Study On The Mechanism And Technology Of Phosphorus Recovery From Biogas Slurry By Biochar Of Straw Enhanced Magnesium Lanthanum Hydrotalcite(LDH)

Phosphorus is a non-renewable resource,and global reserves may be depleted within 50-100 years.At the same time,biogas slurry is a recyclable agricultural waste resource with an abundant amount of phosphorus.Accordingly,recovering phosphorus from biogas slurries could be a promising strategy for nutrient management and utilization,in addition to controlling water eutrophication and reducing the risk of phosphorus resource shortages.Therefore,it is necessary to develop new technologies for recovering phosphorus from biogas slurry,which is crucial for the sustainable development of green agriculture.Although many phosphorus recovery technologies are available,their application in biogas slurry treatment is not yet mature,and their practical value is limited.Adsorption technology is a promising technology for nutrient recovery due to its simplicity,low cost,high selectivity,small sludge production,and environmental friendliness.Compared to membrane concentration and crystallization technologies,inexpensive and eco-friendly adsorption materials have gained more interest.Currently,hydrotalcite（LDH）and straw-derived biochar are often used as cheap and environmentally friendly adsorption materials for treating phosphorus-containing wastewater.Therefore,researching the mechanism and process optimization of straw-derived biochar coupled with enhanced magnesium lanthanum hydrotalcite（LDH）for phosphorus recovery from biogas slurry holds great theoretical and practical significance.The main objective of this study is to recover phosphorus from biogas slurry using adsorption technology.A novel method for preparing straw-derived biochar composite hydrotalcite materials was proposed to effectively recover phosphorus nutrients from biogas slurry.The phosphorus recovery mechanism was analyzed,and the factors affecting phosphorus recovery based on adsorption materials and existing processes were also studied.Moreover,effective methods to recover phosphorus and the use of acidification technology to strengthen the phosphorus recovery process were explored.Furthermore,the study revealed the potential application of straw-derived biochar-reinforced magnesium lanthanum oxide in the phosphorus recovery process from biogas slurry.The main research contents and results are summarized as follows:（1）The first part of the study focused on the synthesis method and phosphorus adsorption characteristics of magnesium lanthanum hydrotalcite materials.Three types of composite materials,including lanthanum magnesium hydrotalcite（Mg/La-LDH）,magnesium aluminum hydrotalcite（Mg/Al-LDH）,and magnesium ferric hydrotalcite（Mg/Fe-LDH）,were synthesized by the intercalation co-precipitation method for phosphorus recovery.Based on the adsorption effect of the composite materials on phosphorus,the hydrotalcite materials with the best adsorption effect were initially screened out.The synthesis ratio of primary materials was optimized,and the phosphorus adsorption characteristics of hydrotalcite materials with the best ratio were analyzed.The results showed that Mg/La-LDH had the best adsorption effect on phosphorus,and the adsorption rate of phosphorus was 96.60%.Among the synthesized materials with different La concentration ratios,La_0.1 has the best adsorption effect,with a phosphorus adsorption capacity of 378.80 mg/g and an adsorption rate of 99.15%.The main mechanism of phosphorus adsorption by La_0.1 is the formation of a stable complex（La PO₄）between phosphate and carbonate in the hydrotalcite layer under the action of ion exchange,in addition to the formation of Mg₃（PO₄）₂ complex through the complexation reaction.The unique surface cluster structure of La_0.1 hydrotalcite also promoted phosphate adsorption.Therefore,La_0.1 was considered the most appropriate adsorption material for phosphorus.（2）The second part of the study aimed to investigate the mechanism by which straw-derived biochar strengthens magnesium lanthanum oxide(Mg/La_0.1-LDH)for phosphorus recovery in biogas slurry.The hydrotalcite material(Mg/La_0.1-LDH)synthesized under the control of magnesium lanthanum metal ions had abundant interlayer ions and nanoparticles,and they exhibited excellent performance in phosphorus adsorption.However,the dispersion of recovery material and the high cost of raw material has reduced its application value.To address this issue,crop straw（rice,corn）was fortified with Mg/La_0.1-LDH,and the fortified straw was used in the preparation of straw-derived biochar magnesium lanthanum bimetallic oxide nanocomposites（BC-LDO）through co-pyrolysis at different temperatures（400,500,and 600°C）.The recovery ability and mechanism of BC-LDO,prepared under different conditions,were investigated.The results showed that high-temperature pyrolysis（600°C）was the most effective in fixing nanoparticles onto straw-derived biochar.Straw-derived biochar-reinforced magnesium lanthanum oxide composite（6YBC-LDO）contained abundant mesoporous and microporous materials,which were more likely to bond with phosphate to form La PO₄ precipitation and ligand exchange to form Mg₃（PO₄）₂ inner sphere complex.6YBC-LDO has highly selective adsorption of phosphate and can be reused many times.It can recover 90%of phosphorus in biogas slurry within 30 min,with a saturated adsorption capacity of 366.39 mg/g.By further optimizing the ratio of straw-derived biochar magnesium lanthanum oxide composite material,the cost of the raw material used in magnesium lanthanum hydrotalcite preparation was reduced by about 50%.The 6YBC-LDO composite material was applied in the phosphorus recovery treatment of biogas slurry;adding 2 g/L of 6YBC-LDO was able to adsorb more than 90%of phosphate.（3）Optimization of the phosphorus recovery process from biogas slurry by straw-derived biochar-reinforced magnesium lanthanum hydrotalcite was the main aim of the third part.By designing a packed adsorption column and a phosphorus recovery system device,the dynamic continuous recovery of phosphorus from biogas slurry was simulated,and the developed straw-derived biochar magnesium lanthanum oxide（6YBC-LDO）composite material was used as the adsorption filler.The influencing factors of phosphorus concentration,biogas slurry flow velocity,and packing height were studied using the single-factor test and Response Surface Methodology（RSM）.The optimized process conditions were as follows:The height of the 6YBC-LDO column was 1.60 cm,the concentration of phosphorus was 106.24 mg/L,and the flow rate of biogas slurry was 2.44 m L/min.Under these conditions,the predicted phosphorus adsorption capacity was 287.58 mg/g,and the measured value was 303.45 mg/g.The RSM model was found to be highly consistent with the experimental results.Furthermore,correlation analysis of influencing factors showed that high chemical oxygen demand（COD）and total suspended solids（TSS）are the key factors affecting the flow velocity and phosphorus concentration of biogas slurry.Moreover,the contents of COD and TSS directly or indirectly affect the phosphorus recovery process.Therefore,it was necessary to strengthen the phosphorus adsorption process further to improve the phosphorus recovery in biogas slurry.（4）The fourth part addresses a study on the effect of straw-derived biochar magnesium lanthanum hydrotalcite on phosphorus recovery from acidified biogas slurry.Acidification pretreatment of biogas slurry using H₂SO₄ was employed to reduce the concentration of TSS and COD and enhance the phosphorus recovery process.H₂SO₄ was used to dissolve solid-phase phosphorus in biogas slurry,transforming it into liquid-phase phosphorus,promoting the transformation of phosphorus form and increasing the phosphorus content of 6YBC-LDO.The study explores the impact of different acidification conditions on the transformation of phosphorus form,the physical and chemical properties of biogas slurry,and the recovery effect of 6YBC-LDO on phosphorus recovery of acidified biogas slurry.The primary mechanism of phosphorus recovery of acidified biogas slurry was also examined,and the correlation between the phosphorus recovery and the characteristics of acidified biogas slurry was analyzed.Finally,the study clarified the factors that influenced the acidification of biogas slurry on phosphorus recovery.The results showed that soluble orthophosphate（Ortho-P）was the main form of phosphorus in the recovery of biogas slurry using6YBC-LDO adsorption materials.When H₂SO₄ was added to adjust the p H of the biogas slurry to 6.0,6.5,and 7.0,the mass concentration of Ortho-P increased by 20,9,and 4 times compared to the original biogas slurry,respectively.The phosphorus recovery capacities of 6YBC-LDO for p H 6.0,p H 6.5,and p H 7.0 were 150.99 mg/g,144.73 mg/g,and 40.32 mg/g,and the phosphorus recovery rates were 64%,85%,and99%,respectively.The results have also shown that the p H value of biogas slurry acidification is the main factor affecting the phosphorus recovery efficiency,and p H 7.0acidified biogas slurry treatment has a significant improvement effect on the phosphorus recovery process.Meanwhile,the phosphorus recovery products of straw-derived biochar magnesium lanthanum hydrotalcite for p H 7.0 acidified biogas slurry are mainly phosphate complexes,with a higher purity of the recovered substances,which has a significant market application prospect.By independently researching and developing straw-derived biochar composite magnesium lanthanum hydrotalcite（LDH）adsorption material recovery,this topic aimed to address the issue of biogas slurry phosphorus resource loss.The approach involved strengthening the phosphorus recovery technology through acidizing biogas slurry,which effectively optimized the phosphorus recovery.This solution fills the gap in current biogas slurry treatment technology and broadens the idea of high-value utilization for biogas slurry resources.The main innovations were as follows:Innovation point 1:A novel material preparation method of straw-derived biochar composite hydrotalcite for effective recovery of phosphorus nutrients in biogas slurry was proposed creatively.The surface structure and element composition changes were investigated before and after the composite adsorption.The mechanism of phosphorus recovery of magnesium lanthanum hydrotalcite enhanced by straw-derived biochar was explained,and it was found that it had higher adsorption capacity and adsorption selectivity for phosphate.Innovation point 2:Using the developed composite material as the adsorption filler,the important influence of the key factors on the phosphorus recovery process effect was clarified,the process parameters of phosphorus recovery were obtained,and an effective way to optimize the process was proposed.Innovation point 3:A technical method for enhancing phosphorus recovery from acidified biogas slurry was proposed,and the synergistic mechanism between the acidification and the phosphorus recovery processes was revealed.The optimal process parameters for this method were obtained.