Comparing CLE-AdCSV applications using SA and TAC to determine the Fe binding characteristics of model ligands in seawater

Competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV) is used to determine the conditional concentration ([L]) and the conditional binding strength (logKcond) of dissolved organic Fe-binding ligands, which together influence the solubility of Fe in seawater. Electrochemical applications of Fe speciation measurements vary predominantly in the choice of the added competing ligand. Although different applications show the same trends, [L] and logKcond differ between the applications. In this study, binding of two added ligands in three different common applications to three known types of natural binding ligands are compared. The applications are: 1) Salicylaldoxime (SA) at 25 micromol (SA25) and short waiting time, 2) SA at 5 micromol (SA5) and 3)2-(2-thiazolylazo)-rho-cresol (TAC) at 10 micromol, the latter two with overnight equilibration. The three applications were calibrated under the same conditions, although having different pH values, resulting in the detection window centers (D) DTAC > DSA25 >/= SA5 (as log D values with respect to Fe3+: 12.3>11.2>/= 11). For the model ligands, there is no common trend in the results of logKcond. The values have a considerable spread, which indicates that the error in logKcond is large. The ligand concentrations of the non humic model ligands are underestimated by TAC and overestimated by SA25 which we attribute to the lack of equilibrium between Fe-SA species in the SA25 application. The application TAC more often underestimated the ligand concentrations and Application the application SA5 was best in estimating over and under estimated the ligand concentration correctly. The trends between these model ligand concentrations were similar for all three applications. The estimated ligand concentrations for the humic and fulvic acids differed approximately by a factor 22 fold between TAC and SA5 and another factor of 2 between SA5 and SA25. The use of SA above 5 micromol suffers from the formation of the species Fe(SA)x (x>1) that is not electro-active as already suggested by Abualhaija and Van den Berg (2014). Moreover, we found that the reaction between the electro-active and non-electro-active species is probably irreversible. This undermines the assumption of the CLE principle, causes overestimation of [L] and could result in a false distinction into more than one ligand group. For future electrochemical work it is recommended to take the above limitations of the applications into account. Overall, the uncertainties arising from the CLE-AdCSV approach mean we need to search for new ways to determine the organic complexation of Fe in seawater.

竞争性配体交换-吸附阴极溶出伏安法（Competitive ligand exchange-adsorptive cathodic stripping voltammetry, CLE-AdCSV）可用于测定溶解态有机铁结合配体的条件浓度([L])与条件结合强度(logKcond)，二者共同影响铁在海水中的溶解度。铁形态测定的各类电化学应用，其核心差异主要在于所选用的外加竞争配体。尽管不同应用体系呈现出相似的变化趋势，但各方法测得的[L]与logKcond存在显著差异。本研究对比了三种常见应用体系中两种外加配体与三种已知天然结合配体的结合行为。三种应用体系分别为：1) 25 μmol浓度水杨醛肟(Salicylaldoxime, SA，SA25)且采用短时间孵育；2) 5 μmol浓度SA(SA5)；3) 10 μmol浓度2-(2-噻唑偶氮)-间甲酚(2-(2-thiazolylazo)-rho-cresol, TAC)，后两种体系均采用过夜平衡。尽管三种体系的pH值存在差异，但均在相同条件下完成校准，其检测窗口中心(D)满足DTAC > DSA25 ≥ DSA5（以相对于Fe³+的logD值计：12.3 > 11.2 ≥ 11）。对于模型配体，logKcond的测定结果未呈现统一趋势，数值离散程度较大，表明logKcond的误差较高。TAC会低估非腐殖质型模型配体的浓度，而SA25则会高估其浓度，我们将此归因于SA25体系中Fe-SA物种未达到充分平衡。TAC体系更易出现配体浓度低估的问题，而SA5体系在准确估算配体浓度（即同时避免高估与低估）方面表现最佳。三种应用体系的配体浓度变化趋势整体相似。腐殖酸与富里酸的估算配体浓度在TAC与SA5体系间相差约22倍，SA5与SA25体系间亦存在2倍左右的差异。当SA浓度高于5 μmol时，会生成Fe(SA)ₓ(x>1)物种，此类物种不具备电活性，这与Abualhaija与Van den Berg(2014)的研究结论一致。此外，我们发现电活性与非电活性物种间的反应大概率不可逆，这破坏了CLE方法的基本假设，导致[L]被高估，还可能造成将配体错误划分为多于一类的假象。对于后续的电化学研究，建议充分考虑上述应用体系的局限性。总体而言，CLE-AdCSV方法存在的不确定性意味着我们需要探索测定海水中铁有机络合作用的新途径。