Synthesis, Crystal Structure, and Excellent Selective Pb2+ Ion Adsorption of New Layered Compound (NH4)In3(SO4)2(OH)6

Abstract

(NH4)In3(SO4)2(OH)6, a new indium incorporated synthetic compound belonging to the alunite supergroup, is successfully prepared by hydrothermal reaction. The crystal structure is determined by single‐crystal X‐ray diffraction. (NH4)In3(SO4)2(OH)6 crystallizes in space group R3m of a = b = 7.69(8) Å, c = 17.71(2) Å and Z = 3, and it adopts a layered structure with alternating corrugated two‐dimensional 2∞[In3(SO4)2(OH)6–] anionic network and interstitial NH4+ ions. The measurements of X‐ray powder diffraction pattern, Fourier transform infrared spectroscopy spectrum, differential Scanning Calorimeter and X‐ray photoelectron spectroscopy have been performed on this new compound. The title compound is air‐ and water‐stable and starts thermal decomposition to In2O3 at 620 K. Interstitial NH4+ ions between two‐dimensional 2∞[In3(SO4)2(OH)6–] layers are exchangeable by cations like Pb2+. The maximum adsorption capacity of Pb2+ ions reaches 990 mg/g at pH 6.5, and the selectivity is observed with a very high distribution coefficient Kd of 1.1 × 106 mL/g compared with other completive cations. The unique layered framework composed of [InO6]9– octahedra and [SO4]2– tetrahedra is chemically stable to enable excellent cycling performance. This investigation will advocate the alunite supergroup minerals for efficient and recyclable adsorption of toxic metal ions in contaminated water.