Adsorption Isotherm and Removal Efficiency: How Are They Related?

Abstract

Adsorption processes and adsorbent materials are studied in many courses within the fields of chemistry, chemical engineering, environmental science, materials science, and related disciplines. Laboratory experiments are usually run to obtain adsorption isotherms and quantify removal efficiency (or adsorption percentage). When analyzing the results, students have sometimes difficulty understanding the relationships between adsorption extent, adsorption isotherm, and removal efficiency. A higher adsorption extent does not necessarily indicate a higher removal efficiency, which can cause confusion. This paper describes a laboratory experiment that is regularly conducted as part of a PhD course on surface and materials chemistry with a thorough discussion of the results. Four adsorption isotherms of paracetamol on activated carbon are analyzed, with the adsorbent dose and initial concentration of paracetamol systematically varied from one isotherm to another. The experimental data and simple isotherm equations clearly demonstrate that adsorption isotherms are independent of the adsorbent dose and the initialconcentration of the adsorbing substance, while removal efficiency depends on these parameters. A relationship between the adsorption extent and removal efficiency is presented, showing that the normal trend is for an increase in the adsorption extent to be accompanied by a decrease in the removal efficiency, even though this may seem contradictory. Additionally, a simple method toevaluate an adsorbent performance is proposed using removal efficiency vs dose plots or equilibrium solute concentration vs dose plots.