EXPERIMENTAL ASSESSMENT OF CHROMIUM IMMOBILIZATION IN CONTAMINATED SOIL BY IN SITU CHEMICAL REDUCTION

As a result of its high water solubility, Cr (VI) may easily move into the soil, thus inducing groundwater pollution. Due to its relatively low toxicity, the extraction of Cr (III) is not generally required, while Cr (VI) removal represents the main environmental concern. Among the technologies available for Cr (VI) removal from polluted soils and groundwater, chemical reduction is known to remove Cr(VI) rapidly and effectively using reducing agent such as ferrous sulfate, sulfur dioxide, or sodium bisulfate followed by precipitation as Cr(III). The main disadvantages of this method are the low efficiency in the presence of low levels of Cr (VI) in the soil, and the costs of the chemicals used in the process. The aim of the present research was to compare the effectiveness of iron sulfate and nano zero valent iron in the chemical reduction of Cr (VI). The experiments were performed on a soil collected at an industrial site: it also resulted polluted by nickel, as a result of a long-term industrial activity. The experimental tests were performed under several operating conditions: the main parameter investigated were the amount of chemical with respect to chromium content, the presence of oxygen in the slurry, and the liquid (L) vs. solid (S) ratio. The soil was fully characterized after each tests, to perform mass balances, and sequential extractions were carried out to assess the metals mobility induced by the washing with the solution containing the reducing agent. Results show that iron sulfate successfully reduced Cr (VI): within 16 hours of treatment the residue amount of all the investigated metals was below the threshold allowed by Italian Environmental Regulation for an industrial reuse. Due to Cr (VI) solubilization, reduction and the following Cr (III) precipitation, in both cases, the treatment resulted in an increase of the amount of chromium bound to the oxide-hydroxide fraction, thus confirming a mechanism of chromium-iron hydroxides precipitation. In addition, depending upon the US adopted in the tests, heavy metals (mainly Ni, Pb and Zn) solubilization was also observed during the experiments. In the tests performed using Fe-0 nanoparticles, prepared by iron chloride reduction with borohydride, the reduction of Cr(VI) was found to be time dependant, and increasing with the concentration of nanoparticles.