T167 Charting Soils at the Atomic Scale (Posters): A Tribute to the Careers of David L. Bish and Jeffrey E. Post

Tuesday, 11 October 2022
9:00 AM - 1:00 PM
Exhibit Hall F

KINETIC ANALYSIS OF HEMATITE AND GOETHITE CRYSTALLIZATION AT PH 2-13: AN IN SITU TIME-RESOLVED SYNCHROTRON X-RAY DIFFRACTION STUDY

Paper number: T167-1
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

Abstract

Hematite (α-Fe2O3) is thermodynamically more stable than goethite (α-FeOOH) in oxic soils and solutions at Earth's surface, and it is unclear why goethite co-precipitates, in violation of classical thermodynamic theory. The formation of hematite and/or goethite is affected by solution pH, Eh, temperature, time, foreign ions, clay minerals, organic content, and microbial activity. Of these parameters, temperature and pH have the most significant impact on the nucleation and growth of hematite and goethite. In this study, we investigated the kinetics of two-line ferrihydrite (Fe3+10O14(OH)2) to hematite and goethite transformation under hydrothermal conditions over a broad pH range of 2 to 13 and temperatures of 80 to 180 °C using time-resolved synchrotron X-ray (TRXRD) diffraction.

Only hematite formed at pH 2-5 and at temperatures higher than 90 °C. Hematite and goethite precipitated simultaneously from ferrihydrite at higher pH concentrations. Changes in mass abundance of hematite and goethite with time exhibited sigmoidal growth kinetics with three stages of transformation: nucleation, crystal growth, and cessation due to exhaustion of the ferrihydrite. No phases other than hematite and goethite were identified, and in our TRXRD experiments, hematite:goethite ratios, defined as wt% hematite/(wt% hematite + wt% goethite), initially were equal and rapidly (within minutes) evolved to persistent steady-states.

We measured hematite and goethite crystallization rates as a function of temperature and pH using both JMAK and linear growth models. Increased temperatures accelerated both hematite and goethite crystallization. The rate of hematite crystallization was slowest at neutral pH. The rate constants for goethite formation were of the same magnitude as those for hematite. Using the Arrhenius approach, we calculated the activation energies for hematite crystallization as a function of pH, and these ranged from 100-120 kJ/mol at pH 2 to 5, 120-160 kJ/mol at pH 6-8, and 70-90 kJ/mol at pH 9-12. The transformation mechanism was temperature-independent in our experimental temperature range. However, the mechanism changed with pH, as suggested by variations in reaction order, with the highest refined reaction orders at pH 7-8.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379274

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.