NICKEL ADSORPTION TO HYDROUS FERRIC-OXIDE IN THE PRESENCE OF EDTA - EFFECTS OF COMPONENT ADDITION SEQUENCE

Nickel, EDTA, and hydrous ferric oxide were combined in different sequences to study the effect on equilibration. In this system, the fraction of nickel adsorbed to the hydrous ferric oxide depended on the component addition sequence, but the fraction of EDTA adsorbed did not. In one sequence, nickel and EDTA were combined to preform a NiEDTA2- complex. The complex had adsorption characteristics similar to EDTA with high adsorption at pH <7 and suppressed adsorption at pH > 7. Equilibrium adsorption of the complex could be modeled using the diffuse layer model. When nickel or EDTA was individually equilibrated with HFO prior to the addition of EDTA or nickel, respectively, the system did not attain equilibrium. Rapid EDTA adsorption at pH <7 prevented the formation of the ternary Ni-EDTA surface complex. At pH >7, nickel adsorption was faster than the formation of the NiEDTA2- complex, but once adsorbed, nickel was slowly desorbed through the formation of solution NiEDTA2-.