Phosphorus Adsorption Effect And Influencing Factors Of Bamboo-Based Biochar-Loaded Nano-Zero-Valent Iron

Phosphorus（P）is the key factor driving eutrophication of water,and how to efficiently and safely reduce P content in water has become an important research direction for current researchers.Biochar（BC）has the characteristics of large specific surface area,porous structure,rich surface functional groups,stable structure and low cost,and is widely used in environmental pollution remediation.However,the surface of biochar is usually negatively charged,and it often shows poor adsorption effect on anionic pollution in the environment（such as phosphate pollution）.As an important biochar modification method,after the surface of biochar loading with metal nano（such as nano-zero valent iron,nZVI）,the surface characteristics of biochar（such as surface area and number and type of functional groups）are changed,and the aggregation process of metal nanomaterials in the environment is also inhibited,and the adsorption performance of metal nanomaterials is improved.At present,the study of biochar prepared from bamboo as raw material is relatively rare compared with other plant-based biochar research reports.In fact,China is the world’s largest country in bamboo resources,of which Phyllostachs edulis accounts for90%of China’s bamboo resources,which provides rich raw materials for biochar preparation.Based on this,biochar was prepared using Phyllostachs edulis as raw material,and using it as a carrier,three kinds of biochar（nZVI-BC）loaded with nano-zero-valent iron（nZVI-BC）with iron/biochar（Fe/BC）mass ratios of 1:1,1:3 and 1:10 were prepared by liquid phase reduction,named 1:1,1:3,1:10-nZVI-BC,respectively,and their adsorption characteristics on P were analyzed,and the scanning electron microscopy-energy spectroscopy（SEM-EDS）,Fourier infrared spectroscopy（FTIR）,and P were analyzed.X-ray photoelectron spectroscopy（XPS）,X-ray diffraction（XRD）and other characterization methods explored its adsorption mechanism on P.In addition,the effects of typical environmental factors,such as solution p H,dissolved organic carbon（DOC）,and coexisting ions on the P adsorption performance of nZVI-BC were explored,and the interaction between typical environmental factors on the adsorption performance of nZVI-BC was analyzed by constructing a response surface model.The main results are as follows:（1）Compared with BC,the hydrophilicity and polarity of the materials after the surface of biochar were loaded with nZVI,the aromatity was weakened,the specific surface area decreased,and the average pore size increased.The hydrophilicity,polarity,and p H_PZC（zero potential corresponding to p H）of nZVI-BC were positively correlated with the Fe/BC mass ratio,while its aromaticity was negatively correlated with the Fe/BC mass ratio.The results of SEM-EDS showed that the surface of BC was smooth and flat and had some pores,while some nanoscale chain spherical and needle-like structures were distributed on the surface and pores of nZVI-BC,and the content of Fe and O elements on the surface of nZVI-BC was significantly increased compared with BC itself.FTIR results showed that COOH,C-O-C,C=C,Fe-O functional groups or bonds on the surface of nZVI-BC increased comparing to BC.XPS and XRD results showed that Fe elements on the nZVI-BC mainly existed in the form of Fe,Fe₂O₃ and Fe₃O₄ on the surface of nZVI-BC.（2）The Fe/BC mass ratio significantly affected the adsorption characteristics of P of nZVI-BC.The results of adsorption kinetic experiments showed that the adsorption process of P of 1:3 and 1:10-nZVI-BC was more in line with the pseudo-secondary-order model,while the adsorption process of P of 1:1-nZVI-BC was more in line with the pseudo-first-order model model.The results of isothermal adsorption experiments showed that the isothermal adsorption curves of 1:1,1:3 and 1:10-nZVI-BC were more in line with the Langmuir model,which predicted that the P saturated adsorption capacity of 1:1-nZVI-BC could reach 272.5 mg/g,which was higher than the P saturated adsorption capacity of1:3 and 1:10-nZVI-BC（88.4 and 9.23 mg/g,respectively）.Through the analysis of various characterization methods,it is found that the adsorption mechanism of P of nZVI-BC may mainly include:a)adsorption of P in aqueous environment by electrostatic action;b)The surface C=C,COOH,C-O-C functional groups Complexed with P;c)Fe⁰ occurred oxidation to form Fe²⁺,Fe³⁺,reacting with H₂PO₄^-and PO₄^3-to form precipitation,which were Fe PO₄·2H₂O,Fe₃（PO₄）₂·8H₂O,Fe（H₂PO₄）₃;d)Fe ions are hydrolyzed to generate Fe₄（PO₄）₃（OH）₃ through co-precipitation.（3）As the p H of the solution increasing from 3 to 10,the removal rate and adsorption capacity of 1:1-nZVI-BC decrease first and then increase,while the removal rate and adsorption capacity of 1:3 and 1:10-nZVI-BC decrease first and then remain unchanged.In this study,the adsorption capacity of P of nZVI-BC is proportional to the ion concentration in solution（taking Na Cl as an example）.In addition,the effects of different coexistence ion species（NO₃^-,Cl^-,CH₃COO^-,HCO₃^-,K⁺）on the P adsorption performance of nZVI-BC were monitored,and it was found that NO₃^-significantly promoted the adsorption capacity of P of nZVI-BC.CH₃COO^-and HCO₃^-significantly inhibited the adsorption capacity of P of nZVI-BC,and inhibition was CH₃COO^-greater than HCO₃^-.K⁺and Cl^-did not significantly affect the adsorption capacity of P of nZVI-BC.While p H was at 3and 7,dissolved organic carbon（DOC,10 mg/L）significantly increased the adsorption capacity of P of nZVI-BC,but when p H was at 10,DOC did not significantly affect the adsorption capacity of P of nZVI-BC.Through the optimization and analysis of response surface,it was found that the optimal adsorption environment conditions for 1:1-nZVI-BC on P were p H at 4.6,DOC concentration at 15.8 mg/L,ion concentration at 0.08 mol/L,while the adsorption capacity of P of nZVI-BC was at 25.5 mg/g（the initial P concentration of solution was 12.91 mg/L）.The influence of DOC concentration,solution p H and ion concentration on P adsorption capacity was as follows:solution p H>ion concentration>DOC concentration,and the interaction between solution p H,ion concentration,DOC concentration and ion concentration had a significant effect on the adsorption capacity of P of 1:1-nZVI-BC.