ADSORBIMENTO DI IONI METALLICI IN SOLUZIONE ACQUOSA: APPLICAZIONI AL CASO DI CROMO, ARSENICO E MERCURIO

The subject of this Ph.D. thesis is the study of the thermodynamic aspects on the adsorption of metallic ions in aqueous solutions by the use of granular activated carbons. In particular, this work is focused on the cases of arsenic, chromium and mercury which are universally considered among the most dangerous water pollutants. The research activity is consisted of the realization of adsorption isotherms and is focused on the analysis of the effects of the main process variables, which are: metal concentration, pH, salinity and temperature. In this sense, experimental activities have been planned in order to have an appreciable number of experimental data covering a significative range of all the investigated parameters. Experimental results have shown that the activated carbon has an appreciable adsorption capacity, which is strongly dependent on the value of the process parameters. In particular, the adsorption of arsenic and chromium have presented a similar qualitative dependence on the pH and the salinity. In both cases a maximum value of adsorption capacity in almost neutral pH solutions and in presence of small quantity of sodium chloride can be observed. The effect of temperature on adsorption capacity has been much more evident for arsenic ions meaning that a higher temperature, results a higher adsorption capacity. The adsorption of mercury ions has shown different behaviours, mainly related to the differences in adsorption mechanisms of cationic and anionic species by the activated carbon respect to. In this case, a maximum adsorption capacity in conjunction with acid pH and a negligible dependence on the solution salinity can be shown. Experimental results have also shown that the adsorption capacity is strongly dependent on temperature. In particular, the adsorption capacity decreases from 10 to 25°C and then arises from 25 to 55°C. The experimental evidence have also highlighted a crucial role of chloride concentration on the mercury adsorption phenomena. A descriptive model of metallic ions adsorption has been developed in order to give a physical interpretation of the experimental results. The methodological approach on the basis of this model has been the analysis of the chemical equilibrium in solution in correspondence to each experimental point the correlation of the ionic species concentration in solution to the value of adsorption capacity with the use of an additive/competitive Langmuir model. The model has been applied on all three metallic ions studied, giving a good description of the dependence among the adsorption capacity and the value of the investigated parameters