Determination of the Redox Potential for Aqueous Fe(Ⅱ)-Goethite Heterogeneous Systems by Potentiometric Method
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摘要:
溶解性二价铁(Feaq 2+)-铁氧化物非均相体系的氧化还原能力显著影响含水层中有毒重金属和有机污染物的迁移转化行为。然而，非均相体系氧化还原电位(Eh)的测定存在平衡缓慢和结果不稳定等难题，故亟待开发与优化电化学法快速准确测定Eh，实现Feaq 2+-铁氧化物非均相体系氧化还原能力的定量表征。本文选取针铁矿作为常见的铁氧化物，探讨了介导和非介导电位法测定Feaq 2+-针铁矿非均相体系Eh的优化实验条件，研究了介导物质的类型与添加顺序以及工作电极预处理方法等条件对Eh测定的影响，并通过能斯特方程验证电位法的可行性。结果表明：工作电极的表面状态、介导物质的选择以及针铁矿-工作电极之间平衡的建立对Eh 测定十分关键。介导法能有效地缩短测定时间，但需要选择合适的介导物质；非介导法需确保工作电极与针铁矿之间平衡的建立。同时，利用非介导法测定了不同pH和Feaq 2+浓度下不同粒径针铁矿与Feaq 2+非均相体系的Eh，根据能斯特方程和最小二乘法多元线性拟合得到不同粒径针铁矿的标准氧化还原电位(Eh0)，对于粒径为200、700和1000nm的针铁矿Eh0分别为815、802和782mV，验证了优化后电位法的可行性。本文的结论可为矿物非均相体系Eh的测定提供方法学参考，也为预测Feaq 2+-铁氧化物非均相体系参与的污染物衰减速率提供理论支撑。
Abstract:The redox properties of Feaq 2+-iron oxide heterogeneous systems play an important role in the environmental behaviors of heavy metal and organic pollutants. However, the determination of Eh for heterogeneous systems is still challenging due to the redox equilibration between the electrode and suspension is sluggish. In this study, goethite was chosen as the representative of iron oxide in the aquifer. Both mediated and non-mediated potential methods were conducted to obtain the Eh of Feaq 2+-goethite heterogeneous system through the optimizing of measurement conditions. Moreover, the effect of the working electrode surface state, the kind of mediators, and the equilibrium time on the Eh measurement were investigated. The results indicated that the working electrode surface state, the suitable mediators, and the establishment of equilibrium were crucial for obtaining accurate Eh. Compared with the non-mediated potentiometric method, the mediated method can shorten the determination time, but the appropriate mediator must be chosen. In non-mediated potentiometric method, the equilibrium between the working electrode and goethite must be established. The standard redox potential (Eh0) of goethite with different particle sizes was obtained, which demonstrated the practicability of this method. The results provide the method for the Eh measurement of mineral heterogeneous systems, and also provide theoretical support for predicting the pollutant abiotic attenuation rate induced by Feaq 2+-iron oxide heterogeneous systems.
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Key words:
- dissolved bivalent iron /
- iron oxide /
- heterogeneous system /
- redox potential /
- goethite
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图 1 介导电位法中不同介导物质测定Feaq 2+-针铁矿非均相体系的Eh(a)；不同介导物质循环伏安曲线(b)；介导物质CMV(c)和AQS(d)添加时间对Feaq 2+-针铁矿非均相体系Eh测定的影响
Figure 1.
图 3 电极钝化及电极预处理时间对电位法测定Fe2+-针铁矿非均相体系Eh的影响：(a)不加介导物质；(b)加入 CMV; (c)加入 AQS；(d)电极预处理时间对介导电位法测定Fe2+-针铁矿非均相体系Eh的影响(AQS为介导物质)
Figure 3.
表 1 不同pH和Feaq 2+浓度下应用介导电位法与非介导电位法测定Eh的对比
Table 1. Comparison of Eh values obtained by mediated and non-mediated potentiometric method under different pH and Feaq 2+ concentrations
实验条件 非介导法
Eh测定值
(mV)介导法Eh测定值
(mV)Eh理论值
(Eh0=804mV)差异(mV) Ru CMV AQS 非介导法 Ru CMV AQS Feaq 2+ 0.75mmol/L，pH 6 −90 +21 / / −56 −34 77 / / Feaq 2+ 0.38mmol/L，pH 7 −211 / −176 −185 −215 +4 / +39 +30 注：“/”表示未取得实验测定值。 
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