Organically modified silica gel and flame atomic absorption spectrometry: employment for separation and preconcentration of nine trace heavy metals for their determination in natural aqueous systems

A 5-formyl-3-(1'-carboxyphenylazo) salicylic acid-bonded silica gel (FCPASASG) chelating adsorbent was synthesized according to a very simple and rapid one step reaction between aminopropyl silica gel (APSG) and 5-fonnyl-3-(1'-carboxyphenylazo) salicylic acid (FCPASA) and its adsorption characteristics were studied in details. Nine trace metals viz.: Cd(11), Zn(II), Fe(III), Cu(11), Pb(11), Mn(II), Cr(111), Co(II) and Ni(II) can be quantitatively adsorbed by the adsorbent from natural aqueous systems at pH 7.0-8.0. The adsorbed metal ions can be readily desorbed with I M HNO3 or 0.05 M Na(2)EDTA. The distribution coefficient, K-d and the percentage concentration of the investigated metal ions on the adsorbent at equilibrium, C-Mseqm % (Recovery, R%) were studied as a function of experimental parameters. The logarithmic values of the distribution coefficient, logK(d), are 3.7-6.4. Some foreign ions caused little interference in the preconcentration and determination of the investigated nine metals by flame atomic absorption spectrometry (AAS). The adsorption capacity of FCPASASG was 0.32-0.43 meq g(-1). C and N elemental analyses of the adsorbent (FCPASASG) allowed us to calculate a surface converge of 0.82 mmol g-1. This value compares well with the best values reported for the azo compounds. The adsorbent and its formed metal chelates were characterized by IR (absorbance and/or reflectance) and UV spectrometry, potentiometric titrations and thermogravimettic analysis (TGA and DTG). The mode of chelation between the FCPASASG adsorbent and the investigated metal ions is proposed to be due to reaction of those metal ions with the salicylic and/or the carboxyphenylazo chelation centers of the FCPASASG adsorbent. Nanogram concentrations (0.07-0.14 ng ml(-1)) of Cd(II), Zn(II), Fe(111), Pb(11), Cr(111), Mn(11), Cu(11), Co(11) and Ni(11) can be determined reliably with a preconcentration factor of 100. (C) 2004 Elsevier B.V. All rights reserved.