Influence of dissolved organic matter on iron speciation and scavenging in the Tedori River estuary

Abstract

 Iron and Fe(III)-binding ligand size distributions in dissolved organic matter from the Tedori River (Japan) estuary were measured using size fractionation and electrochemical techniques. Estuary water samples showed natural concentration levels of particulate and dissolved iron. Dissolved iron (‹ 0.45 μm) was removed from the water column at high salinity (› 0.05%) and found to strongly correlate with Fe(III)-binding ligands at low salinity (‹ 0.05%). It is found that salinity and Fe(III)-binding ligands are related to the competitive complexation of iron with major seawater ions and with dissolved organic matter in the estuary. In dissolved iron, the colloidal form (10 kDa-0.45 μm) was prevalent instead of the truly dissolved form (‹ 10 kDa). Colloidal iron concentrations decreased significantly in saline regions. On the other hand, particulate iron (› 0.45 μm) concentrations were nearly constant regardless of salinity changes in the estuary. Colloidal iron likely converts to particulate iron, which is then rapidly removed via the precipitation and subsequent sinking of large particles.