Silver adsorption on biochar produced from spent coffee grounds: validation by kinetic and isothermal modelling

This study investigates silver adsorption on biochar produced from pyrolysis of spent coffee grounds (SCGs). Biochars were produced from SCGs at temperatures between 500 and 1000 degrees C. SCG-derived biochars were then characterised by different analytical methods, such as Brunauer-Emmet-Teller (BET), Fourier transform infrared (FTIR), X-ray diffraction (XRD), and investigated for silver removal. The results revealed that the biochar produced at 500 degrees C offered a maximum surface area of 40.1 m(2)/g with a yield of 23.48% biochar and the highest silver adsorption capacity of 49.0 mg/g with 99.9% silver removal efficiency. The morphology of adsorbed silver on biochar was determined using scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS), and XRD analyses, which showed an even distribution of silver on the biochar surface. Furthermore, X-ray photoelectron spectroscopy (XPS) confirmed that part of the silver ions was reduced to form metallic silver (Ag-0)/silver nanoparticles (Ag NPs) during adsorption. The kinetics and isothermal evaluation suggested that silver adsorption was dominated by the pseudo-second-order model and Langmuir isotherm, which means that silver adsorption was mainly dominated by chemisorption and monolayer on biochar surface. Overall, this study suggests that 500 degrees C was the most feasible pyrolysis temperature to produce SCG-derived biochar with suitable physicochemical properties that can efficiently adsorb silver species from wastewater.