Effect of divalent cations on the kinetics of Fe(III) complexation by organic ligands in natural waters
Effect of divalent cations on the kinetics of Fe(III) complexation by organic ligands in natural waters
复制标题
DOI：
10.1016/j.gca.2007.12.017
复制
发表时间：
2008-03-01
影响因子：
5
通讯作者：
Omura, Tatsuo
中科院分区：
文献类型：
--
作者：
Fujii, Manabu;Rose, Andrew L.;Omura, Tatsuo
本文研究了黄腐酸、柠檬酸和乙二胺四乙酸（EDTA）等有机配体对铁（III）的络合动力学。特别注意了在典型海水浓度下竞争性二价阳离子（Me: Ca2+和Mg2+）对聚合速率的影响。所有实验均在0.5 M NaCl溶液中进行，溶液中有2 mM碳酸氢盐缓冲，pH为8.0，不存在Me （25 μ M-250 mM）。采用竞争配体（5-磺基水杨酸）法结合可见分光光度法测定的络合物形成速率常数范围为3.3 × 10(4) ~ 3.2 × 10(6) M-1 s（-1）。基于动力学模型对络合反应机理进行了研究。当EDTA作为配体使用时，浓度与配体相当的Me明显延缓了铁配合物的形成速度，因为辅助途径占主导地位（铁配体复合物通过铁与Me配体复合物的直接结合形成）。相比之下，只有当Me浓度超过配体的10-1000倍以上时，才会观察到Me对柠檬酸盐和黄腐酸与铁络合的竞争效应。动力学模型表明，黄腐酸与铁的络合主要通过解离途径发生（配体在Me与Me-配体络合物解离后发生铁的络合），其浓度为二价阳离子和自然水体（包括海水和淡水）中典型的天然有机物。2007爱思唯尔有限公司版权所有。
We have investigated the kinetics of Fe(III) complexation by several organic ligands including fulvic acid, citrate and ethylenediaminetetraacetic acid (EDTA). Particular attention was given to examination of the effect of competitive divalent cations (Me: Ca2+ and Mg2+) at concentrations typical of seawater on the cornplexation rate. All experiments were conducted in 0.5 M NaCl solution buffered with 2 mM bicarbonate at pH 8.0 in the absence and presence of Me (25 mu M-250 mM). The rate constants of complex formation determined by using the competitive ligand (5-sulfosalicylic acid) method combined with visible spectrophotometry ranged from 3.3 x 10(4) to 3.2 x 10(6) M-1 s(-1). The mechanism of complexation was then examined based on a kinetic model. When EDTA was used as a ligand, Me at concentrations comparable to the ligand markedly retarded the rate of iron complex formation due to the predominance of an adjunctive pathway (where iron-ligand complex is formed via direct association of iron to Me-ligand complex). In contrast, the competing effect of Me on iron complexation by citrate and fulvic acid was observed only when the Me concentration was in excess of the ligand by more than a factor of 10-1000. The kinetic model suggests that iron complexation by fulvic acid occurs predominantly via a disjunctive pathway (where iron complexation by ligand occurs after dissociation of Me from Me-ligand complex) at concentrations of divalent cations and natural organic matter typical of natural waters including seawater and freshwater. 2007 Elsevier Ltd. All rights reserved.