Irreversible Trace Metal Binding to Goethite Controlled by the Ion Size

Area of Science:

• Environmental Science
• Geochemistry
• Mineralogy

Background:

• Trace metal dynamics at mineral surfaces control environmental fate and bioaccessibility.
• Adsorption-desorption hysteresis on iron (oxyhydr)oxide surfaces suggests metal entrapment after aging.
• Desorption experiments can perturb metal coordination and mineral properties, complicating labile fraction interpretation.

Purpose of the Study:

• Investigate the irreversible binding of nickel, zinc, and cadmium to goethite.
• Quantify the nonlabile fraction of these metals after aging using isotope exchange.
• Determine the influence of ionic radius on metal incorporation into the goethite structure.

Main Methods:

• Isotope exchange experiments to differentiate labile and nonlabile metal pools.
• Aging goethite surfaces with Ni, Zn, and Cd for 2-120 days.
• Spectroscopy and extraction techniques to characterize metal binding and incorporation.

Main Results:

• Dissolved and adsorbed metal pools exchanged rapidly (<90 min half-times).
• All studied metals formed a solid-associated fraction inaccessible to isotope exchange.
• The nonlabile pool size correlated inversely with ionic radius (Ni > Zn > Cd).
• Spectroscopy and extractions indicated metal incorporation into the goethite structure.

Conclusions:

• Irreversible binding of trace metals to goethite forms a nonlabile pool, sequestering contaminants.
• This process limits micronutrient availability and influences metal isotope signatures.
• Adsorbed metals can sustain dissolved pools, affecting environmental responses to uptake or fluid flow.