## 4 Results and Discussion
### Environmental Availability
Conversely, As, Cr, Cu, Ni, Sb, and Tl were generally most abundant in the EDTA extracts from the control soil samples. Furthermore, the comparison also revealed a generalized significant decrease (\(p<0.01\)) in the PHE levels extracted with EDTA from the deeper sample (60-70 cm depth) of the amended soil. To give a clearer picture of the effects of soil amendment on environmental availability of PHE with time, Figure 9 shows a bar chart comparing water-soluble and EDTA-extracted concentrations of PHE in the surface layer (0-10 cm depth) of the untreated soil, one year after and twelve years after amendment application. The extractable levels of PHE did differ statistically between the three groups of samples (one-way ANOVA, \(p<0.01\)). Before soil amendment, substantial amounts of PHE were released from the mine soil, as indicated in previous work [24]. The leachability risk was particularly high for Zn and Cu. Given that metal sorption and many metal precipitation processes increase with pH [11; 46], amelioration of soil acidity by use of MWS amendment was the major factor driving the immobilization of contaminants, by reducing the activity of H\({}^{+}\) ions in soil solution and increasing negatively charged sites on soil particles. This in turn boosted the adsorption capacity of amended soil as the water-soluble levels of PHE and their uptake by plants dropped following MWS addition, except for As. Increased As mobility observed in the treated soil one year after amendment may also be linked to the increase in the negatively-charged surfaces of soil particles after acid neutralization [47], thus preventing the adsorption of water-soluble As oxyanion species. When comparing soil treated with MWS at years one and twelve after amendment application, a salient decrease in the water-soluble concentrations (Figure 9a) of As, Cu, and Ni was observed with treatment time (\(p<0.01\)), while the opposite trend was apparent for Zn, Pb, and Cd (\(p<0.01\)). A plausible mechanism for reduced As leaching could be  
Figure 9: Bar chart comparing trace element availability before and after marble slurry waste application, at years one and twelve after soil amendment. (**a**) Water-extracted concentration; (**b**) EDTA-extracted concentration. formation of low-solubility calcium arsenates [48; 49; 50], due to reaction of dissolved calcium with arsenic, as reported in other acid mine soils reclaimed with sugar beet lime and compost biosolids [41]. Decreased Cu and Ni availability can be ascribed to adsorption onto organic matter or formation of stable complexes with organic ligands [51]. On the contrary, the release of Zn, Pb, and Cd from the amended soil increased after 12 years. The competition with Ca\({}^{2+}\) ions for sorption sites could have displaced such PHE from the soil binding sites increasing their labile fractions, although they were limited in comparison with those determined in the unamended soil. The extractable amounts of PHE by EDTA (Figure 9b) showed a similar pattern of results for the same time period excepting that of Cu, which increased from 12 to 22% with treatment. This could be explained by the fact that EDTA is a chelating agent that has the ability to release the fraction of Cu associated through complexation with various forms of organic material [52].