Environ. Sci. Techno/. 1995,29, 2953-2958
Bioavailability of Naphthalene Sorbed to Cationic Surfactant-Modified Smectite FIONA H. CROCKER,+ WILLIAM F. GUERIN, AND STEPHEN A . B O Y D * Department of Crop a n d Soil Science, Michigan State University, East Lansing, Michigan 48824-1325
The bioavailability of naphthalene sorbed to hexadecyltrimethylammonium (HDTMA)-modified smectite clay was evaluated by modeling naphthalene mineralization kinetics in dilute clay slurries and in clayfree 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-AIk, 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 (0.25I - m m 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 abilityto directly utilize the sorbed contaminants.
Introduction Enhanced immobilizationof common organic groundwater contaminants in soils and subsoils can be achieved by modifying these materials with cationic surfactants (1-3). Quaternary ammonium cations of the form [(CH&NRl+, where R is a large (>Clo)alkyl hydrocarbon, readily replace native inorganic cations on the exchange sites of soil clays ( 4 ) ,resulting in the formation of effective sorptive phases for nonionic organic contaminants (NOCs). In such surfactant-modified subsoils, the soil-water distribution coefficients for NOCs, such as ethylbenzene (2) and * Corresponding author telephone: (517)353-3993;fax: (517) 3535174. + Present address: Department of Biological Science, Florida State University, Taliahassee, FL 32306.
0013-936X/95/0929-2953$09.0010
1995 American Chemical Societv
tetrachloroethylene (1,2),increased by more than 100times over those observed in native B horizon soils. These results suggest that aquifer materials or subsoils could be modified in situviainjections of cationic surfactants to create sorptive zones that could intercept and immobilize advancing contaminant plumes. The feasibility of this approach has been demonstrated in recent experiments by Burris and Antworth (5). Although this technology is potentially very useful for managing contaminant plumes by minimizing further contamination of the aquifer materials and reducing downgradientcontaminant concentrations in groundwater, it does not permanently remove contaminants from the environment. Coupling contaminant immobilizationwith in situ biodegradation would provide a comprehensive soil restoration technology to effectively eliminate target contaminants (3, 6). The bioavailability of NOCs sorbed to surfactant-modified soils and clays is a critical aspect of this proposed technology. The influence of sorption on the biodegradation of organic contaminants has been recognized as an important, albeit, poorly understood, issue in bioremediation (7- 19). Factors such as the compound’s chemical structure, the nature of the sorbent, the residence time of the sorbed compound, and the desorption rate may influence the biodegradation of sorbed compounds. The fractions of 2,4-dichlorophenoxy acetic acid (2,4-D) (8) and polyaromatic hydrocarbons (PAHs) (9) sorbed to soil, and diquat (10) sorbed to clay, were completely unavailable for degradation. In contrast, toluene (111, PAHs (91, polychlorinated dibenzo-p-dioxins (PCDDs) (121,naphthalene (13), 2,4-D (141, and phenol (15) sorbed to soils and sediments, and benzylamine (16),and phenol (17) sorbed to clays were available for biodegradation, probably following desorption into the aqueous phase. Differences in the organic carbon (OC) contents between two soils and the duration of soil-PAH contact were postulated to explain why PAHs were degraded in one soil (1%OC) and not another soil (13.6% OC) (9). The bioavailability of sorbed compounds may also be affected by the microorganisms themselves. Guerin and Boyd (18,191 recently used a kinetic method to show that the ability to directly utilize soil-sorbed naphthalene is a species-specificcharacteristic. Pseudomonasputidu strain 17484was able to directly access labile sorbed naphthalene and to promote the desorption of nonlabile naphthalene from the interior of soil particles. In contrast to strain 17484, Alcaligenes sp. strain NP-Alk utilized only aqueous phase naphthalene, and most of the soil-sorbed fraction remained unavailable. In this study we examine the bioavailability of naphthalene sorbed to a model sorbent, hexadecyltrimethylammonium (HDTMA)-modifiedsmectite, using the same two organisms. Our aim was to evaluate the coupled immobilization- biodegradation technologyproposed and to gain further insights about the sorption-desorption behavior of NOCs with the modified smectite.
Materials and Methods Preparation of HDTMA Modified-Smectite. Smectite (Wyoming bentonite) with a cation-exchange capacity (CEC) of 90 mequiv (100 g)-l was obtained from the American Colloid Co. (ArlingtonHeights, IL). The smectite was dispersed in distilled water (10 g L-I), and the clay size fraction was separated by gravity sedimentation. VOL. 29, NO. 12. 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1
Properties of 50% HDTMA-Smectite Preparations sample
%oca
Kpb
freeze-dried ove n-dried
