ADSORPTION AND DESORPTION OF DIFFERENT ORGANIC-MATTER FRACTIONS ON IRON-OXIDE

Natural organic matter (NOM) is a complex mixture of different organic components (or fractions), yet few studies have examined the fraction al adsorption of NOM on mineral surfaces. In this study, we fractionat ed NOM into hydrophobic (HbA) and hydrophilic (HL) subcomponents and t wo size fractions (with nominal molecular weights cut off at 3000 (3 K ) dalton in an attempt to elucidate the adsorption and desorption mech anisms of NOM on iron oxide surfaces. Results indicated that, on a C w eight basis, larger size HbA fraction was preferentially adsorbed (wit h a higher adsorption affinity and capacity) over smaller size HL frac tion. However, on an O weight basis, less HbA fraction was adsorbed re lative to the HL fraction, because HbA contained about 1.34 times more C but 0.82 times less O than the HL. These observations are consisten t with results which indicate that only limited adsorption sites are a vailable on the iron oxide surfaces and that the mechanism of HbA and HL adsorption was dominated by surface complexation-ligand exchange. F TIR and NMR spectroscopy and studies with several substituted benzoic acids/phenols further indicated that carboxyl and hydroxyl functional groups of these NOM fractions were actively involved in the reactions, and the steric arrangement of these functional groups may have played an important role in determining the adsorption of NOM fractions. Des orption studies indicated that the adsorbed NOM macromolecules on iron oxide surfaces were strongly bound at a given pH and ionic compositio n, resulting in a strong adsorption-desorption hysteresis. One possibl e explanation for the observed hysteresis is that the solution composi tion and equilibria are not identical between adsorption and desorptio n phases of the experiment because of preferential or selective adsorp tion of certain NOM fractions. This study implies that, due to the pol ydispersity of NOM, the competitive and fractional adsorption-desorpti on of NOM subcomponents must be considered in order to better predict NOM partitioning between the solution and solid phases, and, therefore , the transport behavior of NOM in the subsurface soil environment.