Degradation of organic pollutants by ferrate/biochar: Enhanced formation of strong intermediate oxidative iron species

Ferromanganese oxide-functionalized TiO2 for rapid catalytic ozonation of PPCPs through a coordinated oxidation process with adjusted composition and strengthened generation of reactive oxygen species

Ferromanganese oxide (MFOx) was first utilized to functionalize TiO2 and an MFOx@TiO2 catalyst was developed for catalytic ozonation for rapid attack of pharmaceutical and personal care products (PPCPs) with adjusted reactive oxygen species (ROSs) composition and strengthened ROSs generation. Unlike Al2O3, which strongly relied on adsorption and was significantly influenced by MFOx loading, synergistic catalytical effects of MFOx and TiO2 were observed, and optimal MFOx doping of 2 wt% and MFOx@TiO2 dosage of 500 ppm were obtained for catalyzing ozonation. In ibuprofen (IBP) degradation, MFOx@TiO2-catalyzed ozonation (MFOx@TiO2/O3) obtained 2.0-, 4.7- and 6.9-folds the kobs of TiO2/O3, MFOx/O3 and bare ozonation (B/O3). Stronger O3 decomposition was observed by MFOx@TiO2 over bare TiO2 with the participation of redox pairs Fe(II)/Fe(III) and Mn(II)/Mn(III)/Mn(IV) and increased surface oxygen vacancies (SOVs) from 9.8 % to 33.7 % was detected. The results revealed that Fe(II), Mn(II) and Mn(III) with low valance accelerated Ti(III) generation from Ti(VI), obtaining an unprecedented high Ti(III) composition occupying 35.3 % of the total Ti atoms. Ti(III) catalyzed the direct reduction of SOVs-O2 to •O2-, and it accelerated the formation of Ti(VI)-OH and Ti(VI)-O which catalyzed O3 decomposition into •O2-. •O2- was found to primarily initiate IBP degradation with nucleophilic addition and dominated over 66 % IBP removal. The enhanced •O2- generation further strengthened •OH and 1O2 production. MFOx@TiO2/O3 obtained 17 %, 21 % and 30 % higher TOC removal over TiO2/O3, MFOx/O3 and B/O3, respectively. Acute toxicity tests confirmed the effective toxicity control of organics by MFOx@TiO2/O3 process (inhibition rate: 10.9 %). Degradation test of atenolol and sulfamethoxazole confirmed the catalytic effects of MFOx@TiO2. MFOx@TiO2 performed strong resistance to water matrix in application test and showed good stability and reusability. The study proposed an effective catalyst for strengthening the ozonation process on PPCPs degradation and provided an in-depth understanding of the mechanisms and characteristics of the MFOx@TiO2 catalyst and MFOx@TiO2/O3 process.

A complete review on the oxygen-containing functional groups of biochar: Formation mechanisms, detection methods, engineering, and applications

Biochar is a porous carbon material generated by the thermal treatment of biomass under anaerobic or anoxic conditions with wealthy Oxygen-containing functional groups (OCFGs). To date, OCFGs of biochar have been extensively studied for their significant utility in pollutant removal, catalysis, capacitive applications, etc. This review adopted a whole system philosophy and systematically summarizes up-to-date knowledge of formation, detection methods, engineering, and application for OCFGs. The formation mechanisms and detection methods of OCFGs, as well as the relationships between OCFGs and pyrolysis conditions (such as feedstocks, temperature, atmosphere, and heating rate), were discussed in detail. The review also summarized strategies and mechanisms for the oxidation of biochar to afford OCFGs, with the performances and mechanisms of OCFGs in the various application fields (environmental remediation, catalytic biorefinery, and electrode material) being highlighted. In the end, the future research direction of biochar OCFGs was put forward.

Novel enhanced defluorination of perfluorooctanoic acids by biochar-assisted ultrasound coupling ferrate: Performance and mechanism

An ultrasound (US)/biochar (BC)/ferrate (Fe (VI)) system was firstly proposed to enhance perfluorooctanoic acid (PFOA) defluorination. It achieved 93 % defluorination optimally, higher than the sum of 77 % (28 % and 49 % for US/BC and US/Fe (VI) respectively), implying synergistic effect. Besides, the mechanism study confirmed that, this system can not only increase the specific surface area of BC and the generation of reactive oxidant species (ROS), enriching the active sites and forming new oxygen-containing functional groups, but also promote the formation of intermediate iron species. The PFOA degradation in the US/BC/Fe (VI) was probably an adsorption-degradation process, both ROS and electron transfer promoted the defluorination. Additionally, its sustainability was also demonstrated with 14 % reduced defluorination percentage after five cycles of BC. Overall, the synergistic effect of the US/BC/Fe (VI) and its enhancing mechanism for PFOA defluorination were clarified firstly, which contributes to the development of biochar for assisting polyfluoroalkyl substances degradation.

Research and application of active species based on high-valent iron for the degradation of pollutants: A critical review

Fe(VI), as a new green treatment agent, has two indispensable processes in water treatment: coagulation and oxidation. Fe(VI) has a strong oxidation ability. The intermediate iron species (Fe(V) and Fe(IV)) and reactive radical species (H2O2, •OH, and O2•-) produced by decomposition and reduction reaction have strong oxidation ability, in addition, the hydrolyzed product formed in situ with core (γ-Fe2O3)-shell (γ-FeOOH) structure also has good coagulation effect. Because Fe(VI) is easy to decompose and challenging to preserve, it limits the application and sometimes significantly reduces the subsequent processing effect. How to make Fe(VI) more efficient use is a hot spot in current research. This article summarizes the distribution of active substances during the hydrolysis of Fe(VI), distinguish the differences mechanisms in the similar regulation methods, reviews the current preparation methods of Fe(VI), and finally reviews the applications of Fe(VI) in the field of environmental remediation.

Ferrate(VI)/percarbonate for the oxidation of micropollutants: Interactive activation and release of low-concentration hydrogen peroxide for efficient electron utilization

A novel "ferrate/percarbonate (Fe(VI)/SPC) co-oxidation process" was used to treat ciprofloxacin (CIP) and various micropollutants (MPs), which owned better performance than mixture of Fe(VI), Na2CO3 and H2O2. The mechanism investigation found that the low-concentration H2O2 (1-2 µM) released by SPC can promote the high-valent iron intermediates (Fe(IV)/Fe(V)) of Fe(VI) to the MP oxidation, and Fe(VI) products can also activate SPC to produce hydroxyl radical (·OH). The interactive activation of Fe(VI) and SPC was realized, which retained the high selectivity of Fe(VI) to electron-rich pollutants, and also made up the oxidation of electron-deficient pollutants through •OH, improving the degradation effect of various MPs by 20-30%, and the rate constant was increased by 1 to 3 times. Moreover, non-purgeable organic carbon (NPOC) determination confirmed that •OH participation reduced the NPOC value of CIP from 5.43 mg/L to 4.37 mg/L. The transformation pathway of CIP showed that Fe(VI)/SPC resulted in more hydroxylation intermediates of CIP than Fe(VI) alone. Acute toxicity assays found that the photoinhibition rate of CIP treated with Fe(VI) alone was 14.5%, while the sample treated with Fe(VI)/SPC showed no significant photoinhibition effect, which proved that the new process had good detoxification properties for CIP.

Role of bipyridyl in enhancing ferrate oxidation toward micropollutants

Enhancing Fe(VI) oxidation ability by generating high-valent iron-oxo species (Fe(IV)/Fe(V)) has attracted continuous interest. This work for the first time reports the efficient activation of Fe(VI) by a well-known aza-aromatic chelating agent 2,2'-bipyridyl (BPY) for micropollutant degradation. The presence of BPY increased the degradation constants of six model compounds (i.e., sulfamethoxazole (SMX), diclofenac (DCF), atenolol (ATL), flumequine (FLU), 4-chlorophenol (4-CP), carbamazepine (CBZ)) with Fe(VI) by 2 - 6 folds compared to those by Fe(VI) alone at pH 8.0. Lines of evidence indicated the dominant role of Fe(IV)/Fe(V) intermediates. Density functional theory calculations suggested that the binding of Fe(III) to one or two BPY molecules initiated the oxidation of Fe(III) to Fe(IV) by Fe(VI), while Fe(VI) was reduced to Fe(V). The increased exposures of Fe(IV)/Fe(V) were experimentally verified by the pre-generated Fe(III) complex with BPY and using methyl phenyl sulfoxide as the probe compound. The presence of chloride and bicarbonate slightly affected model compound degradation by Fe(VI) in the presence of BPY, while a negative effect of humic acid was obtained under the same conditions. This work demonstrates the potential of N-donor heterocyclic ligand to activate Fe(VI) for micropollutant degradation, which is instructive for the Fe(VI)-based oxidation processes.

Mechanical energy triggered piezo-catalyzation of Bi2WO6 nanoplates on ferrate (Fe(VI)) oxidation in alkaline media: Performance and mechanism

Piezo-electricity, as a unique physical phenomenon, demonstrates high effectiveness in capturing the environmental mechanical energy into polarization charges, offering the possibility to activate the advanced oxidation processes via the electron pathway. However, information regarding the intensification of Fe(VI) through piezo-catalysis is limited. Therefore, our study is the first to apply Bi2WO6 nanoplates for piezo-catalyzation of Fe(VI) to enhance bisphenol A (BPA) degradation. Compared to Fe(VI) alone, the Fe(VI)/piezo/Bi2WO6 system exhibited excellent BPA removal ability, with the degradation rate increased by 32.6% at pH 9.0. Based on the experimental and theoretical results, Fe(VI), Fe(V), Fe(IV) and •OH were confirmed as reaction active species in the reaction, and the increased BPA removal mainly resulted from the enhanced formation of Fe(IV)/Fe(V) species. Additionally, effects of coexisting anions (e.g., Cl-, NO3-, SO42- and HCO3-), humic acid and different water matrixes (e.g., deionized water, tap water and lake water) on BPA degradation were studied. Results showed the Fe(VI)/piezo/Bi2WO6 system still maintained satisfactory BPA degradation efficiencies under these conditions, guaranteeing future practical applications in surface water treatment. Furthermore, the results of intermediates identification, ECOSAR calculation and cytotoxicity demonstrated that BPA degradation by Fe(VI)/piezo/Bi2WO6 posed a diminishing ecological risk. Overall, these findings provide a novel mechanical energy-driven piezo-catalytic approach for Fe(VI) activation, enabling highly efficient pollutant removal under alkaline condition.

Novel manganese and nitrogen co-doped biochar based on sodium bicarbonate activation for efficient removal of bisphenol A: Mechanism insight and role analysis of manganese and nitrogen by combination of characterizations, experiments and density functional theory calculations

A novel porous manganese and nitrogen co-doped biochar (Mn-N@SBC) was synthesized via one-step pyrolysis, utilizing loofah agricultural waste as the precursor and NaHCO3 as the activator. The behavior of bisphenol A adsorbed on Mn-N@SBC was evaluated using static batch adsorption experiments. Compared to direct manganese-nitrogen co-doping, co-doping based on NaHCO3 activation significantly increased the specific surface area (231 to 1027 m2·g-1) and adsorption capacity (15 to 351 mg·g-1). Wide pH (2-10) and good resistance to cation/anion, humic acid and actual water demonstrated the robust adaptability of Mn-N@SBC to environmental factors. The significantly reduced specific surface area after adsorption, adverse effects of ethanol and phenanthrene on the removal of bisphenol A, and theoretically predicted interaction sites indicated the primary adsorption mechanisms involved pore filling, hydrophobicity, and π-π-electron-donor-acceptor interaction. This work presented an approach to create high-efficiency adsorbents from agricultural waste, offering theoretical and practical guidance for the removal of pollutants.

Nanomaterials and biochar mediated remediation of emerging contaminants

The unrestricted release of various toxic substances into the environment is a critical global issue, gaining increased attention in modern society. Many of these substances are pristine to various environmental compartments known as contaminants/emerging contaminants (ECs). Nanoparticles and emerging sorbents enhanced remediation is a compelling methodology exhibiting great potential in addressing EC-related issues and facilitating their elimination from the environment, particularly those compounds that demonstrate eco-toxicity and pose considerable challenges in terms of removal. It provides a novel technique enabling the secure and sustainable removal of various ECs, including persistent organic compounds, microplastics, phthalate, etc. This extensive review presents a critical perspective on the current advancements and potential outcomes of nano-enhanced remediation techniques such as photocatalysis, nano-sensing, nano-enhanced sorbents, bio/phyto-remediation, which are applied to clean-up the natural environment. In addition, when dealing with residual contaminants, special attention is paid to both health and environmental implications; therefore, an evaluation of the long-term sustainability of nano-enhanced remediation methods has been considered. The integrated mechanical approaches were thoroughly discussed and presented in graphical forms. Thus, the critical evaluation of the integrated use of most emerging remediation technologies will open a new dimension in environmental safety and clean-up program.

How Should We Activate Ferrate(VI)? Fe(IV) and Fe(V) Tell Different Stories about Fluoroquinolone Transformation and Toxicity Changes

Many studies have investigated activation of ferrate (Fe(VI)) to produce reactive high-valent iron intermediates to enhance the oxidation of micropollutants. However, the differences in the risk of pollutant transformation caused by Fe(IV) and Fe(V) have not been taken seriously. In this study, Fe(VI)-alone, Fe3+/Fe(VI), and NaHCO3/Fe(VI) processes were used to oxidize fluoroquinolone antibiotics to explore the different effects of Fe(IV) and Fe(V) on product accumulation and toxicity changes. The contribution of Fe(IV) to levofloxacin degradation was 99.9% in the Fe3+/Fe(VI) process, and that of Fe(V) was 89.4% in the NaHCO3/Fe(VI) process. The cytotoxicity equivalents of levofloxacin decreased by 1.9 mg phenol/L in the Fe(IV)-dominant process while they significantly (p < 0.05) increased by 4.7 mg phenol/L in the Fe(V)-dominant process. The acute toxicity toward luminescent bacteria and the results for other fluoroquinolone antibiotics also showed that Fe(IV) reduced the toxicity and Fe(V) increased the toxicity. Density functional theory calculations showed that Fe(V) induced quinolone ring opening, which would increase the toxicity. Fe(IV) tended to oxidize the piperazine group, which reduced the toxicity. These results show the different-pollutant transformation caused by Fe(IV) and Fe(V). In future, the different risk outcomes during Fe(VI) activation should be taken seriously.

Enhanced dewaterability of food waste digestate by biochar/potassium ferrate treatments: Performance and mechanisms

The combined process of biochar (BC) and potassium ferrate (PF) offers a fascinating technique for efficient dewatering of digestate. However, the effects of BC/PF treatment on the dewaterability and mechanisms of FWD are still unknown. This study aimed to reveal the impact mechanisms of BC/PF treatment on digestate dewatering performance. Experimental results indicated that BC/PF treatment significantly enhanced the dewaterability of digestate, with the minimum specific resistance to filtration of (1.05 ± 0.02) × 1015 m·kg-1 and water content of 57.52 ± 0.51% being obtained at the concentrations of 0.018 g·g-1 total solid (TS) BC300 and 0.20 g·g-1 TS PF, which were 8.60% and 13.59% lower than PF treatment, respectively. BC/PF treatment proficiently reduced the fractal dimension, bound water content, apparent viscosity, and gel-like network structure strength of digestate, as well as increased the floc size and zeta potential of digestate. BC/PF treatment promoted the conversion of extracellular polymeric substances (EPS) fractions from inner EPS to soluble EPS, increased the fluorescence intensity of the dissolved compounds, and enhanced the hydrophobicity of proteins. Mechanisms investigations showed that BC/PF enhanced dewatering through non-reactive oxygen species pathways, i.e., via strong oxidative intermediate irons species Fe(V)/Fe(IV). BC/PF treatment enhanced the solubilization of nutrients, the inactivation of fecal coliforms, and the mitigation of heavy metal toxicity. The results suggested that BC/PF treatment is an effective digestate dewatering technology which can provide technological supports to the closed-loop treatment of FWD.

Iron-doped carbon nanotubes with magnetic enhanced Fe(VI) degradation of arsanilic acid and inorganic arsenic: Role of intermediate iron species and electron transfer

Arsanilic acid (p-AsA), a prevalently used feed additive, is frequently detected in environment posing a great threat to humans. Potassium ferrate (Fe(VI)) was an efficient way to tackle arsenic contamination under acid and neutral conditions. However, Fe(VI) showed a noneffective removal of p-AsA under alkaline conditions due to its oxidation capacity attenuation. Herein, a magnetic iron-doped carbon nanotubes (F-CNT) was successfully prepared and further catalyzed Fe(VI) to remove p-AsA and total As species. The Fe(VI)/F-CNT system showed an excellent capability to oxidize p-AsA and adsorb total As species over an environment-related pH range of 6-9. The high-valent iron intermediates Fe(V)/Fe(IV) and the mediated electron-transfer played a significant part in the degradation of p-AsA according to the probes/scavengers experiments and galvanic oxidation process. Moreover, the situ formed iron hydroxide oxide and F-CNT significantly improved the adsorption capacity for total As species. The electron-donating groups (semiquinone and hydroquinone) and high graphitization of F-CNT were responsible for activating Fe(VI) based on the analysis of X-ray photoelectron spectroscopy (XPS). Density functional theory calculations and the detected degradation products both indicated that the amino group and the C-As bond of p-AsA were main reactive sites. Notably, Fe(VI)/F-CNT system was resistant to the interference from Cl-, SO42-, and HCO3-, and could effectively remove p-AsA and total As species even in the presence of complex water matrix. In summary, this work proposed an efficient method to use Fe(VI) for degrading pollutants under alkaline conditions and explore a new technology for livestock wastewater advanced treatment.

Study on improvement of copper sulfide acid soil properties and mechanism of metal ion fixation based on Fe-biochar composite

In this study, Fe modification of bamboo biochar (BC) with ferrate was used to construct a composite soil amendment based on K2FeO4-biochar (Fe-BC) system. Based on soil culture experiments, Fe-BC combined with organic-inorganic materials at the application levels of 3%, 5% and 10% to copper sulfide contaminated acid soil was studied. Adsorption kinetics experiment was used to investigate the adsorption capacity of Fe-modified biochar to heavy metal Cu. The results showed that the pH value of bamboo biochar could be increased by 1.12 units after K2FeO4 modification. Compared with the BC, the adsorption capacity of Cu2+ increased from 190.48 to 276.12 mg/g, which was mainly reflected in single-layer surface adsorption and chemisorption. Pore diffusion, electrostatic interaction and surface interaction are the possible mechanisms of Fe-BC interaction with Cu2+ ions. And the contents of Pb, Cu and Zn in soil leaching state decreased by 59.20%, 65.88% and 57.88%, respectively, at the 10% application level of Fe-BC. In general, the composite modifier based on ferrate and biochar has a positive effect on improving the characteristics of acidic soil in copper mining area.

Piezoelectricity ameliorates high-valent iron oxo species production in peroxymonosulfate activation for refractory atrazine remediation

Over the past few years, high-valent iron oxo species (Fe(IV)) have shown considerable promise. However, an improved solution is needed for the bottleneck of unsatisfactory electron transfer efficiency in Fe-based catalyst/PMS systems. In this study, Enteromorpha-derived biochar was pyrolyzed with iron and barium titanate (FeBCBa). Under ultrasonic treatment, it removes 94.5% of atrazine (10 mg/L) within 60 min, and is environmentally friendly. BaTiO3's piezoelectricity enhances Fe(IV) production in FeBCBa, resulting in superior performance. In the ultrasonic condition, the apparent reaction rate was 1.42 times higher than in the non-ultrasonic condition. Using density functional theory calculations, it can be shown that due to the Fe dopant, electrons in ATZ's LUMO are more easily transferred to the catalyst's HOMO, which is beneficial for ATZ removal. The results of this study provide new guidance for constructing stable and efficient catalysts for environmental remediation.

Visible light activation of ferrate(VI) by oxygen doped ZnIn2S4/black phosphorus nanolayered heterostructure: Accelerated oxidation of trimethoprim

The increasing consumption of antibiotics and their subsequent release to wastewater or groundwater and ultimately to the water supply (or drinking water) has great concerns. This paper presents a visible light (VL) activated ferrate(VI) (FeVIO42-, Fe(VI)) system to degrade the selected antibiotic, trimethoprim (TMP), efficiently. An oxygen doped ZnIn2S4 nanosheet (O-ZIS) coupled with a black phosphorus (BP) heterostructure (O-ZIS/BP), is fabricated by a simple electrostatic self-assembly method. The O-ZIS/BP photocatalyst is comprehensively characterized by surface and analytical techniques, which show superior separation efficiency of the photoinduced charge carriers in the heterostructure. A VL-O-ZIS/BP-Fe(VI) system achieves more than 80% removal in 1.0 min and complete removal of TMP in 3.0 min. Comparatively, only ⁓7% and ⁓24% of TMP are degraded by O-ZIS/BP and Fe(VI) in 1.0 min, respectively. The degradation experiments using probe molecules of reactive species and electron paramagnetic resonance (EPR) measurements reveal involvement of superoxide (O2-•), hydroxyl radical (•OH), and iron(V)/iron (IV) (FeV/FeIV) species in the mechanism of TMP degradation. Oxidized products of TMP are identified and reaction pathways are given. Theoretical calculations predict the initial attack on the TMP molecule by the reactive species in the VL-O-ZIS/BP-Fe(VI) system. The activation of Fe(VI) by VL-heterostructure photocatalysts accelerates the degradation of antibiotics, demonstrating its potential for water depollution.

Enhanced ferrate(VI) oxidation of organic pollutants through direct electron transfer

Fe(VI) is a versatile agent for water purification, and various strategies have been developed to improve its pollutant removal efficiency. Herein, it was found that in addition to intermediate iron species [Fe(IV)/Fe(V)], direct electron transfer (DET) played a significant role in the abatement of organic pollutants in Fe(VI)/carbon quantum dots (CQDs) system. Around 86, 83, 73, 64, 52, 45 and 17% of BPA, DCF, SMX, 4-CP, phenol, p-HBA, and IBP (6 μM) could be oxidized by 30 μM of Fe(VI), whereas with the addition of CQDs (4 mg/L), the oxidation ratio of these pollutants increased to 98, 99, 80, 88, 87, 66 and 57%, respectively. The negative impact induced by solution pH and background constituents on Fe(VI) abatement of pollutants could be alleviated by CQDs, and CQDs acted as catalysts for mediating DET from organic pollutants to Fe(VI). Theoretical calculation revealed that iron species [Fe(VI)/Fe(V)/Fe(IV)] was responsible for the oxidation of 36% of phenol, while DET contributed to the oxidation of 64% of phenol in the Fe(VI)/CQDs system. Compared with iron species oxidation, the CQDs mediated DET from pollutants to Fe(VI) was more efficient for utilizing the oxidation capacity of Fe(VI). The DET mechanism presented in the study provides a prospective strategy for improving the pollution control potential of Fe(VI).

Mn-modified biochars for efficient adsorption and degradation of cephalexin: Insight into the enhanced redox reactivity

Mn-modified biochars (BCs) were developed by pre-treatment of feedstock (MBCs) or post-modification of biochar (BCM), for simultaneous adsorption and degradation of a model pollutant, cephalexin. The apparent removal rates of cephalexin in the presence of MBCs (2.49 - 6.39 × 10-2 h-1) and BCM (13.3 × 10-3 h-1) were significantly higher than that in the presence of biochar prepared under similar conditions (4.2 × 10-3 h-1). While the •OH generated from the activation of dissolved O2 by the persistent free radicals (PFRs) and phenolic -OH on BC could cause degradation of cephalexin, its removal was drastically enhanced through direct oxidation by the MnOx and related Mn species on Mn-modified BCs. The removal of cephalexin by MBCs decreased as the solution pH was raised from 5.0 to 9.0, which supports the critical role played by Mn3O4 in its oxidation. Removal of cephalexin in the presence of MBCs and Mn3O4 was enhanced with the introduction of Mn(II) ions, suggesting that the Mn3O4 present on MBCs facilitates the re-oxidation of Mn(II) to highly reactive Mn(III). While MnO2 anchored on BCM also enhanced the cephalexin oxidation, the active sites of BC and MnO2 were partially destroyed during post-modification of BC, compromising the redox cycling of Mn(II)/Mn(III) and the generation of •OH. As a result, the performance of BCM in oxidizing cephalexin was inferior to that of MBCs. These findings shed new light on the development of environmentally benign sorbents capable of simultaneously adsorbing and oxidizing organic pollutants.

Distinct chromium removal mechanisms by iron-modified biochar under varying pH: Role of iron and chromium speciation

Iron-modified biochar (Fe-biochar) has been widely developed to attenuate Cr(VI) pollution in both acid and alkaline environments. However, there are few comprehensive studies on how the iron speciation in Fe-biochar and chromium speciation in solution influencing the removal of Cr(VI) and Cr(III) under varying pH. Here, multiple Fe-biochar containing Fe3O4 or Fe(0) were prepared and applied to remove aqueous Cr(VI). Kinetics and isotherms suggested that all Fe-biochar could efficiently remove Cr(VI) and Cr(III) via adsorption-reduction-adsorption. The Fe3O4-biochar immobilized Cr(III) by forming FeCr2O4, while amorphous Fe-Cr coprecipitate and Cr(OH)3 was formed when using Fe(0)-biochar. Density functional theory (DFT) analysis further indicated that pH increase caused more negative adsorption energies between Fe(0)-biochar and the pH-dependent Cr(VI)/Cr(III) species. Consequently, the adsorption and immobilization of Cr(VI) and Cr(III) species by Fe(0)-biochar was more favored at higher pH. In comparison, Fe3O4-biochar exhibited weaker adsorption abilities for Cr(VI) and Cr(III), which were in consistent with their less negative adsorption energies. Nonetheless, Fe(0)-biochar merely reduced ∼70% of adsorbed Cr(VI), while ∼90% of adsorbed Cr(VI) was reduced by Fe3O4-biochar. These results unveiled the importance of iron and chromium speciation for chromium removal under varying pH, and might guide the application-oriented design of multifunctional Fe-biochar for broad environmental remediation.

Insights into oxidation of pentachlorophenol (PCP) by low-dose ferrate(VI) catalyzed with α-Fe2O3 nanoparticles

In this study, the catalytic performance of α-Fe₂O₃ nanoparticles (nα-Fe₂O₃) in the low-dose ferrate (Fe(VI)) system was systematically studied through the degradation of pentachlorophenol (PCP). Based on the established quadratic functions between nα-Fe₂O₃ amount and observed pseudo first-order rate constant (k_obs), two linear correlation equations were offered to predict the optimum catalyst dosage and the maximum k_obs at an applied Fe(VI) amount. Moreover, characterization and cycling experiments showed that nα-Fe₂O₃ has good stability and recyclability. According to the results of reactive species identification and quenching experiment and galvanic oxidation process, the catalytic mechanism was proposed that Fe(III) on the surface of nα-Fe₂O₃ may react with Fe(VI) to enhance the generation of highly reactive Fe(IV)/Fe(V) species, which rapidly extracted a single electron from PCP molecule for its further reaction. Besides, two possible PCP degradation pathways, i.e., single oxygen transfer mediated hydroxylation and single electron transfer initiated polymerization were proposed. The formation of coupling products that are prone to precipition and separation was largely improved. This study proved that nα-Fe₂O₃ can effectively catalyze PCP removal at low-dose Fe(VI), which provides some support for the application of Fe(VI) oxidation technology in water treatment in the context of low-carbon emissions.

Reduction of byproduct formation and cytotoxicity to mammalian cells during post-chlorination by the combined pretreatment of ferrate(VI) and biochar

Ferrate [Fe(VI)] can efficiently degrade various pollutants in wastewater. Biochar application can reduce resource use and waste emission. This study investigated the performance of Fe(VI)/biochar pretreatment to reduce disinfection byproducts (DBPs) and cytotoxicity to mammalian cells of wastewater during post-chlorination. Fe(VI)/biochar was more effective at inhibiting the cytotoxicity formation than Fe(VI) alone, reducing the cytotoxicity from 12.7 to 7.6 mg-phenol/L. The concentrations of total organic chlorine and total organic bromine decreased from 277 to 130 μg/L and from 51 to 39 μg/L, compared to the samples without pretreatment. Orbitrap ultra-high resolution mass spectrometry revealed that the number of molecules of DBPs decreased substantially from 517 to 229 by Fe(VI)/biochar, with the greatest reduction for phenols and highly unsaturated aliphatic compounds. In combination with the substantial reduction of 1Cl-DBPs and 2Cl-DBPs, 1Br-DBPs and 2Br-DBPs were also reduced. Fluorescence excitation-emission matrix coupled with parallel factor analysis suggested that fulvic acid-like substances and aromatic amino acid was obviously reduce likely due to the enhanced oxidation of Fe(IV)/Fe(V) produced by Fe(VI)/biochar and adsorption of biochar. Furthermore, the DBPs generated by electrophilic addition and electrophilic substitution of precursors were reduced. This study shows that Fe(VI)/biochar pretreatment can effectively reduce cytotoxicity formation during post-chlorination by transforming DBPs and their precursors.

Fe(VI) activation system mediated by a solar-driven TiO2 nanotubes electrode for CLQ degradation: Performances, mechanisms and pathways

Ferrate (Fe(VI), FeO₄^2-) has been widely used in the degradation of micropollutants with the advantages of high redox potential, no secondary pollution and inhibition of disinfection byproducts. However, the low transformation of Fe(V) and/or Fe(IV) by Fe(VI) and incomplete mineralization of pollutants limit their application. In this work, we designed a photo electric cell with TiO₂ nanotubes (TNTs) and Pt serving as the anode and cathode to enhance the utilization of Fe(VI) (Fe(VI)-TNTs system). TNTs accelerated the generation of •OH via h_VB⁺ oxidation of OH^- and photogenerated electrons at Pt boosted the transformation of Fe(VI) to Fe(V) and/or Fe(IV), resulting in a 22.2 % enhancement of chloroquine (CLQ) removal compared to Fe(VI) alone. The results from EPR and quenching tests showed that Fe(VI), Fe(V), Fe(IV), •OH, O₂^•- and h_VB⁺ coexisted in the Fe(VI)-TNTs system, among which Fe(V) and Fe(IV) were testified as the primary reactive substances accounting for 59 % of CLQ removal. The performance tests and recycling tests demonstrated that the Fe(VI)-TNTs system maintained excellent performance in an authentic water environment. The plausible degradation pathway of CLQ oxidized in the Fe(VI)-TNTs system was proposed with nine identified oxidation products via N-C cleavage, electrophilic addition and carboxylation processes. Based on the ECOSAR calculation, the constructed reaction system allowed a decrease in acute and chronic toxicity. Our findings provide a highly efficient and cost-effective strategy to enhance Fe(VI) application for micropollutant degradation in the future.

Selective Removal of Emerging Organic Contaminants from Water Using Electrogenerated Fe(IV) and Fe(V) under Near-Neutral Conditions

Fe(IV) and Fe(V) are promising oxidants for the selective removal of emerging organic contaminants (EOCs) from water under near-neutral conditions. The Fe(III)-assisted electrochemical oxidation system with a BDD anode (Fe(III)-EOS-BDD system) has been employed to generate Fe(VI), while the generation and contributions of Fe(IV) and Fe(V) have been largely ignored. Thus, we examined the feasibility and involved mechanisms of the selective degradation of EOCs in the Fe(III)-EOS-BDD system under near-neutral conditions. It was found that Fe(III) application selectively accelerated the electro-oxidation of phenolic and sulfonamide organics and made the oxidation system be resistant to interference from Cl^-, HCO₃^-, and humic acid. Several lines of evidence indicated that EOCs were decomposed via direct electron-transfer process on the BDD anode and by Fe(IV) and Fe(V) but not Fe(VI), besides HO^•. Fe(VI) was not generated until the exhaustion of EOCs. Furthermore, the overall contributions of Fe(IV) and Fe(V) to the oxidation of phenolic and sulfonamide organics were over 45%. Our results also revealed that Fe(III) was oxidized primarily by HO^• to Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system. This study advances the understanding of the roles of Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system and provides an alternative for utilizing Fe(IV) and Fe(V) under near-neutral conditions.

Activated carbon modified titanium dioxide/bismuth trioxide adsorbent: One-pot synthesis, high removal efficiency of organic pollutants, and good recyclability

Considerable endeavors have focused on tightly combining adsorption with photocatalysis in designing composite materials for environmental pollution treatment. Recent advances in coupling titanium dioxide/bismuth trioxide (TiO₂/Bi₂O₃) with activated carbon (AC) show significantly enhanced photocatalytic performance but face critical limitations including low adsorption capacity and multi-step synthesis. In this work, we introduce a one-pot synthesis of activated carbon modified TiO₂/Bi₂O₃ composite materials (TiO₂/Bi₂O₃/AC). Thanks to the integrated adsorbent/photocatalyst system, TiO₂/Bi₂O₃/AC shows a drastically enhanced removal efficiency for sulfamethazine (>81%), far beyond the corresponding value of the reported AC/TiO₂/Bi₂O₃ adsorbent (<40%). Notably, the removal rates of other typical pollutants including tetracyclines, methyl orange, and rhodamine B are as high as >98%. Furthermore, TiO₂/Bi₂O₃/AC obtains >80% of its adsorption rate for the fifth cycle after simple photo-regeneration without any other post-treatments. Kinetic analysis and photoelectric characterization are carried out to provide insight into adsorption mechanism. Therefore, this work demonstrates a considerable potential to design and construct other multifunctional adsorbents with advanced performance.

Improvement of Fe(VI) oxidation by NaClO on degrading phenolic substances and reducing DBPs formation potential

Ferrate(VI) is a green oxidant and can effectively oxidize micropollutants. However, the instability of Fe(VI), i.e., self-decomposition, in the aqueous solution limited its application. Herein, it was found that the degradation of phenolic substances had been substantially improved through the combination of Fe(VI) with NaClO. At the condition of pH 8.0, 50 μM of Fe(VI) degraded 18.66 % of BPA (bisphenol A) at 0.5 min or 21.67 % of phenol at 2 min. By contrast, Fe(VI)/NaClO (50/10 μM) oxidized 38.21 % of BPA at 0.5 min or 38.08 % of phenol at 2 min with a synergistic effect. At the end of the reaction, the concentration of Fe(VI) in Fe(VI)/NaClO (50/10 μM) was 28.97 μM for BPA degradation, higher than the 25.62 μM of Fe(VI) group. By active species analysis, intermediate iron species [i.e., Fe(V) and Fe(IV)] played a vital role in the synergistic effect in Fe(VI)/NaClO system, which would react with the applied NaClO to regenerate Fe(VI). In natural water, the Fe(VI)/NaClO could also degrade phenolic substances of natural organic matter (NOM). Although the NaClO reagent was applied, disinfection by-products (DBPs) formation potential decreased by 22.75 % of the raw sample after Fe(VI)/NaClO treatment. Significantly, THMs, mainly caused by phenolic substances of NOM, even declined by 29.18 % of raw sample. Based on that, this study explored a novel ferrate(VI) oxidation system using the cheap NaClO reagent, which would present a new insight on ferrate(VI) application.

Enhanced visible light assisted peroxymonosulfate process by biochar in-situ enriched with γ-Fe2O3 for p-chlorophenol degradation: performance, mechanism and DFT calculation

In this study, a novel γ-Fe₂O₃/biochar (BF_γ) composite by a plant in-situ enrichment and one-step pyrolysis strategy was prepared, which was applied as a photocatalyst to activate peroxymonosulfate (PMS) for the degradation of p-chlorophenol (4-CP) under visible light irradiation (BF_γ/PMS/Vis) system. The characterization results exhibited that γ-Fe₂O₃ with localized carbon doping was evenly embedded in biochar during the pyrolysis. BF_γ exhibited better photoresponse properties than biochar (BC) and γ-Fe₂O₃. The removal efficiency of this system for 4-CP reached 96.41% under optimal conditions. This system showed high removal efficiency with a wide pH range (3.0-13.0) and under conditions of different organic pollutants. It also showed strong resistance to interference with co-existing inorganic ions and humic acid (HA). Electron paramagnetic resonance (EPR) and radical scavenging experiments revealed that the reactive oxygen species (ROS) in this system included SO₄^-·, ·OH, ·O₂^- and ¹O₂. The density functional theoretical (DFT) calculations further revealed the promotion of localized carbon doping in γ-Fe₂O₃ on electron transfer and photoresponse, including C-O bond (d=1.29 Å), C-Fe bond (d=1.80 Å) and band gap value (E_gap < 0.72 eV). This study provides new insights into constructing environmentally-friendly catalysts and the possibility of the solid waste recycling for other wetland plants.

A novel role of various hydrogen bonds in adsorption, desorption and co-adsorption of PPCPs on corn straw-derived biochars

The effect of various hydrogen bonds with different strength on the environmental behaviors of PPCPs remains unclear. In this study, three pharmaceutical pollutants including clofibric acid (CA), sulfamerazine (SMZ), and acetaminophen (ACT) with different functional groups and pK_a, were selected as representative of PPCPs to investigate the pivotal role of hydrogen bonds in adsorption/desorption and co-adsorption behaviors of PPCPs on two corn straw-derived biochars prepared at 300 °C (BCs-300) and 600 °C (BCs-600), respectively. The results indicated that charge-assisted hydrogen bond (CAHB) and ordinary hydrogen bond (OHB) with different intensities were the pivotal mechanisms responsible for the adsorption of three PPCPs on biochars, which was further confirmed by FTIR, but their immobilization effects of PPCPs on biochars were completely different. Compared with OHB formed between CA and BCs-600, the stronger CAHB (formed between CA and BCs-300, and SMZ/ACT and BCs-300/BCs-600) with covalent bond characteristics that derived from the smaller |ΔpK_a| (<5.0), resulted in the greater adsorption capacity (Q_s) and affinity (K_f) of the three PPCPs on BCs-300 (Q_s ≥ 195 μmol·g^-1, K_f ≥ 1.9956) than that on BCs-600 (Q_s ≤ 92 μmol·g^-1, K_f ≤ 0.5192), thereby making the better immobilization effect of PPCPs by biochar. In addition, in the coexisting systems, either SMZ coexisting with CA/ACT on BCs-300, or ACT coexisting with CA/SMZ on BCs-600, both implied that when the |ΔpK_a| between the target PPCPs and biochar is smaller than that between the coexisting compound and biochar, the target PPCPs can preferentially occupy the shared hydrogen bond sites on the biochar surface, and hard to be replaced by the coexisting compound. This work not only expand the application of designed biochar as engineering adsorbents to control and removal of the specific PPCPs in the environment, but also facilitate accurate assessment of the environmental risk of co-existing PPCPs.

The efficient degradation of diclofenac by ferrate and peroxymonosulfate: performances, mechanisms, and toxicity assessment

In this study, the degradation efficiency and reaction mechanisms of diclofenac (DCF), a nonsteroidal anti-inflammatory drug, by the combination of ferrate (Fe(VI) and peroxymonosulfate (PMS) (Fe(VI)/PMS) were systematically investigated. The higher degradation efficiency of DCF in Fe(VI)/PMS system can be obtained than that in alone persulfate (PS), Fe(VI), PMS, or the Fe(VI)/PS process at pH 6.0. DCF was efficiently removed in Fe(VI)/PMS process within a wide range of pH values from 4.0 to 8.0, with higher degradation efficiency in acidic conditions. The increasing reaction temperature (10 to 30 ℃), Fe(VI) dose (6.25 to 100 µM), or PMS concentration (50 to 1000 µM) significantly enhanced the DCF degradation. The existences of HCO₃¯, Cl¯, and humic acid (HA) obviously inhibited the DCF removal. Electron paramagnetic resonance (EPR), free radical quenching, and probing experiments confirmed the existence of sulfate radicals (SO₄^•¯), hydroxyl radicals (^•OH), and Fe(V)/ Fe(IV), which are responsible for DCF degradation in Fe(VI)/PMS system. The variations of TOC removal ratio reveal that the adsorption of organics with ferric particles, formed in the reduction of Fe(VI), also were functioned in the removal process. Sixteen DCF transformation byproducts were identified by UPLC-QTOF/MS, and the toxicity variation was evaluated. Consequently, eight reaction pathways for DCF degradation were proposed. This study provides theoretical basis for the utilization of Fe(VI)/PMS process.

Integrating Biochar, Bacteria, and Plants for Sustainable Remediation of Soils Contaminated with Organic Pollutants

The contamination of soil with organic pollutants has been accelerated by agricultural and industrial development and poses a major threat to global ecosystems and human health. Various chemical and physical techniques have been developed to remediate soils contaminated with organic pollutants, but challenges related to cost, efficacy, and toxic byproducts often limit their sustainability. Fortunately, phytoremediation, achieved through the use of plants and associated microbiomes, has shown great promise for tackling environmental pollution; this technology has been tested both in the laboratory and in the field. Plant-microbe interactions further promote the efficacy of phytoremediation, with plant growth-promoting bacteria (PGPB) often used to assist the remediation of organic pollutants. However, the efficiency of microbe-assisted phytoremediation can be impeded by (i) high concentrations of secondary toxins, (ii) the absence of a suitable sink for these toxins, (iii) nutrient limitations, (iv) the lack of continued release of microbial inocula, and (v) the lack of shelter or porous habitats for planktonic organisms. In this regard, biochar affords unparalleled positive attributes that make it a suitable bacterial carrier and soil health enhancer. We propose that several barriers can be overcome by integrating plants, PGPB, and biochar for the remediation of organic pollutants in soil. Here, we explore the mechanisms by which biochar and PGPB can assist plants in the remediation of organic pollutants in soils, and thereby improve soil health. We analyze the cost-effectiveness, feasibility, life cycle, and practicality of this integration for sustainable restoration and management of soil.

Immobilization of Cd and Pb in soil facilitated by magnetic biochar: metal speciation and microbial community evolution

The preparation of magnetic biochar from sewage sludge and rice straw for heavy metal contaminated soil remediation has greater application prospects, but its remediation mechanism was rarely considered by combining soil physicochemical properties with microbial community. In this study, the effects of magnetic sewage sludge biochar (SSB) and rice straw biochar (RSB) on Cd and Pb immobilization in paddy soil were compared and analyzed by 60-day soil incubation experiments. The results illustrated that DTPA-Cd and DTPA-Pb were reduced by 51.53% (43.07%) and 53.57% (50.47%), while the percentage of residual fraction of the BCR procedure was enhanced by 31.27% (30.78%) of Cd and 27.25% (23.22%) of Pb in the SSB (RSB) treatment, respectively. Fe was detected on both SSB and RSB surfaces, but SSB had rougher and a larger specific surface area compared to RSB. The addition of SSB and RSB in paddy soil increased soil pH and TOC content, and affected the diversity and species of soil microbial community. Compared with the CK group, the relative abundance of Proteobacteria, Bacteroidota, and Lysobacter decreased, and the relative abundance of Actinobacteriota, Pontibacter, and Alkaliphilus increased with SSB and RSB treatments, all of which reflected the bioavailability of Cd and Pb reduction.

Fixed bed column performance of Al-modified biochar for the removal of sulfamethoxazole and sulfapyridine antibiotics from wastewater

In this study, biochar derived from bamboo pretreated with aluminum salt was synthesized for the removal of two sulfonamide antibiotics, sulfamethoxazole (SMX) and sulfapyridine (SPY), from wastewater. Batch sorption experiments showed that Al-modified bamboo biochar (Al-BB-600) removed both sulfonamides effectively with the maximum sorption capacity of 1200-2200 mg/kg. The sorption mechanism was mainly controlled by hydrophobic, π-π, and electrostatic interactions. Fixed bed column experiments with Al-modified biochar packed in different dosages (250, 500 and 1000 mg) and flow rates (1, 2 and 4 mL/min) showed the dosage of 1000 mg and flow rate of 1 mL/min performed the best for the removal of both SMX and SPY from wastewater. Among the breakthrough (BT) models used to evaluate the fixed bed filtration performance of Al-BB-600, the Yan model best described the BT behavior of the two sulfonamides, suggesting that the adsorption process involved multiple rate-liming factors such as mass transfer at the solid surface and diffusion Additionally, the Bed Depth Service Time (BDST) model results indicated that Al-BB-600 can be efficiently used in fixed bed column for the removal of both SMX and SPY in scaled-up continuous wastewater flow operations. Therefore, Al-modified biochar can be considered a reliable sorbent in real-world application for the removal of SMX and SPY from wastewater.

Mechanistic insight of weak magnetic field trigger transformation of amorphous Fe(III)-(oxy)hydroxide for enhanced ferrate (VI) towards selective removal of natural organic matter

It was important to regulate the formation of Fe-hydroxyl during ferrate (Fe(VI)) oxidation and hydrolysis which was beneficial for interfacial adsorption of natural organic matter (NOM). Based on the influence of weak magnetic field (WMF) on the physical and chemical characteristics of particles in chemistry. This study investigated the effect of WMF on Fe(VI) oxidation and Fe(III) flocculation performance by regulating iron species during hydrolysis, for NOM removal. Results indicated WMF efficiently accelerate the removal of NOM that the reactions rate constants in magnetization system was twice as much as the control group. With the structure and electrochemical analysis, WMF enhanced Hydrogen-bond and caused much polar hydroxyl groups combined with iron ions, further triggered Fe(III) transformed to amorphous Fe-hydroxide and ferrihydrite with large specific surface area and high surface activity which removed the pollutants by adsorption and co-precipitation, instead of crystalloid Fe₂O₃ and Fe₃O₄. In addition, the nucleation aggregation behavior and interaction energy of Fe-(oxy)hydroxide revealed that the lower free energy obtained in magnetization system, could lead to higher nucleation rate, and promoted the aggregation. WMF increased hydrophobicity of Fe-(oxy)hydroxides, further more easily adsorbed with humic acid (HA) and bovine serum albumin (BSA) with lower interaction energies than in control group. The selective removal mechanism of Fe-(oxy)hydroxide hardly to aggregate with pollutants which caused by the difference of electrostatic interaction, was illustrated that electronegativity HA and SA were liable to electrostatically attract with Fe-(oxy)hydroxide and removed while the low electronegativity BSA was difficult to remove which its attraction was weakened.

Degradation of Organic Contaminants by Reactive Iron/Manganese Species: Progress and Challenges

Many iron(II, III, VI)- and manganese(II, IV, VII)-based oxidation processes can generate reactive iron/manganese species (RFeS/RMnS, i.e., Fe(IV)/Fe(V) and Mn(III)/Mn(V)/Mn(VI)), which have mild and selective reactivity toward a wide range of organic contaminants, and thus have drawn significant attention. The reaction mechanisms of these processes are rather complicated due to the simultaneous involvement of multiple radical and/or nonradical species. As a result, the ambiguity in the occurrence of RFeS/RMnS and divergence in the degradation mechanisms of trace organic contaminants in the presence of RFeS/RMnS exist in literature. In order to improve the critical understanding of the RFeS/RMnS-mediated oxidation processes, the detection methods of RFeS/RMnS and their roles in the destruction of trace organic contaminants are reviewed with special attention to some specific problems related to the scavenger and probe selection and experimental results analysis potentially resulting in some questionable conclusions. Moreover, the influence of background constituents, such as organic matter and halides, on oxidation efficiency of RFeS/RMnS-mediated oxidation processes and formation of byproducts are discussed through their comparison with those in free radicals-dominated oxidation processes. Finally, the prospects of the RFeS/RMnS-mediated oxidation processes and the challenges for future applications are presented.

Review of Advanced Oxidation Processes Based on Peracetic Acid for Organic Pollutants

In recent years, the removal of organic pollutants from water and wastewater has attracted more attention to different advanced oxidation processes (AOPs). There has been increasing interest in using peroxyacetic acid (PAA), an emerging oxidant with low or no toxic by-products, yet the promotion and application are limited by unclear activation mechanisms and complex preparation processes. This paper synthesized the related research results reported on the removal of organic pollutants by PAA-based AOPs. Based on the research of others, this paper not only introduced the preparation method and characteristics of PAA but also summarized the mechanism and reactivity of PAA activated by the free radical pathway and discussed the main influencing factors. Furthermore, the principle and application of the newly discovered methods of non-radical activation of PAA in recent years were also reviewed for the first time. Finally, the shortcomings and development of PAA-based AOPs were discussed and prospected. This review provides a reference for the development of activated PAA technology that can be practically applied to the treatment of organic pollutants in water.

Enhanced ferrate oxidation of organic pollutants in the presence of Cu(II) Ion

In this study, we found that the introduction of Cu(II) (several μM, close to the concentration level of some real water/wastewater) in ferrate (Fe(VI)) oxidation can remarkably accelerate the abatement of various organic pollutants under slightly alkaline conditions. The results show that 5 μM sulfamethoxazole (SMX) can be completely degraded by Fe(VI) (50 μM) in the presence of 20 μM Cu(II) within 10 min at pH 8.0, which was 1.65 times higher than that by Fe(VI) alone. High-valent iron intermediates (i.e. Fe(V), Fe(IV)) and Cu(III) were generated as reactive species in the Cu(II)/Fe(VI) system, all of which contributed to the enhanced oxidation of SMX. Common water components, except for HCO₃^- and humic acid, exhibited no influence on SMX removal. Additionally, the enhanced removal of SMX by Cu(II)/Fe(VI) was also observed in real water with the benefit of total removal of Cu(II) by the ferrate resultant particles. Due to the presence of highly reactive and selective oxidant, the Cu(II)/Fe(VI) system could react readily with organic pollutants containing electron-rich moieties, such as phenol, olefin or amino groups. This study provided a simple, selective, and practical strategy for the abatement of organic pollutants and a simultaneous removal of Cu(II).

Comparative study on heterogeneous activation of peroxydisulfate and peroxymonosulfate with black carbon derived from coal tar residues: Contribution of free radical, 1O2 and surface-bound radicals

Carbon materials draw increasing attention as metal-free catalysts for persulfates activation. Herein, the potential of black carbon (BC) derived from coal tar residues on heterogeneous activation of peroxydisulfate (PDS) and peroxymonosulfate (PMS) to eliminate organic pollutants was investigated. Compared with UV/persulfates systems, persulfates/BC systems degraded 3 selected phenolic compounds (i.e. phenol, 4-chlorophenol (4-CP) and bisphenol A (BPA)) with an order of magnitude higher oxidation rates, and removed dissolved organics (DOC) with over 27% higher efficiency. In the PDS/BC system, ¹O₂ and surface-bound radicals were proved to be the dominant active species, while free radicals, ¹O₂, and surface-bound radicals were responsible for organics oxidation in the PMS/BC system. Relative contribution of different reactive species in persulfates/BC systems was pH-dependent. Surface oxygen functionalities of BC were involved in ¹O₂ generation, and its structural defects played a critical role in forming free radicals and surface-bound radicals. This study provided an in-depth insight into carbon-driven persulfates activation processes.

Insight into metal-free carbon catalysis in enhanced permanganate oxidation: Changeover from electron donor to electron mediator

Reports that the exploitation of metal-free carbon materials to enhance permanganate (PM) oxidation to abate organic pollution in water have emerged in recent publications. However, the activation mechanism and active sites involved are ambiguous because of the intricate physicochemical properties of carbon. In this study, reduced graphene oxide (rGO) as a typical carbon material exhibits excellent capability to boost permanganate oxidation for removing a wide array of organic contaminants. The simultaneous two reaction pathways in the rGO/PM system were justified: i) rGO donates to electrons to decompose PM and produce highly reactive intermediate Mn species for oxidizing organic contaminants; ii) rGO mediates electron transfer from organics to PM. Oxygen-containing groups (hydroxyl, carboxyl, and carbonyl) were justified as electron-donating groups, while structural defects (vacancy and edge defects) were shown to be critical for rGO-mediated electron transfer. Therefore, the oxidation pathway of the rGO/PM system can be controlled by regulating oxygen functional groups and structural defects. The changeover from electron donor to electron mediator by decorating surface active sites of carbon materials will be of great help to the design and application of carbocatalysts.

Degradation of mineral-immobilized pyrene by ferrate oxidation: Role of mineral type and intermediate oxidative iron species

Ferrate (Fe(VI)) salts like K₂FeO₄ are efficient green oxidants to remediate organic contaminants in water treatment. Minerals are efficient sorbents of contaminants and also excellent solid heterogeneous catalysts which might affect Fe(VI) remediation processes. By targeting the typical polycyclic aromatic hydrocarbon compound - pyrene, the application of Fe(VI) for oxidation of pyrene immobilized on three minerals, i.e., montmorillonite, kaolinite and goethite was studied for the first time. Pyrene immobilized on the three minerals was efficiently oxidized by Fe(VI), with 87%-99% of pyrene (10 μM) being degraded at pH 9.0 in the presence of a 50-fold molar excess Fe(VI). Different minerals favored different pH optima for pyrene degradation, with pH optima from neutral to alkaline following the order of montmorillonite (pH 7.0), kaolinite (pH 8.0), and goethite (pH 9.0). Although goethite revealed the highest catalytic activity on pyrene degradation by Fe(VI), the greater noneffective loss of the oxidative species by ready self-decay in the goethite system resulted in lower degradation efficiency compared to montmorillonite. Protonation and Lewis acid on montmorillonite and goethite assisted Fe(VI) oxidation of pyrene. The intermediate ferrate species (Fe(V)/Fe(IV)) were the dominant oxidative species accountable for pyrene oxidation, while the contribution of Fe(VI) species was negligible. Hydroxyl radical was involved in mineral-immobilized pyrene degradation and contributed to 11.5%-27.4% of the pyrene degradation in montmorillonite system, followed by kaolinite (10.8%-21.4%) and goethite (5.1%-12.2%) according to the hydroxyl radical quenching experiments. Cations abundant in the matrix and dissolved humic acid hampered pyrene degradation. Finally, two different degradation pathways both producing phthalic acid were identified. This study demonstrates efficient Fe(VI) oxidation of pyrene immobilized on minerals and contributes to the development of efficient environmentally friendly Fe(VI) based remediation techniques.

Electroactive Fe-biochar for redox-related remediation of arsenic and chromium: Distinct redox nature with varying iron/carbon speciation

Electroactive Fe-biochar has attracted significant attention for As(III)/Cr(VI) immobilization through redox reactions, and its performance essentially lies in the regulation of various Fe/C moieties for desired redox performance. Here, a series of Fe-biochar with distinct Fe/C speciation were rationally produced via two-step pyrolysis of iron minerals and biomass waste at 400-850 °C (BCX-Fe-Y, X and Y represented the first- and second-step pyrolysis temperature, respectively). The redox transformation of Cr(VI) and As(III) by Fe-biochar was evaluated in simulated wastewater under oxic or anoxic conditions. Results showed that more effective Cr(VI) reduction could be achieved by BCX-Fe-400, while a higher amount of As (III) was oxidized by BCX-Fe-850 under the anoxic environment. Besides, BCX-Fe-400 could generate more reactive oxygen species (e.g.,^•OH) by reducing the O₂, which enhanced the redox-related transformation of pollutants under the oxic situation. The evolving redox performance of Fe-biochar was governed by the transition of the redox state from reductive to oxidative related to the Fe/C speciation. The small-sized amorphous/low-crystalline ferrous minerals contributed to a higher electron-donating capacity (0.43-1.28 mmol g^-1) of BCX-Fe-400. In contrast, the oxidative surface oxygen-functionalities (i.e., carboxyl and quinoid) on BCX-Fe-850 endowed a stronger electron-accepting capacity (0.71-1.39 mmol g^-1). Moreover, the graphitic crystallites with edge-type defects and porous structure facilitated the electron transfer, leading to a higher electron efficiency of BCX-Fe-850. Overall, we unveiled the roles of both Fe and C speciation in maneuvering the redox reactivity of Fe-biochar, which can advance our rational design of electroactive Fe-biochar for redox-related environmental remediation.

Efficient activation of ferrate(VI) by colloid manganese dioxide: Comprehensive elucidation of the surface-promoted mechanism

Current research focuses on introducing additional energy or reducing agents to directly accelerate the formation of Fe(IV) and Fe(V) from ferrate (Fe(VI)), thereby ameliorating the oxidation activity of Fe(VI). Interestingly, this study discovers that colloid manganese dioxide (cMnO₂) can remarkably promote Fe(VI) to remove various contaminants via a novel surface-promoted pathway. Many lines of evidence suggest that high-valent Fe species are the primary active oxidants in the cMnO₂-Fe(VI) system, however, the underlying activation mechanism for the direct reduction of Fe(VI) by cMnO₂ to generate Fe(IV)/Fe(V) is eliminated. Further analysis found that Fe(VI) can combine with the vacancies in cMnO₂ to form precursor complex (cMnO₂-Fe(VI)*), which possesses a higher oxidation potential than Fe(VI). This makes cMnO₂-Fe(VI)* is more vigorous to oxidize pollutants with electron-rich moieties through the electron transfer step than alone Fe(VI), resulting in producing Fe(V) and Fe(IV). The products of Fe(VI) decay (i.e., Fe(II), Fe(III), and H₂O₂) are revealed to play vital roles in further boosting the formation of Fe(IV) and Fe(V). Most importantly, the catalytic stability of cMnO₂ in complicated waters is superior to popular reductants, suggesting its outstanding application potential. Taken together, this work provides a full-scale insight into the surface-promoted mechanism in Fe(VI) oxidation process, thus providing an efficient and green strategy for Fe(VI) activation.

Reactive High-Valent Iron Intermediates in Enhancing Treatment of Water by Ferrate

Efforts are being made to tune the reactivity of the tetraoxy anion of iron in the +6 oxidation state (Fe^VIO₄^2-), commonly called ferrate, to further enhance its applications in various environmental fields. This review critically examines the strategies to generate highly reactive high-valent iron intermediates, Fe^VO₄^3- (Fe^V) and Fe^IVO₄^4- or Fe^IVO₃^2- (Fe^IV) species, from Fe^VIO₄^2-, for the treatment of polluted water with greater efficiency. Approaches to produce Fe^V and Fe^IV species from Fe^VIO₄^2- include additions of acid (e.g., HCl), metal ions (e.g., Fe(III)), and reductants (R). Details on applying various inorganic reductants (R) to generate Fe^V and Fe^IV from Fe^VIO₄^2- via initial single electron-transfer (SET) and oxygen-atom transfer (OAT) to oxidize recalcitrant pollutants are presented. The common constituents of urine (e.g., carbonate, ammonia, and creatinine) and different solids (e.g., silica and hydrochar) were found to accelerate the oxidation of pharmaceuticals by Fe^VIO₄^2-, with potential mechanisms provided. The challenges of providing direct evidence of the formation of Fe^V/Fe^IV species are discussed. Kinetic modeling and density functional theory (DFT) calculations provide opportunities to distinguish between the two intermediates (i.e., Fe^IV and Fe^V) in order to enhance oxidation reactions utilizing Fe^VIO₄^2-. Further mechanistic elucidation of activated ferrate systems is vital to achieve high efficiency in oxidizing emerging pollutants in various aqueous streams.

Sustainable ferrate oxidation: Reaction chemistry, mechanisms and removal of pollutants in wastewater

This review is intended to evaluate the use of ferrate (Fe(VI)), being a green coagulant, sustainable and reactive oxidant, to remove micro pollutants especially pharmaceutical pollutants in contaminated water. After a brief description of advanced oxidation processes, fundamental dimensions regarding the nature, reactivity, and chemistry of this oxidant are summarized. The degradation of contaminants by Fe(VI) involves several mechanisms and reactive agents which are critically evaluated. The efficiency and chemistry of Fe(VI) oxidation differs according to the reaction conditions and activation agent, such as soluble Fe(VI) processes, which involve Fe(VI), UV light, and electro-Fe(VI) oxidation. Fe(VI) application methods (including single dose, multiple doses, chitosan coating etc), and Fe(VI) with activating agents (including sulfite, thiosulfate, and UV) are also described to degrade the micro pollutants. Besides, application of Fe(VI) to remove pharmaceuticals in wastewater are intensely studied. Electrochemical prepared Fe(VI) has more wide application than wet oxidation method. Meanwhile, we elaborated Fe(VI) performance, limitations, and proposed innovative aspects to improve its stability, such as the generation of Fe(III), synergetic effects, nanopores entrapment, and nanopores capsules. This study provides conclusive direction for synergetic oxidative technique to degrade the micro pollutants.

Stabilization and passivation of multiple heavy metals in soil facilitating by pinecone-based biochar: Mechanisms and microbial community evolution

Soil contamination by multiple heavy metals and As is one of the major environmental hazards recognized worldwide. In this study, pinecone-biochar was used for stabilization and passivation of Pb, Cu, Zn, Cr, and As in contaminated soil around a smelter in Hubei province, China. The stabilization rate of heavy metals in soil can exceed 99%, and the leaching amount can meet the national standard of China (GB/T 5085.3-2007, less than 5, 100, 100, 15, and 5 mg/L, respectively.) within 90 days. The study confirmed that the addition of pinecone-biochar and the coexistence of indigenous microorganisms can effectively reduce the bioavailability of heavy metals. Among the heavy metals, As(III) can be oxidized to As(V) and then stabilized, and other heavy metals can be stabilized in a complex and chelated state characterized by X-ray photoelectron spectroscopy. After pinecone-biochar was added, the abundance of microbial community and intensity of metabolic activities became vigorous, the types and contents of dissolved organic matter increased significantly. A novel innovation is that the addition of pinecone-biochar increased the Bacillus and Acinetobacter in soil, which enhanced the function of inorganic ion transport and metabolism to promote the passivation and stabilization of heavy metals throughout the remediation process.

Using potassium ferrate control hazardous disinfection by-products during chlorination

The generation of hazardous disinfection by-product is one of the major problems in drinking water chlorination. This study aims to investigate the potential of potassium ferrate (K₂FeO₄) on by-product control. Filtered raw water from a water treatment plant in Jinan was used to evaluate the effects of K₂FeO₄ dose, pH, ammonia nitrogen, and Br^- concentration on trihalomethane formation potential (THMFP) and haloacetic acid formation potential (HAAFP). The results present that 3 mg/L K₂FeO₄ effectively reduced ultraviolet absorbance at 254 nm (UV₂₅₄) by 45%, but removed little dissolved organic carbon (DOC) by 12% at pH 7.40, since K₂FeO₄ tends to attack the electron-rich part of organic matter molecules but with restricted mineralization ability. Fluorescence excitation-emission matrix (EEM) analyses indicate the effective removal of fulvic acid and humic acid. Increasing K₂FeO₄ dose reduced THMFP but increased HAAFP, due to their precursors reacting with K₂FeO₄ in different pathway, while the rising pH or Br^- concentration increased THMFP but decreased HAAFP. Both THMFP and HAAFP decrease with increasing ammonia nitrogen concentrations. Additionally, it was found that under alkaline conditions, trihalomethanes (THMs) were dominated by haloacetic acids (HAAs).

Comprehensive evaluation of different strategies to recover methanogenic performance in ammonia-stressed reactors

In this study, strategies for recovery of ammonia-stressed AD reactors were attempted, by addition of preserved bioaugmentation consortium in gel (BioG), fresh consortium in liquid medium (BioL), woodchip biochar (BW), and straw biochar (BS). In comparison to control group with ammonia, effective treatments, i.e., BioG, BioL, BW and BS raised the maximum methane production rate by 77%, 23%, 35%, and 24%, respectively. BW possibly acted as interspecies electrical conduits for Direct Electron Transfer based on conductivity and SEM analysis. BioG facilitated slow release of bioaugmentation inocula from gel into the AD system, which protected them from a direct environmental shock. According to microbial analysis, both BioG, BioL and BW resulted in increased relative abundance of Methanothermobacter thermautotrophicus; and BS induced selective raise of Methanosarcina thermophila. The increase of methanogens via these strategies led to the faster recovery of the AD process.

Straw biochar enhanced removal of heavy metal by ferrate

This study demonstrated that As(III) was appreciably removed by ferrate in the presence of straw biochar. Removal efficiency of As in ferrate/biochar system was over 91%, increased by 34% compared with ferrate alone ([biochar]₀ = 10 mg/L, [ferrate]₀ = 6 mg/L, [As(III)]₀ = 200 μg/L). In the reaction process, As(III) was oxidized to As(V) mainly by ferrate, while ferrate was reduced into ferric (hydr)oxides and coated on the biochar. Biochar was oxidized in the reaction and its surface area, pore volume and the amount of Lewis acid functional groups were substantially improved, which provided interaction sites for As adsorption. Analysis of hydrodynamic diameter and zeta potential revealed that biochar interacted with the ferrate resulted ferric oxides and enlarged the Fe-C-As particle/floc, which promoted their settlement and thus the liquid-solid separation of As. As(V) was adsorbed on the surface of biochar and ferric (hydr)oxides through hydrogen bond, electrostatic attraction and As-(OFe) bond. Ferrate/biochar was not only effective for As removal, but removed 73.31% of As, 50.38% of Cd, and 75.27% of Tl when these hazardous species synchronously existed in polluted water (initial content: As, 100 μg/L; Cd, 50 μg/L; Tl, 1 μg/L). The combination of ferrate with biochar has potential for the remediation of hazardous species polluted water.

Enhanced Oxidation of Organic Contaminants by Iron(II)-Activated Periodate: The Significance of High-Valent Iron-Oxo Species

Potassium periodate (PI, KIO₄) was readily activated by Fe(II) under acidic conditions, resulting in the enhanced abatement of organic contaminants in 2 min, with the decay ratios of the selected pollutants even outnumbered those in the Fe(II)/peroxymonosulfate and Fe(II)/peroxydisulfate processes under identical conditions. Both ¹⁸O isotope labeling techniques using methyl phenyl sulfoxide (PMSO) as the substrate and X-ray absorption near-edge structure spectroscopy provided conclusive evidences for the generation of high-valent iron-oxo species (Fe(IV)) in the Fe(II)/PI process. Density functional theory calculations determined that the reaction of Fe(II) with PI followed the formation of a hydrogen bonding complex between Fe(H₂O)₆²⁺ and IO₄(H₂O)^-, ligand exchange, and oxygen atom transfer, consequently generating Fe(IV) species. More interestingly, the unexpected detection of ¹⁸O-labeled hydroxylated PMSO not only favored the simultaneous generation of ^·OH but also demonstrated that ^·OH was indirectly produced through the self-decay of Fe(IV) to form H₂O₂ and the subsequent Fenton reaction. In addition, IO₄^- was not transformed into the undesired iodine species (i.e., HOI, I₂, and I₃^-) but was converted to nontoxic iodate (IO₃^-). This study proposed an efficient and environmental friendly process for the rapid removal of emerging contaminants and enriched the understandings on the evolution mechanism of ^·OH in Fe(IV)-mediated processes.