4.2.3. Hybrid Nanomaterials

Hybrid nanomaterials can include only metallic materials and a mixture between metallic and non-metallic. Hybrid nanomaterials are strongly pursued due to the fact that these hybrid nanomaterials are able to exhibit improved, synergistic, or new properties that are not found in singular nanomaterials, metallic or non-metallic. Sharma et al. synthesized a metallic hybrid consisting of ZnO and TiO2 hybrid monolith absorbents via nanocasting, followed with calcination at 400 ◦C for 5 h. The hybrid nanomaterial proved to be a better adsorbent when compared to ZiO or TiO2 individually, as it improved the adsorption of Pb (II) and Cd (II) by more than 50% [83]. A hybrid material was developed with different ratios of clay, activated carbon and zeolite showed promising results for the adsorption of three different types of HM (Cd (II), Ba (II), and Cu (II)), which was produced via calcination [82]. Fu et al. developed a hybrid nanomaterial consisting titanate and lignin for the adsorption of Pb (II), Cu (II), and Cd (II) [120]. The hybrid nanomaterial exhibited an improved OH functional group when compared to singular nanomaterial, while exhibiting impressive reusability for HM adsorption. On the other hand, Yarandpour et al. developed a unique mesoporous poly (acrylic acid) (PAA)/dextran-polyaniline (PANI) core-shell nanofiber via the spinning process [121]. The hybrid nanofibers exhibited a fibrous morphology with a flake-like structure, developing a highly porous network throughout the nanomaterial. Subsequently, the nanofibers were able to adsorb more than 95% of Pb (II) ions from aqueous solution. Another hybrid material consisting of zeolite and silica oxide (SiO2) was developed via wet chemical synthesis for the removal or HM in low concentrations [122]. The study suggests that both SiO2 and zeolite work in tandem, where the outer layer consisting of SiO2 rapidly adsorbs Zn (II) and Pb (II), and it then transports it via diffusion to its zeolite core, greatly enhancing the adsorption capacity. The employment of a non-metallic material that can produce a porous network was also the theme of the study done by Lui et al., where a biomimetic SiO2@chitosan composite was effectively able to adsorb As (V) and Hg (II) when compared to singular nanomaterial [123].

#### *4.3. Synthesis and Modification of Nanomaterials*

The synthesis technique that was chosen for the formation of nanomaterials plays a significant role in the final characteristics exhibited. Generally, the synthesis route of nanomaterials follows two ways, which are top-down or bottom-up [110]. Figure 6 illustrates the building mechanism of the nanomaterial on both routes.

**Figure 6.** Nanomaterial synthesis route of nanomaterials following top-down, or bottom-up. Adapted from [107], with permission from CHEMIK, 2014.

The synthesis technique controls the design and building mechanism of nanoparticles to produce materials of precise structure, shape, and size. Some of the synthesis technique includes wet chemical synthesis, the solution combustion technique, hydrothermal technique, sol-gel technique, and mechanical milling [110]. However, there are three prominent methods i.e., sol-gel, hydrothermal, and wet chemical synthesis. These methods are prominent due to the fact that they are easily produced and facile, do not require expensive equipment's, and are highly reproducible. Sol-gel is one of the cheapest ways to produce nanomaterials with precise control on the stoichiometry of nanomaterial. The nanomaterials that were synthesized in this method generally had a small particle size and narrow size distribution due to their linear growth rate across the gel. As the name suggests, the precursors, commonly being metal alkoxides or metal chlorides, are hydrolyzed with water and alcohol and are then mixed and allowed to form a gel-like structure, before proceeding with calcination or sintering to remove the gel structure, leaving defined nanomaterials. Luu et al. synthesized a hybrid material that consisted of iron and TiO2 via the sol-gel technique. The formed nanoparticle exhibited a size of 19.5 nm and a surface area of 42.9 m<sup>2</sup>/g [124]. Another study for the formation of the same nanomaterial that was produced by Luu et al. exhibited a particle size of in the range of 19 to 7 nm and a surface area of between 84 and 182 m<sup>2</sup>/g. The results indicated that the particle size highly governed the surface area, where smaller nanoparticles possessed a larger surface area [125]. Aware et al. produced a hybrid material of Zn and TiO2 via the sol-gel method. The nanoparticle that was produced exhibited a particle size in the range of 12.6 to 18.1nm and a surface area value between 43.376 and 63.667 m2/g [126]. Another popular method in nanomaterial synthesis is a hydrothermal technique. This is a versatile and simple method in the synthesis of nanomaterial under the high pressure and temperature closed condition. Precursors are commonly stirred with a strong alkali solution (NaOH) and are stirred until the precursors have reacted with the alkali reagent. The materials are then transferred into a Teflon lined autoclave stainless steel block and reacted under a predetermined temperature and time. The produced sample will then be washed with water to remove the excess sodium hydroxide (NaOH), and then dried and ground to obtain a high yield of nanomaterials. This method is popular because of its high degree of controllability [127]. Nanomaterials of different physical properties, including size, structure, and surface area can be easily obtained by small tweaks in the synthesis parameters. TiO2, which has a globular shape, can be changed into TiO2 nanotubes via the hydrothermal method, increasing its surface area from 40 m<sup>2</sup>/g to 200 m<sup>2</sup>/g [128]. Hydrothermal can also be used to produce hybrid nanomaterials in a facile way. The versatility of the hydrothermal technique allows it to form metal and non-metallic hybrid nanomaterials. Zhang et al. produced a hybrid nanomaterial that consists of Ag and nanocellulose, which exhibited high bactericidal efficiency against both bacteria and fungus with an average nanoparticle size of 86 nm [129]. Hydrothermal can also provide an alternative for synthesis of novel material, such as graphene oxide (GO), which is commonly produced via the Hummer's method [130]. In line with the simplicity of the synthesis method, wet chemical synthesis, or also known as the liquid phase synthesis technique, has also garnered the interest of researchers due to its simplicity while providing a large degree of control on the final physical characteristics of nanomaterials produced [131]. The process commonly involves the mixture of solutions of different ions at quantified volumes or molarity and provided a controlled amount of heat to initiate the formation of nanomaterials via precipitation. The excess solution is then washed away, and the precipitated nanomaterial is dried and ground/milled. This method offers control in nanomaterial stoichiometry, similar to the sol-gel method, and it can be conducted at a low temperature, reducing the energy consumption [131]. Table 3 shows some of the recent literature on the synthesis of hybrid nanomaterials using various techniques.


**Table 3.** Synthesis of hybrid nanomaterials using different techniques.

#### **5. Recent Progress and Performance Evaluation**

#### *5.1. Adsorption of Heavy Metal*

Research that was conducted by Mousavi et al. was successful in developing a novel bifunctional ordered mesoporous silica via wet chemical synthesis, creating a porous nanomaterial with a highly arranged network of compact silicone group (S-O) [138]. The developed nanomaterial was able to adsorb 98.6% of Cu (II) and 98.09% of Zn (II) ions. Sharma et al. successfully synthesized a hybrid ZnO and TiO2 monolith via the nanocasting technique. The hybrid material was able to adsorb Cd (II) via monolayer adsorption with an adsorption capacity of 786 mg/L, as compared to 643 mg/<sup>L</sup> for ZnO only monolith [83]. Castro et al. produced a biogenic iron compound via HM adsorption using biogenic iron sourced from mining wastewater that was able to absorb As (97.9 mg/L), Cr (20.1 mg/L), Zn (60.3 mg/L) and Cu (95.5 mg/L) in the optimum condition [84]. Activated carbons that were produced from nutshell removed almost 100% of Pb (II), 90–95% of Cu (II), and 80–90% of Zn (II), while biochar that was produced by pyrolysis of waste sludge was able to remove Cu (II), Cd (II), and Pb (II) by more than 90%. Based on the current research trends, researchers are exploring new and environmentally friendly carbon source to produce nano-sized carbon-based adsorbents that can improve HM adsorption due to the key features, such as the high surface area and porosity, ease for surface alkali functionalization, and being economically and environmentally viable. Deng et al. prepared a review on the utilisation of waste, such as palm shell, waste plastic, biomass, and so on to develop carbon based nanomaterials, including CNT and graphene [139]. Another emerging trend among research in the development of efficient adsorbents are the fabrication of various hybrid materials, which is expected to provide a synergistic effect in enhancing particle HM adsorption capacity. Kumar et al. developed hybrid carbon nanofibers and TiO2 polyacrylonitrile (PAN) membranes via electrospinning to adsorb HM ions, such as Pb (II), Cu (II), and Cd (II) [140]. The developed CNFs/TiO2–PAN hybrid membranes exhibited maximum adsorption of around 87%, 73%, 66%, for Pb (II), Cu (II), and Cd (II) metal ions, respectively. Another hybrid material comprised of Fe3O4 and *Raphiafarinifera*, a mangrove plant developed via the chemical co-precipitation method exhibited superior adsorption for Pb (II), Cu (II), Ni (II), Zn (II), and Cd (II) [141]. The hybrid material showed greater HM ion adsorption when compared to the singular particles, exhibiting the synergistic effect of the hybrid nanomaterial. Jiang et a. developed a hybrid graphitic carbon nitride nanosheet for the adsorption of both cationic and anionic HMs from wastewater [89]. The maximum adsorption capacities of Cd (II), Pb (II), and Cr (VI) on the g-C3N4 nanosheets are 123.205 mg/g, 136.571 mg/g, and 684.451 mg/g, respectively. The developed adsorbent can also be reused up to 10 times while achieving an adsorption capacity of 80% or more in 10 cycles.

Table 4 tabulates the synthesis technique and features that are imparted due to the formation of nano-sized adsorbents, which includes improved electrical conductivity, partial cation exchange, highly selective adsorption, high particle porosity, and surface chemical activation. An increase in surface area allows for more adsorption sites, which directly increases adsorption capacity. Porous structure also exhibits the same feature, in which the availability of binding sites for HM ions increases with the increasing porous structures. Nanoparticles are also functionalized to produce highly selective adsorbents, which are capable of adsorbing particular HM ions in a mixed metal water system. Chen et al. demonstrated that, by doping SiO2 with polythiophene (PTh), the composite recorded an impressive selectivity towards Zn (II) ions in multiple ion solutions (with *p*-value > 0.8 as compared to 0.3 of Pb (II)), which has also increased the adsorption capacity [142]. It was postulated that PTh has an a ffinity towards Zn (II) ions, which contributed to the overall selectivity. These features are important in improving the adoption of cationic HMs, such as increasing the strength of adsorption and increasing the adsorption capacity due to the porous structure of nanoparticles as adsorbents. Additionally, literature showed that there are prominent metal ions that are consistently used as model ions, including Cu (II), Pb (II), Co (II), Cr (II), and Zn (II) [143]. Other notable HM ions that are studied upon include Li (I), As (II), and Ni (II). In regards to adsorption capacity, the results vary, which is expected when presuming that all of the nanomaterials produced have varying features that highly influence the adsorption of specific HMs. The employment of hierarchically mesoporous carbon was able to adsorb Cu (II) up to 215.0 mg/g, while PTh/SiO2 nanoparticles were only able to adsorb 35.3 mg/g of Cu (II) [142].

A hybrid nanoadsorbent consisting of Fe and GO was synthesized to produce a magnetic graphene oxide (MGO) [37]. The MGO exhibited superior adsorption of Cd (II) and As (V) as compared to singular GO. The superiority of MGO was mainly attributed to its high dispersibility, thin nanosheets that were exhibited by GO, the synergistic e ffect that resulted from the electrostatic attraction o ffered by Fe, and various O-containing functional groups due to the surface functionality of GO. Marciniak et al. synthesized oxidized mesoporous carbon nanoparticle via the hart template method for the adsorption of Ni (II) and Cd (II) [144]. The nanoparticles were oxidized in di fferent degrees to vary its oxidizing functional group. The results indicated that the nanoparticle with highest surface functional groups of acidic character was able to adsorb more HM ions. Similarly, research that was conducted by Li et al., where they prepared a sludge-based activated carbon impregnated with HNO3 for the removal of Pb (II), was highly governed by the surface functional group [145]. The results indicated that modified adsorbent rich with carboxyl group (R-COO−) was able to adsorb up to 98% of Pb (II), as compared to an unmodified particle which was only able to adsorb 83% of Pb (II). Huang et al. prepared a core-shell Fe3O4@polytetramethylene terephthalate (PTMT) composite magnetic microspheres for the adsorption of HM in highly saline water [38]. Surface amine functionalization greatly reduced particle agglomeration, which was complicated by nanomaterials with large surface area, while the incorporation of magnetite enhanced the recovery e fficiency of the adsorbent while using a simple magnet. In addition, the shell structure also increased the particle surface area and porosity, increasing the adsorption sites, which improved the adsorption capacity of Hg (II) and Pb (II). Evidence of the impact that unique structure has on the adsorption of HM can also be seen in the study done by Ma et al., where a waste cotton fabric based double network hydrogel was developed for HM removal [146]. The experimental data suggested that the porous and sheet-like laminar structures that were exhibited by the nanostructured adsorbent were the reason behind the fast kinetics of Cu (II) and Cd (II) sorption equilibrium displayed. Similarly, Wang et al. developed a hybrid graphene oxide/silk fibroin hybrid aerogel [147]. The synthesized hybrid nanomaterial exhibited a porous network, which helped the HM ions to easily di ffuse into the aerogels, while the silk fibroin exhibited grea<sup>t</sup> chelating feature, holding onto the HM ions for excellent adsorption capacity (Ag (II): 195.8 mg/g, Cu (II): 72.1 mg/g, Cu (II): 83.4 mg/g). Table 4 shows a compilation of recent literature on the development of various types of adsorbent for the removal of various HM ions from wastewaters.


**Table 4.** Recent literature on the development of various types of adsorbent for the removal of HM ions from wastewaters.

#### *5.2. Photocatalysis of Heavy Metal*

Recently, an iron oxide (II) bismuth carbonate hybrid photocatalyst was developed and it showed excellent photocatalytic activity towards the reduction of carcinogenic and mutagenic Cr (VI) to nontoxic Cr (II) [151]. Another study that was done by Du et al. showed excellent photocatalytic Cr (VI) reduction to Cr (III) [152]. Kumar et al. employed a hybrid WO3/reduced graphene oxide (rGO) nanocomposites photocatalyst that exhibited a photocatalytic reduction of Cr (VI) following first-order kinetics and rate constants were found to be 0.0084 min−<sup>1</sup> [153]. Even though traditional semiconducting photocatalyst exhibited good HM ion reduction, they are exclusively responsive towards dedicated UV light sources due to their large band gap. In order to overcome this, traditional photocatalyst were doped/modified with metallic or non-metallic materials to impart visible light sensitivity, producing nanocomposite photocatalyst. The presence of a froing atom on the surface photocatalyst also reduces the band gap, which reduces the energy that is required to excite an electron for photocatalyst initiation. It also increases the rate of photocatalysis due to the presence of a charge carrier [154]. This allows the photocatalyst to absorb photons from visible light sources to excite an electron and initiate the photocatalytic reaction. Figure 7 shows the photoreduction performance of Cr (VI) in a single system and in coexistence with an organic pollutant.

Lie et al. conducted a study on the simultaneous photoreduction of Cr (VI) and the photodegradation of Rhodamine B (RhB) to in an independent and dependent system. The results that are depicted in Figure 7a,b indicates that the photoreduction of Cr (VI) in the presence of RhB is significantly better when compared to photoreduction in a solution only containing Cr (VI) ions [155]. The research work for the simultaneous degradation of phenol and photoreduction of Cr (VI) that was conducted by Yu et al. explained that the presence of phenols improves the photoreduction of Cr (VI) as the electron-hole is consumed by phenol. This suppresses the recombination of electron and holes, which in turn accelerates the photoreduction of Cr (VI) [48]. The same reasoning has been used in other research, where the photoreduction of Cr (VI) was accompanied by the degradation of various types of organic pollutants to sustain the reaction, such as 2-Mercaptobenzothiazole (MBT) [156], sulfamethoxazole [157], RhB [98,136], and chlorinated phenols [158]. The photoreduction of both Pb (II) and Cu (II) was attempted together with the photodegradation of RhB while using a modified Chitosan-Gelatin @ zirconium (IV) selenophosphate nanocomposite ion exchanger. The hybrid photocatalyst was able to retain more than 90% of the HMs, while degrading more than 80% of RhB within 120 min [159]. Another example was exhibited in a research work that was conducted by Du et al. Their investigation showed that the addition of different organic compounds, like citric acid, oxalic acid, and diclofenac sodium mimics the function of a hole scavenger, which increases the photocatalytic Cr (VI) reduction activity. The ability of hole scavengers to consume the photoinduced holes that are produced by the photocatalyst upon light irradiation is accepted as a plausible reason. This allows for more electrons to escape from pair recombination and become available for the reduction of Cr (VI) ions [152]. Photocatalysis have also shown the potential to photoreduce Cr (VI) into Cr (III) while exhibiting potential to kill harmful bacteria, such as *E. coli* (89%) and *S. aureus* (81%) within 10 min of irradiation on photosensitized TiO2 nanofibers [160]. In recent years, the development of binary (bi-material) and ternary (tri-material) photocatalyst has intensified in search of combinations to produce photocatalyst that is responsive towards visible light, low recombination rates, and increased surface reactive sites.

**Figure 7.** (**a**) Photoreduction of Cr (VI) in the presence and absence of Rhodamine B (RhB) and (**b**) removal rate of both Cr (VI) and RhB at different individual cycles. Adapted from [155], with permission from Elsevier, 2019.

An aggregation free CeO2/SnO2/rGO was developed via the hydrothermal process for the simultaneous degradation of MB and photoreduction of Pb (II) and Cd (II). The photocatalyst exhibited close to 100% degradation of methylene blue (MB) and a HM ion reduction of close to 80%. The rGo acts as a charge suppressor and, due to its excellent electrical conductivity, it can trap the electron that is excited from the conduction band (CB) of SnO2 and CeO2, allowing for longer photocatalyst activation. The exploitation of rGO, as an excellent charge separator and trapping site to sustain photocatalytic formation of ROS, was also explored by Bai et al., where a Red Phosporus (RP)-MoS2/rGO ternary photocatalyst was developed via a facile two-step hydrothermal [161]. RP and MoS2 exhibited excellent photocatalytic activity and increased the number of excited electrons/holes under visible light irradiation, while rGO was responsible for enhancing charge separation to sustain and provide stability in electron recombination. These factors synergistically assisted RP-MoS2/rGO to perform impeccably, reducing Cr (VI) by up to 95%. Another nanomaterial that exhibits charge trapping capability is carbon nitride (C3N4), which is a relatively new nanomaterial. Liu et al. developed a binary RP/g-C3N4 photocatalyst for visible light photoreduction of Cr (VI). The catalyst exhibited good stability for removing 85% Cr (VI) and 90% RhB, even after four times of recycling, owing to the effective separation and rapid transfer of e<sup>−</sup>/h<sup>+</sup> pairs between RP and g-C3N4. With the low band gap exhibited (1.35 eV), -O2– can be generated in this system for the lower CB position of RP, where the excited electron can jump into the CB of RP, sustaining photocatalytic activity for a longer period. The synergistic redox reaction allows for Cr (VI) to reduce into Cr (III) [155]. Ye et al. managed to synthesize a

ternary Ag/Bi4O7/g-C3N4 nanosheets Z-scheme heterojunction photocatalyst for the photoreduction of Cr (VI) while using a combination of thermal polymerization, hydrothermal, and calcination [162]. The structure exhibited the deposition of Ag and Bi4O7 on sheets of g-C3N4, similarly to the work that was conducted by Liu et al. The relatively low band gap of both Bi4O7 (1.89 eV) and g-C3N4 (2.97 eV) enabled the ternary photocatalyst to exhibit excellent visible light responsive capabilities. Additionally, the presence of g-C3N4 sheets o ffered excellent charge separation capabilities, thus enhancing the photoreduction of Cr (VI) [163]. Jing et al. developed a three-dimensional PANI/MgIn2S4 nanoflower photocatalyst via electrostatic adsorption between PANI and MgIn2S4. PANI is known to be a good polymeric electric conductor due to the presence of heteroconjugated π bond, making it a suitable candidate as a dopant for photocatalysts [164]. The ternary photocatalyst was able to reduce Cr (VI) by up to 95% within 15 min of light irradiation. The photogenerated electrons in the valence band of MgIn2S4 and the highest occupied molecular orbital (HOMO) of PANI could be excited to the corresponding conduction band and the lowest unoccupied molecular orbital (LUMO) when it assimilates photons with energy larger than the band gap, while the holes were generated in the valence band of MgIn2S4 and the HOMO of PANI. This interaction accelerated the photoreduction of Cr (VI) and the overall photoactivity [165]. Table 5 shows the development of nanocomposite photocatalyst for the reduction of various types of HM.


**Table 5.** Development of nanocomposite photocatalyst for the reduction of various types of HM.

#### *5.3. Membrane Composite for Removal of Heavy Metal*

Nanomaterials are commonly added into polymeric membranes during preparation, where it is dispersed into the dope solution before the membranes are formed via the dry-wet phase inversion technique. The dispersion of nanomaterials across the membrane matrix or deposition on the membrane surface, as in the case of thin-film nanocomposite (TFN) membrane, has significantly altered the permeation and rejection performance of the membranes. Lakhotia et al. developed a TFN membrane with FeO nanoparticle dispersed on the membrane selective layer. As the concentration of FeO nanoparticles increased on the membrane surface, the hydrophilicity and surface charge of the nanocomposite membranes were e ffectively enhanced. The membrane flux was enhanced from 27.46 to 36.85 <sup>L</sup>/m<sup>2</sup> h and high rejection of Mg (II) and Na (I) rejection (>90%), whilst membrane without the FeO nanoparticle rejected salt in the range of 65%. An AlTi2O6 incorporated polysulfone (PSF) composite membrane exhibited improved hydrophilicity with the water contact angle being reduced from 73◦ to 51◦ [173]. The presence of AlTi2O6 hindered the flow of non-solvent during membrane casting, which created more membrane pores on the surface. Polypyrrole (PPy)@Al2O3 was added into a polyethersulfone (PES) membrane matrix for the removal of Cu (II). The addition of PPy@Al2O3 improved the membrane water transport capacity, reduced membrane surface roughness, and eventually mitigated membrane fouling [174]. The nanocomposite membrane also exhibited improved Cu (II) rejection (25% to 81%). Ghaemi et al. developed a polyaniline modified GO nanoparticle that was incorporated into a PES membrane for the remediation of water laden with Pb (II), which also showed good metal ion removal because the incorporating of PANI modified GO increased the viscosity of the membrane dope solution, which in turn reduced the mean pore size of nanocomposite membranes [175]. The filtration of HM-laden water produced clear water containing trace levels of HM as permeate, in which the HMs were concentrated in the retentate.

A study done by Nasir et al. showed that the incorporation of hydrous iron manganese nanoparticle into a PSF membrane was able to adsorb As (II) up to 4.1. mg/g [176]. A zirconia polyvinylidene fluoride (PVDF) composite membrane that was developed by Zheng et al. was able to adsorb As (V) at 25.5 mg/g, whilst maintaining membrane flux of 177.6 <sup>L</sup>/m<sup>2</sup> h [69]. Another work that was done by He et al. incorporated zirconia into a PSF hollow fiber membrane for the removal of As (V) [177]. This work exhibited a much higher adsorption capacity of 131.8 mg/g at zirconia/polymer ratio of 1.5. In a separate study, zirconia was modified while using phosphate solution to produce a PVDF membrane with surface coated nanomaterial that performed excellently for the removal of Pb (II) ions [178]. The adsorption capacity that was recorded by the optimized membrane was 121.2 mg/g at the pH of 5.5. The modification also allowed for selectively improving the membrane adsorption to favor Pb (II) over Zn (II) ions. Delavar developed HMO nanoparticle incorporated membrane for the removal of Cd (II) and Cu (II), where it was revealed that the loading of nanomaterial into the membrane matrix largely influences the adsorption performance. The polymeric structure of the membrane only serves as a pathway for untreated water to pass and to immobilize the adsorbents. Membrane that was loaded with 3 wt% adsorbed 31.3 mg/g of Cd (II) and 29.7 mg/g of Cu (II), whilst 10% loaded membrane adsorbed 33.18 mg/g and 30.6 mg/g of Cd (II) and Cu (II), respectively, asserting this notion [179]. In addition to this, another work on the employment of HMO as a nanoadsorbent, immobilized in a PES membrane, was able to adsorb Pb (II) at an uptake capacity of 204 mg/g, highlighting the potential of HMO as an excellent candidate for HM adsorption [180]. Table 6 shows the details of membranes incorporated with nanomaterials that were developed for the rejection of HMs.


#### **6. Conclusions and Future Perspective**

The advances of nanomaterial in terms of development of materials with novel structure, characteristics, and hybrid nanomaterials have further revealed unique and amplified properties. The ability to develop nanomaterials atom by atom with highly controllable methods is behind the rise of novel nanomaterials. The review highlights the roles of nanomaterials in heightening the efficiency of the above-mentioned technologies for HM removal. However, there is plenty of room for future development and research to discover new hybrid nanomaterials for the removal of HM from wastewaters. Removing HM ions from water sources is a critical task. Removing and destroying HM is difficult due to its stability in ionic form. However, recent advances in using various types of nanomaterials that were developed with HM removal indicate the potential that researchers see in nanomaterials. Of all the proven methods to remove HM, three prominent ways i.e., adsorption, photocatalysis, and membrane separation have been comprehensively reviewed. Undeniably, nanomaterials have enabled significant breakthrough that was made in these strategies. For the adsorption of HM, researchers have developed plenty of nanomaterials, singular or hybrid, which work as nanoadsorbent to trap HM from water bodies, as displayed in Table 4. Even though all of the nanomaterials excel as absorbents, more emphasis needs to be placed in the development of functional HM adsorbents using waste products via the facile synthesis method. The reason behind this notion is that greater research emphasis is required to produce nanomaterials that can be easily scaled up for real-life usage. For instance, producing a very large amount of absorbents for effective treatment of large water bodies, such as lakes and rivers. Current trends in developing adsorbents from waste, such as from biochar or sludge waste, can be studied in depth in terms of upscaling. It needs to

be highlighted that, whilst it is grea<sup>t</sup> for researchers to pursue unique and novel nanomaterial with new features that can aid in HM removal, it is important to consider the e ffectiveness of the approaches for the large-scale removal of HM. Photocatalysis has shown grea<sup>t</sup> promise in reducing HM ions while destroying organic pollutants, which exhibits the versatility of semiconducting nanomaterial as the total remediation of wastewater. Great strides have been taken in sensitizing traditional semiconductors to respond towards visible light source by doping. However, doping needs to be focused on utilizing non-noble metals and non-metal dopants, which are much more abundant in nature when compared to rare and noble metals. In addition, future research to be conducted can place better emphasis on the doping method to improve the precision of dopant incorporated into semiconductor nanomaterials. On the other hand, the feasibility of employing plasmonic metal nanomaterial for the reduction of HM ions should also be explored. Plasmonic metal nanoparticles are known as light harvesting materials that exhibit the ability to harvest visible light photons through the excitation of localized surface plasmon resonance (LSPR). Currently, the studies on this nanomaterial to reduce HM ions in waterways are very scarce. The unique characteristic of the plasmonic metal nanoparticle is expected to spur interests in this field [188,189]. The incorporation of functional nanomaterials has enhanced polymeric membrane separation and permeation e fficiency, where membrane separation or adsorption are two viable ways of removing HM ions from water sources. However, the agglomeration of nanomaterial in the membrane matrix is a common problem, which limits the loading of nanomaterial and reduces the efficiency of nanomaterials. To counter this, steps to functionalise the nanomaterial surface to inhibit agglomeration (employing silane agents, improved surface charge) is welcome progress in further maximizing the synergistic of nanomaterial and membrane technology.

**Author Contributions:** M.N.S. and P.S.G. conceived, designed and wrote the manuscript, while W.J.L. and A.F.I. were responsible to visualization, editing and funding acquisition.

**Funding:** This research was funded by the Ministry of Higher Education (HiCOE Grant, Vot no; 4J183, Fundamental Research Grant Scheme (FRGS, Vot no; 5F005).

**Acknowledgments:** The authors would like to express their sincere gratitude to Advanced Membrane Technology Research Center (AMTEC) and University Teknologi Malaysia for their support.

**Conflicts of Interest:** The authors declare no conflict of interest.
