Research On Green Treatment And Resource Recovery Of Heavy Metal Wastewater Based On Bioelectrochemical System

Bioelectrochemical system（BES）is a promising bioengineering technology that utilizes the metabolic activities of microorganisms to convert the chemical energy in substrates into electrical energy and uses heavy metals,nutrients,and other toxic chemicals as electron acceptors in the cathode chamber,thereby reducing their toxicity.Currently,lower electricity generation is one of the major hurdles to real-world applications of BES technology for wastewater treatment.To this end,BES was employed in this study to treat heavy metals and nutrient-contaminated wastewater,with a focus on the following aspects:utilization of novel exoelectrogens for electricity generation and heavy metal removal from wastewater,treatment of Ni（Ⅱ）-contaminated wastewater with single-chamber microbial fuel cells（SCMFC）,utilization of nitrate-resistant electricigenic bacteria as inoculums to treat nitrate and heavy metals contaminated wastewater,and investigation of the impacts of Co（Ⅱ）on BES anodic electrogenic biofilm during wastewater treatment.The study’s findings are summarized below:（1）The inability of most exoelectrogens to generate electricity in BES is a key barrier to its application for wastewater treatment.As a result,three novel facultative exoelectrogenic strains designated Castellaniella sp.A5,Castellaniella sp.B3,and Castellaniella sp.A3 were isolated from a microbial fuel cell（MFC）.An SCMFC inoculated with the mixed culture of these strains attained the highest electricity generation（i.e.,320 m W/m² power density and 3.19 A/m² current density）,chemical oxygen demand removal efficiency（91.15±0.05%）,and Coulombic efficiency（54.81±4.18%）.In addition,the BES containing biofilms of the mixed culture achieved the highest Cu,Cr,and Cd removal efficiencies of99.89±0.07%,99.59±0.53%,and 99.91±0.04%,respectively.XPS results showed that Cr（VI）and Cu（Ⅱ）in the simulated industrial wastewater were recovered via microbial electrochemical reduction as Cr（ⅡI）and Cu（0）,respectively,whiles Cd（Ⅱ）precipitated as Cd（OH）₂ or Cd CO₃.These results suggest that a mixed culture inoculum of Castellaniella sp.A5,Castellaniella sp.B3,and Castellaniella sp.A3 has great potential as a biocatalyst in BES for heavy metals removal from wastewater.（2）Due to growing concerns about rising energy demands,the feasibility of treating Ni（Ⅱ）-contaminated wastewater with SCMFC was investigated.Results revealed that Ni（Ⅱ）decreased power output from 673.24±12.40 m W/m² at 0mg/L to 179.26±9.05 m W/m² at 80 mg/L.The metal and COD removal efficiencies of the MFCs declined as Ni（Ⅱ）concentration increased from 10 to 30mg/L.FTIR analysis revealed the involvement of various microbial biofilm functional groups in Ni（Ⅱ）uptake.Cyclic voltammetry（CV）and electrochemical impedance spectroscopy（EIS）demonstrated that the electron transfer performance of the biofilms was negatively impacted by Ni（Ⅱ）.High-throughput sequencing results revealed a decrease in Geobacter and an increase in Desulfovibrio in response to the various Ni（Ⅱ）concentrations.The various Ni（Ⅱ）concentrations decreased the expression of electron transport genes,as revealed by phylogenetic investigation of communities by reconstruction of unobserved states and real-time reverse transcription polymerase chain reaction.This work advances our understanding of Ni（Ⅱ）inhibition on biofilms,as well as the concurrent removal of organic matter and Ni（Ⅱ）from wastewater using SCMFCs.（3）The use of BES in the treatment of NO₃^--N contaminated wastewater is still limited to laboratory-scale applications,owing to NO₃^--N inhibition of most bacteria.Thus,the ability of a mixed electricigenic Castellaniella species to perform denitrification and anode respiration coupled with cathodic metals recovery was investigated.Results showed that 500 mg/L NO₃^--N significantly decreased power generation,whereas 100 and 250 mg/L NO₃^--N had a lesser impact.SCMFCs fed with 100 and 250 mg/L NO₃^--N achieved removal efficiencies greater than 90%.EPS protein content and enzymatic activities of the biofilms decreased significantly at 500 mg/L NO₃^--N concentration.CV and EIS results revealed that the 500 mg/L NO₃^--N concentration decreased the redox activities of biofilms.BES inoculated with the Castellaniella species was able to attain Cu（Ⅱ）,Hg（Ⅱ）,Pb（Ⅱ）,and Zn（Ⅱ）removal efficiencies of 99.86±0.10%,99.98±0.014%,99.98±0.01%,and 99.17±0.30%,respectively,from real wastewater.Over 89%of NO₃^--N was removed from the BES anolyte during the recovery of the metals.This research reveals promising denitrifying exoelectrogens for enhanced power generation,NO₃^--N removal,and heavy metals recovery in BES.（4）While Co（Ⅱ）is widely present in industrial effluents,its impacts on electrogenic biofilms during wastewater treatment in BES has never been studied.Results showed that as the concentration of Co（Ⅱ）increased in the wastewater,the power generation decreased from 345.43±4.07 m W/m² at 0 mg/L to 160.51±0.86 m W/m² at 30 mg/L.However,at 5 mg/L Co（Ⅱ）,the impact on power generation was less severe.The COD and Co（Ⅱ）removal efficiencies of the biofilms were found to be dose-dependent.CV and EIS tests revealed that Co（Ⅱ）concentrations of 10,20,and 30 mg/L drastically diminished the electrocatalytic activities of the biofilms.The 5 mg/L Co（Ⅱ）enhanced the proliferation of Anaeromusa-Anaeroarcus,whereas 10,20,and 30 mg/L Co（Ⅱ）reduced its population.Metatranscriptomic analyses showed that the higher Co（Ⅱ）concentrations substantially suppressed the expression of genes related to EET in biofilms.This research sheds light on the effect of Co（Ⅱ）on electrogenic biofilms,thereby establishing the groundwork for the cost-effective treatment of Co（Ⅱ）-contaminated wastewater in the anodes of BES.（5）The selection of active and prospective electrogenic microorganisms is a vital step in achieving maximum power production from the MFC.In this research,novel Gram positive electricigenic strains named Enterococcus sp.AMZ3,Enterococcus sp.AMZ8,and Enterococcus sp.AMZ5 were isolated from the electrogenic biofilm of an MFC.SCMFCs inoculated with a mixed inocula of these strains produced the highest power and current densities of 439.78 m W/m² and5.31 A/m²,respectively.CV and EIS results showed that the mixed culture enhanced the electrocatalytic performance of the SCMFCs.Coculturing these strains increased the synthesis of EPS protein,cytochrome c,and NADH in the biofilms.BES inoculated with the mixed Enterococcus species achieved Cu（Ⅱ）and Ni（Ⅱ）removal efficiencies of 99.99±0.0071%and 99.96±0.02%,respectively,from simulated electroplating wastewater.X-ray photoelectron spectroscopy and X-ray powder diffraction results revealed that the metals were bio-electrochemically reduced to their nontoxic states.These results indicate that the mixed Enterococcus species is promising for the improvement of BES power generation and heavy metals recovery capacity.