MINERALOGY OF IRON PRECIPITATES IN A CONSTRUCTED ACID-MINE DRAINAGE WETLAND

Precipitates forming at various stages of acid mine drainage treatment in a high metal load (approximate to 1000 mg L(-1) Fe) and low pH (approximate to 3) constructed wetland were characterized by chemical dissolution, x-ray diffraction, thermal analysis, and scanning electron microscopy. Minerals precipitating in flumes and in entry wetland cells lacking vegetation included poorly crystalline ferrihydrite, lepidocrocite, goethite, possibly an akaganeite-like mineral, and high Fe/S ratio Fe oxyhydroxysulfates (similar to schwertmannite). Within vegetated wetland cells Lined with crushed limestone, well-crystallized gypsum, lepidocrocite, and Fe-oxyhydroxysulfate minerals with low Fe/S ratios were accompanied by gradual reductions in ferrihydrite and akaganeite. The Fe/S molar ratios of Fe oxyhydroxysulfates in flume precipitates averaged 5.2 +/- 0.3, while those of cell precipitates averaged approximate to 3.5 +/- 0.5. The oxalate-extractable (Fe-ox) to total (Fe-t) Fe fraction of the precipitates was considerably higher in wetland cells (1.1 +/- 0.3), where organic C was 10-fold higher than in entry flumes (0.7 +/- 0.1). Scanning electron micrographs of flume precipitates showed a fiber-like morphology of densely aggregated spherical particles, 1.5 to 2.0 mm in diameter, with a closely packed microcrystalline matrix. Precipitates collected from vegetated wetland cells formed aggregates of somewhat smaller diameter spherical particles with grassy surfaces or finger-like projections entangled with bacterial cells. The overall composition of the precipitates suggested that the Fe chemistry is controlled primarily by the solubility of Fe oxyhydroxides in flumes and by S-enriched Fe oxyhydroxysulfates inside the wetland cells. Although jarosite and goethite are thermodynamically favored in the wetland cells, their formation appeared to be inhibited by the presence of organics and the precipitation of Fe oxyhydroxysulfates and gypsum.