Contribution of boron-specific resins containing N-methylglucamine groups to immobilization of borate/boric acid in a permeable reactive barrier comprising agglomerated MgO

• PRB column tests for removal of borate were performed using MgO for 40 weeks.
• Hydration of MgO did not always accompany with integration of borate.
• 11B-MAS NMR shows that trigonal B is the predominant immobilized species in MgO.
• Tetragonal B is considerably trapped in boron-selective resin amended.
• Immobilized B in N-methylglucamine resin was found as bischelate and monodentate.

Permeable reactive barrier (PRB) column tests for removal of borate were carried out over 287 days corresponding to 76 pore volumes (pv), using MgO agglomerates as the primary reactive material, with and without boron-specific resin (CRB05). Adding 3 (% v/v) CRB05 increased the column shelf life (the time until effluent contains the maximum boron concentration for drinking water) by about 50%, and increased boron accumulation within the column by 15% over 76 pv. The quantities of boron captured were 15–17 mg/g-CRB05, and 1–2 mg-B/g-MgO, decreasing with increasing distance along the column. Rietveld refinement of X-ray diffraction patterns from solid residues after 76 pv revealed that 69–78% of the MgO was transformed into Mg(OH)2 independent of distance along the column, implying that hydration of MgO is not always associated with integration of borate. 11B-NMR spectra demonstrated that trigonal B ([3]B) is predominantly immobilized in Mg(OH)2 in both columns, while tetragonal B ([4]B) is largely trapped in the added CRB05. The complexed forms of [4]B with the N-methylglucamine group after 76 pv were mostly bischelate, with some monodentate. CRB05 contributed to immobilization of [4]B (which is difficult to react with MgO), however, the resin surface was gradually covered with Mg(OH)2 and inactivated.

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