PROCESSES CONTROLLING MANGANESE REMOVAL BY CLAY MINERALS IN STREAMS IMPACTED BY ACID MINE DRAINAGE

Many streams in the Rocky Mountains of Colorado are impacted by acid mine drainage. When the pH of these streams is low, iron is the key pollutant, but as the pH increases the iron precipitates and is no longer an issue. In streams with near neutral pH values manganese becomes the main pollutant. Within these streams the rocks are characteristically coated with black manganese dioxide and a chocolate colored floc is found on the streambed. The kinetics of manganese oxidation and precipitation are extremely slow, and manganese dioxide would not be expected to precipitate in the conditions found in these streams. Previous research has found that clay minerals present in weathering rinds on the streambed cobbles, and in stream sediments, play a role in removing manganese from solution in a controlled labratory environment. Research results presented in this paper will address the mechanisms controlling the removal of manganese from solution by clay minerals.