Determination of Cr (Ⅵ) in Coal Ashby Microwave Alkaline Digestion and Inductively Coupled Plasma-Optical Emission Spectrometry

Quantitative determination of Cr (Ⅵ) in coal ash is very important for controlling the emission of highly toxic Cr (Ⅵ) produced by coal combustion. Available quantification determination methods are mainly used for detecting high concentration Cr (Ⅵ) in water, soil and solid waste, which are not suitable for analyzing low concentration Cr (Ⅵ) in coal ash. The sample processing methods are time consuming and the detection limits are relatively high. Thus, it is necessary to develop a convenient, effective and highly sensitive determination method for Cr (Ⅵ) in coal ash. In this study, alkaline solvent and the microwave digestion system were used for pretreatment of coal ash. Through the experiments, the digestion conditions, which included sample amount, microwave digestion time, and microwave digestion temperature were optimized. The chemical co-precipitation method was used to separate Cr (Ⅲ) and Cr (Ⅵ) in the extracted solution. The concentration of Cr (Ⅵ) was determined by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). The results show that the optimum digestion condition is 0.2 g sample digested under 9℃ for 60 minutes. The determination limit is 0.00134 μg/mL (in Table 1), and the average matrix spike recovery of practical samples is 87.2% (in Table 2). The detection limit of this method is 0.00033 μg/mL (in Table 1), suggesting that the sensitivity of Cr (Ⅵ) was improved by 10 times than that of the traditional 1, 5-diphenylcarbohydrazide spectrophotometric method (0.001 μg/mL).