Elucidation of degradation mechanism of dioxins during mechanochemical treatment

Model dioxin compounds 4-chlorobiphenyl (4CB), octa-chlorodibenzo-p-dioxin (OCDD), and octachlorodibenzofuran (OCDF) were degraded by a mechanochemical (MC) process that involved milling with calcium oxide by use of a planetary ball mill. The degradation of 4CB produced mainly chloride ions and biphenyl, with the chlorine removal efficiency reaching about 100%. Biphenyl was transformed into terphenyls, quaterphenyls, cyclohexylbenzene, and bicyclohexyl through polymerization and hydrogenation reactions. Measurements of chloride ions after MC treatment of OCDD and OCDF showed about 100% dechlorination of both compounds; tetra- to heptachlorinated dibenzo-p-dioxins/furans (T4-H7CDD/Fs) were detected only at trace levels, and no other chlorinated organic compounds were observed. The residue after MC treatment was gray in color, indicating the possibility of carbonization, but the presence of amorphous graphite could not be confirmed.