Competitive Adsorption of Cu(II)–EDTA and Cd(II)–EDTA onto TiO2

Cu(II), EDTA, Cu(II)–EDTA, Cd(II)–EDTA, and Cu(II)/Cd(II) and Cu(II)–EDTA/Cd(II)–EDTA competitive adsorption onto TiO2 has been studied with variation of pH and concentration. For Cu(II) and EDTA, typical cationic and anionic types of adsorption are noted, respectively. Ligand-type adsorption is found for Cu(II)–EDTA and Cd(II)–EDTA under both single and competitive conditions. Surface complexation modeling considered inner-sphere complexation and the diffuse layer model employing MINTEQA2; surface complexes used include Ti–(OH2)O–Cu+, Ti–(OH)EDTAH−22, Ti–(OH)EDTA–Cu−2, and Ti–(OH)EDTA–Cd−2. Experimental and model predictions suggest no competitive adsorption between Cu(II) and Cd(II) at 5 × 10−5 M. On the other hand, adsorption data and model predictions indicate that Cd(II)–EDTA adsorption is favored over that of Cu(II)–EDTA with some competition for adsorption sites. Cd(II)–EDTA adsorption was only slightly affected by the presence of Cu(II)–EDTA; however, Cu(II)–EDTA adsorption was strongly influenced by the presence of Cd(II)–EDTA, especially as the molar ratio of Cd(II)–EDTA/Cu(II)–EDTA increased. A modified surface complexation constant for Cd(II)–EDTA is required to explain the competitive data, suggesting surface site heterogeneity.