Key Takeaways & Executive Findings
- •• • HCBD is unintentionally formed during chlorination stages of carbon tetrachloride, dichloroacetylene, tri-/tetrachloroethylene, and chlorobenzene production, via free-radical mechanisms, and is released through waste gas, wastewater, and solid waste, necessitating multi-pathway emission controls. • • HCBD undergoes long-range atmospheric transport and adsorbs onto soil and sediments, leading to multi-media contamination; its bioaccumulation and food-chain magnification amplify ecological risks, with documented toxicity to aquatic organisms and hepatic/renal damage in mammals. • • Chronic toxicity studies in rats (Kociba et al., 1977) provide critical dose-response data for HCBD, underpinning risk assessment and regulatory limit setting for occupational and environmental exposure. • • Effective mitigation requires integrated process optimization and end-of-pipe treatment technologies, coupled with stringent emission standards and life-cycle management, as demonstrated by the reviewed literature.
Abstract
Hexachlorobutadiene (HCBD) is a persistent organic pollutant (POP) regulated under the Stockholm Convention. Chlorinated chemical production processes are major sources of unintentional HCBD emissions, posing potential threats to ecosystems and human health. This study systematically reviews the formation, emission, and environmental impact of HCBD from such processes. HCBD is widely generated as a by-product during chlorination stages of producing carbon tetrachloride, dichloroacetylene, tri-/tetrachloroethylene, and chlorobenzene, via free-radical mechanisms. It is released through waste gas, wastewater, and solid waste. In the environment, HCBD exhibits multimedia distribution, undergoing long-range atmospheric transport and adsorbing onto soil and sediments, thereby becoming secondary pollution sources. HCBD shows significant bioaccumulation and food-chain magnification; it is toxic to aquatic organisms and causes hepatic and renal damage with potential carcinogenicity in mammals. Effective pollution control requires combined process improvements and end-of-pipe treatments, supplemented by stringent emission standards and life-cycle management. Future research should focus on developing precise emission inventories, elucidating multi-media transport and transformation mechanisms, and assessing composite ecotoxicological effects, thereby providing scientific support for implementing international conventions and formulating effective prevention strategies.
1. Introduction
Chlorinated chemical manufacturing remains a cornerstone of the global chemical industry, yet it inadvertently generates persistent organic pollutants (POPs) such as hexachlorobutadiene (HCBD). Despite regulatory attention under the Stockholm Convention, commercial production routes for chlorinated solvents and intermediates continue to release HCBD as an unintended by-product, particularly during high-temperature chlorination steps. Existing abatement strategies have been hampered by the compound's volatility, chemical stability, and tendency to partition across waste streams, leading to incomplete removal and secondary contamination. The lack of systematic data on formation mechanisms and emission factors across different production processes has further impeded the development of targeted control measures.
This review addresses these bottlenecks by synthesizing current knowledge on HCBD generation pathways, emission routes, and environmental fate. It critically evaluates evidence from field studies and toxicological assays, including chronic rat bioassays, to establish dose-response relationships and ecological risk thresholds. By identifying key process parameters that influence HCBD yields and tracing its multimedia transport, this work provides a scientific basis for optimizing chlorination conditions, designing effective end-of-pipe treatment systems, and implementing robust regulatory frameworks. The findings aim to support industry compliance with international POPs reduction commitments while minimizing environmental and health impacts.
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XU Tianyi, TANG Junhao, ZHANG Haiyan, LI Gang, YANG Qiuting, GAO Ruoran, CHEN Siyao, ZHENG Minghui, LIU Guorui (2026). Formation and Emission of Hexachlorobutadiene during Chlorinated Chemical Production and Its Impact on the Surrounding Environment. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025101901
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Frequently Asked Questions
What are the primary formation mechanisms of HCBD during chlorinated solvent production, and how can process parameters be optimized to minimize its generation?
HCBD forms via free-radical reactions during chlorination of hydrocarbons, particularly in processes for carbon tetrachloride, dichloroacetylene, tri-/tetrachloroethylene, and chlorobenzene. Yields are influenced by temperature, chlorine partial pressure, and residence time. Lowering reaction temperature and using selective catalysts can reduce HCBD formation, but quantitative data on yield reduction are not provided in the abstract; detailed process optimization requires site-specific kinetic studies.
What are the key environmental transport pathways of HCBD, and how does its multimedia partitioning affect long-range contamination?
HCBD is released via waste gas, wastewater, and solid waste. In the atmosphere, it undergoes long-range transport due to its persistence and semi-volatility. It adsorbs onto soil and sediments, where it can accumulate and act as secondary sources. Its octanol-water partition coefficient (log Kow ~4.78) indicates high hydrophobicity, favoring bioaccumulation. The review highlights that HCBD's multimedia distribution necessitates integrated monitoring across air, water, and soil compartments.
What are the critical toxicological endpoints for HCBD, and how do they inform regulatory risk assessment?
HCBD is toxic to aquatic organisms and causes hepatic and renal damage in mammals, with potential carcinogenicity. A two-year chronic toxicity study in rats (Kociba et al., 1977) provides dose-response data, identifying the kidney as a primary target organ. These data underpin reference doses and environmental quality standards, such as those set by the Stockholm Convention and national regulations.
What are the most effective control technologies for reducing HCBD emissions from chlorinated chemical plants, and what are their removal efficiencies?
Effective control combines process optimization (e.g., modifying chlorination conditions) and end-of-pipe treatments such as adsorption on activated carbon, catalytic oxidation, or thermal destruction. However, specific removal efficiencies are not quantified in the abstract. The review emphasizes that stringent emission standards and life-cycle management are essential to achieve significant reductions.
What are the major research gaps in HCBD emission inventories and environmental fate modeling, and how can they be addressed?
Current gaps include lack of accurate emission factors for various production processes and limited understanding of multi-media transport and transformation mechanisms. Future research should focus on developing precise emission inventories through on-site measurements and using multimedia fate models to predict environmental concentrations. Additionally, composite ecotoxicological effects, including interactions with other pollutants, need further investigation.
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