Research paperDispersed air flotation of Chlorella saccharophila and subsequent extraction of lipids – Effect of supercritical CO2 extraction parameters and surfactant pretreatment
Graphical abstract
Introduction
Biodiesel is a renewable fuel source that has the potential to meet the increasing global energy demands for diesel, without the adverse environmental impacts that are associated with fossil fuels [1,2]. Biodiesel can be generated using a number of feedstocks such as palm oil, soybean, corn and coconut, however the lipids from microalgae biomass can also be used, with the advantage that microalgae are capable of producing up to nine times greater oil yields per hectare, compared to other leading crops [2]. Additionally microalgae are considered a nonfood-based feedstock, are capable of recycling CO2 [3,4], and can be used to generate additional value-added products, such as pigments and fatty acids (docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA)) [5].
Currently, only a few microalgae species are used commercially, and this is mainly for pigment production [5]. Examples include astaxanthin from Haematococcus pluvialis [6], β-carotene from Dunaliella salina [7] and lutein from Scenedesmus sp [8]. Products that have high market value, such as those for human nutrition (>$10,000/t USD), can withstand the higher processing costs associated with microalgae, however commodities such as biodiesel cannot withstand these high expenses [9]. The baseline economics of biodiesel production from microalgae (achieving a 10% return) was analyzed by Davis et al. [10], and it was determined that the cost per gallon would be $9.84 and $20.53 USD using open pond and photobioreactors, respectively. At present, the cost of diesel fuel per gallon in Canada is $3.75 USD [11]. Thus, a reduction in processing costs for microalgae-derived biodiesel is necessary to be competitive with fuel prices.
Foam-aided dispersed air flotation (DiAF) is a promising, environmentally sustainable, low energy and low cost harvesting technique for microalgae that can achieve high cell recoveries and concentration ratios; it is simple to operate and is suitable for processing large volumes [12]. Recent studies have investigated the use of surfactants as agents for DiAF to recover microalgae cells from dilute liquid suspensions [[13], [14], [15], [16], [17], [18]]. The surfactant foaming agent facilitates the formation of electrostatic links between the microalgae cells and gas bubbles, so that the cells can be carried by the bubbles to the surface of the liquid suspension chamber, where they are recovered for further processing [19]. Surfactants such as cetyl trimethylammonium bromide (CTAB) can be used [20].
The recovery of lipids from microalgae biomass is a significant challenge for the commercialization of microalgal biodiesel [21,22]. Microalgae oil extraction is most commonly performed using organic solvents or enzyme assisted methods that are either toxic or are energy intensive and inefficient [21,23]. Supercritical CO2 (SC–CO2) extraction is a technique that has emerged in recent years and has been investigated for microalgae oil extraction [21,23,24]. It is regarded as an environmentally benign, “green” technique for lipid recovery with no adverse impacts on the recovered extracts [21,23,25]. An additional advantage associated with using SC-CO2 is that no further processing step is required after extraction, due to its gaseous nature at ambient conditions, leaving only the extract behind [25]. Effective SC-CO2 lipid extraction is dependent on a number of variables such as treatment temperature, pressure, moisture content, time and flow rate [[26], [27], [28], [29]]. The resultant extract consists primarily of triglycerides, free fatty acids, as well as other minor components such as carotenoids, squalene, and sterols [28]. Furthermore, pre-treatment techniques can be applied to disrupt the integrity of the cell wall, thus improving the efficiency of the extraction technique and increasing product recovery [21,30].
The overall goal of this study was to develop an environmentally sustainable approach for recovering lipids from Chlorella saccharophila for biodiesel production using foam-aided dispersed air flotation (DiAF) and SC-CO2 extraction. The first objective was to investigate the fatty acid composition of lipids obtained under different SC-CO2 extraction conditions, measured analytically as total fatty acid methyl ester (T-FAME) and biodiesel dominant FAME (BD-FAME) content (% dry cell weight (dcw)). Here, Response Surface Methodology (RSM) was used to determine the SC-CO2 extraction conditions that would maximize the T-FAME and BD-FAME content from Chlorella saccharophila harvested by foam-aided dispersed air flotation. The second objective was to investigate the effect of surfactant exposure on the microalgae, prior to SC-CO2 extraction, on the yield and composition of the extracted lipids. This was done by comparing three types of biomass: microalgae harvested by centrifugation (i.e. no surfactant exposure), microalgae harvested by DiAF (no additional surfactant pretreatment), and microalgae harvested by DiAF with an additional 24 h of surfactant pretreatment.
Section snippets
Microalgae cultivation
Chlorella saccharophila (ATCC® 30408™, Manassas, VA, USA) was grown in an open pond system [31] for a period of 10 days. A Fitzgerlad medium [32] was used for cultivation, supplemented with 85.7 mg of NH4NO3, 194.3 mg of (NH4)2HPO4, 94.3 mg of NH4SO4 and 1.3 g of sodium bicarbonate per litre of distilled water. A total of 10 L was grown in each batch for experiments. Biomass growth occurred at room temperature and followed a photoperiod of 14 h light, 10 h dark cycle. A total of 9 cultivation
Microalgae Harvesting/pre-extraction conditions
As dispersed air flotation is a harvesting technique that uses surfactant as a processing aid, the microalgae would be exposed to surfactant through this harvesting process. Therefore, to study the effect of surfactant exposure on microalgae, three different harvesting/pre-extraction conditions were investigated: (1) biomass without surfactant treatment; (2) biomass from dispersed air flotation with no hold time and (3) biomass from dispersed air flotation with 24 h hold time. Fig. 2 summarizes
Effect of SC-CO2 extraction conditions on fatty acid composition
Box Behnken experiments were performed on Chlorella saccharophila biomass harvested by DiAF to determine the relationship between the T-FAME and BD-FAME content in lipids and the SC-CO2 extraction parameters (reaction time, pressure, temperature and biomass moisture content). Table 2 summarizes the experimental matrix and both the experimental and model-predicted values obtained from RSM. The experimental results show that the T-FAME and BD-FAME varied from 2.8 to 17.6% and 0.8–8.8%,
Conclusions
An environmentally sustainable approach was developed for recovering lipids from Chlorella saccharophila using foam-aided dispersed air flotation (DiAF) prior to supercritical CO2 (SC–CO2) extraction. Response surface methodology was used to investigate the extraction conditions affecting the fatty acid composition within the lipids. It was found that greater FAME recovery could be achieved using higher temperatures, longer reaction times, lower pressure and higher moisture contents. Although
Acknowledgements
Financial support through the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
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