Quantitative Analysis of Iron Bound to Humic Substances in Surface Water

Water-soluble iron species in surface water are predominantly composites of Fe(III) and natural organic matter (Fe(III)NOM), such as fulvic acid and humic acid (HA). Fe(III)NOM can promote the growth of microorganisms and plants; therefore, it has significant impact on aquaculture and agriculture businesses. In this paper, a simple method of concurrent pretreatment and colorimetry by using deferoxamine (DFO), L-ascorbic acid, and dye nanoparticle-coated test strip (DNTS) for Fe(II) has been proposed for the on-site and direct detection of Fe(III)NOM in surface water. DFO, well-known as a siderophore, shows sufficient intake ability of Fe(III) (Fe(III)DFO K: 1031) from Fe(III)NOM, but unfortunately it interfered with the color detection of Fe( II) with DNTS loading bathophenanthroline sulfonate. However, the addition of a large excess of L-ascorbic acid and the following pH decrease certainly solved the problem by decomposition of the Fe(III)DFO complex and the following reduction, thereby causing a color change corresponding to the Fe concentration. A quantitative analysis of Fe(III)HA in the range from 15 ppb to 1 ppm has been demonstrated to be surely successful by a comparison with two calibration curves of Fe(II) and Fe(III)- humic acid.