Treatment of three polyaromatic hydrocarbons (PAHs), naphthalene, phenanthrene and pyrene, contained on field-contaminated natural sediments were studied. At low solid to liquid ratios, degradation of PAHs appears to be dependent on desorption rates. However, at higher ratio, rate of degradation is kinetic limited. This is unlike other treatment systems, which are often mass transfer limited.

Next, the effect of sonication on the distribution of PAHs in the bioaccessible and less bioaccessible fractions of three field-contaminated sediments was examined. The bioaccessible fraction of phenanthrene and pyrene was found to increase or remain unchanged after 20 min of sonication, while there was a significant decrease in the less bioaccessible fractions. This result suggests that ultrasonic irradiation of sediment may release the PAHs from the less bioaccessible sites to the bioaccessible sites for treatment or effect of ultrasound may preferentially target the less bioacesssible site. Although PAHs sorbed on the less bioaccessible sites are considered to be less available for uptake by biota, they are potential sources for PAH release and are difficult to treat due to limited desorption rates. Thus sonication may be a possible technology for release of PAHs from less bioaccessible sites for a more complete remediation of natural sediments.

Lastly, activated carbon was used as a media to adsorb the released PAHs. This was done to exploit the enhanced desorption resulting from sonication. Contaminants adsorbed on carbon have been shown in previous work to have lower bioavailability compared to contaminants sorbed on natural sediment. From our data, it was shown that with 40 min sonication in the presence of granular activated carbon (GAC), the bioavailability of the sediment was reduced by 58% and 33% for phenanthrene and pyrene, respectively.

Experiments were conducted to investigate the effect of sonochemistry coupled with powedered activated carbon (PAC) on the sequestration of PAHs in contaminated sediments obtained from Little Scioto, OH; Gary, Indiana; and Eagle Harbor, Washington. Without carbon amendments, the availability of the PAHs increased upon sonication of the sediments. Ultrasound coupled with PAC was shown to reduce the availability of PAHs to a greater extent than mixing coupled with PAC. In an ultrasound system, microjets and shockwaves from cavitation bubbles allow for faster release of PAHs for readsorption on PACs compared to a system using mixing at 300 rpm. However, at longer sonication times (1 hr-3 hr), the reduction in PAH availability with time was lower but trended similarly to mixing. This result suggests that after 1 hr, the bulk of the available PAHs exist in the micropores of the sediment, which are inaccessible to the cavitation bubbles. In addition, at 60 mins, the mean particle size of the sediment was less than 10 mm. Particles, this small, do not aid in the production of microjets or shockwaves that can break up the particles. Hence the switching of PAHs from sediments occurred when PAHs desorb as a result of fluid flow on the surface and within the micropores of the sediment. This fluid flow exists in both ultrasound and mixing systems. Eagle Harbor sediment, which has the largest mean particle diameters (560 mm), appeared to show the greatest reduction in PAHs availability with sonication, as compared to mixing, followed by Gary, Indiana (306 mm) and Little Scioto River (74.5 mm) sediments. This trend seems to suggest that sediment with larger initial particle size will benefit greatest when ultrasound is used. This trend may also be explained by aging of the PAHs on the three sediments or the types of natural organic matter (NOM) present. However, information related to aging or NOM composition was not available.