Heavy Metals Adsorption by Nanosheet: Mechanism and Effective Parameters

Abstract

Among the methods for heavy metals removal such as precipitation, evaporation, electroplating and ion exchange, which have many disadvantages, adsorption is the cost effective and environmental friendly technique. Using nanosheets as the base materials for the adsorption because of their large surface area and high adsorption capacity is broadened. Carbon products (Graphene), boron nitride materials (BNM), transition metal dichalcogenides (TMDs), layered double hydroxiades (LDHs) and MXene are most well-known nanosheets, which have been used for heavy metal ions removal from aqueous solutions. In this review, experimental and simulation studies on nanosheet adsorbents are presented to pinpoint the importance of this group of nano-materials on water/wastewater treatment technology. Molecular dynamics (MD) and density functional theory (DFT) are the most common simulation methods for demonstration of adsorption mechanism of nanosheets. In addition, synthesis methods, adsorption mechanism, adsorption performance, and effective parameters of nanosheets and novel techniques to improve the adsorption capability and regeneration of adsorbents are introduced. The adsorption of heavy metals to nanosheet mechanisms is also introduced and isotherm/ kinetics models for each nanosheet are evaluated. With all the advantages of nanosheets, it should be noted that their usage in larger industrial scales should be further investigated.

1. Introduction

Replace the entirety of this text with the introduction to your chapter. The introduction section should provide a context for your manuscript and should be numbered as first heading. When preparing the introduction, please bear in mind that some readers will not be experts in your field of research.

From the time when nanosheets were introduced to scientific society, scientists have been achieving more properties and applications of these nanoparticles in every aspect of science such as engineering, environment, chemistry, physics, and so on [1, 2, 3, 4]. Strong adsorption capability of nanosheets are due to their high surface area, porosity, specific surface charge, surface functionality, and ions adsorption characteristics [5, 6, 7, 8] originating from their small size structures have drawn the attention for energy and water and wastewater treatment applications [9, 10, 11, 12].

Today, expansion of various industries including mining industry, sewage irrigation, metal plating, and electronic industry has contaminated the water and wastewater with heavy metals such as cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), and mercury (Hg) [13]. Human health and the ecosystem are in danger as a result of the heavy metal entering the environment. The presence of heavy metal in water in excess could cause serious health problems for living thing because they are non-biodegradable and may be carcinogenic [14]. As a result, various methods for removing heavy metal ions have been developed, including coagulation and flocculation, precipitation, ion exchange, membrane technologies, electrochemical technologies, and adsorption [15, 16]. There are only a few treatment methods that are used due to various economic and technological factors. These processes have some drawbacks (high cost, process complexity, low efficiency, etc.), despite the fact that they can remove various pollutants from water and wastewaters. It is interesting to note that the adsorption method, which is regarded as the safest, cleanest, and most practical and technically feasible process, was deemed to be the most effective for removing heavy metals [17]. Various adsorbents, such as fly ash [18, 19], activated carbon [20, 21], zeolite [22, 23, 24], montmorillonite [25, 26, 27], and chitosan [28, 29, 30], have been reported for heavy metal removal from aqueous solutions. Nanomaterials frequently have unique characteristics under the nanoscale, for instance a surface effect, small-scale impact, quantum effect, and macro-quantum tunnel effect in contribution with their extraordinary adsorption capacity and reactivity cause them to be the proper materials for removal of heavy ions [31, 32]. A new class of nanomaterials with sheet-like morphologies called 2D nanosheets is made up of layer structures with lateral sizes greater than 100 nm and thickness smaller than 10 nanometers. Physical and chemical characteristics of nanosheets are dependent on formation of their structures from inorganic compounds. Due to their mechanical strength, flexible structure, chemical inertness, and separation performance, nanosheets have shown promise as building blocks or fillers for membranes [33, 34].

Carbon products (graphene), boron nitride materials (BNM), transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), and MXene are most common 2D nanosheets [35] and some novel nanosheets [36, 37, 38, 39, 40, 41, 42, 43, 44, 45] implemented in water and wastewater treatment. The novel carbon nanomaterial adsorbents include carbon nanotubes, graphene, and other nano-adsorbents with carbon element and sp2 as the main hybrid form. Graphene has a two-dimensional structure, which is the foundation for creating graphitic materials in all dimensions. For instance, it can be folded into on-dimensional carbon nanotubes, enveloped in zero-dimensional fullerene, or layered into three-dimensional graphite. Its crystal lattice is composed of a single layer crystalline carbon atoms and has a hexagonal lattice shape. Strong adsorption capacity, high removal efficiency, fast equilibrium speed, low cost, no secondary pollution, and good recyclability are only a few of the notable benefits of using graphene as an adsorbent to remove heavy metals [46]. There are typically four main forms of crystalline BNMs, including hexagonal/rhombohedral BNMs that resemble graphite and have dense phases with sp2 hybridized B-N bonds and cubic BNMs that resemble wurtzite and diamond and have low density phases with sp3 hybridized links. Its several physical forms of BN nanomaterials are 0D, 1D, 2D, and 3D shapes like nanoparticles/fullerenes, nanotubes/nanofibers/nanoribbons, nanosheets/nanomeshes, and abd nanoflowers/hollow spheres, respectively. Particularly, BNMs and BN-based nanomaterials have been thoroughly studied for possible environmental applications such as adsorption, photocatalytic degradation, and membrane separation to remove pollutants [47]. TMDs, including MoS₂, WS₂, MoSe₂, and WSe₂, have been known as semiconducting 2D layered materials. The elements are described as MX₂, where M is a transition metal form (Group 4 to 10), and X stands for chalcogen elements of S, Se, or Te, which can crystallize in either a non-layer form or a layered structure resembling graphite (mainly the Groups 4–7 TMDs). Numerous effective methods, including heteroatom doping, vacancy engineering, and the addition of a second component into or onto 2D TMD nanosheets, have been proposed to further explore the possibilities of these materials. Due to their extensive nanocapillary channels, TMDs nanosheet-based membranes have demonstrated exceptional performance for nanofiltration and desalination in recent years. TMD membranes, as stated, offer more benefits than other 2D nanosheets, such as high water flux and water stability. Additionally, the laminar TMD membranes in water have free spacing that is about sub -1 nm, which aids in molecular separation [48, 49]. LDHs are a broad category of positively charged metal hydroxide layers that resemble brucite and charge-balancing intercalated anions. LDHs have substantial specific surface areas and strong ion and molecule adsorption capacities, much like other two-dimensional materials. Inorganic metal hydroxide layers’ chemical composition, structure, size, shape, and interlayer gallery anions all affect how well LDHs adsorb substances. The chemical identity of the metal ions and the size of the intercalated anions govern the cationic layer thickness, which in turn controls the chemical reactivity of LDHs. The application of original LDHs in sensing and adsorption are constrained by the functional groups and simple structural components, despite the low cost and simplicity of their preparation [50, 51, 52]. Due to their exceptional properties, MXenes is one of the newest and most adaptable two-dimensional; nanomaterials have attracted a lot of attention in recent years. One of the four primary structures M₂AX, M₃, M₄, M₅AX₄, where M is an early transition metal group, A is a group A element (e.g., Al, Si, or Ga), and X is carbon and/or nitrogen, produces MXenes, which are conversion metal carbides, nitrides, and carbonicides from densely layered ceramic precursors known as MAX phases. MAX phases themselves have been used in water treatment applications because of their excellent environmental stability, nontoxicity, and simplicity of preparation. However, the selective etching of an element from the MAX phase to produce a colloidal MXene solution with an accordion shape can produce many distinctive surface characteristics, including hydrophilic surfaces, large surface areas, high electronic conductivity, an abundance of surface functional groups, redox properties, and high capacity for absorption. Adsorption of organic and inorganic contaminants, photocatalysis, water treatment membranes, capacitive deionization, and antimicrobial applications are just a few of the many environmental remediation used for MXenes [53].

In present work, a brief review on heavy metal removal using 2D nanosheets is investigated from experimental and simulation aspect of view. Studies are about common and novel nanosheets, which were prepared in laboratories and tested to evaluate and compare the adsorption capability of heavy metal ions from aqueous solutions. In addition, DFT and MD simulations have been done to study the adsorption mechanism of the nanosheets and the effective parameters of process improvement. Finally, a conclusion on technical challenges using nanosheets in current and future is presented.

2. 2D nanosheets used for heavy metal removal

2.1 Carbon products (graphene)

Using a modified Hummers process, a few layered graphene oxide (GO) nanosheets could be made from graphite and utilized as sorbents to remove Cd(II) and Co(II) ions from large volumes of aqueous solutions. The results of tests on effective parameters including pH, ionic strength, and acid on Cd(II) and Co(II) sorption showed that the sorption was significantly influenced by pH and minimally related to ionic strength. Cd(II) and Co(II) sorption were affected by the abundant oxygen-containing functional groups on the surface of graphene oxide nanosheets [54].

The colloidal behavior of GO showed that common cations (Ca²⁺, Mg²⁺, Na⁺, and K⁺) might destabilize GO solution less aggressively than heavy metal cations (Cr³⁺, Pb²⁺, Cu²⁺, Cd²⁺, and Ag⁺) (Figure 1). Additionally, heavy metal cations can readily cross the electric double-layer (EDL), attach to GO surface, and subsequently change the surface potential, which is a more effective mechanism for GO aggregation. According to aggregation kinetics, Cr³⁺> > Pb²⁺ > Cu²⁺ > Cd²⁺ > Ca²⁺ > Mg²⁺> > Ag⁺ > K⁺ > Na⁺ is the order of destabilizing ability of cations. The destabilizing capacity of metal cations is consistent with their tendency to bind to GO, which is governed by their electronegativity and hydration shell thickness [55]. Sunlight-assisted surface nanostructure tailoring involving controllable morphological and structural alterations of graphene could improve the ion adsorption and filtration. Notably, lighting control could be used to sufficiently adjust the interlayer spacing of single-layer graphene to encourage selective interactions between ions and nanosheets [56].

Figure 1.

Bulk flocculation of GO in aqueous solution containing monovalent cations (A), divalent cations (B), and trivalent cations (C) with different concentrations to determine their destabilization capability (https://pubs.acs.org/doi/10.1021/acs.est.6b04235. Note: Further permissions related to the material excerpted should be directed to the ACS) [55].

Due to its hydrophilic nature, GO can be dispersed in aqueous media. As a result, it is challenging to remove it from the solution using conventional separation methods after the adsorption process, which could raise the cost of industrial application and/or result in re-pollution of the treated water. With the use of magnetic technology (Figure 2), the issue may be resolved [58]. MnFe₂O₄ [57, 59], sulfonated magnetic [60, 61], and Fe₃O₄ [57, 62, 63, 64, 65, 66] were added to the graphene oxide adsorbents, making it easier for them to be trapped from the medium with the help of an external magnetic field. To improve effectiveness of heavy metal removals, scientists have added other materials including Chitosan [67, 68, 69], Cds [70], silica [71, 72], Mg(OH)₂ [73], and CdO [74] to GO nanosheets. Due to its impressive properties, graphydine, a new type of carbon allotrope made up of sp- and sp²-hybridized carbon atoms, has attracted significant interest. Given that graphydine’s acetylenic links interact with metal ions strongly, it is thought to be a promising candidate for the sorption of heavy metals [75, 76, 77, 78].

Figure 2.

(a) Schematic illustration of PEHA-Phos-GO/MnFe₂O₄ nanohybrids preparation and application; SEM images of (b) GO, (c) Phos-GO, and (d) PEHA-Phos-GO/MnFe₂O₄; TEM images of (e) GO, (f) Phos-GO, and (g) PEHA-Phos-GO/MnFe₂O₄ [57].

Molecular dynamics (MD) simulations, density functional theory (DFT), and Monte Carlo calculations (MMC) used to investigate the ability of nanoporous graphene (NPG) membranes to desalinate heavy metal ions (Figure 3). These studies could give us molecular insight regarding the adsorption sites, interaction type, and adsorption energetics of nanosheets toward heavy metal ions [79]. Nanoporous graphene membrane functionalized with different chemicals (hydroxyl, nitrogen, fluorine, boron, etc.) were studied, and the results showed that NPG functionalized with these groups have high water permeation and ion rejection for different conditions [80, 81, 82, 83]. MD simulation studied two graphene membranes that are oppositely exposed to two external electric fields used to separate positive and negative ions from salt water. Ion separation was seen to noticeably improve as the strength of applied electric filed increased [84]. A combination of simulation strategy, DFT, MD, and MMC, was used to show that carbon graphite (111), as a heavy metal sorbent, has an efficient capability of removing silver (Ag), mercury (Hg), cadmium (Cd), palladium (Pd), and zinc (Zn). The results showed that the adsorption process is spontaneous and exothermic in nature. The maximum adsorption can be achieved in neutral to low acid medium [85]. The single layer C₂N nanosheet is more effective at purifying water because of its high porosity compared to other carbon-based membranes of a similar thickness and the presence of nitrogen atoms at the edges of each pore. An external voltage should be applied across the membrane to separate heavy metal ions through the nanosheet. According to the findings of radial pair distribution function analysis, the hydration shell of the cations like Cu²⁺ is responsible for the energy barrier that prevents ions from passing [86].

Figure 3.

MD, DFT, and MMC investigation on the ability of nanoporous graphene (NPG) membranes to desalinate heavy metal ions. (a) Side and top views of the equilibrium adsorption configurations of the dichromate ions (one dichromate ion +100 water molecules) onto the GO model (5 × 5 graphene layer containing: An epoxy, carboxy, and hydroxyl functional groups on its surface obtained using MC simulations) (b) adsorption energy distributions for dichromate ions (c) four different explored adsorption sites for the dichromate ion on the GO surface and the corresponding adsorption energy values [79].

2.2 Boron nitride materials (BNM)

A unique hierarchical boron nitride structure that resembles an urchin created initially using a sample two-step process that involves the production of a comparable “core-shell” structured boron-containing precursor and heat catalytic chemical vapor deposition. This assembly is made up of free-rowing boron nitride nanotubes and crapy boron nitride nanosheets (Figures 4 and 5). To regulate the creation of BN hierarchical structure, a mixed growth mechanism of vapor–liquid–solid and vapor–solid is proposed. The distinctive structure demonstrates excellent Pb²⁺ and Cu²⁺ removal abilities in aqueous solutions. The superior adsorption ability of product is primarily due to the significant lattice defect, high-density edge active sites, extended interplanar space, and distinctive structural features. They contribute to structural stability and provide space for the heavy metal ions that have been absorbed [87].

Figure 4.

An illustration of the BN hierarchical development process of structure using nanotubes and nanosheets [87].

Figure 5.

(a) FESEM image of the primary product obtained at 1200°C and (b) corresponding EDS mapping images (scale bars represent 500 nm) (c) FESEM image of the primary product obtained at 1300°C (d) XRD patterns of the primary product obtained at 1200 and 1300°C [87].

The process of making hierarchical porous boron nitride nanosheets (hp-BNNSs) involved pyrolyzing a solution of boron acid and urea at 1000°C for 5 h. With pores that ranged in size from 1.1 nm to 40 nm and a larger specific surface area of 1145 m²/g, the produced hp-BNNSs displayed a hierarchical porous structure. Heavy metal ions (Cu²⁺ and Ni²⁺), industrial dyes (methylene blue and rhodamine B), and antibiotics (tetracycline) were all adsorbable on hp-BNNSs. This outstanding performance of the hp-BNNSs revealed their enormous potential for flexible water treatment [88].

Nanosheet-structured boron nitride spheres (NSBNSs) is created from solid B powders using a catalytic thermal evaporation process. The ultrathin nanosheets that make up NSBNSs are oriented radially, with the sheet edges toward the surface. The NSBNSs have a flexible capacity for adsorption and excel in removing heavy metal ions from water, oil, and dyes. The NSBNSs have adsorption capabilities for Cu²⁺ and Pb²⁺ that are equal to or greater than those of the adsorbents previously described [89].

A low temperature synthesis technique was employed to create new, few-layered boron nitride (BN-550) nanosheets, which were then used to quickly and effectively adsorb lead ions (Pb²⁺). Through experiments regeneration and pH, easy recyclability and stability of the BN-550 were demonstrated. According to interference assays, the adsorbents had a significant affinity for Pb²⁺ even when other heavy metal ions including Ni²⁺, Cu²⁺, and Cd²⁺ ions interfered [90]. A straightforward two-step approach was used to easily create activated oxygen-rich porous boron nitride nanosheets (OBNNSs), which were rich in B-O bonds and boron atom vacancies. Unlike traditional doping and activation techniques, the oxygen dopants and boron atom vacancies in OBNNSs are mostly injected in situ and chemically activated during the production process. Due to their distinct polarity of B-O bonds and boron atom vacancies, OBNNSs have a high capacity and rate for absorbing metallic ions, outperforming both bulk and activated BN as well as many other absorbents. Aside from that, the extraordinary anti-oxidation, corrosion resistance, and structural stability of the employed OBNNSs make it simple to regenerate them using acidic elution [91].

Two researches were done using pyrolyzing a solution of melamine and boric acid to create boron carbon nitride nanosheets (BCN NSs) [82] and poly(ethyleneimine)-modified h-BNNSs (PEI-h-BNNSs) [83]. A number of products were produced by adjusting the molar ratio and heating temperature in order to improve the synthesis conditions, and 3:3 molar ratio at 550°C temperature was selected for the best Hg²⁺ and Pb²⁺ removal efficiency. BCN NSs have the highest possible adsorption capacity and demonstrated good adsorption performance toward Hg²⁺ and Pb²⁺ thanks to the effectively introduction of abundant function group, increased specific surface area, hydrophilicity, and electrostatic attraction ability. Additionally, BCN NSs exhibit exceptional chemical stability, and even after six cycles of adsorption/desorption, the adsorption capabilities were still greater than 90% [92]. Hexagonal boron nitride nanosheets (h-BNNSs) based on magnetic hybrid aerogels (MHAs) as a lightweight adsorbent were fabricated for Cr(VI), As(V), methylene blue (MB), and acid orange (AO) removal from aqueous solutions. Magnetic nanoparticles (Fe₃O₄ NPs) could be decorated on PEI-h-BNNSs because of PEI-h-BNNSs formation. Magnetic hybrid aerogel with large porous structures were generated by lyophilization treatment of PEI-h-BNNSs@Fe₃O₄ NPs- loaded PVA hydrogels, which has diverse and numerous functional groups, good super-paramagnetic, and a zero net surface charge. Due to these characteristics, this absorbent could be utilized to efficiently remove Cr(VI), As(V), MB, and AO with highest adsorption capacity [93].

Application of functionalized boron nitride nanosheets (BNNS-Fe₃O₄ nanocomposite) by Fe₃O₄ for As(III) and As(V) removal from contaminated water was studied. The nanocomposite could be easily separated from the solution under an external magnetic field because of its supermagnetic characteristic at ambient temperature. Figure 6 shows that the highest adsorption capacity of the nanocomposite for As(III) and As(V) ions is determined to be 4 and 5 times more than that of the unmodified BNNSs. DFT simulations and experiments demonstrate that As(OH)₃ and As(OH)₅ bind to the BNNS-Fe₃O₄ nanocomposite more effectively than the raw BNNSs, which it showed higher adsorption capacity of the nanocomposite [94, 95].

Figure 6.

Adsorption isotherm of (a) BNNSs (b) BNNS-Fe₃O₄ nanocomposite. Structures obtained from DFT optimization: (c) BNNSs-As (OH)₃ and BNNS-Fe₃O₄-As (OH)₃ nanocomposites (d) BNNSs-As (OH)₅ and BNNS-Fe₃O₄-As (OH)₅ nanocomposites (a, c): https://pubs.acs.org/doi/10.1021/acsomega.9b04295. Note: Further permissions related to the material excerpted should be directed to the ACS) [94, 95].

Molecular dynamics simulation of hexagonal boron nitride (h-BN) showed that rationally designed h-BN membranes have excellent permeability, selectivity, and controllability for water desalination. Two key important features play an important role in controlling the water flow and ion rejection, which are size and chemistry of the pores. Nitrogen on the edges of pores were allowed higher flows than boron-lined pores. Particularly, two-dimensional h-BN with medium sized N4 pores exhibit exceptional water permeability and completely ion rejection that is several orders of magnitude higher than of standard membranes. Additionally, Figure 7 shows that mechanical strain affects monolayer h-BN desalination performance with relatively small N3 pores, indicating that tensile strain can perfectly control desalination process [96].

Figure 7.

(a) The number of water molecules passing through the strained N3 membrane at 100 MPa. (b) Water flux vs. strain at 100 MPa. The inset shows the enlarged pore, with a shadow indicating the pore surface area (S), and the values of S are provided in parentheses when tensile strain is applied. (c) PMF of water across the N3 membrane without strain (0%) and with a strain of 3, 6, and 9%. (d) Salt rejection vs. pressure by the N3 membrane under different tensile strains, and the inset plots show the water permeability under 3 ~ 6% strain [96].

Boron nitride nanosheet with functionalized pore obtained by passivating the atoms at the edges with fluorine and hydrogen atoms were simulated with molecular dynamics method [97, 98]. Cu²⁺, Hg²⁺, Pb²⁺, and Cd²⁺ cations pass selectively through the functionalize pore of the boron nitride nanosheet because of an external voltage which was applied along the simulation. The results showed that heavy metal ions met an energy barrier and could not pass through the pores, but external voltage causes them to overcome the energy barriers and crossed the functionalized pores.

The adsorption capability of boron nitride and graphene nanosheets for arsenic [99, 100], Ni²⁺ [83], and Zn²⁺ [82] removal was done using molecular dynamics. Results for arsenic adsorption simulation show that the h-BN nanosheet has a strong interaction with arsenic. However, desorption of arsenic on the h-BN nanosheet displayed higher energies barriers than that of graphene. As a result, when compared to graphite nanosheets, the residence time of arsenic is approximately three times higher on h-BN nanosheet in comparison with graphene. Also, we should note that the adsorption behavior of ions is influenced by presence of limited charges on B and N atoms in the h-BN [99]. However, for simultaneous adsorption of methylene blue and arsenic, graphene indicated better efficiency for wastewater treatment compared to hexagonal boron nitride and hexagonal boron carbon nitride [100]. To remove Ni²⁺ and Zn²⁺ from aqueous solution, molecular dynamic simulation was done with graphene and boron nitride with the functionalized pores under an external voltage. Studies showed that these functionalized nanosheet membranes have great functionality for heavy metal ions removal [82, 83].

2.3 Transition metal dichalcogenides (TMDs)

Molybdenum disulfide (MoS₂) has recently noted as promising heavy metal removal from aqueous solution nano-based adsorbent, which has the potential to provide an alternative to conventional water decontamination technologies. The trade-off between mercuric removal capacity and overall MoS₂ adsorbent stability driven by MoS₂ generation parameters was studied. Flower-like MoS₂ films onto planar alumina support were grown with a bottom-up hydrothermal synthesis setup at different growth temperature. The results showed that low growth temperature could generate amorphous supported MoS₂ enhancing mercuric removal, but lowering the chemical stability, which has considerable byproduct molybdate leaching [101].

A novel removal mechanism was clarified that includes a reduction–oxidation (redox) reaction between heavy metal ions and ce-MoS₂. Ag⁺ ions were used as model species, and to demonstrate this novel removal mechanism, after removal experiments, the Ag contained in MoS₂ was separated using centrifugation and then characterized. The Ag peaks in XRD patterns, indicating a redox reaction between Ag⁺ and ce-MoS₂. Ag⁺ is a mild oxidant and could oxidize the MoS₂ nanosheets to soluble molybdate and sulfate/sulfite ions with reduce itself to metallic Ag(0) particle. Because the key oxidation products are liquid, non-toxic molybdate, and sulfur species, which do not shape an insulating oxide layer to passivate the membrane surface, the surface is still active to effectively recover metallic particles. Hence, precious heavy metals can be effectively removed from wastewater and recovered using MoS₂ nanosheets membrane [102].

An ultrathin MoS₂ nanosheets (MoS₂ NS) were created with combination of quenching process and liquid-based exfoliation. The resulted MoS₂ Ns retained hexagonal phase (2H-MoS₂) and demonstrated thin layer structure with hidden wrinkle. The adsorption tests showed that this ultrathin MoS₂ NS revealed significant adsorption on heavy metals and dyes. The main adsorption mechanism implicated of physical hole-filling effects and electrostatic interactions [103]. Additionally, the MoS₂ NS can be combined with a facile one-step hydrothermal method without addition of templates or surfactants. Enlarged interlayer spacing and multiple defects on the basal planes characteristics of the products proved to be crucial in the removal of Cr(VI). Adsorption and reduction are the synergism of removal process, which the MoS₂ NS can adsorb Cr(VI) and reduce some fraction of adsorbed Cr(VI) to low toxic Cr(III) simultaneously. This process not only remove the Cr(VI) from aqueous solution but also diminished the toxicity of Cr(VI) [104].

MoS₂ NS has the capability of mercury removal, but successful attempt to apply MoS₂ NS for mercury removal are completely few, because the most of sulfur atoms are located inside the bulk of MoS₂ and therefore inaccessible for mercury ions. To resolve this problem, MoS₂ NS with widened interlayer spacing are able of mercury removal [105].

MFe₂O₄ (M = Mn, Co)-MoS₂-carbon dots (CDs) nanohybrid composites (MnFMC and CoFMC) [106] and millimeter-sized nanocomposite MoS₂–001 [107] were used to Pb²⁺ removal. Exploit of high surface available of MoS₂ for enhancing sequestration of target metal pollutants and cation substitution of abundant functional groups on the surface of CDs and MFe₂O₄ nanoparticles, the product composites demonstrate high adsorption performance and preferential Pb²⁺ sorption with high concentration of competing cations (Ca²⁺/Mg²⁺) [106]. MoS₂ nanosheet was loaded into a polystyrene cation exchanger D-001 by a facial hydrothermal method. The as-prepared nanocomposite demonstrated extraordinary adsorption capacity and high selectivity for Pb²⁺ sorption. In addition, NaCl-EDTANa₂ solution can regenerate this nanocomposite without any loss in adsorption capacity [107]. Besides the experiments, which is showing the strong selectivity/affinity of MoS₂ nanosheets toward Pb²⁺, a DFT simulation was conducted to study the adsorption mechanism (Figure 8a). The results demonstrated that the cation selectivity of MoS₂ is in sequence of Pb²⁺ > Cu²⁺ > > Cd²⁺ > Zn²⁺, Ni²⁺ > Mg²⁺, K⁺, Ca²⁺ (Figure 8b). The membrane made of layer-stacked MoS₂ nanosheets had a high water flux, but it is also effectively reduced the concentration of Pb²⁺ in drinking water [108].

Figure 8.

Investigation of Pb²⁺ adsorption. a) DFT simulation b) the maximum bond energy for the binding between cations and MoS₂ [108].

Metallic phase (1 T) and semiconducting phase (2H) of MoS₂ nanosheet were studied experimentally and theoretically for uptake of Pb²⁺, Cu²⁺, and Hg0 ions. Computational results showed that 1 T-MoS₂ could remove Pb²⁺ and Cu²⁺ more preferably that 2H-MoS₂. Facile hydrothermal reaction is the synthesis mechanism of both 1 T and 2H MoS₂ nanosheets. In addition, 1 T-MoS₂ has demonstrated superior properties over 2H-MoS₂, such as ultrafast adsorption kinetics and strong anti-interference activity toward other existing cations [109]. Also, mechanism of 1 T-MoS₂ in Hg0 uptake was studied by DFT simulation, and the results indicated that the adsorption between Hg0 atoms and the nanosheet is chemisorption mechanism and the strongest adsorption configuration is on the top of the Mo atom position [110].

Theoretical and experimental showed that MoS₂ surface could be more reactive in the presence of S-vacancy defects, and this leads to strong adsorption energy of H₂o, Hg²⁺ and O₂ on MoS₂ surface. In addition, surface oxidation can occur when there are enough reaction sites for the Hg²⁺ adsorption and surface oxidation simultaneously [111]. MoS₂ nanosheet with various pores were investigated to evaluate their selectivity to heavy metal ions (Figure 9a, c). When the pore size of membrane matched that of ion hydration shell, the ions were rejected. Figure 9b showed that because of fish-bone structure of MoS₂, the molybdenum pore (pore3) had a high adsorption, and water molecules could pass through it under external pressure. According to simulations, Hg²⁺ ions encountered a significant energy barrier and were unable to pass through created pores. The finding indicted that the type and size of pores affected the ability of reject mercury ions of MoS₂ nanosheet [112].

Figure 9.

Studying the adsorption capability of MoS₂ with different pores. (a) Different pores of MoS₂ (b) the water flux in various applied pressure for designed pores (c) schematic representation of the simulation framework [112].

2.4 Layered double hydroxides (LDHs)

A nanocomposite of graphene/MgAl-layered double hydroxides (G-MgAl-LDH) was created by hydrothermally reacting Al(NO₃)₃.9H₂O, Mg(NO₃)₂.6H₂O, graphene oxide (GO), and urea. In addition to efficiently reducing GO, urea can also cause the formation and in situ development of LDH crystallites on graphene nanosheets. In the nanocomposite, the LDH and graphene layers were both exfoliated. By heating G-MgAl-LDH to a higher temperature, it was simple to make the calcined G-MgAl-LDH, which could be employed as a nanoadsorbent to remove hexavalent chromium from an aqueous solution. As eliminating Cr(VI) from aqueous solution, calcined G-MgAl-LDH shown greater adsorption effectiveness with a lower dose when compared to virgin Mg-Al-LDH. The findings of the adsorption kinetics, isotherm, and thermodynamics showed that the surface adsorption of graphene, the memory effect of calcined LDH, and the synergistic contributions from each component were engaged in the adsorption process of Cr(VI) [113]. A study demonstrates the topological change of 3D hierarchical MgAl-layered double hydroxides in the presence of Fe₃O₄ nanoparticles to produce highly distributed magnetic hetero-nanosheets. The 10 nm Fe₃O₄ magnetic nanoparticles were disseminated throughout the surface of the MgAl-LDH lamella to create the platelet-like hierarchical nano-heterostructures. The biphasic self-assembled nanocomposite was effectively employed as an adsorbent for the ambient temperature decontamination of wastewater spiked with cadmium. The synthesized materials are good candidates for adsorbents in Cd(II) adsorption procedures with the highest adsorption capabilities, according to thorough evaluation (Figure 10). The regeneration research also showed how well the adsorbents performed following three cycles of adsorption/desorption [114].

Figure 10.

(a) Mechanism of adsorption for composite MgAl-LDH + Fe₃O₄ (b) the attraction effect of the magnetic field on magnetic MgAl-LDH + Fe₃O₄ composite. The magnetic fields are generated by NdFeB magnets [114].

By self-assembling between oppositely charged 2D nanosheets of LDH and –MnO₂ nanosheets in the presence of zwitterionic histidine biomolecules, layer-by-layer LDH/histidine/MnO₂ (LDH/his/MnO₂) nanohybrids were created. Exfoliation of highly crystalline MgAl-LDH and bulk MnO₂, respectively, resulted in the preparation of single-layer LDH and –MnO₂ nanosheets. The massive capacities for Pb(II) and As(V) and extremely high distribution coefficients (Kd) for Pb(II) and As(V) amply show the effectiveness of the suggested method for layer-by-layer synthesis of LDH/His/MnO₂ material. The results clearly demonstrate that layer-by-layer building of organic molecules and inorganic 2D nanosheets improved the simultaneous removal of the heavy metal ions Pb(II) and oxyanion of As(V) [115]. In a new study, MgAl-layered double hydroxide nanosheets were created using a straightforward hydrothermal method and assembled on graphene oxide and its magnetic product (Fe₃O₄@GO). The adsorbents combined diverse functional groups with sizable distinct surface area, according to characterization results. Under neutral and weakly alkaline conditions, GO and Fe₃O₄@Go successfully removed heavy metals ions. The primary mechanisms are the invertible substitution of divalent metal cations, metal hydroxides and metal carbonates precipitation reactions, and surface complexation of hydroxyl groups. So, new knowledge about heavy metal environmental remediation is provided by the improved adsorption capacity and mechanistic study [116].

A layered double hydroxide (MgAl-LDH) nanosheets that could effectively and simultaneously capture heavy metal cations and oxyanions from wastewater were created using pinewood sawdust-derived engineered biochar. The adsorption performance of loaded MgAl-LDH and biochar toward cationic and anionic contaminants, such as Pb²⁺ and CrO₄²⁻, is significantly improved by the synergetic effect. Complexations with surface functional groups were primarily responsible for the elimination of Pb²⁺. While CrO₄²⁻ can be transformed into Cr³⁺ by functional groups for removal of oxyanions, the resulting Cr³⁺, which could then replace Al³⁺ through morphic substitution, formed an MgCr-LDH structure. The prepared nanocomposites were found to be suitable for the maximum adsorption capacity of Pb²⁺ ions from an inaqueous solution due to their distinctive 3D porous network and abundance of oxygen containing functional groups. After 180 minutes of desorption experiments at pH 4.5, equilibrium was attained. Desorption experiments made it possible to regenerate adsorbents for use in next time and to recover copper ions [117].

An ultrathin MgFe-LDH nanosheets provide excellent Cu²⁺ ion mineralization in aqueous solutions and soils. Surface adsorption and isomorphous substitution are used in the removal of Cu²⁺ to transform MgFe-LDH into Cu(Mg). By adsorption, the Cu(Mg) Fe-LDH product can either be used directly to get rid of phosphate anions and zero dyes, or it can be converted into metallic copper through leaching-electroreduction. Even in acidic soils, it has been demonstrated that the release of Mg²⁺ ions by MgFe-LDH during Cu²⁺ removal promotes crop growth by acting as magnesium fertilizer [118].

Nanosheets conforming of two-dimensional nanomaterials made up of Ca²⁺, and Y³⁺ cations and carbonate(CO₃²⁻) anions in the interlayer with a invariant consistence and lengths of around 10 μ m have been successfully synthesized in a hydrotalcite subcaste structure, known as a layered double hydroxide, using a facile hydrothermal system. The as-prepared CaY-CO₃²⁻ layered double hydroxide materials demonstrate outstanding affinity and selectivity for metal ions including Cr³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺, Cd²⁺, and Hg²⁺ as well as metalloid As³⁺. The adsorption of all of the heavy metal ions from the aqueous solution was set up to be exceptionally rapid and highly selective, with more than 95% removal achieved within 30 min. The as-prepared adsorbent has excellent chemical stability, which is included they retain their well-defined lamellar shapes indeed under mildly acidic conditions [119].

Thio-functionalized adsorbents are different from conventional ones that are based on are oxygen group. In order to create layered double hydroxide (LDH) as Mn-MoS₄, MoS₄²⁻ anions were inserted into lamellar layers of LDH. This adsorbent was used to remove heavy metals because it was highly stable, effective, and selective. Mn-MoS₄ was ranked at the most effective adsorbent for the removal of such metals so far due to its record high distribution coefficient, fast kinetics, and enormous saturated uptake capacities for Hg²⁺, Ag⁺, and Pb²⁺. Additionally, Mn-MoS₄ is able to effectively control the concentrations of Hg²⁺, Ag⁺ ions from tap water, lakes, and industrial wastewater far below the limits for drinking water. It can also completely withstand the effects of extremely high background electrolytes (Ca²⁺, Mg²⁺, Na⁺, Cl⁻, NO³⁻, and SO₄²⁻). The dominant chemical adsorption mechanism, which coordinates the inner spheres of thio group as soft Lewis acids and Hg²⁺, Ag⁺, and Pb²⁺, is where these remarkable characteristics come from. More intriguingly, intercalated MoS₄ is protected by LDH levels. Mn-MoS₄ is made easier to store and use than other adsorbents owing to the ability to protect MoS₄²⁻ anions from oxidation [120]. A hierarchical porous hybrid monolith, also known as NiFe-MoS₄²⁻-LDH/CF, is created by homogenously immobilizing double hydroxide (LDH) nanosheets on a carbon foam (CF) substrate and then intercaling MoS₄²⁻ ions into the interlayers. The developed NiFe-MoS₄²⁻-LDH/CF hybrid monolith, which has ultrahigh sorption capacities, is found to be highly effective for sequestering Hg²⁺, Pb²⁺, and Cu²⁺ due to its abundance of binding sites, strong affinity and excellent pore accessibility. As evidenced by more than 99% removal rates within 5 min, the metal ions uptake kinetics of this nanoadsorbent are extremely high. More importantly, in the presence of different interfering ions, the developed adsorbent exhibits exceptional sensitivity for the target heavy metals with high distribution coefficients [121].

Due to poor adsorbent-metal interaction, typical sorption technologies are ineffective. In order to achieve this, a simple complexation technique was used to extract dangerous metals from an aqueous solution to create layered cationic platform material loaded with phosphonate. Sample experiments, detection methods, and simulation calculation were used to examine the interactions between clay and water in order to learn the removal mechanisms. The functionalized layered double hydroxide, in particular, had excellent chelation adsorption properties with Zn²⁺ and Fe³⁺, and model fitting showed that chemisorption and monolayer interaction were responsible for the process. The interaction region indicator, a novel concept, was used to describe poor interaction and coordinate bonds after the molecular dynamics simulation (Figure 11) of the interfacial interaction between the phosphonate and clay surfaces was evaluated. The orbital interaction diagram graphically conveyed the in-depth understanding of the chelation mechanism. Severe toxicity analysis, the adsorption column test, and the regeneration of the spent adsorbent all showed that the synthesized material has enormous potential for use in real-world toxic pollution treatment [122]. Phosphonates are environmental-friendly materials and can be possible employed to improve the loss efficiency of the clay materials. In an experimental and DFT study, Zn-Al-layered double hydroxide (LDH) intercalated with amino trimethylene phosphonic acid (ATMP) by a facile technique and employed as an adsorbent for Cu²⁺ and Pb²⁺ from wastewater. The weak interaction investigation indicated that interactions between ATMP and LDH are primarily dominated by H-bond. These benefits recommended that modified LDH can be a tempting adsorbent for adsorption of harmful metal ions [123].

Figure 11.

MD simulation of adsorbent of functionalized layered double hydroxide (a) final configuration of MD model (b) penetration analysis of the ESP on the vdW surface [122].

2.5 MXenes

Application of two-dimensional metal carbides and nitrides (MXenes) in water/wastewater treatment has been receiving more attention. Theoretical and experimental studies were done on mercury and copper ions with MXene nanosheets [124, 125, 126, 127]. The functionalization of MXenes to growth their balance at the same time as demonstrating excessive pollutant elimination can facilitate sustainable water/wastewater treatment processes. A quite solid magnetic titanium carbide (Ti₃C₂Tx) MXene nanocomposite (Figure 12a) (MGMX nanocomposite) changed into correctly synthesized via a facile hydrothermal method and changed into examined for aqueous-phase adsorptive elimination of mercuric ions. Great Hg(II) adsorption in a high concentration of pH conditions and an outstanding experimental Hg(II) uptake capacity were exhibited with the MGMX nanocomposite. In the adsorption/desorption investigation, the MGMX nanocomposite showed reusable capacity for 5 cycles. The MGMX nanocomposite is an efficient sorbent for the removal toxic Hg(II) for water purification due to stability, hydrophilic nature, available adsorptive surfaces, and easy separation after reaction [124]. Using a simple technique, two-dimensional titanium carbide MXene core (Ti₃C₂Tx) shell aerogel spheres (MX-SA) for mercuric ion removal were created (Figure 12b). The synthesized microspheres make an excellent adsorbent for the removal of heavy metals from water due to their distinctive internal structures, deep porosities, big specific surface areas, oxidized functional groups of MXene nanosheets, and available active binding sites. The adsorbent has excellent single- and multi-component removal efficiencies, with Hg²⁺ being 100% efficient and five heavy metal ions being more than 90% efficient. Under extremely low pH conditions (0.5–1.0 M HNO₃), the synthesized materials are highly effective at removing Hg²⁺ and have excellent reproducible qualities. This adsorbent can also be used in column-packed devices due to its small size and spherical shape [125].

Figure 12.

Experimental studies on Ti₃C₂Tx for Hg(II) removal. a) Magnetic titanium carbide (Ti₃C₂Tx) MXene nanocomposite b) two-dimensional titanium carbide MXene core (Ti₃C₂Tx) shell aerogel spheres (MX-SA) [124, 125].

Multilayered oxygen-functionalized Ti₃C₂ (also known as M-Ti₃Ox) nanosheets were ready to uptake Hg(II) from water. The M-Ti₃C₂ has shown to be extremely fast adsorption kinetics, impressively high capacity, high selectivity, and a wide working pH range (3–12) characteristics. This exceptional Hg(II) removal is explained by the distinct interaction (e.g., adsorption coupled with catalytic reduction), according to density functional theory calculations and experimental characterizations. In particular, Ti atoms on {001} facets of M-Ti₃C₂ prefer to adsorb Hg(II) in the form of HgClOH, which then undergoes hemolytic cleavage to form radical species (e.g., OH and HgCl). On the edges of M-Ti₃C₂, the HgCl radicals immediately dim the crystallize to form Hg₂Cl₂. Simple thermal treatment can effectively recover up to 95% of dimeric Hg₂Cl₂. Notably, M-Ti₃C₂ has oxidized to TiO₂/C nanocomposites as a result of the adsorbed OH and energy released during the distinct interaction. Additionally, TiO₂/C nanocomposites have demonstrated superior performance to Degussa P25 in terms of photocatalytic degradation of organic pollutants. M-Ti₃C₂ is a superb candidate for quick or urgent Hg(II) removal and recovery due to these exceptional qualities and its mercuric recyclable nature [126].

Due to their large specific surface area, hydrophilicity, and distinctive surface functional properties, delaminated (DL)-Ti₃C₂T_x displayed excellent Cu removal ability. Reductive adsorption of Cu²⁺, which resulted in the formation of Cu₂O and CuO species, was facilitated by oxygenated moieties in MXene layer structure. When compared to multilayer (ML)-Ti₃C₂T_x, DL showed a higher and quicker Cu uptake. The endothermic nature of the adsorption process was discovered by thermal analysis. Adsorption capacity of DL-Ti₃C₂T_x was 2.7 times greater than an activated carbon that was readily available on the market. These results show how tempting 2D MXene nanosheet are for removing hazardous metals from water [127].

The adsorption characteristics of Pb and Cu on recently synthesized two-dimensional materials MXenes, such as Ti₃C₂, V₂C₁, and Ti₁C₁, are studied using density functional theory calculation (Figure 13a). The majority of the studied MXenes have excellent capability to adsorb Pb and Cu, particularly Ti₂C₁. The practical groups attached to surface of MXenes are sensitive to both the adsorption energies and capacities. Ti₂C₁ remains stable at ambient temperature after adsorbing Pb atoms (Figure 13b), according to ab initio molecular dynamics (ab-initio MD) simulations. These results indicated that these newly emerging two-dimensional MXenes are promising candidates for wastewater treatment and ion separation [128].

Figure 13.

DFT simulation of different two-dimensional materials MXenes, such as Ti₃C₂, V₂C₁, and Ti₁C₁. (a) Schematic of fully Pb adsorbed Ti₂C₁ and Ti₂C₁H₂ that endure 15 ps ab-init MD simulation at 300 K. (b) Comparison of migration energy barrier Em of a single Pb atom on bare and H-decorated samples [128].

Not only nanosheets have advantages in heavy metal removal compared to other adsorbents, but also there are some disadvantages for each kind of nanosheets. Table 1 illustrates advantages and disadvantages of different nanosheets described in this chapter for heavy metal removals.

Nanosheet	Advantages	Limitations	Ref.
Carbon products	1.sp2 domains are re-established; 2.Electron transport property was improved; 3.Colloidal stability; 4.Higher dispersibility in water; 5.The abundance of oxygenated groups	1.Low colloidal stability; 2.Oxygen-containing functional groups are found to be of low density; 3. Limited sorption sites	[129]
BNM	1.Large surface area; 2.Polarity 3.Hydrophobic; 4.Excellent corrosion and oxidation resistance; 5.Very high permeability; 6.Stable under thermal and chemical shocks	1.Poor dispensability; 2.Low chemical reactivity	[130, 131, 132, 133]
TMDs	1.Large number of adsorption sites; 2.Low water affinity; 3.Photocatalytic properties; 4.High surface area; 5.Multilayer filtration; 6.Easy to functionalize; 7.High antibacterial activity with no toxicity; 8.Medium permeability; 9.Stable in acidic and alkaline solutions	1.Ineffective control over interlayer spacing; 2.Hydration of membrane is required at all times for efficient water transport; 3.Functionalization is required to achieve at least 50% salt rejection	[134, 135, 136]
LDH	1.Selective absorbability; 2.Liquid–solid separation; 3.Stability; 4.Recyclability	1.The lack of functional groups and structural components of pristine LDH; 2. Low surface area; 3. Small particle size	[137, 138]
MXene	1.Surface tenability; 2.Electrical conductive properties; 3.Hydrophilicity of MXene flakes; 4.High permeability; 5.Stable under thermal and chemical shocks	1.The inferior durability; 2.lesser-biocompatibility; 3.High tendency for aggregation; 4.Improper reusability; 5.Ineffective control over interlayer spacing	[139]

Table 1.

Advantages/disadvantages of nanosheets using in heavy metal removal.

3. Adsorption mechanism

Adsorption method is widely used for the removal of heavy metals from water and wastewater. Adsorption has some benefits over traditional techniques, such as reducing chemical and biological waste, being less expensive, being more effective, being able to regenerate the adsorbents, and possibly recovering metals [140, 141]. There are three main mechanisms for sorption of heavy metals and are physical sorption, ion exchange, and electrical interactions [142]. Study of adsorption isotherms and kinetics could introduce the sorption mechanism of heavy metals using nanosheet. Adsorption isotherm shows the adsorption amount of adsorbate using equilibrium concentration. Freundlich and Langmuir isotherms are the most famous isotherms, which are used to investigate the adsorption equilibrium data [143].

$$\frac{C_e}{q_e}=\frac{C_e}{q_m}+\frac{1}{K_L{q}_m}$$E1

$$\log q_e=\log K_f+\frac{1}{n}\log C_e$$E2

Eqs. (1) and (2) are the linear equation of Langmuir and Freundlich isotherms. In these equations$C_e$(mg/L) is the concentration of heavy metals ion after equilibrium, $q_e$(mg/g) is the adsorption capacity of the adsorbents at equilibrium,$q_m$(mg/g) is the maximum adsorption capacity,$K_L$(L/mg) is the Langmuir constant,$K_F$ is the Freundlich constant related to the sorption capacity, and n is the Freundlich constant associated to the adsorption intensity. Table 2 illustrates the result of adsorption study of different nanosheets for heavy metal removal.

Nanosheet	Target heavy metal	Langmuir isotherm			Freundlich isotherm			Ref.
		qm	KL	R^2	Kp	n	R^2
		mg.g^−1	L.mg^−1		mg.g^−1
Few layered magnetic graphene oxide	Cu(II)	2667.89	0.0178	0.984	14.372	0.862	0.999	[144]
	Cd(II)	2416.86	0.0571	0.959	48.811	0.508	0.995
Boron Nitride	Cd(II)	236	0.112	0.984	1.27	4.367	0.987	[145]
1 T-MoS2	Pb(II)	129.92	2.475	0.974	60.03	6.094	0.897	[109]
	Cu(II)	73.75	3.115	0.991	38.19	7.727	0.882
2H-MoS2	Pb(II)	55.11	2.385	0.802	23.71	5.838	0.997
	Cu(II)	41.80	0.0038	0.996	0.72	1.679	0.977
MgAl-LDHs	Cr(VI)	63.8	38.808	0.999	51.800	11.062	0.753	[146]
	Ni(II)	92.3	1.874	0.999	64.096	8.440	0.796
Two-Dimensional Ti3C2Tx MXene	Cu(II)	86.53	0.034	0.900	20.29	4.05	0.962	[127]

Table 2.

Study of isotherm of heavy metal removal with nanosheets.

Table 2 shows the correlation coefficient (R²) for Langmuir and Freundlich isotherm methods. According to the results, the Langmuir model is well fitted for MgAl-LDHs, and it shows that the adsorption of heavy metals had occurred uniformly and following the monolayer adsorption instead of multilayer adsorption [146]. The R² value of the Langmuir model was higher than Freundlich model in the cases of Pb(II) and Cu(II) adsorption onto 1 T-MoS₂ and of Cu(II) adsorption onto 2H-MoS₂, indicating the adsorption process was more likely to be the monolayer accompany with a fraction of multilayer chemical adsorption. However, when Pb(II) was adsorbed onto 2H-MoS₂, the higher R² value of the Freundlich model indicates the existence of heterogeneous surface adsorption and non-uniform active sites [109]. On the other hand, the correlation coefficient of Freundlich isotherm model for few layered magnetic graphene oxide, boron nitride, and two-dimensional MXene nanosheets is higher than Langmuir model. The Freundlich model assumes that the interactions at solid–liquid interfaces involve the homogenous adsorption of the adsorbate on the heterogeneous and multilayer surface of the adsorbent [127, 144, 145].

The adsorption procedure generally consists of four stages. A mass transfer from the outside takes place first (heavy metal ions from bulk solution to the surrounding layer of adsorbent). The external diffusion to the exterior surface occurs in step two, and the internal diffusion of the adsorbate to the internal adsorption location occurs in step three. Adsorption, which takes place on the surface of the adsorbents, is the last procedure. A novel assumption of the rate-determining phase leads to the development of a unique kinetic model. In order to understand the kinetics of heavy metal adsorption on adsorbents, pseudo-first order and pseudo-second order kinetic models are frequently used [143].

$$\log \left(q_e-q_t\right)=\log q_e-\frac{K_1}{2.303}t$$E3

$$\frac{t}{q_t}=\frac{1}{K_2{q}_e{}^2}+\frac{t}{q_e}$$E4

Eqs. (3) to (4) are the pseudo-first order and pseudo-second order. $q_t$ (mgg⁻¹) is the adsorption capacity at respected time,$K_1$(1/min) and$K_2$(gmg⁻¹ min⁻¹) are rate constants of pseudo-first order, pseudo-second order models. Table 2 shows the result of adsorption kinetics study of heavy metal removal using different nanosheets.

The pseudo-first order kinetic model implies that physisorption—in which sorption happens only through feeble Van der Waals forces and without the presence of chemical bonds—is the mechanism by which adsorption takes place. This form of adsorption is simpler to regenerate because the adsorption is also readily reversible. According to the pseudo-second order, two responses can happen one after the other or concurrently. The initial response occurs quickly and quickly achieves equilibrium. The second reaction, in contrast, proceeds slowly and takes longer to achieve balance. Pseudo-second order suggests that chemisorption is how the adsorption takes place. From pseudo-first order kinetics, it is inferred that the bonding results from electronic sharing and that the transfer between adsorbents and adsorbate is comparatively stronger than that of the physisorption way [143].

According to the correlation coefficients in Table 3, it can be concluded that the majority of the heavy metal adsorption on nanosheet adsorbents had fit the pseudo-second order kinetic model well. This suggests that the surfaces of these nanosheets are heterogeneous and that chemisorption is likely to be the rate-limiting step during the adsorption processes.

Nanosheet	Target heavy metal	Pseudo-first order			Pseudo-second order			Ref.
		K1	qe	R^2	K2	qe	R^2
		min^−1	mg.g^−1		mg^−1. g.min^−1	mg.g^−1
Few layered magnetic graphene oxide	Cu(II)	1.361	3345.63	0.878	0.0034	1146.91	1.000	[144]
	Cd(II)	1.119	30.47	0.766	0.0683	403.23	1.000
Boron Nitride	Cd(II)	0.524	111.7	0948	0.0001	193.1	0.999	[145]
1 T-MoS2	Pb(II)	1.243	55.20	0.997	0.088	55.68	0.999	[109]
	Cu(II)	0.364	50.66	0.935	0.011	53.42	0.995
2H-MoS2	Pb(II)	0.014	43.56	0.991	0.0003	49.85	0.998
	Cu(II)	0.153	4.18	0.961	0.041	4.59	0.985
MgAl-LDHs	Cr(VI)	0.040	4.253	0.913	0.021	89.286	1.000	[146]
	Ni(II)	0.032	2.238	0.984	0.013	70.423	1.000
Two-Dimensional Ti3C2Tx MXene	Cu(II)	0.408	1.147	0.24	0.0132	39.22	0.999	[127]

Table 3.

Study of kinetics of heavy metal removal with nanosheets.

Typically, electrostatic interaction, hydrophobic interaction, chelation, ion exchange, hydrogen bonding, precipitation, reduction, complexation, interaction, or weak Van der Waals interaction are involved in the absorption of heavy metals on adsorbents. One or a mixture of two or more of these interactions drive the adsorption process. The pH of the solution, the adsorbent’s textural characteristics, and the chemical structure of the target compounds are just a few of the variables that affect the process [147].

Table 4 shows the interaction mechanisms involved in the removal of heavy metals using various nanosheets as the adsorbents. Although there are many interaction mechanism between nanosheet adsorbents and heavy metals in aqueous solutions but the adsorption of heavy metals mainly involves the electrostatic attraction between the opposite charges, ion exchange, and surface complexation.

Nanosheet	Mechanism	Ref.
Few layered magnetic graphene oxide	The coordination of the lone-pair electrons in C═O- and C–O-related groups toward Cd(II) and Cu(II)	[144]
Boron Nitride	complexation between surface –NH2 functional groups and metal ions, ion exchanges between H of –OH and metal ions, and electrostatic interactions associated with combinations between -O- and metal ions	[148, 149, 150]
MoS2	Reduction-oxidation (redox) reaction	[109]
MgAl-LDHs	The synergistic effect of outer-sphere surface complexes on the adsorbent surface and ion exchange in the interlayer space	[146]
Two-Dimensional Ti3C2Tx MXene	The contribution of the surface functional groups	[127]

Table 4.

Study of mechanism of heavy metal removal with nanosheets.

4. Conclusion

Nowadays, application of nanosheets for water/wastewater treatment especially for adsorption of heavy metals has attracted more attentions. Because of diversity of nanosheets, which is due to different synthesis methods and novel nanoparticles, studying heavy metal removal improving. Due to their large surface area, porosity, distinct surface charge, surface functionality, and ion adsorption characteristics, nanosheets have strong adsorption capabilities. In this review, mechanism and effective parameters of using nanosheets sorbents in removing heavy metals from aqueous solutions using experimental and simulation methods studied. Synthesis, adsorption mechanism, adsorption performance, and effective parameters of carbon products (graphene), boron nitride materials (BNM), transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), and MXene as most common 2D nanosheets are studied.

Adsorption is the main mechanism of heavy metal ions removal in 2D nanosheets. Selectivity, adsorption/desorption performance, adsorption capacity, and regeneration experiments are the most characteristics parameters, which researchers have been studying using experiments and simulation methods. The adsorption behavior of nanosheets can improve by functionalizing with magnetic particles. In nanosheets with pores, size and chemistry of the pores exhibit to play an important role in controlling the water flux and ion rejection. For instance, nitrogen on the edges of pores were allowed higher fluxes than boron-lined pores. However, novel synthesis methods could improve the adsorption capability of the serpents. Oppositely charged 2D nanosheets of LDH and –MnO₂ nanosheets assembled in the presence of zwitterionic histidine biomolecules and a novel layer-by-layer LDH/histidine/MnO₂ (LDH/his/MnO₂) nanohybrids were created, which has a massive capacity of Pb(II) and As(V) removal. Novel nanosheets should be designed and prepared with improved adsorption/desorption cycles, easier separation from aqueous solution, non-toxic, and easier production method.

Density functional theory and molecular dynamics, as useful tools for simulation, can develop our insight about interaction between adsorbent and heavy metal ions and effective parameters. The superiority of this method in pre-production simulation has made it possible to rely on the use of this method in the design of new adsorbents.

Isothermal and kinetics models for heavy metal removal were studied, and Langmuir Freundlich models were best fitted to the adsorption mechanism of nanosheets. In addition, pseudo-second order kinetic model showed that chemisorption is the main rate-limiting step of adsorption process. Electrostatic attraction, ion exchange, and surface complexation are the main involving adsorption mechanism through heavy metal removal from aqueous environments.

Combination of experimental approaches and simulations for design of new nanoadsorbents has been growing, and study of interaction between nanosheets and heavy metal ions with consideration of effective parameters including environmental temperature, pH, material types, and heavy metal concentration on thermodynamics resulted of adsorbent mechanism should be organized.