The treatment of contaminated water is currently a major concern worldwide. This work was directed towards the preparation of a composite hydrogel by entrapping cherry stones powder on chitosan, which is known as one of the most abundant natural polymers. The synthesized material was characterized by scanning electron microscopy, by Fourier transform infrared spectroscopy, and by the point of zero charge determination. Its ability to remove two azo dyes models (Acid Red 66 and Reactive Black 5) existing in single form and in binary mixture was evaluated. Response Surface Methodology–Central Composite Design was used to optimize three parameters affecting the process while targeting the lowest final contaminant concentrations. The best results were obtained at pH 2, an adsorbent dose of 100 g/L, and a temperature of 30 °C, when more than 90% of the pollutants from the single component systems and more than 70% of those of the binary mixtures were removed from their aqueous solutions. The adsorption process was in accordance with Elovich and pseudo-second-order kinetic models, and closely followed the Freundlich and Temkin equilibrium isotherms. The obtained results led to the conclusion that the prepared hydrogel composite possesses the ability to successfully retain the target molecules and that it can be considered as a viable adsorbent material.

In the present study, using a combination of several analytical techniques such as conductometry/total dissolved solids (TDS), pH-metry, ICP-MS and UV-Vis spectrometry, 38 parameters (Na, K, Mg, Ca, Al, Fe, B, Li, Cr, Mn, Ni, Cu, Zn, Se, Sr, Ag, Cd, Ba, Pb, Be, V, Co, Ga, As, Rb, Cs, Hg, Tl, U, ${\mathrm{F}}^{-}$, ${\mathrm{Cl}}^{-}$, ${\mathrm{SO}}_4{}^{2-}$, ${\mathrm{NO}}_3{}^{-}$, ${\mathrm{NO}}_2{}^{-}$, ${\mathrm{HCO}}_3{}^{-}$, ${\mathrm{CO}}_2$, electric conductivity (EC)/TDS and pH) for seven natural mineral waters (springs 1 bis, 5, 10, 14, 15, Sonda 2 and Sfantul Spiridon) from the Slanic Moldova area (Romania) were evaluated. Our data were compared with the historical chemical analyses records and also with the limits established by international and Romanian regulations for qualitative evaluation of natural mineral waters. In the case of the Evolution of the mineralization degree over time, it was observed that, in the interval 1933–2021, for all the studied sources, there were variations of mineralization that could be attributed to climatic and geological changes, mode of exploitation, as well as to analysis techniques used. Although decreases in mineralization were observed between 1981 and 2006 for water sources 1 bis, 10, 15 and S2, with a slight recovery and stability period between 2006 and 2021, they have retained their characteristics over time. Moreover, spring 14 retains its status as the most mineralized spring of the seven, although it also recorded a decrease in the mineralization degree between 2013 and 2018. Even if the concentration of major and minor ions showed some variation, highlighting the diversity of the water intakes and its changes over time for some of the springs, it is noticeable that these springs have kept their characteristics over time. It was identified that Sodium (${\mathrm{Na}}^{+}$) was present in all natural mineral waters but predominated in sodium chloride, sodium bicarbonate and sodium sulfate. The concentration of potassium ion has shown a fairly significant fluctuation, in 2006 being registered the lowest values for most sources: 1 bis (88.00 mg/L), 5 (6.00 mg/L), 10 (81.00 mg/L), 14 (115.00 mg/L), 15 (45.20 mg/L), S2 (11.00 mg/L). By means of ICP-MS, trace elements that have never before been tested or reported were identified, thus completing the chemical fingerprint of these natural mineral waters to increase their value for routine or therapeutic uses or further sustainable exploitation.