Layered Yttrium Hydroxide l-Y(OH) Luminescent Adsorbent for Detection and Recovery of Phosphate from Water over a Wide pH Range.

Layered yttrium hydroxide, l-Y(OH), has been explored as a representative member of the layered rare earth hydroxide family (l-RE(OH); RE = rare earths) for removal and recovery of phosphate from aqueous solution. Compared to the hexagonal form, h-Y(OH), which has a weakly positive surface charge only at low pH, the layered polymorph composed of hydroxocation layers exhibited a high point of zero charge (pH ∼ 11) and significantly enhanced adsorptive ability for anions over a wide pH range. The Langmuir isotherm model and pseudo-second-order kinetic model were adopted to explain the phosphate adsorption on l-Y(OH). This new adsorbent revealed high capacity, efficiency, stability, selectivity, and reusability in adsorption of phosphate from a single electrolyte as well as natural waters containing competing anions. Essentially complete phosphate recovery from aqueous solutions at low phosphate concentrations (2.0 mg of P/L) was demonstrated with an adsorbent dosage of 0.025-0.5 g/L. The adsorption of phosphate was accompanied by an increase in the solution pH, suggesting a release of OH ions during the adsorption reaction. In particular, when Ce and Tb were co-doped (l-Y(OH):Ce,Tb), phosphate adsorption led to the characteristic D → F (J = 6, 5, and 4) emissions of Tb under commercial 312 nm UV irradiation. The photoluminescence of phosphate-adsorbed l-Y(OH):Ce,Tb provided evidence of the inner-sphere complexing mechanism involving the formation of Y(Ce,Tb)-O-P bonds through which the energy transfer can occur. The "luminescence-on" behavior of l-Y(OH):Ce,Tb by phosphate adsorption was employed to detect and recover phosphorus at low concentrations in deionized water, mineral water, tap water, and river water.