| Abstract: |
The adsorption of aqueous Pb(II), EDTA, and Pb(II)-EDTA complexes onto TiO2were studied at both stoichiometric and nonstoichiometric Pb(II)/EDTA concentrations. For Pb(II)-TiO2and EDTA-TiO2, a typical cationic and anionic-type of adsorption was noted, respectively. For 10−3and 10−4 M Pb(II)-EDTA systems, near-equal adsorption of Pb(II) and EDTA indicated that the complex adsorbs as a single species. Also, a ligand-type Pb(II)-EDTA adsorption, i.e., decreasing adsorption with an increase in the pH, was noted. Systems with EDTA greater than Pb(II) showed near-zero lead removal; competitive adsorption of EDTA and Pb(II)-EDTA onto TiO2was suggested to cause this effect. For Pb(II) concentrations (5 × 10−4and 10−3 M) higher than EDTA (10−4 M), significantly higher EDTA adsorption at high pH as compared to individual 10−4 M EDTA and 10−4 M Pb(II)-EDTA systems was noted. Adsorption modeling was completed employing the geochemical speciation model MINTEQA2 employing the diffuse layer model. Inner-sphere complexation was considered to occur between Pb(II), EDTA, Pb(II)-EDTA, and the TiO2surface sites. Surface complexes used in the modeling included Ti-O-Pb+, Ti-EDTAH2−, Ti-EDTA-Pb−, and Ti-O-Pb-EDTA3−. The cationic-type complexation, Ti-O-Pb-EDTA3−, was postulated to explain and model the anomalous EDTA adsorption as noted for Pb(II) > EDTA studies. Results from the present study show that the adsorption behavior in aqueous metal/EDTA systems will change with any variation in the contaminant concentration ratios. |
