## 3 Materials and Methods
### Experimental Methods
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Step** & **Speciation** & **Pretreatment** \\ \hline \multirow{6}{*}{\(1\)} & & The soil sample was weighed at 1.00 g and transferred into a 250 mL polyethylene centrifuge tube with a lid. In total, 40 mL of 0.1 mol/L HAC solution was added to the sample, then shaken on a reciprocating shaker at 22 \(\pm\) 5 \({}^{\circ}\)C and at a speed of 200 r/min for a duration of 16 h. Subsequently, the mixture was centrifuged at 3000\(\times\)\(g\) for 20 min to extract the solution. The resulting solution was filtered through a 0.22 \(\upmu\)m filter membrane and the supernatant was transferred into a 10 mL polyethylene tube. Then, 1 mL of concentrated HNO\({}_{3}\) was introduced and stored in a refrigerator until analysis was conducted. Additionally, 20 mL of ultrapure water was added to the centrifuge tube, agitated for an additional period of 15 min, followed by another round of centrifugation for 20 min in order to discard the supernatant. \\ \hline \multirow{6}{*}{\(2\)} & & In total, 40 mL of 0.5 mol/L NH\({}_{2}\)OH-HCl solution was added to the centrifuge tube and shaken for 16 h at 22 \(\pm\) 5 \({}^{\circ}\)C and 200 r/min. The mixture was then centrifuged for 20 min, and the supernatant was transferred to a 10 mL polyethylene tube. A total of 1 mL of concentrated HNO\({}_{3}\) was added, and it was stored in the refrigerator for \\  & & testing. After shaking the centrifuge tube with 20 mL of ultrapure water for 15 min, it was centrifuged for another 20 min before discarding the supernatant. \\ \hline \multirow{6}{*}{\(3\)} & & A total of 10 mL of 8.8 mol/L H\({}_{2}\)O\({}_{2}\) solution (pH = 2.0) was slowly added into the centrifuge tube, covered with a lid and disintegrated at room temperature for 1 h, shaking manually every 10 min during the disintegration process. Then it was placed in an adjustable electrothermal thermostatic water bath (85 \(\pm\) 2 \({}^{\circ}\)C) to disintegrate for 1 h, the lid was removed, and heating continued until the volume was less than 3 mL. The centrifuge tube was then removed, allowed to cool down, and another 10 mL of 8.8 mol/L H\({}_{2}\)O\({}_{2}\) solution (pH = 2.0) was added, dissolved in the water bath for 1 h. \\  & & The lid was again removed and heating continued until the volume was about 1 mL. \\  & & After removing the centrifuge tube, 50 mL of 1 mol/L NH\({}_{4}\)AC solution (pH = 2.0) was added again, sealed and shaken for 16 h (22 \(\pm\) 5 \({}^{\circ}\)C, 200 r/min), centrifuged for \\  & & 20 min, and the extracted solution was filtered through 0.22 \(\upmu\)m filter membrane, and then the supernatant was transferred to a 10 mL polyethylene tube, and added with \\  & & 1 mL of concentrated HNO\({}_{3}\), and then kept in refrigerator to wait for the \\  & & measurement. Add 20 mL of ultrapure water to the centrifuge tube, shake for 15 min and centrifuge for 20 min, then discard the supernatant. \\ \hline \multirow{6}{*}{\(4\)} & & \multirow{6}{*}{residual fraction (F4)} & The centrifuge tube containing the solid residue was placed in a water bath at 60 \({}^{\circ}\)C \\  & & until evaporation, and after constant weighing at 60 \({}^{\circ}\)C, it was transferred to a sample bag and preserved by grinding. A total of 0.1 g of the solid residue was taken and eliminated by HCl-HNO\({}_{3}\)-HF-HClO\({}_{4}\) method and left to be measured. \\ \hline \multirow{6}{*}{\(5\)} & & \multirow{6}{*}{water-soluble fraction (F5)} & In total, 1.00 g of sample was weighed into a 100 mL capped polyethylene centrifuge tube, 20 mL of boiled and cooled ultrapure water pH = 7.0 was added, sealed and shaken for 16 h and then centrifuged for 30 min at a force of 4000 g. The extract was filtered through a 0.45 \(\upmu\)m membrane, and the supernatant was pipetted into a 10 mL polyethylene tube and added with 1 mL of concentrated HNO\({}_{3}\), stored under refrigeration and stored for measurement. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The experimental steps of improved BCR method.