Charuvila T. Aravindakumar

Oxidative degradation study on antimicrobial agent Ciprofloxacin by electro-Fenton process: Kinetics and oxidation products

Oxidative degradation of the antimicrobial agent ciprofloxacin hydrochloride (CIP) has been investigated using electro-Fenton (EF) treatment with a constant current in the range 60-500 mA. The process generates highly oxidant species OH in situ via electrochemically monitored Fenton reaction. The EF experiments were performed using cells with a carbon felt cathode and Pt anode. Effect of applied current and catalyst concentration on the kinetics of oxidative degradation and mineralization efficiency have been investigated. Degradation of CIP followed pseudo-first order reaction kinetics. The rate constant of the oxidation of CIP by OH has been determined to be (1.01 ± 0.14) × 10(10) M(-1) s(-1) by using competitive kinetics method. An optimum current of 400 mA and a catalyst concentration of Fe(2+) at 0.1mM are found to be optimal for an effective degradation of CIP under our operating conditions. A remarkably high degree of mineralization (>94%) was obtained at 6h of treatment under these conditions. A number of stable intermediate products have been identified using HPLC and LC-MS/MS analyses. Based on the identified reaction intermediates, a plausible reaction pathway was proposed for the mineralization process. The high degree of mineralization obtained in this work highlights the potential application of EF process in the efficient removal of fluoroquinolone based drugs in aqueous medium.

Exploring the interaction of bisphenol-S with serum albumins: a better or worse alternative for bisphenol a?

The interaction of bisphenol-S (BPS) with serum albumins using steady-state, synchronous, time-resolved, and circular dichroism spectroscopies has been investigated. The binding interactions have also been investigated in the case of bisphenol A (BPA). The fluorescence quenching pathways are different for both of these endocrine disrupting compounds. Steady-state and time-resolved studies reveal static quenching at lower concentrations of BPS and dynamic quenching at higher concentrations. CD results also maintained the concentration dependent variation with a complete distortion of α-helices at 10(-5) M BPS. Besides this, addition of sodium dodecyl sulfate (SDS) results in the further unfolding of protein in the case of BPS, whereas time-resolved studies indicated refolding for BPA denatured human serum albumin (HSA). The entire study indicates an irreversible binding of BPS with HSA. Hence, these results reveal the possible involvement of BPS in the physiological pathway raising a health threat as already their presences in body fluids are known.

Photoproduction of hydroxyl radicals from Fe(III)-hydroxy complex: a quantitative assessment

Abstract Photolysis of aqueous solutions of ferric perchlorate in the presence of methanol, benzene and 2-deoxy- d -ribose at low pH has been investigated using sunlight and UV light in order to quantitatively assess the involvement of hydroxyl radicals ( OH) in the photolysis of Fe(III)-hydroxy complexes. Time-dependent formation of formaldehyde, phenol and thiobarbituric acid-reactive substance (TBA-RS) was observed in the presence of methanol, benzene and 2-deoxy- d -ribose, respectively. The observed quantum yields of formaldehyde, phenol and TBA-RS are 0.025, 0.0098 and 0.0038, respectively, using UV light. A slightly lower quantum yields are obtained with sunlight owing to its lower intensity. The mechanistic aspects of these reactions are proposed based on the formation of OH from the photo-excited Fe(OH)2+ complex and their subsequent reactions with methanol, benzene and 2-deoxy- d -ribose. The quantum yield of OH is calculated from the observed quantum yields of formaldehyde, phenol and TBA-RS based on the reported percentage contribution of OH in the formation of these products using radiation chemical techniques from the respective reactions. The calculated quantum yields of OH were in the range 0.018–0.025 and 0.014–0.018 with UV light and sunlight, respectively. The results obtained with the photolytic studies have been compared with those from radiation chemical studies.

Influence of inorganic ions and selected emerging contaminants on the degradation of Methylparaben: A sonochemical approach

The study on the possible pathway of hydroxyl radicals mediated sonolytic degradation of paraben in water is reported. Methylparaben (MPB) which is the most utilized of paraben family is selected as a model emerging pollutant. The influence of common anions and some selected emerging contaminants that may coexist in typical water matrix on the degradation pattern is analyzed alongside. Among the anions, carbonate presents a negative influence which is attributed to the competition for OH radical. Some emerging contaminants also showed negative impact on degradation as was clear from HPLC data. The intermediates, analyzed by LC-Q-TOF-MS include hydroxylated and hydrolytic products. Three major steps (aromatic hydroxylation, hydroxylation at the ester chain and hydrolysis) are proposed to involve in the reaction of OH radical with MPB which ultimately leads to mineralization. The intensity of formation and decay of mono and dihydroxy products of MPB in the presence of additives have also been evaluated. COD analysis indicates a percentage reduction of 98% at 90 min of sonolysis and further increase in the degradation time resulted complete mineralization, which became evident from the mass spectrometric data. MTT assay revealed considerable decrease in the potential cytotoxicity.

Oxidative degradation of fensulfothion by hydroxyl radical in aqueous medium

Oxidative degradation of fensulfothion, a model organophosphorus compound, has been investigated by pulse radiolysis and H2O2/UV photolysis. A nearly complete transformation of fensulfothion was observed within 4min of irradiation. Very little Total Organic Carbon (TOC) reduction was obtained at this time scale. When the product studies at this stage were conducted using LC-MS/MS analyses, nearly 20 transformation products were obtained. The entire products were identified as from the reaction of OH with fensulfothion or with some of its initially transformed products. Nearly 80% reduction in TOC was observed when photolysis was conducted using higher concentrations of H2O2 at longer time scale. A reaction rate constant (bimolecular) of 1.10×10(10)dm(3)mol(-1)s(-1) was obtained for the reaction of OH with fensulfothion using pulse radiolysis technique. The transient absorption spectrum obtained from the reaction of OH has a maximum at 280nm and a weak, broad maximum around 500nm along with a small shoulder around 340nm. The intermediate spectrum is assigned to the radical cation of fensulfothion (3) and the hydroxyl radical adducts (1 and 2). This assignment is supported by the intermediate spectrum (λmax at 280nm) from the reaction of sulfate radical anion (SO4(-)) (k2=3.20×10(9)dm(3)mol(-1)s(-1)) which is a one electron oxidant. It is thus demonstrated that the combination of both pulse radiolysis and the product estimation using LC-MS/MS is ideal in probing the complete mechanism which is very important in the mineralization reactions using Advanced Oxidation Processes.

Oxidation Reactions of 1- and 2-Naphthols: An Experimental and Theoretical Study

The transients formed during the reactions of oxidizing radicals with 1-naphthol (1) and 2-naphthol (2) in aqueous medium have been investigated by pulse radiolysis with detection by absorption spectroscopy and density functional theory (DFT) calculations. The transient spectra formed on hydroxyl radical ((•)OH) reactions of 1 and 2 exhibited λ(max) at 340 and 350 nm at neutral pH. The rate constants of the (•)OH reactions of 1 (2) were determined from build-up kinetics at λ(max) of the transients as (9.63 ± 0.04) × 10(9) M(-1) s(-1) ((7.31 ± 0.11) × 10(9) M(-1) s(-1)). DFT calculations using the B3LYP/6-31+G(d,p) method have been performed to locate favorable reaction sites in both 1 and 2 and identification of the pertinent transients responsible for experimental results. Calculations demonstrated that (•)OH additions can occur mostly at C1 and C4 positions of 1, and at C1 and C8 positions of 2. Among several isomeric (•)OH adducts possible, the C1 adduct was found to be energetically most stable both in 1 and 2. Time-dependent density functional theory (TDDFT) calculations in the solution phase has shown that the experimental spectrum of 1 was mainly attributed by 1a4 (kinetically driven (•)OH-adduct) formed via the addition of (•)OH at the C4 position which was 0.73 kcal/mol endergonic compared to 1a1 (thermodynamic (•)OH adduct), whereas 2a1 (thermodynamic/kinetic (•)OH-adduct) was mainly responsible for the experimental spectrum of 2. Naphthoxyl radicals of 1 and 2 have been predicted as the transient formed in the reaction of (•)OH at basic pH. In addition, the same transient species resulted from the reactions of oxide radical ion (O(•-)) at pH ≈ 13 and azide radical (N3(•)) at pH 7 with 1 and 2. Further, UV photolysis of aqueous solutions of 1 and 2 containing H2O2 (UV/H2O2) were used for the (•)OH induced oxidation product formations up on 60% degradations of 1 and 2; profiling of the oxidation products were performed by using an ultraperformance liquid chromatography quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) method. According to the UPLC-Q-TOF-MS analyses, the preliminary oxidation products are limited to dihydroxy naphthalenes and naphthoquinones with N2-saturation, while some additional products (mainly isomeric monohydroxy-naphthoquinones) have been observed in the degradations of 1 and 2 in the presence of O2. We postulate that dihydroxy naphthalenes are derived explicitly from the most favorable (•)OH-adducts speculated (preference is in terms of the kinetic/thermodynamic dominancy of transients) by using theoretical calculations which in turn substantiate the proposed reaction mechanisms. The observations of (•)OH-adducts for an aromatic phenol (herein for both 1 and 2 at pH 7) rather than phenoxyl type radical in the pulse radiolysis experiments is a distinct and unique illustration. The present study provides a meaningful basis for the early stages associated with the (•)OH initiated advanced oxidation processes of 1- and 2-naphthols.

Studies on structure–reactivity in the reaction of OH radicals with substituted halobenzenes in aqueous solutions

The reaction of OH radicals with a number of substituted halobenzenes (C6H5 –nXnY, where X = F, Cl or Br and Y = CH3, CH2Cl, CHCl2, CF3 or OCH3) produces a transient absorption band whose λmax is in the range 310–340 nm. This band is assigned to the OH adduct and decayed with second-order kinetics, the bimolecular rate constant being in the region of 0.6 × 109– 5.0 × 109 dm3 mol–1 s–1. The OH radical reacts mainly by addition to the benzene ring. The rate constants for the reaction of OH radicals, as determined from formation kinetics studies, are between 1.7 and 9.3 × 109 dm3 mol–1 s–1, and are found to depend on the nature of the substituents. A linear correlation is observed with the Hammett parameter. The distribution pattern for the OH adduct is discussed. The OH adduct can be oxidized by K3[Fe(CN)6]. The rate constant for the oxidation depends on the position of OH addition and the extent of oxidation on the nature of the substituents. The reaction of O– with 2-chlorotoluene is by H-atom abstraction (14%), producing an absorption band with λmax= 258 nm, and by addition to the benzene ring (86%), producing absorption bands at 290, 320 and 410 nm. The bimolecular rate constant for the reaction of O– with 2-chlorotoluene is 2.0 × 109 dm3 mol–1 s–1 and Iµ320nm= 2.6 × 103 dm3 mol–1 cm–1. For the reaction of eaq– with 2-chlorobenzyl chloride, 44% of the eaq– react with the Cl of the CH2Cl group producing an absorption band at 258 nm and 56% react with the Cl of the benzene ring producing an absorption band at 320 nm. The bimolecular rate constant as determined from the decay of eaq– is 2.2 × 1010 dm3 mol–1 s–1.

Oxidation Reactions of Thymol: A Pulse Radiolysis and Theoretical Study

The reactions of (•)OH and O(•-), with thymol, a monoterpene phenol and an antioxidant, were studied by pulse radiolysis technique and DFT calculations at B3LYP/6-31+G(d,p) level of theory. Thymol was found to efficiently scavenge OH radicals (k = 8.1 × 10(9) dm(3) mol(-1) s(-1)) to produce reducing adduct radicals, with an absorption maximum at 330 nm and oxidizing phenoxyl radicals, with absorption maxima at 390 and 410 nm. A major part of these adduct radicals was found to undergo water elimination, leading to phenoxyl radicals, and the process was catalyzed by OH(-) (or Na(2)HPO(4)). The rate of reaction of O(•-) with thymol was found to be comparatively low (k = 1.1 × 10(9) dm(3) mol(-1) s(-1)), producing H abstracted species of thymol as well as phenoxyl radicals. Further, these phenoxyl radicals of thymol were found to be repaired by ascorbate (k = 2.1 × 10(8) dm(3) mol(-1) s(-1)). To support the interpretation of the experimental results, DFT calculations were carried out. The transients (both adducts and H abstracted species) have been optimized in gas phase at B3LYP/6-31+G(d,p) level of calculation. The relative energy values and thermodynamic stability suggests that the ortho adduct (C6_OH adduct) to be most stable in the reaction of thymol with OH radicals, which favors the water elimination. However, theoretical calculations showed that C4 atom in thymol (para position) can also be the reaction center as it is the main contributor of HOMO. The absorption maxima (λ(max)) calculated from time-dependent density functional theory (TDDFT) for these transient species were close to those obtained experimentally. Finally, the redox potential value of thymol(•)/thymol couple (0.98 V vs NHE) obtained by cyclic voltammetry is less than those of physiologically important oxidants, which reveals the antioxidant capacity of thymol, by scavenging these oxidants. The repair of the phenoxyl radicals of thymol with ascorbate together with the redox potential value makes it a potent antioxidant with minimum pro-oxidant effects.

Oxidation reactions of 2-thiouracil: a theoretical and pulse radiolysis study.

The reaction of hydroxyl radical ((•)OH) with the nucleic acid base analogue 2-thiouracil (1) has been studied by pulse radiolysis experiments and DFT. The generic intermediate radicals feasible for the (•)OH reactions with 1, namely, one electron oxidation product (1(•+)), (•)OH-adducts (3(•), 4(•), and 5(•)), and H-abstracted radicals (6(•) and 7(•)), were characterized by interpreting their electronic and structural properties along with calculated energetics and UV-vis spectra. Pulse radiolysis experiments showed that the transient formed in the reaction of (•)OH with 1 in water at pH 6.5 has λ(max) at 430 nm. A bimolecular rate constant, k(2) of 9.6 × 10(9) M(-1) s(-1), is determined for this reaction via competition kinetics with 2-propanol. The experiments suggested that the transient species could be a dimer radical cation 2(•+), formed by the reaction of 1 with the radical cation 1(•+). For this reaction, an equilibrium constant of 4.7 × 10(3) M(-1) was determined. The transient formed in the reaction of 1 with pulse radiolytically produced Br(2)(•-) at pH 6.5 as well as Cl(2)(•-) at pH 1 has also produced λ(max) at 430 nm and suggested the formation of 2(•+). The calculated UV-vis spectra of the transient species (1(•+), 3(•), 4(•), 5(•), 6(•), and 7(•)) showed no resemblance to the experimental spectra, while that of 2(•+) (λ(max) = 420 nm) agreed well with the experimental value and thus confirmed the formation of 2(•+). The 420 nm peak was due to σ → σ* electronic excitation centered on a 2-center-3-electron (2c-3e) sulfur-sulfur bond [-S∴S-]. 2(•+) is the first reported example of a dimer radical cation in a pyrimidine heterocyclic system. Further, 5-C and 6-C substituted (substituents are -F, -Cl, -NH(2), -N(CH(3))(2), -OCH(3), -CF(3), -CH(3), -CH(2)CH(3), n-propyl, phenyl, and benzyl) and 5,6-disubstituted 2-thiouracil systems have been characterized by DFT and found that the reaction (1 + 1(•+) → 2(•+)) is exergonic (1.12-13.63 kcal/mol) for many of them.

Sonochemical degradation of Coomassie Brilliant Blue: effect of frequency, power density, pH and various additives.

Coomassie Brilliant Blue (CBB), discharged mainly from textile industries, is an identified water pollutant. Ultrasound initiated degradation of organic pollutants is one among the promising techniques and forms part of the Advanced Oxidation Processes (AOPs). Ultrasonic degradation of CBB under different experimental conditions has been investigated in the present work. The effect of frequency (200 kHz, 350 kHz, 620 kHz and 1 MHz) and power density (3.5 W mL(-1), 9.8 W mL(-1) and 19.6 W mL(-1)) on the degradation profile was evaluated. The optimum performance was obtained at 350 kHz and 19.6 W mL(-1). Similar to other sonolytic degradation of organic pollutants, maximum degradation of CBB was observed under acidic pH. The degradation profile indicated a pseudo-first order kinetics. The addition of ferrous ion (1×10(-4) M), hydrogen peroxide (1×10(-4) M), and peroxodisulphate (1×10(-4) M) had a positive effect on the degradation efficiency. The influence of certain important NOM like SDS and humic acid on the sonolytic degradation of CBB was also investigated. Both the compounds suppress the degradation efficiency. LC-Q-TOF-MS was used to identify the stable intermediate products. Nearly 13 transformed products were identified during 10min of sonication using the optimized operational parameters. This product profile demonstrated that most of the products are formed mainly by the OH radical attack. On the basis of these results, a degradation mechanism is proposed.