Study On Preparation,Efficiency And Mechanism Of Biogas Residue Based Biosorbent And Catalyst

With the promotion of biogas engineering technology in China,a large amount of biogas residue is inevitably produced.The biogas residue cannot be disposed and utilized by traditional treatment methods in time,which becomes one of the bottlenecks in sustainable development of biogas fermentation industry.In this work,the biogas residue produced by anaerobic fermentation of crop straw was used as raw material to synthesize the carboxylated biogas residue(BR-COOH)via chemical modification.The preparation process was optimized by changing the reaction condition,and the basic properties of BR-COOH were characterized by using the modern analytical instruments and methods.The adsorption behavior and mechanism of copper adsorbed onto BR-COOH was investigated by batch adsorption experiments.The adsorption experiments in copper-zinc ions mixed systems were conducted to explore the competitive adsorption behaviors,properties and mechanisms of divalent metal ion captured by BR-COOH.Subsequently,copper oxide-loaded biochar(Cu@NBC)and single Cu atoms decorated biochar(SACu@NBC)were obtained by in-situ N-doping and pyrolysis using BRCOOH adsorbed copper and copper/zinc ions,then they were applied for the activation of persulfate(PS)to decompose bisphenol A(BPA)in simulated wastewater.The effects of the content doped N and adsorbed copper ions on the activity of biosorbent-based catalyst were investigated by measuring the BPA removal in different systems,and the BPA degradation mechanism in Cu@NBC/SACu@NBC-PS system was also clarified.The way of in-situ pyrolysis after enriching heavy metal ions to the surface biogas residue-based biosorbent could achieve the recycling of heavy metal ions and high value utilization of biogas residue.The main research contents and findings are as follows:(1)The biogas residue-based biosorbent was synthesized by introducing carboxyl functional groups into the fiber structures of biogas residue.The adsorbent structure was controlled by changing the amount of alkali in the alkalization process,the amount of chloroacetic acid and the reaction temperature in the carboxylation process.The obtained target biosorbent was characterized by modern instrumental analytical methods such as scanning electron microscopy,infrared spectroscopy,and thermogravimetric analysis.The results showed that the best adsorption capacity of BR-COOH towards Cu2+was 30.49 mg/g,and the optimal preparation conditions were:3.0 g of digestate was mixed with 18.66%mass fraction of NaOH and 44.1 7%mass fraction of chloroacetic acid-isopropanol solution,and the reaction temperature was 66.22℃.After modification,a large number of negatively charged carboxyl functional groups were successfully introduced into BR-COOH,and the surface pore channels and specific surface area were significantly increased,which provided many potential adsorption sites for the heavy metal ions removal.(2)The adsorption behavior,performance and mechanism of Cu2+ adsorbed onto BRCOOH were investigated by using the above obtained biosorbent to treat Cu2+simulated wastewater.The results showed that Cu2+ could be removed effectively by BR-COOH,and maximum adsorption capacity of BR-COOH towards Cu2+ was 76.92 mg/g at 40℃.The adsorption of Cu2+onto BR-COOH was a chemical adsorption-controlled monolayer adsorption process,and it was a spontaneous heat absorption reaction.Cu2+ adsorption mainly relied on the chelation by oxygen-containing functional groups in BR-COOH.And the adsorption capacity of BR-COOH could be maintained at more than 85%after eight cycles of adsorption and desorption.(3)The competitive adsorption behavior and performance of the BR-COOH for the Cu2+and Zn2+were investigated by using the above obtained biosorbent to treat simulated wastewater containing Cu2+and Zn2+,and the mechanism of competitive adsorption of divalent metal ions by BR-COOH was explored by combination of the density functional theory calculation and correlation analysis.The results showed that both Cu2+ and Zn2+ were removed by oxygen-containing functional groups in BR-COOH through chelation,and BR-COOH preferred to adsorb Cu2+.The carboxyl functional groups in BR-COOH played an important role in the competitive adsorption,and the adsorption energy of adsorbing Cu2+ by BR-COOH was smaller than that of adsorbing Zn2+by BR-COOH.The competitive adsorption of divalent metal ions by BR-COOH was mainly influenced by the electronegativity of the adsorbed metal ions,and the stronger the electronegativity of the metal ions,the easier they were adsorbed by the carboxylated biosorbent.(4)Cu@NBC were obtained by in-situ N-doping and pyrolysis process using the abovementioned BR-COOH adsorbed with the Cu2+as raw material.The activation performance of Cu@NBC,the main active sites in Cu@NBC,and the mechanism of peroxydisulfate(PDS)activaion were investigated in the BPA simulated wastewater treatment process.The results showed that the Cu2+in the BR-COOH was gradually transformed into the oxidized state of copper during the pyrolysis process,and Cu2O was the main active site in Cu@NBC;the immobilization of Cu in the Cu@NBC and morphological transformation to Cu2O could be promoted by N doping,which could improve the activity of catalyst.The activation of PDS by Cu@NBC was achieved by forming Cu@NBC-PDS*complex with high oxidation potential,and the Cu@NBC-PDS*complex could extract electrons directly from BPA during the oxidation of BPA.In the simulated polycarbonate wastewater treatment process,the Cu@NBC/PDS system could resist the interference of inorganic salt ions and has good cyclic stability.Meanwhile,the Cu@NBC/PDS system could effectively remove tyrosine and tyrosine-like proteins from the simulated wastewater,and the humic acid would be decomposed into short-chain fulvic acid-like substances.(5)SACu@NBC were obtained by in-situ N-doping and pyrolysis process using the above-mentioned BR-COOH adsorbed with the Cu2+ and Zn2+ as raw material.And we investigated the activation performance of SACu@NBC,the main active sites in SACu@NBC,the mechanism of peroxymonosulfate(PMS)activation,and the feasibility of coupling SACu@NBC into ultrafiltration purification process were also explored.The results showed that during the high temperature pyrolysis(T>900℃),the adsorbed Zn2+would volatilize from the carbon layer,while the adsorbed copper ions transformed into monatomic Cu and coordinated with N elements to form Cu-N4 structure in the carbon layer.The Cu atom sites were the main active sites in SACu@NBC,and the oxidation capacity of the SACu@NBC/PMS system could be improved by increasing the content of Cu atoms.The degradation of BPA in SACu@NBC/PMS system was via electron transfer process.BPA and PMS were adsorbed onto the surface of SACu@NBC firstly,and PMS was activated to form the SACu@NBC-PMS*complex.Subsequently,the SACu@NBC-PMS*complex with higher potential seized the electrons of BPA to achieve the oxidation of BPA,while PMS was converted to SO42-.The hydrophilicity and flux of the polysulfone(PSF)membrane were significantly improved after loading SACu@NBC,and the SACu@NBC/PSF membrane could effectively remove higher concentrations of BPA under high water flux conditions.