Copper removal and elemental sulfur recovery from fracturing flowback water in a microbial fuel cell with an extra electrochemical anode

Fracturing flowback water (FFW) from the shale gas exploitation resulted in environmental burden. FFW could be treated by a microbial fuel cell (MFC), but the challenge for the precipitation of ultrafine particles due to the supersaturation of sulfide remains to be addressed. Herein, we reported a Dual-anode MFC (DA-MFC), in which the FFW remediation and elemental sulfur recovery could be performed by regulating potential of the electrochemical anode. The removal of COD and sulfate was 70.0 ± 1.2% and 75.5 ± 0.4% in DA-MFCs by controlling potential at -0.1 V (vs. SHE) for 36 h. Meanwhile, the efficiency of copper removal and elemental sulfur recovery was up to 99.9 ± 0.5% and 75.6 ± 1.8%, respectively, which was attributed by the electrochemical oxidation of sulfide to elemental sulfur. Trichococcus, unclassified Prolixibacteraceae and unclassified Cloacimonadales enriched on the bioanodes of DA-MFCs were sensitive to potential regulation and favorable for degrading complex organics. UnclassifiedSynergistaceae, Desulfobacterium, Desulfovibrio, unclassified bacteria and Syner-01 was conducive to sulfate removal. Moreover, the elimination of Azoarcus due to potential regulation suppressed the biological oxidation of sulfide. Thus, organics were efficiently removed through the biological oxidation and sulfate reduction on bioanode, the copper ions were combined with the sulfide from sulfate reduction to precipitate effectively, and then the excessive sulfide in the system was converted into elemental sulfur attached on the electrochemical anode. The results provide new sights on bio-electrochemical technology for treatment of wastewater containing complex organics, heavy metals and sulfates.

Simultaneous denitrification and desulfurization-S0 recovery of wastewater in trickling filters by bioaugmentation intervention based on avoiding collapse critical points

In order to better achieve efficiently simultaneous desulfurization and denitrification/S⁰ recovery of wastewater, the intervention of sulfur oxidizing bacteria (SOB) and denitrifying bacteria (DNB) was employed to avoid the collapse critical points (the dramatically decrease of S/N removal efficiency) under the fluctuated load. With the assistance of DNB and SOB, collapse critical point of trickling filter (TF) was delayed from the P₈ (105-114 d) to P₁₀ stage (129-138 d). The treatment efficiency of nitrogen and sulfur was the highest with the S/N ratio of 3:1. The bioaugmentation of DNB and SOB at collapse critical point could effectively regulated collapse situation, which further increased the maximum system utilization/elimination capacity to 4.50 kg S m^-3·h^-1 and 0.90 kg N m^-3·h^-1 (increased by 56.89% and 65.56% in comparison to control). High-throughput sequencing analysis indicated that Proteobacteria (average 78.59%) and Bacteroidetes (average 9.30%) were dominant bacteria in the reactor at all stages. As the reaction proceeds, the microbial community was gradually dominated by some functional genera such as Chryseobacterium (average 2.97%), Halothiobacillus (average 22.71%), Rhodanobacter (average 14.02%), Thiobacillus (average 9.01%), Thiomonas (average 16.70%) and Metallibacterium (average 21.63%), which could remove nitrate or sulfide. Both of Principal Component Analysis (PCA) and Canonical Correlation Analysis (CCA) demonstrated the important role of DNB/SOB during the long-term run in the trickling filters (TFs).

Sulfate reduction and elemental sulfur recovery using photoelectric microbial electrolysis cell

Elemental sulfur recover from sulfate-rich environment has great significance for the sustainable development of environment and energy. This study aimed to realize simultaneous sulfate reduction and elemental sulfur recovery using a novel photoelectricity microbial electrolysis cell (PMEC) under low applied voltages. At an applied voltage of 1.2 V, the sulfate reduction rate in the PMEC reached 200 ± 2.3 mg L^-1 d^-1, and 46.3 ± 7.9% of the reduced sulfate converted to elemental sulfur. With increasing voltages from 0.8 to 1.5 V, the sulfate reduction rates enhanced from 37.8 ± 12.4 to 236 ± 18.1 mg L^-1 d^-1. The recovery efficiency of elemental sulfur from removed sulfate decreased to 35% at 1.5 V, which was attributed to the higher concentration of dissolved oxygen diffusing from the anode side. Sulfate reducing bacteria (including Desulfovibrio and Desulfomicrobium) cooperated with sulfur oxidizing bacteria (including Thiomonas and Acinetobacter) for recovering elemental sulfur that could be regulated by cathode configuration. The study provided an alternative to apply solar energy in biological sulfur recovery and reduce energy consumption of wastewater treatment.

Recirculation ratio regulates denitrifying sulfide removal and elemental sulfur recovery by altering sludge characteristics and microbial community composition in an EGSB reactor

Expanded granular sludge blanket (EGSB) is regarded as a promising reactor to carry out denitrifying sulfide removal (DSR) and elemental sulfur (S⁰) recovery. Although the recirculation ratio is an essential parameter for EGSB reactors, how it impacts the DSR process remains poorly understood. Here, three lab-scale DSR-EGSB reactors were established with the different recirculation ratios (3:1, 6:1 and 9:1) to evaluate the corresponding variations in pollutant removal, S⁰ recovery, anaerobic granular sludge (AGS) characteristics and microbial community composition. It was found that an intermediate recirculation ratio (6:1) could facilitate long-term reactor stability. Adequate recirculation ratio could enhance S⁰ recovery, but an excessive recirculation ratio (9:1) was likely to cause AGS fragmentation and biomass loss. The S⁰ desorbed more from sludge at higher recirculation ratios, probably due to the enhanced hydraulic disturbance caused by the increased recirculation ratios. At the low recirculation ratio (3:1), S⁰ accumulation as inorganic suspended solids in AGS led to a decrease in VSS/TSS ratio and mass transfer efficiency. Although typical denitrifying and sulfide-oxidizing bacteria (e.g., Azoarcus, Thauera and Arcobacter) were predominant in all conditions, facultative and heterotrophic functional bacteria (e.g., Azoarcus and Thauera) were more adaptable to higher recirculation ratios than autotrophs (e.g., Arcobacter, Thiobacillus and Vulcanibacillus), which was conducive to the formation of bacterial aggregates to response to the increased recirculation ratio. The study revealed recirculation ratio regulation significantly impacted the DSR-EGSB reactor performance by altering AGS characteristics and microbial community composition, which provides a novel strategy to improve DSR performance and S⁰ recovery.

Study of MoO3-γAl2O3 catalysts behavior in selective catalytic reduction of SO2 toxic gas to sulfur with CH4

In the present study, a detailed investigation was carried out on MoO₃ alumina-supported catalysts behavior in selective catalytic reduction of SO₂ to sulfur with CH₄. At first, four different molybdenum catalysts with weight rates of 0, 5, 10, and 15 were impregnated on γ-alumina to be characterized using XRD, SEM, BET, BJH, and N₂ adsorption. Then, to find the most active catalyst, temperature dependency test was performed on all of the prepared catalysts and the result representing Al2O3-Mo10 as the best catalyst. In next step, the effects of feed gas composition, space velocity, and long-term activity, as an important industrial factor, were tested on Al₂O₃-Mo10. It was revealed instantaneously from the beginning, MoO₃ specie started to convert mainly into MoS₂ and MoO₂, and a minor part into Mo₂C, which is terminated after 750 min achieving a stable condition. Thereafter, SO₂ conversion and sulfur selectivity increased from 85.8 to 89.4% and 99.4 to 99.7%, respectively. XRD graph of the used catalyst and TPO thermogravimetric/mass-spectra proved possible happening of the proposed mechanism in long-term activity. At the end, mean activation energy was determined based on Arrhenius model in temperature range of 550 to 800 °C, with a value of 0.33 eV for Al₂O₃-Mo10.

Metabolic divergence in simultaneous biological removal of nitrate and sulfide for elemental sulfur production under temperature stress

The simultaneous removal of NO₃^- and HS^- at temperature stress (25-10°C) is evaluated here. An expanded granular sludge bed (EGSB) reactor was run over 120days at N/S molar ratio of 0.35 (for S⁰ production) under constant sulfur loading rate of 0.4kgS/m³d. The simultaneous removal of NO₃^- and HS^-, was achieved at applied conditions. Average HS^--S removal varied from 98 (25°C) to 89.2% at 10°C, with almost complete NO₃^- removal. Average S⁰ yield ranged from 83.7 at 25°C to 67% at 10°C. The temperature drop caused a decrease in granular sludge accumulated S⁰ fraction by nearly 2.5 times. Decreased temperature caused metabolic pathway change observed as higher SO₄^2- production, apparently allowing the biomass to obtain more energy per HS^- consumed. It is hypothesized that the metabolic shift is a natural response to compensate for temperature-induced changes in energy requirements.

Elemental sulfur recovery and spatial distribution of functional bacteria and expressed genes under different carbon/nitrate/sulfide loadings in up-flow anaerobic sludge blanket reactors

To characterize the impact of influent loading on elemental sulfur (S⁰) recovery during the denitrifying and sulfide oxidation process, three identical, lab-scale UASB reactors (30cm in length) were established in parallel under different influent acetate/nitrate/sulfide loadings, and the reactor performance and functional community structure were investigated. The highest S⁰ recovery was achieved at 77.9% when the acetate/nitrate/sulfide loading was set to 1.9/1.6/0.7kgd^-1m^-3. Under this condition, the genera Thauera, Sulfurimonas, and Azoarcus were predominant at 0-30, 0-10 and 20-30cm, respectively; meanwhile, the sqr gene was highly expressed at 0-30cm. However, as the influent loading was halved and doubled, S⁰ recovery was decreased to 27.9% and 45.1%, respectively. As the loading was halved, the bacterial distribution became heterogeneous, and certain autotrophic sulfide oxidation genera, such as Thiobacillus, dominated, especially at 20-30cm. As the loading doubled, the bacterial distribution was relatively homogeneous with Thauera and Azoarcus being predominant, and the nirK and sox genes were highly expressed. The study verified the importance of influent loading to regulate S⁰ recovery, which could be achieved as Thauera and Sulfurimonas dominated. An influent loading that was too low or too high gave rise to insufficient oxidation or over-oxidation of the sulfide and low S⁰ recovery performance.

Enhanced elementary sulfur recovery with sequential sulfate-reducing, denitrifying sulfide-oxidizing processes in a cylindrical-type anaerobic baffled reactor

Simultaneous removal of COD, SO4(2-) and NO3(-) and recovery of elemental sulfur (S(0)) were evaluated in a four-compartment anaerobic baffled reactor (ABR) with separated functional units of sulfate reduction (SR) and denitrifying sulfide removal (DSR). Optimal SO4(2-)-S/NO3(-)-N ratio was evaluated as 5:5, with a substantial improvement of S(0) recovery maintained at 79.1%, one of the highest level ever reported; meanwhile, removal rates of COD, SO4(2-) and NO3(-) were approached at 71.9%, 92.9% and 98.6%, respectively. Nitrate served as a key factor to control the shift of SR and DSR related populations, with the possible involvement of Thauera sp. during SR and Sulfurovum sp. or Acidiferrobacter sp. during DSR, respectively. DsrB and aprA genes were the most abundant during SR and DSR processes, respectively. Cylindrical-type ABR with the improved elemental sulfur recovery was recommended to deal with sulfate and nitrate-laden wastewater under the optimized SO4(2-)/NO3(-) ratio.

GeoChip-based analysis of the microbial community functional structures in simultaneous desulfurization and denitrification process

The elemental sulfur (S°) recovery was evaluated in the presence of nitrate in two development models of simultaneous desulfurization and denitrification (SDD) process. At the loading rates of 0.9 kg S/(m³·day) for sulfide and 0.4 kg N/(m³·day) for nitrate, S° conversion rate was 91.1% in denitrifying sulfide removal (DSR) model which was higher than in integrated simultaneous desulfurization and denitrification (ISDD) model (25.6%). A comprehensive analysis of functional diversity, structure and metabolic potential of microbial communities was examined in two models by using functional gene array (GeoChip 2.0). GeoChip data indicated that diversity indices, community structure, and abundance of functional genes were distinct between two models. Diversity indices (Simpson's diversity index (1/D) and Shannon-Weaver index (H')) of all detected genes showed that with elevated influent loading rate, the functional diversity decreased in ISDD model but increased in DSR model. In contrast to ISDD model, the overall abundance of dsr genes was lower in DSR model, while some functional genes targeting from nitrate-reducing sulfide-oxidizing bacteria (NR-SOB), such as Thiobacillus denitrificans, Sulfurimonas denitrificans, and Paracoccus pantotrophus were more abundant in DSR model which were highly associated with the change of S(0) conversion rate obtained in two models. The results obtained in this study provide additional insights into the microbial metabolic mechanisms involved in ISDD and DSR models, which in turn will improve the overall performance of SDD process.