First order multivariate optimization
The MW-ASE results together with design matrices of 23 full factorial design for each MW-ASE reagent [H2O, (5 M) HCl and (2 M) HNO3] are shown in Tables S1–S3 (Appendix A). The investigated factors were coal amount (A), microwave temperature (B) and time (C). Additionally, 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. The ANOVA information propagated in terms of Pareto charts was used to evaluate influential parameters and their interactions (AB, BC and AC). The charts for metal determination in water extracts are given in Fig. 1 and those of (5 M) HCl and (2 M) HNO3 are illustrated in Figs. S1,S2 (Appendix A), respectively. It has to be noted that, the bar length of the Pareto chart is directly proportional to the absolute value of the estimated effects. The ANOVA results observed in Fig. 1, show that the other two parameters (coal amount and temperature) exceeded the p-value (i.e. their bar length exceeded the red reference line) for Sr and Ba, demonstrating that coal amount and temperature are significant at 95% confidence level for the extraction of these two metals. Similar results were observed for (5 M) HCl (Fig. S1) and (2 M) HNO3 (Fig. S2) extracting reagents for extraction of most metals, hence time (C) was fixed at 5 min, since it was the only insignificant factor. Therefore, the significant parameters were further optimised by using RSM optimisation tool.
Second order multivariate optimization
In order to obtain optimum conditions for the other two significant factors (microwave temperature and coal amount), second order multivariate optimization strategy was conducted, which involved the application of response surface methodology based on the central composite design. The matrix of the central composite design contained 14 experiments with responses (% recovery) correlating to each and every experimental run (see Tables S4–S6). The ANOVA data acquired and the quadratic equations (not included in the paper for simplicity reasons) of the models for each extraction reagent were used to interpret the relationship between analytical response and the evaluated parameters (microwave temperature and coal amount).
The three dimensional response surface plots for water extracts are illustrated in Fig. 2, the plots for 5 M HCl and 2 M HNO3 reagents are shown in Figs. S3,S4 (see Appendix), respectively. These three dimensional response surface plots show that, maximum analytical responses for most of the investigated metal ions (for all the examined extracting reagents) were achieved at 200 °C and 0.1 g for temperature and time, respectively. Hence, the overall interpretation of the multivariate optimization revealed that, the optimum conditions of the proposed MW-ASE were: 0.1 g sample amount, 200 °C microwave temperature and 5 min extraction time for H2O, HCl and HNO3. Additionally, the analytical responses achieved were compared with the predicted values of the RSM model and there were no significant difference at a 95% confidence levels. Therefore, the obtained optimum factors were used during validation and application stages.
Validation of the proposed MW-ASE method
The optimum conditions of the proposed MW-ASE method were then applied in three coal CRMs (SARM 18, 19 and 20) and the results are shown in Fig. 3. The latter demonstrates that, Ga, Sr and Ba were easily extracted with pure water, therefore, these three metals can be regarded as highly mobile metal ions and can be easily introduced into the water bodies during rainy seasons. Recently, our research group reported MW-ASE method for determination of sulphur forms in coal samples and it was also observed that pure water can effectively extract ± 20% of sulphate ions in SARM 2028. Therefore, it can be concluded that Ba, Ga and Sr can be associated with the sulphate, carbonate and phosphate ions of the coal. Additionally, several research groups have also identified these three metal ions as easily mobile and therefore, are a threat to the environment5,8,28. From Fig. 3, it can also be observed that most metal ions showed strong affinity towards the HCl acidic environment, except for Ga. These observations suggest that most of metal ions are associated with the sulphate minerals of the coal and can only be mobile in acid conditions. The diluted hydrogen peroxide extracts showed minimum amount of metal ions, these results show that there are lighter interactions between elemental impurities and the organic content of the coal.
The proposed sequential extraction was also compared with the sequential extraction method reported by Laban et al.5 and comparison results are illustrated in Fig. 4. The analytical results shown in Fig. 4 illustrate that the proposed the proposed MW-ASE procedure is quite comparable with the literature reported results described by Laban and Atkins for all investigated metal ions, expect for Pb. The latter showed results that were below detection limits in the literature report. This is because, the literature work used less sensitive spectrometric technique (ICP-AES) as compared to the ICP-MS used in the current study. Therefore, it can be concluded that, the proposed method is more sensitive to Pd detection as compared to Laban and Atkin’s work. However, the rest of the metal ions show similarities, especially Cr which showed some strong interaction with the organic matter of the coal in all the investigated CRMs (SARMs 18–20). It has to be noted that, even though the pattern of extraction was similar, but the proposed method showed moderate extraction recoveries (79–89%), while the literature report showed excellent extraction efficiencies (≥ 100%). The strong affinity of Cr towards organic components was also reported by several researchers5,8,29,30. Cobalt also reveals strong interaction with organic components of SARM 18, but was more bound with sulphate minerals in SARM 19 and 20. The rest of the metal ions were also sulphate bonded, except for strontium in SARM 18. Trace elements such as Mn and Pb are known to dominate in coal as carbonate or sulphide minerals, hence are easily solubilized in acidic medium5,31. The rest of the metal ions were mostly sulphate bonded, except for Sr in SARM 18. In overall, Laban and Atkin’s extraction recoveries were higher as compared to those of the current study. This is due to the kaolinite and quartz bonded minerals that could not be distracted with the chemical reagents used in the proposed MW-SAE method. It has to be noted that, HF reagent used for the destruction of the organic minerals by Laban and Atkin, can also decomposed clay minerals. The current study replaced notorious HF with environmentally friendly dilute H2O2 for the decomposition of organic matter26,28. Therefore, Cr partially remained in the clay part of the coal; hence cannot be quantitatively extracted without the use of hydrofluoric acid (HF)5,8,29,32.
Application of the proposed MW-SAE method in real coal samples
Table 5 shows the ICP-OES/MS results that were obtained from the sequential extraction of three coal samples (CSA, CSB and CSC) received from one of the South African coal mines. From Table 5 results it can be observed that, Ga, Ba and Sr were the only metals that showed solubility toward water, this means that during wet weather conditions, these metals can be easily leached to the environment. Additionally, Ba and Sr concentration results show that, these two metals were most dominating across the whole spectrum of the three coal samples. It is also worthy to mention that, the sum concentration of most of the investigated metal ions correlated well with the concentration levels that were previous reported using total digestion methods26,27. This correlation confirms that, the accuracy and reliability of the proposed MW-ASE method was excellent. It worthy to indicate that, Be, Sc, Pb and Th in CSA showed slightly lower extraction efficiencies as compared to one literature report27. The similar trend was observed for Be and Th in CSB and for Be and V in CSC. The lower recoveries of the few metal ions (Be, Sc, V, Pb and Th) might be due to the strong affinity of these metal ions with the quartz minerals of the coal33,34,35. However, most of the challenging metal ions were not detected on the previously reported workor they were reported at lower extraction recoveries26. It has to be noted that, 3 mL of 7 mol−1 HNO3 was only added in order to enhance the extraction efficiencies of the investigated elements. Therefore, the proposed MW-ASE method reported 50% less consumption of HNO3 when compared with literature reported studies36,37,38. Therefore, the proposed MW-ASE procedure can be used as an alternative environmentally friendly method for sequential extraction of metal ions from various coal samples and related matrices.
Comparison of current MW-ASE with literature reports
The proposed MW-ASE method was compared with other sequential extraction methods that were performed in coal related matrices for determination of various metal distribution. All the ten compared methods and the current method are illustrated in Table 6. The latter shows that, seven publications described the metal mobility in coal samples3,5,11,12,13,39,41, and the other three studies reported metal distribution in coal fly ash matrices7,9,40. Furthermore, this table shows that, all the published literature methods reported the use of either large reagent volumes (≥ 20 mL), notorious concentrated inorganic acids or both3,5,7,9,11,12,13,39,40. However, with the proposed MW-ASE method, it was possible to use reduced diluted reagent volumes of 12 mL and environmentally friendly reagents (H2O and diluted H2O2). This was due to the rapid interaction of reagents and the coal, which was enhanced by the use of microwaves, as reported by Laban and Atkins5. Another impressive feature about the proposed methods was its short extraction time (0.34 h) as compared to the published literature reports, with extraction time ranging from 2.5 to 86 h. It has to be noted that, 2.5 h sequential extraction was also facilitated by the use of microwave system5. However, the only limitation about the proposed MW-ASE method was the use of high temperatures (200 °C). The latter were also reported for all the sequential extraction methods that involved the use of microwave systems3,5,12. The mostly investigated metal ions on various sequential extraction reports were transition metals and good recoveries (≤ 100%) were obtained. However, all the literature reported sequential extraction methods showed poor precision (≥ 10%), except for the work reported by Yang et al.3, Dahl et al.9 and the proposed MW-ASE method.