Influence of soil washing with a chelator on subsequent chemical immobilization of heavy metals in a contaminated soil.

To assess the influence of soil washing with a chelator on the chemical immobilization of heavy metals, batch experiments were performed on the fine fraction of a contaminated soil under various operating conditions. Results show that pre-washing with EDTA facilitated the chemical immobilization of Cu and Cr, while an opposite effect for Pb and Zn was observed, in particular when Ca(OH)(2) was added as the immobilizing agent. Metal fraction analyses of the soils indicate that soil washing can reduce the metal mobility by removing the labile fractions, while it may also destabilize some strongly bound fractions, reversely increasing the mobility and thus compromising the subsequent immobilization performance to some extents. To secure an effective combination of soil washing and chemical immobilization for the remediation of heavy metal-contaminated sites, a comprehensive study on metal fraction distribution in the soil is needed.

Influence of EDTA washing on the species and mobility of heavy metals residual in soils.

Aiming to estimate the potential risk of ethylenediaminetetraacetic acid (EDTA)-enhanced soil washing, the heavy metal species and their mobility in the washed soil under different combinations were investigated by batch leaching tests and the sequential extraction procedure. Results demonstrate that the metal removal efficiency was rather low (less than 12%), partially due to the significant Ca dissolution and strong bonding between metals and the soil as well as the insufficient EDTA dosage. The washing combination of 0.0005 M EDTA and half-an-hour washing can enhance the instant mobility of Ni, Zn and Pb possibly owing to the slow detachment of EDTA-destabilized metals. Metal fractionation also exhibits the corresponding increase in their labile exchangeable fractions. Therefore, a more concentrated EDTA solution for a longer duration often decreased their mobility. The increase in some fractions of a curtain metal implies the redistribution of this metal during the EDTA soil washing. The pathway of such a redistribution may vary for different metals, but the redistribution to organic matter is often a slow process, while that to carbonates or Fe/Mn oxides is a faster one and even may occur in a half hour washing with 0.0005 M EDTA solution. These redistribution processes may also increase the metal chemical availability. Therefore, we should prudently control the chelating reagent concentration and washing duration to finally minimize the mobility and availability of the remaining heavy metals when designing the soil washing for the remediation of metal-contaminated soils.