Heavy metal contaminants removal by soil washing

The feasibility of soil washing for decontaminating a silty sand spiked with cadmium, chromium, lead, and zinc was evaluated in laboratory-scale batch and column experiments. Soil samples were subjected to chelant extraction using a solution of disodium salt of ethylenediaminetetraacetic acid (Na(2)EDTA), sodium metabisulfite (Na(2)S(2)O(5)) solution (an inexpensive reducing reagent), and a solution containing a mixture of the two reagents. Batch and column washing of the contaminated soil with deionized water (DI water) revealed that approximately 70% of the cadmium in the sample is weakly bound and readily mobilized in aqueous solution at neutral pH, followed by approximately 25%-30% of zinc, approximately 20%-25% chromium, and only approximately 10% of lead. Of the washing reagents tested, Na(2)EDTA solutions were generally more effective than Na(2)S(2)O(5) for removing heavy metals from the soil samples. Na(2)EDTA preferentially extracted lead over zinc and cadmium but exhibited little impact on chromium removal. Cadmium and, especially zinc, removal by a 0.01-M Na(2)EDTA solution were enhanced considerably by inclusion of 0.1 M Na(2)S(2)O(5), suggesting that a mixture of the two reagents may provide an economically optimum solution for certain contaminated soils.