Equilibrium Biosorption of Zn²⁺ and Ni²⁺ Ions from Monometallic and Bimetallic Solutions by Crab Shell Biomass

This work explored the technical feasibility of using crab shell (CS) as a promising, low-cost biosorbent to individually and simultaneously remove Zn²⁺ and Ni²⁺ from aqueous solutions. It was found that in both monometallic and bimetallic systems, Zn²⁺ and Ni²⁺ biosorption by CS was strongly dependent on the solution pH, with the optimum biosorption occurring at a pH of 6.0 for both heavy metals. The obtained isotherms for Zn²⁺ and Ni²⁺ biosorption onto CS in monometallic and bimetallic systems demonstrated that CS has a higher affinity for Zn²⁺ than for Ni²⁺ . The experimental equilibrium data for the bimetallic system revealed that when one heavy metal is present in the system, there is a decrease in the equilibrium biosorption capacity for the other heavy metal; therefore, the combined action of Zn²⁺ and Ni²⁺ was antagonistic. The Sips and Redlich– Peterson isotherm models best fitted the equilibrium biosorption data for Zn²⁺ and Ni²⁺ in the monometallic systems, while the modified Sips model best fitted the binary biosorption equilibrium data. DRIFTS analyses indicated that carbonate ion, chitin, and proteins are mainly involved in the biosorption of Zn²⁺ and Ni²⁺ by CS from aqueous solutions, as confirmed using a range of analytical techniques.