Dimethylphenol Isomers Research Articles

Influence of dimethylphenol isomers on electrochemical degradation: Kinetics, intermediates, and DFT calculation

Dimethylphenol isomers (DMP) pose a great threat to the environment, and the electrooxidation (EO) process proves to be an extraordinarily effective method to degrade DMP. However, the EO performance is affected by the molecular structure of DMP and the adopted experimental parameters. In this study, the effects of 2,4-DMP and 2,6-DMP on the working potential, limiting current density (Jlim), and pH were systematically analysed, with Ti-mesh plates used as the cathode and Ti/PbO2 as the anode. The peak potentials of 2,4-DMP and 2,6-DMP were determined to be 0.83 V and 0.77 V by cyclic voltammetry, with Jlim were 2.5 mA·cm−2 and 2.0 mA·cm−2, respectively. The whole process exhibited pseudo-first-order kinetics, and the kinetic constants (K) for the degradation of 2,4-DMP and 2,6-DMP were determined to be 0.0041 min−1 and 0.0150 min−1, respectively. Additionally, the optimal initial pH value for 2,4-DMP and 2,6-DMP was 5.0, where the highest hydroxyl (OH) radical density, as determined by the electron spin technique (ESR), was achieved at a higher current density. Comparatively, the OH radical density in the 2,6-DMP solution was lower than that in 2,4-DMP. In situ Fourier infrared (FT-IR) spectroscopy, GC–MS, and density functional theory (DFT) were employed to explore three possible degradation pathways. The main intermediates for 2,4-DMP degradation were determined to be quinone and ether, while that for 2,6-DMP degradation was quinone. According to the results of this study, the molecular structure (different methyl group positions on the benzene ring) has a great influence on the EO process.

Changes in bacterial diversity and catabolic gene abundance during the removal of dimethylphenol isomers in laboratory-scale constructed wetlands.

Constructed wetlands (CWs) are well-established wastewater treatment technologies and applied for bioremediation of contaminated water. Despite the optimal performance of CWs, the understanding of the bacterial processes in the rhizosphere, where mainly microbial degradation processes take place, is still limited. In the present study, laboratory-scale CWs planted with Juncus effusus and running under controlled conditions were studied in order to evaluate removal efficiency of dimethylphenols (DMPs), also in comparison to an unplanted bed. Next to removal rates, the bacterial community structure, diversity, and distribution, their correlation with physiochemical parameters, and abundance of the phenol hydroxylase gene were determined. As a result, better removal performance of DMP isomers (3,4-, 3,5-, and 2,6-DMP added as singles compounds or in mixtures) and ammonium loads, together with a higher diversity index, bacterial number, and phenol hydroxylase gene abundance in Juncus effusus CW in comparison with the non-planted CW, indicates a clear rhizosphere effect in the experimental CWs. An enhancement in the DMP removal and the recovery of the phenol hydroxylase gene were found during the fed with the DMP mixture. In addition, the shift of bacterial community in CWs was found to be DMP isomer dependent. Positive correlations were found between the bacteria harboring the phenol hydroxylase gene and communities present with 3,4-DMP and 3,5-DMP isomers, but not with the community developed with 2,6-DMP. These results indicate that CWs are highly dynamic ecosystems with rapid changes in bacterial communities harboring functional catabolic genes.

Delftia sp. LCW, a strain isolated from a constructed wetland shows novel properties for dimethylphenol isomers degradation

BackgroundDimethylphenols (DMP) are toxic compounds with high environmental mobility in water and one of the main constituents of effluents from petro- and carbochemical industry. Over the last few decades, the use of constructed wetlands (CW) has been extended from domestic to industrial wastewater treatments, including petro-carbochemical effluents. In these systems, the main role during the transformation and mineralization of organic pollutants is played by microorganisms. Therefore, understanding the bacterial degradation processes of isolated strains from CWs is an important approach to further improvements of biodegradation processes in these treatment systems.ResultsIn this study, bacterial isolation from a pilot scale constructed wetland fed with phenols led to the identification of Delftia sp. LCW as a DMP degrading strain. The strain was able to use the o-xylenols 3,4-DMP and 2,3-DMP as sole carbon and energy sources. In addition, 3,4-DMP provided as a co-substrate had an effect on the transformation of other four DMP isomers. Based on the detection of the genes, proteins, and the inferred phylogenetic relationships of the detected genes with other reported functional proteins, we found that the phenol hydroxylase of Delftia sp. LCW is induced by 3,4-DMP and it is responsible for the first oxidation of the aromatic ring of 3,4-, 2,3-, 2,4-, 2,5- and 3,5-DMP. The enzyme may also catalyze both monooxygenation reactions during the degradation of benzene. Proteome data led to the identification of catechol meta cleavage pathway enzymes during the growth on ortho DMP, and validated that cleavage of the aromatic rings of 2,5- and 3,5-DMPs does not result in mineralization. In addition, the tolerance of the strain to high concentrations of DMP, especially to 3,4-DMP was higher than that of other reported microorganisms from activated sludge treating phenols.ConclusionsLCW strain was able to degraded complex aromatics compounds. DMPs and benzene are reported for the first time to be degraded by a member of Delftia genus. In addition, LCW degraded DMPs with a first oxidation of the aromatic rings by a phenol hydroxylase, followed by a further meta cleavage pathway. The higher resistance to DMP toxicity, the ability to degrade and transform DMP isomers and the origin as a rhizosphere bacterium from wastewater systems, make LCW a suitable candidate to be used in bioremediation of complex DMP mixtures in CWs systems.

Removal of dimethylphenols and ammonium in laboratory-scale horizontal subsurface flow constructed wetlands.

Phenolic compounds in industrial wastewaters are toxic pollutants and pose a threat to public health and ecosystems. More recently, focus is being directed toward combining the treatment of these compounds with a cost-effective and environmentally sound technology. The removal efficiency of dimethylphenol and ammonium nitrogen was studied, for the first time, in three different laboratory-scale horizontal subsurface flow constructed wetlands planted with Juncus effusus. Two of the wetlands used were filled with gravel. One of these was planted and the second left without vegetation. The third wetland was a hydroponic system. It was found that the removal efficiencies of dimethylphenol was dependent on the inflow loading of the contaminant and was higher in the planted systems. Both planted systems yielded 99% removal efficiency up to loads of 240mg/d, compared to only 73% for the unplanted constructed wetland. Factors and processes such as redox dynamics, methanogenesis, reduction of ammonium and low nitrate and nitrite concentrations imply simultaneous aerobic and anaerobic dimethylphenol transformations. A significant surplus of organic carbon was detected in the planted wetlands, which may originate from intermediates of the dimethylphenol transformation processes and/or organic plant root exudates. The present study demonstrates that horizontal subsurface flow constructed wetlands are a promising alternative system for the treatment of effluents contaminated with dimethylphenol isomers.

Mechanism and Kinetics of the Atmospheric Oxidative Degradation of Dimethylphenol Isomers Initiated by OH Radical

Dimethylphenols are highly reactive in the atmosphere, and their oxidation plays a vital role in the autoignition and combustion processes. The dominant oxidation process for dimethylphenols is by gas-phase reaction with OH radical. In the present study, the reaction of OH radical with dimethylphenol isomers is studied using density functional theory methods, B3LYP, M06-2X, and MPW1K, and also at the MP2 level of theory using 6-31G(d,p) and 6-31+G(d,p) basis sets. The activation energy values have also been calculated using the CCSD(T) method with 6-31G(d,p) and 6-311+G(d,p) basis sets using the geometries optimized at the M06-2X/6-31G(d,p) level of theory. The reactions subsequent to the principal oxidation steps are studied, and the different reaction pathways are modeled. The positions of the OH and CH3 substituents in the aromatic ring have a great influence on the reactivity of dimethylphenol toward OH radical. Accordingly, the reaction is initiated in four different ways: H-atom abstraction from the phenol group, H-atom abstraction from a methyl group, H-atom abstraction from the aromatic ring by OH radical, or electrophilic addition of OH radical to the aromatic ring. Aromatic peroxy radicals arising from initial H-atom abstraction and subsequent O2 addition lead to the formation of hydroperoxide adducts and alkoxy radicals. The O2 additions to dimethylphenol-OH adduct results in the formation of epoxide and bicyclic radicals. The rate constants for the most favorable reaction pathways are calculated using canonical variational transition state theory with small curvature tunneling corrections. This study provides thermochemical and kinetic data for the oxidation of dimethylphenol in the atmosphere and demonstrates the mechanism for the conversion of peroxy radical into aldehydes, hydroperoxides, epoxides, and bicyclic radicals, and their lifetimes in the atmosphere.

Determination of Gas-Phase Acidities of Dimethylphenols: Combined Experimental and Theoretical Study

The gas-phase acidities of the six dimethylphenol isomers were determined experimentally, by using the kinetic method, and theoretically, through quantum chemistry calculations. The experimental values, relative to the gas-phase acidity of phenol, are (in kJ mol(-1)): -1.76+/-0.76 (2,3-Me2C6H3OH), 1.78+/-0.29 (2,4-Me2C6H3OH), 0.83+/-0.58 (2,5-Me2C6H3OH), -4.39+/-0.89 (2,6-Me2C6H3OH), 5.38+/-1.08 (3,4-Me2C6H3OH), and 1.88+/-0.08 (3,5-Me2C6H3OH). This trend was discussed by considering the substituent effects on the thermodynamic stabilities both of the parent phenols and the corresponding phenoxide ions. The above acidity data, the literature values for 2-, 3-, and 4-methylphenol, and the substituent effects analysis allowed to develop a simple empirical method to estimate the acidity of any methyl-substituted phenol.

Immunomodulatory effects of mono-, di-, and trimethylphenols in mice

The relationship of air pollutants with the increasing prevalence of allergic diseases is a matter of concern in developed countries. In this study, the immunomodulatory effects of mono-, di-, and trimethylphenols in mice were examined as regards two aspects. First, whether or not these chemicals act as sensitizers was evaluated by local lymph node assay. Of the 13 methylphenols tested, three dimethylphenol isomers (2,4-DMP, 2,5-DMP, and 3,4-DMP) were found to induce auricular lymphocyte proliferation after dermal application on both ears of mice. Cytokine production patterns in the supernatants of cultured auricular lymphocytes from mice showed these methylphenols to be contact sensitizers. Second, the effects of methylphenols on cytokine production profiles were examined using cultured splenocytes from immunologically naive mice. Under subtoxic conditions, eight methylphenols inhibited interferon-γ (IFN-γ) production significantly, while the effect on intreluekin-4 (IL-4) production was moderate, resulting in higher IL-4/IFN-γ ratios in all of the tested chemicals, with the most prominent effect shown by 2,6-DMP. These results suggest that several methylphenols, especially dimethylphenol isomers, have potencies that affect the immune system, being immunogens themselves or modulators of the Th1/Th2 cytokine balance.

Kinetics of the Gas-Phase Reactions of OH and NO3Radicals with Dimethylphenols

Rate coefficients for the reactions of hydroxyl (OH) and nitrate (NO3) radicals with the six dimethylphenols have been determined at 295 ± 2 K and atmospheric pressure using the relative rate technique. Experiments were performed in a 3.91 m3 atmospheric simulation chamber using in situ FTIR spectroscopy and gas chromatography for chemical analysis. The reactivity of the dimethylphenols is compared to other phenolic compounds and it is shown that the rate coefficients strongly depend on the number and position of the OH and CH3 substituents around the aromatic ring. In addition, the dimethylphenol isomers appear to exhibit an opposite trend in reactivity for reaction with OH and NO3 radicals. The rate coefficient data for the dimethylphenols and related phenolic compounds are explained in terms of known mechanistic features of the reactions. The atmospheric implications are also discussed.

Occupational exposure to aromatic hydrocarbons at a coke plant: Part I. Identification of hydrocarbons in air and their metabolites in urine by a gas chromatography-mass spectrometry method.

A method for the qualitative analysis of aromatic hydrocarbons in air and their various urinary metabolites is presented. The air was sampled in charcoal tubes and extracted with carbon disulfide. The hydrocarbons were identified as being aliphatic hydrocarbons (C(9)-C(19)), aromatic hydrocarbons and heterocyclic compounds. The urinary metabolites after enzymatic hydrolysis were analyzed by solid-phase extraction with a styrene-divinylbenzene resin, silylation with N,O-bis(trimethylsilyl)acetamide and GC/MS for separation and detection. Satisfactory separation of all compounds investigated was achieved without interference due to matrix peaks. The following compounds were identified in the urine of workers: dimethylphenol isomers, 4-ethyl-1,3-benzenediol, 2-ethoxybenzoic acid and methoxyphenols. Trimethylsilyl derivatives of aromatic hydroxyacids and hydroxymethoxyacids were found in the urine of occupationally exposed workers by means of a silylation procedure.

Degradation of Monochlorinated and Nonchlorinated Aromatic Compounds under Iron-Reducing Conditions

The capacity for Fe(sup3+) to serve as an electron acceptor in the microbial degradation of monochlorinated and nonchlorinated aromatic compounds was investigated in anoxic sediment enrichments. The substrates tested included phenol, benzoate, aniline, their respective monochlorinated isomers, o-, m-, and p-cresol, and all six dimethylphenol isomers. Phenol and 2-, 3-, and 4-chlorophenol were utilized by anaerobic microorganisms, with the concomitant reduction of Fe(sup3+) to Fe(sup2+). The amount of Fe(sup2+) produced in the enrichments was 89 to 138% of that expected for the stoichiometric degradation of these substrates to CO(inf2), suggesting complete mineralization at the expense of Fe reduction. Under Fe-reducing conditions, there was initial loss of benzoate and 3-chlorobenzoate but not of 2- or 4-chlorobenzoate. In addition, there was initial microbial utilization of aniline but not of the chloroaniline isomers. There was also initial loss of o-, m-, and p-cresol in our enrichments. None of the dimethylphenol isomers, however, was degraded within 300 days. Furthermore, we tested the capacity of an Fe-reducing, benzoate-grown culture of Geobacter metallireducens GS-15 to utilize monochlorinated benzoates and phenols. G. metallireducens was able to degrade benzoate and phenol but none of their chlorinated isomers, suggesting that the degradation of chlorophenols in our sediment enrichments may be due to novel Fe-reducing organisms that have yet to be isolated.

Crown ether substituted polysiloxanes as stationary phases for open tubular column gas chromatography

AbstractTwo new crown ether substituted polysiloxanes, 4‐propylbenzo‐15‐crown‐5polysiloxane(PSO‐B‐15C5) and propyloxydibenzo‐14‐crown‐4polysiloxane (PSO‐DB‐14C4), were prepared and used as stationary phases coated on fused‐silica open tubular columns. The chromatographic characteristics, including column efficiency, polarity, phase transition temperature, allowable operating temperature range and selectivity, were studied. A unique selectivity of these columns was found for the separation of all six dimethylphenol isomers. The separation mechanism is discussed.

Crown ethers as stationary phases in gas chromatography : Comparison between dibenzo-18-crown-6, dibenzo-24-crown-8 and dicyclohexano-24-crown-8 with respect to polarity, selectivity and stability

Chromatogrpahic characteristics, including thermal stability, polarity and selectivity, of dibenzo-18-crown-6 (DB18C6), dibenzo-24-crown-8 (DB2C8) and dicyclohexano-24-crown-8 (DCH24C8) were compared. Phase transition studies were carried out by plotting graphs of log(specific retention volume) against the inverse of absolute temperature. Nitrochlorobenzene, nitrophenol, nitroaniline, chloroaniline, cresol, chlorophenol and dimethylphenol isomers were injected onto these crown ether stationary phases at 3%, 10% and 20% loadings. Dimethylphenol and nitrochlorobenzene isomers were separated well on 10% DB24CB and 10% DCH24C8 columns. DB18C6 is useful for the direct analysis of nitrophenol and nitroaniline isomers without any derivatization.