Structural aspects of polysaccharide hydrogels based on sodium alginate and divalent cations Ba2+, Ca2+, Sr2+, Cu2+, Zn2+, Ni2+ and Mn2+ was studied using data on hydrogel elemental composition and combinatorial analysis of the primary structure of alginate chains. It was shown that the elemental composition of hydrogels in the form of freezing dried microspheres gives information on the structure of junction zones in the polysaccharide hydrogel network, the degree of filling of egg-box cells by cations, the type and magnitude of the interaction of cations with alginate chains, the most preferred types of alginate egg-box cells for cation binding and the nature of alginate dimers binding in junction zones. It was ascertained that metal–alginate complexes have more complicated organization than was previously desired. It was revealed that in metal–alginate hydrogels, the number of cations of various metals per C12 block may be less than the limiting theoretical value equal to 1 for completely filled cells. In the case of alkaline earth metals and zinc, this number is equal to 0.3 for calcium, 0.6 for barium and zinc and 0.65–0.7 for strontium. We have determined that in the presence of transition metals copper, nickel and manganese, a structure similar to an egg-box is formed with completely filled cells. It was determined that in nickel–alginate and copper–alginate microspheres, the cross-linking of alginate chains and formation of ordered egg-box structures with completely filled cells are carried out by hydrated metal complexes with complicated composition. It was found that an additional characteristic of complex formation with manganese cations is the partial destruction of alginate chains. It has been established that the existence of unequal binding sites of metal ions with alginate chains can lead to the appearance of ordered secondary structures due to the physical sorption of metal ions and their compounds from the environment. It was shown that hydrogels based on calcium alginate are most promising for absorbent engineering in environmental and other modern technologies.

The elemental composition and structural features of the junction zones of a strontium–alginate hydrogel and their alteration under the intercalation of multi-walled carbon nanotubes into the hydrogel structure were studied. It was shown that the crosslinking with Sr2+ cations due to electrostatic interactions leads to the association of polymer chains into junction zones with incompletely filled cells. It was found that in strontium alginate, the average cell occupation number of Sr2+ cations is less than 1 and approximately equal to 0.64. In nanocomposite hydrogels including multi-walled carbon nanotubes, its increase to 0.81 indicates the appearance of a more ordered structure of alginate chains in junction zones. The information about the most preferred types of egg-box cells for binding with Sr2+ cations was analyzed. The existence of Sr2+ cations in nonequivalent positions was established. The possibility of separating the contributions of chemical adsorption due to ionic bonds with alginate chains and physical adsorption due to the appearance of local energy minima near alginate chains, leading to the appearance of ordered secondary structures, was demonstrated. It has been shown that the addition of carbon nanotubes to a hydrogel changes their sorption capability, leading, first of all, to an increase in the possible sites of physical adsorption.