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

We synthesized Zr(4+) incorporated MgAl-layered double hydroxides, Mg(AlZr)-LDH(A) (where A denotes a counteranion in the interlayer space and is expressed as CO(3) for carbonate and Cl for chloride ions), with different molar ratios of Mg/(Al+Zr). Then we characterized their uptake behavior toward phosphate ions. CO(3)-type tertiary LDH materials synthesized at room temperature show low crystallinity, whereas the highly crystalline Cl-type tertiary LDH, [Mg(0.68)Al(0.17)Zr(0.14)(OH)(2)][Cl(0.26)(CO(3))(0.04)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(-), NO(-)(3)<SO(2-)(4)<<HPO(2-)(4) for CO(3)-type materials and SO(2-)(4)<HPO(2-)(4)<NO(-)(3) 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 investigated in detail. The CO(3)-type materials have higher phosphate uptakes than the Cl-type materials. The maximum phosphate uptake of the CO(3)-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 contrast to the case of the usual binary MgAl-LDH(CO(3)): 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 CO(3)-type tertiary LDH materials is well explained by complex formation of phosphate ions directly with Zr(IV) centers in the layers.