Abstract
The adsorption kinetics and mechanism of two dissolved Cu (II) species, i.e. hydrated Cu (II) (HyCu (II) ) and its humic acid complexed (HA-Cu (II) ), on silica gel (SG) have been studied. The adsorption medium containing both Hy-Cu (II) and HA-Cu (II) was first prepared by introducing humic acid (HA) previously immobilized on SG into aqueous solution containing Hy-Cu (II) . The acidity of the solution was adjusted to pH 4.0 in order the Hy-Cu (II) to be proportionally distributed to HA-Cu (II) . The adsorption of the Cu (II) species on SG showed that Hy-Cu (II) was adsorbed much faster than HA-Cu (II), with adsorption rate constant (kad) of 0.48 min-1 for Hy-Cu (II) and 0.05 min-1 for HACu (II) . Since HA-Cu (II) complex should be anionic in nature, strong adsorption of the species on SG could not be expected. The adsorption energy (E) of 24.82 kJ mol-1 for HA-Cu (II) complex which is significantly higher than that of 12.56 kJ mol-1 for Hy-Cu (II) suggests that HA-Cu (II) complex is chemically adsorbed through ligand exchange after the rapid electrostatic interaction through outersphere complexation of Hy-Cu (II) on SG. This evidence indicates that the adsorption of the Cu (II) species proceeds sequentially, where the Hy-Cu (II) is adsorbed first and it then acts as a bridge for the adsorption of HA-Cu (II) .
