BACKGROUND The substitution of Ca(2+) in Ca-hydroxylapatite by toxic Cd(2+) can cause the forming of Cd-hydroxylapatite and is a significant issue in a great variety of research areas, which hence needs an understanding of the essential physicochemical characteristics. Unfortunately, the solubility product and thermodynamic data for Cd-hydroxylapatite in water under a variety of conditions now are lacking. Little information has been reported by previous researchers. Additionally, the dissolution mechanism of Cd-hydroxylapatite has never been studied. RESULTS Dissolution of the synthetic cadmium hydroxylapatite [Cd-HAP, Cd5(PO4)3OH] in HNO3 solution (pH = 2), ultrapure water (pH = 5.6) and NaOH solution (pH = 9) was experimentally studied at 25, 35 and 45°C. Characterization by XRD, FT-IR and FE-SEM proved that Cd-HAP solids showed no recognizable change during dissolution. For the Cd-HAP dissolution in aqueous acidic media at initial pH 2 and 25°C, the solution cadmium and phosphate concentrations increased rapidly and reached the peak values after 20-30 days and 10 days reaction, respectively. Thereafter, the Cd-HAP dissolution rate decreased slowly, whereas the solution Cd/P molar ratio increased constantly from 1.65-1.69 to 6.61-6.76. The mean K sp values for Cd5(PO4)3OH were determined to be 10(-64.62) (10(-64.53)-10(-64.71)) at 25°C, 10(-65.58) (10(-65.31)-10(-65.80)) at 35°C and 10(-66.57) (10(-66.24)-10(-66.90)) at 45°C. Based on the obtained solubility data from the dissolution at initial pH 2 and 25°C, the Gibbs free energy of Cd5(PO4)3OH forming [Formula: see text] was determined to be -3,970.47 kJ/mol (-3,969.92 to -3,970.96 kJ/mol). Thermodynamic parameters, ΔG (0), ΔH (0), ΔS (0), and [Formula: see text] for the dissolution process of Cd-HAP in aqueous acidic media at initial pH 2 and 25°C were calculated 368,710.12 J/K mol, -158,809.54 J/mol, -1,770.20 and -869.53 J/K mol, respectively. CONCLUSIONS Based on the experimental results of the present work and some previous researches, the cadmium hydroxylapatite (Cd-HAP) dissolution in aqueous media is considered to have the following coincident processes: the stoichiometric dissolution coupled with protonation and complexation reactions, the non-stoichiometric dissolution with Cd(2+) release and PO4 (3-) sorption and the sorption of Cd(2+) and PO4 (3-) species from solution backwards onto Cd-HAP surface. The obtained solubility products (K sp) 10(-64.62) (10(-64.53)-10(-64.71)) for Cd-HAP was approximately 7.62-5.62 log units lower than 10(-57)-10(-59) for calcium hydroxylapatite (Ca-HAP).Graphical abstractDissolution of cadmium hydroxylapatite [Cd5(PO4)3OH].

中文翻译：

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合成镉羟基磷灰石[Cd5（PO4）3OH]在25-45°C下的表征，溶解和溶解度。

背景技术有毒的Cd（2+）替代Ca-羟基拉美石中的Ca（2+）会导致Cd-羟基拉美石的形成，并且在许多研究领域中都是重要的问题，因此需要了解基本的理化性质。特征。不幸的是，现在缺少各种条件下Cd-羟基磷灰石在水中的溶解度积和热力学数据。以前的研究人员报道的信息很少。另外，从未研究过Cd-羟基磷灰石的溶解机理。结果分别在25、35和35℃的温度下，通过实验研究了合成的羟基磷灰石镉[Cd-HAP，Cd5（PO4）3OH]在HNO3溶液（pH = 2），超纯水（pH = 5.6）和NaOH溶液（pH = 9）中的溶解度。 45℃。通过XRD进行表征 FT-IR和FE-SEM证明Cd-HAP固体在溶解过程中没有可识别的变化。对于在初始pH 2和25°C下Cd-HAP在酸性介质中的溶解，溶液中镉和磷酸盐的浓度迅速增加，并分别在20-30天和10天反应后达到峰值。此后，Cd-HAP溶解速率缓慢降低，而溶液中Cd / P摩尔比从1.65-1.69不断增加至6.61-6.76。Cd5（PO4）3OH的平均K sp值在25°C下确定为10（-64.62）（10（-64.53）-10（-64.71）），10（-65.58）（10（-65.31）- 35°C时为10（-65.80）），45°C时为10（-66.57）（10（-66.24）-10（-66.90））。根据在初始pH 2和25°C下从溶解获得的溶解度数据，确定形成Cd5（PO4）3OH [分子式：见正文]的吉布斯自由能为-3,970.47 kJ / mol（-3,969。92至-3,970.96 kJ / mol）。计算了Cd-HAP在初始pH 2和25°C下在水性酸性介质中的溶解过程的热力学参数ΔG（0），ΔH（0），ΔS（0）和[公式：参见文本] 368,710.12 J / K mol分别为-158,809.54 J / mol，-1,770.20和-869.53 J / K mol。结论基于目前工作的实验结果和一些先前的研究，羟基磷灰石镉（Cd-HAP）在水性介质中的溶解被认为具有以下同时发生的过程：化学计量的溶解，质子化和络合反应，非化学计量的溶解与Cd（2+）释放和PO4（3-）吸附，以及Cd（2+）和PO4（3-）从溶液向Cd-HAP表面的吸附。所获得的溶解度乘积（K sp）10（-64.62）（10（-64.53）-10（-64。