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pH dependence of ferrous sorption onto two smectite clays
1USEPA, Office of Research and Development, National Risk Management Research Laboratory, Cinncinati, OH, USA.
Chemosphere
|November 3, 2004
Summary
Smectite clays like montmorillonite can adsorb large amounts of ferrous iron, especially at higher pH. This significant, bio-available iron pool in sediments is often missed by standard dissolved iron measurements.
Area of Science:
- Geochemistry
- Mineralogy
- Environmental Science
Background:
- Smectite minerals are key components in many sediments.
- Understanding iron adsorption by clays is crucial for geochemical processes.
Purpose of the Study:
- To quantify ferrous iron adsorption by SWa-1 and Wyoming montmorillonite.
- To investigate the influence of pH on ferrous iron sorption.
- To determine the mechanisms of ferrous iron binding to clay surfaces.
Main Methods:
- Sorption isotherm and sorption edge experiments were conducted.
- Two smectite minerals (SWa-1 and Wyoming montmorillonite) were used.
- Ferrous iron concentrations ranged from 0.037 mM to 2.5 mM across pH 4.0 to 8.0.
Main Results:
- At pH 4.0-6.75, ferrous iron sorption was constant (~1000 L kg⁻¹), attributed to cation exchange.
- Above pH 6.75, sorption dramatically increased, reaching 6600-8000 L kg⁻¹ at pH 8.
- Increased sorption at higher pH is linked to specific interactions at mineral edges.
Conclusions:
- Smectite clays possess a substantial capacity for sorbing ferrous iron.
- A large, redox-labile, and bio-available pool of sorbed iron exists in clay-rich sediments.
- Standard dissolved iron measurements may underestimate the total available iron in these environments.