Removal of Cr (VI) from aqueous solution using ultrafine coal fly ash

Coal fly ashes WSRA and BQRA were ball milled for 5 h to produce their ultrafine coal fly ashes WSUA and BQUA, respectively. Batch kinetic, isotherm and pH effect on adsorption were studied to evaluate removal of Cr (VI) from aqueous solutions by ultrafine coal fly ashes comparing with raw coal fly ashes. The kinetics of adsorption indicates the process to be intraparticle diffusion controlled and follows the Lagergren first-order kinetics for all coal fly ashes. The first-order rate constants (k ₁) of Cr (VI) adsorption onto WSRA, WSUA, BQRA and BQUA are 1.981, 1.497, 2.119 and 1.500 (×10⁻²) min⁻¹, respectively. The adsorption capacities of WSUA and BQUA are much better than those of WSRA and BQRA. Equilibrium adsorption data of all coal fly ashes well satisfy the Langmuir isotherm. The adsorbed amounts of Cr (VI) onto WSUA and BQUA decrease from pH 2 to pH 6 and then increase up to pH 12.