Enhanced adsorption of synthetic anionic dyes toward residual coffee ground biochars via incorporation of Si-rich red mud: A mechanistic study

This study investigated the effects of red mud (RM) pre-treatment on the adsorption performance of residual coffee grounds biochar (RCGB) toward anionic azo dyes (Sunset Yellow FCF (SYF) and Methyl Orange (MO)). The results show that the RM-pretreated RCGB (RM@RCGB) removed SYF (Qe = 15.64 mg/g) and MO (Qe = 16.88 mg/g) more effectively than RCGB (Qe of SYF = 0.37 mg/g; Qe of MO = 0.57 mg/g), attributed to a significant improvement in the specific surface area (RCGB: 1.5 m2/g; RM@RCGB: 160.1 m2/g) following the RM pre-treatment as well as the deposition of Si components on the RCGB surface. Through adsorption kinetics, intra-particle diffusion, and isotherm experiments, chemisorption and liquid-film diffusion demonstrated significant involvement in the adsorption process. According to field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) analyses revealed that SYF and MO adsorption on RCGB occurs mainly via pore-filling effects, while adsorption on RM@RCGB is strongly influenced by Si-O ligand interactions. When NaOH functioned as a desorbing reagent, the reusability efficiency of RM@RCGB after four cycles remained above 39.2 % (SYF Qe = 31.4 %, MO Qe = 47.0 %). Therefore, RM pre-treatment shows strong potential to enhance RCGB adsorption efficiency for synthetic anionic dyes.