Hydrodechlorination of diuron in aqueous solution with Pd, Cu and Ni on activated carbon catalysts

The hydrodechlorination (HDC) of diuron with hydrogen in aqueous phase was studied in a trickle bed reactor using different Pd, Cu and Ni catalysts supported on activated carbon. The reactor was fed with a diuron aqueous solution (2–30 mg/L) and a H2/N2 (1:1) gas stream. Wide ranges of temperature (35–100 °C) and space–time (64.8–647.5 kgcat h/mol) were studied at a pressure of 1.4 bar. HDC of diuron with Pd on activated carbon (Pd/AC) catalyst with 0.5 wt.% Pd can be conducted successfully at 50 °C, reaching conversion values above 95%. The porous structure of the catalysts showed a significant effect on their activity. A nearly complete removal of diuron was achieved with the 0.5 wt.% Pd catalyst of high external (or non-microporous) area, whereas conversion values of around 70% were obtained with the highly microporous catalyst. The resulting effluents showed lower ecotoxicity values than the inlet diuron solution, thus indicating the evolution to less toxic species. Though the Pd catalysts showed a higher activity, the Cu and Ni ones also led to high conversions well above 80% at 2.5 wt.% metal load. Nevertheless, opposite to the Pd catalysts, a significant metal leaching was observed from the Ni and Cu ones, which affected negatively to the ecotoxicity of the final effluents, showing higher values than for the own initial diuron solution. To improve the anchorage of the active phase to the support, activated carbon was subjected to an oxidative treatment with HNO3. A significant decrease of leaching was observed, leading to ecotoxicity values comparable to those obtained with the Pd catalysts.