Abstract
Some metal pollutants are corrosive in nature, are associated with fouling and slagging challenges of the coal boilers, are highly volatile and might cause air pollution and are catalyst poisoners during Fischer–Tropsch catalytic reaction. Therefore, this work describes an improved microwave-assisted sequential extraction (MW-ASE) method followed by ICP-OES/MS analysis for metal distribution determination in South African coal samples. The multivariate optimum conditions for each sequential step were 0.1 g, 200 °C and 5 min for sample amount, microwave temperature and extraction time, respectively. Under the optimum conditions, Ga, Sr and Ba were the only metals that showed solubility towards water, therefore, these metals are classified as highly mobile and eco-toxic under wet environmental conditions. Additionally, all the investigated metals showed solubility towards acidic conditions (HCl and HNO3). These results suggest that, most metal ions are predominantly bonded to sulphate, sulphide, and carbonate coal minerals. Alternatively, Ce, Cr and Y showed total extraction recoveries of ≤ 90%, confirming their strong affinity towards quartz coal minerals. In overall, the proposed MW-ASE method reported short extraction time (0.34 h), environmentally friendly reagents (H2O and diluted H2O2) and rapid multivariate optimization with acceptable extraction efficiencies (79–98%) and reproducibility (RSD ≤ 5%).
Highlights
Some metal pollutants are corrosive in nature, are associated with fouling and slagging challenges of the coal boilers, are highly volatile and might cause air pollution and are catalyst poisoners during Fischer–Tropsch catalytic reaction
The three real coal samples assigned as coal sample A (CSA), coal sample B (CSB) and coal sample C (CSC) were collected from one of the South African coal mines and were sieved to ensure that they match the same particle size as the CRMs
The investigated factors were coal amount (A), microwave temperature (B) and time (C). These factors were examined by using analysis of variance (ANOVA) and the percent recoveries obtained for all the investigated heavy metals were used as analytical responses
Summary
Some metal pollutants are corrosive in nature, are associated with fouling and slagging challenges of the coal boilers, are highly volatile and might cause air pollution and are catalyst poisoners during Fischer–Tropsch catalytic reaction. This work describes an improved microwave-assisted sequential extraction (MW-ASE) method followed by ICP-OES/MS analysis for metal distribution determination in South African coal samples. It has to be noted that, coal elemental constituents can be associated with organic matter, mineral matter and pores of the coal, depending on their physiochemical properties The latter is the main determining factor for metal occurrence within the coal structure. Laban et al developed a microwave assisted sequential extraction method using different acids ( H2O, HCl, H NO3 and H NO3-HCl-HF) for the determination of metal association with water soluble phases, pyrite and organic matter, respectively[5]. The current study aimed at developing a rapid and eco-friendly microwave based sequential extraction procedure followed by ICP-OES/MS metal determination for evaluation of heavy metal distribution and mobility in coal samples. This is because, this certified reference material contains certified known amount of the selected heavy metals under the study
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