Speciation of chromium in waters via sol-gel preconcentration prior to atomic spectrometric determination

Abstract

Chromium occurs in natural samples in two relatively stable valence states, i.e. in the form of Cr(III) and Cr(VI) species. Their concentration in natural waters is usually very low, in the order of a few .g/L. This low concentration necessitates either the use of very sensitive analytical techniques or the application of suitable preconcentration methods prior to instrumental determination. In the present study, a resin having immobilized mercapto or amino functional groups was prepared by reacting silica gel with 3-mercaptopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, L-glutamic acid dimethyl ester, aminophenol, aminothiophenol and aminobenzothiazol for the preconcentration of Cr(III) and Cr(VI) species in waters prior to their determination by flame atomic absorption spectrometry (FAAS). Characterization of the novel sorbents were performed using scanning electron microscopy (SEM/EDS), thermo gravimetric analysis (TGA) and elemental analysis. Among the sorbents synthesized, amino sol-gel and silica amino have been shown to be efficient in terms of sorption capacity. The applicability of the new sorbents for the preconcentration of Cr(III) and Cr(VI) species in waters was examined by batch method. After pH 4, silica amino could quantitatively sorb both Cr(III) and Cr(VI) whereas amino sol-gel sorbed only Cr(III). Effect of the amount of silica amino and amino sol-gel was examined at the optimum pH. The optimum amount of silica amino for quantitative sorption was found to be 0.1 g for 20.0 mL of 1.0 mg/L Cr(III) and Cr(VI) solutions whereas that of amino sol-gel was 0.05 g for 20.0 mL of 1.0 mg/L for both species. The effect of Cr(III) and Cr(VI) concentrations on amino sol-gel and silica amino were investigated at the initial concentrations of 1.0, 10.0, 25.0 and 50.0 mg/L. Elution was realized with 2.0 M HCl, and the percent recovery values changed between 60 and 80%.