Transient-Stage (<200 PV) Modeling of Sr, Cs and I Desorption From Hyper-Alkaline Weathered Hanford Sediments.

We evaluated the initial desorption of Sr, Cs, and I from Hanford Site (WA, USA) sediments subjected to hyperalkaline (pH >13) Na-Al-NO3-OH solutions for 182 days under two pCO2 regimes (undetectable at <10ppmv and atmospheric, 385 ppmv). Desorption was monitored in flow-through experiments conducted after removal of the hyperalkaline waste source and modeled using the reactive transport model CrunchFlow. The initial (<200 PV) Sr and Cs desorption was governed largely by ion exchange processes with Ca²⁺ displacing Sr²⁺ and K⁺ displacing Cs⁺. However, the breakthrough of the desorption curves were altered significantly by an evolving milieu of aqueous ions—including the contaminant I—resulting from the concurrent dissolution of labile minerals formed during the sediment weathering processes. To model this system, we coupled equilibrium ion exchange and kinetic mineral dissolution routines with steady-state 1D transport in the reactive-transport model, Crunchflow.