## 5 Discussion
### Occurrence Modes of As in the Coal  
The determination of the occurrence mode of arsenic in the coal from the Santanghu Coalfield is the main research target in this study. The occurrence modes of arsenic in coal from the Hanshuiquan mine measured by the sequential chemical extraction experiment (SCEE) and two-orbit atomic fluorescence (AFS) methods indicate that the occurrence modes of arsenic were divided into sulfide form state, residual form state, organic state, and adsorption state [25] (Table 3). The average proportion of arsenic in coal in a sulfide form state was 73.09% [25]. The electron probe micro analysis (EPMA) results show that arsenic exists in the pyrite, but the maximum content in pyrite is less than 0.5% (wt. %) (Figure 4) [26]. Although there are content differences among the various areas [38; 39], the extremely low arsenic content in pyrite from the Hanshuiquan mining area indicates that there are differences in enrichment states compared to the coal from Wulantuga in China [40; 41] and Sarawak in Malaysia [42]. The pyrite form state and arsenopytire form state are not the main states for arsenic in the inorganic state; there is another inorganic state for arsenic in the coal of the Hanshuiquan mine. Even though there are differences in the occurrence modes, arsenic in coal from several regions (Santanghu, Ningwu, Wulantuga, and Guizhou) mainly occurs in pyrite (Table 3).  
Figure 4: Images of pyrite in sample STD9-1 ([PERSON] et al., 2018) [26]. (**a**) Filling status of pyrite; (**b**) pyrite in fracture. Clay—Clay; Ct—Telocollinite; Cg—Corpocolinite; Sf—Semifusinite; Py—Pyrite; Vd—Vitrodetrinite; Id—Interodottrinite.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Sample**} & **adsorption** & **Organic** & **Residual** & **Sulfide** & **Sulfide** & **The Result of Reductive Difference** & **Percentage** & **Sum of** & **The Total** \\  & **Arsenic** & **Bound** & **Form of** & **Bound** & **Method** & & **of As\({}^{\text{\text{\text{\text{\textasci  
An arsenate form state has been reported. The arsenate form state is partly found in Guizhou, China [28; 44], and arsenic can become enriched in the arsenate form state from the high content of arsenic in the coal of western Guizhou [31; 45]. It is speculated whether arsenic can also exist in the form of arsenate. The SEM-EDS can directly observe the combination state and distribution of the arsenic in the coal [46; 47]. The element distribution density identified via SEM-EDS shows that the surface of the organic matter, where a large amount of silicate minerals are enriched, also contains a certain amount of arsenic (Figure 5a). Arsenic is also present in the kaolinite, showing that the arsenic occurs in the residual state in line with previous results (Figure 5b). Figure 5c also shows that there is a certain relationship between the silicate minerals and arsenic contents (Figure 5c).  
The XRD results show that there is little ojuelaite and scorodit in the coal, while it is not possible to observe the relationship between Fe and the As via SEM-EDS. The distribution of the Fe is unordered, which relates to one of the properties of the element. As an active element, in fact, the modes of occurrence for Fe in coal are various. Fe can not only become enriched in the iron-rich minerals, but also occur in various components. XRD can help to identify the compounds of the materials accurately, and is a reliable way to be sure of the compounds in the coal [1; 2; 3; 4; 5; 6]. There is a better relationship between the As and Zn, which secure the ojuelaite in the coal. Except for the silicate minerals, there is also a good relationship between the As and Zn content in the organic matter (Figure 5a). It can be predicted that the form of Zn-arsenate is a mode of occurrence for arsenic in the coal, and that Zn-arsenate is distributed in the organic matter [24; 48].