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Prabuju Vairakannu for their insightful advice and suggestions throughout the research. I would like to thank the former and current head of the Faculty of Energy and Engineering, prof. Lepakshi Barbori, and all the staff members of the Faculty of Energy and Engineering for their constant motivation and support.


Here, a new solid carbon-based acid catalyst was synthesized by carbonization of digested microalgae biomass followed by sulfonation. The dissolved solid acid catalyst based on microalgal biomass (DMB) was mainly composed of carboxylic, phenolic and sulfonic acids.





132 Figure 5.6 Raman spectrum of biochar and DMB catalyst 133 Figure 5.7 XPS analysis of DMB catalyst (the inset is the S2p. spectrum of the DMB catalyst).

Figure 3.5  FESEM  and  microscopic  images  (40x)  of  T.  obliquus  KMC 24 (a, d: cells grown in control medium; b, c, e, f:
Figure 3.5 FESEM and microscopic images (40x) of T. obliquus KMC 24 (a, d: cells grown in control medium; b, c, e, f:

Symbols and Units

1 Introduction

  • Current World Energy Scenario
  • Renewable Energy
  • Biodiesel
    • Microalgae as 3 rd generation biodiesel feedstock
    • Microalgal lipid enhancement strategies
    • Stress-induced reactive oxygen species (ROS) generation and its putative role in lipid accumulation
  • Harvesting
    • Microalgal harvesting strategies and its challenges
    • Flocculation
  • Microalgal Biomass to Biodiesel Conversion
    • Transesterification
    • Catalyst for transesterification
    • Carbon based catalyst

The Government of India has identified Pongamia pinnata (karanja) and Jatropha curcas oils (JCO) as the most promising feedstocks for biodiesel [12]. The emphasis then shifted to third generation raw materials, i.e. micro and macroalgae to minimize the potential shortage of soil and feed. Reaction parameters such as sulfonating agent, sulfonation time, and carbon precursor affect the catalytic activity of the catalyst [82].

Figure 1.1. Energy scenario in the world by (A) sector and (B) source in 2018 (data  source: International Energy agency, 2021)
Figure 1.1. Energy scenario in the world by (A) sector and (B) source in 2018 (data source: International Energy agency, 2021)

Literature Review and Objectives

Microalgae Strain Selection

Cultivation Strategies for Improved Biomass and Lipid Content

In their study, lipid productivity was significantly increased from 31.5 mg L−1d−1 to 71 mg L−1d−1 [98]. In another study, supplementation of a high concentration of NaCl in late log phase increased lipid productivity during two-phase culture of Monoraphidium dybowskii LB50 [99].

Microalgae Biochemical Composition

Carbohydrates also include structural and storage components. Structural components such as pectin, cellulose and sulfated polysaccharides are generally present in the cell wall, while storage components such as starch accumulate inside or outside the chloroplast. Microalgae protein contains a number of different amino acids, such as arginine and leucine. The protein content in microalgae is believed to be proportional to the fat content.

Lipids in Microalgae

In general, microalgae have a total carbohydrate content of about 20% of dry weight [104]. Protein synthesis is the most complex mechanism in all cells.

Environmental Stresses affecting Microalgal Lipid Content and Composition

  • Salinity stress
  • pH stress
  • Temperature stress

It has been reported that an increase in iron concentration resulted in a concomitant increase in the growth rate and lipid content of Botryococcus braunii KMITL 2 [126]. In another study, Praveenkumar et al. 2012) reported that iron deprivation caused a slight decrease in biomass without significant increase in lipid content of Chlorella sp.

Possible Link between ROS Generation and Lipid Accumulation

Physical Properties of Microalgae

Algal cell diameter plays a key role in the settling process, as cell size affects the resistance of algal cells in the medium. Microalgal cells, which contain vesicles with high lipid and gas content, do not settle under the influence of any gravitational force, which increases the complexity of the accumulation process [149].

Microalgae Harvesting Techniques

  • Chemical method
  • Biological methods .1 Autoflocculation
  • Electrical based harvesting techniques
  • Magnetic particle assisted harvesting

During the sedimentation process, gravitational forces cause the suspended algal cells to settle out of the medium of different densities. Studies have shown that for most microalgal species, recovery of microalgal biomass through flotation is relatively rapid compared to sedimentation [158]. Flocculation flotation is more beneficial than sedimentation for microalgal biomass restoration due to the low density of microalgal flocs compared to microalgal cells [180].

In bacteria-mediated flocculation, microalgae flocculate using the extracellular polysaccharides (EPS) and gamma glutamate secreted by bacteria [224].

Biomass as a Precursor for Carbon-Based Catalyst

In the case of surface-functionalized magnetite, there are two strategies for tagging polyelectrolyte, the "attached to" and "immobilized to" strategy. In the "attached-to" approach, the surface of microalgae cells is coated with a polymer binder that aids in bonding with the magnetic particles. In the case of an "immobilized" strategy, the surface of the uncoated magnetic particles is functionalized with a polyelectrolyte that promotes binding to the algal cells [251].

The "immobilized-on" approach based chitosan-Fe3O4 nanoparticle composites was able to harvest 99% of Chlorella sp.

Biomass-Derived Heterogeneous Catalyst in Biodiesel Production

  • Transesterification reaction by heterogeneous base catalysts

In another study, Mansir et al. 2018) achieved a FAME yield of 90% in 3 h using Gallus domesticus shells derived CaO catalyst doped with molybdenum and zirconium (Mo-Zr). Direct conversion of biomass to basic catalyst involves basic oxide synthesis, such as CaO catalyst derived from biomass through calcination. A maximum FAME yield of 91.5% was obtained using rice husk ash-supported CaO catalyst at 7 wt%.

The direct sulfonation method does not require complicated pre-treatment, making the process economical [277].

Development and characterization of a cheap and environmentally friendly catalyst for the transesterification of microalgal lipids into biodiesel.

Process Development and Optimization of Nutrient Starvation for Enhanced Microalgal

Growth and Lipid Accumulation


In this chapter, we isolated a new microalga Tetradesmus obliquus KMC24 and briefly exposed it to nutrient stress (nitrogen and/or phosphorus) by two-step cultivation to obtain maximum biomass and lipids. Effect of nutrient deficiency on morphology, biomass concentration, photosynthetic activity and biochemical composition of Tetradesmus obliquus KMC24 was evaluated. Several recent reports have shown that oxidative stress-resistant microalgae are very effective for biofuel production. The responses of various stress biomarkers such as reactive oxygen species (ROS), cellular enzymatic antioxidants such as catalase (CAT), ascorbate peroxidase (APX), and non-enzymatic scavengers such as polyphenols were studied to study the role of oxidative stress due to nutrient deficiency. also specified.

The influence of nutrient starvation on the fatty acid composition of Tetradesmus obliquus KMC24 and subsequently on the biodiesel quality was also investigated.

Materials and Methods

  • Microalgae isolation and growth conditions
  • Experimental conditions
  • Analytical procedures
  • Statistical analysis

The DCW was calculated by the difference in weight of the blank filter paper and the filter paper loaded with the microalgae culture and expressed in g L-1. The neutral lipid intensity in the microalgal cells was determined with a minor modification of the method described by Anand et al. A 1 ml aliquot of the supernatant was mixed with 1 ml of thiobarbituric acid (TBA) solution containing 20.0% (w/v) trichloroacetic acid (TCA), 0.01% butylated hydroxytoluene and 0.65% TBA.

A 1 ml aliquot was mixed with 0.5 ml of 1 N Folin-Ciocalteu reagent in a test tube and incubated for 3 minutes.

Results and Discussion

  • Isolation and identification of microalgal strains
  • Influence of nutrient starvation on biomass production
  • Morphological changes due to nutrient starvation
  • Microalgal lipid composition
  • Cell viability

A significant difference in carbohydrate content was not found when the duration of nitrogen starvation was increased for more than two days. The decrease in protein content under phosphorus starvation may be due to the limitation in the biosynthesis of RNA and ATP. Carbohydrate content in all four phosphorus-starved cultures was significantly higher (P < 0.05) than the control and was highest in the first and second. the day of hunger. respectively on the first day of starvation -N-P, which continued to decrease in the following days.

A gradual decrease in fluorescence intensity with increasing duration of -N-P starvation was observed.

Figure 3.11. The correlation between lipid content and ROS level of T. obliquus KMC24  under various nutrient-starved conditions
Figure 3.11. The correlation between lipid content and ROS level of T. obliquus KMC24 under various nutrient-starved conditions


Development of a Sustainable and Efficient Harvesting Technique


Food waste is common solid waste generated by households and the food industry in thousands of tons annually. To manage these wastes, researchers have tried to valorize them as adsorbents, catalysts, biomuds, etc. Recently, some researchers have promoted the concept of green chemistry and circular bioeconomy by valorizing bioflocculant derived from eggshells [55,333].

Eggshells are calcium-rich solid wastes with high cation density and multiple functional groups such as –OH, -C=O and –PO4 [334].

Materials and Methods .1 Microalgae as an adsorbent

  • Eggshell-derived bioflocculant as an adsorbate
  • Optimization of bioflocculant concentration, pH, and temperature
  • Harvesting efficiency of microalgal biomass
  • Analytical methods
  • Determination of adsorption kinetics
  • Determination of adsorption thermodynamics
  • Recycling of harvested medium

These eggshell characteristics facilitate the adsorption and destabilization of the negatively charged microalgae cells. The effect of temperature on harvesting efficiency was also studied by varying the temperature (25°C, 35°C, 45°C, 55°C) of the culture medium. The morphology of the samples was determined by FESEM (Zeiss Sigma-300 Field Emission Scanning Electron Microscopy).

The spontaneity of adsorption of eggshell-derived bioflocculant onto microalgae cells was estimated using the Van't Hoff equation.

Results and Discussion

  • Microalgal growth
  • Effect of bioflocculant concentration
  • Effect of pH value
  • Effect of temperature
  • Microscopic, FESEM-EDX and flame photometer analysis
  • Kinetic studies
  • Growth of T. obliquus KMC24 in the recycled medium
  • Biomass, lipid content and FAME analysis
  • Eggshell-derived bioflocculant - A better alternative

The inoculum concentration was adjusted to have similar initial cell densities in the recycled culture medium for growth studies. The flocculation efficiency of the eggshell-derived bioflocculant at different pH values ​​was further elucidated based on the zeta potential value of the culture medium (Figure 4.2b). From the above results, it can be concluded that the pH of the culture medium plays an important role in the harvest of T.

The rate of a chemical reaction can be determined from the activation energy of the system.

Figure 4.1. (a) Harvesting efficiency of T. obliquus KMC24 at various bioflocculant  concentrations; (b) Relationship between bioflocculant concentration and zeta potential
Figure 4.1. (a) Harvesting efficiency of T. obliquus KMC24 at various bioflocculant concentrations; (b) Relationship between bioflocculant concentration and zeta potential


Development and Characterization of Low-cost and Eco-friendly Catalyst for Microalgal Lipid

Transesterification to Biodiesel


Biodiesel production from microalgal biomass generates a significant amount of deoiled microalgal biomass as a waste product [362]. Therefore, in the present study, an integrated biorefinery concept was attempted by valorizing deoiled microalgal biomass as a solid acid catalyst for the transesterification of microalgal oil. The deoiled microalgae biomass-based (DMB) solid acid catalyst was developed by carbonization of the organic compounds followed by sulfonation.

The catalyst obtained from the digested biomass was further characterized in order to evaluate its physicochemical properties.

Materials and Methods

  • Materials
  • De-oiling of microalgal biomass
  • Synthesis of DMB catalyst
  • Characterization
  • Reaction study
  • Catalyst reusability study
  • Analysis of FAME and biodiesel properties

The structural analysis of the catalyst was determined by X-ray diffraction (XRD) technique using X-ray diffractometer (TTRAX III 18 kW; Rigaku Corporation, Japan) with Cu Kα radiation (λ = 1.5406 Å). The total acid density of the catalyst was determined by the titration method as reported in the literature [78]. Subsequently, desorption of the physisorbed NH3 was carried out from ambient temperature to 800 °C at a heating rate of 10 °C min-1 with a flow of He (30 ml min-1). The desorbed NH 3 was detected using a TCD detector.

At the end of the esterification process, the catalyst was recovered via centrifugation and the unreacted methanol was evaporated.

Results and Discussion

  • Optimization of catalyst synthesis conditions
  • Characterization of DMB catalyst .1 XRD analysis
  • Optimization of FAME yield
  • Reusability study
  • FAME composition analysis
  • Biodiesel properties
  • Comparative study of DMB catalyst with other bio-based solid acid catalyst The catalytic activity of the DMB catalyst for FAME yield was compared with other

The catalytic efficiency of the DMB catalyst was compared to the conventional acid-base catalyst. The water contact angle of the DMB catalyst was found to be 0°, indicating the hydrophilicity of the catalyst. Thus, the catalytic activity of the DMB catalyst was comparable to the conventional acid-base catalyst.

The gradual decrease in the catalytic activity of the DMB catalyst after the fourth cycle was probably due to the leaching of –SO3H.

Figure 5.1. Effect of (a) pyrolysis temperature, (b) sulfonation time, and (c) H 2 SO 4
Figure 5.1. Effect of (a) pyrolysis temperature, (b) sulfonation time, and (c) H 2 SO 4


Overall Conclusion and Future Scope


In the next part of the study, bioflocculant from waste eggs was successfully applied for T harvesting. In the final phase of the current research, a sustainable catalyst was developed using deoiled microalgae biomass as a carbon support. Hydrophobic carbon support suppressed the leaching of -OH and -SO3H groups. Thus, the high catalytic efficiency of the DMB catalyst for the transesterification reaction process signified the potential for sustainable biodiesel production.

Recyclability of the used medium will certainly contribute to lowering the water footprint of the microalgae biodiesel production system.

Future scope

Furthermore, the conversion of microalgal lipid to biodiesel and de-oiled microalgal biomass to catalyst will help maximize the economic value of microalgae within a biorefinery concept, leading to a circular bioeconomy for the production of biodiesel from microalgae.

Ishikawa, Determination of optimal microalgae cultivation strategy for biodiesel production using flow cytometric monitoring and mathematical modeling, Biomass and Bioenergy. Forde, Microalgal culture dewatering for biodiesel production: investigation of polymer flocculation and tangential flow filtration, J. Díaz, Recent development of heterogeneous catalysts for biodiesel production by oil esterification and transesterification reactions: a review, Renew.

Lee, Synthesis of biomass as a heterogeneous catalyst for use in biodiesel production: state of the art and fundamental review, Renew.


Table 5.3 FAME  composition  of  AO  using  DMB  catalyst  and  its  comparison with conventional acid-base catalyst
Figure No.  Figure Caption  Page No.
Figure 3.5  FESEM  and  microscopic  images  (40x)  of  T.  obliquus  KMC 24 (a, d: cells grown in control medium; b, c, e, f:
Figure 4.7  Growth of T. obliquus KMC24 in the recycled medium  116  Figure 4.8  Biomass  concentration  and  lipid  content  of  T


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