Hydrothermal treatment and strontium ion sorption properties of fibrous cerium(IV) hydrogenphosphate

Abstract

Fibrous cerium hydrogenphosphate, CeP, has been treated hydrothermally in 1 mol dm ^-3 phosphoric acid solution. CEP and its hydrothermally treated product, CeP(HT), have been characterized by X-ray powder diffractometry, scanning electron microscopy, solid-state ³¹ P MAS NMR spectroscopy, FTIR spectroscopy, chemical and thermal analyses. A poorly crystalline fibrous CeP with a d-spacing of 1.1 nm converted into a highly crystalline CeP(HT) with platelet morphology by hydrothermal treatment. Solid-state ³¹ P MAS NMR and FTIR measurements confirmed that one kind of phosphate (H ₂ PO ₄ ) is present in CeP and two kinds of phosphate (HPO ₄ , PO ₄ ) are present in CeP(HT), in which the integrated intensity ratio of HPO ₄ to PO ₄ is 2:1. From chemical and thermal analyses, structural formulae for CeP and CeP(HT) are assumed to be CeO(H ₂ PO ₄ ) ₂ 2H ₂ O and Ce(HPO ₄ ) (PO ₄ ) _0.5 (OH) _0.5 , respectively. The Na ⁺ exchange capacity of CeP amounted to 4.5 mmol g ^-1 at pH 11 while that of CeP(HT) was less than 1.0 mmol g ^-1 in the pH range 2–12. The pH dependence of the metal ion distribution coefficients exhibited ideal ion-exchange behaviour on CeP while metal ion distribution coefficients on CeP(HT) scarcely depended on pH. The metal ion selectivities of CeP and CeP(HT) increased in the order: Na ⁺ <Ca ²⁺ <Sr ²⁺ <K ⁺ , and Na ⁺ <K ⁺ <Ca ²⁺ <Sr ²⁺ , respectively. The distribution coefficient for the Sr ²⁺ ion of CeP(HT) was higher than that of CeP under hydrothermal conditions.