Design criteria and required chemistry for removing manganese in acid mine drainage using subsurface flow wetlands

Manganese (Mn) is a difficult metal to remove in acid mine drainage because of high pH requirements for oxidation of Mn to form Mn oxide precipitates, However, Mn removal can be quickened in a gravel system providing a large surface area and high pH for Mn oxidation to occur with microbiological mediation. An experiment was designed with 12 gravel bed mesocosms that were 9.8 m(3) X 6.7 m(3) x 0.6 m(3) to determine the best design criteria for optimizing Mn removal. Treatments consisted of two Mn loading rates (1.1 and 2.7 g/m(2).d) and two gravel types (limestone and river gravel) and were replicated three times, Water flowed through the experimental wetlands for 734 days. Manganese removal was more effective in limestone than in river gravel. Manganese removal was not affected by water temperature ranging from 5 to 30 degreesC in either rock material. Manganese removal rates ranged from 100 to 600 m/a in the limestone wetlands and 10 to 60 m/a in the river gravel wetlands. Greater pH in limestone (approximately 6.9) compared with river gravel (approximately 5.5) favored Mn oxide precipitation. Greater pH, coupled with oxidation-reduction potential (redox) values greater than 430 mV in the limestone. resulted in water chemistry near conditions predicting manganite to be the dominant Mn phase. Ideal pH and redox conditions for Mn removal are pH from 6.8 to 7.2 and redox greater than 500 mV. The range of dissolved oxygen (DO) required to remove Mn in the various wetlands ranged from 3 to 5 mg/L, with approximately one-half of the DO loss caused by Mn hydroxide formation and one-half ascribed to biological consumption, The influent DO should be at least 0.35 mg/L for every I mg/L Mn removed. Removal rates for Mn ranged from I to 17 g/m(2).d in limestone and from I to 2 g/m(2).d in river gravel. Limestone is the material of choice for subsurface flow wetlands for Mn removal. Removal rates and required chemistry determined in this study can be used to design subsurface flow wetlands for optimum Mn removal.