Enhancement of desorption with ultrasound has recently been reported by several workers and exploited for fast regeneration of adsorbent resins in industrial applications (Breitbach and Bathen, 2001; Breitbach et al., 2003). In this work, application of focused ultrasound transmitted from a probe for desorption of metals from Chelex-100 was attempted. The aim is to investigate also the possible degradation of the resin due to the application of high intensity ultrasound. A t test was applied to these data and showed that there were no significant differences between elution procedures (Ultrasound and magnetic stirring). After a number of sorption/desorption cycles it could be observed that no significant differences between results obtained by repetitive usage of the same batch occurred, meaning that the resin did not become damaged and kept its complexing properties. The performance of ultrasound-assisted elution was also tested for other divalent metal ions such as Cu2+, Ni2+ and Cd2+. Like Pb, these metals could be eluted in a short time (1 min) by applying ultrasound with good performance. Therefore, not only the elution time is considerably reduced (from an hour with magnetic stirring to a minute with the ultrasounds probe) but the necessary acid volume is also reduced (only with 1 mL), so higher preconcentration factors can be achieved. A speciation method based on Chelex-100 titration and ultrasound-assisted elution has been developed to study the distribution of Pb among different fractions in seawater. Complexation mechanisms of Pb were investigated from the sorption curves of Pb on Chelex-100 according to the Gibbs-Donnan model. Three complexes were formed. Assuming that inorganic species are quantitatively retained by the resin, it was possible to distinguish between labile inorganic species (completely retained) and labile organic species. The variation of the labile Pb fraction (inorganic+organic) with pH was obtained from the difference between Pb concentration retained on the resin and free ion concentration. Likewise, variation of inert Pb fraction with pH was obtained by subtraction of the Pb concentration retained on the resin from the total Pb concentration. The method has been applied to seawater samples from Ria de Vigo (Galicia, Spain). References [1] Breitbach, M.and Bathen, D., 2001. Influence of ultrasound on adsorption processes. Ultrason. Sonochem., 8:277-283. [2] Breitbach, M., Bathen, D. and Schmidt-Traub, H., 2003. Effect of Ultrasound on Adsorption and Desorption Processes. Ind. Eng. Chem. Res., 42:5635-5646.

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