Oxidation degradation of 2,2',5-trichlorodiphenyl in a chelating agent enhanced Fenton reaction: Influencing factors, products, and pathways.

The sodium pyrophosphate (SP)-enhanced Fenton reaction has been proven to have promising potential in remediation of polychlorinated biphenyls in soils by keeping iron ions soluble at high pH and minimizing the useless decomposition of H2O2. However, little information can be obtained about the effect of environmental factors on its remediation performance. Thus, the effect of environmental factors on the degradation of 2,2',5-trichlorodiphenyl (PCB18), one of the main PCB congeners in Chinese sites, was investigated in this study. PCB18 degradation was sensitive to pH, which ranged from 39.8% to 99.5% as increased pH from 3.0 to 9.0. ·OH was responsible for PCB18 degradation at pH 5.0, while both ·OH and O2- resulted in PCB 18 degradation at pH 7.0 with the calculated reaction activation energy of 73.5 kJ mol-1. Bivalent cations and transition metal ions decreased PCB18 degradation markedly as their concentrations increased. The addition of humic acid had an inhibitory on PCB18 degradation, but no obvious inhibition of PCB18 removal was observed when the same concentration of fulvic acid was added. The addition of 1 and 10 μM model humic constituents (MHCs) promoted PCB18 degradation, but the addition of 100 μM MHCs decreased PCB18 removal. Biphenyl, two dichlorobiphenyl, and two hydroxy trichlorobiphenyl derivatives were identified as the major degradation products of PCB18 in the Fe2+/SP/H2O2 system at pH 7.0. Thus, an oxidative pathway contributed by OH and a reductive pathway induced by O2- were proposed as the main mechanisms for PCB18 degradation in the SP-enhanced Fenton reaction.