Removing of Cr(III) and Cr(VI) compounds from aqueous solutions by shale waste rocks

Abstract

Shale is a sedimentary rock often occurring in gangue that accompanies coal and other minerals. It is extracted with them and often treated as waste rock. In the paper, the shale was tested for sorption of Cr(III) and Cr(VI) ions from aqueous solutions. The samples were taken from three different Polish mines. The structural and surface properties of the shales were determined via nitrogen adsorption/ desorption. The specific surface area of the shales ranges from 10.3 to 12.3 m2/g. In sorption tests, the effect of pH was tested. The initial concentrations of chromium ions fell in range 1–500 mg/L for Cr(III) and 1–100 mg/L for Cr(VI). Several adsorption models were considered. The adsorption follows the Langmuir isotherm. The sorption capacity in the Langmuir model is 7.67–9.85 mg/g for Cr(III), whereas only 0.56–4.66 mg/g for Cr(VI). The sorption percentage equals 75%–90% for low concentrations of chromium ions, and drops with an increase in the initial concentration. There is a significant difference between the tested shales in chromium sorption, especially for Cr(VI). One of the tested shales turned out to have best performance for both Cr(III) and Cr(VI).