Redox Behavior of Secondary Solid Iron Species and the Corresponding Effects on Hydroxyl Radical Generation during the Pyrite Oxidation Process

Abstract

During the pyrite oxidation process, aqueous ferrous/ferric ions (Fe 2+ /Fe 3+ ), as well as surface-adsorbed Fe 2+ /Fe 3+ , have been widely recognized to dominate hydroxyl radical ( • OH) generation, while this study reveals that the secondary solid iron species also play nonnegligible roles. Based on the different forms and the presence of sites, the secondary solid iron species were classified as Fe coat (iron-containing coating on the pyrite surface) and Fe dep (ex situ-deposited iron (oxyhydr)oxide that is not in contact with pyrite). Instead of participating in building a stubborn passivation layer on the pyrite surface, Fe coat is easy to fall off from the pyrite surface as the oxidation of pyrite deepens, while large fractions of Fe dep and Fe coat are found to be extractable with nitrilotriacetic acid (NTA). Achieved by cyclically oxidizing pyrite within different NTA levels (0/0.1/10 mM), Fe coat and Fe dep were proved to have distinct redox behavior during the pyrite oxidation process. Amorphous Fe dep , originated from the hydrolyzation of dissolved Fe 3+ , accelerates the nonradical decay of hydrogen peroxide (H 2 O 2 ); as a result, the accumulation of Fe dep always decreases the • OH production during the pyrite oxidation process. However, part of Fe dep adsorbs on the pyrite surface through electrostatic attraction and converts into Fe coat . The electron conduction between Fe coat and pyrite was verified, which accelerates the oxidative dissolution of pyrite, produces reactive Fe(II), and therefore favors • OH generation. This study improves our understanding of the redox behavior of pyrite in complex media such as natural processes and practical engineering systems.

Redox Oscillations Activate Thermodynamically Stable Iron Minerals for Enhanced Reactive Oxygen Species Production

“…Redox oscillations could activate thermodynamically stable pyrite to produce RAMPs. Given that the crystallinity of iron mineral controls the redox activity following the order of adsorbed Fe > amorphous Fe > crystalline Fe, the above results suggest that redox fluctuations likely promote the production of reactive Fe­(II) and amorphous Fe on the surface of pyrite . Specifically, the abiotic oxidation of pyrite by air and hydrogen peroxide could produce metastable secondary ferric oxyhydroxides coated on the surface of native pyrite at low tide .…”

“…Given that the crystallinity of iron mineral controls the redox activity following the order of adsorbed Fe > amorphous Fe > crystalline Fe, 39 the above results suggest that redox fluctuations likely promote the production of reactive Fe(II) and amorphous Fe on the surface of pyrite. 40 Specifically, the abiotic oxidation of pyrite by air and hydrogen peroxide could produce metastable secondary ferric oxyhydroxides coated on the surface of native pyrite at low tide. 40 Afterward, the amorphous ferric precipitates were readily reduced into dissolved ferrous ions by microbial anaerobic respiration at high tide.…”

“…40 Specifically, the abiotic oxidation of pyrite by air and hydrogen peroxide could produce metastable secondary ferric oxyhydroxides coated on the surface of native pyrite at low tide. 40 Afterward, the amorphous ferric precipitates were readily reduced into dissolved ferrous ions by microbial anaerobic respiration at high tide. The dissolved ferrous ions could be reoxidized into metastable ferric oxyhydroxides precipitates upon shifting to low tide.…”

Surface-Modified Pyrite with a Schwertmannite Shell: Enhancing the Application Performance of Pyrite-Mediated Advanced Oxidation Processes in a Continuous-Flow System

“…Huminicki and Rimstidt 21 proposed that the formation of iron oxide surface layers during pyrite oxidation is originated from the growth of deposited Fe oxyhydroxide colloids and it occurs only if the growth rate of these colloids exceeds approximately ∼10 −8 mol•s −1 , which is consistent with our recent study. 22 Accordingly, the modification of iron oxide shell layers on pyrite surfaces is inseparable from necessary artificial intervention.…”

“…Note: Unless otherwise specified, all experiments were repeated three times and error bars in all figures represented the standard error between the results of parallel experiments. The determination of the characteristic peak in XPS S2p and Fe 2p 3/2 spectra and the corresponding reference is presented in our previous study 22.…”