This study deals with the feasibility of the oxidative leaching of molybdenum and cobalt sulphides contained in a spent hydrodesulphurisation catalyst using H2O2 in acidic medium. The study was first carried out with MoS2 in order to define the important parameters of the leaching procedure and then applied to an unroasted CoMo/Al2O3 spent catalyst containing 11 wt % of Mo and 2.7 wt % of Co. In both cases, a 23 factorial design was used. The results obtained with MoS2 highlighted the necessity to limit the increase in temperature of the medium to minimize the decomposition of H2O2 and the necessity to keep pH constant to avoid an important acidification of the medium which could cause dissolution of alumina matrix when leaching will be applied to spent catalysts. The oxidative leaching of the unroasted CoMo/Al2O3 spent catalyst was performed without previous grinding. In a single-step, at pH = 1.3 with a stoichiometric factor equal to 2.4, an L/S ratio equal to 7.5, and an H2O2 concentration of 3.75 mol·L−1, it was possible to recover molybdenum and cobalt simultaneously with leaching yields of 90% and 83%, respectively, without dissolving more than 8% Al. In these conditions, the maximal temperature recorded during the experiment was about 60 °C.

Electrochemical tests of nitrate reduction on Boron-Doped Diamond cathode are investigated through a Design of Experiments (DOE) method. The results show good reduction of nitrate into almost exclusively N2. In the studied domain, the best experimental conditions are high initial nitrate content, low acidic pH values and low working current densities. The application of DOE conclusions on an agro-industrial wastewater gives really satisfying results: final nitrate contents lower than 50 mg/L without nitrite or ammonium formation, and with low energy consumption (under 25 kWh/kgNO3).