Comparison of diffusive gradients in thin films and equilibrium dialysis for the determination of Al, FežIII/ and Zn complexed with dissolved organic matter

The distinction between ‘free’ metals and organically complexed metals in aqueous solutions is important for research involving the mobility or bioavailability of metals in the environment. In this study, the applicability of equilibrium dialysis Žmolecular weight cut-off 1000 Da. and diffusive gradients in thin films ŽDGT. to determine ‘free’ Al, FeŽIII. and Zn in four forest soil solutions was compared. The ‘free’ metals as measured by both methods, consist of hydrated metal cations and soluble inorganic metal complexes. In addition, dialysis measures any organic complexes 1000 Da and DGT measures a portion of smaller labile organic complexes. The four soil solutions were prepared by water extraction of an organic soil horizon ŽH. from a Fimic Anthrosol, and contained either 20 or 75 mg C l dissolved organic matter at pH 4.0 and pH 7.0. To test the performance of both methods and optimize experimental volume and time, experiments using metal nitrate solutions were carried out. In the solutions at pH 4.0, no significant differences in average ‘free’ metal contents were found for Al, FeŽIII. or Zn. This makes DGT a viable alternative for equilibrium dialysis for the study of the complexation of all three metals in acidic soils. At pH 7.0, again no significant difference between both methods were found for Al and FeŽIII., but the DGT results for Zn were significantly higher. This was likely caused by labile organic complexes that were only detected by DGT.