where qe (mol/g) is the metal ion concentration on the adsorbent at
equilibrium, Xm (mol/g) is the maximum adsorption capacity, β is the
activity coefficient related to the mean free energy of adsorption and ε
is Polanyi potential. The Polanyi potential (ε) can be expressed as,
ε = RTIn 1+1=Ce ð Þ ð4Þ
where R (J/mol) is the universal gas constant, T is the absolute
temperature in Kelvin and Ce (mol/L) is the metal ion concentration in
the solution at equilibrium. A graph was plotted between lnqe versus
ε2 gives a linear relationship with slopes of β and intercepts of Xm
(figure not shown). The activity coefficient β was further used to
calculate the adsorption mean free energy ‘E’ (kJ/mol) which is the
free energy for the transfer of 1 mol of metal ions from the infinity in
the solution to the surface of the adsorbent.