**3. Occurrence of acid mine drainage**

AMD normally has a lower value of pH, higher specific conductivity, high concentration of heavy metals such as iron, aluminum, and manganese, and low concentration of heavy metals viz. chromium, nickel, cobalt, arsenic, and so on. The pyrite mineral which is responsible for occurrence of AMD is shown in **Figure 4**. In the current scenario, AMD is left untreated due to inadequate, underdeveloped technologies and or infeasible processes (expensive) in various parts of the globe. The acid generation reaction due to pyrite oxidation, which is widely known as one of the sulphide minerals is given in Eq. (1). The oxidation reaction results in dissolved Fe, sulphate, and hydrogen as reaction products [9, 10].

$$\text{FeS}\_2 + \frac{7}{2}\text{.}\text{O}\_2 + \text{H}\_2\text{O} \rightarrow \text{Fe}^{2+} + 2\text{.}\text{SO}\_4^{2-} + 2\text{.}\text{H}^+\tag{1}$$

As the reaction indicated in Eq. (1) moves in the forward direction, the reaction products ferrous iron, sulphate, and hydrogen cataion increase the total dissolved solids (TDS) and hence acidity by lowering pH of solution [11]. If the adjacent surroundings get sufficiently oxidized (depending on oxygen concentration, pH, and microbial activity), much of the Fe2+ will be oxidized into Fe3+ as expressed in Eq. (2).

**Figure 4.** *Pyrite mineral.*

*Water Quality - Factors and Impacts*

$$\mathrm{Fe^{2+}} + \frac{1}{4}\mathrm{.}O\_{2} + \mathrm{H^{+}} \rightarrow \mathrm{Fe^{3+}} + \frac{1}{2}\mathrm{.}H\_{2}\mathrm{O} \tag{2}$$

For pH equals 2.3 and 3.5, the ferric iron (Fe3+) precipitates as Fe(OH)3 and [KFe3(SO4)2(OH)6], respectively, a low Fe3+ retains in solution which lowers the pH.

$$\text{Fe}^{3+} + \text{3.}H\_2O \xrightarrow{} \text{Fe(OH)}\_3 \text{ solid} + \text{3H}^+ \tag{3}$$

The leftover Fe3+ in Eq. (2) which remains unreacted in Eq. (3) might promote oxidation of additional pyrite as per Eq. (4).

$$\rm FeS\_2 + 14Fe^{3+} + 8.H\_2O \rightarrow 15.Fe^{2+} + 2.SO\_4^{2-} + 16H^+ \tag{4}$$

The aforementioned basic reactions suggest that the acid generation produces ferric iron which gradually precipitates into Fe(OH)3 and may be represented as Eq. (5) which is a combined reaction of Eqs. (1) and (3).

$$\text{FeS}\_2 + \frac{15}{4}\text{.}\text{O}\_2 + \frac{7}{2}\text{.}\text{H}\_2\text{O} \rightarrow \text{Fe(OH)}\_3 + 2\text{.}\text{SO}\_4^{2-} + 4\text{H}^+\tag{5}$$

In another way, Eq. (6) represents the overall reaction for stable Fe3+ used to oxidize additional pyrite.

$$\mathrm{FeS}\_2 + \frac{15}{8} \mathrm{.} O\_2 + \frac{13}{2} \mathrm{.} \mathrm{Fe^{3+}} + \frac{17}{4} \mathrm{.} H\_2O \to \frac{15}{2} \mathrm{.} \mathrm{Fe^{2+}} + 2 \mathrm{.} \mathrm{SO\_4^{2-}} + \frac{17}{2} \mathrm{.} H^+ \tag{6}$$

In all of the above equations except Eqs. (2) and (3), the oxidant and oxidized mineral are presumed as oxygen and pyrite, respectively. However, pyrrhotite and chalcocite minerals contain altered proportions of metal sulfide and also metals excluding iron [12].

When the water is adequately acidic, acidophilic microbes that flourish at low pH can build up themselves. The microorganism "Thiobacillus Ferroxidans" is assumed to take a huge part in accelerating the synthetic response occurring in mine water circumstances, i.e., these microbes catalyze the oxidation of Fe2+. Another microorganism "Ferroplasma Acidarmanus" has recently been found to play an important role in acid generation in the source water.

Although the formation of H+ as a result of certain metals precipitations expressed in Eqs. (7) and (8) are not the major acidity sources, these also are considered as treatment alternatives [13].

$$\text{Fe}^{+3}/\text{Al}^{+3} + \text{3.H}\_2\text{O} \rightarrow \text{Fe}(\text{OH})\_3/\text{Al}(\text{OH})\_3 + \text{3.H}^+\tag{7}$$

$$\text{Fe}^{+2}/\text{Mn} + + 0.25\text{O}\_2\text{ (aq.)} + 2.5\text{H}\_2\text{O} \rightarrow \text{Fe}(\text{OH})\_3/\text{Mn}(\text{OH})\_3 + 2\text{H}^+\tag{8}$$

Different metals are normally found in AMD because they are available in rocks, like pyrite. There are different metal sulphides viz. ZnS, PbS, NiS, CdS, CuS, etc. which may deliver metal particles into solution but may not produce acidity. The key factors determining the acid generation rate are as follows.


*Performance Evaluation of Waste Materials for the Treatment of Acid Mine Drainage… DOI: http://dx.doi.org/10.5772/intechopen.99669*


In the special case where microbial acceleration is significant, some other factors such as activation energy (biological), population density (microbes), and growth rate determine the activity of bacterial. The growth rate depends on pH, temperature, and the presence of various nutrients like nitrate, potassium, ammonia, phosphorous and CO2 content.
