Nanobioremediation-Based Removal of Pollutants

Nanobioremediation-Based Removal of Pollutants: Comparison

Please note this is a comparison between Version 2 by Camila Xu and Version 1 by Vishnu D. Rajput.

pollution
heavy metals and metalloids
phytoremediation potential

1. Introduction

Rapidly increasing anthropogenic/technogenic activities are adding potentially toxic metals, agrochemicals, and an excess of nutrients to the soil ^[1]. Soil is a basis of crop production as it supports plants to uptake nutrients ^[2]. In fact, agriculture sustains and defines human lives; however, it is often disruptive of natural ecosystems. Humans’ voracious appetites for getting the benefits from natural resources grow in tandem with population growth. The conflict between the benefits and the sustainable management of agricultural land and its conservation has been reported in the literature for a long time [3,4]^[3][4]. Land pollution is a threat to livelihoods, quality of life, and sustainable development ^[5]. Thus, conserving soil is the utmost requirement for the current era owing to the pressures of increasing population and the shrinking of arable lands by technogenic activities.

The advancements in nanotechnology open a window globally to remediate or restore polluted soil in an effective way [6,7]^[6][7]. It has been claimed that nanotechnology has great potential as an environmentally cleaner technology, including by alleviation of the toxicities of various metals/metalloids [8,9]^[8][9]. Besides, nanotechnology has been recognized as a potential method for the remediation of pollutants in a variety of environmental matrices, including soils ^[6]. In this context, soil remediation is one of the main domains where nanotechnological approaches have been widely used. The uses of NPs have been explored lately to remove contaminants in a variety of ways, including by adsorption, redox reactions, precipitation, and co-precipitation, all of which are aided by their enormous specific surface area ^[9].

With the help of NPs, hyperaccumulators and indigenous soil microbes could enhance biodegradation processes, thereby increasing the potential extent of remediation. This could be called nano-phytoremediation and microbial-mediated nano-remediation. The use of NPs with bioremediation approaches may result in a lot of benefits as these particles are small (1–100 nm) with a larger surface area and reactivity ^[10]. The broad range of studies indicated that the foliar use of NPs alleviates metal-pollutant toxicity and enhances plant growth, resulting in a high accumulation of elemental toxic content in plant tissues [11,12,13]^[11][12][13]. The degree of contamination, the bioavailability, and the accumulation of metals by the plants are decisive for the efficiency of nano-phytoremediation as a way of removing heavy metals (HMs) from contaminated sites [14,15]^[14][15].

The utilization of NPs based on a metal–organic framework (MOF) has received a lot of important attention lately, but it is primarily employed for drinking water and wastewater treatment [16,17,18]^[16][17][18]. Thus, there is a massive opportunity for scientists to envisage potential uses of MOF in soil remediation. However, concerns related to the safe use of NPs to remediate polluted soils, and their release into ecosystems, are still less explored, and this becomes a matter of concern [19,20]^[19][20].

2. An Appraisal of Nanobioremediation-Based Removal of Pollutants; Special Emphasis on Microbe-Mediated Remediation

Soil rich in vital nutrients and micronutrients is believed to best support the optimum health of the plant and its growth ^[21]. Human-made activities have currently polluted the soil with a variety of persistent organic compounds, viz., polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), volatile organic compounds (VOCs), HMs (Hg and Pb), agrochemicals (pesticides, fungicides, and fertilizers), and also sometimes with excess nutrients ^[22]. At the same time, urbanization and industrialization have also added solid wastes, varieties of chemicals, and solvents to the environment and agricultural soil ^[23].

Nanobioremediation is a cost-effective technique of utilizing plants and microbes for the breakdown of pollutant compounds, ultimately improving soil quality and reducing pollution. By breaking down contaminants in the soil, the process may be able to eradicate, retain, or reduce the amount of pollutants present [24,25]^[24][25]. The efficiency of bioremediation has been studied and enhanced in the past using chemical additives or biotechnology ^[26] but nanotechnology further improved the process with a newer aspect ^[27]. A summary of different NP-mediated removal of pollutants from contaminated media is elaborated in Table 1.

Table 1. Summary of different nanoparticles-mediated removal of different pollutants from contaminated media.

Nanoparticles	Remediated Contaminant(s)	Operational Conditions and Removal Efficiency	References
Polyvinylpyrrolidone (PVP) coated iron oxide nanoparticles	Cd and Pb	NPs applications were integrated with the process of bioremediation mediated by Halomonas sp. In the removal setup of Cd and Pb, Halomonas sp. was inoculated for 48 h at 180 rpm, 28 °C. The 100% removal was recorded after 24 h, while for Cd, it was observed after 48 h.	[44]	^[28]
Zero-valent iron (nZVI) commercial suspension at two doses (1% and 10%)	As	pH was set at 12.2 ± 0.1 of the nZVI suspension. To avoid the aggregation of nZVI in the suspension, polyacrylic acid was used as a stabilizer. Maximal immobilization of As in brownfield soil was recorded at 10% of nZVI.	[45]	^[29]
Graphene oxide nanoparticles (nGOx) and nZVI	Metals, viz., Cd, Pb, Zn, Cu, and As in the As-Metals polluted soil	Applications of nZVI and nGOx to the polluted soils considerably impacted the availability of As and metals. Cu, Pb, and Cd were immobilized by nGOx, while mobilized As and P. In a turn of nZVI, it immobilized the effectively As and Pb, and poorly Cd but enhanced availability of Cu. This study revealed that both NPs applications might be act as strategies for the immobilization and stabilization that can later be utilized for phytoremediation.	[46]	^[30]
Titanium oxide nanoparticles-bonded-chitosan nanolayer (NTiO	₂	-NCh)	Cd and Cu	The pH was set at 7.0 during the experimentation. The removal was assisted by 60–70 s heating by using microwave–enforced sorption approach. Application of NTiO	₂	-NCh was found to eliminate Cu and Cd by 88.01% and 70.67%, respectively.	[47]	^[31]
Palladium (Pd), Pd NPs	Cr	The use Pd NPs as bionanocatalyst has been explored. It was found that Pd NPs completely reduced Cr	⁶⁺	in 12 h. 6.3 mg of PdNPs was used to reduce 5.0 µmol of Cr	⁶⁺	.	[48]	^[32]
Magnetic iron oxide nanoparticles (Fe	₃	O	₄	NPs) treated with	Staphylococcus aureus	, and surface encapsulated with phthalic acid (n-Fe	₃	O	₄	-Phth-	S aureus	)	Cu, Ni, Pb	n-Fe	₃	O	₄	-Phth-S was found to remediate 83.0–89.5%, for Cu	²⁺	, 99.4–100%, for Pb	²⁺	, and 92.6–7.5% for Ni	²⁺	. The study also identified n-Fe	₃	O	₄	-Phth-	S. aureus	as an excellent biosorbent for the removal of divalent ions from an aqueous medium.	[49]	^[33]
ZnO NPs	Cu, Cd, Cr, and Pb	The applications of ZnO-NPs at 5 mg L	⁻¹	with	Bacillus cereus	and	Lysinibacillus macroides	showed the maximal removal of Cr, Cu, and Pb which was 60%, 70%, and 85%, respectively. The optimal pH for efficient removal was 8.0. The removal was less in the case of bacteria-mediated remediation which was found to be 83 and 70% in	B. cereus	and 60 and 65% in	L. macroides	.	[50]	^[34]

Nanobioremediation implies both nanotechnology and bioremediation together, where the process is executed at the nanoscale. The target pollutants are adsorbed, degraded, or modified owing to the unique physicochemical properties of the NPs, which also act as catalysts and help to reduce the activation energy required for breaking down the compounds [28]^[35]. The nanobioremediation process has been explored and studied, and the most exploited NPs are carbon- and metal-based [29,30]^[36][37]. Polymeric NPs in the form of nanocapsules or nanospheres are also exceptional in the elimination of persistent pesticide compounds and long-chain hydrocarbons [31]^[38]. However, in the case of HMs, the challenge is entirely different as they are non-biodegradable, as well as very prone to entering biological systems and food chains [32]^[39].

Biosorption and bioaccumulation using plants and microbes are traditional methods to remove HMs from polluted soils. However, recent pieces of evidence have reported the use of NPs in HM remediation with remarkable outcomes [33]^[40]. Nanoparticles are reported to have been applied in combination simultaneously or sequentially with specific microbes and the results have been convincing [34]^[41]. They could help to speed up the elimination of HMs by acting as nanocarriers of microbes or microbial biosorbents [35]^[42]. A pictorial diagram that represents the process of nanobioremediation, especially for biogenic NPs, is depicted in the Figure 1.

Figure 1. An overview of the processes of nanobioremediation using biogenic nanoparticles.

The synergy of NPs and bacterial degradation has also gained attention; however, the availability of a handful of published papers would not permit the making of a categorical review. Many authors have given their efforts to nanobioremediation, and yet it is still too insignificant for making any conclusive decision ^[10].

Integration of NPs with microbes for bioremediation is a two-phasic process that involves overlapping abiotic and biotic processes (Figure 1) [36]^[43]. In the first phase, after the entry of NPs into the system, pollutants undergo varieties of physicochemical processes and modifications depicting abiotic processes such as absorption, adsorption, dissolution, and chemical catalysis of photocatalytic reactions [37]^[44]. The second phase includes biotic processes such as biocides, bioaccumulation, biostimulation, and biotransformation [38,39]^[45][46]. These biotic processes play a crucial role in the removal of pollutants from the system.

2.1. Nanobioremediation of Heavy Metals

The existence of HMs in the environment is largely due to increased anthropogenic activities. However, disturbed biogeochemical cycles are also responsible for their release into the environment as pollutants. Elements like As, Cd, Cr, Hg, and Pb have no biological functions to perform in the biological system. Heavy metals comprise major inorganic pollutants as they exhibit substantial toxic impacts on biota even at the lowest concentrations [40,41]^[47][48]. The toxicity of HMs also rests on their bioavailability and absorption [42]^[49]. Acidic environments instigate the toxicity of HMs, especially if the soil structure is poor and has low nutrients (e.g., mining areas) [43]^[50].

Heavy metals primarily affect the plants and lower soil organisms by inducing the generation of reactive oxygen species (ROS), which further results in the damage of macromolecules such as proteins and nucleic acids [41]^[48]. The existence of HMs in the soil affects crops and vegetation, their nutritional quality, and the ecological aspects associated with them. The effect of HMs on crops varies depending on the crop species, soil physicochemical characteristics, and HM type ^[51]. The general mechanism of toxicity exerted by HMs on crop plants includes ROS generation, which affects the cell organelles, macromolecules such as proteins and nucleic acids, and other components of the plant’s structure and function [51,52]^[51][52]. It has also been reported to affect respiration and photosynthesis, reduce enzyme activities, elevate oxidative stress, reduce biomass, diminish crop yield, and affect the abundance, activity, diversity, and genetic makeups of useful soil microflora [53,54]^[53][54].

One of the key methods for the elimination of HMs includes site stabilization that immobilizes them at a specific site to decreases mobility and availability in the soil, and stops them from leaching across the sites ^[55]. The use of various NPs, including biogenic, has been gaining a lot of attention for the removal of HMs ^[56]. Biogenic NPs are those that are synthesized using biological organisms. The commonly known biogenic NPs, such as Ag NPs, are formed by Morganella psychrotolerans [57,58]^[57][58].

Nanoparticles of FeO coated with polyvinylpyrrolidone (PVP) have been successfully used for improving the bioremediation of the soil contaminated with Pb and Cd by a Gram-negative bacteria, Halomonas sp. This approach has significantly removed nearly 100% of Pb after 24 h, and Cd after 48 h, as compared to removal by bacteria or only NPs ^[59]. A biosorbent of magnetic Fe₃O₄ NPs treated with S. aureus, with a surface encapsulated with phthalic acid (as a n-Fe₃O₄-Phth-S complex), was used for the removal of Cu, Ni, and Pb, and the adsorptive removal of 795, 1355, and 985 µmol g⁻¹ for Cu, Pb, and Ni was achieved, respectively. In terms of percentage, the recovery rates of 83.0–89.5% for Cu²⁺, 99.4–100% for Pb²⁺, and 92.6–7.5% for Ni²⁺ were observed. The comparative study with dried S. aureus and n-Fe₃O₄-Phth-S for HM removal inferred that the n-Fe₃O₄-Phth-S core of the NPs, as well as the functional groups present on the microbial surface, played a key role in the removal of HMs [49]^[33]. Thus, ithis work r revealed that the core of the NPs, as well as functional groups present on the microbial surface, had an important impact on the elimination of the contaminants.

A recent study on the removal of Cu, Cd, Cr, and Pb using HM-resistant bacteria such as B. cereus (PMBL-3) and L. macroides (PMBL-7) evidently confirmed that ZnO NPs at 5 mg L⁻¹ synergistically removes the Cr by 60%, the Cu by 70%, and the Pb by 85%, as compared to B. cereus (80 and 60%) and L. macroides (55 and 50%) at neutral pH, respectively [46]^[30]. At neutral pH the surface of ZnO NPs exhibit negative charges that promote electrostatic interactions with metal cations; however, at lower pH, the HMs get precipitated as hydroxides and then hydrogen ions compete for binding with adsorbents ^[60]. The strain XMCr-6 of B. cereus has also been reported to reduce the Cr⁶⁺ through an enzyme-mediated process. The reduced Cr³⁺ was observed to have a binding affinity to cells using coordination bonds with the functional group present on the surface of the bacterial cell wall. The formation of Cr₂O₃ NPs was found on the cell surface as a by-product ^[61].

The use of probiotic bacteria (L. casei and L. fermentum) to absorb Cd from water in association with Se⁵⁺ and Se NPs was also investigated. The higher absorption of Cd by L. casei with Se⁴⁺ ions (65%), compared to Se NPs (55.90%), was discovered in this study, and it was correlated to the higher solubility of Se⁵⁺ compared to Se NPs. When comparing L. fermentum and L. casei, the efficiency of Cd absorption was significantly higher in L. fermentum (50.87%) than L. casei (43.78%). The percentage of Cd adsorption by L. casei when used in conjunction with Se NPs shows no significant change. However, with increased Se NPs ratio percentages, Cd absorption was slightly increased from 5.49 to 16.54 in the presence of L. casei with Se NPs, compared to L. casei ^[62].

A threefold approach is now gaining popularity as the HM pollutants can be used by selective microbes to synthesize biogenic NPs (resource recovery), thereby removing them from the environment (remediation) and yielding value for the waste (effective waste utilization). A study using Enterococcus faecalis for biorecovery of Pd as Pd NPs reported the synthesis of intra- and extra-cellular (membrane-bound). The range of Pd NPs was observed as 10 nm by transmission electron microscopy; however, the size of the Pd NPs was dependent on environmental conditions such as temperature, pH, and biomass. The obtained Pd NPs have great use as a bionanocatalyst that shows good catalytic efficiency (6.3 mg Pd NPs completely reduced 5.0 µmol Cr⁶⁺ in 12 h) and the application is potentially useful to treat industrial effluents [44]^[28].

A similar study produced Te NPs from anaerobic sludge based upon supplementation with riboflavin ^[63]. It formed insoluble elemental tellurium (Te⁰ NPs) using pollutant tellurite Te⁴⁺ oxyanions present in the wastewater. It has been reported that 2-Hydroxy-1,4-naphthoquinone promotes the reduction of Te⁴⁺ and the quantity of Te⁰NPs synthesis ^[64]. The process is supported by Rhodobacter capsulatus, where malate is the electron-donating substrate ^[65], and riboflavin speeds up the rate of Te⁴⁺ reduction by anaerobic methanogenic granular sludge ^[66].

2.2. Degradation of Persistent Organic Pollutants

The pollution posed by POPs has been shown to have a negative influence on both the environment and human health, as certain POPs have been found to bioaccumulate in adipose tissue and to have the potential to act as carcinogens. Therefore, their remediation is a major challenge and is obligatory. A Gram-negative bacterial strain (NM05 of Sphingomonas) was earlier reported to degrade the pesticide hexachlorocyclohexane (HCH) ^[67] upon treatment with Pd/Fe0 bimetallic NPs (CMC-Pd/nFe0), showing the synergistic effect on the degradation of HCH that was enhanced by nearly 1.7–2.1-fold compared to the controls that had the Sphingomonas sp. strain NM05 or CMC-Pd/nFe0 alone ^[60]. The degradation process was found to be affected by experimental conditions (pH, temperature, HCH concentrations, etc.) ^[68].

The perovskite (LaFeO₃) NPs and biochar from water caltrop (Trapa natans) shells studied on marine sediment reported enhanced degradation of PAHs. The study used lignocellulosic fiber-reinforced biodegradable composites (LFBC) at 0.75 g L⁻¹ and pH 6.0 to activate the peroxymonosulfate (3 × 10⁻⁴ M) that helped in the oxidation of oxidizing PAHs in the sediments.

Up to 90% of total degradation was achieved; however, individually 2-ring PAHs 52%, 3-ring PAHs 61%, 4-ring PAHs 66%, 5-ring PAHs 56%, and 6-ring PAHs 29% were observed ^[69]. The process also reported improved microbial diversity of sediment and the major phylum Proteobacteria was observed initially, but after the process, Hyphomonas was predominantly observed ^[70]. In a continuous-flow experiments system for the degradation of naphthalene in the groundwater, 400 mg L⁻¹ of synthesized CaO₂ NPs degraded the naphthalene of optimum concentration 20 mg L⁻¹. This study highlights complete remediation of naphthalene in the presence of CaO₂ NPs and microbes (an abundance of Coccobacilli) from column effluent within 50 days ^[71].

In the case of soil, improving the microbial community by application of NPs is another way to reduce/remove the toxic pollutant loads from it. Si NPs have been reported to improve microbial colonization and biomass, including the rhizospheric microbes that are helpful for improving soil health [72,73]^[72][73]. However, prolonged exposure and accumulation of these NPs in soil may affect the nutrient and organic matter content.

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