Micron resorcinol-formaldehyde functionalized spheres as a new non-porous boron selective resin

A micron-sized, non-porous boron-selective resin based on resorcinol–formaldehyde spheres is presented. Resorcinol-formaldehyde spheres, produced within minutes by a scalable ultrasonic synthesis, were functionalized by grafting poly(epichlorohydrin) brushes from surface hydroxyl groups, followed by attachment of N -methyl-D-glucamine (0.323 g/g, i.e., 1.65 mmol/g), as verified by XPS, FT-IR, and independent chlorine quantification. Because the brush architecture exposes the chelating groups on the external surface of the non-porous particles ( S BET = 3 m 2 /g), boron uptake is exceptionally fast: 89–99% of boron is removed within 30 s, and equilibrium is reached within 5 min. The adsorption capacity of 12.79 mg/g more than doubles that of commercial Amberlite IRA-743 (5.62 mg/g) under identical conditions, and adsorption is spontaneous and exothermic ( ΔG° ≈ −23 kJ/mol, ΔH° = −33 kJ/mol). 11 B MAS NMR and XPS identified predominantly tetradentate/bischelate (~80%) and monochelate boron complexes. The resin maintains 85–98% removal across pH 2–11, ensuring stable and efficient boron complexation across a wide pH range. Furthermore, an experimental test using a real water sample containing co-ions demonstrated strong adsorbent selectivity for boron removal. Finally, the adsorbent was successfully regenerated 10 times without any decrease in boron removal capacity, and post-regeneration analysis showed no evidence of adsorbent degradation. Collectively, these results demonstrate that a non-porous adsorbent with long functional brushes can maintain high adsorption performance even under acidic conditions, where other boron selective resins often fail.