Simultaneous uptake of Ni²⁺, NH⁺₄ and PO^3-₄ by amorphous CaO-Al₂O₃-SiO₂ compounds

Simultaneous uptake of Ni²⁺, NH⁺₄, and PO^3-₄ 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)₂ṡ4H₂O, Al(NO₃)₃ṡ9H₂O, 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 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 Ni²⁺ and PO^3-₄ were high, but were relatively low for NH⁺₄. The uptake abilities for Ni²⁺ 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 Ni²⁺ 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 < 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.