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

Abstract

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 Al 3+–FA and Cu 2+–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 Al 3+) and oxine-derivatised controlled-pore glass (for Cu 2+). 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 (Cu 2+–FA) and slowly labile (Al 3+–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 Al 3+–FA system, the DGT–labile Al 3+ equated to the sum of the labile+moderately labile fractions determined by FIA. For the Cu 2+–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.