Revisiting the Oxidizing Capacity of the Periodate-H2O2 Mixture: Identification of the Primary Oxidants and Their Formation Mechanisms.

This study reexamined the mechanisms for oxidative organic degradation by the binary mixture of periodate and H2O2 (PI/H2O2) that was recently identified as a new advanced oxidation process. Our findings conflicted with the previous claims that (i) hydroxyl radical (•OH) and singlet oxygen (1O2) contributed as the primary oxidants, and (ii) •OH production resulted from H2O2 reduction by superoxide radical anion (O2•-). PI/H2O2 exhibited substantial oxidizing capacity at pH < 5, decomposing organics predominantly by •OH. The likelihood of a switch in the major oxidant under varying pH conditions was revealed. IO4- as the major PI form under acidic conditions underwent one-electron reduction by H2O2 to yield radical intermediates, whereas H2I2O104- preferentially occurring at pH > 7 caused 1O2 generation through two-electron oxidation of H2O2. PI reduction by O2•- was suggested to be a key reaction in •OH production, on the basis of the electron paramagnetic resonance detection of methyl radicals in the dimethyl sulfoxide solutions containing PI and KO2, and the absence of deuterated and 18O-labeled hydroxylated intermediates during PI activation using D2O and H218O2. Finally, simple oxyanion mixing subsequent to electrochemical PI and H2O2 production achieved organic oxidation, enabling a potential strategy to minimize the use of chemicals.

Chloride-Mediated Enhancement in Heat-Induced Activation of Peroxymonosulfate: New Reaction Pathways for Oxidizing Radical Production.

This study is the first to demonstrate the capability of Cl- to markedly accelerate organic oxidation using thermally activated peroxymonosulfate (PMS) under acidic conditions. The treatment efficiency gain allowed heat-activated PMS to surpass heat-activated peroxydisulfate (PDS). During thermal PMS activation at excess Cl-, accelerated oxidation of 4-chlorophenol (susceptible to oxidation by hypochlorous acid (HOCl)) was observed along with significant degradation of benzoic acid and ClO3- occurrence, which involved oxidants with low substrate specificity. This indicated that heat facilitated HOCl formation via nucleophilic Cl- addition to PMS and enabled free chlorine conversion into less selective oxidizing radicals. HOCl acted as a key intermediate in the major oxidant transition based on temperature-dependent variation in HOCl concentration profiles, kinetically retarded organic oxidation upon NH4+ addition, and enabled rapid organic oxidation in heated PMS/HOCl mixtures. Chlorine atom that formed via the one-electron oxidation of Cl- by the sulfate radical served as the primary oxidant and was involved in hydroxyl radical production. This was corroborated by the quenching effects of alcohols and bicarbonates, reactivity toward multiple organics, and electron paramagnetic resonance spectral features. PMS outperformed PDS in degrading benzoic acid during thermal activation operated in reverse osmosis concentrate, which was in conflict with the well-established superiority of heat-activated PDS.

Electrochemical Oxidation-Membrane Distillation Hybrid Process: Utilizing Electric Resistance Heating for Distillation and Membrane Defouling through Thermal Activation of Anodically Formed Persulfate.

This study reports distillation-based salt removal by Ohmic heating in a hybrid process, in which electrochemical oxidation (EO) and direct contact membrane distillation (DCMD) are performed sequentially. In addition to anodically destructing the organics, the hybrid process also separated the sulfate-based electrolytes from treated water through distillation, without consuming external energy, owing to the temperature of the aqueous sulfate solution being elevated to 70 °C via resistive heating. The hybrid process treated organic compounds in a nonselective fashion, whereas DCMD alone did not completely reject (semi)volatile organics. Integrating EO with DCMD made the hybrid process resistant toward the wetting phenomenon; the process exhibited a steady distillate flux and salt rejection as the initial loading of amphiphilic sodium dodecyl sulfate was increased to 0.3 mM. Anodic persulfate formation from the sulfate and Ohmic heating caused an in situ yield of the sulfate radical in the feed solution; this eliminated membrane fouling, according to the observation that the water flux, which was drastically reduced upon adding alginate, was recovered immediately after an electric current was applied. The hybrid process concurrently decomposed spiked organics and removed naturally present inorganic ions in actual flue gas desulfurization wastewater, without an external supply of electrolyte and heat energy.

Activation of Hydrogen Peroxide by a Titanium Oxide-Supported Iron Catalyst: Evidence for Surface Fe(IV) and Its Selectivity.

Iron immobilized on supports such as silica, alumina, titanium oxide, and zeolite can activate hydrogen peroxide (H2O2) into strong oxidants. However, the role of the support and the nature of the oxidants produced in this process remain elusive. This study investigated the activation of H2O2 by a TiO2-supported catalyst (FeTi-ox). Characterizing the catalyst surface in situ using X-ray absorption spectroscopy (XAS), together with X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR), revealed that the interaction between H2O2 and the TiO2 phase played a key role in the H2O2 activation. This interaction generated a stable peroxo-titania ≡Fe(III)-Ti-OOH complex, which reacted further with H2O to produce a surface oxidant, likely ≡Fe[IV] ═ O2+. The oxidant effectively degraded acetaminophen, even in the presence of chloride, bicarbonate, and organic matter. Unexpectedly, contaminant oxidation continued after the H2O2 in the solution was depleted, owing to the decomposition of ≡Fe(III)-Ti-OOH by water. In addition, the FeTi-ox catalyst effectively degraded acetaminophen over five testing cycles. Overall, new insights gained in this study may provide a basis for designing more effective catalysts for H2O2 activation.

Differential Microbicidal Effects of Bimetallic Iron-Copper Nanoparticles on Escherichia coli and MS2 Coliphage.

Bimetallic iron-copper nanoparticles (Fe/Cu-NPs) were synthesized by a single-pot surfactant-free method in aqueous solution [via the reduction of ferrous ion to zerovalent iron nanoparticles (Fe-NPs) and the subsequent copper-coating by metal ion exchange]. The produced Fe/Cu-NPs formed aggregates of spherical nanoparticles (approximately 30-70 nm) of Fe-Cu core-shell structures with 11 wt % copper content. The microbicidal effects of Fe/Cu-NPs were explored on Escherichia coli and MS2 coliphage, surrogates for bacterial and viral pathogens, respectively. Fe/Cu-NPs exhibited synergistically enhanced activity for the inactivation of E. coli and MS2, compared to single-metal nanoparticles (i.e., Fe-NPs and Cu-NPs). Various experiments (microbial inactivation tests under different conditions, fluorescence staining assays, experiments using ELISA and qRT-PCR, etc.) suggested that Fe/Cu-NPs inactivate E. coli and MS2 via dual microbicidal mechanisms. Two biocidal copper species [Cu(I) and Cu(III)] can be generated by different redox reactions of Fe/Cu-NPs. It is suggested that E. coli is strongly influenced by the cytotoxicity of Cu(I), while MS2 is inactivated mainly due to the oxidative damages of protein capsid and RNA by Cu(III).

Synchronized methylene blue removal using Fenton-like reaction induced by phosphorous oxoanion and submerged plasma irradiation process.

In this study, a combination of phosphorus (PP) oxoanions in a submerged plasma irradiation (SPI) system was used to enhance the removal efficiency of dyes from wastewater. The SPI system showed synergistic methylene blue removal efficiency, due to the plasma irradiation and Fenton-like oxidation. The ferrous ions released from the iron electrode in the SPI system under plasmonic conditions form complexes with the PP anions, which can then react with dissolved oxygen (O2) or hydrogen peroxide (H2O2) via Fenton-like reactions. The experimental results revealed that a sodium triphosphate (TPP) combined SPI system has a higher dye removal efficiency than a tetrasodium pyrophosphate (DP) or a sodium hexametaphosphate (HMP) combined SPI system under similar dissolved iron ion concentrations. To confirm the accuracy of the proposed removal mechanism via Fenton-like oxidation, it was compared to SPI systems under an oxygen environment (TPP/SPI/O2 (k = 0.0182 s-1)) and a nitrogen environment (TPP/SPI/N2 (k = 0.0062 s-1)). The results indicate that the hydroxyl radical (OH) in the TPP/SPI/O2 system is the major oxidant in methylene blue removal, because the dye degradation rates dramatically decreased with the addition of radical scavengers such as tert-butanol (k = 0.0023 s-1) and methanol (k = 0.0021 s-1). On the other hand, no change was observed in the methylene blue removal efficiency of the TPP/SPI/O2 system when it was subjected to a wide range of pHs (3-9). In addition, it was proved that this system could be used to eliminate six different commercial dyes. The results of this study indicated that the TPP/SPI/O2 system is a promising advanced oxidation approach for dye wastewater treatment.

Activation of Oxygen and Hydrogen Peroxide by Copper(II) Coupled with Hydroxylamine for Oxidation of Organic Contaminants.

This study reports that the combination of Cu(II) with hydroxylamine (HA) (referred to herein as Cu(II)/HA system) in situ generates H2O2 by reducing dissolved oxygen, subsequently producing reactive oxidants through the reaction of Cu(I) with H2O2. The external supply of H2O2 to the Cu(II)/HA system (i.e., the Cu(II)/H2O2/HA system) was found to further enhance the production of reactive oxidants. Both the Cu(II)/HA and Cu(II)/H2O2/HA systems effectively oxidized benzoate (BA) at pH between 4 and 8, yielding a hydroxylated product, p-hydroxybenzoate (pHBA). The addition of a radical scavenger, tert-butyl alcohol, inhibited the BA oxidation in both systems. However, electron paramagnetic resonance (EPR) spectroscopy analysis indicated that (•)OH was not produced under either acidic or neutral pH conditions, suggesting that the alternative oxidant, cupryl ion (Cu(III)), is likely a dominant oxidant.

Activation of Persulfates by Graphitized Nanodiamonds for Removal of Organic Compounds.

This study introduces graphited nanodiamond (G-ND) as an environmentally friendly, easy-to-regenerate, and cost-effective alternative catalyst to activate persulfate (i.e., peroxymonosulfate (PMS) and peroxydisulfate (PDS)) and oxidize organic compounds in water. The G-ND was found to be superior for persulfate activation to other benchmark carbon materials such as graphite, graphene, fullerene, and carbon nanotubes. The G-ND/persulfate showed selective reactivity toward phenolic compounds and some pharmaceuticals, and the degradation kinetics were not inhibited by the presence of oxidant scavengers and natural organic matter. These results indicate that radical intermediates such as sulfate radical anion and hydroxyl radical are not majorly responsible for this persulfate-driven oxidation of organic compounds. The findings from linear sweep voltammetry, thermogravimetric analysis, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy analyses suggest that the both persulfate and phenol effectively bind to G-ND surface and are likely to form charge transfer complex, in which G-ND plays a critical role in mediating facile electron transfer from phenol to persulfate.

Enhanced Inactivation of Escherichia coli and MS2 Coliphage by Cupric Ion in the Presence of Hydroxylamine: Dual Microbicidal Effects.

The inactivation of Escherichia coli and MS2 coliphage by Cu(II) is found to be significantly enhanced in the presence of hydroxylamine (HA). The addition of a small amount of HA (i.e., 5-20 µM) increased the inactivation efficacies of E. coli and MS2 coliphage by 5- to 100-fold, depending on the conditions. Dual effects were anticipated to enhance the biocidal activity of Cu(II) by the addition of HA, viz. (i) the accelerated reduction of Cu(II) into Cu(I) (a stronger biocide) and (ii) the production of reactive oxidants from the reaction of Cu(I) with dissolved oxygen (evidenced by the oxidative transformation of methanol into formaldehyde). Deaeration enhanced the inactivation of E. coli but slightly decreased the inactivation efficacy of MS2 coliphage. The addition of 10 µM hydrogen peroxide (H2O2) greatly enhanced the MS2 inactivation, whereas the same concentration of H2O2 did not significantly affect the inactivation efficacy of E. coli Observations collectively indicate that different biocidal actions lead to the inactivation of E. coli and MS2 coliphage. The toxicity of Cu(I) is dominantly responsible for the E. coli inactivation. However, for the MS2 coliphage inactivation, the oxidative damage induced by reactive oxidants is as important as the effect of Cu(I).

Oxidizing capacity of periodate activated with iron-based bimetallic nanoparticles.

Nanosized zerovalent iron (nFe0) loaded with a secondary metal such as Ni or Cu on its surface was demonstrated to effectively activate periodate (IO4-) and degrade selected organic compounds at neutral pH. The degradation was accompanied by a stoichiometric conversion of IO4- to iodate (IO3-). nFe0 without bimetallic loading led to similar IO4- reduction but no organic degradation, suggesting the production of reactive iodine intermediate only when IO4- is activated by bimetallic nFe0 (e.g., nFe0-Ni and nFe0-Cu). The organic degradation kinetics in the nFe0-Ni(or Cu)/IO4- system was substrate dependent: 4-chlorophenol, phenol, and bisphenol A were effectively degraded, whereas little or no degradation was observed with benzoic acid, carbamazepine, and 2,4,6-trichlorophenol. The substrate specificity, further confirmed by little kinetic inhibition with background organic matter, implies the selective nature of oxidant in the nFe0-Ni(or Cu)/IO4- system. The comparison with the photoactivated IO4- system, in which iodyl radical (IO3•) is a predominant oxidant in the presence of methanol, suggests IO3• also as primary oxidant in the nFe0-Ni(or Cu)/IO4- system.

Microbial inactivation by cupric ion in combination with H2O2: role of reactive oxidants.

The cupric ion mediated inactivation of Escherichia coli was enhanced by the presence of hydrogen peroxide (H2O2), with increasing inactivation efficacy observed in response to increasing concentrations of H2O2. The biocidal activity of the Cu(II)/H2O2 system is believed to result from the oxidative stress caused by reactive oxidants such as the hydroxyl radical ((•)OH), cupryl species (Cu(III)), and the superoxide radical (O2(•-)), which are produced via the catalytic decomposition of H2O2. In E. coli cells treated with Cu(II) and H2O2, the intracellular level of (•)OH and Cu(III) increased significantly, leading to complete disruption of cell membranes. On the basis of experimental observations made using an (•)OH scavenger, copper-chelating agents, and superoxide dismutase, it is concluded that Cu(III) is the predominant species responsible for the death of E. coli cells. It was also found that the production of Cu(III) was promoted by the reactions of copper with intracellular O2(•-). MS2 coliphage was found to be even more susceptible than E. coli to the oxidative stress induced by the Cu(II)/H2O2 system.

Hand-ground fullerene-nanodiamond composite for photosensitized water treatment and photodynamic cancer therapy.

The unique capability of fullerene (C60) to absorb light and generate reactive oxygen species (ROS) has been extensively studied for photosensitized water treatment and cancer therapy. Various material synthesis strategies have been proposed in parallel to overcome its intrinsic hydrophobicity and to enhance availability in water and physiological media. We present here a strikingly simple approach to make C60 available to these applications by hand-grinding dry C60 powder with nanodiamond (ND) using a mortar and pestle. The resulting ND-C60 composite was found to form a stable aqueous colloidal suspension and efficiently drive photosensitized production of ROS under visible light illumination. ND-C60 rapidly adsorbed and oxidized organic contaminants by photogenerated ROS. In the experiments for photodynamic cancer therapy, ND-C60 was internalized by cancer cells and induced cell apoptosis without noticeable toxicity. Treatment of tumor-bearing mice with ND-C60 and light irradiation resulted in tumor shrinkage and prolonged survival time.

Accelerated oxidation of microcystin-LR by Fe(II)-tetrapolyphosphate/oxygen in the presence of magnesium and calcium ions.

Fe(II)-tetrapolyphosphate complexes are known to activate molecular oxygen (Fe(II)-TPP/O2) to produce reactive oxidants (most likely, Fe(IV)-TPP complexes) that are capable of degrading refractory organic contaminants in water. This study found that magnesium and calcium ions (Mg2+ and Ca2+) accelerate the degradation of micfrocystin-LR (MC-LR), the most toxic and abundant cyanotoxin, by the Fe(II)-TPP/O2 system. The addition of Mg2+ and Ca2+ increased the observed rate constant of MC-LR degradation by up to 4.3 and 14.8 folds, respectively. Mg2+ and Ca2+ accelerated the MC-LR degradation in the entire pH range, except for the alkaline region with pH > ca. 10. The addition of Mg2+ and Ca2+ also reshaped the pH-dependency of the MC-LR degradation, greatly increasing the rate of MC-LR degradation at neutral pH. It was found that Mg2+ and Ca2+ accelerate the reaction of Fe(II)-TPP complexes with oxygen, resulting in faster production of reactive oxidants. The findings from cyclic voltammetry and potentiometric titration suggest that Mg2+ and Ca2+ form ternary complexes with Fe(II)-TPP, which exhibit higher reactivity with oxygen. Due to the effects of Mg2+ and Ca2+, the rate of MC-LR degradation by the Fe(II)-TPP/O2 system was even higher in natural water than in deionized water.

Nickel-Nickel oxide nanocomposite as a magnetically separable persulfate activator for the nonradical oxidation of organic contaminants.

The nanocomposite of metallic nickel and nickel oxide (denoted as Ni-NiO), synthesized by a simple sol-gel method, was found to activate peroxydisulfate (PDS), resulting in the effective oxidation of phenolic compounds and selected pharmaceuticals. A nonradical mechanism was proposed to explain the activation of PDS by Ni-NiO, in which organic contaminants are believed to be oxidized through an electron abstraction pathway mediated by the reactive complexes formed between PDS and the Ni-NiO surface. This mechanism was supported by multiple lines of evidence including radical scavenger experiments, the oxidation products, linear sweep voltammetry, and electron paramagnetic resonance spectroscopy. The Ni-NiO/PDS system exhibited a PDS utilization efficiency (expressed by the ratio of degraded organic contaminant to decomposed PDS) that was over 80%, and Ni-NiO showed a greater activity for PDS activation than a commercial nanoparticulate nickel oxide. This improved performance of Ni‒NiO was attributed to the disproportioned incorporation of the metallic Ni into the NiO matrix, creating more sites with oxygen vacancy. Also owing to the metallic Ni, Ni-NiO possessed magnetic properties and therefore could be easily separated and reused.

Chloride-enhanced oxidation of organic contaminants by Cu(II)-catalyzed Fenton-like reaction at neutral pH.

The Cu(II)-catalyzed Fenton-like reaction was found to be significantly accelerated in the presence of chloride ion (i.e., the Cu(II)/H2O2/Cl- system), enhancing the oxidative degradation of organic contaminants at neutral pH. The degradation of carbamazepine (a select target contaminant) by the Cu(II)/H2O2 system using 1µM Cu(II) and 10mM H2O2 was accelerated by 28-fold in the presence of 10,000mg/L Cl- at pH 7. The observed rate of carbamazepine degradation generally increased with increasing doses of Cu(II), H2O2, and Cl-, and exhibited an optimal value at around pH 7.5. Various other organic contaminants such as propranolol, phenol, acetaminophen, 4-chlorophenol, benzoic acid, and caffeine were also effectively degraded by the Cu(II)/H2O2/Cl- system. Experiments using oxidant probe compounds and electron paramagnetic spectroscopy suggested that cupryl (Cu(III)) species are the major reactive oxidants responsible for the degradation of these organic contaminants. The enhanced kinetics was further confirmed in natural seawater.

Adsorption of As(V) by boehmite and alumina of different morphologies prepared under hydrothermal conditions.

Morphology-controlled materials at the micro- and nanoscale levels are of great significance to the design and application of materials. Stable and well-dispersed boehmite and alumina with different morphologies were fabricated under hydrothermal conditions. The nitrate, chloride, and sulfate aluminum salts yielded nanoplate, microspindle, and microsphere morphologies, respectively. Calcination of the prepared boehmite samples yielded alumina samples with retention of the morphologies. In comparisons of samples with identical morphologies, alumina exhibited better uptake of As(V) than boehmite; the As(V) concentration was analyzed by the standard molybdenum blue method. The adsorption capabilities of the morphologically controlled materials are ranked microspindle > microsphere > nanoplate. The impacts of process parameters, such as reaction time; initial As(V) concentration; solution pH; competing ions (Ca2+, Mg2+, NO3-, PO43-), which are common in most aquatic ecosystems; and co-contaminants (Cr(VI), Pb(II)), on removal efficiencies were examined. A well-defined mesostructure, superior surface area, chemical and electrostatic interaction, and surface charge distribution over the aluminol surface sites could be factors in the uptake of As(V). The design and synthesis of functional hierarchical micro- and nanostructured materials with the desired adsorptive properties, which are suitable for water treatment applications, can be achieved through environmentally benign hydrothermal fabrication.

Methylene blue removal by submerged plasma irradiation system in the presence of persulfate.

The submerged plasma irradiation (SPI) system is utilized in applications and implications of many environmental fields as an advanced oxidation technology (AOT). However, ramifications of the SPI-based technologies for water treatment are constrained by the different inadequacies. To overcome this matter, in this study, the influence of added persulfate such as peroxydisulfate (PDS) or peroxymonosulfate (PMS) on the removal efficiency of methylene blue (MB) in the SPI system was investigated. The SPI-PMS system was the most effective for MB removal. Also, the pseudo first order rate constants of MB degradation increased with the increase of persulfate dose, applied voltage, and initial solution pH values. The radical species responsible for both the PDS and PMS-activated systems are likely to be peroxomonosulfate radicals (i.e., SO5 (·-)), sulfate radicals (SO4 (·-)), and hydroxyl radicals ((·)OH). Additionally, the persulfate-based SPI system is a novel AOT capable of producing SO4 (·-)or (·)OH and oxidizing water pollutants at near neutral pH.

Enhanced methylene blue oxidative removal by copper electrode-based plasma irradiation with the addition of hydrogen peroxide.

Submerged plasma irradiation (SPI)-based advanced oxidation processes have been studied for the oxidation of recalcitrant organic compounds because of their various physical and chemical properties. However, SPI technologies still have a few drawbacks such as relatively low efficiency for wastewater treatment and high energy consumption. In order to overcome these drawbacks, in this study, we proposed the combination of SPI and the Cu(II)-catalyzed Fenton-like system. The removal of methylene blue (MB) by the SPI system was significantly enhanced upon the addition of H2O2. The pseudo-first-order rate constants of MB removal increased with the increase of applied voltage. In addition, the optimum H2O2 dose and initial solution pH were 100 mM and 9, respectively. The reactive oxidants responsible for MB removal in copper electrode-based SPI/H2O2 systems are likely to be hydroxyl radicals (OH) or cupryl ion (Cu(III)), wherein Cu(III) is especially important. Furthermore, the copper electrode-based SPI/H2O2 system is a novel advanced oxidation process capable of oxidizing water recalcitrant and toxic organic pollutants at neutral pH.