Dispersed air flotation of Chlorella saccharophila and subsequent extraction of lipids – Effect of supercritical CO₂ extraction parameters and surfactant pretreatment

Highlights

• Lipids were extracted from surfactant-exposed microalgae with supercritical CO₂.

• Fatty acid methyl ester (FAME) content was optimized based on extraction parameters.

• Maximum FAME recoveries obtained at longer extraction times and moisture removal.

• Good yields also obtained without additional drying at shorter extraction times.

Abstract

An environmentally sustainable approach was developed for recovering lipids from Chlorella saccharophila for biodiesel production. This involved harvesting the microalgae via foam-aided dispersed air flotation (DiAF) prior to supercritical CO₂ (SC–CO₂) extraction. Response surface methodology was used to investigate the extraction conditions that would affect the fatty acid composition within the lipids, measured analytically as total fatty acid methyl ester (T-FAME) and biodiesel dominant FAME (BD-FAME) content (% dcw). SC-CO₂ extraction parameters for maximum T-FAME (20.4 ± 1.9% dcw) and BD-FAME (9.8 ± 0.4% dcw) content in the extracted lipids were 90 min, 241 bar, 73 °C and 89% moisture (w/w), and 86 min, 241 bar, 73 °C and 86% moisture (w/w), respectively. Further analysis indicated that good T-FAME and BD-FAME content could be achieved when the extraction time was reduced to 30 min without drying, which may be more industrially favorable, as they would reduce processing expenses. Furthermore, the exposure of biomass to surfactant was found to have a significant effect on the composition of FAME in the extracted lipids. Surfactant exposed biomass, resulted in T-FAME and BD-FAME contents of 5.3 and 2.4% (dcw), respectively, and a longer exposure time resulted in higher contents. Centrifuged biomass had a FAME profile most suited for biodiesel. However, surfactant exposure resulted in higher levels of polyunsaturated fatty acids, which could be recovered as high value-added byproducts, and potentially improve the overall process economics. These findings suggest that the lipid composition can be tailored to suit specific applications by adjusting the extraction parameters and surfactant exposure.

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.