Fe(II) uptake on natural montmorillonites. II. Surface complexation modeling

Area of Science:

• Geochemistry
• Environmental Science
• Clay Mineralogy

Background:

• Montmorillonite clays are key components in geological formations and engineered barriers.
• Understanding metal sorption onto clays is crucial for predicting contaminant transport and fate.
• Structural iron content in clays can influence their surface reactivity and sorption properties.

Purpose of the Study:

• To investigate the sorption behavior of Fe(II) on montmorillonites with varying structural iron content.
• To evaluate the impact of redox conditions on Fe(II) sorption mechanisms.
• To model Fe(II) sorption using a surface complexation approach.

Main Methods:

• Sorption edge and isotherm measurements were conducted on low (STx) and high (SWy) structural Fe-content montmorillonites.
• Experiments were performed under anoxic and strongly reducing conditions.
• Sorption data were modeled using the two-site protolysis nonelectrostatic surface complexation and cation exchange (2SPNE SC/CE) model.

Main Results:

• Fe(II) sorption was higher on SWy than STx under anoxic conditions.
• Sorption levels were similar for both montmorillonites under reducing conditions.
• A surface complexation reaction involving Fe(II) oxidation to Fe(III) was necessary to model the data.

Conclusions:

• Structural iron in montmorillonite plays a significant role in Fe(II) sorption, particularly under anoxic conditions.
• The surface oxidation of sorbed Fe(II) to Fe(III) is a key process influencing sorption.
• The 2SPNE SC/CE model, with an added oxidation reaction, effectively describes Fe(II) sorption on montmorillonite.