Environmental Engineering Reference
In-Depth Information
with or without cell disruption (Viswanath et al.
2010
). To identify and quantify microalgal lipids,
other analytical methods such as thin layer
chromatography (TLC), high-performance liquid
chromatography (HPLC), and gas- chromatography
mass-spectrometry (GC-MS) are used (Elsey et al.
2007
). The choice of method depends on the effi -
ciency, accuracy, and cost effectiveness, ease of
use, high throughput capability, robustness, exact-
ness, and reproducibility (Viswanath et al.
2010
).
Screening of a large number of lipid-producing
microalgal samples under laboratory conditions is
diffi cult and time consuming. Hence, to overcome
this problem, attention is increasingly focused on
in situ measurement of lipid content, such as Nile
Red-NR staining (Fig.
1e
), time-domain nuclear
magnetic resonance (TD-NMR), and boron-
dipyrromethene (Bodipy) (Mutanda et al.
2011
).
due to a high energy requirement (Halim et al
.
2011
). Ethanol, octonol, or 1,8-diazabicyclo-
(5.4.0)-undec-7-ene (DBU) yield lower FAME
than n-hexane extraction (Samorì et al
.
2010
).
The advantage of using solvents for lipid extrac-
tion is that they are inexpensive and very effi cient
for oil extraction. For the separation of other
valuable products, such as beta-carotene,
astaxanthin, and other essential fatty acids, from
microalgae, solvent extraction is used extensively
(Grima et al.
2003
).
13.3
Supercritical CO
2
Extraction
Supercritical fl uid extraction is one of the most
effective methods for the extraction of oil from
microalgae (Mendes et al.
1995
). CO
2
is fi rst
heated and compressed until it reaches liquid-gas
state or above its critical point. Harvested micro-
algae are added to act as a solvent. Limited meth-
ods have been studied for the recovery of lipids
from microalgae and to transform them to bio-
diesel (Halim et al.
2011
). Certain studies recov-
ered lipid content of up to 26 % (g lipid/g dry
weight) from
Nannocloropsis
sp. (Andrich et al.
2005
). Halim et al. (
2011
) used supercritical CO
2
extraction at 60 °C, and pressure of 30 MPa, to
extract lipids from
Chlorococcum
sp. and
obtained higher lipid contents of 5.8 % g lipid/g
dry weight, whereas using hexane Soxhlet extrac-
tion obtained only 3.2 % g lipid/g dry weight.
Using wet algae obtained a maximum lipid yield
of 7.1 % g lipid/g dry weight, which was lower
than other
Botryococcus
species, as the lipid is
high 28.6 % g lipid/g dry weight (Lee et al.
2010
).
This indicates that the supercritical CO
2
lipid
extraction was enhanced in the presence of water
with the blend of microalgae. The main advan-
tage of this method is that it is not toxic, is easy
to recover, is usable at low temperatures, and is
relatively rapid because of the low viscosities and
high diffusivities (Andrich et al.
2005
). The dis-
advantage is that it requires expensive equipment
(Perrut
2000
) and a huge amount of energy to
reach high pressure (Tan and Lee
2011
).
13.1
Extraction Technique
In order to produce biodiesel from microalgae,
several methods can be used for the extraction of
biofuel and high-value products. The important
lipid-extraction techniques are chemical solvents,
supercritical CO
2
, physico-chemical, biochemi-
cal and direct trans-esterifi cation.
13.2
Solvent Extraction
Microalgal oil can be extracted using chemicals
such as n-hexane, chloroform, benzene, diethyl
ether, and ethanol. In large-scale studies, Yaguchi
et al
.
(
1997
) used choloroform-methanol blends
for the extraction of lipid and obtained high lipid
yields of up to 83 % (g lipid/g dry weight).
n-Hexane is the most commonly used solvent for
the extraction of lipids due to its lower toxicity
and its affi nity for non-lipid contaminants (Halim
et al
.
2011
). For example, Miao and Wu (
2006
)
used hexane as a solvent for the extraction of lip-
ids from
Ch. Protothecoides
; the yield of lipid
contents was up to 55 % (g lipid/g dry weight).
Soxhlet extraction is not used at industrial scales
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