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Review

Carbon-Based Nanomaterials in Water and Wastewater Treatment Processes

by
Krzysztof Piaskowski
,
Renata Świderska-Dąbrowska
and
Tomasz Dąbrowski
*
Faculty of Civil Engineering, Environmental and Geodetic Sciences, Koszalin University of Technology, 75-453 Koszalin, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(16), 7414; https://doi.org/10.3390/su17167414 (registering DOI)
Submission received: 30 June 2025 / Revised: 5 August 2025 / Accepted: 14 August 2025 / Published: 16 August 2025

Abstract

The observed increase in the diversity and level of pollutant content in the water environment forces the development of more effective technologies for their removal. Using nanomaterials in water and wastewater treatment offers numerous opportunities to remove organic and inorganic contaminants that are hardly removable in conventional processes. In this group, carbon-based nanomaterials, mainly carbon nanotubes (CNTs), graphene (Gr), and graphene oxide (GO), are very popular. This review aims to present the directions and diversity of applications of carbon-based nanomaterials (CNMs) in water and wastewater technology, as well as the challenges and environmental dangers that new solutions entail. Authors also present the results of the research on the changes in properties of GO produced in the laboratory as water suspension and a freeze-dried product over time. The results confirm the significant influence of the form of graphene oxide and its storage time on the structural properties, hydrophilicity, and stability of GO. Therefore, they should be considered when selecting an adsorbent or reaction catalyst in environmental applications for developing new greener and sustainable methods of treatment and purification, which use fewer reagents and release safer products.

1. Introduction

Nanomaterials applied to water and wastewater treatment are based on materials with sizes < 100 nm, i.e., the size of atoms and molecules. In this size range, the structure of nanomaterials is characterized by specific physical, chemical, and biological properties. Nanoparticles are capable of deeper penetration, purifying water or wastewater more effectively, which is generally impossible using conventional technologies. A higher surface-to-volume ratio increases their reactivity with pollutants present in the environment. In addition, chemical or physical modifications to the surface of carbon nanomaterials enable their use in removing specific contaminants from water and sewage [1,2,3]. Their chemical activity and sorption capacity increase as the size decreases [4]. Increasingly, technologies based on nanotechnology also enable their reuse, efficient use of space, and the generation of less harmful intermediate products compared to traditional technologies [5]. Nanomaterials can be successfully applied as efficient, economical, and environmentally friendly materials to remove various substances from wastewater [6]. They are increasingly effective in eliminating various complex and hard-to-remove contaminants from different sewage sources, including pathogens, toxins, inorganic and organic solvents, dyes, and heavy metals [7,8].
This review presents a comprehensive overview of carbon-based nanomaterials (CNMs), which have gained increasing importance in recent years in research on the removal of emerging micropollutants (EMPs) from water and wastewater, particularly in light of the new EU Wastewater Directive which implies a more sustainable approach towards energy efficiency and water recycling. The paper covers several aspects related to CNMs, highlighting, in particular, the versatility and high potential for modifying nanomaterials for various applications. The authors also present their research findings on the effect of time on the chemical stability of graphene oxide (GO), as well as the potential impact of CNMs on the environment and living organisms. Their properties, raw materials used for production and a wide range of environmental applications, make them green and sustainable products.

2. Carbon-Based Nanomaterials

All nanomaterials with carbon atoms are called carbon-based nanomaterials (CNMs). Most often, carbon nanomaterials are distinguished by their geometric structure. Carbon nanostructures contain particles in the shape of tubes, horns, spheres, or ellipses [9]. There are spherical fullerenes (where carbon atoms in hexagons and pentagons form a hollow polyhedron), carbon nanotubes (CNTs), including single-wall carbon nanotubes (SWNTs) and multi-wall carbon nanotubes (MWNTs), graphene (a single layer of atoms arranged in a hexagonal lattice), and carbon black (amorphous) [2,10]. The Graphene-family (GFNs) includes pure graphene (Gr), graphene oxide (GO), and reduced graphene oxide (rGO) [10]. Table 1 presents various forms of occurrence of widely used groups of CNMs, along with their modifications, based on the available literature. It demonstrates not only the dynamic growth of research on CNMs but also their potential for an efficient treatment of water and wastewater [11].
Graphene-family nanomaterials (GFNs) exhibit a range of chemical compositions, sizes (from nanometers to micrometers), shapes, and forms, including single-layer or multi-layer graphene and graphene ribbons. They are available in the form of suspension and powder. The properties of graphene nanomaterials, including high mechanical strength, surface charge, hydrophobic surface, stability, and antimicrobial character, are desirable, among others, in wastewater and water treatment applications [128,129].
Changes or enrichment of the surface are introduced, and composites (hybrids) with other inorganic or organic particles are formed to increase the reusability, separation, and removal effectiveness of GNMs. In this way, they become more desirable variants in removing pollutants from water or wastewater [130]. Carbon nanotubes (CNTs) and graphene (Gr) can exist in either functionalized or non-functionalized forms. Such modifications can be carried out by oxidation, activation with alkali, introducing substances with magnetic properties, introducing catalysts (metals or metal oxides), hybridization using other CNMs, and derivatizing using specific chemical molecules. For example, CNTs can be enriched with hydroxyl (-OH), carbonyl (C=O), and carboxyl (-COOH) groups using oxidation methods to create highly functional, water-dispersible carbon nanotubes compared to original CNTs [2,131]. Correct functionalization may make the aggregation of nanomaterials more difficult. This, in turn, increases the adsorption capacity or adsorption dynamics and improves selectivity by impacting hydrophobic interactions and π-π bonds. The functionalized CNTs also have better selectivity and higher desorption hysteresis [131,132,133].
Carbon nanotubes and graphene-based products are prone to modification. Combining graphene with many nanomaterials yields interdependent effects during organic pollutants removal. Graphene and GNMs can effectively decrease the concentrations of organic pollutants by adsorption and photodegradation [134]. They may react with different biological, organic, and inorganic pollutants, including microbes, pharmaceuticals, hydrocarbons, dyes, heavy metals, pesticides, and radionuclides [131].
GO and rGO are the essential groups of materials obtained from graphene at lower production costs. GO, which features a substantial quantity of functional groups in its carbon structure (carboxyl, epoxy, carbonyl, and hydroxyl groups), enables the creation of permanent suspensions in an aqueous solution. rGO is a form of graphene oxide, occurring after reducing the oxygen content by chemical or thermal methods [135,136]. rGOs thus acquire properties similar to graphene, which cannot be obtained from graphite. Compared to CNTs, GO nanomaterials show more potent adsorption properties towards many pollutants [6].
Chemical exfoliation of graphene allows effortless synthesis of GO and rGO, requiring no complicated equipment or metallic catalysts. The resulting graphene product contains no catalyst remnants and does not require refinement [136]. The polar groups in the GO structure make it hydrophilic, allowing for better interfacial bonding with various fibers and polymers. Because functional groups can interact, polar groups may be modified [137]. The GO surface has strong functional groups that make this material a possible adsorbent for the complexation of metal ions. Nanoplatelets of GO and GN doped with metal oxides create composite nanomaterials [136].
Distribution of superficial oxygen groups impacts the catalytic and adsorption character of GO. It is necessary to determine the transformations of the GO structure in different conditions of its preparation and storage. Commercial use of GO is only possible if controlled oxygen functionality, repeatable synthesis, and high stability are achieved.

2.1. Quality and Durability of Carbon-Based Nanomaterials—GO

An essential condition for using CNMs for treatment technologies is their durability and qualitative stability during production and storage. Muzyka et al. [138] drew attention to the possibility of developing preliminary criteria for selecting the carbon material and method of its oxidation to obtain product with expected composition and structural properties. Among the types of graphite tested (natural—scale and flake—as well as synthetic and electrode), the most suitable for obtaining rGO (after exfoliation of graphite oxide) turned out to be flake graphite, which was the most fragmented and had the smallest number of defects. When using the modified Tour’s method (H2SO4, H3PO4, KNO3, KMnO4), graphite oxides contained more oxygen groups than when oxidizing using the Hummers method (H2SO4, NaNO3, KMnO4) [138].
Perera et al. [139] studied the creation of various oxygen functional groups in GO during its oxidation. The improved Hummers method was used, and H3PO4 was added. The process took between 1 and 4 h. According to X-ray photoelectron spectroscopy (XPS) analysis, the ratio of carbon from oxygen groups to graphitic carbon remained consistent in all analyzed samples. In contrast, the percentage of individual oxygen groups changed with the oxidation time. The hydroxyl and the ether groups had the highest share (41–46%), regardless of the oxidation time. As oxidation progressed, the proportion of the carbonyl group increased until it reached a maximum of 9.6% after two hours. The carboxyl group percentage increased from 1.1% after the first hour to 5.1% after 4 h [139].
Luo et al. [140] demonstrated that the oxidation temperature affects the type and content of oxygen groups in GO. They synthesized GO using the modified Hummers method, varying the reaction temperature over a wide range (0 °C to 100 °C). As the temperature increased, graphite gradually oxidized, reaching the highest oxidation state at 50 °C (C/O = 1.85). Under such conditions, GO mainly contained hydroxyl groups (approx. 29%), and the spaces between the oxide layers were the largest. At a higher temperature (80 °C), part of the hydroxyl groups turned into epoxy groups. The newly created carboxyl groups changed the color of the sample to brown. However, at the highest temperature (100 °C), the content of hydroxyl groups and the degree of graphite oxidation decreased. A change in the color and degree of water dispersion of the graphite suspension accompanied the increase in the temperature of the oxidation reaction. The color varied from brown-black (0 °C) through light brown at 30–50 °C, to yellowish at 60–70 °C, and, finally, to dark brown at 100 °C [140].
If the obtained carbon nanomaterial is to be used in industry, it has to maintain its desired properties during transport, storage, and use. Nuncira et al. [141] controlled the stability of an aqueous suspension of GO using UV-vis light, particle size, and zeta potential measurement using the dynamic light scattering (DLS) technique. The ninety-day studies revealed that the GO particles had an average thickness equivalent to two layers of graphite, with an approximate 1 nm distance between layers. In addition, the surface of GO had a high negative charge (the zeta potential of the GO particles was approximately −65 mV at a concentration of 200 ppm); it guarantees high stability of the colloidal suspension. After 60 days, the apparent particle size decreased by 43%, due to the rearrangement of the suspension nanolayers, and not due to particle precipitation (confirmed by UV-vis) [141].
A more extended observation period—2 years—was used by Li et al. [142]. The solid GO samples obtained with the modified Hummers method were stored at room temperature. Over time, the surface of GO underwent a gradual transformation, altering its properties. Desorption of oxygen functional groups caused an increase in the C/O ratio. At the same time, the average distance between GO layers and the density of structural defects decreased (an increase in ID/IG ratio, determined by Raman spectra, was observed). The authors also observed the negative impact of increased temperature (above 60 °C, GO decomposed and carbon dioxide was released) and humidity (contact with air moisture decreased the activation energy from 150 to 134 kJ/mol) [142].
Graphene oxide is also sensitive to light. Xue et al. showed that the photochemical reduction in GO to rGO starts at a UV light intensity of 100 mW/cm2 and room temperature [143].
In turn, Chlanda et al. [144] tested stability of GO flakes stored in the form of an aqueous suspension for 14 months without light access. They noticed that impurities in the suspension (residues from GO synthesis using the modified Hummers method) led to increased GO flake thickness, from 0.9 to 1.7 nm in a pure GO suspension to as much as 30 nm. Additionally, the increase in impurity adsorption on the flake’s surface over time was noticed. GO flakes stored in a purified suspension showed no significant changes in properties [144].

2.2. Preliminary Research on Changes in the Properties of Graphene Oxide During Its Storage (Aging) in the Form of an Aqueous Suspension or Solid Form After Freeze-Drying

The tested GO was synthesized using modified Hummers method as described in [145,146]. The obtained light-brown aqueous GO suspension (GO-s) was stored for 20 months in closed glass containers at room temperature, without exposure to light. Part of the GO sample was subjected to freeze-drying. The material was frozen using a temperature reduction rate of 100 °C/h and then dried for 18 h at 25–29 °C under a pressure of 133–266 Pa. The lyophilisate (GO-L) was stored for 12 months in closed Petri dishes at room temperature, protected from light. Samples of both stored materials were analyzed after specific periods. GO-s samples were analyzed after 1, 2, 3, 9, and 20 months of storage (later referred to as GO-s_1, GO-s_2, GO-s_3, GO-s_9, and GO-s_20). GO-L samples were analyzed after 1, 2, 3, and 12 months of storage (marked as GO-L_1, GO-L_2, GO-L_3, and GO-L_12).
After freeze-drying, the GO-L sample was in the form of a light, spongy cake. Its apparent density was 14–22 mg/cm3 [145]—Figure 1.
SEM micrographs of GO-L confirm its high porosity, showing large, charged GO lobes (Figure 2). The EDS displayed the presence of carbon and oxygen atoms on the GO-L surface (C/O = 1.7), resulting from the formation of oxygen functional groups during the oxidation of graphite (Table 2). However, the small sulfur content can be attributed to the residue remaining after GO synthesis, during which sulfuric acid (VI) is used.
During the storage of GO water suspension (GO-s) and freeze-dried (GO-L), changes in the color of the samples were observed. They ranged from light brown for fresh samples to dark brown for the most extended storage periods (Figure 3 and Figure 4).
The darkening of GO samples may result from the decreased number of hydroxyl groups (which combine into epoxy groups) and the slow desorption of oxygen-containing functional groups from the GO surface. This leads to a decreased hydrophilic character, causing water molecules to leave the space between the GO layers. Consequently, the degree of dispersion of the suspension decreases, and the color changes to dark brown.
Changes in the concentration of oxygen functional groups, which are responsible for the negative GO surface charge, can be monitored by zeta potential. Measurements were conducted at a temperature of 25 °C with a ZetaPALS, Brookhaven Instruments Corporation, Nashua, NH, USA. The electrokinetic potential of GO-s at 0.1 mg/cm3 of concentration and a solution pH of 3–3.5, stored for 1–9 months, was stable (ranging from −45 mV to −41 mV). The potential significantly increased to the average value of −25 mV in the sample after 20 months (Figure 5A). An increase in particle size was also noted, indicating aggregation (Figure 5B).
In the case of freeze-dried graphene oxide (GO-L), solid samples were dissolved in distilled water (0.1 mg/cm3) and stirred for 10 min at a pH range of 3.0–3.5 before measuring the zeta potential.
The average zeta potential values of GO-L were much higher compared to GO-s and ranged from −24 mV to −18 mV (Figure 6A). Simultaneously, the measured particle size was almost ten times larger (Figure 6B). This may be caused by incomplete dispersion of GO-L (too short mixing time) or fewer oxygen functional groups on the GO-L surface. Additionally, Ham et al. showed that some oxygen groups are removed from graphene oxide during freeze-drying. They observed C/O ratio increase (from 1.6 to 2.1) and the ID/IG ratio (based on Raman spectra) from 0.84 to 0.99 [147].
Raman spectroscopy is applied to determine the structural configuration of CNMs. The ID/IG parameter, calculated as the ratio of the D to G peak intensities in the Raman spectrum, enables the assessment of the degree of ordering in carbon crystal structures. Its increase may indicate a decreasing number of superficial oxygen groups. ID/IG for GO, according to [148], is approximately 0.94. In turn, I2D/IG correlates with the number of layers, and this relationship is inversely proportional [138,148]. Figure 7 shows Raman spectra for GO-s samples, and Figure 8 shows spectra of GO-L samples. The ID/IG and I2D/IG values calculated based on these are given in Table 3. Spectra were obtained using a DXR Ramana Microscope (Olympus), at 455 nm of wavelength and 4 W of laser power.
Based on the ID/IG ratio value, it can be concluded that GO-s contains a slightly larger amount of superficial oxygen functional groups (higher ratio values indicate a greater number of defects), which persist virtually throughout the entire storage period. A slight increase in ID/IG was recorded for the sample stored for the most extended period, 20 months (GO-s_20), which correlates with this sample’s increase in zeta potential. The ID/IG values obtained for GO-L are more stable throughout the observation period, ranging from 0.83 to 0.85. Their higher value than the GO-s samples is also confirmed by zeta potential measurements—similar potential values were obtained for all samples, but they were lower than those for the GO-s.
However, the lower values of the I2D/IG coefficient obtained for freeze-dried GO_L indicate more material layers than GO-s. Their practically constant value confirms the greater stability of the material during its storage and correlates well with particle size measurements (Figure 6B).
The preliminary research results presented above confirm the significant influence of the GO form and storage time on the structural properties, hydrophilicity, and stability of GO. Therefore, they should be considered when selecting an adsorbent or reaction catalyst in environmental applications.

3. Selected Applications of Carbon-Based Nanomaterials

CNMs have uncommon physical and chemical properties. This makes them applicable for the following [9]:
  • adsorption;
  • disinfection;
  • membrane processes;
  • photocatalytic processes.
The growing interest in research on CNMs for use in water and wastewater treatment, as well as related innovations, is mainly at the laboratory testing stage. A barrier to industrial scale is the high production costs of nanomaterials, which depend on the type of raw material, synthesis methods, energy consumption, production volume, and final processing [149]. For example, the prices of SWCNTs and MWCNTs are higher than those of popular activated carbon [150]. The type of CNT also affects production costs, as SWCNT is more expensive to produce than MWCNT due to its complex structure and material purity requirements (50–100 USD/g) [151]. Since the properties of nanomaterials are superior to those currently used in water and wastewater treatment technology, this determines the search for more cost-effective synthesis methods and raw materials in the form of, for example, waste materials (plastics, biowaste), which may enable the widespread use of CNMs in the effective removal of various micropollutants in the environment [152,153]. The production of nanomaterials from waste materials is one of the pathways towards sustainable water purification and wastewater treatment.

3.1. Adsorption

Adsorption is commonly used for the removal of pollutants during water and wastewater treatment. This is determined by several factors, including the method’s high flexibility and potential for modernization, its simplicity in design and operation, cost-effectiveness, environmental friendliness, as well as the possibility of reusing and regenerating the adsorbent. Traditional adsorbents, including activated carbon (AC), clay, zeolite, and bentonite, are limited because of their low adsorption capacity and kinetics, small specific surface area, no selectivity, and limited active sites [132,154,155].
The disadvantages of conventional adsorbents can be mitigated by newly developed nanomaterials, which increase the removal efficiency of contaminants several times [156].
Carbon nano-adsorbents, such as CNT, GO, graphene, and their derivatives, compared to conventional adsorbents, have a larger specific surface area, more active sites, adjustable pore size, higher reactivity, shorter internal diffusion time, higher process kinetics and high affinity for pollutants, enhanced by modification of nano-adsorbents surface [132,155,156,157]. In addition, CNTs are stable thermally and chemically, containing numerous p-p bonds between carbon atoms, making them applicable for remediation, such as the removal of pharmaceutical contaminants.
CNT surface modification (formation or introduction of assorted functional groups on side walls and edges of CNTs) can adjust the CNT to the chemical properties of the removed contaminants [131,158]. Most often, concentrated inorganic acids (HNO3, H2SO4), or mixtures of sulfuric acid (VI) with potassium manganate (VII) or hydrogen peroxide, are used to create functional groups on the CNT surface [115].
The introduction of a heteroatom, a group of atoms, or an entire molecule outside or inside the nanotube, is relatively easy due to its substantial surface area. The connection to the p-conjugated carbon nanotube framework can be achieved through covalent bonding, noncovalent functionalization with particles, or saturating the nanotubes by deposition [135].
Nonpolar organic pollutants exhibit a strong affinity for CNTs impacted by possible interactions of aromatic rings. However, the CNT surface requires functionalization for hydrophilic ions or molecules [131,158]. The iron oxide attachment, oxidation, nonmagnetic metal oxides coating, introduction of thiol group, or functionalization with sulfur increases the affinity for heavy metal ions. Thus, the degree of metalloid adsorption is improved [155]. CNTs functionalized with bimetallic Pd-Fe, Zr, Fe, Ti, Ag, and Ce have been successfully used to remove toxic elements (As, F, Cu, Cd) and organic substances (2,4-dichlorophenol) from water. An additional advantage is the simplicity of the modification method, which frequently entails the CNTs’ oxidative functionalization and subsequent alkaline settling of the metal oxide on the nanotube surface [131].
Various polymers can be applied for modifying CNTs. In addition to improving their affinity for a specific substance, natural biopolymers may increase the hydrophilicity of the adsorbent surface and enhance the biocompatibility of carbon nanotubes [131]. Such CNTs can be used to remove heavy metals, mycotoxins, antimicrobials, antibiotics, and even uranium during wastewater treatment [133].
Functionalized CNTs exhibit excellent adsorption capabilities and effectively remove organic, inorganic, and biological pollutants from the aquatic environment. Sorption can occur due to electrostatic interactions between adsorbate and adsorbent, as well as the effects of surface complexation and bonding of metal and functional groups [155].
The adsorption of pharmaceutically active compounds (PhACs) on carbon nanomaterials results from possible hydrophobic effect, electrostatic, and covalent interactions [132]. The total adsorption effectiveness is determined by the properties of carbon nanomaterials and the molecules removed (e.g., PhAC). Therefore, properties such as pore size and distribution, specific surface area, and surface-active groups significantly influence the adsorption efficiency of PhAC in the aqueous phase. In the case of CNTs, sorption first occurs on the outer surfaces and grooves of CNT [132]. Kuśmierek et al. found out that pH significantly impacts the efficiency of adsorption of paracetamol and ibuprofen on CNTs and functionalized CNTs (CNT-OH and CNT-COOH) [84].
In various studies, CNTs have been shown to effectively adsorb benzene [100], 1,2-dichlorobenzene [96], trihalomethanes [94], and polycyclic aromatic hydrocarbons (PAHs) [99]. In turn, high natural organic matter (NOM) removal efficiency was found in the combined coagulation–adsorption process rather than in adsorption alone. Maximum NOM removal was obtained during alum coagulation with SWCNTs (86%) and FeCl3 coagulation with SWCNTs (87%) [159].
The pH significantly impacts adsorption of heavy metal ions on CNTs. Stafiej and Pyrzyńska [47] showed that the adsorption characteristics of divalent ions (Cu, Co, Cd, Zn, Mn, Pb) change with pH. The affinity of metal ions at pH 9 for CNTs was as follows: Cu2+ > Pb2+ > Co2+ > Zn2 + > Mn2+. Ruparelia et al. [160] showed in their studies that the adsorption of metal ions on CNTs took place in the order Pb2+ > Ni2+ > Zn2+ > Cd2+. Yang et al. [57] found that the adsorption efficiency of Ni2+ on oxidized MWCNTs increases along with pH (range 2–9) from 0 to approximately 99%. Oxidized MWCNTs are highly efficient in solidifying and preconcentrating Ni2+. However, the affinity order of heavy metal ions adsorbed by CNTs also depends on their properties. Yue et al. [54] employed a low-density macroporous structure of CNTs incorporated into a polymer hydrogel composed of sodium alginate and polyacrylamide. The adsorption capacity of this structure was 38.9 mg/g for Cu2+ ions, 1.28 times bigger than of the polymer hydrogel.
Numerous research results also confirm the high effectiveness of CNTs in removing anions from water and wastewater, such as phosphates (V), which are responsible for the eutrophication of water bodies. Yang et al. [121] functionalized the surface of CNTs with La2(CO3)3, resulting in an adsorbent with a sorption capacity of 178.6 mg/g for PO43− ions, which is effective over a wide pH range (3–7). Phosphate removal occurred through a combination of physical and chemical processes, resulting from ligand exchange, complexation of both the external and internal surfaces of carbon nanotubes, and precipitation reactions. An important aspect of the research, in terms of applying the obtained nanomaterial in industrial installations, was to determine the influence of the ionic strength of the solution and the competitiveness of other ions typically present in real wastewater. The authors highlighted the significant influence of the concentration of carbonate ions alone, which substantially reduced the effectiveness of the tested adsorbent. The problem of competitiveness of components contained in the matrix—in water or wastewater—was also thoroughly investigated by You et al. [61]. The research focused on the use of CNTs modified with La(OH)3 and CaO2 for the simultaneous removal of phosphate ions and Cr (VI) from industrial wastewater. Despite the high effectiveness of the adsorbent in an acidic environment, observed for both pollutants (a synergistic effect was demonstrated), and the high stability of the nanomaterial, the presence of other ions was only insignificant in the case of Cr (VI). Cr (VI) was reduced to Cr (III) using calcium peroxide, while phosphates were removed by ion exchange and complexation reactions on the inner and outer surfaces of CNTs. The high phosphate recovery rate (over 70%) demonstrates the application potential of the obtained adsorbent.
Ceroni et al. [161] demonstrated that the functionalization of MWCNT surfaces enhances the rate of dye adsorption from wastewater, allowing the dye to be regenerated. The synthesized MWCNT containing benzenesulfonate groups (MWCNT-S) disperses well in water and exhibits a two-fold higher adsorption efficiency of methylene blue (MB) than insoluble MWCNT. The process is reversible, and it is possible to recover the adsorbent (with 75% efficiency) and the dye, requiring treatment with 1 M NaCl for 1 h.
After remediation treatment, CNTs that remove pollutants from aqueous media are frequently subjected to regeneration or separation. Nanoparticles can be separated from a solution using filtration or centrifugation; however, these methods are costly and energy-intensive. Subsequently, interest is growing in the magnetic features of the adsorbent. Such a property of spent adsorbents can be easily and cheaply separated [131]. The synthesis of magnetic nano-adsorbents enabled the precise and rapid separation of the nano-adsorbents. Despite this, they maintain the majority of their surface area in comparison to other immobilization techniques. Such environmental remediation with such nanomaterials is much closer to engineering applications [132]. Additionally, the magnetic component of the composite nano-adsorbents prevents the unwanted mobility of nanoparticles in the environment, thereby keeping all components together [131].
Magnetic carbon nanotubes (MCNTs), as a nanocomposite material, disperse well in water. They can be reused after separation or regeneration with a magnet. Such a feature makes them more environmentally friendly and sustainable. MCNTs are used to remove petroleum, sulfates, dyes (methyl orange, crystal violet, Janus green B, and methylene blue), toluene, ethylbenzene, thiazine, xylene, Cu (II), Zn, Pb (II), etc., from water [133].
An interesting proposal for the use of magnetic MWCNT for the remediation of oily wastewater was presented by Won et al. [109]. They developed the spatial structure of the sorbent (macronization), using m-cresol and salts (sodium chloride and sodium L-glutamate) as dispersants, enabling the creation of open pores with a size of 200–800 μm. This increased the specific surface area of the sorbent and, compared to the sorbent before macronization, its sorption capacity (depending on the type of oil; 2.37–13.60 g of oil/g), which was maintained for ten consecutive cycles. The possibility of reusing the sorbent and its easy recovery (by magnet) makes it an attractive nanomaterial for industrial applications.
Graphene (Gr) has more advantages as an adsorbent than CNT. Single-layer graphene materials have two primary planes that are available for adsorbates, whereas the inner walls of CNTs are not accessible. GO and rGO can be easily synthesized without additional purification steps [136]. Compared to raw graphene, GO has a higher affinity for metal ions because it contains a significant amount of superficial oxygen groups. GO and graphene nanosheets doped with metal oxides constitute enhanced composites [136].
Liu et al. [24] used Gr to adsorb methylene blue dye from water. Adsorption capacity depends on the temperature (153.85 mg/g at 293 K to 204.08 mg/g at 333 K). The highest adsorption efficiency (~99.68%) was achieved at a pH of 10.
Wang et al. [29] synthesized a magnetic-sulfone graphene nanocomposite (Gr-SO3H/Fe3O4) for removal of cationic and anionic dyes from water. Over 93% of removal efficiency was obtained for cationic dyes within the first 10 min of the process. The adsorption capacity of Gr-SO3H/Fe3O4 was worse for anionic dyes.
Bu et al. [32] used thiosemicarbazide-functionalized GO for the removal of methyl blue cationic dye. The amino groups and the C=S bond in thiosemicarbazide caused a several times higher sorption capacity of the nanocomposite than GO alone (596.642 mg/g vs. 196.8 mg/g). In comparison to GO, the new adsorbent exhibited an increase in specific surface area, volume, and pore size. The nanocomposite removed the dye through a pseudo-second-order reaction, following the Langmuir adsorption isotherm model. The high efficiency of dye adsorption on the nanocomposite surface was caused by hydrogen bonds, p-p, and electrostatic interactions of the negative surface of GO and cationic dye [32].
Graphene/GO functionalized with metal oxides (Fe3O4, MnO2, TiO2, and ZnO) has unique features, enhancing the efficiency of heavy metal removal. For example, magnetic graphene produced by microwave modification of GO, thanks to ferrocene precursors, removes As (V), Pb (II), and Cr (VI) at 99% efficiency. The porous iron-iron oxide matrix on graphene effectively removes Cr (VI), Hg2+, and Pb2+ ions. Magnetic cyclodextrin–chitosan nanocomposites with functionalized GO showed high Cr (VI) ion removal dynamics. The magnetic beta-cyclodextrin/GO nanocomposite was used to remove Cr (VI) ions rapidly [162]. They are also able to remove radionuclides [157].
The pH impacts sorption of pollutants by functionalized GO nanomaterials. The superficial carboxyl groups have a pH of approximately 4–5. Therefore, most chemicals are removed at a pH greater than 5. Then, the carboxyl groups are deprotonated, generating a negative surface charge on the surface. For example, at pH < 4, Pb2+ sorption was approximately 60%, and, at pH > 4, it increased to 80–90% [2].
Studies have also demonstrated the high effectiveness of GO in adsorption of tetracycline antibiotics from water. Gao et al. [92] achieved sorption capacity of 313.48 mg/g for tetracycline, 212.31 mg/g for oxytetracycline, and 398.41 mg/g for doxycycline.
Pavagadhi et al. [125] proved that GO can be used to remove algal toxins. GO showed a higher adsorption capacity than commercial activated carbon, 1.15 times higher for micocystin-LR and 1.82 times higher for micocystin-RR. Additionally, for all tested doses (500–900 µg/L), after 5 min, over 90% of micocystin-LR and micocystin-RR were removed.
MnFe2O4/GO was applied for the sorption of Pb (II) ions and neutral red (NR) dye. The chemisorption process was most effective at pH 6.0 and a process time of 120 min for Pb (II) ions (capacity of 636.94 mg/g) and 30 min for the NR dye (capacity of 46.08 mg/g). After each cycle, the adsorbent was removed from the solution with a magnet and regenerated with 0.2 molar HCl. The stable operation of MnFe2O4/GO has been demonstrated over the subsequent five cycles, resulting in a reduction in total water purification costs [40].
Wei et al. [163] functionalized magnetic graphene (MGO) with chitosan, cyclodextrin, and EDTA, improving its selectivity and adsorption capacity. MGO functionalized using EDTA enabled the removal of heavy metals at levels of 96.2% Pb (II), 95.1% Hg (II), and 96.5% Cu (II).
Polyethyleneimine-grafted GO was also highly efficient in the removal of Pb (II) ions. The optimal process parameters were a pH of 6 and a process duration of 120 min. The mechanism of the process depended on the concentration of Pb (II) ions: at low concentrations, chemisorption dominated, whereas at high concentrations, physical sorption prevailed. The process was carried out according to the Freundlich model, achieving an adsorption capacity of 64.94 mg/g [81].
Aerogels are another type of carbon nanomaterial applicable in the adsorption process. CNT and Gr aerogels, with their large specific surface area and porous structure, are useful adsorbents [97]. Ye et al. synthesized mechanically compressible low-density graphene aerogels (3.3 mg/cm3) for water treatment from GO, poly(vinyl alcohol) (PVA), and glutaraldehyde. Different mass ratios of PVA to GO enabled the production of aerogels that selectively absorbed hydrophilic organic dye, hydrophobic organic solvents, and oils from wastewater [34].
Hao et al. [164] used aerogel from GO, montmorillonite, and sodium alginate to selectively remove Cu2+ from wastewater. It removed copper ions at an efficiency of 95.1%, due to significant specific surface area (266.3 m2/g) and selectivity.
Sharma et al. [165] prepared silica aerogel doped with GO (GO-SA) using the supercritical fluid deposition method. GO-SA was used to remove anionic and cationic dyes, as well as the antibiotic sulfamethoxazole. Process efficiency was 98.2–98.7% for dyes and 94.5% for the antibiotic. Additionally, GO-SA is reusable, maintaining high efficiency (>85%) after five cycles.
Fan et al. [42] developed a magnetic nanocomposite with a three-dimensional aerogel structure in which rGO acted as a carrier of zero-valent nanoiron for sorption of methyl orange and methylene blue. The adsorption of dyes on the nanocomposite was highly efficient over a pH range of 1–10. The highest capacity was achieved for methylene blue (3918 mg/g). For methyl orange, it was 667 mg/g. The equilibrium was reached after 30 min of process, described with the pseudo-second-order kinetic model. Moreover, the tested adsorbent was stable, and dyes were removed by over 90% even after five cycles.
A promising trend in treatment processes of water and wastewater is the use of adsorbents derived from biomass. They are cheap and sustainable, but typically have low adsorption capacity and, in the case of lignins, poor dispersibility in water, as independent adsorbents. Cellulose and lignins are the most common components of biomass. However, it is possible to modify the surface of biomass-derived adsorbents and/or combine them with other, more efficient, but also more expensive, adsorbents. Chen et al. [35] used aminated lignin airgel doped with only 0.1% of GO to remove malachite green. They achieved an adsorption capacity of 113.5 mg/g and an efficiency of 91.72% at optimal conditions. Synergistic interactions between the carboxyl groups of GO and the amino groups of lignin significantly improve the performance of the process. The results are described using the Langmuir isotherm and the pseudo-second-order kinetic model. An additional advantage of the bio-adsorbent was high adsorption efficiency (89.8%) after five cycles.
Yu et al. [16] developed hybrid aerogels from cellulose nanofibers, CNTs, and Gr. This material efficiently removed cationic and anionic organic dyes.
Vo et al. [33] synthesized a hydrogel of GO nanosheets cross-linked with chitosan chains. The resulting three-dimensional structure facilitates the spread of adsorbate in the nanocomposite. It increases the adsorption capacity for anionic (methyl orange and Congo red) and cationic (methyl blue and rhodamine B) dyes. A higher GO share in the nanocomposite promoted cationic dyes removal. A higher chitosan share promoted anionic dyes. This cost-effective and biocompatible hydrogel material can also be applied to a filtration column with high processing capacity. After four cycles (with ethanol washing), the dye removal capacity stayed constant.
Chitosan, a biodegradable and biocompatible polysaccharide, in combination with GO (2% addition), was also used by Li et al. [68] for adsorption of heavy metals from model wastewater. The resulting composite combined the features of GO (a large specific surface area and a high number of active sites) with those of chitosan (chelating ability). Moreover, the presence of chitosan reduced the agglomeration of GO. The material exhibited a high adsorption capacity for Cu (II) (60.7 mg/g), Pb (II) (48.7 mg/g), and Cd (II) (32.3 mg/g); initial dose: 70 mg, pH 6; contact time: 90 min; and temperature: 20 °C.
While carbon nanomaterials exhibit promising potential as adsorbents, numerous critical technical obstacles still need to be overcome. First and foremost, the primary focus of research has been on the adsorption of a single solute in synthesized wastewater or deionized water. The adsorption behavior in actual water or wastewater is, however, more complex due to the presence of contaminants that always coexist, including NOM, inorganic ions, or other inorganic materials. Finally, unless they are magnetic, it is difficult to remove them from the solution after the process is complete. The commonly used method for separation is ultracentrifugation, which is expensive and costly [132]. The adsorbent’s regeneration and reuse are also significant from the practical and cost perspectives. Despite their high adsorption capacity and applicability, metal oxide nanoparticles and CNMs show disadvantages. For example, the nano-granular size increases the surface area. However, it may cause destabilization and aggregation in the aqueous environment. Consequently, the adsorption rate shows a decreasing trend [166].

3.2. Disinfection

Many engineered nanomaterials are efficient disinfectants, such as fullerol, CNTs, and fullerene nanoparticles (nC60). The use of antimicrobial nanomaterials is associated with a low degree of reactivity in water, resulting in a mild oxidizer. The safety profile of these materials ensures their utilization while also precluding the formation of toxic byproducts during the disinfection process [167,168].
Generally, we can distinguish several ways of using and operating various nanomaterials used in water disinfection [169]:
  • direct impact on bacterial cell structures);
  • penetration through the micro-organism’s cell membrane;
  • oxidation of selected cellular components;
  • hydroxyl radicals (as part of the reaction of NPs as photocatalysts;
  • production of dissolved metal ions that may contribute to the destruction of some cellular components.
Graphene-based products can potentially be used as a powerful cytotoxic product against bacteria (both Gram-positive and Gram-negative) and fungi. At the same time, it is a low cytotoxic product against human cells and animal forms [170]. This action is mainly based on the physical and chemical nature of graphene products (sheet morphology, defect density, oxygen group density, mobility of electrons, and carbon radicals) [171]. The impact of graphene derivatives on micro-organisms can be attributed to several mechanisms, including mechanical interaction, which may result in damage to cell walls, cell wrapping, the extraction of phospholipids, and chemical oxidation. These processes lead to the creation of reactive oxygen species (ROS), which in turn may induce oxidative stress within the cell. [170,171]. Graphene may also prevent the adhesion of bacterial cells to the substrate, thereby effectively inhibiting the formation of biofilms [170].
Among graphene derivatives, GO had the best antimicrobial activity in the suspension test. The activity decreased in the following order: rGO, graphite (Gt), and graphite oxide (GtO) [172]. Given that graphene oxide is composed of layered sheets containing substantial amounts of oxygen functional groups, bacterial cell membranes exhibit signs of damage when exposed to its walls [173].
Studies have shown that GO is an efficient disinfectant against Pseudomonas aeruginosa and Staphylococcus aureus, surpassing the efficacy of benzalkonium chloride, a commonly used surface disinfectant. Reduced graphene oxide (rGO), when utilized as an antibacterial surface, exhibits remarkable efficacy in eradicating bacteria activated with near-infrared solar radiation [170]. Multilayer GO and rGO plates and flakes have a cytotoxic effect on bacterial cells. They do not create reactive oxygen species (ROS) [170].
CNT-derived materials are utilized as disinfectants against E. coli, Salmonella, and various viruses. Their exclusive physical and cytotoxic properties, surface functionalization, fibrous shape, tube size and length, and number of layers (single- or multi-wall) contribute to their antimicrobial nature. Despite the significantly slower rate of bacterial inactivation in contact with CNTs, it may be sufficient to prevent the formation of biofilm, compared to conventional disinfectants, and the subsequent biofouling of membranes used for water filtration [1,174,175].
CNT filters are easier to regenerate than granular activated carbon filters. They can also remove multiple contaminants, including bacteria and viruses, thereby potentially replacing conventional adsorbents and disinfectants in point-of-use systems [176]. Single-walled carbon nanotubes (SWCNTs) have antimicrobial properties and more substantial toxicity towards bacterial cells. Direct contact with SWCNTs causes the cell membrane surface to undergo mechanical and oxidative stress, leading to subsequent cell death [171,176]. SWCNTs with hydroxyl and carboxyl groups on their surface are more antibacterial against bacteria than –NH2-functionalized SWCNTs [167]. The cytotoxicity of SWCNTs involves the following steps: preliminary contact of the micro-organism with SWCNT, effects on the cell membrane, and oxidation of the micro-organism [171]. The simultaneous use of various bactericidal agents can enhance the antibacterial effect of SWCNTs. Lilly [177] demonstrated that SWCNT (100 μg/mL) in combination with 1.5% H2O2 or 0.25% NaOCl is more sporicidal against B. anthracis than SWCNT or the reagents alone. SWCNTs and oxidizing reagents ensured a synergistic effect of two antibacterial mechanisms.
However, unfunctionalized CNTs hardly disperse in water. Then, the contact between micro-organisms and CNTs is insufficient. To increase disinfection efficiency, the reactor surface can be modified with CNTs, or SWCNTs can be immobilized on the membrane filter. Removal of E. coli bacteria reaches 87% within 2 h [167,174]. Furthermore, carbon nanomaterials in their original form are small in size. This makes them difficult to recover after water treatment. Therefore, they can be introduced onto membranes and other materials, embedded in polymer beads, or modified with magnetic compounds, facilitating their recovery after the process [2,167].
One of the directions of disinfection that has been intensively developed recently is the use of nanocomposites, which consist of two or more different materials, one of which has a nanometric dimension [178].
CNMs such as CNTs, GO, and carbon nanofibers are often used as carriers for Ag nanoparticles. Nanosilver is a very commonly used bacteriostatic and bactericidal material. It has low toxicity and inactivates microbes in water [179]. Carbon nanocomposites of CNTs and GO with Ag, due to their enhanced nanostructure, are efficient disinfectants. Their antibacterial effect depends on the properties of silver nanoparticles (AgNPs), such as shape, size, adhesion, and dispersion. It also depends on the chemical properties of GO and CNTs [180]. GO-Ag and CNT-Ag nanocomposites exhibit more effective bactericidal efficiency than Ag-NPs alone [180]. Effectiveness of SWCNTs-Ag was 70.2%, and SWCNTs 38.9% in E.coli (Gram-negative) disinfection tests. For S. aureus bacteria (Gram-positive), the results obtained were as follows: SWCNTs-Ag (95.8%) and SWCNTs (−131.4%); the reproduction rate was higher than the disinfection rate. The SWCNTs-Ag nanocomposite is more effective against S. aureus because SWCNTs tend to accumulate these bacteria close to AgNPs [178]. Also, AgNPs increased the bactericidal activity of MWCNT to 96.66% ± 1.99% against Gram-positive bacteria and 94.59% ± 1.39% against Gram-negative bacteria [181]. MWCNT has demonstrated high efficacy against various pathogens, including E. coli O157, Enterococcus faecalis, Salmonella enterica, Staphylococcus aureus, Rhizomucor miehei, and Rhizopus oryzae. MWCNTs produced using chemical vapor deposition and ferrocene catalyst demonstrated high efficiency as a disinfectant in wastewater treatment and microbiological control, as a microbial growth and biofilm formation inhibitor [182].
The process of bacterial inactivation by GO-Ag remains incompletely understood. Probable causes include a large surface area and higher sorption capacity of the CMN surface, the Ag-NPs’ catalytic activity (resulting in high dispersibility as well as stability of the conditions), and direct contact of the cell membrane with GO-Ag [180]. Other authors point out that the antimicrobial character of GO-Ag is caused by free radicals damaging the membrane, altered permeability (resulting from Ag interaction), irregular cavities in the outer bacterial membrane, and the leakage of cellular matter [179].
Another component of nanocomposites used in water disinfection is iron oxide. Graphene and iron oxide nanocomposites are antibacterial (property of GO) and easy to separate (magnetic properties of iron oxide). Magnetic graphene oxide (M-GO) contains iron oxide spread on GO nanosheets. Such a composite exhibits excellent bactericidal properties against E. coli. M-GO separation after the process is easy with the use of an external magnet [168]. The CNT0–60/PPy/AgNPs nanocomposite was also utilized to remove bacteria from water, demonstrating 100% efficiency against E. coli [178]. Polyvinylcarbazole (PVK) combined with SWCNT (PVK-SWNT) or GO (PVK-GO) nanocomposites showed 80% efficiency against Gram-positive and Gram-negative bacteria [2]. Recent reports prove that GO-based nanocomposites are able to adsorb viruses, which is important in the spread of viral diseases. For example, Ferré-Pujol [183] synthesized GO-NH2C8OH (GO grafted with 1,8-aminooctanol), which enables the concentration of viruses from wastewater to be at a level that allows for the detection of pathogens by PCR.

3.3. Membrane Processes

In wastewater treatment technology, membrane processes are used to remove and separate pollutants. Conventional polymer nanofiltration membranes exhibit excellent separation efficiency and have a broad range of applications. Their production is uncomplicated and relatively cheap. However, polymer membranes also have downsides, including low thermal and chemical durability, vulnerability to contamination, and physical aging in challenging operating conditions [184].
Membrane filters modified with carbon materials exhibit significantly improved properties compared to polymer membranes. Such membranes may become widely used for removing various pollutants due to their flexibility in adapting to contaminants [185]. Membranes functionalized with engineered nanomaterials (ENMs) are an increasingly important trend in the research and development of new membranes. Since ENMs have photocatalytic, antimicrobial, and hydrophilic properties, membranes produced with their addition are resistant to fouling, which is one of the problems that emerge when using membranes to purify water or wastewater [172].
The biological fouling (biofouling) of membranes occurs when micro-organisms grow on the membrane, decreasing water purification efficiency [128]. CNMs introduced into membranes can enhance their permeability, mechanical, chemical, and thermal stability, as well as resistance to contamination. Such a membrane possesses antifouling and bactericidal properties and can separate mono- and divalent ions [167,186,187,188]. Additionally, carbon nanomaterial functionalization (e.g., CNT) ensures the hydrophilicity of the membrane by introducing functional groups on its surface, which causes changes in porosity, surface roughness, permeability, selectivity, and fouling resistance [4].
CNTs are increasingly important nanomaterials for synthesizing composite membranes with polymer. Such composites feature low mass density, substantial tensile strength and modulus, flexibility, and a high shape ratio, increasing performance [189]. However, pure CNTs often aggregate, reducing water flow and contaminant removal. Therefore, CNTs should, first, be functionalized to enhance fouling resistance, mechanical strength, thermal stability, permeability, impurities degradation, and self-cleaning of the composite membrane, thereby decreasing the energy requirements. CNT membranes’ functionality may be improved by grafting Cu, Ag, Au, Pt, Pd, TiO2, polymers, and biomolecules (enzymes, DNA, and proteins) [190]. There are two categories of CNT membranes [2,186]:
  • Membranes based on nonporous polymer and aligned CNTs, which make composite membranes permeable.
  • Polymer-based membranes where blended CNTs modify the physicochemical characteristics of the composite membrane.
Aligned CNT membranes may be applied in high-flux desalination. The permeability of these membranes was several times higher compared to polycarbonate membranes [176]. Filtration membranes with radially organized CNTs can rapidly remove bacteria and viruses and are biofouling resistant. The SWCNTs and polyvinylcarbazole (PVK) composite membrane showed high bacterial inactivation in direct contact [1]. Vertically aligned CNT (VA-CNT) membranes contain nanotubes perpendicular to the membrane surface, bound by the filler. The structure of mixed-CNT membranes resembles that of a composite reverse osmosis thin-film membrane. The top layer consists of a mixture of CNTs and polyamide (PA) [186]. The advantage of CNTs and polymer composite membranes is their higher tensile strength compared to conventional membranes. For example, the tensile strength of MWCNT/polyacrylonitrile (PAN) membrane is 97% higher, and MWCNT/chitosan composite membrane (at 2 wt% of MWCNT addition) is 90% higher. As mentioned before, CNTs also improve permeability [176].
CNTs can also be covalently attached to polymer surfaces, as in, e.g., Ag/polyacrylic acid (PAA)-CNTs hybrid membrane, PSf/Pebax composite membrane coated with functional multiwalled CNTs, CNT/polymer membrane with PVA coating, and a CNT-based hybrid film doped with gold nanoparticles on a polystyrene (PS) film. Such membranes are separate emulsions of oil in water stabilized with surfactants. Catalytic composite membranes can decompose up to 92.6% of nitrophenol in oil emulsion [191].
In turn, graphene-based materials enable the production of mechanically durable, ultra-thin, high-flow, highly selective, and pollution-resistant separation membranes, such as those used for water desalination. Next-generation graphene and GO membranes are cheaper than thin-film composite polyamide membranes [192]. GO membranes are smoother and more hydrophilic thanks to GO properties and the smooth surface of their sheets [172]. On the other hand, pure graphene and polymer composites are challenging to create due to the hydrophobic properties of graphite and graphene, which limit their use in water filtration [191].
GO-modified membranes may be applied for nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), photocatalysis, membrane bioreactors, and pervaporation. They enhance the removal of dyes, the separation of monovalent and divalent ions, and the dehydration of water-solvent mixtures [193]. Reduced GO (rGO) can also be used as an additive to create ultrathin negatively or positively charged membranes for ultrafast nanofiltration of organic diluents. rGO is resistant to organic diluents and severe conditions [184]. Thin-layer nanofiltration (TFN) nanocomposite membranes with GO can also be utilized to separate phosphorus from water [194].
The following techniques are employed for the production of GO nanocomposite membranes: filtration, spray coating, evaporation, drop casting, spin coating, layer-by-layer construction of GO nanosheets, dip coating, and electric field induction [195,196]. Graphene-based separation membranes (GBSMs) offer high water permeability, as well as antibacterial and anti-pollution properties [195]. GO in the membrane matrix improves water permeation, antibiofouling, and antimicrobial effects in UF, and reversed and forward osmosis (RO and FO) [197]. According to Meng et al. [172], GO is preferred for the production of membranes in practice. The suspension test showed that it has higher antimicrobial properties than rGO, graphite, and graphite oxide. Membranes modified with highly negatively charged GO nanosheets show significant potential to improve water permeability and remove salt more effectively [197]. GO membranes modified with hyperbranched polyethyleneimine (PEI) offer high water permeability and significant discard of Mg2+, Pb2+, Ni2+, Cd2+ and Zn2+ over 90% [128].

3.4. Catalytic Processes

An important advanced oxidation process (AOP) is heterogeneous photocatalysis. Numerous organic contaminants in water and wastewater are efficiently oxidized by it. Three major benefits of the catalytic process are its inherent destructiveness, the absence of mass transfer, and its capacity to operate in ambient conditions (oxygen from the air is an adequate oxidant). Additionally, it has the potential to finalize mineralization into CO2 [198].
Photocatalytic nanomaterials can be divided into zero-, one-, two-, and three-dimensional (e.g., 3D graphene). Selection is based on the application and required properties. Zero-dimensional nanomaterials are most often used, especially as core–shell nanoparticles. Fe3O4 core structures are most widely studied due to their magnetic character, which is vital for separating the nanocomposite from water [199].
Gr and GFNs have been utilized as catalysts to degrade organic pollutants in water. These materials function as adsorbents, activating the oxidizing agent to facilitate the degradation process. The presence of graphene-based materials substantially increases the efficiency of the oxidizing agent. Graphene materials are utilized as photocatalysts for degrading organic dyes, pharmaceuticals, and toxic chemicals [200].
A novel heterojunction, the semiconductor–graphene heterojunction, is generating interest. The electron flow is increased, enhancing the photocatalytic performance, due to the ultrahigh electronic Gr conductivity. Moreover, the presence of extensive π–π conjugation on the Gr surface could facilitate the adsorption of various substances during the process. However, a weight share of Gr over 1% causes a substantial decrease in photocatalytic activity caused by the light-shielding effect, limiting the semiconductor’s radiation. In recent years, numerous semiconductor–graphene heterojunctions have been examined, including TiO2-graphene, ZnO-graphene, CdS-graphene, and C3N4-graphene [199].
Various semiconductors and carbon supports may noticeably enhance the photocatalytic degradation. They are characterized by interfacial charge limitation, efficient p-n semiconductor nanojunction formation, enhanced visible light absorption, and high surface area. GO-TiO2, GO-WO3, and Co3O4–C3N4 composites enhance photodegradation of antibiotics by •OH effect on the benzene ring or isoxazole [201]. Graphene-TiO2 heterojunction photocatalytic performance is significantly higher than TiO2 alone [202,203].
Magnetite Fe3O4 is the most studied photocatalyst among ferritic nanomaterials. Magnetite’s properties provide ease of magnetic separation and long-term stable reactivity. [204]. The Fe3O4–CuO–ZnO-Nano graphene nanocomposite achieved 93% photodegradation efficiency of the Methylene blue dye [205], and Fe3O4@Bi2O3 30%rGO reached 98.3% photodegradation of Ciprofloxacin [206], while the Fe3O4–ZnO–rGO nanocomposite effectiveness was 83.5% for Methylene violet [207].
CNMs can enhance TiO2 activity as co-catalysts. CNTs and TiO2 photoactive hybrid are a typical example of photoactive heterostructures. A substantial amount of adsorption and catalysis sites, the induction of catalysis under visible light, and the delay in recombination of electrons and holes are features that enhance the decomposition of organic contaminants in photocatalytic processes [208].
CNTs have also been used as carriers for metals/metal oxides in Fenton-like catalysts. Their porous structure enables the good dispersion of the catalyst and prevents its aggregation on the carrier surface. During a Fenton-like process, in the presence of an oxidant (H2O2), hydroxyl radicals with high oxidizing potential are generated. Strong interactions between transition metal ions and the carrier enhance the electron transfer required in this reaction, thereby improving their catalytic properties [209]. Many researchers have confirmed the high effectiveness of iron oxides (Fe3O4) deposited on CNTs as a Fenton-like catalyst for the oxidation of organic pollutants in water and wastewater, including phenols [103]. CNT impregnated with Fe and Cu also showed catalytic activity when used for the oxidation of paracetamol [90]. In addition, a synergistic effect was observed between the Cu+/Cu2+ and Fe2+/Fe3+ systems, which contributed to over 90% process efficiency after 5 h of contact in a solution with a near-neutral pH.
Due to the low stability of free radicals, the Fenton process can be supported by UV radiation or an electric field [209,210].
CNMs, CNTs in particular, are also used in the process of anodic oxidation of organic pollutants. The high efficiency of the electrochemical reactions is due to the good electrical conductivity of CNMs and their large specific surface area. Compared to glass electrodes, CNT electrodes are characterized by very low overpotential and higher peak current values. For this reason, the anodic oxidation processes of many organic compounds are highly efficient (approximately 90%), and their degradation occurs much faster than in biological processes traditionally used in wastewater treatment plants. Furthermore, they are also effective against poorly biodegradable substances [211].
One of the combined methods commonly used is photocatalytic ozonation. Catalytic ozonation with the addition of CNTs may effectively degrade substances that are resistant to decomposition, such as pharmaceuticals. Unfortunately, there is still a lack of knowledge about catalytic processes, and the production of ozone is expensive [132].
ZnO-based graphene, GO, and rGO nanocomposites showed high efficiency during photocatalysis of harmful metal ions and micro-organisms. ZnO-GO showed 75% efficiency of photodegradation of methylene blue [212], Au-ZnO-rGO showed 94% photodegradation efficiency of Rhodamine B [213], ZnO-graphene showed 99% photodegradation efficiency of Deoxynivalenol [214], and CdS-ZnO-graphene showed 98% photodegradation efficiency of 4-nitroaniline [214].
Three-dimensional graphene (3DG) has been developed in recent years. It has a foam-like macrostructure, significantly increased surface area, and controlled shape and size. It can be easily applied and reclaimed [199,215]. 3D ZnO-GO nanocomposites have garnered particular interest due to their high photocatalytic activity, which is attributed to the efficient transfer of charges between ZnO and 3D GO [216]. The photocatalytic removal of pollutants present in wastewater can also be effectively achieved on CNT buckypapers (BPs) modified with TiO2. The obtained TiO2-BPs membrane achieved an efficiency of over 90% in removing diclofenac (DF), carbofuran (CB), and methylene blue (MB) (UV-Vis exposure: 90 min; photocatalyst dose: 0.6 mg) [217].

4. Impact of CNMs on the Aquatic Environment

Continuous growth in the production and use of CNMs results in an inevitable increase in their presence in the environment [218,219]. The release of CNM nanoparticles may result in unintentional environmental changes; therefore, risk assessment and management strategies are necessary [11]. Increasing amounts of CNMs are detected in the water environment [10]. CNMs enter the environment through various pathways, including manufacturing (as dust and wastewater), transportation, removal, direct applications (with fertilizers during remediation), or through effluents from WWTPs [129]. CNTs are emitted mainly during production, use, and disposal. Approximately 90% of CNTs end in landfills, 10% in soils, and the rest in sediments and air [220]. Nanomaterials can easily be transferred to living organisms and have toxic effects [3]. In aquatic organisms, exposure to MWCNTs may be associated with cytotoxic and genotoxic effects [221].
Most wastewater containing nanoparticles ends up in municipal WWTPs. Micro-organisms used there also spontaneously synthesize nanoparticles and are considered natural “nano factories.” Therefore, effluent from WWTPs introduces nanomaterials into the environment (soil, surface waters, and seas) [222]. In WWTPs, GO is proposed for numerous applications due to its ease of dispersion and formation of stable suspensions [129]. However, it may decrease the effectiveness of activated sludge in the treatment process and impact the quality of the effluent [129,223]. GO may damage the cell membranes of micro-organisms during the activated sludge process and negatively affect anaerobic denitrification. GO and rGO also participate in the formation of trihalomethanes. At a low GO/biomass ratio, it will be effectively retained in the secondary settler [224].
Nanomaterials’ advantages, including large surface areas and pore sizes, facilitate the reactions with other pollutants in water ecosystems [225]. Nanomaterials can agglomerate into particles or form long fiber chains, modifying their properties and potentially negatively impacting organisms. Particles that arise during the disintegration of nanomaterials exhibit nanostructure-dependent biological activity, which can also be harmful to organisms [226]. Nanomaterials can remove and recover substances influencing their activity and bioavailability [225]. In water, abiotic and biotic transformations, heteroaggregation, complexation, and adsorption impact GNM toxicity, decreasing it through aggregation or increasing it through solubility. The most critical parameters are size and shape of particles, chemical composition, crystal structure, area, charge, and chemistry of the surface, as well as physicochemical changes in the abiotic and biotic environments [129]. CNMs have a particular toxic effect on reproductive systems and the development of offspring in various animal ecosystems. This effect depends on the structure of carbon crystals [227].
The most essential mechanisms of GNMs’ toxicity are oxidative stress and damage to the cell membranes of bacteria. Sharp edges of GNMs may physically damage living organisms (including bacterial and viral cells). However, micro-organisms can convert GO into graphene (also changing the surface charge), influencing its fate and toxicity [129].
GO may pose a potential threat to human health and the environment. Its cytotoxicity depends on size, dose, and oxidation state (C/O ratio). Higher oxygen content in GO decreases its toxicity [128,134]. Graphene can be internalized into cells via passive or active internalization (endocytosis). The primary mechanism of graphene toxicity involves the production of intracellular reactive oxygen species (ROS), which damage proteins and DNA, ultimately leading to cell death through apoptosis or necrosis [223].
Studies of Daphnia magna showed that the mortal GO concentration was greater than 2 mg/L. At a concentration of 0.5 mg/L, there was a negative impact on growth and offspring amount. It was found that GO may be toxic to Zebrafish, causing several cellular changes [129,228,229]. Not all CNTs exhibit toxicity to mammalian cells; this depends on the physical tube size, dispersion or agglomeration, the method of CNT processing, and the presence of other contaminants within the structure. Dispersed CNTs are less toxic than agglomerates, which contain a higher fraction of soot-free CNTs [167].
Sunlight can also induce specific physicochemical changes in nanomaterials in water. Therefore, nanomaterial fate in real water systems is hard to predict. For example, solar radiation caused the degradation and reduction in GO. GO present in water treatment systems, in addition to exposure to sunlight, can also interact with chemicals used in the process. Chlorination may potentially impact the morphology of GO and functional groups containing oxygen. Chemical changes can lead to shifts in nanosheet physics, such as fragmentation into much smaller sizes. They can also affect the media’s pH, and the size distribution and surface charge of nanomaterials, as well as their antibacterial activity [218]. In sunlight, GO decomposes to CO2, rGO-like materials and low-molecular-weight compounds with different toxicity than the substrate. Organisms in the aquatic food chain that GNMs impact most are algae [128]. Also, fullerene C60 is harmful to organisms in water. The lipophilic character causes its accumulation in lipid-rich parts of the cell, such as the membranes [167].
Reported CNM concentrations in water is given in ng/L or even lower [10]. CNMs in rivers may accumulate in sediments or be transported to marine ecosystems when suspended. Contamination of water with CNMs is rarely reported; however, C60 has been detected in effluent from WWTPs at concentrations of ng/L. The concentration of C60 in surface waters in Europe is 0.017 ng/L and CNTs 0.004 ng/L [10].
There are still gaps in knowledge about interactions between nanoparticles and the environment. Therefore, much more research is needed to determine how CNM products influence living organisms [226]. Two key elements in the risk assessment of CNMs are their potential toxicity to organisms and their potential for bioaccumulation. There is limited evidence on the ecotoxicological impact of CNMs on waterborne organisms. Additionally, information on how co-occurring pollutants impact toxicity and bioavailability is limited [10]. The primary reasons for this are the limited methodologies and underdeveloped techniques for isolating and quantifying CNMs in the environment, as well as the difficulty in distinguishing natural carbon from CNMs derived from [230].
Most toxic effects of CNMs were observed during brief exposures at high concentrations. The degree of toxicity varies depending on the species, exposure time, and technique used in CNM production. CNMs can have synergistic or antagonistic interactions with other micropollutants. These interactions primarily depend on the specific chemical properties of the micropollutants. CNMs can modify the initial toxicity of the contaminant, acting as a carrier or a sorbent. Therefore, it is necessary to investigate the interactions of CNMs with other micropollutants (at environmentally relevant concentrations) and their long-term impact [10]. They should be analyzed in the form in which they are released into the environment and in the form in which they occur after undergoing biochemical transformations as a result of various environmental factors [231].
Previous studies have revealed a lack of comprehensive assessment of the impact of nanomaterials on the environment and humans, as well as the possibility of monitoring and establishing risk assessment protocols [152]. The release of nanoparticles containing pollutants into the environment should be a more important factor in the development of methods for recovering nano-adsorbents, thereby reducing their presence in the environment [153]. The search for more environmentally friendly synthesis methods and raw materials for the production of nanomaterials may also reduce environmental impact, provided that they consider the full life cycle of nanoparticles. The current sustainable approach in research involves obtaining raw materials for the production of efficient and high-quality CNTs from plastic waste and biowaste, thereby enabling the reuse of solid waste and reducing carbon dioxide emissions. Increasing attention is also being paid to the development of an environmentally friendly and sustainable synthesis method, known as green synthesis, as current methods often require hazardous substances and special safety precautions during production [149,232,233].

5. Conclusions

Nanomaterials used in water and wastewater treatment research offer several key advantages. The most important ones include a significant specific surface area, mechanical strength, thermal stability, strong binding to aromatic substances, and antibiotic activity. Thanks to this, they may soon become an alternative to currently used conventional methods and technologies, increasing the efficiency and possibilities of removing various pollutants in adsorption, filtration, and catalysis processes of difficult-to-remove and biodegradable dyes, a wide range of heavy metals, pharmaceuticals (PhACs), and organic micropollutants (including NOM, PHs). These materials can be utilized for their bacteriostatic and bactericidal properties in the disinfection process, to protect filter membranes against fouling, and in effective water desalination. Newer methods of modifying CNTs are expanding the range of applications and contaminants that can be removed highly effectively, as well as enhancing the recovery and reuse of nanomaterials.
However, challenges remain that limit the commercial application of CNTs on an industrial scale. These include high production and modification costs, which can also pose safety concerns. Most applications of CNMs in water and wastewater treatment are limited to laboratory-scale testing, often under controlled conditions and in single-component solutions. A major challenge will be to verify the developed nanomaterials under real-world conditions on a pilot scale. The development of a standardized methodology for toxicity assessment and analytical methods for determining CNMs in the environment will be an urgent task. The small size of nanomaterials and their high surface reactivity may pose a threat due to their ease of biotransformation and increased interactions with the environment, as well as their impact on living organisms. Further research is needed to determine the long-term environmental effects of CNMs within the life cycle of carbon nanoparticles. However, this may not be easy due to the growing diversity of synthesized and modified nanomaterials. CNMs offer sustainable solutions that are in line with current economic trends and pro-environmental activities. These solutions include higher efficiency of water and wastewater treatment, adsorption or degradation of micropollutants hard to remove in current methods, properties allowing reuse of spent CNMs, and use of waste materials, such as plastics and biowaste, which are increasingly being utilized for the production of CNTs, thereby enabling their reuse. This reduces the production costs of nanomaterials and contributes to a reduction in carbon dioxide emissions. In this direction, sustainable methods of nanomaterial synthesis (green synthesis) are being sought, which will be part of the increasingly popular circular economy.

Author Contributions

Conceptualization, K.P. and R.Ś.-D.; methodology, K.P. and R.Ś.-D.; formal analysis, K.P. and R.Ś.-D.; investigation, and resources, K.P., R.Ś.-D. and T.D.; data curation, K.P. and R.Ś.-D.; software, K.P.; writing—original draft preparation, K.P. and R.Ś.-D.; writing—review and editing, K.P., R.Ś.-D. and T.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Graphene oxide sample: 1—after lyophilisation (GO-L), 2—optical microscope photo of GO-L, and 3—SEM micrographs of GO-L (from the authors’ sources).
Figure 1. Graphene oxide sample: 1—after lyophilisation (GO-L), 2—optical microscope photo of GO-L, and 3—SEM micrographs of GO-L (from the authors’ sources).
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Figure 2. SEM micrograph of GO-L (from the authors’ sources).
Figure 2. SEM micrograph of GO-L (from the authors’ sources).
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Figure 3. Color changes in GO-s samples in time: 1—1 month (GO-s_1); 2—2 months (GO-s_2); 3—3 months (GO-s_3); 4—9 months (GO-s_9); and 5—20 months (GO-s_20) (from the authors’ sources).
Figure 3. Color changes in GO-s samples in time: 1—1 month (GO-s_1); 2—2 months (GO-s_2); 3—3 months (GO-s_3); 4—9 months (GO-s_9); and 5—20 months (GO-s_20) (from the authors’ sources).
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Figure 4. Color changes in GO-L samples in time: 1—1 month (GO-L_1); 2—2 months (GO-L_2); 3—3 months (GO-L_3); and 4—12 months (GO-L_12) (from the authors’ sources).
Figure 4. Color changes in GO-L samples in time: 1—1 month (GO-L_1); 2—2 months (GO-L_2); 3—3 months (GO-L_3); and 4—12 months (GO-L_12) (from the authors’ sources).
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Figure 5. Zeta potential (A) and particle sizes (B) of GO-s samples.
Figure 5. Zeta potential (A) and particle sizes (B) of GO-s samples.
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Figure 6. Zeta potential (A) and particle sizes (B) of GO-L samples.
Figure 6. Zeta potential (A) and particle sizes (B) of GO-L samples.
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Figure 7. Raman Spectra of GO-s samples after background correction.
Figure 7. Raman Spectra of GO-s samples after background correction.
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Figure 8. Raman Spectra of GO-L samples after background correction; GO-standard is commercial GO.
Figure 8. Raman Spectra of GO-L samples after background correction; GO-standard is commercial GO.
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Table 1. Examples of CNMs applicable for treatment of water and wastewater, based on the available literature.
Table 1. Examples of CNMs applicable for treatment of water and wastewater, based on the available literature.
Carbon-Based Nanomaterials (CNMs)
Carbon nanotubes (CNTs):
Single-Walled (SWCNTs)
Multi-Walled (MWCNTs)
Graphene (Gr)Graphene-family nanomaterials (GFNs):
Graphene oxide (GO),
Reduced graphene oxide (rGO)
Removal/degradation of dyes:
CNT/TiO2 [12]
Graphene G-CNT [13]
CNT/PANI polyaniline [14]
CNTs/Fe-Ni/TiO2 [15]
Cellulose Nanofibrils CNF-CNT [16]
MnOx@polyvinylidene fluoride MnOx@PVDF/MWCNTs [17]
Titanium Dioxide with Silver Ag-TiO2/MWCNT [18]
Titanium Dioxide with Palladium Pd-TiO2/MWCNT [18]
MWCTs entrapped in gelatin with embedded magnetic nanoparticles Gel-CNT-MNPs [19]
Palladium-doped–ZrO2 Pd-ZrO2-MWCNTs [20]
MWCNTs encapsulated in alginate microvesicles with Ba2+ Ba2+-ALG/MWCNT [21]
TiO2/graphene-MWCNT nanocomposite immobilized in poly(vinyl alcohol) PVA/TiO2/Gr-MWCNT [22]
Magnetic Chitosan Enwrapping Nanosized
γ-Fe2O3 m-CS/γ-Fe2O3/MWCNTs [23]
Gr [24]
ZnO and graphene composites ZnO/Grs [25]
cobalt (Co) and nickel (Ni) metal–organic frameworks CNMs–Gr [26]
SiO2/Cu2O-graphene [27]
Graphene hydrogel GH [28]
Graphene aerogel GA [28]
cellulose nanofibrils graphene nanoplates CNF-GnP [16]
magnetic-sulfonic graphene nanocomposite Gr-SO3H/Fe3O4 [29]
Sulfonated Graphene Nanosheets GNSs [30]
Graphene nanoplatelets GNP-SiO2 [31]
Thiosemicarbazide functionalized GO-TSC-GO [32]
Chitosan GO-CTS [33]
Poly(vinyl alcohol) GO/PVA [34]
GO/aminated lignin aerogel GALA [35]
GO-TiO2 [36]
Poly(N,N-dimethyl amino ethylmethacrylate) GO-PDMAEMA [37]
Poly(2-hydroxyethyl methacrylate) GO-PHEMA [38]
Fly ash with chitosan FCGO [39]
MnFe2O4/GO [40]
Poly(vinyl alcohol)/poly(acrylic acid) PVA/PAA/TiO2/GO [41]
Aerogel with zero-valent iron rGOA-nZVI [42]
Cu2O-rGO/Fe3O4@SiO2 [43]
2D bimetallic sulfides/N-doped FeCoS/N-rGO [44]
Reduced graphene oxide/halloysite nanotubes polydopamine PDA/rGO/HNTs [45]
Nicandra physaloides (L.) Gaertn seed, gum NPG/GO [46]
Removal of metal ions:
CNTs [47,48]
Hydroxyl Functionalized CNT-OH [49]
Carboxyl Functionalized CNT-COO [49]
Amide Functionalized CNT-CONH2 [49]
Supported-epoxidized carbon nanotube SENT [50]
MnO2/CNTs [51,52]
Poly-amidoamine Dendrimer PAMAM/CNT [53]
Polyacrylamide-Sodium Alginate PAAM-SA-CNT [54]
Oxidized CNTs [55]
Thiol-derivatized SWCNT-SH [56]
MWCNTs [57]
Magnetic MWCNT MMWCNT [58]
Fe3O4 nanoparticles with 3-aminopropyltriethoxysilane MWCNTs/Fe3O4-NH2 [59]
Oxidized Oxi-MWCNTs [60]
Carboxyl Functionalized MWCNT-COOH [48]
Hydroxyl Functionalized MWCNT-OH [48]
La(OH)3 and CaO2 fabricated CNT La-Ca-CNT [61]
Graphene/MgO [62]
SiO2/Graphene [63]
Graphene hydrogel GH [28]
Graphene aerogel GA [28]
Graphene nanosheets GNSs [64]
Sulfonated Graphene Nanosheets GNSs [30]
Gr/MnO2-QD quantum dot [65]
Graphene nanoplatelets GNP [48]
Amination GO-NH2 [66]
2,2′-dipyridylamine GO-DPA [67]
Chitosan GO-CTS [68]
Few-layered FGO [69]
GO aerogels [70]
2-pyridinecarboxaldehyde thiosemicarbazone GO/2-PTSC [71]
Mn-doped Fe(III) oxide nanoparticle implanted graphene GMIO [72]
Glycol-GO [73]
Cyclodextrin–chitosan CCGO [74]
Manganese Ferrite MnFe2O4/GO [40]
Chitosan-Poly(vinyl alcohol) GO-CS-PVA hydrogels [75]
Fe3O4 1,2-diaminocyclohexanetetraacetic acid Fe3O4/GO/DCTA [76]
GOx-microbots [77]
magnetic chitosan Pb2+ Pb-MCGO [78]
Aminated Fe3O4 AMGO [79]
Polystyrene PS@ + rGO@GO@Fe3O4 PG-Fe3O4 [80]
Polyethyleneimine-Grafted PEI/GO [81]
Thymine-Grafted rGO-Thy [82]
Halloysite nanotubes polydopamine PDA/rGO/HNTs [45]
Triethylenetetramine-magnetic TET-MrGO [83]
Removal/degradation of pharmaceuticals:
CNTs [84]
Copper alginate CA-CNTs [85]
CeO2@CNT membrane [86]
Alumina Hybrid CNTs/Al2O3 [87]
Modified MWCNTs [88]
Carboxyl Functionalized MWCNT-COOH [84,89]
Hydroxyl Functionalized MWCNT-OH [84]
Alumina Hybrid MWCNTs/Al2O3 [87]
Fe-Cu Doped MWCNTs Fe-Cu/CNT[90]
Graphene hydrogel GH [28]
Graphene aerogel GA [28]
Graphene nanoplatelets GNP [91]
GO [92]
Co3O4/rGO [93]
2D bimetallic sulfides/N-doped FeCoS/N-rGO [44]
Removal/degradation of organic substances:
CNTs [94]
CNT/ZnO/TiO2 [95]
Metallic SWNTs [96]
PVA-based Polymer Nanoparticles PNP/CNTs [97]
SWCNTs [98,99]
MWCNTs [98,100]
Magnetic MWCNT MMWCNT [58]
ZnO nano particles on Multiwall Carbon Nanotubes MWNT/ZnO [101]
iron oxide and manganese dioxide MWCNTs/Fe3O4-MnO2 [102]
Fe3O4 nanoparticles with 3-aminopropyltriethoxysilane MWCNTs/Fe3O4-NH2 [59]
Fe3O4 decorated MWCNT Fe3O4/MWCNT [103]
Sulfonated Graphene Nanosheets GNSs [30]
Au nanoparticles anchored on the Ionic Liquid of 3,4,9,10-perylene tetracarboxylic acid-noncovalent functionalized graphene Au/PDIL-GS [104]
GO [105]
rGO [105]
2D bimetallic sulfides/N-doped FeCoS/N-rGO [44]
Silver nanospheres (Ag-NSs) rGO nanosponge RGONS/Ag-NSs [106]
Separation of oil/water:
Ferric Oxide Nanoparticles Doped CNT/Fe2O3 [107]
Silver Nanoparticles Polyacrylic Acid Ag/PAA-CNTs [108]
Magnetic CNT with macropores formed by salts [109]
Graphene aerogel GA [110]
nanoporous graphene NPG [111]
Polyethersulfone GO-PES [112]
Poly(arylene ether nitrile) halloysite nanotubes polydopamine (PEN)/HNTs@GO-PDA [113]
Halloysite nanotubes polydopamine PDA/rGO/HNTs [45]
Other:
Chromium Oxide-decorated Cr2O3-CNT [114]
Oxidized CNTs [115]
Vertically Aligned Carbon Nanotubes on Anodized Aluminum Oxide AAO-CNT [116]
vertically aligned VA CNT [117]
Plasma Induced Grafting Carboxymethyl Cellulose MWCNT-g-CMC [118]
Polymerized Citric Acid (CA), Acrylic Acid (AA) and Acrylamide (AAm) modified MWCNT/PCA, PAA, PAAm [119]
TiO2-MWNTs [120]
Lanthanum Carbonate CNT LC-CNT [121]
La(OH)3- and CaO2-fabricated CNT La-Ca-CNT [61]
graphene aerogel (GA) decorated with platinum nanoparticles GA/Pt [122]
graphene-poly(acrylonitrile-co-maleimide) G-PANCMI [123]
poly(N-vinylcarbazole)-graphene (PVK-G) [124]
GO [125]
GO-Ag [126,127]
Poly(N-vinylcarbazole) PVK-GO [124]
Table 2. Results of EDS analysis of GO-L (from the authors’ sources).
Table 2. Results of EDS analysis of GO-L (from the authors’ sources).
ElementWeight%Atomic%
C55.1062.53
O43.0636.69
S1.850.78
Totals100.00
Table 3. ID/IG and I2D/IG ratios based on Raman spectra (Figure 7 and Figure 8) for GO-s and GO-L samples; values in brackets do not fit the trend and are not taken into consideration.
Table 3. ID/IG and I2D/IG ratios based on Raman spectra (Figure 7 and Figure 8) for GO-s and GO-L samples; values in brackets do not fit the trend and are not taken into consideration.
SampleID/IGI2D/IG
GO-s_1(0.91)0.09
GO-s_20.81(0.74)
GO-s_90.800.17
GO-s_200.830.15
GO-L_10.830.10
GO-L_20.850.095
GO-L_30.840.090
GO-L_120.830.094
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MDPI and ACS Style

Piaskowski, K.; Świderska-Dąbrowska, R.; Dąbrowski, T. Carbon-Based Nanomaterials in Water and Wastewater Treatment Processes. Sustainability 2025, 17, 7414. https://doi.org/10.3390/su17167414

AMA Style

Piaskowski K, Świderska-Dąbrowska R, Dąbrowski T. Carbon-Based Nanomaterials in Water and Wastewater Treatment Processes. Sustainability. 2025; 17(16):7414. https://doi.org/10.3390/su17167414

Chicago/Turabian Style

Piaskowski, Krzysztof, Renata Świderska-Dąbrowska, and Tomasz Dąbrowski. 2025. "Carbon-Based Nanomaterials in Water and Wastewater Treatment Processes" Sustainability 17, no. 16: 7414. https://doi.org/10.3390/su17167414

APA Style

Piaskowski, K., Świderska-Dąbrowska, R., & Dąbrowski, T. (2025). Carbon-Based Nanomaterials in Water and Wastewater Treatment Processes. Sustainability, 17(16), 7414. https://doi.org/10.3390/su17167414

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