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Geochemical characterization of soil contaminated with hexavalent chromium and investigation of its adsorption capacity in hexavalent chromium

Panagopoulos Dimitrios

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Summary

In the geo-environment, at greater or lesser concentrations, chromium compounds are detected. Chromium, in addition to its indigenous existence, is widely used in industrial activities. On the one hand, its presence is due to geological formations such as hyper basic rocks and ophiolithic soils. However, human activity contributes to the accumulation of chromium compounds in aquifers due to uncontrolled waste disposal. The main forms of appearance of chromium in the environment, based on its oxidative status, are two. Trivalent and Hexavalent chromium. Trivalent chromium Cr (III) is necessary for the human body as it strengthens the metabolic process whereas hexavalent Cr (VI) is a major threat when it is at greater than the permissible concentrations, as it underlies the risk of skin diseases, cancers, reproductive issues and more.In the context of this diploma thesis, the adsorption of hexavalent chromium, in soil from three pilot drillings from the Asopos area, was studied, where large concentrations of hexavalent chromium in groundwater have already been identified. At the same time, the geochemical characterization of the soil, which was studied in terms of its adsorption capacity, was carried out in order to establish mineralogical, chemical and other parameters that could function or inhibit the above process.To investigate the adsorption capacity of the soil in question, batch experiments were carried out, and the effect of pH on adsorption was examined at a concentration of 1 ppm and 5 ppm but also on the variation of the adsorption curve in relation to the granulometry. For this reason, five different granulometries were selected and the same batch experiments were repeated for the same concentrations. At the same time, the initial concentration of hexavalent chromium in each pilot borehole was taken into account.It has been found, therefore, that a change in pH values has a great effect on the adsorption of hexavalent chromium, and more specifically, for a pH range of 4 to 8, it has been observed that as the pH increases, adsorption decreases. Also, the initial concentration of Cr (VI) was found to significantly affect the adsorbed concentration of hexavalent chromium. Of the three pilot drillings, the two were highly polluted, with Cr (VI) concentrations of 600-900 ppb while the third was not contaminated. In the latter, due to its unsaturation regarding the Cr (VI), removals were measured up to 40% higher than the other two.Finally, data from the geochemical characterization were taken into account in relation to the adsorption curves for the two different initial concentrations but also the different granulometries in order to determine the granulometric fraction controlling the adsorption process but also if there are specific chemical elements likely to affect the process. Amorphous iron oxides and increased organic carbon affect the process, because the low concentration of the former reduces soil adsorption capacity, while increased organic carbon concentration intensifies the reduction of Cr(VI) to Cr(III).

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