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

Rosette-like layered double hydroxide (LDH) crystals with interlayer CO anions were synthesized by the reaction of Mg(NO), Al(NO), 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 CO interlayer ions with ClO 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 COLDH, ClOLDH, and ClLDH, 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 (HPO) and nitrate (NO) was conducted by using ClOLDH and ClLDH. The results indicate that both anions were adsorbed through an ion-exchange mechanism. The maximum HPO adsorption capacity at equilibrium on ClOLDH was 1.6 mmol g (49.6 mg P g), which corresponds to approximately 75% of the total positive layer charge. ClLDH showed a similar adsorption capability. Commercially available platelike LDH particles were essentially impermeable to water flow due to clogging, while the LDH 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 ClOLDH (mean particle diameter ∼ 38 μm) in a packed bed showed good uptake of HPO and NO from aqueous solutions. These results demonstrate the potential of LDH materials to serve as a column filler adsorbent of the hazardous anions.