Desorption of four different metals, Pb²⁺, Hg²⁺, Ni²⁺, and Cr³⁺, from a model inorganic sediment, Al₂O₃, has been conducted under similar conditions using sonication and hydrodynamic mixing. For Pb²⁺ and Hg²⁺ with very fast water exchange rates (k_Pb²⁺_-H2O = 7 x 10⁹ s^-1 and k_Hg²⁺_-H2O = 2 x 10⁹ s^-1, respectively), during the first few minutes of sonication, desorption by sonication was enhanced compared to hydrodynamic mixing. However, with longer sonication times, the release by sonication decreased with time, while the desorption by hydrodynamic mixing appeared to reach equilibrium. At 60 min, less desorption occurred by sonication than that by hydrodynamic mixing, especially for Hg²⁺. Comparatively, for metals with slower water exchange kinetics namely Ni²⁺ (k_Ni²⁺_-H2O = 3 x 10⁴ s^-1) and Cr³⁺ (k_Cr³⁺_-H2O = 5 x 10^-7 s^-1), sonication also enhanced their desorption compared to hydrodynamic mixing. Unlike Pb²⁺ and Hg²⁺, the reduced desorption with longer sonication times was not observed for Ni²⁺ and Cr³⁺ even at 3 hr.

The enhanced release by sonication at short times may be attributed to the extreme conditions generated during the violent collapse of cavitation bubbles. The reduced desorptions observed with Pb²⁺ and Hg²⁺ at longer sonication times were likely due to the fast resorption. Due to the relatively slow water exchange kinetic constants of Ni²⁺and Cr³⁺ compared to Pb²⁺ and Hg²⁺, Ni²⁺ and Cr³⁺ do not show the fast resorption, which leads to the decrease in Pb²⁺ and Hg²⁺ release with longer sonication. Considering their different water exchange kinetics, which usually determine the adsorption rates, it is obvious that ultrasound is capable of releasing metals from Al₂O₃. The net desorption is kinetically determined by: 1) the desorption effects of sonication and 2) the resorption combined with the particle changes induced by ultrasound.

Sonication also enhanced the release of the four metals (Pb²⁺, Hg²⁺, Ni²⁺, and Cr) from standard sediment PACS-2 (purchased from NRC) at pH 8.0 compared to hydrodynamic mixing.  However, the difference between different metals is not as obvious as that from Al₂O₃ particles.  In acidic condition (pH 3.0), sonication enhanced Pd²⁺ and Ni²⁺ releases compared to those at pH 8.0, whereas decreased Hg²⁺ and Cr release compared to those at pH 8.0.  The interesting thing was that at pH 3.0, ultrasonic release of Pb²⁺ was lower that by hydrodynamic mixing.  Longer sonication times (3 hr) led to decreases in the release of metals except for Ni²⁺ release at pH 3.0.  Sonication significantly increased organic matter release at pH 8.0.  But at pH 3.0, ultrasonic release of organic matter was not obviously higher than that by hydrodynamic mixing, even lower than that by hydrodynamic mixing at pH 8.0.

The different releases of various metals at different pH values were likely due to the differences in their contents, different sediment compositions, sorption modes, metal species, and the effects of organic matters in sediment.  For example, Hg²⁺ release was likely controlled by organic matter; Cr release was influenced by its species; whereas Pb^{2 +}and Ni²⁺ seemed to be affected by inorganic mineral phases.

The characterization of sediments and experiments on real sediments from Ashtabula River is being carried out to investigate the effects of organic matter and different compositions of sediment on the release of various metals. Molecular level examination of metal complexation and adsorption modes will also help to understand the mechanism of sonolytic release.

The application of ultrasound combined with biomass (transgenic Chlamydomonas reinhardtii, a green alga) for the removal of Hg from model and real sediments (Al₂O₃, a-HgS, and PACS-2 marine sediment) was investigated. A transgenic Chlamydomonas reinhardtii (2AMT-2) expressing a plasmamembrane-anchored metallothionein polymer effectively recovered Hg(II) released into the aqueous phase by sonication over a broad pH range from 2.0 to 9.0.  The results showed that this combined technique of ultrasound and alga biomass (2AMT-2) was effective to remove Hg from solids and sediments, especially from Al₂O₃ and a-HgS with no natural organic matter.  An implication of the results of this study is that the application of ultrasound and biomass (transgenic C. reinhardtii) has the potential for in-situ Hg removal from contaminated inorganic sediments.

To effectively recover Hg from organic sediments, the competition for Hg binding between algae and NOM must be considered. Effective means for containing algal cells but allowing direct interaction of alga and Hg-NOM complexes would likely greatly improve algal uptake of Hg.  Further work is needed to determine parameters for optimum operations such as biomass replacement and sonication conditions. 

Desorption of three polyaromatic hydrocarbons (PAHs), naphthalene, phenanthrene and pyrene, from natural sediment will be studied. The sonochemical degradation of these PAHs in aqueous phase was investigated to determine the importance of factors such as temperature, concentration, Henry's constant and reactivity with hydroxyl radicals. The result obtained suggested that Henry's constant cannot fully account for the trend observed in semi-volatile contaminant. PAHs are able to reactant through the hydroxyl radical pathway in the bulk as well as through pyrolysis in the bubble.

In order to study the desorption mechanism, humic acid aged a-Al₂O₃ were used as a model sediment. PAHs were added to the humic acid laden a-Al₂O₃ and allowed to sorb for one week. Automated Soxhlet Extraction was used to extract the PAHs from the sediment. Recovery ranges from 87-93%. This model sediment will be sonicated to determine the rate of desorption and degradation of PAHs.