A new statistical physics model for the ternary adsorption of Cu2+, Cd2+ and Zn2+ ions on bone char: Experimental investigation and simulations

A new statistical physics model is developed to elucidate the ternary adsorption of some heavy metal ions, i.e. copper (Cu2+), cadmium (Cd2+) and zinc (Zn2+) on bone char. The model is derived by the partition function describing the statistical properties in thermodynamic equilibrium of the investigated adsorption system. Ternary equilibrium adsorption isotherms of these heavy metals on bone char are determined and analyzed at different temperatures (30, 40 and 50 °C). The study of the adsorption capacity shows that the bone char is more effective to remove Cu2+ ions, in both single-compound and ternary systems, while Cd2+ and Zn2+ are adsorbed at lower but similar extent. Moreover, in ternary system, all the adsorption capacities are reduced and Cd2+ and Zn2+ experienced the highest competition effects, as their adsorption capacity significantly decreased with respect to single-compound counterparts. On the other hand, the ternary statistical physics model includes several physicochemical parameters that can attribute microscopic insights to the investigated system. The application of this model indicates that all heavy metal ions are mainly bonded by a horizontal position on bone char. By analyzing and comparing the evolution of the number of ions bonded per adsorbent receptor site (RS), an inhibition effect is observed. The new model is also adopted to characterize the energy of the system, in terms of the interactions between heavy metal ions and adsorbent surface. In particular, both single-compound and ternary adsorption on bone char are endothermic and mainly characterized by physisorption, with a tendency to chemisorption, as confirmed by model outcomes.