**2. Processes involving in mobility of heavy metals in natural waters**

The mobility of metals and metalloids in aquatic compartments is a very complex phenomenon as it involves a great variety of physical, chemical and biological processes mainly determined by pH values, precipitation and dissolution of secondary minerals, sorption–desorption reactions, hydrolysis processes, by oxidation– reduction processes and co-adsorption processes, for instance: graphene oxide a very soluble chemical specie, it is commonly found in aquatic environments, adsorbed on inorganics contaminants such as the hematite and goethite, which co-adsorbed graphene oxide about 92% at pH 3-5 [10].

The degree sorption/desorption of metals depend on time of contact between sorbate and sorbent and oxide aging due to the weathering. The adsorption and precipitation can be simultaneous but can dominate a mechanism due to reaction conditions and metal involved. When the precipitate consists of species derived from both aqueous solution and dissolution of the mineral, it is referred to as a coprecipitate [7].

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

Overall, Zn, Cd, Cu and Al cations have high concentrations in acidic water and undergone high mobility, while oxyanions such as SO4 <sup>2</sup>�, AsO4 <sup>3</sup>�, MoO4 <sup>2</sup>�, CrO4 2� increase their mobility when water is neutral or alkaline [4, 11].

#### **2.1 Cation exchange adsorption**

Non-specific or cation exchange adsorption, also known as physical adsorption, is an electrostatic phenomenon that occur on the surface of clays (kaolinite, illite, montmorillonite, vermiculite, smectite and chlorite) and it is caused by the weathering of olivine, augite, pyroxene, mica and feldspar. Structure of clays are characterized by thick microcrystalline sheets composed of tetrahedral layers of silica and octahedral layers of aluminum in 1: 1 or 2:1 proportion [1].

Cation exchange is carried out by less selective outer-sphere clusters. The cations are bound to the surface of the negatively charged clay through weak covalent bonding independent of the aqueous pH value. This type of phenomenon is reversible in nature, and occurs very quickly, typically controlled by diffusion and electrostatic reactions, while the smallest ions of the aqueous phase are exchanged for larger ions on the surface of clay, for example, Mg2+ exchange Al3+ and Al3+ exchange Si4+. The number of cations reversibly adsorbed per unit weight of adsorbent (e. g. clay) is called cation exchange capacity (CEC) [2].

### **2.2 Specific adsorption**

The specific adsorption is also known as chemisorption or surface complexation. It is mainly carried out in oxides of Fe, Al, and Mn. The ions, either cations or anions, are highly bonded on the surface of the oxides by covalent bonding with oxygen atom or OH groups. The Fe or Mn oxide are electrically charged by the adsorption or release of H<sup>+</sup> ions, from the oxygen atoms at the interface between the mineral and the solution. Because oxides are amphoteric chemical compounds, they have negative and positive charges on their surface, and the net charge is largely symmetric about at zero point, at a characteristic pH value. The pzc, stand for zero point of charge varies between several oxides compound; Fe oxides have a pzc between pH 7.0 and 8.5 which implies a positive charge on their surface. While the pzc of Mn oxides varies between pH 1.5 and 4.6, which indicates that they have a net negative charge on their surface [1]. It is carried out on the surface of Fe and Mn oxides with variable charges and complexation with MO functional groups, by weak electrostatics charges with pH-dependent bonding.

#### **2.3 Hydrolysis**

In pure water at 25°C, [H3O<sup>+</sup> ] = [OH�] = 1.0 x 10�<sup>7</sup> M and pH = 7.0 (neutral pH); when dissolving NaCl in water there is no an appreciable hydrolysis reaction and the pH of the solution remains at a value of 7.0. However, when ammonium chloride (NH4Cl) is added to the water, pH drops below 7.0, it means that [H3O<sup>+</sup> ] > [OH�], and when sodium acetate (NaC2H3O2) is dissolved in water, pH increases above 7.0, it means that [OH�] > [H3O<sup>+</sup> ]. In general, salts containing an ion of an alkali or alkaline earth metal (except Be2+) do not significantly hydrolyze; thus, when a substance is added to water and dissociated, one of its ions causes a change in the pH of water (pH 6¼ 7), it is at that moment when we speak of a hydrolysis reaction. In fact, all positive ions react with water to produce an acidic solution [12, 13].

$$\text{4Fe}^{2+} + \text{O}\_2 + \text{10H}\_2\text{O} \rightarrow \text{Fe}(\text{OH})\_3 \tag{1}$$

$$\text{Fe}^{3+} + \text{3H}\_2\text{O} \longleftrightarrow \text{Fe(OH)}\_{3\text{ (s)}} + \text{3H}^+ \tag{2}$$

In deposits of tailings containing high levels of sulfides such as pyrite and pyrrhotite, during complex oxidation process, Fe2+ dissolves and when reacting with water, carries out a hydrolysis reaction, increasing the acidity of the medium by decreasing the pH value [Eq. (1)] [8, 9]. Likewise, when Fe2+ is oxidized to Fe3+ and pH values >5, a hydrolysis reaction occurs, releasing H<sup>+</sup> and ferric hydroxide (reaction 2) that generates an oxyhydroxide [4]. However, the increase in acidity in a mining waste, even though it reaches the aquatic environment, does not necessarily implies that the water has low pH values since the presence of carbonate minerals can neutralize the acidity.
