Exchange of Adsorbed Pb(II) at the Rutile Surface: Rates and Mechanisms

Area of Science:

• Environmental Science
• Geochemistry
• Environmental Chemistry

Background:

• Lead (Pb(II)) adsorption onto mineral surfaces influences its environmental mobility.
• Adsorbed Pb(II) contributes significantly to bioaccessible lead in contaminated soils.
• Understanding the lability of adsorbed Pb(II) is crucial for assessing environmental risk.

Purpose of the Study:

• To evaluate the exchangeability of Pb(II) adsorbed onto rutile surfaces.
• To determine the rate and extent of Pb(II) isotope exchange across varying pH conditions.

Main Methods:

• Utilized Pb(II) isotope exchange experiments with 207Pb-enriched dissolved lead.
• Investigated Pb(II) adsorbed to rutile at pH 5, 6, and 7.
• Employed Pb LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy.

Main Results:

• Approximately 95% of adsorbed Pb(II) on rutile was found to be exchangeable.
• Exchange occurred in two phases: a rapid initial exchange (<1 hour) and a slower phase (hours to days).
• EXAFS spectra showed consistent binding mechanisms across pH and loading, ruling out coordination changes as the cause for differing exchange rates.

Conclusions:

• The majority of adsorbed Pb(II) is labile and can rapidly replenish dissolved lead pools.
• Slower exchange at pH 5 may be linked to diffusion processes in aggregated particles.
• Environmental Pb(II) dynamics are significantly influenced by the rapid exchange between adsorbed and dissolved phases.