Synergetic effect of alkaline earth metal oxides and iron oxides on the degradation of hexachlorobenzene and its degradation pathway

The degradation of hexachlorobenzene (HCB) was carried out over physical mixtures of a series of alkaline earth metal oxides (MO: M = Mg, Ca, Sr, Ba) and iron oxides with different crystal types (FexOy:Fe2O3 or Fe3O4) at 300 °C. These physical mixtures all showed a synergetic effect toward the degradation of HCB. A range of degradation products were identified by various methods, including tri- to penta-chlorobenzenes by gas chromatography/mass spectrometry (GC–MS), tri- to penta-chlorophenols, tetrachlorocatechol (TCC) and tetrachlorohydroquinone (TCHQ) by GC–MS after derivatization, and formic and acetic acids by ion chromatography. Two degradation pathways, hydrodechlorination and oxidative degradation, appear to occur competitively. However, more sequential chlorinated benzene and phenol congeners were formed over mixed MO/Fe3O4 than over mixed MO/Fe2O3 under the same conditions. The oxidative reaction dominated over mixed MO/Fe2O3 and was promoted as the major reaction by the synergetic effect, while both the oxidative and hydrodechlorination reactions were important over mixed MO/Fe3O4, and both pathways are remarkably promoted by the synergetic effect. The enhanced hydrodechlorination may be attributed to free electrons generated by the transformation of Fe3O4 into Fe2O3, and hydrogen provided by water adsorbed on the MO.

► Hexachlorobenzene was degraded over alkaline earth metal oxide (MO)/iron oxide.
► Chlorobenzenes, chlorophenols, and others were identified as degradation products.
► Oxidative reaction was promoted by synergetic effect over MO/Fe2O3.
► Both oxidation and hydrodechlorination reactions are promoted over MO/Fe3O4.