Abstract
Information on metal binding with fluorescent substances has been widely studied. By contrast, information on metal binding with non-fluorescent substances remains lacking despite the dominance of these substances in aquatic systems. In this study, the metal binding properties of both fluorescent and non-fluorescent substances were investigated by using metal titration combined with two-dimensional correlation spectroscopy (2D–COS) analysis. The organic matters in the eutrophic algae-rich lake, including natural organic matters (NOM) and algae-induced extracellular polymeric substances (EPS), both contained fluorescent and non-fluorescent substances. The peaks in the one-dimensional spectra strongly overlapped, while 2D–COS can decompose the overlapped peaks and thus enhanced the spectral resolution. Moreover, 2D FTIR COS demonstrated that the binding susceptibility of organic ligands in both NOM and algal EPS matrices followed the order: 3400>1380>1650 cm−1, indicative the significant contribution of non-fluorescent ligands in metal binding. The modified Stern-Volmer equation also revealed a substantial metal binding potential for the non-fluorescent substances (logKM: 3.57∼4.92). As for the effects of organic ligands on metal binding, EPS was characterized with higher binding ability than NOM for both fluorescent and non-fluorescent ligands. Algae-induced EPS and the non-fluorescent substances in eutrophic algae-rich lakes should not be overlooked because of their high metal binding potential.
Highlights
Dissolved organic matter (DOM) in aquatic environments can interact with metals and affect their toxicity, mobility and bioavailability [1,2]
The synchronous fluorescence (SF) results indicated that the algal extracellular polymeric substances (EPS) mainly contained fluorescent protein-like substances, while protein-like and humic-like substances were observed in natural organic matters (NOM)
In the previous studies aiming at lab-cultured cynobacterium M. aeruginosa, Xu et al [18] detected the protein-like substances in all EPS fractions but the humic-like substances in only the soluble EPS fractions
Summary
Dissolved organic matter (DOM) in aquatic environments can interact with metals and affect their toxicity, mobility and bioavailability [1,2]. DOM in waters is generally characterized with heterogeneous organic compositions including fluorescent (i.e., protein-, humic-, and fulvic-like) and non-fluorescent (i.e., carbohydrates, lipids, and lignins) substances [3,4,5]. A comprehensive study on metal-DOM interaction should include related information on both fluorescent and non-fluorescent substances. Fluorescence quenching titration is a popular method in the study of metal-ligand interaction [6,7,8]. Through SF quenching titration technique, the heterogeneous metal binding behaviors between leaf litter and soil DOM can be explored [7,8]. SF technique is not applicable for non-florescent substances, even with these making up a significant proportion of aquatic DOMs [11]
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