Evidence for strong but dynamic iron-humic colloidal associations in humic-rich coastal waters.

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.