Lability of metal ion-fulvic acid complexes as probed by FIA and DGT: a comparative study

Two kinetic-based analytical techniques with very different time constants (flow injection analysis, FIA and diffusive gradients in thin films, DGT) have been compared in a study of metal ion lability in the systems Al3+-FA and Cu2+-FA (fulvic acid, FA). Flow injection analysis used an in-line micro-column of chelating ion exchanger to capture the labile metal fraction during the short contact time, 1-3 s, with the injected sample. The moderately labile and inert metal fractions were rejected by the ion exchanger but the former gave a colorimetric reaction down-line. The labile fraction was quantified by subsequent elution of captured metal ions and down-line analysis. The chelating resins used were oxine-derivatised fractogel (for Al3+) and oxine-derivatised controlled-pore glass (for Cu2+). Diffusive gradients in thin films utilised cross-linked polyacrylamide films as the diffusive barrier and Chelex-100 to capture the diffusing metal ions that are labile on the experiment time-scale (min). Diffusion coefficients, D, for metal-FA systems at different metal:FA ratios were measured independently in a diffusion cell. They indicated that for both the labile (Cu2+-FA) and slowly labile (Al2+-FA) systems the metal ion diffuses at a similar velocity to the FA, even when the total metal ion concentration exceeds the capacity of FA to bind metal ions in non-labile forms (FA complexation capacity, CC). Complexation capacity was used as a basis for comparison of the two techniques. It was observed that for the less labile Al3+-FA system, the DGT-labile Al3+ equated to the sum of the labile + moderately labile fractions determined by FIA. For the Cu2+-FA system the CC determined by DGT was smaller than that determined by FIA (significant at 1S.D.). Further, the results indicated that D values measured for labile metal-FA complexes in a diffusion cell may not be appropriate for interpretation of diffusion processes that occur in the DGT experiment. (C) 2003 Elsevier B.V. All rights reserved.