Effect of humic substances on the iron speciation in natural waters by CLE/CSV

Abstract

Humic substances (HS), the main hydrophobic component of dissolved organic matter, are present in all natural waters and are known to interact strongly with trace metals by complexation and co-precipitation. Traditionally, the role assigned to HS in iron cycling was as scavengers via flocculation in brackish waters. Their iron binding properties have recently been studied by competing ligand equilibration with detection by cathodic stripping voltammetry (CLE/CSV), the standard method to obtain complexing capacities and conditional stability constants. According to the stability and solubility of Fe–HS complexes obtained in seawater, HS could have a crucial role in iron cycling in deep oceanic and coastal seawaters. An attempt to study HS iron complexing characteristics in seawater with a variety of different artificial iron ligands (AL) revealed unexpected complications which have implications for previous complexation studies of fresh, brackish and coastal waters. For some AL, the sensitivity can be enhanced catalytically via addition of an oxidant, usually bromate, which was found to cause interference in the form of an overlapping peak due to Fe–HS complexes. Our data shows that iron complexation with HS is not detected by CSV in the presence of either NN (1-nitroso-2-naphthol), due to out-competition at the NN concentration required by the analytical method, or of TAC (2-(2-thiazolylazo)-4-methylphenol) probably due to interactions of the HS binding groups with TAC. The ligands SA (salicylaldoxime) and DHN (dihydroxynaphthalene), without bromate, were found to be suitable for the measurement of iron complexation in the presence of HS, giving similar results. DHN offers a better signal/noise ratio at lower ligand concentrations and might for this reason be preferable for the study of the iron complexing properties of HS in seawater. However, to minimise interference due to a slow, gradual, oxidisation of the DHN, it is necessary to carry out the equilibration of the titration overnight under refrigeration in the dark, or to minimise the length of the equilibration period.