Dear Thangavel,

This is most probably due to the internal pore strain. Both the specific surface area and pore volume decreased due to incorporation of something (?) from your effluent into the large size (3.7 nm) pores of your material, this incorporation leads to the internal pore strain thus increasing the pore diameter.

Regards

https://www.researchgate.net/post/What_is_the_relationship_between_surface_area_BET_and_average_pore_diameter_4V_A_by_BET_of_the_nanoparticles

few answers may explain

Pls download the papers through the link by Ahmad

This is completely normal and meaningful (including your data). Smaller means more surface area, be it pores or particles. Pore volume also decreases with larger pores (hence smaller number of pores).