Dissolution and hydration kinetics of MgO

O. Fruhwirth, G.W. Herzog, I. Hollerer, A. Rachetti
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引用次数: 127

Abstract

The dissolution and hydration kinetics of MgO single crystals and powder samples were investigated with regard to the H+ and Mg2+ concentrations and the temperature. The rate of dissolution of rotating MgO discs in buffered solutions was determined from measurements of [Mg2+] and those of the crystals and powder fractions were determined by pH and conductivity analysis. The degree of hydration was analysed by means of a thermogravimetric method. Several rate-controlling processes depending on pH were present at room temperature.

(1) At pH < 5 the rate-controlling step was proton attack followed by desorption of Mg2+ of OH- depending on the value of [Mg2+]. The rate was proportional to either -pH or pMg-pH. These processes are part of the overall neutralization reaction. MgO + 2H+→Mg2+ + H2O

(2) At pH ≈ 5 the rate-controlling step was a diffusion-limitation process due to protons. The rate was proportional to the proton concentration.

(3) At pH > 7 the rate-controlling step was OH- adsorption followed by Mg2+ and OH- desorption leading to a rate maximum. These processes are part of the overall dissolution reaction. MgO + H2O→Mg2+ + 2OH- The neutralization processes are interpreted in terms of irreversible thermodynamics yielding a linear dependence of the rate on pH or pMg-pH. It is concluded from conductivity and scanning electron microscopy measurements during and after hydration experiments that the hydration rate is controlled by the dissolution rate under given conditions. After a supersaturation period Mg(OH)2 precipitates preferentially at the MgO surface, so that an MgO lattice reaction can be excluded. All processes undergo an Arrhenius acceleration with increasing temperature (activation energy, 70 kJ mol-1) and the overall reactions are then limited by proton and OH- diffusion.

氧化镁的溶解和水化动力学
研究了H+、Mg2+浓度和温度对MgO单晶和粉末样品溶解和水化动力学的影响。旋转MgO圆盘在缓冲溶液中的溶解速率由[Mg2+]测定,晶体和粉末组分的溶解速率由pH和电导率分析测定。用热重法分析了水化程度。在室温下,根据pH值存在几种速率控制过程。根据[Mg2+]的值,控制反应速率的步骤是质子攻击,然后是OH-的Mg2+解吸。速率与-pH或pMg-pH成正比。这些过程是整个中和反应的一部分。MgO + 2H+→Mg2+ + H2O(2)在pH≈5时,由于质子的作用,控制速率的步骤是一个限制扩散的过程。反应速率与质子浓度成正比。速率控制步骤是OH-吸附,然后是Mg2+和OH-解吸,从而达到速率最大值。这些过程是整个溶解反应的一部分。MgO + H2O→Mg2+ + 2OH-中和过程用不可逆热力学解释,其速率与pH或pMg-pH呈线性关系。通过水化实验时和实验后的电导率和扫描电镜测量得出,在一定条件下,水化速率受溶解速率控制。过饱和后,Mg(OH)2优先在MgO表面析出,因此可以排除MgO晶格反应。随着温度的升高(活化能为70 kJ mol-1),所有的反应都经历了Arrhenius加速,整个反应受到质子和OH-扩散的限制。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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