Synthesis of a high-density phosphoric acid functional mesoporous adsorbent: Application to chromium(III) removal

(Chemical Equation Presented) A high-capacity phosphonic acid functional adsorbent is prepared by co-condensing oligomers of tetraethoxysilane (TEOS) and trimethoxysilylpropyl diethylphosphonate (DEPPS). Hydrolysis/condensation reactions for both silanes are monitored in the sol-state using ²⁹Si NMR spectroscopy, and the dependency of the resulting physicochemical properties on these reaction times is investigated. The molar ratio of TEOS to DEPPS is varied to determine the ratio at which high DEPPS incorporation could be attained without evidence of structural perturbation. The phosphonic acid adsorbent is characterized by solid-state ²⁹Si and ³¹P NMR spectroscopy, nitrogen sorption, and elemental analysis. The adsorbent exhibits a ligand density of 10.0 mmol P/g, and maximum values of S_BET of 437 m²/g, D_p of 59 Å, and V_p of 0.65 cm ³/g. Further characterization shows an equilibrium chromium(III) adsorption capacity of 82 mg/g at pH 3.6 and rapid adsorption kinetics. Column adsorption at a high space velocity of 156 h^-1 is achieved with a minimum effluent chromium concentration of 0.01 mg/L. The chromium-loaded adsorbent is stripped using 12 M HCl. A viable synthesis method for the preparation of a high-density functional mesoporous adsorbent is demonstrated.