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 range from 7 to 9 when these aromatic rings contained adjacent carboxyl and hydroxyl groups, the level of calcium loaded was always greater than with compounds that did not contain this arrangement. Also with these compounds, the presence of calcium increased the loading of the organic onto GAC. When rings contained only carboxyls, only hydroxyls or carboxyl and hydroxyl groups not adjacent to one another, the calcium loading was much lower. For the 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 no organic compound was present in the system. It was noted that there was not a relationship between the ability of a ligand to form aqueous complexes and the effect of that ligand on calcium loading. Specifically phthalate was the strongest complexing ligand evaluated, but it did not cause significant calcium loading onto GAC. Calcium loading was found to increase in relation to the acidity constants of the organic ligands: increasing calcium loading was observed as the value (pKa2–pKa1) increased. These results are consistent with the formation of hydrophobic Ca-ligand assemblages within the GAC.