Release of Pb in soils washed with various extractants

• Residual Pb in washed soils could be activated, especially under flood condition.
• Dissolution of FeMn oxides under flood is the main cause for Pb release.
• Residual Pb release should be considered in assessing the suitability of soil washing.
• Citric acid solution is unsuitable for washing Pb-contaminated agricultural soil.

The suitability of an agent for washing heavy metal contaminated soils depends not only on the efficiency of heavy metal removal but also on the persistence of the washing effect, especially for agricultural soil. Redistribution of residual heavy metals in washed soil from inert pools to labile pools can reverse the initial washing effect. In order to study the redistribution of residual Pb in Pb-contaminated soils washed with citric acid (CA), ethylenediaminetetraacetic acid (EDTA), FeCl3, and HCl solutions, washed soils were incubated under both flood and 70% field water capacity (FWC) conditions for 180 days. The Pb availability in CA-, EDTA-, FeCl3-, and HCl-washed soils varied with incubation time. Diethylenetriaminepentaacetic acid (DTPA)-Pb generally increased with incubation time, except in CA-washed soil under flood incubation condition. Pb associated with the exchangeable/acid extractable fraction (F1) increased, whereas Pb in the easily reducible fraction (F2) decreased over time, for all washed soils under flood incubation condition; the changes in the amount of Pb associated with each fraction were much smaller under 70% FWC incubation. The transformation of Pb from F2 to F1 during flood incubation was largely caused by the mobilization of Fe and Mn oxides. The mobility factors (MF) of Pb for samples under flood incubation increased with increasing incubation time while those for samples under 70% FWC incubation changed only slightly, which implies increasing environmental risks of Pb from washed soils that are flooded. Therefore, the redistribution and potential environmental risks posed by heavy metals in washed soils, especially in paddy rice soils, should be taken into account when the suitability of a washing agent is evaluated. In the present study, CA was found to be unsuitable for washing Pb-contaminated agricultural soil, whereas the FeCl3 solution was the most stable washing agent tested, as it had high removal efficiency and low release of Pb after washing.

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