Sorption of thorium onto hematite colloids

A potential role on the radionuclide migration is generally assigned to colloids present in natural waters. As iron is one of the most abundant elements in earth crust, hematite (alpha-Fe2O3) colloids have been chosen as model colloids for a systematic and detailed study on their retention properties of metal ions. The present work focuses on the Th(IV)/hematite system. The influence of different physico-chemical parameters such as pH (2 to 10) and ionic strength (0.01 to 0.1 M) on Th sorption (use of Th-228 at tracer levels, i.e. 0.25 nM) has been investigated through batch experiments. Results obtained (log K-d as a function of pH) are presented and discussed. Two models based on surface complexation theory have been considered to describe the sorption data: the Kurbatov model (without electrostatic term) and the diffuse layer model (with an electrostatic term). Both models allow the description of Th retention onto hematite colloids by considering equilibria which involve different hydroxo surface complexes, >FeOTh(OH)(n-1)(4-n) (n = 1 to 5) and >FeOH-Th4+. The retention reactions obtained through these models, as well as the associated surface complexation constants, are compared to each other and to Literature. A discussion is made on both approaches.