Growth reaction hydroxyapatite

Humic substances. Analogous to the reactions described above, humic substances (the polymeric pigments from soil (humus) and marine sediments) can be formed by both enzymatic and non-enzymatic browning. High concentrations of free calcium and phosphate ions and supersaturation with respect to hydroxyapatite can sustain in soil, because adsorption of humic acids to mineral surfaces inhibits crystal growth (Inskeep and Silvertooth, 1988). A similar adsorption to tooth mineral in a caries lesion can be anticipated for polycarboxylic polymers from either the Maillard reaction or enzymatic browning. 

Lower et al. (1998b) can shed more light on this phenomenon. They again used AFM, SEM, TEM, SEM-EDS, electron diffraction, and XRD to study the reactions between 0.5 and 500 mg/L of Pb with hydroxyapatite at pH 6 and a reaction temperature of 22 °C. A commercial hydroxyapatite was used at sorbent concentrations of 0.5 g/L. Reactions were observed over a 2 h period. At high initial Pb concentrations, Pb solution concentrations dropped from 500 mg/L to <100 mg/L. At concentrations of 0.5-100 mg Pb/L, after reaction, Pb levels dropped to less than 15 pg/L. In both cases, hydroxyapatite dissolved and hydroxypyromorphite formed. The authors applied some nucleation and crystal growth theory developed... 

Some work has been reported on deposition of hydroxyapatite under hydrothermal conditions, that is much above 100 °C. This includes a study by Liu, Savino and Yates (2011) who coated hydroxyapatite on titanium, stainless steel, aluminium and copper substrates by a seeded hydrothermal deposition method. The deposition strategy included an electrochemical reaction to form quickly a thin layer of HAp seed crystals. Subsequent hydrothermal crystal growth from the seed layer resulted in dense and durable HAp films. In a typical hydrothermal synthesis, a solution of Na2EDTA (0.20 M) and Ca(NOs)2 (0.20 M) was prepared in 15 ml water and a solution of (NH4)2HP04 (0.12 M) in 15 ml water was prepared in a second container. The two source solutions were mixed together after the pH of each solution was raised to 10.0 with ammonium hydroxide. The resulting combined solution was stirred at room temperature for about 20 min and then transferred to a Teflon-lined stainless steel pressure vessel of 40 ml internal volume. 

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