## 4 Discussion  
According to Table 2, the content of Zn, Pb, and Cu were lower than exploitable concentrations in ores. Nevertheless, the content of these three trace elements were over the background levels and maximum allowable limits of the region of Murcia (Spain) (Table 3) [29]. Comparing data from Tables 2 and 3, RD would be classified as a contaminated soil due to the high concentrations of Zn, Pb, As, and Cu. These metals were in the list of seventeen trace elements that have been identified in soils as very toxic due to their high degree of mobilization (Co, Ni, Cu, Zn, As, Se, Pd, Ag, Cd, Sn, Sb, Te, Pt, Hg, Tl, Pb, and Bi). Most of them are incorporated in the list of priority pollutants established by the US Environmental Protection Agency [31]. The level of Cu is also higher than the Cu concentration in the mine wastes of this area. Differences in the H/C ratio indicated that carbon in VC was in the form of more aliphatic carbon structures than in CA (Table 4). The two carbon-based materials showed similar alkaline pH, whereas the Eh of AC was higher than that of VC probably due to the presence of some oxygenated functional groups generated during the activation process. Finally, important differences were related to surface area and porosity. As expected, AC shows higher S\({}_{\text{BET}}\) and porosity than VC. Sulfuric acid was selected as leaching solution owing to its effectiveness for the selective dissolution of Zn compounds over Pb and Ca [32]. Instead, lead oxides and carbonates present in the sample can dissolve in sulfuric acid solution and then precipitate as lead sulfate. [PERSON] et al. [33] studied the use of 17 different inorganic and organic leaching agents for the recovery of Zn from a carbonate-type Pb-Zn ore flotation tailing. These authors concluded that sulfuric acid resulted in 91 wt% of Zn extraction with a high selectivity against Pb. The main influence of carbon materials in the initial leaching solutions is observed in the Eh values (Table 5). The addition of VC and specially AC decreased the initial Eh of the leaching solutions that continues along leaching time. This result was according to previous works and according to the low Eh values of carbon materials [25; 26]. The main reason for Eh decrease was related to low Eh of carbon-based materials due to their high C content (Table 4). Table 4 shows that Eh of AC was higher than that of VC. However, when AC was added to leaching solutions, the decrease of Eh was higher than after VC addition. This fact could be related to different interactions between both carbon-based materials and oxidant species and minerals present in the systems. In fact, [PERSON] [26] observed that Eh decreased in the leaching of chalcopyrite catalyzed by charcoal and attributed this effect to galvanic interactions between mineral and charcoal in the acidic medium. With respect to pH evolution along leaching time, the pH slightly increased with time (from 1 to 6 h) when sulfuric solution 1 M was used as leaching system whereas there were no significant differences in case the of 0.5 or 0.25 M. In general, there were no significant differences in the solution pH between leaching systems with or without the addition of carbon materials. This result was different to those obtained previously using activated carbon or charcoal in the leaching of sulfide minerals with ferric as oxidant [25]. In this case [25], the addition of carbon-based adsorbent significantly decreased the pH of the solution and it was proposed that this could be due to the reaction of surface functional groups of carbon-based adsorbents with the oxidizing agent (Fe\({}^{3+}\)/H\({}_{2}\)SO\({}_{4}\)). Fe\({}^{3+}\) can react with different oxygenated groups generating organic acids and releasing protons to the medium.