Sonolytic Desorption of Mercury from Aluminum Oxide ZIQI HE, † SAMUEL J. TRAINA, ‡ JERRY M. BIGHAM, § AND LINDA K. WEAVERS* ,† Department of Civil and Environmental Engineering and Geodetic Science and School of Natural Resources, The Ohio State University, Columbus, Ohio 43210, and Sierra Nevada Research Institute, University of California, Merced, P.O. Box 2039, Merced, California 95344 As discrete particles and/or as coatings on other mineral surfaces in natural systems, aluminum (hydr)oxides are efficient sinks for Hg(II). Ultrasound at 20 kHz was applied to enhance the desorption of Hg(II) from aluminum oxide particles (5.0 μmol of Hg g -1 ). Results showed that at short times ultrasound enhanced Hg(II) release at pH 4.0 compared to both that from hydrodynamic mixing and that expected on the basis of the Hg(II) sorption isotherm. The higher the input power of sonication, the higher the desorption of Hg(II). However, with longer times, much less desorption occurred by ultrasound than by hydrodynamic mixing, with mass balance measurements demonstrating that the desorbed Hg(II) was resorbed back to the particles. The particles were characterized to explore the mechanism for resorption of Hg(II) by prolonged sonication. No surface area change was observed even though ultrasound dramatically reduced the particle size and changed the surface morphology. Although a decrease in the point of zero charge (PZC) due to sonication was observed, it was excluded as the primary mechanism for Hg(II) resorption. Hg(II) occlusion by aluminum hydroxide precipitation was supported by X-ray photoelectron spectroscopy results and the formation of solutions supersaturated with Al. Experiments on presonicated particles verified the occlusion theory by ruling out the effects of the surface area and PZC. Introduction Mercury may undergo several different biogeochemical processes in aquatic systems including volatilization and deposition at the water-air interface driven by oxidation/ reduction reactions, complexation/decomplexation with ligands, adsorption/desorption to particulate matter and sediments, precipitation/dissolution as metacinnabar (mer- curic sulfide), leaching and transport to groundwater, and methylation/demethylation (1-4). The key factor determin- ing the concentration of Hg in biota is the methylmercury concentration in water, which is controlled by net meth- ylation and demethylation processes. Complexation and adsorption of the precursor, Hg(II), by ligands and sediments may inhibit the production of methylmercury (1). In fact, the major fraction of Hg in an aqueous system, inorganic Hg, is stored in sediments, and persists as a source of methyl- mercury under different environmental conditions (5). Therefore, the treatment and removal of Hg from sediments are necessary for control of methylation and bioaccumu- lation. Numerous studies have been conducted to examine Hg(II) sorption and/or release from natural and synthetic particles, including clays (6), soils (7-9), metal sulfides (10, 11), and metal (hydr)oxides (12-15). As discrete particles and/or as coatings on other mineral surfaces in natural systems, especially in well-weathered soil and sediments with low natural organic matter, crystalline and amorphous aluminas play significant roles in geology and environmental sciences (16, 17). Because of their chemical properties and physical structure, aluminum (hydr)- oxides are efficient sinks for many contaminants including cations of Pb, Zn, Cd, Sr, and Hg (12, 13, 18-20). The influence of pH, ionic strength, and ligands (Cl - , SO4 2- , PO4 2- ) on the sorption of Hg(II) by alumina has also been investigated to better understand the Hg(II) sorption process (12, 21). In addition to Hg speciation, surface characteristics (surface area, porosity, pore size distribution, and PZC) can have a significant impact on adsorption (17). Trivedi and Axe (19, 22) have studied the sorption of metal ions to hydrous metal oxides and concluded that intraparticle surface diffusion is an important and rate-limiting step, requiring a lengthy period of time for sorption. Desorption of heavy metals from sediments can be much slower and/or nonreversible (8, 23). This slow process may lead to long times for the cleanup of dredged metal-contaminated sediments by washing, chemi- cal extraction, thermal extraction, and bioremediation (24). Sonochemical techniques involve the use of sonic or ultrasonic waves to produce an oxidative environment via cavitation bubbles generated during the rarefaction period of sound waves. The formation, growth, and collapse of cavitation bubbles, leading to high local temperatures and pressures, are considered the main mechanism through which chemical reactions occur in sonochemistry. In a homogeneous aqueous system, three different reaction sites have been postulated: (i) the gaseous interiors of collapsing cavities where both temperature and pressure are extremely high (up to and above 5000 K and 1000 atm, respectively), resulting in dissociation of chemical compounds including water (25); (ii) the interfacial liquid region between cavitation bubbles and the bulk solution where high temperature (ca. 1000-2000 K) and high temperature gradients exist; (iii) the bulk solution region at ambient temperature where small amounts of • OH radicals diffusing from the interface may contribute to oxidation and organic contaminant destruction reactions. Studies have been conducted on the application of ultrasound to degrade a variety of contaminants in water (26-29). Besides chemical effects, the collapse of cavitation bubbles results in a variety of physical and mechanical effects on the surface of solids. When solid particles exist in the vicinity of cavitation bubbles, the collapse may occur symmetrically or asymmetrically depending on the proximity and size of the solids. Symmetric cavitation generates shock waves which result in particle-particle collisions and cause extremely turbulent flow at the liquid/solid interface, referred to as microstreaming (increasing the rate of mass transfer near the surface). When solids are in close proximity to the bubbles, asymmetric collapse occurs, leading to the formation of microjets of solvent which bombard the solid surface, * Corresponding author phone: (614)292-4061; fax: (614)292-3780; e-mail: weavers.1@osu.edu. † Department of Civil and Environmental Engineering and Geo- detic Science, The Ohio State University. ‡ University of California, Merced. § School of Natural Resources, The Ohio State University. Environ. Sci. Technol. 2005, 39, 1037-1044 10.1021/es049431y CCC: $30.25  2005 American Chemical Society VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1037 Published on Web 01/13/2005