Adsorption of organic macromolecules by granular activated carbon .1. Influence of molecular properties under anoxic solution conditions

The adsorption of nine different natural and synthetic macromolecular dissolved organic materials (DOMs) by granular activated carbon (GAG) was investigated under anoxic conditions to elucidate the relative influence of different molecular properties. In particular, attempts were made to better understand the respective roles of DOM molecular size and chemical structure. Anoxic conditions were chosen because of the additional complications caused by the presence of dissolved oxygen; the latter issue is addressed in a companion paper. For a chemically homogeneous model DOM, polystyrene sulfonate (PSS), adsorption was observed to decrease with increasing macromolecular size. Similar results were obtained for three natural humic materials (Laurentian, peat, and Aldrich humic acids) that were fractionated into different size regimes by ultrafiltration. The similarity between the results for PSS and among the various size fractions of each of these three humic acids suggests that the chemistry of humic macromolecules, particularly that which affects adsorption by GAG, is relatively uniform from one fraction to another, and that differences observed in GAC uptake of different fractions can be attributed principally to physical size effects. Examination of the adsorption of additional DOMs isolated from different sources to GAG, however, reveals the importance of chemical structure, particularly the presence of acidic functional groups. The adsorption affinity of GAC for different DOMs was found to correlate directly with the number of acidic functional groups normalized by macromolecule size, up to a limit beyond which further increases in acidity enhanced the solubility of the organic macromolecules too much to allow additional adsorption to occur.