Abstract
In this work, the residues of green coffee waste were probed for elimination of the toxic anionic dyes Congo red and Eriochrome Black-T taken from the water phase after washing with de-ionised water, and followed by drying at up to 150°C in an air oven for 3 h. In their dried form, the residues of green coffee waste, before and after adsorption of anionic dyes, were characterised by Fourier transformed infrared spectroscopy (FT-IR), scanning electron spectroscopy (SEM), and energy dispersive x-ray analysis (EDX). The FT-IR findings showed that the surface of green coffee residues exhibited sufficient oxygen features (O - H, C - O, C = O, C - O - C). The SEM and EDX analyses showed positive adsorption of both the Congo red and Eriochrome Black-T on the green coffee residues. The adsorption output showed that the percentage of removal of CR and EBT had a substantial effect on the pH solution and the maximum removal of both dyes (CR and EBT) obtained at pH-4 and pH-6 respectively. Pseudo-first-order models for CR and pseudo-second-order models for EBT were better suited to the kinetic tests. The EBT adsorption was well defined by the Freundlich isotherm model, while CR adsorption was best suited to the Langmuir isotherm model for green coffee waste residues. CR and EBT have been found to have maximum adsorption capacities of 20.04 and 60.60 mg/g respectively. The EBT and CR adsorption mechanisms mainly involved electrostatic attraction, chemical reactions, pi-pi interactions and hydrophobic force growth. Compared with other crop residues, the adsorbent showed better removal efficiency of dyes, suggesting possible and beneficial use of green coffee waste residues to effectively decontaminate dye-contaminated wastewater.