Describe the reductive dechlorination process for chlorinated solvent bioremediation.

Reductive dechlorination is the primary mechanism for biodegrading chlorinated solvents like PCE and TCE. Process: Under anaerobic conditions, specialized bacteria (Dehalococcoides, Dehalobacter, Desulfitobacterium) use chlorinated compounds as electron acceptors for respiration (organohalide respiration). Sequential dechlorination: PCE -> TCE -> cis-DCE -> VC -> ethene. Each step removes one chlorine atom, adding a hydrogen. Requirements: Strongly reducing conditions (sulfate-reducing to methanogenic). Electron donors - hydrogen generated from fermentation of organic substrates (lactate, vegetable oil, molasses). Absence of competing electron acceptors (oxygen, nitrate). Challenges: cis-DCE and vinyl chloride (VC) may accumulate as toxic intermediates; complete dechlorination to ethene requires Dehalococcoides ethenogenes or mccartyi which may be absent from some sites. Biostimulation adds electron donor to stimulate indigenous dehalogenators. Bioaugmentation with Dehalococcoides cultures (KB-1, SDC-9) when native populations insufficient. Enhanced reductive dechlorination now standard technology for chlorinated solvent sites.