Abstract

Environmental risks associated with water and soil pollution are considered great threats to the ecological system. Environmental quality and ecological integrity demand unity of effort and approach to ensuring safe one health system. Several approaches have been developed to ameliorate the impact of heavy metals and toxic chemical pollutants in the environment. The review is aimed to report existing knowledge on microbe-mediated nanomaterials synthesis and biofunctionalized processes for a sustainable, effective and advantageous bioremediation technique for heavy metals, hydrocarbons and chemicals. Nanobioremediation has become an emerging field ensuring complete removal/clean-up of polluted soil and water bodies, thereby improving environmental health. It becomes highly recommendable that large scale applications of these methods are full of potential in repairing and restoring the ecological space thus, creating a sustainable environment. Therefore, there is a need in the progressive improvement, development and devise of new and sustainable microbe-nanotechnology-based methods for the effective restoration of the ecological system.

1. Loss of Ecological Integrity

The quality and integrity of the environment and ecological system demands unanimity of effort towards ensuring one perfect health system which includes climate stability, safe, healthy and pest-free environment, constant nutrient cycles and vital energy flow that aids availability of food for human survival ¹. However, such expectations are beginning to decline across the globe as a great challenge is progressively posed on two of the most vital life-supporting systems of the ecosystem which are soil and water ². The pollution of the environment in the exploration and handling of crude oil, heavy metals and toxic chemicals is a smouldering public health threat. These threats are more on life forms that play important role in maintaining the food chain thereby disrupting energy level in the environment ³. The level of imbalance in the ecosystem has been observed to increase as consequences of various man-made activities ⁴.

Due to the increasing population growth and geometric global industrialization, the ceaseless release of heavy metals and toxic chemicals has led to environmental pollution, and its exposure has jeopardized public health and safety, inducing several illnesses in human ⁵. One of which is the serious eutrophication threat to water resources as a result of the release of a wide range of nutrient rich pollutants which encourages the dense growth of plants and algae in the water bodies ⁴. Likewise, the release of heavy metals, chemicals and agro-industrial wastes into the soil increases the level of imbalance in the ecosystem such as the proliferation of resistant-microbial community, further causing deleterious effects in humans ⁶. Consequently, a challenging task to tackle and remediate a wide range of environmental pollutants has been created ⁷. Ecological restoration approach is essential to repair damaged resources by enhancing their ability to restore/change as the environment changes. Removal of heavy metals and toxic chemicals from the soil and water is important and needful for the revival and restoration of the ecosystem.

2. Eco-restoration: Importance and Approach

Ecological restoration emphasizes the recovery approaches required for a damaged ecosystem which could include the interruption of human activities causing degradation, treatments and continuous monitoring of the ecological space. There are few established approaches in eco-restoration which includes reclamation (abatement of acute threats, such as toxic waste clean-up, bank stabilization), rehabilitation, revegetation, habitat creation, mitigation (selected improvements to structure and function), repair (restoration of critical structural and functional components), reestablish pre-disturbance condition (restoration of all structural and functional components to historic conditions. Since the ecological system is complex and dynamic, a single and broad approach solution for ecosystem restoration may be insufficient and inappropriate. Therefore, ecological restoration is often designed to repair damaged natural resources and, in another way, enhance/equip their ability as the environment changes. Since humans and human values are responsible for the degrading ecosystem, they also play a central role in the restoration and development of balance and self-sustaining ecosystem, by applying series of approaches one of which is remediation (physical, chemical and biological) ⁸.

Putting into consideration the advancement and dynamic of approach, biological-based methods (bioremediation) have been promising, sustainable and the most sought-after method, offering an eco-friendly and cost-effective option to remove pollutants from the environment using microbes, plants and enzymes ^{9, 10}. The use of microorganisms in bioremediation has been highlighted ¹¹. They are aimed to degrade, detoxify, and transform pollutants into less hazardous waste for easy natural degradation ¹². However, lengthy treatment processes, chanced technologies and low effectiveness in the high concentration of polluted sites have been a major setback to tackle or remediate various forms of environmental pollution ⁷. The use of nanomaterials in bioremediation has been suggested as efficient, cost-effective and environmentally-friendly alternatives to existing treatment materials and environmental remediation. Besides, they can be synthesized from a variety of bulk materials and converted into nanosized materials which are more reactive and more mobile in nature ¹³.

3. Nanotechnology and Environmental Remediation

Nanotechnologies have been reported as present solution vectors in the economic environment ¹⁴. Nanotechnology which had gained lots of application in several fields such as agriculture, medicine, consumer goods and electronics has also found its usefulness in the environment (Figure 1). Nanoparticles and other forms of nanomaterials (nanotubes, nanofibers, nanorods, nanowires, nanoshells, nanobeads, nanoarrays, nanogel, nanoemulsion, dots and dust) have played a major role in the removal of pollutants in the environment ^{15, 16, 17, 18}. Nanobioremediation-a newly emerging technique for remediation of pollutants using biosynthetic nanoparticles is a rapidly developing research area in green nanotechnology with potential application in environmental cleanup and removal of environmental pollutants (such as heavy metals, toxic chemicals) from polluted soil and water ¹⁹. The biosynthetic route of nanoparticle synthesis could emerge as better and safer alternative methods. The optimization of the processes can result in the synthesis of nanoparticles with desired morphologies and controlled sizes, quickly and cleanly ²⁰. The incorporation of nanomaterials in bioremediation launches the application of both physicochemical and biological methods as a tool for the remediation of polluted environments ¹⁰. Nanobioremediation technique primarily uses nanomaterials to break and reduce the pollutants to favourable substances, accommodating natural biodegradation ²¹.

Extensive research on nanotechnology as a modern and multidisciplinary approach has played a vital and innovative role in tackling and reducing pollutants in the environment ^{22, 23}. The application of nanomaterials in bioremediation has shown to be promising, cost-effective, efficient and sustainable when compared with existing treatment and remediation approaches of the environment ¹³. The use of nanomaterials focuses on treatment and remediation ²⁴, sensing and detection ²⁵, and pollution prevention ²⁶.

Some nanoparticles which have become increasingly explored due to their specificity, reactivity, increased surface-volume ratio/charge, crystal behaviour, cost-effective and eco-friendliness includes Iron, Silver, Gold, Zinc, Copper and many more (Table 1). Iron oxide (IONPs) and magnetic nanoparticles (MNPs) being one of the first studied nano-bioremediated clean-up approach showed promising results in the treatment and removal of pathogenic bacteria through electrostatic interaction and magnet capture thereby, enhancing water purification ²⁸. Stabilization of magnetic nanoparticles also informed its greater affinity, enormous surface charge, high surface to volume ratio and abundant reactive groups on its shell as well as its core for various application ^{17, 28}. Monodispersity of MNPs and pairing mechanism with other nanoparticles such as gold and silver have been reported as an outstanding approach, enhancing its effectiveness, applicability as well as supplementing its efficacy as a bioremediating agent ^{29, 30}.

In recent years, the synthesis of nanoparticles via biological methods also known as the green methods has emerged as a sustainable and dependable method ⁴¹. Synthesizing nanoparticles by biological methods using microorganisms have been suggested as a possible eco-friendly alternative to chemical methods ⁴². The methods involving biosynthesis of nanoparticles were found appealing, gaining acceptance with its ease of preparation, tolerable/controlled toxicity, low-cost, and desired morphological characteristics ²⁰. Besides the aforementioned advantages, the deposition of extracellular enzymes which are utilized for the production of relatively pure nanoparticles is of utmost advantage including the ability to pattern the production and organization of nanoparticles into stabilized structures ^{43, 44}. Few of the explored bio-materials for the synthesis of nanoparticles include plant extract ⁴⁵, bacteria ⁴⁶, fungi ⁴⁷, enzymes ⁴⁸, algae ⁴⁹ and actinomycetes ⁵⁰.

Bacteria have been reported to be the most prevalent microbial agent engaged for nanomaterial synthesis ^{51, 52}. This is predicated on the attraction of bacteria for metals and their excellent binding properties with metals hence, enhancing them as the most preferred agents for the biosynthesis of nanoparticles. Bacillus spp have received wide attention in the synthesis of silver nanoparticles from cell pellets, cell filtrate and bioflocculant ^{53, 54, 55, 56, 57}. Studies which explored Bacillus for silver nanoparticles include B. subtilis ^{58, 59, 60}, B. megaterium ⁶¹, B. flexus ⁶², B. methylotrophicus ⁶³, B. cereus ^{64, 65}, B. licheniformis ⁶⁶, B. stearothermophilus ⁶⁷ and B. marisflavi ⁶⁸. Other bacteria investigated for silver nanoparticles synthesis are Weissella oryzae ⁶⁹, Pseudomonas deceptionensis ⁷⁰, Pseudomonas aeruginosa ⁷¹, Escherichia coli ⁷² and Streptomyces sp. ⁷³.

On the other hand, a work documented the possibility of biosynthesizing gold nanoparticles (AuNPs) from varying groups of bacteria which include: Firmicutes, cyanobacteria, glidobacteria, alphaproteobacteria, gammaproteobacteria, deltaproteobacteria and actinobacteria ⁵². Another study explored the production of gold nanoparticles using Bacillus cereus ⁷⁴. An eco-friendly approach for the synthesis of AuNPs was devised using Brevibacillus formosus ⁷⁵. Synthesis of stable cadmium sulfide nanoparticles using surfactin produced by Bacillus amyloliquifaciens was studied ⁷⁶, establishing the versatility of the Genus Bacillus. The single-step formation of iron oxide nanoparticles from soil bacteria Streptomyces sp. was investigated ⁷⁷. Magnetic iron oxide nanoparticles were produced from iron-reducing bacteria-Thiobacillus thioparus and Microbacterium marinilacus isolated from iron ore mining sites ⁷⁸ and river sediment ⁷⁹ respectively.

Apart from bacteria, fungi and yeast are also being used for biosynthesis of nanoparticles ¹³. Fungi are an excellent source of various extracellular enzymes which influences nanoparticles synthesis. In addition to this, nanoparticles with well-defined dimensions can be obtained using fungi. They have been widely used for the biosynthesis of nanoparticles and the mechanistic aspects guiding the nanoparticle emergence for some of them have also been documented ^{52, 74}. They have been shown to synthesize large amounts of nanoparticles due to their characteristic larger volumes of proteins ⁸⁰. Fungal species can readily synthesize metal nanoparticles both intra- or extracellularlly using high levels of secreted proteins and/or enzymes ^{81, 82}. However, extracellular synthesis has advantages over an intracellular one in that, fungal nanoparticles are mostly precipitated outside the cells and do not require lysis of fungal cells. Fungal biomass can withstand flow pressure and agitation conditions in bioreactors and other chambers; it can be easily cultured, handled and, therefore, has advantages as it ensures recovery and purification of nanoparticles. It is extremely cost-effective and less time-consuming ⁸³. In the synthesis of numerous enzymes and rapid growth with the use of simple nutrients, yeast strains possess certain benefits over bacteria facilitating the preference for yeast in the synthesis of metallic nanoparticles.

Study reported extracellular biosynthesis of silver nanoparticles (AgNPs) by Aspergillus fumigatus ⁸⁴. Details on the green synthesis of silver nanoparticles from a plethora of Pleurotus spp (oyster mushroom) was reported ⁸⁵. Mycosynthesis of nanoparticles carefully reviewed ⁵² established the importance of fungi in gold nanoparticles synthesis. A green method for the synthesis of gold nanoparticles was exploited by ⁸⁶ and ⁷⁴ using Penicillium crustosum and Fusarium oxysporum respectively. Lead resistant marine yeast Rhodosporidium diobovatum was reported to be responsible for the synthesis of lead sulfide nanoparticles ⁸⁷. Similarly, fungi such as Pleurotus sp., Alternaria alternata were utilized for the synthesis of iron oxide nanoparticles ^{88, 89, 90}. The use of Aspergillus niger and ethanol under supercritical condition for the synthesis of magnetic iron and magnetite nanoparticles ⁹¹. Utilization of yeast mediated silver nanoparticles synthesis approaches have been documented which include psychrotrophic yeast Yarrowia lipolytica ⁹², Kluyveromyces marxianus and Candida utilis ^{93, 94}.

The ability of algae to hyper-accumulate and reduce metal ions has been reported to enhance its opportunity to biosynthesize nanoparticles ⁹⁵. Varying active compounds such as polysaccharides, peptides, and minerals, fatty acids, bioactive compounds such as antioxidants, pigments, chlorophylls, and phycobilins are responsible for the chemical transformation of metal ions ⁹⁶. In other words, the bioactive compounds act as reducing and stabilizing agents ^{97, 98}. Few compounds were identified as the reducing/capping agents responsible for gold synthesis by algae. Study reported the role of nitrate reductase enzyme as a bio-reducing agent produced by algae for nanoparticle synthesis ⁹⁹. The presence of the hydroxyl group in the extract of brown algae Turbinaria conoides and Sargassum tenerrimum facilitated the synthesis of spherical shaped gold nanoparticle ¹⁰⁰. Similarly, polysaccharides and polyphenols which contains hydroxyl group were further suggested as the capping agent for gold nanoparticles synthesis from brown algae Cystoseira baccata ¹⁰¹. It can be concluded that the presence of functional groups such as carboxylic and aromatic groups in algae enhances their ability to reduce silver ion to silver nanoparticles ¹⁰². Phytochemicals such as steroid, saponins, terpenoids, and tannins were also found to be responsible for the bio-reduction of silver ions ¹⁰³. Algae were acknowledged as relatively easy to handle, grow at low temperature, less toxic and biosynthesize nanoparticles within a short time when compared with other groups of microorganisms ^{104, 105}.

Consequently, algal groups that have been exploited in the biosynthesis of nanoparticles include Chlorophyceae, Phaeophyceae, Cyanophyceae, Rhodophyceae, and others (diatoms and euglenoids) ¹⁰⁶. Algal-mediated biosynthesis of nanoparticles has been reported in the synthesis of gold nanoparticles from Tetraselmis kochinensis ¹⁰⁷, Fucus vesiculosus ¹⁰⁸, gold and silica nanoparticles from Navicula atomus, Diadesmis gallica ^{109, 110}. Gold nanoparticles synthesis has also been reported from aqueous extract of brown algae Laminaria japonica ¹¹¹, Microalgae Chlorella vulgaris ¹¹², Sargassum marginatum ¹¹³, red algae Seaweed Corallina officinalis ¹¹⁴, and green algae Pithophora Oedogonia ¹¹⁵. The synthesis of silver nanoparticles has been reported from Lyngbya majuscule, Ulva fasciata, Spirulina platensis, and Chlorella vulgaris ^{116, 117, 118}, Sargassum wightii and Fucus vesiculosus ^{108, 119}. Investigation on the aqueous algal extract of microalga Scenedesmus sp., Pithophora oedogonia, red alga Hypnea musciformis and green algae Caulerpa serrulata for the synthesis of silver nanoparticles was confirmed by ¹²⁰, ¹⁶ and ¹²¹ respectively. Other algae reported for silver nanoparticle synthesis include brown algae Cystophora moniliformis ¹²² and marine algae Caulerpa racemosa ¹²³. On the other hand, few studies established the synthesis of iron nanoparticles from algae. Report on the possibility of biosynthesizing iron nanoparticles from Chlorococcum sp., further ascertaining the importance of carbonyl and amine groups’ presence in polysaccharides and glycoproteins encoded in algae cell wall as the bio-reducing agent ¹²⁴.

The synthesis of nanoparticles has been reported from various conventional methods; nonetheless, few studies have established the importance of biosynthesized nanomaterials in bioremediation techniques ^{5, 125}. The removal of pollutants such as heavy metals, hydrocarbons, organic and inorganic chemicals and toxins through enhanced microbial activity using nanoparticles/nanomaterials shaped by plants, fungi, and microbes with the assistance of nanotechnology is widely known as nanobioremediation ¹²⁶. Several nano-applications for eco-remediation are quickly growing from pilot-scale to full-scale accomplishment in treating the devalued environment ¹³.

In furtherance to accelerate, manage cost and overcome unending release and piled pollutants, this has prompted the choice of combine/synergistic applications of new and existing technologies, which is becoming increasingly important over the use of a single method ^{15, 127}. Surface modification, immobilization and functionalization of nanomaterials with polymers, microbes and other materials were found to have improved particles’ properties and enhance processes when considered for industrial applications ¹²⁸. On the other hand, the approach encourages the functionalization of microbial cell surfaces either with nanoparticles and/or a polymer which has an affinity to bind to microbial cells ^{129, 130}. Biofunctionalization approach-an aspect of functionalizing nanomaterials with biological agents has consistently received a whooping global market value for wastewater remediation in the middle-east ¹³¹ and thus, projecting its usefulness across the globe.

Microbes have been employed to sharpen nanoparticles utilization for nanobioremediation process as several researchers reported positive results in the application of microbe-mediated nanoparticles in remediation processes. The wide acceptance of microbes for this scientific approach was established due to their unique physical, biological, chemical, and optical properties such as sensitive affinity membranes, super-hydrophobic and filtering nature, adjustable functionality, and high surface-to-volume ratio ^{30, 132}.

Microbe-mediated nanobioremediation of heavy metals as displayed (Table 2) validates the potential of microbes in environmental clean-up. Nanoparticles efficiency in bioremediation was conducted through the in-situ synthesis of palladium (Pd) nanoparticles from Pd (II) ions mediated by Clostridium pasteurianum obtained from sandy aquifer material. Biosynthesized Pd nanoparticles recorded positive remediation in the conversion of hexavalent chromium Cr (VI) to insoluble Cr (III) and thus, leading to the production of hydrogen gas ¹³³. This study documented a removal rate of 7.2 g Cr (VI) hence, demonstrating how nanocatalysts significantly improve Cr (VI) removal rates over traditional in situ biostimulation strategies. Similar approach conducted was channeled towards Cr (VI) reduction using polyvinyl alcohol (PVA), sodium alginate, and carbon nanotubes (CNTs) matrix immobilized on Pseudomonas aeruginosa ¹³⁴. The biological reduction to soluble Cr (III) by the immobilized cells was responsible for 84 % reduction of 80 mg/L Cr (VI), further suggesting a complete reduction of Cr (VI) when interacted for 24 hours. Immobilization of Shewanella oneidensis on calcium alginate beads stabilized CNTs was also demonstrated as a good biofunctionalized mechanism for the reduction of toxic Cr (VI) to Cr (III) in wastewater ¹³⁵. This setup was reported to have shown a four times higher reduction rates when compared with microbes and calcium alginate beads only, establishing the incorporation of the 3 constituents for an effective biological reduction processes in environmental bioremediation and wastewater treatment. The biogenic role of a different microbe (Lysinibacillus sphaericus) in the synthesis of magnetic oxide nanoparticles aimed at removing hexavalent chromium in the environment ¹³⁶. Their study documented the use of exopolysaccharides (EPS) matrix of biofilm obtained from Lysinibacillus sphaericus as a better reducing, stabilizing and capping agent, possessing several binding sites for various metal ions. The EPS functionalized magnetic oxide nanoparticles showed enhanced capability to adsorb Cr (VI). On the other hand, algae have also shown their relevance in nanobioremediation. Iron nanoparticles production mediated by Chlorococcum sp. showed an appreciable reduction of about 92 % of 4 mg/L Cr (VI) to Cr (III) ¹²⁴. Phyco-synthesized iron nanoparticles mediated by biomolecules from this algal cell showed high reactivity, greater stability and efficient reduction of toxic pollutants in the environment.

The incorporation of Chlorella vulgaris as a functionalized agent in ultrafine bi-metallic (TiO₂/Ag) chitosan nanofiber mats explained the importance of algae in photo-removal approach of Cr (VI) under UV irradiation ¹³⁷. The release of organic substances (chlorophylls, carboxylate acids) by Chlorella vulgaris was reported to have enhanced the photocatalytic reduction of Cr (VI) on TiO₂/Ag chitosan nanofiber mats, affirming the synergistic approach of algae and TiO₂/Ag hybrid nanomaterial. A related approach was conducted on pharmaceutical effluents containing heavy metals identified majorly as Chromium (Cr) and Lead (Pb) ¹³⁸. In this study, green synthesized silver nanoparticles facilitated by Bacillus cereus was aided with alumina for an effective bioremediation process. The nano-adsorbent method mediated by bacterial cell certified the removal of 98.13% and 98.76% of Cr and Pb respectively, which were released as waste effluents from pharmaceutical industries. The production of lead sulfide (PbS) nanoparticles from Rhodosporidium diobovatum, substantiating the possibility of an easy breakdown of Pb (II) ions pollutants present in the environment into less toxic and useful forms by fungi ⁸⁷. Research findings showcase the potential and ability to biosynthesize chitosan nanoparticles (NCt) from Cunninghamella elegans using a bioactive polymer (chitosan) ¹³⁹. It was reported that the produced NCt particles exhibited a higher biosorption and bioremediation capacity on Pb (II) and Cu (II) ions in polluted environment than bulk chitosan. The successful removal of cadmium in cadmium contaminated water showed the prowess of Pseudomonas aeruginosa enhanced bioreduction of cadmium which in turn accelerated the biosynthesis of Cadmium sulphide nanoparticles ¹⁴⁰. Similarly, assessing a study on the removal and bioremediation of cadmium polluted soil, it was observed that the co-treatment of Bacillus subtilis and nanohydroxyapatite (NHAP), effectively removed cadmium ¹⁴¹. Subsequently, promoting the proliferation of rhizosphere community and bacterial diversity in the remediated soil.

The assessment of slightly different biofunctionalized strategy involving polyvinylpyrrolidone (PVP)-coated iron oxide nanoparticles interacted with Halomonas sp. sourced from oil-polluted soil has been reported ¹⁴². A recent study has reported the possibility of bioremediating both cadmium and lead in the soil ¹⁴³. This microbe mediated approach recognized the combined application of Escherichia coli and metallic nanoparticles towards the removal of the heavy metals. Mercury polluted soil was found to have been effectively bioremediated with selenium nanoparticles ¹⁴⁴ whereby the selenium nanoparticles were produced with the presence of Citrobacter freudii. The conversion of elemental mercury (Hg⁰) to insoluble mercuric selenide (HgSe) mediated by biogenic nano-selenium assessed under aerobic and anaerobic conditions reported a bioremediation value of 39.1-48.6 % and 45.8-57.1 % respectively. The nanobioremediation of industrial effluents containing nickel electroplating has been reported. In this study nickel compound removal in the effluent was facilitated with the introduction of Microbacterium sp. leading to the production and recovery of nickel oxide nanoparticles ¹⁴⁵. A study on Hypocrea lixii revealed the potential of fungi to biologically reduce toxic metals especially nickel in pollutants, further devising the biosynthesis of nickel oxide nanoparticles from the waste for further application ¹⁴⁶.

Microbe-mediated nanobioremediation of hydrocarbon otherwise referred to as persistent organic pollutants has been reported (Table 3). Harnessing the electrostatic interaction of Rhodococcus erythropolis-functionalized magnetic nanoparticles ¹⁴⁷. This mechanism substantively bio-desulfurize 56 % of dibenzothiopene (DBT) present in hydrocarbon hence, authenticating the advantage of Rhodococcus erythropolis functionalized magnetic nanoparticles over the single application of each constituent for bioremediation. The effective synergistic effect of non-zerovalent iron (nZVI) and Sphingomonas sp. as an effective pair towards the debromination and gradual degradation of polybrominated diphenyl ethers (PBDEs) in aqueous solution ¹⁴⁸. On the other hand, the viability of the combined use of Sphingomonas wittichii and bimetallic (Pd/nFe) nanoparticles for the bioremediation of hydrocarbon (2,3,7,8-tetrachlorodibenzo-p-dioxin) was established ¹⁴⁹. Integrated hybrid (nano-bioredox) degradation process facilitated active dechlorination to form dibenzo-p-dioxin. Further study on the use of Sphingomonas sp. was found useful wherein, it was assessed as a biofunctionalized tool for carboxymethyl cellulose (CMC) stabilized bimetallic (Pd/Fe) nanoparticles ¹⁵⁰. The nanocomposite was found to be successful for the degradation of gamma-hexachlorocyclohexane (γ-HCH) commonly known as lindane used majorly in cosmetics.

In a study conducted for the removal of Aroclor 1248- a congener of PCBs, the significant dechlorination and conversion of the pollutant when treated with bimetallic (Pd/Fe) nanoparticles under anoxic conditions to biphenyls were displayed ¹⁵¹. Progressive bioremediation of the resulting biphenyls with Burkholderia xenovorans reported further catalytic decrease of the persistent Aroclor 1248 from 33.8 x 10^-5 μg/g to 9.5 x 10^-5 μg/g. Investigation on PAH (benzo[a]pyrene) clean up using silica nanoparticles biofunctionalized with lipid bilayers of Pseudomonas aeruginosa ¹⁵². The lipid (1,2-dimyristoylsn-glycero-3-phosphocholine) molecule was found to play an active role, improving the adsorption/sequestration of the PAH when conjugated with silica nanoparticles. Graphene oxide nanoparticles biofunctionalized with laccase enzyme produced from Trametes versicolor have been studied for their potential and combine increase for the biodegradation of PAH (anthracene) ¹⁵³. The incorporation of laccase enzyme produced by fungi as conjugant was reported to have improved the degradation ability than their single application and also enhance their long term stability. Polymer (polyallylamine hydrochloride) layered magnetic nanomaterial functionalized with Alcanivorax borkumensis proved the possibility of an active degradation of hydrocarbon ¹⁵⁴. Unique features such as the formation of neutral lipid inclusions in Alcanivorax borkumensis biofilms, biosurfactant micelle formations ascertain the possibility of the hydrocarbon decomposition.

The importance of Bacillus licheniformis mediated biodegradation process was reported ¹⁵⁵. The method was assessed to biofunctionalized Zn₅OH₈Cl₂ modified Fe₂O₃ nanoparticles with Bacillus licheniformis for the breakdown of crude oil into natural degradable substances. In addition, show some prospect on the possible enhancement of microbial biosurfactant for effective bioremediation of large scale oil pollution. The synergistic effect involving iron oxide nanoparticle and Alkaligenes faecalis which enhances biodegradation of crude oil polluted environment has been reported ¹⁵⁶. Their study observed that assessment of different concentrations of Alkaligenes faecalis and iron oxide nanoparticle at 200 mg is effective in the clean-up of crude oil polluted environment.

Dye has been widely known as an essential component in varying industries such as textile and cosmetics. However, it is disposed-off majorly as liquid effluent into the environment which has been found to be toxic to living organisms ¹⁵⁷. Microbe mediated nanobioremediation of dyes used in textile industries has been reported (Table 4). A study established the combined effect of silver nanoparticles biofunctionalized with Chromobacterium violaceum as a biosorption approach for remediating washing water used for the processing of cotton fabrics ¹⁵⁸. The method ascertained the effective removal of both organic compounds and dyes used in fabrics production. This treatment further showed its efficiency for the elimination of used silver nanoparticles as well as the recovery of bacteria thus, posing lesser damage to the environment. Congo red used as dye on cotton fabrics have also been remediated from wastewater with the application of silver nanoparticles synthesized from Bacillus pumilis ¹⁵⁹. This method was found efficient for the removal of Congo red (less resistant to light and washing) with maximum recovery of silver material leached into the effluents to avoid damage on the environment. A study reported the effective catalysis of Congo red dye using silver nanoparticles synthesized from green algal species Caulerpa serrulata ¹²¹. Although, methyl orange dye is seldom used in textile due to its sensitivity to acids, however, they still find usefulness as a dye for wool hence, a form of wastewater pollutant. Nanobioremediation mechanism assessed the consortium of Micrococcus lylae, Micrococcus aloeverae and Cellulosimicrobium sp. for the production of titanium oxide nanoparticles ¹⁶⁰. Active degradation of methyl orange dye under the influence of UV light was achieved in a reactor. These rhizospheric roots associated microbes showed the possibility and effectiveness of natural sources of nanoparticles biosynthesis and about ~99 % photocatalytic degradation of methyl orange dye, an indication of the importance of photocatalytic approach for a safe ecological and adequate nanobioremediation strategy. A similar finding was documented for algal-mediated (Hypnea musciformis [wulfen] J.V Lamouroux) silver nanoparticles synthesis and its active usefulness in degrading methyl orange solution under visible light illumination ¹⁶.

An attempt on the bioreduction of Azo dyes -important synthetic colourants which have been widely used in textile, printing, paper manufacturing was carried out using palladium nanoparticles produced from Klebsiella oxytoca ¹⁶¹. The synthetic organic colourants were adequately bioreduced and recovered from the liquid effluents. The synergistic effect of Chlamydomonas reinhardtii and fabricated polysulfone nanofibrous web was found effectual for the removal of reactive dyes from wastewater ¹⁶². The biosynthesis of palladium nanoparticles from Saccharomyces cerevisiae had been shown to ensure adequate photocatalytic degradation of direct blue dye in liquid effluents ¹⁶³. Photocatalytic degradation of methylene blue and the treatment of effluent containing hazardous dye using silver nanoparticles synthesized from microalgae Chlorella pyrenoidosa ¹⁶⁴ and Caulerpa racemosa ¹⁶⁵ were reported to have produced substantial results, reducing the level of the pollutants in their controlled experiments.

The biosynthesis of manganese oxide nanoparticles from Pseudomonas putida recognized bacteria potential for adequate removal of organic micropollutants ¹⁶⁶ (Table 5). Another research established the combined effect of silica nanoparticles biofunctionalized with an enzyme (laccase) derived from Thielavia sp. for the removal of bisphenol A ¹⁶⁷. Bisphenol A (BPA) widely recognized as an important industrial chemical used for the manufacturing of plastics and resins required for food and beverages storage has become an irritant to the environment. The removal of bisphenol A with an approach focused on the application of manganese oxide nanoparticles biosynthesized from algae (Desmodesmus sp.) ¹⁶⁸. A catalytic study targeted on nitrate removal from liquid effluents demonstrated the application of Chlorella vulgaris ¹⁶⁹. Dual role played by the algae which includes biogenic production of palladium nanoparticles as well as its immobilization on electrospun nanofiber mats enhances the catalytic activity of the complex towards nitrate removal from liquid effluents. Nitro compounds commercially produced for solvents or chemical intermediates generate quite a substantial volume in industrial effluents. Nanobioremediation mechanism assessed Tubinaria conoides; Sargassum tenerrimum for the biogenic production of gold nanoparticles, exploited for the catalytic reduction of nitro compounds in wastewater ¹⁰⁰.

The frequent discharge of antibiotics and antiseptics constituent in wastewater is of major concern. This emanates both from domestic and industrial sections of the populace. Not only has it polluted the environment but, has also proliferated the emergence of antibiotic-resistant microbes in wastewater ¹⁷⁰. However, the possibility of eradicating these pharmaceutical compounds using nanobioremediation approach was assessed in the following studies (Table 6). The efficiency of platinum nanoparticles biosynthesized from Desulfovibrio vulgaris for the removal of effluents containing pharmaceutical compounds. 1,2,4-triazole used majorly for the production of pesticides often contributes to the pollution of groundwater through leaching ¹⁷¹. A consortium of bacteria dominated by Pseudomonas and Bacillus spp responsible for the biosynthesis of manganese oxide was found to effectively remove 1,2,4-triazole from wastewater ¹⁷². This study demonstrated the potential of biogenic manganese oxide nanoparticles for the removal of various recalcitrant pollutants from biotreated chemical industrial wastewater. Likewise, 2,4,6-trinitrophenol (TNP) commonly known as picric acid is a useful component in antiseptic production yet, posing threat to the environment as a pollutant in aqueous solution. Study demonstrated the progressive application of Pseudomonas aeruginosa mediated Fe₃O₄ nanoparticles as a fragment of multi-walled carbon nanotubes for the production of nanocomposite relatively used for nanobioremediation of picric acid ¹⁷³. Triclosan micro-accumulation which has been implicated as a risk factor of cancer has often found its use in antibacterial and antiseptics agents, however, the importance of fungi (Trametes versicolor) as an important biofunctionalized agent for bimetallic (Pd/Fe) nanoparticles for the removal of triclosan in liquid effluents was established ¹⁷⁴. Laccase enzyme derived from Trametes versicolor was found to have played a significant role in the two step redox approach which include anaerobic dechlorination and sequential oxidation of 2-phenoxyphenol.

Similarly, the biofunctionalization of magnesium oxide (MgO) nanoparticles by yeast (Candida sp.) was described ¹⁷⁵. This myco-nano strategy was found to have accelerated the degradation and treatment process of Cefdinir in an aqueous medium. Turbinaria conoides - an alga, mediated the biosynthesis of both gold and silver nanoparticles which found its usefulness as an antimicrofouling agent ¹⁷⁶. Hydrogen peroxide -a common pharmaceutical constituent yet an environmental pollutant was efficiently removed ¹⁷⁷. This was achieved by the electrocatalytic reduction of the compound in industrial waste effluents facilitated by palladium nanoparticles synthesized from Sargassum bovinum.

4. Conclusion

Advances in nanotechnology have shown its importance as a veritable tool towards ecological restoration and improved economic environment. A substantial number of nanomaterials have found their usefulness as pollutants removal from soil and water. This study further informed on the importance of microorganisms and their significance in biosynthesis and biofunctionalization of nanomaterials in nanobioremediation processes. Since it is necessary to progressively improve and develop new methods and approaches for an effective bioremediation, careful consideration and inclusion of microbes is essential, due to their eco-friendliness, simplicity and cost-effectiveness. Moreover, better understanding must be examined to advance and create suitable microbe-biofunctionalized approaches to achieve effective and sustainable environmental clean-up strategies.

Competing Interest

The authors have no competing interests.

Authors’ Contributions

‘N. Torimiro, OB. Daramola’ conceptualized the review work. ‘OB. Daramola, OD. Oshibanjo, OF. Otuyelu, BA. Akinsanola, OO. Yusuf, OT. Ore’ managed the literature searches. ‘OB. Daramola’ wrote the first draft of the manuscript. ‘N. Torimiro, RK. Omole, OB. Daramola’ reviewed and edited the manuscript. All authors read and approved the final manuscript.

References