Evolving reactivity and products of natural iron-bearing minerals toward sulfide

Abstract

Iron minerals play dynamic roles in the sulfur cycle in natural environments and engineered systems. Although pure or synthesized iron minerals are widely studied for their importance in the sulfur cycle, heterogeneous iron-bearing materials are important since they are found naturally and may be economically feasible for engineered sulfur treatment. In this study, reaction capacities, products, and kinetics of two natural iron-bearing minerals derived from taconite towards aqueous sulfide were studied using batch and column reactors. Siderite (SR)- and iron oxide (IO)-rich natural minerals were selected based on distinction in iron mineralogical characteristics and quantity. The siderite rich SR reacted with more sulfide per gram (FeR), over the iron oxide rich IO. SR produced more solid phase sulfur, liberated by 9 N HCl (AVS) and chromic acid, where IO produced more elemental sulfur and thiosulfate. Sulfate as an oxidation product was minimal for both materials in batch and column reactors. Reaction kinetics were dependent upon the molar ratio FeR/HSinitial. Generally, initial reaction rate increased with increasing FeR/HSinitial, or as iron becomes the dominant reactant. SR maintained an appreciable reaction rate with each sequential spike and differing particle sizes, reaction rate constant (k) falling only slightly below the expected relationship. IO exhibited a high initial reaction rate, but its reaction rate constant decreased substantially with sequential sulfide spikes and varied greatly with larger particle sizes. In continuous flow column reactors, both materials FeR and AVS increased over batch reactor values. A reactive-transport model was developed using batch reactor kinetic results to establish reaction rate, k, dependence on FeR/HSinitial, and was used to describe sulfide evolution through a column. In summary, the selected materials generally reacted predictably with sulfide with respect to their iron mineralogy and experienced predictable sulfur reaction kinetics and capacities across different loading scenarios. These results represent an important step toward elucidating the reactivity of heterogeneous iron-bearing materials at the interfacial processes with the cycles of Fe and S and sulfur treatment systems.