Hexachlorobenzene In Soil Research Articles

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.

Degradation behaviors and accumulative effects of coexisting chlorobenzene congeners on the dechlorination of hexachlorobenzene in soil by nanoscale zero-valent iron.

It is well known that many chlorinated organic pollutants can be dechlorinated by nanoscale zero-valent iron. However, in the real chlorinated organic compounds contaminated soil, the congeners of high- and low-chlorinated isomer often coexist and their dechlorination behaviors are poorly known, such as hexachlorobenzene (HCB). In this work, the 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 by nanoscale zero-valent iron were studied. The amount and extent of accumulated coexisting chlorobenzenes was analyzed under different environmental conditions. The results indicate that nanoscale zero-valent iron can improve the degradation efficiency of highly toxic chlorinated benzenes and reduce the accumulative effects of highly toxic chlorinated benzenes on dechlorination of HCB. The accumulative effects of three coexisting chlorobenzene congeners on the dechlorination of HCB were ranked as follows: 1,2,4-TCB > 1,2,4,5-TeCB > PeCB.

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.

Effects of biochar on dechlorination of hexachlorobenzene and the bacterial community in paddy soil

Anaerobic reductive dechlorination is an important degradation pathway for chlorinated organic contaminants in paddy soil. This study investigated the effects of amending paddy soil with wheat straw biochar on both the dechlorination of hexachlorobenzene (HCB), a typical highly chlorinated contaminant, and on the structure of soil bacteria communities. Soil amendment of 0.1% biochar did not significantly affect the dechlorination of HCB in the soil. However, biochar amendment at higher application levels (5%) stimulated the dechlorination of HCB in the first month of anaerobic incubation and inhibited the dechlorination of HCB after that period. The stimulation effect may be ascribed to the graphite carbon and carbon-centered persistent radicals, which are redox active, in biochar. The inhibiting effect could be partly ascribed to the reduced bioavailability of HCB in biochar-amended soils. High-throughput sequencing revealed that the amendment of biochar changed the soil bacterial community structure but not the bacterial abundances and diversities. The relative abundance of Dehalococcoidaceae in the tested soils showed a significant relationship with the dechlorination percentages of HCB, indicating that Dehalococcoidaceae may be the main HCB-dechlorinating bacteria in the studied paddy soil. The results indicated that low application levels of biochar did not affect the dechlorination of HCB in the paddy soil, while high application levels of biochar mainly inhibited the dechlorination of HCB due to the reduced bioavailability of HCB and the reduced abundances of certain dechlorinating bacteria in the biochar-amended paddy soil.

Bioavailability assessment of hexachlorobenzene in soil as affected by wheat straw biochar

Biochar incorporation with soil could increase sorption of organic contaminants, thereby reducing their bioavailability. In this study, the effects of wheat straw biochar on the sorption, dissipation and bioavailability of hexachlorobenzene (HCB), a typical persistent organic pollutant (POP), were investigated in laboratory experiments. We observed that HCB sorption by biochar was 42 times higher than that by soil and the sorption isotherm was linear for the concentration range studied. Biochar amendments reduced HCB dissipation, volatilization and earthworm (Eisenia foetida) uptake of HCB from soil. Hydroxypropyl-β-cyclodextrin extraction correlated better with the earthworm bioassay than butanol extraction of HCB in biochar-amended soil. The results of both chemical extraction and earthworm bioassay indicate that biochar amendment of soil resulted in a rapid reduction in the bioavailability of HCB, even for the 0.1% biochar application rate. This suggested that wheat straw biochar could potentially be used in immobilizing POPs in contaminated sites.

Effect of Initial Concentration on Adsorption-Desorption Characteristics and Desorption Hysteresis of Hexachlorobenzene in Soils

Adsorption and desorption are important processes that influence the transport, transformation, and bioavailability of hexachlorobenzene (HCB) in soils. To examine the adsorption-desorption characteristics of HCB, equilibrium batch experiments were carried out using two soils (red soil and paddy soil) with different initial HCB concentrations (0.25, 0.50, 0.75, 1.00, 1.50, 2.50, 3.50, and 5.00 mg L −1) by using 0.01 mol L −1 calcium chloride as the background solution. The successive desorption experiments (48, 96, 144, 192, and 240 h) were conducted after each adsorption equilibrium experiment. The results revealed that adsorption and desorption isotherms of HCB on two soils were nonlinear, which can be best described by the Freundlich equation with the square of the correlation coefficient ( r 2) ranging from 0.97 to 0.99. Desorption of HCB from the two soils exhibited hysteresis at all HCB concentrations because the Freundlich desorption coefficients were always higher than the Freundlich adsorption coefficients. The hysteretic effect was enhanced with increasing initial HCB concentration, and positive hysteresis was observed at different concentrations.

Effect of cyclodextrin on the remediation of hexachlorobenzene in soil by electrokinetic Fenton process

Integration of electrokinetics with chemical oxidation especially Fenton's process is a widely accepted technique for the degradation of organic compounds in soil. This paper presents a study on the effect of cyclodextrin as an enhancing agent during the electrokinetic Fenton process for the remediation of soil contaminated with a hydrophobic organic compound. Kaolin was used as the model clay and was spiked with hexachlorobenzene (HCB) to get an initial concentration of 100 mg/kg. An attempt was made to evaluate the performance of high concentration hydrogen peroxide (H 2O 2) to oxidize the contaminants in the sorbed state in the presence and absence of cyclodextrin. The treatment was carried out for 15 consecutive days. The results clearly demonstrate the efficiency of the process while at the same time, the distribution of the residual contaminant in the soil is greatly influenced by the presence of cyclodextrin. There was no soluble iron addition during the process and the system used the inherent iron in kaolin to catalyze the Fenton-like reactions. A maximum of 76% removal of the contaminant from the soil was observed when 30% H 2O 2 was used in the absence of cyclodextrin.

Remediation of hexachlorobenzene in soil by enhanced electrokinetic Fenton process

The feasibility of enhanced electrokinetic Fenton process for the remediation of Hexachlorobenzene (HCB) in low permeability soil was investigated. Kaolin was artificially contaminated with HCB and treated by electrokinetic and electrokinetic Fenton processes. β -Cyclodextrin was used to enhance the solubility of HCB in pore fluid. Three tests were carried out, of which two were electrokinetic experiments to observe the suitability of β -cyclodextrin as a flushing solution for these processes. The third experiment was the electrokinetic Fenton test using β -cyclodextrin as an enhancing agent. Results show that the removal efficiency depends on the choice of a suitable flushing solution and physical parameters like pH, electric current and electro-osmotic flow.

A Simplified Method for Determination of Pentachlorophenol and Hexachlorobenzene in Soil Contaminated by Industrial Chemical Residues

A Simplified Method for Determination of Pentachlorophenol and Hexachlorobenzene in Soil Contaminated by Industrial Chemical Residues

Short-term toxic effects of chlorobenzenes on broadbean (Vicia faba) seedlings

The root growth, changes in superoxide dismutase (SOD, EC 1.15.1.1) activity, malonyldialdehyde (MDA) and total soluble protein level of broadbean (Vicia faba) seedlings were researched at different soil concentrations of chlorobenzene (CB), 1,2,4-trichlorobenzene (TCB) and hexachlorobenzene (HCB). The results showed that root growth of seedlings was interrupted after 5 d of 50-200 microg x g(-1) TCB treatment. During a 3 d recovery period, root growth was, however, restored to some extent although there was a delay in returning to the control level. The total soluble protein content in seedlings increased with TCB concentration and duration of exposure. Effect of TCB stress on SOD activity in seedlings displayed a significant dose-effect relationship for 1-5 d of 50-200 microg x g(-1) treatment. When broadbean seedlings were placed in clean tap water for 3 d following exposure to 5 d of TCB stress to clear tap water for 3 d, SOD activity at 50 microg x g(-1) TCB recovered towards control level (P>0.05) while a significant increase in SOD activity was observed at 100 and 200 microg x g(-1) TCB compared to control (P<0.05). The experiments also revealed that a significant increase of MDA level in seedlings occurred after 3 and 5 d of 100 and 200 microg x g(-1) TCB treatment (P<0.05 and P<0.01), and there was a positive correlation between TCB concentration and MDA level. All the above results showed that SOD activity and MDA level of broadbean seedlings might be proposed as the biomarkers for short-term TCB contamination in soil. Compared to TCB, the toxicity of 50-1000 microg x g(-1) CB or HCB in soil to broadbean seedlings was not observed after a 3 d exposure.

Stimulation of reductive dechlorination of hexachlorobenzene in soil by inducing the native microbial activity

The reductive dechlorination and behaviour of 14C-hexachlorobenzene (HCB) was investigated in an arable soil. The activity of the native anaerobic microbial communities could be induced by saturating the soil with water. Under these conditions high rates of dechlorination were observed. After 20 weeks of incubation only 1% of the applied 14C-HCB could be detected in the fraction of extractable residues. Additional organic substances, like wheat straw and lucerne straw, however considerably delayed and reduced the dechlorination process in the soil. The decline of HCB was not only caused by dechlorination but also by the formation of non-extractable residues, whereby their amounts varied with time depending on the experimental conditions. Several dechlorination products were detected, indicating the following main HCB transformation pathway: HCB → PCB → 1,2,3,5-TeCB → 1,3,5-TCB → 1,3-DCB, with 1,3,5-TCB as main intermediate dechlorination product. The other TeCB-, TCB- and DCB-isomers were also detected in low amounts, showing the presence of more than one dechlorination pathway. Since the methane production rates were lowest when the dechlorination rates were highest, it can be assumed that methanogenic bacteria were not involved in the dechlorination process of HCB. The established 14C-mass balances show, that with increasing dechlorination and incubation times, the 14C-recoveries decreased.

Organochlorine Pesticide Residues in Styrian Soils and Seeds of the Styrian Oil Pumpkin: An Inventory and a Concept for their Reduction

The fungicide hexachlorobenzene and other organochlorine pesticides have been detected in agricultural soils of the Austrian federal state Styria and in the seeds of the Styrian oil pumpkin grown on these fields. For monitoring the exact amounts of these contaminants in the single ecological compartments effective extraction and sample pre-treatment techniques have been developed, thus leading to a significant correlation between the concentration of hexachlorobenzene (HCB) in soil and its content in the oil seeds grown there, the factor ranging between 5 and 25. The efficiency of a new concept for the reduction of the pesticide contamination of the pumpkin seeds involving the application of activated charcoal powder could be demonstrated in a field-study. Thus the bioavailability of HCB in soil could be reduced significantly leading to lower HCB-contaminated oil seeds.

Oxidation of sorbed hexachlorobenzene in soils using catalyzed hydrogen peroxide

The Fenton-like oxidation of a highly hydrophobic and biorefractory compound, hexachlorobenzene, was investigated in silica sand and a natural soil using a number of process conditions including catalysis by soluble iron and naturally-occurring iron minerals coupled with a range of hydrogen peroxide concentrations from 3 mM to 300 mM (100 mgl−1 to 10 000 mgl−1). In addition, hexachlorobenzene desorption rates were quantified and compared to the rates of oxidation. Using soluble iron and peroxide concentrations ⩾ 100 m1M, sorbed hexachlorobenzene was oxidized more rapidly than the measured desorption rate, indicating that it may have been, in part, oxidized on the surface of the silica sand. The results suggest that hydroxyl radicals may cross the liquid-solid interface under aggressive reaction conditions. Alternatively, the aggressive Fenton-like reaction conditions may alter the hexachlorobenzene sorption characteristics and increase its rate of desorption resulting in an enhanced coupled desorption-oxidation mechanism.In the second phase of study using natural iron minerals as the catalyst, hexachlorobenzene was oxidized more slowly than it was desorbed. The probable mechanism for mineral-catalyzed oxidation is desorption followed by oxidation through a Fenton-like process at the mineral surface. In the treatment of a natural sandy loam soil, hexachlorobenzene was oxidized at rates slower than it was desorbed. The most efficient process condition (mineral-catalyzed oxidation) was optimized using a central composite rotatable factorial design analysis. These experiments showed that maximum hexachlorobenzene removal occurred with H2O2 concentrations of 3 to 5 mM and slurry volumes of 25 to 30 times the field capacity of the soil.

Hexachlorobenzene: Its Vapor Pressure and Vapor Phase Diffusion in Soil

AbstractThe vapor pressure of hexachlorobenzene (HCB), measured with the gas saturation technique, was related to the temperature by the equation log10p = 12.94 − (5279/T) giving HCB vapor pressure values from 0.420 × 10−5 to 22.4 × 10−5 mm Hg over the temperature range 15 to 45° C. The heat of vaporization of HCB was calculated to be 24.2 kcal/mol. The vapor pressure data was used to measure the vapor phase diffusion of HCB in soil using a vapor‐diffusion cell. The steady‐state vapor diffusion of HCB in soil was directly related to soil air‐filled porosity and was therefore greatly reduced by increased soil bulk density and increased soil water content. The apparent steady‐state HCB diffusion coefficient was linearly related to the soil porosity term, Pa10/3 / PT2 where Pa is the soil air‐filled porosity and PT is the total soil porosity. The calculated diffusion coefficient in air for HCB was 1.0 × 104 cm2/day.

Hexachlorobenzene in soils and crops after soil treatment with pentachloronitobenzene

Hexachlorobenzene is found as an admixture in technical pentachloronitrobenzene, which has widespread use as a soil fungicide. Soils that had been treated in the past with this pesticide contain measurable levels of hexachlorobenzene. For different crops and parts of the plant, there were great differences in the amounts taken up from soil. The lowest relative contents in crop to soil were found for sugarbeet crowns and leaves, and for grass blades more than 5 cm above the ground. A clear accumulation was found in carrots and grass roots. Pentachlorobenzene behaves almost like hexachlorobenzene, whereas relative contents for pentachloronirobenzene are usually lower.