Speciation analysis and preconcentration of chromium using chemically modified silica

Abstract

The sorption properties of the chemically modified silica gel having amidoxime and benzothiazolyl moieties toward trivalent and hexavalent chromium were studied. The determination of chromium was carried out on flame automic absorption spectrometry (FAAS). Cr(III) sorption on Si-Benzothiazole and Cr(VI) sorption on Si-Amidoxime were quantitative within the pH ranges 4.0-7.0 , respectively. Adsorption behavior of Cr(III) and Cr(VI) obeyed Langmuir model. The maximum sorption capacities of 3.59 and 8.24 mg g⁻¹ were obtained for Cr(III) and Cr(VI), respectively. A sample method for the preconcentration and speciation of chromium was developed using dual mini-column system (2.79 mm i.d.) packed with 20 mg of Si - Amidoxime and 50 mg of Si-Benzothiazole , respectively. The results showed the optimum conditions for the proposed procedure as follows : pH = 4.0 , sample solution flow rate = 0.5 mL min⁻¹ , sample volume = 100 mL , eluent = 50. mL of 10% H₂O₂ in 0.1 M NaOH and eluent flow rate = 0.5 mL min⁻¹. The preconcentration factor of 20 was obtained . No interferences from Na⁺ , K⁺ and Ca²⁺ for the adsorption of Cr(III) was observed. Cl⁻ and O₋₃ slightly affected the adsorption of Cr(VI) while SO₄²⁻ significantly quenched Cr(VI) adsorption. The method detection limits were 2.4 and 2.1 microgram L⁻¹ for Cr(III) and Cr(VI) , respectively. The spiked synthetic samples showed high accuracy and precistion. The application of this method to determine Cr(III) and Cr(VI) in drinking water gave high accuracy (86-95% recovery) with % RSD less than 10 (n = 6)