Preparation of K2SbPO6-Loaded Porous Geopolymer Particles for Efficient Sr(II) Removal: Adsorption Performance and Mechanism

To achieve efficient separation of Sr2+ under complex ionic-strength conditions, porous geopolymer particles (PGs) were used as a support to construct a K2SbPO6-loaded porous geopolymer composite, denoted as K2SbPO6@PGs, via in situ loading of one-dimensional K2SbPO6 by a high-temperature solid-state route. Its adsorption performance and mechanism were systematically compared with those of pristine PGs. Structural characterization (SEM/EDS, XRD, FTIR, XPS, and BET) confirmed that the K2SbPO6 crystalline phase was uniformly anchored onto the PGs framework while preserving interconnected mesoporous channels. K2SbPO6@PGs exhibited excellent Sr2+ removal over a wide pH range (3–12), with a removal efficiency of approximately 92% at pH 3, which was significantly higher than that of PGs (approximately 5%). The isotherm data were better fitted by the Sips model (R2 = 0.982), and the maximum adsorption capacity reached 189.35 mg·g−1 (theoretical qm = 201.14 mg·g−1). Kinetic fitting showed that PGs followed the pseudo-first-order model, whereas K2SbPO6@PGs were better described by the pseudo-second-order model, indicating that chemical adsorption dominated the process through K+/Sr2+ exchange and surface complexation. Coexisting-ion experiments demonstrated strong resistance to monovalent ions, whereas Ca2+ and Mg2+ caused more pronounced competitive effects. The results indicate that PGs mainly provide interconnected mass-transfer pathways and granular structural support, whereas K2SbPO6 provides selective exchange sites with high affinity for Sr2+. The synergy between these two components endows the composite with good pH adaptability and enhanced adsorption performance and suggests its potential for subsequent continuous-flow separation studies.