Computational Study of the Gas-Phase Thermal Degradation of Perfluoroalkyl Carboxylic Acids.

Perfluoroalkyl carboxylic acids (PFCAs) are persistent and ubiquitous pollutants. Environmental remediation is often achieved by absorption on matrices followed by high-temperature thermal treatment to desorb and decompose the PFCAs. Detailed product studies of the thermal degradation of PFCAs have been hampered by the complex nature of product mixtures and associated analytical challenges. On the basis of high-level computational studies, we propose reaction pathways and mechanisms for the high-temperature mineralization of a series of linear PFCAs with a backbone length from C-4 to C-8. The favored initial reaction pathways are nonselective C-C bond homolytic cleavages (with bond dissociation energies of ∼75-90 kcal/mol), resulting in carbon-centered radicals which can undergo ß-scissions (Ea ≈ 30-40 kcal/mol) which can be preceded by F atom shifts (Ea ≈ 30-45 kcal/mol). In competing barrierless processes, the carbon-centered radicals can lose •F, resulting in the formation of volatile perfluoroalkenes (ΔH ≈ 50-80 kcal/mol). A variety of competing fragmentation processes yield shorter chain perfluorinated PFCAs, isomeric alkenes, alkenoic acids, alkyl, and alkyloic acid radicals. The results provide the energetics for primary, secondary, and tertiary reaction products and insight into the fundamental understanding of the pyrolytic pathways of PFCAs leading to their mineralization.

Fundamental Studies of the Singlet Oxygen Reactions with the Potent Marine Toxin Domoic Acid.

Domoic acid (DA), a potent marine toxin, is readily oxidized upon reaction with singlet oxygen (1O2). Detailed product studies revealed that the major singlet oxygenation reaction pathways were the [2 + 2] cycloaddition (60.2%) and ene reactions (39.8%) occurring at the Z double bond. Diene isomerization and [4 + 2] cycloaddition, common for conjugated diene systems, were not observed during the singlet oxygenation of DA. The bimolecular rate constant for the DA reaction with 1O2 determined by competition kinetics was 5.1 × 105 M-1 s-1. Based on the rate constant and steady-state concentrations of 1O2 in surface waters, the environmental half-life of DA due to singlet oxygen-induced transformations is between 5 and 63 days. The 1O2 reaction product mixture of DA did not exhibit significant biological activity based on ELISA studies, indicating that singlet oxygenation could be an important natural detoxification process. The characteristic oxidation products can provide valuable markers for the risk assessment of DA-contaminated natural waters.

ß-Cyclodextrin Reverses Binding of Perfluorooctanoic Acid to Human Serum Albumin.

Perfluorooctanoic acid (PFOA), a persistent organic pollutant known to cause adverse health effects, strongly binds to human serum albumin (HSA). ß-Cyclodextrin (ß-CD), a nontoxic cyclic sugar, strongly complexes PFOA in a host-guest complex and has been proposed for environmental remediation of PFOA. The interactions between HSA, PFOA, and ß-CD were investigated in order to determine if ß-CD can reverse the binding of PFOA to HSA, with potential therapeutic applications toward exposure to PFOA. 19F Nuclear magnetic resonance (NMR), circular dichroism, and fluorescence spectroscopies were used to study these interactions. Multiple PFOA binding sites to HSA, one with strong affinity and others with low affinity, are evident from changes in the fluorescence emission spectra of HSA and the fluorescence lifetimes of the single Trp residue in HSA with increasing PFOA concentration. Structural changes in the protein are also evident from changes in the circular dichroism spectra of HSA upon titration of PFOA. Addition of ß-CD to PFOA and HSA reversed these changes, indicating that formation of the ß-CD:PFOA host-guest complex is favored even in the presence of HSA. Equimolar ß-CD to PFOA (1:1 ß-CD:PFOA ratio) causes dissociation of the weakly bound PFOA from HSA, whereas excess ß-CD relative to PFOA (5:1 ß-CD:PFOA ratio) leads to the complete disassociation of the strongly bound PFOA molecule from HSA. The 19F NMR studies further suggest that the 2:1 ß-CD:PFOA complex inhibits PFOA binding to HSA. These data demonstrate that ß-CD has potential to be used in therapeutic applications for PFOA in human blood.

Oxidative remediation of 4-methylcyclohexanemethanol (MCHM) and propylene glycol phenyl ether (PPh). Evidence of contaminant repair reaction pathways.

A large spill of 4-methylcyclohexanemethanol (MCHM) and propylene glycol phenyl ether (PPh) into the Elk River near Charleston, West Virginia on January 9, 2014 led to serious water contamination and public concerns about appropriate remediation. To assess the feasibility of advanced oxidation processes (AOPs) for remediation of waters contaminated with these compounds, we induced hydroxyl radical (HO˙) reactions using time-resolved and steady-state radiolysis methods. Detailed product analyses showed initial HO˙ attack was at the benzene ring of PPh, and occurred through H-atom abstraction reactions for MCHM. Pulse radiolysis and steady state radiolysis experiments conducted using pure compound solutions, mixtures of the compounds and real water solvents allowed us to obtain mechanistic insights of hydroxyl radical attack and establish the fate of the compounds using AOP remediation technologies. These results demonstrate that hydroxyl radical induced oxidization of PPh can lead to "repair-type" reactions, which regenerates this contaminant. The study further highlights the importance of such counterproductive reactions for the quantitative estimate of the required amount of oxidant in any large-scale treatment approaches.

Photochemical Transformation of Aminoglycoside Antibiotics in Simulated Natural Waters.

Aminoglycoside antibiotics are widely used in human therapy and veterinary medicine. We report herein a detailed study on the natural-organic-matter- (NOM-) photosensitized degradation of aminoglycosides in aqueous media under simulated solar irradiation. It appears that the direct reaction of the excited states of NOM ((3)NOM*) with aminoglycosides is minor. The contributions of reactive oxygen species (ROSs) in the bulk solutions are also unimportant, as determined by an assessment based on steady-state concentrations and bimolecular reaction rate constants in a homogeneous reaction model. The inhibition of the photodegradation by isopropamide is rationalized through competitive sorption with aminoglycosides on the NOM surface, whereas the addition of isopropanol negligibly affects degradation because it quenches HO(•) in the bulk solution but not HO(•) localized on the NOM surface where aminoglycosides reside. Therefore, a sorption-enhanced phototransformation mechanism is proposed. The sorption of aminoglycosides on NOM follows a dual-mode model involving Langmuir and linear isotherms. The steady-state concentration of HO(•) on the surface of NOM was calculated as 10(-14) M, 2 orders of magnitude higher than that in the bulk solution. This fundamental information is important in the assessment of the fate and transport of aminoglycosides in aqueous environments.

Ozonation of Cylindrospermopsin (Cyanotoxin): Degradation Mechanisms and Cytotoxicity Assessments.

Cylindrospermopsin (CYN) is a potent toxic alkaloid produced by a number of cyanobacteria frequently found in lakes and reservoirs used as drinking water sources. We report for the first time detailed pathways for the degradation of CYN by treatment with ozone. This was accomplished by use of ultra-high-performance liquid chromatography (UHPLC)-quadrupole time-of-flight mass spectrometry (QTOF MS), which revealed that CYN is readily degraded by ozone with at least 36 transformation products. Structural similarities among the major products indicated that the carbon-carbon double bond in the uracil ring of CYN was most susceptible to attack by ozone. Furthermore, the nitrogen functionality associated with the tricyclic guanidine moiety is also involved via a degradation pathway that has not been previously observed. To assess the potential toxicity of ozonation products of CYN, the cytotoxicity of CYN and the mixture of its ozonation products was measured in a human hepatoma cell line (HepG2). The IC50 for CYN at 24 and 48 h incubations was approximately 64.1 and 12.5 µM, respectively; however, the ozonation products of CYN did not exhibit measurable cytotoxicity to human cells. The results indicate ozone is an effective and practical method for CYN attenuation in water treatment without formation of overtly toxic transformation products.

Selective reduction of Cr(VI) in chromium, copper and arsenic (CCA) mixed waste streams using UV/TiO2 photocatalysis.

The highly toxic Cr(VI) is a critical component in the Chromated Copper Arsenate (CCA) formulations extensively employed as wood preservatives. Remediation of CCA mixed waste and discarded treated wood products is a significant challenge. We demonstrate that UV/TiO2 photocatalysis effectively reduces Cr(VI) to less toxic Cr(III) in the presence of arsenate, As(V), and copper, Cu(II). The rapid conversion of Cr(VI) to Cr(III) during UV/TiO2 photocatalysis occurs over a range of concentrations, solution pH and at different Cr:As:Cu ratios. The reduction follows pseudo-first order kinetics and increases with decreasing solution pH. Saturation of the reaction solution with argon during UV/TiO2 photocatalysis had no significant effect on the Cr(VI) reduction demonstrating the reduction of Cr(VI) is independent of dissolved oxygen. Reduction of Cu(II) and As(V) does not occur under the photocatalytic conditions employed herein and the presence of these two in the tertiary mixtures had a minimal effect on Cr(VI) reduction. The Cr(VI) reduction was however, significantly enhanced by the addition of formic acid, which can act as a hole scavenger and enhance the reduction processes initiated by the conduction band electron. Our results demonstrate UV/TiO2 photocatalysis effectively reduces Cr(VI) in mixed waste streams under a variety of conditions.

Degradation mechanism of cyanobacterial toxin cylindrospermopsin by hydroxyl radicals in homogeneous UV/H2O2 process.

The degradation of cylindrospermopsin (CYN), a widely distributed and highly toxic cyanobacterial toxin (cyanotoxin), remains poorly elucidated. In this study, the mechanism of CYN destruction by UV-254 nm/H2O2 advanced oxidation process (AOP) was investigated by mass spectrometry. Various byproducts identified indicated three common reaction pathways: hydroxyl addition (+16 Da), alcoholic oxidation or dehydrogenation (-2 Da), and elimination of sulfate (-80 Da). The initiation of the degradation was observed at the hydroxymethyl uracil and tricyclic guanidine groups; uracil moiety cleavage/fragmentation and further ring-opening of the alkaloid were also noted at an extended reaction time or higher UV fluence. The degradation rates of CYN decreased and less byproducts (species) were detected using natural water matrices; however, CYN was effectively eliminated under extended UV irradiation. This study demonstrates the efficiency of CYN degradation and provides a better understanding of the mechanism of CYN degradation by hydroxyl radical, a reactive oxygen species that can be generated by most AOPs and is present in natural water environment.

Cr(VI) adsorption and reduction by humic acid coated on magnetite.

Easily separable humic acid coated magnetite (HA-Fe3O4) nanoparticles are employed for effective adsorption and reduction of toxic Cr(VI) to nontoxic Cr(III). The adsorption and reduction of Cr(VI) is effective under acidic, neutral, and basic pH conditions. The chromium adsorption nicely fits the Langmuir isotherm model, and the removal of Cr(VI) from aqueous media by HA-Fe3O4 particles follows pseudo-second-order kinetics. Characterization of the Cr-loaded HA-Fe3O4 materials by X-ray absorption near edge structure spectroscopy (XANES) indicates Cr(VI) was reduced to Cr(III) while the valence state of the iron core is unchanged. Fe K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) and X-ray diffraction measurements also indicate no detectable transformation of the Fe3O4 core occurs during Cr(VI) adsorption and reduction. Thus, suggesting HA on the surface of HA-Fe3O4 is responsible for the reduction of Cr(VI) to Cr(III). The functional groups associated with HA act as ligands leading to the Cr(III) complex via a coupled reduction-complexation mechanism. Cr K-edge EXAFS demonstrates the Cr(III) in the Cr-loaded HA-Fe3O4 materials has six neighboring oxygen atoms likely in an octahedral geometry with average bond lengths of 1.98 Å. These results demonstrate that easily separable HA-Fe3O4 particles have promising potential for removal and detoxification of Cr(VI) in aqueous media.

Oxidation of microcystin-LR by ferrate(VI): kinetics, degradation pathways, and toxicity assessments.

The presence of the potent cyanotoxin, microcystin-LR (MC-LR), in drinking water sources poses a serious risk to public health. The kinetics of the reactivity of ferrate(VI) (Fe(VI)O4(2-), Fe(VI)) with MC-LR and model compounds (sorbic acid, sorbic alcohol, and glycine anhydride) are reported over a range of solution pH. The degradation of MC-LR followed second-order kinetics with the bimolecular rate constant (kMCLR+Fe(VI)) decreasing from 1.3 ± 0.1 × 10(2) M(-1) s(-1) at pH 7.5 to 8.1 ± 0.08 M(-1) s(-1) at pH 10.0. The specific rate constants for the individual ferrate species were determined and compared with a number of common chemical oxidants employed for water treatment. Detailed product studies using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) indicated the oxidized products (OPs) were primarily the result of hydroxylation of the aromatic ring, double bond of the methyldehydroalanine (Mdha) amino acid residue, and diene functionality. Products studies also indicate fragmentation of the cyclic MC-LR structure occurs under the reaction conditions. The analysis of protein phosphatase (PP1) activity suggested that the degradation byproducts of MC-LR did not possess significant biological toxicity. Fe(VI) was effective for the degradation MC-LR in water containing carbonate ions and fulvic acid (FA) and in lake water samples, but higher Fe(VI) dosages would be needed to completely remove MC-LR in lake water compared to deionized water.

Molecular insights into the bonding mechanisms between selenium and dissolved organic matter.

Natural organic matter (NOM) plays a critical role in the mobilization and bioavailability of metals and metalloids in the aquatic environment. Selenium (Se), an environmental contaminant of aquatic systems, has drawn increasing attention over the years. While Se is a vital micronutrient to human beings, animals and plants, excess Se intake may pose serious long-term risks. However, the interaction between Se and dissolved organic matter (DOM) remains relatively unexplored, especially the reaction mechanisms and interactions of specific NOM components of certain molecular weight and the corresponding functional group change. Herein, we report an investigation on the interactions between Se and DOM by focusing on the mass distribution profile change of operationally defined molecular weight fractions of humic acid (HA) and fulvic acid (FA). The results showed that across all molecular weights studied, HA fractions were more prone to enhanced aggregation upon introduction of Se into the system. For FA, the presence of Se species results in aggregation, dissociation, and redox reactions with the first two being the major mechanisms. Total organic carbon analysis (TOC), UV-vis spectroscopy (UV-vis), and Orbitrap MS data showed that [10, 30] kDa MW fraction had the largest aromatic decrease (CRAM-like, lignin-like and tannin-like) upon addition of SeO2 via dissociation as the dominant mechanism. Fourier transform infrared spectroscopy (FT-IR) revealed that Se based bridging or chelation of functional groups from individual DOM components through hydrogen bonding in the form of SeO⋯H and possibly Se⋯H and/or attractive electrostatic interactions lead to aggregated DOM1⋯Se⋯DOM2. It was concluded from two-dimensional correlation analyses of excitation emission matrix (EEM) and FT-IR that the preferred Se-binding follows lipid âž” peptide âž” tannin âž” aromatic functionalities. These results provide new understanding of Se interactions with various NOM components in aquatic environments and provide insight for Se assessing health risk and/or treatment of Se contaminated water.

Detail study on the interaction between perfluorooctanoic acid (PFOA) with human hemoglobin (Hb).

Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) are often referred to as legacy perfluoroalkyl substances (PFAS). Human exposure to PFAS leads to severe negative health impacts including cancers, infertility, and dysfunction in the kidneys. Steady-state absorbance, fluorescence, and circular dichroism (CD) methods were used to study the interactions between PFOA and Hb. The results demonstrate the presence of multiple PFOA binding sites on the Hb protein. The detailed analysis of the ferric hemoglobin protein (met Hb) absorbance data as a function of PFOA concentration indicates the presence of at least two binding sites with equilibrium dissociation constants of 0.8 ± (0.2) × 10-6 M and 63 ± (15) × 10-5 M. A competitive binding study with 1,8-ANS showed PFOA can bind to the same binding site as 1,8-ANS on the Hb protein. The titration curve for PFOA binding to Hb in its CO bound form (CO-Hb) yields a single equilibrium dissociation constant of 139 ± (20) × 10-6 M. PFOA binding at low concentrations occurs at the high-affinity sites leading to the destabilization of the protein structure as reflected by changes in the CD spectrum. PFOA interactions with Hb also interfere with the kinetics of CO association to this protein. The rate for CO association to Hb increases at low PFOA concentrations, whereas at elevated PFOA concentrations, the ligand association is biphasic as a new kinetic process with a different rate constant was observed. Overall, this study provides a detailed explanation of PFOA-induced structural and conformational changes to the Hb protein based on the spectroscopy data.

Ferrate(VI) mediated degradation of the potent cyanotoxin, cylindrospermopsin: Kinetics, products, and toxicity.

The presence of cylindrospermopsin (CYN), a potent cyanotoxin, in drinking water sources poses a tremendous risk to humans and the environment. Detailed kinetic studies herein demonstrate ferrate(VI) (FeVIO42-, Fe(VI)) mediated oxidation of CYN and the model compound 6-hydroxymethyl uracil (6-HOMU) lead to their effective degradation under neutral and alkaline solution pH. A transformation product analysis indicated oxidation of the uracil ring, which has functionality critical to the toxicity of CYN. The oxidative cleavage of the C5=C6 double bond resulted in fragmentation of the uracil ring. Amide hydrolysis is a contributing pathway leading to the fragmentation of the uracil ring. Under extended treatment, hydrolysis, and extensive oxidation lead to complete destruction of the uracil ring skeleton, resulting in the generation of a variety of products including nontoxic cylindrospermopsic acid. The ELISA biological activity of the CYN product mixtures produced during Fe(VI) treatment parallels the concentration of CYN. These results suggest the products do not possess ELISA biological activity at the concentrations produced during treatment. The Fe(VI) mediated degradation was also effective in the presence of humic acid and unaffected by the presence of common inorganic ions under our experimental conditions. The Fe(VI) remediation of CYN and uracil based toxins appears a promising drinking water treatment process.

Hydroxyl radical oxidation of cylindrospermopsin (cyanobacterial toxin) and its role in the photochemical transformation.

Cylindrospermopsin (CYN), an alkaloid guanidinium sulfated toxin, is produced by a number of cyanobacteria regularly found in lakes, rivers, and reservoirs. Steady-state and time-resolved radiolysis methods were used to determine reaction pathways and kinetic parameters for the reactions of hydroxyl radical with CYN. The absolute bimolecular reaction rate constant for the reaction of hydroxyl radical with CYN is (5.08 ± 0.16) × 10(9) M(-1) s(-1). Comparison of the overall reaction rate of CYN with hydroxyl radical with the individual reaction rate for addition to the uracil ring in CYN indicate the majority of the hydroxyl radicals (84%) react at the uracil functionality of CYN. Product analyses using liquid chromatography-mass spectrometry indicate the major products from the reaction of hydroxyl radical with CYN involve attack of hydroxyl radical at the uracil ring and hydrogen abstraction from the hydroxy-methine bridge linking the uracil ring to the tricyclic guanidine functionality. The role of hydroxyl radical initiated pathways in the natural organic matter (NOM) photosensitized transformation of CYN were evaluated. Scavenger and trapping experiments indicate that hydroxyl radical mediated transformations account for approximately ~70% of CYN destruction in surface waters under solar irradiation in the presence of NOM. The absence of solvent isotope effect indicates singlet oxygen does not play a significant role in the NOM sensitized transformation of CYN. The primary degradation pathways for HO• mediated and NOM photosensitized destruction of CYN involve destruction of the uracil ring. The fundamental kinetic parameters determined from these studies are critical for the accurate evaluation of hydroxyl-radical based technologies for the remediation of this problematic cyanotoxin in drinking water and important in the assessment of the environmental oxidative transformation of uracil based compounds.

Application of dimensional analysis in sorption modeling of the styryl pyridinium cationic dyes on reusable iron based humic acid coated magnetic nanoparticles.

Cationic dyes exist in various industrial wastewaters and removal prior to discharge is necessary due to their carcinogenic behavior which poses a serious threat to human health. Iron based humic acid coated magnetic nanoparticles (HA-MNPs) were evaluated for the removal of 2-[4-(dimethylamino) styryl]-1-methylpyridinium iodide (2-ASP) as a model compound for cationic styryl pyridinium dyes from aqueous media. HA-MNPs were prepared by co-precipitation and characterized. The adsorption of 2-ASP, measured by fluorescence, demonstrates HA-MNPs are efficient for the 2-ASP removal with a maximum adsorption capacity of ~8 mg/g. Kinetic behavior and equilibrium studies showed the adsorption process fits with pseudo 2nd order and Langmuir isotherm models. The adsorption is relatively fast with ~70% of the adsorption complete within 30 min. The overall removal increases by increasing solution pH. The observed increase in adsorption can be assigned to an enhanced electrostatic attraction between the positively charged 2-ASP and the increase in the negative charge on the HA-MNPs surface as a function of increasing solution pH. Effective and repetitive regeneration of the HA-MNPs was achieved using NaOH treatment of saturated sorbent. Regeneration of HA-MNPs showed that removal efficiency remains consistently high after five consecutive cycles. Dimensional analysis suggested that initial concentration/sorbent dose ratio should be considered for accurate sorption modeling confirmed by experimental data. Then generalized empirical models for isothermal study and removal efficiency prediction were accurately deduced. This finding will help researchers in sorption studies to design their experiments more efficiently and to develop improved empirical models in removal prediction.

Elucidation of specific binding sites and extraction of toxic Gen X from HSA employing cyclodextrin.

The presence of per and poly-fluoroalkyl substances (PFAS), commonly referred to as forever chemicals, in aquatic systems is a serious global health problem. While the remediation of PFAS from aqueous media has been extensively investigated, their interactions with and removal from biological systems have received far less attention. We report herein structural alterations to human serum albumin (HSA) upon addition of perfluoro(2-methyl-3-oxahexanoic) acid (Gen X) monitored by changes to the fluorescence and circular dichroism (CD) spectra of HSA. The equilibrium association constant for Gen X binding to HSA is 7( ± 1) × 103 M-1 determined from changes in HSA fluorescence emission data during titration. Site-specific HSA binding fluorophores, 8-anilinonaphthalene-1-sulfonic acid (1,8-ANS), warfarin and dansyl-L-proline were used to investigate the specific binding sites of Gen X on HSA. A competitive displacement study yields association constants for Gen X to HSA at the 1,8-ANS, warfarin, and dansyl-L-proline binding sites to be 6.25 ( ± 0.5) × 104 M-1, 1.1 × 106 M-1, and 2.5( ± 0.2) × 109 M-1 respectively. Addition of ß-cyclodextrin (ß-CD) and heptakis(6-deoxy-6-amino)-ß-cyclodextrin heptahydrochloride to the HSA:Gen X complex leads to the effective extraction of Gen X from the complex with the return of HSA in its native form. Gen X also leads to displacement of site-specific binding fluorophores bound to HSA, while subsequent addition of ß-CD extracts Gen X from HSA with the return of the characteristic fluorescence of the HSA bound site-specific agent. These results illustrate the strong and specific binding sites of Gen X on HSA and demonstrate the principles for the potential application of ß-CD for the remediation of PFAS from biological systems.

Making waves: Defining advanced reduction technologies from the perspective of water treatment.

Studies related to advanced reduction technologies (ARTs) have grown exponentially since the term was first coined in 2013. Despite recent interests in ARTs, the conditions and requirements for these processes have yet to be defined and clarifed. In comparision to well defined advanced oxidation technologies/processes (AOTs/AOPs) which involve the generation of hydroxyl radical as the common characteristic, ARTs function by electron donation from a variety of reducing agents and activators. Based on an extensive literature review, we propose that ARTs be defined as processes employing strong chemical reductants with E° ≤ -2.3 V vs. normal hydrogen electrode at 25 ºC. While extensive studies have revealed critical fundamental details of AOTs/AOPs mediated processes, there are still significant gaps in elucidation of the mechanistic details of reductive degradation/transformation of highly toxic compounds by ARTs. A significant number of pollutants and toxins resistant to AOTs/AOPs treatment are effectively degraded by ARTs. A great leap is needed on understanding ARTs to fully utilize their potential to efficiently remediate recalcitrant compounds of different sources and structures.

TiO(2) Photocatalytic Degradation of Phenylarsonic Acid.

Phenyl substituted arsenic compounds are widely used as feed additives in the poultry industry and have become a serious environmental concern. We have demonstrated that phenylarsonic acid (PA) is readily degraded by TiO(2) photocatalysis. Application of the Langmuir-Hinshelwood kinetic model for the initial stages of the TiO(2) photocatalysis of PA yields an apparent rate constant (k(r)) of 2.8 µmol/L·min and the pseudo-equilibrium constant (K) for PA is 34 L/mmol. The pH of the solution influences the adsorption and photocatalytic degradation of PA due to the surface charge of TiO(2) photocatalyst and speciation of PA. Phenol, catechol and hydroquinone are observed as the predominant products during the degradation. The roles of reactive oxygen species, •OH, (1)O(2), O(2) (-•) and h(VB) (+) were probed by adding appropriate scavengers to the reaction medium and the results suggest that •OH plays a major role in the degradation of PA. By-products studies indicate the surface of the catalyst plays a key role in the formation of the primary products and the subsequent oxidation pathways leading to the mineralization to inorganic arsenic. TiO(2) photocatalysis results in the rapid destruction of PA and may be attractive for the remediation of a variety of organoarsenic compounds.

Selective oxidation of H1-antihistamines by unactivated peroxymonosulfate (PMS): Influence of inorganic anions and organic compounds.

The rapid and selective peroxymonosulfate (PMS) induced transformation of H1-antihistamines cetirizine (CET) and diphenhydramine (DPH) can be influenced by the presence of common organic and inorganic water constituents. Presence of HCO3- and/or CO32-, which often exhibit powerful inhibition on the advanced oxidation processes (AOPs), can enhance the PMS mediated transformation of CET/DPH. The observed promotion is demonstrated by the changed solution pH through detailed kinetic studies. The impact of halide ions is remarkable, with I- inhibiting the process through consumption of PMS, while Cl- increases slightly the transformation kinetics through the formation and subsequent reactions of HOCl. The CET/DPH degradation in the Br-/PMS system is influenced by the generation of reactive species such as HOBr which leads to different reaction pathways as compared to PMS alone. The results demonstrated the performance of PMS can be tailored through varying the experimental parameters. In addition, the presence of model organic constituents found in water, e.g., humic acid, phenol, pyridine or sorbate, has a minimal effect on the PMS mediated oxidation processes, highlighting the strong application potential of PMS in water treatment.

Rapid transformation of H1-antihistamines cetirizine (CET) and diphenhydramine (DPH) by direct peroxymonosulfate (PMS) oxidation.

With growing interest in advanced oxidation processes (AOPs), the number of research studies on peroxymonosulfate (PMS) mediated pollutant degradation has increased significantly due to its high radical generation potential upon activation. However, rare studies have focused on the non-radical based PMS reactions. In this study, degradation of model H1-antihistamines cetirizine (CET) and diphenhydramine (DPH) by unactivated PMS was investigated. Addition of scavengers to the reaction mixture ruled out the involvement of hydroxyl radical (OH), sulfate radical (SO4-), singlet oxygen (1O2) and superoxide anion radical (O2-), indicating direct PMS oxidation as the predominant reaction path. Such a mechanism was further supported by the N-oxide products identified by mass spectrometry and nuclear magnetic resonance (NMR) analyses. Solution pH had a pronounced influence on the degradation kinetics regardless the presence or absence of transition metal Fe(II). The highest species dependent second order rate constants were kHSO5-/DPH0 of 175 ± 15.9 M-1 s-1 and kHSO5-/CET- of 36.6 ± 0.16 M-1 s-1. The addition of 100 µM Fe(II) promoted OH mediated degradation of H1-antihistamines and their N-oxide products. This study demonstrated selective transformation with the potential for extensive degradation employing both the direct and catalytic PMS oxidative processes.