English

Abstract

Oxygen has been considered the main driver of the weathering processes in mining wastes, omitting the role of microbes. Among many approaches to control oxygen access to the wastes, in situ treatment of the mineral surface has been tried since the late eighties. Various materials including NPR (Natural Phosphate Rock) were added with the expectation of finding an iron-phosphate coating. Irregular and inconsistent results were obtained when the effluents were evaluated according to NPR stoichiometry; however, the lower dosages showed some improvements in the effluents. Since the results did not consistently produce iron phosphate, any positive effects on effluents were considered accidental and the approach abandoned. We suspected microbes at work based on basic ecological considerations. Hence 1991, we began experimenting on tailings and waste rock with additions of NPR, postulating that if chemo-lithotrophic microbes on the mineral surface accelerate oxidation, then heterotrophic (oxygen-consuming) microbes would reduce oxidation. Samples from tailings plots where NPR was tilled into the surface were tested for pore-water quality after eight years. Effluents from waste rock exposed outdoors in drums were monitored for 2.7 years. Repeatedly, the one-time addition of NPR produced effluents with elevated pH and low metal acidity. Later, microscopic investigations of the rocks found an organic layer on the mineral surfaces. Investigations by scientists in 6 different universities confirmed the presence of a biofilm as the cause of the reduced acid generation. In 2013, heterotrophs were identified and quantified as they covered the surface of German lignite, following a bioleach testing protocol starting at pH around 1. These findings conclusively showed that the development of heterotrophic biofilms and improved effluents from sulfidic mine wastes are a consequence of adding waste NPR. We conclude that sufficient evidence has been gathered to prove that the geo- microbial control or in situ control of sulfide oxidation is a viable concept. It needs to be pursued to control or curtail acid mine drainage now and in the future. In this paper, we document the evolving ecological thought process over 23 years of research, which lead step by step toward understanding of the effects of NPR on the reduction of sulfide oxidation.