Abstract: The binding constant(K_DOM) of pyrene onto different molecular weight(M_w) fractions of a dissolved organic matter(DOM) extract from an estuarine sediment was measured via a fluorescence quenching method, and the possible binding mechanisms were discussed. The influence of water chemical parameters on pyrene binding was studied to elucidate the effects of DOM steric conformation. DOM fraction with larger M_w(>14000) showed a greater K_DOM(2.02×10⁵) than that with smaller M_w did, and the K_DOMs were 1.16×10⁵ and 1.13×10⁵ for the fractions with M_w of 1000-14000 and <1000, respectively. The K_DOMs of different M_w DOM fractions were correlated positively with the atomic ratio of C/H and absorbance at 280 nm; while a negative correlation existed between K_DOM and (O+C)/N (atomic ratio), and K_DOM and O-containing groups. These correlations indicate the positive influence of aromatic structure and the negative effect of polarity on DOM binding capacity. Infrared spectroscopy revealed that specific interactions existed between pyrene and the DOM fraction with larger M_W besides hydrophobic partition, i.e., NH-π interaction in DOM moiety(1000-14000) and π-π electron donor acceptor(EDA) forces in DOM moiety (>14000). K_DOM varied in a complex pattern with increasing cation concentration and pH, which could be explained by the change in DOM steric conformation. Formed aggregates favored the lipophilic partition of pyrene. However, the accessibility to some interior binding sites became low when the aggregate was too large, leading to a reduced K_DOM.

Key words: Dissolved organic matter, Conformation, Water chemistry, Pyrene, Binding constant