Sulfide immobilization with taconite tailings: products, capacity, and stability

Abstract

Excess sulfate (SO42-) may negatively impact certain freshwater ecosystems. In Minnesota, wild rice (zizania palustris) is threatened by excess sulfate. Several technologies are being researched and developed to remove sulfate from waters where wild rice grows. One of which is a two-part active treatment system that first converts sulfate to sulfide (H2S, HS-, S2-) and then immobilizes that sulfide via reactions with Fe-bearing minerals in tailings from the local mining industry. This dissertation presents studies of the reaction between aqueous sulfide and Fe2+-rich minerals found in taconite tailings and the stability of the reaction products. Chapter 1 introduces the importance of wild rice, the regulatory environment surrounding sulfate, and current sulfate treatment strategies, including the one presented here. The tailings used in this study are from Mesabi Select, a beneficiated tailings product made of primarily siderite and quartz, as well as Fe-rich clay minerals. Chapter 2 contains a study of the reaction of aqueous sulfide with siderite and emphasizes the nature of the reaction product and the determinants of overall sulfide immobilization capacity. In Chapter 3, the sulfidation of Fe-rich clays in Mesabi Select are explored further, firstly by their selective dissolution with various acids and secondly by the reactivity of these acid-treated fractions with sulfide. By comparison, the contributions of individual silicate mineral phases to sulfide immobilization were assessed. Chapter 4 provides an analysis of the stability in oxidizing conditions of tailings that have been sulfidated by a pilot-scale biological sulfate reduction reactor. More specifically, the impacts on sulfate mobility due to differences in tailings mineralogy and total sulfur content pre- and post-sulfidation are discussed. Finally, Chapter 5 provides background on the quantification of aqueous sulfate using a myriad of techniques, both standardized and not, along with recommendations for their application in field- and bench-scale assessments. Taken together, this dissertation will provide the reader with a new understanding of the reactivity between sulfide and Fe2+-rich minerals as it relates to sulfide immobilization for the purpose of sulfate remediation, the products of this reaction, and the stability of those products in oxidizing conditions. More broadly, this work encourages an appreciation of the complex geochemistry of engineered environments and the equal complexity of those environments’ restoration.