Synthesis, structural characterization, and statistical modeling of methylene blue adsorption onto phosphosilicated apatites powders

Abstract

In this work, three compounds of Sr-La-phosphosilicates apatites, with the chemical formula Sr10-xZn0.66Ndx(PO4)6-x(SiO4)xO (where x = 0, 2 and 4), were synthesized using mechanochemical synthesis and solid-state reaction techniques. The prepared compounds were characterized using powder X-ray diffraction (XRD), Infrared spectroscopy (FT-IR), 31P and 29Si MAS NMR, Scanning Electron Microscopy (SEM), Thermogravimetric analysis (TGA), and Brunauer-Emmett-Teller (BET). The compounds were composed of a single pure apatitic phase crystallizing in a hexagonal system with the space group P63/m. The calcined compounds showed an increase of the crystallinity degree and the size of the crystals. Solid 31P and 29Si NMR data displayed a single crystallographic site for phosphor element. The single resonance peak at −76.21 ppm, in the 29Si specruim, confirmed to Q0 type silicon specie. The compounds were further applied for the adsorption of Methylene Blue from water. The highest adsorption capacity was reached using Milled-4 sample (515 mg/g). It was 363 mg/g for the calcined-4 one. The statistical physics (AMSP) investigation suggested that the adsorption of methylene blue was physical with energies lower than 30 kJ/mol.