Simultaneous uptake of Ni 2+, NH +4 and PO 3−4 by amorphous CaO–Al 2O 3–SiO 2 compounds

Abstract

Simultaneous uptake of Ni 2+, NH + 4, and PO 3− 4 by amorphous CaO–Al 2O 3–SiO 2 (C-A-S) compounds was investigated using batch and column methods. Fifteen different C-A-S samples with systematically varied chemical compositions were prepared by coprecipitation from ethanol–water solutions containing Ca(NO 3) 2⋅4H 2O, Al(NO 3) 3⋅9H 2O, and Si(OC 2H 5) 4, using NH 4OH as the precipitating agent. The resulting precipitates were dried and heated at various temperatures to produce the C-A-S samples, which were then characterized by XRD, FTIR, solid state MAS NMR, DTA-TG, and N 2 adsorption. All the C-A-S samples prepared at 600–900 °C were amorphous, apart from the CaO-rich samples. Simultaneous uptake of Ni 2+, NH + 4, and PO 3− 4 was determined by a batch method using a solution with an initial concentration of 2 mM. In these experiments, the uptake abilities of the C-A-S samples for Ni 2+ and PO 3− 4 were high, but were relatively low for NH + 4. The uptake abilities for Ni 2+ and PO 3− 4 increased but that for NH + 4 decreased as the silica content in the C-A-S decreased, suggesting that similar uptake mechanisms (ion substitution and/or precipitation) are operating for Ni 2+ and PO 3− 4, but the uptake mechanism for NH + 4 is different (physical adsorption). The column experiments indicate that the order of uptake ability of C-A-S for the three ions is NH + 4 ≪ PO 3− 4 < Ni 2+. Although the silica content of the C-A-S does not have the expected influence on the uptake of these three ions, for NH + 4 it plays an important role in the formation of the amorphous phase and also in the suppression of Ca 2+ and/or Al 3+ release from the C-A-S during the uptake experiments. The optimum uptake properties of the C-A-S can thus be controlled by adjusting the chemical compositions and heating conditions under which the samples are prepared.