## 5 Discussion
### Formation and Evolution of Secondary Minerals  
The soil mineralogy of the marshland impacted by AMD is consistent with a depleted carbonate buffering system, in which jarosite and hematite are the most stable phases under the prevailing geochemical conditions. The sulfurization process provides an acidic and oxidizing environment suitable for jarosite (or natrojavosite) formation, at pH less than 3.1 and ~5000 mg L\({}^{-1}\) of dissolved sulphate concentration according to [PERSON] et al. [34] through the reaction:  
\[\text{K}^{+}\left(\text{Na}^{+}\right)+3\text{Fe}^{3+}+2\text{SO}_{4}\text{$ {}^{2-}$}+6\text{H}_{2}\text{O}\rightarrow\text{K}\left(\text{Na}\right)\text{ Fe}_{3}\text{${}^{3+}$}\left(\text{SO}_{4}\right)_{2}\left(\text{OH}\right)_{6}+6\text{H}^{+} \tag{11}\]  
Jarosite-group minerals can scavenge and act as sinks for PTEs in AMD-impacted areas [35]. The high contents of As, Sb, and Tl linked to jarosite-rich soil horizons suggest that these contaminants were sequestered by jarosite through structural incorporation or surface adsorption [36], thus enhancing the natural attenuation processes. Gypsum is another sulphate relatively widespread as a newly-formed mineral, although its relative abundance in the soil is rather low. Its origin is conditioned by the prior dissolution of carbonates that provides the Ca\({}^{2+}\) ions necessary to combine with the sulphate ions released into solution by sulphide oxidation, according to the reactions:  
\[\text{CaCO}_{3}+2\text{H}^{+}\rightarrow\text{Ca}^{2+}+\text{H}_{2}\text{O}+ \text{CO}_{2} \tag{12}\]  
\[\text{Ca}^{2+}+\text{SO}_{4}\text{${}^{2-}$}+2\text{H}_{2}\text{O}\rightarrow \text{CaSO}_{4}\text{${}^{2}$H}_{2}\text{O} \tag{13}\]  
The evolution from Fe-sulphate minerals to Fe oxyhydroxides occurs by a combination of dehydration, oxidation and neutralization reactions [16]. As the concentration of dissolved Fe\({}^{3+}\) decreases with increasing pH, Fe\({}^{3+}\) solubility is limited by the precipitation of ferric oxyhydroxides, such as goethite:  
\[\text{Fe}^{3+}+2\text{H}_{2}\text{O}\rightarrow\text{FeOOH}+3\text{H}^{+} \tag{14}\]  
Other poorly-crystallized secondary products such as ferrihydrite and schwertmannite might have previously formed, depending on the geochemical conditions of the AMD system [37; 38]; however, these precursors are thermodynamically metastable with respect to goethite [39]. Accordingly, goethite might have formed both by direct precipitation and by transformation of the metastable phases, and over time converted to hematite, which is the dominant Fe phase in the furthest reddish-ochre zone:  
\[2\text{FeOOH}\rightarrow\text{Fe}_{2}\text{O}_{3}+\text{H}_{2}\text{O} \tag{15}\]  
Most of the Pb released by dissolution of galena (Equation (9)) has precipitated at low pH in the form of anglesite, and has thereby been immobilized at or near the source of contamination:  
\[\text{Pb}^{2+}+\text{SO}_{4}\text{${}^{2-}$}\rightarrow\text{PbSO}_{4} \tag{16}\]  
Our SEM observations revealed that anglesite has grown epitaxially on the surface of pre-existing barite crystals, with the latter acting as substrate for heterogeneous nucleation. The removal of Pb\({}^{2+}\) ions from the aqueous solution and its persistent storage in anglesite seems to be an effective mechanism of natural attenuation. During prolonged dry weather, efflorescent blooms of hydrated sulphate salts are formed as products of evaporation of the acidic sulphate-rich solutions derived from oxidizing pyrite. Although the neoformed sulphate minerals are ephemeral, they provide important clues about the precipitation reactions from which they arise and the subsequent mineral transformations occurring with increasing temperature and/or decreasing water activity [27; 34; 40].