Impacts of cryogenic sampling processes on iron mineral coatings in contaminated sediment

• Published in: The Science of the total environment Vol. 765; p. 142796
• Main Authors: Hua, Han, Yin, Xin, Renno, Maria Irianni, Sale, Thomas C., Landis, Richard, Dyer, James A., Axe, Lisa
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 15.04.2021
• Subjects: Cryogenic coring; FESEM; Iron sulfide coatings; Surface mineral coating characterization; XRD; Cryogenic coring; Iron sulfide coatings; Surface mineral coating characterization; XRD; FESEM
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2020.142796

This study focused on comparing iron mineral coatings found in contaminated sediments from a cryogenic (Cryo Core) core versus an Anaerobic Core (collected under oxygen-free and ambient conditions). After thawing the Cryo Core in an oxygen-free glovebox, a suite of analyses was applied on sediments from both cores: pH, redox potential, X-ray fluorescence (XRF), X-ray diffraction (XRD), and field-emission scanning electron microscopy (FESEM) with energy dispersive X-ray analyzer (EDX). Among the iron minerals identified, crystalline pyrite was found throughout the Cyro Core sediment samples, which is in contrast to that observed for the Anaerobic Core. Moreover, mackinawite and greigite that were ubiquitous in the Anaerobic Core were not observed in Cryo Core samples. To better understand why the metastable minerals were not present, a freeze/thaw process was simulated on Anaerobic Core samples using a liquid‑nitrogen quench with surface coatings characterized by FESEM/EDX. In these quenched samples, mackinawite was no longer observed, and in its place was pyrite. In addition, both greigite and pyrite were found to be unique morphologically after quenching. Dissolution and re-precipitation of iron sulfide coatings during the freeze/thaw process appears to affect the geochemistry of the pore water through two main mechanisms of freeze-concentration and freezing potential. [Display omitted] •Iron mineral coatings were compared between a cryogenic and anaerobic core.•Pyrite concentrations in the Cryo Core were elevated in redox transition zones.•Metastable forms of iron sulfides minerals were not observed in Cryo Core samples.•Liquid nitrogen quench appears to impact surface coating mineralogy.•Dissolution-reprecipitation explains the transformation of surface iron sulfides.