Introduction

One of the environmental concerns due to the industrialization is a serious water pollution that can jeopardize public health and environmental sustainability. Among the wide diversity of pollutants affecting water resources, heavy metals are of particular concern because they are not biodegradable and highly toxic even at very low concentrations. Because of their high solubility in aquatic environment, the presence of heavy metals deteriorates the quality of potable water [1]. Their pathogenic properties and tendency toward bioaccumulation in living organisms lead to several acute and chronic health problems [2,3,4,5]. Heavy metals are not spontaneously wiped out in a normal state but very stable in nature and even can become more toxic and dangerous compounds over time [6]. Common heavy metal pollutants such as chromium, arsenic, cadmium, lead, copper, zinc, mercury, and nickel are usually present in wastewater, originated from the artificial fertilizer and herbicide production, dyeing, textile, paints, pigments and glass industries, mining operations, metal plating, and battery manufacturing processes [2, 4].

Various treatment methodologies are available to remove heavy metals such as chemical precipitation, electrochemical purification, ion exchange, reverse osmosis, membrane filtration, coagulation, and adsorption by activated carbon [2, 6]. Generally, these methods are less effective in removing heavy metals at low concentrations and are not economically viable [6, 7]. Adsorption seems to be more attractive because the process is simple, less expensive, and flexible in scaling-up process [7, 8].

Adsorption is a phenomenon where the solute (adsorbate) molecules accumulate at the surface of solid adsorbent. The presence of unbalanced forces at the surface of liquid or solid phase triggers the tendency to attract and retain the molecular species in contact with the solid surface. Adsorbent is a solid material that has the ability to extract certain substances (adsorbates) from a media by causing them to adhere to its surface without changing the physical properties. Adsorbate is the substance to be adsorbed on the surface of solid adsorbent [9]. The adsorption of heavy metals onto different natural adsorbents such as clay, seaweed, and biomass and synthetic adsorbents such as activated carbon, polymer resin, and mesoporous silica has been intensively studied and reported in literature [2, 10]. Often, these adsorbents are unsuccessful to remove heavy metals at low concentration level. As a result of slow removal rates and capacities, the adsorbents usually fail to meet the desired standard of water pollutant control requirements [11, 12].

Particle size is an important feature of adsorbent. The smaller the size of the particle, the greater the reactivity due to the higher cross-sectional area of the particle [6]. For the same material in the bulk form, the material that is in a smaller size can possess different physical and chemical properties. A small size enables the particle to access a variety of biological environments [13]. Materials with particle size between 1 and 100 nm are defined as nanomaterials [12].

In this chapter, nanomaterials for heavy metal adsorption are introduced, with special emphasis on the preparation methods and material characteristics. The hazardous properties of common heavy metals and measurement techniques are highlighted. Also, current studies on heavy metal adsorption by nanomaterials and factors affecting the adsorption performance together with the removal mechanisms are discussed.

Nanomaterials

In order to be used as adsorbent for heavy metal removal from wastewater, the porous nanomaterials should satisfy the following criteria: (1) inert and nontoxic, (2) high adsorptive capacities and selectivity, and (3) the adsorption process is reversible from which the adsorbents could be infinitely recycled [12, 14].

Types of Nanomaterials

A variety of nanomaterials have been studied for the removal of heavy metals from water. Figure 1 shows some examples of nanomaterials for heavy metal adsorption. Basically, these nanomaterials are categorized into three groups, i.e., metal oxide nanoparticles, carbon nanomaterials, and nanocomposites. The common synthesis methods, advantages, and disadvantages of nanomaterials are summarized in Table 1.

Fig. 1
figure 1

Nanomaterials for heavy metal adsorption

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Table 1 Classification of nanomaterials
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Metal oxide nanoparticles (MONs) are potential heavy metal adsorbents because of their large surface area and high surface activities due to the size-quantization effect. Among various MONs, ferric oxide nanoparticles such as hematite (α-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4) are extensively used in removing heavy metals. They are low-cost materials because of the facileness of resources and ease in synthesis [15]. The magnetic property of ferric oxide nanoparticles renders ease of recovery and reusability, hence reducing the economic load [18]. Another example of MONs is magnesium oxide nanoparticles, which are nontoxic and possess good ductility and better mechanical properties over polymeric materials [6].

Carbon nanomaterials (CNMs) exhibit excellent traits of adsorbent due to their well-developed internal pore structures and unique surface morphology [16]. Carbon nanotube (CNT) is a carbon nanomaterial with unique one-dimensional hollow nanostructure and physicochemical properties and has been broadly used in the field of heavy metal adsorption [12, 13]. Graphite oxide (GO) possesses two-dimensional structure, leading to enhanced mechanical and thermal properties. Oxidation treatment of graphite to produce GO introduces oxygen-containing functional groups on its surface, increasing its hydrophilicity so as to improve the adsorption capability of heavy metal ions. The presence of surface functional groups making GO as an attractive adsorbent for metal aqua ions and positively charged pollutants in wastewater treatment [2, 12, 19].

MONs and CNMs offer promising and effective adsorption toward heavy metals. Nevertheless, the materials are difficult to be separated from water upon adsorption and can cause excessive pressure drop in flow-through systems due to their ultrafine particles [15]. In order to overcome these drawbacks, a new class of nanomaterials, namely, nanocomposites (NCs), is introduced. Nanocomposites are compounding nanoparticles such as MONs into or onto larger size porous supports such as synthetic polymer host and biochar. The reactivity of some nanoparticles is considered very high, e.g., some MONs have the ability to self-ignite when exposed to air. Generally, the supporting matrix is required to conserve the chemical properties of nanoparticles by inhibiting the oxidation process before they come in contact with the targeted pollutants [17].

Preparation and Characterization of Nanomaterials

Table 2 shows the preparation methods of nanomaterials and the characteristics of nanomaterials. MONs can be synthesized by three methods, i.e., physical, chemical, and biological syntheses. The methods of chemical synthesis are famous and widely reported, which include chemical coprecipitation, thermal decomposition, sol-gel, and hydrothermal. Among these, chemical coprecipitation is the simplest and most efficient method for bulk production of MONs. The preparation process involves aqueous raw materials such as FeCl3, FeCl2, and SiO2, in the presence of a base (NaOH, NH3) or ethanol. The formation of MONs is complete when a certain pH is achieved [8, 18, 19]. Thermal decomposition is advantageous because of the controllable size and shape of MONs, but the high-temperature environment is always required. The synthesis conditions such as chemical involved, temperature, and time have a great effect on the materials produced [20,21,22].

Table 2 Preparation and physicochemical properties of nanomaterials
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The hydrothermal reaction is progressively discovered and reported in recent literature. This reaction is performed in water by applying high pressure of higher than 2000 psi at a minimum temperature of 200 °C. Generally, MONs are formed by hydrolysis and oxidation, followed by neutralization of mixed metal hydroxides. The particle size is controlled mainly through the rate processes of nucleation and grain growth, which is greatly affected by the chemical involved and hydrothermal time [18, 20, 23]. The sol-gel method is a simple chemical method which involves hydroxylation and condensation of the molecular precursor in a solvent as well as evaporation of the solvent to form MONs of uniform size and shape, but it requires longer preparation time [7]. The electrical wire explosion method is a physiochemical method to produce MONs using pulse of high-density current. This method is relatively clean and highly productive. It was found that the explosion product of pure iron wire in air is a mixture of Fe3O4 and γ-Fe2O3 [24].

Chemical vapor deposition (CVD) is a famous and broadly used technique to synthesize CNT because of its low setup cost, high production yield, and ease in scaling-up. Figure 2 shows the schematic diagram of CVD method. The process involves passing a hydrocarbon vapor through a tubular furnace as the raw material or precursor to synthesize CNT. High temperature (>600 °C) with the presence of a catalyst on the substrate is essential to decompose the hydrocarbon into carbon and hydrogen. Then, the carbon gets dissolved into the catalyst until the carbon-solubility limit in the catalyst is reached; the dissolved carbon precipitates out and crystallizes in the form of a cylindrical network [25].

Fig. 2
figure 2

Schematic diagram of a CVD setup

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The common precursors used in the synthesis of CNT are methane, ethylene, acetylene, benzene, xylene, and carbon monoxide. The molecular structure of the precursor may influence the morphology of the CNT grown. The light hydrocarbons such as methane generally produce straight hollow CNT, while the cyclic hydrocarbons such as benzene produce curved or hunched CNT. A catalyst is required in the CVD to decompose hydrocarbon at a lower temperature. Two main criteria of catalyst selection are high solubility and diffusion rate of carbon in the catalyst. Typically, the common catalysts used are metal nanoparticles such as Fe, Co, and Ni. The substrate is basically used as a support for the metal catalyst, in which the catalyst-substrate interaction gives the effects on the yield and quality of CNT. Different substrates could lead to the growth of CNT in different forms, such as powder, thin or thick and straight or coiled. Graphite, quartz, silicon, alumina, and zeolite are the common substrates used in CVD for the synthesis of CNT [16, 25 ]. There are two typical growth mechanisms for CNT, i.e., tip-growth model and base-growth model. When the interaction of catalyst-substrate is weak, hydrocarbon vapor decomposes on the surface of metal catalyst, from which the carbon diffuses down through the catalyst and precipitates across the bottom of the catalyst. This mechanism is demonstrated in Fig. 3a that is known as tip-growth model. While, for a strong catalyst-substrate interaction, the carbon precipitation is difficult to push the catalyst upward, the CNT grows up with the catalyst rooted on the base. The so-called base-growth model is shown in Fig. 3b [25].

Fig. 3
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Growth mechanisms of CNT: (a) tip-growth model, (b) base-growth model (Adapted from [25])

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Activation of CNT plays an important role in enhancing the surface morphology and surface functional groups. During activation, the metallic impurities from catalyst and substrate are dissolved and consequently may alter the surface characteristics by introducing new functional groups. An additional chemical used in activation such as HNO3 functionalizes the CNT by adding oxygenated surface groups on the surface [16].

In 1958, Hummers reported an effective method to synthesize GO from graphite by using KMnO4 and NaNO3 in concentrated H2SO4. The basic chemical reaction involved in the Hummers method is the oxidation of graphite by concentrated H2SO4 with catalysts, KMnO4 and NaNO3. The evolution of toxic gases such as NO2 and N2O4 in oxidation process is the significant drawback of the method. To overcome this, the oxidation has been done by using modified Hummers method, i.e., H2SO4 and H3PO4 instead of NaNO3 [26, 27].

NCs are prepared by two methods, i.e., direct compounding and in situ synthesis. In direct compounding, nanoparticles and polymer matrix are prepared separately and then mixed or compounded by fusion or mechanical forces. In in situ synthesis, metal ions that served as nanoparticle precursors are preloaded within the polymer matrix and uniformly distributed through chemical reaction or coprecipitation [17].

According to Zhou et al. [8], the surface of nano-MnO2-biochar is more porous compared to its precursor (biochar) and exhibits a larger specific surface area of 80 m2/g than that of biochar (61 m2/g). The pore width of nano-MnO2-biochar is 3.9 nm that is about 3 times smaller than that of biochar and 1.5 times smaller than that of MnO2 nanoparticles. The MNO2 nanoparticles densely grown and formed microspherical structure on the surface of biochar, consequently, enlarged the surface area of the NC. The surface area of commercial Dowex 50 WX4 resin is 72 m2/g with a total pore volume of 0.02 cm3/g, while the surface area of magnetite Dowex 50 WX4 resin nanocomposite increased to 160 m2/g with a total pore volume of 0.067 cm3/g, indicating that the surface structure of nanocomposite is more porous than that of pure resin [10].

Heavy Metal Removal

Heavy Metals and Their Toxicity

Heavy metals are generally considered to be those whose density exceeds 5 g/cm3. Heavy metals, such as zinc, copper, chromium, iron, and manganese in very small amounts, are essential for the human body to function. But, if the metals accumulate in the body to a certain amount, they can cause poisoning, resulting in serious implications to human health. Unlike organic pollutants, heavy metals do not degrade biologically. The presence of heavy metals even at trace level is believed to be a risk to the ecological environment as well as to human health [1].

Wastewater regulations are established to minimize human and environmental exposure to hazardous chemicals. This includes limits on the types and concentrations of heavy metals that may be present in the discharged wastewater. The maximum contaminant level (MCL) is the standard set by the US Environmental Protection Agency (EPA) for drinking water quality, which indicates the highest level of a contaminant that is allowed in drinking water [34]. The World Health Organization (WHO) also recommended a maximum level of the contaminant in drinking water, which is known as the maximum permissible limit (MPL) [35]. Table 3 shows some heavy metals which are relevant to the environmental context.

Table 3 Toxicity of common hazardous heavy metals
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Mercury is an important and useful industrial material. Mercury is used as pigments in inks; as aids to early metallurgy; in the electrolytic production of chlorine and sodium hydroxide; in electrical appliances, dental amalgams, oil refinery, battery manufacturing, and gold extraction; and as raw material for various mercury compounds [45, 46]. According to the United Nations Environment Program (UNEP), about 730 tons per year of mercury is released to the environment through gold extraction processes, which is about 35% of the global amount of mercury released [46]. The WHO has declared mercury as one of the most dangerous pollutants for human health. Mercury is present in the inorganic form in the surface water and groundwater at the concentration usually below 0.5 μg/L. Although nearly all mercury in water is thought to be in the form of Hg2+, methylation of mercury could occur to form organomercury compounds, such as methyl mercury and ethyl mercury, which are readily absorbed by living organism, causing numerous adverse health effects. In human, acute oral poisoning results primarily in hemorrhagic gastritis and colitis, and then the ultimate damage is to the kidney. Mercury also affects the central and peripheral nervous system as well as respiratory and renal systems but does not cause mutations [35, 46].

Cadmium is widely used in steel industry, plastics, batteries, pigments, alloys, fertilizers, and metal plating [47]. Cadmium is a nonessential element and well-known nephrotoxic agent. Cadmium accumulates primarily in the kidney with the development of chronic disease that has a long biological half-life in humans of 10–35 years. Cadmium damages cells by strong affinity to glutathione and sulfhydryl group in proteins and displacement of zinc, iron from protein. Cadmium may damage oxidative DNA and decrease genetic stability which results in the enhanced probability of mutations. However, there is no evidence of carcinogenicity by oral route and no clear evidence for the genotoxicity of cadmium. Food is the main source of daily exposure to cadmium. The daily oral intake is 10–35 μg/L. High exposure and excessive intake of cadmium can result in serious illnesses such as itai-itai disease and affect the reproductive hormone levels [35, 47, 48].

Arsenic is used as alloying agents in the manufacturing of transistors, lasers, and semiconductors [35]. In nature, arsenic can be found in organic and inorganic forms. It is widely distributed throughout the earth’s crust, most often as arsenic sulfide or as metal arsenates and arsenides. In well-oxygenated surface waters, arsenic appears mainly in the form of As5+, while in groundwater, it can be reduced to As3+ under anaerobic condition [31]. Arsenic is not essential in humans. Arsenic water pollution has been reported in the USA, China, Chile, Bangladesh, Mexico, Argentina, Poland, Canada, Hungary, New Zealand, Japan, Vietnam, Nepal, India, Cambodia, Vietnam, Pakistan, and India [28, 31]. Water contamination by arsenic causes life-term diseases such as cancer, neurological disorder, nausea, hyperkeratosis, muscular weakness, and loss of appetite [18, 28]. Generally, As3+ exists mainly as a nonionic arsenous acid (H3AsO3) form at pH ranging from weakly acidic to weakly alkaline, so As3+ does not have high affinity to the surface of various adsorbents compared to As5+. Thus, the oxidation of As3+ to As5+ as pretreatment by pH adjustment of water prior to adsorption is necessary for the effective removal of As3+. However, it is impractical to conduct such a pretreatment and after-treatment pH adjustment for the treatment of large natural water bodies contaminated with As3+. Thus, it is desirable to develop adsorbents which are favorable to the adsorption of both As3+ and As5+ without pretreatment [28]. The particle size of adsorbent has a considerable effect on the removal efficiency of arsenic. When the particle size is reduced from 300 to 12 nm, the adsorption capacities increase nearly 200 times for As3+ and As5+ [30].

Chromium is also widely distributed in the earth’s crust. It can exist in valences of +2 to +6. It is used in leather industry, electroplating, pigment production, and battery production [21]. Chromium toxicity causes liver damage, pulmonary congestion, edema, skin irritation, allergic dermatitis, kidney damage, and respiratory impairment [6, 49]. Cr3+ has no evidence of carcinogenicity via oral route, but Cr6+ is classified as human carcinogen [35].

Lead is mainly used in the production of batteries, solder, alloys, wood, and paper, painting and dyeing, and electroplating and as lubricating agents in petrol [7, 35]. Lead is a toxicant that can accumulate in the human skeleton, muscles, bones, kidney, liver, and brain and can cause damage to these human organs and reproductive and nervous systems [3, 6, 50]. The biological half-life is considerably longer in children than in an adult. Prenatal exposure to lead may have early effects on mental development that does not persist to the age of 4 years. Lead is classified as the possible human carcinogen [35] .

Nickel is used in the production of stainless steel and nickel alloys, mining, electrolysis, electroplating, battery dye metallurgy, pesticide, and reactor materials. It is also widely used in research and medical applications [35, 51, 52]. Nickel is a nonbiodegradable toxic metal ion present in wastewater, which may cause adverse health effects such as anemia, diarrhea, encephalopathy, hepatitis, lung and kidney damage, gastrointestinal distress, pulmonary fibrosis, renal edema, skin dermatitis, and central nervous system dysfunction [51, 53]. Among these health effects, skin dermatitis is the most prevalent effect of nickel. There is limited evidence to show that nickel is a carcinogen by oral exposure, but it is carcinogenic to humans via inhalation route [35].

Copper is an essential trace nutrient that is required in 5–20 μg/g by humans but can be toxic if exceeding 20 μg/g [54]. The most bioavailable and therefore most toxic form of copper is the cupric ion (Cu2+). Cupric ion is a common metal ion found in effluents of a large number of industries such as electrical wire manufacturing, electroplating, and metal surface treatment, paint and pigment industry, wood processing industry, and mining [8, 18, 55]. It is used to make pipes, valves, and fittings and as a part of alloys and coating [35]. Excess copper exposure in short term causes irritants, dermatitis, and gastrointestinal distress, while long-term exposure causes liver and kidney diseases, Wilson’s disease, coma, and eventual death. Copper can affect all fish as well as marine plants miles downstream from the source of copper released in water [8, 34, 56] .

Zinc is an essential trace element found in virtually all food and potable water in the form of salts or organic complexes and acts as the micronutrient. The daily requirement for an adult man is 15–20 mg/day. The containment level of zinc is normally below than 0.01 mg/L in surface water and 0.05 mg/L in groundwater [35, 57 ]. Zinc is mainly applied in mining, metal cleaning, plating baths, pulp, paper board mills, fertilizer industries, brass and bronze alloys manufacturing, paints, rubber, plastics, cosmetics, and pharmaceuticals [57, 58]. Excess exposure of zinc can cause serious health effects such as anemia, massive gastrointestinal bleeding, vomiting, skin dermatitis, and nausea [58].

Measuring Techniques of Heavy Metals in Water

Atomic absorption spectrometry (AAS) is a very common and reliable technique for detecting metals and metalloids in environmental samples. It is an analytical technique that measures the concentration of elements. AAS is sensitive, in a way that it can measure down to parts per billion (μg/dm3) of a sample. The underlying principle is based on the use of wavelengths of light specifically absorbed by elements. For example, a lamp containing lead emits light from the excited lead atoms that produce the right mix of wavelengths and be absorbed by any lead atoms from the sample. The sample is firstly atomized and vaporized. Then, a beam of electromagnetic radiation emitted from the excited lead atoms is passed through the vaporized sample. Some of the radiation will be absorbed by the lead atoms in the sample. The amount of light absorbed is proportional to the number of lead atoms. A calibration curve is established by running several samples of known lead concentration under the same condition. The comparison between the amount the standard absorbs and the calibration curve is performed to calculate the lead concentration in the unknown sample [59, 60]. Figure 4 shows the principle operation of AAS.

Fig. 4
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Principle operation of AAS

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Inductively coupled plasma (ICP) is used to measure trace metals in a variety of solutions. Two commonly used ICP techniques are inductively coupled plasma-optical emission spectroscopy (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS). An ICP-MS combines a high-temperature ICP source with a mass spectrometer. A sample is converted by ICP source from atoms to ions. These ions are then separated and detected by mass spectrometer. The liquid or dissolved solid sample is typically introduced into the plasma and is converted into aerosol using a laser. The elements in the sample aerosol are converted first into gaseous atoms and then ionized toward the end of the plasma. Once the ions enter the mass spectrometer, they are separated by mass-to-charge ratio. A mass spectrum is a plot of ion signal as a function of mass-to-charge ratio, which is used to determine the elemental or isotopic signature of a sample, the mass of particles and of molecules, and to explain the chemical structure of molecules [61, 62]. Figure 5 shows the principle operation of ICP-MS .

Fig. 5
figure 5

Principle operation of ICP-MS

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Laser-induced breakdown spectroscopy (LIBS) is a type of simple atomic emission spectroscopy technique used for the analysis of heavy metals. LIBS can be applied to determine the elemental compositions of a wide range of materials in the solid, liquid, or gaseous phase. A high energy and short laser pulse is used to create a micro-plasma on the sample surface, resulting in vaporization and atomization of a small amount of target material. The electrons of the atoms in the sample at excited electronic states will fall down into natural ground states and results in the emission of light with discrete spectral peaks. The emitted light is collected and coupled with spectrograph detector to identify the elemental composition of the sample [63, 64]. Table 4 shows the comparison, advantages, and disadvantages among the measuring techniques of heavy metals. Figure 6 shows the principle operation of LIBS .

Table 4 Typical measuring techniques of heavy metals
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Fig. 6
figure 6

Principle operation of LIBS

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Possible Mechanisms of Heavy Metal Adsorption

In adsorption, the force of attraction exists between the adsorbate and the adsorbent. This force of attraction can be due to the van der Waals of attraction, which is a weak force or chemical bond, which is a strong force of attraction. On the basis of the attraction forces, the adsorption can be classified into physisorption and chemisorption. In general, the process of adsorption is governed by four sequential steps:

  1. 1.

    Bulk diffusion describes the diffusion of the adsorbate from the bulk solution to the surface of adsorbent.

  2. 2.

    Film diffusion is the diffusion of adsorbate through the boundary layer.

  3. 3.

    Pore diffusion or intraparticle diffusion is where the adsorbate is diffused from the surface into the adsorbent pores.

  4. 4.

    Adsorption is the binding of adsorbate to the active sites on the adsorbent surface [9].

The adsorption mechanisms of heavy metal ions onto porous nanomaterials can occur through ion exchange and π-cation electrostatic interactions as shown in Fig. 7. The adsorption of metals in water contributes to the release of H+ ion and possibly other exchangeable species such as Na2+, Ca2+, Mg2+, and K+ around the adsorbent surface. Ion exchange involves electrostatic interactions between heavy metal ions and the charged particles on the adsorbent surface, which implies that any ions leaving the surface are replaced by an equivalent number of ions from the bulk solution. The stoichiometric process is usually rapid, diffusion-controlled, and reversible. However, the selectivity of certain ions toward the surface may inhibit the process stoichiometry. π-cation electrostatic interaction occurs when heavy metal ions act as a central group to form close association with delocalized π-electrons and oxygenated functional groups on the adsorbent surface such as the phenolic, carboxylic, and carbonyl functional groups [65, 66].

Fig. 7
figure 7

Possible mechanisms of heavy metal adsorption onto porous nanomaterials (Adapted from [67])

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Adsorption of Heavy Metals by Nanomaterials and Factors Affecting the Removal Performance

Table 5 shows the adsorption of heavy metals by nanomaterials under specified adsorption conditions. MONs, i.e., TiO2, α-Fe2O3, and Fe3O4 nanoparticles, demonstrate Cd2+ removal percentage of 64.9%, 71.0%, and 96.8%, respectively [1, 23, 68]. The maximum adsorption capacity of Cd2+ by Fe3O4 nanoparticles, GO, and magnetite Dowex 50WX4 resin nanocomposite are 77.7 mg/g, 205 mg/g, and 398 mg/g, respectively [1, 10, 19]. Compared with CNMs and NCs, MONs showed a less effective adsorption of Cd2+.

Table 5 Adsorption of heavy metals by nanomaterials
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Ferric oxide nanoparticles are promising adsorbents for As3+ and As5+ removal. Of these, hematite (α-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4) are widely used to adsorb As3+ and As5+ from water. α-Fe2O3 adsorbent displays the maximum adsorption capacity of 95.0 mg/g and 47.0 mg/g for As3+ and As5+, respectively [28]. γ-Fe2O3 shows a comparable performance of 67.0 mg/g and 95.4 mg/g, respectively [29], while Fe3O4 is the least performed with 8.2 mg/g and 6.7 mg/g for As3+ and As5+, respectively [30].

For the removal of Cr6+, magnetite Dowex 50WX4 resin (400 mg/g) demonstrates a better adsorption capacity than MnO nanoparticles (16.4 mg/g) [6, 10], while GO shows a moderate adsorption performance of Cr2+ with a 63% removal percentage and a maximum capacity of 60.2 mg/g [19].

Compared to MONs, CNMs seem to be more suitable as Pb2+ adsorbent. The maximum adsorption capacities of Pb2+ by MnO and TiO2 nanoparticles are 21.8 mg/g and 7.4 mg/g, respectively, which are much lower than that CNT (207 mg/g) and GO (556 mg/g) [6, 7, 16, 19]. Magnetite Dowex 50WX4 resin exhibits a comparable result of 380 mg/g for Pb2+ removal [10].

NCs show the highest removal percentage of Ni2+ compared to MONs: 91.8%, 70.3%, and 56% for silica/GO composite, α-Fe2O3, and SiO2 nanoparticles, respectively [19, 68]. The maximum adsorption capacity of Ni2+ by GO is 250 mg/g, that is, comparable to 384 mg/g by magnetite Dowex 50WX4 resin nanocomposite [10, 19].

For the removal of Cu2+, MONs give a better adsorption performance compared to CNMs. The maximum Cu2+ adsorption capacities of Fe3O4 (30.0 mg/g) and MnO2 (93.9 mg/g) are higher than that of CNT (0.68 mg/g) and oxidized CNT (15.3 mg/g) [1, 8, 11, 31]. Magnetite Dowex 50WX4 resin nanocomposite and nano-MnO2-biochar offer the best Cu2+ adsorption performance with 416 mg/g and 142 mg/g, respectively [8, 10].

TiO2 nanowires show a lower percentage of Zn2+ removal than GO [19, 23]. A better adsorption capacity of Zn2+ can be achieved by CNT (65.4 mg/g) and GO (323 mg/g) [16, 19].

Solution pH is among the significant factors affecting the adsorption behavior of metal ions in aqueous solution. The H+ ion concentration affects the solubility of the metal ions in the solution, competes with heavy metal ions for the adsorption sites, and changes the surface electric charge density which can facilitate or reduce the electrostatic interactions [8, 22]. Generally, poor adsorption of heavy metals is observed at low pH. At acidic environment, the surface of nanomaterials is surrounded by H+ ions, thereby inhibiting heavy metal ions from approaching the binding sites. In contrast, as the pH increases, the surface becomes negatively charged, thus facilitating greater heavy metal removal. The maximum adsorption observed at certain pH and the subsequent reduction in adsorption capacity is probably due to precipitation of metals, thereby reducing the adsorption capacity of nanomaterials [11].

As3+ exists predominantly as non-charged H3AsO3 when pH is less than 9.2, while the predominant As5+ species exists as negatively charged H2AsO4− or HAsO22− in the range of pH between 2.2 and 11.5. At pH 6–7, the surface of α-Fe2O3 is negatively charged; consequently the adsorption of As3+ occurs due to a less repulsive interaction between the nanoparticles and the non-charged As3+. However, there is a coulomb repulsive force between the negatively charged surface of α-Fe2O3 and the negatively charged As5+ and also between the readily adsorbed As5+ and As5+ in the solution. Therefore, by increasing the concentration of arsenic, the amount of As3+ adsorbed at equilibrium is more than the amount of As5+ adsorbed at equilibrium [28, 30].

At pH 3–6, a poor Cu2+adsorption performance is observed due to competition with H+. The pHPZC of nano-MnO2-biochar is 11.0; hence, the surface is positively charged because the solution pH is lesser than pHPZC. However, a considerable capacity of 142 mg/g is shown at pH 6. It is speculated that metal deposition may play some role in Cu2+adsorption onto nano-MnO2-biochar outweighs that of electrostatic attraction at the cation exchange sites [8].

A low adsorption rate of Pb2+ by TiO2 nanoparticles is observed at pH lower than 3 due to the competition between H+ and Pb2+ for the adsorption sites of TiO2 nanoparticles [7]. At pH 3–6, the adsorption of Pb2+ increases with increasing pH since the competition between cations subsided. However, beyond pH 6, the adsorption capacity decreased with further increase of pH due to the precipitation of metal aqua ions [7]. Likewise, the adsorption of Cd2+ ions onto CuO nanoparticles is not accurate beyond pH 8 because Cd2+ ions started to precipitate as hydroxides although the solution remains or becomes clear [22]. According to Al-Saad et al. [68], the maximum removal percentage of Al3+ is at pH 5 and decreased at higher pH due to the formation of Al(OH)4 in the alkali media.

The removal percentage of heavy metals onto nanomaterials decreases with the increase in initial concentration of heavy metals. At low concentration, the removal percentage is high and gradually decreased with the increase of concentration. Sufficient adsorption sites are available to accommodate low concentration of heavy metals. However, as the concentration increases, more surface sites are occupied, and finally, the nanomaterial-based adsorbents get exhausted. Therefore, the removal percentage of heavy metals is concentration-dependent [1, 19, 21]. Sheet et al. [19] showed the decrease of removal percentage of heavy metals onto GO at concentrations of 30 mg/L and 200 mg/L (Table 5).

Usually, there is a sharp increase in the removal percentage at the beginning of contact between nanomaterials and heavy metals in solution. This is followed by a gradual increase with the increase in contact time. A rapid adsorption at the early contact time is due to the counter attraction between the negatively charged surface of the adsorbent and the positively charged of heavy metal ions as well as the number of vacant sites available on the nanomaterials surface. With increasing of time, a repulsive force between the readily adsorbed heavy metals and the remaining heavy metals in the solution is initiated, thus slowing down the adsorption process [1, 11].

CNT demonstrates a higher sorption capacity for Cd2+, Pb2+, Ni2+, and Zn2+. Adsorption of heavy metals onto CNT may be due to the interaction of heavy metals and the acidic oxygenated functional groups that are introduced on the CNT surface by activation or oxidation during the post-treatment process. CNT displayed a – descending order of Pb2+ (207 mg/g), Cd2+ (80.3 mg/g), Zn2+ (65.4 mg/g), and Ni2+ (41.9 mg/g) removal due to metal ion penetration to reach the adsorption sites. The effectiveness of penetration depends upon the ionic radius, which is also in descending order of Pb2+ (119 pm), Cd2+ (97 pm), Zn2+ (74 pm), and Ni2+ (69 pm). A larger ionic radius has a lower hydrated ionic radius, which facilitates the penetration effectiveness into the porous matrix. Compared to commercial activated carbon, CNT is more effective in the removal of heavy metals [16].

The chemical structure of the nanomaterials is also a determining factor in heavy metal adsorption. The presence of oxygen atoms on graphite oxide in the forms of epoxy, hydroxyl, and carboxyl groups is highly favorable to the positively charged heavy metal ions due to strong electrostatic interaction [19]. The presence of sulfur functional groups on nanomaterials displays a positive influence on Pb2+ adsorption [11]. Olanipekun et al. [2] showed the higher adsorption capacity of GO than that of graphite due to the presence of various surface functional groups on GO, especially that of – COOH groups .

Conclusion

A large amount of heavy metal-contaminated water is discharged daily to the environment worldwide due to inevitable industrial development. Excess exposure to toxic heavy metals may lead to severe health problems. Adsorption is regarded as the promising technique to remove heavy metals from wastewater, and porous nanomaterials are potential candidates of heavy metal adsorbents. In this chapter, we presented recent progress in porous nanomaterials as heavy metal adsorbents. The rationale, preparation methods and the characteristics of three types of nano-sized materials have been highlighted. The adsorption performance of heavy metals by nanomaterials is influenced by several factors including pH, initial heavy metal concentration, contact time, as well as surface properties of the nanomaterials.