An integrated two-stage process for effective dechlorination of polychlorinated biphenyls in subcritical water in the presence of hydrogen donors

In this study, a novel two-stage process, i.e. first stage for hydrogen species generation at 150 °C and second stage for PCB dechlorination at 330 °C, was developed for efficient PCB degradation in subcritical water with nanoscale Ni/Fe particles. The obtained results showed that over 92% of PCBs was completely converted into 1-alkyl-benzenes during this process. The addition of hydrogen donors could greatly enhance PCB dechlorination in descending order: oxalic acid ≈ HAc > NaH2PO4 ≈ KH2PO4. The anion introduced along with H+ played the crucial role in overall dechlorination process due to the undesired reaction between anion and reactive nanoscale Ni/Fe. Oxalic acid was quite effective for the generation of hydrogen species by supply more available H+ without undesired side reaction. In contrast, phosphate could react with Ni/Fe rapidly to form complex precipitates on particle surface and thereby block the reactive sites of particle following suspension of dechlorination. In addition, low 150 °C at first stage as compared to 330 °C at second stage favored the dissolution and adsorption of hydrogen on Ni surface for catalyzing to form more reactive hydrogen species used for rapid dechlorination.

Effect of residence time on dechlorination during two-stage reaction process. X axis: residence time at first stag, CPCBs = 5.25 mg/L, CNi/Fe(s) = 6.25 g/L, Tfirst stage = 150 °C, Tsecond stage = 330 °C.Figure optionsDownload as PowerPoint slideHighlights
► Corrosion of Fe0 and dechlorination are conducted at different temperature, respectively.
► Corrosion of Fe0 conducted at 150 °C favors product of reactive hydrogen species.
► Slight addition of oxalic acid enhances utility efficiency of Fe0 without dislodgment of Ni from Ni/Fe.