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The rate of ferrihydrite transformation to goethite via the Fe(II) pathway

¹ Department of Earth and Environmental Sciences, Rutgers, The State University of New Jersey, Newark, New Jersey 07102, U.S.A.
² Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, U.K.
³ School of Earth Sciences, University of Leeds, Leeds LS2 9JT, U.K.

Correspondence: ^* E-mail: nyee{at}andromeda.rutgers.edu

In this study, we quantified the rate of ferrihydrite conversion to goethite via the Fe(II) pathway using synchrotron radiation-based energy dispersive X-ray diffraction (ED-XRD). Ferrihydrite transformation experiments were conducted in oxygen-free solutions at neutral pH with synthetic 2-line ferrihydrite reacting with 100 mM Fe(II). The kinetics of goethite crystallization was measured in situ at temperatures ranging from 21 to 90 °C. The results showed that in the presence of ferrous iron, the transformation of poorly ordered ferrihydrite into crystalline goethite is rapid and highly dependent on temperature. The time-resolved peak area data fitted using a Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetic model yielded rate constants of 4.0 x 10^–5, 1.3 x 10^–4, 3.3 x 10^–4, 2.27 x 10^–3, and 3.14 x 10^–3 1/s at reaction temperatures of 21, 45, 60, 85, and 90 °C respectively. The activation energy for the transformation was determined to be 56 ± 4 kJ/mol. Comparison with the activation energy predicted for the phase conversion in the absence of ferrous iron indicates that Fe(II) acts as a catalyst that decreases the activation energy barrier by approximately 38 kJ/mol. The kinetic parameters derived from the experimental data suggest that goethite crystallization is controlled by a 1-D phase boundary growth mechanism with a constant nucleation rate occurring during the reaction.

Key Words: Crystal growth • goethite • geomicrobiology • ferrihydrite • kinetics • transformation

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