**1. Introduction**

In a bid to bolster its economic growth, heavy industrialization across developing countries is rampant. One hindsight with this phenomenon is the creation of many types of poorly managed waste that eventually seep deep into the environment via air, soil, and water. The damage of natural resources takes place as the direct consequence of the release of hazardous substances [1]. In particular, water is the prominent recipient of many types of pollutant. The continuous pollution of water, in tandem with the increased demand for fresh water due to industrialization and population growth, has strained water resource to a breaking point. Various types of pollutants, such as natural organic matters (NOM), oil, pathogens, and heavy metals (HM) have badly a fflicted water. HM pollutions has been a prominent issue, as the HM infusing into the water sources can be produced from various human activities, including mining, agriculture, and electronic industries [2]. It is imperative that HM contents in waterways are controlled e fficiency in view of their poisonous and toxic nature towards all organisms. In a broad context, HMs are a group of trace metals with an atomic density of less than 5 ± 1 g/cm<sup>3</sup> [3]. Cadmium, manganese, iron, arsenic, and mercury are some of the prominent HMs that are commonly found in wastewater [4]. These metals usually exist in the form of ions in waterways and soil. They can pose a health hazard to both humans and the ecosystem via avenues, such as direct ingestion or in contact with contaminated water or soil, drinking water that has been contaminated with HM, ingestion of foodstu ff laden with HM (plants or aquatic life), as well as the accumulation of HM via the food chain. Critically, organisms are unable to metabolize and excrete HMs out of the body. The presence of HM ions in water samples imparts grea<sup>t</sup> ecological impact due to its toxicity and bioaccumulation, as aquatic life is known to accumulate significant concentration of metals in water where the presence of such metal in water samples are below the detection levels [5].

The sources of HM ions in the environment can be focused on two origins, i.e., natural sources and human activities. The former source includes phenomena, such as landslides, weathering, and volcanic eruption. These activities have significantly contributed to HM pollution, as these events release trapped HM ions into the environment. Anthropogenic activities are also a major source of HM pollution. Activities, such as mining, smelting operations, industrial production and manufacturing industry, and agricultural use of HM in the form of pesticides are major causes of HM pollution on soil and in water. Controlling the leaching of HM into the environment is important. Various legislations and laws have been enacted to control the number of pollutants that are released by industry players into the environment. Table 1 shows the maximum level of HM content in water samples across different local and global agencies.


**Table 1.** Maximum level of heavy metal (HM) content in water samples. Table was reproduced from [6], with permission from EDP Sciences, 2017.

> Remark: NM refers to 'not mentioned'.

On the other hand, the remediation of these HMs is also essential in reducing the impact of HM on human health. As of now, numerous strategies have been developed to separate these HMs from water sources. Remediation techniques include the employment of absorbents, coagulation of HM ions, chemical precipitation, membrane filtration, electrodialysis, and photocatalysis. Each of these techniques has its unique advantages as well as disadvantages that are particularly associated to their efficiency for large scale HM remediation. Unique nanomaterials that were developed by researchers are at the center of these treatment methods. The discovery and the subsequent extensive research on nanomaterials developed in different geometries have brought about different physicochemical properties that cannot be expressed by bulk materials. Nanomaterials, such as carbon nanotubes (CNT) [7], graphene [8], titania nanotubes (TNT) [9], hybrid metal-non-metal nanomaterials, such as graphite silica [10] and graphene oxide-magnetite nanocomposite [11], and other metal oxide-based nanomaterials have been extensively used to remove HM ions from polluted waters, with a varying degree of success. These nanomaterials can be advantageously used to remove HM ions via adsorption, owing to their large adsorption capacities. Especially, nanomaterials of different geometries have exhibited high HM removal efficiency due to unique properties, such as large surface area, specific

surface charge values, surface functionality, and HM ions binding capabilities. These nanomaterials can also be incorporated with other treatment techniques, such as adsorptive membranes and composite membranes to work synergistically and to further improve HM ion removal efficiency.

Over the years, a number of comprehensive reviews have been made in regard to nanomaterials and their applications for environmental remediation. Khin et al. compiled a comprehensive review of the application of nanomaterials as a viable solution in the removal of various types of pollutants and biological contaminants [12]. Azzouz et al. published a more recent review on the utilization of nanomaterials as sorbents in a solid-phase pollutant extraction system for environmental samples [13]. The author discussed the use of various types of nanomaterials as potential sorbents for analytical applications. Jeevanandam et al. prepared a review on the history, sources, toxicity, and regulations of nanoparticles and nanostructured materials [14]. Despite the efforts that have been made in this area, very small number of reviews has been made to discuss the significant role of nanomaterials in removing pollutants from wastewater [15]. It is worth providing insights regarding the functionality of nanomaterials of different compositions, characteristics and structures, as well as their key roles in the removal of HM ions, particularly in different remediation techniques from various sources.

In this paper, an overview of prominent sources and types of HM containments in the water sources is first provided. Next, various nanomaterial synthesis techniques were used to prepare unique nanomaterials especially for HM ions removal from water sources are discussed. In the main body of this review, the performances of nanomaterials in various HM removal strategies are evaluated. Finally, the current hurdles and future directions of the HM removal strategies that are based on nanomaterials are highlighted.

#### **2. Heavy Metal Ions in the Environment**

#### *2.1. Sources of Heavy Metal*

HMs have grea<sup>t</sup> ecological consideration, due to their toxicity and accumulation. Fish might accumulate significant concentrations of metals in water in which those metals are below the limit of detection in a routine water sampling and analysis [16]. The sources of HM in the ecosystem are focused on human activities and natural phenomena. Some of the sources include the usage of HM-laden pesticides and naturally occurring HM from the Earth's core via activities, such as soil erosion and volcanic eruption.

Pesticides (ethylene dibromide, and methyl bromide fungicides), insecticides (dithiocarbamates, and captan), and herbicides (paraquat, diquat, and 2, 4-dichlorophenoxyacetic acid) have been used for many decades as means to improve the survival of vegetations that are planted for commercial purpose and used to kill off various kinds of pest that harm the quality of crops. HMs are usually used as active compounds in the aversion of pest. Copper, usually in the form of copper sulfate and mercuric chloride, is used for its anti-fungicidal properties. Sodium, in the form of sodium dichromate works as a cotton defoliant [17]. Zinc phosphide is used as a rodenticide, whist cadmium chloride is used as a fungicide [18]. These pesticides laden with HM are usually prepared in ionic form, which later dissociates after being dispersed into the soil. Their nature allows for them to reside in water and travel far away from the point of origin. Pesticides can be taken up by plants, dissociate into soil, or carried away by residual water into other water bodies in contact, such as rivers and lakes [19]. Absorption of pesticides by plants removes them from the environment. However, plants cannot metabolize these compounds and stay in the plants, which can be transferred to other organisms that consume these plants, such as animals and humans itself [20]. Evidence of HM ending up in plant specimens is aplenty [21–23]. Via phytoextraction, plants can absorb HMs, which are essential in plant growth [24]. However, they also absorb HMs, such as cadmium, chromium, and lead, which do not serve them in any biological function. Consequently, the bioaccumulated HM will be passed along the food chain [25].

#### *2.2. E*ff*ect of Heavy Metal*

#### 2.2.1. E ffect of Heavy Metal Ions Towards the Environment

The largest contributor of HM in the air is the usage of hydrocarbons, such as gasoline, diesel, and petrol. HM, such as arsenic, lead, and cadmium, are emitted when these hydrocarbons are combusted [26]. Volcanic eruptions produce hazardous impacts to the environment, as the deterioration of social and chemical conditions and the gases (carbon dioxide, sulfur dioxide, carbon monoxide, and hydrogen sulfide) that are released during eruptions, various organic compounds and HMs, such as mercury, lead, and gold, are also released [4]. HMs enter plant and animal tissues via air inhalation, diet, and manual handling. Motor vehicle emissions are a major source of airborne contaminants, including arsenic, cadmium, cobalt, and nickel. HMs leaching from industrial and consumer waste can pollute water sources (groundwater, lakes, streams, and rivers); acid rain can exacerbate this process by releasing HMs that are trapped in soils. However, the risk of these metals entering the food chain is highly dependent on the mobility of the metal cations and its bioavailability in soil. The metal cations are bound to negatively charged particles in soil, such as clay and organic matter. When these metal cations detach from the negatively charged particles, they become available to be absorbed by plants and other organisms that live in the soil [25]. Plants are exposed to HMs through the uptake of water, and are stored until animals, which then transfer the HM into the animal's body, consume these plants. The ingestion of plant and animal-based foods that are laden with HM is one of the worrying sources of HMs in humans. The presence of such inorganic pesticide can also degrade the soil due to the accumulation of compounds at undesirable levels [27]. Absorption through skin contact with soil is another potential source of HM contamination. Studies have also shown that HMs can be accumulated in the plant tissues of *Sebera acuminate* and *Thlaspi caerulescens,* as they cannot be metabolized [21]. Arsenic poisoning is one of the most prevalent HM cases across the globe, which usually occurs by drinking water that is contaminated with arsenical pesticides, natural mineral deposits, or inappropriate disposal of arsenical chemicals. A work done by Sim et al. revealed that rivers in Sarawak have experienced extensive land use and logging, and hence su ffer from contaminations of HMs, such as arsenic, chromium, and copper [28]. Another similar research also revealed that River Pra and its tributaries displayed an enrichment in HM ions [2]. Figure 1 shows the route of absorption, distribution, and excretion that are related to the exposure of HMs and inorganic pesticides [29].

**Figure 1.** Common route of absorption, distribution, and excretion related to the exposure of HMs and inorganic pesticides. Adapted from [29], with permission from Frontiers, 2017.

#### 2.2.2. Effect of Heavy Metal Ions Towards Humans

Human can be afflicted with HM poisoning due to the consumption of food or water that is laden with HM. They stay in the human body system and result in constant accumulation of different types of HM, ince humans are not able to metabolize it. Exposure to As (V) leads to an accumulation of As (V) in tissues, such as skin, hair, and nails, resulting in various clinical symptoms, such as hyperpigmentation and keratosis. There is also an increased risk of skin, internal organ, and lung cancers. Lead is known to disrupt the balance between the production of free radicals and the generation of antioxidants to detoxify the reactive intermediates or to repair the resulting damage [30]. Reactive oxidation species (ROS) may cause structural damage to cells, proteins, nucleic acid, membranes, and lipids, resulting in a stressed situation at cellular at very high concentrations [31]. Lead is also known to disrupt biological processes, such as cell adhesion, intra- and inter-cellular signaling, protein folding, maturation, apoptosis, ionic transportation, enzyme regulation, and release of neurotransmitters [32]. Aluminium is a common HM that is used in the production of carbonated drink cans and cooking utensils [33]. The WHO postulated that aluminium exposure is probably a risk factor in the onset of Alzheimer disease in humans, whist reports have also suggested that humans can also be afflicted with contact dermatitis and irritant dermatitis [30,34]. Mercury, as a type of HM that is commonly found in many types of seafood and being previously used in dental amalgam, can also potentially harm

humans due to their acute toxicology. Mercury tends to be tightly bound in the brain, spinal cord, ganglia, autonomic ganglia, and peripheral motor neurons upon entering into the human body [35,36].

#### *2.3. The Chemistry of Heavy Metal Ions*

HM ions tend to present in salt or oxide forms. The HM ions ionic values are generally between +2 to +6, which indicates that metals placed in group 2 to group 6 of the periodic table of elements are categorised as HMs. Differing ionic states of HMs can significantly affect the suitability of different HM removal techniques and their conditions or parameters. For instance, the ionic state of HM can be altered in different pH conditions, which in turn affects the electrostatic interaction between the nanoadsorbent and the HM ions [37]. For the removal of HM via adsorption, the pH condition between 4 and 7 is favourable, as it greatly improves surface coordination, electrostatic attraction, and co-precipitation, which will result a higher removal rate. Increment from pH 4 to 7 reduces protonation phenomena, as it increases the H<sup>+</sup> availability. As the pH increases, the overall charge positivity increases and it also increases the interaction between the HM and adsorbent. This also increases the formation of chelate complexes between metal cations and the lone pair of electrons on the sulfur and nitrogen atom [38]. HM ions, such as Pb (II) and Hg (II) are efficiently adsorbed by neutrally charged adsorbents in the pH range between 4 and 7. On the other hand, when the absorbent surface is charged, the pH can also be manipulated to attain a parameter where the greatest electrostatic interaction between adsorbent and HM occur. Besides, the surface functional group also plays a role in the adsorption capacity. Al-Senani et al. exhibited that the removal of Co (II) and Cd (II) was effective above pH 9, exhibiting a removal rate of more than 98% [39]. This is due to the functional group exhibited by the adsorbent, which includes carboxyl, hydroxyl, and amine, all being involved in the binding mechanism between adsorbent and HM.

#### **3. Nanomaterials-Assisted Approach for Heavy Metal Removal**

The removal of HM can be carried out by several traditional techniques. Some of the methods include electrochemical treatment, photocatalysis, coagulation, adsorption, chemical precipitation, and membrane technologies, such as reverse osmosis and nanofiltration. However, in this review paper, three main HM removal technologies that have portrayed the greatest potential for large scale application have been focused upon. The HM removal techniques in focus are adsorption, photocatalytic reduction and membrane filtration/adsorption.

#### *3.1. Adsorption of Heavy Metals*

Adsorption is a process where a surface holds a molecule onto it. Adsorption happens via two phenomena, which are physisorption or chemisorption [40]. IUPAC defines adsorption as the increase in concentration of a dissolved substance at the interface of a condensed and a liquid phase due to the operation of surface forces [41]. Physisorption occurs when forces, such as intermolecular forces, are used to attach the absorbate onto the absorbent meanwhile chemisorption involves valence forces of the same kind as those operating in the formation of chemical compounds [42]. For any adsorption process, kinetic and isotherm studies are performed to evaluate the adsorption phenomena, rate, and efficiency [43]. Some of the common parameters that affect adsorption include the pH of the aqueous solution, the interaction between the adsorbent and adsorbate, surface charge, the surface area of adsorbate, and the size of adsorbent and adsorbate. Adsorption is a viable way of removing HM from water bodies. Many researchers have conducted a study investigating the ideal pH value, adsorbate surface area, and porosity and surface charge for effective adsorption of various types of HM ions [44]. Figure 2 illustrates the possible interaction of HM ions with the surface if adsorbents, such as polyaniline/TiO2 composites [45], cation exchanged porous zeolite [46], and binary metal adsorption by biochar derived from activated sludge [47].

**Figure 2.** Schematic illustration of the adsorption of HM via the surface of (**a**) hybrid polyaniline/TiO2 nanocomposite adsorbents. Adapted from [45], with permission from Elsevier, 2018. (**b**) cation exchange by hierarchically porous zeolite for improved adsorption of cationic HMs. Adapted from [46], with permission from Elsevier, 2019. and (**c**) selective HM ion adsorption by biochar in a single and binary metal system. Adapted from [47], with permission from Elsevier, 2019.

#### *3.2. Photocatalytic Reduction of Heavy Metal*

Among many types of wastewater remediation techniques that have been discovered by researchers, photocatalysis remains one of the best methods, simply because it is able to destroy or reduce the pollutant, rather than just mitigate, trap, or isolate them. The application of various types of semiconductor materials, including titanium dioxide (TiO2) and zinc oxide (ZnO) as a light responsive material to treat wastewater that is laden with organic have garnered much of the attention of emerging researchers. These semiconductors can produce strong oxidative free radicals that are capable of destroying a large range of organic pollutant and reduce HM ions when it is irradiated with light sources. These semiconductors harvests the photons and excite electrons into a higher energy state when light is irradiated, producing electron pair holes that are transferred on the surface of the semiconductor, which in turn, produces these ROS such as OH- and O2-−. Figure 3 shows a brief illustration on the excitation of an electron in a structure of photocatalyst and the subsequent creation of ROS species.

Many types of research have been focused on the fine-tuning and the modification of these semiconductors for efficient degradation of organic pollutants since semiconductors are able to efficiently degrade NOM. The possibility of using photocatalyst in the removal of HM is less explored, as it is impossible to degrade metallic ions. However, photocatalyst has shown promise as a means to reduce the harmful effects of HM by reducing the metal ions into less harmful by-products. The reduction of HM ions is a viable means for the treatment of HM pollution. Cr (VI) is significantly harmful to organisms, even in small trace amount, as compared to Cr (II). Hence, the common practice

for remediation of Cr (VI) is reducing it to Cr (II). This is where photocatalysts can play a prominent role in HM waste remediation. Based on the current research trend, the employment of photocatalyst has been commonly used as means to reduce Cr (VI), which can be abundantly found in contaminated water, sourced from industries, such as electroplating, pigmentations, and so on [48]. The photoreduction of Cr (VI) is elucidated in Equation (1) and Equation (2) [49];

$$2\text{ Cr}\_2\text{O}\_7\text{}^{2-} + 14\text{H}^+ + 6\text{e}^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}\_2\text{O} \tag{1}$$

$$\text{H}\_2\text{H}\_2\text{O} + 2\text{h}^+ \rightarrow \text{H}\_2\text{O}\_2 + 2\text{H}^+ \tag{2}$$

Cr (VI) has been extensively researched, because the hexavalent equivalent is much more poisonous than its divalent variant, Cr (III) counterpart. When the photocatalyst is irradiated with photons, they absorb it and excite an electron towards the particle surface. The Cr (VI) ions consume these electrons, allowing single photoreduction. This, in turn, makes the catalyst reactive due to the continuous presence of an electron-hole pair. To exploit this mechanism, research regarding photoreduction of Cr (VI) metal ions are paired with an organic pollutant, as the organic pollutant becomes an electron source for the electron deprived photocatalyst, which initiates a chain reaction on continuous photoreduction of Cr (VI) and the degradation of an organic pollutant. This mechanism provides further proof that photocatalysis can work for the photodegradation and photoreduction of wastewater mixed with HM ions and organic pollutants, hence enhancing its versatility [49].

**Figure 3.** Excitation of an electron in a structure of photocatalyst and subsequent creation of ROS. Adapted from [48], with permission from Elsevier, 2018.
