Ball-Milled Spent Coffee Ground Biochar Effectively Removes Caffeine from Water

Caffeine in aquatic ecosystems is an emerging contaminant causing significant environmental concern. In this work, spent coffee ground (SCG) was pyrolyzed at 300, 450, and 600 °C to produce pristine SCG biochars (CG), which were then ball-milled to produce ball-milled SCG biochars (BMCG). A batch experiment with ball-milled and pristine biochars showed that ball-milled biochars pyrolyzed at 450 °C and 600 °C had the highest capacities to adsorb caffeine. Subsequently, ball-milled CG450 (BMCG450) was selected for further analysis. The results showed that ball milling dramatically augmented the specific surface area and oxygen-containing functional groups of the biochar. The Langmuir maximum caffeine adsorption capacity was 82.65 mg/g. Both solution pH and ionic strength affected caffeine removal by BMCG450. As pH increased, increased electrostatic repulsion limited caffeine adsorption onto the biochar. However, an increase in ion strength slightly enhanced caffeine adsorption because of the electrostatic screening effect of cations. The ball-milled SCG biochar also showed high adsorption efficiency in a completely mixed flow reactor under continuous flow conditions. Our study indicates that ball-milled SCG biochar at 450 °C can serve as a viable sorbent for the removal of caffeine from water.