Abstract
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 SUVA254 (R2 = 0.82-0.93) but negatively with Slope275-295 (R2 = 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.
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