New insights into Th(IV) speciation on sepiolite: Evidence for EXAFS and modeling investigation

•No effect of ionic strength on Th(IV) removal was observed at pH >5.0.•Th(IV) sorption on sepiolite at pH 7.5 was surface precipitation by EXAFS analysis.•Th(IV) sorption can be satisfactorily fitted by surface complexation modeling.

The interaction mechanism of Th(IV) on sepiolite was investigated by batch, EXAFS and surface complexation modeling. The batch experiments revealed that the sorption of Th(IV) on sepiolite remarkably decreased with increasing of ionic strength at pH <5.0, while no effect of ionic strength on Th(IV) removal was observed at pH >5.0. The removal kinetics and isotherms of Th(IV) on sepiolite can be satisfactorily simulated by the pseudo-second-order and Langmuir model, respectively. The analysis of EXAFS spectra for sepiolite-Th(IV) at pH 5.0 and 7.5 indicated that the sorption of Th(IV) on sepiolite at pH 5.0 and 7.5 was inner-sphere surface complexation and surface precipitation, respectively. The results of surface complexation modeling showed that Th(IV) sorption on sepiolite can be fitted by a cation exchange sites (X4Th species) at pH <4.5, an inner-sphere surface complexation (SOTh3+ species) at near neutral and a surface precipitation at pH >7.0 very well. These findings are crucial for the evaluation of fate and transport of Th(IV) at water-mineral interface.

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