Sorption of borate onto layered double hydroxides assembled on filter paper through in situ hydrothermal crystallization

Boric acid is known to be difficult to immobilize because it has a pK_a of 9.23, which means it is present in a molecular form in most aqueous environments and can be changed in the structure and nature with the change of the environmental conditions. Layered double hydroxides (LDHs) are potential sorbents for borate. However, the drawbacks of using LDHs for borate removal are that they are affected by the presence of other anions and are difficult to separate from water. In the present work, novel composites of Mg-Al type of LDH, synthesized onto filter papers and then intercalated with gluconate (F-LDH-G) were prepared. LDH with interlayer of carbonate form was first immobilized onto the surface of filter paper (F-LDH-CO₃) by in situ hydrothermal crystallization, and then ion-exchanged sequentially with chloride (F-LDH-Cl) followed by gluconate. The influence of the molar ratio of gluconate/LDH and reaction time on F-LDH-G synthesis was explored. Products were characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy and nuclear magnetic resonance as well as measurements of borate sorption. The optimal molar ratio of gluconate/LDH and reaction time for preparing F-LDH-G from F-LDH-Cl were 40 and 24h (F-LDH-G-40-24h), respectively. F-LDH-G-40-24h had higher sorption density and greater stability than both the original filter paper and the filter papers containing other LDHs, even in the presence of other anions. ¹¹B NMR and XRD results indicate that the principal mechanism for borate immobilization on F-LDH-G-40-24h is complexation of gluconate with borate in both bischelate and monodentate forms. The novel composite is boron-specific and facilitates solid/liquid separation.