Homo-Conjugation of Low Molecular Weight Organic Acids Competes with Their Complexation with Cu(II)

Dissolved organic matter (DOM) controls the bioavailability and toxicity of heavy metals in aquatic environments. The observation of decreased conditional binding constants with increasing DOM concentration is not well documented, which may result in significant uncertainties in heavy metal behavior modeling and risk assessment. We used eight low molecular weight organic acids (LMOC) with representative structures to simulate DOM molecules. The interactions between LMOC molecules resulted in the decreased Cu(II)-LMOC binding with increasing LMOC concentrations, but higher pH values than theoretical calculation after mixing LMOC solutions of different pHs. We thus proposed homoconjugation between LMOC molecules through negative charge-assisted H-bond ((-)CAHB). A mathematic model was developed to describe Cu(II)-LMOC complexation ( K_C) and LMOC homoconjugation ( K_LHL). The increased competition of LMOC homoconjugation over Cu(II)-LMOC complexation, as suggested by the increased ratios of K_LHL/ K_C, resulted in the apparently decreased Cu(II)-LMOC binding with the increased LMOC concentration. Similar concentration-dependent binding was also observed for DOM. With the identified homoconjugation between DOM molecules, some of the literature data with concentration-dependent behavior may be re-evaluated. This is the first work that quantitatively identified homoconjugation among organic molecules. Both the modeling concepts and results provide useful information in investigating the environmental roles of DOM in mediating metal bioavailability and transport.