Geochemical Reactions in the Unsaturated Zone of the Pyritic Mine Wastes

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

ABSTRACT Geochemical Reactions in the Unsaturated Zone of Pyritic Mine Wastes By J. Donald Rimstidt There are over 60 billion tons of mine wastes from non-fuel mineral production in the US today and more are being produced at a rate of about 2 billion tons annually. Although most of these wastes are relatively benign, consisting of common rocks and soils, a significant fraction contains iron sulfide minerals, especially pyrite, that can oxidize to produce an acidic sulfate solution, often containing high concentrations of toxic metals. Discharge of this acid mine drainage (AMD) into nearby streams and lakes is responsible for the ecological impairment of thousands of miles of streams. Furthermore, AMD reduces water quality often making it unfit for human consumption or for use as industrial process water. Thus, AMD is a multi-billion dollar societal problem that can benefit significantly from an organized scientific and engineering research program. We propose to study chemical reactions that occur at the air/mineral and air/brine/mineral interfaces in the unsaturated parts of pyritic waste piles. Specifically, we will measure the rates of oxidation of pyrite in moist air as a function of the partial pressure of oxygen, relative humidity, and temperature. This oxidation produces sulfuric acid and ferrous sulfate and is the primary mode of acid production. Furthermore, we will investigate the evolution of the ferrous sulfate as it evolves via oxidation, dehydration, and neutralization through a series of ferrous/ferric hydroxysulfate minerals. These minerals are a secondary source of acid production and are hosts of many of the trace metals that cause environmental degradation. They are of particular concern because they are quite soluble so they dissolve rapidly during rain events to release large amounts of acidity and trace metals to nearby receiving waters. Our goal is to provide details about the thermodynamic stability and rates of transformation of these sulfate minerals so we can predict the evolution of this reservoir of latent acidity and trace metals. We believe that this information will provide valuable guidance to the engineers and geochemists that are designing new waste disposal facilities or who are preparing remediation plans for abandoned mine wastes.