## 2 Materials and Methods  
### Study Area and Sample Collection  
This study was carried out in the XKS mining area (111\({}^{\circ}\)25\({}^{\prime}\) E-111\({}^{\circ}\)31\({}^{\prime}\) E, 27\({}^{\circ}\)43\({}^{\prime}\) N-27\({}^{\circ}\)49\({}^{\prime}\) N), located in the central part of Hunan Province, China (Figure 1), which has the largest antimony reserves and production in the world. It has been officially mined for more than 120 years since 1897 [28] and has a total area of about 26 km\({}^{2}\). The area has a subtropical continental monsoon climate with an average annual rainfall of 1354 mm and an average annual temperature of about 16.7 \({}^{\circ}\)C [2,29]. Intensive mining activities have led to the accumulation of a large amount of solid waste, and the total cumulative amount of waste slag generated in the mining area reaches more than 7500 \(\times\) 10\({}^{4}\) t, with about 350 slag heaps, mainly including smelting slag and tailings, etc. [30]. Recently, the local government has greatly improved the ecological environment of the mining area through the program of ecological remediation of a large number of historical slags, and the vegetation coverage of the ecological remediation area has been significantly increased.  
Based on the time series of the slag ecological remediation project, we selected the unremediated smelter slag (marked as US), unremediated tailings (marked as UT), the smelter slag heap after 5 years of remediation (marked as RS5), the smelter slag heap after 10 years of remediation (marked as RS10), and the tailings piles after 10 years of remediation (labeled as RT10), respectively, for sample collection. In addition, native vegetation areas (marked BS) were also selected for sampling to represent pre-disturbance levels.  
The ecological remediation of the historical smelter slag heap and tailings piles followed standardized protocols implemented by the local government. For all remediation areas included in this study, the same core procedures were applied. Revegetation primarily utilized native and locally adapted plant species selected for their tolerance to the challenging substrate conditions and potential for stabilizing contaminants. In order to maintain consistency, the dominant species across all remediated sampling sites was _Misanthus_ sp., a perennial grass known for its robust growth, extensive deep root system, and reported tolerance to metal(loid)s (can accumulate a great amount of metal, such as the average concentrations of As and Sb in plant roots reaching 38.5 and 177.3 mg/kg, respectively) [31, 32, 33]. Prior to planting, the substrate surface was mechanically leveled and loosened. Plant establishment relied solely on natural precipitation; no supplemental irrigation was applied after the revegetation process. _Misanthus_ sp. were established primarily through seeding.  
In July 2024, five composite samples were collected from each of the study areas using a mixed multipoint sampling method, with three replicate samples collected from each sample site and mixed into one composite sample using the quadratic method. A total of 30 composite samples were collected; all samples were taken from near-surface depths (0-20 cm), and the collected samples were placed in sample bags and immediately transported back to the laboratory. After natural drying, milling, and sieving, they were used for subsequent geochemical characterization, total metal(loid)s, and metal(loid)s speciation analyses. All chemicals and reagents used in this study were at least of analytical grade.