arumugham2021

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Supercritical carbon dioxide extraction of plant phytochemicals for biological and environmental applications e A review

Thanigaivelan Arumugham a

^,^***

, Rambabu K a , Shadi W. Hasan a , Pau Loke Show b

^,^*

, J€ org Rinklebe c

^,

d , Fawzi Banat a

^,^**

aDepartment of Chemical Engineering, Khalifa University, 127788, Abu Dhabi, United Arab Emirates

bDepartment of Chemical Engineering, Faculty of Science and Engineering, University of Nottingham Malaysia, 43500, Selangor Darul Ehsan, Malaysia

cUniversity of Wuppertal, School of Architecture and Civil Engineering, Institute of Foundation Engineering, Water- and Waste-Management, Laboratory of Soil- and Groundwater-Management, Pauluskirchstraße 7, 42285, Wuppertal, Germany

dDepartment of Environment, Energy and Geoinformatics, Sejong University, Seoul, 05006, Republic of Korea

h i g h l i g h t s g r a p h i c a l a b s t r a c t

Sc-CO2 assisted SFE application for plant metabolites extraction is reviewed.

Biological properties of extracts are preserved by optimized SFE conditions.

Addition of co-solvents (modiﬁers) enhances the biological activity of the extract.

Micro-pollutant and toxic metal recovery rely on their solubility in Sc- CO₂.

Possible challenges and future perspectives for SFE technology are discussed.

a r t i c l e i n f o

Article history:

Received 6 September 2020 Received in revised form 17 November 2020 Accepted 29 December 2020 Available online 2 January 2021 Handling Editor: Derek Muir

Keywords:

Supercritical CO2extraction Bioactive compounds Therapeutic properties

a b s t r a c t

Recently, supercriticalfluid CO2extraction (SFE) has emerged as a promising and pervasive technology over conventional extraction techniques for various applications, especially for bioactive compounds extraction and environmental pollutants removal. In this context, temperature and pressure regulate the solvent density and thereby effects the yield, selectivity, and biological/therapeutic properties of the extracted components. However, the nature of plant matrices primarily determines the extraction mechanism based on either density or vapor pressure. The present review aims to cover the recent research and developments of SFE technique in the extraction of bioactive plant phytochemicals with high antioxidant, antibacterial, antimalarial, and anti-inflammatory activities, influencing parameters, process conditions, the investigations for improving the yield and selectivity. In another portion of this review focuses on the ecotoxicology and toxic metal recovery applications. Nonpolar properties of Sc-CO2 create strong solvent strength via distinct intermolecular interaction forces with micro-pollutants and toxic metal complexes. This results in efficient removal of these contaminants and makes SFE technology

*Corresponding author.

**Corresponding author.

***Corresponding author.

E-mail addresses: [email protected] (T. Arumugham), rambabu.

[email protected] (R. K), [email protected] (S.W. Hasan), PauLoke.

[email protected], [email protected] (P.L. Show), rinklebe@uni- wuppertal.de(J. Rinklebe),[email protected](F. Banat).

Contents lists available atScienceDirect

Chemosphere

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h e m o s p h e r e

https://doi.org/10.1016/j.chemosphere.2020.129525

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Antioxidant activity Environmental applications

as a superior alternative for conventional solvent-based treatment methods. Moreover, a compelling assessment on the therapeutic, functional, and solvent properties of SFE is rarely focused, and hence this review would add signiﬁcant value to the SFE based research studies. Furthermore, we mention the limitations and potential of future perspectives related to SFE applications.

1. Introduction

In recent years, extraction of effective and bioactive plant- derived phytochemicals from horticulture and medicinal crop sources has gained signi

ﬁ

cant attention due to growing concern on healthy lifestyle (Chew, 2020; Cruz et al., 2020; Hauthal, 2001;

Kayathi et al., 2020; Tejedor-Calvo et al., 2020; Zanqui et al., 2020).

Even though plant extracts substitute various synthetic and hazardous chemicals in the food and cosmetic industries, their medicinal properties are still an emerging topic of interest in pharma industries. Most importantly, the attention of numerous researches has focused on

“

bioactive compounds extraction

”

from plants due to a growing number of health-conscious consumers amidst the coronavirus disease (COVID-19) pandemic. In general, plants contain a wide range of valuable biochemical compounds such as Flavonoids (Lei-Xiong et al., 2019; Tian et al., 2019), Essential oil/

terpenoids (Cao et al., 2019; García Sarri o et al., 2018; Pavela et al., 2019; Ubessi et al., 2019; Yang et al., 2019), Polyphenols (Casagrande et al., 2018; Cilla et al., 2018; Zhou et al., 2018), Fatty acids (AL Juhaimi et al., 2018; Lessa et al., 2018; Petropoulos et al., 2018), Pigments (Backes et al., 2018; Melini et al., 2019), Sterol- cholesterols (Chanioti and Tzia, 2019; Maniet et al., 2019; MS et al., 2018), Alkaloids (Belyagoubi-Benhammou et al., 2019; Jiang et al., 2019), among others. Also, different therapeutic bene

ﬁ

ts, including antioxidants (Cappato et al., 2018; Rios Romero et al., 2018), anti-in

ﬂ

ammatory (Villarreal-Soto et al., 2019; Zengin et al., 2019), and antimicrobial (Alexandre et al., 2019; Putnik et al., 2019) ability of these plant-derived phytochemicals have been reported.

Conventional techniques such as soaking (Roy et al., 2019), maceration (Uysal et al., 2019), pressurized hot water (Biel-Maeso et al., 2017), and Soxhlet solvent extraction (Mukhopadhyay et al., 2020) are the most frequently used methods to recover bioactive compounds from the biomass. Usage of carcinogenic organic solvents (Akinyemi et al., 2016; Martins et al., 2012; Yara-Var on et al., 2016), speci

ﬁ

cally hexane, toluene, chloroform, methanol, and acetone in these techniques alter the chemical nature of these compounds and make them toxic for human consumption. Also, the practical use of these extraction methods is limited due to the utilization of large quantity of solvent, low extraction rate, longer extraction time, energy-intensive nature, solvent contamination, thermal degradation of thermolabile active compounds, and de- natured quality of

ﬁ

nal products (P

ł

otka-Wasylka et al., 2017).

These signi

ﬁ

cant shortcomings have urged researchers to develop novel, ef

ﬁ

cient, economic, green, and safe extraction techniques to recover the bioactive compounds without losing their quality and properties. According to the literature (Antonetti et al., 2016; Chen et al., 2012; Pumure et al., 2010), microwave extraction (ME), supercritical

ﬂ

uid extraction (SFE), and ultrasonic-assisted extraction (UAE) have been reported as novel and green non-thermal meth- odologies. Among them, supercritical carbon dioxide (Sc-CO

₂

) based extraction technology is generally recognized as safe and has been utilized as a green approach for the productive extraction and recovery of valuable compounds from various plant materials (Corzzini et al., 2017). The concept of using supercritical

ﬂ

uids for

the separation of natural products was

ﬁ

rst reported at the end of the 1980s. Since 2015 to 2020, based on the key term search of

“

supercritical CO

2 ﬂ

uid extraction

”

in scopus database (on November 16, 2020), more than 2000 research publications were reported. Especially, most of the publications in the corresponding SFE

ﬁ

eld are receiving paramount importance as inferred from the increase in the number of publications from 279 in 2015 to 431 in 2020 (refer Fig. 1). In the search results, documents were limited to article or reviews. Recently, SFE has been also implemented in various new applications such as drug delivery (Pascoal et al., 2017), solvents for the chemical reaction (L opez-Domínguez et al., 2017), and environmental applications (Ghoreishi et al., 2016).

The rapid growth in urbanization and industrialization has led to unavoidable toxic metal and organic contamination of both living and non-living eco-systems worldwide. The use of toxic organic solvents for environmental pollutant mitigation is strictly not advisable due to the rising environmental concerns. Recently, SFE based toxic pollutant removal has been focused on replacing conventional extraction techniques such as PUREX (Plutonium Uranium Reduction Extraction) (Wagner et al., 2016). Also, the SFE approach has been used to reduce secondary solvent contamination and enhance toxic metal removal using chelating agents through forming a neutral metal complex, which can bring strong solute-solvent interactions in the non-polar Sc-CO

2

medium. This approach facilitates the adoption of SFE in environmental research applications, especially towards pollution mitigation (Amezcua- Allieri et al., 2012; Aslanidou et al., 2013; Berg et al., 1999;

Kawashima et al., 2009; Kosyakov et al., 2013; J.-H. Park et al., 2013a, 2013b; Wang and Chiu, 2009).

In the last

ﬁ

ve years (2015

e

2020), most of the review works

Fig. 1.Annual number of publications on the“supercritical CO2ﬂuid extraction”since 2015 to 2020. (data obtained from Scopus on November 16, 2020).

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have been published regarding the extraction of valuable compounds from microalgal biomass (Yen et al., 2015), Supercritical

ﬂ

uid extraction of essential oils and bioactive compounds (da Silva et al., 2016; Youse

ﬁ

et al., 2019), and supercritical

ﬂ

uid technologies for e-waste treatment (Li and Xu, 2019). Importantly, those review articles cover only the aspects of Sc-CO2 based on extraction technique or speci

ﬁ

c plant source materials, etc. Especially, the topic in the view of the bio/therapeutic values and Sc-CO2 solvent properties for bioorganic molecules, simple/complex organic/inor- ganic pollutants removal has not received the same level of interest in the past years. Thus, this review aims to provide detailed insights into Sc-CO2 extraction technique in the therapeutic and pollutants abatement applications. In this review, we present an overview of the recent research progress of SFE in both therapeutic/biological and environmental applications. Besides, the necessary informa- tion on the operation principle, process parameters, in

ﬂ

uencing factors, selectivity, and Sc-CO

2

solubility is also discussed. Finally, critical insights on the future trends of SFE

ﬁ

eld are provided.

2. Importance of plant metabolites

“

Plant metabolites

”

is an important source of active pharma- ceuticals. It has been an exciting topic of research in the past 50 years (David et al., 2015). In general, the plant body naturally consists of two types of metabolites: primary and secondary. The primary metabolites (carbohydrates, lipids, nucleic acids) are mainly involved in plant growth and reproduction. However, the secondary metabolites (such as phenolic compounds: simple phenols and

ﬂ

avonoids, terpenes: monoterpene, sesquiterpenes, ste- rols, alkaloids, etc.) mainly contribute to plant protection against diseases, environmental adaptability and other life-support func- tions (Rosado-Aguilar et al., 2017). For example, phenolic com- pounds in pomegranate (Punicagranatum L.) fruit peel contain:

ﬂ

avonoids (anthocyanins such as pelargonidin, delphinidin, cyanidin along with their derivatives and anthoxanthins such as catechin, epicatechin, and quercetin), tannins (ellagitannins and ellagic acid derivatives such as punicalagin, punicalin, and pedun- culagin) and phenolic acids (such as chlorogenic, caffeic, syringic, sinapic, p-coumaric, ferulic, ellagic, gallic and cinnamic acid) (Singh et al., 2018). Similarly, a signi

ﬁ

cant quantity of terpenoid- metabolites with remarkable biological properties in pumpkin (Montesano et al., 2018), carbazole alkaloids and coumarins in Clausena plants (Huang et al., 2017), and plant sterols (PS) from soybean oil, crude tall-oil (CTO) and pine tree pulp (Olkkonen et al., 2017) have also been reported.

Recently, several research studies and reviews have covered the biological effects of plant-derived secondary metabolites (Ramachandra Rao and Ravishankar, 2002; Sakamoto et al., 2018;

Shitan, 2016; Zhao et al., 2005; Zhou and Memelink, 2016). Some critical reviews highlighted the treatment of Candida infections using plant-derived bioactive compounds (Martins et al., 2015), promissory effects of phenolic compounds on oxidative-stress diseases (Martins et al., 2016), and sweet potato as a functional food (de Albuquerque et al., 2019), etc. Besides, numerous research studies have been performed to evaluate the biochemical proper- ties such as antioxidants (Lizcano et al., 2010), anti-in

ﬂ

ammatories (Shukla et al., 2010), anti-cancerous (Siriwatanametanon et al., 2010), anti-diabetic (El-Sayed, 2011), and antifungal activities (Jainkittivong et al., 2009) of the secondary metabolites, on human health. Therefore, the growing necessity for the development of proper extraction or isolation methods for these inherent bioactive chemicals from plants intends to receive more attention among researchers and industries.

3. Supercriticalﬂuid extraction (SFE) technology

The term

“

supercritical state

”

was

ﬁ

rst coined by Baron Gag- niard de la Tour in 1822. According to Hannay and Hogarth (1879), supercritical

ﬂ

uids are those that have pressure-dependent solvating power. However, a tremendous interest from industries and researchers on SFE was evident, only after the 1980s through pat- ents and more research publications (Sihvonen et al., 1999). Now, most of the SFE technology uses CO

2

as the preferred choice of solvent. CO

2

is typically odorless, colorless, non-

ﬂ

ammable, non- toxic, inert nature, safe, inexpensive, and recyclable.

The critical region of CO

2

originates at 31.1

C and 73 atm. In this supercritical state, CO

2

behaves like both gas and liquid. In detail, CO

₂

possesses high density like liquid, diffusivity like gas, and viscosity like gas-liquid. The distinctive characteristics of supercritical CO

2

(Sc-CO

2

) have gained much attention to use it as a solvent in extracting valuable bioactive compounds from plant biomass matrices. Usually, density is taken into consideration to determine the solvating power of solvents. In the SFE process, CO

2

initially diffuses into entire plant matrices and then dissolve valu- able phytochemicals using its solvent density properties. Some key advantages of the supercritical phenomenon, including solvating strength, diffusion coef

ﬁ

cient, and lower viscosity, greatly favor more ef

ﬁ

cient extraction of the components than the subcritical process.

The Sc-CO

2

extraction process requires moderate temperature and pressure for the separation of bioactive compounds (from plant materials) without any loss in their functional properties. Also, a simple depressurizing setup allows the

ﬁ

nal product to be solvent- free at the end of the extraction step. Therefore, Sc-CO

2

based extraction techniques have been widely employed to extract many valuable bioactive compounds from plant materials. Several research studies have focused on Sc-CO

2

based extraction techniques (Bagheri et al., 2014; Naziri et al., 2016; S anchez-Camargo et al., 2011). For example, Sc-CO

2

assisted phytochemicals extraction from cacao pod husk (Valadez-Carmona et al., 2018), Draco- cephalumkotschyi seed oil (Sodei

ﬁ

an et al., 2017), Persimmon (Diospyros kaki L (Zaghdoudi et al., 2016), sugar beet and sugar cane molasses (Varaee et al., 2019) and cha~ nar (Geof- froeadecorticans) almond oil (Salinas et al., 2020) have been reported. However, the polar/non-polar nature of the extracted bioactive compounds imposed severe restrictions on the single- step SFE process.

Sequential or multistep extraction has been performed with the

nonpolar type super-critical CO

2

along with or without other co-

solvents, such as ethanol (polarity

¼

5.2) and water

(polarity

¼

9.0) (Garmus et al., 2014). Moreover, the idea of

sequential extraction was emphasized to demonstrate the

maximum amount of various types of bioactive compounds

extraction from a single plant source. For example, most of the

recent research works are related to sequential SFE extraction that

has been adopted as an effective technique to selectively extract as

well as to improve the product yield of speci

ﬁ

c compounds (de

Aguiar et al., 2019; Monroy et al., 2016a; S€ okmen et al., 2018). For

instance, Juliane Vigan o et al. (2015) implemented a three-stage

SFE setup which was operated at 60

C/17 MPa, 50

C/17 MPa and

60

C/26 MPa as a sequential Sc-CO

2

extraction system. It was

observed that the density of Sc-CO

2

was increased from 669.1

(Stage-I) to 803.4 kg/m

³

(Stage-III) across the stages. The major

extracted compounds were tools, fatty acids, and carotenoids in the

ﬁ

rst, second, and third stages, respectively. Moreover, as compared

to a single-stage process, the overall product yield was signi

ﬁ

cantly

increased from 4.39 to 20.34% (Vigan o et al., 2016). Similarly, in

another research study to extract different polar bioactive

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compounds, the

ﬁ

rst and second stages of the SFE unit was operated at 60

C/10 MPa and 40

C/20 MPa. The results revealed that geranylgeraniol and tocotrienols compounds were obtained as major compounds in the

ﬁ

rst and second stages, respectively (Vardanega et al., 2019).

Even other extraction techniques such as pressurized solvent processes, subcritical CO

2

(SubC-CO

2

) extractions, subcritical water extraction, and UAE have been used in integration with multi-step supercritical CO

2

extraction. A research study employed supercritical extraction as a pretreatment step, followed by subcritical (SubC-CO

₂

). Many lipophilic characteristics of Sc-CO

₂

were not appropriate for polar moieties extraction. Implementing SubC-CO

2

based extraction with 10% co-solvent of ethanol was more effective for selective extraction of cyanidin glycosides, ellagic acid, and quercetin glycosides from Bilberry (Babova et al., 2016). As the same, the pressurized solvent (ethanol) process was used with Sc- CO

2

based extraction to separate high total phenolics contents from mango peel waste (Mangifera indica L) (Garcia-Mendoza et al., 2015).

Further, some modern extraction techniques such as UAE were also employed in the sequential extraction process (Guandalini et al., 2019; Zekovi c et al., 2015). In the ultrasonic-assisted extraction (UAE) process, the formation of a cavitation bubble and its collapse increases the mass transfer rate by penetrating the solvent deep into the cell matrices of the plant biomass. Typically, the increase in temperature lowers the density and viscosity of the solvent, which facilities cavitation bubble formation in the extraction medium to enhance the mass transfer rate towards higher product yield. For instance, Carla Da Porto et al. (2015) employed the combined extraction process of UAE followed by SFE for grape marc. The total polyphenolic yield was high for the combined process as compared to the individual Sc-CO

2

extraction technique.

During the UAE process, ultrasonic waves create many cavitation bubbles within low viscosity of solvent at high temperature. This results in increases of cohesive forces to improve the mass transfer rate for extracting more polyphenolic compounds. Thus, the integrated process was found to be more effective for extracting the proanthocyanidin compounds (Da Porto et al., 2015).

4. Critical operational parameters for the Sc-CO2based extraction

Highly compressible gases trapped in liquid solvents (co-solvents) result in a new class of tunable solvents called Gas-expanded liquid (GXL), which consists of bene

ﬁ

cial extraction properties. The usage of CO

₂

gas to prepare GXL solvents is safer and more economical. To improve the extractability of CO

2

based SFE processes, a small amount of organic solvent(s) is mixed with a larger quantity of CO

₂

as a

“

modi

ﬁ

er

”

which improves the mass transfer rate and gas solubility of the GXL solvent, resulting in high extraction ef

ﬁ

ciency (Lee et al., 2010; Pan et al., 2012; Shi et al., 2013). On the other hand, major operating parameters like solvent type, temperature, pressure, and CO

2ﬂ

ow rate also plays a critical role in the enhancement of selectivity and global yield of SFE (Domingues et al., 2012; Rebolleda et al., 2012; Saravana et al., 2017).

The addition of polar co-solvents to Sc-CO

2

based extraction process could bring major in

ﬂ

uence by altering the solvating properties of non-polar Sc-CO

₂

. Most well-known polar solvents such as water (Yang et al., 2013) and alcohols (methanol and ethanol) (Ghafoor et al., 2012; Hedayati and Ghoreishi, 2015) have been studied for the improvement in the selective solvating power of Sc-CO

2

for effective extraction of polar compounds. Several re- searchers have proposed the use of individual solvents such as

ethanol and methanol or mixtures of these solvents to facilitate the extraction of highly polar natured phenolic phyto-compounds. The high solvating power in high-density solvents strengthens the bonds between the solute and plant matrix, followed by the dissolution of solutes into them and outbound carriage from the plant matrix. This entire extraction phenomenon by the modi

ﬁ

ed Sc-CO

2

solvent is characterized by a relatively low mass transfer resistance as compared to the unmodi

ﬁ

ed Sc-CO

₂

based extraction.

Several research studies have recommended the use of small amounts of the co-solvent (1e15% of ethanol to obtain more effective extraction (Da Porto et al., 2014; Pimentel et al., 2013). For example, Carla Da Porto et al. (2015) observed that using 10%

ethanol-water mixture at 8 MPa and 40

C yielded about 57% of polyphenols from spent

Rosa damasceneﬂ

owers (Porto et al., 2015).

In contrast, other studies have reported the preference for a high concentration of solvents. For example, the usage of 40% of ethanol- methanol mixture was reported optimal for guaran a (Paulliniacu- pana) seeds. The obtained extract possessed a total phenolics content (TPC) of 105.7 pyrogallol equivalent per gram of guaran a seeds (Marques et al., 2016). Alternatively, the authors have claimed that the use of water as a ternary solvent could reduce 88% of ethanol consumption in the solvent mixtures. Therefore, the ternary solvent mixture was labeled as green and cost-ef

ﬁ

cient for the extraction of bioactive compounds (Kühn and Temelli, 2017).

Temperature is one of the crucial factors that in

ﬂ

uence the product yield ef

ﬁ

ciency of the SFE process, based on the plant matrix nature. Only a few studies are available for the effect of temperature on the product yield of Sc-CO

₂

based SFE processes (Joki c et al., 2017; Liza et al., 2010; Pavli c et al., 2018; Sonsuzer et al., 2004). For example, in babassu seeds (Orbignyaphalerata) oil extraction, the increase in temperature from 60 to 80

C for the SFE process has enhanced the oil yield from 86.56 to 89.86%. Higher operational temperatures showed to improve the formation of lighter phase oil through volatilization when compared to heavier oil fractions formed in the lower temperature extract (de Oliveira et al., 2019). In another research study, lycopene yield (from to- mato skin) showed an incremental trend with an isobaric increase in temperature (Fig. 2). With the increase in temperature, super- critical CO

2

attained low-density solvent properties on one hand, whereas the volatility of the phyto-compounds was improved on the other. Further, some degradation effect at high temperature was re

ﬂ

ected in the yield of lycopene (1.17 and 1.18 mg of lycopene/g of the sample at 363

e

373 K/a constant pressure of 40 MPa, respectively). Finally, it was noticed that lycopene solubility was high due

Fig. 2.Yield vs. CO2consumed at constant pressure (40 MPa) and different operating temperatures at a solventﬂow rate of 2.5 mL/min (Reproduced with permission from (Topal et al., 2006), copyright year: 2006, publisher: American Chemical Society).

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to the dominant volatility effects than the solvent density effects (Topal et al., 2006).

Moreover, some bioactive compounds are heat sensitive, which means that high temperatures may lead to a destructive effect on the compound structure, and

ﬁ

nally, would result in lower product yield (Sharif et al., 2015). For example, adverse negative temperature effect was seen in milk thistle seed oil extraction. In this work, the authors have studied the oil extraction at various temperatures of 40, 60, and 80

C by keeping the pressure and

ﬂ

ow rate constant at 200 bar and 4 mL/min CO

2

, respectively. The results showed that 40

C was an optimized temperature that demonstrated the highest overall oil yield of 129.64 mg/g along with silybin A yield of 0.93 mg/g and silybin B yield of 1.21 mg/g in the

ﬁ

nal extract. An increase in temperature to 80

C from the optimized condition led to a tremendous decrease (~29-folds) of solvent density. This dominant solvent density lowering factor, as compared to the rise in oil vapor pressure, resulted in decreased oil yield (Fig. 3a) (Çelik and Gürü, 2015).

The operational pressure of the SFE process also attributes to the solvent behavior and overall performance of the extraction process.

In some cases, high pressure at constant temperature leads to the domination of the solvent density effect compared to the vapor pressure effect. This means that solvent density has a linear rela- tionship with the extraction yield. For example, Perva-Uzunali c et al. (2004) reported that increasing the pressure from 200 to 400 bar (at a constant temperature of 80

C) improved the CO

₂

density from 595 to 824 kg/m

³

. The result implied that the extraction performed at 80

C/400 bar resulted in a high yield of

12.8% with more than 96% of capsaicinoids in the obtained product (Perva-Uzunali c et al., 2004). The plausible reason was that high operational pressure could be more effective in mitigating the external and internal mass transfer resistance for the extraction process. Although high-pressure favors high product yield, some studies have reported different results (Ahmed et al., 2012; Akay et al., 2011). For instance, SFE of loquat seed oil reported that the overall yield was independent of the operational pressure, for the extraction performed at a constant temperature of 40

C. Possible reasons consisted of the limited solubility of the oil components of the seeds as well as the strong packing of plant matrices, which limited the penetration ability of the Sc-CO

2

(Machmudah et al., 2008).

Other than temperature and pressure, solvent

ﬂ

ow rate has a signi

ﬁ

cant in

ﬂ

uence on the SFE. A few research studies have observed the effect of

ﬂ

ow rate on the yield of the SFE process (Sodei

ﬁ

an et al., 2016; Wang et al., 2012; Zhang et al., 2010). In most cases, an increase in the

ﬂ

ow rate of CO

2

improved the interactions between the solute components of the plant matrix and the solvent due to turbulence. This phenomenon helped to reduce the external mass transfer resistance and resulted in higher product yield (Bernardo-Gil et al., 2011). In contrast, some results showed a negative or null impact when the solvent

ﬂ

ow rate goes beyond the optimum value. For example, in the case of milk thistle seed oil extraction, no signi

ﬁ

cant amounts of the oil and total silybin were obtained for CO

₂ﬂ

ow rate

>

4 mL/min, as depicted in Fig. 3b (Çelik and Gürü, 2015).

5. Biological applications of Sc-CO₂based SFE

Plant extract obtained through Sc-CO

2

extraction are enriched with a wide range of phytochemicals and exhibit antioxidant properties along with various other therapeutic properties, such as antimicrobial, anti-in

ﬂ

ammatories, anti-diabetic, and anti- cancerous activities. For example, Daniel Ribeiro Grij o et al.

(2019) observed that the supercritical extracts of cannabis

ﬂ

owers with a high concentration of neutral cannabinoids exhibited superior anti-tumor activity for cervical cells (Ribeiro Grij o et al., 2019). Similarly, a variety of polyphenolic compounds, including hydroxytyrosol, chlorogenic acid, and caffeic acid, obtained from SFE extracts of olive oil residues, possessed high antioxidant, anti- cancer, and anti-diabetic capacities (Caballero et al., 2020). In general, the selectivity of the supercritical extraction

ﬂ

uid is an important factor in determining the functional properties of the

ﬁ

nal extract. Changing the operating conditions, including temperature, pressure, and modi

ﬁ

er solvent type/concentration, could achieve selective extraction of the targeted compound(s) from the biomass. Generally, the volatility of solute molecules (in the plant matrices) and solvent density may decrease or increase with regard to the variations in the operational temperature and pressure.

Achievement of selective extraction of compounds as well as high product yield, is still considered a signi

ﬁ

cant challenge in the SFE process. Enrichment of speci

ﬁ

c type of compound(s) in the extract yield usually results in higher selective products for speci

ﬁ

c biological/therapeutic applications.

5.1. Cytotoxic, antiproliferative and anti-inﬂammatories activity

Bioactive alkaloids (such as pyrrolizidine, quinoline, isoquinoline, pyridine, indole, and phenanthrene) are used as anti-tumor agents owing to their cytotoxic potential achieved by the cell membrane disruption phenomenon in eukaryotic cells. For example, isoquinoline and indole types of alkaloids were extracted from

Melocactus zehntneri

aerial parts (young plant) using Sc-CO

2

.

Fig. 3.Effect of (a) Temperature and (b) CO2ﬂow rate on amounts of silybin A, silybin

B, total silybin and oil (Reproduced with permission from (Çelik and Gürü, 2015), copyright year: 2015, publisher: Elsevier).

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The supercritical extraction conditions were optimized to produce alkaloids enriched

ﬁ

nal product, which was free of polysaccharides.

However, the increase in the extraction temperature at constant pressure has adversely affected the concentration of the alkaloids in the extract, thus showing less activity against HMVII (an epithelial and tumoral eukaryotic and mitochondrial cell line). The cytotoxicity of the extract was signi

ﬁ

ed by conventional plasmatic mem- brane disintegration as well as interactional effects of other potential alkaloids present in the extract with the cell contents (Brand~ ao et al., 2017).

High levels of lipid-soluble antioxidants all-trans-zeaxanthinall- trans-b-carotene, and a-tocopherol), total phenols, and total

ﬂ

a- vonoids present in the

L. europaeum

oil SFE extract, showed signi

ﬁ

cant

in vitro

inhibitory effect on the growth of colon adenocarcinoma cells (Rosa et al., 2017). Pressure and temperature effects were studied for anti-tumor enriched SFE extract production from

Schinus terebinthifolius

raddi fruits. These extracts were investigated for their antiproliferative activity using nine types of tumor cells. Among the examined cell lines, kidney cancer cells showed total growth inhibition value lower than the positive control for all the extracts obtained at 50e60

C, independent of pressure effects. From the Gas chromatography

e

mass spectrom- etry (GC-MS) results, the sesquiterpenes based metabolites, such as d -3-carene, a -phellandrene, limonene, germacrene D, and caryophyllene present in SFE extract were identi

ﬁ

ed as anti-tumor activity contributors. At high pressure, solvent density has increased, which caused a reduction in the decreased d -3-carene metabolite due to preferential extraction of high molecular weight components (triterpenes, acids, phenolic compounds, and carotenoids) (Fig. 4 a). However, global yield was found to increases from 13 to 13.7% under high pressure due to the enhancement of solvent density or strength (Fig. 4 b) (Silva et al., 2017).

Cytotoxicity and maturation analysis of Bone Marrow-derived Dendritic Cells (BMDC) were studied using radish leaves extracts acquired through SFE. The extracts showed a maximum phenolics contents of 1375 and 1455 mg gallic acid equivalents/100 g dried mass at operating conditions of at 35

C/400 bar and 40

C/400 bar, respectively. The extracts obtained at different conditions showed a reduction in the cell viability with an increase in the extract concentration. Also, it was observed that the extracts did not promote the Dendritic Cell maturation due to their high antioxidative and anti-in

ﬂ

ammatories activities (Goyeneche et al., 2018). Some re- ports showed that the addition of modi

ﬁ

er solvents could improve the biological properties of the resultant extract. For example, ethanol derived SFE extracts of

L. rivularis stalks

showed signi

ﬁ

cant anti-in

ﬂ

ammatory properties. In this process, the addition of an optimal concentration of 1.0 wt% of ethanol to Sc-CO

2

, resulted in extracts supplemented with caryophyllene oxide (sesquiterpene), catechin, quercetin, and resveratrol (

ﬂ

avonoids). Consequently,

these bioactive compounds were capable of inhibiting enzymes such as a -amylase and a -glucosidase, which can cause type-2 diabetes (Uquiche et al., 2019). Interestingly, SFE process with Sc-CO

2

resulted in extracts with comparable cytotoxic activity with other extraction processes. For example, Limonoid extracts obtained from

Citrus aurantifolia

by SFE and solvent extraction methods showed almost similar IC

50

values of 8.5 and 9 m g/mL, respectively, for their cytotoxic activity assessment against L5178Y lymphoma cells (Castillo-Herrera et al., 2015).

5.2. Antimicrobial activity

Several research studies reported the antibacterial activity of the Sc-CO

2

assisted SFE extracts. Essential oils that were derived from the

Thymus

genus showed strong antimicrobial potential due to the high quantity of monoterpenoid phenols (thymol and carvacrol) or monoterpenic alcohols (geraniol and linalool) present in it. Besides, the high thymol content of the produced extract imparted better antimicrobial activities to it than commercial fungicides like bifo- nazole and ketoconazole, for all the tested microbial strains except for

A. fumigates

(Petrovi c et al., 2016). Usually, a minimum inhibitory concentration (MIC) value determines the antibacterial activity of extracts. The MIC values must be

<

500 m g/mL for strong inhibitors, 600

e

1500 m g/mL for moderate inhibitors and

>

1600 m g/

mL for weak inhibitors. For instance, butia seed SFE extracts exhibited moderate MIC value (587 m g/mL) against the

B. cereus

and strong MIC values (196 m g/mL to 98 m g/mL) against

E. coli. Apart

from fatty acids, the extracts consisted of some considerable amount of phenolic moieties that exhibited inhibitory effects on gram-positive and gram-negative bacteria. Especially, high con- centrations of cinnamic acid and caprylic acid components in the extract had less interaction with lipophilic membrane proteins of gram-positive bacteria. But, this was reversed in the case of gram- negative bacteria where phenolics in the extract had strong hydrogen bond interaction with the cell wall, which resulted in severe cell destruction (Cruz et al., 2017).

In general, gram-negative bacteria are structurally different from the gram-positive ones. The double-walled cell membrane of gram-negative bacteria provides strong obstruction for foreign substances to interact with it. But, in the case of gram-positive bacteria, there is no cell wall, and the lipophilic character of this bacteria facilitates high penetration of foreign substances having the same lipophilic characteristics. A schematic representation of this phenomenon is illustrated in Fig. 5. For example, after the supercritical extraction of

chia

seeds, the

ﬁ

nal extract had a fatty acid chemical pro

ﬁ

le composed of high content of linoleic acids (~19%). The MIC studies on this extract revealed no activity for gram-negative

E. coli,

whereas weak inhibitory effects were observed for gram-positive

B. cereus

(Guindani et al., 2016). In

Fig. 4. Effects of Sc-CO2density on (a)d-3-carene mass content and (b) global yield (Reproduced with permission from (Silva et al., 2017), copyright year: 2017, publisher: Elsevier).

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another study, SFE extracts of

C. xanthocarpa

consisted of hydrophobic constituents such as a -eudesmol, b -eudesmol, g -eudesmol, caryophyllene (E), a -sabinene, b -sabinene, germacrene B, d -cadinene, humulene, and Selina-3,7(11)-diene which were completely resistant to gram-negative bacteria (E. coli,

Pseudomonas aerugi- nosa,

and

Salmonella typhimuruim). The outer layers of these gram-

negative bacteria were made up of lipopolysaccharide, which possessed strong hydrophilic characteristics and prevented the penetration of the active hydrophobic components of the extract.

However, the extracts were effective against the gram-positive bacteria, speci

ﬁ

cally the extract obtained at low temperature and pressure of 40

C/15 MPa as compared to extracts acquired at other pressure and temperature conditions (Czaikoski et al., 2015).

The zone of inhibition test is one of the easiest methods to evaluate the antimicrobial activity of the plant extracts. Mostly, a clearance zone size of

>

21 mm represents a high antibacterial activity of the extract. In the case of SFE derived plant extracts, operational temperature and pressure are the signi

ﬁ

cant parame- ters which in

ﬂ

uence the antimicrobial activity of the extracts.

A. lappa

leaves extracts obtained using lower temperature showed a maximum of inhibition zone of 31.6 mm against

Staphylococcus aureus

for the extracts. On the other end, extracts obtained under high temperatures showed a relatively smaller inhibition zone size of 24.5 mm. The major antibacterial compounds present in the extract were lupeol acetate, amyrin acetate, diisooctyl phthalate, and phytol. However, variations in operational pressure produced no changes in the antibacterial effects of the resultant extracts (de Souza et al., 2018).

It was observed that a generalization rule on the operational temperature and pressure effects on the antimicrobial activity of SFE extract is complicated, and there is a need for in-depth in- vestigations to understand the trend better. Also, the chemical nature of plant biomass, as well as the interactional mechanism of SFE solvent with plant matrices play a critical role in determining the antibacterial activity of the product extract. Santos et al. (2019) reported on the bactericidal evaluation of feijoa peel extract ob- tained using Sc-CO

₂

and ethanol solvent mixture. It was found that the presence of high ferulic acid content in the extract, which was derived from the plant biomass, was the major reason to induce the bacteriostatic and bactericidal effect against both gram-positive and gram-negative bacterial strains (Santos et al., 2019).

5.3. Antimalarial activity

The World Health Organization (WHO) has announced that malaria is one of the life-threatening diseases, which is caused by female anopheles mosquitoes. Plasmodium parasites are typically injected into blood through the mosquito bite, which causes severe acute febrile illness. WHO in 2006 declared artemisinin-based combination therapies (ACT) as remedial measures for treating malaria.

Artemisiaannua

L. leaves one of the plant biomass that possesses natural chemical ingredients like sesquiterpenoids (artemisinin),

ﬂ

avonoids, and coumarins exhibit strong antima- larial activity. Since

A. annua

leaves consist of suf

ﬁ

cient artemisinin, several research studies have been focusing on the extraction of such bioactive compounds from the leaves. Many extraction techniques, such as soxhlet, supercritical, ultrasound-assisted, and microwave-assisted methods have been reported for the artemisinin recovery using various types of non-polar organic solvents.

Among them, SFE using Sc-CO

₂

has received speci

ﬁ

c interest due to its solvent strength (density) and less solvent contamination.

Recently, Martinez-Correa et al. (2017) reported on Sc-CO

2

based SFE extraction of

Artemisiaannua

L. leaves extract using a single and multi-step approach. The product yield of 5.47 mg/g of leaves with a high artemisinin concentration of 95.1 mg/g of extract was obtained for the single-step Sc-CO

₂

facilitated SFE operated at 60

C/

40 MPa. The extract exhibited strong antimalarial activity with 50 and 100% inhibition of

P. falciparum

parasite growth for concen- trations

<

0.1 and 0.39 m g/mL. The antimalarial effect results of the extracts obtained for the single-step Sc-CO

2

and normal ethanolic extraction process were comparable, as shown in Fig. 6. However, SFE using Sc-CO

2

as solvent was more selective in Artemisinin extraction, and the adoption of a multi-step approach resulted in the extract of a higher content of phenolic compounds in the second stage (Martinez-Correa et al., 2017a).

Also, Martinez-Correa et al. (2017) have selected

Curcuma longa

L. leaves to study the relationship between antimalarial activity and extraction methods. The results obtained in this study were similar to the author

’

s previous research work on

Artemisiaannua

L. The single-step extraction (ethanol and Sc-CO

2

) exhibited notable antimalarial activity against

P. falciparum

parasite growth with an IC

₅₀

value of ~16 m g/mL. However, based on the curcumin content in both extracts, it was understood that single step Sc-CO

2

extraction was more selective for non-polar antimalarial compounds present in the

Curcuma longa

L leaves (Martinez-Correa et al., 2017b).

Additionally, Table 1 provides an overall summary of recently published research works on the therapeutic based functional properties of Sc-CO

2

assisted SFE extracts derived from various plant sources.

5.4. Antioxidant properties

Biologically, antioxidants are the class of bioactive compounds that inhibit or slow down the oxidation of various substances such as DNA, lipids, and food materials. Antioxidants are very essentials for humans to prevent as well as to cure several disorders. Human ingestion of antioxidant compounds mostly happens through di- etary supplements, nutraceuticals, and functional food ingredients.

In this aspect, selective isolation and puri

ﬁ

cation of these antioxidants from several plant sources have gained much importance in food chemistry and technology. Recently, supercritical extraction has been exploited to extract bioactive compounds with a wide range of antioxidant properties from different plant sources. Most common methods, such as 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) reduction, Ferric ion reducing antioxidant power (FRAP) assay, and 2,2

⁰

-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid

Fig. 5. Anti-bacterial activity of plant extract against gram-positive and gram-negative

bacteria.

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(ABTS) protocol have been widely adopted to estimate the antioxidant properties of these extracts (Fig. 7). Phenolics,

ﬂ

avonoids, terpenoids, and carotenoids are the most common bioactive com- pounds which exhibit the antioxidant properties in plant extracts derived by SFE technology. For example, fatty acids (punicic acid) in Pomegranate (Punicagranatum L.) seed, Phenolics (hydroxytyrosol) in Olive (Olea europaea) residues, monoterpenoid phenols (Thymol) in

Thymus praecox

are some of the recently reported plant materials for antioxidants rich extract production using Sc-CO

₂

based SFE technique (Petrovi c et al., 2016). Interestingly, the Sc-CO

2

extracted oleoresin class of antioxidants from the tomato (Lycopersicum

esculentum

L.) peels waste exhibited a competitive antiradical activity as compared with the synthetic antioxidant, butylated hydroxytoluene (Kehili et al., 2017).

In general, the radical scavenging activity of a given plant extract is interpreted from its IC

50

value through which better antioxidant activity can be obtained at less values. The antioxidant activity of SFE derived plant extract is mainly in

ﬂ

uenced by the extraction conditions. Antioxidant activity of Rowanberry (Sorbus aucuparia L.) pomace extract derived through Sc-CO

2

based SFE was lower than pressurized liquid extraction techniques (Bobinait_ e et al., 2020). In another study, the addition of 15% of ethanol during the SFE of uvaia (Eugenia pyriformis Cambess.) leaves demonstrated high antioxidant activity of the resultant extract. High polarity of the ethanol modi

ﬁ

er enabled the solubilization of potentially bioactive compounds, which elevated the antioxidant capacity of the extract (Klein et al., 2019). In another work with radish leaves, the DPPH radical scavenging effects of SFE extracts derived for different extraction conditions showed signi

ﬁ

cant variations among them.

DPPH based antioxidant activities were estimated as 359 and 403 mg Trolox equivalents/100 g of dry leaves mass for SFE extract of the leaves obtained at 35

C/400 bar and 40

C/400 bar, respectively (Goyeneche et al., 2018). Some SFE studies reported a linear increase in the antioxidant activity for incremental variations in the operating pressure of the extraction process (Uquiche et al., 2019) (de Souza et al., 2018). In addition to phenolics, certain other lipid-based bioactive phytoconstituents contribute to the antioxidant activity of the extract. For example, SFE extracts obtained from sage herbal dust were assessed for their antioxidant ability through DPPH and FRAP assay. The highest antioxidant ability was reported for the extract obtained at operational conditions of 50

C and 300 bar. It was also observed that the extended extraction of diterpenes, in addition to phenolics at these operating conditions, enhanced the overall antioxidant activity of SFE extract (Pavli c et al., 2018). Moreover, Table 2 summarizes recent research works related to the antioxidant activities of several Sc-CO

₂

based SFE extracts obtained from a wide range of plant biomass.

6. Environmental applications of Sc-CO2based SFE

The tremendous increase in the CO

₂

emissions in the past few decades has led to an imbalance on earth

’

s biosphere in terms of drastic shifts in climate change, melting of polar icecaps, oxygen depletion, etc. To annul these harmful effects of the CO

₂

emission without any comprise on industrialization and urbanization, most of the countries try to adopt effective and novel measures in mitigating the CO

₂

emissions through various approaches (Heck et al., 2018; Ouyang and Lin, 2017; Thompson et al., 2016). On the other hand, the massive trend of industrial growth has resulted in severe air, water, and soil contamination by toxic heavy metals and persistent organic pollutants dispersion through the industrial ef-

ﬂ

uents (Cetin et al., 2019; Costa-B€ oddeker et al., 2020; Jia et al., 2019). Also, substantial amounts of heavy metal and organic micro-pollutants generated from natural resources contribute to environmental pollution (Li et al., 2014; Quilliam et al., 2013). Uti- lization of CO

2

as a solvent in the SFE technology for the removal of these hazardous metals and organic pollutants would address the problems of both CO

₂

mitigation and environmental pollution due to heavy metals/organic pollutants (Meskar et al., 2018; Salda~ na et al., 2005; Wang et al., 2018). Also, usage of CO

2

gas generated in process industries such as food and pharma, as a solvent medium in SFE technology results in closed-cycle operations and signi

ﬁ

- cantly reduce the CO

2

emissions. Most of the current research work on toxic metal recovery and toxins elimination from soils have emphasized the usage of Sc-CO

₂

based SFE as an ef

ﬁ

cient non- conventional technique for a given application.

6.1. Toxic micro-pollutant removal application

SFE technology, either as a standalone or as an integrated pro- cess, has emerged as an effective method for the extraction of different analytes from the soil and marine sediment samples.

Examples of these hazardous analytes include polynuclear aro- matic hydrocarbons (PAHs) (Librando et al., 2004), organophos- phorus pesticides (Naeeni et al., 2011), organochlorine pesticides (Quan et al., 2010), and various other priority pollutants in soils (Meskar et al., 2018; Salda~ na et al., 2005; Wang et al., 2018).

Removal of PAHs-based micro-pollutants by SFE facilitated by an online carbon nanotubes (CNTs) network has been studied using spiked soil samples. The CNTs acted as an ef

ﬁ

cient trapping sorbent for the removal of PAHs and their nitro-, oxy-, and alkyl-derivatives.

The recovery of the PAHs and their derivatives were obtained in the

range of 62.9e111.8%. The integrated SFE process exhibited good

selectivity and sensitivity for the removal of 16 types of standard

PAHs as well as 15 types of their derivatives (Han et al., 2015). Sc-

CO

2

based SFE extraction of n-aliphatic hydrocarbons from ground

Fig. 6.Antimalarial activity ofA. annuaextracts: (a) Sc-CO2and (b) Ethanolic extract (Reproduced with permission from, (Martinez-Correa et al., 2017a), copyright year: 2017, publisher: Elsevier).

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samples of Marcellus shale has been reported. The extraction was performed to 80

C and 21.7 MPa to represent

in situ

reservoir conditions. Aliphatic hydrocarbon yields of 0.3e12 mg/g TOC of the feed were obtained. The SFE technique proved to be effective for the mobilization of residual organic content trapped in overmature shales (Jarboe et al., 2015). Interestingly, the Sc-CO

2

based SFE

approach was used to clean the trapped micro-pollutants (oil) from micro-porous membrane surfaces. Powerful

“

greener

”

solvents such as ethyl alcohol or isopropyl alcohol combined with Sc-CO

2

gave satisfactory results in oil removal from the membrane surfaces (Micha

ł

ek et al., 2015).

Ben Said et al. (2015) studied the application of Sc-CO

2

based SFE

Table 1

Summary of the research works published on therapeutic effects of Sc-CO2assisted palnt extracts.

Plant material Major Compound Therapeutic effects Optimum extraction

condition

Ref.

Cannabis sativaL.

(genus cannabis) ﬂower extracts

D⁹Tetrahydrocannabinol content Cytotoxicity and anti-tumor activity against cervical tumor cells (50% inhibition of cells (CC50) for Hela, SiHa and C33 is 2.7±0.1, 3.1±0.7 and 2.8±0.4 mg mL1, respectively)

Pre-heating: 140C for 30 min, 70C and 40 MPa, r¼0.857 g mL¹

Ribeiro Grijo et al.

(2019)

Melocactus zehntneri (Cactaceae) aerial parts

Alkaloid (isoquinoline and indole type) Cytotoxic activity against HMVII andTrichomonas vaginalis55C and 30 MPa, r¼850.22 kg/m³

Brand~ao et al. (2017)

Lycium europaeum (European Wolfberry) fruit

Carotenoids (all-trans-zeaxanthin and all-trans- bcarotene)

Cytotoxicity against Caco-2 cell (L. europaeum oil: 50mg/mL (35% reduction) by MTT test and 100mg/mL (28% reduction) by AlamarBlue assay.

)

40C and 30 MPa Rosa et al.

(2017)

Schinus terebinthifolius Raddi fruits (Anacardiaceae family)

Volatile compounds:d-3-carene,a-phellandrene, limonene, germacrene D, and caryophyllene

Antiproliferative activity against human tumor cell Lines.

Total growth inhibition (TGI): Kidney (TGI¼0.8mg/mL), prostate (TGI¼4mg/mL), ovarian (TGI¼4.4mg/mL), and multidrug-resistant ovarian (TGI¼1.2mg/mL) cancer cells, and glioma (TGI¼3.9mg/mL).

50C and 20 MPa Silva et al.

(2017)

Citrus aurantifolia swingle (Key lime) seeds

Limonoids Cytotoxic activity against L5178Y leukemia lymphoblasts.

a signiﬁcant cytotoxic effect at concentrations>10mg/mL and IC50¼8.5mg/mL

60C and 44 MPa Castillo- Herrera et al. (2015) Raphanus sativusL.

(Radish leaves)

Phenolics and falvonoids Antioxidant capacity and cytotoxicity (dendritic cell) 35-40C and 40 MPa Goyeneche et al. (2018) Leptocarpha

rivularis(Stalks plant)

Phenolics (caryophyllene oxide, quercetin, kaempferol and resveratrol)

Anti-inﬂammatory activity, IC50¼1.66 mg/mL at co-solvent concentration of 1.5 wt%).

Inhibition ofa-amylase anda-glucosidase, two hydrolytic enzymes associated with diabetes mellitus type 2.

IC50fora-glucosidase (2.7 mg/mL) thana-amylase (15.1 mg/mL).

60C and 40 MPa with 1 wt% of ethanol as co- solvent

Uquiche et al. (2019)

Thymus praecoxssp.

Polytrichus (Thymus praecox)

Monoterpenoid phenols (Thymol) Antibacterial and antifungal activity.

(Minimum inhibitory concentration MIC: 38e200mg/mL, Minimal bactericidal concentration MBC: 75e300mg/mL for bacteria; and Minimum fungicidal concentration MIC: 17 e150mg/mL, MFC 35e300mg/mL for fungi).

40C and 10 MPa Petrovic et al. (2016)

Butia catarinensis (Butia) seed

Phenolics Strong antibacterial potential.

Minimum inhibitory concentration (MIC): 50C/10Mpa (196mg/mL) and at 60C/30 Mpa (98mg/mL) strongly inhibitedE. coli.

50C/10 MPa and 60C/30 MPa

Cruz et al.

(2017)

Salvia hispanica (Chia)

Fatty acids and phenolics Antimicrobial activity.

No extract showed inhibition againstE. colibacteria (gram negative) (MIC values>50,000.00mg/mL), whereas MIC (mg/mL) for B. cereus¼3125mg/mL (a weak inhibitory activity).

50C and 30 MPa with 5 wt% of ethanol as co- solvent

Guindani et al. (2016)

Campomanesia xanthocarpa (Guabiroba)

a-eudesmol,b-eudesmol,g-eudesmol, caryophyllene (E),a-sabinene,b- sabinene, germacrene B,d-cadinene, humulene and selina- 3,7(11)-diene

Antibacterial activity.

Effective inhibition zone of 21.34 mm forS. aureuswhereas S. typhimuriumandE. coli, were completely resistant.

40C and 15 MPa Czaikoski et al. (2015)

Arctium lappa (burdock) leaves

Phenolics Antimicrobial activity.

Diameter of inhibition zone (mm) is 31.6±1.20 mm for Staphylococcus aureus.

60C and 25 MPa with ethanol as co-solvent

de Souza et al. (2018)

Acca sellowiana (O.

Berg) Burret (feijoa) leaves

Phenolics Antimicrobial activity.

bacteriostatic and bactericidal effects against typical foodborne pathogens, at 6750 and 13,500mg$mL1.

55C and 30 MPa with 5% ethanol as co- solvent

Santos et al.

(2019) Artemisia annuaL.

(sweet wormwood) leaves

Sesquiterpene (Artemisinin) phenolic,ﬂavonoids Antimalarial activity.

Minimal inhibition concentration (MIC) of 0.39mg/mL, a 100% inhibition of P.falciparum parasite growth occurred.

Also, IC50values less than 0.1mg/mL showed high antiplasmodial activities.

60C and 40 MPa Martinez- Correa et al.

(2017a)

Curcuma longaL (Turmeric)

Phenolics, Flavonoids and curcumin Antimalarial activity.

high antiplasmodial activity due to IC50approximately 16mg/mL.

60C and 40 MPa Martinez- Correa et al.

(2017b)

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for the removal of model contaminants present in the recycled post-consumer polypropylene. Two different shapes of the food- grade polypropylene viz pellet (diameter of 2.5 mm) and

ﬁ

lm (thickness of 100

e

300 m m) were examined to understand the shape effects on the SFE removal of the pollutants. It was observed that the Sc-CO

2

based SFE of the contaminants was strongly dependent on the operational conditions of the process, with pressure being the signi

ﬁ

cant parameter. Complete removal of the contaminants was observed at 90

C and 200 bar for a contact time of 7.5 h from the pellet matrix, even with the heaviest contaminant concentration of 807 g/mol (Ben Said et al., 2016). Extensive use of electrical/electronic products has resulted in tremendous volumes of electronic wastes (e-wastes), whose safe disposal is highly challenging and has dragged the global attention. Recently, huge efforts have been implemented on identifying sustainable ways for the safe disposal of e-waste through Sc-CO

2

technology. Bromi- nated

ﬂ

ame retardants (BFRs) and polyvinyl chloride (PVC) are the major halogenated compounds that are widely used as

ﬂ

ame retardants in electronic display housing plastics. A recent study on the removal of these halogenated compounds from the plastic wastes by improved Sc-CO

2

based SFE has been reported. In this research, the extracted BFR additives were decomposed with PVC to generate the HBr and HCl, which were further eliminated by conventional halide stripping techniques. The results of the response surface methodology analysis showed a very high debromination and dechlorination ef

ﬁ

ciencies of 99.5 and 99.1%, respectively. The free radical mechanism (chain initiation, growth, and termination) was elucidated in the

ﬁ

nal formation of dehalo- gentaed carbon materials and organic chemical feedstocks using the SFE technology (Zhang and Zhang, 2020).

Sc-CO

2

based SFE processes have been investigated in textile applications for fabric cleaning and disinfection operations to

replace the conventional methods. A tunable green process of textile cleaning was reported by Aslanidou et al. (2016). The work involved the application of the Sc-CO

2

based SFE process and an aqueous suspension as co-solvent to clean the silk sample, which was contaminated with oil, rabbit skin glue, beetroot paste, and

Aspergillus Niger

fungus. High cleaning ef

ﬁ

ciency in the range of 97

e

99% was obtained under optimal operational conditions of 150 bar and 40

C in the presence of 5% w/w of Ca(OH)

2

. The complete absence of organic solvents in this cleaning process resulted in no color fading or migration in the textile fabrics (Aslanidou et al., 2016). Ammonium per

ﬂ

uorooctanoate (APFO) is a well-known additive that is commonly used in

ﬂ

uoropolymer synthesis. Usually, after the polymer production process, this APFO gets immobilized in the

ﬁ

nal

ﬂ

uoropolymer materials and subse- quently leads to serious environmental issues. Sc-CO

2

based SFE process was examined to eliminate this APFO residue from polymer surfaces. By altering the operational pressure and temperature in the range of 12e20 MPa and 60e100

C, the optimum extraction conditions were achieved for the effective removal of APFO. Thus, the high extraction ef

ﬁ

ciency of 92.3% of APFO was achieved at 12 MPa and 60

C. The reason for the high extraction ef

ﬁ

ciency was due to the high solubility of APFO residue in Sc-CO

2

at these conditions, which was accomplished via strong intermolecular interaction forces, as illustrated in Fig. 8 (He et al., 2020).

6.2. Toxic heavy metal recovery application

Recently, increased attention is paid for research investigation

on the usage of SFE technology for the removal of toxic heavy metal

ions. Most of the complexing agents employed in the SFE process

were adopted from the conventional metal removal processes

owing to their established complexation action (Falciglia et al.,

Fig. 7.DPPH, FARP and ABTS assay for antioxidant activity assessment of SFE extracts.

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Table 2

Antioxidant properties of plant extracts derived from Sc-CO2based extraction.

Plant material Major compound Antioxidant activity Extraction condition Ref.

Zea maysL. (Cob and pericarp of purple corn) Anthocyanins,ﬂavonoids and phenolics

DPPH method.

EC50(aqueous extract, 3rd Step)¼35.3 and 33.4 mg/mL for the cob and pericarp.

Sequential extraction step, 1st SFE:

50C and 40 MPa, 2nd Ethanol and 3rd aqueous extraction.

Monroy et al.

(2016a) P. edulis sp (Passion fruit bagasse) Tocols, fatty acids or carotenoids High antioxidant activity atﬁrst step, 60C and 17 MPa due to high

tocols content. DPPH value is 203.899 mg TE/100 g for the extract.

Sequential SFE process (60C/17 MPa;

50C/17 MPa; 60C/26 MPa)

Vigano et al. (2016)

Mangifera indicaL. (mango peel) Carotenoids % of antioxidant activity (%AA) is<20% for stage 1 and>70% for stage 2

extracts.

Two stage process: (i) Sc-CO2at 40C and 30 MPa (ii) pressurized ethanol

Garcia-Mendoza et al. (2015) Arctium lappa(burdock) leaves Diisooctyl phthalate, amyrin acetate,

lupeol acetate and phytol

DPPH and phosphomolybdenum reduction method.

%AA (250mg/mL) is 29.58%, IC50(mgextractmL¹) is 0.467±0.003 and AAphosphor(mgtocopherolgextract1 ) is 133.9±3.78.

80C and 15 MPa with Ethanol

0.8 gEtOHgCO21 de Souza et al.

(2018)

Lycopersicum esculentumL. (Tomato) peels Lycopene andb-carotene Highest DPPH quenching activity, equal to 84.51±5.25%. 50C and 30 MPa Kehili et al. (2017)

Pentaclethra

Macroloba(Pracaxi) seed

hydrophilic (phenolic compounds) and lipophilic antioxidants (b- carotene anda-tocopherol)

DPPH method showed>39% of inhibition and FRAP was>2.1mM Fe^2þ/ KgOil

40 and 60C and 20 MPa Teixeira et al.

(2020) Camellia sinensisvar.Assamica(Assam tea

seeds)

Fatty acids (oleic acid) and phenolics DPPH antioxidant capacity (IC50¼11.22 g/L oil sample)

ABTS antioxidant capacity (IC50) of the extracted oil was 9.65 g/L oil sample.

40C and 20 MPa (DPPH) 60C and 17.5 MPa (ABTS)

Muangrat and Jirarattanarangsri (2020)

Beta vulgaris(Beetroots) leaves Phenolics Antioxidant activity by DPPH was 506.6±2.3mg TE/g d.m. 35C and 40 MPa; Ethanol co-solvent Goyeneche et al.

(2020) Spondias tuberosa(Umbu) seed Phenolics and fatty acids Antioxidant activity (%) was only 17.1±1.7% and EC50(mg/mL) was

>5000.

40C and 15 MPa Dias et al. (2019)

Berberis vulgaris(Barberry fruit) Anthocyanins, phenolic compounds, and vitamin C

Antioxidant activity (AA) is 85.9%. 60C and 20 MPa Shariﬁet al. (2019)

Beta vulgarisL. (Beetroot aerial parts leaves) Phenolics DPPH EC50(mG/mL)¼150±7, ABTS TEAC (mmolTrolox/gextract)¼99±24 and FRAP PR(mmolTrolox/gextract)¼240±3.

40C and 25 MPa; 10% EtOH:H2O as co-solvent.

Lasta et al. (2019)

Glycine max(Soybean) residue Phenols andﬂavonoids DPPH inhibition percentages such as 39±2% and 27±3%. 35 and 40C and 40 MPa; 25% ethanol

as co-solvent

Alvarez et al. (2019) Oenocarpus distichusMart. (Bacaba-de-leque)

fruit residue

Unsaturated fatty acids with polyphenolics.

DPPH (mM Trolox/g) is 1019.28±66.77. 60C and 46 MPa Cunha et al. (2019)

Arbutus unedo(Strawberry-tree) extracts Phenolics (galloyl hexoside and 5-O- galloylquinic acid)

Oxygen Radical Absorbance Capacity (ORAC, mg TE/g d.w.) is 320.2±45.7 due to high TPC (mg GAE/g d.w.) of 37.0±3.1.

70C and 25 MPa Alexandre et al.

(2018)

Vaccinium myrtillusL. (Bilberry) seed Fatty acid&Vitamin E EC50(mg mL¹) is 9.5±0.1. 40C and 20 MPa Gustinelli et al.

(2018) Rosmarinus eriocalyx(Salvia jordanii) leaves a-tocopherol Trolox equivalent (TE) antioxidant concentration, TEACDPPHmmol TE/g

dw is 10.13±0.77, TEACABTSmmol TE/g dw. Is 203.17±3.5 and TEACFRAPmmol TE/g dw is 121.85±1.89.

70C and 45 MPa Bendif et al. (2018)

Allium sativum(Garlic Husk) Phenolics (garlic acid, 4-hydrobenzoic

22 acid, caffeic acid, p-coumaric acid, andtrans-ferulic acid)

Antioxidant activity (IC50) is 0.69 mg/mL. 200C and 10 MPa Chhouk et al. (2017)

Odontonema strictum(Acanthaceae) leaves Flavonoids DPPH antioxidant activity is 49.21%. 60e65C and 20 MPa; 15% methanol

as co-solvent

Ouedraogo et al.

(2018) Citrus ichangensisC. reticulate (Citrus fruits)

leaves

Carotenoids Antioxidant activity (DPPH assay (IC50values) is 0.98±0.01 mg cm³). 45C and 25 MPa Ndayishimiye and

Chun (2017) Eugenia involucrata(Cherry of the Rio Grande)

leaves

b-elemene (BE) and bicyclogermacrene (BG)

Antioxidant activity: 93.82±1.35% 40C and 20 MPa Ciarlini et al. (2017)

Crocus sativus(Saffron) petals Flavonoid and Polyphenols Antioxidant activity: 74.5±1% 62C and 16.4 MPa Ahmadian-

Kouchaksaraie and Niazmand (2017)

Vitis viniferaL. (Grape) seed Vitamin E

Polyphenols Carotenoids

Lipophilic antioxidant activity.

Trolox equivalent antioxidant capacity (TEAC) assay is 2.7emg/kg.

50C and 50 MPa Ben Mohamed et al.

(2016) Hylocereus polyrhizus(red pitaya) Flesh and Peel Betacyanins (Pigments) Antioxidant activity: 78.96% at 25 MPa; 50/50% EtOH-water mixture

(Flesh)

86.19% at 25 MPa; 70/30% EtOH-water mixture (Peel).

25 MPa; EtOH-water mixture as co- solvent

Fathordoobady et al. (2016) (continued on next page)

T.Arumugham,R.K,S.W.Hasanetal.Chemosphere271(2021)129525

11

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2016; Hutchison et al., 2008; Saleh et al., 2016; Sharma et al., 2017).

However, chemical composition and the structural nature of these complexing agents in

ﬂ

uence their solubility in Sc-CO

2

(Pitchaiah et al., 2017). Few of the notable complexing agents that have been reported as ligand molecules for Sc-CO

2

based SFE of metal ions recovery include calixarenes (Rathod et al., 2015), carbamate- conjugated catechol (Park et al., 2020), tributyl phosphate (Park et al., 2020), N,

N-Dialkylamides (Pitchaiah et al., 2019), Cyanex

921(Sovizi and Dehghani, 2016), etc. For example, tributyl phos- phate was used with Sc-CO

2

to form a non-polar complex with selected rare earth metals like cerium, lanthanum, and neodymium (Nd) (Song et al., 2020).

SFE extraction process for metal recovery applications is signi

ﬁ

cantly in

ﬂ

uenced by the solubility, stability, and chemical structure of metal chelates. Hung et al. (2016) reported the use of trioctylamine and 2-ethylhexyl 2-ethylhexylphosphonic acid as complexing agents for effective removal of molybdenum (Mo). In this study, the trioctylamine was found to be inef

ﬁ

cient due to the lack of stability in its complex form in Sc-CO

₂

. The other complexing agent, 2-ethylhexyl 2-ethylhexylphosphonic acid, resulted in stable complex formation with greater solubility. As a result, a high Mo removal ef

ﬁ

ciency of 90% was achieved for 2-ethy lhexyl 2- ethylhexyl phosphonic acid. In addition, the selectivity studies were also performed for Mo removal in the presence of iron (Fe) and zirconium (Zr) metal ions. The results showed that the Mo selectivity of the solvent system was good in the presence of Fe ions but got affected negatively in the case of Zr ions (Hung et al., 2016).

Solvent strength and diffusivity of Sc-CO

₂

are also other in

ﬂ

u- ential factors on the metal-complex stability obtained in the course of metal recovery. To improve the solvent strength of Sc-CO

2

, penta-deca-

ﬂ

uoro-octanoic acid was used as a modi

ﬁ

er along with CalixOctyl (25,27-Bis (1-octyloxy)calix[4]arene-crown-6, 1,3- alternate) for cesium metal recovery from concrete rubbles. This process resulted in a product yield of

<

30%. However, prior leaching of concrete rubble with nitric acid improved the yield to attain ~55%

(Leybros et al., 2018). Some studies reported using malic and citric acid (common organic acids) as chelating agents along with Sc-CO

2

for neodymium (Nd) extraction from hard disk drives. In this study, the SFE extraction process was conducted at 90

C/120 bar using malic and citric acids as co-solvents for Nd recovery from the unroasted NdFeB permanent magnets of the hard drives. Nd extraction ef

ﬁ

ciencies of 99.9 and 86% were achieved for malic acid and citric acid, respectively, under identical experimental conditions. Due to the enhanced extraction results of malic acid, SFE process was also performed with roasted NdFeB powder at 90

C and 120 bar to produce 99.5% of the Nd extraction. Meanwhile, typically atmospheric pressure Sc-CO

2

leaching provided a low Nd recovery of only 21% after 360 min of extraction time (Reisd€ orfer et al., 2020).

Table2(continued) PlantmaterialMajorcompoundAntioxidantactivityExtractionconditionRef. EuterpeedulisMart.(Jucara)residuesAnthocyaninsandphenolicsDPPH(mmolTE/gdr)is63.6±0.6.60Cand20MPa/10%ethanolasco- solventGarcia-Mendoza etal.(2017) Brassicaoleracea(BroccoliLeaves)PhenolicsEC50¼0.1275glyophilisedsample/mgDPPH.35Cand150bars;20%ethanolasco- solventArn aiz

etal.(2016) ZeamaysL.(Purplecorncob)PhenolicsDPPH(EC50)values31mg/mL50Cand40Mpa;70%ethanolasco- solventMonroyetal. (2016b) Eremanthuserythropappus(Candeia)Sesquiterpenes(a-bisabolol, eremanthinandcostunolide)IC50(mgmL1)is604.42±14.18.70Cand24MpaSantosetal.(2016) LippiasidoidesCham.(Pepper-rosmarin)leavesPhenolicsandﬂavonoidsMaximumantioxidantactivitywas95%.60Cand400barGarmusetal. (2015) SecalecerealeL.(Ryebran)UnsaturatedfattyacidsOxygenradicalabsorbance(ORAC)andDPPHscavengingcapacities were683.8±45and62.28±1.2mMtroloxequivalents/g.70Cand55MPaPovilaitisand Venskutonis(2015) Leptocarpharivularis(Asteraceae)Terpenoids(a-thujone,b- caryophylleneandcaryophyllene oxide)

IC50valueofsupercriticalextractwas1.99±0.32mg/mL52Cand19.2MPaUquicheetal. (2015)

Fig. 8.APFO extraction fromﬂuoropolymer surfaces using the Sc-CO2 based SFE process (Reproduced with permission from (He et al., 2020), copyright year: 2020, publisher: Elsevier).

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Another research study on metal recovery has reported the use of highly Sc-CO

₂

soluble, open-chain crown ether bridged di- phosphates as chelating ligands to extract the lanthanide metal ions (La

^3þ

, Ce

^3þ

, Pr

^3þ

, Nd

^3þ

, Sm

^3þ

, Gd

^3þ

, Er

^3þ

, and Yb

^3þ

) from cellulose paper matrix. SFE experiment was carried out at 40

C and 20 MPa, wherein the extraction ef

ﬁ

ciencies of the metals varied from 68 to 94% depending on the stability and solubility of the produced metal complexes. Also, the steric hindrance branched topology, and masking effects of the ligands also played a signi

ﬁ

- cant role in the extraction of the metals (Duan et al., 2019). Calgaro et al. (2015) found that supercritical extraction was more effective than the typical atmospheric pressure extraction technique for metal removal applications. The authors successfully removed 90%

of copper (Cu

^2þ

) metal from the printed circuit boards (PCBs) through the Sc-CO

2

based SFE process operated with a 1:20 solid- liquid ratio and 20% of H

2

O

2

and H

2

SO

4

as modi

ﬁ

ers. Overall, the usage of the Sc-CO

2

driven SFE technique seems to be a promising eco-friendly approach for PCBs recycling (Calgaro et al., 2015).

7. Future trends

Although Sc-CO

2

based SFE extraction has been established as a non-conventional extraction technique, the only limited focus has been paid to optimizing various in

ﬂ

uential factors on the selectivity and yield of the target product. There is a synergic effect among the operational parameters to in

ﬂ

uence the chemical constituents in the

ﬁ

nal extract. In this regard, more detailed investigations are required for the optimization framework of the Sc-CO

2

based SFE to transfer the technology from lab scale to pilot/industrial scales module with signi

ﬁ

cant emphasis on the process economics.

Currently, co-solvents as a modi