dechlorination

Dechlorination is the removal or replacement of chlorine atoms in organic or inorganic compounds. In environmental chemistry it is a key mechanism for reducing toxicity and enabling degradation of chlorinated pollutants. It also refers to neutralizing residual chlorine in drinking water and wastewater treatment.

Biological dechlorination, or reductive dechlorination, is carried out by anaerobic microbes that use organochlorines as electron acceptors. In groundwater remediation, chlorinated solvents such as perchloroethylene (PCE) and trichloroethylene (TCE) are often transformed stepwise to lesser chlorinated products and ultimately to ethene. Organisms such as Dehalococcoides species are prominent dehalogenators.

Abiotic or chemical dechlorination uses reducing agents or catalysts to break C–Cl bonds. Zero-valent iron (ZVI) and related materials can abiotically reduce chlorinated solvents; hydrogenolysis and palladium-catalyzed processes are also employed in specialized settings.

For drinking water, residual chlorine is commonly neutralized with reducing agents such as sodium bisulfite, sodium sulfite, or ascorbic acid to form chloride. Activated carbon can remove some chlorinated organics, often as part of a treatment train.

Applications and concerns: Dechlorination is central to environmental cleanup of groundwater contaminated with chlorinated solvents and to pretreatment of industrial effluents. Care is needed to avoid formation of toxic intermediates such as vinyl chloride; complete dechlorination to ethene or other nonhazardous products is preferred. It is a subset of dehalogenation.