Rosette-like Layered Double Hydroxides: Adsorbent Materials for the Removal of Anionic Pollutants from Water

Rosette-like layered double hydroxide (roseLDH) crystals with interlayer CO₃^2- anions were synthesized by the reaction of Mg(NO₃)₂, Al(NO₃)₃, and hexamethylenetetramine (HMT) at 140 °C over 4 days. Crystals as large as 20 μm were produced when using a specific range of HMT concentrations. The substitution of CO₃^2- interlayer ions with ClO₄^- or Cl^- anions was achieved by the addition of perchloric acid or hydrochloric acid, respectively, to dispersion of material in methanol. The products were denoted as CO₃^2-roseLDH, ClO₄^-roseLDH, and Cl^-roseLDH, respectively. These LDHs were characterized using X-ray diffraction under controlled relative humidity, as well as by Fourier transform infrared spectroscopy and scanning electron microscopy. Adsorption experiment with anions such as phosphate (HPO₄^2-) and nitrate (NO₃^-) was conducted by using ClO₄^-roseLDH and Cl^-roseLDH. The results indicate that both anions were adsorbed through an ion-exchange mechanism. The maximum HPO₄^2- adsorption capacity at equilibrium on ClO₄^-roseLDH was 1.6 mmol g^-1 (49.6 mg P g^-1), which corresponds to approximately 75% of the total positive layer charge. Cl^-roseLDH showed a similar adsorption capability. Commercially available platelike LDH particles were essentially impermeable to water flow due to clogging, while the roseLDH crystals showed excellent permeability, an order of magnitude higher than that exhibited by the platelike LDH synthesized using a homogeneous precipitation method with different growth conditions. Anion adsorption during batch and flow-through test with the ClO₄^-roseLDH (mean particle diameter ∼ 38 μm) in a packed bed showed good uptake of HPO₄^2- and NO₃^- from aqueous solutions. These results demonstrate the potential of roseLDH materials to serve as a column filler adsorbent of the hazardous anions.