Selenium Modified Nickel Foam Cathode Of Microbial Electrolysis Cell Enhances Hydrogen Production From Waste Activated Sludge And Its Mechanism Research

Municipal sewage treatment plants using biological treatment technology will produce a large number of waste activated sludge,which not only contains many organic substances,but also contains nitrogen,phosphorus and heavy metal pollutants,causing harm to human beings and the environment.How to effectively release carbon sources from excess sludge and convert them int o resources is the development direction of sludge treatment technology.In order to solve the problem that excess sludge as a substrate in microbial electrolysis cell?MEC?system is difficult to produce hydrogen continuously,potassium ferrate combined with ultrasound?K2Fe O4/ULT?pretreatment method was developed to hydrolyze excess sludge effectively to provide sufficient carbon source for MEC system.Se@NF cathode material with three-dimensional structure was developed to inhibit the Methane production process in MEC system.The electrochemical,material and microbiological methods were used to develope the characteristics of Se@NF cathode and its effect on sludge treatment and hydrogen production in MEC system.The mechanism of methane production inhibition by biofilm formed on Se@NF cathode was clarified,and the control strategy of continuous hydrogen production in treating residual sludge by MEC system was established.The K₂FeO₄/ULT pretreatment process for hydrolysis of excess activated sludge was studied.Several commonly used pretreatment methods were compared,including alkali treatment,potassium ferrate treatment,alkali-ultrasonic combined treatment and potassium ferrate-alkali combined treatment.The results showed that K₂FeO₄/ULT pretreatment had the best sludge hydrolysis performance.Soluble chemical oxygen demand?SCOD?and volatile fatty acids?VFAs?of sludge pretreated with K₂FeO₄/ULT reached 10206 mg/L and 9160 mg/L respectively,which were more than 4 times higher than that of raw slu dge.After 90 minutes of reaction,the degree of sludge degradation?DD?reached 40%,and the degradation efficiency was more than 9 times that of the raw sludge.More than 90% of the sludge particle size decreased from 116.4 ?m to 61.2 ?m,and the removal rate of volatile suspended solid?VSS?reached 18%,more than three times that of the raw sludge.At the same time,the mechanism of K₂FeO₄/ULT pretreatment on sludge hydrolysis was revealed.Fe?VI?in potassium ferrate was reduced to Fe?II?and Fe?III?after oxidizing organic matter,which could catalyze the activities of a large number of hydroxyl radical formed in ultrasound,forming a synergistic effect to hydrolyze sludge more effectively.The sludge pretreated with K₂FeO₄/ULT produced 728.2 m L?7.3 g sludge?methane gas,which was 1.5 times as much as the raw sludge.The molecular ecology analysis represented by Hiseq sequencing and Weighted Uni Frac clustering showed that the communities in sludge pretreated with K₂FeO₄/ULT were suitable for anaerobic fermentation to produce biogas.The dominant archaea were Methanomassiliicoccus and Methanobacterium.Selenium modified nickel foam cathode material with three-dimensional structure was developed,and MEC reactor was operated at 0.6 V external voltage.The effects of annealing and bonding methods for preparing the new cathode on the reactor efficiency were compared.The results show that annealing method has more advantages in hydrogen production.The maximum hydrogen productio n rate can reach 0.341±0.017 L/L·d,which is 2.6 times higher than that of rolled activated carbon cathode MEC reactor.The highest hydrogen content can reach 81.8%,and methane can hardly be detected in the whole operation process?< 0.2%?.At the same time,the highest COD removal rate of MEC reactor with Se@NF cathode can reach 76.8%.The electrochemical analysis showed that the current of MEC with Se@NF cathode was 5.8-9.2 times higher than that of MEC with rolled activated carbon cathode and foamed nickel cathode material.It was verified that the current output of MEC could be effectively increased by Fe?III?which was reduced from potassium ferrate.X-ray diffraction results showed that selenium was highly crystallized in cathode materials.Clear Brag g reflection is related to hexagonal lattice?triangular crystal system?.It is proved that only short-range ordered selenium existed in nickel foam,but no long-term ordered selenium is observed.According to the microbial community analysis,the dominant bacteria on anode biofilm of MEC with Se@NF cathode were Geobacter,followed by Petrimonas and Anaerovirgula,which had the ability to produce acid by fermentation.Methanobacterium with relatively high abundance was found in both rolled activated carbon cathode and foamed nickel cathode biofilm,but no Methanobacterium were detected in Se@NF cathode biofilm,which confirmed that Se@NF cathode had the ability to inhibit methanogenesis.The hydrogen produced by MEC reactor with Se@NF cathode was not utilize d by methanogenic archaea,thus obtaining a higher hydrogen yield.The effects of hydrogen and methane production by Se@NF cathode and traditional platinum carbon cathode?Pt@CC?under 0.8 V applied voltage in MEC system were compared.The maximum hydrogen production rate of MEC with Se@NF cathode is 0.522±0.057 L/L·d.The average hydrogen production is more than 0.4 L/L·d.It is much higher than that of PT@CC cathode MEC which produces a lot of methane in the experimental process.Meanwhile,the content of hydrogen is 80.67%?89.79%,while that of methane is only 0.11%?0.94%.Se@NF cathode has higher hydrogen production ra te and hydrogen content than MEC using excess activated sludge as substrate up to now.The maximum current density of MEC on Se@NF cathode is 152.4±32.6 A/m3.X-ray photoelectron spectroscopy?XPS?analysis showed that in the Ni2 p and Se3 p regions,the cathode surface mainly contained high oxidation state of Ni2+ and Se6+ ions.Illumina Hiseq sequencing showed that Methanobacterium was dominant in the Pt@CC cathode group,while Methanobacterium could hardly be detected in the Se@NF cathode group.The results of macrogenomic analysis showed that all the methanogenic modules?methanol,acetate,carbon dioxide and methylamine/dimethylamine/trimethylamine?in the Se@NF cathode group lacked the necessary enzymatic genes?mtd,hmd,rud F,mtr H,mfn A,mfn D and mta C?,which interrupted the pathway of methanogenic metabolism and thus inhibited methane production.In addition,the hydrogen production efficiency of the reactor after inoculating methanogens with Se@NF cathode was also investigated.The results showed that the Se@NF cathode could eliminate the inoculated methanogens in a short time without affecting the hydrogen production rate.Se@NF cathode has simple fabrication process and low material cost,which is suitable for extended application.This study provide d sufficient carbon source through K₂FeO₄/ULT pretreatment of hydrolyzed sludge,promoted MEC current output,and inhibited methanogenesis process by Se@NF cathode MEC,avoided hydrogen being utilized by methanogenesis process,enhanced hydrogen production efficiency of excess sludge,and provided theoretical basis for treating excess sludge in MEC system in the future and sustained hydrogen production at the same time.