Adsorption processes occupy a special place in a variety of technologies for purifying waters of biologically harmful non-degradable components [1]. In the Republic of Moldova there are significant deposits of mineral adsorbents, such as diatomite, tripoli and clay minerals [2], while fruit processing enterprises can provide sufficient raw materials (as fruit stones, nut shells, grape seeds) for obtaining active carbons [3].
As a support for adsorbents adsorption, there were selected the local Vascauti and Ghidirim diatomites as well as activated carbons CAS-23 and CAN-8, prepared from peach pits and nut shells in the Institute of Chemistry. As the objects of this research were selected synthetic colorants - red congo and direct blue, e.g. compounds with the same structural skeleton, but different functional groups, that exhibit a different chemical behavior.
It has been established that the adsorption of direct blue dye on the Ghidirim diatomite is about 2-3 times lower than that of the congo red dye. This phenomenon is explained by a larger number of -SO3Na groups in the direct blue dye structure, which causes a higher total negative charge resulting from its dissociation, leading to their rejection by the negative electric adsorbent particles and minimizing this way the adsorption value. The dye adsorption on mineral adsorbents is, therefore, a complex phenomenon, comprising the immobilization in the pores and electrostatic interactions. In the case of carbon adsorbents the important factor for immobilization of dye molecules is the mesoporous adsorbent volume value, the pollutant adsorption being almost directly proportional to the mentioned structure parameter. The analysis of obtained data from adsorption isotherms of congo red and direct blue dyes on the same type of active carbon shows a pretty large difference in the dye adsorption values, in spite of the fact that both dyes have the same structural skeleton. Perhaps, in this case the steric factor occurs, e.g. the size of molecules and their varying degree of hydration, due to the number and nature of different functional groups (SO3H, OH, NH2) of colorants that capture the H2O molecules. It is also possible the influence of electric charges, arising from dissociation of both functional groups of colorants and those on the carbon surface (OH, COOH), similar to the case of mineral sorbents.
The obtained experimental data allow us to conclude that: a) the local adsorbents, both the mineral and especially the carbon ones, exhibit very good adsorption properties compared to the synthetic dyes and can be used for purification of waste waters of textile enterprises, b) studied adsorption of dyes depends largely on the origin of adsorbents, their structural features and the nature of functional groups.