| Powdered activated carbon (PAC) is an adsorbent widely used in drinking water treatment for organic micropollutant removal. Nonetheless, the presence of natural organic matter (NOM) in source water can reduce its efficiency for micropollutant removal. It has been shown that NOM inhibit micropollutant by two major mechanisms: direct competition for available adsorption sites which ultimately reduce adsorption capacity, and blocking of pore entrance which reduces micropollutant adsorption rate. The objective of this study was to elucidate the effect of two factors on competitive adsorption, which included background compound molecular size and adsorbent pore size distribution (PSD). Atrazine, an herbicide of environmental concern, was chosen as the target contaminant. To evaluate the effect of background compound molecular size, five synthetic compounds, Methyl Orange (MO, 304.3 Da), Brilliant Yellow (BY, 578.6 Da), Congo Red (CR, 650.7 Da), Xylenol Orange (XO, 668.7 Da) and Evans Blue (EB, 868.9 Da), were used as NOM surrogates, where their effects on atrazine (215.7 Da) adsorption equilibrium and kinetics on two PAC were evaluated. MO, the smallest surrogate, caused the highest reduction in atrazine adsorption but without producing pore constriction which affected adsorption kinetic. CR and BY, with intermediate MW and elongated molecular structures, caused reductions in both atrazine adsorption capacity and kinetics, decreasing the atrazine diffusion rate by about two logarithmic orders of magnitude at the highest surrogate loadings investigated. In contrast, XO, a surrogate with similar MW to those of CR and BY but with a more compact molecular structure (higher molecular width and thickness), was less effective in reducing atrazine adsorption capacity or inducing pore constriction, even though its adsorption capacity was higher. Our result indicated that in additional to molecular size, molecular dimensions also played a role in the extent of competitive adsorption.;For the effect of PSD, four commercially available PAC produced by Norit Netherlands, which included W20, W35, SA Super and SA UF, as well as one proprietary adsorbent which consisted of extensive secondary micropore and mesopore volumes, labeled as Pellet II, were evaluated for atrazine adsorption in the presence of CR in order to assess the role of PSD on competitive adsorption. The capacity for CR adsorption correlated well with pore volume between 13 and 350 A. Atrazine adsorption capacity reduction caused by CR was similar for the four Norit PAC, but was significantly higher for Pellet II. Kinetic experiments showed that increasing secondary micropore and mesopore volumes could alleviate pore blockage effect, that is, an adsorbent could adsorb more CR before the decrease in atrazine Ds become significant. At the highest CR surface loading achieved, atrazine Ds decreased by more than two logarithmic orders of magnitude for the Norit PAC, but only one logarithmic order of magnitude for Pellet II. Overall, our result suggested that increasing the secondary micropore and mesopore to a certain extent could shift the mechanism of competition from pore blockage to direct competition. |
