The Sulfur Cycling And Regulation For Bioelectrochemically Enhanced Removing Organic Matter And Iron From Flowback Water

Shale gas exploitation has become an important national energy strategy.A high volume of flowback water（FW）is generated during this exploitation and contains high salinity(such as Cl^-and SO₄^2-),complex organics and heavy metals,thereby posing a significant challenge for ecological protection during shale gas extraction.In this thesis,sulfur-cycle mediated bioelectrochemical processes were selected to remove complex organics and iron from FW.The mechanism for the enhanced sulfur cycling during this process was studied.The regulation and control of sulfur cycle by anode potential in bioelectrochemical system was also studied to enhance complex organics removal but decrease sulfide production avoiding the production of black odor water.This study will help to realize the regulation and enhancement of FW treatment by bioelectrochemical technology in terms of the mechanism.Three parts were included in this thesis as follows:1.To investigate the impact of sulfate concentration on the performance of FW treatment by microbial fuel cell（MFC）,sulfate concentration in the synthetic FW was intermittently changed,and those results were attained as follows:（1）Sulfate reduction coupled microbial fuel cell enhanced the removal of complex organics in FW.COD removal increased with the rise of sulfate concentration accompanying with higher power production.The MFC injected with stable-high-sulfate-containing synthetic FW achieved an effective FW treatment including an average COD removal of 69.8±9.7%and power production of 2164±396m W/m³.（2）The intermit change of sulfate concentration in the synthetic FW did not influence the complex organics removal in MFC.This removal recovered when FW with high sulfate was injected again,but the power production was obviously inhibited resulting in a much lower value of 385±112 m W/m³.（3）After being acclimated with low sulfate-containing FW,the microbial community on the anode changed to adapt to the low condition and showed a low tolerance to sulfide.The MFC treated with stable-high-sulfate-containing synthetic FW showed stable power generation and high tolerance to sulfide,but the power generation was also inhibited by excessive sulfate reduction.2.By investigating the performance of sulfur-cycling mediated bioelectrochemical processes for removing complex organics and iron from FW,those results were attained as follows:（1）Simultaneous removal of COD and iron from the synthetic FW was realized in a sulfur-cycle-mediated MFC,with the removal efficiencies of 90.6±8.7%for iron and72±6%for COD,and a power density of 2636±476 m W/m3.Furthermore,real FW was more effectively treated,with higher total iron removal efficiency and power generation.（2）The dominant microorganisms of Sulfurovum and unclassified Desulfuromonadales in the anolyte and on the anode,respectively,played essential roles in biological and electrochemical oxidation of sulfide to sulfate,thus accelerating the sulfur cycle and improving iron removal.（3）Sulfur-oxidizing bacteria（SOB）enrichment in the anolyte contributed to stable and long-term high power generation through avoiding severe elemental sulfur accumulation on the anode.（4）Bacteroidetes,Firmicutes,Proteobacteria,and Chloroflexi,which were enriched in this sulfur-cycle-mediated MFC,were responsible for the efficient removal of complex organics from FW.3.To investigate the mechanism of regulation and control on sulfur cycle by the electrode potential,four different potentials（-0.2,-0.1,0.0 and+0.2 V vs.SHE）were applied on the anodes,and those results were attained as follows:（1）Electrode-potential-control（EPC）bioelectrochemical systems mediated by sulfate-reducing bacteria（SRB）could effectively remove organics from FW but decrease sulfide production.（2）The potential shaped a cryptical sulfur cycle in EPC systems with thiosulfate as an important intermediate.The anode with-0.1 and-0.2 V might overcome Dsr MKJOP as electron acceptor of Tp Ic₃ and Hmc and inhibit Dsr C-S⁰trisulfide reduction,and thus likely contributed to thiosulfate production.（3）Biological thiosulfate oxidation to sulfate was enhanced under-0.1 V with the accumulation of Sox complex coding genes,thus forming a novel sulfur cycle of sulfate-sulfite/thiosulfate-sulfate.（4）Abundant genes coding thiosulfate reductase and dissimilatory sulfite reductase under-0.2 V suggested preferential thiosulfate and sulfite reduction.The obvious accumulation of SQR coding gene in EPC systems likely also played an important role in decreasing sulfide concentrations.（5）The enrichment of Rhodobacteraceae,Desulfobulbaceae and Desulfuromonadales on the anode,and Helicobacteraceae and Halothiobacillaceae in the anolyte of EPC systems likely contributed to the low sulfide production but along with efficient COD removal.The research results provide new insights into the mechanism by which sulfate-mediated bioelectrochemical processes strengthen organic matter and iron.A new cryptical sulfur cycling of sulfate-sulfite/thiosulfate-sulfate was proposed on the aspect of bioelectrochemistry.This study also provide scientific basis and technology choice for the development of new bioelectrochemical treatment of other sulfate-containing wastewater with high salinity and complex organics and the process regulation.