Kinetic and thermodynamics of chromium ions adsorption onto low-cost dolomite adsorbent

The chromium bearing wastewater in this study was used to simulate the low concentration discharge from a major aerospace manufacturing facility in the UK. Removal of chromium ions from aqueous solutions using raw dolomite was achieved using batch adsorption experiments. The effect of; initial Cr(VI) concentration, amount of adsorbent, solution temperature, dolomite particle size and shaking speed was studied. Maximum chromium removal was found at pH 2.0. A kinetic study yielded an optimum equilibrium time of 96 h with an adsorbent dose of 1 g/L. Sorption studies were conducted over a concentration range of 5–50 mg/L. Cr(VI) removal decreased with an increase in temperature (qmax: 20 °C = 10.01 mg/g; 30 °C = 8.385 mg/g; 40 °C = 6.654 mg/g; and 60 °C = 5.669 mg/g). Results suggest that the equilibrium adsorption was described by the Freundlich model. The kinetic processes of Cr(VI) adsorption onto dolomite were described in order to provide a more clear interpretation of the adsorption rate and uptake mechanism. The overall kinetic data was acceptably explained by a pseudo first-order rate model. Evaluated ΔGo and ΔHo specify the spontaneous and exothermic nature of the reaction. The adsorption takes place with a decrease in entropy (ΔSo is negative).

► This study assesses the feasibility of chromium uptake onto dolomite.
► Evaluated ΔGo and ΔHo specify that the reaction is spontaneous and exothermic.
► External mass transfer was the initial controlling process in the adsorption system.
► Freundlich isotherm fitted the experimental data for all solution temperatures.