Preparation Of Biochar@Hydrogel And Its Effect On The Promotion Of Anaerobic Digestion

In recent years,the output of wastewater and landfill leachate in my country has increased year by year.They contain a large amount of heavy metals,such as Ni²⁺and Cd²⁺,which have a certain impact on the anaerobic digestion process.Studies show that hydrogels can adsorb heavy metals in wastewater very well,but their anaerobic digestion effects are seldom studied.In addition,biochar is so available and economical as to considered to be a potential promoter of anaerobic digestion.Therefore,biochar@hydrogel（PCH）was developed to adsorb the heavy metals in the system and improve the anaerobic digestion performance in this paper.Firstly,the effect of PCH concentration on anaerobic production of methane in the presence of ethanol as an anaerobic digestion substrate in a system containing heavy metals Ni²⁺and Cd²⁺was explored.The results show that PCH can not only effectively alleviate the inhibition of heavy metals on the system,but also promote methane production;2 g/L biochar@hydrogel（PCH2）promotes the methane production best,and provides 52.87%higher acetic acid production,79.65%higher cumulative methane production（415±27 m L/g VS）,and 69.23%higher the maximum daily methane production than the control group（CM group）and the average methane content as high as 67.51%.The surface of PCH is rough,porous,and hydrophilic with the contact angle of 50^°.In addition,the functional group such as-OH in PCH can improve the buffering capacity of the system.PCH2 added to the system increases the conductivity by 36.54%,the contents of Firmicutes and Bacteroidetes by 10.16%and 18.20%,respectively,and the content of Methanosaeta by 20.35%,indicating that PCH can improve the community richness and diversity of bacteria and archaea.Secondly,the reusability of 2 g/L biochar@hydrogel under the condition that Ni²⁺and Cd²⁺inhibit the anaerobic system was explored in the presence of ethanol as a substrate.The results show that the Cd²⁺and Ni²⁺adsorption process of PCH2 conforms to the second-order kinetic model(Cd²⁺and Ni²⁺have R² of 0.996 and 0.995,respectively),and PCH2 can be effectively adsorbed heavy metals and promoted methane production four times.The SEM image shows that after four use cycles of PCH2,the specific surface area decreases by 45.40%,the Ni²⁺and Cd²⁺adsorption rates decrease to97.32%and 95.78%,respectively,the cumulative methane production promotion effect remains 35.63%,the maximum daily methane production promotion effect remains48.64%,and the average methane content remains 69.76%since acid leaching and cleaning operations lead to the collapse of the pore structure of PCH2.Finally,2 g/L biochar@hydrogel was used in the heavy metals-containing leachate and excess activated sludge system to study its effect on gas production and its reusability.The results show that PCH2 added to the leachate and excess activated sludge system increases the COD degradation rate to 75.41%and the VS removal to 63.67%and provides 66.27%higher acetic acid production and higher propionic acid degradation rate than the control group.Modified Gompertz model show that in its four use cycles,PCH2has 37.74%lower averageλvalue than the control group,and the R² between 0.977 and0.998,indicating that it promotes the gas production of anaerobic system very well and provides 52.38%,52.78%,38.36%,and 29.58%higher cumulative methane production and 40.60%,87.00%,44.25%,and 34.19%higher daily maximum methane production than the RN,RN-2,RN-3 and RN-4 group,and the average methane content of64.69%,64.02%,67.89%,and 69.91%,respectively.During the fourth cycle,the Ni²⁺and Cd²⁺adsorption and desorption rates decrease to 96.24%,96.78%,87.99%and 92.30%,respectively.The above results show that PCH2 not only can promote anaerobic production of methane by effectively adsorbing heavy metals,but also will play a role in the field of heavy metals-containing wastewater treatment as a green and efficient new material.