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

Abstract

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