Adsorption Characteristics of MetaI-EDTA Complexes onto Hydrous Oxides A. R. BOWERS l Department of Civil and Environmental Engineering, Vanderbilt University, Nashville, Tennessee 37235 AND C. P. HUANG Environmental Engineering Program, Department of Civil Engineering, University of Delaware, Newark, Delaware 19716 Received December 31, 1984; accepted August 12, 1985 The adsorption characteristics of a variety of metal-EDTA complexes onto hydrous oxides, principally aluminum oxide (7-AlzOs),were examined in aqueous solution. Adsorption of these complexes increased with increasing proton concentration due to the formation of surface complexes between EDTA and the surface hydroxo groups, specifically the AIOH~ surface groups. The pH-dependent adsorptive behavior and the magnitude of adsorption of the "free" EDTA specieswere similar to those of the metal complexes. The results also showed that the adsorption of "free" EDTA was exothermic, while the adsorption of Ni(II)-EDTA complexes was endothermic in the lower pH region (~3.5) and exothermic at higher pH values (~6.0). This implied that the surface preferred the NiHEDTA-1 species rather than the NiEDTA -2 species. Specific adsorption of the metal complexes was evidenced by the charge reversal exhibited by the "y-AI203 particles at the highest surface loadings. A quantitative model was formulated based on the pH-dependent speciation of the oxide surface, speciation of the metal complexes in solution, and ~" potential measurements. This model proved valid over a wide range of pH (3-10) and for both high (>50% coverage) and low (<10% coverage) surface loadings. © 1986 Academic Press, Inc. INTRODUCTION The presence of organic or inorganic metal- complex forming agents in solution may se- riously affect the adsorption characteristics of various metals onto hydrous oxides. Typical "free" metal/oxide adsorptive behavior is such that the metals are increasingly adsorbed at higher pH values, with a rather steep "pH- adsorptive edge" occurring at the onset of ad- sorption (1-7). However, this "free" metal adsorptive pattern is often significantly modi- fied when complex forming agents are pres- ent (5-7). In general, the same adsorptive forces which exist for the "free" metal-solid system apply 1To whom correspondence should be addressed. Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986 to the metal complexes as well. However, the magnitude and direction of the coulombic in- teractions may change, since the overall charge on the metal-ligand species may be different from that of the "free" metal species, depend- ing on the charge and the extent of coordi- nation of that ligand. Furthermore, additional bonding possibilities arise due to ligand-sur- face bonds, and in the case of organic com- plexes, an increased tendency to form hydro- gen or hydrophobic bonds exists (8, 9). Leckie and co-workers postulated that a nonsym- metrical metal-ligand system may allow the formation of three surface-adsorbate config- urations as shown in Scheme 1 (5-7). The Type I configuration would result in a "metal- like" adsorptive behavior (adsorption increas- ing as pH increases), while the Type II config- 575 0021-9797/86 $3.00 Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved.