**2.4 Oxidation-reduction**

Sulfide minerals in mining waste pose several relative resistances to weathering (**Table 1**) [9]. However, due to physicochemical processes, the weathering process presents alterations that influence the composition of water that can drain or percolate in the mining waste.

The oxidation of sulfur minerals is due frequently to their deposition in aerated places where sulfide minerals are thermodynamically unstable. Therefore, pyrite and pyrrhotite, which generally dominate sulfide deposits are the first sulfides to undergo oxidation [Eqs. (3) and (4)] due to their chemical composition. During oxidation process, one mole of pyrite and pyrrhotite with atmospheric oxygen, is the main electron acceptor, and water form Fe2+, 4 and 2 moles of acid, respectively [9]. However, sphalerite, chalcopyrite, galena and arsenopyrite also contribute to the generation of acid drainage [4, 14].

$$\text{FeS}\_2 + \frac{15}{4}\text{O}\_2 + \frac{7}{2}\text{H}\_2\text{O} \rightarrow 2\text{SO}\_4^{2-} + \text{Fe(OH)}\_3 + 4\text{H}^+\tag{3}$$

$$\text{Fe}\_{1-x}\text{S} + \left(2 - \frac{\varkappa}{2}\right) \text{O}\_2 + \varkappa \text{H}\_2\text{O} \rightarrow \text{SO}\_4^{2-} + (1-\varkappa) \text{Fe}^{2+} + 2\varkappa \text{H}^+ \tag{4}$$


#### **Table 1.**

*Relative resistances of sulfide minerals and magnetite in oxidized tailings taken from [9].*

*Mobility of Heavy Metals in Aquatic Environments Impacted by Ancient Mining-Waste DOI: http://dx.doi.org/10.5772/intechopen.98693*

Heat is released when oxidation reaction of metal sulfides occurs. When pyrite is oxidized and form acid, this reaction releases 1440 KJ.mol�<sup>1</sup> , heat that is hardly released by an oxidation reaction [15]. The heat generated by that exothermic reaction of pyrite, as long as there is high permeability in the tailings cover, it transports oxygen by convection, increasing oxidation rate of sulfides. However, when a reservoir is saturated with water, advective transport is reduced and the main oxygen transport mechanism is diffusion [14, 16]. In addition, oxygen can be supplied by wind in lateral parts of deposit, where oxygen can migrate upward or reach basal regions by advection, convection and diffusion, [17]. Therefore, low concentrations of oxygen are due to its high consumption in oxidation of sulfides minerals and to a limited supply due to the low permeability that governs waste materials.

When the cover of the tailings dam is depleted in oxygen and the air supply is insufficient, high oxidation of sulfides causes water to be characterized by pH <3, high concentrations of sulfates and metals, for instance Zn, Fe, Pb. Under these scenarios, Fe3+ ion remains in solution and becomes the dominant oxidant of metal sulfides, generating acidity in the environment, Eqs. (5) and (6) show generation of acid from oxidation of pyrite and pyrrhotite [9].

$$\rm FeS\_2 + 14Fe^{3+} + 8H\_2O \to 15Fe^{2+} + 2SO\_4^{2-} + 16H^+ \tag{5}$$

$$Fe\_{(1-x)}\mathbb{S} + (\mathbb{S} - 2\mathbb{x})Fe^{3+} 4H\_2O \to (\mathbb{S} - 3\mathbb{x})Fe^{2+} + \mathbb{S}O\_4^{2-} + 8H^+ \tag{6}$$

## **2.5 Geological factors influencing mobility of heavy metals**

Geological formations are volumes of rocks confined to a certain space, they are formed by different types of rocks (igneous, sedimentary, and metamorphic) of different nature, they can enclose mineral deposits of great economic value. Metallic mineral deposits are generally enriched in sulfides such as: pyrite (FeS2), pyrrhotite (Fe(1-x) S), sphalerite (ZnS), chalcopyrite (FeCuS2), arsenopyrite (FeAsS), galena (PbS) and cubanite (CuFe2S3). From the metallurgical method, the recovery of high economic value metals such as Pb, Zn and Cu is carried out, releasing to the environment waste ground material commonly called "tailings" enriched in gangue minerals that do not represent any economic interest for its exploitation as: pyrite (FeS2), arsenopyrite (FeAsS), pyrrhotite (Fe(1-x) S), calcite (CaCO3), quartz (SiO2) and K feldspars (AlSi3O8) [3, 18].

Once the deposition of tailings is concluded, either if oxygen and water flow through pores or if there is ferric ion, oxidation of sulfides occur, generating acidity through the release of protons H+ (Eqs. (7)–(10)]. In deposit of tailings, As is associated with pyrite, Zn and Cd with sphalerite, and they are released when these minerals are dissolved. Despite that galena has a reactivity similar sphalerite, it does dissolve very slowly because secondary mineral of anglesite (PbSO4) generally precipitates on its edges, which avoid its dissolution even in highly oxidized environments removed from sphalerite. Cu in acidic environments is released from chalcopyrite, while Co and Ni are generally derived from the oxidation of pyrite and pyrrhotite [3, 9].

$$\text{FeS}\_{2(s)} + \text{3.5O}\_{2(ac)} + H\_2\text{O}\_{(l)} \to \text{Fe}^{2+}\_{(ac)} + 2\text{SO}\_4^{2-} + 2\text{H}^+\_{(ac)}\tag{7}$$

$$\text{ZnS}\_{(s)} + \text{2O}\_{2(ac)} \rightarrow \text{Zn}^{2+}\_{(ac)} + \text{SO}^{2-}\_{4(ac)}\tag{8}$$

$$\text{PbS}\_{(s)} + \text{2O}\_{2(ac)} \rightarrow \text{Pb}^{2+}\_{(ac)} + \text{SO}^{2-}\_{4(ac)}\tag{9}$$

$$\text{FeS}\_2 + \text{14Fe}^{3+}\_{(ac)} + \text{8H}\_2\text{O}\_{(l)} \rightarrow \text{15Fe}^{2+}\_{(ac)} + \text{2SO}\_4^{2-} + \text{16H}^+\_{(ac)}\tag{10}$$

If the oxidation of sulfides remains, the acidity would increase at pH <4, generating acid mine drainage (AMD). However, if the mineralogy of the encasing rock has enough carbonate as calcite (CaCO3), hydroxide and silicate, when dissolution occurs, acid is consumed and neutralization of the AMD is completed [Eq. (11)], originating secondary gypsum precipitates and other metal sulfates [9, 11].

$$\text{CaCO}\_{3\text{ (s)}} + \text{H}\_2\text{SO}\_{4\text{ (ac)}} + \text{H}\_2\text{O}\_{(l)} \to \text{CaSO}\_4 \cdot 2\text{H}\_2\text{O}\_{(s)} + \text{CO}\_2\text{}\_{(g)}\tag{11}$$

However, when acidity-consuming minerals are insufficient, neutralization is not achieved, so continuous generation of AMD dissolves mineral phases, causing supersaturation of ions with high electrical conductivity and generating the precipitation of secondary minerals such as oxyhydroxides of Fe3+ (goethite), hydroxysulfates (jarosite, scorodite and beudantite). Those highly oxidized tailings, are rich in sulfides; the precipitation of these minerals' forms cemented layers known as hardpads, of low porosity and high density that serve as hydraulic barriers and temporary sinks of metals and metalloids, where the precipitation of beudantite and scorodite limits the mobility of As and Pb, and Fe oxyhydroxides adsorb Cd, Pb, Cr, Zn (despite their high mobility) and As where substitution process in jarosite is common [3, 6, 8]. Although Fe oxyhydroxides have a high adsorption capacity for highly toxic metals, it decreases as increasing crystallinity, the larger grain size the lower surface area. Although high concentrations of metals and metalloids reach the aquatic environment by runoff, their mobility will depend on the nature of the sediments and minerals that predominate in the aquatic environment [9].
