tratamento redutivo de solo e água subterrânea contaminados com ...

‘ABSTRACTREDUCTIVE TREATMENT OF SOIL AND GROUNDWATER CONTAMINATED WITHHEXAVALENT CHROMIUMChemical reduction of the hexavalent chromium, Cr(VI), present in contaminated soiland groundwater was investigated using several different reductants (batch and semibatch)in order to evaluate the extension of Cr(VI) reduction and the immobilization(precipitation) of trivalent chromium, Cr(III). A comparative study concerning the use ofdifferent reductants revealed that Fe(II) and ZVIcol (nanoparticles stabilized withcarboximethylcellulose) present the most promising findings for the Cr(VI) removal,since it was found that a quantitative reduction of this specie is accompanied by itsimmobilization. Application of Fe(II) and ZVIcol using the 1[Cr(VI)]:20[Fe(II)] and1[Cr(VI)]:4[ZVIcol] molar ratios resulted in a redox conversion degree higher than 98%and in the immobilization of the labile and the exchangeable Cr(VI) fractions. It wasverified for Fe(II) that increasing the volumetric flow rate (G) both the pore volumenecessary for the redox treatment and the residual Cr(VI) decrease. The theoreticalhydrodynamic study concerning the packed column reactor containing soil (porousmedium) permitted to evaluate the reductant dispersion degree inside the column inrelation to the plug-flow model as functions of G by measuring the volumetric massdispersion coefficient (k VD ), which was calculated from simulation of the experimentalsaturation curve. In all cases, it was found the kinetics of the Cr(VI)/Cr(III) redox processfollows a pseudo-first order rate law. The complexicity concerning the kinetic processwas evidenced by the dependence of the overall pseudo-first order kinetic rate constant(k*) on reaction time, G and the initial concentration of the reductant. Aphenomenological kinetic model was proposed in order to obtain a relation capable torepresent k* for the different experimental conditions investigated, as well for thepossible limit cases. An overall reaction scheme was proposed in order to describe thedifferent elementary processes concerning the Cr(VI) and Cr(III) species, which takeplace inside the porous soil matrix (soil/groundwater). A pilot scale test was carried outin situ taking into account the best experimental scenario obtained in laboratory.- xix -