Arsenic determination in complex mining residues by ICP OES after ultrasonic extraction

The impact caused by mining residue deposits with high amounts of arsenic is a potential environmental problem and motive for investigations. Arsenic is always associated with gold ores and is present in mining areas, probably due to sulfide oxidation and the high pH range where arsenic is soluble. It has been shown that some samples of residues coming from the studied deposits presented arsenic concentrations above 2500 mg kg(-1) The aim of this work is to develop methods for arsenic extraction in mining residues employing ultrasound assistance extraction and analyte determination by hydride generation ICP OES. The analytical method provided analysis precision and a better evaluation of environmental impact of mining residue deposits. Optimal conditions for ultrasound assisted extraction were obtained by evaluation of parameters such as the position of the sample flask inside the ultrasound bath and temperature as a function of the immersion time. These tests identified the ideal position of the sample flask and indicated that the maximum time that the sample could be analyzed without arsenic lost due to increase of the temperature was 180 minutes. With these conditions, an experimental design was developed to obtain the optimal conditions of arsenic extraction, evaluating the type of extraction solution and the sonication time. A 2(2) Central Composite Design (CCD) was employed to evaluate the acid concentration (HCl, HNO3 and H3PO4) and the sonication time (maximum 180 minutes). The quadratic equation of response surface generated by CCD, employing HNO3, indicated that the best condition for arsenic extraction is using an acid concentration of 2.7 mol L-1 and sonication for 136 minutes to give an average concentration of 1847 +/- 17 mg kg(-1). Recovery studies were carried out and the obtained values were close to 100%. Optimal conditions of arsenic extraction were obtained by evaluating the operational parameters of ultrasound bath and the analysis conditions with the experimental design. The method developed contributes significantly to a more precise evaluation of the real environmental impact caused by the mining residues deposit. (C) 2012 Elsevier B.V. All rights reserved.