Predicting chromium (VI) adsorption rate in the treatment of liquid-phase oil-based drill cuttings

Abstract

The adsorption rate of chromium (VI) on commercial activated carbon during the treatment of the flocculation effluent of liquid-phase oil-based drill-cuttings has been investigated in terms of contact time and initial chromium (VI) ion concentration. Homogenizing 1 g of the activated carbon with 100 ml of the flocculation effluent of known initial concentrations (1.25 → 6.25 mg/l, in turn) in a flask, at a constant stirring speed of 80 rpm at 25^oC for 180 min, the pseudo-second-order kinetics was observed to be more suitable in predicting the adsorption rate of chromium (VI) ion in the treatment process as experimental data fitted the model relatively better than the pseudo-first-order kinetics with R² = 0.9999. Adsorption took place in two steps: an initial high rate step before reaching a plateau at equilibrium in the low rate step. Equilibrium was attained in a contact time of 60 min and the equilibrium adsorption capacity of the activated carbon was 78.6%. Equilibrium adsorption data fitted the Freundlich isotherm well with R² = 0.981. The intensity of adsorption was 1.32, which indicates a strongly favourable adsorption. This showed that a large proportion of the chromium (VI) ion was adsorbed at low concentration of the adsorbate in solution.

Key words: Carbon adsorption, kinetic models, drill cuttings, chromium removal.