Fe-impregnated walnut shell biochars using iron mine tailing wastes as an efficient adsorbent for removal of synthetic dyes

Abstract

In this study, the adsorption of Amaranth (AR) and Fast Green FCF (FG) by walnut shell biochar (WB) and Fe-impregnated WB with iron mine tailing wastes (MWB) was examined to identify the differences in their adsorption mechanisms. The physico-chemical properties of WB and MWB were analyzed using ultra-high-resolution scanning electron microscopy equipped with energy-disper-sive X-ray spectroscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffractometer. The adsorption of AR and FG using WB and MWB fit the pseudo-second-order kinetic (R2: WB = 0.997–0.999, MWB = 0.994–0.998) and Freundlich isotherms (R2: WB = 0.738–0.927, MWB = 0.983–0.993) better than the pseudo-first-order kinetic (R2: WB = 0.657– 0.969, MWB = 0.982–0.987) and Langmuir isotherms (R2: WB = 0.711–0.806, MWB = 0.965–0.988). This result suggests that the adsorption of AR and FG onto WB and MWB is governed by chemisorption and multilayer adsorption. Based on the differences in the XPS spectra of WB and MWB before and after the adsorption of AR and FG, the adsorption mechanism of AR and FG onto WB and MWB suggests that the hydrophobic interaction and electrostatic force governs the adsorption of AR and FG. Even following three adsorption–desorption cycles with MeOH, MWB continued to exhibit excep-tional reusability (AR = 61.03%, FG = 75.42%). This finding implies that the impregnation of iron materials on the WB surface benefits the adsorption of synthetic dyes in aqueous solutions.