Removal of Phenolics From Olive Mill Wastewater Using Activated Spent Coffee Grounds: Toward an Understanding of Adsorption Behavior

Abstract This research investigates the adsorption mechanisms of phenolics from olive mill wastewaters (OMW) onto physically (thermal treatment at 100, 150, 200, and 250 °C) and chemically (treatment with methanol or sodium hydroxide, and coating with milk proteins) activated spent coffee grounds (SCG), utilizing high-performance liquid chromatography (HPLC) and Fourier-transform infrared spectroscopy (FT-IR). The study examined two optimal conditions for adsorption (A: temperature ( T ) = 30 °C, pH = 8.0, biosorbent dosage ( r ) = 0.02 g/mL, initial phenolic concentration in OMW ( C 0 ) = 250 mg/L; B: temperature ( T ) = 50 °C, pH = 4.0, biosorbent dosage ( r ) = 0.02 g/mL, initial phenolic concentration in OMW ( C 0 ) = 200 mg/L), employing activated SCG. HPLC was used to quantify significant phenolics – namely hydroxytyrosol, tyrosol, gallic acid, caffeic acid, p-coumaric acid – before and after the adsorption. The results indicated that NaOH-activated SCG significantly improved the adsorption of gallic acid and hydroxytyrosol. FT-IR analysis revealed the formation of new hydroxyl (–OH) groups, which enhanced the chemical interactions facilitating phenolic binding. In contrast, physical activation primarily contributed to an increase in pore size. The adsorption mechanism was found to be affected by the molecular structure, concentration, and competitive dynamics of the phenolics.