Study On The Remediation Of Cadmium And Phenol Contaminated Soil By Layered Double Hydroxide/Biochar Composites

In recent years,the soil pollution is increasingly fierce,and the remediation of contaminated soil is imminent.At present,most of the researches on soil pollution remediation are focused on the remediation of multiple heavy metals or multiple organic compounds,and there are few studies on stabilization of heavy metals and oxidation of organic compounds by using the same material.Among many materials,layered double hydroxide（LDHs）,which can not only adsorb heavy metals,but also degrade organic pollutants,can be defined as an excellent soil pollution repair agent.At the same time,the large specific surface area and microporous structure of biochar are conducive to adsorbing heavy metals and organic pollutants.Meanwhile,biochar can effectively avoid layered double hydroxide aggregation and improve the soil remediation effect.Therefore,layered double hydroxide/carbon composites were prepared and used to remove cadmium and phenol,which provided a theoretical basis for the application of composite materials in actual soil remediation.In this study,the biochar（BC）was prepared,which was made of rice husks.Fe-Al LDHs and Fe-Al LDHs/BC were synthesized by hydrothermal method and coprecipitation method.Compared the performance differences of composite materials,BC and Fe-Al LDHs for removing cadmium and phenol,meanwhile the optimum amount of BC was determined.The removal rates of cadmium and phenol in the aqueous phase of the composite waere investigated under different pollutant concentration,p H,reaction time and H₂O₂ content.The experimental result indicated that the maximum adsorption capacity of Fe-Al LDHs/BC for Cd²⁺could reach 154.94 mg/g,as well as the removal rate of Cd²⁺reached over 86.96%in a wide p H range.When the p H was 3 and the concentration of H₂O₂ was 400 mg/L,the total removal rate of 80 mg/L phenol could reach 82.38%.Through analysis methods such as SEM,BET,FTIR and XRD,the changes of the physical and chemical properties of the composite material before and after the reaction were compared,and the mechanisms of action of the composite material on cadmium and phenol were investigated.The results show that the coordination reaction is beneficial to the removal of cadmium.The hydroxyl and carbonyl groups on the surface of the material can coordinate with Cd²⁺to form a stable complex.Fe²⁺and Fe³⁺can be used as catalytic sites for phenol degradation.The layered structure of the composite and the abundant hydroxyl groups are also lucrative for the catalytic activity.The artificially simulated cadmium and phenol contaminated soil was prepared.Composite materials with good removal performance of cadmium and phenol in solution were selected as the remediation agent.The remediation effects of the composite were investigated under different pollutant concentration,reaction time and H₂O₂ content.The repairing effect was evaluated by the change of soil p H,diethylenetriamine pentaacetic acid（DTPA）leaching method,toxicity characteristic leaching experiment（TCLP）,improved BCR continuous extraction method and the change of phenol content in soil.The results demonstrated the solidification rate of available cadmium in 5,10,20 mg/kg polluted soil could reach more than 90%by adding 6%composite material.When the concentration of hydrogen peroxide was 400 mg/L,the removal rate of phenol in 50,100,200 mg/kg polluted soil could reach65.85%,62.1%and 48.58%.In the remediation of combined polluted soil,different concentrations of cadmium and phenol combined contaminated soil were prepared.By comparing the remediation effects of single cadmium,phenol contaminated soils and compound contaminated soils,the interaction between phenol and cadmium was investigated and the remediation performance of combined contaminated soils was evaluated.The experimental results show that the effective cadmium content and leaching toxicity of the composite contaminated soil samples after stabilization are increased compared with the single cadmium contaminated soil samples.The available cadmium content in 10 mg/kg cadmium contaminated soil samples increased from 0.41713mg/kg to 0.52547,0.65421 and 0.84042 mg/kg,respectively.Compared with the single phenol-contaminated soil sample,the residual phenol content in the compound contaminated soil increased.The phenol in the soil can be adsorbed on the surface of composite materials through hydrophobic bonds,increasing the hydrophobicity of soil surface and hindering the solidification of Cd.Covering the surface of the composite with Cd precipitates can affect the formation of Fenton-like system,resulting in the reduction of phenol removal rate.