Nano-Microbial Remediation of Polluted Soil: Comparison
Please note this is a comparison between Version 1 by Behnam Asgari Lajayer and Version 2 by Peter Tang.

Nanobiotechnology has been used to bio-remediate or reclaim soil contaminated with organic and inorganic pollutants. The removal of pollutants from industrial wastes is a major challenge. The utilization of nanomaterials is gaining popularity, which might be accredited to their enhanced physical, chemical, and mechanical qualities. The development of advanced nanobiotechnological techniques involving the use of nanomaterials for the reclamation of polluted soils has indicated promising results and future hope for sustainable agriculture. By manufacturing environment-friendly nanomaterials, the industrial expenditure on decreasing the load of pollution might be reduced. A potential emerging domain of nanotechnology for eco-friendly production and cost reduction is “green biotechnology”, alongside the utilization of microorganisms in nanoparticle synthesis.

  • bioremediation
  • environmental pollution
  • nanomaterials
  • remediation

1. Introduction

Agriculture is considered one of the most important human activities, as it is the main source of food, feed, fuel, and fiber [1]. This activity can cause many environmental problems, especially when insecticides and mineral fertilizers are used in excess [2][3][2,3]. Therefore, agricultural contamination might refer to several activities that lead to the destruction or pollution of agroecosystems and affect human well-being [4]. In other words, agricultural soil, soil health, and fertility have been drastically impacted by many different types and classes of pollutants [5]. Some contaminants have a longer lifespan and are recalcitrant. They persist in the soil for many years, disrupting the food chain and causing biological imbalances in the soil, ultimately endangering human health [6]. Pesticides, fertilizers, household and industrial wastewater, industrial activities, and automobile pollution are the major anthropogenic sources of hazardous toxic metals and/or metalloids in the soil [7].
The pressing need is environmental remediation, which must be addressed as a priority [8][9][10][8,9,10]. In recent decades, various techniques have been used for this purpose, such as mycoremediation [11], phytoremediation [12][13][12,13], vermiremediation [4], bioremediation [6], remediation by using biosorbent materials such as biochar [14], fly ash and organic fertilizers [15], humic substances [16], and nanomaterials (NMs) via green remediation; or combined remediation [17].
The notion of sustainable remediation has recently gained much attention [13], as it essentially aims to reduce the concentration of contamination to risk-free levels while avoiding additional environmental impacts [6]. Several recent developments in this field have combined multiple technologies into a system that provides a cost-effective and time-saving way to disinfect a site while being able to restore the site’s quality. As an economical and environmentally sound means of remediating polluted areas, bioremediation is one of the solutions to problems of pollution [18]. The use of microorganisms to remove contaminants from the soil is the key principle of bioremediation [19]. As defined by the Environmental Protection Agency, bioremediation involves the biodegradation of hazardous pollutants to reduce their toxicity or intensity. It offers a number of benefits over physicochemical approaches, including high selectivity, specificity, cost and energy performance, and low demand. However, bioremediation has the disadvantage that it takes longer to degrade toxic compounds, usually several months to a year. It also limits the use of sites that are heavily contaminated with toxic pollutants, resulting in a loss in terms of resource utilization [4][15][4,15]. Nanoparticles are used in many scientific fields including automobiles, cosmetics, agriculture, foods, textiles, aviation, defense, engineering, medicine, and the environment [20][21][22][20,21,22]. According to the National Nanotechnology Initiative of the United States (NNI), there are relatively few studies on using nanotechnology in the analysis and manipulation of materials up to 100 nanometers in size, where unique phenomena enable novel applications of nanotechnology [23]. As an integrated field of nanoscale science, technology, and engineering, nanotechnology consists of viewing, analyzing, modeling, and manipulating materials within this size range. In recent years, nanotechnology has been increasingly used to remove contaminants due to its smaller particulate matter, high surface-to-volume ratio, ease of deployment at impact sites, flexibility, and other advantages [24]. The utilization of nanotechnology for environmental remediation has attracted much attention [23]. Ongoing research and many publications show how nanotechnology can tackle remediation duties and challenges [25][26][25,26]. Nanoremediation is a technology that has been recognized as environmentally beneficial by the Environmental Protection Agency. It is acknowledged as a viable strategy for traditional site cleaning [27]. Various techniques for using NMs for soil and water reclamation (nanoremediation) have been reported, such as nano-phytoremediation [28][29][28,29], nano-bioremediation [30], nano-Fe3O4 [31], nano zero-valent iron [32], nano-hydroxyapatite [33], nano zeolite [34], nano zero-valent iron [35], ZnO-nanoparticles (NPs) [36], nano-TiO2 [37], stabilized NPs [38], and nano-silica [39].

2. An Evaluation of Nanobioremediation-Based Pollutant Reduction with a Focus on Microbe-Mediated Remediation

Previous techniques for removing heavy metals (HMs) from contaminated soils include biosorption and bioaccumulation utilizing crops and bacteria. However, recent evidence has revealed that the use of NPs in the remediation of HMs has produced impressive results [40]. It has been found that the use of NPs in conjunction with specific microbes, either sequentially or simultaneously, has provided promising results [41]. Not only can they aid in the removal of HMs, but they can also act as nanocarriers for microbial populations or microbial adsorbents [42]. The integration of NPs with microbes for bioremediation is a two-phase procedure that combines biotic and abiotic factors [43]. After entering the system, the contaminants encounter a series of physical methods and revisions that include abiotic mechanisms such as uptake, adsorption, and dissolution, as well as synthetic catalyst supports for photocatalysis during the first stage [44]. Biocides, bioaccumulation, biostimulation, and biotransformation are examples of biotic systems in the second stage [45]. These biotic systems are essential for removing pollutants from the mechanism. Table 1 provides an overview of various NP-mediated pollutants removed from contaminated media.
Table 1.
Summary of the various nanoparticle-mediated pollutants removed from contaminated media.

Nanoparticles

Contaminant Remediated

Factors of Performance and Removal Efficiency

References

Iron oxide nanoparticles with a polyvinyl pyrrolidone coating

Cd, Pb

The use of nanoparticles was combined with a bioremediation process driven by Halomonas sp. Halomonas sp. was inoculated for 48 h at 180 rpm and 28 °C in the Cd and Pb removal system. After 24 h, 100% removal was detected, whereas it took 48 h for Cd.

[46]

Industrial suspension of zero-valent iron (nZVI) at two dosages (1% and 10%)

As

The pH of the nZVI suspension was adjusted to 12.2 ± 0.1. Polyacrylic acid was utilized as a stabilizer to prevent the accumulation of nZVI in the suspension. The maximum amount of As immobilized in brownfield soil was 10% of nZVI.

[47]

Graphene oxide nanoparticles (nGOx) and nZVI

Metals such as Cd, Pb, Zn, Cu, and As were found in As- and metal-contaminated soil.

The application of nZVI and nGOx to contaminated soils had a significant influence on the availability of As and metals. nGOx immobilized Cu, Pb, and Cd while mobilizing As and P. In the case of nZVI, it successfully immobilized As and Pb (but not Cd) while increasing Cu’s availability. This study discovered that both NPs may work as techniques for immobilization and stabilization, which can then be used for phytoremediation.

[48]

Titanium oxide nanoparticles bonded to a chitosan nanolayer (NTiO2–NCh)

Cd and Cu

During the experiment, the pH was adjusted at 7.0. The elimination was aided by a microwave-enforced sorption technique that lasted 60–70 s. Cu and Cd were eliminated at a rate of 88.01% and 70.67%, respectively, when NTiO2–NCh was used.

[49]

Palladium (Pd), Pd NPs

Cr

Pd NPs were investigated as a bionanocatalyst. Pd NPs were shown to decrease Cr6+ completely in 12 h. To decrease 5.0 mol of Cr6+, 6.3 mg of Pd NPs was utilized.

[50]

Magnetic iron oxide nanoparticles (Fe3O4 NPs) were treated with Staphylococcus aureus and had their surfaces encapsulated in phthalic acid (n-Fe3O4–Phth–S. aureus)

Cu, Ni, Pb

The remediation efficiency of n-Fe3O4–Phth–S. aureus was reported to be 83.0–89.5% for Cu2+, 99.4–100% for Pb2+, and 92.6–7.5% for Ni2+. The researchers also discovered that n-Fe3O4–Phth–S. aureus was an effective biosorbent for removing pollutants.

[51]

ZnO NPs

Cu, Cd, Cr, and Pb

The maximum removal of Cr, Cu, and Pb by ZnO-NPs at 5 mg L−1 with Bacillus cereus and Lysinibacillus macroides was 60%, 70%, and 85%, respectively. The ideal pH for effective removal was 8.0. The elimination was reduced in the case of bacteria-mediated remediation, which was determined to be 83% and 70% with B. cereus, and 60% and 65% for L. macroides.

[52]

3. Soil Nanoremediation

Nanoremediation is a virtually new application of nanotechnology for addressing environmental pollution issues [53][75]. Recently, this technique has been used to treat hazardous waste. Although it is a new technical sector, the application of nanotechnologies for environmental remediation has recently attracted a lot of attention from the scientific community [54][76]. The use of zero-valent iron (ZVI) as a permeability barrier was the first research idea developed by Gillham [55][77] based on their experience with NPs in decontamination of water-halogenated contaminants [56][78]. Many researchers utilized chemical synthesis, whereas others use green leaf extracts similar to those used to remove pollutants in aqueous solutions to create zero-valent iron NPs. The use of NPs can effectively degrade numerous pollutants such as organic halocarbons [57][79], nitrates, HMs [58][80], pesticides, and dyes [59][81]. There have been very few studies that have applied NP technology for the remediation of contaminants in soil; studies in this field have instead used for the decontamination of water or aqueous solutions [60][82].
Studies have reported that NPs can adsorb pollutants and facilitate their destruction through redox reactions, surface reactions, ion exchange, surface complexation, electrostatic contact, and adsorption [61][83]. A bentonite matrix was used by Shi et al. [60][82] to remove Cr (VI) from water and soil solutions using ZVI nanoparticles (nZVI) and Fe NPs with zero valency (B-nZVI). They discovered that the use of bentonite (B-nZVI) as a carrier material increased the effectiveness of nZVI nanoparticles, resulting in reduced aggregation and improvements in the active surface area. Likewise, the temperature was directly proportional to the amount of Cr (VI) removed, as were pH and total B-nZVI, which decreased with an increase in pH [62][84]. B-nZVI NPs have large surface areas and are highly reactive, enabling them to work as excellent adsorbents of Cr (VI) [60][82]. A wide variety of contaminants have been studied using NPs, including chlorinated organic compounds, insecticides, phenols and amines, organic acids, and chlorinated organic compounds [62][84]. Two decades ago, experiments showed that NPs, when injected into the soil, could remain effective for up to 56 days and could travel up to 20 m through the groundwater [63][85]. Zhang [63][85] reported that it was possible to remove over 99% of trichloroethene (TCE) from polluted locations within a few days.
Studies have shown that zero valent iron NPs trapped in silica microspheres can decompose polybrominated diphenyl ethers, a type of environmental pollutant that can readily accumulate in the soil [64][65][86,87]. Tetrahydrofuran (THF) was used by Qiu et al. [64][86] to degrade decabromodiphenyl ether from an aqueous solution. The researchers discovered that it was efficient in a solution of THF and water when exposed to environmental and temperature stress. Moreover, the study of Xie et al. [65][87] suggested that the removal efficiency or elimination efficiency for decabromodiphenyl ether in soil achieved by this degradation process was 78%. It was more significant than the biomass of plants treated with NPs. Additionally, Cr (VI) phytotoxicity was investigated, and iron NPs supported by bio-carbon were tested on cabbage mustard, which showed increased growth and lowered Cr (VI) levels. With the injection of 8 g per kg of soil, the immobilization efficiency for Cr (VI) and overall chromium (Cr) was 100% and 91.94%, correspondingly, in remediation experiments [66][88]. A lipid derivative of choline-coated silica NPs was used for bioremediation of polycyclic aromatic hydrocarbons (PAHs). Other NMs that have been used include iron sulfide stabilized by carboxymethylcellulose, which was tested for the consolidation of mercury in soils that were heavily contaminated with this metal [67][89].
According to various publications in 2016, the experimental processes and parameters for the synthesis of NPs differ, making it difficult to compare the efficiency gains due to variability in their structures, compositions, and morphologies, all of which impact the adsorption capacities for comparable pollutants. Currently, there is a lack of information on how they break down various types of toxins. The need for comprehensive studies on NMs is underlined by the lack of knowledge on their mechanisms of recovery and reuse, as well as their widespread application and effectiveness for the remediation of industrial effluents and polluted soils. Nevertheless, the reported results have indicated that this remediation technique is valuable compared with conventional techniques.
The effects of nanomaterials on various ecosystems, and their function, life cycle, and release of metal ions are still largely unexplored. Nanoremediation offers several advantages, including lower costs and shorter clean-up times for polluted areas, as well as the possibility to apply it on a large scale. However, to avoid negative impacts on the environment, detailed studies are needed to examine the effects of nanoremediation at the ecosystem level.

4. Microorganism-Assisted Nanoremediation

The use of nanoremediation is more sustainable and environmentally friendly if the NPs are biologically produced and microbes are used at the same time. Chemically produced NMs may have many disadvantages in terms of chemical consumption and self-agglomeration in aqueous solutions. In this regard, the utilization of plant extracts, and fungal and bacterial enzymes for green NP production might be a promising option. In this process, metallic NPs are created due to their reducing effect on the metal complex salts. Co-precipitation, or the addition of proteins and bioactive components to the outer surfaces of the NPs, greatly increased their strength in an aqueous environment. Mahanty et al. [68][92] found Aspergillus tubingensis (STSP 25) biofabricated iron oxide NPs from the rhizosphere of Avicennia officinalis in Sundarbans, India. About 90% of the HMs (Ni (II), Cu (II), Pb (II), and Zn (II)) in wastewater were eliminated or removed by the synthesized NPs, which had a regeneration potential of up to five cycles. The metal ions were chemically bound to the surface of the NPs by an endothermic reaction [68][92]. The co-precipitation of iron oxide NPs and exopolysaccharides (EPS) from Chlorella vulgaris has been described in other studies. The effective alteration of NPs by EPS functional groups was demonstrated by FT-IR spectroscopy. It was also demonstrated that the nanocomposite could remove 85% of NH4+ ions and 91% of PO43− ions [69][93].
It has been claimed that using bacteria to produce NPs is a practical and beneficial method for the environment. A copper-resistant Escherichia species, SINT7, was used to synthesize copper NPs. Biogenic NPs were observed to degrade azo dyes and textile effluents. Consequently, at a lower concentration of 25 mg/L, 83.6%, 90.6%, 97.1%, and 88.4% of reaction black-5, malachite green, Congo red, and direct blue-1 were lowered, respectively. When the concentration was increased to 100 mg/L, they reduced by 76.84%, 31.1%, 83.90%, and 62.32%, respectively. Additionally, treated samples of industrial sewage contained less phosphate and chloride ions, along with the suspended particles. The performance of biogenic NPs such as these may boost cost-effective and long-term industrial manufacturing [70][94]. Cheng et al. [71][95] used no additional sulfur to make iron-sulfur NPs. These NPs had the ability to annihilate Naphthol Green B dye through the extracellular transfer of electrons. The utilization of Pseudoalteromonas sp. CF10-13 in manufacturing NPs offers an environmentally acceptable biodegradation method. The manufacturing of toxic gases and metal complexes was constrained by the endogenous creation of NPs.
The use of biological particles is a more effective way to remediate industrial wastewater. As well as the direct production of NPs from microbes, there are several other ways in which microorganisms can contribute to the advancement of nanotechnology. In addition to NPs, microorganisms can also provide catalytic enzymes that help in wastewater treatment.

5. Utilization of Nanomaterials for Micro-Remediation of Polluted Soils

Bioremediation using of microorganisms has been proposed as a supposedly efficient approach to remediating contaminated sites [72][105]. Microorganisms that are capable of modifying soils contaminated with HMs and organic pollutants have attracted much attention. Volatilization, metal-binding, alteration, and chemical precipitation are some of the techniques used for the remediation of HMs using microorganisms [73][74][106,107]. According to Xu et al. [75][108], the following elements bind metals to microbial cells: CrO42−, Cu2+, Hg2+, Au3+, Cd2+, Ni2+, Pd2+, and Zn2+. The mobility of these metals and their harmful consequences were diminished by this metal-binding. Furthermore, Polti et al. [76][109] studied the use of microorganisms for the bioremediation of Cr (VI)-contaminated soils. Soil samples showed that the Streptomyces species MC1 was able to reduce Cr (VI) to Cr (III), the latter being more stable and less hazardous than the former. Metals that are volatile, such as Hg, can be volatilized by microbes [77][110]. On the other hand, organic pollutants can be destroyed by a variety of microorganisms or enzymes. Certain microorganisms can use the nitrogen and carbon in organic contaminants, leading to soil decontamination.
For example, four microbes were isolated from soils planted with bamboo, pine, and rice to treat polluted soils, including Rhodotorula glutinis 4CD4, Pseudomonas nitroreducens 4CD2, Pseudomonas putida 4CD1, and Pseudomonas putida 4CD3. All the isolated microorganisms effectively broke down p-hydroxybenzoic acid, ferulic acid, p-coumaric acid, and p-hydroxybenzaldehyde using phenols as a carbon source [78][111]. Pseudomonas stutzeri OX1 has also been shown to be able to break down tetrachloroethylene. Due to the production of toluene-xylene monooxygenase, which induces the aerobic breakdown of pollutants in bacteria, researchers ascribed this degradation to Pseudomonas stutzeri OX1 [79][112]. The effectiveness of micro-remediation for remediating pollutants is impacted by the potential impact of NMs on microorganisms. Shrestha et al. [80][113] investigated the influence of NMs on the architecture and function of soil microbial communities using MWCNTs. According to pyrosequencing research, applying 10 g kg−1 MWCNTs increased the abundance of many bacterial taxa such as Cellulomonas, Pseudomonas, Nocardioides, and Rhodococcus, which are thought to be potential degraders of resistant pollutants. NMs were also observed to affect the level of microbial assembly in the soil in favor of species that were more resilient to NMs or were capable of rapid degradation, which was advantageous for soil micro-remediation. Research on the breakdown of 2,4-dichlorophenoxyacetic acid in soils was carried out utilizing Fe3O4 NPs in combination with soil microorganisms. The addition of Fe3O4 NPs to the soil increased the microbial diversity and enzymatic activity (e.g., acid phosphatase, amylase, urease, and catalase), resulting in greater organic waste degradation efficiency than when the soil was treated with microorganisms alone [81][114]. Tilston et al. [82][115] discovered that the use of nZVI coated with polyacrylic acid (PAA) altered the composition of the bacterial community in the contaminated soil and reduced the efficiency of chloroaromatic mineralizing microorganisms. Populations of Dehalococcoides, a bacterium capable of dechlorinating chlorinated organic pollutants, were similarly reduced with 0.1 g L−1 nZVI [78][111].
The adverse upper effect of NMs on microorganisms prevented the biodegradation of pollutants in polluted soils. Furthermore, the impact of NMs on micro-remediation of polluted soils differs depending on the type and concentration of NMs. With increased CNT concentrations, extractability and microbial degradation of PAHs were assumed to decrease with an increasing CNT concentration. Compared with MWCNTs, SWCNTs had a stronger influence on the mineralization and extraction of PAHs [83][116]. Furthermore, a high concentration of MWCNTs significantly inhibited the development of phenanthrene-catabolizing bacteria as well as the growth of phenanthrene-degrading bacteria in the soil, while fullerene and low levels of CNTs had no negative influence on microbial activity [84][117]. When discussing bioremediation, it is important to remember that the pollutants that NMs ingest affect their bioavailability to microorganisms. The reduced bioavailability of contaminants affects the microbial remediation power in the polluted soils. MWCNTs adsorbed on phenanthrene were studied for their biodegradation and mineralization by Agrobacterium. It was found that the use of MWCNTs as contaminants significantly reduced the bioavailability of hydrophobic organic compounds in the environment [85][118]. Other research used 14C-2,4-DCP as the target contaminant to explore the mineralization, breakdown, and residual distribution of radioactively tagged 2,4-dichlorophenol (14C-2,4-DCP) in conjunction with SWCNTs and MWCNTs. In contaminated soils, SWCNTs at a concentration of 2 g kg−1 significantly reduced microbial mineralization and the breakdown of 14C-2,4-DCP. The reduced bioavailability of 2,4-DCP, the potential microbial toxicity of CNTs, and the reduced activity of native soil microorganisms all had inhibitory effects [86][119]. Overall, NMs have both beneficial and detrimental effects on the micro-remediation of contaminated soils (Figure 1).
Figure 1.
Beneficial and negative impacts of nanomaterials on micro-remediation of soil. Green arrows indicate upregulation. Red arrows indicate downregulation.