Abstract: In nature, pyrite is initially oxidized by atmospheric O2, releasing acidity and Fe2+. At pH below 3.5, Fe2+ is rapidly oxidized by T. ferrooxidans to Fe3+, which oxidizes pyrite at a much faster rate than O2. Commonly, limestone is used to prevent pyrite oxidation. This approach, however, has a short span of effectiveness because after treatment the surfaces of pyrite particles remain exposed to atmospheric O2 and oxidation continuous abiotically. Currently, a proposed mechanism for explaining non-microbial pyrite oxidation in high pH environments is the involvement of OH- in an inner-sphere electron-OH exchange between pyrite/surface-exposed disulfide and pyrite/surface-Fe(III)(OH)n3-n complex and/or formation of a weak electrostatic pyrite/surface-CO3 complex which enhances the chemical oxidation of Fe2+. The above infer that limestone application to pyritic geologic material treats only the symptoms of pyrite oxidation through acid mine drainage neutralization but accelerates non-microbial pyrite oxidation. Therefore, only a pyrite/surface coating capable of inhibiting O2 diffusion is expected to control long-term oxidation and acid drainage production. The objective of this study was to examine the feasibility in controlling pyrite oxidation by creating, on pyrite surfaces, an impermeable phosphate or silica coating that would prevent either O2 or Fe3+ from further oxidizing pyrite. The mechanism underlying this coating approach involves leaching mine waste with a coating solution composed of H2O2 or hypochlorite, KH2PO4 or H4SiO4, and sodium acetate (NaAC) or limestone. During the leaching process, H2O2 or hypochlorite oxidizes pyrite and produces Fe3+ so that iron phosphate or iron silicate precipitates as a coating on pyrite surfaces. The purpose of NaAC or limestone is to eliminate the inhibitory effect of the protons (produced during pyrite oxidation) on the precipitation of iron phosphate or silicate and to generate iron-oxide pyrite coating, which is also expected to inhibit pyrite oxidation. The results showed that iron phosphate or silicate coating could be established on pyrite by leaching it with a solution composed of: (1) H2O2 0.018-0.16 M; (2) phosphate or silicate 10-3 to 10-2 M; (3) coating-solution pH [approximate]5-6; and (4) NaAC as low as 0.01 M. Leachates from column experiments also showed that silicate coatings produced the least amount of sulfate relative to the control, limestone and phosphate treatments. On the other hand, limestone maintained the leachate near neutral pH but produced more sulfate than the control.

Abstract: Sulfate reducing passive bioreactors are increasingly being used to remove metals and raise the pH of acidic waste streams from abandoned mines. These systems commonly use a variety of organic substrates (i.e. manure, wood chips) for sulfate reduction. The effectiveness of these systems decreases as easily accessible reducing equivalents are consumed in the substrate through microbial activity. Using column studies at room temperature (23-26 degrees C), we investigated the addition of lactate and methanol to a depleted manure substrate as a method to reactivate a bioreactor that had lost >95% of sulfate reduction activity. A preliminary experiment compared sulfate removal in gravity fed, flow through bioreactors in which similar masses of each substrate were added to the influent solution. Addition of 148 mg/l lactate resulted in a 69% reduction in sulfate concentration from 300 to 92 mg/l, while addition of 144 mg/l methanol resulted in an 88% reduction in sulfate concentration from 300 to 36 mg/l. Because methanol was found to be an effective sulfate reducing substrate, it was chosen for further experiments due to its inherent physical properties (cost, low freezing point and low viscosity liquid) that make it a superior substrate for remote, high elevation sites where freezing temperatures would hamper the use of aqueous solutions. In these column studies, water containing sulfate and ferrous iron was gravity-fed through the bioreactor columns, along with predetermined methanol concentrations containing reducing equivalents to remove 54% of the sulfate. Following an acclimation period for the columns, sulfate concentrations were reduced from of 900 mg/l in the influent to 454 mg/l in the effluent, that reflects a 93% efficiency of electrons from the donor to the terminal electron acceptor. Iron concentrations were reduced from 100 to 2 mg/l and the pH increased nearly 2 units. (C) 1999 Elsevier Science Ltd.