High-Capacity Be(II) Adsorption by a Multidentate TFP-HEDA Adsorbent: Mechanistic Insight and Statistical Validation

The selective removal of beryllium from aqueous matrices remains a critical environmental and industrial challenge due to beryllium's extreme toxicity, strong hydration chemistry, and the difficulty of separating Be²⁺ from chemically similar cations such as Al³⁺. In this study, a novel multidentate Schiff-base porous organic adsorbent, TFP-HEDA, was synthesized by condensation of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde (TFP) with N-(2-hydroxyethyl)ethylenediamine (HEDA) followed by urethane post-functionalization and systematically characterized by FTIR, ¹H/¹³C NMR, MALDI-TOF MS, elemental analysis, BET surface area analysis (617 m² g^-1), PXRD, and XPS. Batch adsorption experiments demonstrated rapid Be²⁺ uptake, achieving 90% removal within 20 min and equilibrium within 30 min. Among the isotherm models evaluated, the Langmuir model yielded the highest statistical consistency (R² = 0.9835, RMSE = 5.15 mg g^-1, χ² = 1.137) with a predicted maximum adsorption capacity of 163.93 mg g^-1 agreeing closely with the experimental value of 163.67 ± 6.42 mg g^-1 (deviation < 0.2%); this mathematical adequacy is interpreted as compatibility with a finite, saturable set of inner-sphere coordination sites rather than confirmation of a flat, energetically uniform surface, with chemisorption independently and more rigorously established by Dubinin-Radushkevich analysis (E = 28.87 kJ mol^-1) and post-adsorption FTIR and XPS evidence. Dubinin-Radushkevich analysis confirmed a chemisorption mechanism with mean adsorption energy E = 28.87 kJ mol^-1, consistent with inner-sphere Be²⁺-O/N coordination. Process optimization using response surface methodology based on a central composite design achieved 99% Be²⁺ removal at pH 5, an adsorbent dose of 60 mg/20 mL, and a contact time of 30 min (R² = 0.9892). Post-adsorption FTIR, XPS, BET, and TGA characterization confirmed framework integrity and the inner-sphere multidentate coordination mechanism. TFP-HEDA retained 82.4% of its initial capacity after nine adsorption-desorption cycles, demonstrating practical regenerability for Be²⁺ recovery applications.