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Colloidal particles in lake waters interact inevitably with cyanobacterial extracellular polymeric substance (EPS), which will change their behavior and fate. Quantitative prediction of the effects of cyanobacterial EPS on colloidal behavior is difficult due to its variability and heterogeneity. To explore the effects of molecular weight (MW) fractions and chemical properties of cyanobacterial EPS on aggregation kinetics of colloidal particles, time-series cyanobacterial samples were collected in Lake Taihu, China, from April to November (during blooming and maintenance period), with the bulk EPS matrix fractionating into low MW (LMW–, <1 nm) and high MW (HMW–, 1 nm–0.45 μm) fractions. HMW–EPS was generally characterized with higher absorbance and predominant distribution of protein-like substances, while LMW–EPS contained mainly the humic- and fulvic-like substances. The absorbance, molecular size, and humification degree for each MW fraction consistently increased from April to November, showing obvious temporal variations from blooming period to maintenance period. As for the MW-dependent aggregation behaviors, the HMW–EPS provided better stability against aggregation than the LMW–EPS, and the bulk EPS matrix that consisted of HMW– and LMW–fractions exhibited the effects intermediate between that of each fraction alone. Regardless of MW fractions, the effects of EPS-induced stability enhancement were more evident in maintenance period than in blooming period. Further analysis showed that the colloidal stability was correlated positively with SUVA 254 (R 2 = 0.82–0.93) but negatively with Slope 275–295 (R 2 = 0.53–0.91) of UV–Vis absorption spectra, indicating that aromaticity and MWs were two critical parameters controlling colloidal aggregation. Therefore, cyanobacterial EPS can exhibit variable effects on colloidal stability, and characterization of MW distribution is strongly required in predicating the behavior and fate of colloidal particles in water environments. Unlabelled Image • Effects of MWs and chemical properties of cyanobacterial EPS on colloidal stability were studied. • The concentration and chemical properties in each MW fraction showed obvious temporal variations. • HMW–EPS provided better stability against aggregation than bulk and LMW–EPS. • Colloidal stability was positively correlated with aromaticity and MWs for each MW fraction. • Characterization of MW distribution was required in future studies on colloidal stability. [ABSTRACT FROM AUTHOR]

Cyanobacterial extracellular polymeric substances (EPSs) in aquatic environments are easily adsorbed onto colloidal particles, whereas the adsorption behavior as affected by molecular weight (MW) properties remained unknown till now. Herein, the bulk cyanobacterial EPS matrix (<0.45 μm) was fractionated into high MW (HMW-, 1 kDa~0.45 μm) and low MW (LMW-, <1 kDa) fractions, with MW-dependent adsorption heterogeneities onto TiO 2 colloids exploring through batch experiment, UV–Vis and fluorescence spectroscopy, and two dimensional Fourier transform infrared correlation spectroscopy (2D–FTIR–COS). About two-thirds of total organic matters within bulk EPS matrix were distributed in the HMW fraction, leaving one-third in the LMW fraction. Compared to LMW–EPS, the HMW counterpart exhibited higher aromaticity and richness of autochthonous protein-like substances, showing evident MW-dependent differences in abundance and composition. The adsorption capacity based on the measurement of total abundance, UV–Vis and fluorescent spectra all decreased in sequence of HMW- > Bulk > LMW-EPS, demonstrating obvious MW-dependent adsorption heterogeneities. During adsorption, the values of SUVA 254 in residual supernatants exhibited an initial decrease followed by gradual increase for all samples, suggesting that the preferentially adsorbed aromatic substances can be subsequently replaced by the non-aromatic moieties. 2D–FTIR–COS further revealed that the carboxylic groups of proteins were preferentially adsorbed onto colloidal surface, followed by the C C functional groups and then the C H groups of polysaccharides, which accounted for the variations of SUVA 254 values in the supernatants. This study demonstrated that the adsorption behavior of EPS matrix was highly MW-dependent, and detailed characterization on size fractionation is thus needed in future studies. Unlabelled Image • MW-dependent adsorption behavior of cyanobacterial EPS on colloidal particles was studied. • Two-thirds of organic matters within EPS matrix were located in the HMW fraction. • Compared to LMW-EPS, the HMW-EPS exhibited higher aromaticity and richness of protein-like substances. • HMW-EPS exhibited higher adsorption capacity, followed by the bulk and LMW-EPS. [ABSTRACT FROM AUTHOR]