Enhanced adsorption of Sr2+ from water by K/N co-doped K2Ti4O9/ZrO2 nanocomposite: Structure, adsorption performance, and mechanisms

Abstract

Development of efficient nanocomposite adsorbents is of particular interest for treatment of radioactive wastewater by adsorption. In this study, a novel K/N co-doped K₂Ti₄O₉/ZrO₂ nanocomposite (denoted as NTZ-X-Y, X: temperature, Y: time) was synthesized by a solvent thermal coupling solid-phase melting calcination strategy. Detailed analysis indicated that calcination temperature and time, as well as K⁺/-NH₂ doping precisely engineered the mesoporous architecture (BET surface area: 117.46 m²/g; pore size: 7.96 nm) and enriched its active functional groups (Zr-O, Ti-O, O-H, C-N), thereby enhancing its adsorption performance. The optimized NTZ-500–3 exhibited exceptional Sr²⁺ adsorption capacity (31.76 mg/g). In binary systems with a molar ratio of Sr²⁺/Na⁺ or Cs⁺ of 1:10, the Sr²⁺ adsorption capacity decreased by only 3.81 and 4.39 mg/g, respectively, only about 1/5 of that of the control group. Thermodynamic analysis confirmed that the adsorption process was spontaneous and endothermic, while kinetic and isotherm modeling, specifically pseudo-second-order (R² = 0.997) and Freundlich (R² = 0.995) models, indicated the dominance of chemisorption. Density functional theory calculations further revealed that the adsorption mechanism involved synergistic contributions from ion exchange, surface complexation (predominantly via Zr-O coordination sites), electrostatic attraction, and pore filling. Our findings validated that the dual-functionalization strategy of K⁺/-NH₂ doping combined with controlled calcination created a hierarchical architecture with enhanced accessibility to active sites. This design leveraged the K₂Ti₄O₉ layered structure for rapid ion exchange and the high-affinity surface sites of ZrO₂ for selective complexation, offering a promising solution for efficient Sr²⁺ remediation in radioactive wastewater treatment.