The transport of metal ions in hydrogels containing humic acids as active complexation agent

The complexation and transport/immobilization abilities of humic acids are the most important environmental qualities of these substances. Therefore, the study of these properties is important for understanding their role both in natural systems and in human-driven applications. This contribution is focused on the diffusion of metal ions (Co, Cu and Ni) studied by means of a diffusion couple, and their immobilization in humic hydrogels. It was found that the diffusion characteristics of metal ions are strongly affected by their reactivity and their initial concentrations. The concentration dependence of Cu diffusivity exhibited a different trend when compared to Co and Ni, which the result of the high affinity of Cu to humic acids. When a metal ion is first complexed by humic acids it is bound in an exchangeable mode, which can be transferred to a nonexchangeable one. Our results indicate that the dynamic equilibrium established in the diffusion process includes mainly Cu ions in an exchangeable mode as a result of their high affinity and the high amount of bound Cu. Their transformation of bound Cu ions in strong Cu-humic complexes is very slow. In contrast, Co and Ni have much lower affinities to humic acids, thus their diffusivities are less affected by their reaction with humic acids.