## 4 Results and Discussion  
### Mineral Composition and Soil Reaction  
The soil affected by AMD discharges from the sulfide mine wastes is silt loam in texture (\(\sim\)68% silt, 25% sand and 7% clay), and mineralogically composed of quartz, clay minerals (illite and kaolinite) and jarosite, with minor feldspars, gypsum and hematite, as determined by XRPD analysis of control samples (Figure 2). Figure 2: XRPD diffractograms of topsoil samples of amended soil (T10) and control soil (ST10). In addition, a suite of accessory minerals was identified by ESEM-EDS, including barite, monazite, zircon, rutile, plumbogummite, cuprospinel and poorly crystallized iron oxides. Consistently, the contaminated mine soil has a poor buffering capacity to neutralize acid that is being produced, or has been produced, by AMD because it is completely free of carbonates and depleted in other acid-neutralizing minerals. Jarosite typically occurs as clusters of euhedral crystals of less than 1 \(\upmu\)m in size, coexisting with iron oxides nanoparticles and soil clays, as observed by ESEM at high magnification (Figure 3a,b). The EDS spectra of some isolated crystals of a jarosite-group mineral that appear brighter in the ESEM-BSE images revealed a remarkable amount of Pb, by virtue of which they can be ascribed to plumbojarose (Figure 3c). Twelve years after amendment with MWS, the soil showed a silt loam texture with high contents of fine-grained calcite in the topsoil layer (0-20 cm) and lower proportions of the other constituents relative to control soil, with the exception of gypsum. It is interesting to note that most calcite crystals remain unaltered, maintaining their original rhombohedral morphology despite the long time since marble slurry application. Nevertheless, some of them showed signs of partial dissolution on their surface (Figure 3d), and some others appear to be coated by iron oxyhydroxides (Figure 3e). Direct precipitation of gypsum (Figure 3f) was induced by partial dissolution of the carbonates added to the soil, thus providing the Ca\({}^{2+}\) ions necessary to combine with the sulfate ions. The neutralization reaction can explain not only the increase in gypsum concentration observed in the amended soil layer but also the occurrence of metal oxide coatings, as the Fe\({}^{3+}\) solubility is controlled by the precipitation of ferric iron oxyhydroxides [2]. Soil reaction measured on control samples was ultra-acid, with pH values ranging from 3.1 to 3.5 (Table 1), whereas in the amended plot the soil samples had pH values in the range of 5.8-6.4, with an average of 6.2. Therefore, the soil solution appears to be neutralized to slightly acidic reaction throughout the entire sampled profile (0-70 cm) in the response to MWS amendment. Moreover, the long-term measurement of soil redox potential changed noticeably from strongly oxidizing conditions (Eh = 535-647 mV) to moderately oxidizing conditions (Eh = 343-525 mV) after treatment. The mean values of soil electrical conductivity were 2.62 mS cm\({}^{-1}\) in the amended soil and 3.04 mS cm\({}^{-1}\) in the unamended soil, indicating a low soluble salt content in both soil solutions. Figure 3: ESEM-BSE images showing salient mineralogical and textural features of the amended soil after 12 years of amendment application: (**a**) aggregate of jarosite with pseudocubic habit of euhedral crystals; (**b**) iron oxide nanoparticle aggregate; (**c**) isolated pseudocubic crystal of plumbojarose; (**d**) calcite grain with dissolution grooves on its surface; (**e**) iron oxide coating calcite grain; and (**f**) newly-formed crystal of gypsum. As compared to soil electrochemical properties after one year of treatment, the soil pH declined but not to detrimental levels, from 6.8 to 6.3, and the electrical conductivity remained practically constant at 3.3-3.5 mS cm\({}^{-1}\) twelve years after amendment application.