Reductive destruction of chlorinated organics in molten salt
The destruction of hazardous chemicals, and in particular halogenated compounds, has become of significant interest in the last twenty years, as public awareness of Green issues has become more prominant. This work investigates the use of the DuPont process, for the non-oxidative destruction of organohalogens, as an alternative method of disposal for these environmentally hazardous compounds. A small-scale bath was constructed for the generation of a 2% sodium hydride solution in molten sodium hydroxide, by the reaction of hydrogen and metallic sodium. A variety of different halogenated compounds, both aliphatic and aromatic, were immersed in the molten salt of interest, along with respective non-halogenated analogues. The reaction products were analysed using a variety of different analytical techniques including FTIR, IC, GC and GCMS. By identifying these reaction products, possible reaction mechanisms have been postulated. For PVC, it is shown that the immersed polymer undergoes some form of base-accelerated thermal degradation, whilst the immersion of 4-chlorostyrene and poly-4-chlorostyrene apparently leads to an elimination-addition reaction. This results in the formation of the dechlorinated monomer, styrene, which then undergoes further reduction, giving ethyl benzene. The immersion of volatile halogenated compounds, such as chlorobenzene, leads to nucleophilic substitution. However, the presence of the hydride ion is shown to be essential, as the substitution reaction is in competition with volatilisation: no substitution is observed in molten sodium hydroxide alone. The rate of nucleophilic substitution in the presence of hydride is comparable to the rate of volatilisation, resulting in only 50% dechlorination. With non-volatile compounds, 100% dechlorination is observed in both the presence and absence of the hydride ion. However, the rate of dechlorination is significantly faster when the hydride ion is present. The ability of the 2% sodium hydride bath to dechlorinate poly-halogenated compounds is of particular interest, as the DuPont process could be used for the destruction of the more hazardous halogenated compounds such as PCBs and dioxins. Furthermore, due to the non-oxidative properties of the molten salt, no chlorinated gaseous products are observed, including HCl. This would be of major benefit to a method of disposal for halogenated hydrocarbons.