Evaluation of Dunaliella salina microalgae for biodiesel production a case study (Botswana Ash Company)

Production of biodiesel from microalgae has received worldwide recognition as an alternative to petroleum diesel. The process involves culturing, harvesting, drying, extraction, transesterification and characterisation of the microalgae biomass to biodiesel. The research aimed to investigate the potential of Dunaliella salina microalgae inhabiting in Botash Company’s solar ponds Botswana for biodiesel production. Optimum salinity and nutrients concentrations that produced maximum biomass and lipids were investigated. Furthermore, the lipids and the biodiesel produced were identified, quantified and characterised.

D. salina microalgae were isolated from the solar ponds and cultured in f/2 media under optimum conditions. One Factor at A Time (OFAT) design was used to determine the optimum salinity for the growth and lipid accumulation of the microalgae. A salinity concentration of 2.5 moles produced the highest biomass concentration of 2480 mg L-1, biomass productivity of 58.40 mgL-1d-1, lipid content of 22.65 % and lipid productivity of 14.43 mgL-1d-1. In regard to nutrient optimisation, Plackett-Burman and Box-Behnken designs of response surface methodology (RSM) were used to optimise the culture media by selecting significant nutrients and their concentrations for culturing D. salina and promotion of lipids accumulation. In the first stage, Plackett-Burman method identified NaHCO3, NaH2PO4.2H2O and NaNO3 as factors that significantly influenced the biomass concentration, lipid content and lipid production. MgSO4.7H2O and micronutrients solution had a significance effect on the lipid content. The relationship on biomass concentration, lipid content and lipid production were predicted using a model that had an R2 value of 99.3% with a P-value of 0.000. In the second stage using Box-Behnken design, the nutrients selected in the first stage were further optimised and a predicted model was generated with an R2 value of 95.8%. The optimum medium that constituted 1302.4 mgL-1 NaHCO3, 22.5 mgL-1 NaH2PO4.2H2O, 181.1 mgL-1 NaNO3, 44.7 mgL-1 CaCl2, 2355.0 mgL-1 MgSO4.7H2O, 5.6 mlL-1 of micronutrients solution produced maximum lipid production of 181.1 mgL-1, which is 12.3% more than the 127.0 mgL-1 produced using the un-optimised medium.

After approximately 36 days of culturing the cells were harvested and used to extract lipids by solvent extraction method, Thin Layer Chromatography (TLC) as a screening and the extracted lipids were approximated to linoleic acid, oleic acid, and some triglycerides. Biodiesel was produced from the extracted lipids using alkali transesterification process. Fourier Transform Infrared (FTIR) identified chemical bonds in biodiesel with a strong peak at 1743 - 1742 cm-1 wavelength as triglycerides ester carbonyl functional group. Furthermore, the D. salina biodiesel’s chemical composition was quantified using Gas Chromatography (GC) and Mass Spectrometry (MS) spectrum that verified the presence of monounsaturated methyl esters.

The physiochemical characterisation of D. salina biodiesel was in the recommended specification of the EN 14214, ASTM D6751 and SANS 1935:2004 standards indicating a good quality of the fuel. The D. salina biodiesel when compared to sunflower biodiesel and petroleum diesel had the highest ignition temperature, burnout temperature, ignition index and also the least comprehensive performance reflecting good thermal properties of the fuel. The thermal kinetics of D. salina biodiesel show a high pre-exponential factor (A) and Gibbs energy (∆G) but low activation energy (E) and entropy (∆S). The enthalpy (∆H) and higher heating value (HHV) were comparable to the ones for sunflower biodiesel and petroleum diesel. Based on the results of physio-chemical and thermal characterisation, the D. salina biodiesel could be used in boilers such as the ones in Botash Company for ignition and steam production purposes. In conclusion, the study was able to enhance lipids production during culturing stage, produce high biodiesel through an effective selection of the best catalyst during the transesterification stage, identified and characterise the biodiesel.