Reuse of sludge from photo-Fenton process and further application as a catalyst in electrochemical processes for the treatment of landfill leachate.

To evaluate the potential of sludge from solar photo-Fenton to be transformed into a catalyst for the heterogeneous electro-Fenton (EF) process, some parameters were investigated, including (i) the impact of heat treatment on catalytic activity through structural and compositional analysis of the iron species (100, 600 and 750 °C); (ii) analysis of the effect of pH and current density (j) on the decolorization and decarbonization efficiency; and (iii) the generation of high-value organic acids. The calcination of sludge to 600 and 750 °C presents changes in the multiferroic structures but does not enhance its catalytic activity, given the similarity of the treatment results of landfill leachate with the catalyst with and without calcination. The treatment of leachate via heterogeneous EF using sludge dried at 100 °C and under pH 3 achieves better results than the treatment without pH adjustment. At the evaluated j values (30, 60 and 90 mA cm), Chemical Oxygen Demand removal efficiencies of 47 %, 48 % and 50 % were respectively found, and Total Organic Carbon abatements of 36 %, 36 % and 42 % were reached. Despite the low effluent mineralization, high value-added products (carboxylic acids) are formed during 240 min of EF, with the possibility of selective electroconversion into acetic and formic acid. Regarding decolorization, a successful result was found with the j of 90 mA cm, which achieved almost 90 % color removal. Finally, a final treatment strategy is proposed using the sludge only dried to 100 °C, followed by heterogenous EF with acidification to pH 3 and j of 60 mA cm.