Reductive dechlorination of polychlorinated biphenyl compounds in supercritical carbon dioxide

Abstract

Polychlorinated biphenyl mixtures (Aroclor 1242 and 1248) are dechlorinated efficiently (but not quantitatively) with heated columns of zero-valent metal or bimetallic mixture in a continuous process. Supercritical carbon dioxide transfers the substrates through the reactor and cumulative ten minute fractions of eluate are trapped in hexane during 1 h of continued operation. The extent of dechlorination is influenced appreciably by the identity of the zero-valent (ZV) metal (Fe⁰ > Ni⁰ > Zn⁰ > Cu⁰), by the temperature (400 » 300 > 200 °C) and pressure (⩽4500 psi) within the reactor column(s) and especially by the composition of the feedstock. Although water is both unnecessary and detrimental to the dechlorination yields, the inclusion of methyl ketone(s) in the feedstock solution appreciably improves the extent of the dechlorination. Two columns (25 × 1 cm) of Ag⁰/Fe⁰ or Ag⁰/Ni⁰ bimetallic mixture result in a virtually quantitative dechlorination of substrate Aroclor 1242 (≡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 could be detected in the PCB eluting region by GC-MS.