When the first adsorption test was carried out, the removal value was high, but why after 8 months of adsorption testing again, the removal value obtained was very different?
a) You are using the same sample -> something in it aged, e.g. by oxidation or hydrolysis
b) You reprepared by the same method -> the preparation is not reproducible, e.g. because of supplier variation, temperature control or lab humidity management
What are you testing and do you the stability profile of it?
With age, sorbents change and it's not uncommon, that's why many list expiration dates. This may be due to many factors such as sorption of the gas in which it is stored or impurities in the gas in contact with the sorbent, or microscopic rearrangement of the molecules within the sorbent that alter its characteristics.