The role of micro-organisms in the formation of acid mine drainage in the North Eastern Coalfield of India

Abstract

The paper deals with the nature of mine drainage in the North Eastern Coalfield, Margarita, Assam (India). The drainages from the Margerita group of mines are found to be acidic with high sulphate ranging up to 1500 ppm and iron content rising up to 40 ppm. The total sulphur in coals is up to 7% out of which 50–80% is non-reactive organically found sulphur. The acidity mainly arises from the oxidation of pyrites of coal. Microscopic studies reveal that both the reactive (size 0–5 micron) as well as stable (50 micron) pyrites are present in coal samples studied. Leaching studies indicate that oxidation of reactive pyrites followed by dissolution of sulphate sulphur is sufficient to produce the observed acidity and organic sulphur does not seem to play any significant role in acid production in mine drainages. The presence and chemical activity of iron oxidising; sulphur oxidising; and iron sulphur oxidising bacteria in mine water was ascertained by the chemical oxidation of ferrous iron to ferric iron; decrease in pH and production of acid. Iron and sulphur oxidising bacteria which are having maximum activity were examined under electromicroscope for morphological study. The rod shaped bacteria with rounded ends, size varying from 1.20–3.20 micron in length and 0.51–1.25 micron in breadth are identified asThiobacillus ferrooxidans. This bacteria is found to accelerate the sulphur leaching rate from coal and is indigenous to mine drainages.