Speciation of cesium cation in natural seawater

Cesium radioactive isotopes (¹³⁴Cs and ¹³⁷Cs) are key fission products released during nuclear accidents such as Fukushima and through routine operations of nuclear power plants. This study presents a combined experimental and computational investigation of the speciation of the cesium cation in natural seawater, with the aim of elucidating its local hydration structure under environmentally relevant conditions. X-ray Absorption Fine Structure (XAFS) measurements at the Cs L_III edge were performed on cesium solutions prepared in distilled water, natural seawater and NaCl saturated seawater. Complementary molecular dynamics (MD) simulations were carried out using four model systems of increasing ionic complexity to characterize Cs⁺ solvation. In both distilled water and natural seawater, a coordination environment consisting of eight oxygen atoms at a Cs-O distance of approximately 3.11-3.12 Å is consistent with the known chaotropic character of Cs⁺ and its loosely ordered first hydration shell. Molecular dynamics simulations confirm these findings and demonstrate that Cl^- anions remain outside the first coordination sphere of Cs⁺ under natural seawater conditions. In NaCl saturated seawater, EXAFS data are similar and the possible entry of one or two Cl^- into the Cs⁺ coordination sphere cannot be clearly inferred (but not excluded either). These results establish that cesium speciation in natural seawater is dominated by water coordination and is not significantly perturbed by the ionic matrix of seawater, providing a structural foundation for modeling cesium bioavailability and environmental fate in marine ecosystems.