Mechanisms of pore water organic matter adsorption to montmorillonite

The extent and mechanisms of adsorption of marine pore water organic matter to montmorillonite were studied in a series of batch and sequential adsorp tion experiments. Pore water natural organic matter (pNOM) and easily extra cted natural organic matter (eNOM) were collected from Liberty Bay (Puget S ound, WA, USA) sediments. The pNOM and eNOM were each divided into two size fractions using a 1000 D ultrafilter. Batch adsorption isotherms were appr oximately linear, and the > 1000 D fractions of both pNOM and eNOM had larg er partition coefficients (K-d) than the < 1000 D fractions. A two-componen t fit of the sequential adsorption data indicated that pNOM and eNOM contai ned a similar amount of NOM (30%) that was not surface reactive toward mont morillonite. After correcting the batch adsorption K(d)s for the non-reacti ve components, the K(d)s estimated by batch and sequential adsorption were identical (similar to 2.7 l/kg for > 1000 D pNOM and eNOM, and similar to 1 .6 l/kg for < 1000 D pNOM and eNOM). Mechanisms of adsorption were investig ated by systematically changing conditions (pH, temperature and ionic compo sition) of > 1000 D fractions during batch isotherm experiments. Adsorption of NOM was found to decrease with increased temperature, suggesting that h ydrophobic effects were not the dominant adsorption mechanisms in this syst em. Ion exchange was also not an important adsorption mechanism because ads orption increased with ionic strength. The observed enhancement in adsorpti on with ionic strength indicated that van der Waals interactions were impor tant in the adsorption of NOM. Ligand exchange was found to be a significan t mechanism since the presence of SO42- in solution reduced the amount of N OM adsorbed. Ca2+ enhanced adsorption slightly more than Na+, suggesting th at cation bridging was involved. The relative contributions of van der Waal s interactions, ligand exchange and cation bridging were estimated to be ap proximately 60%, 35% and 5%, respectively, for adsorption of NOM in a CaCl2 solution. (C) 2000 Elsevier Science B.V. All rights reserved.