Simultaneous uptake of Ni2+, NH+4 and PO3−4 by amorphous CaO–Al2O3–SiO2 compounds

Simultaneous uptake of Ni2+, NH+4, and PO3−4 by amorphous CaO–Al2O3–SiO2 (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(NO3)2⋅4H2O, Al(NO3)3⋅9H2O, and Si(OC2H5)4, using NH4OH 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 N2 adsorption. All the C-A-S samples prepared at 600–900 °C were amorphous, apart from the CaO-rich samples. Simultaneous uptake of Ni2+, NH+4, and PO3−4 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 Ni2+ and PO3−4 were high, but were relatively low for NH+4. The uptake abilities for Ni2+ and PO3−4 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 Ni2+ and PO3−4, 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 ≪ PO3−4 < Ni2+. 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 Ca2+ and/or Al3+ 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.

Graphical abstractAmorphous CaO–Al2O3–SiO2 compounds show good simultaneous uptake abilities for cations and anions and the uptake is NH+4 ≪ PO3−4 < Ni2+ in order.Figure optionsDownload full-size imageDownload as PowerPoint slide