Does temperature increases or decreases
adsorption at constant conditions?
Understanding what really happens in the adsorption process is very complicated.
The adsorption process is dependent on many environmental conditions, namely, pH, contact time, adsorbent time, and temperature.
Generally, adsorption is an exothermic process, which means that the adsorption capacity decreases with the increases in temperature, However, in many studies, it was found that the adsorption process is endothermic (high temperature is more beneficial for the adsorption of molecules on the surface of adsorbent). This can be explained by the fact that when the temperature increases, an enhancement in the diffusion rate of the molecules adsorbed on the adsorbent surface through the external boundary layer and into the internal pores can occur, leading to high adsorption rate.
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If adsorption is physical (physisorption) it is strictly exothermic. That is, heat is released during adsorption. This heat is at the level of condensation heat for gases. If the adsorption is chemical (chemical adsorption), the event may be endothermic or exothermic, depending on the nature of the process.
As mentioned above, adsorption is exothermic and therefore adsorption decreases with increasing temperature. For a few systems, the entropy of adsorption may be bigger than the enthalpy. Therefore it may happen, that adsorption is increased with increasing temperature. But this is very seldom and from my point of view can only appear with big molecules (e.g. polymeres). Before you go with this theory repeat your experiments multiple times.
When pH, dose, and time are kept constant in adsorption studies, the effect of temperature becomes a crucial parameter to investigate. Here are some key points regarding the effect of temperature on adsorption:
-Adsorption capacity: Generally, an increase in temperature leads to an increase in the adsorption capacity of the adsorbent. This is because higher temperatures provide more kinetic energy to both the adsorbate molecules and the adsorbent surface, facilitating stronger interactions and enhancing the adsorption process.
-Rate of adsorption: Higher temperatures often result in faster adsorption kinetics. The increased thermal energy enhances the diffusion of adsorbate molecules to the surface of the adsorbent, allowing for quicker adsorption.
-Thermodynamic parameters: Temperature influences the thermodynamic parameters of adsorption, such as the equilibrium constant and free energy change. The adsorption equilibrium constant may decrease with increasing temperature, indicating a decrease in adsorption affinity. However, the free energy change of adsorption usually becomes more negative with higher temperatures, indicating that the process is more spontaneous.
-Competitive adsorption: Temperature can also affect the selectivity of adsorption when multiple contaminants are present. Different adsorbates may exhibit varying affinities for the adsorbent at different temperatures, leading to changes in their competitive adsorption behavior.
Adsorption occurs in 2 ways, it's either physical (physisorption) which occurs at lower energy values usually Ea40 kJ/mol. The study on the effect of temperature usually determines the nature of the process whereupon changes in enthalpy, entropy and Gibbs free energy are studied together with the activation energy based on the Arrhenius principle
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