The role of pore size distribution in competitive adsorption on activated carbon

The objective of this study was to develop a conceptual model of how molecules of different size compete in different size pores, so as to gain insight into the mechanisms by which natural organic matter (NOM) molecules reduce the adsorption capacity and adsorption rate of organic micropollutants in natural water.; Rather than using conventional activated carbons, a series of phenolic resin-based activated carbon fiber (ACF) adsorbents with different micropore size distributions were used. The pore shape and pore structure of these materials is well-characterized. At low to moderate degrees of activation, ACFs have a telescopic pore structure, with branched pore tree structure development at high levels of activation. In order to eliminate the size heterogeneity associated with NOM, various dye molecules of different size and shape were used. The target micropollutant was the herbicide atrazine. Batch adsorption isotherms and kinetic tests included single solute, simultaneous adsorption, dye preloading and atrazine preloading.; When the competing adsorbate was similar or smaller in size than atrazine, direct competition for adsorption sites in accessible micropores was observed. Strong adsorption was observed in primary micropores (< 8 Å) due to overlapping pore wall potentials. Subsequent displacement or desorption was found to be a very slow process. This hysteresis behavior was not observed in carbons containing larger micropores. Significantly increasing the size of the competing adsorbate resulted in the following transition in competition mechanism with increasing pore size: surface pore blockage, pore constriction without atrazine capacity reduction, and direct competition for adsorption sites. By manipulating the molecular structure of the competing adsorbate, evidence for pore mouth blockage was also found, highlighting the importance of molecular structure on competition. Shifting from a telescopic pore structure to a pore-tree structure with small micropores branching off from large micropores and mesopores showed that micropore constriction by large competing adsorbates was important at high loadings.; Based on these competitive adsorption mechanisms it was proposed that in order to minimize the impact of NOM on micropollutant adsorption capacity and adsorption kinetics, the adsorbent must have a wide range of micropore sizes, including pores of similar size to the target compound.