Mathematical modeling of adsorption isotherms of water vapour on polymeric ionites used in agrochemical practice

Polymeric ion-exchange materials (ionites) are used in agriculture to make substrates for complete and long-term plant nutrition. The main nutrients in bound form are contained on the ion-exchange groups of the ionite. The degree of dissociation of these groups, and thus the release of ions into the soil, depends on the moisture content. In case of heavy rainfall and during drought, the exchange properties of the ionite will be different. Therefore, the study of water adsorption in such ionites is a rather urgent problem. In this work, the water vapour adsorption by ionites is studied using a thermodynamic model that incorporates the interplay between changes in the energy of dissociation of ion-exchange groups and the osmotic effect. Within the framework of this approach adsorption isotherms are built in the range of air humidity from 0,1 to 1 at physically reasonable values of system parameters. Isotherms have a characteristic inflection point in the area of medium humidity, which allows them to be attributed to isotherms of the fifth type. The obtained model isotherms describe adsorption in mesoporous and microporous materials, which are ionites, and fully correspond to experimental adsorption isotherms of this type.