Trace Element Cycling during Iron(II)-activated Recrystallization of Iron(III) Oxide Minerals

Biogeochemical iron cycling initiates secondary abiotic reactions between aqueous Fe(II) and Fe(III) oxide minerals, which results in dynamic recrystallization via simultaneous Fe(II) oxidative adsorption and Fe(III) reductive dissolution. Fe(III) oxide minerals are abundant in soils, sediments, and groundwater systems, and often control the fate and transport of trace elements. A robust understanding of their reactivity with Fe(II) and how associated trace elements are affected during Fe(II)-activated recrystallization is required to predict the effect of biogeochemical processes on contaminant fate and micronutrient availability. The main objective of the research presented in this dissertation is to characterize how Fe(II)-activated recrystallization of iron oxide minerals affects the cycling and fate of associated trace elements. The specific foci are to: 1) obtain a general description of redox-inactive trace element cycling through iron oxide minerals, 2) examine the chemical controls on net trace element release from goethite and hematite, 3) explore surface passivation and trace element release inhibition during Fe(II)-activated recrystallization of iron oxides containing insoluble elements, and 4) determine the fate of redox-sensitive metals that are structurally incorporated in iron oxides during reaction with Fe(II). Compositional measurements and spectroscopic results show that Ni is cycled through the minerals goethite and hematite during Fe(II)-activated recrystallization. Adsorbed Ni becomes progressively incorporated into the minerals while Ni pre-incorporated into iron oxides is released to solution. The kinetics of Ni and Zn release to solution are primarily controlled by the amount of Fe(II) sorption. Furthermore, these structurally-incorporated trace elements are mobilized from iron oxides into fluids without net iron reduction. The Fe(II)-activated release of Ni and Zn from goethite and hematite is substantially inhibited when the insoluble elements Al, Cr, and Sn are co-substituted within the mineral structures. Incorporation of Al into goethite substantially decreases the amount of Fe atom exchange between aqueous Fe(II) and Fe(III) in the mineral and, consequently, the amount of Ni release from the structure. This implies that the mechanism for trace element release inhibition, following substitution of insoluble elements, is a decrease in the amount of mineral recrystallization. Reaction of Cu(II)-, Co(III)-, and Mn(III,IV)-substituted goethite and hematite with Fe(II) results in the reduction and release of Cu, Co, and Mn to solution. This work suggests that important proxies for ocean composition on the early Earth may be invalid, identifies new processes that affect micronutrient availability, contaminant transport, and the distribution of redox-inactive trace elements in natural and engineered systems, and shows that redox-sensitive elements are susceptible to reduction and release to solution despite being incorporated within a stable mineral structure. Furthermore, this work illustrates that naturally occurring iron oxides that contain insoluble impurities are less susceptible to Fe(II)-activated recrystallization and exhibit a greater retention of trace elements and contaminants than pure mineral phases. These discoveries demonstrate that, in the presence of Fe(II), iron oxide minerals are not passive surfaces that merely adsorb ions but rather their entire volume equilibrates with fluids. Such advances expand our view on the potential impacts of iron cycling on the fate of trace elements and contaminants