Reductive dechlorination of polychlorinated biphenyl compounds in supercritical carbon dioxide

Polychlorinated biphenyl mixtures (Aroclor 1242 and 1248) are dechlorinated efficiently (but not quantitatively) with heated columns of zero-valent me tal or bimetallic mixture in a continuous process. Supercritical carbon dio xide transfers the substrates through the reactor and cumulative ten minute fractions of eluate are trapped in hexane during 1 h of continued operatio n. The extent of dechlorination is influenced appreciably by the identity o f the zero-valent (ZV) metal (Fe-0 > Ni-0 > Zn-0 > Cu-0), by the temperatur e (400 >> 300 > 200 degrees C) and pressure (less than or equal to 4500 psi ) within the reactor column(s) and especially by the composition of the fee dstock. Although water is both unnecessary and detrimental to the dechlorin ation yields, the inclusion of methyl ketone(s) in the feedstock solution a ppreciably improves the extent of the dechlorination. Two columns (25 x 1 c m) of Ag-0/Fe-0 or Ag-0/Ni-0 bimetallic mixture result in a virtually quant itative dechlorination of substrate Aroclor 1242 (similar to 28 mg min(-1)) . Moreover, during 1 h of continued operation, the dechlorinations are very repeatable. In preliminary treatments of 30% (v/v) acetone-hexane extracts (ca. 600 ppm PCBs) of a spiked sandy loam soil, no chlorinated organics co uld be detected in the PCB eluting region by GC-MS.