Evaluation of modified mineral performance for chromate sorption from aqueous solutions

The removal of Cr(VI) from aqueous solutions was investigated using various minerals, such as zeolite (clinoptilolite), bentonite, exfoliated vermiculite and attapulgite. These minerals were modified using hexadecylammonium bromide (HDTMA-Br). The parameters under investigation were the HDTMA-Br concentration (0.36–8 g L−1), the pH (3–9) at which the modification process was conducted, the pH (3–9) of the adsorption solution, the modified mineral type and its concentration (1.5–10 g L−1), the mineral–metal contact time, the chromium initial concentration (10–500 mg L−1) and the system equilibrium time with respect to chromate adsorption. FTIR, SEM-EDX, XRD and electrokinetic zeta measurements were conducted for the natural and modified minerals which confirmed the modification of the minerals. The optimum pH for Cr(VI) adsorption was 4, in all the examined cases. The maximum adsorption capacity followed the order vermiculite (27 mg g−1) > bentonite (24 mg g−1) > attapulgite (15 mg g−1) > zeolite (13 mg g−1). Freundlich was the isotherm that provided the best fit to the experimental data when attapulgite was employed, while Langmuir was the most suitable one when the other three minerals were employed. Kinetics examination revealed a rapid adsorption of the metal during the initial stages. The pseudo-second-order equation fitted well to the experimental data.

► HDTMA-Br modified minerals can effectively remove Cr(VI) from aqueous solutions.
► The modification process conditions impact on the subsequent adsorption efficiency.
► Langmuir was the best isotherm equation for all minerals except attapulgite.
► Zeta potential values were used to determine the adsorption efficiency of minerals.