Effect of Exhausted Coffee Ground Particle Size on Metal Ion Adsorption Rates and Capacities

Spent coffee grounds (SCGs) are common waste products that can be used as low-cost adsorbents to remove contaminants from water. SCGs come in a range of particle sizes based on how they were ground to brew coffee. However, few studies have investigated how SCG particle size influences the adsorption rate and capacities of metal ions. In this study, SCGs were washed under alkaline conditions, creating exhausted coffee grounds (ECGs). ECGs were sieved into four particle size ranges (106-300, 300-500, 500-710, and 710-1000 μm). Monocomponent batch adsorption experiments were conducted with each size fraction using 0.3 mM Pb²⁺, Cu²⁺, Zn²⁺, and Ni²⁺ at pH 5.5 to examine the effect of particle size on the adsorption rates and capacities. The initial adsorption rates for all the four metal ions were 8-12 times higher for the smallest ECGs compared to the largest ECGs. Slower initial adsorption rates with increasing particle size were due to intraparticle diffusion of metal ions into the porous structure of ECGs. However, the equilibrium adsorption capacities for each metal ion and the surface acidic group concentrations were similar across the range of particle sizes studied, suggesting that grinding ECGs does not substantially change the number of adsorption sites. The equilibrium adsorption capacities for Cu²⁺ and Pb²⁺ were 0.18 and 0.17 mmol g^-1, respectively. Zn²⁺ and Ni²⁺ had lower adsorption capacities of 0.12 and 0.10 mmol g^-1, respectively. The time needed to reach equilibrium ranged from less than 2 h for Zn²⁺ and Ni²⁺ adsorption onto the smallest ECGs to several hours for Pb²⁺ or Cu²⁺ adsorption onto the largest ECGs. Future adsorption studies should consider the effect of ECG particle size on reported adsorption capacities, particularly for shorter experiments that have not yet reached equilibrium.