Irreversible and reversible adsorption of some heavy transition metals on graphitic carbons from dilute aqueous solutions Original Research Article

The adsorption of various heavy transition metal compounds was studied using two different flow adsorption methodologies: flow injection adsorption and flow equilibrium adsorption analyses. In both cases the determinations were made of the heats of adsorption and the amounts of adsorption from dilute aqueous solutions. The carbons used for the work included two different types of graphitised carbon black and active carbons prepared from coconut shells and coal. The work shows that in every case a significant proportion of the solutes was adsorbed irreversibly. At the same time reversible adsorption of the solute also took place. The two graphitised carbons differed in respect of the extent of the irreversible adsorption of Hg(NO3)2 and KPtCl6, which appeared to be related to small differences in the amounts of polar sites on the surface of these carbons. There were also indications of the decomposition of the adsorbates soon after saturation of the surfaces took place. These effects were much more pronounced for the adsorptions on active carbons which contained from 2–5% of their total surfaces in the form of hydrophilic sites. Generally the heats of irreversible adsorptions were very high, even on graphitised carbon blacks, approaching −200 kJ mol−1 at surface coverages below 1% of their BET N2 surface.