## 2 Materials and Methods
### Selective Extractions and pH-Dependent Leaching Test  
Selective extractions and pH-dependent leaching tests were conducted to identify whether these Fe-Mn nodules could be potential \"Arsenic main sources\" in soil [39; 40]. Three extractants were utilized for the selective extractions of soil As, namely, acidified hydrogen peroxide (H2O2-HNO3), hydroxylamine hydrochloride (NH2 OH-HCI), and citrate-bicarbonate-dithionite (CBD), aiming to elucidate the association of As with the Fe, Mn, and Al (oxyhydr) oxides in the Fe-Mn nodules [41; 42]. Additionally, a control experiment was conducted using 0.5 mol/L HNO3, which may dissolve some Al-containing minerals [35]. Hydroxylamine hydrochloride, known for its aggressive extraction properties,  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Fraction** & **Process** \\ \hline F1 & water-soluble & 2.5 g of sample and 25 mL of distilled water were mixed, shaken well, shaken well, shaken for 30 min, centrifuged for 20 min, and filtered * \\ F2 & exchangeable & 25 mL MgCl2 was added to the residue, shaken well, shaken for 30 min, and centrifuged for 20 min \\ F3 & bound to carbonates & 25 mL NaOAc was added to the residue, shaken well, shaken for 1 h, and centrifuged for 20 min \\ F4 & weakly bound to organic matter & 50 mL Na4 PO7 was added to the residue, shaken well, shaken for 40 min, left for 2 h, and centrifuged for 20 min \\ F5 & Fe/Mn oxide-bound & 50 mL 0.25 M NH2 OH-HCI mixture was added to the residue, shaken well, shaken for 1 h, and centrifuged for 20 min \\ F6 & strongly bound to organic matter & H2O2 + 2.5 mL NH4 OAc-HNO3, diluted to 25 mL, left for 10 h, and centrifuged for 20 min \\ F7 & residual & the residues were dried, ground, and weighed; 20 g of the residue was digested with the 5 mL HCl-HNO3-HCIO4 mixture + 5 mL HF \\ \hline \hline \end{tabular}  
* Centrifuge: Centrifugation for 20 min, 4000 rpm. Filter: The clear solution was filtered with a 0.45 μm filter membrane aperture, and the filtrate was used in a 25 mL colorimetric tube. Cleaning: Add about 100 mL of water to the residue, centrifuge for 10 min, discard the water phase, and retain the residue.  
\end{table}
Table 1: Fractionation and seven-step sequential extraction procedures.  
targets As associated with easily reducible Fe and Mn oxides [43]. Acidified hydrogen peroxide extraction primarily involves dissolved Mn oxides [35]. CBD demonstrates a strong extraction capability for As bound to both amorphous and crystalline iron oxides [44].  
The pH-dependent leaching test was adapted from the CEN/TS 14997 methodology [45]. A pH ranging from 2 to 11 was chosen for the experiment to analyze how As is released under different pH conditions. A total of 1 gram of the sample was placed in a 50 mL centrifuge PP tube, and 9.6 mL of deionized water was added to keep the ratio of liquid to solid (L/S) at 9.6, reaching a final value of roughly 10 when the acid and base were added. Ten pH values--2, 3, 4, 5, 6, 7, 8, 9, 10, and 11--were tested, including an extreme pH higher than that of typical soil. The pH levels were adjusted using HNO\({}_{3}\) (1 mol/L or 0.1 mol/L) and NaOH (2 mol/L or 1 mol/L). In every instance, a control experiment was carried out. For 48 h, the reactors were agitated. ICP-MS was then used to measure the leachate filtrate (Millipore(r) 0.45 \(\upmu\)m) [28; 35; 39]. Table 2 describes the selective extraction procedure. Every extraction was carried out twice, using procedural blanks. The extracts underwent analysis for Fe, Al, Mn, and As using inductively coupled plasma-optical emission spectrometry (ICP-OES, iCAP 6500, Thermo Fisher Scientific Co., Taufkirchen, Germany), after being filtered through a 0.45 \(\upmu\)m membrane filter.