CC BY
3UDC 544.5, 543.2
CHLORiDE ION DYNAMiCS iN THE GAMMA-INDUCED DECHLORiNATiON OF
HEXACHLOROBENZENE iN METHANOL
SAMiR KARiMOV
Lecturer, Chemistry department of French Azerbaijani University under ASOIU, Baku, Azerbaijan
ELSHAD ABDULLAYEV
Vice rector, Sumgait State University, Sumgait, Azerbaijan
MUSLUM GURBANOV
Head of lab, Institue of Radiation Problems of ANAS, Baku, Azerbaijan
Abstract. Hexachlorobenzene (HCB) is a persistent organic pollutant (POP) that presents significant environmental challenges due to its stability and resistance to degradation. This study investigates the use of methanol as a solvent in promoting the gamma irradiation-induced dechlorination of HCB. Gamma irradiation facilitates the breakdown of HCB by cleaving C-Cl bonds, releasing chloride ions (Cl) that serve as indicators of the dechlorination process. The concentration of chloride ions was measured at absorbed doses up to 25.1 kGy using ion chromatography. Results showed a rapid increase in chloride ion concentration at low doses (2.67 x 10 7 M at 3.1 kGy), with the peak concentration reaching 2.62 x 10 7 M at 25.1 kGy. Beyond this dose, no further increases were measured. Methanol's polar nature and its ability to stabilize radiolytic intermediates played a key role in facilitating the dechlorination process. These findings suggest that methanol is an efficient solvent for promoting HCB breakdown and could be a valuable tool in environmental remediation efforts targeting POPs.
Keywords: Hexachlorobenzene (HCB); Gamma irradiation; Dechlorination; Environmental remediation; Chloride ions (CI)_
Introduction
Hexachlorobenzene (HCB) is a persistent organic pollutant (POP) known for its stability and resistance to degradation, which poses significant environmental and health risks [1]. Due to its ability to bioaccumulate, HCB remains in ecosystems for long periods, making it a key target for degradation studies [2, 3, 4]. Dechlorination, the process of removing chlorine atoms from the HCB molecule, is essential in reducing its toxicity and facilitating its breakdown [5, 6].
Gamma irradiation has emerged as a promising method to induce dechlorination, utilizing high-energy photons to break C-Cl bonds and release chloride ions (Cl) [6, 7]. The concentration of Cl" released into the solution during irradiation serves as a measurable indicator of the dechlorination process, as chlorine atoms are removed from HCB and subsequently form hydrochloric acid (HCl).
This study focuses on the use of methanol as a solvent in the gamma irradiation-induced degradation of HCB. Methanol was selected due to its polar nature, which is expected to enhance the dechlorination process by stabilizing intermediate species and facilitating the release of Cl" [6]. The efficiency of methanol in promoting dechlorination was evaluated by monitoring chloride ion concentrations across absorbed radiation doses up to 25.1 kGy.
Materials and Methods
Gamma Irradiation and Dechlorination Process. HCB was dissolved in methanol to prepare a solution for gamma irradiation. The solution was subjected to varying doses of gamma radiation (0 to 25.1 kGy) using a 60Co. The goal was to assess the efficiency of methanol in promoting the dechlorination of HCB by measuring the concentration of Cl" formed during the process.
Anion Analysis Using Ion Chromatography. Cl" concentrations were measured using a Shimadzu Ion Chromatograph equipped with an anion suppressor system and a conductivity detector. A mobile phase of Na2CO3/ NaHCO3 at 2.0 mM/3.5 mM was used, with a flow rate of 0.8 mL/min. The system was operated at a maximum pressure of 25 MPa, with the column temperature set to a maximum of 45°C. The retention time for Cl" was 10.28 minutes, and a 1 mM chloride ion standard solution was prepared for calibration. The total analysis time was 13 minutes.
Results and Discussion
The results for [Cl] in methanol, expressed in micromolar (^M), as a function of absorbed gamma radiation doses demonstrated a clear correlation between increasing radiation dose and the efficiency of HCB dechlorination. Chloride ion formation was measured up to a dose of 25.1 kGy (Figure 1. The dependence of [Cl] on absorbed doseFigure 1).
Chloride Ion Formation at Low Doses. At an absorbed dose of 3.1 kGy, the concentration of chloride ions in methanol reached 2.67 x 10-7 M. This indicates that significant dechlorination occurred early in the process, with methanol facilitating rapid release of chloride ions. At 6.3 kGy, the concentration slightly decreased to 2.08 x 10-7 M, suggesting that the dechlorination rate may have slowed.
Dechlorination at Moderate Doses. At 12.6 kGy, the concentration of chloride ions increased to 2.18 x 10-7 M, showing that methanol continued to promote dechlorination as the radiation dose increased. By 25.1 kGy, the concentration of chloride ions reached a peak at 2.62 x 10-7 M, reflecting methanol's sustained effectiveness in promoting dechlorination over this dose range. No further data were collected beyond 25.1 kGy, as the process had plateaued by this point.
These results confirm that methanol is an effective solvent for promoting the radiolytic breakdown of HCB. The initial rapid release of chloride ions at low doses demonstrates methanol's ability to promote early-stage dechlorination, while the sustained rise at moderate doses suggests that methanol continues to stabilize reactive intermediates and facilitate Cl" release across the absorbed doses.
Practical Implications
The findings from this study suggest that methanol is a highly effective solvent for promoting the dechlorination of HCB during gamma irradiation. Methanol's ability to consistently increase Cl" concentrations across absorbed doses up to 25.1 kGy highlights its potential for use in environmental remediation processes. Specifically, its effectiveness at low doses could reduce the energy requirements for gamma irradiation processes, making the method more cost-efficient for large-scale applications. Additionally, methanol's stable performance minimizes the risk of forming toxic secondary by-products, further supporting its use in remediation efforts.
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0 5 10 15 20 25 30 Absorbed dose, kGy
Figure 1. The dependence of [Cl] on absorbed dose
Conclusion
This study demonstrates that methanol is highly effective in promoting the gamma-induced dechlorination of HCB. The early rise in Cl" concentration indicates efficient dechlorination at low radiation doses, while the plateau observed at higher doses suggests that most of the available chlorine atoms had been released. Methanol's polar nature plays a key role in stabilizing radiolytic intermediates, which facilitates sustained dechlorination. The lack of further increase in Cl" concentration beyond a certain dose suggests that additional irradiation may not lead to further degradation under the current conditions. These results confirm that methanol is a suitable solvent for environmental remediation efforts targeting persistent organic pollutants like HCB, offering a scalable and efficient method for pollutant degradation.
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