Preparation Of Modified Hard Biochar Composites And Adsorption On Nitrate And Phosphate In Water

Nitrate and phosphate are the non-renewable and irreplaceable natural resource,the important raw material for industrial and agricultural production,and the necessary nutrient element for crop growth.When industrial wastewater and domestic sewage containing phosphate(PO₄^3-)and nitrate（NO₃^-）are discharged directly into surface water,it leads to water eutrophication and threatens the ecological environment.Therefore,effective methods are required to remove nitrate and phosphate from contaminated wastewater prior to its discharge,among which adsorption method is one of the preferred technologies.Biochar has also become a hot adsorption material in current research and development due to its carbonaceous porous structure and rich natural endowment of oxygen-containing groups on the surface.However,the biochar obtained by simple pyrolysis of natural biomass often does not exert its natural endowment,and needs modification to improve its structural properties,so as to improve the adsorption properties of pollutants.We found that biochar derived from hardwood and hard fruit shells（hereinafter referred to as hard biochar）showed poor texture properties and low adsorption capacity for pollutants.However,hard biochar usually appears granular（column or sheet）after pyrolysis,facilitating the separation after its use and recovery,and the ability to maintain head pressure in continuous sewage treatment applications.Therefore,it is necessary to further study the methods to enhance the adsorption performance of hard biocarbon and promote its application in the field of environmental pollution control.Therefore,the main work of this paper is almond shell biochar（a kind of hard biochar）as the research object,through different modification means to improve the adsorption capacity of phosphate and nitrate,for the high-value utilization of natural organic biomass and effective removal of phosphate and nitrate pollutants in wastewater to provide important reference.The specific research contents are as follows:（1）Mg/Al-Layered Double Hydroxide Modified Biochar for Simultaneous Removal Phosphate and Nitrate from Aqueous SolutionIn this study,three layered double layer metal hydroxide modified almond shell biochar（ASB-x）with different Mg/Al atomic ratios（x=2,3,4）were prepared by two-step wet impregnation method.The structural characteristics of the adsorbents were characterized by XRD,SEM/EDS,FTIR,elemental composition analysis,hydrophobicity analysis and N₂ adsorption/desorption.Through adsorption experiments,the removal performance of NO₃^-and PO₄^3-was evaluated,and a material with good adsorption performance was selected for detailed study.The results showed that the two Mg/Al hydroxides were successfully introduced into ASB surface with low crystallinity structure and enhanced hydrophilicity and polarity,and the appropriate dosage of adsorbent was 2.5～15 g·L^-1.When x=3（ASB-3）,the best adsorption rates for phosphate and nitrate are 47.5%and 27.7%,respectively.The specific surface area(4.51 m²·g^-1)and total pore volume(0.023cm³·g^-1)of ASB-3were larger than that of ASB(2.18 m²·g^-1,0.012 cm³·g^-1).Adsorption kinetics and isotherm studies also showed that the adsorption rate,affinity and capacity of ASB-3for phosphate and nitrate were increased.ASB-3 can provide a potential carbon adsorbent replacement,in order to improve the removal efficiency of phosphate and nitrate ions in water.（2）Egg white-mediated fabrication and phosphate adsorption of Mg/Al-LD H-hard biochar compositeIn this study,a method of adhesion in composite suspension was proposed to prepare Mg/Al layered double hydroxides（LDH）supported ASB composites（ASBC）.The materials were characterized by XRD,SEM/EDS,FTIR,XPS,elemental composition analysis,hydrophobicity analysis,p H_ZPC analysis and N₂adsorption/desorption.The adsorption performance of phosphate was evaluated by adsorption experiment.The adsorption behavior of phosphate was studied by kinetic adsorption model and isothermal adsorption model.The results showed that the adhesion between ASB and LDH particles was achieved by using egg white as adhesive while maintaining the crystallinity of LDH as raw material.The modified ASBC has enhanced hydrophilicity and increased polar groups on its surface,adsorbing phosphate in water through ion exchange mechanism.Compared with ASB（ASB=7.0%）,ASBC phosphate removal rate is significantly improved(η_ASBC=96.4%),in line with the quasi-secondary kinetic model,the maximum adsorption amount is 133.13 mg·g^-1,adsorption thermodynamics in line with the Langmuir model,indicating that the phosphate adsorption rate of ASBC material is mainly controlled by the chemical adsorption mechanism,adsorption process may be monolayer adsorption.（3）Comparison of nitrate and phosphate adsorption of typical mineral loaded and CO₂-activated biochar in waterA new type of inorganic mineral/hard charcoal composite adsorbent（ASB-S,ASB-Si,ASB-Al,ASB-Mg,ASB-Ca,ASB-Fe）was prepared by the co-modification of biochar with typical mineral loading and CO₂ activation.The physical and chemical properties of the adsorbent were characterized by XRD,SEM/EDS,FTIR,XPS,elemental composition analysis,hydrophobicity analysis and N₂adsorption/desorption.The adsorption performance was evaluated by the removal effect of phosphate and nitrate.The adsorption behavior was studied by adsorption kinetics and isothermal adsorption model.The results showed that the surface and pore structure of biochar were reconstructed under the combined activation of mineral loading and CO₂ atmosphere,and rich pore structure and large specific surface area were formed.ASB-Fe has the best adsorption performance(η（NO₃^-）=40.29%;η(PO₄^3-)=29.37%),and the ion exchange action occurs in the adsorption process.The kinetic results show that for ASB-S,ASB-S and ASB-Si fit the quasi-primary kinetic model,ASB-Al,ASB-Mg,ASB-Ca,ASB-Fe,indicating that the nitrate adsorption is controlled by the chemoradsorption mechanism.For phosphate,ASB-S,ASB-Si,ASB-Al,ASB-Mg,ASB-Ca,and ASB-Fe similarly fit into quasi-secondary kinetic models,indicating that the adsorption of phosphate is controlled by chemical action.The maximum adsorption amount of ASB-Fe to phosphate(q_m=6.73 mg·g^-1)and nitrate(q_m=8.33 mg·g^-1)was higher than the others.The adsorption of two inorganic ions,ASB-S,ASB-Si,ASB-Al,ASB-Mg,ASB-Ca and ASB-Fe,all agreed to the Langmuir model as monolayer.