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%.