Trichloroethylene adsorption by activated carbon preloaded with humic substances: effects of solution chemistry

Trichloroethylene (TCE) adsorption by activated carbon previously loaded ("preloaded") with humic substances was found to decrease with increasing concentrations of monovalent ions (NaCl), calcium (until solubility was exceeded), or dissolved oxygen in the preloading solution. For a given percentage of organic carbon removal during humic acid loading, greater reductions in TCE adsorption occurred with increasing monovalent ion concentration and calcium concentration at constant ionic strength. However, this effect was related primarily to the amount of humic material adsorbed--the reduction in TCE adsorption was independent of the ionic composition of the preloading solution when compared at similar humic acid loading. Experiments were performed which showed that calcium ions can associate with humic material after the humic has been adsorbed, which subsequently reduces TCE uptake, but this effect does not dominate when calcium is present during humic loading. At sufficiently high calcium concentrations (approaching solubility), aggregation or co-precipitation of humic acid mitigated the effects of preloading. In contrast to the effects of ionic composition, the presence of dissolved oxygen did fundamentally change the mechanism by which organic macromolecules compete with TCE. TCE uptake was lower when preloading by poly(maleic acid) (PMA) occurred in the presence of dissolved oxygen, even when the amount loaded was the same. One explanation invokes a coupling mechanism promoted by the carbon surface, which results in either additional blockage of TCE sorption sites, additional site competition, or both. In all experiments, the effects of preloading were consistent with those reported previously, which have been interpreted as a loss of high-energy sites available to TCE, causing a significant reduction in the site-energy heterogeneity, and reduced uptake in the low concentration region.