Calcium and TOC loading: effect of hydroxyl and carboxyl substituents

Calcium loading onto granular activated carbon (GAC) was evaluated in batch reactor tests for a series of single ring aromatic compounds. In the pH ra nge from 7 to 9 when these aromatic rings contained adjacent carboxyl and h ydroxyl groups, the level of calcium loaded was always greater than with co mpounds that did not contain this arrangement. Also with these compounds, t he presence of calcium increased the loading of the organic onto GAC. When rings contained only carboxyls, only hydroxyls or carboxyl and hydroxyl gro ups not adjacent to one another, the calcium loading was much lower. For th e compounds evaluated with only carboxyl groups (benzoic acid and phthalic acid), calcium loading onto GAC at pH greater than 8 was the same as when n o organic compound was present in the system. It was noted that there was n ot a relationship between the ability of a ligand to form aqueous complexes and the effect of that ligand on calcium loading. Specifically phthalate w as the strongest complexing ligand evaluated, but it did not cause signific ant calcium loading onto GAC. Calcium loading was found to increase in rela tion to the acidity constants of the organic ligands: increasing calcium lo ading was observed as the value (pKa(2)-pKa(1)) increased. These results ar e consistent with the formation of hydrophobic Ca-ligand assemblages within the GAC. (C) 2001 Elsevier Science B.V. All rights reserved.