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 PO, PO, or PO ions, namely, ACF-P1, ACF-P2, and ACF-P3, were prepared by phosphorylating ACF with trisodium phosphate (NaPO), sodium dihydrogen pyrophosphate (NaHPO), and sodium tripolyphosphate (NaPO), 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 at 25 °C and pH 7 which is higher than that of ACF (5.634 mg g), ACF-P1 (19.38 mg g), and ACF-P2 (30.12 mg g) 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 NaPO 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 (PO, PO, or PO) 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.