Abstract
Upon exposure to sunlight extracellular polymeric substances (EPS) were partially transformed into transparent exopolymer particles (TEP) and unstable flocs of different sizes without the addition of any precursors. Parallel factor (PARAFAC) modelling of the sample fluorescence spectra identified humic-like and protein-like or tyrosine-like components in both untreated and irradiated EPS samples. After 58 hours of solar irradiation, humic-like substances were entirely decomposed, while the regenerated protein-like substance from EPS was the key component in the irradiated samples. Degradation and reformation of EPS occurred which was confirmed by the results of size exclusion chromatography, dissolved organic carbon, total protein and total polysaccharide analyses. Irradiated EPS was composed of –COOH or C = O (amide I band) and –NH and –CN (amide II band), while Fourier transform infrared spectroscopy (FTIR) of TEP revealed more acidic –COOH and –C–O groups, indicating typical acidic protein-like TEP. The regenerated protein-like substances could form complexes with free metals originating from degraded EPS in irradiated samples, which could be responsible for the formation of TEP/floc in the aqueous media. These results suggest that TEP/floc formation from EPS could occur by a complexation mechanism between dissolved organic matter and metals, thereby causing ionic charge neutralisation upon sunlight exposure.
Highlights
Bacterial biofilms are formed by communities that are embedded in a self-produced matrix of extracellular polymeric substances (EPS)[1], which is a term encompassing a large group of very different biopolymers
We obtained in this study, we propose a conceptual model for the formation of Transparent exopolymeric particles (TEP)/floc from the photoinduced EPS (Fig. 7)
The molecular compositions of EPS, when decomposed, can form new protein-like components. Such newly formed protein-like components can form complexes with free metals produced from the photodegradation of EPS and its molecular components
Summary
Bacterial biofilms are formed by communities that are embedded in a self-produced matrix of extracellular polymeric substances (EPS)[1], which is a term encompassing a large group of very different biopolymers. TEP are polysaccharide particles, formed by the aggregation of polymers exuded by phytoplankton and are strongly involved in organic matter sedimentation[13]. The study solely identified turbidity as a precursor to floc formation and many important characteristics behind the flocculation process, i.e., particle size, surface charges, DOC changes and fluorescent component changes were overlooked. It is vital to identify physical, chemical and fluorescent characteristics during the transformation processes of EPS to TEP/flocs during natural sunlight exposure. TEP/floc formation was carried out on treated EPS before and after exposure to sunlight at different intervals across the five consecutive days of irradiation. New insights into the mechanism of photo-flocculation are comprehensively discussed based on the evidences provided from different physical, chemical and spectroscopic analyses of EPS and TEP under natural sunlight conditions
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