Green Synthesis Of LDH/BC And Its Resource Utilization For Nitrogen And Phosphorus Recovery From Biogas Slurry

The issues of biogas slurry treatment and the misuse of chemical fertilizers have become increasingly severe in China due to the growth of biogas engineering and agriculture.Biochar presents a promising solution to these challenges by adsorbing and recovering N and P from biogas slurry.Additionally,the saturated biochar can serve as a substitute for chemical fertilizers and a soil amendment in agriculture.This approach reduces the reliance on chemical fertilizers,improves soil fertility,and alleviates the pressure of sewage N and P treatment.Consequently,it results in a win-win scenario of sewage treatment and agricultural crop yield enhancement,promoting sustainable development.The present study focuses on optimizing the synthesis method of the layered double hydroxide-biochar composite（LDH/BC）for efficient N and P adsorption and recovery from biogas slurry.The adsorbed LDH/BC,which is rich in N and P,can be applied to the soil to reintroduce these nutrients into the agricultural ecosystem for utilization.The present study reports the following findings:1.A green synthesis method has been developed for the preparation of LDH/BC composites.The method involves the direct anchoring of MgFe-LDH on corn stover via co-precipitation with Fe Cl₃ and Mg（OH）₂,followed by pyrolysis.The use of Mg（OH）₂ maintains the p H of the mixture at around 10,eliminating the need for cumbersome p H control steps of the traditional co-precipitation method.Additionally,the rate of solid-liquid reaction is low,resulting in high LDH crystallinity and purity.These improvements have enhanced the properties of the MgFe-LDH/biochar composite after pyrolysis,including increased anchoring of MgFe-LDH,larger specific surface area,more stable layered structure,and customizable LDH.An adsorption isotherm study revealed that the theoretical maximum adsorption capacity of phosphorus for the composite was 379.4 mg/L,and the dominant adsorption mechanism was chemical adsorption accompanied by physical adsorption.2.MgFe-LDH/BC composites were synthesized using three different methods,including the green synthesis method,traditional pre-pyrolysis,and post-pyrolysis method.The results showed that the physicochemical properties,surface morphology,and properties of the MgFe-LDH/BC composites were significantly influenced by the different synthesis methods employed.The N@MgFe-LDH/BC composite synthesized using the green synthesis method exhibited the largest specific surface area,highest Mg and Fe contents,and excellent magnetic response properties.Moreover,N@MgFe-LDH/BC showed the best adsorption and recovery performance for AN and P in biogas slurry,with removal efficiencies of 30%and 81.8%,respectively.The main recycling mechanisms involved were memory effects,ion exchange,and co-precipitation.3.The dynamic adsorption performance of N@MgFe-LDH/BC,prepared from biomass with different particle sizes,was studied by simulating a fixed bed column system.The results indicate that,although the utilization rate and maximum adsorption capacity of N@MgFe-LDH/BC modified by crushed biomass are higher than those modified by non-crushed large-particle biomass,its surface properties（pore structure）and adsorption efficiency are not as good as the latter.In the dynamic adsorption experiment,N@MgFe-LDH/BC modified by large particle biomass demonstrated excellent performance in the fixed bed column with high flow rate.Compared to low flow rate,the adsorption capacity of ammonia nitrogen decreased by 5.9%,while the adsorption capacity of phosphorus increased by 24.8%.The scale-up design showed that the fixed bed column（N@MgFe-LDH/BC）treated by this adsorption process can process 108 t of biogas slurry per day,and remove about 35.3%of AN and 24.4%of P,taking into account both performance and efficiency.4.The substitution of chemical fertilizers with 2%saturated N@MgFe-LDH/BC has significantly improved soil properties and fertility,increased crop nutrient composition,and resulted in a 139.3%increase in crop yield.This eco-agricultural system,which centers on N@MgFe-LDH/BC preparation and biogas slurry resource utilization,offers substantial environmental benefits by enabling the internal carbon cycle of the system and reducing the generation of CO₂ and PM2.5 from straw burning.Additionally,the efficient recovery of nitrogen and phosphorus from biogas slurry by the N@MgFe-LDH/BC adsorption process can reduce the nitrogen and phosphorus load in sewage treatment plants.The application of saturated N@MgFe-LDH/BC as a fertilizer and soil amendment reduces the use of chemical fertilizers and promotes crop production,further demonstrating the potential of this approach as a sustainable alternative to traditional agricultural practices.To conclude,the green synthesis method of LDH/BC is highly versatile and has great potential for practical applications.It expands the current research scope for the synthesis of ideal forms of LDH and its composites.Saturated N@MgFe-LDH/BC can be used as a soil amendment and a substitute for traditional chemical fertilizers,promoting the bio-earth cycle of carbon,nitrogen,and phosphorus.The agricultural resource circulation system,with N@MgFe-LDH/BC at its core,is of significant importance for the development of biogas engineering,reduction of agricultural chemical fertilizer,and resource recovery and utilization in China.This system has broad application prospects and represents a sustainable solution to the challenges posed by the current agricultural practices.