Implementation of a Sulfide-Air Fuel Cell Coupled to a Sulfate-Reducing Biocathode for Elemental Sulfur Recovery.

Bio-electrochemical systems (BES) are a flexible biotechnological platform that can be employed to treat several types of wastewaters and recover valuable products concomitantly. Sulfate-rich wastewaters usually lack an electron donor; for this reason, implementing BES to treat the sulfate and the possibility of recovering the elemental sulfur (S0) offers a solution to this kind of wastewater. This study proposes a novel BES configuration that combines bio-electrochemical sulfate reduction in a biocathode with a sulfide-air fuel cell (FC) to recover S0. The proposed system achieved high elemental sulfur production rates (up to 386 mg S0-S L-1 d-1) with 65% of the sulfate removed recovered as S0 and a 12% lower energy consumption per kg of S0 produced (16.50 ± 0.19 kWh kg-1 S0-S) than a conventional electrochemical S0 recovery system.

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 ±â€¯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 ±â€¯1.9% of all sulfate removed.

Treatment of real flue gas desulfurization wastewater in an autotrophic biocathode in view of elemental sulfur recovery: Microbial communities involved.

Sulfur oxide emissions can lead to acidic precipitation and health concerns. Flue gas desulfurization (FGD) systems treat these emissions generating a wastewater with high-sulfate content. This work is the first attempt to treat this effluent with bioelectrochemical systems (BES) in order to recover elemental sulfur, a technology that allows the treatment of several wastewaters that lack of electron donor. The sulfate treatment and elemental sulfur recovery have been studied in a biocathode with simultaneous sulfate reduction to sulfide and partial sulfide oxidation, comparing the performance obtained with synthetic and real wastewater. A decrease of the sulfate removal rate (SRR) from 108 to 73mgS-SO42-L-1d-1 was observed coupled to an increase in the elemental sulfur recovery from 1.4 to 27mgS-S0L-1d-1. This elemental sulfur recovered as a solid from the real wastewater represented a 64% of the theoretical elemental sulfur produced (the elemental sulfur corresponded to a 72% of the solid weight). In addition, microbial communities analysis of the membrane and cathode biofilms and planktonic biomass showed that the real wastewater allowed a higher growth of sulfur oxidizing bacteria (SOB) adapted to more complex waters as Halothiobacillus sp. while decreasing the relative abundance of sulfate reducing bacteria (SRB).

Evaluation of key parameters on simultaneous sulfate reduction and sulfide oxidation in an autotrophic biocathode.

Bioelectrochemical systems (BESs) are being studied as an alternative technology for the treatment of several kinds of wastewaters with a lack of electron donor such as high-strength sulfate wastewaters. This study evaluates different parameters that influence the simultaneous sulfate reduction and sulfide oxidation in an autotrophic biocathode: ion-exchange membrane (IEM), cathodic pH and cathode potential. Two different membranes were studied to evaluate sulfate and sulfide adsorption and diffusion from the cathode to the anode, observing that a cation-exchange membrane (CEM) widely decreased these effects. Three different cathode pH (5.5, 7 and 8.5) were studied in a long-term operation observing that pH = 7 was the optimal for sulfate removal, achieving reduction rates around 150 mg S-SO42- L-1 d-1. Microbial community analysis of the cathode biofilm demonstrated a high abundance of sulfate-reducing bacteria (SRB, 67% at pH 7, 60% at pH 8.5 and 42% at pH 5.5), mainly Desulfovibrio sp. at pH 5.5 and 7 and Desulfonatronum sp. at pH 8.5. The cathode potential also was studied from -0.7 to -1.2 V vs. SHE achieving sulfate removal rates higher than 700 mg S-SO42- L-1 d-1 at cathode potentials from -1.0 to -1.2 V vs. SHE. Also, the highest cathodic recovery and the highest sulfur species imbalance were observed at a cathode potential of -1.0 V vs. SHE, which indicated a higher elemental sulfur production.

Treatment of high-strength sulfate wastewater using an autotrophic biocathode in view of elemental sulfur recovery.

Treatment of high-strength sulfate wastewaters is becoming a research issue not only for its optimal management but also for the possibility of recovering elemental sulfur. Moreover, sulfate-rich wastewater production is expected to grow due to the increased SO2 emission contained in flue gases which are treated by chemical absorption in water. Bioelectrochemical systems (BESs) are a promising alternative for sulfate reduction with a lack of electron donor, since hydrogen can be generated in situ from electricity. However, complete sulfate reduction leads to hydrogen sulfide as final sulfur compound. This work is the first to demonstrate that, in addition to an efficient sulfate-rich wastewater treatment, elemental sulfur could be recovered in a biocathode of a BES under oxygen limiting conditions. The key of the process is the biological oxidation of sulfide to elemental sulfur simultaneously to the sulfate reduction in the cathode using the oxygen produced in the anode that diffuses through the membrane. High sulfate reduction rates (up to 388 mg S-SO42- L-1 d-1) were observed linked to a low production of sulfide. Accumulation of elemental sulfur over graphite fibers of the biocathode was demonstrated by energy dispersive spectrometry, discarding the presence of metal sulfides. Microbial community analysis of the cathode biofilm demonstrated the presence of sulfate-reducing bacteria (mainly Desulfovibrio sp.) and sulfide-oxidizing bacteria (mainly Sulfuricurvum sp.). Hence, this biocathode allows simultaneous biological sulfate reduction and biological sulfide oxidation to elemental sulfur, opening up a novel process for recovering sulfur from sulfate-rich wastewaters.