Abstract:
The surface reactivity of iron (hydr)oxides plays a crucial role in controlling their interfacial reactions, for which various surface complexation models have been developed. The diversity of mineralogical properties of iron (hydr)oxides has resulted in a redundancy of model parameters, which hampers the modeling of iron (hydr)oxides in soils and sediments, where goethite, hematite and ferrihydrite dominate the iron (hydr)oxide mass fraction. To capture their combined surface reactivity, optimized generic protonation parameters of the Charge Distribution-Multisite Complexation (CD-MUSIC) extended-Stern-Gouy-Chapman (eSGC) model were derived by reanalyzing literature datasets and tested with some newly synthesized iron (hydr)oxides. It was observed that the proton and monovalent ion affinity constants of the different iron (hydr)oxides were located in a narrow range. For the singly- and triply-coordinated hydroxyl sites the obtained generic log(affinity constants) were 8.3 and 11.7 for the protonation reaction and -0.5 for the reaction with the monovalent background ions. Their combination with fixed site densities of singly-/triply-coordinated hydroxyl sites of 3.45/2.70, 5.00/2.50, and 5.80/1.40 sites/nm2 for goethite, hematite, and ferrihydrite, respectively, provided good results. The Stern layer capacitances of the inner and outer Stern layers were set equal and could be acquired by an empirical correlation with the sample specific surface area (SSA). The CD-MUSIC-eSGC model with the generic model parameters enables good quality predictions of the proton reactivity of iron (hydr)oxides in 1:1 electrolyte solutions regardless of the sample heterogeneity. The advantages of the generic CD-MUSIC-eSGC model are twofold: (1) protonation of iron (hydr)oxides can be described without making use of spectroscopic measurements and proton titrations, and (2) the model calculations are greatly simplified.
摘要:
铁(氢)氧化物的表面反应性在控制其界面反应中起着至关重要的作用。为此,已经开发了各种表面络合模型。铁(hydr)氧化物的矿物学性质的多样性导致了模型参数的冗余,这阻碍了土壤和沉积物中氧化铁的建模,在针铁矿,赤铁矿和水铁矿占铁(氢)氧化物质量分数。为了捕获它们的结合表面反应性,通过重新分析文献数据集得出电荷分布-多位点络合(CD-MUSIC)扩展-Stern-Gouy-Chapman(eSGC)模型的优化的通用质子化参数,并用一些新合成的铁(氢)氧化物进行了测试。观察到不同的铁(氢)氧化物的质子和一价离子亲和常数位于狭窄的范围内。对于单配位和三配位的羟基位点,质子化反应获得的通用log(亲和常数)为8.3和11.7,与单价背景离子反应获得的通用log为-0.5。它们与针铁矿的单/三配位羟基位点的固定位点密度为3.45/2.70、5.00/2.50和5.80/1.40位点/nm2的组合,赤铁矿,和水铁矿,分别,提供了良好的结果。内部和外部Stern层的Stern层电容设置为相等,并且可以通过与样品比表面积(SSA)的经验相关性来获取。具有通用模型参数的CD-MUSIC-eSGC模型可以对1:1电解质溶液中铁(hydr)氧化物的质子反应性进行高质量的预测,而与样品的异质性无关。通用CD-MUSIC-eSGC模型的优点是双重的:(1)可以在不使用光谱测量和质子滴定的情况下描述铁(氢)氧化物的质子化,(2)模型计算大大简化。