At equilibrium, the amount of Cd2+ sorbed by both sorbents was
similar (5.22 and 5.24 mmol/g, respectively for CS and CC), while
the amount of Hg2+ sorbed ranged between 0.31 mmol/g for the CC
and 0.55 mmol/g for the CS. The equilibrium selectivity of the
system (S) is given by the ratio between the distribution
coefficients (K) of both metals at equilibrium, and the distribution
coefficient is defined as qA,e/CA,e. The value of the equilibrium
selectivity (SCd:Hg) found for the CS sorbent is close to the unit (1.7)
while the value found for the CC sorbent was 8.4, indicating that
the crab carapace powder is very selective for cadmium and it is
more selective than the clam shell power.