Chlorophenols and other related derivatives of environmental concern: properties, distribution and microbial degradation processes

Chlorophenols are chlorinated aromatic compound structures and are commonly found in pesticide preparations as well as industrial wastes. They are recalcitrant to biodegradation and consequently persistent in the environment. A variety of chlorophenols derivatives compounds are highly toxic, mutagenic and carcinogenic for living organisms. Biological transformation by microorganisms is one of the key remediation options that can be exploited to solve environmental pollution problems caused by these notorious compounds. The key enzymes in the microbial degradation of chlorophenols are the oxygenases and dioxygenases. These enzymes can be engineered for enhanced degradation of highly chlorinated aromatic compounds through directed evolution methods. This review underscores the mechanisms of chlorophenols biodegradation with the view to understanding how bioremediation processes can be optimized for cleaning up chloroaromatic contaminated environments.     

Assessment of the impact of the emission of certain organochlorine compounds on the aquatic environment: Part I: Monochlorophenols and 2,4-dichlorophenol

The consequences of contamination of the aquatic environment by chlorophenols (2-chlorophenol, 3-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol), have been evaluated by extensive review of the available scientific data. The chlorophenols generally exert moderate toxic effects to mammalian and aquatic life, although toxicity to fish upon long-term exposure may be considerable, as has been found for 2,4-dichlorophenol. Persistence is low when adapted microflora is present, capable of biodegrading these compounds, but may become moderate to high depending on the environmental conditions. This is most pronounced for 3-chlorophenol. Bioaccumulation is expected to be low. A striking feature of these chlorophenols is their strong organoleptic effect.     

Mechanism of the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans from chlorophenols in gas phase reactions

The pyrolysis of chlorinated phenates at a temperature of about 280 degrees C results in the formation of definite chlorinated dibenzodioxin (PCDD) congeners [1-3]. It is shown that in gas phase reactions chlorophenols react in the presence of oxygen above 340 degrees C not only to PCDD but also to chlorinated dibenzofurans (PCDF). The mechanism of this reaction of chlorophenols to PCDD and PCDF was elucidated. In a first step phenoxyradicals are formed which are capable of forming PCDDs and PCDFs. This is confirmed by the oxygen dependency of the reaction. In an argon atmosphere no dimerization of chlorophenols could be observed at 420 degrees C. By the identification of intermediates and by analyzing the PCDF isomers formed from individual chlorophenols the reaction pathway is elucidated. As intermediates in the formation of PCDFs polychlorinated dihydroxybiphenyls (DOHB) were identified. These are most likely formed by the dimerization of two phenoxy radicals at the hydrogen substituted carbons in ortho-positions under simultaneous movement of the hydrogen atoms to the phenolic oxygen PCDDs are formed in the gas phase via ortho-phenoxyphenols (POP) analogous to the pyrolysis of phenates, but due to the radical mechanism in the first condensation step to POPs not only a chlorine atom is capable for substitution but also the hydrogen atoms. The formation of the DOHBs and their condensation to PCDFs and hydroxylated PCDFs as well as the ratio of PCDD to PCDF formed show a strong dependency on the reaction temperature, the substitution pattern of the chlorophenols and the oxygen concentration.     

Contribution of free radicals to chlorophenols decomposition by several advanced oxidation processes

The chemical decomposition of aqueous solutions of various chlorophenols (4-chlorophenol (4-CP), 2,4-dichlorophenol (2-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP)), which are environmental priority pollutants, is studied by means of single oxidants (hydrogen peroxide, UV radiation, Fenton's reagent and ozone at pH 2 and 9), and by the Advanced Oxidation Processes (AOPs) constituted by combinations of these oxidants (UV/H2O2 UV/Fenton's reagent and O3/UV). For all these reactions the degradation rates are evaluated by determining their first-order rate constants and the half-life times. Ozone is more reactive with higher substituted CPs while OH* radicals react faster with those chlorophenols having lower number of chlorine atoms. The improvement in the decomposition levels reached by the combined processes, due to the generation of the very reactive hydroxyl radicals. in relation to the single oxidants is clearly demonstrated and evaluated by kinetic modeling.     

Formation of hydroxylated and dimeric intermediates during oxidation of chlorinated phenols in aqueous solution

The oxidation of selected chlorophenols (2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachlorophenol) was studied in aqueous solutions using UV/H2O2 and O3 methods. The formation of oxidation intermediates was measured to elucidate their importance in the treatment of chlorophenols. Results indicated that chlorophenols can be treated efficiently by the methods studied, but the dechlorination of the compounds was insufficient. Analysis of intermediates in the acetylated extraction fractions showed that hydroxylation of chlorophenols and formation of dimeric products were involved in the oxidation of chlorophenols in both treatment processes. The majority of the intermediates detected were transient, and thus were not detectable after an extended treatment time. The presence of a complicated mixture of intermediates suggests the need for toxicity testing to confirm the detoxifying effect of the chemical oxidation of chlorophenols.     

Organic chlorine compounds in lake sediments. III. Chlorohydrocarbons, free and chemically bound chlorophenols

Chlorohydrocarbons and free and chemically bound chlorophenols, catechols and guaiacols were measured from lake sediments in Central Finland. Chlorohydrocarbons occurred at low backround levels except chlorocymenes from pulp mills and PCBs near the city of Äänekoski, where a local leakage occurrs. Free chlorophenols (hexane extractables) were found in all 18 areas studied, but the bound ones (extracted with strong alkali) only in the pulp mill reciepient lakes where they dominated. Preliminary analyses of chemically bound chlorophenols in high molecular fractions of spent bleach liquors and pulp mill effluents indicated that majority of chlorocatechols and guaiacols are bound already in the factory.     

Environmental fate and bioavailability of wood preservatives in freshwater sediments near an old sawmill site.

Impacts of an old contaminated sawmill site located in Eastern Finland were studied, with emphasis on transportation and bioaccumulation of wood preservatives in the surrounding water system. To assess the transportation of chlorophenols and chromated copper arsenate (CCA) from the sawmill to the nearby lake, the concentrations of these compounds in selected sediment samples were analyzed. To assess the contribution of a pulp mill further upstream, the concentration of extractable organic halogens (EOX) was analyzed. Bioaccumulation of wood preservatives from sediments was examined using Lumbriculus variegatus as test organism. In sediments collected from the sawmill area, concentrations of chlorophenols, arsenic, chromium and copper were high. In the surrounding area the concentrations of these compounds were slightly elevated at some sampling points but were mostly within the natural range of variation. Thus, it can be concluded that transportation of wood preservatives from the sawmill area to its surroundings is fairly low. However, 60 microg/l of arsenic and 50 microg/l of copper were found in water taken from a brook that runs through a landfill area of the sawmill to the nearby river, and the concentration of arsenic in the surface sediment at one sampling point in the lake was slightly elevated. The total amount of organohalogens in sediment was higher in the river and the lake than in the sawmill area. Of all the wood preservatives studied, only arsenic was found to bioaccumulate in present conditions, reaching a tissue concentration of 362 microg/g dw in organisms exposed for 28 days to sediment from the brook. High concentration of arsenic in oligochaeta tissue was related to high concentration of arsenic in the pore water.     

Acute toxicity of chlorophenols to earthworms using a simple paper contact method and comparison with toxicities to fresh water organisms

An acute toxicity test of chlorophenols on earthworms (Eisenia fetida) was performed using a simple paper contact method proposed by OECD testing guideline no. 207, that were applied as an earthworm toxicity test. The median lethal concentration, EC50, had significant correlation with logP(ow) (1-octanol/water partition coefficient) of the chemicals. The toxicity of chlorophenols on E. fetida was compared with toxicities for other species: an algae (Selenostrum capricornutum), a crustacean (Daphnia magna), and a fish (Oryzias latipes). It was found that the toxicity of chlorophenols was almost same for E. fetida and for fresh water organisms. These results suggest the possibility of drawing correlations between the effects of pollutants on living things in different environments, fresh water and soil.     

Environmental VOSCs--formation and degradation of dimethyl sulfide, methanethiol and related materials

Volatile organic sulfur compounds (VOSCs) play a major role in the global sulfur cycle. Two components, dimethyl sulfide (DMS) and methanethiol (MT) are formed in large amounts by living systems (e.g. algae, bacteria, plants), particularly in marine environments. A major route to DMS is by action of a lyase enzyme on dimethylsulfoniopropionate (DMSP). DMSP has other roles, for instance as an osmoprotectant and cryoprotectant. Demethiolation of DMSP and other materials leads to MT. A major transport process is release of DMS from the oceans to the atmosphere. Oxidation of DMS in the atmosphere by hydroxyl and nitrate radicals produces many degradation products including CO2, COS, dimethyl sulfoxide, dimethyl sulfone, organic oxyacids of sulfur, and sulfate. These materials also have roles in biotic processes and there are complex metabolic interrelationships between some of them. This review emphasizes the chemical reactions of the organic sulfur cycle. For biotic reactions, details of relevant enzymes are provided when possible.     

New results on the inhibiting action of chloro- and nitro-substituted phenols on the biodegradation of phenol

Substituted phenols were tested, by kinetic measurements, as inhibitors of the phenol degradation by an activated sludge. Inhibiting actions were expressed as pI₅₀. Results of these measurements (with nitrophenols and chloronitrophenols) and of previous measurements (with chlorophenols) on the same sludge were correlated with structural properties (logP, ∑σ) of the inhibitors. The QSARs referring to the present activated sludge are compared with previous correlations of data obtained with a different sludge.     

Qualitative and quantitative cancer risk assessment of 2, 3, 7, 8-tetrachloro-dibenzo-p-dioxin (2, 3, 7, 8-TCDD)

The carcinogenicity of 2,3,7,8-TCDD at multiple organ sites in animals has been well established by several cancer bioassays. Results of two of the most notable of these, the Kociba et al. (1978) rat feeding study and the National Toxicology Program (1980) gavage study in rats and mice showed hepatocellular carcinomas in two strains of female rats and male and female mice. Other tumor sites included carcinomas of the lung, tongue, hard palate and nasal turbinates, thyroid, and subcutaneous tissue. The evidence for carcinogenicity of 2,3,7,8-TCDD in animals is regarded as “sufficient” using the classification system of the International Agency for Research on Cancer (IARC).

Two Swedish epidemiologic case-control studies (Hardell and Sandstrom, 1979; Eriksson et al. 1979, 1981) reported a significant five- to sevenfold excess risk of soft-tissue sarcomas (STS) from occupational exposure to chlorinated phenoxyacetic acid herbicides and/or chlorophenols. Additionally, several small cohort studies collectively exhibited an unusual cluster of STS, significantly increased over combined expected incidence. Problems with these studies do not appear to be sufficient to discount this excess risk. The human evidence alone for the carcinogenicity of 2,3,7,8-TCDD is “inadequate” using the IARC classification. However, for 2,3,7,8-TCDD in combination with chlorinated phenoxyacetic acid herbicides and/or chlorophenols, the human evidence is considered to be “limited.” The overall evidence for carcinogenicity considering both animal and human studies would place 2,3,7,8-TCDD alone in the IARC category 2B, meaning that the substance is probably carcinogenic in humans. The overall weight of evidence for 2,3,7,8-TCDD in combination with chlorinated phenoxyacetic acid herbicides and/or chlorophenols is regarded as IARC category 2A, also meaning that they are probably carcinogenic for humans.

Using current EPA methodology for quantitatively estimating cancer risks, several animal data sets have been analyzed. Comparing the results, the upper-limit incremental unit risk estimate is 1.6 × 10⁻² for a lifetime exposure of 1 ng/kg/day. This estimate is derived from a lifetime feeding study (Kociba et al., 1978) in which 2,3,7,8-TCDD induced tumors of the liver, lungs, hard palate, and nasal turbinates in female rats. Incremental unit cancer risks are also extrapolated for lifetime 2,3,7,8-TCDD exposures in water and air. Based on continuous lifetime exposure to 1 ng/L 2,3,7,8-TCDD in drinking water, the upper-limit estimate of extra cancer risk per individual is 4.5 × 10⁻³. For lifetime exposure to 1 pg 2,3,7,8-TCDD/m³ in the ambient air, the upper-limit individual risk is 3.3 × 10⁻⁵.     

Oxidation of chlorophenols in soil at natural pH by catalyzed hydrogen peroxide: the effect of soil organic matter.

This investigation reports on the effects of soil organic matter (SOM) during the oxidation of chlorophenols with Fe2+-catalyzed H2O2 (Fenton oxidation) system. The soil pH was 7.1 and was not altered. Sorption experiments of soil pre-treated under various oxidation conditions were performed. Concentrations of organic matter in the liquid phase and soil before and after oxidation were analyzed. The results were correlated to the observation in batch Fenton oxidation tests. They showed that the oxidation of chlorophenols at natural soil pH depended on the dose of H2O2 and Fe2+. The soil organic content did not vary significantly after various Fenton treatments, while the sorption of chlorophenols was 10-25% less by the oxidation. The concentration of chlorophenols in the liquid phase exhibited a "decrease and rebound" phenomenon in the batch Fenton oxidation tests. It appeared that the oxidation of SOM resulted in the release of sorbed chlorophenols which were then oxidized by the excess H2O2. An "oxidation-desorption-oxidation" scheme was proposed to describe one of the interaction mechanisms among the oxidant, SOM, and chlorophenols during oxidation.     

Oxidative degradation of chlorophenol derivatives promoted by microwaves or power ultrasound: a mechanism investigation

Background, aim, and scope

Phenols are the most common pollutants in industrial wastewaters (particularly from oil refineries, resin manufacture, and coal processing). In the last two decades, it has become common knowledge that they can be effectively destroyed by nonconventional techniques such as power ultrasound (US) and/or microwave (MW) irradiation. Both techniques may strongly promote advanced oxidation processes (AOPs). The present study aimed to shed light on the effect and mechanism of US- and MW-promoted oxidative degradation of chlorophenols; 2,4-dichlorophenoxyacetic acid (2,4-D), a pesticide widespread in the environment, was chosen as the model compound.

Materials and methods

2,4-D degradation by AOPs was carried out either under US (20 and 300 kHz) in aqueous solutions (with and without the addition of Fenton reagent) or solvent-free under MW with sodium percarbonate (SPC). All these reactions were monitored by gas chromatography–mass spectrometry (GC–MS) analysis and compared with the classical Fenton reaction in water under magnetic stirring. The same set of treatments was also applied to 2,4-dichlorophenol (2,4-DCP) and phenol, the first two products that occur a step down in the degradation sequence. Fenton and Fenton-like reagents were employed at the lowest active concentration.

Results

The effects of US and MW irradiation were investigated and compared with those of conventional treatments. Detailed mechanisms of Fenton-type reactions were suggested for 2,4-D, 2,4-DCP, and phenol, underlining the principal degradation products identified. MW-promoted degradation under solvent-free conditions with solid Fenton-like reagents (viz. SPC) is extremely efficient and mainly follows pyrolytic pathways. Power US strongly accelerates the degradation of 2,4-D in water through a rapid generation of highly reactive radicals; it does not lead to the formation of more toxic dimers.

Discussion

We show that US and MW enhance the oxidative degradation of 2,4-D and that a considerable saving of oxidants and cutting down of reaction times is thereby achieved. The results support the interpretation of previously published data and improve the understanding of the factors of direct degradation along different pathways.

Conclusions

Oxidative pathways for 2,4-D, 2,4-DCP, and phenol were proposed by a careful monitoring of the reactions and detection of intermediates by GC–MS.

Recommendations and perspectives

The understanding of the factors that affect chlorophenols degradation along different pathways may facilitate the optimization of the treatment. Type of energy source (US or MW), power, and frequency to be applied could be designed in function of the operative scenario (amount of pollutant in soil, water, or oils).     

Bioaccumulation, bioavailability and environmental fate of chlorophenol impurities, polychlorinated hydroxydiphenylethers and their methoxy analogues

The bioaccumulation potential and environmental fate of polychlorinated hydroxydiphenyl ethers (HO-PCDEs; polychlorinated phenoxyphenols, PCPP), the major impurities of chlorophenol formulations and their methoxy analogues (MeO-PCDEs; polychlorinated methoxyanisoles, PCPAs) were investigated. Oligochaete worms (Lumbriculus variegatus) exposed to sediment spiked with a model substance of one HO-hexaCDE (4'-HO-PCDE 161) or its methoxy analogue (4'-MeO-PCDE 161) clearly accumulated the test compounds revealing the potential for environmental risk of HO-PCDEs and MeO-PCDEs. The HO-PCDE tested has earlier been reported as an abundant component in a Finnish chlorophenol formulation (Ky-5) and its methoxy analogue is recognized as an abundant MeO-PCDE in sawmill soil contaminated by the formulation. The occurrence of 4'-HO-PCDE 161 and its methoxy analogue among other HO-PCDEs and MeO-PCDEs in lake mussels (Anodonta piscinalis) incubated in a river contaminated via the manufacture of Ky-5 showed that these compounds are bioavailable and transported in the aquatic environment. Mussel comparison with sediment data pointed to a higher accumulation potential for MeO-PCDEs than for HO-PCDEs. The finding of HO-PCDEs in groundwater samples collected from a groundwater reservoir, which had been contaminated by chlorophenols, points to potential of HO-PCDEs for transport with water in soil.     

Modeling the formation of PCDD/F in solid waste incinerators

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) appear in unacceptable amounts in the gaseous emissions during the incineration of wastes containing significant quantities of chlorine and metals, such as MSW and medical waste. They are formed both in the gas phase at temperatures above 600 degrees C and on the surface of the solid phase (flyash) in the temperature range 400-225 degrees C. Both the precursor (from existing smaller chlorinated molecules) and de novo (from elemental carbon) routes are involved. An empirically derived global model for their de novo formation on flyash in MSW and medical waste incinerators has now been extended to include the precursor mechanism, and a gas phase formation component, with separate rate expressions for PCDD and PCDF. Homogeneous PCDD formation is governed by the concentration of chlorophenols and PCDF by that of chlorophenols and chlorobenzenes. The result is more complete system which distinguishes between the gas and solid phase contributions to the I-TEQ. An additional step for the adsorption of gaseous PCDD/F back onto the solid phase during cooling suggests this should be minimal in the gas ducts of an incinerator. The extended model has been tested against experimental data collected from a well-controlled pilot incinerator and commercial incinerators, and found to adequately describe the measured outputs. With the model it should be possible to predict the PCDD/F emissions from commercial incinerators, provided that the ash properties and the overall temperature-time profiles are known.     

Uptake and transformation of phenol and chlorophenols by hairy root cultures of Daucus carota, Ipomoea batatas and Solanum aviculare

Hairy root cultures of Daucus carota L., Ipomoea batatas L. and Solanum aviculare Forst were investigated for their susceptibility to the highly toxic pollutants phenol and chlorophenols and for the involvement of inherent peroxidases in the removal of phenols from liquid media. Roots of D. carota grew normally in medium containing 1000 micromol l(-1) of phenol, whilst normal growth of roots of I. batatas and S. aviculare was only possible at levels up to 500 micromol l(-1). In the presence of chlorophenols, normal root growth was possible only in concentrations not exceeding 50 micromol l(-1), except for I. batatas which was severely affected at all concentrations. Despite the reduction in biomass, the growth of S. aviculare cultures was sustained in medium containing up to 2000 micromol l(-1) of phenol or 2-chlorophenol, and up to 500 micromol l(-1) of 2,6-dichlorophenol. The amounts of phenol removed by the roots within 72 h of treatment were 72.7%, 90.7% and 98.6% of the initial concentration for D. carota, I. batatas and S. aviculare, respectively. For the removal of 2,6-dichlorophenol the values were, respectively, 83.0%, 57.7% and 73.1%. Phenols labelled with 14C were absorbed by the root tissues and condensed with highly polar cellular substances as well as being incorporated into the cell walls or membranes. The results suggest that S. aviculare, an ornamental plant, would be best suited for remediation trials under field conditions.     

Chemical speciation and bioavailability of rare earth elements (REEs) in the ecosystem: a review

Rare earths (RE), chemically uniform group of elements due to similar physicochemical behavior, are termed as lanthanides. Natural occurrence depends on the geological circumstances and has been of long interest for geologist as tools for further scientific research into the region of ores, rocks, and oceanic water. The review paper mainly focuses to provide scientific literature about rare earth elements (REEs) with potential environmental and health effects in understanding the research. This is the initial review of RE speciation and bioavailability with current initiative toward development needs and research perceptive. In this paper, we have also discussed mineralogy, extraction, geochemistry, analytical methods of rare earth elements. In this study, REEs with their transformation and vertical distribution in different environments such as fresh and seawater, sediments, soil, weathering, transport, and solubility have been reported with most recent literature along key methods of findings. Speciation and bioavailability have been discussed in detail with special emphasis on soil, plant, and aquatic ecosystems and their impacts on the environment. This review shows that REE gained more importance in last few years due to their detrimental effects on living organisms, so their speciation, bioavailability, and composition are much more important to evaluate their health risks and are discussed thoroughly as well.     

Organic Compounds in Urban Atmospheres: A Review of Distribution,Collection and Analysis

An attempt has been made to summarize the present state of knowledge concerning the occurrence, sampling, and analytical procedures for organic components in the atmosphere. This review places special emphasis on classification strategy and draws attention to classes of compounds which may be potentially toxic. Methods currently used for collection and analyses are briefly described and evaluated.     

Isolation and physiological characterization of the pentachlorophenol degrading bacterium Sphingomonas chlorophenolica

Many chlorophenols tend to persist in the environment, and they may become public health hazards. Among chlorophenols, pentachlorophenol (PCP) is a priority pollutant that has been used widely as a general biocide in commercial wood treatment. Owing to the rapid industrial growth, serious soil and water pollutions by chlorophenols has been reported in Taiwan. In this study, 10 indigenous PCP-degrading bacterial strains were isolated from a PCP-degrading mixed culture, and the potential of both the pure and mixed cultures for PCP degradation compared. Moreover, the physiological characteristics and optimum growth conditions of the PCP-degrading bacteria were investigated. One of the isolated bacterial strains with good potential for PCP degradation was characterized and identified as Sphingomonas chlorophenolica by 16S rDNA gene analysis. The result of the optimum growth temperatures revealed that this organism was a mesophile. The optimum pH for PCP removal by S. chlorophenolica was between 6.9 and 7.6. Increase in concentration of PCP has a negative effect on the biodegradation potential of S. chlorophenolica and PCP concentration above 600 mg l(-1) was inhibitory to its growth. The results of this study indicate that this S. chlorophenolica strain has a better potential for PCP degradation compared to the enriched mixed culture. The physiological characterization of the isolates also indicates the possible application of this strain for bioremediation of sites contaminated with PCP.