why the extent of adsorption decreases with an increase in temperature in adsorption in solution phase

Dear student!

Normally, in the process of adsorption, there is a decrease in surface energy which appears as heat and so adsorption is considered as invariably an exothermic process. In other words, ΔH of adsorption is always  negative. 

So, adsorption is accompanied by evolution of heat, so according to the Le-Chatelier’s principle, the magnitude of adsorption should decrease with rise in temperature to maintain the balance between the system and surroundings.