Preparation and characterization of β-FeOOH-coated sand and its adsorption of Cr(VI) from aqueous solutions

Batch adsorption experiments were conducted to explore the adsorption of Cr(VI) in aqueous solutions by beta-FeOOH-coated sand. We investigated the key factors which affected the adsorption process such as adsorbent dosage, initial pH, initial Cr(VI) ion concentration, contact time and temperature. The uptake of Cr(VI) was very rapid and 44.3%, 51.6%, 58.9% of the adsorption happened during the first 180 minutes at 293K, 303K and 313K, respectively. The pseudo-second-order rate equation successfully described the adsorption kinetics. To study the adsorption isotherm, two equilibrium models, the Langmuir and Freundlich isotherms, were adopted. At 293K, 303K and 313K, the adsorption capacities obtained from the Langmuir isotherm were 0.060, 0.070 and 0.076 mg Cr(VI) per gram of the adsorbent, respectively. Thermodynamic parameters such as the change of energy, enthalpy and entropy were calculated using the equilibrium constants. The negative value of Delta G (0) and the positive value of Delta H (0) showed that the adsorption of Cr(VI) in aqueous solutions by beta-FeOOH-coated sand was spontaneous, endothermic and occurred by physisorption.