Nanoparticles
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Contaminant Remediated
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Factors of Performance and Removal Efficiency
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References
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Iron oxide nanoparticles with a polyvinyl pyrrolidone coating
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Cd, Pb
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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.
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[46]
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Industrial suspension of zero-valent iron (nZVI) at two dosages (1% and 10%)
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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.
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[47]
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Graphene oxide nanoparticles (nGOx) and nZVI
|
Metals such as Cd, Pb, Zn, Cu, and As were found in As- and metal-contaminated soil.
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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.
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[48]
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Titanium oxide nanoparticles bonded to a chitosan nanolayer (NTiO2–NCh)
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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.
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[49]
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Palladium (Pd), Pd NPs
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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.
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[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)
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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.
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[51]
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ZnO NPs
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Cu, Cd, Cr, and Pb
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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.
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[52]
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3. Soil Nanoremediation
Nanoremediation is a virtually new application of nanotechnology for addressing environmental pollution issues
[53]. 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]. The use of zero-valent iron (ZVI) as a permeability barrier was the first research idea developed by Gillham
[55] based on their experience with NPs in decontamination of water-halogenated contaminants
[56]. 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], nitrates, HMs
[58], pesticides, and dyes
[59]. 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].
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]. A bentonite matrix was used by Shi et al.
[60] 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]. B-nZVI NPs have large surface areas and are highly reactive, enabling them to work as excellent adsorbents of Cr (VI)
[60]. 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]. 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]. Zhang
[63] 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]. Tetrahydrofuran (THF) was used by Qiu et al.
[64] 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] 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]. 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].
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] 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]. 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 NH
4+ ions and 91% of PO
43− ions
[69].
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]. Cheng et al.
[71] 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]. 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]. According to Xu et al.
[75], the following elements bind metals to microbial cells: CrO
42−, Cu
2+, Hg
2+, Au
3+, Cd
2+, Ni
2+, Pd
2+, and Zn
2+. The mobility of these metals and their harmful consequences were diminished by this metal-binding. Furthermore, Polti et al.
[76] 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]. 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].
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]. The effectiveness of micro-remediation for remediating pollutants is impacted by the potential impact of NMs on microorganisms. Shrestha et al.
[80] 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 Fe
3O
4 NPs in combination with soil microorganisms. The addition of Fe
3O
4 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]. Tilston et al.
[82] 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].
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]. 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]. 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]. 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]. 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.