Chemical speciation of iron in seawater using catalytic cathodic stripping voltammetry with ligand competition against salicylaldoxime
Chemical speciation of iron in seawater using catalytic cathodic stripping voltammetry with ligand competition against salicylaldoxime
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DOI：
10.1016/j.marchem.2014.06.005
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发表时间：
2014-08-20
期刊：
影响因子：
3
通讯作者：
van den Berg, Constant M. G.
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文献类型：
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作者：
Abualhaija, Mahmoud M.;van den Berg, Constant M. G.
海水中铁的化学形态通常通过阴极溶出伏安法 (CSV) 测定，利用添加的电活性配体之间的配体竞争以获得 CSV 信号和天然配体，以确定天然物种的复合稳定性。不同的程序在选择的添加配体方面有所不同。最近的研究结果表明，其中一些程序受到腐殖质的干扰，目前已知腐殖质在沿海和海洋水域中普遍存在。我们在这里使用海水中的水杨醛肟 (SA) 重新优化铁形态形成的 CSV，发现与现有方法的差异，以及对电活性物质的不同解释。主要发现是，在 -5 倍更少的 SA 下获得最佳灵敏度，负责在电极上吸附的络合物是 FeSA，FeSA(2) 物质不会吸附，并且在溶解氧 (DO) 存在的情况下，通过催化作用（Fen 充当 DO 还原的催化剂），该方法的灵敏度大大提高。在 pH 8 海水中，Fe' 与 SA 的络合物（FeSA 和 FeSA2）的络合物稳定性是在 1 至 40 μM SA（针对 EDTA）和 1 至 100 μM SA（不含 EDTA）的 SA 浓度范围内进行校准。 EDTA 数据的数据拟合得出 log K'(Fe'SA) = 6.50 +/- 0.04 和 log Bi-Fe'SA2 = 10.85 +/- 0.08。数据拟合符合电活性物质 FeSA 的形成，该电活性物质 FeSA 在 [SA] >5 μ M 时被非电活性 FeSA(2) 取代。仅与 Fe-III 的氢氧化物物质竞争形成 FeSA（1 至 100 μM SA 之间，不含 EDTA），对这些稳定常数进行独立校准，得出 log K'(Fe'SA) = 6.52 +/- 0.01 和 log B'(Fe') 值(sA2) = 10.72 +/- 0.03。这些是我们建议的常数值，因为它们独立于 EDTA 形态形成的任何不确定性。这些常数与通过 EDTA 校准确定的常数的相似性表明，Fe 与 SA 和 EDTA 的形态已得到很好的理解。重新优化的方法适用于混合深度凯尔特海样品和来自大西洋的两个 GEOTRACES 样品，SA 浓度为 5 μM。GEOTRACES 水中的配体浓度为 1.47 和 1.49 nM（log K'(Fe/1) 值为 11.1 和 11.9），凯尔特海水中的配体浓度为 2.53 nM（log Ki(FerL) = 11.5）。将该方法应用于添加到海水中的配体，腐殖酸（Suwannee River）的 log Ki(Fe),(L) 值为 11.6 +/- 0.1，铁载体（去铁敏 B）的 log Ki(Fe),(L) 值为 12.2 +/- 0.3。对凯尔特海水中 Fe 与天然配体的络合物解离速率的测量得出 k(FeL) = 0.00133 +/- 0.0002 s(-1) 值。该反应的半衰期为 8.7 分钟。这意味着在分析之前添加 SA 后需要 I h 的反应时间。 (C) 2014 Elsevier B.V. 保留所有权利。
The chemical speciation of iron in seawater is typically determined by cathodic stripping voltammetry (CSV) making use of ligand competition between an electroactive ligand added to obtain the CSV signal and the natural ligand to determine the complex stability of the natural species. Different procedures differ in the added ligand that is selected. Recent findings have suggested that several of these procedures suffer from interference by humic substances, which are now known to be ubiquitous in coastal and ocean waters. We re-optimise here CSV of iron speciation using salicylaldoxime (SA) in seawater, finding differences with the pre-existing method, and a different interpretation for the electroactive species. The main findings are that optimum sensitivity is obtained at -5 x less SA, that the complex responsible for adsorption on the electrode is FeSA, that the FeSA(2) species does not adsorb, and that the sensitivity of the method is much improved in the presence of dissolved oxygen (DO) through a catalytic effect (Fen acts as catalyst for the reduction of DO). The complex stability for complexes of Fe' with SA (FeSA and FeSA2), in pH 8 seawater, is calibrated over a range of SA concentrations between 1 and 40 mu M SA against EDTA and between 1 and 100 mu M SA without EDTA. Data fitting of the EDTA data gave log K'(Fe'SA) = 6.50 +/- 0.04 and log Bi-Fe'SA2 = 10.85 +/- 0.08. The data fits agree with the formation of an electroactive species FeSA which is superseded by a non-electroactive FeSA(2) at [SA] >5 mu M. Independent calibration of these stability constants on the basis of the formation of FeSA in competition only with the hydroxide species of Fe-III, between 1 and 100 mu M SA, without EDTA, gave values of log K'(Fe'SA) = 6.52 +/- 0.01 and log B'(Fe') (sA2) = 10.72 +/- 0.03. These are the values we propose for the constants as they are independent of any uncertainties in the speciation with EDTA. The similarity of these constants to those determined via calibration against EDTA shows that the speciation of Fe with SA and EDTA is well understood. The re-optimised method is applied to a mixed depth Celtic Sea sample, and two GEOTRACES samples from the Atlantic, at a SA concentration of 5 mu M. Ligand concentrations were 1.47 and 1.49 nM in the GEOTRACES water (log K'(Fe/1) values of 11.1 and 11.9) and 2.53 nM in the Celtic Sea water (log Ki(FerL) = 11.5). Application of the method to ligands added to seawater gave log Ki(Fe),(L) values of 11.6 +/- 0.1 for humic acid (Suwannee River) and 12.2 +/- 0.3 for a siderophore (desferrioxamine B). Measurement of the rate of dissociation of the complex of Fe with the natural ligand in Celtic seawater gave a value of k(FeL) = 0.00133 +/- 0.0002 s(-1). The half-life of this reaction is 8.7 minutes. This means that a reaction time of I h is required after the addition of SA prior to analysis. (C) 2014 Elsevier B.V. All rights reserved.