On the thermodynamics and kinetics of scorodite dissolution
On the thermodynamics and kinetics of scorodite dissolution
复制标题
DOI：
10.1016/j.gca.2019.09.012
复制
发表时间：
2019-11
影响因子：
5
通讯作者：
Xiangyu Zhu;D. Nordstrom;R. McCleskey;Rucheng Wang;Xiancai Lu;Siliang Li;H. Teng
中科院分区：
文献类型：
--
作者：
Xiangyu Zhu;D. Nordstrom;R. McCleskey;Rucheng Wang;Xiancai Lu;Siliang Li;H. Teng
臭葱石（FeAsO 4·2 H2O）-水的相互作用是至关重要的，在地表环境中的分布和存储，但由于在大气条件下的矿物的稳定性和风化速率的模糊性，是不充分的理解。在本研究中，我们试图通过实验确定计算臭葱石在25 °C下的溶解度和溶解速率所需的热力学和动力学参数。实验在低pH（1.15）溶液中进行，使用特别合成的大臭葱石晶体（平均直径为27.9 μm）。这样的实验条件保证了实验结果不受二次含铁相析出和晶粒尺寸效应的影响。首次用测定的Fe和As的平衡浓度，沿着和新近发表的Fe-As络合物缔合常数，测定了50-90 °C时的溶度积Ksp和溶解速率rnat。然后使用获得的Ksp Δ T和rn Δ T依赖性来推导溶解反应的吉布斯自由能、焓、熵、热容和活化能。最后，外推至25 °C，得到了溶解度和溶出速率，考察了pH值对溶出的影响，并分析了溶出反应的ΔG_r关系。我们的研究结果表明，文献数据可能高估了臭葱石溶解度在pH > 4 - 4.5，由于忽略了铁的水解的影响。估计的环境条件下的溶解速率是一个数量级低于较早的报告，暗示尺寸效应的重要性，但比常见的造岩矿物高1至2个数量级，警告臭葱石的固定和存储的建议使用。所确定的rn ΔG关系不能完全由过渡态模型拟合，特别是在接近平衡时，这表明本研究中的溶解可能是由缺陷辅助的表面反应控制的。
Scorodite (FeAsO4·2H2O)–water interaction is critical to As distribution and storage in surface environment but is inadequately understood due to ambiguities in the mineral’s stability and weathering rate at atmospheric conditions. In the present study we attempted to experimentally determine the thermodynamic and kinetic parameters needed to compute solubility and dissolution rate of scorodite at 25 °C. Experiments were carried out in low pH (1.15) solutions using specially synthesized large scorodite crystals (with Mean Diameter of 27.9 μm). Such experimental conditions ensured the results were not subject to the influence of secondary Fe-bearing phase precipitation and grain size effect. Measured equilibrium concentrations of Fe and As, along with newly published Fe-As complexes association constants, were first used to determine the solubility products Kspand dissolution rates rnat 50–90 °C. The obtained Ksp∼ T and rn∼ T dependence was then used to derive the Gibbs free energy, enthalpy, entropy, heat capacity, and activation energy for the dissolution reaction. Finally, we extrapolated the measurements to 25 °C and obtained room temperature solubility and dissolution rate, scrutinized the pH effect on dissolution, and analyzed the ΔG∼rnrelation of the dissolution reaction. Our results show that literature data are likely overestimated scorodite solubility at pH > 4 ∼ 4.5 due to neglecting the effect of ferric iron hydrolysis. Estimated ambient condition dissolution rate is an order magnitude lower than the earlier report, implicating the importance of size effect, but is one to two orders of magnitude higher than that of common rock-forming minerals, cautioning the proposed use of scorodite for As fixation and storage. The determined rn∼ ΔG relation cannot be fully fitted by the transition state model, particularly at near equilibrium, suggesting dissolution in this study may be controlled by defect-assisted surface reactions.