Modeling selenium (IV and VI) adsorption envelopes in selected tropical soils using the constant capacitance model

Area of Science:

• Environmental Chemistry
• Soil Science
• Geochemistry

Background:

• Selenium's environmental and health significance necessitates understanding its soil adsorption behavior.
• Soil adsorption influences selenium bioavailability and potential toxicity.
• Differentiating adsorption mechanisms for different selenium species is crucial for environmental management.

Purpose of the Study:

• To evaluate the adsorption of selenite (Se(IV)) and selenate (Se(VI)) on Brazilian soil samples.
• To investigate the influence of soil pH on selenium adsorption.
• To model selenium adsorption using the constant capacitance model.

Main Methods:

• Adsorption experiments were performed on 15 soil samples with controlled Se(IV) or Se(VI) concentrations.
• Soil pH was adjusted across a range of 2.5 to 10.
• Experimental data were fitted to the constant capacitance model, optimizing complexation and protonation constants.

Main Results:

• Selenite (Se(IV)) adsorption was high across all soils and decreased with increasing pH, correlating with iron (Fe) and aluminum (Al) oxide content.
• Selenate (Se(VI)) adsorption was minimal in typical agricultural soils but notable in Rhodic Acrudox.
• The constant capacitance model effectively described both Se(IV) and Se(VI) adsorption.

Conclusions:

• Soil pH and mineral content significantly control selenium species adsorption.
• Selenite is more readily adsorbed than selenate in most soil conditions.
• The constant capacitance model provides a robust framework for predicting selenium adsorption in soils.