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The formation of energetically favourable and metastable mineral phaseswithin the Fe-H2O system controls the long-term mobility of iron complexes aswell as other aqueous phase constituents in natural aquifers, soils and otherenvironmentally and industrially relevant systems. The fundamental mechanismcontrolling the formation of these solid phases has remained enigmatic. Here,we develop a general state-of-the-art partial equilibrium model and succeed inpredicting the rate of amorphous 2-line ferrihydrite precipitation, dissolutionand overall transformation to crystalline goethite at alkaline pH. Allmechanistic steps constituting the transformation mechanism accurately predictthe experimentally measured solid and aqueous phase composition over time,involving only a single kinetic rate constant each. It is found that theprecipitation of goethite (i) occurs from solution and (ii) is limited by thecomparatively slow dissolution of the first forming amorphous phase 2-lineferrihydrite. A generalised transformation mechanism applicable to near-neutraland mildly acidic pH further illustrates that differences in the kinetics ofFe(III) precipitation are controlled by the coordination environment of thepredominant Fe(III) hydrolysis product. Findings provide a framework for themodelling of other iron(bearing) phases across a broad range of aqueous phasecompositions.

Speciation controls the kinetics of iron hydroxide precipitation and transformation