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Enhanced production of reactive oxidants by Fenton-like reactions in the presence of carbon materials
Abstract
The production of the reactive oxidant (i.e., hydroxyl radical) by the Fenton-like reaction (Fe(III)/H2O2) was greatly improved by the addition of carbon materials such as powdered activated carbon (PAC) and carbon nanotubes (CNT). The presence of carbon materials enhanced the oxidative conversion of methanol into formaldehyde by the Fe(III)/H2O2 system. CNT exhibited a slightly higher activity than PAC. In the presence of the carbon materials, the reduction of Fe(III) to Fe(II) was accelerated while the decomposition rate of H2O2 increased, which appears to be responsible for the enhanced oxidant production. The reducing power of carbon materials was also shown by the reductive conversion of polyoxomolybdate. Increasing the addition of carbon materials exhibited saturated (or slightly decreased) oxidant yields, while decreasing the H2O2 utilization efficiency. The enhanced oxidant production by carbon materials was more pronounced in the absence of dissolved oxygen, which was explained by the reactions of organo-radicals produced by methanol oxidation.
... In addition, the carbon-centered PFRs on the surface of hydrochar may act as electron donors, transferring electrons from hydrochar to an electron acceptor, namely Fe(III), thereby mediating the reduction of Fe(III) (Qin et al., 2017). The significantly decreased removal of triclosan under alkaline conditions (pH = 9.7) could be explained by the limited solubility of iron and the nonradical decomposition of H 2 O 2 at high pH conditions (Seo et al., 2015). Note that the pK a of triclosan is 7.8 and it mainly exists as an anionic species (98.7%) at a pH of 9.7. ...
... The reduction reaction of Fe (III) played a critical role at this stage. Previous studies have shown that carbon materials with carbonyl/quinone groups can promote the reduction of Fe(III) ions by accelerating the electron transfer from Fe(III) to Fe(II) (Seo et al., 2015;Peng et al., 2017). In minutes 5-30, the concentration of Fe(II) increased continuously, but the growth rate slowed down, and the concentration was slightly higher than the leaching concentration. ...
... It was found that the binding energy of O-C=O of HC 240-4 decreased from 288.14 to 287.90 eV after the Fenton-like reaction, which indicated that the electron density around Fe(III) and carbonyl changed. Previous studies showed that the bonding of Fe(III) and carbonyl lowered the electron transfer energy from H 2 O 2 to Fe(III) and accelerated Fe(III)/Fe(II) cycle (Seo et al., 2015;Yang et al., 2018). The oxygen-containing groups on the surface of hydrochar and PFRs could form a complex with iron (Qin et al., 2017;Qian et al., 2018), resulting in combined Fe(III) being easier to reduce than dissolved Fe(III) . ...
The traditional Fenton system is subject to the low efficiency of the Fe(III)/Fe(II) conversion cycle, with significant attempts made to improve the oxidation efficiency by overcoming this hurdle. In support of this goal, iron-enriched sludge-derived hydrochar was prepared as a high-efficiency catalyst by one-step hydrothermal carbonization and its performance and mechanisms in mediating the oxidation of triclosan were explored in the present study. The hydrochar prepared at 240 °C for 4 h (HC240-4) had the highest removal of triclosan (97.0%). The removal of triclosan in the HC240-4/H2O2 system was greater than 90% in both acidic and near-neutral environments and remained as high as 83.5% after three cycles, indicating the broad pH applicability and great recycling stability of sludge-derived hydrochar in Fenton-like systems. H2O2 was activated by both persistent free radicals (PFRs; 19.7%) and iron (80.3%). The binding of Fe(III) to carboxyl decreased the electron transfer energy from H2O2 to Fe(III), making its degradation efficiency 2.6 times greater than that of the conventional Fenton reaction. The study provides a way for iron-enriched sludge utilization and reveals a role for hydrochar in promoting iron cycling and electron transfer in the Fenton reaction.
... For instance, Pourzamani et al. demonstrated that MWCNTs could improve the efficiency of electro-Fenton process by adsorbing pollutants and accelerating the production of ROS [17]. Seo et al. reported that carbon materials possessed reducing ability, which could accelerate the reduction of Fe (III) to Fe(II), thus facilitating the decomposition of H 2 O 2 and the removal of target compounds [18]. Peng et al. found that the reduction of Fe(III) could also be promoted at a much lower MWCNTs dosage (2.0 mg/L) [16]. ...
... Surprisingly, almost no ATL was removed in the Xan/XOD system even after 24 h (Fig. 3c), ruling out the possibility of O 2 ⋅attacking ATL directly. Previous studies have demonstrated that surface Fe(III) could be reduced by CNTs [18], H 2 O 2 [12], or O 2 ⋅ - [54], so we further analyzed the role of O 2 ⋅from its reducing capacity. According to the limited number of electrons theoretically available from MWCNTs (Text S9), it was unlikely to be the primary electron donor for the reduction of Fe(III) to Fe(II) (Eq. ...
The effects of multi-walled carbon nanotubes (MWCNTs) on the removal of contaminants mediated by Fenton reactions and how the functional groups on MWCNTs cooperate to modulate the cycling of Fe(III)/Fe(II) remain unclear. Here, we showed that MWCNTs significantly enhanced the removal of atenolol (ATL) in Fenton-like system by facilitating the generation of ·OH and O2·–, in which ·OH directly attacked ATL, while O2·– promoted the cycling of Fe(III)/Fe(II). Moreover, we found that in addition to –COOH, –OH on MWCNTs played an important role in ATL removal, that is, Fe(III) was likely to interact with –OH by H-bond and O–H···π interactions, which significantly reduced the reduction potential of Fe(III)/Fe(II), thus promoting the removal of ATL. Our findings revealed the role of –OH and –COOH on MWCNTs in promoting the removal of ATL in Fe(III)/H2O2 system, which is helpful to rationally design the CNTs-based materials applied for wastewater treatment.
... The multi-valence state of Mo in MoO 2 (Mo 4+ to Mo 5+ and Mo 6+ ) was attributed to accelerating Fe 3+ / Fe 2+ redox cycles and efficient PMS activation. Similarly, carbon and nitrogen-based (CNTs, biochar, rGO, activated carbon, etc.) electronrich materials have also been tested to accelerate the Fe 3+ /Fe 2+ redox cycles of the catalysts during treatment [1,84,[115][116][117][118]. Functionalized multi-walled CNT (FCNT-H), for instance, was proposed by Yang et al. to reduce Fe 3+ in the Fe 2+ /H 2 O 2 system through direct electron transfer or complexation as FCNT-H-Fe 3+ [84]. ...
... It has been found that the addition of different carbon materials can enhance the Fenton reaction because the presence of carboxyl, carbonyl, and quinone groups present in the carbon material surface accelerates the reduction of Fe 3+ to Fe 2+ (Figure 4) [102][103][104][105][106]. On the other hand, carbon materials have been also used as support of the Fenton catalyst. ...
Currently, the presence of emerging contaminants in water sources has raised concerns worldwide due to low rates of mineralization, and in some cases, zero levels of degradation through conventional treatment methods. For these reasons, researchers in the field are focused on the use of advanced oxidation processes (AOPs) as a powerful tool for the degradation of persistent pollutants. These AOPs are based mainly on the in-situ production of hydroxyl radicals (OH•) generated from an oxidizing agent (H2O2 or O2) in the presence of a catalyst. Among the most studied AOPs, the Fenton reaction stands out due to its operational simplicity and good levels of degradation for a wide range of emerging contaminants. However, it has some limitations such as the storage and handling of H2O2. Therefore, the use of the electro-Fenton (EF) process has been proposed in which H2O2 is generated in situ by the action of the oxygen reduction reaction (ORR). However, it is important to mention that the ORR is given by two routes, by two or four electrons, which results in the products of H2O2 and H2O, respectively. For this reason, current efforts seek to increase the selectivity of ORR catalysts toward the 2e− route and thus improve the performance of the EF process. This work reviews catalysts for the Fenton reaction, ORR 2e− catalysts, and presents a short review of some proposed catalysts with bifunctional activity for ORR 2e− and Fenton processes. Finally, the most important factors for electro-Fenton dual catalysts to obtain high catalytic activity in both Fenton and ORR 2e− processes are summarized.
... CNTs bearing negatively charged surface groups (e.g., carboxyl, hydroxyl, and ether) have shown great potential for enhancing the heterogeneous Fenton reactivity by combi- nation with iron ions [55]. Several studies have confirmed that CNTs can be regarded as electron-transfer (ET) catalysts, involving the reduced and oxidized catalyst states, which affect catalytic activity. ...
Carbon-based nanomaterials (CBM) have shown great potential for various environmental applications because of their physical and chemical properties. The unique hybridization properties of CBMs allow for the tailored manipulation of their structures and morphologies. However, owing to poor solar light absorption, and the rapid recombination of photogenerated electron-hole pairs, pristine carbon materials typically have unsatisfactory photocatalytic performances and practical applications. The main challenge in this field is the design of economical, environmentally friendly, and effective photocatalysts. Combining carbonaceous materials with carbonaceous semiconductors of different structures results in unique properties in carbon-based catalysts, which offers a promising approach to achieving efficient application. Here, we review the contribution of CBMs with different dimensions, to the catalytic removal of organic pollutants from wastewater by catalyzing the Fenton reaction and photocatalytic processes. This review, therefore, aims to provide an appropriate direction for empowering improvements in ongoing research work, which will boost future applications and contribute to overcoming the existing limitations in this field.
... The peak at 284.42 eV in C1s spectra displays the sp 2 graphitic structure, which is a pattern that is anticipated for biochar [34]. The relative proportion of sp 3 C-C/sp 2 C--C of C1s spectra in the used FSBC-10 decreased from 34.48 to 9.33 % compared to fresh FSBC-10.The decreased fractions of sp 2 C--C after the reaction implies that they may be active sites that can transfer single electrons to H 2 O 2 to generate •OH [35,36], or accelerate the reduction of Fe(III) to Fe(II) [37]. Similarly, the O1s spectrum (Fig. 6 g) shows deconvoluted binding energy peaks at 530.11 eV for Fe-O due to unavoidable O element present (partial oxidation) during catalyst synthesis due to the exposure to the air. ...
... The concentration of H 2 O 2 was determined by titanium salt spectrophotometry. Fe 2+ and Fe 3+ were determined by 1,10- Phenanthroline spectrophotometry [55,57]. A chemical oxygen demand (COD) detector (Multi-direct, Lovibond) was employed to detect COD [58]. ...
The Fenton reaction as an effective advanced oxidation technology has been widely used in wastewater treatment for its stable effluent quality, simple operation, mild condition, and higher organic degradation with non-selectivity. However, the traditional Fenton reaction is limited by the sluggish regeneration of Fe2+, resulting in a slower reaction rate, and it is necessary to further increase the dosage of Fe2+, which will increase the production of iron sludge. Activated carbon (AC) has a strong adsorption property, and it cannot be ignored that it also can reduce Fe3+. In this study, the degradation of acid red G (ARG) by adding AC to the Fe3+/H2O2 system, the role of the reducing ability, and the reason why AC can reduce Fe3+ were studied. By adding three kinds of ACs, including coconut shell-activated carbon (CSPAC), wood-activated carbon (WPAC), and coal-activated carbon (CPAC), the ability of ACs to assist the Fe3+/H2O2 Fenton-like system to degrade ARG was clarified. Through the final treatment effect and the ability to reduce Fe3+, the type of AC with the best promotion effect was CSPAC. The different influence factors of particle size, the concentration of CSPAC, concentration of H2O2, concentration of Fe3+, and pH value were further observed. The best reaction conditions were determined as CSPAC powder with a particle size of 75 μm and dosage of 0.6 g/L, initial H2O2 concentration of 0.4 mmol/L, Fe3+ concentration of 0.1 mmol/L, and pH = 3. By reducing the adsorption effect of CSPAC, it was further observed that CSPAC could accelerate the early reaction rate of the degradation process of ARG by the Fe3+/H2O2 system. FT-IR and XPS confirmed that the C-O-H group on the surface of CSPAC could reduce Fe3+ to Fe2+. This study can improve the understanding and role of AC in the Fenton reaction, and further promote the application of the Fenton reaction in sewage treatment.
... The similar co-catalytic effect on enhancing the cOH yield from H 2 O 2 decomposition has been observed in the combination of biochar with iron minerals, 43,46 and in the Fenton-like systems with presence of activated carbon, carbon nanotubes and reduced graphene oxide. 47,48 The EPR spectra conrm the production of both cOH and O 2 c À in most of the reaction systems. The cOH signals with characteristic 1 : 2 : 2 : 1 pattern (Fig. 2(c)) can be ranked by relative intensity in an order similar to the cOH production shown in Fig. 2(a). ...
Activation of hydrogen peroxide (H2O2) with biochar is a sustainable and low-cost approach for advanced oxidation of organic pollutants, but faces the challenge of a low yield of hydroxyl radical (˙OH). Herein, we hypothesize that the activation efficiency of H2O2 can be enhanced through co-catalysis of trace dissolved iron (Fe) with biochar. Two biochar samples derived from different feedstock, namely LB from liquor-making residue and WB from wood sawdust, were tested in the co-catalytic systems using trace Fe(iii) (0.3 mg L-1). The cumulative ˙OH production in [Fe(iii) + LB]/H2O2 was measured to be 3.28 times that in LB/H2O2, while the cumulative ˙OH production in [Fe(iii) + WB]/H2O2 was 11.9 times that in WB/H2O2. No extra consumption of H2O2 was observed in LB/H2O2 or WB/H2O2 after addition of trace Fe(iii). Consequently, the reaction rate constants (k obs) for oxidation of pollutants (2,4-dichlorophenoxyacetic acid and sulfamethazine) were enhanced by 3.13-9.16 times. Other iron species including dissolved Fe(ii) and iron minerals showed a similar effect on catalyzing 2,4-D oxidation by biochar/H2O2. The interactions involved in adsorption and reduction of Fe(iii) by biochar in which the defects acted as electron donors and oxygen-containing functional groups bridged the electron transfer. The fast regeneration of Fe(ii) in the co-catalytic system resulted in the sustainable ˙OH production, thus the efficient oxidation of pollutants comparable to other advanced oxidation processes was achieved by using dissolved iron at a concentration as low as the concentration that can be found in natural water.
... Such modified CNTs and GAC can also serve as catalysts or reductants that mineralize the target contaminants [24]. Some studies have previously verified their role as electron donors, exhibiting metal reduction [25][26][27] under certain conditions. However, systematic studies on estimating the reducing power are lacking. ...
Determining the degree of the reducing power of multi-walled carbon nanotubes (MWCNTs) and granular activated carbon (GAC) enables their effective application in various fields. In this study, we estimate the reducing power of carbon nanotubes (CNTs) and GAC by measuring the reduction degree of various compounds with different reduction potentials. MWCNTs and GAC materials can reduce Cr(VI), Fe(III) and PMo12O403−, where the reduction potentials range from +1.33 V to +0.65 V. However, no reduced forms of PW12O403− and SiW12O404− compounds were detected, indicating that the reducing power of MWCNTs and GAC is insufficient for reduction potentials in the range +0.218 V to +0.054 V. MWCNTs exhibit a short reduction time (5 min), whereas GAC exhibits a gradually increasing reduction degree of all the compounds assessed until the end of the reaction. This indicates a higher reduction degree than that of MWCNTs systems. Acidic initial pH values favor reduction, and the reduction degree increases as the pH becomes lower than 4.0. Moreover, large quantities of MWCNTs and GAC increase the concentrations of the reduced compounds.
Co-activation of H2O2 with biochar and iron sources together provides an attractive strategy for efficient removal of refractory pollutants, because it can solve the problems of slow Fe(Ⅱ) regeneration in Fenton/Fenton-like processes and of low •OH yield in biochar-activated process. In this study, a wood-derived biochar (WB) was modified by heteroatom doping for the objective of enhancing its reactivity toward co-activation of H2O2. The performance of the co-activated system using doped biochars and trace dissolved Fe(Ⅲ) on oxidation of organic pollutants was evaluated for the first time. The characterizations using X-ray photoelectron spectroscopy (XPS), Raman spectra and electrochemical analyses indicate that heteroatom doping introduced more defects in biochar and improved its electron transfer capacity. The oxidation experiments show that heteroatom doping improved the performance of biochar in the co-activated process, in which the N,S-codoped biochar (NSB) outperformed the N-doped biochar (NB) on oxidation of pollutants. The reaction rate constant (kobs) for oxidation of sulfadiazine in NSB + Fe + H2O2 is 2.25 times that in NB + Fe + H2O2, and is 72.9 times that in the Fenton-like process without biochar, respectively. The mechanism investigations indicate that heteroatom doping enhanced biochar's reactivity on catalyzing the decomposition of H2O2 and on reduction of Fe(Ⅲ) due to the improved electron transfer/donation capacity. In comparison with N-doping, N,S-codoping provided additional electron donor (thiophenic C-S-C) for faster regeneration of Fe(Ⅱ) with less amount of doping reagent used. Furthermore, co-activation with NSB maintained to be efficient at a milder acidic pH than Fenton/Fenton-like processes, and can be used for oxidation of different pollutants and in real water. Therefore, this research provides a novel, sustainable and cost-efficient method for oxidation of refractory pollutants.
The heterogeneous Fenton-like systems induced by Fe-containing minerals have been largely applied for the degradation of organic pollutants. However, few studies have been conducted on biochar (BC) as an additive to Fenton-like systems mediated by iron-containing minerals. In this study, the addition of BC prepared at different temperatures was found to significantly enhance the degradation of contaminants in the tourmaline-mediated Fenton-like system (TM/H2O2) using Rhodamine B (RhB) as the target contaminant. Furthermore, the hydrochloric acid-modified BC prepared at 700 °C (BC700(HCl)) could achieve complete degradation of high concentrations of RhB in the BC700(HCl)/TM/H2O2 system. Free radical quenching experiments showed that TM/H2O2 system removed contaminants mainly mediated by the free radical pathway. After adding BC, the removal of contaminants is mainly mediated by the non-free radical pathway in BC700(HCl)/TM/H2O2 system which was confirmed by the Electron paramagnetic resonance (EPR) experiments and electrochemical impedance spectroscopy (EIS). In addition, BC700(HCl) had broad feasibility in the degradation of other organic pollutants (Methylene Blue (MB) 100%, Methyl Orange (MO) 100%, and tetracycline (TC) 91.47%) in the tourmaline-mediated Fenton-like system. Possible pathways for the degradation of RhB by the BC700(HCl)/TM/H2O2 system were also proposed.
Abstract Carbon materials (e.g., pyrogenic carbon (PyC)) are widely used in agricultural soils and can participate in various biogeochemical processes, including iron (Fe) cycling. In soils, Fe(II) species have been proposed as the main active contributor to produce reactive oxygen species (ROS), which are involved in various biogeochemical processes. However, the effects of PyC on the transformation of different Fe species in soils and the associated production of ROS are rarely investigated. This study examined the influence of PyC (pyrolyzed at 300–700 °C) on Fe(II)/Fe(III) cycling and hydroxyl radical (·OH) production during redox fluctuations of paddy soils. Results showed that the reduction of Fe(III) in soils was facilitated by PyC during anoxic incubation, which was ascribed to the increased abundance of dissimilatory Fe(III)-reducing microorganisms (biotic reduction) and the electron exchange capacity of PyC (abiotic reduction). During oxygenation, PyC and higher soil pH promoted the oxidation of active Fe(II) species (e.g., exchangeable and low-crystalline Fe(II)), which consequently induced higher yield of ·OH and further led to degradation of imidacloprid and inactivation of soil microorganisms. Our results demonstrated that PyC accelerated Fe(II)/Fe(III) cycling and ·OH production during redox fluctuations of paddy soils (especially those with low content of soil organic carbon), providing a new insight for remediation strategies in agricultural fields contaminated with organic pollutants. Graphical Abstract
Low-cost stainless-steel electrodes can activate hydrogen peroxide (H2O2) by converting it into a hydroxyl radical (•OH) and other reactive oxidants. At an applied potential of +0.020 V, the stainless-steel electrode produced •OH with a yield that was over an order of magnitude higher than that reported for other systems that employ iron oxides as catalysts under circumneutral pH conditions. Decreasing the applied potential at pH 8 and 9 enhanced the rate of H2O2 loss by shifting the process to a reaction mechanism that resulted in the formation of an Fe(IV) species. Significant metal leaching was only observed under acidic pH conditions (i.e., at pH <6), with the release of dissolved Fe and Cr occurring as the thickness of the passivation layer decreased. Despite the relatively high yield of •OH production under circumneutral pH conditions, most of the oxidants were scavenged by the electrode surface when contaminant concentrations comparable to those expected in drinking water sources were tested. The stainless-steel electrode efficiently removed trace organic contaminants from an authentic surface water sample without contaminating the water with Fe and Cr. With further development, stainless-steel electrodes could provide a cost-effective alternative to other H2O2 activation processes, such as those by ultraviolet light.
This review presents an exhaustive overview on the mechanisms of Fe3+ cathodic reduction within the context of the electro-Fenton (EF) process. Different strategies developed to improve the reduction rate are discussed, dividing them into two categories that regard the mechanistic feature that is promoted: electron transfer control and mass transport control. Boosting the Fe3+ conversion to Fe2+ via electron transfer control includes: (i) the formation of a series of active sites in both carbon- and metal-based materials and (ii) the use of other emerging strategies such as single-atom catalysis or confinement effects. Concerning the enhancement of Fe2+ regeneration by mass transport control, the main routes involve the application of magnetic fields, pulse electrolysis, interfacial Joule heating effects, and photoirradiation. Finally, challenges are singled out, and future prospects are described. This review aims to clarify the Fe3+/Fe2+ cycling process in the EF process, eventually providing essential ideas for smart design of highly effective systems for wastewater treatment and valorization at an industrial scale.
The exploitation of effective modification means to get highly active and stable catalytic materials for heterogeneous photo-Fenton-like (HPF-like) process has been enormously stimulated by the globally increased demands for water purification. Oxygen vacancies (OVs) modulation is regarded as a prevalent strategy to improve the performance of heterogeneous catalysts. Herein, the polyvinylpyrrolidone (PVP) modified CuFeO2 (PVP/CFO) catalysts were prepared to degrade ofloxacin (OFX) target pollutant in HPF-like process. The modification of PVP introduced the rich OVs and more Fe²⁺ active sites on PVP/CFO surface, while the aggregation of CuFeO2 particles was inhibited by the modification of PVP. The PVP/CFO catalyst with the addition of 4 g PVP in the synthesis (PVP/CFO-4) possessed more superior catalytic performance for OFX degradation than other prepared catalysts in the present work. Additionally, the as-prepared PVP/CFO-4 catalyst possessed good stability for OFX degradation in HPF-like system. The introduced OVs by PVP modification and the generated photoelectron on PVP/CFO by light excitation effectively accelerated in-situ recycling of Cu²⁺/Cu⁺ and Fe³⁺/Fe²⁺, which was beneficial for •OH/•O2– formation and OFX mineralization. The quenching experiment and EPR spectra revealed that •OH, •O2⁻ and e⁻ were the major active species, and h⁺ was auxiliary species for OFX degradation. The OFX degradation pathways were proposed by density functional theory calculation and liquid chromatography mass spectrometry. The overall toxicity of intermediates was relieved after HPF-like degradation of OFX based on quantitative structure–activity relationship predictions.
Fenton chemistry has been widely studied in a broad range from geochemistry, chemical oxidation to tumor chemodynamic therapy. It was well established that Fe3+/H2O2 resulted in a sluggish initial rate or even inactivity. Herein, we report the homogeneous carbon dot-anchored Fe(III) catalysts (CD-COOFeIII) wherein CD-COOFeIII active center activates H2O2 to produce hydroxyl radicals (•OH) reaching 105 times larger than that of the Fe3+/H2O2 system. The key is the •OH flux produced from the O-O bond reductive cleavage boosting by the high electron-transfer rate constants of CD defects and its self-regulated proton-transfer behavior probed by operando ATR-FTIR spectroscopy in D2O and kinetic isotope effects, respectively. Organic molecules interact with CD-COOFeIII via hydrogen bonds, promoting the electron-transfer rate constants during the redox reaction of CD defects. The antibiotics removal efficiency in the CD-COOFeIII/H2O2 system is at least 51 times large than the Fe3+/H2O2 system under equivalent conditions. Our findings provide a new pathway for traditional Fenton chemistry.
Fenton-like technology has been widely used in the field of water treatment as an advanced oxidation processes (AOPs), and a multitude of non-homogeneous catalysts have been reported effectively to enhance it. Herein, nitrogen and sulfur co-doped nanocarbon materials (NSC) was designed and synthesized, which formed a unique mesoporous structure and abundant functional groups on the surface. It exhibits superior catalytic performance to enhance the Fe(III)/H2O2 system for sulfamethoxazole (SMX) degradation, kobs is 18 times higher than Fe(III)/H2O2 system after the introduction of NSC under the same experimental conditions. Also, the working pH of Fe(III)/H2O2 system was broadened by NSC. Quenching and probe experiments demonstrate that hydroxyl radical (∙OH) was the dominant active species. Both dissociative and surface-bound iron species were observed, which indicates direct reduction and in-situ binding of Fe(III), respectively. Further investigation found nitrogen and sulfur co-doped attribute to the formation of abundant functional groups, the C[sbnd]O bond and C[sbnd]S[sbnd]C bond on the surface of NSC result in the directly reduce of Fe(III), while –COOH contributes to the in-situ binding. The intermediates and possible degradation pathway of SMX degradation were proposed based on the results of liquid chromatography-quadrupole time of flight-mass spectrometer (LC-QTOF-MS). The results of cycling and co-existing ions experiments demonstrate that NSC has outstanding potential for application. This work provides a novel insight into the sustained removal of micropollutants from the aqueous environment based on the nanocarbon materials enhancing the Fenton-like oxidation.
High energy consumption in pyrolyzing precursors for catalyst preparation would limit the application of nitrogen-doped carbon-based single-atom catalysts in actual pollutant remediation. Herein, we report an Fe single atom (7.67 wt %) loaded polyaniline catalyst (Fe-PANI) prepared via a simple impregnation process without pyrolysis. Both experimental characterizations and density functional theory calculations demonstrated that isolated -N═ group sites can fasten Fe atoms through Fe-N coordination in PANI, leading to a high stability of Fe atoms in a heterogeneous Fenton reaction. Highly dispersive yet dense -N═ groups in PANI can be protonated to be adsorption sites, which largely reduce the migration distance between reactive radicals and organics. More significantly, frontier molecular orbitals and spin-density distributions reveal that electrons can transfer from reduction groups of PANI to an Fe(III) site to accelerate its reduction. As a result, a remarkably boosted degradation behavior of organics under near-neutral conditions (pH 6), with low H2O2 concentration, was achieved. This cost-effective Fe-PANI catalyst with high catalytic activity, stability, and adsorption performance has great potential for industrial-level wastewater treatment.
The sluggish Fe³⁺/Fe²⁺ cycle was the rate-limiting step in the Fenton-like reaction, and metal-free carbonaceous materials are considered as emerging alternatives to solve this problem. However, the effect of carbon material properties on the distribution of reactive species remains poorly understood. This study investigated the possibility and mechanism of using biochar to accelerate the Fe³⁺/Fe²⁺ cycle to overcome the low efficiency of Fe³⁺/persulfate (PS) catalytic oxidation of phenanthrene. More importantly, the contribution of reactive species in the reaction systems with the variation of biochar pyrolysis temperatures was quantitatively studied. The results showed that medium-temperature derived biochar (BC500) had the greatest ability to enhance the Fenton-like system compared to the low- and high-temperature (BC350/700), and the first-order rate constant achieved 5.2 and 35.7-fold increase against the biochar/PS and Fe³⁺/PS systems, respectively. Using electrochemical evidence, sulfoxide probe tests, and steady-state concentration calculations, radicals yields were found to rise and then reduce with decreasing pyrolysis temperature, while the nonradical contribution of Fe(IV) increased to 56.3%. Electron paramagnetic resonance, Boehm titration, and Raman spectroscopy unraveled that the enhanced effect of biochar resulted from itself persistent free radicals, phenolic-OH, and edge defects, which enabled electron transfer between Fe³⁺ and biochar. Fe²⁺ was thus continuously generated and effectively activated the PS. This work enables a better understanding of the Fe³⁺-mediated Fenton-like reaction in the presence of biochar and provides a sustainable green strategy for Fenton chemistry with potential applications.
Zeolite imidazole framework-8 (ZIF-8) is a promising template to obtain porous nanocarbons. In this study, microporous nitrogen-doped nanocarbons from the carbonization of ZIF-8 (ZCN) was prepared as an efficient metal-free catalyst to improve several micropollutants degradation in Fe(III)/H2O2 process. The sulfamethoxazole (SMX) ratio was increased from 20% to 100% with the addition of ZCN (50 mg/L) in Fe(III)/H2O2 within 20 min, and the working pH was endowed. The direct reduction for Fe(III) resulting from carbonyl on ZCN's surface was revealed. Hydroxyl radical (•OH) was determined to be the main reactive species, and the evolution of different Fe species during the reaction was discussed by monitoring the mass balance of Fe species. We found that part of the iron was bound to the surface of ZCN during the reaction. Additionally, the dissociative Fe was captured by ZCN to form Fe-Nx bonds. Surface-bound Fe with a lower energy barrier was more likely to react with H2O2 to generate Fe(II). Our work revealed that in addition to the direct reduction by ZCN, another catalytic reduction pathway for the sustainable conversion of Fe(III) to Fe(II) in the ZCN/Fe(III)/H2O2 process was operative.
Due to slow recovery of Fe²⁺ and a narrow pH range of the Fenton and Fenton-like reactions, a single-atom Fe implanted N-doped carbon material ([email protected]/SA-Fe) was used as the co-catalyst in the Fe³⁺/H2O2 system in this paper. This material greatly accelerated the Fe³⁺/Fe²⁺ cycle and thus promoted Rhodamine B (RhB) degradation. RhB was almost completely removed even at pH=7. A systematic comparison was conducted between [email protected]/SA-Fe and [email protected] (without Fe). The results proved the vital importance of Fe single atoms, which not only improved the electron conductivity of [email protected]/SA-Fe, but also promoted the combination of carbon materials and Fe³⁺ with a higher oxidation potential. Moreover, the enhanced H2O2 adsorption on single-atom Fe sites facilitated the participation of H2O2 in the reaction. In addition, [email protected]/SA-Fe exhibits excellent catalytic activity in various water bodies. The material still possessed good re-usability and stability after five catalytic cycles, which demonstrated its great application potential in wastewater treatment.
Biochar-supported copper/manganese oxide composite (CuOx-MnOy@BC) as a useful Fenton-like catalyst was synthesized and characterized. The nanocatalyst was applied to eliminate metronidazole (MNZ) from an aqueous wastewater. 99.7% of MNZ and 55.2% of the total organic carbon (TOC) removal were achieved at 20 mM of H2O2, 0.75 mg/L of CuOx-MnOy@BC, and 20 mg/L of MNZ under neutral pH, and the catalyst remained active under a wide range of pH. Free radicals quenching studies verified the dominant role of hydroxyl radicals produced through activation of H2O2 with the synergistic effect of Cu2+, Mn2+, persistent free radicals, and visible light. The reusability of catalyst was confirmed through five repeated uses.
Pyrite and engineering carbon materials have received increasing attention for their catalytic potential in Fenton reactions due to their extensive sources and low cost. However, effects of carbon materials on the degradation of pollutants by pyrite-catalyzed heterogeneous Fenton oxidation have not been fully understood. In this study, the performance of pyrite-catalyzed heterogeneous Fenton system on the degradation of ciprofloxacin (CIP) was investigated in the presence of activated carbon (AC), biochar (BC), and carbon nanotubes (CNTs). Synchronous and asynchronous experiments (adsorption and catalysis) were conducted to elucidate the roles of the carbon materials in pyrite-catalyzed Fenton reactions. The results demonstrated that all the three carbon materials accelerated the pyrite-catalyzed Fenton oxidation of CIP. Under the experimental conditions, the reaction rates, which were obtained by fitting the synchronous experimental results with the pseudo-first-order kinetic model, of pyrite/AC, pyrite/BC and pyrite/CNTs with H2O2 for the removal of CIP were 8.28, 3.40 and 3.37 times faster than that of pyrite alone. Adsorption experiments and characterization analysis showed that AC had a higher adsorption capacity than BC and CNTs for CIP, which enabled it to distinguish itself in assisting the pyrite-catalyzed Fenton oxidation. In the presence of the carbon materials, the adsorption effect should not be neglected when studying the catalytic performance of pyrite. Free radical quenching experiments and electron spin-resonance spectroscopy (ESR) were used to detect and identify free radical species in the reactions. The results showed that hydroxyl radicals (•OH) contributed significantly to the degradation of CIP. The addition of carbon materials promoted the production of •OH, which favored the degradation of CIP. The results of this study suggested that the synergistic effect of oxidation and adsorption promoted the removal of CIP in pyrite/carbon materials/H2O2 systems, and coupling pyrite and carbon materials shows great potential in treating antibiotic wastewater.
Black liquor and Fenton sludge are common industrial wastes that come from the conventional kraft pulping process and the Fenton wastewater treatment process, respectively. In this study, the biochar-supported iron-based catalysts were synthesized through a simple one-step pyrolysis method using acid-precipitated black liquor (APBL) as carbon source and Fenton sludge as iron source, and were then applied for Fenton-like removal of rhodamine B (RhB) dyes. The optimized catalyst ([email protected]FAK), pyrolyzed at 900 ºC with KOH as the activator, was mesoporous biochar-supported nanoscale zero-valent iron (nZVI). Under the optimal conditions (50 mg L⁻¹ RhB, initial pH 3, 2 mM H2O2 concentration, catalyst dosage of 0.2 g L⁻¹), the [email protected]FAK/H2O2 system achieved almost 100% RhB dye removal within 10 min. Moreover, the [email protected]FAK/H2O2 system also exhibited excellent removal efficiencies for malachite green, crystal violet, and methylene blue. The [email protected]FAK had a good magnetic separation ability (146.4 emu g⁻¹) and maintained a high removal efficiency of RhB (83.8%) in the presence of H2O2 after five times of recycling/reuse. The degradation of RhB dye was mainly attributed to •OH, including surface-bound •OH and free •OH. Besides the excellent catalytic ability and Fe(III) reduction ability of nZVI itself, the biochar supports with high specific surface area and mesoporous structure also played important roles in the removal of RhB dye, such as adsorbing RhB dyes, alleviating nZVI aggregation, and accelerating the reduction of Fe(III) to Fe(II).
Traditional Fenton reaction refers to the activation of H2O2 by Fe²⁺ to generate hydroxyl radicals, but Fe²⁺ is easily oxidized to Fe³⁺ and the reduction of Fe²⁺ is a difficult challenge. Recently, more and more scientists have been trying to resolve this problem by adding inorganic heterogeneous cocatalyst into the reaction and establish heterogeneous co-catalytic system. We reviewed the latest evolution of inorganic heterogeneous co-catalytic Fenton methods in wastewater treatment. The two distinct kinds of heterogeneous cocatalyst, transition metals (Single-atom, zero-valent metal, metal sulphide, and metal phosphide) and non-metal (carbon, boron, and phosphorus materials) cocatalysts are summarized, and the reaction mechanisms of each type are discussed, then the advantages and disadvantages of practical applications are assessed. Four optimization methods for the co-catalytic system are proposed, including material optimization, operational optimization, synergy with other advanced oxidation processes, and novel Fenton-like process construction. Finally, future research needs for Fenton-like systems are presented.
In this study, algae of Aegagropila linnaei (AL) was selected as low-cost raw material to synthesize biochar/iron oxide composites for the removal of bisphenol-A (BPA). Pristine biochar (BC) with olive-shaped pores and KOH-activated acid washed biochar (ABC) with novel interconnected structure were successfully prepared and further functionalized with ferrous sulfate for effective soldering of Fe3O4 nanoparticles onto the surface of acid biochar through hydrothermal method. The iron modified acid biochar composite (FABC) was found to be with the optimal physicochemical property such as smaller crystalline size (100-150 nm), larger surface area (144.62 m²/g), larger pore volume (0.259 cm ³/g), lower capacitance decrease (2.3%), and lower resistance (0.73 Ω) as compared to other modified biochar. The removal rates of BPA by BC, ABC, and FABC were 43.2 ± 0.5%, 52.6 ± 0.3%, and 69.8 ± 2.3%, respectively, when the concentration of BPA was 10 mg/L (pH 3.0, 303k). Under acidic conditions (at pH 3.0), the reaction activity and the adsorption of hydrogen ions onto Fe3O4 nanoparticles in FABC was prompted, and further increasing the removal rate of BPA. The result suggests that adsorption played main role, while active species (⋅OH and ⋅O2−) produced by the nano-iron oxide on the surface of the composites played some effect in the removal of BPA.
In this study, copper nanoparticles supported on carbonized cotton as a heterogeneous Fenton-like three-dimensional (3D) structure catalyst were constructed for catalytic degradation of polyvinyl alcohol (PVA), Rhodamine-B, and Reactive Red X-3B. Herein, cotton in natural fibers was used as a carbon source, which provides a new method for the preparation of catalysts supported by carbon materials. In [email protected] cotton nanoparticles (CCCNs), the carbon layer is produced during the calcination process, which not only enhances the conversion of Cu⁰ to Cu⁺/Cu²⁺, but also accelerates the electron transfer and promotes the catalytic decomposition of hydrogen peroxide (H2O2) to generate ·OH radicals. In a wide pH range (3–7), the CCCNs/H2O2 system could effectively degrade PVA, and the removal rates of PVA and TOC were found to be 99.5 and 98.68%, respectively. The optimized experimental conditions are as follows: pH, 7; mass ratio of Cu(OH)2CO3 to cotton, 10:4; catalyst dosage, 2.6 g L⁻¹; reaction temperature, 65 °C; H2O2 dosage, 0.16 g mL⁻¹, and without adjusting pH. In addition to the high catalytic activity, CCCNs exhibited good stability and reusability due to the strong 3D carbon layer. The CCCN composite material has been proven to be ideal catalyst to enhance the removal rate of the heterogeneous Fenton-like reaction system in removing organic pollutants present in wastewater.
The waste sulfuric acid from the alkylation process contains numerous organic pollutants and seriously threatens the ecological environment. This study focused on the effective utilization of waste sulfuric acid to produce poly aluminium sulfate and deep degradation of organic pollutants in the Al2(SO4)3 solution through the advanced oxidation process. The coconut shell-derived and H3PO4-steam activated biochar (CBC) was used as the catalyst in the advanced oxidation process. The results revealed that the H3PO4-steam activation could introduce abundant acidic functional groups and enrich metal ions on the CBC surface, which served as the active sites and promoted H2O2 to form •OH and •HO2 radicals for efficient organic pollutant degradation. The treated Al2(SO4)3 solution with the colour removal of 96.2% and total organic carbon removal of 93.8% under the optimal reaction conditions was used to prepare the certified poly aluminium sulfate product. This study is of great significance in the valorization of waste sulfuric acid and the application of recycled agricultural residues in the advanced oxidation process.
Sulfate radicals based AOPs (SR-AOPs) in practical application are suppressed by slow Fe(III)-to-Fe(II) conversion cycle, seeking an efficient strategy to enhance PMS activation for the desirable efficacy of organic pollutant (OP) degradation is thus of great interest. Herein we report a transition metal sulfide-enhanced PMS activation to fast and completely oxidize BPA with an improved Fe(III)/Fe(II) cycle via the co-catalytic effect of metal sulfides. Among these metal sulfide co-catalyzed PMS systems considered, the WS2/Fe(II)/PMS system showed the highest efficiency for BPA degradation. In this AOP, 99.4% of BPA could be degraded within 20 min, and WS2 exhibited an exceptional reusability, which could be extendable to degrade other refractory organic pollutants and treat actual leachate. Quenching tests and ESR analysis indicated that sulfate radical (·SO4⁻) and hydroxyl radical (·OH) mainly contributed to the desirable degradation of BPA. The generated singlet oxygen (¹O2) by WS2-induced PMS activation would react with Fe(II) to form ·O2⁻, which also led to BPA degradation. Combining density functional theory (DFT) calculations with intermediate products detection, elaborate degradation pathways of BPA included hydroxylation and ring-opening reaction. Protonation of WS2 c-surfaces induced sulfur vacancies, resulting in highly exposed W(IV) sites where Fe(III)-to-Fe(II) cycle was enhanced. WS2 exhibited excellent stability and reusability even after five cycles. This cocatalytic effect of WS2 can inspire other advanced Fe(II)-based PMS AOPs through surface vacancy engineering of transition metal sulfides.
Nanomaterials coated ceramic membranes have become one of the most promising methods to degrade organic pollution. In this study, low crystalline metal nanomaterials were coated on ceramic membranes for the first time. This research fabricated a ceramic membrane with low crystalline Fe2O3@carboxylic multi-walled carbon nanotubes (CMWCNTs) and evaluated its aromatic compounds-degrading ability and self-cleaning ability. The coated membrane demonstrated an excellent removal rate (＞90%) of phenol and atrazine in 120 min’s continuous flow filtration via surface photo-Fenton reaction. Quenching experiments reveal the active substances of the reaction system were singlet oxygen (¹O2) and a small amount of hydroxyl radical (·OH). With humic acid (HA) and bovine serum albumin (BSA) as model pollutants, the polluted membranes recovered 84.4% and 77.0% flux through the photo-Fenton reaction respectively, which proved the self-cleaning performance of the coated membrane. After 6 cycles, an average degradation efficiency of 92% for phenol was maintained in continuous flow filtration. These results indicated the efficacy of the photo-Fenton coupling coated membrane for treating organically polluted water and its relevance for the design and application of future photo-Fenton membrane filtration system.
The sluggish kinetics of Fe(II) recovery strongly impedes the scientific progress of Fenton reaction (Fe(II)/H2O2) towards practical application. Here, we propose a novel mechanism that metal-free nitrogen-doped carbon nanotubes (NCNT) can enhance Fenton chemistry with H2O2 as electron donors by elevating the oxidation potential of Fe(III). NCNT remarkably promotes the circulation of Fe(III)/Fe(II) to produce hydroxyl radical (•OH) with excellent stability for multiple usages (more than 10 cycles) in the NCNT/Fe(III)/H2O2 system. Although carbonyl on NCNT can act as the electron supplier for Fe(III) reduction, the behavior of NCNT is distinct from common reductants such as hydroxylamine and boron. Electrochemical analysis and density functional theory calculation unveil that nitrogen sites of NCNT can weakly bind with Fe(III) to elevate the oxidation potential of Fe(III) (named near-free Fe(III), primarily FeOH²⁺) at pH ranging from 2.0 to 4.0. Without inputs of external stimulations or electron sacrificers, near-free Fe(III) can promote H2O2 induced reduction of Fe(III) to initiate Fenton chain reactions for long-lasting generation of •OH. To our delight, it is a common property of N-doped carbon materials (e.g., graphene, carbon nanofibers, and acetylene black), our research thus provides a novel, sustainable, and green strategy for promoting Fenton chemistry.
Volatile organic compounds (VOCs) emissions are regarded as a worth concerned threat to human health. The UV-Fenton coupled with mass transfer enhanced process shows promising effects on VOCs treatment. However, the detailed mechanism and mathematical model for this method have not been established. This work focuses on the hypothesis and validation of a mathematical model for UV-Fenton removal of VOCs using activated carbon particles to enhance mass transfer efficiency. Based on the mathematical model of reaction-diffusion-mass transfer, a mathematical model is established by using a series of important parameters such as ub, Dg, Dl, Kial, Kla and hydroxyl radical lifetime. The proposed model in this study introduces the key parameter of synergistic factor, which greatly improves the consistency between the model predicted results and the experimental data (the determination coefficient R² distribution range changed from 0.71∼0.98 to 0.95∼0.98). Moreover, it can also explain reasonably the steady trend of outlet VOC concentration after 30 minutes of reaction. The mathematical model confirms that the addition of activated carbon during the UV-Fenton reaction ensures mass transfer efficiency and considerably improves (growth from 2% to 54%) the VOCs removal efficiency due to the synergy between UV-Fenton oxidation and mass transfer enhancement. Meanwhile, it provides insight into fruitful utilization of the oxidation capacity in the oxidation reaction,and achieves the purpose of predicting the efficiency of VOC removal in the Fenton process.
While biochar supported iron materials have been widely studied in advanced oxidation processes (AOPs), little is known about the effect and mechanism of goethite/biochar in sulfate radical (SO4⁻) based AOPs. Herein, a novel goethite/biochar composite was applied as peroxymonosulfate (PMS) activator for tetracycline (TC) degradation in the water. The superior catalytic efficiency of goethite/biochar was achieved through radical (OH and SO4⁻) and non-radical (¹O2) processes according to the radicals quenching experiments and electron paramagnetic resonance analysis. Carbonyl group and Fe species were the main active sites on the surface of goethite/biochar, which was demonstrated by combining Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and reaction kinetic experiments. Furthermore, nine main by-products of TC degradation were detected by liquid chromatography-mass spectrometry and the reasonable degradation pathway was proposed according to the molecular structure analysis. Overall, the goethite/biochar materials could be applied to activate PMS for TC degradation, and this study will benefit the application of iron/biochar materials in practical water treatment.
In this study, we propose a heterogeneous Fenton-like system for selective oxidation towards target compounds based on the recognition of electrostatic attraction. The catalyst Fe-OCNT was fabricated by binding Fe ions onto the surface of oxidized carbon nanotubes, and the resultant Fe-OCNT/H2O2 system could oxidize methylene blue and chrysoidine G only among six test compounds, both of which are positively charged and could be adsorbed on the negatively charged surface of the catalyst. Also, the selective oxidation system features a broad pH suitability from 4.0 - 9.0, excellent resistance to traditional free HO• quenching agents and halogen ions, and satisfactory reusability. After the exclusion of the contribution of various reactive species, e.g., free HO•, FeIV=O, ¹O2, and surface-bound O*, we suggest that HO• adsorbed on the CNT surface is responsible for such selective oxidation. This work presents a proof-of-concept design of highly selective Fenton-like system in water treatment.
A series of goethite (Gt)-graphene (rGO) composites (Gt-rGO) having different rGO contents (2%-10%) was biologically prepared under mild conditions with Acidovorax sp. BoFeN1 and exhibited comparable or even higher catalytic efficiencies upon sulfonamides degradation than most known chemically synthesized catalysts. Pseudo-first-order rate constant of sulfanilamide degradation (60 μM, 0.971 h⁻¹) in the system mediated by Gt-rGO with the optimal rGO content of 6% was 6.7, 15.4 and 168.1 folds higher than those in the control rGO/H2O2, Gt/H2O2 and H2O2 systems, respectively. Excellent synergistic catalytic effects between Gt and rGO in Gt-rGO were identified in four continuous cycles. The Gt-rGO systems exhibited more efficient •OH generation, H2O2 decomposition and Fe(II) accumulation rates than the control Gt or rGO systems. Fast Fe(III)/Fe(II) cycling was obtained in the Gt-rGO systems, which might be due to the strong Fe−C coordination and the decrease of rGO aggregation and Gt particle sizes. Additionally, Gt particles in Gt-rGO exposed more defects as active sites for H2O2 activation. High-performance liquid chromatography-mass spectrometer analysis suggested that sulfanilamide was gradually degraded through hydroxylation, C−N cleavage and benzene ring opening. The results provided a new approach for the tailored design of eco-friendly, cost-effective and efficient iron (oxyhydr)oxides-graphene catalysts for contaminants elimination.
Conventional water treatment methods are difficult to remove stubborn pollutants emerging from surface water. Advanced oxidation processes (AOPs) can achieve a higher level of mineralization of stubborn pollutants. In recent years, the Fenton process for the degradation of pollutants as one of the most efficient ways has received more and more attention. While homogeneous catalysis is easy to produce sludge and the catalyst cannot be cycled. In contrast, heterogeneous Fenton-like reaction can get over these drawbacks and be used in a wider range. However, the reduction of Fe (III) to Fe(II) by hydrogen peroxide (H2O2) is still the speed limit step when generating reactive oxygen species (ROS) in heterogeneous Fenton system, which restricts the efficiency of the catalyst to degrade pollutants. Based on previous research, this article reviews the strategies to improve the iron redox cycle in heterogeneous Fenton system catalyzed by iron materials. Including introducing semiconductor, the modification with other elements, the application of carbon materials as carriers, the introduction of metal sulfides as co-catalysts, and the direct reduction with reducing substances. In addition, we also pay special attention to the influence of the inherent properties of iron materials on accelerating the iron redox cycle. We look forward that the strategy outlined in this article can provide readers with inspiration for constructing an efficient heterogeneous Fenton system.
In this review, we systematically summarized the origins, development, basic mechanism and limits of reductant-enhanced iron-based peroxide activation processes (including H2O2, peroxymonosulfate and peroxydisulfate). Notably, the use of reductants would simultaneously accelerate the reduction of Fe³⁺ and quench the production of reactive species. This trade-off between Fe³⁺ reduction and reactive species consumption is mainly determined by two aspects, i.e. (i) intrinsic chemical properties like electron transfer pathways and reaction rates with reactive species, and (ii) experimental conditions like reagent dosage ratio, dissolved O2 and pH. Therefore, we first reviewed the characteristics of different iron/peroxide/reductant systems in terms of the above two aspects. Then, we evaluated the practical application potential of different reductants based on their toxicity, costs and self-transformation products. Finally, newly-emerging enhancement strategies like multiple dosing and regulation of phase and crystalline degree were summarized, and future research needs were proposed.
Fenton reaction with zerovalent iron (ZVI) and ions was studied to treat pharmaceutical wastewaters (PhWW) including antibiotics and non-biodegradable organics. Incremental biodegradability was assessed by monitoring biochemical oxygen demand (BOD) changes during Fenton reaction. Original undiluted wastewater samples were used as collected from the pharmaceutical factory. Experiments were carried out to obtain optimal conditions for Fenton reaction under different and ion salts (ZVI and ) concentrations. The optimal ratio and dosage of /ZVI were 5 and 25/5 g/L (mass basis), respectively. Also, the optimal ratio and dosage of ions were 5 and 35/7 g/L (mass basis), respectively. Under optimized conditions, the chemical oxygen demand (COD) removal efficiency by ZVI was 23% better than the treatment with ion. The reaction time was 45 min for ZVI and shorter than 60 min for ion. The COD and total organic carbon (TOC) were decreased, but BOD was increased under the optimal conditions of /ZVI = 25/5 g/L, because organic compounds were converted into biodegradable intermediates in the early steps of the reaction. The BOD/TOC ratio was increased, but reverse-wise, the COD/TOC was decreased because of generated intermediates. The biodegradability was increased about 9.8 times (BOD/TOC basis), after treatment with ZVI. The combination of chemical and biological processes seems an interesting combination for treating PhWW.
Kinetic data for the radicals H⋅ and ⋅OH in aqueous solution,and the corresponding radical anions, ⋅O− and eaq−, have been critically pulse radiolysis, flash photolysis and other methods. Rate constants for over 3500 reaction are tabulated, including reaction with molecules, ions and other radicals derived from inorganic and organic solutes.
Fenton chemistry encompasses reactions of hydrogen peroxide in the presence of iron to generate highly reactive species such as the hydroxyl radical and possibly others. In this review, the complex mechanisms of Fenton and Fenton-like reactions and the important factors influencing these reactions, from both a fundamental and practical perspective, in applications to water and soil treatment, are discussed. The review covers modified versions including the photoassisted Fenton reaction, use of chelated iron, electro-Fenton reactions, and Fenton reactions using heterogeneous catalysts. Sections are devoted to nonclassical pathways, by-products, kinetics and process modeling, experimental design methodology, soil and aquifer treatment, use of Fenton in combination with other advanced oxidation processes or biodegradation, economic comparison with other advanced oxidation processes, and case studies.
The catalytic properties of granular activated carbon (GAC) in GAC/iron oxide/hydrogen peroxide (H2O2) system was investigated in this research. Batch experiments were carried out in de-ionized water at the desired concentrations of ethylene glycol and phenol. Rate constants for the degradation of hydrogen peroxide and the formation rate of iron species were determined and correlated with mineralization of ethylene glycol at various GAC concentrations. The observed first order degradation rate of hydrogen peroxide in the absence of iron oxide and organic matter increases linearity with the increasing of the GAC concentration. The decomposition rate of hydrogen peroxide was suppressed significantly as the solution pH became acidic or by reducing the surface area of the GAC. The reduction of the surface area was obtained by loading an organic compound (such as phenol) on the GAC or by using the oxidizing agent (H2O2). The addition of both chemicals, phenol and H2O2, affects mainly the surface area of the small pores, resulting in reducing the catalytic activity inside the micropores.The catalytic properties of the GAC were used to accelerate the formation rate of the ferrous ions, which is known in the literature to be the limiting rate reaction in the classic Fenton like reagent. It was shown that the ethylene glycol mineralization rate was increased by more than 50%.Finally, optimization of the GAC consumption leading to the fastest mineralization of the ethylene glycol, resulting in decreasing of the decomposition rate of H2O2 while enhancing the generation rate of ferrous ions.
Identical values of the bimolecular rate constant of the ferrous ion – hydrogen peroxide reaction were obtained from intercomparisons of the methods previously used in following this reaction. In perchloric acid the bimolecular rate constant is unaffected by acid concentration; in sulphuric acid it increases slightly in acid concentrations above 10−2N. The results agree with and explain the differences between those obtained by Baxendale and by Dainton, but are only in marginal agreement with those recently reported by Weiss.
Each of the 24 chapters of this book on standard electrode potentials in aqueous solutions was prepared by knowledgeable specialized experts and reviewed by referees who are credited herein. For the sake of space, discretion was exercised as to which half-reactions would be included for a given element, data have been limited to a single temperature, 25/sup 0/C, and no attempt was made to give complete citation to all pertinent publications. Separate abstracts have been prepared for three chapters.
This study compares two different solar photo-Fenton processes, conventional photo-Fenton at pH3 and modified photo-Fenton at neutral pH with minimal Fe (5 mg L(-1)) and minimal initial H(2)O(2) (50 mg L(-1)) concentrations for the degradation of emerging contaminants in Municipal Wastewater Treatment Plants effluents in solar pilot plant. As Fe precipitates at neutral pH, complexing agents which are able to form photoactive species, do not pollute the environment or increase toxicity have to be used to keep the iron in solution. This study was done using real effluents containing over 60 different contaminants, which were monitored during treatment by liquid chromatography coupled to a hybrid quadrupole/linear ion trap mass analyzer (LC-QTRAP-MS/MS) operating in selected reaction monitoring (SRM) mode. Concentrations of the selected contaminants ranged from a few ng L(-1) to tens of μg L(-1). It was demonstrated in all cases the removal of over 95% of the contaminants. Photo-Fenton at pH3 provided the best treatment time, but has the disadvantage that the water must be previously acidified. The most promising process was photo-Fenton modified with Ethylenediamine-N,N'-disuccinic acid (EDDS), as the pH remained in the neutral range.
The oxidation of alcohols in Fe2+-H2O2 systems has been reexamined and a new kinetic analysis developed. Relative reactivities of methyl, ethyl, isopropyl, and tert-butyl alcohols are in good agreement with those from radiolysis experiments. The analysis and product studies indicate attack on both α- and β-hydrogens of ethyl and isopropyl alcohols. The β-hydroxyalkyl radicals are not oxidized by Fe3+, but dimerize, terminating kinetic chains. They are, however, oxidized by Cu2+ to yield glycols with chain propagation. Thus the oxidation of tert-butyl alcohol is converted to a long-chain process by Cu2+. We suggest that, in these systems, radical oxidation by Fe3+ is an electron-transfer process, but, with Cu2+, involves either ligand transfer or an organocopper intermediate. The Fe3+ oxidation is evidently reversible in some cases, since solvolysis of ethyl trifluoromethanesulfonate in the presence of Fe2+ yields significant amounts of Fe3+ and butane.
The reactions between ferrous ion, FeS , and the simple complexes of Fe(III), FeOHS , and FeSO4 with superoxide and perhydroxyl radicals O2 /HO2 have been studied as a function of pH (1-7) in aqueous sulfate and formate media. The measured rate constants for the various reactions are k17(HO2 + FeS ) = (1.2 +/- 0.2) x 10W M s (in good agreement with earlier reported values/sup 6d/); k18(O2 + FeS ) = (1.0 +/- 0.1) x 10X M s ; k20(O2 + FeSO4 ) approx. = k22(O2 + FeOHS ) = (1.5 +/- 0.2) x 10Y M s . The reduction of FeSO4 by HO2 is relatively slow, k19(HO2 + FeSO4 ) less than or equal to 10T M s . At neutral pH the superoxide radical is catalytically decomposed to dioxygen and hydrogen peroxide by trace amounts of Fe(II)/Fe(III), if the overall reaction is completed before Fe(III) can polymerize/precipitate. The corresponding second-order rate constant is k/sub cat/ = (1.3 +/- 0.2) x 10X M s . The results from the kinetic studies of reactions 20 and 22 corroborate the mechanism for the catalytic decomposition of hydrogen peroxide by Fe(II)/Fe(III) advanced by Barb, Baxendale, George, and Hargrave in 1951. 13 references, 2 figures, 6 tables.
This paper describes a kinetic model for the decomposition of hydrogen peroxide by ferric ion in homogeneous aqueous solution (pH < 3). The reaction was investigated experimentally at 25.0 °C and I = 0.1 M (HClO4/NaClO4), in a completely mixed batch reactor and under a wide range of experimental conditions (1 ≤ pH ≤ 3; 0.2 mM ≤ [H2O2]0 ≤ 1 M; 50 μM ≤ [Fe(III)]0 ≤ 1 mM; 1 ≤ [H2O2]0/[Fe(III)]0 ≤ 5000). The results of this study demonstrated that the rate of decomposition of hydrogen peroxide by Fe(III) could be predicted very accurately by a kinetic model which takes into account the rapid formation and the slower decomposition of Fe(III)−hydroperoxy complexes (FeIII(HO2)2+ and FeIII(OH)(HO2)+). The rate constant for the unimolecular decomposition of the Fe(III)−hydroperoxy complexes was determined to be 2.7 × 10-3 s-1. The use of the kinetic model allows a better understanding of the effects of operational parameters (i.e., pH and [H2O2]0/[Fe(III)]0) on the complex kinetics of decomposition of H2O2 by Fe(III).
The advances in acid catalysis by heteropolyacids are examined and data on the acidity of heteropolyacids are surveyed. The characteristics of homogeneous and heterogeneous acid catalysis by heteropolyacids are discussed and its applied aspects are noted.
The bibliography includes 116 references.
A reaction mechanism has been validated for the photochemical generation of hydroxyl radicals by ultraviolet or visible irradiation of oxalato iron(III) complexes (ferrioxalate) in the presence of hydrogen peroxide and 2-propanol as a model substrate. A kinetic simulation program incorporating the set of reactions was written to predict the behavior of this photochemical system. The program calculates the quantum yield of oxidation of 2-propanol used as a hydroxyl radical scavenger. The scavenger's oxidation product, 2-propanone, was analyzed by gas chromatog-raphy after controlled exposure of the solution to a calibrated light source. The theoretical quantum yields of 2-propanol oxidation, ΦRH , agreed reasonably well with experimentally determined ΦRH values under a variety of initial reaction conditions. The value of ΦRH , which under appropriate conditions is directly proportional to the quantum yield for the generation of hydroxyl radicals, Φ OH , was considerably greater than unity in most cases (often ΦRH = 3.0-4.0), indicative of a chain mechanism involving iron cycling between the II and III oxidation states.
The catalytic behavior of the Fe3+/Fe2+ system in the electro-Fenton degradation of the antimicrobial drug chlorophene has been studied considering four undivided electrolytic cells, where a Pt or boron-doped diamond (BDD) anode and a carbon felt or O2-diffusion cathode have been used. Chlorophene electrolyses have been carried out at pH 3.0 under current control, with 0.05 M Na2SO4 as supporting electrolyte and Fe3+ as catalyst. In these processes the drug is oxidized with hydroxyl radical (OH) formed both at the anode from water oxidation and in the medium from electrochemically generated Fenton's reagent (Fe2+ + H2O2, both of them generated at the cathode). The catalytic behavior of the Fe3+/Fe2+ system mainly depends on the cathode tested. In the cells with an O2-diffusion cathode, H2O2 is largely accumulated and the Fe3+ content remains practically unchanged. Under these conditions, the chlorophene decay is enhanced by increasing the initial Fe3+ concentration, because this leads to a higher quantity of Fe2+ regenerated at the cathode and, subsequently, to a greater OH production from Fenton's reaction. In contrast, when the carbon felt cathode is used, H2O2 is electrogenerated in small extent, whereas Fe2+ is largely accumulated because the regeneration of this ion from Fe3+ reduction at the cathode is much faster than its oxidation to Fe3+ at the anode. In this case, an Fe3+ concentration as low as 0.2 mM is required to obtain the maximum OH generation rate, yielding the quickest chlorophene removal. Chlorophene is poorly mineralized in the Pt/O2 diffusion cell because the final Fe3+–oxalate complexes are difficult to oxidize with OH. These complexes are completely destroyed using a BDD anode at high current thanks to the great amount of OH generated on its surface. Total mineralization is also achieved in the Pt/carbon felt and BDD/carbon felt cells with 0.2 mM Fe3+, because oxalic acid and its Fe2+ complexes are directly oxidized with OH in the medium. Comparing the four cells, the highest oxidizing power regarding total mineralization is attained for the BDD/carbon felt cell at high current due to the simultaneous destruction of oxalic acid at the BDD surface and in the bulk solution.
A study was made to establish proper conditions for the selective determination of Fe(II) by the 1,10-phenanthroline method in the presence of large amounts of Fe(III). It was shown that fe(III) is effectively masked by fluoride. The pH of the solution to be masked should be below 2·5 in order to prevent acceleration by the fluoride of aerial oxidation of Fe(II).ZusammenfassungGeeignete Bedingungen zur selektiven Bestimmung von Fe(II) mit dem 1,10-Phenanthrolin-Verfahren in Gegenwart großer Mengen Fe(III) wurden ermittelt. Es wurde gezeigt, daß Eisen(III) durch Fluorid wirksam maskiert wird. Der pH-wert der Lösung, in der maskiert werden soll, sollte unter 2,5 liegen, damit nicht die Luftoxidation von Fe(II) durch Fluorid beschleunigt wird.RésuméOn a effectué une étude pour établir les conditions convenables pour le dosage sélectif de Fe(II) par la méthode à la 1,10-phénanthroline en la présence de grandes quantités de Fe(III). On a montré que Fe(III) est efficacement dissimulé par le fluorure. Le pH de la solution à dissimuler doit être au-dessous de 2,5 afin d'éviter l'accélération par le fluorure de l'oxydation de Fe(II) par l'air.
In the literature, numerous discrepancies occur in rate constant data for reactions of the hydroxyl radical with inorganic and organic compounds. Some of these have been redetermined, using a pulse radiolysis competition technique in which the ferrocyanide ion is used as a reference solute. The validity of the method has been established by comparison of the data with rate constants determined by direct measurement using pulse radiolysis. The results have been used to correct literature data obtained by other indirect competition methods using thymine, paranitrosodimethylaniline and thiocyanate ion as reference solutes. Satisfactory self-consistency has been achieved.
This study focuses on the removal of 32 selected micropollutants (pharmaceuticals, corrosion inhibitors and biocides/pesticides) found in an effluent coming from a municipal wastewater treatment plant (MWTP) based on activated sludge. Dissolved organic matter was present, with an initial total organic carbon of 15.9 mg L(-1), and a real global quantity of micropollutants of 29.5 μg L(-1). The treatments tested on the micropollutants removal were: UV-light emitting at 254 nm (UV(254)) alone, dark Fenton (Fe(2+,3+)/H(2)O(2)) and photo-Fenton (Fe(2+,3+)/H(2)O(2)/light). Different irradiation sources were used for the photo-Fenton experiences: UV(254) and simulated sunlight. Iron and H(2)O(2) concentrations were also changed in photo-Fenton experiences in order to evaluate its influence on the degradation. All the experiments were developed at natural pH, near neutral. Photo-Fenton treatments employing UV(254), 50 mg L(-1) of H(2)O(2), with and without adding iron (5 mg L(-1) of Fe(2+) added or 1.48 mg L(-1) of total iron already present) gave the best results. Global percentages of micropollutants removal achieved were 98 and a 97% respectively, after 30 min of treatments. As the H(2)O(2) concentration increased (10, 25 and 50 mg L(-1)), best degradations were observed. UV(254), Fenton, and photo-Fenton under simulated sunlight gave less promising results with lower percentages of removal. The highlight of this paper is to point out the possibility of the micropollutants degradation in spite the presence of DOM in much higher concentrations.
Ferric ion (Fe[III]) catalyzes the decomposition of hydrogen peroxide (H(2)O(2)) into strong oxidants such as hydroxyl radical ((•)OH) and ferryl ion (Fe[IV]) through the redox cycling of the iron couple (Fe[II]/Fe[III]). The use of these reactions for the catalytic oxidation of organic compounds is usually limited to the acidic pH region due to the low solubility of Fe(III) and the low efficiency of oxidant production at neutral pH values. The addition of phosphotungstate (PW(12)O(40) (3-)), a polyoxometalate extends the working pH range of the Fe(III)/H(2)O(2) system up to pH 8.5. PW(12)O(40) (3-) forms a soluble complex with iron that converts H(2)O(2) into oxidants. The coordination of Fe(II) by PW(12)O(40) (3-) also alters the mechanism of the reaction of Fe(II) with H(2)O(2) at neutral pH, resulting in formation of an oxidant capable of oxidizing aromatic compounds. The base-catalyzed hydrolysis of PW(12)O(40) (3-) gradually results in inactivation of the catalyst. In the absence of Fe(III), PW(12)O(40) (3-) was completely hydrolyzed after 1 day at pH 7.5, whereas the Fe(III)-PW(12)O(40) (3-) complex was active for at least four days under the same conditions.
Iron oxides catalyze the conversion of hydrogen peroxide (H(2)O(2)) into oxidants capable of transforming recalcitrant contaminants. Unfortunately, the process is relatively inefficient at circumneutral pH values because of competing reactions that decompose H(2)O(2) without producing oxidants. Silica- and alumina-containing iron oxides prepared by sol-gel processing of aqueous solutions containing Fe(ClO(4))(3), AlCl(3), and tetraethyl orthosilicate efficiently catalyzed the decomposition of H(2)O(2) into oxidants capable of transforming phenol at circumneutral pH values. Relative to hematite, goethite, and amorphous FeOOH, the silica-iron oxide catalyst exhibited a stoichiometric efficiency, defined as the number of moles of phenol transformed per mole of H(2)O(2) consumed, which was 10-40 times higher than that of the iron oxides. The silica-alumina-iron oxide catalyst had a stoichiometric efficiency that was 50-80 times higher than that of the iron oxides. The significant enhancement in oxidant production is attributable to the interaction of Fe with Al and Si in the mixed oxides, which alters the surface redox processes, favoring the production of strong oxidants during H(2)O(2) decomposition.
The degrdn. of herbicides 4-chlorophenoxyacetic acid (4-CPA), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aq. medium of pH 3.0 was comparatively studied by anodic oxidn. and electro-Fenton using a B-doped diamond (BDD) anode. All solns. are totally mineralized by electro-Fenton, even at low current, being the process more efficient with 1 mM Fe2+ as catalyst. This is due to the prodn. of large amts. of oxidant hydroxyl radical (OH.bul.) on the BDD surface by H2O oxidn. and from Fenton's reaction between added Fe2+ and H2O2 electrogenerated at the O2-diffusion cathode. The herbicide solns. are also completely depolluted by anodic oxidn. Although a quicker degrdn. is found at the 1st stages of electro-Fenton, similar times are required for achieving overall mineralization in both methods. The decay kinetics of all herbicides always follows a pseudo 1st-order reaction. Reversed-phase chromatog. allows detecting 4-chlorophenol, 4-chloro-o-cresol, 2,4-dichlorophenol and 2,4,5-trichlorophenol as primary arom. intermediates of 4-CPA, MCPA, 2,4-D and 2,4,5-T, resp. Dechlorination of these products gives Cl-, which is slowly oxidized on BDD. Ion-exclusion chromatog. reveals persistent oxalic acid in electro-Fenton by formation of Fe3+-oxalato complexes, which are slowly destroyed by OH.bul. adsorbed on BDD. In anodic oxidn., oxalic acid is mineralized practically at the same rate as generated. [on SciFinder (R)]
In the presence of oxygen, organic compounds can be oxidized by zerovalent iron or dissolved Fe(II). However, this process is not a very effective means of degrading contaminants because the yields of oxidants are usually low (i.e., typically less than 5% of the iron added is converted into oxidants capable of transforming organic compounds). The addition of polyoxometalate (POM) greatly increases the yield of oxidants in both systems. The mechanism of POM enhancement depends on the solution pH. Under acidic conditions, POM mediates the electron transfer from nanoparticulate zerovalent iron (nZVI) or Fe(II) to oxygen, increasing the production of hydrogen peroxide, which is subsequently converted to hydroxyl radical through the Fenton reaction. At neutral pH values, iron forms a complex with POM, preventing iron precipitation on the nZVI surface and in bulk solution. At pH 7, the yield of oxidant approaches the theoretical maximum in the nZVI/O2 and the Fe(II)/O2 systems when POM is present, suggesting that coordination of iron by POM alters the mechanism of the Fenton reaction by converting the active oxidant from ferryl ion to hydroxyl radical. Comparable enhancements in oxidant yields are also observed when nZVI or Fe(II) is exposed to oxygen in the presence of silica-immobilized POM.
Nanotube/nanoparticle hybrid structures are prepared by forming Au and Pt nanoparticles on the sidewalls of single-walled carbon nanotubes. Reducing agent or catalyst-free electroless deposition, which purely utilizes the redox potential difference between Au3+, Pt2+, and the carbon nanotube, is the main driving force for this reaction. It is also shown that carbon nanotubes act as a template for wire-like metal structures. The successful formation of the hybrid structures is monitored by atomic force microscopy (AFM) and electrical measurements.
Activated carbon (AC) has a long history of applications in environmental technology as an adsorbent of pollutants for the purification of drinking waters and wastewaters. Here we describe novel role of AC as redox mediator in accelerating the reductive transformation of pollutants as well as a terminal electron acceptor in the biological oxidation of an organic substrate. This study explores the use of AC as an immobilized redox mediator for the reduction of a recalcitrant azo dye (hydrolyzed Reactive Red 2) in laboratory-scale anaerobic bioreactors, using volatile fatty acids as electron donor. The incorporation of AC in the sludge bed greatly improved dye removal and formation of aniline, a dye reduction product. These results indicate that AC acts as a redox mediator. In supporting batch experiments, bacteria were shown to oxidize acetate at the expense of reducing AC. Furthermore, AC greatly accelerated the chemical reduction of an azo dye by sulfide. The results taken as a whole clearly suggest that AC accepts electrons from the microbial oxidation of organic acids and transfers the electrons to azo dyes, accelerating their reduction. A possible role of quinone surface groups in the catalysis is discussed.
Photocatalytic reduction of mercury in aqueous solutions using PW12O40(3-) or SiW12O40(4-) as photocatalysts has been studied as a function of irradiation time, concentration of Hg(II), polyoxometalate, and organic substrate in the presence or absence of dioxygen. The photocatalytic cycle starts with irradiation of polyoxometalate, goes through the oxidation of, for instance, propan-2-ol (used as sacrificial reagent), and closes with the reoxidation of reduced polyoxometalate by Hg2+ ions. Mercury(II) is reduced to mercury(I) and finally to Hg(0) giving a dark-gray deposit, following a staged one-by-one electron process and a first-order kinetics in [Hg2+]. The process is slightly more efficient in the absence of dioxygen, while the increase of either catalyst or propan-2-ol concentration results in the augmentation of the rate of reduction till a certain point where it reaches a plateau. The results show that this method is suitable for a great range of mercury concentration from 20 to 800 ppm achieving almost complete recovery of mercury up to nondetected traces (<50 ppb). In addition, this homogeneous process demonstrates advantages such as the lack of necessity for separation of the zero state metal from the catalyst and ensures that the precipitation of metal will not poison the catalyst or hinder its photocatalytic activity.
Aniline degradation at pH 2 by Fenton and electro-Fenton processes was kinetically investigated in this study. Electro-Fenton process was found to be superior to ordinary Fenton process with the current impacts of 1.2 to 3.1 for removal efficiency and 1.2 to 5.8 for degradation rate depending on initial Fe2+ concentration. This is mainly due to the rapid electrochemical regeneration of Fe2+. Overall rate equations for aniline degradation by Fenton and electro-Fenton processes (in units of molar and minute) are: [EQUATION: SEE TEXT]. With current application, aniline degradation rate seems to be autonomous from Fenton's reagent concentrations and approaching a half order with respect to aniline. In addition, for complete removal of 0.01 M aniline, the delay in current supply at the initial stage could save up to one-third of the total energy required by the ordinary electro-Fenton process. As a result, significant reduction in energy consumption and operating cost could be obtained by the current-delay operating mode.
The objective of this work was to study the abatement of 200mgL(-1) sulfamethoxazole (SMX) solution by means of photo-Fenton process. Biodegradability of the treated solutions was followed by the ratio biochemical oxygen demand at five days/chemical oxygen demand (BOD(5)/COD) and toxicity by Microtox and inhibition tests. Experiments with different initial concentration of H(2)O(2) were carried out. The initial amount of Fe(2+) and pH of the solution were set at 10mgL(-1) and 2.8 respectively. The temperature of the reactor was kept constant in all the experiments (25+/-0.8 degrees C). Photo-Fenton process is thought to be a successful treatment step to improve the biodegradability of wastewater containing SMX. The complete antibiotic removal was achieved for a H(2)O(2) dose over 300mgL(-1). Biodegradability (BOD(5)/COD) rose from zero (SMX solution) to values higher than 0.3 (treated solutions). Toxicity and inhibition tests pointed out in the same direction: oxidized intermediates for initial H(2)O(2) dose over 300mgL(-1) showed no toxicity effects on pure bacteria and no inhibition on activated sludge activity.
The corrosion of zero-valent iron (Fe0(s)) by oxygen (O2) can lead to the oxidation of organic compounds. To gain insight into the reaction mechanism and to assess the nature of the oxidant, the oxidation of methanol, ethanol, 2-propanol, and benzoic acid by the reaction of nanoparticulate zero-valent iron (nZVI) or ferrous iron (Fe[II]) with O2 in the absence of ligands was studied. At pH values below 5, Fe0(s) nanoparticles were oxidized by O2 within 30 min with a stoichiometry of approximately two Fe0(s) oxidized per O2 consumed. The yield of methanol and ethanol oxidation products increased from 1% at acidic pH to 6% at pH 7, relative to nZVI added. Product yields from 2-propanol and benzoic acid were highest under acidic conditions, with little oxidation observed at neutral pH. At pH values below 5, product formation was attributable to hydroxyl radical (OH.) production through the Fenton reaction, involving hydrogen peroxide and Fe(II) produced during nZVI oxidation. At higher pH values, the oxidation of Fe(II), the initial product of nZVI oxidation, by oxygen is responsible for most of the oxidant production. Product yields at circumneutral pH values were consistent with a different oxidant, such as the ferryl ion (Fe[IV]).