Cadmium Release from Four Sorbents during Treatment of Contaminated Soils by Catalyzed H2O2 Propagations (Modified Fenton’s Reagent)

The release of heavy metals from soils and subsurface solids is a significant concern during in situ chemical oxidation treatment. The release of cadmium sorbed on four solids of varied properties was investigated in slurries treated by catalyzed H2O2 propagations (CHP—modified Fenton’s reagent). Cadmium was released in all four solids in CHP reactions conducted at pH 3; however, aqueous cadmium concentrations decreased in CHP reactions conducted at pH 7. Cadmium release at pH 3 was directly proportional to the soil organic carbon (SOC) content of the four solids (R2=0.99), and may have been due to destruction of the SOC by hydroxyl radical in the reactions at pH 3. Scavenging of hydroxyl radical minimized the release of cadmium in CHP systems at pH 3, which is consistent with cadmium release resulting from destruction of SOC by hydroxyl radical. Scavenging of hydroxyl radical in CHP systems at pH 7 resulted in increased cadmium release, compared to pH 7 reactions without the hydroxyl radical scavenger isopropanol. Superoxide, the conjugate base of perhydroxyl radical (pKa=4.8), was proposed as the desorbing species in the scavenged CHP reactions at pH 7. The results of this research demonstrate that cadmium release will likely be minimal in CHP reactions conducted at pH 7 if compounds that increase the activity of superoxide are not present at significant concentrations.