Oxyanion Surface Complexes Control the Kinetics and Pathway of Ferrihydrite Transformation to Goethite and Hematite

The rate and pathway of ferrihydrite (Fh) transformation at oxic conditions to more stable products is controlled largely by temperature, pH, and the presence of other ions in the system such as nitrate (NO3-), sulfate (SO42-), and arsenate (AsO43-). Although the mechanism of Fh transformation and oxyanion complexation have been separately studied, the effect of surface complex type and strength on the rate and pathway remains only partly understood. We have developed a kinetic model that describes the effects of surface complex type and strength on Fh transformation to goethite (Gt) and hematite (Hm). Two sets of oxyanion-adsorbed Fh samples were prepared, nonbuffered and buffered, aged at 70 +/- 1.5 degrees C, and then characterized using synchrotron X-ray scattering methods and wet chemical analysis. Kinetic modeling showed a significant decrease in the rate of Fh transformation for oxyanion surface complexes dominated by strong inner-sphere (SO42- and AsO43-) versus weak outer-sphere (NO3-) bonding and the control. The results also showed that the Fh transformation pathway is influenced by the type of surface complex such that with increasing strength of bonding, a smaller fraction of Gt forms compared with Hm. These findings are important for understanding and predicting the role of Fh in controlling the transport and fate of metal and metalloid oxyanions in natural and applied systems.

Automated summary

The transformation rate and pathway of ferrihydrite under oxic conditions are influenced by temperature, pH, and the presence of other ions such as nitrate, sulfate, and arsenate. The effects of surface complex type and strength on the transformation process have been partially understood. A kinetic model has been developed to describe the impact of surface complex type and strength on the transformation of ferrihydrite to goethite and hematite. The results show that the rate of transformation decreases significantly for oxyanion surface complexes dominated by strong inner-sphere bonding. The type of surface complex also affects the transformation pathway, with a smaller fraction of goethite forming compared to hematite as the strength of bonding increases.