Solid State Structural Transformation of Tetraborate into Monoborate in the Interlayer Galleries of Reconstructed ZnAl Layered Double Hydroxide

Remediation of borate in wastewaters using adsorbents is important from an environmental perspective. Understanding the nature of borate species and their relative abundance is essential in its remediation which varies with the local environment offered by the adsorbent. ZnAl oxide (ZnAl2-Cl CLDH) obtained by calcination of ZnAl layered double hydroxide (LDH) with Zn/Al atomic ratio of 2.0 and chloride as interlayer anion (ZnAl2-Cl LDH) was studied for borate uptake. Uptake of borate reconstructed the oxide phase to tetraborate containing LDH phase. On slow drying of this sample, a solid state structural transformation of tetraborate to monoborate occurred in the intergalleries of LDH. This transformation in solid state was probed by powder X-ray diffraction (PXRD), Fourier-transformed infrared (FT-IR) spectroscopy, 11B and 1H magic-angle spinning (MAS), nuclear magnetic resonance (NMR), and thermogravimetric analysis. Hydrolytic dissociation of intercalated tetraborate (B4O72–) into monoborate species comprising tetrahedral [B­(OH)4] and trigonal B­(OH)3 occurred as evidenced from the decrease in the gallery height measured through PXRD and from the change in the trigonal to tetrahedral ratio monitored by 11B MAS NMR. The structural transformation involved interplay of intercalated water molecules and the local hydroxide environment of the layered lattice. Triple-quantum 11B magic-angle spinning (3QMAS) experiments further substantiated the interconversion between two monoborate species in the interlayers of LDH. Washing the sample dried for 38 days with water removed the weakly held boric acid as supported from 11B MAS NMR and FT-IR measurements. Interestingly, such transformation did not occur for a rapidly dried sample. This study shows the nature of borate speciation depending on the environment and offers a perspective of its remediation and re-entry into aquatic systems using solid adsorbents.