Mechanisms of Sr sorption in peaty-podzolic-gleyic and alluvial soddy-gleyic soils

Radiostrontium (⁹⁰Sr), a highly mobile radionuclide of environmental concern, can be strongly retained by soils <i>via</i> interactions with organic and mineral components. This study investigated sorption patterns and mechanisms of ⁹⁰Sr in two contrasting soil types-peaty-podzolic-gleyic (PPG) and alluvial soddy-gleyic (ASG)-through batch experiments with varying experimental conditions and sequential extraction. Despite differences in pH, organic carbon, and clay content, all soil horizons showed high Sr sorption (85-96%) at near-native pH. In organic-rich horizons, Sr was predominantly bound to organic matter <i>via</i> complexation, while in mineral and organo-mineral horizons, ion exchange with clay minerals was the dominant retention mechanism. pH-dependent sorption revealed that Ca²⁺ competition significantly reduces Sr retention at low pH. Sequential extraction confirmed that Sr is mostly exchangeable in mineral horizons but more strongly retained in organic layers. These findings highlight the contrasting sorption mechanisms in different soil compartments and provide key insights for predicting Sr mobility in contaminated environments.