Chemical Stability of Large Organic Molecule-Metal Complexes Dissolved in Natural Water as Studied by Size Exclusion Chromatography/Inductively Coupled Plasma Mass Spectrometry

Abstract

Abstract The chemical stability of large organic molecule-metal complexes (LOMMCs) dissolved in pond water were investigated by a hyphenated system of size exclusion chromatography/UV absorption detection/inductively coupled plasma mass spectrometry (SEC/UV/ICP-MS) after ultrafiltration using a filter with a molecular permeation limit larger than 10000 Da. LOMMCs dissolved in the preconcentrated pond water sample were observed at the retention times corresponding to molecular weights larger than 300000 Da (Peak I) and that of 10000—50000 Da (Peak II) in the chromatograms measured by UV absorption and ICP-MS. When the pH of the sample solution was varied from 7.0 to 0.5 by adding the HNO3 solution, the peak intensities of both Peak I and Peak II in these chromatograms decreased with the decrease in pH of the sample solution. These results indicate that LOMMCs were decomposed by addition of HNO3. Peak I was rapidly decomposed below pH 4, while Peak II was gradually decomposed over the pH range from 7.0 to 0.5. The decomposition curve of Peak I was similar to the solubility characteristics of hydroxides of Fe and Al. On the other hand, that of Peak II suggested the dissociations of metal complexes (Cu and Zn) with some biogenic organic molecules. It was concluded from these results that some colloidal particles of hydroxides of Fe and Al may be core materials of LOMMCs for Peak I, while some metal complexes with biogenic protein-like compounds may be LOMMCs for Peak II. The possible models for LOMMCs in pond water were proposed as ``a string-ball model'', which might interpret as the removal and preservation mechanisms for trace elements in natural water.