Pentachlorophenol reductive dechlorination

Chlorinated dibenzo ip-dioxins are contaminants of phenol-based pesticides and may enter the environment where they are subject to the action of sunlight. Rate measurements showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is more rapidly photolyzed in methanol than octachlorodi-benzo-p-dioxin. Initially TCDD yields 2,3,7-trichlorodiben-zo-p-dioxin, and subsequent reductive dechlorination is accompanied by ring fission. Pure dibenzo-p-dioxin gave polymeric material and some 2,2 -dihydroxybiphenyl on irradiation. Riboflavin-sensitized photolysis of the potential precursors of dioxins, 2,4-dichlorophenol and 2,4,5-trichloro-phenol, in water gave no detectable dioxins. The products identified were chlorinated phenoxyphenols and dihydroxy-biphenyls. In contrast, aqueous alkaline solutions of purified pentachlorophenol gave traces of octachlorodibenzo-p-dioxin on irradiation. 

Bouchard B, R Beaudet, R Villemur, G MscSween, F Lepine, J-G Bisaillon (1996) Isolation and characterization of Desulfitobacterium frappieri sp. nov., an anaerobic bacterium which reductively dechlorinates pentachlorophenol to 3-chlorophenol. Int J Syst Bacteriol 46 1010-1015. 

Mikesell MD, S A Boyd (1986) Complete reductive dechlorination and mineralization of pentachlorophenol by anaerobic microorganisms. Appl Environ Microbiol 52 861-865. 

Photochemical decomposition can also be carried out in the presence of a suspension of photoactive material such as Ti02 where substrate absorption onto the uv activated surface can initiate chemical reactions e. g. the oxidation of sulphides to sul-phones and sulphoxides [37]. This technology has been adapted to the destruction of polychlorobiphenyls (PCB s) in wastewater and is of considerable interest in environmental protection. Using pentachlorophenol as a model substrate in the presence of 0.2 % TiOj uv irradiation is relatively efficient in dechlorination (Tab. 4.5) [38]. When ultrasound is used in conjunction with photolysis, dechlorination is dramatically improved. This improvement is the result of three mechanical effects of sonochemistry namely surface cleaning, particle size reduction and increased mass transport to the powder surface. 

The decomposition of pinacyanol chloride dye using octahedral layered mixed-valent manganese oxides has been published [81]. Catalytic reduction reactions using birnessite have been tried for removing pentachlorophenol (PCP) from soil and water (detoxyfication) [82]. Transformation and dechlorination of PCP incubated with peroxidase, laccase, or birnessite is decreased in the presence of humic monomers as cosubstrate. The dehalogenation number for birnessite is 3.3, compared with 3.5 for peroxidase and 1.5 for laccase [82]. 

Electrochemical degradation utilizes the high redox potential at an anode and the low redox potential at a cathode to oxidize or reduce contaminants. Most applications of electrochemical degradation involved cathodic reduction or anodic oxidation of chlorinated organic contaminants. Schmal et al, (55) showed that pentachlorophenol was sequentially dechlorinated (i.e. reduced) at a carbon fiber cathode. Experiments conducted by Cheng et al. (34) indicated that the composition of the cathode was critical for cathodic reduction of 4-... 

---