Reactivity and transport mapping of Cu(II) ions in humic hydrogels

International audience; Reactivity and transport properties of metal ions are important both for evaluating and understanding role of humic substances in natural systems and human-driven applications as solving their structural questions. This paper focuses on metal ions diffusion and immobilization in humic hydrogels with various contents of reactive functional groups of humic substances in order to study an effect of their selective blocking on the mobility and immobilization of Cu(II) ions. Cu(II) ions were chosen as an example of reactive metal for the diffusion experiments. The environment for the diffusion experiment was prepared from the hydrogel form of humic acids. Humic acids were modified by the methylation with trimethylsilyl-diazomethan. Various ratios between untreated and methylated humic acids were used in order to regulate the content of carboxylic groups in humic hydrogels. The hydrogels were pressed gently in glass tubes and used for diffusion experiments (method of instantaneous planar source). After the diffusions the hydrogel from the tubes was sliced and Cu(II) ions were extracted separately from the slices of hydrogels by the 1 M HCl. The concentration of Cu(II) ions in extractants was determined by means of ultraviolet/visible spectrometry. Previous works showed that transport of metal ions in humic gels is strongly influenced by reactivity of humic acids, therefore changes in the content of carboxylic groups result in changes of diffusivity. The diffusion coefficients of Cu(II) ions, determined in this work, showed the significant dependence on reaction ability of humic hydrogels. Lower amounts of the carboxylic groups caused the decrease of the diffusion rate. The reason could be that the interactions of Cu(II) ions with HA in gel cause their immobilization. The immobilized metal ions are strongly bonded with HA and are not able to diffuse. It results in the increase of the concentration gradient of movable Cu(II) ions, which is the driving force of their transport. The used mathematical description is discussed in terms of the influence of reaction of Cu(II) ions with humic acids and their structural arrangement in hydrogels with various content of reactive functional groups. In general, diffusion experiments seem to provide valuable method for reactivity mapping studies on humic substances.