Speciation\nControls\nthe Kinetics of Iron Hydroxide\nPrecipitation and Transformation at Alkaline pH

The formation of\nenergetically favorable and metastable\nmineral\nphases within the Fe–H<sub>2</sub>O system controls the long-term\nmobility of iron complexes in natural aquifers and other environmentally\nand industrially relevant systems. The fundamental mechanism controlling\nthe formation of these phases has remained enigmatic. We develop a\ngeneral partial equilibrium model, leveraging recent synchrotron-based\ndata on the time evolution of solid Fe(III) hydroxides along with\naqueous complexes. We combine thermodynamic considerations and particle-morphology-dependent\nkinetic rate equations under full consideration of the aqueous phase\nin disequilibrium with one or more of the forming minerals. The new\nmodel predicts the rate of amorphous 2-line ferrihydrite precipitation,\ndissolution, and overall transformation to crystalline goethite. It\nis found that the precipitation of goethite (i) occurs from solution\nand (ii) is limited by the comparatively slow dissolution of the first\nforming amorphous phase 2-line ferrihydrite. A generalized transformation\nmechanism further illustrates that differences in the kinetics of\nFe(III) precipitation are controlled by the coordination environment\nof the predominant Fe(III) hydrolysis product. The framework allows\nmodeling of other iron(bearing) phases across a broad range of aqueous\nphase compositions.