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Definition

The indiscriminate disposal of non-biodegradable, heavy metal ionic pollutants from various sources, such as refineries, pulp industries, lead batteries, dyes, and other industrial effluents, into the aquatic environment is highly dangerous to the human health as well as to the environment. Among other heavy metals, lead (Pb(II)) ions are some of the most toxic pollutants generated from both anthropogenic and natural sources in very large amounts. Adsorption is the simplest, efficient and economic water decontamination technology. Hence, nanoadsorbents are a major focus of current research for the effective and selective removal of Pb(II) metal ions from aqueous solution. Nanoadsorbents based on graphene and its derivatives play a major role in the effective removal of toxic Pb(II) metal ions.

1. Introduction 

Water decontamination is one of the most serious challenges among scientists globally due to the increasing population, pollution, and global warming [1,2,3,4,5]. Wastewater from developing industries, such as chemical manufacturing, metallurgical, battery manufacturing, papermaking, and mining industries produce a very large amount of various toxic pollutants in the form of heavy metal ions [6,7,8,9]. Excess heavy metal ions concentration in wastewater is a serious risk to public health as well as to other living organisms on Earth [10,11,12]. These toxic heavy metal pollutants are widely found in the Earth’s crust which tends to bioaccumulate in living organisms; they are non-biodegradable, which can cause various diseases, genetic disorders, and lethal ecological effects [12,13,14,15]. Heavy metal ions in aqueous media pose several toxic threats to the human health as well as to the other living organisms even at low concentrations. As per the regulatory system of the WHO (World Health Organization), the permissible concentration of Pb(II) (Lead (II)) in wastewater is approximately 0.01mg/L [16,17,18]. Moreover, the excess exposure of Pb(II) can lead to irreversible brain damage, cardiovascular disease, cognitive impairment, encephalopathy disease symptoms and even death [19,20,21,22]. Pb(II) is also extremely toxic to the nervous system, kidneys, and may lead to a wide range of human health issues such as nausea, anemia, infertility, coma, convulsions, hemochromatosis, renal failure, cancer, adverse effects on the metabolism and intelligence, and dermatitis brass chills, and cramps in the calves [12,14,23,24,25,26,27].

The current tight regulatory systems do not allow the release of heavy metal ions-contaminated wastewaters into the environment and require the removal of all toxic pollutants before discharging. Therefore, the development of efficient and economical novel materials and technologies for the effective removal of metal ion pollutants are required. Several traditional techniques have been used for the removal of heavy metal ions from aqueous medium, including reverse osmosis, precipitation, biosorption, ion-exchange, electrochemical processes, membrane filtration, irradiation, coagulation, and adsorption [28,29,30,31,32,33,34,35,36,37,38,39,40]. Table 1 lists the techniques used for Pb(II) ions removal from the wastewaters. The main disadvantages of conventional techniques include high cost, metallic sludge generation, incomplete removal, and disposal of secondary waste. Therefore, a cost effective, potential and convenient decontamination technology for wastewater treatment is highly desirable. The high efficiency, cost-effective, and simple operation makes adsorption technology one of the most promising technology for the effective removal of the heavy metal ions from the aqueous solution which adsorb metallic ions at solid-liquid interfaces [41,42,43,44,45,46,47,48,49]. On the other hand, adsorption technology has several challenges, such as selective recovery and reuse of valuable adsorbents and pollutants in the presence of humic substances [50,51]. Adsorbents developed with favorable structures, morphologies, superior adsorption capacities, and ease of separation are a major focus of the current research [26,52,53,54,55,56,57,58,59,60]. Recently, the development of nanotechnology has attracted considerable attention due to the remarkable potential of the nanoadsorbents for the selective and effective removal of ionic pollutants from aqueous solution when compared with the traditional adsorbents [13,33,61,62,63,64,65,66]. The nanoadsorbents play a major role in the separation of heavy metal ions because they are effective sorbents due to their unique structural properties and specific adsorption tendency. Nanoadsorbents have several unique advantages over conventional adsorbents due to their large specific surface area to volume ratio, tunable pore size, regeneration, reusability etc. [33,56,67,68,69,70,71].

Carbon (carbon nanotubes (CNTs), fullerenes, graphene, and graphene derivatives) based adsorbent materials are being used for the effective removal of the heavy metal ions because of their extraordinary high surface area to volume ratio and light weight as compared to other materials [66,96,97]. Currently, adsorbents based on the carbon nanotubes [98,99], activated carbon [100,101,102,103,104,105], graphene/ graphene oxide (GO) [60,67,106,107,108,109,110,111,112,113,114,115,116,117], graphene magnetite’s [118,119,120,121,122,123] polymeric adsorbents [124,125,126,127,128,129,130,131,132] and other type of adsorbents [133,134,135,136,137] have attracted substantial attention for the removal of heavy metal ions from the wastewaters. Carbon nanotubes have significant adsorption capacities for the effective removal of heavy metal ions from aqueous solutions as compared to non-functionalized and activated carbon-based adsorbents [99,138,139,140,141,142]. Although, carbon nanotubes-based materials are the promising adsorbents but their high cost and lower availability, limit the large-scale applications as adsorbents. In addition to this, water permeation through CNTs membranes is a major concern which is still unclear even after both the experimentally and molecular dynamic studies. Therefore, economic, readily available, and effective adsorbents are the prime concern of the 21^st century for the effective removal of heavy metal ions from wastewaters. Although, the presence of humic substances has strong complexation binding ability with metal ions because of their abundant oxygen-containing functional groups, yet graphene-based adsorbents have been proven the best substituents for the removal of heavy metal ions from aqueous medium [66,67,143]. Therefore, the adsorption behavior of heavy metal ions onto the surface of graphene-based nanoadsorbents is the chief concern of this study.

Graphene, a flat, single-atom thick sheet of sp²-hybridized carbon atoms with a two-dimensional honeycomb densely packed lattice arrangement, is an essential non-classical carbon adsorbent [144]. Graphene has a variety of real-time applications because of its extraordinary excellent thermal, mechanical and electrical properties [145]. Graphene is considered a promising material for the comprehensive adsorption of wide variety of pollutants from aqueous systems owing to its theoretically large specific surface area to volume ratio (≈2675m²/g), high Young’s Modulus (1.06 × 10³ Gegapascals) and high thermal conductivity (3 × 10³ Watt per meter kelvin) [113,146]. The wet chemical redox process is one of the most effective method for the production of graphene from graphite, where GO is an intermediate with a high density of negatively-charged reactive functional groups, viz. hydroxyl, carboxylic, and epoxy groups [147,148,149]. The presence of several functional groups makes GO soluble in polar and non-polar solvents as the oxygen functionality makes GO hydrophilic and the graphene domain makes GO hydrophobic. All the functional groups present on the edges and on the basal plane of GO play an important role in the heavy metal ions removal process [150]. Therefore, GO could be the best potential scavenger for the effective removal of cationic pollutants through electrostatic interactions between the positively-charged pollutants and negatively-charged functional groups of GO [27,151]. The relative high surface area, surface functionality, and better conductivity of graphene sheets play a key role in the better adsorption of several heavy metal ions through the preconcentration of aqueous medium [67,144]. Nair et al. reported that a few layered thick GO sheet membrane can completely block the passage of pollutants in the form of liquids and gases in a dry state while facilitating the permeation of water vapors [152]. On the other hand, aggregation, which is a major disadvantage of graphene layers, could be prevented partially through its composite formation [41,153]. In addition, composite formation may impart enhanced efficiency for the removal of heavy metal ions from wastewater due to the synergistic effects of the materials used. Electrosorption, a simple, novel, ecofriendly and recently attractive heavy metal ions adsorption technique which does not requires toxic or non-toxic chemicals and involves ideal nanostructured materials with a high surface area (such as activated carbon, carbon nanotubes, and carbon aerogels) onto which an external electric field is applied to remove metal ions from aqueous solutions [93,154,155,156,157]. Hence, this review includes various surface modification approaches and post synthesis, assembly steps, which will enable the exploitation of GO as a novel adsorbent material for cost effective water purification and the removal of heavy metals through graphene and its composites, which may be helpful in the purification of potable and safe drinking water as well as the removal of heavy metal ions from wastewaters and to clean up the environmental problems.

2. Removal of Lead

Pb(II) is one of the most useful heavy metal with wide spectrum applications worldwide. On the other hand, the presence of Pb(II) in aqueous system is a major threat to mankind as well as to the ecosystem owing to its high toxicity. Therefore, removal of Pb(II) ions from the wastewater aqueous system is quite essential to diminish the toxic threats. An adsorption process is largely dependent on the surface area and pore size of the adsorbents, whereas, the adsorption of metal ions is based on chemical adsorption onto specific adsorption sites, i.e., adsorption capacity of an adsorbent increases with the increase of its functional groups [158,159]. Graphene is thanked to have several active functional groups such as carboxylic, hydroxyl, and epoxy functional groups acting as better adsorption sites [160,161,162,163]. Hence, graphene and graphene derivatives due to their easy preparation, surface modifications, bulk availability and high adsorption capacities have been extensively studied for the removal of Pb(II) ions from aqueous system.

2.1. Removal of lead using functionalized GO/ RGO/ GO-aerogel
An adduct of GO and EDTA, was prepared using silylation by reacting N-(trimethoxysilylpropyl) ethyledinediamine triacetic acid (EDTA-silane) with the -OH functionality of the GO layers [164]. Hummer’s and Offeman’s modified double stage oxidation process was used to produce GO. In the first preoxidized step, natural graphite was treated with potassium persulfate and phosphorous pentaoxide in sulfuric acid followed by the oxidation with potassium permanganate, conc. sulfuric acid and hydrogen peroxide. GO was filtered and washed with 0.1 M HCl and deionized water. Resulting GO was then reacted through silylation process with EDTA-silane and was filtered followed by repeatedly washing with methanol and water. For comparison, a reduced GO adduct, EDTA-RGO black was prepared by direct thermal reduction treatment at 300 ◦C for 30 min. Adsorption isotherms analysis was performed in a plastic vial using 10 mg or 25 mg adsorbent to a 100 or 200 mL solution of Pb(II) ions at room temperature. It is evident that the adsorption of positively-charged metal ions depend on the charge present on the surface of the adsorbents as functionalities [165,166]. Zeta potential of the adsorbents at different pH was determined using Nano-ZS, Zeta Sizer and correlated with the adsorption of Pb(II) ions. At a particular pH, more negative zeta potential indicated the highest carboxylic and hydroxyl functionalities on the surface of EDTA-GO and GO adsorbents than activated carbon and EDTA-RGO was found to be consistent with Boehm’s titration results [167–170]. The enhanced adsorption for Pb(II) ions from the aqueous solution was observed with an EDTA-GO adduct (Figure 1) when compared with GO and EDTA-RGO adduct. The maximum adsorption capacities of the EDTA-GO adduct and GO for Pb(II) were found to be 479±46 and 328±39mg/g, respectively, which were greater than those of oxidized carbon nanotubes and activated carbon-based adsorbents [164]. The superior performance of EDTA-GO was correlated with the chelating characteristics of EDTA with functionalized graphene sheets. Mainly two adsorption process were considered to be responsible for the improved efficiency of removal of Pb(II) ions with the EDTA-GO adduct, i.e., the ion-exchange reaction process (Scheme 1) between Pb(II) and carboxylic and hydroxyl functional groups responsible for chelate complex formation between Pb(II) and EDTA onto the GO surface, as shown below

The second adsorption mechanism involves the stable complex formation of EDTA with Pb(II) ions for the complete removal of Pb(II) ions from aqueous solution (Scheme 2). After adsorption, Pb(II) equilibrium concentration reached lower than food and drug administration drinking water concentration i.e < 10ppm. Higher adsorption capacity was attributed with the higher stability constant (Log K≈18.0) of Pb(II)-EDTA complex. Langmuir and Freundlich adsorption isotherms exhibited good agreement with the experimental data having a correlation coefficient (R2) 0.975 and 0.933 at pH 6.8 respectively. The adsorption phenomenon rate for Pb(II) ions onto the surface of EDTA-GO was found to be pH dependent. It was noted that as pH of the aqueous medium increased from 5.0 to 8.0, adsorption capacity of EDTA-GO also increased due to chelate formation of Pb(II) with the hydroxyl, carboxylic functional groups of GO and EDTA. Low adsorption at high acidic medium was attributed due to neutralization of COO- and O- surface charges and relative competition between proton and Pb(II) ions. Lower zeta potential until pH 8.0 indicated that there were enough negative charge density to provide the strong electrostatic attraction between adsorbent’s functional group and Pb(II) ions. However, as the pH of the medium increased from 8.0, zeta potential rise indicated that negative charge density decreased and Pb(II) ions hydrolzse to give Pb(OH)+, Pb(OH)2, [Pb(OH)3]-, [Pb3(OH)4]2+ [Pb2(OH)2]2+ [Pb6(OH)8]4+ and [Pb4(OH)4]4+ [171–173]. As it is desirable to have appropriate both the adsorption and desorption capacities to be an economical and ideal adsorbent, desorption experiments
at different pH were carried out using Pb(II) pretreated EDTA-GO to determine desorption capacity. Desorption rate of Pb(II) was found to be maximum in acidic medium as consistent with zeta potential indicated that at lower pH, neutralization of functional groups leads to the competition between proton and Pb(II) ions adsorption [174]. Atomic absorption spectroscopy results were found to be in good agreement indicated that Pb(II) ions could be repeatedly desorbed from EDTA-GO adsorbent even after 10 cycles.

3. Conclusions and future perspective

Graphene based nanoadsorbents have been proven to play the vital role in the selective adsorption of heavy metal ions, such as Pb(II), from wastewater. The selective recovery and reuse of valuable adsorbents and pollutants are the important challenges for the adsorption technology using nanoadsorbents with favorable structures, morphology, superior adsorption capacities, and ease of separation. Graphene is always thanked for its unique property of functionalization, used to alter its properties and, consequently, its applications. This review exploited recently-developed graphene-based novel nanoadsorbents for the effective and selective removal of Pb(II) ions from wastewaters. In addition to the various own functionalities, GO was further surface modified with the negatively-charged functional groups to enhance the selective and effective Pb(II) ion adsorption from wastewaters. Due to wide varieties of surface functional groups, high surface area, and a preponderance of exposed edges planes, GO exhibited remarkable superior adsorption of Pb(II) ions. In addition, GO composites with ion scavengers (for example, EDTA), metal nanoparticles, magnetites, polymers, and aerogels have been reviewed for the adsorption of Pb(II) ions under the influence of temperature, pH of the medium, adsorbate, and sorbent loadings, etc., and exhibited superior adsorption than pristine GO for Pb(II) ion adsorption from an aqueous solution. One of the major advantages revealed by the graphene-based nanoadsorbents is the recovery of adsorbents and adsorbates even after a series of life cycles. Although simple and effective routes for GO-based nanoadsorbents with high surface area and pore size for superior adsorption of heavy metal ions are still in demand, graphene-based nanoadsorbents have proven their potential adsorbability for the expediency of mankind and the environment.

Author Contributions: S.H.H., M.K. put forwarded the idea and prepared the manuscript. J.S.C. and S.H.H. has edited and approved the manuscript.

This entry is adapted from the peer-reviewed paper 10.3390/app9142925