Journal
ENVIRONMENTAL SCIENCE & TECHNOLOGY
Volume 44, Issue 22, Pages 8485-8490Publisher
AMER CHEMICAL SOC
DOI: 10.1021/es101081c
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Funding
- UHI Millennium Institute
- Highland and Islands Enterprise
- Scottish Funding Council
- European Regional Development Fund
- Natural Environment Research Council [NE/C510532/1]
- Royal Society of Edinburgh
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This study investigated the physicochemical forms of dissolved iron in the coastal plume (salinity = 28-35) of a small river draining a peat-rich catchment. Speciation information was obtained through a combination of fractionation by crossflow filtration (CFF) along with voltammetric detection of either naturally occurring iron-humic complexes (July survey) or known, synthetic complexes (September survey) formed by titrating the samples with the competing ligand 2-(2-thiazolylazo)-p-cresol (TAC). The majority of colloidal iron (>5000 Da) was present as iron-humic complexes supplied by the river and showing uniform conditional stability constants throughout the plume (log K'(Fe'HS) 11.3 +/- 0.1, i.e. log KFe3+HS = 21.3 +/- 0.1). Noncolloidal or soluble iron was strongly complexed to ligands of marine origin with log K'(Fe'HS) = 11.9 +/- 0.1. Equilibrium of the total iron pool with the added TAC ligand was achieved in all but the highest salinity sample, albeit more slowly for colloidal than for soluble iron. In addition, measurements of humiclike fluorescence suggested that the conformation of colloids could change over time as a result of dissociation of the iron-humic associations. These results are consistent with the concept that iron in coastal waters is strongly but reversibly bound to humic substances and therefore may be available for complexation by siderophore-type ligands released by microorganisms.
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