Synthesis and phosphate uptake behavior of Zr4+ incorporated MgAl-layered double hydroxides

We synthesized Zr4+ incorporated MgAl-layered double hydroxides, Mg(AlZr)-LDH(A) (where A denotes a counteranion in the interlayer space and is expressed as CO3 for carbonate and Cl for chloride ions), with different molar ratios of Mg/(Al + Zr). Then we characterized their uptake behavior toward phosphate ions. CO3-type tertiary LDH materials synthesized at room temperature show low crystallinity, whereas the highly crystalline Cl-type tertiary LDH, [Mg0.68Al0.17Zr0.14(OH)(2)][Cl-0.26 center dot(CO3)(0.04)center dot 1.24H(2)O], was synthesized for the first time using a hydrothermal treatment at 120 degrees C. The distribution coefficients (K-d) of oxo-anions were measured with a mixed solution containing trace amounts of the anions. The selectivity sequences were Cl-, NO3- < SO42- << HPO42- for CO3-type materials and SO42- < HPO42- < NO3- for the crystalline Cl-type material. The uptake of phosphate ions from model wastewater (2.0 mg-P/dm(3)) and phosphate-enriched natural seawater (0.33 mg-P/dm(3)) was investig ated in detail. The CO3-type materials have higher phosphate uptakes than the Cl-type materials. The maximum phosphate uptake of the CO3-type material with a molar ratio of Mg/(Al + Zr) of 3 is 30 mg-P/g at pH 8.7 with the wastewater, and 16 mg-P/g at pH 8.1 with the seawater, in coritrast to the case of the usual binary MgAl-LDH(CO3): 10 Mg-P/g with the wastewater and less than 1 mg-P/g with the seawater. The large uptake and high selectivity of the CO3-type tertiary LDH materials is well e,xplained by complex formation of phosphate ions directly with Zr(IV) centers in the layers. (c) 2007 Elsevier Inc. All rights reserved.