Washing remediation of polluted Cr(Ⅵ)-containing soil using chelators compounded with low-molecular weight organic acids
To explore the role of the biodegradable chelating agent nitrilotriacetic acid(NTA)for the substitution of ethylenediaminetetraacetic acid disodium(EDTA)in mixed reagents,this study evaluated the washing kinetics of these mixed reagents,alongside the effects of concentration,solid-liquid ratios,pH values,and washing times,on their corresponding removal efficiencies of Cr(Ⅵ)-polluted soil,specifically by treating soil with NTA and EDTA compounded with oxalic acid and tartaric acid,respectively.In addition,the BCR sequential extraction procedure was used to analyze the differences in chemical states,mobility,and stability of heavy metals within the soil before and after washing.The corresponding results demonstrated that a maximum Cr(Ⅵ)removal efficiency of 82.94%could be achieved by using a mixed solution of 0.25 mol·L-1 NTA and 0.3 mol·L-1 oxalic acid in equal volume with a solid-liquid ratio of 1∶9,pH of 3,and washing time of 240 min,which was consistent with that of EDTA.The Pseudo-second-order kinetic equation could be applied in the washing kinetic process.Meanwhile,the BCR results demonstrated that the Cr contents of the mild acid-soluble,oxidizable,and residual fractions in the soil were reduced from 6 346.67,4 280.00 mg·kg-1,and 2 387.67 mg·kg-1 to 1 161.67,433.35 mg·kg-1,and 741.00 mg·kg-1,respectively,after washing;in contrast,the reducible fraction increased from 2 746.67 mg·kg-1 to 6 016.67 mg·kg-1 following washing,thereby indicating that heavy metal mobility was inhibited in the contaminated soil.Additionally,the organic matter content in the soil rose significantly after washing(from 13.47%to 30.56%),as the content of urease and dehydrogenase declined from 46.32 μg·d-1·g-1 and 329.09 μg·d-1·g-1 to 29.36 μg·d-1·g-1 and 131.98 μg·d-1·g-1,respectively;nonetheless,soil composition predominantly remained the same.Therefore,this compound of NTA and oxalic acid is an efficient eluant which has little effect on the soil.
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