Determining Fe concentration and speciation in waters with high natural organic matter content

Valency, solubility and size of Fe are important controls on quality of natural waters. Recent recognition of the ubiquitous influence of natural organic matter on these, means that wider monitoring of Fe species in waters containing organic matter is required. We assess and develop methods that explicitly recognize this need. We show that for water samples prepared by acidification and filtration for inductively coupled plasma – atomic emission spectroscopy (I.C.P.-A.E.S.) analysis, only 50%–80% of Fe was recovered. This was because acidification coagulated the natural organic matter; Fe could then attach to its surface and/or be entrained as it deposited, both processes prevent Fe entering the I.C.P.-A.E.S. We show that this discrepancy in analysis of Fe – which could also occur and be more significant in analysis of trace metals - is likely to be undetected as the coagulate is unlikely to be observed prior to analysis. Accurate Fe analysis was possible, by I.C.P.-A.E.S. if H₂O₂ and U.V. light were used to remove organic carbon (O.C.), or by use of proprietary Fe chelators that produce colored complexes (e.g. Palintest LR, Palintest, UK). We also show that reliable resolution of Fe(II) concentration into free Fe(II) and Fe(II) bound to O.C. can be achieved by monitoring the time taken for ferrozine color development. This occurs within 2 s for free Fe(II) but up to 3 days for Fe(II) bound to O.C.