Abstract
In recent years, several novel cation-independent bioflocculants have been reported, which can avoid the secondary contamination caused by addition of cations. However, compared with cation-dependent bioflocculants, the flocculating mechanism of cation-independent bioflocculants is largely unknown. In this study, a cation-independent bioflocculant MBF-W6 produced by Chryseobacterium daeguense W6 was used as a model to investigate the flocculating mechanism. The results showed that the major flocculating component of MBF-W6 is a complex of proteins and polysaccharides. The zeta potential results indicated that kaolin clay particles were not precipitated due to charge neutralization and the bridging mediated by cations did not play a major role in the flocculating process. These results are consistent with the fact that MBF-W6 is a cation-independent bioflocculant. Further scanning electron microscopic observation showed that MBF-W6 induced flocs formed tight packed structure, suggesting that the kaolin clay particles maybe directly attached and bridged by bioflocculant MBF-W6. In addition, we also found out that Fe 3+ ions inhibit the flocculating activity of MBF-W6 by affecting –COO - and –NH groups. Therefore this study can improve our understanding on flocculating mechanism of cation-independent bioflocculants.
Highlights
Flocculants are widely used in various industrial processes, such as wastewater treatment, drinking water purification and downstream fermentation processes [1,2,3]
We found that (i) the major flocculating component of MBF-W6 is a complex mixture of proteins and polysaccharides; (ii) the charge neutralization and bridging mediated by cations are not the underlying flocculating mechanism of MBF-W6, which is consistent with the fact that the flocculant of MBF-W6 is cation-independent; (iii) kaolin clay particles may get attached and bridged by MBF-W6 directly and form a tight packed structure; (iv) Fe3+ ions can inhibit the flocculating activity of MBF-W6 by influencing –COO− and –NH groups of these polymers
Previous study has reported that the bioflocculant produced by Klebsiella pneumoniae is mainly composed of extracellular polysaccharides that exhibited significant resistance to heat [22]
Summary
Flocculants are widely used in various industrial processes, such as wastewater treatment, drinking water purification and downstream fermentation processes [1,2,3]. Bridging mediated by cations and charge neutralization are the two main mechanisms for these cation-dependent bioflocculants [17,18]. The treatment of kaolin suspension by a bioflocculant secreted by Bacillus mucilaginosus GY03 served as a model for studying flocculating mechanism characterized by bridging mediated by cations and charge neutralization [17]. Flocculation of kaolin suspension with bioflocculant HBF-3 served as a model to investigate the flocculating mechanism in which bridging mediated by Ca2+ was proposed [6]. F19 produces a cation-independent bioflocculant, whose flocculating activity is inhibited by the presence of Fe3+ ions [3]. The bioflocculants produced by Klebsiella pneumoniae and Aspergillus flavus show a good flocculating activity in kaolin clay suspension without cation addition [19,20]. The flocculating mechanism of these cation-independent bioflocculants is largely unexplained
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