RETRACTED ARTICLE: Phycoremediation of automobile exhaust gases using green microalgae: a twofold advantage for pollutant removal and concurrent biomass/lipid yields

Abstract

In the present study, an effort has been made to sequester automobile exhaust gases from a petroleum based engine using microalgae Scenedesmus quadricauda. The automobile selected for the present study was a two wheeler motorcycle. S. quadricauda was subjected to un-suspended/attached non-enclosure cultivation onto a silicone matrix coated stainless steel pipe, protracted to the motorcycle silencer outlet through which, the exhaust gases passed out. The automobile exhaust gases initially contained 13% of carbon dioxide, 2.4% of carbon monoxide, 0.2% of nitric oxide, 0.08% of nitrogen dioxide, 0.1% of sulfur dioxide and 0.05% of sulfur trioxide concentrations, which were removed up to 62, 57, 65, 66, 64 and 65% respectively by S. quadricauda from the attached growth experiment. The present study results showed that the exhaust gases played a crucial role in inducing biomass and lipid yields up to ≥2 and ≥ 0.6 g L^− 1 respectively. The tolerance of S. quadricauda to the exhaust gas components’ toxicity and its efficiency in treating them followed by the biomass and lipid productivities were better than anticipated in the present study.

Introduction

The global climate change, increasing pollution, deteriorating environment, land and water degradation have encouraged governments, policy makers, scientists and researchers in finding ways to develop mitigation through Phycoremediation technologies (i.e., sequestration through microalgae). Increasing carbon dioxide content in the atmosphere has become a universal problem as it was deliberated to be the main cause of global warming. Currently, the transportation and energy sectors are considered to be the major anthropogenic sources, responsible for more than 20 and 60% of greenhouse gases emissions [1]. Roughly the oceans can absorb up to one-third of the CO₂ emitted each year through human activities [1]. According to the Intergovernmental Panel on Climate Change full report [2], the CO₂ concentration in the air was up to 270 ppm in the nineteenth century at the time of Industrial Revolution. By the year 2000, it has increased up to 350 ppm and by 2015 it reached 400 ppm as a result of over exploitation of fossil fuels. Generally, 31% of CO₂ comes from thermal power plants, 17% from transportation sector, especially from automobiles and 12% from public utility apart from electrical energy. The global average temperature would rise by 1.5–3.0 °C by the year 2030 if the CO₂ emissions continue to increase at the present pace [2].

According to air pollution studies, there are certain constituents that come from the automobile exhaust gases which react with the atmosphere forming smog type pollutants. Henceforth, substantial efforts have been made to determine these exhaust gas compositions, laying specific emphasis on trace constituents, such as hydrocarbons and oxides of nitrogen. In general, the exhaust gases are emitted through combustion processes. The composition of the exhaust gases released from a typical petroleum or diesel-based engine contains nitrogen, oxygen, water vapor, carbon dioxide, carbon monoxide, oxides of nitrogen (NOx – NO and NO₂), oxides of sulfur (SOx - SO₂, and SO₃), lead, hydrocarbons (HC) and particulate matter (PM) [3].

Oxides of nitrogen and oxides of sulfur that have been linked to acid rain [4], usually released through the internal combustion processes in the petroleum engines. Though few components of these exhaust gases are harmless, there are few others that are harmful and are considered as major pollutants. One of the most hazardous gases is CO, which has the potential to kill people and animals if its concentrations are high enough. The nitrogen emitted from a petrol engine usually contributes 71% of the total exhaust gas, while CO₂ being 14%, H₂O 13% and the other components on an average contribute 1–2% approximately. Whereas, the diesel engine emits 67% of N₂, 12% CO₂, 11% H₂O, 10% O₂ and the rest of the components up to 0.3% [3, 5].

A worldwide acknowledged natural CO₂ sequestration process is the biological CO₂ fixation by photosynthetic microalgae which have about 10–50 times greater CO₂ fixation efficiencies when compared to those of higher plants [6]. Microalgae have simple growing requirements that include light, sugars, CO₂, nitrogen, phosphorous and potassium to produce lipids in larger amounts over shorter periods of time [7]. Hence, the microalgae biomass can be used as feedstock for a variety of biofuels [8, 9]. Microalgae can typically be used to capture CO₂ and other toxic elements from various sources [10, 11]. The screening of appropriate microalgae strains for CO₂ mitigation has a substantial effect on the efficiency and cost affordability of the entire bio-mitigation process.

There have been numerous studies [11, 12] since a decade on the sequestration of industrial flue gases containing toxic components, especially carbon dioxide, nitrogen oxides and sulfur oxides. Particularly the post-combustion capturing has gained lot of interest due to its flexibility and low operational costs compared to pre-combustion methods and oxy-fuel combustion methods [12]. Even today, much emphasis is being laid on industrial emissions and flue gas mitigation technologies but there are only a limited number of studies on treating automobile exhaust gases and their toxicity. Recently few technologies emerged involving high speed electrons, accelerated by a strong electric field generated in the thin discharge gap of an automobile engine to dissociate and ionize the exhaust gases into molecules [13]. Another development is the use of plasma-assisted catalyst technology to reduce diesel exhausts [14]. Biological treatments included bio-catalytic converters containing chambers for algae through which the engine exhaust gases pass and are sequestered [15]. Another recent study focused on Chlorella sorokiniana to produce algal slurry-diesel emulsions using surfactant pack made of butanol, CTAB and span80 to reduce diesel engine emissions especially NOx [16]. Likewise, the scrubbing of industrial flue gases also made use of different species of microalgae inoculated into a liquid medium through which the flue gases pass and get sequestered to some extent [17,18,19,20]. However, there has been no study in particular which discusses microalgae-based cleansing of automobile exhaust gases without overhead and energy demanding till now.

Recently an attractive option of cultivating microalgae on surfaces as un-suspended or attached growth has been given utmost importance due to the promising results obtained. When compared to conventional methods of cultivation, the attached systems have been reported to offering notable biomass yields, good light distribution, effortless economical scale up and effective harvesting processes with minimal water use and contamination issues [21]. There are again two options in this un-suspended or attached growth which are, enclosure (microalgae enclosed into matrix) and non-enclosure (microalgae biofilm onto the surface) attachments [22]. However, the non-enclosure attached growth has gained more attention comparatively, due to the drawbacks in the enclosure method.

Until now there were only few studies, reported on this type of approach. In this attached mode of growth, there would be dense accumulation of microalgae inside the reactors. In non-suspended/attached non-enclosure method of cultivation, it is much easier to harvest fully grown microalgae biomass from the medium especially on small area just by scratching off and drying [23, 24]. Meanwhile, there was no study reported on attached non-enclosure cultivation of microalgae in dry conditions with direct exposure to flue gases without liquid medium. Specifically, it was nowhere reported on the sequestration of vehicular exhaust gases through un-suspended cultivation of algae though very few suspended cultivation studies were reported. The interest towards un-suspended/attached non-enclosure growth of algae in dry conditions with periodical exposure to the liquid/nutrient medium for onsite sequestration in the present study was actually inspired from epiphytic lichens (algae-fungi associations), which directly come in contact with the constituents of air and uptake significant nutrients through gaseous absorption onto their entire surface areas.

Within this context, in the present study, S. quadricauda was cultivated as un-suspended attached non-enclosure culture onto a stainless steel pipe with the help of a silicone matrix, where it comes in direct contact with the outlet exhaust gases released from the automobile engine. The aim of this work was to evaluate the mitigation potential of Scenedesmus quadricauda as well as its biomass yielding capacity, assessing the influence of exhaust gases on the entire process. This indorsed to a quantitative evaluation of the benefits over coupling biomass production to remediation.

Methodology

Sampling

Microalgae culture

The micoalga S. quadricauda was obtained from the mother cultures maintained as quadrant streak plates in a freezer from the previous studies [25]. The isolated colonies were looped out and sub-cultured into freshly prepared Bold’s basal medium (BBM from Sigma-Aldrich) in a 500 mL Erlenmeyer flask with 300 mL working volume. The culture was maintained at 30 °C temperature in a laboratory culture rack under fluorescent lights illuminating 99 μmol m^− 2 s^− 1 light intensity with a photoperiod of 12:12 Day/Light. Initially the culture pH was found to range between 7.2–7.5 and after 24 h it started varying with algal biological activity as the experiment progressed.

Exhaust gas collection

The motorcycle exhaust gas was collected into Tedlar bags through gas sampling [26] by connecting one end of a 100 cm long Polytetrafluoroethylene (PTFE) Teflon® tube to the vehicle silencer outlet. The other end of the tube was connected to the inlet of a vacuum pump. The vacuum pump outlet was further connected to a 5 L capacity Tedlar bag with a PVC tube. The motorcycle engine was ignited and run for about 10 min after which, the gas was collected. These collected samples were proceeded for analyses using gas chromatography (GC).

Experimental design

Motorcycle engine specifications

In the present research work, a petrol engine based two-wheeler motorcycle was selected as the exhaust gas source. The engine specifications of the motorcycle were as follows: Engine type DTSI, 4-stroke; Displacement-149 cc; Air cooling; Maximum power-15.06 @ 9000 (Ps @ RPM); Maximum torque-12.5 @ 6500 (Nm @ RPM); No. of cylinders-01; 103.47 BHP t^− 1 power to weight Ratio; 86.80 NM t^− 1 torque to weight ratio; Specific output-100 BHP L^− 1.

Unconventional attached growth cultivation setup

The basic design of the motorcycle silencer was slightly altered in order to carry out the experiments planned in the present study. The outlet of the silencer pipe with inner diameter 3.5 cm, was extended by fitting it with a fabricated stainless steel (SS) pipe of length 20 cm with inner diameter 4 cm (Fig. 1). In point of fact, the pipe was initially a 2 mm thick SS sheet which was coated with silicone matrix available in the market. A thin layer of silicone paste was evenly spread all over the SS sheet using a glass rod. The silicone coated SS sheet was let dry slowly in a hot air oven at 30 °C for 24 h. The 30 °C oven temperature was due to the fact that, higher temperatures would disturb the solidification property. After solidification, the silicone coated SS sheet was weighed. Then the sheet was rolled into a pipe and secured intact with the help of Teflon tapes.

Fig. 1

Schematic representation of the silicone coated SS pipe extension to the motor vehicle silencer

In order to verify the exact volume of liquid medium this pipe could accommodate, a simple experiment was carried out. The empty SS pipe was closed on one edge with the help of a Teflon sheet and temporary glue. Then the pipe was filled with distilled water until top. Then, the water from the SS pipe was measured using a 1000 mL measuring cylinder which showed the amount of water that occupied the pipe to be 250 ± 5 mL. Thus, the volume of this silicone coated SS pipe was practically found to be 0.25 L capacity. Then the silicone coated SS pipe is fixed to the automobile silencer outlet.

Cultivation of microalgae with exhaust gases

S. quadricauda culture was inoculated into two 1 L Erlenmeyer flasks each containing 800 mL BBM medium working volume. The culture flasks were maintained at 27 °C temperature and 99 μmol m^− 2 s^− 1 light intensity in a culture rack. After 36 h, the cultures turned to pale green indicating the growth of microalgae. Optical density (OD) readings were taken on a daily basis using a spectrophotometer (Shimadzu, 2450) at 680 nm to know the growth rate and growth pattern (curve) of S. quadricauda in both the flasks. After the 36-h acclimatization, experiments were planned as described below.

Onsite automobile attached growth testing

The silicone coated SS pipe was immersed into the 1 L flask at aseptic conditions and left for 12–24 h to let the microalgae cells get attached onto the silicone matrix. After 24 h, the SS pipe was taken out from the flask and dried under light conditions in a laminar air flow. The outer lining of the SS pipe was cleaned and fixed to the motorcycle silencer. The motorcycle engine was ignited and left for 1 h with 5 min of acceleration at four intervals (once in every 15 min). The experiment was planned for 10 h with alternative1 h of consecutive acceleration for 5 min at four-time intervals and 1 h of engine idle with no acceleration. Thus, the exhaust gases from the silencer passed through the SS pipe containing attached microalgae (Fig. 2). Further the microalgae interacted exhaust gas was collected into a 10 L Tedlar bag through polyurethane piping, connected to the SS pipeline outlet. The collected gas samples were subjected to GC analyses.

Fig. 2

Interpretation of the extended SS pipe interior

After completion of the 10 h experiment, the SS pipe was detached from the silencer pipe and placed (after cleaning the outer layer of the SS pipe) into a fresh I L Erlenmeyer culture flask containing growth medium for overnight to uptake nutrients (Fig. 3). The culture flask containing nutrients was supplied with air bubbling and the head space gases were monitored regularly for any traces of CO₂, CO, NO, NO₂, SO₂ and SO₃. These steps were followed as a daily routine until the microalgae growth reached late stationary phase, i.e., the OD curve showed decline and the green color perished in the flask as well as in the SS pipe lining.

Fig. 3

Representing the cultivation of S. quadricauda (a) Nutrient aid for attached growth experiment (b) Exhaust gas experimental setup in the laboratory

Laboratory experiment with exhaust gases

The exhaust gases were fed to a freshly inoculated microalgae culture medium through bubbling at 25 mL min^− 1 flow rate (0.1 vvm). A 10 m long PTFE Teflon® tube was used to connect the silencer to the laboratory flask in the culture rack. Just like in the above experiment, the exhaust gas bubbling was supplied for 10 h (Fig. 3). The head space gases were collected into Tedlar bags and monitored regularly for evaluating CO_2, CO, NO, NO₂, SO₂ and SO₃ percentages through GC. The results were compared to understand the feasibility quotient of the two experiments.

Analysis

Gaseous concentrations analyses (initial and final)

The motorcycle exhaust gas collected in Tedlar bags were analyzed in the laboratory for the percentage compositions of CO₂, CO, NO, NO₂, SO₂ and SO₃, using GC (GC-4890, Agilent Technologies, USA) equipped with a thermal conductivity detector and DB-XLB capillary column of 30 m (length) × 0.25 mm (inner diameter) × 1.0 μm (thickness) dimensions. The injector, detector and oven temperatures were 100, 80 and 100 °C respectively. The carrier gas was nitrogen, purged at a flow rate of 1.2 mL min^− 1. For the estimation of nitrogen oxides, helium was used as carrier gas at the same flow rate. The final concentrations of CO₂, CO, NO, NO₂, SO₂ and SO₃ in the gas samples collected from both the experiments were analyzed.

The initial and final concentrations were precisely determined by comparing the peaks against their respective reference standard peaks. All the gaseous standards were procured from Sigma-Aldrich, i.e., ≥ 99.8% CO₂ (295108), ≥ 99.0% CO (295116), 98.5% NO (295566), ≥ 99.5% NO₂ (295582), ≥ 99.9% SO₂ (295698) and ≥ 99% SO₃ (227692). The initial concentrations of CO₂, CO, NO, NO₂, SO₂ and SO₃ from the automobile exhaust gas samples were found to be 13, 2.4, 0.2, 0.08, 0.1 and 0.05%, respectively. And the difference between the initial and final concentrations of the gaseous components was directly proportional to the percentage removal. The percentage concentrations of the gaseous samples against their reference standards was calculated as following:

$$ \mathrm{Percentage}\ \mathrm{of}\ \mathrm{sample}=\frac{\mathrm{sample}\ \mathrm{area}}{\mathrm{standard}\ \mathrm{area}}\times \mathrm{purity}\ \mathrm{of}\ \mathrm{standard}\ \left(99\%\right) $$

(1)

Determination of total biomass yields

The attached microalgae culture from the automobile experiment was estimated for its total biomass yield by evaluating the initial (silicone coated SS pipe before cultivation) and final (microalgae inhabited silicone coated SS pipe) weights of the SS pipe. Beforehand, the SS pipe was dried in a hot air oven at 50 °C temperature (the temperature that did not allow biological activity and biomass property loss) and cooled to room temperature in a desiccator so as to get the biomass dry weight. Then, the silicone coat to which the microalgae were attached was carefully peeled off from the SS pipe. The attached biomass was easily detachable from the silicone matrix in dry condition. The biomass was gently transferred into a freshly weighed 50 mL round bottom flask using a spatula for lipid extractions.

Similarly, the fully-grown cultures from the laboratory flasks in both the experiments were filtered through pre-weighed Whatman No. 1 filter papers. The filter papers with microalgae were dried in a hot air oven as mentioned above. The fully dried filter papers containing moisture free microalgae cells were then weighed to calculate their respective dry biomass weights.

Extraction of oil/lipid from dried algae biomass

The lipids were extracted from powdered biomass through solvent extraction procedure using hexane [25]. The solvent was introduced into the 50 mL round bottom flask containing the biomass along with a stir bar to reflux heat for 60 min at 70 °C. Solvent (mL) to algae (g) ratio was taken to be 30:1 to ensure efficient extraction. A condenser was connected to the top of the round bottom flask. Cold water was running over the condenser throughout the extraction processes. The entire setup was placed in a hot water bath for controlling the temperature and to ensure uniform heating during the course of the experiment. After 60 min, the round bottom flask was separated from the extraction setup and allowed to cool. After cooling down, the algae cells were removed by filtering through Whatman #1 filter paper, layered on a glass funnel. The biomass debris was stuck on the filter papers while the solvent containing lipids was transferred into a fresh pre-weighed round bottom flask. Then, the solvent was evaporated out in a rotavapor at its boiling point. The remaining compound left in the flask was the lipid sample whose final weight was estimated and stored for further procedures.

Characterization of the extracted lipids for fatty acid compositions

The presence of various fatty acid components in the hexane extracted lipid samples were confirmed by qualitatively analyzing through a GC-mass spectroscopy (GC-MS 6890 N, Agilent Technologies, USA), equipped with 40–350 °C Inert Mass selective quadruple detector, HP-5 MS column (Agilent Technologies, USA) of dimensions: 30 m × 0.25 mm × 0.25 μm length × inner diameter × thickness respectively with helium (He) as carrier gas. The lipid components were confirmed based on the MS references from National Institute Standard and Technology mass spectral database libraries.

Process parameters

Parameters like pH, dissolved oxygen (DO) (HQ40D Portable pH & DO Meter), dissolved free CO₂ (CO_2(aq)) (4500-CO₂ C, American Public Health Association (APHA)), bicarbonate (HCO₃⁻) and carbonate (CO₃^− 2) system (2320B-Alkalinity, APHA [27]) were monitored periodically in the laboratory flask experiment only as it was not possible in the automobile attached growth experiment. The results were analyzed to get an understanding on how the automobile exhaust gases influenced the growth pattern and carbon sequestration capability of S. quadricauda.

Results and discussion

Biomass and lipid yields of S. quadricauda

Microalgae S. quadricauda was able to adapt and grow in the presence of toxic gases like CO and SOx in both the experimental setups. The growth patterns were normal and the yields were better than anticipated. Following are the details of the discussion:

Yields obtained from automobile attached growth experiment

The attached growth was found to be favorable for curbing the exhaust gases. Various factors were taken into consideration during the entire cultivation period as follows: The flask in which the SS pipe was placed to allow attached growth of microalgae, also developed some growth in the liquid medium. The culture medium in the flask was examined for OD values and based on these values a growth curve was drawn. Initially when the SS pipe was placed in the flask, after the day 1 exposure to exhaust gases, no color development was seen indicating a possible lag phase where the microalgae took time to adjust and acclimatize. The following 9-d, a rapid color development was noticed indicating the rapid linear growth phase followed by a 1-d stationary phase, during which there was no color development. And then, the bright green colored medium started exhibiting a yellowish-brown shade indicating a possible decline phase. Thus, the microalgae grew efficiently for a period of 12 d with day time exposure to exhaust gases and night time nutrient supply. On the other hand, the SS pipe was viewed for color development manually. According to the manual assessment (observations based on color, brightness and turbidity) there was a prolonged lag phase for about 3 d, i.e., no color development followed by a linear growth phase for 8 d, i.e., gradual darkening of color and turbidity. Then there was a slight shift in the green color to yellowish brown indicating the beginning of the decline phase. The SS pipe was detached for biomass withdrawal, when the OD readings in its corresponding laboratory flask started showing slight inclination towards the decline phase. The total biomass yield obtained from the attached growth in the SS pipe was evaluated based on the following considerations:

The initial weight of the silicone coated SS pipe was subtracted from the final weight after algae growth, which showed a biomass weight of 0.65 g. And the biomass yield obtained from the nutrient flask in which the SS pipe was placed during the nights, was found to be 0.3 g L^− 1. The volumetric capacity of the pipe was 0.25 L liquid medium, and the amount of biomass produced per this 0.25 L capacity pipe was 0.65 g. That means for a liter capacity the biomass yield becomes 2.6 g L^− 1. Thus, the biomass yield for the SS pipe experiment was substantiated to be 2.6 g L^− 1 and the lipid yield obtained after extraction was equivalent to 0.7 g L^− 1 (0.18 g lipid from 0.65 g of attached algae biomass).

The 0.3 g L^− 1 yield produced from the flask was not included into the final yield of the exhaust pipe experiment due to the reason that the flask was not given any additional supply of gases except ambient air in the night time and that the flask was only used as a nutrient aid for the attached growing algae during the nights. Another benefit was to understand the attached algae growth pattern through OD values (Fig. 4a). As the attached growing algae interacted with the medium, it led to the growth in the flask. Hence the 0.3 g L^− 1 biomass yield was considered as a spin-off in the present research work. This biomass sample was stored as an automobile exhaust gas acclimatized mother culture in BBM broth at 4 °C for future studies.

Fig. 4

S. quadricauda growth pattern and biomass yields as observed from (a) Nutrient flask in automobile attached growth experiment (b) Laboratory flask experiment with automobile exhaust gas inputs

Yields obtained from laboratory flask experiment with microalgae

The growth phase of S. quadricauda in the laboratory flasks went on for a period of 15 d under the influence of automobile exhaust gas. The biomass growth curve based on OD values showed a 1-d lag phase followed by a 10-d rapid growth phase followed by a 1-d stationary phase (Fig. 4b). The total biomass yield was found to be 2.2 g L^− 1 and lipid yield 0.6 g L^− 1.

The biomass productivities of S. quadricauda from both the experiments in the present study were around 0.15–0.3 g L^− 1 d^− 1 according to the OD values and the final dry weight yields. These results were in accordance with Yoo et al. [28], who cultivated Scenedesmus sp. with 10% CO₂ and reported a biomass productivity of up to 0.218 g L^− 1 d^− 1 combined with CO₂ reduction and lipid production. Furthermore, the lipid yields obtained from both the experiments in the present study were clearly showing that the total lipid content accumulated by S. quadricauda utilizing automobile exhaust gases was around 28–33%. These results were in agreement with the reports of Liu et al. [29], who argued that the lipid content of Scenedesmus sp usually ranges between 20 and 50% of biomass dry weights.

Treatment of exhaust gases by S. quadricauda

Removal percentages in automobile attached growth experiment

The samples collected in the Tedlar bags were analyzed on a daily basis in the evening time after the completion of the experimental duration. The GC results have shown a linear decrease in the gaseous percentages gradually with increasing microalgae growth. The concentration of CO₂ has been removed up to 62%, CO 57%, NO 65%, NO₂ 66%, SO₂ 64% and SO₃ 65% (Fig. 5). From these dropping concentrations, it was understood that the microalgae S. quadricauda has utilized these gases for its vital activities and growth purposes. S. quadricauda was able to sustain the toxicity of CO₂, CO and SOx concentrations and was capable to survive in dry conditions for about 10 h a day. These observations were in correlation with the findings of Li et al. [30], who reported that Scenedesmus obliquus tolerated an exposure to flue gas CO₂ up to 12% and was efficient in removing 67% of it. The motorbike engine selected for the study was of old model and the petrol we used to run the motorbike to collect the exhaust gases in the present study was from a local petrol bunk nearby who might have mixed low quality kerosene. The smoke that came out while conducting the experiments was seemingly very turbid. The NOx values are in the range of general NOx compositions reported by Sassykova et al. [31]. Only the sulphur dioxide concentration was slightly higher and the possible reason must be kerosene mixed petrol or any other deposited impurities due to low maintenance of the petrol containers.

Fig. 5

Phycoremediation of exhaust gas constituents by S. quadricauda from automobile attached growth experiment (a) CO₂ (b) CO (c) NO (d) NO₂ (e) SO₂ (f) SO₃

Removal percentages in laboratory flask experiment

Just like in the automobile experiment, the samples collected in the Tedlar bags in this experiment were analyzed and the GC results showed a notable decrease in the gaseous percentages gradually with increasing microalgae growth. The concentration of CO₂ was removed up to 65%, with CO 58%, NO 68%, NO₂ 68%, SO₂ 65% and SO₃ 66% (Fig. 6). These results were almost similar to those obtained from attached growth experiments.

Fig. 6

Phycoremediation of exhaust gas constituents by S. quadricauda from laboratory flask experiment (a) CO₂ (b) CO (c) NO (d) NO₂ (e) SO₂ (f) SO₃

Assessment of various process parameters in laboratory flask experiment

pH, DO, dissolved free CO₂, bicarbonate and carbonate system, were analyzed in the laboratory flask experiments which explained the effect of exhaust gases on the growth and toxicity removal efficiencies of S. quadricauda.

Initially, the pH values were acidic scale of 3.8–5.0 until the microalgae hit exponential growth phase where the pH shifted towards acidic-neutral scale, i.e., 5.0–6.5. During the stationary phase the pH was neutral, 6.5–7.2 and finally reached slightly acidic range of 5.4–6.7 as the decline started indicating a cessation in the biological activities of S. quadricauda. The reason for the starting day pH being less than 4 was due to the formation of nitric and sulfuric acids in the nutrient medium as a result of the exhaust gas interactions with the liquid medium.

In the sequence of the 12-d growth of S. quadricauda, the DO concentrations ranged from 1 to 6.5 ppm. On the initial day, before the exhaust gas inputs, the DO value was 1 ppm and by the evening after the exhaust gas runs, it was 1.4 ppm. Next morning it was 1 ppm again. From this, it was understood that the DO values were reduced overnight due to the respiration activity of the microalgae. The DO concentrations increased linearly with time and reached up to 6.5 ppm on the final day of the experiment even on the onset of decline, indicating the buildup of microalgae toxins once it stopped growing.

The CO_2(aq) results showed that from the initial day to the final day, the concentration decreased gradually (Fig. 7). Actually, the pH, alkalinity, CO_2(aq) and bicarbonate are all interrelated and one effects the other in terms of concentrations. Therefore, all the constituents showed a linear decrease in their respective concentrations towards the end of the experiment. When at pH is below 8, bicarbonate alkalinity is total alkalinity. Hence in Fig. 7, bicarbonate alkalinity was only shown along with CO_2(aq) concentrations.

Fig. 7

Bicarbonate and dissolved free CO₂ reduction as achieved by S. quadricauda from laboratory flask experiment with exhaust gas inputs

Throughout the study, there was no alkaline pH (> 8) reported and no accumulation of carbonates and hydroxides was seen due to the continuous inputs of exhaust gases which rapidly interacted with the nutrient medium, forming soluble acids.

Hence from all the above observations it was clear that, microalgae S. quadricauda was capable of growing in harsh and dry conditions in two different cultivation strategies, subsequently treating the significant constituents that headed from the automobile exhaust gases.

Fatty acid compositions

Generally, the fatty acid composition of green microalgae derived lipids comprises of phospho-lipids, glyco-lipids, betaine lipids and gycero-lipids which are again polar, non-polar, saturated, unsaturated, poly-unsaturated, free fatty acids and neutral lipids along with some microalgae type specific insignificant class of lipids. There are various market benefits of these fatty acids. One such important application is fatty acid methyl esters (FAME)/biodiesel. However, not all the fatty acid components produced by the microalgae are useful to produce FAMEs. Only a particular fatty acid group ranging C14 to C22 are significant in FAME production. The lipid samples from the two experiments were analyzed for the presence of diesel producing fatty acids, especially saturated and unsaturated fatty acid components (Fig. 8). Significant long chain groups like C16:0 (palmitic acid), C18:0 (stearic acid), C18:1 (oleic acid), C18:2 (linoleic acid), and C18:3 (linolenic acid) were present in the GC-MS profiles of the hexane extracted lipid samples from the present study (Table 1). The fatty acid compositions were almost similar in both the lipid samples as they belong from the same species. Only a minor variation in purities and retention times was seen which might be due to the differences in their respective growth conditions or equipment handling conditions in between the runs. Giving strong point to this statement, Fuentes-Grunewald et al. [32], have reported that no matter how different the cultivation techniques were, the fatty acid profiles of a particular species remained almost unaffected.

Fig. 8

GC-MS chromatograms of the lipids extracted from the biomass of S. quadricauda (a) From automobile attached growth (b) From laboratory flask experiment

Table 1 List of significant fatty acid components present in the hexane extracted lipid samples of S. quadricauda (a) From automobile attached growth (b) From laboratory flask experiment

Conclusions

In the present study the approach of growing green microalgae using exhaust gases was positively implemented. Another different strategy was to grow the microalgae in un-suspended or substrate attached conditions allowing direct exposure to exhaust gases in dry conditions during inputs. Though much emphasis is needed into this study in different viewpoints, this concept was a downright new modus operandi for microalgae cultivation, which has not been reported anywhere else until now as per our knowledge. Therefore, from the present study observations, it can be agreed that S. quadricauda stands as a promising microalgae species that could withstand toxic exhaust gaseous concentrations and produce fruitful results. The reports made clear that the high CO₂ percentages and other toxic gases like CO, SOx and NOx have played a substantial role in inducing biomass and lipid yields from S. quadricauda. The treatability efficiency of the same was significant and adequate.