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

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.