Efficient sequestration of cesium ions using phosphoric acid-modified activated carbon fibers from aqueous solutions

In this study, acid-modified activated carbon fibers (ACF-Ps) were synthesized by phosphorylation. Three different types of ACF-based adsorbents functionalized with PO₄^3-, P₂O₇^4-, or P₃O₁₀^5- ions, namely, ACF-P1, ACF-P2, and ACF-P3, were prepared by phosphorylating ACF with trisodium phosphate (Na₃PO₄), sodium dihydrogen pyrophosphate (Na₂H₂P₂O₅), and sodium tripolyphosphate (Na₅P₃O₁₀), respectively, and utilized as adsorbents to remove cesium ions (Cs⁺) from aqueous solutions. Among the tested adsorbents, ACF-P3 exhibited the highest Cs⁺ adsorption capacity of 37.59 mg g^-1 at 25 °C and pH 7 which is higher than that of ACF (5.634 mg g^-1), ACF-P1 (19.38 mg g^-1), and ACF-P2 (30.12 mg g^-1) under the same experimental conditions. More importantly, the Cs⁺ removal efficiencies of ACF-P3 (82.90%), ACF-P2 (66.2%), ACF-P1 (34.2%) were 29.3-, 23.4-, and 12.11-fold higher than that of un-treated ACF (2.83%). The results suggested that the phosphorylation with Na₅P₃O₁₀ is highly suitable for Cs⁺ adsorption which effectively functionalizes ACF with a greater number of phosphate functional groups. Adsorption and kinetic data well-fitted the Langmuir isotherm and pseudo-second-order model, respectively, which indicated the monolayer adsorption of Cs⁺ onto ACF-P1, ACF-P2, and ACF-P3 which were largely controlled by chemisorption. Overall, phosphoric acids containing different phosphate-based polyanions (PO₄^3-, P₂O₇^4-, or P₃O₁₀^5-) enriched -OH and/or -COOH surface functional groups of ACF in addition to P-containing surface groups (PO, C-P-O, C-O-P, and P-O) and facilitated the Cs⁺ adsorption through surface complexation and electrostatic interactions.