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Bioflocculation of Hematite Suspensions with Products from Yeast Cell Rupture.

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Short Research Communication
Bioflocculation of Hematite Suspensions with
Products from Yeast Cell Rupture
1. A. H. Schneiderl, M. Misra and R. W. Smith*
Mackay Schoolof Mines, University of Nevada, Reno, Nevada 89557, USA
The flocculation of -38 pm hematite aqueous suspensions by the yeast Candida
parapsilosis, and by products derived from the ultrasonic rupture of this microorganism
was studied. Results obtained are compared to the flocculation of comparable
suspensions by cooked corn starch and a polyacrylamide (average molecular weight
106).
Introduction
Finely divided mineral particles can be flocculated by microorganisms provided
these organisms adhere to the mineral particles and reduce the elecmstatic charge on
the particles, or they are sufficiently hydrophobic to promote hydrophobic interaction
between the now hydrophobic mineral particles, or by some combination of these
two mechanisms [1-31. The use of microorganisms as flocculants for fine mineral
suspensions has been reported [1-61. Mycobactenum phlei is a highly hydrophobic
bacterium and has been shown to be a good flocculating agent for a number of
mineral suspensions. Recent work has shown that the yeast Candida parapsilosis is
another good flocculant. In previous studies it was found that the relative particle
sizes of the organism cells and of the mineral particles affected the flocculation. This
paper compares the flocculation of frne (- 38 p)hematite by whole C. parapsilosis
cells, ruptured cells, and the soluble fraction of the organism derived from its
rupture.
Experimental Procedures
The hematite studied was a specular hematite from Ironton, Michigan. A wild strain
of C. parapsilosis was grown in a culture medium consisting of: 10 g/l dextrose, lg/l
beef extract, 1 g/l yeast e x m t and 2 g/l enzymatic-hydrolysatecasein. The pH of the
medium was adjusted to pH 7.0 before inoculation. Cultures were grown in 300 ml
flasks, continuously shaken in a water bath held at 35°C.After 48 hours, cells were
harvested by centrifugation. The cells were ruptured by ultra sound by placing a
suspension of 100 ml containing 16 g of biomass in a beaker, and sonication for up
to 20 minutes. Figure 1 shows the percent weight of soluble fraction
produced/weight of biomass as a function of time. These results indicate that the
organism is composed of about 40wt8 soluble fraction. The temperature of the
suspension rose to approximately 70°C indicating an energy consumption of about
15.2 kJ. Based on the data of Figure 1, a sonication rime of 12 minutes was chosen
Universidade Federal do Rio Grande do Sul, Port0 Alegre, RS, Brazil.
* Author for correspondence.
248
Bwfloccuhtion of Hematite Surpenswnswith Produtsfrom Yeast Cell Rupture
saaiootiosrtfmeo
Figure 1. Proportion of soluble fraction released to total weight of biomass as a
function of sonication time at 20,000 H r (temperature rise as a function of
sonication time is also included).
for sample preparation. A calculation based on examination of scanning electron
micrographs indicated that rupturing the cells increased the surface area of wall
structures from about 2.8 to 9.0 m2/g.
Sedimentation tests were performed by preparing 200 ml suspensions containing
2wt% solids. The suspensions were mixed by stirring at 250 rpm for 1 minute,
followed by stirring for 2 minutes at 50 rpm.The suspensionswere then transferred
to graduated cylinders. Samples were collected at set time intervals at a distance 15
cm down from the top of the water surface. The solid concentration was determined
gravimetrically. The flocculation efficiency E(%) was calculated using the
relationship:
E(%) =lo0 x ( Ci -Cf)/ Ci
where Ci= initial concentrationof solids, and C, = concentration of solids after time t.
Experimental Results
Figure 2 indicates the effect of flocculation efficiency on hematite suspensions ( pH
6.5) by whole yeast cells and by the soluble portion of the cells as a function of time.
The curves are compared to a similar curve obtained in the absence of a flocculant.
In the presence of a flocculant, near maximum flocculation is obtained within 30
seconds after the beginning of settling. Therefore, 45 seconds was chosen as a base
time parameter and was the time of settling used in subsequentexperimentation.
Figure 3 shows the flocculation efficiency as a function of concentration for
whole C. parapsilosis cells, for ruptured cells, and the soluble fraction extracted
from the microorganism. Hematite concentration was 2wt%; time of settling was 45
seconds, and the pH was maintained at pH 6.5. The soluble part of the organism
flocculated the suspensions at slightly lower concentrations than ruptured cell
249
I. A . H.Schneider, M. Mima and R. W.Smith,
fragments, and at much lower concentrations than whole cells. There is a critical
concentfarion of broken cells and of the soluble fraction (at a lower concentration)
above which flocculationdecreases with increasing concentration.
0
2
3
&ttling-e(minl
1
5
4
6
Figure 2. Flocculation eficiency of hematite suspenswns in the presence of whole
cells (60 kglt), the soluble cellfiactwn (1.2 kglt), andin the absence qfjlocculants.
loo
8
80
P
0
r
E
40
20
0
0.01
. .-.....'
0.1
'
soluble fractlon
.....*.' . -----..' .
1
10
.....'.I
100
.
- - - m
1000
Conc-~wtI
Figure 3. Flocculation efficiency of hematite suspensions as a function of
concentration of whole cells, ruptured cell fragments, and the soluble cell fiaction.
(2 wt% s o l a ; 4.5 seconds settling time; height = 1.5 cm down fiom water swface;
pH 6 5 02)
*
250
Bwfloccukzticm @Hematite Suspemwns with Products porn Yeast Cell Rupture
100
1:
1
0
2
6
4
8
10
12
14
pa
Figure 4. Flocculatwn flciency of hematite suspenswns as a function of pH for
whole celh (60 kglt),for ruptured cell fragments (3.0 kglt). the soluble cell fraction
(1.2 kglt), and in the absence offlocculants. (2 WHOsolids: 45 secondr settling time;
height = 15 cm down from the water surface)
I
Polyacrylamide
>
Soluble lractkn
Ruptured c d s
I0
Figure 5. Concentration range for flocculation of a 2 ~ 1 %solids suspension of
-38 pm hematite particles by polyacrylm*&. starch, and C. parapsilosis and its
products.
Figure 4 shows flocculation efficiency as a function of pH for whole cells
(concentration 60 kg/ton) and for the soluble fraction (concentration 1.2 kg/ton);
ruptured cells gave similar results if cell concentrations were 3 kg/ton. Also shown
are data for hematite suspension flocculation in the absence of the microorganism or
its derivatives. At the concentrations of organic material studied, the suspensions
were readily flocculated at pH values approximately pH 2.8 to 11.5, with an
optimum pH of about pH 6.
I. A . H. Schneider, M. Misra. and R. W . Smith
Figure 5 indicates the concentration ranges required for flocculation of -38 p
hematite for C. parapsilosis, its sonication products, cooked corn starch, and a
polyacrylamide(molecular weight 1P).
Discussion and Conclusions
The results show that C. parapsilosis and substances derived from the
micraorganism will adhere to hematite and will promote flocculation of fine hematite
suspensions. The mechanism of flocculation with microorganism cells has been
discussed elsewhere [1,51, and basically appears to occur by cell bridging. The
dosages generally required for flocculation are quite high (from 50-100 kg/t). Whole
cells are poor flocculants for very fme minerals, such as -1 p clay particles, and
one reason is related to the relative sizes of the microorganism cells and the clay
particles. It appears that if the microorganism cells are smaller in size than the
mineral particles to be flocculated, then the adhering cells surround the mineral
particles. In addition, if the cells' surface is hydrophobic they will agglomerate the
mineral particles in the suspension through a bridging mechanism. However, if the
mineral particles are smaller than the 1 to 2 p n organism size, then the hydrophilic
mineral particles surround the cells. Ruptured cells also have a greater surface area
and more sorption sites than whole cells. Therefore, they can flocculate mineral
suspensions at lower concentrationsthan whole cells (approximately 0.8-20 kg/t).
The soluble fraction derived from rupture of the cells can flocculate mineral
suspensions at even lower concentrations (0.4-10 kg/t). This fraction has about the
same flocculating ability as cooked corn starch. A long chain polyacrylamide
(average molecular weight 106) flocculates some suspensions at concentrations of
less than 0.1 kg/t. The relative ability of these different substances to selectively
flocculatea particular mineral in a mixed suspension has not yet been measured, and
will be the subject of a subsequentpaper.
References
1. Smith, R.W., Misra, M. and Dubel, J. 1992. Bactaial Flocculation of Phosphate Wastes
Using a Hydrophobic Bacterium. Residues and Efluents, Processing and Environmental
Cmiderafim, eds. R. G. Reddy. W. Imbrie and P. Queneay TMS,747-756.
2. Dubel, J.. Smith. R.W.. Miaa. M. and Chen, S. 1992. Microorganisms as Chemical
Reagents: the Hematite System. Minerals Engineering, 5,547-556.
3. Misra, M., Dubel, J.. Chen. S. and Smith, R.W. 1993. Selective Flocculaticmof Fine Coal
with HydrophobicMycobacteriumphlei. Minerals and Metall. Process., 10.20-23.
4. Smith, R.W.. Misra, M. and Chm, S. 1993. Adsorption of a Hydrophobic Bacterium onto
Hematite: Implications m the Froth Flotation of Hematite.J. Soc. I d . Microbwl., 11, 6367.
5. Smith, R.W. and Misra, M. 1993. Recent Developments in the Bioprocessmg of Minerals.
Minerd Procasing and Extractbe Metalh~rgyReviews. 12.37-60.
6. Schneidex, I.A.H.. Misra, M. and Smith R.W. 1993. Flocculation of Mineral Fines with
Ruptured Organisms. Biohy&ometaUurgical Technologies, Volume II, eds. A. E. Torma,
M. L. Ape1 and C. L. Brierley, TMS, 197-207.
Received: 21 September 1993;Accepted afier revision:9 February 1994.
252
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