Question

Question: Acid mine drainage (AMD) occurs when geologic deposits containing pyriteare exposed to oxygen and moisture. AMD is generated in a multistep process catalysed by acidophilic (acid-loving) bacteria. Balance each step and identify those that increase acidity:

1. ${\mathbf{\text{FeS}}}_{\mathbf{\text{2}}}{\mathbf{\text{(s) +O}}}_{\mathbf{\text{2}}}\mathbf{\text{(g)}}\mathbf{\to }{\mathbf{\text{Fe}}}^{\mathbf{\text{2 +}}}\mathbf{\text{(aq) + S}}{{\mathbf{\text{O}}}_{\mathbf{\text{4}}}}^{\mathbf{\text{2 -}}}\mathbf{\text{(aq)}}$
2. ${\mathbf{\text{Fe}}}^{\mathbf{\text{2 +}}}{\mathbf{\text{(aq) +O}}}_{\mathbf{\text{2}}}\mathbf{\text{(g)}}\mathbf{\to }{\mathbf{\text{Fe}}}^{\mathbf{\text{3 +}}}{\mathbf{\text{(aq) +H}}}_{\mathbf{\text{2}}}\mathbf{\text{O(l)}}$
3. ${\mathbf{\text{Fe}}}^{\mathbf{\text{3 +}}}{\mathbf{\text{(aq) +H}}}_{\mathbf{\text{2}}}\mathbf{\text{O(l)}}\mathbf{\to }{\mathbf{\text{Fe(OH)}}}_{\mathbf{\text{3}}}{\mathbf{\text{(s) + 12H}}}^{\mathbf{\text{+}}}\mathbf{\text{(aq)}}$
4. ${\mathbf{\text{FeS}}}_{\mathbf{\text{2}}}{\mathbf{\text{(s) + Fe}}}^{\mathbf{\text{3 +}}}\mathbf{\text{(aq)}}\mathbf{\to }{\mathbf{\text{Fe}}}^{\mathbf{\text{2 +}}}\mathbf{\text{(aq) + S}}{{\mathbf{\text{O}}}_{\mathbf{\text{4}}}}^{\mathbf{\text{2 -}}}\mathbf{\text{(aq)}}$

Short answer

Steps (1), (3), and (4) enhance acidity, because ${\mathrm{H}}^{+}$is generated

Step-by-step solution

 Concept Introduction

Acid mine drainage (AMD) is the discharge that occurs when water interacts with accessible rocks containing sulphur-bearing minerals, causing sulphuric acid and dissolved iron to develop. Heavy metals such as copper, lead, and mercury are dissolved in the acidic run-off, polluting ground and surface water.

 Identify the steps which increase the acidity

Let’s consider each steps:

$2{\text{FeS}}_2(\text{s})+7{\text{O}}_2\left(g\right)+2{\text{H}}_2\text{O}\left(\text{l}\right)\to 2{\text{Fe}}^{2+}\left(aq\right)+4{\text{SO}}_4^{2-}\left(aq\right)+4{\text{H}}^{+}\left(aq\right)$ (1)

$4{\text{Fe}}^{2+}\left(aq\right)+{\text{O}}_2\left(g\right)+4{\text{H}}^{+}\left(aq\right)\to 4{\text{Fe}}^{3+}\left(aq\right)+2{\text{H}}_2\text{O}\left(\text{l}\right)$ (2)

$4{\text{Fe}}^{3+}\left(aq\right)+12{\text{H}}_2\text{O}\left(\text{l}\right)\to 4\text{Fe}(\text{OH}{)}_3\left(s\right)+12{\text{H}}^{+}\left(aq\right)$ (3)

${\text{FeS}}_2\left(s\right)+14{\text{Fe}}^{3+}\left(aq\right)+8{\text{H}}_2\text{O}\left(\text{l}\right)\to 15{\text{Fe}}^{2+}\left(aq\right)+2{\text{SO}}_4^{2-}\left(aq\right)+16{\text{H}}^{+}\left(aq\right)$ (4)

Therefore, the steps which increase the acidity are 1, 3 and 4