Effect of grain size variation on strontium sorption to heterogeneous aquifer sediments

Strontium-90 (⁹⁰Sr) is a major contaminant at nuclear legacy sites. The mobility of ⁹⁰Sr is primarily governed by sorption reactions with sediments controlled by high surface area phases such as clay and iron oxides. Sr²⁺ adsorption was investigated in heterogeneous unconsolidated aquifer sediments, analogous to those underlying the UK Sellafield nuclear site, with grainsizes ranging from gravels to clays. Batch sorption tests showed that a linear K_d adsorption model was applicable to all grainsize fractions up to equilibrium [Sr] of 0.28 mmol L^-1. Sr²⁺ sorption values (K_d; Langmuir q_max) correlated well with bulk sediment properties such as cation exchange capacity and surface area. Electron microscopy showed that heterogeneous sediments contained porous sandstone clasts with clay minerals (i.e. chlorite) providing an additional adsorption capacity. Therefore, gravel corrections that assumed that the > 2 mm fractions are inert were not appropriate and underestimated K_d(bulk) adsorption coefficients. However, K_{d (<2 mm)} values measured from sieved sediment fractions, were effectively adjusted to within error of K_{d (bulk)} using a surface area dependant gravel correction based on particle size distribution data. Amphoteric pH dependent Sr²⁺ sorption behaviour observed in batch experiments was consistent with cation exchange modelling between pH 2-7 derived from the measured cation exchange capacities. Above pH 7 model fits were improved by invoking a coupled cation exchange/surface complexation which allowed for addition sorption to iron oxide phases. The overall trends in Sr²⁺ sorption (at pH 6.5-7) produced by increasing solution ionic strength was also reproduced in cation exchange models. Overall, the results showed that Sr²⁺ sorption to heterogeneous sediment units could be estimated from K_{d (<2 mm)} data using appropriate gravel corrections, and effectively modelled using coupled cation exchange and surface complexation processes.