Effect of rhamnolipid biosurfactants on performance of coal and mineral flotation Khoshdast, 2011

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International Biodeterioration & Biodegradation 65 (2011) 1238e1243
Contents lists available
International Biodeterioration & Biodegradation
journal homepage: www.elsevier .com/locate/ ibiod
Effect of rhamnolipid biosurfactants on performance of coal and mineral flotation
Hamid Khoshdast a, Abbas Sam a, Hojatollah Vali b, Kambiz Akbari Noghabi c,*
aMining Engineering Department, Faculty of Engineering, Shahid Bahonar University, Jomhouri Eslami Blvd, P.O. Box 76169-133, Kerman, Iran
bDepartment of Earth and Planetary Sciences, McGill University, Montreal, Canada
cNational Institute of Genetic Engineering and Biotechnology (NIGEB), Karaj-Tehran Highway, Pajohesh Blvd, P.O. Box 14155-6343, Tehran, Iran
a r t i c l e i n f o
Article history:
Received 8 July 2011
Received in revised form
6 October 2011
Accepted 8 October 2011
Available online 2 November 2011
Keywords:
Pseudomonas aeruginosa
Rhamnolipid biosurfactant
Froth flotation
* Corresponding author. Tel.: þ98 21 44580352; fax
E-mail addresses: Akbari@nigeb.ac.ir, Kambizakb@
0964-8305/$ e see front matter � 2011 Elsevier Ltd.
doi:10.1016/j.ibiod.2011.10.003
a b s t r a c t
The effect of a concentrated rhamnolipid biosurfactant (purity> 97%) produced by Pseudomonas aeruginosa
MA01 on flotation performance of coal and minerals was studied and compared with those from synthetic
flotation reagents in both single and mixed systems. Results of phosphate flotation tests showed that
phosphate content in the floated product decreased by rhamnolipid concentration. However, final recovery
of phosphate at appropriate dosage of reagentwas insignificant. This could be ascribed to the increase of iron
recovery tofinal concentrate likely due to collecting action of rhamnolipid for ironminerals. In coal flotation,
the increase of rhamnolipid concentration inmixturewith pine oil reduced both ash content and coal yield of
concentrates. In desulfurization of a sample iron concentrate using reverse flotation, rhamnolipid addition
successfully decreased the sulfur content of the iron concentrate. The flotation responses of the studied
sample ores showed that rhamnolipid can be used as a promising frother or co-frother inmineral processing
practices.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Today, it is well known that some microorganisms are able to
produce surface-active compounds that can vary in their chemical
structure and size. Since the last decade of the past century the
concern about environmental protection has lead to an increased
interest in the production and properties of these natural products.
These surfactants, often called microbial surfactants or bio-
surfactants, are considered nowadays as potential substitutes of
synthetic chemicals frompetrochemical origin. Themain advantages
of biosurfactants over their chemical counterparts are their lower
toxicity, better environmental compatibility, biodegradability, and
effectiveness in a wide range of temperatures and pH. Last, but not
least, their production by renewable resources provides further
impetus for serious consideration of biological surfactants as possible
alternatives of the commonly used industrial chemicals (Cohen and
Exerowa, 2007; Vilinska et al., 2008).
Among the various species of biosurfactants much work has been
done on rhamnose containing microbial surfactants (namely rham-
nolipid) produced by Pseudomonas aeruginosa strains. This ubiqui-
tous environmental bacterium can be isolated from many different
habitats includingwater, soil and plants. Rhamnolipids are usually as
: þ98 21 44580395.
yahoo.com (K.A. Noghabi).
All rights reserved.
a mixture of two or four species. They differ by the length of hydro-
phobic chains (from C8 to C12) some of which are unsaturated with
onedouble bond.However, under usual growth conditions, twomain
homologues are primarily obtained: monorhamnolipid (RL-1) and
dirhamnolipid (RL-2) (Fig. 1) (Price et al., 2009). Rhamnolipids have
more and more environmental applications in recent years. In this
field,most of the studies focused on their function in the remediation
of wastewaters and soils contaminated by heavy metals and hydro-
phobic organic compounds, such as petroleum hydrocarbons and
pesticides (e.g., Zhang andMiller 1992; Torrens et al., 1998; Mulligan
and Wang 2006; Fu et al., 2007; Jayabarath et al., 2009; El Zeftawy
and Mulligan, 2011). Recently, Fazaelipoor et al. (2010) studied the
possible application of rhamnolipids as a frothing reagent in coal
flotation and reported that the use of rhamnolipid as the sole frother
of the process yielded products of 72e79 percent recovery with
10e15.5 percent ash content supporting 55e57.5 percent efficiency.
However, low purity of the applied rhamnolipid product ((Zeiss model Axioskop 40, Germany) at magnifications
of�40 and�100, by carefully transferring a loopful from crystalline
zone on a glass slide and the thin films were photographed.
2.4. Structural analysis of crude biosurfactant
Fourier-transform infrared (FTIR) spectroscopy spectra were ob-
tained in a Nicolet 6700 Fourier-transform infrared spectrometer
(FTIR) (Madison, WI). A sample of each purified biosurfactant was
placed in between two CaF2 windows, without spacers, and the set
was mounted in a thermostated cell holder. Each spectrum was
obtained by collecting 256 interferograms with a nominal resolution
of 2 cm�1. The equipment was continuously purged with dry air in
order to minimize the contribution peaks of atmospheric water
vapor. The sample holder was thermostated at 25 �C using a Peltier
device (Proteus system from Nicolet).
The chemical structure of the purified biosurfactants was deter-
mined by ES-MS in an Agilent LC-MSD-Trap-VL equipment. Full scan
data were obtained by scanning from 950 to 150 m z�1. The acqui-
sitionparameterswere: ionpolarity negative, ESI ion source type, dry
gas flow-rate 6 l min�1, nebulizer pressure 30 psi, temperature
325 �C, and the capillary exit was held at �127.3 V. The trap drive
values were close to 50, indicating an intermediate stability for our
compounds.
2.5. Surface tension measurement and pH-stability
A crude rhamnolipid solution was prepared at a concentration of
250 ppm. Then, it was appropriately serially diluted (250e0 ppm),
and the surface tension values of the prepared dilutions were
measured using a duNouy ring-type tensiometer (Laudamodel TD1-
C, Germany). The tested solutionwas placed into a clean glass beaker
(50 ml) specific for the surface tensiometer (Lauda). Before con-
ducting the experiment and between each pair ofmeasurements, the
sample cupwaswashed three timeswith distilledwater and acetone
in series and then allowed to dry; the platinumringwasalso similarly
treated, and then it was flamed till redness and left to cool.
The critical micelle concentration (CMC) of rhamnolipid was
estimated from the intercept of two straight lines extrapolated
from the concentration-dependent and concentration-independent
sections of a curve plotted between rhamnolipid concentration and
surface tension values. CMC values were used for the tentative esti-
mation of the percentage purity of rhamnolipid (RL) (Abdel-
Mawgoud et al., 2009):
Purityð%Þ ¼ ðCMCstandard RL=CMCtest RLÞ � 100 (1)
Information for standard rhamnolipid was that reported by
Abdel-Mawgoud et al. (2009).
H. Khoshdast et al. / International Biodeterioration & Biodegradation 65 (2011) 1238e12431240
In order to evaluate the stability of rhamnolipid at different pH
values, rhamnolipid polluted solutions (6, 25, and 50 ppm) were
adjusted at pH values of 2, 4, 7, 9, and 11 using HCl or NaOH as acidic
and alkaline pH-regulators, respectively.
2.6. Batch flotation tests
Three sets of flotation experiments were planned to investigate
the possible application of rhamnolipid in coal and mineral flotation
as a frothing reagent and/or co-reagent. Except frother concentration
to be assessed, all other parameters including solid percentage, pH of
pulp, reagent type, conditioning time, impeller speed, and froth-
collecting period, were set on the basis of operating conditions
applied in industrial practices. Table 1 gives the operating conditions
used in this study. All the tests were carried out in a Denver D-12
flotationmachine. For each semi-batch test, requisite amount of bulk
sample was transferred into the cell and additional water was added
to maintain required pulp density. The impeller speed of the flota-
tion chamber was set and pulp was allowed to condition for 5 min.
The pH, when required, was adjusted during pulp conditioning
period. Then, the required volume of reagents (except frother) from
a stock solution was added to the cell and conditioned for other
minutes. Next, frother or frother mixture was added and after
conditioning for a minute, the air inlet valve was opened and the
froth-off was scrapped every 15 s and collected after minutes. After
the final froth sample was collected, the machine was stopped. The
froth products and the tailings (the part that remained inside the
machine) were dried, weighed, and analyzed.
The efficiency of the mineral flotation process was evaluated in
terms of final recovery and grade of Fe, P2O5 and S using the
equation (Wills and Napier-Munn, 2006):
R ¼ C
c
f
� 100 (2)
where R (%) is element recovery, C is the fraction of the total feed
weight that reports to the concentrate, f and c are element grade (%) of
feed and concentrate, respectively. The yield of coal and ash
(noncombustible materials) content of concentrate were calculated
as the process response for coal flotation experiments (Gupta et al.,
2009):
Y ¼ TA � FA
TA � CA
� 100 (3)
where Y is the percent yield of coal, and FA, CA and TA are the
percentage of ash in feed, concentrate and tailings, respectively.
Table 1
Operating conditions applied in flotation experiments.
Parameter Phosphate ore Coal sample
Source of sample Esfordi Phosphate Complex, Yazd, Iran Zarand Coal W
Particle size d80 z 85 mmshowed that the bulk sample contains 70.68% Fe and
0.332% sulfur.
Iron concentrate
ashing Plant, Kerman, Iran Gol-e Gohar Iron Ore Complex, Sirjan, Iran
d80 z 75 mm
tion Reverse flotation
1000 rpm
180 s
20%
ral pH of tap water used 2.5 � 0.1, regulated by H2SO4
collector Potassium Amyl Xanthate (PAX) as collector
120 g t�1
3 min
Methyl isobutyl carbinol (MIBC)
:15, 0:20, 0:50 100:0, 75:25, 50:50, 25:75, 0:100, 0:150, 0:200
Fig. 2. Crystalline appearance of the produced rhamnolipid: (a) presence of pure dendrite rhamnolipid inside the container; at magnifications of (b) þ7; (c) �40; and (d) �100.
H. Khoshdast et al. / International Biodeterioration & Biodegradation 65 (2011) 1238e1243 1241
3. Results and discussion
3.1. Physical properties
Physical state, color, and odor were determined for crude rham-
nolipid residue obtained after extraction. The rhamnolipid product
appeared as viscous sticky oily residue with amber to yellowish
brown color and fruity pineapple like odor. Crystalline rhamnolipid
grown inside the glass container was an evidence of high purity of
the product. From Fig. 2, it appears that the rhamnolipid product is
characterized by its dendrite type crystals.
Table 2
Main monorhamnolipid and dirhamnolipid species produced by Pseudomonas
aeruginosa MA01 as determined by ES-MS [M � H]� e pseudomolecular ion.
[M � H]� (m/z) Fragments (m/z) Probable assignation
503 334, 164 R1C10C10
529 334, 164 R1C10C12:1
531 334, 164 R1C10C12
649 479, 310 R2C10C10
675 479, 310 R2C10C12:1
677 479, 310 R2C10C12
3.2. Structural analysis of crude biosurfactant
A classical technique for structural analysis of organic compounds
is FTIR. Characteristic absorption bands corresponding to functional
groups typically forming part of rhamnolipids could be observed for
the samples: the OeH stretching bands (free hydroxyl groups of
rhamnose rings) around 3385e3390 cm�1, the stretching bands of
the methylene and terminal methyl groups of the acyl chains
between 2850 and 2930 cm�1, the stretching band of the ester C]O
groups at ca.1740 cm�1, the freeeCOOe band (free carboxyl group of
the second fatty acid) around 1560e1580 cm�1, and the CeOeC
vibrations (rhamnose rings) between 1020 and 1030 cm�1. This
analysis confirmed the glycolipid nature of our biosurfactants which
could in fact correspond to rhamnolipids (data not shown here).
ES-MS analysis of the biosurfactant purified from the crude extract
was carried out at �127 V to observe the pseudomolecular ion, and
fragmentation was performed by helium injection inside the source
at the same voltage. As detailed in Table 2, analysis was compatible
with the presence of three major monorhamnolipid species:
Rha1C10C10, Rha1C10C12:1, and Rha1C10C12; as well as another three
major dirhamnolipid species: Rha2C10C10, Rha2C10C12:1, and
Rha2C10C12.
3.3. Surface tension measurement and pH-stability
One of the main characteristics of surfactants is their tendency to
adsorb at interfaces in an oriented fashion as a consequence of their
amphipathic structure. As the surfactant concentration increases, the
surface tension of the surfactant solution decreases up to a certain
value and then becomes almost constant due to the interface satu-
rationwith the surfactantmolecules. The surfactant concentration at
which this phenomenon occurs is known as the CMC and is deter-
mined from the break point of the surface tension versus
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80 90 100
Concentration (ppm)
S
u
r
f
a
c
e
 t
e
n
s
io
n
 (
m
N
/m
)
 pH 2
 pH 4
 pH 7
 pH 9
 pH 11
CMCtest: 10.1 ppm
Fig. 3. A plot of surface tension as a function of concentration of crude rhamnolipid at
different pH values.
9.44 10.51 9.34 9.00 11.71
70.69 73.36
65.88 63.26
77.17
0
10
20
30
40
50
60
70
80
90
20 : 0
(Practical)
10 : 10 5 : 15 0 : 20 0 : 50
Pine oil to Rhamnolipid Ratio (g t-1 : g t-1)
P
e
r
c
e
n
t
a
g
e
 Ash Content Yield
Fig. 5. Effect of rhamnolipid addition onflotationperformanceof the studied coal sample.
H. Khoshdast et al. / International Biodeterioration & Biodegradation 65 (2011) 1238e12431242
concentration curve. For practical purposes, it is important to
distinguish between an effective biosurfactant and an efficient bio-
surfactant. Effectiveness ismeasured by theminimumvalue towhich
the surface tension can be reduced, whereas efficiency is measured
by the biosurfactant concentration required to produce a significant
reduction in the surface tension of water. The latter can be known
from the CMC of the biosurfactant (Abdel-Mawgoud et al., 2009).
From Fig. 3, it appears that rhamnolipid lowered the surface
tension of water from about 70 to 30 mN m�1. The CMC value for
rhamnolipid was found to be 10.1 ppm. Although purity can be
more accurately calculated using chromatographic methods, it was
calculated by comparing the CMC values between the test and
a standard sample using the following proposed relations:
Percentof impurities¼PurityStd�½ðCMCTest=CMCStdÞ�1��100
(4)
Percent of purity ¼ ð100� Percent of impuritiesÞ (5)
Purity and CMC for standard (Std) rhamnolipid are 98% and
9.85 ppm. Using the above equation, it was found that the purity of
test rhamnolipid was about 97.5%.
The stability of test rhamnolipid at different pH values is also an
important issue that can affect its application spectrum. As seen in
Fig. 3, the test rhamnolipid showed an almost stable surface activity
0
20
40
60
80
100
 
 5
0
0
 :
 5
0
5
0
0
 :
 0
(
P
r
a
c
t
ic
a
l)
4
0
0
 :
 1
0
0
2
5
0
 :
 2
5
0
 2
5
0
 :
 5
0
 1
5
0
 :
 3
0
 
 0
 :
 5
0
0
 
 0
 :
 2
0
0
 
 
 0
 :
 5
0
Flo-Ys 20 to Rhamnolipid Ratio (g t-1 : g t-1)
P
e
r
c
e
n
t
a
g
e
 Phosphate Recovery Phosphate Grade Iron Recovery Iron Grade
Fig. 4. Effect of rhamnolipid addition on flotation performance of the studied phos-
phate ore.
profile at acidic pH values. But, a more pronounced reduction in
surface activity was observed at alkaline pH values. Ishigami et al.
(1987) and Champion et al. (1995) have shown that the
morphology of rhamnolipid biosurfactants is a function of pH,
changing from lamellar, to vesicular and ultimately to micellar as the
pH increases as a consequence of interfacial forces such as strong
hydration forces as well as electrolytic forces (e.g., Naþ in this study)
(Aratono et al., 2003), resulting in gradually reduction of RL activity
by increasing pH values.
3.4. Phosphate ore flotation tests
Fig. 4 shows phosphate and iron grades of concentrates obtained
at different Flo-Ys 20 (F-20) to rhamnolipid ratios. As seen, in
experiments with total reagent dosage of 500 g t�1 (i.e., 500:0,
400:100, and 250:250), the decrease in F-20 and the increase in RL
concentration reduced phosphate grade; whereas iron content was
significantly increased. Such a behavior was observed in the first test
with F-20 to RL ratio of 500:50 g t�1. By decreasing total reagent
dosage (i.e., 250:50and150:30), the approachof grade variationswas
conversed. These results showed that the increase of RL dosage has
negative effect on phosphate flotation but positive on iron ore
flotation. In the case of three last tests without F-20 addition, iron
content was gradually reduced by decreasing the RL concentration;
however, the variations of phosphate contentwere insignificant. This
shows that RL has a collecting effect on iron ores (mainly magnetite
and hematite). Iron recovery calculations (Fig. 4) confirmed the
conclusion. As Fig. 4 shows, the phosphate recovery values did not
change significantly except tests runwith the lowest F-20 to RL ratios,
i.e., 150:30 and 0:50.
0
20
40
60
80
100
120
100 : 0
(Practical)
 75 : 25 50 : 50 25 : 75 0 : 100 0 : 150 0 : 200
MIBC to Rhamnolipid Ratio (g t-1 : g t-1)
Ir
o
n
 r
e
s
p
o
n
s
e
 (
%
)
0
2
4
6
8
10
12
14
16
18
20
S
u
lf
u
r
 c
o
n
t
e
n
t
 (
%
)
 Fe Recovery Fe in Tailings Sulfur in Tailings
Fig. 6. Effect of rhamnolipid addition onflotation performance of the studied iron
concentrate.
H. Khoshdast et al. / International Biodeterioration & Biodegradation 65 (2011) 1238e1243 1243
3.5. Coal flotation tests
Ash content of concentrates at different pine oil to rhamnolipid
ratios is shown in Fig. 5. As seen, rhamnolipid addition decreased
coal recovery (yield) and in some extent, the ash content of
concentrates. The reduction in coal recovery can likely explained as
following: diesel oil as coal collector by van der waals bonding
improved the hydrophobic properties of coal particles. Rhamnolipid
contains two long hydrocarbon chains which are able to interact
with the surface of coal particles and/or the adsorbed diesel oil
molecules through van der waals bonding. In contrast, the presence
of multiple oxygenated group in rhamnolipid structure increases
hydrogen bonding between rhamnolipid and water molecules;
thus, rhamnolipid can act as depressant for coal particles preventing
coal particle-bubble contacting probability. This, in turn, would
decrease the coal separation efficiency. Fairly decrease of ash
content by rhamnolipid concentration can likely ascribed to
decreasing the recovery of noncombustible materials involved to
coal particles due to incomplete liberation degree. In the last test
applying maximum pine oil/rhamnolipid ratio (0:50), the increase
in both ash content and coal yieldmay be due to increasing carrying
capacity of bubbles by frothing agent concentration. Khoshdast et al.
(2011) showed that rhamnolipids have very high frothing capacity
which can significantly increase carrying capacity of bubbles. In
addition, at high concentrations, pulp will be saturated by rham-
nolipidmolecules compensating depressing action of rhamnolipids.
This, in turn, increases the separation efficiency of coal particles.
3.6. Iron concentrates flotation tests
In reverse flotation process, product floated to the froth is
considered as tailings and remaining product inside the cell is
accepted as concentrate. Fig. 6 shows the metallurgical results for
iron concentrate reverse flotation at different MIBC to rhamnolipid
dosage ratios. Rhamnolipid addition in these experiments showed
complicated effects. As rhamnolipid concentration increases, sulfur
content of the floated tailings increased up to MIBC:RL ratio of
0:100 at first, and then decreased at 0:150 and 0:200 ratios. In
contrast, tailings iron content was negligible up to ratio 0:100, and
then increased. The increase of tailings sulfur grade by rhamnolipid
concentration can be ascribed to increasing of frothing capacity of
system, since rhamnolipid acts as a powerful frothing agent. In the
absence of MIBC (i.e., 0:100, 0:150 and 0:200), metallurgical
responses of iron and sulfur can be explained as follows. Khoshdast
et al. (2011) showed that MIBC is a “selective” frother suitable for
flotation of particles with low density whereas, rhamnolipid can be
categorized as “powerful” frother which is more efficient for
flotation of particle with high density. The studied sample was
a magnetite (Fe3O4) concentrate containing 0.332% sulfur included
in pyrite (FeS2) mineral. As rhamnolipid concentration increases
(Fig. 6), sulfur grade deceased but iron grade increased in the
floated tailings, simultaneously. Therefore, the increase in iron
content can be ascribed to the improved flotation rate of magnetite
particles with higher density (about twice) compared to pyrite
particles. However, there may be other reasons for the complicated
flotation response of iron concentrate in the presence of rhamno-
lipid which rise from pulp chemistry, such as rhamnolipid inter-
action with collector, mineral surface, etc.
4. Conclusions
In this study, a strain of P. aeruginosa was used to produce
rhamnolipid biosurfactant from soybean oilemineral salts medium.
The rhamnolipid product was investigated in terms of its physical/
chemical properties and applicability as a frothing reagent inmineral
flotation. Surface tensionmeasurements showed that rhamnolipid is
of 97.5% purity which can reduce surface tension of water from 72 to
30 mN m�1 with a critical micelle concentration of about 10 mg l�1.
Rhamnolipid showed an almost stable surface activity profile at pH
values less than neutral (from 7 to 2). A more pronounced reduction
in surface activity observed at alkaline pH values which could be
ascribed to the formation of vesicles at these pH values. The case
studies on coal and mineral flotation at different operating condi-
tions showed that the biosurfactant could be successfully applied to
coal and mineral flotation. However, more detailed fundamental
researches are required to support the statementmade about the use
of rhamnolipids for mineral flotation.
Acknowledgements
The authors acknowledge the technical assistance of Esfordi
Phosphate Complex (Yazd, Iran), Zarand Coal Washing
Plant (Kerman, Iran), and Gol-e Gohar Iron Ore Complex (Sirjan,
Iran).
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	Effect of rhamnolipid biosurfactants on performance of coal and mineral flotation
	1 Introduction
	2 Materials and methods
	2.1 Bacterial strain and culture medium
	2.2 Biosurfactant production
	2.3 Physical characterization
	2.4 Structural analysis of crude biosurfactant
	2.5 Surface tension measurement and pH-stability
	2.6 Batch flotation tests
	2.7 Phosphate flotation related analytical measurements
	2.8 Ash content analysis
	2.9 Iron flotation related analytical measurements
	3 Results and discussion
	3.1 Physical properties
	3.2 Structural analysis of crude biosurfactant
	3.3 Surface tension measurement and pH-stability
	3.4 Phosphate ore flotation tests
	3.5 Coal flotation tests
	3.6 Iron concentrates flotation tests
	4 Conclusions
	Acknowledgements
	References