Dechlorination of Lindane, Dieldrin, Tetrachloroethane, Trichloroethene, and PVC in Subcritical Water

Abstract

Pure water has been used to dechlorinate aliphatic organics without the need for catalysts or other additives. Dehydrohalogenation (loss of HCI with the formation of a double bond) occurred at temperatures as low as 105-200 degrees C for 1,1,2,2-tetrachloroethane, lindane (1,2,3,4,5,6-hexachlorocyclohexane, gamma-isomer), and dieldrin (1,2,3,4,10,10-hexachloro-6,7-epoxy-1,4,4a,5,6,7,8,8a-octahydro-endo, exo-1,4:5,8-dimethanonaphthalene). Complete loss of the parent compounds was achieved in less than 1 h at 150, 200, and 300 degrees C for 1,1,2,2-tetrachloroethane, lindane, and dieldrin, respectively. The initial dechlorination of lindane had an activation energy of 84 kJ mol(-1) with an Arrhenius pre-exponential factor of 1.5 x 10(6) s(-1). Dehydrohalogenation of lindane formed trichlorobenzenes, followed by subsequent hydrolysis and hydride/chloride exchange to form chlorophenols, lower chlorobenzenes, and phenol as the major final product. Reaction of poly(vinyl chloride) at 300 degrees C for 1 h formed aromatic hydrocarbons ranging from benzene to anthracene and a char residue with a ca. 1:1 carbon-to-hydrogen ratio (mol/mol). The residue contained <1 wt % of chlorine compared to 57 wt % chlorine in the original polymer. All compounds tested yielded chloride ion as the major product (at higher temperatures), indicating that complete dechlorination of some aliphatic organochlorines may be feasible.

Catalytic dechlorination of polychlorinated biphenyls in subcritical water by Ni/Fe nanoparticles

“…According to the Arrhenius equation, rate constant increases with temperature. The activation energy for PCBs dechlorination in subcritical water was higher than that for chlorinated aliphatic and aromatic hydrocarbons [26], hence, enhancement of 50 • C from 200 • C to 250 • C only results in limited increase of rate constant. In this study, the pressure in autoclave was maintained by the steam/water equilibrium, thus the autoclave was saturated with aqueous and gaseous phase, and the volume of each phase changed with temperature.…”

Liquid phase hydrodechlorination of dieldrin and DDT over Pd/C and Raney-Ni

“…There are very few data on the transformations of ''drins'' or DDT in catalytic reduction systems. The known techniques for dieldrin (or other ''drins'') dechlorination refer mainly to the photolysis [11], hydrolysis [12], or reduction under harsh reduction conditions, e.g. using metallic sodium [13], lithium and tert-butanol [14], etc.…”

Dechlorination of lindane in the multiphase catalytic reduction system with Pd/C, Pt/C and Raney-Ni

“…Lindane decomposition via sonochemical [14] or gamma ray [15] induced dechlorination/destruction has also been demonstrated. It has been reported recently [16] that lindane undergoes the processes of dehydrochlorination and hydrolysis in subcritical water to give less chlorinated benzenes, phenols, and phenol as the major final product. The use of just water under comparatively mild conditions (100-250 • C and the corresponding equilibrium pressures) is attracting, however, the drawback is low selectivity and formation of toxic chlorinated phenols as byproducts.…”