Amorphous High-Surface-Area Aluminum Hydroxide–Bicarbonates for Highly Efficient Methyl Orange Removal from Water

Amorphous high-surface-area aluminum hydroxide–bicarbonates were synthesized starting from AlCl₃, base, and bicarbonate in water. Composites with a chemical formulas of [Al₁₃O₄(μ–OH)₂₄(H₂O)_6.5(OH)_5.5]­(HCO₃)_1.5 (I-NaOH) and [Al₁₃O₄(μ–OH)₂₄(H₂O)₆(OH)₆]­(HCO₃) (I-NH₃) were obtained by the use of NaOH/NaHCO₃ and NH₃/NH₄HCO₃ as base/bicarbonate, respectively. The surface area of the composites was highly dependent on the pH level of the synthetic solution, and composites with high surface areas (ca. 200 m² g^–1) were obtained around pH 7–8. Pore-size distributions determined from the N₂ adsorption isotherms showed that I-NH₃ and I-NaOH possess mainly large (pore radius r_p > 3 nm) and small (r_p < 3 nm) pores, respectively, despite similar surface areas. While SEM images showed that both I-NH₃ and I-NaOH were aggregates of nanoparticles, the particles were more fused in I-NaOH, which is in line with the existence of small pores and the use of a stronger base (NaOH), which would facilitate the dehydration condensation reaction. The composites were applied as adsorbents to remove methyl orange (MO) from water. The time course of MO adsorption was readily fitted with a pseudo-second-order model, and over 90% MO removal was attained within 10 min with I-NH₃, while I-NaOH showed much less MO removal (26%). The MO adsorption isotherm of I-NH₃ was reproduced with a Langmuir model with an adsorption capacity of 154 mg g^–1. Notably, the aluminum hydroxide–bicarbonates could not absorb methylene blue, which is a cationic dye, while anions (MO and PO₄^3–) were readily absorbed. Solid-state ²⁷Al MAS NMR spectra showed that the concentration of 5-coordinated aluminum species, which may serve as guest binding sites, was higher for I-NH₃. These results show that electrostatic interaction between anionic MO and coordinatively unsaturated 5-coordinated cationic aluminum species and the large external surface area of I-NH₃ contribute to the highly efficient MO adsorption.