Interfacial mechanisms of heterogeneous Fenton reactions catalyzed by iron-based materials: A review

The heterogeneous Fenton reaction can generate highly reactive hydroxyl radicals(·OH)from reactions between recyclable solid catalysts and H_2O_2 at acidic or even circumneutral pH.Hence,it can effectively oxidize refractory organics in water or soils and has become a promising environmentally friendly treatment technology.Due to the complex reaction system,the mechanism behind heterogeneous Fenton reactions remains unresolved but fascinating,and is crucial for understanding Fenton chemistry and the development and application of efficient heterogeneous Fenton technologies.Iron-based materials usually possess high catalytic activity,low cost,negligible toxicity and easy recovery,and are a superior type of heterogeneous Fenton catalysts.Therefore,this article reviews the fundamental but important interfacial mechanisms of heterogeneous Fenton reactions catalyzed by iron-based materials..OH,hydroperoxyl radicals/superoxide anions(HO_2./O_2~-.)and high-valent iron are the three main types of reactive oxygen species(ROS),with different oxidation reactivity and selectivity.Based on the mechanisms of ROS generation,the interfacial mechanisms of heterogeneous Fenton systems can be classified as the homogeneous Fenton mechanism induced by surface-leached iron,the heterogeneous catalysis mechanism,and the heterogeneous reaction-induced homogeneous mechanism.Different heterogeneous Fenton systems catalyzed by characteristic iron-based materials are comprehensively reviewed.Finally,related future research directions are also suggested.     

Assessing the effects of surface-bound humic acid on the phototoxicity of anatase and rutile TiO2 nanoparticles in vitro

In this study,the cytotoxicity of two different crystal phases of TiO₂ nanoparticles,with surface modification by humic acid（HA）,to Escherichia coli,was assessed.The physicochemical properties of TiO₂ nanoparticles were thoroughly characterized.Three different initial concentrations,namely 50,100,and 200 ppm,of HA were used for synthesis of HA coated TiO₂ nanoparticles（denoted as A/RHA50,A/RHA100,and A/RHA200,respectively）.Results indicate that rutile（LC50（concentration that causes 50%mortality compared the control group）=6.5）was more toxic than anatase（LC₅0=278.8）under simulated sunlight（SSL）irradiation,possibly due to an extremely narrow band gap.It is noted that HA coating increased the toxicity of anatase,but decreased that of rutile.Additionally,AHA50 and RHA50had the biggest differences compared to uncoated anatase and rutile with LC₅0of 201.9 and21.6,respectively.We then investigated the formation of reactive oxygen species（ROS）by TiO₂ nanoparticles in terms of hydroxyl radicals（OH）and superoxide anions（O₂^-）.Data suggested that O₂^- was the main ROS that accounted for the higher toxicity of rutile upon SSL irradiation.We also observed that HA coating decreased the generation of OH and O₂^- on rutile,but increased O₂^- formation on anatase.Results from TEM analysis also indicated that HA coated rutile tended to be attached to the surface of E.coli more than anatase.     

Quantum chemical investigation on photodegradation mechanisms of sulfamethoxypyridazine with dissolved inorganic matter and hydroxyl radical

Sulfamethoxypyridazine(SMP) is one of the commonly used sulfonamide antibiotics(SAs).SAs are mainly studied to undergo triplet-sensitized photodegradation in water under natural sunlight with other coexisting aquatic environmental organic pollutants.In this work,SMP was selected as a representative of SAs.We studied the mechanisms of triplet-sensitized photodegradation of SMP and the influence of selected dissolved inorganic matter,i.e.,anions(Br~-,Cl~-,and NO~-_3) and cations ions(Ca~(2+),Mg~(2+),and Zn~(2+)) on SMP photodegradation mechanism by quantum chemical methods.In addition,the degradation mechanisms of SMP by hydroxyl radical(OH·) were also investigated.The creation of SO_2 extrusion product was accessed with two different energy pathways(pathway-1 and pathway-2) by following two steps(step-I and step-II) in the tripletsensitized photodegradation of SMP.Due to low activation energy,the pathway-1 was considered as the main pathway to obtain SO_2 extrusion product.Step-II of pathway-1 was measured to be the rate-limiting step(RLS) of SMP photodegradation mechanism and the effect of the selected anions and cations was estimated for this step.All selected anions and cations promoted photodegradation of SMP by dropping the activation energy of pathway-1.The estimated low activation energies of different degradation pathways of SMP with OH·radical indicate that OH·radical is a very powerful oxidizing agent for SMP degradation via attack through benzene derivative and pyridazine derivative ring.     

Assessing the effect of different natural dissolved organic matters on the cytotoxicity of titanium dioxide nanoparticles with bacteria

Titanium dioxide nanoparticles(TiO_2 NPs) are among the most widely manufactured nanomaterials on a global scale. However, prudent and vigilant surveillance, incumbent upon the scientific community with the advent of new technologies, has revealed potentially undesirable effects of TiO_2 NPs on biological systems and the natural environment during their application and discharge. Such effects are likely best evaluated by first assessing the fate of the TiO_2 NPs in natural environments. In this study, the effects of terrestrial humic acid(HA) and tannic acid(TA), two major members of the collective:dissolved organic matter(DOM), on the cytotoxicity of TiO_2 NPs to Escherichia coli were investigated in the presence and absence of natural sunlight. Qualitative(transmission electron microscopy(TEM)) and quantitative(LC50) analyses were employed in this study. In addition, the production of reactive oxygen species(ROS) in the form of UOH was further assessed—as HA or TA increased the production of ROS decreased. The inhibition of bacterial viability in the light treatment groups, with respective treatment organics at concentrations of 10 ppm, was less in TA than in terrestrial HA. SAS was used to analyze the treatment effect of individual factors of light irradiation, DOM, and concentration of TiO_2 NPs.     

Impact of dissolved organic matter on the photolysis of the ionizable antibiotic norfloxacin

Norfloxacin (NOR), an ionizable antibiotic frequently used in the aquaculture industry, has aroused public concern due to its persistence, bacterial resistance, and environmental ubiquity. Therefore, we investigated the photolysis of different species of NOR and the impact of a ubiquitous component of natural water — dissolved organic matter (DOM), which has a special photochemical activity and normally acts as a sensitizer or inhibiter in the photolysis of diverse organics; furthermore, scavenging experiments combined with electron paramagnetic resonance (EPR) were performed to evaluate the transformation of NOR in water. The results demonstated that NOR underwent direct photolysis and self-sensitized photolysis via hydroxyl radical (·OH) and singlet oxygen (¹O₂) based on the scavenging experiments. In addition, DOM was found to influence the photolysis of different NOR species, and its impact was related to the concentration of DOM and type of NOR species. Photolysis of cationic NOR was photosensitized by DOM at low concentration, while zwitterionic and anionic NOR were photoinhibited by DOM, where quenching of UOH predominated according to EPR experiments, accompanied by possible participation of excited triplet-state NOR and ¹O₂. Photo-intermediate identification of different NOR species in solutions with/without DOM indicated that NOR underwent different photodegradation pathways including dechlorination, cleavage of the piperazine side chain and photooxidation, and DOM had little impact on the distribution but influenced the concentration evolution of photolysis intermediates. The results implied that for accurate ecological risk assessment of emerging ionizable pollutants, the impact of DOM on the environmental photochemical behavior of all dissociated species should not be ignored.     

Multiple transformation pathways of p-arsanilic acid to inorganic arsenic species in water during UV disinfection

p-Arsanilic acid(p-ASA) is widely used in China as livestock and poultry feed additive for promoting animal growth.The use of organoarsenics poses a potential threat to the environment because it is mostly excreted by animals in its original form and can be transformed by UV–Vis light excitation.This work examined the initial rate and efficiency of p-ASA phototransformation under UV-C disinfection lamp.Several factors influencing p-ASA phototransformation,namely,p H,initial concentration,temperature,as well as the presence of Na Cl,NH4+,and humic acid,were investigated.Quenching experiments and LC–MS were performed to investigate the mechanism of p-ASA phototransformation.Results show that p-ASA was decomposed to inorganic arsenic(including As(Ⅲ) and As(V))and aromatic products by UV-C light through direct photolysis and indirect oxidation.The oxidation efficency of p-ASA by direct photosis was about 32%,and those by HOU and1O2 were 19% and 49%,respectively.Cleavage of the arsenic–benzene bond through direct photolysis,HOU oxidation or1O2 oxidation results in simultaneous formation of inorganic As(Ⅲ),As(IV),and As(V).Inorganic As(Ⅲ) is oxidized to As(IV) and then to As(V) by1O2 or HOU.As(IV) can undergo dismutation or simply react with oxygen to produce As(V) as well.Reactions of the organic moieties of p-ASA produce aniline,aminophenol and azobenzene derivatives as main products.The photoconvertible property of p-ASA implies that UV disinfection of wastewaters from poultry and swine farms containing p-ASA poses a potential threat to the ecosystem,especially agricultural environments.