ADSORPTION OF NITRILOTRIACETATE (NTA), CO AND CONTA BY GIBBSITE

Adsorption of Co2+, nitrilotriacetic acid (NTA) and equal-molar Co2+ a nd NTA by a low surface area (LSA) commercial gibbsite (3.5 m(2) g(-1) ) was investigated in batch as a function of pH (4.5 to 10.5), adsorba te (0.5 to 10 mu M) and adsorbent (0.5 to 75 g L-1) concentrations and ionic strength (0.01 to 1 M NaClO4). The adsorption of Co2+ (Co-only) and the acid form of NTA (NTA-only) by gibbsite in 0.01 M NaClO4 exhi bit cation-like and anion-like adsorption edges, respectively. For the equal-molar CoNTA chelate, Co and NTA adsorption edges were similar b ut not identical to the Co-only and NTA-only edges. Differences sugges t the existence of a ternary CoNTA surface complex with the Co in the intact chelate coordinated to surface hydroxyls. NTA-only adsorption w as insensitive to ionic strength variation, indicating weak electrosta tic contributions to surface coordination reactions. This is consisten t with the formation of inner-sphere surface NTA complexes and ligand exchange reactions in which monodentate, bidentate and binuclear NTA s urface complexes form. Cobalt adsorption increases (edge shifts to low er pH by 1 pH unit) on LSA gibbsite as ionic strength increases from 0 .01 to 1 M NaClO4. For the same ionic strength change, a similar shift in the Co-only edge was observed for another commercial gibbsite (16. 8 m(2) g(-1)); however, no change was observed for delta-Al2O3. Ionic strength shifts in Co2+ adsorption by gibbsite were described as an ou ter-sphere CoOH+ surface complex using the triple-layer model. Results suggest that, at waste disposal sites where Co-60 and NTA have been c o-disposed, NTA will not promote ligand-like adsorption of Co for acid conditions, but will reduce cation-like adsorption for basic conditio ns. Thus, where gibbsite is the dominant mineral sorbent, NTA will not alter Co-60 mobility in acidic pore waters and groundwaters; however, NTA could enhance Co-60 mobility where alkaline conditions prevail, u nless microbial degradation of the NTA occurs.