Synergies of metallic catalysts in the Fenton and photo-Fenton processes applied to the treatment of pulp bleaching wastewater

This work studied the incorporation of Co2+, Cu2+, and Mn2+ as Fenton co-catalysts, seeking synergetic effect that could allow for overall reduction of chemical demand. Focus was on removing recalcitrant pollutants AOX from pulp bleaching wastewater, but side effect on wastewater COD and colour was also assessed, and operating cost of treatment was quantified. Optimal operating condition of Fe-catalysed (photo-)Fenton processes was found by response surface methodology, and then the incorporation of Cu2+, Co2+, and Mn2+ as (co)catalysts was studied - the optimal dose of catalyst was kept but supplied as a combination of iron with the aforementioned metals, under increasing molar ratios. Although Fe2+ showed the highest catalytic activity, up to 25% of Co2+, Cu2+, or Mn2+ were successfully included in the catalyst combination, with similar performance and operating cost, particularly using Cu2+ and Mn2+. Regardless of the catalyst combination, Fenton process yielded higher AOX and COD removal than photo-Fenton, highlighting the lower photoactivity of Co2+, Cu2+ and Mn2+, compared with Fe2+. Colour removal was highly favoured by the substitution of iron by other transition metals. Synergy between metals is challenged by complex wastewaters, therefore some discussion was left on the viability of full-scale incorporation of such catalyst solution.