Determination of hexavalent chromium in lime-treated sewage sludge by anion-exchange fast protein liquid chromatography with electrothermal atomic absorption spectrometry detection

Abstract

The applicability of an anion-exchange fast protein liquid chromatographic–electrothermal atomic absorption spectrometric procedure (FPLC-ETAAS) was investigated for the determination of trace amounts of Cr(VI) in lime-treated sewage sludge. Samples of sewage sludge and sludge mixed with sawdust (volume ratio 1 + 1) with a pH of 8.0 and 7.8, respectively, as well as their mixtures with 5% of quicklime with a pH of 12 were analysed. Liming was applied for sludge stabilisation and disinfection. TRIS–HCl buffer (0.005 mol dm⁻³, pH 8.0) and the same buffer with NaCl (0.5 mol dm⁻³) were employed in gradient elution (15 min, flow rate 1 cm³ min⁻¹). The separated Cr species were determined ‘off line’ by ETAAS in 0.5 cm³ fractions. It was experimentally proven that Cr(VI) was reproducibly and quantitatively eluted from 12.0 to 13.0 min with a maximum peak at 12.5 min. The repeatability of measurement for a standard solution of 25.0 ng cm⁻³ of Cr(VI) was found to be 4.0% and the LOD 1.5 ng cm⁻³ of Cr(VI). The technique was successfully applied for the determination of Cr(VI) in lime-treated sewage sludge. In aqueous extracts of untreated sewage sludge Cr(VI) was not detected. In aqueous extracts of lime-treated sewage sludge Cr(VI) was determined 48 h after liming [the concentration was about 200 ng g⁻¹ Cr(VI)] and was monitored over a time span of six months. The validation of the procedure was performed by analysis of a certified reference material, spiking of samples (recoveries from 93 to 101%) and comparative analysis of Cr(VI) in the fraction eluted at 12.5 min by the ion chromatography thermal lens spectrometry technique. The applied FPLC-ETAAS procedure was found to be a reliable and sensitive technique for the determination of traces of Cr(VI) in lime-treated sewage sludge and investigation of oxidation–reduction processes of Cr. The LOD for sludge samples analysed was 20 ng g⁻¹ of Cr(VI) and the repeatability of measurement 6.5%.