Algae and Fungal Synthesis of Metal-based Nanoparticles for Bioremediation

Algae and fungi are the alternative microbes, next to bacteria, for nanosized metal-based particle synthesis including metals, oxides of metals and composites. Algae and fungi possess a wide variety of biomolecules which can be extracted in large quantities. Thus, these microbial mediated synthesis approaches are highly beneficial in the up-conversion of nanoparticle production, in addition to reduction in toxicity towards the environment (Zhao et al. 2018).

Algal Synthesis of Metal and Metal Oxide Nanoparticles

Algae, intracellular methods and extracellular extracts are highly beneficial in nanosized metal particles synthesis, similar to bacteria. Gold nanoparticles are synthesized using algae, namely Sargassurn wightii, S. incisifolium, S. muticum, Porphyrra species, Laminaria japonica, Padina pavonica and Rhizoclonium hieroglyphicum. It is noteworthy that the nanosized gold obtained from these algal species is below 50 nm in size. In addition, blue-green algae such as Plectonema boryanum, Spirulina platensis, Phormidium valderianum, Calothrix species and Microcoleus chthonoplastes were also utilized for 10-100 nm sized nano-gold particle fabrication (Khan et al. 2019). However, gold nanoparticles are used to detect toxic heavy metals or pollutants and are not used to eliminate them, as mentioned in the earlier section. Nanosized silver particles are the most significant standalone nanosized metallic particles that are synthesized via algal extracts for bioremediation applications. Aziz et al. (2015) fabricated nano-silver using Chlorella pyrenoidosa extracts. The result showed that the algal extracts offered an exceptional consistency in nanoparticles’ morphology with intrinsic crystallinity and functional moieties for surface stabilization. In addition, the study also showcased the photocatalytic efficiency of algal extract synthesized silver nanoparticles, which will be useful for the treatment of wastewater (Aziz et al. 2015). Likewise, Kumar et al. (2013) fabricated nano-silver using extract of Ulva lactuca at room temperature. They also proved that these biogenic, spherical shaped, -50 nm sized silver nanoparticles possess ability to photocatalytically degrade toxic methyl orange dye in water via silver as nanocatalyst under the illumination of visible light (Kumar et al. 2013).

Metal oxides such as zinc, magnesium, copper and iron are also synthesized via algal extracts for enhanced bioremediation application. In recent times, Khalafi et al. (2019) synthesized highly pure and stable zinc oxide nanoparticles using microalgal extracts of Chlorella. The characterization results showed that 20 nm sized, monodispersed ZnO with hexagonal Wurtzite structure was formed. The gas chromatography results revealed that the oxide nanoparticle possess enhanced photocatalytic activity of Dibenzothiophene degradation (97%) as an organosulfur contaminant prototype at neutral pH. Further, the biogenic oxide nanoparticle exhibited exclusive recyclability after five runs and rapid separation of contaminants which proved their durability (Khalafi et al. 2019). Similarly, magnesium oxide (MgO) nanoparticles were synthesized using the algal extract of marine brown algae named Sargassurn wighitii.

The results revealed that the average size of the obtained nanoparticle with structure of cube with face-centers is 68 nm and possess enhanced antibacterial and antifungal properties against human pathogens. In addition, these MgO nanoparticles also possess a photocatalytic ability to degrade toxic organic methylene blue dye after exposing it to sunlight and ultraviolet irradiation (Pugazhendhi et al. 2019). Likewise, Gu et al. (2018) synthesized nanosized oxides of copper particles by using brown alga Cystoseira trinoids and ultrasonication. The results demonstrated that spherical shaped, 6-8 nm sized copper oxide nanoparticles were fabricated via algal extract with enhanced antibacterial and l,l-Diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging property. Furthermore, these nanoparticles also possess ability to degrade toxic methylene blue dye via sunlight and ultraviolet light mediated catalytic property at acidic pH 4.0 (Gu et al. 2018). It is noteworthy that 18-25 nm sized, cubic shaped, nanosized oxides of iron particles can be fabricated using Sargassum muticum, which can be utilized as magnetic waste water treatment agent, photocatalytic toxic heavy metal and dye degradation application as mentioned in the earlier section (Mahdavi et al. 2013, Fawcett et al. 2017).

Fungal Synthesis of Metal and Metal Oxide Nanopartieles

Fungal synthesis, specifically dead biomass extracts, is extensively utilized for nanosized metal particle fabrication for bioremediation applications (Rai et al. 2016). Filamentous fungi including Aspergillus niger, A. fumigatus, Verticillium species and Phoma glomerata are used for silver nanoparticle synthesis, whereas Rhizopus oryzae, Verticillium luteoalbum, Aureobasidium pullulans and Colletotrichum species are utilized for gold nanoparticles synthesis. In addition, copper nanoparticles are synthesized via Hypocrea lixii and Trichoderma koningiopsis, whereas cadmium nanoparticles were synthesized using Fusarium oxysporum extracts (Salvadori et al. 2018). Fungi also serve as a pollutant or contamination in the environment (Andersen et al. 2017). Thus, the utilization of fungal dead biomass as a source for nanoparticle preparation will serve as a better bioremediation approach in industries to avoid fungal contaminants towards the environment. Similarly, He et al. (2017) reported that white rot fungi group, such as Phaenerochaete chrysosporium, Trametes versicolor, Pleurotus sajor-caju, Schizophyllum commune, P. ostreatus and Stereum hirsutum, are also extensively used for the fabrication of nano-metal particles such as silver, copper, gold, selenium and cadmium sulfide nanoparticles. Also, these fungal extracts’ fabricated nanosized particles were proved to possess enhanced bioremediation properties (He et al. 2017). Moreover, Narayanan et al. (2015) demonstrated a novel fabrication method of nanosized gold particles by intracellular approach using Flammulina velutipes mushroom. The result showed that the fungi lead to the development of 20 nm sized nano-gold particles and possess the ability of heterogeneous catalyst to reduce the toxic organic pollutant such as methylene blue dye and 4-nitrophenol, which can serve as a better bioremediation agent (Narayanan et al. 2015). Furthermore, Roy et al. (2014) fabricated nanosized silver particles via Saccharomyces cerevisiae yeast extract. The result reported that the yeast extract yielded spherical shaped, quantum sized nano-silver particles with 10 nm of average size. These biogenic nanoparticles were proved to possess enhanced catalytic degradation ability against organic methylene blue dye under irradiation of visible sunlight for a few hours (Roy et al. 2015). Recently, porous silver nanoparticles with 2.35 nm as pore width were synthesized via Aspergillus foetidus and were utilized for the efficient reduction of arsenic in contaminated aqua environment (Mukherjee et al. 2017).

Iron ore tailing is a mining waste, which causes hazard to the environment and is required to be remediated from the contaminated sites. The fungal species that are present in these sites, especially Aspergillus aculeatus, possess enhanced leaching efficiency and produce iron containing nanoparticles under optimum condition, which reduces contamination in the site. Further, these iron nanoparticles are capped with proteins of fungi, which make them highly biocompatible and serve as a bioavailable iron source for the enhanced growth of plants from the seed of mungbean (Bedi et al. 2018). Likewise, nanosized copper and copper oxide particles are synthesized by the extracellular biomolecules of a white rot fungus from Chilean forests, namely Stereum hisutum (Cuevas et al. 2015), whereas both extra and intracellular extracts from the dead fungal biomass of Hypocrea lixii were utilized for the synthesis of nanosized oxides of nickel particles (Salvadori et al. 2015). Both these nanoparticles possess enhanced bioremediation and photocatalytic degradation ability against organic pollutants (Bokare et al. 2008, Gong et al. 2011, Devi and Singh 2014).

Metal Nanoeomposites Synthesized via Algae and Fungi

In recent times, numerous metal nanocomposites were developed using algae and fungal extracts for bioremediation applications. Recently, titanium dioxide (Ti02) nanoparticles were synthesized via extracts of green algae named Chlorella pyrenoidosa and were deposited over graphene oxide sheets to form nanocomposites. The result revealed that the Ti02 nanoparticles possess enhanced photocatalytic activity, which elevates the potential of nanocomposite to degrade crystal violet dye in aqueous medium under irradiation of visible light (Sharma et al. 2018). Likewise, platinum nanoparticles were synthesized using the extracts of Indian brown seaweed named Padina gymnospora and were embedded with polyvinylpyrrolidone (PVP) for the formation of nanocomposites. The results showed that the seaweed extract yielded 5-50 nm sized nanoparticles with truncated octahedral shape and the nanocomposite possessed enhanced antibacterial activity against seven disease causing bacteria which can contaminate air, water and soil (Ramkumar et al. 2017). Moreover, Gurusamy et al. (2019) utilized chemical synthesized, 12 nm sized titanium dioxide - zinc oxide nanocomposite for biodiesel production from Ulva lactuca seaweeds. Further, the leftover biomass of U. lactuca was utilized to synthesize 12 nm sized silver nanoparticle with enhanced antibacterial activity against Proteus vulgaris (Gurusamy et al. 2019). Likewise, silver nanoparticles were synthesized by encapsulating alginate from the cell wall of brown algae to form the silver-alginate nanocomposites. The resultant nanocomposites were 8 nm in size with spherical shape and a crystal structure of face cube. Further, the results emphasized that these nanocomposites possess catalytic ability to reduce organic dye pollutants such as methylene blue, 4-nitrophenol and reactive red with the existence of sodium borohydride (Thangaraj et al. 2018). In recent times, a novel hydrogel was fabricated from the green algae Enteromorpha biomass polymerization with acrylic acid for three different applications. These hydrogels possess enhanced swelling property, which makes them act as an effective adsorbent for copper based heavy metal removal application. The adsorbed metal particles were below 100 nm in size, which makes them metal-hydrogel nanocomposites. These nanocomposites were reported to possess catalytic activity to reduce organic contaminants, including methylene blue and p-nitrophenol with catalytic activity (80-90%) after five times of reuse and storage for 30 days (Su et al. 2018). Similarly, it was recently reported that the fungal derived chitosan-based metal nanocomposites are widely used in the heavy metal biosorption application to eliminate contaminants of heavy metals from the environmental entities. It was emphasized that the biosorption property of individual fungal derived chitosan and metal nanoparticles will increase drastically, when they are synthesized as a nanocomposite (Pattnaik and Busi 2018). Furthermore, trimetallic structure with lanthanum, copper and zirconium was reinforced with algal biochar via microwave method to form nanocomposite. These nanocomposites were employed for the remediation of green malachite dye from the contaminated site, which revealed their exclusive adsorption and photocatalytic ability against environmental contaminants (Sharma et al. 2019). Even though all these studies supported fungal and algal synthesized metal nanoparticles for bioremediation process, the time taken for the nanoparticle production, typically 24-120 hours, and intracellular approach hindering the downstream process still remain as limitations to develop simple and cheap biogenic metal nanoparticles for bioremediation purpose (Jeevanandam et al. 2016).

Plant Extracts Mediated Synthesis of Metal-based Nanoparticles for Bioremediation

Plant extracts in the form of phytochemicals are the most common and widely utilized reducing and stabilizing agent for the formation of nanosized biogenic metal particles. The major advantage of this approach is the wide availability of plants and the extract can be obtained from several parts of the plants (Dauthal and Mukhopadhyay 2016). It is noteworthy that the phytochemicals can be extracted from the plant parts including leaves, fruits and other parts such as root, stem and bark for less toxic nanometal particle synthesis for bioremediation applications.

Leaf Extracts

Leaves are the most common plant part that was used for the extraction of phytochemicals to serve as a better agent with reducing and stabilizing property for nanosized particle formation (Kumar et al. 2012). Recently, aqueous leaf extracts of Aloysia triphylla for nanogold particle fabrication were shown to exhibit antibacterial and catalytic property. The results emphasized that the extracted phytochemicals yielded 40-60 nm sized, spherical shaped nanogold particles with the ability to catalytically reduce organic pollutants within 10 mins (Lopez-Miranda et al. 2019). Likewise, spherical shaped, 4-13 nm sized nanogold particles were prepared via mangrove leaf Avicennia marina extract for bioremediation application. These nanoparticles were encapsulated with sodium alginate to form nanobeads and were employed for 4-nitrophenol organic pollutant elimination with their enhanced heterogeneous catalytic property (Nabikhan et al. 2018). Similarly, aqueous leaf extract of Cressa cretica was utilized for the fabrication of hexagonal, spherical, pentagonal and rod shaped, 15-22 nm sized gold nanoparticles. These biogenic nanoparticles were proved to possess exclusive catalytic activity to reduce 4-nitrophenol under the influence of sodium borohydride solution (Balasubramanian et al. 2019). Moreover, silver and gold nanoparticles were fabricated using an extract from the leaf of medicinal plant named Myxopyrum serratulum A.W. Hill. The resultant nanoparticles were proved to possess bioremediation ability to catalytically reduce organic pollutant dyes such as Congo red, 4-nitrophenol and methylene blue (Vijayan et al. 2018). Nanosized silver particles are the other major metal nanoparticles that are synthesized via leaf extracts. The Mussaenda erythrophylla leaf extract was recently utilized to fabricate nanosized silver particles with the exceptional catalytic degradation ability of azo methyl orange dye along with sodium borohydride (Varadavenkatesan et al. 2016). Similarly, nano-silver particles were prepared via leaf extract of a medicinal plant of India, namely Helicteres isora. These biosynthesized silver nanoparticles have been proven to possess catalytic degradation property against organic dyes such as safranin, methyl orange, methylene blue and methyl violet (Bhakya et al. 2015). Further, aqueous leaf extract of Passiflora edulis f. flavicarpa (P. edulis) was used to prepare silver nanoparticles with effective photocatalytic degradation property against organic pollutants such as methyl orange and methylene blue (Thomas et al. 2019). Furthermore, Justicia adhatoda extract from leaf was utilized as agent with reducing property for nano-silver particle preparation with size dependent photocatalytic activity against methylene blue organic dye (Latha et al. 2019). Apart from noble metals, iron nanoparticles were also synthesized from the leaf extract of Spinacia oleracea and palladium nanoparticles via Delonix regia leaf extract to be useful for wastewater treatment and catalytic pollutant degradation application (Dauthal and Mukhopadhyay 2013, Turakhia et al. 2018).

Oxides of zinc, iron, copper and titanium are the most common metal oxide nanoparticles that are synthesized via plant leaf extracts for bioremediation and environmental applications. In recent times, stable, hexagonal and spherical shaped zinc oxide nanoparticles with an average size of 12 nm are fabricated via Murraya koenigii leaf extracts. These nanostructures exhibited enhanced antibacterial activity against Staphylococcus aureus and Bacillus subtilis, which can be beneficial as a latent agent to eliminate microbial contaminants in the treatment of wastewater facilities (Elumalai et al. 2015). Similarly, zinc sulfide nanoparticles were prepared via leaf extract of Corymbia citriodora to exhibit photocatalytic property. The result revealed that the nanoparticle size was 45 nm with surface plasmon resonance and quantum confinement property, which makes them an excellent photocatalytic degradation agent of organic pollutant methylene blue dye under UV light irradiation (Chen et al. 2016). Further, hexagonal rod shaped, 52 nm sized nano-iron oxide particles were fabricated via extract of Ruellia tuberosa from leaves to exhibit photocatalytic activity. The result demonstrated that these oxide nanoparticles possess ability to photocatalytically degrade 80% of crystal violet dye under the influence of solar irradiation (Vasantharaj et al. 2019b). Moreover, nanosized iron oxide particles are prepared via extract of Amaranthus spinosus from leaf to exhibit improved photocatalytic ability. The result emphasized that the biogenic nanosized iron oxide possesses ability to degrade 75% of methyl orange and 69% of methylene blue under solar irradiation (Muthukumar and Matheswaran 2015). Likewise, leaf extract of Fraxinus chinensis Roxb synthesized magnetic nanosized iron oxide particles were proved to possess ability to degrade toxic crystal violet and eriochrome black T dyes to save environment from these pollutants (Ali et al. 2019). Leaf extracts of R. tuberosa (Vasantharaj et al. 2019b), Azadirachta indica (Thirumurugan et al. 2017) for copper oxide nanoparticles synthesis and Ti02 nanoparticles via Jatropha curcas (Goutam et al. 2018) are the other oxide nanoparticles that are used as photocatalyst and in bioremediation applications.

Nanocomposites were also extensively synthesized using plant leaf extracts for environmental bioremediation applications. Recently, nanosized palladium-reduced GO-oxide of iron composites were synthesized via extract of Withania coagulans from leaf for photocatalytic pollutant degradation. These novel biogenic nanocomposites exhibit enhanced ability as magnetically separable and reusable catalyst to reduce 4-nitrophenol organic dye pollutant from water at room temperature (Atarod et al. 2016b). Similarly, nanosized titanium dioxide-silver particles were synthesized via leaf extract of Euphorbia heterophylla for bioremediation application. The result revealed that the nanocomposite possesses excellent catalytic activity to reduce organic pollutant dyes such as Congo red, 4-nitrophenol, methylene blue and methyl orange in water, which will be beneficial to remediate polluted wastewater (Atarod et al. 2016). Likewise, nanosized silver- reduced GO composites were fabricated via leaf extract from Abutilon hirtum, which exhibited excellent recoverable catalytic reduction property against Rhodomine B, 4-nitrophenol and Congo red in aqueous medium (Maryami et al. 2016). Moreover, copper-reduced graphene oxide- iron oxide nanocomposites from Euphorbia wallichii leaf extract (Atarod et al. 2015), silver-H- zeolite Socony Mobil-5 (HZSM) nanocomposite via Euphorbia heterophylla (Tajbakhsh et al. 2016) and silver-iron oxide nanocomposite from Euphorbia peplus Linn (Sajjadi et al. 2017) are used as enhanced catalytic reduction agents of organic pollutants in water.

5.3.2 Other Extracts

Fruit, root, bark and flower extracts are the other reducing agents that are used to form nanosized metal particles to use them as bioremediation agent. Gold nanoparticles are fabricated using the Banana pith extract to exhibit efficient catalytic reduction ability against organic malachite green pollutant dye in water (Nayak et al. 2018). Nano-silver particles with enhanced stability and an average size of 69 nm are synthesized via extracts of fruits from Bridelia retusa to reduce Congo red dye (Vinayagam et al. 2017). Likewise, nanosized iron and oxides of iron particles from extracts of fruit Vaccinium ftoribimdum (Murgueitio et al. 2018) and Anthemis pseudocotula (Abdullah et al. 2018), respectively, were used for the elimination of total petroleum hydrocarbons from soil and water. Moreover, iron oxide nanoparticles were synthesized by obtaining extracts from traditional plant species such as Pittosporum undulatum, Schinus mode, Melia azedarach and Syzygium paniculaturn. The resultant nanoparticles were proved to possess ability to reduce and act as an excellent bioremediation agent to remove hexavalent chromium contaminants from the environment (Truskewycz et al. 2018). Further, nanocomposites such as reduced graphene oxide-zinc oxide from grapefruit extract to degrade Rhodamine В dye (Ramanathan et al. 2019), iron oxide-silicon dioxide-copper oxide-silver from Crataegus pentagyna fruit extract to degrade methylene blue and Rhodamine b (Ebrahimzadeh et al. 2019), and copper-magnesium oxide from Cassytha filiformis for methylene blue, Congo red, 4-nitrophenol and 2, 4-dinitrophenylhydrazine degradation (Nasrollahzadeh et al. 2018) are fabricated to be a potential bioremediation agent. In addition, root (Yeganeh-Faal et al. 2017), seed (Gopalakrishnan et al. 2015), bark (Amiri et al. 2017) and flower extracts (Rostami-Vartooni et al. 2019) were also used to synthesize nanoparticles for bioremediation applications. However, stability and reusability are the major limitation for utilizing these nanoparticles as commercial and large-scale bioremediation agents.

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