Non-Equilibrium Multi-Ion Biosorption Isotherms for Removal of Heavy Metals from Drinking Water

Biosorption isotherms define the relationship between biosorption capacity of the biosorbent and the equilibrium concentration of the ions in solution, at a constant temperature. Experiments are routinely performed under near-equilibrium because it is impossible to determine the exact time at which equilibrium was attained. A novel attempt to study multi-ion biosorption in non-equilibrium conditions has been made, based on the Probability Isotherm theory. Materials and Methods: Probability Isotherm theory was examined with cucumber and kiwifruit peel beads which are reported to be efficient biosorbents. The peels were incubated in a cocktail of seven ions (As, Cd, Cr, Cu, Hg, Pb and Ni) at the same initial concentration (0.1- 15 mgL-1) and four different temperatures (25-55°C). Non-equilibrium biosorption data were modeled by Langmuir isotherm model. Data were analyzed using a one-way ANOVA coupled with a Bonferroni post-hoc test on GraphPad Prism 8 software. Cd and Ni ions showed the most well-defined trends with Langmuir isotherm model. The binding of ions was physico-chemical with simultaneously occurring physisorption and tchemisorption reactions. Conclusions: Probability Isotherm theory can be applied to multi-ion biosorption in non-equilibrium conditions. The behavior of each ion is unique and no two biosorption systems are alike.