Understanding Fe(II)-catalyzed transformations of Fe(III)-(oxyhydr)oxides is critical for correctly interpreting stable isotopic distributions and for predicting the fate and mobility of trace, heavy and radioactive metal ions in the environment. Recent 57Fe-enriched isotopic tracer experiments have shown that even the most stable iron oxides (hematite, goethite) undergo a rapid low temperature recrystallization when exposed to Fe(II) ions without substantial change in physical characteristics. The proposed explanation is simultaneous oxidation of sorbed Fe(II) and reductive Fe(II) release coupled in a 1:1 ratio by electron conduction through oxide crystallites. Given the availability of a comprehensive atom exchange data set for goethite as a function of particle size and pH (e.g., Handler et al. Environ. Sci. Technol. 2014, 48, 11302–11311), we developed a stochastic atom exchange simulation in an attempt to fit the published data imposing this 1:1 constraint. The simulation is otherwise devoid of mechanistic assumptions, and the scaling of the time step is the only fitting parameter. We found that at pH 7.5 where net Fe(II) sorption is substantial, aggregation is significant, and exchange is complete within 30 d, two stages of recrystallization are evident, a rapid initial exchange followed by slower size-dependent exchange. At pH 5.0 where net Fe(II) sorption and aggregation are insignificant and exchange is slow and incomplete over the same timescale, we show that exchange is proportional to available particle surface area. The entire data set can be explained in terms of a systematic pH dependence of the density of exchange-active surface sites, where Fe(II) sorption and interfacial electron transfer presumably occur. Estimates of the exchange rate via these sites are generally commensurate with molecular simulation predictions of the interfacial electron transfer rate from adsorbed Fe(II) to lattice Fe(III). The findings are consistent with the proposed conduction mechanism of recrystallization, which has implications for the fate and mobility of trace ions and pollutants in the environment.

Mechanisms of Fe(II)-catalyzed goethite recrystallization from stochastic simulation

253rd National Meeting (2017)

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