Wet chemical synthesis of hydroxyapatite (HAp) nanostructures was carried out with different solution pH values (9, 10 and 11) and sintering temperatures (300°C, 500°C, 700°C and 900°C). The effects of pH and sintering temperature on the structural and morphological properties of nanocrystalline HAp powders were presented. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis were performed to obtain the crystalline structure, chemical composition, morphology and particle size of the HAp powders. The TEM analysis is used in order to observe the rod- and flake-like HAp structures. XRD confirms the presence of both HAp hexagonal and monetite phases, although the monetite phase was less abundant in the resultant powders. Increase in pH reduced the monetite phase and enhanced Ca/P ratio from 1.7 to 1.83. Additionally, an increment in sintering temperature increased the crystallite size from 20 to 56 nm. The SEM analysis revealed the formation of semi-spherical and flake-like HAp structures with preferential flake morphology. An increase in pH and sintering temperature resulted in the growth and coalescence of crystals resulting in a porous capsular morphology. The FTIR analysis confirmed the reduction of carbonate stretching modes with an increase in pH and H–O–H antisymmetric stretching mode is eliminated for powders sintered at 900°C confirming the formation of stable and porous HAp powders.

This work shows the physicochemical behavior of two different diatoms from the country of Mexico (State of Jalisco and Hidalgo) with similar compositions. These were used to eliminate toxic cations from a synthetic solution containing 5.270 mg As 3+ /L; 4.280 mg Ag + /L; 3.950 mgNi 2+ /L; 4.090 mg Cr 6+ /L; and 4.081 mg Pb 2+ /L. These diatoms were used as filters, and the quantity of cations remaining in the solution after filtering was measured. According to the most important results found, for the recovery of metals, both minerals achieved arsenic, silver, lead, and nickel recoveries up to 95%, and lower than 10% for chromium. This could be due to the absence of an environment to reduce Cr 6+ to Cr 3+ . On the other hand, it was observed that there was no selectivity during the recovery of the other cations present in the solution. According to efficiency of interchange, the mineral from Hidalgo is slightly better than the mineral from Jalisco for the removal of arsenic, lead, and silver. For nickel, and particularly Cr 6+ , the efficiency is higher for the sample from Jalisco.

Summary. In the present work, a study of H+ effects on the microstructure and morphology of hydroxyapatite synthesized by the hydrothermal method is reported. The synthesis was realized with pH values of 10, 9.6, 9, 8, and 7. To know the particle size distribution, growth habit, chemical composition, and crystalline phases present, SEM-EDS, XRD, AFM, and FTIR spectroscopy techniques were used and completed with Rietveld analysis. The obtained results showed an important effect of H+ on the morphological and crystallographic characteristics of the hydroxyapatite, demonstrating that the shape and nanoparticle size, as well as the number of crystalline phases, can change depending on the pH during the synthesis. It was observed that there is an increase in the formation of the monoclinic phase and a decrease of the hexagonal phase when the pH value diminishes from 9.6 to 7. The crystallite size also decreases from 46.69 to 19.56 nm. An explanation of the role of pH on the final characteristics of the hydroxyapatite was related to the amount of H+ and OH- ions, and it was included in this work.

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