## 2 Materials and Methods
### Geochemical Modeling  
The pH and total dissolved concentrations of major cations and anions, trace elements (Fe, Al, SiO\({}_{2}\) and Mn), Hg, As, and Sb in the eluates were elaborated by the geochemical code PHREEQC version 3.5.0 [33] to estimate the aqueous solution speciation and to construct Eh-pH diagrams. The PHREEQC geochemical code is based on the calculation of equilibrium between aqueous solutions and minerals, gases, solid solutions, exchangers, and sorption surfaces. PHREEQC implements several types of aqueous activity models depending on the applied database; most of them use the Davies equation (B-Dot activity model) as the Lawrence Livermore National Laboratory model [34] and WATEQ4F. The [PERSON] specific ion-interaction and the SIT (Specific Ion Interaction Theory) aqueous models are also available in a specific database. In the present study, the thermodynamic properties of some minerals relevant to our simulations were checked and eventually added to the PHREEQC _minteq.v4_, a database derived from MINTEQA2 version 4 [35,36] that uses the B-dot activity model. The thermodynamic data for the reactions added to the _minteq.v4_ database are reported in Table 2 with the corresponding references. \begin{table}
\begin{tabular}{c c c c} \hline
**Solids Phases and Minerals** & **Reactions** & **Log k** & **References** \\ \hline Scorodite & FeAsO\({}_{4}\) + 3H\({}^{+}\) = Fe\({}^{3+}\) + H\({}_{3}\)AsO\({}_{4}\) + 2H\({}_{2}\)O & 0.4 & [34] \\ Ca\({}_{5}\)(AsO\({}_{4}\))\({}_{3}\)(OH) & Ca\({}_{5}\)(AsO\({}_{4}\))\({}_{3}\)(OH) = 5 Ca\({}^{2+}\) + 3 AsO\({}_{4}^{3-}\) + OH\({}^{-}\) & \(-\)40.12 & [37] \\ Ca\({}_{4}\)(OH)\({}_{2}\)(AsO\({}_{4}\))\({}^{2}\)4H\({}_{2}\)O & Ca\({}_{4}\)(OH)\({}_{2}\)(AsO\({}_{4}\))\({}^{2}\) + 4H\({}_{2}\)O + Ca\({}^{2+}\) + 2 AsO\({}_{4}^{3-}\) + 2 OH\({}^{-}\) + 4H\({}_{2}\)O & \(-\)27.49 & [37] \\ Ca\({}_{3}\)(AsO\({}_{4}\))\({}_{2}\)\({}^{3}\)H\({}_{2}\)O & Ca\({}_{3}\)(AsO\({}_{4}\))\({}^{2}\)3H\({}_{2}\)O = 3 Ca\({}^{2+}\) + 2 AsO\({}_{4}^{3-}\) + 3 H\({}_{2}\)O & \(-\)21.14 & [37] \\ FeAsO\({}_{4}\) & FeAsO\({}_{4}\) = Fe\({}^{3+}\) + AsO\({}_{4}\)\({}^{-}\) & 13.97 & [38] \\ FeHAsO\({}_{4}\) & FeAsO\({}_{4}\) = Fe\({}^{2+}\) + HAsO\({}_{4}\)\({}^{2-}\) & 3.37 & [38] \\ MgHAsO\({}_{4}\) & MgHAsO\({}_{4}\) = Mg\({}^{2+}\) + HAs\({}_{4}\)\({}^{2-}\) & 2.68 & [38] \\ CaHAsO\({}_{4}\) & CaHAsO\({}_{4}\) = Ca\({}^{2+}\) + HAs\({}_{4}\)\({}^{2-}\) & 2.51 & [38] \\ NaHAsO\({}_{4}\) & NaHAsO\({}_{4}\) = Na\({}^{+}\) + H\({}_{2}\)AsO\({}_{4}\)\({}^{-}\) & 1.75 & [38] \\ NaH\({}_{2}\)AsO\({}_{3}\) & NaH\({}_{2}\)AsO\({}_{3}\) = Na\({}^{+}\) + H\({}_{2}\)AsO\({}_{3}\)\({}^{-}\) & 0.25 & [38] \\ KH\({}_{2}\)AsO\({}_{4}\) & KH\({}_{2}\)AsO\({}_{4}\) = K\({}^{+}\) + H\({}_{2}\)AsO\({}_{4}\)\({}^{-}\) & 1.89 & [38] \\ Ca(Sb(OH)\({}_{6}\))\({}_{2}\)\({}^{2}\)6H\({}_{2}\)O & Ca(Sb(OH)\({}_{6}\))\({}_{2}\)\({}^{2}\)6H\({}_{2}\)O = Ca\({}^{2+}\) + 2 Sb(OH)\({}^{-}\)6 + 6H\({}_{2}\)O & \(-\)10.57 & [39] \\ Ca(Sb(OH)\({}_{6}\))\({}_{2}\) & Ca(Sb(OH)\({}_{6}\))\({}_{2}\) = Ca\({}^{2+}\) + 2 Sb(OH)\({}^{-}\)6 & \(-\)12.55 & [40] \\ Romeite & Ca\({}_{2}\)Sb\({}_{2}\)O\({}_{7}\) + 5H\({}_{2}\)O + 2H\({}^{+}\) = 2 Ca\({}^{2+}\) + 2 Sb(OH)\({}^{-}\)6 & \(-\)6.7 & [41] \\ \hline \end{tabular}
\end{table} 
Table 2: Thermodynamic properties included in the _minteq.v4_ database. The solids phases or minerals, the precipitation/dissolution reactions, the Log k and the corresponding reference(s) are reported. The relationships between Hg-Cl-S, As-S, and Sb-S-Ca-Fe were studied through the Pourbaix diagrams (Eh-pH) and calculated according to the point-by-point mass balance method [45]. Following [45], the predominant species at each given point of Eh and pH was identified, taking into account other variables, e.g., temperature, and presence of ligand(s). The predominant species was determined to be the species with the highest contents of the considered element. This predominance involves the absolute element content (in moles) considering both aqueous and solid species.