The role of natural organic matter as a coating on iron oxide and quartz

The general conclusion of this study is that NOM adsorption on iron oxide is largely due to ligand exchange mechanisms. This is supported by the observations that: (1) specifically preadsorbed anions (phosphate and sulfate) inhibited NOM adsorption, whereas the nonspecifically adsorbed anions like Cl- had little influence. (2) adsorption of NOM was exothermic (Qa = 16-46 kJ mol(-1) C of NOM adsorbed). (3) FTIR spectra provided direct evidence of the complexation between hydroxyl and carboxyl groups and the hydroxyl groups of iron oxide. (4) C-13-CPMAS solid-state NMR revealed that fractions rich in hydroxyls were preferentially adsorbed and that the steric arrangement of those hydroxyls influenced NOM adsorption on iron oxide surfaces. Although this NOM sample will not be representative for all aquatic humic substances, general rules may apply since abundant carboxyl and hydroxyl functional groups are usually present. A report on the desorption processes investigated in these experiments and a mathematical model is in press for publication (Gu, SCHMITT, CHEN, LIANG & MCCARTHY 1994).