Chapter 22, Problem 1AQ

Acid mine drainage is in part a chemical process and in part a biological process. Discuss the chemistry and microbiology that lead up to acid mine drainage and point out the key reactions that are biological. What ways can you think of to prevent acid mine drainage? How might you prevent further generation of acid drainage?

Summary Introduction

To explain:

The chemistry and microbiology which is involved in the process of acid-mine drainage. Also, explain the ways by which acid mine drainage and further generation of acid can be prevented.

Concept introduction:

Acid mine drainage is a process of outflow of extreme acidic water from coal mines or metal mines. In the majority of cases, acid primarily comes from the oxidation of iron sulfide which is found in conjunction with other valuable metals. This process is accompanied by both spontaneous and microbiological reactions.

Explanation of Solution

Acid mine drainage is a major environmental problem which is mediated by both spontaneous and microbiological reactions. During acid mine drainage, oxidation of iron sulfide occurs as a combination of both chemical and bacterial mediated reactions in which oxygen and ferric ions act as an electron acceptor.

When iron sulfides are first exposed in a mining process, an initiator reaction starts which oxidize hydrogen sulfide into sulfates and develops acidic conditions as ferric ions are released. Bacteria such as Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans are involved in biological processing of acid mine drainage.

The biological reaction which is involved in the process is described as below as:

${\text{FeS}}_{\text{2}}+{\text{ 14Fe}}^{\text{3+}}+{\text{ 8H}}_{\text{2}}\text{O}\to {\text{15Fe}}^{\text{2+}}+{\text{ 2SO}}_{\text{4}}{}^{\text{2-}}+{\text{ 16H}}^{\text{+}}$

Mixing of acidic mine water into lakes and rivers degrade the quality of water due to the presence of acids and dissolved metals such as aluminum, iron, lead, and cadmium. These compounds are very toxic to aquatic organisms. Various steps which can be taken in order to reduce acid mine drainage are as follows:

(1) Addition of lime or other alkaline compounds can be done in lakes and rivers containing acids. This would help in the neutralization of acids.

(2) Addition of uncontaminated soil and plantation of vegetation is also an effective measure to prevent acid mine drainage.

A further generation of acid drainage can be prevented by keeping a check on mining of coals and by making special treatment plans for neutralization of acidic water.