[Thesis]. Manchester, UK: The University of Manchester; 2017.
The threats of global warming attributed to fossil fuel combustion, combined with
increasing energy demands and a growing population, have generated interests in diversifying
the world energy mix. Biofuels from microalgae offer a sustainable renewable option
and do not suffer the sustainability issues associated with early forms of bioenergy.
However, research efforts of nearly 5 decades have not resulted in any significant
gains and have motivated further investigation into novel techniques. The dilute nature
of microalgae suspensions often requires dewatering and drying, which adds to energy
intensity and costs associated with recovery processes. Curiously, the conventional
recovery techniques do not consider the characteristic tendency of microalgae cells
for surface attachment. This behaviour of cells, coupled with the discovery of a non-porous
adsorbent material, NyexTM particles, has brought to the fore an exciting prospect.
This has motivated the underpinning question behind this research; does the non-porous
characteristic of the NyexTM particles presents an opportunity to recover microalgae
cells from suspension using an adsorption technique?Using Chlamydomonas reinhardtii
as a model microalgae strain, preliminary batch studies revealed a rapid recovery
of the cells onto the NyexTM particles; nearly 90% recovery was attained within one
minute, which depends on suspension concentration. At a correlation coefficient, R2
= 0.99, the Freundlich isotherm was found to give a better description of batch systems
than the Langmuir isotherm, which suggests that cell coverage onto the NyexTM particles
may not be a simple monolayer adsorption. Although a low adsorptive capacity of 0.55
mg/g was measured, the equilibrium parameter (1𝑛⁄) of about 0.6 was well within the
range for favourable adsorption (i.e. 0 - 1). Further studies undertaken suggest that
the recovery of cells could be driven by a hydrophobic-hydrophobic interaction, electrostatic
forces of attraction and the flocculating behaviour of the NyexTM particles.Fixed
bed studies showed that the lack of pores led to an early breakthrough. However, findings
demonstrated that unlike most column studies, the bed capacity was a more valuable
parameter to assess the column performance. Unexpectedly, depressed breakthrough curves,
where bed exhaustion never attained Ct/C0 = 1.0, were observed. Nonetheless, the modified
dose response (MDR) model was found to predict the experimental bed capacity to a
greater degree of accuracy than other models. Furthermore, this research exploited
the logistic features of the Bohart-Adams and the Clark models to adapt them to the
experimental data. The adapted models significantly improved the accuracy of predictions
with R2 values > 0.99 for the depressed breakthrough curves.The conductive nature
of NyexTM particles was explored to electrochemically regenerate the adsorbent and
reuse it to recover more cells. A current density of 32 mA.cm-2 was sufficient to
inactivate the cells, regenerate the adsorbent and attain a maximum percentage recovery.
Interestingly, scanning electron micrograph showed that the membranes of the adsorbed
cells were ruptured, during NyexTM regeneration, potentially leading to lipid release.
The maximum lipids extracted into a hexane solvent was estimated as 30 μg/mL at a
current density of 64 mA.cm-2.Overall, the potential to recover microalgae cells onto
a non-porous adsorbent has been demonstrated. The prospect of rupturing membranes
of adsorbed cells offers the opportunity to use this technique to recover microalgae
cells for potential biofuel applications. The results obtained from this research
can serve as the impetus to further exploit this novel procedure. Future work should
consider high lipid producing varieties of microalgae strains, develop a robust protocol
to account for all forms of lipids released and undertake an energy and cost analysis
to develop the technology further.