Synthesis and phosphate uptake behavior of Zr 4+ incorporated MgAl-layered double hydroxides

Abstract

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 0.68Al 0.17Zr 0.14(OH) 2][Cl 0.26⋅(CO 3) 0.04⋅1.24H 2O], was synthesized for the first time using a hydrothermal treatment at 120 °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.