Recovery of elemental sulfur with a novel integrated bioelectrochemical system with an electrochemical cell.

Several industrial activities produce wastewater with high sulfate content that can cause significant environmental issues. Although bioelectrochemical systems (BESs) have recently been studied for the treatment of sulfate contained in this wastewater, the recovery of elemental sulfur with BESs is still in its beginnings. This work proposes a new reactor configuration named BES-EC, consisting of the coupling of a BES with an electrochemical cell (EC), to treat this type of wastewater and recover elemental sulfur. The reactor consisted of four electrodes: i) an abiotic anode, ii) a biocathode for the autotrophic sulfate reduction, iii) an anode of an electrochemical cell (EC) for the partial oxidation of sulfide to elemental sulfur (the biocathode and the EC anode were placed in the same chamber) and iv) an abiotic EC cathode. Several cathode potentials and sulfate loads were tested, obtaining high sulfate removal rates (up to 888 mg SO42--S L-1 d-1 at -0.9 V vs. SHE with a specific energy consumption of 9.18 ± 0.80 kWh kg-1 SO42--S). Exceptionally high theoretical elemental sulfur production rates (up to 498 mg S0-S L-1 d-1) were achieved with the EC controlled at a current density of 2.5 A m-2. Electron recovery around 80% was observed throughout most of the operation of the integrated system. In addition, short experiments were performed at different current densities, observing that sulfate removal did not increase proportionally to the higher applied current density. However, when the BES was controlled at 30 A m-2 and the EC at 7.5 A m-2, the proportion of elemental sulfur produced corresponded to 92.9 ± 1.9% of all sulfate removed.