## 3 Results and Discussion
### Effect of Revgetation on the Chemical Speciation of Metal(loid)s in Mining Areas  
The environmental behavior of metal(loid)s not only depends on their total concentrations but is also significantly influenced by their chemical speciation [9; 40]. Based on the analysis results of the total concentration of metal(loid)s, As and Sb, which are relatively high in the total content of the samples, were selected as the key elements for further analysis of the metal(loid)s chemical speciation.  
The results of the chemical speciation of As and Sb are shown in Figure 3. In the US, the percentage of the speciation of As was in the order of residual As (63.76%) > reducible As (28.36%) > exchangeable As (3.66%) > oxidizable As (3.23%). Compared with the UT, the percentage of reducible As was relatively higher, which was presumed to be mainly related to the potential presence of As in amorphous glassy phases and microstructural features (pores and defects) formed during high-temperature smelting, making As more susceptible to release under the reducing conditions of the BCR. After revegetation, the percentage of the speciation of As was in the order of residual As (73.70-78.97%) > reducible As (17.99-22.03%) > oxidizable As (2.55-3.96%) > exchangeable As (0.31-0.48%). Compared with the US and UT, the percentage of residual As gradually increased after revegetation, while the percentages of both reducible and exchangeable As decreased to different degrees.  
Figure 3: Distribution of metal(loid) fractions. Exchangeable (EXC), reducible (RED), oxidizable (OXI), and residual (RES) fractions of As (**a**) and Sb (**b**) were determined by the BCR extraction method.  
The chemical speciation of Sb in the US was in the order of residual Sb (85.88%) > oxidizable Sb (12.63%) > reducible Sb (1.03%) > exchangeable Sb (0.46%), and after revegetation, the speciation of Sb changed to residual Sb (91.58-96.61%) > reducible Sb (1.76-4.84%) > oxidizable Sb (1.34-3.19%) > exchangeable Sb (0.29-0.38%). Both As and Sb in all samples were mainly in the residual fraction, indicating that most of the As and Sb are present in the lattice of primary and secondary minerals, which have low mobility and can be stable for a long period of time under the natural condition [27,40]. Compared with other speciation forms, exchangeable As (0.31-0.48%) and exchangeable Sb (0.29-0.76%) were the least fraction after revegetation, whereas the percentage of As and Sb in the reducible fraction was higher than that in the oxidizable fraction and exchangeable fraction, which was mainly attributed to the strong adsorption of As and Sb with the Fe-Mn oxides [27]. Under the influence of plant growth activities, exchangeable As and Sb can be transported by plants, while oxidizable and reducible As and Sb may be transformed by plant activation or by weathering and leaching [40]. In addition, increased weathering and other activities may lead to the combination of As and Sb with amorphous secondary minerals, thus causing an increase in the content of residual fractions [27,40,52].  
In general, after revegetation, the percentage of exchangeable As and Sb in the slag, which are easy to migrate and transform, decreased significantly, while the percentage of residue formation increased gradually, indicating that the bioavailability of As and Sb in the slag decreased significantly with the revegetation. From the perspective of environmental risk, there is no doubt that this is beneficial for the control of metal(loid)s migration in the mining area.