The authors observed that humic acid (HA) mediates transport of tetracycline and pyrene in saturated porous media via distinctively different mechanisms. The presence of HA (20–80 mg C/L) in the influent consistently enhances the transport of tetracycline, whereas for pyrene a critical HA concentration exists (about 10 mg C/L), below which transport is inhibited but above which transport is enhanced. The difference in the HA effect stems from the difference in relative sorption affinity to HA and sand between these two compounds. Because sorption of pyrene is driven primarily by hydrophobic effect, pyrene exhibits much stronger sorption to HA than on sand. Accordingly, pyrene in the influent (or mobile phase) is predominantly associated with HA, and its transport is controlled by the partition of HA between mobile phase and sand. For the polar, ionic, and highly hydrophilic tetracycline, sorption is driven mainly by surface complexation and ligand exchange, so tetracycline exhibits relatively strong adsorption on sand, but has much weaker sorption to HA than pyrene does. For tetracycline, the effect of HA on transport is likely the competition of HA for the available adsorption sites on sand. In addition, tetracycline and pyrene exhibit markedly different breakthrough profiles, both in the presence and in the absence of HA; this can be attributed to the greater degree of adsorption nonequilibrium of tetracycline on sand. Environ. Toxicol. Chem. 2012;31:534–541. © 2011 SETAC
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