Thermodynamic Properties of Heavy Metals Ions Adsorption by Green Adsorbents

Abstract

There is growing interest in the use of cheap organic materials to clean heavy metal pollution by adsorption. This chapter presents the thermodynamic parameters of the adsorption of heavy metals ions adsorption by green adsorbents: the Gibbs free energy, entropy, and enthalpy. Research indicates that the temperature of the adsorption medium is the most important parameter influencing thermodynamic analysis. As a result, the adsorption of heavy metals ions by green adsorbents is spontaneous in most cases, with ΔG° lower than 0. Some researchers found that the uptake of heavy metals ions increased with temperature, an endothermic process. A more enhanced level of uptake in parallel with a temperature rise resembles the nature of a chemisorption mechanism, with ΔH° higher than 0. In contrast, other authors obtained the opposite trends, with ΔH° lower than 0, low temperature caused a high adsorption (exothermic process), and the mechanism was mainly physical adsorption. Some authors reported positive ΔS° values for adsorption of heavy metals ions by green adsorbents, suggesting the affinity of metals ions for adsorbents used. On the other hand, negative ΔS° values were also reported by diverse authors, indicating a decrease in the randomness at the solid/solution interface during the adsorption process. No conclusion should be drawn based on corresponding values of thermodynamic parameters. Since the thermodynamic parameters were evaluated from very different adsorbent/adsorbate combinations, it was not possible to note a correlation between the corresponding enthalpy change and entropy change following adsorption.