Degradation Of Hexachlorobenzene Research Articles

Dithionite promoted microbial dechlorination of hexachlorobenzene while goethite further accelerated abiotic degradation by sulfidation in paddy soil

It is of great scientific and practical importance to explore the mechanisms of accelerated degradation of Hexachlorobenzene (HCB) in soil. Both iron oxide and dithionite may promote the reductive dechlorination of HCB, but their effects on the microbial community and the biotic and abiotic mechanisms behind it remain unclear. This study investigated the effects of goethite, dithionite, and their interaction on microbial community composition and structure, and their potential contribution to HCB dechlorination in a paddy soil to reveal the underlying mechanism. The results showed that goethite addition alone did not significantly affect HCB dechlorination because the studied soil lacked iron-reducing bacteria. In contrast, dithionite addition significantly decreased the HCB contents by 44.0–54.9%, while the coexistence of dithionite and goethite further decreased the HCB content by 57.9–69.3%. Random Forest analysis suggested that indicator taxa (Paenibacillus, Acidothermus, Haliagium, G12-WMSP1, and Frankia), Pseudomonas, richness and Shannon’s index of microbial community, and immobilized Fe content were dominant driving factors for HCB dechlorination. The dithionite addition, either with or without goethite, accelerated HCB anaerobic dechlorination by increasing microbial diversity and richness as well as the relative abundance of the above specific bacterial genera. When goethite and dithionite coexist, sulfidation of goethite with dithionite could remarkably increase FeS formation and then further promote HCB dechlorination rates. Overall, our results suggested that the combined application of goethite and dithionite could be a practicable strategy for the remediation of HCB contaminated soil.

Mechanochemical treatment of hexachlorobenzene-contaminated soil with additives.

The use of mechanochemistry for the remediation of hexachlorobenzene (HCB)-contaminated soil was investigated. Additives such as alkaline materials, neutral materials, natural minerals, and solid waste were studied to explore their effect on the degradation of hexachlorobenzene in soil with single or combined addition by mechanochemical method. The best combination of materials were determined based on HCB destruction percentage by considering the impact on soil quality, the treatment cost, and the availability of additives. Scanning electron microscope (SEM) images and X-ray photoelectron spectrometer (XPS) analysis were conducted for the mechanism studies. The combination of albite and ferric oxide (Fe3O4) was found to achieve the best performance in the degradation of HCB with the destruction percentage from 74.3 to 92.5% after 2-h and 6-h reaction, respectively. The developed fracture structure and complex compositions of albite provided abundant reaction sites for mechanochemical degradation of HCB in soil.

Biochar and biochar-polylactic acid composite enhance biodegradation of hexachlorobenzene in soil by altering microbial community

It is of great scientific and practical importance to explore the mechanisms of accelerated degradation of hexachlorobenzene (HCB) in soil. However, the response of microbial community composition and structure, and their links to anaerobic reductive dechlorination in paddy soils remains unclear yet. We examined impacts of biochar and biochar-polylactic acid composite addition on the diversity, composition, and structure of soil prokaryotic community and their different contribution to HCB anaerobic dechlorination. The results showed that both biochar and biochar-polylactic acid composite addition could promote HCB dechlorination in paddy soils. Soil prokaryotic community richness and diversity were not significantly impacted by biochar but were remarkably affected by biochar-polylactic acid composite amendment. Random forest analysis and structural equation models indicated that the relative abundance of indicator taxa, soil Eh, and Fe2+ content best explained (p < 0.01) the HCB dechlorination with biochar addition, while soil Eh, Fe2+ content, and microbial diversity were the most important predictors of HCB dechlorination with biochar-polylactic acid composite addition. Together, our findings suggest that biochar amendment promoted anaerobic dechlorination of HCB in paddy soils by increasing the relative abundance of specific bacterial genera, while biochar-polylactic acid composite promoted HCB dechlorination by enhancing microbial community diversity. Therefore, biochar and biochar-polylactic acid composite showed distinct effect on the composition and diversity of soil prokaryotic community and different contribution to HCB dechlorination. Overall, our results suggested that biochar-polylactic acid composite could be used as soil remediation material to efficiently accelerate dechlorination of HCB.

Application of Deconica castanella ligninolytic enzymatic system in the degradation of hexachlorobenzene in soil.

Hexachlorobenzene (HCB) is a pollutant still found in the environment despite being widely banned. Considering that basidiomycetes are useful to degrade a variety of organochlorinated pollutants, we therefore report the influence of HCB on the ligninolytic enzymatic system of Deconica castanella. The inoculum was prepared with sugarcane bagasse and soybean flour and was added in soil with and without HCB (2000mgkgsoil-1 ), 5% emulsion containing soybean oil and Tween 20 at proportion 9:1, v:v; with 70% moisture at 25°C. Fungal biomass was quantified by widely acknowledged growth biomarker ergosterol. The extraction of the enzymatic complex was performed and laccase, Mn-dependent peroxidase (MnP), and lignin peroxidase (LiP) activities were determined. Furthermore, HCB and its metabolites were quantified by gas chromatography and chlorides by potentiometric titration. Results evidenced that HCB did not interfere in fungal growth, though the only detected enzymatic activity was laccase. MnP and Lip were not detected during D. castanella growth in soil. The peak of laccase enzymatic activity occurred in the presence of HCB. In addition, the laccase exhibited thermostability. Therefore, we hereby shed light on the role of laccase in the degradation of HCB by an efficient low-cost and environmentally safe detoxification mechanism.

Mechanochemical degradation of hexachlorobenzene with a combined additive of SiC and Fe

Mechanochemical treatment (MTC) with SiC as an additive possessed a good performance on the degradation of hexachlorobenzene (HCB). However, this method consumed the additive of SiC and generated a toxic product of CCl4. This work attempted to add another additive, iron powder, to the MCT and explored the performance of the combined additive of SiC + Fe to solve this problem. Result indicated that CCl4 was not produced and SiC was not consumed when Fe was added in proper proportion. Compared with the single additive of SiC, the addition of Fe can further accelerate the degradation of HCB. The reaction pathway was proposed as: HCB can be mostly dechlorinated after a duration of MCT, and only trace-level low chlorobenzenes were generated under the effect of the combined additive. The left benzene skeletons can be broken to produce graphite and amorphous carbon. The shedding Cl radicals preferentially reacted with Fe to form FeCl3, and the reaction between Cl radicals and SiC and its toxic products can be avoided. Free electrons generated from the additive of SiC, fast reaction between Cl and Fe, and sparks in the MCT were considered the potential factors to accelerate the HCB degradation in MCT.

Enhanced anaerobic degradation of hexachlorobenzene in a Hydragric Acrisol using humic acid and urea

Humic substances acting as an electron shuttle and nitrogen transformation process influence remarkably the electron transfer in anaerobic reaction systems and thus may affect the reductive dechlorination of hexachlorobenzene (HCB). In order to develop an efficient agricultural strategy for the remediation of organochlorine-contaminated soils, a batch incubation experiment was conducted to study the effects of humic acid, urea, and their interaction on the reductive dechlorination of HCB in a Hydragric Acrisol with high iron oxide content. After 44 d of anaerobic incubation, the five treatments, sterile control, control, humic acid, urea, and humic acid + urea decreased HCB residues by 28.8%, 47.8%, 64.7%, 57.8%, and 71.3%, respectively. The amendment of humic acid or urea significantly decreased soil Eh values and accelerated Fe(III) reduction to Fe(II), thus promoting markedly reductive dechlorination of HCB. Humic acid had a larger dechlorination effect than urea. Since there was a synergistic interaction between humic acid and urea that accelerated HCB dechlorination, the treatment having both amendments together was the most efficient for HCB dechlorination. The results showed that the combination of NH4+-N supplied by a fertilizer and humic substance is a feasible strategy for the remediation of organochlorine-contaminated soils with abundant iron oxide.

Alumina-mediated photocatalytic degradation of hexachlorobenzene in aqueous system: Kinetics and mechanism

Five kinds of Al2O3 were characterized by SEM, TEM, FT-IR and BET surface area, and then used as carriers to investigate the photochemical removal of hexachlorobenzene (HCB) in aqueous system. The results showed that HCB coated on the surfaces of all Al2O3 could be photodegraded rapidly, and Neutral-Al2O3 presented the best performance. Meanwhile, the efficient removal of HCB in real water matrices, including tap water, river water and secondary clarifier effluent showed the potential practical application of Al2O3. EPR and theoretical calculation revealed the generation of hydroxyl radicals on Al2O3 surface under 500 W Xe lamp irradiation. Nine intermediates and a small amount of Cl− were identified by GC/MS, LC/MS and IC analysis, which was further verified by transition state calculations. These results can provide a new technique for HCB removal in water and wastewaters, and give more insights into the environmental ecological risk assessment of this pollutant.

Mechanochemical treatment with CaO-activated PDS of HCB contaminated soils

Mechanochemical methods with co-milling reagents have been widely used to degrade organic pollutants. In this study, calcium oxide and persulfate were employed as co-milling reagents in a mechanochemical process that showed highly effective degradation of hexachlorobenzene in contaminated soil. The influences of soil particle size and organic matter content were also investigated. The interaction between different factors was analyzed by response surface methodology, and a multi-variate regression equation was obtained relating the soil-to-oxidant mass ratio, rotation speed and organic matter content. The existence of SO4- and OH during the mechanochemical reaction was proved by the indirect detection of benzoquinone and p-hydroxybenzoic acid for the first time, providing a new method for testing free radicals in solid-phase reactions. Finally, a possible activation mechanism and hexachlorobenzene degradation pathway were proposed. This study successfully presents a mild degradation method in the field of hexachlorobenzene contaminated site remediation.

Mechanism of the rapid mechanochemical degradation of hexachlorobenzene with silicon carbide as an additive.

Mechanochemical treatment (MCT) is a promising method for degrading hexachlorobenzene (HCB). Silicon carbide (SiC) was proposed in this study as a new additive to accelerate the reaction in MCT. The high performance of SiC was verified, and the relevant mechanism was explored. Graphite, amorphous carbon, CCl4, SiO2, and water-soluble chloride were confirmed as predominant products in the proposed method, and only trace-level low chlorinated benzenes were detected. The reaction pathway was revealed as follows: under the attack of free electrons, chlorine atoms were shed from the benzene rings of HCB to form Cl· radicals, which reacted with SiC to form SiCl4 and CCl4 and with the in situ-generated iron powder to produce Fe-based chloride. The left benzene rings were translated to graphite and amorphous carbon. As an intermediate product, SiCl4 further reacted with water vapor in the atmosphere to produce SiO2 and HCl. The in situ-generated iron powder could not remarkably accelerate the degradation reaction. The major contribution of SiC was the supply of free electrons to trigger the reaction. Two sources of free electrons were discussed. Friction heat resulting from hard SiC also contributed to the endothermic reaction of HCB degradation.

Degradation kinetics of hexachlorobenzene over zero-valent magnesium/graphite in protic solvent system and modeling of degradation pathways using density functional theory

Hexachlorobenzene (HCB), like many chlorinated organic compounds, has accumulated in the environment from agricultural and industrial activity. Because of its health risks and adverse impact on various ecosystems, remediation of this contaminant is of vital concern. The objective of this study is to evaluate the proficiency of activated magnesium metal in a protic solvent system to accomplish reductive dechlorination of HCB. Experimental results were compared with those predicted by quantum chemical calculations based on Density Functional Theory (DFT). Multivariate analysis detected complete degradation of HCB within 30 min at room temperature, the reaction having a rate constant of 0.222 min−1. Dechlorination was hypothesized to proceed via an ionic mechanism; the main dechlorination pathways of HCB in 1:1 ethanol:ethyl lactate were HCB → PCBz → 1,2,4,5-TCB; 1,2,3,5-TCB → 1,2,4-TriCB; 1,3,5-TriCB → 1,4-DiCB; 1,3-DiCB. The direct relationship between the decreasing number of Cl substituents and dechlorination reaction kinetics agrees with the ΔG values predicted by the computational model. This methodology shows promise for the development of a practical and sustainable field application for the remediation of other chlorinated aromatic compounds.

Mechanochemical formation of chlorinated phenoxy radicals and their roles in the remediation of hexachlorobenzene contaminated soil

Mechanochemical degradation (MCD) is a promising eco-friendly method to dispose persistent organic pollutants (POPs). Mechanically induced free-radical attack was thought to be one of the key elements in initiating and accelerating the dechlorination and degradation of POPs. In this study, mechanochemical formation of free-radicals and their roles in the remediation of hexachlorobenzene (HCB) contaminated soil were explored using both of experimental analysis and quantum chemical calculations. It was found that chlorinated phenoxy radicals(CB-O) can be produced in the milling process and they played a vital role in the dechlorination of HCB, based on the results of electron spin-resonance (ESR) and X-ray photoelectron spectra (XPS). Two transition states of mechanochemical reaction along the formation of pentachlorinated phenoxy radical (PeCB-O) were located, with the energy barriers of 39.4 and 3.4 kJ/mol. The localized orbital locator (LOL), Mayer bond order and topological analysis were also implemented to depict the process in detail. Free-radical attack dominated dechlorination pathway of HCB in the MCD process was also verified by the Fukui function analysis. The study on the mechanically-induced generation of free-radicals and their associated modes of action on the degradation of HCB will provide a deep insight into mechanochemical remediation mechanism of POPs contaminated soil.

Augmenting atrazine and hexachlorobenzene degradation under different soil redox conditions in a bioelectrochemistry system and an analysis of the relevant microorganisms

Soil microbial fuel cells (MFCs) are a sustainable technology that degrades organic pollutants while generating electricity. However, there have been no detailed studies of the mechanisms of pollutant degradation in soil MFCs. In this study, the effects of external resistance and electrode effectiveness on atrazine and hexachlorobenzene (HCB) degradation were evaluated, the performance of soil MFCs in the degradation of these pollutants under different soil redox conditions was assessed, and the associated microorganisms in the anode were investigated. With an external resistance of 20Ω, the degradation efficiencies of atrazine and HCB were 95% and 78%, respectively. The degradation efficiency, degradation rate increased with decreasing external resistance, while the half-life decreased. There were different degradation trends for different pollutants under different soil redox conditions. The fastest degradation rate of atrazine was in the upper MFC section (aerobic), whereas that of HCB was in the lower MFC section (anaerobic). The results showed that electrode effectiveness played a significant role in pollution degradation. In addition, the microbial community analysis demonstrated that Proteobacteria, especially Deltaproteobacteria involved in current generation was extremely abundant (27.49%) on soil MFC anodes, although the percentage abundances of atrazine degrading Rhodocyclaceae (8.77%), Desulfitobacterium (0.64%), and HCB degrading Desulfuromonas (0.73%), were considerably lower. The results of the study suggested that soil MFCs can enhance the degradation of atrazine and HCB, and bioelectrochemical reduction was the main mechanism for the pollutants degradation.

Theoretical and experimental investigation into the photocatalytic degradation of hexachlorobenzene by ZnCr layered double hydroxides with different anions

In this paper, we synthesized five different types of ZnCr layered double hydroxides with different anions (F−, Cl−, Br−, I− and NO3−). A thorough determination of the structure, chemical composition and other physical and chemical properties of the synthesized LDHs materials were done by techniques such as XRD, FTIR, BET, ICP-AES and UV–vis. The photocatalytic degradation of hexachlorobenzene reaction showed that different anions have largely influence on the reactivity and selectivity of the LDHs. The structure stability difference of the LDHs was explained and compared based on the difference of the following physical properties such as Mulliken bonding population, hydrogen bonding and density of states, which were calculated based on DFT theory. It was found that the photocatalytic performance of these LDHs was highly related to their structure stability. In addition, the HCB degradation mechanism was also analyzed by combining GC–MS data and the dissociation energy calculation.

Structure and function of the bacterial communities during rhizoremediation of hexachlorobenzene in constructed wetlands.

Vertical flow constructed wetlands (VF CWs) are considered to be effective for treating organic pollutants. The rhizosphere of macrophytes such as Phragmites sp., Typha sp. serves as an active and dynamic zone for the microbial degradation of organic pollutants. However, it is still not clear how soil bacterial communities respond to macrophytes and pollutants during the process. For this purpose, the seedlings of Phragmites australis and Typha angustifolia were planted respectively in the VF CWs added with HCB at a dose of 2mg/kg. During 96days of cultivation, we monitored hexachlorobenzene (HCB) removal efficiency by GC/MS and the structure of the rhizosphere bacterial communities in the different VF CWs by denaturing gradient gel electrophoresis (DGGE), and constructed bacterial clone library based on PCR-amplified 16S rRNA gene. As expected, the rhizosphere bacterial communities also remained insensitive to HCB exposure in the wetland soil. The diversity of these microbes presented two stages, from the varied up and down to equilibrium in the entire experimental period. Molecular analysis revealed that the phylum Firmicutes dominated over the bacterial communities. The genera that increased under HCB stress included the well-known HCB-degrading bacteria (Pseudomonas sp. and Alcaligenes sp.) and other common bacteria found in contaminated soil but with lesser known practical functions (Burkholderia sp., Lysinibacillus fusiformis, and Bacillus cereus). Furthermore, there was a certain variance in the relative abundances of the bacterial phyla and HCB removal efficiency among different VF CW treatments. The degradation of HCB in T. angustifolia microcosms was faster than that in P. australis and unvegetated wetlands, and the highest bacterial diversity and richness was found in the VF CWs comprising T. angustifolia.

Novel Co–Fe–O micro/nano structures manipulated by urea and their applications in degradation of hexachlorobenzene

Four microsized composite materials containing oxides of cobalt and iron were prepared via a polyol-mediated process involving aggregation of nanoparticles that formed initially. Two alkoxide precursors with different morphologies were obtained by controlling the amount of urea during the synthesis stage and calcinate each precursor in nitrogen and air afford four products. Hexagonal flower-like particles were formed in the absence of urea, while tetragonal brick-like structures were obtained in the presence of urea. The morphologies were retained during calcination. However, the compositions of the materials depend on the presence of air or nitrogen during calcination. In the absence of urea during initial synthesis, CoFe2O4 was formed when calcined in air, but a mixture of Fe3O4, Fe2O3, CoO and a small amount of Co3O4 was obtained when calcined in nitrogen. In the presence of urea, CoFe2O4/Fe2O3 was formed when calcined in air and a mixture, Co0.62Fe2.38O4/Fe3O4, was formed when calcined in nitrogen. In the catalytic degradation of hexachlorobenzene, the two products synthesised by calcination under nitrogen display much higher catalytic activity that is attributed to the lower valence of the metals. The two products calcined in air possess magnetic properties, while the products synthesised in the presence of urea show weaken magnetic properties because of the partial substitution of CoFe2O4 by Fe2O3.

Novel Biochar-Plant Tandem Approach for Remediating Hexachlorobenzene Contaminated Soils: Proof-of-Concept and New Insight into the Rhizosphere.

Volatilization of semi/volatile persistent organic pollutants (POPs) from soils is a major source of global POPs emission. This proof-of-concept study investigated a novel biochar-plant tandem approach to effectively immobilize and then degrade POPs in soils using hexachlorobenzene (HCB) as a model POP and ryegrass (Lolium perenne L.) as a model plant growing in soils amended with wheat straw biochar. HCB dissipation was significantly enhanced in the rhizosphere and near rhizosphere soils, with the greatest dissipation in the 2 mm near rhizosphere. This enhanced HCB dissipation likely resulted from (i) increased bioavailability of immobilized HCB and (ii) enhanced microbial activities, both of which were induced by ryegrass root exudates. As a major component of ryegrass root exudates, oxalic acid suppressed HCB sorption to biochar and stimulated HCB desorption from biochar and biochar-amended soils, thus increasing the bioavailability of HCB. High-throughput sequencing results revealed that the 2 mm near rhizosphere soil showed the lowest bacterial diversity due to the increased abundance of some genera (e.g., Azohydromonas, Pseudomonas, Fluviicola, and Sporocytophaga). These bacteria were likely responsible for the enhanced degradation of HCB as their abundance was exponentially correlated with HCB dissipation. The results from this study suggest that the biochar-plant tandem approach could be an effective strategy for remediating soils contaminated with semi/volatile organic contaminants.

Influence of Degradation Behavior of Coexisting Chlorobenzene Congeners Pentachlorobenzene, 1,2,4,5-Tetrachlorobenzene, and 1,2,4-Trichlorobenzene on the Anaerobic Reductive Dechlorination of Hexachlorobenzene in Dye Plant Contaminated Soil

The degradation of hexachlorobenzene (HCB) is of great concern and attracts considerable scientific and regulatory interests, due to the high toxicity, great bioaccumulation, and persistence of HCB in the environment. However, in the real HCB-contaminated soil, the effect of coexisting chlorobenzene congeners on the degradation capacity of HCB is poorly known. In this work, the anaerobic degradation behaviors of three coexisting chlorobenzene congeners pentachlorobenzene (PeCB), 1,2,4,5-tetrachlorobenzene (1,2,4,5-TeCB), and 1,2,4-trichlorobenzene (1,2,4-TCB) and the influence of initial pH and reaction temperature on the dechlorination of HCB in HCB-contaminated soil from the dye plant were studied. The amount and extent of accumulated coexisting chlorobenzenes was analyzed under different environmental conditions. The results indicate that the concentrations of three coexisting chlorobenzene congeners change in the form of wave. The anaerobic degradation activity of HCB is reduced due to the feedback inhibition caused by accumulation of coexisting chlorobenzene congeners, and the feedback inhibition varies from environmental conditions.

Mechanochemical degradation of hexachlorobenzene using Mg/Al2O3 as additive

In the present work, we investigate the destruction efficiency of hexachlorobenzene (HCB) by milling with various reagents in a planetary ball mill under different milling conditions. Under the same conditions of mill rotary rate and charge ratio, the mixture of magnesium powder and aluminum oxide (Mg/Al2O3) was found best in promoting the destruction of HCB, which can be completed destroyed after 90 min grinding at a charge ratio of 20:1 (reagent/HCB, m/m), a ball mass/reagent mass ration of 30:1 and a mill rotation speed of 550 rpm. The ground samples were characterized and analyzed by X-ray fluorescence, gas chromatography (GC), X-ray diffraction and ion chromatography. The intermediate products, such as pentachlorobenzene, tetrachlorobenzene, trichlorobenzene isomers, dichlorobenzene and monochlorobenzene were detected by GC. Then the main dechlorination path way for HCB was proposed. With a series of verification experiments, the final degradation products of HCB were amorphous carbon and inorganic chlorine. Based on this study, Mg/Al2O3 has the potential to complete the innocuous treatment of chlorinated compounds.

Kinetics and Product Analysis of the Electrochemical Dehalogenation of Hexachlorobenzene

A method for establishing the kinetics and products of the electrochemical degradation of hexachlorobenzene (HCB) is described. One plausible way of reducing HCB´s toxicity is through cathodic dehalogenation. The characterization of products formed during the electrochemical reduction and the reaction kinetics are both reported. Pentachlorobenzene (PCB), tetrachlorobenzene (TCB) and trichlorobenzene (TriCB) were identified among the degradation products by high performance liquid chromatography (HPLC) with ultraviolet (UV) detection. The kinetics is analyzed under different electrolysis conditions.

Biodegradation of hexachlorobenzene by a constructed microbial consortium.

A consortium comprised of an engineered Escherichia coli DH5α and a natural pentachlorophenol (PCP) degrader, Sphingobium chlorophenolicum ATCC 39723, was assembled for degradation of hexachlorobenzene (HCB), a persistent organic pollutant. The engineered E. coli strain, harbouring a gene cassette (camA (+) camB (+) camC) that encodes the F87W/Y96F/L244A/V247L mutant of cytochrome P-450cam (CYP101), oxidised HCB to PCP. The resulting PCP was then further completely degraded by ATCC 39723. The results showed that almost 40 % of 4 μM HCB was degraded by the consortium at a rate of 0.033 nmol/mg (dry weight)/h over 24 h, accompanied by transient accumulation and immediate consumption of the intermediate PCP, detected by gas chromatography. In contrast, in the consortium comprised of Pseudomonas putida PaW340 harbouring camA (+) camB (+) camC and ATCC 39723, PCP accumulated in PaW340 cells but could not be further degraded, which may be due to a permeability barrier of Pseudomonas PaW340 for PCP transportation. The strategy of bacterial co-culture may provide an alternative approach for the bioremediation of HCB contamination.