Photodegradation of pentachlorophenol and its degradation pathways predicted using density functional theory

Para-Chlorophenol (4-CP) Removal by a Palladium-Coated Biofilm: Coupling Catalytic Dechlorination and Microbial Mineralization via Denitrification

Rapid dechlorination and full mineralization of para-chlorophenol (4-CP), a toxic contaminant, are unfulfilled goals in water treatment. Means to achieve both goals stem from the novel concept of coupling catalysis by palladium nanoparticles (PdNPs) with biodegradation in a biofilm. Here, we demonstrate that a synergistic version of the hydrogen (H₂)-based membrane biofilm reactor (MBfR) enabled simultaneous removals of 4-CP and cocontaminating nitrate. In situ generation of PdNPs within the MBfR biofilm led to rapid 4-CP reductive dechlorination, with >90% selectivity to more bioavailable cyclohexanone. Then, the biofilm mineralized the cyclohexanone by utilizing it as a supplementary electron donor to accelerate nitrate reduction. Long-term operation of the Pd-MBfR enriched the microbial community in cyclohexanone degraders within Clostridium, Chryseobacterium, and Brachymonas. In addition, the PdNP played an important role in accelerating nitrite reduction; while NO₃^- reduction to NO₂^- was entirely accomplished by bacteria, NO₂^- reduction to N₂ was catalyzed by PdNPs and bacterial reductases. This study documents a promising option for efficient and complete remediation of halogenated organics and nitrate by the combined action of PdNP and bacterial catalysis.

Ascorbic acid induced activation of persulfate for pentachlorophenol degradation

In the present study, ascorbic acid (AA) induced persulfate activation was investigated for the further exploration of organic pollutants oxidation by persulfate. We interestingly found that AA showed a significant catalytic activity to persulfate. Under neutral pH and room temperature condition, about 71.3% of pentachlorophenol (PCP, 10 mg L^-1) was decomposed in 180 min with 40 mmol L^-1 persulfate and 1.0 mmol L^-1 AA, while only 15.4% and 3.2% of PCP was removed by alone persulfate and AA respectively. The result of EPR spectra identified sulfate radical (SO₄^•-) and hydroxyl radical (OH) were generated during the reaction between persulfate and AA. Quenching experiments confirmed that both SO₄^- and OH contributed to the decomposition of PCP. With the addition of AA augmented from 0 to 1 mmol L^-1, the PCP degradation ratio continuously increased. However, excess AA could consume the generated reactive oxygen species (ROSs) that led to the inhibition of PCP degradation. Meanwhile, the PCP degradation by persulfate-AA was strongly pH dependent. The PCP degradation rate was declined as the initial pH increased from 3.5 to 10.5. At pH above 12.5, the base activation began to predominate over AA activation of persulfate. Furthermore, it was observed that the AA inducing persulfate activation was related to the extent of AA ionization, while C₆H₈O₆ promoted the highest persulfate activation for the PCP degradation, and C₆H₆O₆^2- induced the lowest persulfate activation. This study indicates the high potential of AA induced persulfate activation for treatment of organochlorine contaminated water.

Direct photolysis mechanism of pesticides in water

Photodegradation is one of the most important abiotic transformations for pesticides in the aquatic environment, and the high energy of sunlight causes characteristic reactions such as bond scission, cyclization, and rearrangement, which are scarcely observed in hydrolysis and microbial degradation. This review deals with direct photolysis via excitation of a pesticide by absorbing natural or artificial sunlight in order to know its basic photochemistry, and indirect photolysis meaning either sensitization by dissolved organic matters or oxidation by reactive oxygen species is basically excluded. Several experimental approaches including spectroscopic techniques together with theoretical calculations are first discussed from the viewpoint of the reaction mechanisms in direct photolysis. Then, the typical photoreactions of pesticides are summarized by chemical classes and/or functional groups and discussed as far as possible in relation to their mechanisms.

Dechlorination of chlorinated phenols by subnanoscale Pd 0 /Fe 0 intercalated in smectite: pathway, reactivity, and selectivity

Smectite clay was employed as templated matrix to prepare subnanoscale Pd(0)/Fe(0) particles, and their components as well as intercalated architectures were well characterized by X-ray energy dispersive spectroscopy (X-EDS) and X-ray diffraction (XRD). Furthermore, as-prepared Pd(0)/Fe(0) subnanoscale nanoparticles were evaluated for their dechlorination effect using chlorinated phenols as model molecules. As a result, pentachlorophenol (PCP) is selectively transformed to phenol in a stepwise dechlorination pathway within 6h, and the dechlorination rate constants show linearly relationship with contents of Pd as its loadings <0.065%. Comparing with PCP, other chlorinated phenols display similar degradation pattern but within much shorter time frame. The dechlorination rate of chlorinated phenols increases with decreasing in number of -Cl attached to aromatic ring, which can be predicted by the total charge of the aromatic ring, exhibiting an inversely linear relationship with the dechlorination rates. While the selectivity of dechlorination depends on the charges associated with the individual aromatic carbon. Chloro-functional groups at the ortho-position are easier to be dechlorinated than that at meta- and para- positions yielding primarily 3,4,5-TCP as intermediate from PCP, further to phenol. The effective dechlorination warrants their potential utilizations in development of in-situ remediation technologies for organic pollutants in contaminated water.

Pentachlorophenol dechlorination and simultaneous Cr6+ reduction by Pseudomonas putida SKG-1 MTCC (10510): characterization of PCP dechlorination products, bacterial structure, and functional groups

It is the first report in which a novel psychrotrophic Pseudomonas putida SKG-1 strain was evaluated for simultaneous bioremediation of pentachlorophenol and Cr(6+) under various cultural and nutritional conditions. Pentachlorophenol (PCP) dechlorination products, bacterial structure, and functional groups were characterized by gas chromatography and mass spectrometry (GC-MS), scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDS), and Fourier-transform infrared (FTIR) techniques. The strain was extremely tolerant to excessively higher individual concentration of PCP (1,400 mg l(-1)) and Cr(6+) (4,300 mg l(-1)). Increasing concentration of PCP and Cr(6+) exerted inhibitory effect on bacterial growth and toxicants' removal. The strain exhibited growth, and concomitantly remediated both the pollutants simultaneously over a broad pH (7.0-9.0) and temperature (28-32 °C) range; maximum growth, PCP dechlorination (87.5%), and Cr(6+) removal (80.0%) occurred at optimum pH 8.0 and 30 °C (from initial PCP 100 mg l(-1) and Cr(6+) 500 mg l(-1)) under shaking (150 rpm) within 72 h incubation. Optimization of agitation (125 rpm) and aeration (0.4 vvm) in bioreactor further enhanced PCP dechlorination by ~10% and Cr(6+) removal by 2%. A direct correlation existed between growth and bioremediation of both the toxicants. Among other heavy metals, mercury exerted maximum and cobalt minimum inhibitory effect on PCP dechlorination and Cr(6+) removal. Chromate reductase activity was mainly associated with the supernatant and cytosolic fraction of bacterial cells. GC-MS analysis revealed the formation of tetrachloro-p-hydroquinone, 2,4,6-trichlorophenol, and 2,6-dichlorophenol as PCP dechlorination products. FTIR spectrometry indicated likely involvement of carbonyl and amide groups in Cr(3+) adsorption, and SEM-EDS showed the presence of chromium on P. putida surface. Thus, our promising isolate can be ecofriendly employed for biotreatment of various industrial wastes contaminated with high PCP and Cr(6+) concentrations.

Field Determinations of Pentachlorophenol in Water Using UV/Vis Spectroscopy

Pentachlorophenol (PCP) is an organochloride pesticide banned in many countries due to its broad-spectrum toxicity. Current methods for monitoring PCP in environmental water require expensive laboratory equipment, limiting field monitoring. Two field methods for screening the concentration of PCP in environmental water are described herein. The first involves filtering the sample and calculating the indicative concentration from the absorbance at 320 nm. Alternatively, the sample can be acidified with concentrated hydrochloric acid to produce a fine suspension. This cloudy solution can be matched to a photo card for field estimation of concentration, or calculated more accurately from the absorbance at 450 nm. The useable ranges for these methods are 2 ppb to 100 ppm for the un-acidified method and 4 ppm to 1000 ppm for the acidified method. Results indicate that aquatic humic substances and natural turbidity present in environmental water do not compromise the results.

Defect-mediated formation of Ag cluster-doped TiO2 nanoparticles for efficient photodegradation of pentachlorophenol

A novel strategy was designed to prepare Ag cluster-doped TiO(2) nanoparticles (Ag/TiO(2) NPs) without addition of any chemical reducing agent and/or organic additive. A defect-rich TiO(x) species was generated by laser ablation in liquid (LAL) of a Ti target. The silver ions could be reduced and deposited on the surface of TiO(2) NPs through the removal of oxygen vacancies and defects; the TiO(x) species evolved into anatase NPs in a hydrothermal treatment process. The derived Ag/TiO(2) NPs are approximately 25 nm in size, with narrow size distribution. The Ag clusters are highly dispersed inside TiO(2) and less than 3 nm in size. The doped amount can be tuned by changing the concentration of Ag(+) ions. The as-synthesized Ag/TiO(2) NPs display improved photocatalytic efficiency toward pentachlorophenol (PCP) degradation.

Degradation of pentachlorophenol by potato polyphenol oxidase

In this study, polyphenol oxidase (PPO) was extracted from commercial potatoes. Degradation of pentachlorophenol by potato PPO was investigated. The experimental results show that potato PPO is more active in weak acid than in basic condition and that the optimum pH for the reaction is 5.0. The degradation of pentachlorophenol by potato PPO reaches a maximum at 298 K. After reaction for 1 h, the removal of both pentachlorophenol and total organic carbon is >70% with 6.0 units/mL potato PPO at pH 5.0 and 298 K. Pentachlorophenol can be degraded through dechlorination and ring-opening by potato PPO. The work demonstrates that pentachlorophenol can be effectively eliminated by crude potato PPO.

Photocatalytic degradation kinetics and mechanism of pentachlorophenol based on superoxide radicals

The micron grade multi-metal oxide bismuth silicate (Bi12SiO20, BSO) was prepared by the chemical solution decomposition technique. Photocatalytic degradation of pentachlorophenol (PCP) was investigated in the presence of BSO under xenon lamp irradiation. The reaction kinetics followed pseudo first-order and the degradation ratio achieved 99.1% after 120 min at an initial PCP concentration of 2.0 mg/L. The pH decreased from 6.2 to 4.6 and the dechlorination ratio was 68.4% after 120 min at an initial PCP concentration of 8.0 mg/L. The results of electron spin resonance showed that superoxide radical (O2*-) was largely responsible for the photocatalytic degradation of PCP. Interestingly, this result was different from that of previous photocatalytic reactions where valence band holes or hydroxyl radicals played the role of major oxidants. Some aromatic compounds and aliphatic carboxylic acids were determined by GC/MS as the reaction intermediates, which indicated that O2*- can attack the bond between the carbon and chlorine atoms to form less chlorinated aromatic compounds. The aromatic compounds were further oxidized by O2*- to generate aliphatic carboxylic acids which can be finally mineralized to CO2 and H2O.

Reduction of pollutants in pulp paper mill effluent treated by PCP-degrading bacterial strains

Two PCP-degrading bacterial strains, Bacillus cereus (ITRC-S6) and Serratia marcescens (ITRC-S7) were used for the treatment of pulp and paper mill effluent at conditions; 1.0% glucose and 0.5% peptone at 30 +/- 1 degrees C at 120 rpm for 168 h of incubation. These two bacterial strains effectively reduced colour (45-52%), lignin (30-42%), BOD (40-70%), COD (50-60%), total phenol (32-40%) and PCP (85-90%) within 168 h of incubation. However, the highest reduction in colour (62%), lignin (54%), BOD (70%), COD (90%), total phenol (90%) and PCP (100%) was recorded by mixed culture treatment. The bacterial mechanism for the degradation of pulp and paper mill effluent may be explained by an increase in the cells biomass using added co-substrates resulting liberation of significant amount of chloride due to bacterial dechlorination of chlorolignins and chlorophenols this showed reduction in colour, lignin and toxicity in the effluent. Further, GC-MS analysis of ethyl acetate-extractable compounds from treated pulp paper mill effluent reinforces the bacterium capability for the degradation of lignin and pentachlorophenol, as many aromatic compounds such as 2-chlorophenol, 2, 4, 6-trichlorophenol and tetrachlorohydroquinone, 6-chlorohydroxyquinol and tetrachlorohydroquinone detected which were not present in the untreated effluent.

Identification and characterization of integron mediated antibiotic resistance in pentachlorophenol degrading bacterium isolated from the chemostat

A bacterial consortium was developed by continuous enrichment of microbial population isolated from sediment core of pulp and paper mill effluent in mineral salts medium (MSM) supplemented with pentachlorophenol (PCP) as sole source of carbon and energy in the chemostat. The consortia contained three bacterial strains. They were identified as Escherichia coli, Pseudomonas aeruginosa and Acinetobacter sp. by 16S rRNA gene sequence analysis. Acinetobacter sp. readily degraded PCP through the formation of tetrachloro-p-hydroquinone (TecH), 2-chloro-1,4-benzenediol and products of ortho ring cleavage detected by gas chromatograph/mass spectrometer (GC-MS). Out of the three acclimated PCP degrading bacterial strains only one strain, Acinetobacter sp. showed the presence of integron gene cassette as a marker of its stability and antibiotic resistance. The strain possessed a 4.17 kb amplicon with 22 ORF's. The plasmid isolated from the Acinetobacter sp. was subjected to shotgun cloning through restriction digestion by BamHI, HindIII and SalI, ligated to pUC19 vector and transformed into E. coli XLBlue1alpha, and finally selected on MSM containing PCP as sole source of carbon and energy with ampicillin as antibiotic marker. DNA sequence analysis of recombinant clones indicated homology with integron gene cassette and multiple antibiotic resistance genes.

Photodechlorination of octachlorodibenzothiophene and octachlorodibenzofuran: comparison of experimental degradation pathways with degradation pathways predicted by DFT

Polychlorodibenzothiophenes (PCDTs) are sulfur analogues of polychlorodibenzofurans (PCDFs) and have been detected in environmental samples. We used density functional theory calculations (Gaussian 98W) to predict the photodechlorination pathways of octachlorodibenzothiophene (OCDT) and octachlorodibenzofuran (OCDF) in hexane, and we compared the predicted pathways with those observed during UV irradiation experiments. OCDT and OCDF were observed to degrade through first-order dechlorination processes, and the rate constant for OCDT was less than one-third that for OCDF. The main experimental photodechlorination pathways of OCDT and OCDF led to hexachlorinated and tetrachlorinated congeners, respectively; that is, the photodechlorination pathway of OCDT differed from that of OCDF. On the assumption that the dechlorination mechanisms involved radical reactions, we used DFT calculations to estimate bond-dissociation energies and single-point energies of OCDT and OCDF and their dechlorinated congeners, and we used the resulting information, along with hypotheses regarding the rate-controlling step of the degradations, to predict theoretical degradation pathways. We propose that reaction of dechlorinated radicals with a hydrogen donor was the rate-controlling step for OCDT and that C-Cl bond dissociation by UV light was the rate-controlling step for OCDF.

Investigation of the biotransformation of pentachlorophenol and pulp paper mill effluent decolorisation by the bacterial strains in a mixed culture

Mixed culture of two bacterial strains Bacillus sp. and Serratia marcescens showed potential pentachlorophenol (PCP) degradation and decolorisation of pulp paper mill effluent. The physico-chemical quality of pulp paper mill effluent has been analyzed after 168 h incubation period degraded by mixed culture. The study revealed that it has decreased high load of BOD, COD, TS, TDS, TSS, sulphate, phosphate, total nitrogen, total phenols, metals and different salts (i.e. chloride, sodium, nitrate, potassium) at 168 h incubation period. PCP degradation in pulp paper mill effluent was confirmed by HPLC analysis. Mixed culture was found to degrade PCP up to (94%) present in pulp paper mill effluent with 1% glucose and 0.5% peptone (w/v) at 30+/-1 degrees C, pH 8.0+/-0.2 at 120 rpm in 168 h incubation period. The simultaneous release of chloride ion up to 1,200 mg/l at 168 h emphasized the bacterial dechlorination in the medium. The pulp paper mill effluent degradation was also supported by decline in pH, AOX (absorbable organic halides), color, D.O., BOD, COD and PCP. The analysis of pulp paper mill effluent degradation products by GC-MS analysis revealed the formation of low molecular weight compound like 2-chlorophenol (RT=3.8 min) and tetrachlorohydroquinone (RT=11.86 min) from PCP extracted degraded sample. Further, mixed culture may be used for bioremediation of PCP containing pulp paper mill waste in the environment.

Photodegradation fates of cis-chlordane, trans-chlordane, and heptachlor in ethanol

cis-Chlordane, trans-chlordane, and heptachlor were photodegraded in ethanol, and their degradation fates and degradation products were determined by a computational chemical method. The most degradable material was heptachlor (first-order reaction constant k=0.13 min(-1)). Chlorine balances changed during UV irradiation, and the chlorine atoms in chlordane and heptachlor were eventually mineralized. cis-Chlordane, trans-chlordane and heptachlor each generated two di-dechlorinated products. Reactivities at various positions in these compounds were predicted on the basis of bond dissociation energies calculated by nonempirical molecular orbital calculation (Gaussian 98W).

Photodegradation of hexachlorobenzene and theoretical prediction of its degradation pathways using quantum chemical calculation

Experimentally determined photodegradation pathways of hexachlorobenzene (HCB), a chlorinated aromatic compound, in hexane, 2-propanol (IPA), and methanol were compared with those predicted by quantum chemical calculation based on density functional theory (DFT), and the adequacy of the prediction method was evaluated. The experimental main degradation pathways of HCB were virtually the same for the three solvents and also agreed with the predicted main degradation pathways. In the DFT method, the main degradation product was the dechlorinated benzene at the position where the C-Cl bond was predicted to have the lowest bond dissociation energy. This result suggested that the photodechlorination pathways of chlorinated aromatic compounds could be predicted by comparing the bond dissociation energies calculated with the DFT method.

The environmental behaviour of polychlorinated phenols and its relevance to cork forest ecosystems: a review

Pentachlorophenol (PCP) has been used as a herbicide, biocide and preservative worldwide since the 1930s and as a result, extensive and prolonged contamination exists. The environmental impact increases when its many degradation products are taken into consideration. A number of chloroanisols and their related chlorophenols have been found in cork slabs collected from Portuguese oak tree forests before stopper manufacturing, and contamination by PCP and polychlorinated anisole (PCA) has been detected in Canadian forests. It is suggested that the use of polychlorinated phenols, in particular PCP, is thought to be a cause of the cork taint problem in wine, a major socio-economic impact not only for industry but on sensitive and highly biodiverse ecosystems. It also highlights particular issues relating to the regional regulation of potentially toxic chemicals and global economics world wide. To fully understand the impact of contamination sources, the mechanisms responsible for the fate and transport of PCP and its degradation products and assessment of their environmental behaviour is required. This review looks at the current state of knowledge of soil sorption, fate and bioavailability and identifies the challenges of degradation product identification and the contradictory evidence from field and laboratory observations. The need for a systematic evaluation of PCP contamination in relation to cork forest ecosystems and transfer of PCP between trophic levels is emphasised by discrepancies in bioaccumulation and toxicity. This is essential to enable long term management of not only transboundary contaminants, but also the sustainable management of socially and economically important forest ecosystems.