Stabilization Of Cadmium Contaminated Soil By Aluminate And Its Mechanisms

At present,soils pollution by heavy metals in China were serious.Cadmium(Cd)was the heavy metal with the highest point over standard rate,and a large number of Cd-contaminated soils required remediation.Stabilization is a widely used soil remediation technology,but the commonly used stabilization materials have disadvantages such as low Cd remediation efficiency and poor long-term stability,which can hardly meet the needs of Cd contaminated soil remediation.Aluminate have the ability to stabilize Cd,and have the advantages of fast hardening and strong corrosion resistance,so it is expected to be an ideal remediation material for Cd contaminated soil.In this work,we analyzed the stabilization effect of aluminate on Cd in soil through bench scale experiment with silicate as comparison,and evaluated the feasibility and safety of soil as grassland soil after remediation through pot experiment.The stabilization mechanism was expounded by SEM-EDS,FTIR,XRD and XPS analysis in the solution system.The main results are as follows:(1)The effects of different environmental factors on the stabilization of aluminate were analyzed through bench scale experiment.The results showed that silicate reduced the 7-d extracted Cd concentration by 63.46%~93.02% and19.05%~94.38%,respectively,aluminate reduced the 7-d extracted Cd concentration by 58.07%~77.75% and 76.97%~93.15%,respectively,when water was used as the extraction agent.Silicate reduced the 7-d extracted Cd concentration by3.25%~90.34% and 77.65%~99.65%,respectively,aluminate reduced the 7-d extracted Cd concentration by 44.17.07%~86.10% and 98.00%~99.77%,respectively,when sulfuric-acid/nitric-acid were used as extraction agent.As the stabilization time increased to 180 d,silicate reduced the 180-d extracted Cd concentration by41.08%~86.63% and-32.68%~49.79%,respectively,while aluminate reduced the 180-d extracted Cd concentration by 28.72%~79.23% and 56.56%~88.87%,respectively.At 4 ℃,aluminate and silicate reduced the 7-d extracted Cd concentration by 97.35%~97.92% and 78.79%~99.72%,respectively.After 16freeze-thaw cycles,silicate and aluminate reduced the extracted Cd concentration by93.99% and 95.48%,respectively.In conclusion,both aluminate and silicate can stabilize Cd in soil,among which aluminate has a better effect and is less affected by environmental factors.(2)The feasibility and safety of the remedied soil as grassland soil were evaluated by pot experiment.The results showed that the germination rate of tall fescue in the soil remediated by silicate and aluminate was higher than that of the control,the growth of tall fescue by silicate was better than that by aluminate.In remediated soil,the content of Cd in the underground part of tall fescue was lower than control,but the content of Cd in the overground part and the transfer coefficient of Cd is higher than control.In the unplanted treatments,silicate and aluminate reduced the extracted Cd concentration from 27.83 μg·L-1 to 22.67 μg·L-1 and 12.97μg·L-1,respectively,and it was no significant difference with planted treatments by silicate and aluminate(26.60 μg·L-1 and 13.42 μg·L-1,respectively).In the planted and unplanted treatment,the total soil enzyme activity reduced by 35.73% and22.58% with silicate,respectively,and increased by 19.93% and 18.71% with aluminate,respectively.In conclusion,planting tall fescue has little influence on the stabilization effect of aluminate,and aluminate is safer and more beneficial to soil than silicate.(3)The stabilization mechanism of aluminate was studied in the solution system.The results showed,in Cd solution(94.02 mg·L-1),aluminate and silicate reduced Cd2+ concentration by 99.92%~99.93% and 99.68%~99.92%,respectively,and increased p H by 6.56~6.81 and 7.01~7.69,respectively.Through SEM-EDS,FTIR,XRD and XPS analysis,it was found that aluminate could form C-S-H and C-A-S-H gels through hydration reaction to physical encapsulating and adsorbing Cd2+.In addition,aluminate could also increase p H to generate Cd(OH)2,Cd O or Cd CO3 to stabilize Cd.In conclusion,aluminate is an ideal Cd stabilization material,with advantages of approving effect,little influence by environmental factors and ecological safety,and has approving prospects in the remediation of Cd contaminated soil.