Phosphorus-modified Biochar Cross-linked Mg–Al Layered Double-hydroxide Composite For Immobilizing Remediation Of Uranium Contaminated Soil

Uranium mining is usually accompanied by the problem of soil contaminated by uranium and associated heavy metals.Immobilization/stabilization technology is an effective remediation solution,but the choice of stabilizers is the key to its cost issue.Biochar,as a type of carbonaceous material,has abundant functional groups on its surface,a porous structure that can be developed,high adsorption efficiencies for uranium and heavy metals,and a low preparation cost.In recent years,biochar has become a popular material for adsorbing and immobilizing uranium in soil.However,a single biochar usually exhibits a comparatively poor BET specific surface area and the type of functional group performance.In this paper,a novel phosphorus-modified biochar cross-linked Mg-Al layered double-hydroxide composite（PBC@LDH）was prepared,which was measured by Fourier-transform infrared spectroscopy,Scanning/Transmission electron microscope equipped with X-ray spectroscopy,BET specific surface area analysis,X-ray diffraction and other characterization methods to characterize the physicochemical properties of original biochar（BC）,phosphorus modified biochar（PBC）,and PBC@LDH.The selection of original biomass and the performance verification of modified biochar were carried out through batch adsorption experiments.Laboratory-scale soil incubation experiments were used to analyze the changes of p H,conductivity（EC）,and redox potential（Eh）values,standardized procedures（SPLP）toxic leaching amount,and Tessier sequential extraction on the BC,PBC and PBC@LDH treatments with different dosage,meanwhile,the immobilization mechanism was analyzed by means of XPS,et al.After that,a dynamic column leaching experiment that simulated acid rain was used to verify the immobilization persistence of the stabilizer on uranium-contaminated soil.Finally,an outdoor pot experiment was also used to verify the enrichment and transport of U and associated heavy metal Pb in Indian mustard after stabilizer remediation.The research results are summarized as follows:（1）With the increase of pyrolysis temperature,the variation for characteristics of bamboo（ZBC）,rice husk（DBC）and Chinese fir（MBC）biochar had been obviously different,e.g.,the DBC had a higher ash content;ZBC had a more developed pore structure.The kinetic curves of ZBC,DBC and MBC capturing U（VI）were more consistent with the pseudo-second-order kinetic model,and adsorption equilibrium can be reached within 3 h.Adsorption isotherms of ZBC,DBC and MBC to U（VI）were more consistent with the Langmuir model.The maximum adsorption capacity of ZBC700 for U（VI）was 18.55 mg/g.（2）PBC and PBC@LDH added–OH,P–O functional groups,PBC@LDH successfully grafted Mg–O functional groups.Phosphate activation greatly increased the specific surface area of BC to 489.87 m²/g.After loading LDH,the BET of PBC@LDH decreased to 445.17m²/g.Compared with BC,the O element content of PBC and PBC@LDH increased by 1.61%and 6.39%,and the C element decreased by 7.02%and 24.52%,respectively.The crystal diffraction peaks between PBC@LDH and LDH were highly consistent,which proved the successful cross-linking.The surface of PBC was rougher than BC,and the pore structure was increased.Layered LDH structure was successfully deposited on the surface of PBC@LDH.HR-TEM analysis proved that the surface of PBC@LDH existed a hexagonal crystal structure and the grain diameter was approximately 15 nm.The lattice fringes with lattice spacing of 0.372 nm and 0.261 nm corresponded to the（006）and（101）crystal planes of LDH,respectively.Thermogravimetric analysis confirmed that bamboo biochar has a hard texture.（3）PBC and PBC@LDH could reach adsorption equilibrium for U（VI）in 2 h,and the pseudo-second-order kinetic model can better fit the adsorption process.Langmuir model could be more consistent with the adsorption process of U（VI）by modified biochar（R²>0.97）.The maximum adsorption capacity of PBC and PBC@LDH for U（VI）was225.991 mg/g and 274.151 mg/g,respectively,all of which were much higher than that of BC（17.744 mg/g）.When solution p H 4～8,the adsorption capacity was much larger.Zeta potential analysis confirmed that phosphorus modification increased the negative charge of BC,and the introduction of LDH led to the increase of the positive charge of PBC.The mechanism of PBC adsorption of U（VI）mainly corresponded to the coordination and reduction between e.g.,P–O,–OH groups and UO₂²⁺ions.The mechanism of PBC@LDH adsorb U（VI）was the synergistic reaction of–OH,P–O,and Mg–O–H groups to U（VI）.Furthermore,the reduction between the oxygen-containing groups and UO₂²⁺as well as the co-precipitation of polyhydroxy aluminum cations capturing U（VI）ion also play an important role.（4）A 10%dose of PBC@LDH（M-10）could greatly increase the p H and EC of UMT soil.After 40 days of incubation,the p H of M-10 was 7.5.The redox potential（Eh）of PBC@LDH treatment was stable as-92 m V,so the PBC@LDH can be an excellent reducing agent.Compared with untreated soil（CK）,after 40 days of incubation,M-10 could reduce80%of SPLP toxic leaching of U and 34%of Pb leaching in soil.Application of 10%dose of BC（B-10）and PBC@LDH could increase the proportion of residual U in soil（14.77%and 14.88%respectively）.The sum of exchangeable and carbonate bound proportion of 10%dosage of PBC（P-10）and M-10 were 38.86%and 43.7%,respectively,all of which was significantly lower than CK（50.22%）.Compared with CK,M-10 could reduce the exchangeable form of Pb in UMT soil by 11.45%,which was the maximum value compared with other treatment schemes.The reaction caused on reduction,complexation,and co-precipitation between surface functional groups on PBC@LDH and U is the main immobilization mechanisms.At the same time,cation-πinteraction and co-precipitation are contributed to PBC@LDH immobilized the associated heavy metal Pb in UMT soil.（5）The peak value of column leaching of U in soil treated by PBC@LDH was 0.13mg/L,which was 73%lower than that of CK（0.49 mg/L）.The peaks of U leaching efficiency in BC,PBC and PBC@LDH treatments were lower than CK and delayed.The peak of PBC@LDH was 1.14%,which was 53%lower than CK（2.43%）.The cumulative leaching loss of U after PBC@LDH remediation was 1.57 mg/kg,which was 53%lower than that of CK（3.37 mg/kg）.Each remediation plan had little effect on the leaching volume of Pb,but PBC@LDH treatment could reduce the cumulative leaching volume of Pb,which was 22.3%less than that of CK.After leaching,the proportion of residual U in the soil treated with PBC@LDH increased by 2.19%compared with that before leaching,but the proportion of exchangeable form in the samples treated by PBC@LDH was the larger reaching 4.87%.Compared with before leaching,the proportion of carbonate bound form in the soil treated with CK,BC,PBC and PBC@LDH increased by 15.9%,9.68%,4.73%and 4.75%respectively.The morphology of Pb did not change much after leaching.（6）After PBC and PBC@LDH remediation,Indian mustard grew well.After PBC@LDH treatment,compared with CK,the biomass of Indian mustard increased by52.7%,and the bioconcentration factors of U and Pb were reduced by 73.4%and 34.2%,respectively;and translocation factors were reduced by 15.1%and 2.4%,respectively.Effective content of U and Pb in UMT soil after PBC@LDH treatment could be reduced by55.97%and 14.1%,respectively.Bio-TEM analysis confirmed that PBC@LDH remediation can reduce the bioconcentration and translocation of U and Pb in UMT soil by Indian mustard,which corresponds to the complexation,precipitation and reduction of the functional groups released by PBC@LDH with U and Pb,on the other hand,–OH groups could coordinate with free uranium ions around the vascular bundle structure of plant root cells,thereby preventing the formation of U-containing vesicle structures and reducing the risk of migrating U to the ground.