BACKGROUND This work studied the treatment of and metal recovery from a synthetic acid mine drainage (AMD) containing 500 mg L−1 copper (Cu2+) and iron (Fe+3), and 50 mg L−1 nickel (Ni2+) and tin (Sn2+) by using a bioelectrochemical system (BES). The presence of electroactive bacteria improved the performance of such reactor configuration, by contrast with systems with abiotic anodes. RESULTS Operating as a microbial fuel cell (MFC), all of the Fe3+ was reduced to Fe2+ in about 24 h and Cu2+ was electrodeposited onto the cathodic surface, a Cu electrode, obtaining pure Cu0. Almost all of the Cu in the catholyte was recovered after four days. The maximum current density and power attained in this stage were 0.136 mA cm−2 and 0.0134 mW cm−2, respectively. Subsequent operation as a microbial electrolysis cell (MEC) allowed simultaneous recovery of the Fe2+, Ni2+ and Sn2+ by fixing the cathode potential at −0.7 V versus Ag/AgCl. The electrode material in this stage was titanium. The tin was completely deposited onto the cathodic surface after one day of electrolysis. After three days, 77% and 60% of Ni and Fe, respectively, was recovered. CONCLUSION It was possible to recover Cu0 while generating electricity at the same time using a BES. The cell voltage required for the metal electrodeposition of Fe2+, Ni2+ and Sn2+ was low in the case of the BES because of the contribution of the electroactive bacteria. Sequential metal deposition is possible by adjusting the operating parameters of the BES reactors. © 2021 Society of Chemical Industry

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BACKGROUND: This work analyzes the synthesis of H 2 O 2 from dilute NaOH solutions under 0.1 MPa O 2 using a batch reactor with a three-dimensional rotating cylinder electrode. The centrifugal force produces a radial co-current flow of the gas and liquid phases. Thus, good mass transfer conditions are achieved and the O 2 reduced to H 2 O 2 is easily replenished in the liquid phase. RESULTS:Experiments with a glassy carbon rotating disc electrode identified 0.5 mol L -1 NaOH at 30 • C as suitable operating conditions. Galvanostatic experiments with three-dimensional rotating electrodes concluded that the best performance was obtained for a reticulated vitreous carbon structure of 100 ppi, at 40 mA cm -2 of macrokinetic current density and 1000 rpm rotation speed. Long-term experiments showed 79% current efficiency and 8.2 kWh kg -1 specific energy consumption until 6 h of electrolysis, with 8.4 g L -1 H 2 O 2 concentration. However, the current efficiency decreases for longer electrolysis times and consequently the specific energy consumption is increased. Thus after 10 h electrolysis the concentrations were H 2 O 2 10.4 g L -1 and NaOH 1.41 mol L -1 . CONCLUSION: A reactor having a three-dimensional rotating cylinder electrode with co-current oxygen and liquid flows inside the structure showed promising performance for H 2 O 2 production. Keywords: hydrogen peroxide; electrochemical reactor; three-dimensional electrode; oxygen reduction ρ electrolyte density (kg m -3 ) ω rotation speed (rpm or s -1 )