Adsorption of Methylene Blue on Activated Charcoal

To determine the correct statement regarding the adsorption of methylene blue from an aqueous solution onto activated charcoal at $25^\circ\text{C}$, we consider the thermodynamic and physical characteristics of the adsorption process:

1. Thermodynamic Nature of Adsorption:

   • Adsorption is a spontaneous surface phenomenon. The change in Gibbs free energy for any spontaneous process at constant temperature and pressure is negative: $\Delta G < 0$
   • When adsorbate molecules (methylene blue) are transferred from the solution and restricted to the surface of the adsorbent (activated charcoal), their degrees of freedom decrease, resulting in a decrease in entropy: $\Delta S < 0$
   • From the relation $\Delta G = \Delta H - T\Delta S$, for $\Delta G$ to be negative when $\Delta S$ is negative, the enthalpy change must also be negative: $\Delta H < 0$
   • A negative enthalpy change ($\Delta H < 0$) indicates that the process is exothermic, meaning it is accompanied by a release of heat and a decrease in enthalpy. Therefore, statement (B) is correct.

2. Analysis of the Other Options:

   • Option (A): The adsorption of methylene blue onto activated charcoal is primarily physical adsorption (physisorption), which involves weak van der Waals forces and does not require appreciable activation energy. Hence, it does not require activation at $25^\circ\text{C}$.
   • Option (C): According to Le Chatelier's principle, since the adsorption process is exothermic ($\Delta H < 0$), the extent of adsorption decreases with an increase in temperature.
   • Option (D): Physisorption is a reversible process; adsorbed methylene blue can be desorbed under appropriate conditions (e.g., increasing temperature or changing the solvent medium).

Thus, the only correct statement is that the adsorption process is accompanied by a decrease in enthalpy.

Correct Option: B