Porous hierarchical Mxene/chitosan cryogel with synergistic phosphate/amidoxime groups for ultra-efficient uranium adsorption under pH-response behavior

• Mxene/chitosan cryogel with multifunctional groups (MXSC-4-AO) was proposed. • MXSC-4-AO demonstrated porous hierarchical structure. • MXSC-4-AO showed ultra-high batch adsorption capacity (1393.15 mg g −1 ). • Excellent dynamic adsorption and efficient recovery capability were also exhibited. • Under pH-response of protonation, synergistic coordination of AO/PO 3 H 2 were revealed. In this work, we present a porous hierarchical adsorbent (MXSC-4-AO) enriched with phosphate and amidoxime groups prepared by amidoximation of MXSC, consisting of phosphorylated MXene obtained by ion-gelation method of the cross-linking agent sodium hexametaphosphate and selectively disubstituted chitosan. MXSC-4-AO realized the insertion-induced multilayer lamellae on the pore wall of three-dimensional porous network structures, exhibited larger specific surface area and more active adsorption sites, and showed fast adsorption rate (86.31 % adsorption rate in the first 60 min) and excellent adsorption performance (1393.15 mg·g −1 ). Adsorption of UO 2 2+ was more consistent with the pseudo-second-order model and Langmuir isothermal model. Especially, density functional theory calculations were used to analyze the adsorption mechanism of MXSC-4-AO in depth, the binding of MXSC-4-AO and uranium ions was combination of electrostatic interaction and synergistic coordination under pH-response behavior. The coordination model of MXSC-4-AO cryogel with UO 2 2+ is closer to the optimized binding model of AO(OH), AO(OH)/PO 3 H 2 , AO (NH 3 + /OH) with UO 2 2+ under the behavioral response of protonation. In particular, MXSC-AO exhibited good selectivity in low concentration wastewater and high dynamic adsorption capacity (269.6 mg·g −1 ), which can realize the rapid recovery and collection of uranium. Overall, MXSC-4-AO could be considered as a promising candidate for treating complex wastewater.