Stability of Organic Iron Complexes in Dajiuhu Peats and Its Ecological Significance
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摘要: 为了研究泥炭沼泽源铁有机配合物的络合稳定性，利用pH电位滴定法和荧光淬灭滴定法测定了大九湖泥炭沼泽中不同分子量段的DOM和Fe2+、Fe3+的络合稳定常数.pH电位滴定法结果（4.0~6.1）和荧光淬灭滴定法（1.5~4.1）差异较大，这与高pH条件下OH被脱质子化及Fe2+的氧化有关.相对而言，pH滴定法更适用于探究不同分子量段DOM与铁的络合稳定性，荧光淬灭法不改变样品酸碱条件，更适于研究不同价态铁与DOM的络合稳定性.研究结果表明：DOM与Fe3+的络合稳定常数大于Fe2+，低分子量段（< 3 kDa）的DOM与Fe2+、Fe3+的络合稳定常数更大.泥炭沼泽源铁有机配合物具有较好的络合稳定性，分子量相对较小的DOM与铁的络合能力更强.即便Fe2+氧化为Fe3+，仍能与DOM络合并保持较强的稳定性，这有利于陆源溶解性铁向水生态系统的输出.沼泽源铁有机配合物的络合稳定性还会影响铁的生物可利用性.Abstract: In order to study the complexing stability of organic iron complexes from peat, the stability of DOM-Fe2+, Fe3+ in different molecular weights of Dajiuhu peatland was determined by potentiometric titration and fluorescence quenching titration. There are obvious differences between the results of potentiometric titration (4.0-6.1) and fluorescence quenching titration (1.5-4.1), which can be explained by the deprotonation of OH in high pH value and the oxidation of Fe2+ during the experiments. Potentiometric titration is suitable for studying the complexing stability of different molecular weights of DOM-Fe; the pH would not change during the experiment using fluorescence quenching titration method. Thus, the fluorescence method can be well applied to the element of variable valency. The results show that the complexing stability constants of DOM-Fe3+ is higher than that of Fe2+, and that of low-molecular-weight DOM with Fe2+ and Fe3+ is higher. The experiment indicates that high stability of DOM-Fe in peat, and relatively low-molecular-weight DOM has stronger ability to complex iron. Even Fe2+ is oxidized to Fe3+, it can still complex with DOM stably, which is conducive to the output of terrestrial dissolved iron to aquatic ecosystem. The complexing stability of iron organic complexes from peat will also affect the bioavailability of iron.
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图 1 大九湖DOM样品三维荧光光谱图（3DEEM）
a.原始DOM样品；b.加入Fe(Ⅱ)后的三维荧光光谱图
Fig. 1. Three-dimensional fluorescence spectrogram (3DEEM) of DOM sample from Dajiuhu
图 2 泥炭DOM与Fe2+、Fe3+的荧光淬灭曲线
图中11条曲线是DOM样品中加入11个浓度梯度的Fe的荧光淬灭曲线
Fig. 2. Fluorescence quenching curves of DOM-Fe2+, Fe3+
图 3 样品A不同分子量DOM与Fe2+、Fe3+络合用荧光淬灭法Stern-Volmer方程拟合曲线
Fig. 3. Fitting curves of Stern-Volmer equation of fluorescence quenching method for complexing sample A with different molecular weights DOM-Fe2+, Fe3+
表 1 泥炭DOM的TOC和总酚含量
Table 1. TOC and total phenol content of DOM from peat
样品编号 TOC（mg/L） 总酚（mg/L） A1 30.62 1.12 A2 6.20 1.03 A3 14.02 2.46 B1 8.81 0.58 B2 2.45 0.42 B3 3.91 0.87 C1 2.20 0.48 C2 1.82 0.46 C3 2.93 0.50 
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表 2 泥炭不同分子量段DOM的加质子常数对数值
Table 2. Logarithm of proton addition constant of DOM in different molecular weight segments of peat
样品编号 A1 A2 A3 B1 B2 B3 C1 C2 C3 lgKH 5.42 5.39 4.93 4.95 5.47 5.29 4.02 4.83 4.67
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表 3 pH电位滴定法测得不同分子量段泥炭DOM与Fe2+、Fe3+的络合稳定常数对数值（lgK）
Table 3. Logarithm of complexing stability constants of DOM, Fe2+, Fe3+ in different molecular weight segments of peat measured by potentiometric titration
金属离子 样品编号 A1 A2 A3 B1 B2 B3 C1 C2 C3 Fe2+ 5.27 4.93 4.07 4.34 4.60 4.37 4.01 4.50 4.46 Fe3+ 5.81 5.75 4.83 5.45 6.10 5.70 4.51 5.57 5.32
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表 4 荧光淬灭法测得的DOM与不同金属离子的lgK值
Table 4. lgK of DOM-metal ions measured by fluorescence quenching method
样品编号 Fe2+ Fe3+ A1 1.63 3.92 A2 1.61 3.96 A3 1.53 3.68 B1 1.61 4.35 B2 1.50 4.32 B3 1.46 3.93 C1 1.53 4.20 C2 1.65 4.36 C3 1.59 4.30
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