Biomedical Engineering Reference
In-Depth Information
3.2. Effect of γ -PGA Concentration and Molecular Weight
When evaluated at different concentrations (5-100 mg/L), the γ-PGA from PY-90 and
IFO3335 strains of B. subtilis showed an optimum flocculation activity of 15 and 10.5 l/OD
respectively, for a 20-mg/L kaolin suspension containing 4.5 and 10 mM CaCl 2 [54,55].
However, Shih et al. [30] found the optimum flocculation (8.5 l/OD) to occur at a γ-PGA dose
of 3.7 mg/L in a 5 g/L kaolin suspension containing 9.0 mM CaCl 2 . This variation in γ-PGA
level may be attributed to different Bacillus species and culture conditions employed in the
latter study [30], in which B. licheniformis (CCRC 12826) and a combination of glutamic
acid, citric acid and glycerol was used as carbon source, as opposed to B. subtilis and only
glutamic acid used in the former study [54,55]. Recently, Wu et al. [31] examined the
flocculation activity at various concentrations of γ-PGA from DYU1 B. subtilis strain (10-80
mg/L) in a reaction mixture containing kaolin and 5 x 10 -3 mM of metal cations (Ca 2+ , Mg 2+ ,
Fe 2+ , Fe 3+ or Al 3+ ) and a level ranging from 10-30 mg/L was found effective in attaining the
optimum flocculation activity (Figure 3).
The molecular weight (MW) of γ-PGA also showed a significant impact on the
flocculation activity. A high MW γ-PGA (2 x 10 6 kDa) from B. licheniformis (CCRC 12826)
exhibited a higher flocculation activity compared to the purified low MW γ-PGA (~1 x 10 5
kDa) from Sigma [30]. Apparently, a high MW polymer contains a large number of free
functional groups, which may act as bridges connecting numerous suspended particles to
form flocs of large size during the flocculation reaction [49].
3.3. Effect of Added Metal Cations and pH
Incorporation of metal cations plays a key role in promoting flocculation, probably
because of decline in charge density by cations leading to inter-particle bridging between
suspended particles. Flocculation studies on bacterial γ-PGA isolate were unequivocal in
reporting the synergistic effect of added metal cations, with the trivalent ions (Al 3+ or
Fe 3+ ) exerting the highest effect, followed by divalent (Ca 2+ , Mg 2+ or Fe 2+ ) and
monovalent (Na + or K + ) ions [30,31,55]. The amount of metal ions required for maximum
flocculating efficiency decreased following an increase in the valency of metal ions. Wu
et al. [31] reported an optimum concentration of >10 mM for monovalent ions, but only
0.10-0.90 mM for divalent ions and <0.005 mM for trivalent ions in a reaction mixture of
kaolin and γ-PGA from B. subtilis (DYU1) (Figure 4). Nonetheless, a high Ca 2+
concentration of 2-8 mM and 13.5 mM was required to attain the maximum flocculation
activity for γ-PGA produced from B. subtilis (PY-90) and B. licheniformis (CCRC
12826), respectively [30,54]. Unlike monovalent and divalent ions, a rise in concentration
of trivalent ions (Al 3+ or Fe 3+ ) sharply reduced the flocculation activity (Figure 4), which
may be ascribed to the drop in solution pH caused by precipitation of these ions.
However, the flocculation activity could be restored by adjusting pH of the reaction
mixture to 7 as reported by Yokoi et al. [55], who proposed the hydroxide precipitates of
trivalent ions and Fe 2+ ions to be effective in stimulating the flocculation activity of γ-
PGA at or near neutral pH. Likewise, for both Ca 2+ and Mg 2+ cations, the pH around 7
was shown to be optimum in achieving the largest flocculation activity and flocculating
Search WWH ::




Custom Search