Development of a novel and green microwave-assisted hydrogen peroxide digestion method for total sulphur quantitative extraction in coal samples prior to inductively coupled plasma-optical emission spectroscopy and ion-chromatography determination

Abstract

A green, novel, fast and cost effective procedure has been developed for determination of total sulphur in coal samples based on the use of diluted hydrogen peroxide and microwave irradiation. The resulting digests were then analysed by using inductively coupled plasma-optical emission spectroscopy (ICP-OES) and ion-chromatography (IC). The investigated factors include hydrogen peroxide concentration, microwave temperature, extraction time and coal amount. A certified reference material (SARM 20) was used for the optimization of the influential experimental parameter. It was observed that quantitative recoveries of total sulphur in coal could be attained when microwave temperature, extraction time, [H₂O₂], and coal amount are 150 °C, 5 min, 3 mol L⁻¹ and 0.05 g, respectively. In addition, the 2⁴ factorial design results demonstrated that the interactions between coal amount and time (BD) as well as coal amount and temperature (BC) were significant at a 95% confidence level. Under optimum conditions, the accuracy of the proposed microwave-assisted hydrogen peroxide digestion (MW-AHPD) method was examined by analysing three coal certified reference materials (SARM 18, 19 and 20) and quantitative recoveries (89–101%) were achieved. Additionally, the precision of the proposed method, expressed in terms of relative standard deviation (n = 15), was satisfactory (≤1.5%). The method detection limit (0.014 μg g⁻¹) and method quantification limit (0.047 μg g⁻¹) for sulphur analysis were relatively good compared to those reported in the literature. The proposed digestion method converted all the sulphur species into sulphates as confirmed by the IC results and it can be routinely applied to replace the standard Eschka method. The P-XRD results confirmed the undigested kaolinite and quartz minerals.