Determination of cobalt in sewage sludge using ultrasonic slurry sampling graphite furnace atomic absorption spectrometry

Cobalt in sludge of domestic and industrial origin, with high iron contents (> 17 g/kg), was determined by slurry sampling graphite furnace atomic absorption spectrometry (GF-AAS). Slurries prepared by ultrasonic stirring were adequately diluted to cover the variation in cobalt content in the sludge samples. The diluent was 5% HNO3. Standard atomisation conditions for cobalt determination were used and no matrix modifier was applied. Slurry sampling GF-AAS results in the sludge were verified by analysing totally digested samples by inductively coupled plasma atomic emission spectrometry (ICP-AES) and by GF-AAS. The procedure was validated by analysing the certified reference material BCR 146 R, a sewage sludge of industrial origin. Recoveries for cobalt in the spiked slurried sludge samples ranged from 92 to 96%, with a relative standard deviation of 10%. Recoveries in the certified sludge using slurry sampling GF-AAS technique were about 103% for a cobalt content of 7.39 mg/kg.