Optimisation of the operational parameters for a comprehensive bioelectrochemical treatment of acid mine drainage.

Bioelectrochemical systems provide a promising tool for the treatment of acid mine drainage (AMD). Biological sulphate reduction powered with electrical energy consumes acidity and produces sulphide, which can precipitate metals. However, the produced sulphide and the changes in pH resulting from the biological processes affect the efficiency and the environmental impacts of this treatment significantly. In this work, the effects of pH and sulphur speciation on the sulphate reduction rate (SRR) and comprehensive AMD treatment were evaluated in two-chamber microbial electrolysis cells at a cathode potential of -0.8 V vs. NHE. The increase of initial sulphate concentration from below 1000 mg to above 1500 mg S-SO42-/L increased SRR from 121 ± 25 to 177 ± 19 mg S-SO42-/L/d. SRR further increased to 347 mg S-SO42-/L/d when the operation mode was changed from batch to periodical addition of sulphate and acidity (363 mg S-SO42-/L/d and 22.6 mmol H+/L/d, respectively). The average SRR remained above 150 mg S-SO42-/L/d even at pH above 8.5 and with the total dissolved sulphide concentration increasing above 1300 mg S-TDSu/L. Operation at pH above 8 enabled the recovery of over 90% of the sulphur as dissolved sulphide and thus assisted in minimising the formation and release of toxic H2S.

The effect of start-up on energy recovery and compositional changes in brewery wastewater in bioelectrochemical systems.

Start-up of bioelectrochemical systems (BESs) fed with brewery wastewater was compared at different adjusted anode potentials (-200 and 0 mV vs. Ag/AgCl) and external resistances (50 and 1000 Ω). Current generation stabilized faster with the external resistances (9 ± 3 and 1.70 ± 0.04 A/m3 with 50 and 1000 Ω, respectively), whilst significantly higher current densities of 76 ± 39 and 44 ± 9 A/m3 were obtained with the adjusted anode potentials of -200 and 0 mV vs. Ag/AgCl, respectively. After start-up, when operated using 47 Ω external resistance, the current densities and Coulombic efficiencies of all BESs stabilized to 9.5 ± 2.9 A/m3 and 12 ± 2%, respectively, demonstrating that the start-up protocols were not critical for long-term BES operation in microbial fuel cell mode. With adjusted anode potentials, two times more biofilm biomass (measured as protein) was formed by the end of the experiment as compared to start-up with the fixed external resistances. After start-up, the organics in the brewery wastewater, mainly sugars and alcohols, were transformed to acetate (1360 ± 250 mg/L) and propionate (610 ± 190 mg/L). Optimized start-up is required for prompt BES recovery, for example, after process disturbances. Based on the results of this study, adjustment of anode potential to -200 mV vs. Ag/AgCl is recommended for fast BES start-up.

The effect of anode potential on bioelectrochemical and electrochemical tetrathionate degradation.

The effect of poised anode potential on electricity production and tetrathionate degradation was studied in two-chamber flow-through electrochemical (ES) and bioelectrochemical systems (BES). The minimum anode potential (vs. Ag/AgCl) for positive current generation was 0.3V in BES and 0.5V in the abiotic ES. The anode potential required to obtain average current density above 70mAm-2 was 0.4V in BES and above 0.7V in ES. ES provided higher coulombic efficiency, but the average tetrathionate degradation rate remained significantly higher in BES (above 110mgL-1d-1) than in the abiotic ES (below 35mgL-1d-1). This study shows that at anode potentials below 0.7V, the electrochemical tetrathionate degradation is only efficient with microbial catalyst and that significantly higher tetrathionate degradation rates can be obtained with bioelectrochemical systems than with electrochemical systems at the tested anode potentials.

Long-term stability of bioelectricity generation coupled with tetrathionate disproportionation.

To prevent uncontrolled acidification of the environment, reduced inorganic sulfur compounds (RISCs) can be bioelectrochemically removed from water streams. The long-term stability of bioelectricity production from tetrathionate (S4O6(2-)) was studied in highly acidic conditions (pH<2.5) in two-chamber fed-batch microbial fuel cells (MFCs). The maximum current density was improved from previously reported 80mAm(-2) to 225mAm(-2) by optimizing the external resistance. The observed reaction products of tetrathionate disproportionation were sulfate and elemental sulfur. In long-term run, stable electricity production was obtained for over 700days with the average current density of 150mAm(-2). The internal resistance of the MFC decreased over time and no biofouling was observed. This study shows that tetrathionate is an efficient substrate also for long-term bioelectricity production.

Electricity generation from tetrathionate in microbial fuel cells by acidophiles.

Inorganic sulfur compounds, such as tetrathionate, are often present in mining process and waste waters. The biodegradation of tetrathionate was studied under acidic conditions in aerobic batch cultivations and in anaerobic anodes of two-chamber flow-through microbial fuel cells (MFCs). All four cultures originating from biohydrometallurgical process waters from multimetal ore heap bioleaching oxidized tetrathionate aerobically at pH below 3 with sulfate as the main soluble metabolite. In addition, all cultures generated electricity from tetrathionate in MFCs at pH below 2.5 with ferric iron as the terminal cathodic electron acceptor. The maximum current and power densities during MFC operation and in the performance analysis were 79.6 mA m(-2) and 13.9 mW m(-2) and 433 mA m(-2) and 17.6 mW m(-2), respectively. However, the low coulombic efficiency (below 5%) indicates that most of the electrons were directed to other processes, such as aerobic oxidation of tetrathionate and unmeasured intermediates. The microbial community analysis revealed that the dominant species both in the anolyte and on the anode electrode surface of the MFCs were Acidithiobacillus spp. and Ferroplasma spp. This study provides a proof of concept that tetrathionate serves as electron donor for biological electricity production in the pH range of 1.2-2.5.