Optimization of growth operational conditions for CO2 biofixation by native Synechocystis sp.

BACKGROUND: A fundamental step in assessing the viability of a CO₂ biofixation system based on microalgae is to identify the maximum CO₂ biofixation yield that can be achieved for this microorganism when it is cultivated under optimum operational growth conditions. Response surface methodology was applied to determine optimum culture conditions for CO₂ biofixation by a recently isolated freshwater cyanobacterium Synechocystis sp. The strain was cultivated in a 1 L bubble column photobioreactor, in semicontinuous mode. RESULTS: Statistical analysis showed that temperature (from 22 to 39 °C), pH (from 7.2 to 8.8) and light intensity (from 928 to 2272 µE m^?2 s^?1), in addition to some of their interactions, had a significant effect on CO₂ biofixation. An optimum CO₂ biofixation rate of 2.07 gCO₂ L^?1culture day^?1 was found within the experimental region, at an average light intensity 686 µE m^?2 s^?1, pH 7.2 and temperature 35.3 °C. CONCLUSIONS: Based on these results, it is concluded that Synechocystis sp. presents a good tolerance to both high temperature and light intensity, characteristics which facilitate its application in outdoor CO₂ biofixation systems. Copyright © 2011 Society of Chemical Industry     

Optimization of CO2 Biofixation by Chlorella vulgaris Using a Tubular Photobioreactor

The effects of various CO₂ concentrations in CO₂ bioconversion by cultivation of microalga Chlorella vulgaris were investigated using a vertical tubular photobioreactor. The response surface technique with central composite design was applied to model the CO₂ biofixation rate, the specific growth rate (SGR), and the biomass productivity of C. vulgaris as function of CO₂ concentration and cultivation time. The developed nonlinear model was employed to determine the optimum CO₂ concentration in an air‐CO₂ mixture and the cultivation time for maximum CO₂ biofixation, SGR, and microalgae biomass productivity. In addition, a multiple responses optimization method was also applied to determine the maximum CO₂ uptake rate, the SGR, and the biomass productivity, simultaneously. The predicted optimum values agreed well with the experimental data.     

Modified Photobioreactor for Biofixation of Carbon Dioxide by Chlorella vulgaris at Different Light Intensities

The performance of a modified bioreactor inside a light enclosure for carbon dioxide biofixation by Chlorella vulgaris was investigated. The influence of different light intensities on the CO₂ biofixation and biomass production rates was evaluated. The results showed that the photon flux available to the microalgal cultures can be a key issue in optimizing the microalgae photobioreactor performance, particularly at high cell concentrations. Although the optimal pH values for C. vulgaris are in the range of 6–8, cell growth can take place even at pH 4 and 10. Batch microalgae cultivation in the photobioreactor was used to investigate the effect of different light intensities. The maximum biomass concentration of 1.83 g L^?1 was obtained at a light intensity of 100 μmol m^?2s^?1 and under aeration with 2 L min^?1 of 2 % CO₂‐enriched air.     

Critical review of strategies for CO2 delivery to large-scale microalgae cultures

Microalgae have great, yet relatively untapped potential as a highly productive crop for the production of animal and aquaculture feed, biofuels, and nutraceutical products. Compared to conventional terrestrial crops they have a very fast growth rate and can be produced on non-arable land. During microalgae cultivation, carbon dioxide (CO₂) is supplied as the carbon source for photosynthesising microalgae. There are a number of potential CO₂ supplies including air, flue gas and purified CO₂. In addition, several strategies have been applied to the delivery of CO₂ to microalgae production systems, including directly bubbling CO₂-rich gas, microbubbles, porous membrane spargers and non-porous membrane contactors. This article provides a comparative analysis of the different CO₂ supply and delivery strategies and how they relate to each other.     

Gasification of Microalgae Using Supercritical Water and the Potential of Effluent Recycling

The gasification of microalgae in supercritical water was investigated in this work. The product gas contained mainly H₂, CO₂, CH₄, and C₂H₆. Operation at high temperatures and lower biomass concentrations resulted in the highest carbon gasification efficiency and the lowest total organic carbon levels in the residual water. Due to its content of inorganic nutrients, the residual water was applied as cultivation medium for microalgae. However, algal growth in the untreated residual water was inhibited by the existence of potentially toxic substances evolved from gasification. Upon treatment by activated carbon filtration and ultraviolet light degradation, these substances were eliminated and cultivation in the residual water was possible. The major fraction of inorganic residues from gasification was recovered by means of water purging, increasing the potential of nutrient recycling for cultivation.     

Microalgal bioremediation of food-processing industrial wastewater under mixotrophic conditions: Kinetics and scale-up approach

The Chlorella microalgae were mixotrophically cultivated in an unsterilized and unfiltered raw food-processing industrial wastewater. Both inorganic carbon (CO₂-air) and organic carbon (wastewater) were provided simultaneously for microalgae growth. The aim of the study is to find out the utilization rates of total organic carbon (TOC) and chemical oxygen demand (COD) under mixotrophic conditions for a given waste water. About 90% reduction in TOC and COD were obtained for all dilutions of wastewater. Over 60% of nitrate and 40% of phosphate were consumed by microalgae from concentrated raw wastewater. This study shows that microalgae can use both organic and inorganic sources of carbon in more or less quantity under mixotrophic conditions. The growth of microalgae in food-processing industrial wastewater with all studied dilution factors, viz. zero (raw), 1.6 (dilution A), and 5 (dilution B) suggests that the freshwater requirement could be reduced substantially (20%–60%). The degradation kinetics also suggests that the microalgae cultivation on a high COD wastewater is feasible and scalable.

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Optimization of the influential factors for the improvement of CO2 utilization efficiency and CO2 mass transfer rate

Microalgae fix CO₂ as energy source and afford biomass and high valued products such as carotenoids, pigments, proteins, and vitamins that can be used for the production of nutraceuticals, pharmaceuticals, animal feed additives, cosmetics, etc. Carbon dioxide is the sole source of carbon and it is supplied continuously for the microalgal cultivation. But undissolved CO₂ is lost by outgassing and sufficient dissolved CO₂ should be provided to avoid carbon limitation. The effect of CO₂ mass transfer with different CO₂ concentrations, aeration rate of gas, bubble size, baffle type and baffle number on the growth of Chlorella sp. AG10002 was investigated and the optimized conditions for the enhancement of biomass productivity were determined. We confirm that these results can be provided as basic data to improve the CO₂ mass transfer ability for the high density culture of Chlorella sp. and some microalgae having commercial value.     

CO2 absorption properties of Brønsted acid-base ionic liquid composed of N,N-dimethylformamide and bis(trifluoromethanesulfonyl)amide

The solubilities of CO₂ and the liquid densities in a Brønsted acid-base ionic liquid, [DMFH][Tf₂N], composed of N,N-dimethylformamide (DMF) and bis(trifluoromethanesulfonyl)amide (HTf₂N) have been investigated at high pressures and at different temperatures. The results were compared with those in DMF and a typical 1-butyl-3-methylimidazolium analogue with the same anion, [BMIM][Tf₂N]. The mole fraction scaled solubilities of CO₂ in the three liquids showed a slight increase in the following order, DMF < [DMFH][Tf₂N] < [BMIM][Tf₂N], whereas more remarkable difference was observed in the volume scaled concentrations of CO₂, [BMIM][Tf₂N] < [DMFH][Tf₂N] « DMF, mainly due to the bulkiness of liquid entities.     

Extension of a compressible lattice model to CO2 + cosolvent + polymer systems

We have recently proposed a compressible lattice model for CO₂ + polymer systems in which CO₂ forms complexes with one or more functional groups in the polymer. Furthermore, we have shown that this model is able to simultaneously correlate phase equilibria, sorption behavior, and glass transition temperatures in such systems. In the present work, we extend the model to ternary CO₂ + cosolvent + polymer systems and demonstrate that cloud point behavior in CO₂ + dimethyl ether + poly (?-caprolactone), CO₂ + dimethyl ether + poly (isopropyl acrylate), and CO₂ + dimethyl ether + poly (isodecyl acrylate) systems can be predicted using parameters obtained from binary data. Our results also suggest that dimethyl ether may form weak complexes with poly (?-caprolactone), poly (isopropyl acrylate), and poly (isodecyl acrylate).     

Supercritical antisolvent precipitation of andrographolide from Andrographis paniculata extracts: Effect of pressure, temperature and CO2 flow rate

Andrographis paniculata extracts were precipitated using the so-called supercritical antisolvent (SAS) technique. Ethanol was used as the solvent and compressed CO₂ as the antisolvent. The effects of process operating conditions (pressure: 5-24 MPa, temperature: 308-328 K and CO₂ flow rate: 0.5-1.5 g/min) on particle size and morphology of precipitated andrographolide were evaluated. X-ray diffraction (XRD) patterns showed significant changes in andrographolide morphology depending on process operating conditions; both column-like and slice-like crystals were observed depending on operating conditions. Crystals with mean diameters of 3.30-228.35 μm were produced, smaller crystals were obtained at high pressure, low temperature and high CO₂ flow rate and vice versa for large crystals. In addition, SAS process also produced high precipitation yields, since solubility of andrographolide is small in the supercritical CO₂ plus ethanol. When operating under subcritical conditions, amorphous particles were produced.     

Sustainable Hydrogen Photoproduction by Phosphorus-Deprived Marine Green Microalgae Chlorella sp.

Previously it has been shown that green microalga Chlamydomonas reinhardtii is capable of prolonged H₂ photoproduction when deprived of sulfur. In addition to sulfur deprivation (-S), sustained H₂ photoproduction in C. reinhardtii cultures can be achieved under phosphorus-deprived (-P) conditions. Similar to sulfur deprivation, phosphorus deprivation limits O₂ evolving activity in algal cells and causes other metabolic changes that are favorable for H₂ photoproduction. Although significant advances in H₂ photoproduction have recently been realized in fresh water microalgae, relatively few studies have focused on H₂ production in marine green microalgae. In the present study phosphorus deprivation was applied for hydrogen production in marine green microalgae Chlorella sp., where sulfur deprivation is impossible due to a high concentration of sulfates in the sea water. Since resources of fresh water on earth are limited, the possibility of hydrogen production in seawater is more attractive. In order to achieve H₂ photoproduction in P-deprived marine green microalgae Chlorella sp., the dilution approach was applied. Cultures diluted to about 0.5–1.8 mg Chl·L⁻¹ in the beginning of P-deprivation were able to establish anaerobiosis, after the initial growth period, where cells utilize intracellular phosphorus, with subsequent transition to H₂ photoproduction stage. It appears that marine microalgae during P-deprivation passed the same stages of adaptation as fresh water microalgae. The presence of inorganic carbon was essential for starch accumulation and subsequent hydrogen production by microalgae. The H₂ accumulation was up to 40 mL H₂ gas per 1iter of the culture, which is comparable to that obtained in P-deprived C. reinhardtii culture.     

Supercritical CO2 extraction of Helichrysum italicum: Influence of CO2 density and moisture content of plant material

Kinetics and selectivity of supercritical carbon dioxide (SC CO₂) extraction of Helichrysum italicum flowers were analyzed at pressures in the range of 10-20 MPa and temperatures of 40 °C and 60 °C (density of SC CO₂ from 290 to 841 kg/m³) and also at 10 MPa and 40 °C using flowers with different moisture contents (10.5% and 28.4%). Increased moisture content of H. italicum flowers resulted in enchased solubility of solute enabling decrease of SC CO₂ consumption necessary for achieving desired extraction yield. The most abundant compounds in the supercritical extracts are sesquiterpenes and waxes while monoterpenes and sesquiterpenes are the main constituents of essential oil obtained by hydrodistillation. The optimal set of working parameters with respect to extraction yield, SC CO₂ consumption and chemical composition of extract were defined related to moisture content of raw material and SC CO₂ density.     

Study on supercritical extraction of lipids and enrichment of DHA from oil-rich microalgae

The present study aims to isolate the lipids from microalgae by supercritical CO₂ (SC-CO₂) extraction followed by a further enrichment of crude lipids to produce high-purity docosahexenoic acid (DHA) by an urea complexation method. Our systematic approach indicates the optimum conditions of supercritical CO₂ extraction were obtained as follows: 35 MPa, 40 °C, ethanol (95%, v/v) as the co-solvent, and the mass ratio of material to co-solvent 1:1. Under these conditions, 33.9% of lipid yield and 27.5% of DHA content were achieved. Despite the relatively low lipid yield, supercritical CO₂ extraction has exhibited many advantages over the Soxhlet extraction for the DHA enrichment such as high DHA purity and superb product quality. Furthermore, urea complexation method on DHA enrichment considerably increased the DHA purity from 29.7% to 60.4% with an enrichment ratio of 60.6%, under the optimum complexation conditions of urea/fatty acid 2:1, complexation time 8 h, and the complexation temperature of −10 °C.     

Liquid CO2 extraction of flowers and fractionation of floral concrete of Michelia champaca Linn

Extraction of the fresh flowers of Michelia champaca L. with liquid CO₂ provided a floral extract in 1.0 ± 0.04 wt% yields. The extract so obtained contains far less waxes and is organoleptically very superior. Similarly extraction with pentane gave the so-called ‘Concrete’ in 1.58 ± 0.06 wt%. While the concrete contains co-extracted floral waxes that make it unsuitable for blending with other perfumes, direct extraction with CO₂ is an expensive process mainly due to low bulk density of flowers and their availability during short flowering season. On the other hand, fractionation of the concrete with liquid CO₂ to separate the waxy components has provided solvent and almost wax free fractions. The duration of extractive fractionation has been optimized for selective extraction with liquid CO₂ at 62 bar. These liquid CO₂ fractions of concrete and liquid CO₂ extract of flowers were analyzed by GC and GC/MS and their composition compared with that of concrete and partially de-waxed absolute obtained in the conventional way. The major fragrance compounds enriched in the direct liquid CO₂ extract were methyl benzoate (11.5 ± 0.8%), phenyl ethyl alcohol (5.0 ± 0.6%), phenyl acetonitrile (10.4 ± 1.1%), indole (1.2 ± 0.3%), methyl anthranilate (1.3 ± 0.5%), E-β-ionone (1.5 ± 0.4%), and Z-methyl jasmonoate (1.0 ± 0.3%). The liquid CO₂ fractionation of concrete is a practical process and the first fraction is comparable with direct liquid CO₂ flower extract in terms of composition of the major compounds.     

Guava (Psidium guajava L.) seed oil obtained with a homemade supercritical fluid extraction system using supercritical CO2 and co-solvent

In this work we designed and built a homemade supercritical fluid extraction (HM-SFE) system, in which pure CO₂ and CO₂ with co-solvents were used. The HM-SFE was made by means of thermal dilatation-contraction (TDC). This HM-SFE system was used for obtaining guava (Psidium guajava L.) seed oil, using supercritical CO₂ adding ethanol as co-solvent (CO₂ SC/EtOH), extractions were performed at 313 K and different pressures (10, 20 and 30 MPa), each one in four stages of 30 min, the extract with higher yield was subjected to transesterification and high-resolution gas chromatography (HRGC) analysis. The highest extraction yield was obtained at 30 MPa (17.30% w/w), this yield was higher than one observed in a previous work using SC-CO₂, and near to the one obtained by Soxhlet extraction (20.2% w/w). HRGC enabled the identification of components of the derivatized extract as methyl esters of palmitic, oleic, linoleic, and stearic fatty acids. The results obtained with HM-SFE system was compared with a commercial SFE system, obtained very similar results. In this work was possible to construct a low cost and simple manner HM-SFE system which was employed for obtaining guava seed oil, using CO₂ SC/EtOH.     

Quantification of specific interactions between CO2 and the carbonyl group in polymers via ATR-FTIR measurements

We have used in situ ATR-FTIR measurements to provide estimates of the strength of specific interactions between carbon dioxide (CO₂) and carbonyl groups in polymers such as poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc), poly(lactide) (PLA) and poly(lactide-co-glycolide) (PLGA). Polymer films were exposed to high pressure CO₂ and the carbonyl stretching vibration at 1700 cm⁻¹ and the CO₂ bending mode at 660 cm⁻¹ were studied. The observed shift in the carbonyl stretching band to higher wavenumber was attributed to dielectric effects according to the Kirkwood-Bauer-Magat (KBM) equation. On the other hand, the splitting of the CO₂ bending mode provided direct evidence of specific interactions between the polymer and CO₂. These interactions were quantified via an equilibrium constant for the association reaction between CO₂ and the carbonyl group.     

Measurements and modeling of high-pressure excess molar enthalpies and isothermal vapor-liquid equilibria of the carbon dioxide + N,N-dimethylformamide system

Mixtures of supercritical CO₂ and N,N-dimethylformamide (DMF) are very often involved in supercritical fluid applications and their thermodynamic properties are required to understand and design these processes. Excess molar enthalpies () for CO₂ + DMF mixtures were measured using an isothermal high-pressure flow calorimeter under conditions of temperature and pressure typically used in supercritical processes: 313.15 and 323.15 K at 9.00, 12.00, 15.00 and 18.00 MPa and 333.15 K at 9.00 and 15.00 MPa. The Peng-Robinson and the Soave-Redlich-Kwong equations of state were used in conjunction with the classical mixing rules to model the literature vapor-liquid equilibrium and critical data and the excess enthalpy data. In most cases, CO₂ + DMF mixtures showed very exothermic mixing and excess molar enthalpies exhibited a minimum in the CO₂-rich region. The lowest value (−4526 J mol⁻¹) was observed for a CO₂ mole fraction value of 0.713 at 9.00 MPa and 333.15 K. On the other hand, at 9.00 MPa and 323.15 and 333.15 K varies linearly with CO₂ mole fraction in the two-phase region where a gaseous and a liquid mixture of fixed composition are in equilibrium. The effects of pressure and temperature on the excess molar enthalpy are large. For a given mole fraction, mixtures become less exothermic as pressure increases or temperature decreases. These excess enthalpy data were analyzed in terms of molecular interactions, phase equilibria, density and critical parameters previously reported for CO₂ + DMF. All throughout this paper, the key concepts and modeling tools originate from the work of van der Waals: the paper is intended as a small piece of recognition of van der Waals overwhelming contributions to thermodynamics.     

Supercritical fluid extraction of bioactive lipids from the microalga Nannochloropsis sp.

Marine microalgae are recognised as an important renewable source of bioactive lipids with a high proportion of polyunsaturated fatty acids (PUFA), which have been shown to be effective in preventing or treating several diseases. For the extraction of oil from microalgae, supercritical CO₂ (ScCO₂) is regarded with interest, being safer than hexane and offering a negligible environmental impact, a short extraction time and a high‐quality final product. Whilst some experimental papers are available on the supercritical fluid extraction (SFE) of oil from microalgae, only limited information exists on the kinetics of the process. In such a contest, a mathematical model able to describe the kinetics of the SFE was applied to the recovery with ScCO₂ of lipids from Nannochloropsis sp., a marine microalga commonly used in aquaculture and characterised by a lipid fraction with a high PUFA content. The aim of this paper was to examine the effect of operating conditions on the kinetics of the SFE, on process yields and on the fatty acid composition of lipid extracts.     

Extraction of sage (Salvia officinalis L.) by supercritical CO2: Kinetic data, chemical composition and selectivity of diterpenes

The supercritical carbon dioxide (SFE) extraction of Dalmatian sage (Salvia officinalis L.) was investigated and compared to extraction performed by Soxhlet ethanol-water (70:30) mixture extraction (SE) and hydrodistillation (HD). The supercritical extraction allowed isolation of wide spectrum of phytochemicals, while other applied methods were limited to either volatiles (HD) or high molecular compounds isolation (SE). The kinetics of the supercritical extraction and fractionation within the pressure range of 10-30 MPa at 50 °C were also analyzed as well as the chemical compositions of total extract and partial or differential fractions isolated at different CO₂ consumption. Volatile fraction could be isolated at low pressure and low CO₂ consumption, whereby the pressures between 10 and 15 MPa followed by increased CO₂ consumption were favourable for obtaining desired selectivity of diterpenes which contain compounds with expressed antioxidative characteristics.     

Fluorescence studies on local density change in supercritical CO2 mixtures using the order parameter model

Supercritical CO₂ has many unusual physicochemical properties in or near the critical region, which is mostly due to the local density enhancements and specific molecular interactions. Compared to pure CO₂, the study on CO₂ mixed with co-solvent is attractive in wide applications but it is hard to quantitatively describe the local density change and intermolecular clusters in CO₂ mixtures. The fluorescence response of a sensitive pyrene probe, empirical Py scale, was determined in CO₂ mixed with pentane at 323.15 K in different phase regions. A variable called the order parameter was introduced to account for possible correlations in the first derivative of Py scale and fluid pressure (dPy/(d(P/P₀ − 1))). This model, avoiding the hard choice of the reference line in CO₂ mixtures, was effective in calculating the local density change and it showed the specific molecular interaction in the critical region. The behaviors of the local density change and the isothermal compressibility of supercritical CO₂ mixtures correlated well with microscopic and macroscopic observations.