LIGANDS, METALS, AND METAL-LIGAND COMPLEXES AS DIFFERENTIAL PROBES OFSOIL ADSORPTIVE HETEROGENEITY

The availability and transport of solutes in the subsurface can be str ongly influenced by the scale-dependent adsorptive heterogeneity of th e aquifer formation. Adsorptive heterogeneity appears at both the micr o (''heterogeneity'') and the macro (''variability'') scale. It has be en suggested in the literature that adsorptive heterogeneity should be defined only for specific adsorbate/adsorbent pairs. We have investig ated the hypothesis that the observed adsorptive heterogeneity is not only an inherent property of the adsorbent but depends in part on the nature and the concentration of the adsorbates. Batch experiments yiel ded adsorption pH edges for a ligand (oxalic acid), a metal (Cu), and a metal-ligand complex (Cu-oxalate) adsorbing onto two related soils, Milford coated (MC) and Milford uncoated (MU), containing different am ounts of metal oxide coating. For the concentration range studied, oxa late revealed appreciable variability between MC and MU. Cu revealed a smaller degree of variability between MC and MU while Cu-oxalate indi cated an intermediate degree of variability. The effectiveness of an a dsorbate to act as a probe of soil variability was found also to depen d on the concentration of the adsorbate. Apparent soil variability inc reased with a higher concentration of oxalate but showed no concentrat ion dependence for Cu. For Cu-oxalate, variability depended on the con centration of both oxalate and Cu. At the microscale, oxalate exhibite d appreciable heterogeneity in both soil samples whereas Cu adsorption on the same samples appeared to be quite homogeneous. Cu-oxalate exhi bited appreciable heterogeneity in MC for lower concentration of oxali c acid. For higher concentration of oxalate (1.0 mM) no appreciable he terogeneity was exhibited. Cu-oxalate adsorption on MU was homogeneous for both oxalate concentrations. These results verify that both the m acro and micro scale adsorptive heterogeneity depends not only on the adsorbent but also on the nature and the total concentration of the ad sorbate used to investigate the adsorption process.