Researchers at the Indian Institute of Technology Guwahati (IIT-G) have conducted a study of coal mines in which they demonstrated the 'bioremediation' of acid mine drainage in the coal mines of northeast India. Bioremediation is a process which uses mainly microorganisms, plants, or microbial or plant enzymes to detoxify contaminants in the soil and other environments. The use of either naturally occurring or deliberately introduced microorganisms to consume and break down environmental pollutants helps clean a polluted site.
The new study marks the first time researchers have demonstrated the bioremediation of acid mine drainage from northeastern coalfields using 'constructed wetlands'.
What Is Acid Mine Drainage?
Acid mine drainage is the formation and movement of highly acidic water rich in heavy metals, forming through the chemical reaction of surface water and shallow subsurface water with rocks that contain sulphur-bearing minerals. This results in the generation of sulphuric acid. The chemical reaction of surface water such as rainwater, snowmelt and pond water leads to the formation of the highly acidic water. In other words, acid mine drainage is the runoff produced when water comes in contact with exposed rocks containing sulphur-bearing minerals. These minerals react with water and air to form sulphuric acid and dissolved iron. Then, the acidic run-off dissolves heavy metals including copper, lead, and mercury which pollute ground and surface water.
What Are Constructed Wetlands?
The acidic wastewater generated from coal mines or polymetallic mines contain high amounts of sulphate, iron and various toxic heavy metals. According to the United States Environmental Protection Agency (EPA), constructed wetlands are treatment systems which use natural processes involving wetland vegetation, soils, and their associated microbial assemblages to improve water quality.
The study provides an efficient sustainable treatment approach to mitigate acid mine drainage pollution and also addresses the long-term operational sustainability issues encountered in constructed wetlands receiving acid mine. The scientists developed a biochemical mechanism to understand the functioning of different fundamental processes which co-occur in constructed wetlands.
What Are The Benefits Of The New Research?
The new research is beneficial because it includes optimisation of parameters with the use of simple organic carbon in order to provide an effective sustainable solution for the mitigation of acid mine drainage pollution. The study, led by Professor Saswati Chakraborty, IIT Guwahati, was recently published in the Chemical Engineering journal. The scientists involved in the research conducted a laboratory-scale study in order to demonstrate the potential of the new method for field-scale applications at the northeastern coalfields.
The researchers developed an experimental methodology for the bioremediation of acid mine drainage. In order to treat acid mine drainage in the coalfields, the scientists tried to optimise the ratio of chemical oxygen demand (COD) to sulphate in the constructed wetlands. The COD is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution. The higher the chemical oxygen demand in water, the greater the levels of oxidisable organic matter and consequently, the lower the amount of Dissolved Oxygen (DO).
How The New Technique Helps Mitigate Acid Mine Drainage Pollution
Optimisation of the ratio helped eliminate high acidity, sulphate and metals. This will help control water pollution and improve water quality through ecosystem restoration of coalfields in northeast India.
According to the study, the implementation of the new technology will ensure ecosystem restoration. Also, constructed wetlands can successfully be used for the treatment of the acidic wastewater generated in coalfields.
The researchers used a low-cost and easily available carbon source called lactate. This helped perform the remediation of acid mine drainage wastewater. Sulphate-reducing bacteria were grown in the constructed wetlands using organics like lactate. The bacteria helped reduce sulphate to sulphide.
The biochemical process resulted in the increase of wastewater pH to 6-6.5. Subsequently, iron and other soluble metals precipitated as metal sulphide and metal hydroxide in the constructed wetlands.
Therefore, the new study provides an excellent experimental explanation for a sustainable long-term treatment approach to mitigate acid mine drainage pollution using a simple organic carbon source in constructed wetlands.