud (RM) and corn straw (CS), a modified red mud biochar catalyst (MRBC) was synthesized through acid pretreatment and pyrolysis (700 °C). MRBC presented excellently in peroxydisulfate (PDS) activation to degrade levofloxacin (LFX). Under the experimental setup involving 8 mM PDS, 10 mg L−1 LFX, and 1.6 g L−1 MRBC, 88.59% of LFX was effectively eliminated within 30 min with an exceedingly low Fe leaching rate of 0.049 mg L−1. Furthermore, even after four cycles of reuse, it was able to sustain a remarkable 61.63% efficiency in LFX degradation. The characterization of catalysts demonstrated the synergistic effect of RM and biomass could be responsible for the excellent catalytic properties of MRBC. The results of quenching experiment and the presence of DMPOX in EPR demonstrated radical pathway mainly contributed to the LFX degradation in MRBC/PDS system. Overall, PDS was activated by the active sites of MRBC such as Fe (II) and persistent free radicals (PFRs) to generate a large amount of SO 4 •− and •OH to remove LFX. This work investigated the synergy of RM and biomass in catalyst preparation, deepened the insights into the application of RM catalysts in PS-AOPs to degrade antibiotic, and contributed greatly to the recovery of waste resources. [Display omitted] • 88.59% removal of LFX within 30 min was achieved by MRBC/PDS system. • The synergistic effect of red mud and biochar enhanced the PDS activation. • The modification of HNO 3 promoted surface structure and iron element proportion. • Results of quenching experiments and EPR confirmed radical pathway degrade LFX. [ABSTRACT FROM AUTHOR]

Highlights • Metribuzin solutions treated by AO-H 2 O 2 , EF and PEF with a BDD/air-diffusion cell. • Two kinetic stages in EF and PEF: uncomplexed and Fe(III)-complexed herbicide. • Overall mineralization by all methods is ensured at current densities ≥100 mA cm−2. • PEF as the best method: total combustion of 0.523 mM metribuzin at pH 3.0 in 420 min. • Heteroaromatic, dimeric and aliphatic by-products degraded by BDD(OH) and OH. Abstract This article reports the electrochemical degradation of the herbicide metribuzin (MTZ) in sulfate medium by advanced oxidation processes like anodic oxidation with electrogenerated H 2 O 2 (AO-H 2 O 2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). A boron-doped diamond (BDD) anode was combined with an air-diffusion cathode with ability to produce H 2 O 2. Unprecedented overall combustion was feasible by all methods at a constant current density (j) ≥100.0 mA cm−2. The total organic carbon (TOC) removal achieved by AO-H 2 O 2 was independent from pH within the range 3.0–9.0, whereas the oscillatory dependence of the pseudo-first-order MTZ decay rate constant with this variable was ascribed to adsorption on the BDD surface. In EF and PEF at pH 3.0, 0.50 mM Fe2+ was determined as optimum catalyst content and the MTZ removal showed two consecutive pseudo-first-order kinetic stages. These were related to the fast reaction of the target molecule with OH formed from Fenton's reaction, followed by a slower attack of physisorbed BDD(OH) onto Fe(III)-MTZ complexes. The effect of j and MTZ content on decay kinetics and TOC removal was examined. PEF was the best treatment due to the decomposition of photoactive intermediates by UVA radiation, yielding total mineralization of a 0.523 mM herbicide solution after 420 min of electrolysis at 100.0 mA cm−2. A thorough reaction pathway for MTZ degradation is proposed from the sixteen heteroaromatic by-products and three aliphatic molecules identified by GC–MS and LC-MS/MS. Oxalic and oxamic acids were detected as final carboxylic acids by ion-exclusion HPLC. [ABSTRACT FROM AUTHOR]

An iron single-atom catalyst, composed of robust FeN 4 moieties anchored on a nitrogen-doped porous carbon matrix (Fe-SAC/NC), has been developed via a surfactant-coordinated metal-organic framework (MOF) approach for application in heterogeneous electro-Fenton (HEF) process. The cohesive interaction between the surfactant and MOF precursor enabled the formation of abundant and stable FeN 4 moieties. The Fe-SAC/NC-catalyzed HEF allowed the complete degradation of 2,4-dichlorophenol with low iron leaching (1.2 mg L-1), being superior to nanoparticle catalyst synthesized without surfactant. The experiments and density functional theory (DFT) calculations demonstrated the dominant role of single-atom FeN 4 sites to activate the electrogenerated H 2 O 2 yielding •OH. The dense FeN 4 moieties allowed harnessing the modulated electronic structure of the SAC to facilitate the electron transfer, whereas the adjacent pyrrolic N enhanced the adsorption of target organic pollutants. Moreover, the excellent catalysis, recyclability and viability of the Fe-SAC/NC were verified by successfully treating several organic pollutants even in urban wastewater. [Display omitted] • Synthesis of FeN 4 moieties on N-doped carbon for effective electro-Fenton treatment. • Simultaneous increase of activity and stability of catalysts by single-atom strategy. • Superiority of SAC over nanoparticle catalyst for degradation of organic pollutants. • H 2 O 2 activation mechanism validated through DFT analysis. • High recyclability of SAC and remarkable performance in urban wastewater matrix. [ABSTRACT FROM AUTHOR]

[Display omitted] • MIL-88B(Fe)/Fe 3 S 4 hybrids for effective water treatment by heterogeneous electro-Fenton. • Hybrid catalyst: highly uniform composition due to excellent distribution of small Fe 3 S 4. • Sulfidation allows breaking the activity-stability trade-off for high performance HEF. • Good recyclability of hybrids and fast trimethoprim removal even in urban wastewater. • Great catalytic activity and degradation mechanism confirmed by DFT calculations. Fe-based metal–organic frameworks are promising catalysts for water treatment, although their viability is hampered by the slow regeneration of active Fe(II) sites. A facile sulfidation strategy is proposed to boost the catalytic activity of MIL-88B(Fe) in heterogeneous electro-Fenton (HEF) treatment of organic micropollutants at mild pH. The synthesized MIL-88B(Fe)/Fe 3 S 4 hybrids possessed numerous and durable unsaturated iron sites, acting the S2− atoms as electron donors that enhanced the Fe(II) recycling. The sulfidated catalyst outperformed the MIL-88B(Fe), as evidenced by the 7-fold faster degradation of antibiotic trimethoprim by HEF and the fast destruction of micropollutants in urban wastewater. The hybrid catalyst was reused, obtaining >90% drug removal after four runs and, additionally, its inherent magnetism facilitated the post-treatment recovery. Electrochemical tests and DFT calculations provided mechanistic insights to explain the enhanced catalysis, suggesting that the accelerated Fe(III)/Fe(II) cycling and the enhanced mass transport and electron transfer accounted for the efficient trimethoprim degradation. [ABSTRACT FROM AUTHOR]

This work reports the novel application of an Fe-based 2D metal−organic framework (MOF), prepared with 2,2′-bipyridine-5,5′-dicarboxylate (bpydc) as organic linker, as highly active catalyst for heterogeneous photoelectro-Fenton (PEF) treatment of the lipid regulator bezafibrate in a model matrix and urban wastewater. Well-dispersed 2D structures were successfully synthesized and their morphological, physicochemical and photocatalytic properties were assessed. UV/Vis PEF using an IrO 2 /air-diffusion cell with an extremely low catalyst concentration (0.05 g L−1, tenfold lower than reported 3D MOFs) outperformed electro-oxidation with electrogenerated H 2 O 2 , electro-Fenton and visible-light PEF. Its excellent performance was explained by: (i) the enhanced mass transport of H 2 O 2 (and organic molecules) at the 2D structure, providing active sites for heterogeneous Fenton's reaction and in-situ Fe(II) regeneration; (ii) the ability of photoinduced electrons to reduce H 2 O 2 to •OH, and Fe(III) to Fe(II); and (iii) the enhanced charge transfer and excitation of Fe–O clusters, which increased the number of electron-hole pairs. LC-QToF-MS and GC-MS allowed the identification of 16 aromatic products of bezafibrate. The complete removal of four micropollutants mixed in urban wastewater at pH 7.4 revealed the great potential of (Fe−bpydc)-catalyzed PEF process. Image 1 • Fe−based 2D MOF degrades various organic micropollutants at mild pH via PEF catalysis. • Complete degradation of bezafibrate by UV/Vis PEF using only 0.05 g L−1 Fe−based 2D MOF. • 16 primary oxidation products of bezafibrate detected by LC-QToF-MS and GC-MS. • Detailed mechanism for PEF with Fe−based 2D MOF: heterogeneous Fenton + photocatalysis. • Thorough study on the stability, recyclability and electronic structure of Fe−based 2D MOF. [ABSTRACT FROM AUTHOR]

• Successful heterogeneous EF process at mild pH using MIL-88B-derived catalysts. • Magnetic N-doped nano-ZVI@C rods by pyrolysis: high activity, stability and recyclability. • Almost overall gemfibrozil degradation in raw urban wastewater with several pollutants. • Identification of 6 aromatic and 6 aliphatic products upon gemfibrozil degradation. • ZVI promoted continuous conversion of Fe(III) to Fe(II): heterogeneous Fenton's reaction. The application of metal-organic frameworks (MOFs) as suspended heterogeneous catalysts in electro-Fenton (EF) process for water treatment is almost inexistent. Fe-MOFs synthesized from phthalic acid-based linkers were spindle-shaped MIL(Fe)-88B and NH 2 -MIL(Fe)-88B crystals, whereas their calcination yielded nano-ZVI@C and nano-ZVI@C-N. The lipid regulator gemfibrozil was spiked into 0.050 M Na 2 SO 4 solutions or urban wastewater and treated in electrolytic cells with an air-diffusion cathode to generate H 2 O 2. The nano-ZVI@C-N catalyst obtained at 800 °C showed the highest activity, with high stability as deduced from the low iron leaching and high recyclability. Almost total drug removal and significant mineralization was feasible in both matrices at near-neutral pH. The presence of core-shell nano-ZVI and Fe 3 O 4 nanoparticles encapsulated in N-doped fusiform porous carbon rods was revealed, ensuring the Fe(III) conversion to Fe(II). Carbon doping with N contributed to the enhanced catalytic activity, and the strong magnetism facilitated the post-treatment catalyst recovery. [ABSTRACT FROM AUTHOR]

• Novel EF process to work at pH 3.0–9.0 upon catalysis with soluble Fe(III)–EDDS complex. • Carbon-felt cathode allowed 50% regeneration of Fe(II), promoting the formation of •OH. • SEM-EDS, XPS and voltammetry: precipitation + adsorption of Fe(III)–EDDS at t > 60 min. • Oxidation and cyclization/dimerization routes of BHA yielded 5 and 6 by-products each. • Degradation mechanism: homogeneous Fenton, heterogeneous Fenton, electrocatalysis. The feasibility of destruction of organic pollutants in water at near-neutral pH by homogeneous electro-Fenton (EF) process employing a soluble Fe(III)–EDDS complex as catalyst is demonstrated for the first time. The performance of the Fe(III)–EDDS-assisted EF process with carbon-felt or air-diffusion cathodes was evaluated from the degradation of butylated hydroxyanisole (BHA) in sulfate medium. The influence of applied current, pH and Fe(III):EDDS ratio and dosage on BHA decay and mineralization was related to the evolution of H 2 O 2 and iron concentrations. Using Fe(III)–EDDS, up to 50% Fe(II) regeneration was achieved in 10 min, whereas only 23% was transformed using hydrated Fe3+. Almost total removal of BHA was achieved thanks to homogenous Fenton, heterogeneous Fenton with cathodically adsorbed Fe(III), and electrocatalysis. The mineralization partly corresponded to the gradual destruction of EDDS by hydroxyl radical (k abs = 5.22 × 109 M−1 s−1), and involved the formation of 5 oxidation and 6 dimerization or cyclization by-products. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe/C@mSiO 2 nanoreactor designed from MOF pyrolysis for the first time as HEF catalyst. • Confinement effect induced by the mesoporous SiO 2 shell boosts BPA degradation. • Enhanced e− transfer for Fe(II) regeneration and mass transport of reactants to the core. • Core-shell strategy breaks the activity-stability trade-off for high-performance HEF. • Fe/C@mSiO 2 behaves as adsorbent-catalyst with high recyclability at near-neutral pH. Despite the high density and uniform distribution of active sites in metal–organic frameworks (MOFs) and their derivatives, the relatively low stability in water still limits their utilization in heterogeneous catalysis. Herein, the confinement of pyrolyzed MIL-88B(Fe) derivatives within mesoporous SiO 2 allowed fabricating core–shell nanoreactors (Fe/C@mSiO 2) that served as heterogeneous electro-Fenton (HEF) catalysts for the first time, revealing an excellent performance. The as-prepared catalysts were featured by high specific surface area and dense active sites. During service as core–shell nanoreactors, they behaved as a dual function adsorbent-catalyst, exhibiting superior catalytic activity and recyclability as compared to HEF catalysts without shell. Using 0.2 g/L of catalyst, the complete removal of bisphenol A at pH 6.2 and 100 mA was achieved at 120 min, with extremely low iron leaching of 0.11 mg/L. The rigid mSiO 2 shell not only protected the iron active sites from leaching, but it also provided porous and permeable channels for efficient mass transport. The unique core–shell architecture concentrates the catalytic sites and reactants within a confined space, promoting the fast degradation of bisphenol A. Furthermore, the defect-rich carbon substrate and the high dispersibility of iron-rich sites favor a fast electron transfer. The efficient treatment of several organic micropollutants in consecutive trials corroborated the high activity and stability of the Fe/C@mSiO 2. This work contributes to the rational design of HEF catalysts, aiming at consolidating their practical application in advanced wastewater treatment. [ABSTRACT FROM AUTHOR]