Competitive adsorption in natural water: role of activated carbon pore size

The impact of pore size on the competition mechanism between natural organic matter (NOM) in Illinois groundwater and the micropollutant atrazine was assessed using activated carbon fibers (ACFs). Two microporous ACFs with narrow and broad pore size distributions, designated ACF-10 and ACF-25, respectively, were used. The average pore sizes of ACF-10 and ACF-25 were 6 and 13.4 Å. Single solute adsorption, simultaneous adsorption and preloading experiments were performed. On ACF-10 it was found that the adsorption of atrazine was reduced significantly in the presence of NOM, even though the NOM loading was very small as a result of pore exclusion. The uptake of atrazine by ACF-10 in the presence of NOM (simultaneous adsorption) was comparable to the NOM-preloaded capacity. In addition, preloaded atrazine was not displaced by subsequently adsorbed NOM. The results support a pore blockage mechanism by which NOM molecules block access to, but do not penetrate into the primary micropores. Atrazine capacity on ACF-25 which has primary micropores as well as a large volume of secondary micropores, was reduced in the presence of NOM; however, the reduction in capacity was much less than that observed with ACF-10. Preloading with NOM showed further capacity reduction compared with simultaneous adsorption. These results combined with the result that preloaded atrazine exposed to NOM showed displacement of atrazine support a direct site competition mechanism in the secondary micropore region. Attempts to regenerate NOM preloaded ACF-10 and ACF-25 using a strong alkali solution failed to recover atrazine capacity, suggesting that NOM was strongly adsorbed at the fiber surface as well as within micropores.