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Leaching of Gallium from Coal Fly Ash, Alumina and Sediment Samples with an Acid Mixture for its Determination by ICP-OES

Leaching of gallium (Ga) from coal fly ash, alumina and sediment is reported for its determination by inductively coupled plasma optical emission spectrometry (ICP-OES). A mixture of acids (H2SO4, HNO3 and HF) was used for the leaching process which was carried out overnight at room temperature or heated for 1 h on a hot plate. HF is essential for the quantitative recovery of Ga from coal fly ash. Leaching with either HCl or HNO3 in combination with HF also yielded quantitative recoveries of Ga from coal fly ash, but not from other matrices. After leaching, HF and HNO3 were evaporated and HCl was added to the acid leach before dilution for analysis. When mixed acid leaching was employed, leaching of Ga was 92-102%. For accuracy, the method was applied to a sediment sample, a candidate material prepared as the in-house CRM, and the values obtained were well within the uncertainty reported. The present procedure was validated using the solvent extraction method. The values by the two methods were in close agreement according to the Student’s t-test with a confidence level of 95%. The limit of detection was 0.45 mg Kg-1. The analytical methodology was applied for the determination of Ga in three coal fly ash samples, two from thermal power plants and a NIST CRM 1633b (not certified for Ga) as well as an alumina sample from an aluminum industry facility. A red mud sample, by-product of Bayer’s process of aluminum extraction, was also analyzed using HCl/HF and HNO3/HF leach.