Enhancing selectivity of phosphate-functionalized hydrogel absorbent for uranium uptake in acidic multi-element solutions: A response surface methodology approach for hydrogel synthesis

• Optimized phosphate-functionalized hydrogel shows 85 % uranium removal in acidic multi-ion solutions. • Response surface methodology maximized hydrogel adsorption capacity to 333.8 mg/g at pH 5. • Hydrogel exhibits strong selectivity, durability, and reusability for uranium uptake in harsh acidic conditions. The effective extraction of uranium from nuclear wastewater poses a major environmental issue due to its harmful impact on ecosystems. This study aims to design and optimize a phosphate-functionalized hydrogel absorbent that exhibits improved selectivity for uranium in acidic multi-element nuclear wastewater, overcoming the central issue of co-adsorption of competing ions found in traditional methods. To achieve this, a polyacrylic acid-co-polyacrylamide (p(AA-co-Am)) hydrogel was synthesized and systematically optimized using response surface methodology (RSM) based on a central composite design (CCD). Key synthesis parameters were identified, including initiator/crosslinker concentration (0.88 mol %), acrylic acid-to-acrylamide molar ratio (6.99), and water content (75 vol. %), which maximized the adsorption capacity to 333.8 mg/g at pH 5. The hydrogel maintained over 80 % capacity (260 mg/g) in acidic conditions (pH=2), demonstrating its durability in harsh environments. To improve selectivity, various phosphorus-containing compounds were introduced as uranium-targeting agents, with phosphoric acid (PA) proving particularly effective. This led to the creation of p(AA-co-Am)-PA hydrogels with different concentrations of phosphate groups (0–0.4 M). Competitive adsorption tests in acidic multi-ion solutions demonstrated that the PA-functionalized hydrogel, at an absorbent mass to solution volume ratio of 1:83, achieved over 85 % uranium removal efficiency along with exceptional selectivity, surpassing non-functionalized versions. The optimized absorbent also demonstrated impressive reusability (up to 4 cycles) and stability, as confirmed by FTIR (which showed retained functional groups) and TGA (indicating thermal stability with uranium retention). This research presents a scalable approach for creating selective uranium absorbents by combining RSM-driven hydrogel optimization with phosphate functionalization, providing a practical solution for the remediation of acidic nuclear wastewater.