Preparation And Effect Of Activated Algal Biochar On Hydrogenotrophic Denitrification Process At Biocathode Using Bioelectrochemical System(BES)

Due to the fabulous development of industry and agriculture without appropriate wastewater treatment,nitrate pollution entered various water environment,resulting in eutrophication of water environment then the blue algal bloom.Therefore,to effectively deal with nitrate pollution are increasing concerned.Recently,treatment of nitrate-containing wastewater using BES（bioelectrochemical system）not only preserves the advantages of biological methods in high efficiency and cleanliness,but also integrates the advantages of electrochemical treatments,achieveing the waste resource utilization.Focused on hydrogentrophic denitrification at biocathode in BES,this study investigated the enhancement of denitrification with the supplementation of activated algal biochar.Firstly,algal biochar is prepared and then activated through different ways as KOH or nitric acid.The surface topography and pore structure of those biochar were analysed by scanning electron microscope（SEM）.Next,Fourier transform infrared spectrometry（FTIR）was adopted to explore the variety of functional groups.Boehm titration,however,was used to quantify the concentration of acidic functional groups.Meanwhile,the specific surface area was calculated through the Brunauer-Emmett-Teller way（BET-N₂）.X-ray photoelectron spectroscopy（XPS）was used to probe into combining form of C and N.Secondly,two groups without microorganisms in the cathode were first set up,and one of them was put in 0.5 g algal biochar.Then,four groups of BES reactors were set up:control group,SBC group（with straw biochar）,GAC group（containing granular activated biochar）and ABC-800 group（with the algal biochar）,to compare the effect of various carbon materials.Next,another four groups of BES reactors were set up:control group,ABC-800（without activation）group,ABC-800N（activated by nitric acid）group and ABC-800K（activated by KOH）group.Meanwhile,the concentrations of nitrate,nitrite and nitrous oxide were estimated to analyze the process of denitrification.In order to find the fate of N₂O,relative enzyme activities of degrading nitric oxide（NOR）and digesting nitrous oxide（NOS）were tested.Electrochemical analysis was used to determine the effect of various biochar materials on the electrochemical performance of BES cathodes.In particular,electrochemical impedance spectroscopy（EIS）was used to gain the equivalent resistance,and cyclic voltammetry（CV）to get the peak current.Chronoamperometry（CA）and chronocoulometry（CC）were utilized to probe into reversibility and stability.Finally,high-throughput sequencing was adopted,to find species related to denitrification or electrochemistry of each BES biocathode.The main results are as follows:（1）By comparing the surface topography of those biochar materials,it is obvious to find that the two kinds of measures as KOH and nintric acid activation increased pore volumes at the same time.The BET result revealed that specific surface areas of the activated biocahr was enlarged multiplied.The tests for the contents of surface functional groups indicated that ABC-800N owned the most conjugated ketones and quinones,which related to biocompatibility and electron transfer.Moreover,XPS test revealed that plenty of graphite-N existed in ABC-800N.（2）Two BES reactors without microorganisms in the cathode were set up to demonstrate that the biochar can enhance the denitrification process.Next,four groups of BES with biocathode were set up,in which cathode was added with 0.5 g of ABC-800,straw biochar（SBC）and granular active carbon（GAC）.The result illustrated that ABC-800 group played the best in nitrate removal.（3）Another four groups of BES with biocathode were set up,which were added with 0.5g of ABC-800,ABC-800N and ABC-800K in each reactor,respectively.The nitrate removal constant of the four groups were 21.79（ABC-800N）,13.89（ABC-800K）,10.81（ABC-800）and 6.94（control）.Furthermore,accumulation of nitrite and nitrous oxide occurred in both group control and ABC-800,but not happened in remaining two groups.According to the test of enzyme activities,the NOS activity affected the fate of N₂O most.（4）Apparently,group ABC-800N was presented with the highest closed area and peak current comparing to other groups in the CV test.EIS fitted curves revealed that ABC-800N owned the smallest charge transfer resistance(R_ct),which was only 4.02Ω.The CA test indicated that group ABC-800N owned the highest electron exchange capacity（EEC）,and CC test illustrate group ABC-800N had the best reversibility and stability.（5）Through Illumina high-throughput sequencing,it was found that group ABC-800N contained more microorganisms related to nitrite reduction and electroactive species.Moreover,the microbial community of group ABC-800N vaired greatly compared to other groups.The highest microbial abundances on the group ABC-800N mainly includes electroactive and hydrogen autotrophy microorganisms,such as Thauera,JGI＿0001001＿H03,Thiobacillus,and Denitratisoma.