## 5 Discussion  
### Enrichment Characteristics of As in the Coal  
There are at least five different modes of occurrence for elements in coal: a water-soluble state, an exchangeable state, an adsorption state, a mineral state, and an organic state [1,3,6]. The adsorption-state arsenic can be adsorbed on the surface of the mineral and the coal matrix, and it can even adsorb into the pores and fractures. The mineral state means that arsenic is an independent mineral, which occurs in the mineral lattice and inclusion. The organic-state arsenic means that the arsenic is combined into the coal molecule [27,28]. There are many studies on the enrichment characteristics of arsenic in coal [27, 28, 29, 30]. Arsenic content in coal has relativity with the coalification [29,30]. Arsenic can be easily combined with the organic matter in low-rank coal, while, with the increased coalition of the coal, polymerization of the aromatic hydrocarbon in the bituminous coal decreases the binding ability with the trace elements, at which point arsenic tends to concentrate  
Figure 3: Microscopic images under oil immersion lens and scanning electron microscope. (**a**) Kaolinite infillings of fusinite (500\(\times\)); (**b**) kaolinite and other clay minerals in fusinite (500\(\times\)); (**c**) pyrite filled into the plant tissue pores; (**d**) subidiomorphic pyrite; (**e**) As distribution map of coal particles under scanning electron microscope; (**f**) silicate minerals distributed on the surface of the organic matter. in the low-rank coal (Table 1). In addition, the distribution of arsenic in Chinese coal is related to the coal accumulation period to some extent. The arsenic content decreases successively from the Triassic period, Tertiary, Jurassic, and Carboniferous to the Permian period (Table 1) [21]. The various coal-forming plants and the different organs of the same plant also contribute to the variety of adsorption capacities of the trace elements [17; 21]. The inferior organisms, algae, and herbs contribute a higher arsenic content than the advanced plants (Spermatophyta, Pteridophyta, and Bryophyta), and arsenic is easily enriched in coal formed in marine sedimentary environments [31; 32; 33]. The coal formed in the Jurassic period from the Xishanyao Formation in the Santanghu Coalfield is of the continental sedimentation with a low coalification degree, and the coal is long-flame coal which belongs to the low rank of sub-bituminous coal. The average arsenic contents in world coal and Chinese coal are 8.30 \(\upmu\)g/g and 3.79 \(\upmu\)g/g, respectively [5; 34]. Based on the enrichment coefficient (CC) proposed by [PERSON] (2012) [5], the enrichment characteristics of the various elements in the coal from the Hanshuiquan mine show that the enrichment of arsenic presents characteristics of significant enrichment. The average arsenic content in coal from the Hanshuiquan mine, compared with the average values of Chinese coal and the world's coal, the CC values are 10.87 and 4.97, respectively (Table 2). Arsenic in the 9# coal seam presents an extremely high content of 108.37 \(\upmu\)g/g, which is approximately 13 times that of world hard coal and 28 times that of Chinese coal [35] (Table 2). The average arsenic content ranges from 0 \(\upmu\)g/g to 5 \(\upmu\)g/g for the raw coal in Santanghu Basin, which is classified as first- and second-grade arsenic coal. In addition, the arsenic content in some individual mines is very high and can even be classified as be third- or fourth-grade arsenic coal [36]. The arsenic content of the raw coal in the 9# coal seam in the Hanshuiquan reconnaissance and survey area ranges from 0 \(\upmu\)g/g to 54 \(\upmu\)g/g [37]. Although an abnormal enrichment of arsenic has been found, there has seldom been any research carried out on it.