Bioavailability of Naphthalene Sorbed to Cationic Surfactant-Modified Smectite Clay

The bioavailability of naphthalene sorbed to hexadecyltrimethylammonium (HDTMA)-modified smectite clay was evaluated by modeling naphthalene mineralization kinetics in dilute clay slurries and in clay-free controls. Sorbed naphthalene was directly available to Pseudomonas putida strain 17484, as evidenced by initial rates and extents of naphthalene mineralization that significantly exceeded predicted values assuming sorbed naphthalene was unavailable. For the soil isolate, Alcaligenes sp. strain NP-Alk, sorbed naphthalene was unavailable, and measured rates agreed closely with predicted rates. For this bacterium, sorbed naphthalene was available only upon its desorption from the HDTMA-modified smectite. This desorption was very rapid from unaggregated HDTMA-smectites and from HDTMA-clay aggregates of less than 0.25-mm diameter. Naphthalene mineralization in the presence of larger clay aggregates (0.25-1-mm diameter) was desorption rate limited. Contaminants sorbed to HDTMA-modified soils or clays should be largely bioavailable to bacteria, since the desorption rates from these materials are high and some degradative bacteria have the ability to directly utilize the sorbed contaminants.