**4. Discussion**

Metals are transported from tailings deposit to the stormwater system through runoff in the soluble or particulate form. Depending on physicochemical conditions of aqueous medium and nature of metal, these can be adsorbed to the colloidal material, precipitate, solubilize or to be complexed. When sequential chemical extraction is applied to sediments impacted by old mining wastes with high concentrations of minerals such as sulfides and significant amounts of carbonates and silicates, it has been determined that Zn, although it has been found in fractions F2 and F3, tends to be more abundant in F1, that is, it has high mobility and availability in the aquatic environment, Cu has more affinity for F3 and F4 fraction [3, 9, 19] and As is generally found adsorbed to the oxyhydroxide fraction [23]. Although in stormwater systems the highest concentrations of heavy metals are found in sediments, rather than in the soluble fraction, these can be available mainly due to changes in pH and cause great effects on aquatic organisms and on the health of the man [24]. On the other head, the fractionation is also a useful tool to determine whether pollutants are of natural origin or anthropogenic. Heavy metals of origin anthropogenic are present mainly in the first fractions, while that the origin lithogenic are in the residual fraction [19].

The old mining residues (tailings), that were deposited near the rivers; they are characterized by having large amounts of sulfides (mainly pyrite and pyrrhotite) with high concentrations of potentially toxic elements (PTEs) [25]. When the sulfides in the tailings are exposed to weathering by the presence of water, atmospheric and dissolved oxygen, oxidation process takes place [26]. The oxidation process, as undergo for pyrite is very complex, when the oxidation reaction is carried out, heat is released, which, by advection, can significantly improve transport of gas in the waste pile, increasing oxidation rate of sulfides [16]. Furthermore, oxygen can also enter lateral parts of reservoir upward and into basal regions. Nevertheless, in the rainy season, tailings deposits can become saturated with water, so diffusion is the main oxygen transport mechanism [14].

In oxidation of pyrite, Fe2+ dissolves and reacts with water through hydrolysis process, it generates acidity [9]. On the other hand, when the mining waste cover has insufficient atmospheric oxygen, a high oxidation of sulfides causes water to have a pH <3, then under these acidic conditions, Fe3+ can remain in solution and become a dominant oxidant for the oxidation of pyrite [4]. The oxidation of pyrite, causes the dissolution of sulfides such as: sphalerite, pyrrhotite, arsenopyrite and chalcopyrite, although galena has reactivity similar to sphalerite, it does not dissolve easily since an anglesite (PbSO4) edge is formed in galena that is almost insoluble in acidic environments.

If oxidation of sulfides continues at values pH<4, the generation of AMD occurs, which can be neutralized if the encasing rock has enough carbonates, hydroxides, and silicates to consume the acidity generated [3]. However, if oxidation persists, and pH <3, precipitation of secondary phases such as ferric

oxyhydroxides and gypsum takes place, which is accumulate, causing cementation and agglomeration of grains called "hardpan", it decreases the porosity below the surface. The formation of hardpans limits water infiltration and vertical oxygen diffusion [27], for this reason they are considered hydraulic and diffusive barriers that protect the non-weathered material from oxidation [28]. In historical residues from New Zealand [29] they found that hardpand is mainly composed of very fine minerals (μm and nm) of Fe-As-S, in which the oxyhydroxides of As, bukovskyite [Fe2(AsO4)(SO4)(OH).7H2O] and scorodite (FeAsO4.2H2O) are the most abundant. The formation of cement is facilitated in dry climates that allow the evaporation process that improves the cementation of minerals. Although, hardpans serve as sinks for PTEs, their function is not permanent, since their layers could undergo fracturing, and as consequence the infiltration of oxic surface water can cause oxidation of sulfides [30]. On the other hand, the aging of oxyhydroxides (ferrihydrite to goethite) reduces the adsorption capacity due to the increase in crystallinity [31].

Furthermore, precipitation of secondary minerals such as jarosite has a great synergistic capacity to simultaneously incorporate Pb (II) and As (V) in its structure, during mineral growth and mineral-water interactions; amount of As (V), which replaces SO4 is greater when Pb (II) is also incorporated, in the same way amount of Pb incorporated in the structure is also greater when As (V) is incorporated, this simultaneity seems to confer less aqueous solubility to jarosite [32]. Despite presence of hardpans, if the oxidation of sulfides continues, acid dissolves mineral species that contain high concentrations of PTEs, and then are available to reach stream water through runoff. PTEs in surface waters are found in their different compartments. However, sediments are considered the main sink and transport medium, since, through adsorption, precipitation, co-precipitation and coadsorption they can remove highly toxic elements; the adsorption and coprecipitation in Fe minerals limit migration of pollutants in aquatic environments [33]. Although [34] have been found by SEM (Scanning Electron Microscope), that johansenite (manganese pyroxene) originates MnO which could be better adsorbent of PTEs than FeO.

In the Yinma River, in Northeast China [35] they found that Pb and Cu had higher adsorption affinity to sediments than Ni and Cd, and they were adsorbed in higher concentrations to Fe and Mn oxyhydroxides, than in matter organic and residual (solid primary minerals). Cu in ferrihydrite is adsorbed by outer sphere interaction weak bonds (ion exchange) and by inner sphere interaction strong bonds (specific adsorption). However, the presence of organic carbon (OC) causes ferrihydrite, although it precipitates at a smaller size (5 nm to 1 nm), forms a layer or cover that inhibits adsorption of Cu, so Cu is contained mainly by coprecipitation, being trapped in cavities, when precipitation takes place [36]. In addition, it have been found that Cu is adsorbed to humic acids, by ionic bonding and complex formation through its carboxylic and phenolic functional groups, sorption capacity is mainly carried out at pH <4, at higher pH, sorption could be complicated [37].

The mobility and fate of As in sediments and groundwater is strongly controlled by the sorption process, and its extent of adsorption is influenced by the presence of OM. Coprecipitation/preadsorption of HA in ferrihydrite inhibits As from binding to Fe oxyhydroxides because OM can compete with As for available binding sites, promoting the mobility of As (v) > As (III) [38]. Likewise, the retention of As (III) and As (V) on goethite surfaces is reduced in the presence of (HA) and (FA) [39]. Furthermore, the sorption of As in sulfurous minerals also influences mobility, since it can co-precipitate in FeS2 or precipitate in sulphides such as rejalgar (As4S4) *Mobility of Heavy Metals in Aquatic Environments Impacted by Ancient Mining-Waste DOI: http://dx.doi.org/10.5772/intechopen.98693*

[40]. Several studies indicate that Zn and Cd are highly mobile, Zn > Cd; [41] found that Zn presented high correlation with aluminosilicates, and the adsorption results indicate that it is mainly adsorbed on clays through weak external sphere bonds. Aquatic environments commonly contain graphene oxide (a very soluble substance), which is characterized by its surface containing carboxyl groups that can form complexes with metal ions and coadsorb (approx 91%) to hematite and/or goethite at pH 3-5, decreasing its adsorption when it increases; this coadsorption is considered an irreversible process [10].
