Characterization, dissolution and solubility of synthetic cadmium hydroxylapatite [Cd5(PO4)3OH] at 25-45°C.
Characterization, dissolution and solubility of synthetic cadmium hydroxylapatite [Cd5(PO4)3OH] at 25-45°C.
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DOI：
10.1186/s12932-015-0025-1
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发表时间：
2015
影响因子：
2.3
通讯作者：
Huang Y
中科院分区：
文献类型：
--
作者：
Zhu Y;Zhu Z;Zhao X;Liang Y;Dai L;Huang Y
Ca-羟基磷灰石中的 Ca2+ 被有毒的 Cd2+ 取代会导致 Cd-羟基磷灰石的形成，这是许多研究领域的一个重要问题，因此需要了解其基本的物理化学特性。不幸的是，目前缺乏各种条件下 Cd-羟基磷灰石在水中的溶度积和热力学数据。之前的研究人员报道的信息很少。此外，Cd-羟基磷灰石的溶解机理从未被研究过。在 25、35 和 45°C 下，对合成羟基磷灰石镉 [Cd-HAP, Cd5(PO4)3OH] 在 HNO3 溶液 (pH = 2)、超纯水 (pH = 5.6) 和 NaOH 溶液 (pH = 9) 中的溶解情况进行了实验研究。 XRD、FT-IR 和 FE-SEM 表征证明 Cd-HAP 固体在溶解过程中没有表现出可识别的变化。对于 Cd-HAP 在初始 pH 2 和 25°C 下在含水酸性介质中的溶解，溶液中的镉和磷酸盐浓度迅速增加，并分别在反应 20-30 天和 10 天后达到峰值。此后，Cd-HAP溶解速率缓慢下降，而溶液Cd/P摩尔比不断增加，从1.65-1.69增加到6.61-6.76。 Cd5(PO4)3OH 的平均 Ksp 值在 25°C 时确定为 10−64.62 (10−64.53–10−64.71)，在 35°C 时为 10−65.58 (10−65.31–10−65.80) 和 10−66.57 (10−66.24–10−66.90)在 45°C 下。根据在初始 pH 2 和 25°C 下溶解获得的溶解度数据，Cd5(PO4)3OH 形成的吉布斯自由能确定为 -3,970.47 kJ/mol（-3,969.92 至 -3,970.96 kJ/mol）。计算出初始 pH 2 和 25°C 下 Cd-HAP 在酸性水溶液介质中溶解过程的热力学参数 ΔG0、ΔH0、ΔS0 分别为 368,710.12 J/K mol、-158,809.54 J/mol、-1,770.20 和 -869.53 J/K mol。羟基磷灰石镉 [Cd5(PO4)3OH] 的溶解。基于本工作和一些先前研究的实验结果，羟基磷灰石镉（Cd-HAP）在水介质中的溶解被认为具有以下同时发生的过程：与质子化和络合反应相结合的化学计量溶解，伴随Cd2+释放和PO43−吸附的非化学计量溶解以及Cd2+和PO43−物质从溶液中向后吸附到Cd-HAP表面上。获得的 Cd-HAP 溶解度积 (Ksp) 10−64.62 (10−64.53–10−64.71) 比羟基磷灰石钙 (Ca-HAP) 的 10−57–10−59 低约 7.62–5.62 个对数单位。
The substitution of Ca2+ in Ca-hydroxylapatite by toxic Cd2+ 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. 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 Ksp 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 was determined to be −3,970.47 kJ/mol (−3,969.92 to −3,970.96 kJ/mol). Thermodynamic parameters, ΔG0, ΔH0, ΔS0, and 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. Dissolution of cadmium hydroxylapatite [Cd5(PO4)3OH]. 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 Cd2+ release and PO43− sorption and the sorption of Cd2+ and PO43− species from solution backwards onto Cd-HAP surface. The obtained solubility products (Ksp) 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).