Surface complexation modeling of americium sorption onto volcanic tuff

Area of Science:

• Geochemistry
• Environmental Science
• Radiochemistry

Background:

• Americium sorption on geological materials is crucial for nuclear waste disposal safety.
• Volcanic rocks, like those at Yucca Mountain, present complex sorption behaviors.
• Understanding americium's interaction with rock surfaces informs environmental risk assessments.

Purpose of the Study:

• To develop and validate a surface complexation model (SCM) for americium sorption on volcanic rocks.
• To assess the applicability of quartz surface properties to model sorption on devitrified and zeolitic tuff.
• To predict americium distribution coefficients (Kd) across a range of pH conditions representative of natural environments.

Main Methods:

• Utilized the PHREEQC software to develop a surface complexation model.
• Based the model on laboratory data for americium sorption on quartz.
• Applied the model to volcanic rock compositions representative of Yucca Mountain, using groundwater chemistry data.

Main Results:

• The SCM successfully modeled americium sorption on quartz using two surface reactions (sulfate and carbonate complexes).
• Calculated distribution coefficients (Kd) for americium on volcanic rocks closely matched experimental values.
• Model predictions showed Kd values decreasing with increasing pH in the 7-9 range.

Conclusions:

• Surface properties of volcanic rocks can be approximated by quartz for modeling americium sorption.
• The derived surface complexation constants enable prediction of americium sorption in complex natural systems.
• The model accounts for uncertainties in geochemical conditions along transport pathways, improving predictive capabilities.