Ultrasonication and Microwave Assisted Lipids Extraction Process (MALE) for Microalgal
Syed Hasnain Shah and Ifikhar A Raja*
Department of Environmental Sciences, COMSATS Institute of Information Technology, Pakistan
Submission: December 25, 2017; Published: March 20, 2018
*Corresponding author: Ifikhar A Raja, Department of Environmental Sciences, COMSATS Institute of Information Technology, Abbottabad, Pakistan.
How to cite this article: Syed Hasnain Shah, Ifikhar A Raja. Ultrasonication and Microwave Assisted Lipids Extraction Process (MALE) for Microalgal
Biodiesel Production. Adv Biotech & Micro. 2018; 8(5): 555746. DOI: 10.19080/AIBM.2018.08.555746
The world fuel demand is mainly supplied by fossil-based fuels . Over exploitation and use of fossil fuels raised serious issues, energy shortage and their detrimental effects on the environment. To overcome these problems there is immense need to replace fossil by renewable energy sources [2,3]. Biomass (organic materials) provides an effective alternate energy source for the production of biofuels (biogas, bioethanol, biodiesel) [4-6]. Among these biodiesel is considered as one of the promising alternative fuel for diesel engines due its renewability and environment friendly nature . Biodiesel can be produced from food crops, edible, non-edible vegetable oils (from soybeans or oil palm), macro and microalgae and aquatic weeds [8,9]. However, the use of food crop for energy production is in competition with food consumption and limited availability of agricultural land. Microalgal lipid emerged as potential source for biodiesel compared to other bioresources . It has no competition with food crop or arable land. Further it has the advantage of adaptability to different environments, high biomass accumulation and lipid yield up to 80% of the dry mass .
Microalgal lipids are used as feed stock for biodiesel production and are derived from oleaginous microalgae [11,12]. For efficient utilization of microalgae for biodiesel production it is important to employ effective and reliable methods for lipid extraction from the microalgal biomass . The production process involves several stages such microalgae cultivation, biomass harvest, treatment and lipid extraction, finally biodiesel synthesis. Ultrasound and microwave techniques are used for lipid extraction. Both physical and chemical process are employed for effective lipids recovery from microalgal biomass and it conversion into biodiesel . Microwave assisted processes are more effective than ultrasound techniques. Different processes.
Ultrasound is an effective method for microalgae cells disruption at lower temperatures as compared with autoclave, thermal and microwave methods. It is preferred as it does not require any addition of chemicals or a bead, therefore processing costs is lower in this method. Based on the suitability it is an effective method for homogenization and cells lysis in aqueous phase . During the process of ultrasonication the sonic waves are generated and transferred to microalgal intact cells. These waves are strong enough for multiple productions of microbubbles in nucleation points in the liquid medium. These waves create a series of microbubbles (cavitation phenomenon), which then burst and release kinetic energy to the cellular surface and finally the cell wall becomes ruptured and lipids are then released off the cell to the exocellular medium .
During the rarefying phase of the sonic waves the microbubbles grow and then collapse in compression phase. When they collapse in strong shock waves they pass through liquid medium. The whole process of microbubbles formation, rarefying and compression is called cavitation. The collapse of these bubbles converts the sonic energy into mechanical energy that is about to 300MPa which is equivalent to several thousand atmospheric pressure. This imparts enough energy for lysis that exceed from the mechanical strength of cell wall. A second factor called shear stress caused by high velocity gradient and vibrating bubbles in ultrasonic process, also increase the extent of cell lysis . Gerde and his team have investigated the effect of ultrasonic waves on the cellular integrity and lipids recovery from heterotrophic and autotrophic microalgae i.e. Schizochytrium limacinum and Chlamydomonas reinhardtii respectively . Cell disruption efficiency was determined by the release of intracellular materials lipids were stained by Nile red fluorescence. Maximum cell disruption was achieved at energy input of 800J/10mL Gerde & Araujo  have reported
five methods of lipids extraction from Chlorella vulgaris in
combination with ultrasonication [18,19]. The results were quite
promising and in first method they have recovered 52.5% (wt/
wt) lipids without addition of silica. According to their findings,
ultrasonication was effective for extracting lipids in integration
with Bligh and Dyer method .
Microwave heating is caused by the ability of the materials
to absorb microwave energy and convert it to heat. There are
different factors which affect the effectiveness of microwave
assisted lipids extraction process (MALE) for cell lysis and lipids
extraction i.e. microwave power, temperature, treatment time,
input energy consumption (kJ), solid–liquid ratio, dielectric
properties, and the nature and concentration of organic solvent
used for lipids extraction . The mechanism for microalgal
cell disruption by microwave process could be explained
based on nature of microwaves and substances present in the
substrate (biomass). Microwave heating of substances (biomass)
occurs due to dipolar and ionic mechanisms. When substrate is
subjected to electromagnetic waves (microwave) the presence
of moisture or water causes dielectric heating due to dipolar
nature of water. Interaction between compounds and incoming
microwaves moisture or water. The permanently polarized
dipolar molecules try to realign in the direction of electric field
in the oscillating electric field incident on water molecules .
At high frequencies (MHz-GHz) the intra-molecular friction
caused by the oscillations/vibrations of polar molecules raises
the number of charged ions result a rapid heating within fraction
of seconds in substrate compounds. Subsequent heating in
intracellular matrix result in the rupture of cell membrane and
release of cellular contents .
According to the findings of Balasubramanian et al.  a
continuous microwave process could potentially be used for
microalgal cells disruption and lipids extraction. According
to their results and findings 77% (wt/wt) lipids recovery was
achieved at higher microwave frequencies and temperature at
95 ◦C for 30 minutes from Scenedesmus obliquus. Temperature
and contact time were the main factors influencing the lipids
yield. From lipids analysis, the presence of unsaturated fatty
acids confirmed the suitability if microwave process for
biodiesel synthesis. Based on their results it is suggested
that microwave process is equally effective for a number of
microalgae strains to extract lipids for biofuels synthesis .
According to the research findings from fractal characterization
and dynamic microstructures of cell wall disruption, the
microwave processing of wet microalgae biomass was found
to effective for lipids extraction in aqueous phase. In another
study the effectiveness and efficiency of microwave processing
for lipids extraction from wet biomass of Chlorella Py-Zu1,
have been investigated Cheng et al. . They have used
microwave frequency at higher temperature (80-120 °C) for 20
to 26 minutes which caused an increased cell fractal dimensions.
About 19% (wt/wt) lipids recovery was achieved in wet phase as
compared to conventional method (20% recovery). The content
of free fatty acids was increased from 88.65% to 91.95% due to
molecular bond vibrations, electromagnetic effect and breakage
by heat which result the fragmentation of high molecular weight
fatty acids . However biofuels are not yet commercially
competetive with fossil fuels.