Ultrasound (US) was studied as an alternative method to remediate contaminated sediments. Different ultrasonic systems were designed and characterized to fundamentally understand ultrasonic effects in porous sediment and to strategically improve the ultrasonic remediation process. These include a flow-through packed-media column coupled with US, the combination of US and persulfate (PS), and a scaled-up design of a multi-stepped horn.
First, 20 kHz US waves propagating in a flow-through column packed with porous media were characterized. Sound penetration and location of cavitation were measured by a hydrophone; enhanced pore flow velocity and faster bromide tracer breakthroughs were observed. In particular, US pressure decreased exponentially with distance from the US source due to wave absorption and scattering by porous media resulting in localized cavitation close to the horn tip. A Darcy model incorporating ultrasonic effects (i.e., acoustic pressure and cavitational heating) revealed that acoustic pressure increased flow velocity at the beginning of sonication while reduced water viscosity due to cavitational heating was accounted for the enhanced pore flow in long-time sonication. Bromide breakthrough tests verified the US accelerated solute transport, as well as the dispersion of tracer in the porous media. Findings of this column study suggest that US may improve remediation through enhancing fluid flow and mass transfer in porous sediments.
To improve contaminant degradation in sediments, sonication combined with PS was examined. PS is an in-situ chemical oxidation (ISCO) method. An ultrasonic reactor coupled with an electron paramagnetic resonance (EPR) spectrometer through a flow-cell was used to gain insight into the mechanisms of ultrasonic activation of PS. The high hydroxyl radical (•OH) yield in the US-PS system was attributed to the rapid reaction between sulfate radical anion (SO4•‒) and water molecules at the bubble-water interface. Likewise, the high dissociation rate of PS, estimated from steady-state approximation, was expected at the high temperature interface. Modeling of temperature and reactivity distribution surrounding a single cavitation bubble indicated a much larger interfacial region as compared to previous results. Addition of tert-Butyl alcohol and nitrobenzene to the US-PS system verified the location of PS dissociation at the interface and elucidated the •OH activation of PS to SO4•‒. The mechanisms unveiled provide mechanistic support to implement US-PS system for sediment remediation.
In addition, a novel ultrasonic horn with a multi-stepped configuration and a cone-shaped tip was designed to enable scaled-up testing of sonication. Hydrophone and sonochemiluminescence experiments showed and verified multiple cavitation zones around the horn neck and tip. Calorimetry and dosimetry results demonstrated higher energy efficiency (31.3%) and faster hydroxyl radical formation rate (0.36 μM min-1) for the new horn, which led to faster degradation of aqueous phenanthrene, a model contaminant. Characterization of the designed horn using COMSOL modeling and acoustic pressure maps in a large water tank exhibited a much improved treatment capacity (~ 5 L) as compared to typical horn systems. The scale-up efforts allows a potential of large-scale performance with the designed horn for remediation of contaminated sediments.