Direct ultrasound-assisted extraction of heavy metals from sewage sludge samples for ICP-OES analysis

Ultrasound-assisted extraction was studied as sample preparation method for inductively coupled plasma optical emission spectrometry (ICP-OES) analysis of eight metals (Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn) in sewage sludge. Microwave digestion was used for comparison. A multivariate approach (experimental design) was used for the optimization of both sample preparation methods. Microwave digestion was completely optimized by means of 2((5-l)) fractional factorial design. The optimum microwave digestion conditions were 120 psi pressure, 30 min time-at-pressure, 3 ml HNO3, 1 ml HF and 1 ml HCl. The utility of a new ultrasound probe system equipped with an all-glass sonotrode, which lowers the possibility of sample contamination, was evaluated for the first time. Ultrasound extraction was optimized by applying three different experimental designs. The optimum conditions were found to be 80% amplitude of the ultrasound energy (400 W), 20 min sonication time, 90degreesC temperature, mixture of 5 ml HNO3 and 5 ml of HCl, bubbling and 10 min magnetic stirring. Both optimized sample pretreatment methods were validated with the BCR reference material no. 146R (sewage sludge of industrial origin) and three real sewage sludge samples, yielding microwave digestion quantitative recoveries for all the elements studied, whereas ultrasound energy quantitatively extracts only Cd, Cu, Pb and Zn. With ultrasounds, recovery values higher than 75, 84 and 81% were obtained for Cr, Mn and Ni, respectively, and Co was not able to be detected. Reproducibility and overall analysis time were similar for ultrasound and microwave extractions. Hence, with some limitations that mainly came from the available technology, the ultrasound extraction procedure was found to be a good alternative to the common microwave digestion since the former is less hazardous (i.e., it works under atmospheric pressure and does not use HF acid) and it is considerably cheaper than the microwave system. (C) 2004 Elsevier B.V. All rights reserved.