Biomedical Engineering Reference
In-Depth Information
activity (5.2 l/OD) and flocculation rate (90%) were attained at 30°C. But, the flocculation
activity dropped by 50% after incubation at 60°C for 15 min, followed by a complete
inactivation of γ-PGA at 120°C. Yokoi et al. [54] also reported a rapid decline in flocculation
activity of γ-PGA from PY-90
B. subtilis
strain at elevated temperatures, diminishing to zero
on heating at 100°C (40 min). Obviously, the bioflocculants composed of protein or peptide
backbone are susceptible to heat treatment, while those containing sugars and polysaccharides
are thermally stable [56].
3.5. Effect of Various Inorganic and Organic Suspensions
The γ-PGA from different
Bacillus
species was not only effective in flocculating kaolin
suspension, but also in organic suspensions and some other inorganic suspensions including
active carbon, solid soil and acid clay (Table 3) [30,55]. The γ-PGA isolate from
B. subtilis
(IFO3335) at 20 mg/L exhibited a greater flocculating efficiency (l/OD) in acid clay
suspension (10.9-34.7) compared to that in active carbon (2.8-3.3) and solid soil (1.0-1.9). In
the absence of any added metal cations, γ-PGA flocculated suspensions of calcium and
magnesium salts, with the highest flocculation activity (l/OD) being shown in Ca(OH)
2
suspension (21.3), followed by MgCO
3
(9.3), Mg(OH)
2
(8.1), Al
2
O
3
(6.1),
[Ca(PO4)
2
]
3
⋅Ca(OH)
2
(5.7) and CaCO
3
(2.0) [30,55]. The γ-PGA from
B.
licheniformis
(80
mg/L) showed a good flocculating effect in organic suspensions (5 g/L) as well, which
equaled to 3.0, 2.6, 4.6 and 2.8 l/OD for cellulose powder, carboxymethylcellulose,
Saccharomyces cerevisiae
and
B. circulans
, respectively (Table 3) [30].
3.6. Effect of Crosslinking Poly(
γ
-glutamic Acid)
Crosslinking of γ-PGA (CL-γ-PGA) with an optimum γ-irradiation dose of 20 kGy was
shown to generate the highest water absorption capacity (1005.6 mL/g) and viscosity (3.31 η)
and possess twice the flocculation activity (5.48 l/OD) when compared to a non-crosslinked
γ-PGA (2.74 l/OD) in kaolin suspension at 30°C and pH 5.0 [57]. However, a pretreatment
with polyacrylamide (PAC) drastically raised the flocculation activity of CL-γ-PGA in
different suspensions [57,58]. Table 4 summarizes the flocculation activity of γ-PGA as
affected by pretreatment with different doses of PAC for various suspensions at 30°C and pH
5.0 [58]. Compared to the control (only CL-γ-PGA), the flocculation activity (l/OD) rose by
39.9, 32.8, 30.0, 23.5, 88.2 and 12.2 for kaolin, bentonite, diatomaceous earth,
E. coli
,
M.
aeruginosa
and Mandai pond (real water sample) suspensions, respectively, for PAC and CL-
γ-PGA at a dose (μg/mL) of 0.5 and 10, 2 and 5, 0.5 and 10, 0.5 and 10, 0.5 and 5, and 2 and
20 [58]. Apparently, the Al
3+
ion in PAC could electrostatically interact with the negatively
charged cell surface of
E. coli
and
M. aeruginosa
or with the silicate anion induced on the
surface of kaolin, bentonite and diatomaceous earth suspensions to form small flocks, which
may eventually become larger following the addition of CL-γ-PGA. Yet, the pretreatment
with PAC did not show any effect on the real sample suspension from Yamato River (a
dirtiest river in Japan). In contrast, a reversed trend was shown in crystal violet (CV) dye
suspension, probably due to electrostatic interaction between the iminium cation in CV and
