Data Sheet 1_Mineral-dependent and calcium-inhibited adsorption of radium-226 onto quartz and kaolinite in a natural sand substrate.docx

The addition of natural sand resulted in rapid removal of dissolved ²²⁶Ra from solution, reaching up to 86% after 48 h depending on ²²⁶Ra and Ca concentrations. Overall, removal decreased significantly with increasing Ca concentration, suggesting that Ca²⁺ reduced ²²⁶Ra²⁺ adsorption, consistent with competition between ²²⁶Ra²⁺ and Ca²⁺ for sorption sites.

The presence of organic matter in the form of algal cells produced a small but consistent additional removal of ²²⁶Ra (∼6%), indicating that interactions between ²²⁶Ra and organic matter were limited relative to mineral processes. Mineral-specific experiments demonstrated that ²²⁶Ra sorption was strongly mineral-dependent, with extensive sorption onto kaolinite (96% ± 2% within 6h; K_d = 49–62 L kg^-1) and minimal sorption onto quartz (10% ± 9% at 48h, K_d = 0–0.55 L kg^-1).

These findings demonstrate that ²²⁶Ra partitioning in sandy aquatic environments is governed primarily by the presence of reactive clay minerals rather than quartz or organic matter, and by aqueous Ca concentration. Under the controlled laboratory conditions of this study, these findings provide insight into the factors governing ²²⁶Ra mobility and bioavailability in sandy sediments where clay content and water chemistry strongly influence the fate of ²²⁶Ra.</p>