Enhanced adsorption of malachite green on hydroxyl functionalized coal: Behaviors and mechanisms

Abstract

The discharge of untreated colored wastewater poses a severe threat to global health. This work reports a simple yet highly efficient way to modify Zhundong coal (MZD) with hydroxyl functional groups for malachite green (MG) removal. The modification significantly improved the adsorption capacity of MG from 63.0 mg/g to 538.7 mg/g. According to Fourier transform infrared spectroscopy, zeta-potential analyzer, X-ray photoelectron spectroscopy, and N2 absorption characterization, the improvement in adsorption capacity was mainly attributed to the enhancement of hydroxyl functional groups on the surface of coal. The adsorption of MG on MZD depended on pH, and the highest removal efficiency reached 99.7% at pH 10 with an initial MG concentration of 500 mg/L. The adsorption process was mainly controlled by chemisorption, and it was endothermic and spontaneous according to thermodynamic analysis. The adsorption data could be well reproduced by the pseudo-second order kinetic model and the Redlich-Peterson isotherm model. The MZD shows good regeneration performance with only a 16.9% reduction of initial adsorption capacity after five circles. Hydrophobic interactions, π-π interactions, and pore-filling contributed to adsorption, whereas electrostatic interactions played a dominant role in the MG removal by MZD. This work provides a new sight to prepare useful and cheap adsorbents from coal to remove contaminants from printing and dyeing wastewater.