Iron binding humics in coastal waters
 
Iron and humics in coastal waters

The last thing you want is to find a peak for natural species of iron or other metals when you are trying to determine their speciation using ligand competition, because the natural peak interferes with the analytical peak for the metal. On the other hand, the natural peak can be used to detect the natural species directly. The presence of a peak for iron with humic and fulvic acid in seawater has given evidence that humic substances may be an important ligand for iron in the marine system. We are currently carrying out experiments to determine ligand concentrations by the competing ligand technique and to compare these to those obtained from the response of iron-binding humics. Secondly we are looking into competition by other metals for the humics. The origin of the humics in ocean waters is still unknown: are they terrestrial or from marine origin?

Scans for substances in samples from LiverpoolBay and for fulvic and humic acid:


This work is being carried out by Mahmoud Alhaija, in collaboration with Abida Mahmood and Sylvia Sander from Otago University.


As part of this work Mahmoud Alhaija has updated the CSV method for Fe speciation in the presence of salicylaldoxime, discovering that the sensitivity is catalytically enhanced in the presence of dissolved oxygen. The complex stability for Fe and SA was calibrated on basis of competition against EDTA and directly from the variation in the response as a function of the concentration of SA. Good agreement between the two  methods indicates that the peak height can be used to calibrate the complex stability, also demonstrating that the FeSA species is not stabilised by the adsorption on the electrode.


Calibration of alpha-FeSA at various concentrations of EDTA:


Relevant references

Laglera, L. M. and C. M. G. van den Berg (2009). "Evidence for geochemical control of iron by humic substances in seawater." Limnology and Oceanography 54(2): 610-619.

Yang, R. and C. M. Van den Berg (2009). "Metal complexation by humic substances in seawater." Environ Sci Technol 43(19): 7192-7197.

Abualhaija, M. and C. M. G. Van Den Berg (2013). "Iron speciation in seawater by catalytic stripping voltammetry using salicyl aldoxime." Marine Chemistry submitted.