Goethite and Hematite Nucleation and Growth from Ferrihydrite: Effects of Oxyanion Surface Complexes

Abstract

The presence of oxyanions, such as nitrate (NO3 –) and phosphate (PO4 3–), regulates the nucleation and growth of goethite (Gt) and hematite (Hm) during the transformation of ferrihydrite (Fh). Our previous studies showed that oxyanion surface complexes control the rate and pathway of Fh transformation to Gt and Hm. However, how oxyanion surface complexes control the mechanism of Gt and Hm nucleation and growth during the Fh transformation is still unclear. We used synchrotron scattering methods and cryogenic transmission electron microscopy to investigate the effects of NO3 – outer-sphere complexes and PO4 3– inner-sphere complexes on the mechanism of Gt and Hm formation from Fh. Our TEM results indicated that Gt particles form through a two-step model in which Fh particles first transform to Gt nanoparticles and then crystallographically align and grow to larger particles by oriented attachment (OA). In contrast, for the formation of Hm, imaging shows that Fh particles first aggregate and then transform to Hm through interface nucleation. This is consistent with our X-ray scattering results, which demonstrate that NO3 – outer-sphere and PO4 3– inner-sphere complexes promote the formation of Gt and Hm, respectively. These results have implications for understanding the coupled interactions of oxyanions and iron oxy-hydroxides in Earth-surface environments.

Effects of Ionic Interferents on Electrocatalytic Nitrate Reduction: Mechanistic Insight

“… − For example, ferrihydrite has a surface area of approximately 610 m 2 g –1 , while the surface area decreases during the transformation from ferrihydrite to goethite as a result of crystal growth . NO 3 – adsorption also affects the crystal phase transformations; e.g., NO 3 – outer-sphere complexes with ferrihydrite promote its transformation to goethite . The facet of the precipitation determines the adsorption preference between NO 3 – and other ions .…”

Understanding Nanoscale Interactions between Minerals and Microbes: Opportunities for Green Remediation of Contaminated Sites

“…Extracellular biosynthesis occurs on microbial cell surfaces or within extracellular polymeric substances secreted by microbes. , Certain proteins or polysaccharides of microbes (e.g., the Mms6 protein isolated from magnetotactic bacteria and lipopolysaccharide on the membrane of Gram-negative bacteria) can selectively bind metal ions and exhibit facet-dependent preferential association with mineral nanoparticles. , Microbes are capable to precisely control the solution chemistry conditions at the mineral surfaces (e.g., altering concentrations of NO 3 –, PO 4 3– , or Cl – , and tuning the pH by producing lactic acid or urease). , Thus, modulating these activities can effectively regulate crystal nucleation and subsequent growth, , and consequently, obtain mineral nanoparticles with desirable surface activities. For instance, tuning the self-assembly of proteins to form aggregates with different spatial structures , can modulate the distribution of binding sites of proteins and consequently, the templates for the formation of mineral nanoparticles.…”