Performance and mechanism of phytic acid-modified mesoporous tobermorite for U(VI) adsorption in water

Tobermorite (TOB) was first prepared with waste fly ash and calcium carbide slag by the hydrothermal method, and then TOB was subjected to thermal activation, hydrochloric acid acidification, and phytic acid modification to synthesize phytic acid-modified mesoporous tobermorite (P-STOB). The adsorption results showed that the maximum adsorption ratio of U(VI) by P-STOB could achieve 99.17 %, significantly higher than that of TOB (79.20 %). Moreover, under conditions of pH=5, T=303 K, and an initial U(VI) concentration of 10 mg/L, the theoretical uptake capacity for U(VI) by P-STOB was 176.66 mg/g. Pseudo-second-order and Langmuir isotherm adsorption models fit the adsorption process well, which indicated that the adsorption process was mainly chemical adsorption and primarily monolayer adsorption. Thermodynamic parameters revealed that the U(VI) adsorption was an endothermic and spontaneous process. The Dubinin-Radushkevich model suggests that the adsorption mechanism is surface chemical complexation. Even after five cycles, the U(VI) removal ratio remained above 74.63 %. Characterizations using SEM, EDS, XRD, BET, XPS and FT-IR confirmed the successful introduction of phytic acid onto the surface of P-STOB. The main mechanism of the adsorbent's interaction with U(VI) is the coordination between P-O and U(VI), the surface complexation between -OH and U(VI), and the electrostatic gravitational force between ions.