Mechanism Of Biochar-Supported Phosphorus-Doped Ferrihydrite For Simultaneous Passivation Of Lead And Cerium In Soil

The quality of soil is related to agricultural product safety and human health.The phenomenon of soil heavy metal pollution in China is prominent,and soil heavy metal pollution has received extensive attention.Rare earth is an important mineral resource,which is widely used in the development of new materials.At the same time,rare earths can also be used as agricultural micro-fertilizers to significantly increase crop yields.However,the extensive use of rare earths causes the ir accumulations and harms in the environmental ecosystem.In addition,most rare earth ores are associated with metallic minerals,such as the typical cerium ore（monazite and bastn?site）that contains galena and cerussite.The problem of compound pollution caused by rare earth and heavy metal mining,smelting and application is worthy of attention.At present,there are few reports about the remediation of rare earth elements and heavy metals compound pollution.Chemical stabilization remediation technology is an important means of remediation of heavy metal-contaminated soils in recent years,and it is of great significance to develop functional remediation materials that simultaneously passivate heavy metals and rare eart hs.Ferrihydrite（FH）is a typical iron oxyhydroxide nano-mineral,which has a high affinity for heavy metals and metal-like substances,which can interact with ferrihydrite by co-precipitation and adsorption.However,the easy agglomeration of ferrihydrit e nanoparticles is not conducive to their adsorption of heavy metal ions.In this study,biochar supported P-doped ferrihydrite（P-FH@BC）was prepared and filtered out by the method of phosphorus doping modification and supporting by biochar,and a representative P-FH@BC pair was investigated through batch adsorption experiments.The adsorption effect and related mechanism of representative P-FH@BC on Pb（II）and Ce（III）were studied through batch adsorption experiments,and the adsorption effect of representative P-FH@BC on lead and cerium compound simulated polluted soil was also discussed.According to the characteristics of P-FH@BC,the passivation mechanism of P-FH@BC on lead and cerium in soil was investigated by magnetic separation,and the effect of P-FH@BC on soil physicochemical properties and microbial community structure was investigated.The main results and conclusions of the study are as follows.（1）P-FH@BC was designed and representative P-FH@BC was selected.Biochar was used as a carrier material,and a new and efficient biochar-supported phosphorus-doped ferrihydrite composite was synthesized by a one-step method,which greatly improved the adsorption capacity of the adsorbent.By comparing the morphologies and crystal characteristics of P-FH and P-FH@BC,it was found that the addition of biochar made nanoparticles（NPs）uniformly dispersed on the surface of biochar,which effectively improved the adsorption effect.Compared with the adsorption capacity of P-FH for Pb（II）in solution,that of P-FH@BC was greatly improved.The effect of biochar pyrolysis temperature on the adsorption capacity of P-FH@BC composites was explored,and it was found that biochar prepared at 800°C had greater advantages as a carrier material compared with other pyrolysis temperatures.The adsorption performance also ha d a certain effect,and the results showed that the synthesized composite ha d the best adsorption effect when the ratio of P-FH to biochar was 10:1.According to the adsorption capacity of the composite material to Pb（Ⅱ）in solution,the biochar-supported phosphorus-doped ferrihydrite prepared when P-FH:BC800=10:1 was screened out as an ideal material.At the same time,the effects of p H and dosage on the adsorption capacity of P-FH@BC were investigated,and it was found that when p H<6,the adsorption capacity increased with the increas ing of p H,and the optimal dosage of P-FH@BC was 40 mg（25℃,250 mg/L）.In addition,the representative material had good reusability,and the adsorption capacity could still reach 252.9 mg/g after 4cycles.（2）The Pb（II）/Ce（III）adsorption capacity of representative P-FH@BC for single and binary systems was further explored.The adsorption capacity of P-FH@BC for Pb（II）/Ce（III）depends on the p H value of the initial solution.When the p H was between 3 and 6,the adsorption capacity of P-FH@BC for Pb（Ⅱ）/Ce（Ⅲ）increased with increasing of p H,which was related to the amount of negative charge on the surface of the material.The adsorption data of Pb（II）and Ce（III）in aqueous solution by P-FH@BC conformed to the pseudo-second-order kinetic model and Langmuir adsorption isotherm model,indicating the homogeneous chemical adsorption of Pb（II）/Ce（III）by P-FH@BC was the main method,and the maximum adsorption capacity could reach 357.8 mg/g and 235.7 mg/g,which was greater than the adsorption effect of representative adsorbents reported in other references.The intraparticle diffusion,Elovich and Bangham models also fit the adsorption process of Pb（II）/Ce（III）well by P-FH@BC,and the adsorption of Pb（II）by P-FH@BC was faster according to the adsorption parameters.In addition,the adsorption thermodynam ic data indicated that the adsorption of Pb（II）/Ce（III）by P-FH@BC was a spontaneous and endothermic process.In the binary system of Pb（II）and Ce（III）,the adsorption capacity of P-FH@BC for lead and cerium was inhibited to different degrees,indicating that there is a competitive adsorption of Pb（II）and Ce（III）in the coexisting system.The adsorption capacity of P-FH@BC for Pb（II）was less affected by the presence of Ce（III）,while the adsorption of Ce（Ⅲ）by P-FH@BC had a greater inhibitory effect by the presence of Pb（II）.Based on the analysis of the material characterization results before and after adsorption,it was found that the same mechanisms of Pb（II）and Ce（III）adsorption by P-FH@BC include electrostatic attraction,pore filling,surface complexation and chemical precipitation.However,the iron-containing functional groups in P-FH@BC,such as Fe-O,had better binding ability to Pb（II）,resulting in ion exchange or insoluble precipitation.（3）The remediation effect of representative P-FH@BC on simulated lead and cerium contaminated soil was studied.The results showed that Si O₂,Al-containing compounds,Al-Fe-Si and other minerals and organic components in the soil c ould passivate with lead and cerium.Therefore,after a period of aging,the lead and cerium in the treatment group without the addition of passivation agent also occurred some passivation.After adding P-FH@BC,through the investigation of soils with different lead and cerium concentrations,passivation time and dosage,it was found that under the conditions of 90d and 3%dosage,500 mg/kg of lead and cerium single or compound in equal proportions soil samples,the weak acid extraction states of Pb（II）and Ce（III）were greatly reduced（single lead decreased by 79.3%,single cerium decreased by 77.4%）,and the content of residual lead and cerium was lower than that without remediation agent,indicated that the addition of P-FH@BC made the lead and cerium in the soil more stable.Through the magnetic separation of the reacted P-FH@BC characterization from the soil,it was found that a large amount of corresponding functional groups-OH,-COOH,C=O,Si-O/C-O-C/-PO₃H₂,Si-O-Si,Fe-O,Fe-OH and P-O peaks intensities on the surface of the P-FH@BC changed,indicating that P-FH@BC and soil components reacted with lead and cerium.Combined with XRD analysis,it was found the appearance of a large number of precipitates(Pb ₃（PO₄）₂,Pb₃P₃O₉2H₂O,Ce PO₄ and Pb Fe1₂O₁₉)and secondary minerals(Pb₂Fe_0.78Al_0.22（HPO₄）₂（OH）₂ 26.54°,ICDD No.84-1779),indicating that P-FH@BC had a significant effect on lead and cerium.The passivation mechanism of P-FH@BC included formation of secondary minerals,chemical precipitation,ion exchange,surface complexation,and physical adsorption.（4）The effects of adding P-FH@BC on soil physicochemical properties and bacterial community structure were explored.Adding 0%,1%,2%,3%,and 4%of P-FH@BC to the simulated polluted soil,after 90 days of soil culture experiments,it was found that the soil p H increased when the dosage≥2%,which was beneficial to improve the stability of lea d and cerium.At the same time,the addition of P-FH@BC could increase the contents of soil organic matter（SOM）and cation exchange capacity（CEC）in the soil,and the value increased with the increasing of the dosage,which is beneficial to the stabiliza tion of lead and cerium.The addition of P-FH@BC had little effect on soil nutrient element N.However,after the addition of P-FH@BC,the content of total phosphorus in the soil increased,which was due to the phosphorus content in P-FH@BC.In addition,the available phosphorus in the remediation soil was reduced compared with the uncontaminated soil samples of the same treatment,which was due to the combination of lead and cerium with phosphorus in the contaminated soil to form less soluble precipitates or secondary minerals.The DNA of soil samples from different treatment groups was extracted for 16 S r DNA high-throughput sequencing,and combined with relevant bioinformatics analysis,it was found that the effect on the changes of soil bacterial community abundance and diversity index after adding P-FH@BC were not obvious compared with the blank control,indicating that P-FH@BC was a non-toxic remediation material to soil microorganisms.Meanwhile,lead and cerium treatments reduced soil bacterial community abundance.However,soil bacterial community abundance was restored after the remediation by P-FH@BC.Further analysis of the soil bacterial community structure in different treatment groups found that th e abundance of heavy metal and rare earth sensitive bacteria genus Sphingomonas was lower in the polluted group than that in CK,but recovered after adding P-FH@BC culture and restoration.P-FH@BC might alleviate the toxicity of lead and cerium by reducing their bioavailability.The P-FH@BC combined the advantages of biochar and phosphorus-doped ferrihydrite was designed in this study,exhibited good adsorption performance on lead and cerium,and greatly improved the passivation effect of lead and cerium in soil.This research was based on the concept of passivation remediation:the“release”of heavy metals and rare earths caused by human activities was passivated in the environment again.The research results have certain guiding significance for the actual remediation of soil polluted by heavy metal and rare earth element.