Bench-Scale Remediation Studies

Ultrasonic Treatment of Sediment Slurries. Few studies have been conducted concerning the use of ultrasound to remove contaminants from sediment slurries in bench-top laboratory systems. Ultrasound applications on sediment slurries include desorption and degradation of 4-chlorobiphenyl (a model PCB) from synthetic sediments, desorption of mercury from model and field-contaminated sediments, and changes in PAH bioavailability with ultrasound and amendment additions.

Studies have also demonstrated high-power ultrasound for degrading hydrophobic organic contaminants in different slurries, such as riverine sediments and heavy clay soil from industrial sites. Ultrasound has been applied in assisted extractions of heavy metals, organic contaminants, and organic matter using either organic solvents or acid. Sediment slurry studies clearly show that organic and metal contaminants desorb more effectively under the presence of ultrasound than in its absence, thus reducing mass transfer as a rate-limiting step.

Although ultrasound decreases contaminant loading in sediment slurries, mechanistic understanding is sought to improve degradation efficiency. Lu and Weavers validated that ultrasound-enhanced desorption occurs prior to aqueous sonochemical oxidation using 4-chlorobiphenyl and model sediments and by monitoring adsorbed, aqueous, and total compound concentrations over sonication time.

This knowledge has been employed to study ultrasonic enhancement of PAH bioaccessibility in different sediments. As shown in Figure 6, PAHs sorbed and entrapped by minerals, SOM, and black carbon undergo accelerated desorption in the presence of sonication. Owing to various physical ultrasonic effects, contaminants are more accessible for treatment.

Figure 6. Ultrasonically enhanced mass transfer of entrapped contaminants

Ultrasonic Treatment of Soils. Enhanced Oil Recovery. Most lab-scale experiments exploring ultrasonic treatment of porous media have studied enhanced oil recovery (EOR). A variety of mechanisms have been proposed for the optimization of EOR. Bulk temperature increases from ultrasound reduce oil viscosity and surface tension, thus enhancing oil percolation rates. Heat generation is attributed to cavitation, boundary friction due to interfacial energy transfer, and dissipation of acoustic waves. Larger power outputs increase viscosity reductions, and lighter fluids have more viscosity reduction than heavier fluids.

Additionally, capillary forces entrap residual oil, or ganglia, in pore spaces. Sound waves cause mechanical vibrations on pore walls, counteracting the pressure gradient of capillary forces resulting in mobilization of ganglia. Ultrasound has a “nudging" effect on the capillary threshold, effectively “unplugging" the ganglia. This effect is proportional to wave amplitude, and inversely related to frequency. Moreover, ultrasound increases oil droplet size during continuous irradiation. Drop coalescence is ascribed to Bjerknes forces.

The droplet’s location in the wave field dictates if the Bjerknes oscillation phase of the droplets is attractive or repulsive. Increased droplet size forms liquid streams, thus enhancing mobility. Oil percolation in porous media can be physically obstructed by adsorbed films and small particles. Mechanical vibrations break adsorbed oil films on pore walls. Increased media permeability from enhanced pore flow removes smaller particles, such as clays and asphaltene. Therefore, ultrasonic effects on EOR are well studied in porous media and can be utilized for remediation efforts.

Lab-Scale Ultrasonic Treatment in Porous Media. Few investigations have studied ultrasonic applications as a remediation technology in soils. Ultrasonic phenomena observed for EOR have been employed in groundwater remediation studies. By altering acoustic intensity and propagation direction, ultrasound can guide plume motion in contaminated aquifers. Sonication effects on DNAPL, such as TCE, have been studied in media packed column experiments. Disperse DNAPL or residual ganglia are entrapped by capillary forces in porous media. Sound waves mobilize and increase transport of residual DNAPL by an order of magnitude during pump and treat, compared to pump and treat alone.

Not only do acoustic waves enhance pure phase NAPL transport, enhanced dissolution occurs by increased NAPL-water mass transfer. This enhancement has been demonstrated with single- and multicomponent NAPLs. Enhanced mass transfer is attributed to increased pore water velocities due to oscillatory flow. Colloids, too, may necessitate remediation since they may act as sorbents for metals and organic compounds. Sonically enhanced colloid transport occurs by enhanced interstitial velocities and dispersion, similar to NAPL transport.