Adsorption of heavy metals from aqueous solutions by waste coffee residues: kinetics, equilibrium, and thermodynamics

Abstract

Waste coffee residues (WCRs) are used to remove heavy metals (Cu²⁺, Pb²⁺, and Zn²⁺) from aqueous solutions. The surface characteristics of WCRs were analyzed by scanning electron microscopy, specific surface area analysis, zeta-potential analysis, and Fourier-transform infrared spectroscopy. The specific surface area, mean pore diameter, and pH_pzc of the WCRs were 0.19 m²/g, 14 nm, and 3.5, respectively. Adsorption experiments are performed to evaluate the kinetics, equilibrium, and thermodynamic parameters. The percentage of Cu²⁺ that was removed from a solution increased as the pH and the WCR dose increased, but it declined as the Cu²⁺ concentration increased. The adsorption kinetics and equilibrium fit pseudo-second-order and Langmuir models, respectively. At pH 5 and 25°C, the maximum adsorption capacities of the WCRs for Cu²⁺, Pb²⁺, and Zn²⁺ were 8.2, 27.6, and 8.0 mg/g, respectively. The ΔH⁰ values for the adsorption of Cu²⁺, Pb²⁺, and Zn²⁺ on WCRs were 9.73, 13.77, and 9.15 kJ/mol, respectively, and the corresponding ΔS⁰ values were 109.4, 127.0, and 96.6 J/mol/K. The adsorption of Cu²⁺, Pb²⁺, and Zn²⁺ on WCRs is an endothermic, spontaneous physisorption process, and the adsorption kinetics is controlled mainly by surface diffusion.

Keywords:

• Adsorption
• Coffee residues
• Heavy metals
• Kinetics
• Equilibrium
• Thermodynamics

Acknowledgment

The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 101-2221-E-151-038-MY3. Mr Chang of Da-Yeh University is appreciated for performing some of the experiments.