Clean-Up and Assessment of Metal Contaminated Soils

Abstract

Leaching is a possible option for the treatment of excavated soilcontaminated by heavy metals. In order to evaluate this potential,a protocol is proposed involving a schedule of combined speciationmethods, coupled with leaching tests. The suitability of the sequentialextn. alone as a decision tool is limited. This could be shown byan example of a Pb-contaminated soil. However, in combination withthe instrumental XAFS-method, which dets. the chem. binding formof heavy metals in soils, it is possible to assess different lixiviantsand leaching parameters. Detailed knowledge of binding forms is mostimportant for hot spots, where metal species form their own solidphases, e. g. as pptd. salts or as a liq. phase (if mercury is present).Only a sophisticated knowledge of the chem. state of these phasesenables a systematic search for appropriate leaching parameters tooptimize the leaching conditions. Leaching processes can lower totalamts. of heavy metals by up to 99 % , but a total sepn. of mobilefractions cannot be guaranteed. This limits the applicability ofthese processes, and a great effort has to be made to control remainingamts. of mobile metals. XAFS-results have shown that a sepn. of justthe mobile metal fractions from the soil is not practicable due toequil. adjustments which happen during extn. processes. The kineticbehavior during extn. has to be investigated exptl. Although thereare sophisticated "Surface Complex Formation" models describing solid-liq.equil., treatment results can hardly be predicted. These models assumethe metal bindings being surface-controlled, but this is not thecase in hot spots. Esp. if soils are highly contaminated, it canbe assumed that particles of pptd. metals are forming their own soil-phases.In extn. processes, these phases behave differently to surface boundmetals and could lead to "mobile " metal compds. remaining. One soil,which was highly polluted with lead, could be leached efficientlywith acetate soln. if a sequence of fourfold acidic steps and a finalrinsing under weak acidic conditions was applied. If citrate is usedinstead of acetate, the whole leaching can be performed at moderatepH conditions (pH 5). This is advantageous because the soil matrixis damaged to a lesser extent. Three mercury-contaminated soils couldbe cleaned hydrometallurgically in a circuit process, if sodium chlorideis used, and oxidizing conditions are chosen. The integrating stepof this process is the electrolysis; the mercury-loaded aq. streamcan be regenerated in one step that combines the cathodic depositionof mercury and the anodic re-oxidn. of the chloride. [on SciFinder(R)]