Continuous ultrasound-assisted extraction of hexavalent chromium from soil with or without on-line preconcentration prior to photometric monitoring

A continuous ultrasound-assisted extractor was coupled to a photometric detector in order to obtain a fully automated approach for the determination of Cr-VI in soil. The use of a flow injection (FI) manifold as interface between the extractor and the photometric detector allowed the monitoring of Cr-VI after extraction in a continuous manner. The coloured complex formed between 1,5-diphenylcarbazide (DPC) and Cr-VI was used as recommended in EPA method 7196A because it is one of the most sensitive and selective reactions for Cr-VI determination. A preconcentration minicolumn packed with a strong anion-exchange resin was placed between the extractor and the detector, providing a more sensitive method. The linear dynamic ranges were 1-10 and 0.25-7.5 mg l(-1) for the methods without (method A) and with preconcentration (method B), respectively. The limits of detection were 4.52 ng for method A and 1.23 ng for method B. Both methods were applied to a natural contaminated soil and the results obtained agreed well with those obtained by the reference EPA method 3060A. The influence of different amounts of Cr-III in the samples was also studied and the results showed that the proposed methods did not disturb the original species distribution.