Nanomaterials—based on graphene oxide and its derivatives—for separation and preconcentration of metal ions

Nanomaterials—based on graphene oxide and its derivatives—for separation and preconcentration of metal ions

C H A P T E R 12 Nanomaterialsdbased on graphene oxide and its derivativesdfor separation and preconcentration of metal ions Mohammad Asaduddin Laska...

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C H A P T E R

12 Nanomaterialsdbased on graphene oxide and its derivativesdfor separation and preconcentration of metal ions Mohammad Asaduddin Laskar1, Sana Siddiqui2 1 2

Faculty of Science, Department of Chemistry, Jazan University, Jazan, Saudi Arabia; Department of Chemistry, Samta College of Arts and Science, Jazan, Jazan University

1. Introduction Adsorption has been a promising process in the quantitative determination of metal ions, especially in wastewater treatment. The determination of metal ions, which are present in trace level in various complex matrices, often requires sample preparation prior to instrumental analysis. The most commonly followed methods of sample preparation are solid-phase extraction (SPE), solid-phase microextraction, cloud point extraction, and electrochemical method. The SPE method is usually associated with passing of sample solution through a cartridge, filled with a solid matrix, whereby the target metal ions is retained or removed from the solution. The solid-phase microextraction generally involves the introduction of a fiber, which is coated with a solid matrix capable of retaining metal ions, into the sample solution. In cloud point extraction, the temperature and the concentration (of surfactant) are so adjusted that the metal ions get transported from the aqueous to a m phase. In the electrochemical method, the composition of the electrodes is manipulated with different materials. Besides others, nanomaterials have eventually emerged as a promising solid matrix that could be applied to sample preparation techniques, due to their high surface area and abundant active sites. Graphene oxide (GO) is a two-dimensional nanomaterial and is obtained by the oxidation of graphene, whereby many functional groups (such as eCOOH, -OH, carbonyl, and epoxy) get introduced into its extended layered structure. GO serves as an excellent candidate for

Graphene-Based Nanotechnologies for Energy and Environmental Applications https://doi.org/10.1016/B978-0-12-815811-1.00012-0

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adsorption by virtue of its large specific surface area and also serves as an excellent platform for modification (derivatization). Hence, GO and its derivatives have been receiving great attention as scavengers of metal ions. The structure and properties of GO vary with the methods of synthesis. The high specific surface area and the abundant oxygen-containing functional groups make it a promising candidate for preconcentration of metal ions. The adsorption mechanisms for GO, and its derivatives, may involve covalent boding, pep interaction, electrostatic interaction, Lewis acidebase interaction, and/or hydrogen bonding. The swelling, intercalating, and ion exchange properties (of GO) play a key role in facilitating the uptake of metal ions. The presence of defects and oxygen-rich functional groups, on GO and reduced GO (rGO), facilitates the anchoring of metal and metal oxides onto their surfaces. The nanocomposites, with GO and/or rGO, could display the individual properties (of the constituents) and also exhibit synergistic properties. Such nature of these nanocomposites has favored their application in the field of sensing and preconcentration. Adsorption of metal ions is generally governed by different environmental parameters, namely pH, background electrolytes, ionic strength, temperature, etc. Therefore, it was thought worthwhile to discuss the different adsorptive properties of GO, and its derivatives, as reported in the recent years.

2. GO in solid-phase extraction In the preparation of GO, the Brodie and Staudenmaier methods involve the oxidation of graphite with KClO3 and HNO3, while Hummer’s method employs KMnO4 and H2SO4. The so-formed oxidized product is then subjected to ultrasonication in order to obtain layered GO sheets. The GO sheets exhibit good mechanical strength, flexibility, and biocompatibility. The GO possesses abundant oxygen-containing functional groups (such as eCOOH, -COH, -COC-) whereby exhibiting excellent hydrophilicity. The hydrophilic nature of GO facilitates excellent contact with the metal ions, thereby favoring proper interaction between the adsorbent and the adsorbate.

2.1 Nascent GO As for instance, a modified Hummer’s method was used for preparing GO, which was used as an adsorbent for Co(II) [1]. The optimum pH for adsorption was found to be 5.0e8.0. It was revealed that Co(II) was retained, onto the GO, through interactions with the functional groups, namely eC-O and eC]O, and pep bonds. A maximum of 21.28 mg g1 of Co(II) was retained at pH 5.5 and 25 C. Similarly, the GO nanosorbent was employed for the removal of Pb(II), Ni(II), and Cr(IV) from wastewater pertaining to pharmaceutical industries [2]. The removal efficiency for Ni(II), however, decreased with increasing concentration of the GO. It was observed that 70 mg of the adsorbent could remove the metals at pH 8.0 from 100 mL of effluent. A filtering medium, consisting of GO [3], exhibited the adsorption capacity (calculated by Langmuir’s isotherm model) for Pb(II) as 227.2 mg g1. An increasing flow rate and a thicker GO bed had, respectively, negative and positive effects on the removal efficiency. The adsorption capacity for Cu2þ and Pb2þ was found to be 26.4 mgg1 and 80.6 mgg1, respectively, for a vortexing time of 30 s [4]. The detection limits were found to be 1.25 mgL1 and 2.56 mgL1 for Cu2þ and Pb2þ, respectively, III. Environment

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and the adsorbent was used for 150 replicate measurements without any mass loss. In another case, the flow rate with which the aqueous solution of Cu(II) and Pb(II) were injected into a fixed-bed sand column (supported with a layer of GO), affecting the latter’s removal efficiency [5]. As for instance, when the flow rate was increased from 1 mL min1 to 5 mLmin1, the removal efficiency for Cu(II) and Pb(II) got reduced from 15.3% to 10.3% and from 26.7% to 19.0%, respectively. Again, when the amount of GO was increased from 10 to 30 mg, the removal efficiency increased sharply, for Pb(II), from 26.7% to 40.5%. The application of a binary mixture resulted in the reduction of the removal efficiency for both metals. The breakthrough curve data of Cu(II) fitted better than Pb(II) when applied to convection dispersion reaction model. In yet another work, GO (as adsorbent) was filled into a microcolumn for the SPE of Pb2þ and Ni2þ from water samples [6]. The linear range of 7e260 and 5e85 mgL1 was observed for Pb2þ and Ni2þ, respectively. A detection limit of 2.1 and 1.4 mgL1 and preconcentration factor of 102.5 and 95 were observed for Pb2þ and Ni2þ, respectively. The RSD for 10 replicate determinations of 50 mgL1 (of Pb2þ and Ni2þ) were 4.1% and 3.8%, respectively. At pH 4.5 and 20 C, the maximum sorption capacity, by GO, for U(VI) was found to be 1330 mg g1 [7]. With the increase in pH, the physical adsorption gradually gave way to the formation of inner-sphere surface complex by virtue of the negatively charged deprotonated groups, namely -O- and -COO-. For Cs(I) [8], the abundant oxygen-containing functional groups of GO rendered a maximum adsorption capacity of 40.00 mg g1 at pH 3.0 and 293 K. The exothermic adsorption was found to be governed by outer-sphere and inner-sphere complexation at pH < 4.0 and pH > 5.0, respectively. In another study [9], Cr(III) was retained on GO, suspended in neutral water, followed by its transfer into a coacervate (of Triton X-45) for direct determination with electrothermal atomic absorption spectrometry. An enrichment factor of 140, with detection limit of 5 ng L1, was achieved with 0.01% Triton X-45 in 10 mL of the sample. In a study, GO was employed for the adsorption of U(VI) and Sr(II) in single and binary mixtures [10]. It was observed that the adsorption capacity for U(VI) was higher than Sr(II), whereby indicating that electrostatic interaction played the main role. The adsorbates formed inner-sphere complex (on the surface) and the process fitted well to pseudo-second-order kinetic model and Langmuir isotherm model. The pH-dependent adsorption of Pb(II) from aqueous solution [11] was found to be facilitated by the constituting eCOOH functional groups. The endothermic adsorption of Cr(III), on GO, followed pseudo-second-order model and Langmuir isotherm model [12]. The maximum adsorption capacity of 92.65 mg g1 was obtained at pH 5.0 and 296 K. The adsorption process was mainly governed by pH as well as the formation of inner-sphere complex (with GO).

2.2 Chelates adsorbed on GO A chelating reagent, namely 3-(1-methyl-1H-pyrrol-2-yl)-1H-pyrazole-5-carboxylic acid, was employed for retaining Mn(II) and Fe(III) ions (from sample solution) and subsequently gets adsorbed onto GO [13]. The detection limits of 145 and 162 ng L1 and the linear range of 0.31e355 mg L1 and 0.34e380 ng L1 were observed for Mn(II) and Fe(III) ions, respectively. The chelates of Co(II) and Ni(II), formed by N-(5-methyl-2hydroxyacetophenone)-N’-(2-hydroxyacetophenone) ethylenediamine, was retained by GO [14]. At pH 6.0, the preconcentration factor of 250 (for 1250 mL sample volume) and the

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detection limits of 0.25 and 0.18 ng mL1 were obtained for Co(II) and Ni(II), respectively. A solution of 3.0 M HNO3 was employed as an eluting agent. A GO packed column was employed for the determination of Mn2þ, Fe3þ, and Cu2þ with 1-phenyl-3(2-thienylmethyl) thiourea as the chelating agents [15]. The detection limit was found to be 0.06e0.49 mgL1 with preconcentration factor of 280.

2.3 Functionalized/grafted GO The anchoring of chelating agents or metal-selective molecules onto GO has been able to impart selectivity and sensitivity in the preconcentration of metal ions. A nanoporous graphene was functionalized with carboxyl group and was subsequently used, at pH 8, for preconcentration and speciation of Hg(II), CH3eHgþ, and C2H5Hgþ in water and caprine blood samples [16]. A 500 mL eluent, consisting of 0.3 M HNO3, could strip the mercury-loaded sorbent. The linear range, for caprine blood samples, was found to be 0.03e6.3 mg L1, while the preconcentration factor and detection limit were observed as 10.4 and 0.0098 mg L-1, respectively. When the epoxy and hydroxyl functional groups, constituting the bulk and edge of GO, were substituted with mercapto groups, a mercury extractant with enhanced hydrophilicity and affinity, as well as large surface area, was achieved [17]. The abundant thiol groups served as the active adsorption sites for Hg(II) and CH3Hgþ. The cloud point extraction-like method could render the enrichment factors of 78 and 77, respectively, for Hg(II) and CH3Hgþ(with 40 mL samples), while the detection limit for high-performance liquid chromatography-inductively coupled plasma mass spectrometry was observed to be 3.8 and 1.3 ngL1 respectively. In order to prevent the loss of small-sized GO nanosheets in high-pressure SPE system, the carboxyl groups of GO were covalently coupled to the amino groups of spherical aminosilica support [18]. This spherical particle covered with GO sheets rendered large surface area and high adsorption capacity by virtue of its wrinkled surface structure. At the optimum pH 5.5, the adsorption capacity for Cu(II) and Pb(II) were found to be 6.0 and 13.6 mgg1, respectively. The softness and flexibility of GO nanosheets facilitate quantitative preconcentration from a large volume of aqueous samples at optimum flow rate. For Cu(II) and Pb(II), the enrichment factors were 200 and 250 with detection limits of 0.084 and 0.27 ng mL1, respectively, A biocomposite, comprising of GO and magnetic chitosan, was further functionalized with the mercapto group [19]. A facile and eco-friendly synthesis procedure was also developed for modification of GO with 3-mercaptopropyltrimethoxysilane. At optimum pH 6.5, an amount of 60 mg of sorbent and 10 min of contact time lead to 250 mL of breakthrough volume and a preconcentration factor of 80 (within the linear range of 0.122e80 ng mL1). A relative standard deviation of 4.7% reflected the good repeatability of the sorbent when 3 mL of an eluent, containing 0.1 M HCl þ2% w/v thiourea, was used. The amount of mercury was determined with continuous-flow cold vapor atomic absorption spectrometry and the limit of detection and quantification were observed as 0.06 ng mL1 and 0.12 ng mL1, respectively. The Arbuzov reaction was employed for incorporating triethyl phosphate onto the surface of GO [20]. The so-prepared modified adsorbent exhibited a maximum adsorption capacity of 251.7 mgg1 for U(VI) at pH 4.0 and 303K. Spectroscopic investigations indicated that chemical adsorption played the main role in the removal of U(VI). In another study, an exfoliated GO was impregnated with trioctylamine (in acetone medium) for the adsorption of Cr(VI) at pH 2.5 [21].

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A maximum adsorption capacity of 232.55 mg g1 was achieved mainly through cationep, lone pairep, and electrostatic interactions. The adsorption process was found to be exothermic and followed Langmuir isotherm model and second-order kinetics. The retained Cr(VI) could be eluted with NH4OH. The electrochemical determination of As(III) was done with a gold microelectrode incorporated with amino-functionalized GO [22]. The adsorption process could tolerate the presence of other commonly coexisting ions. The detection limit was observed to be 0.162 ppb and the sensitivity was found to be 130.631 mA ppb1 cm2. A sulfonated GO, consisting of hydroxyl, carboxyl, and sulfonyl functional groups, could adsorb U(VI) at ultralow pH [23]. The Langmuir isotherm model was found to be the most befitting and the maximum adsorption capacity of 45.05 mg g1 was observed at pH 2.0. In another work, GO was coupled to ethylenediamine so as to use this nanomaterial for removing Fe3þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, and Pb2þ from aqueous medium [24]. The spiked samples exhibited a recovery of 90%e98% (with relative standard deviation of ˂ 6%) and the detection limit of 0.07, 0.10,0.07,0.08,0.06, and 0.10 ngmL1 were observed for Fe3þ, Co2þ, Ni2þ, Cu2þ, Zn2þ and Pb2þ, respectively. A one-pot method was adopted for the modification of GO with sodium dodecyl sulfate (a surfactant) that was subsequently used for the removal of Ni2þ [25]. The adsorption capacity for Ni2þ was found to increase from 20.19 to 55.16 mg g1 (by Langmuir model) after modification. The retention of Ni2þ, onto the adsorbent, might have occurred due a collective phenomenon comprising of electrostatic attraction, chemical interaction, and ion exchange. The chemical modification of GO with hexachloroplatinic acid, in the presence of ethylene glycol, gave a nanostructured adsorbent, that was subsequently used for the determination of arsenic (Ar) [26]. The linear range was found to be 10e100 nM with detection limit of 1.1 nM. Zhang et al. [27]. adopted a facile vacuumassisted filtration method for developing a stable GO framework membrane, involving isophorone diisocyanate as the cross-linking agent. This covalently modified membrane exhibited high water permeability due to the constituting enlarged nanochannels. An external pressure of 1.0 bar could develop a high flux of 80e100 L m2h1 bar1 through the membrane. Moderate retention capacity, of about 40%e70%, was observed for Pb2þ, Cu2þ, Cd2þ, and Cr3þ.

2.4 GO in nanocomposite A nanocomposite, composed of GO and magnetic chitosan, was used as a support for synthesizing Zn(II)-imprinted polymer through coprecipitation [28]. The linear range was found to be 0.5e5.9 mgL1 with maximum adsorption capacity of 71.4 mgg1 for Zn(II). The limit of detection and quantification were observed to be 0.09 mgL1 and 0.3 mgL1, respectively. A nanocomposite, comprising of GO and poly(3,4-ethylenedioxythiophene), was employed for modifying a glassy carbon electrode for electrochemical detection of Hg(II) [29]. The nanocomposite was prepared through liquideliquid interfacial polymerization, whereby poly(3,4-ethylenedioxythiophene)-nanorods were anchored onto the surface of GO nanosheets. The linear range was found to be 10.0 nMe3.0 mM with detection limit of 2.78 nM. A composite, composed of TiO2 and GO, was used as an adsorbent and filled in a microcolumn for determination of rare elements and rendered the detection limits of 2.2, 1.6 and 2.8 gL1 for La, Tb, and Ho, respectively [30]. The enrichment factors were found to be 17.1, 11.1, and 10.2, while the relative standard deviations of 3.6%, 1.3%, and 1.4% were observed for La,

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Tb, and Ho, respectively. A chemical vapor deposition method was adopted for synthesizing a nanoadsorbent, by catalytically growing graphene sheets on silica, for the determination of Pb(II), Cd(II), and Cr(III) [31]. The constituting silica allows the nanoadsorbent to interact with metal ions in both free and bonded forms (in biological samples). The linear range was found to be 0.012e12.5 mgL-1 and the relative standard deviations of 3.1%e3.8% and 6.3%e7.2% was recorded for intraday and interday precisions. A nanocomposite electrode, comprising of iron oxide and graphene, was employed as an electrochemical sensor after further modifying it by in situ plating with bismuth [32]. The synergetic effect, of graphene and iron oxide, facilitated improved electrochemical activity and high sensitivity for Zn(II), Cd(II), and Pb(II). The linear range was found to be 1e100 mg L1 while the limit of detection for Zn(II), Cd(II), and Pb(II) were observed at 0.11 mg L-1, 0.08 mg L1, and 0.07 mg L1, respectively. At pH 3e10, a nanocomposite, containing carboxylic GO and akaganeite (b-FeOOH), exhibited adsorption capacities of 77.5 mgg1 and 45.7 mgg1for As(III) and As(V), respectively [33]. There was no interference, in adsorption, in the presence of 2000-fold of foreign species, namely SO4 2 , NO3  , Cl, and Mg2þ. The retained arsenic could be eluted with 400 mL of 2M NaOH þ2.0% NaBH4 solution and the detection limit was observed to be 29 ng L1. GO was coated with magnetic chitosan, and the resulting composite exhibited a maximum adsorption capacity of 79 mg g1 at pH 5 and 303 K for Pb2þ, within 40 min of contact time [34]. The adsorption process could be fitted well with pseudo-secondorder kinetic model and Langmuir isotherm model. A composite, comprising of GO and polyacrylamide, exhibited dependency on the pH and ionic strength during the adsorption of Sr(II) from aqueous solutions [35]. The endothermic adsorption process followed Langmuir isotherm model. The maximum adsorption capacity was found to be 2.11 mmol g1, at pH 8.5 and 303K. GO and TiO2 powder were thermally treated to prepare a composite for the adsorption of radiocobalt (60Co) [36]. The sorption process was dependent on the ionic strength at pH ˂ 8, while exhibiting independency at pH ˃ 8. At higher pH, the inner-sphere surface complexation played the major role in adsorption, while at lower pH, ion exchange and outer-sphere complexation took over the leading role. When nanosized hydrated manganese oxide was grown in situ on GO, the resulting nanocomposite showed excellent selectivity for Pb(II) [37]. Even in the presence of a highly competing Ca(II) ions, the adsorption capacity was found be more than 500 mg g1. The ample presence of oxygen-containing groups, on GO, facilitated the growth of well-spread hydrated manganese oxide. Besides the specific interactive role of hydrated manganese oxide, the supporting GO contributed to adsorption of Pb(II) through Donnan membrane effect. In another study, the selfpolymerization of monomeric dopamine on the surface of GO rendered a composite, which was subsequently employed for the adsorptive removal of U(VI) [38]. The collective contribution of polydopamine and the constituting functional groups, on the large areal surface of GO, resulted in the maximum adsorption capacity of 145.39 mg g1. The in-situ chemical oxidative polymerization, of pyrrole onto GO, resulted in a nanocomposite, which was further functionalized with phytic acid (through electrostatic attraction) [39]. A high electrochemical conductivity was demonstrated by the electrode (constituting the modified nanocomposite) and the linearity was observed at a range of 5e150 mgL1 for Cd(II) and Pb(II). The dispersive SPE of rhodium was done with a celluloseegraphite oxide composite [40]. A recovery of more than 95% was observed for rhodium (in 30s contact time) with a tolerance limit of 10,000 mgL1, 25,000 mgL1, 10,000 mgL1, and 20,000 mgL1 for Naþ, Kþ, Mg2þ,

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and Ca2þ, respectively. The detection limit was found to be 5.4 mgL1 (RSD 1.6%). The best eluent was 10 mL H2SO4 (2.5 M), with 60 s of optimum contact time. A nanocomposite material, composing of spinel nickel ferriteebased GO, was used as an adsorbent for the removal of Pb(II) and Cr(II) [41]. The heavy metals underwent monolayeric chemisorption through endothermic inner-sphere complexation. The sorption mechanism was well fitted to pseudo-second-order and Langmuir models. A solvothermal process was employed for the synthesis of graphene manganese ferrite (MnFe2O4-G), which was used for the adsorptive removal of Pb(II) and Cd(II) [42]. At 37 C, the adsorption of Pb(II) and Cd(II) were optimum at pH 5 (120 min) and pH 7 (180 min), respectively. A spontaneous and exothermic adsorption was exhibited at the temperature range of 27e47 C. The effect on fluoride adsorption, of the crystalline structure of two modified GO adsorbents, was studied [43]. The two hybrid adsorbents were prepared through in situ hydrolysis procedure by anchoring goethite and akaganeite onto GO. The sodium acetateeinduced modification had significant influence on the adsorption of fluoride due to its role in the structure of iron (oxy) hydroxide. Such influential role reflects the potential of naturally occurring organic ligands (in water or soil matrices) to manipulate the adsorption behavior. A composite comprising of nanosheets of GO was modified by incorporating polyethylenimine into polydopamine-coated GO through Michael addition reaction [44]. This composite exhibited better adsorption behavior than the polyethylenimine-coated GO and pure GO. The adsorption capacities for Cu(II), Cd(II), Pb(II), and Hg(II) were found to be 87, 106, 197, and 110 mg g1, respectively. Additionally, a hydrothermal treatment of the adsorbent composite resulted in an aerogel with high surface area of 373 m2g1. Although the aerosol adsorbent exhibited enhanced regenerability, the adsorption capacity for the metals, however, got reduced. A nanocomposite was prepared by combining diethylenetriamine-functionalized multiwalled carbon nanotubes with GO [45]. It was used as an adsorbent for the SPE of trace amounts of Cr(III), Fe(III), Pb(II), and Mn(II) ions from wastewater, and the maximum adsorption capacities were found to be 5.4, 13.8, 6.6, and 9.5 mgg1, respectively, with respective detection limits of 0.16, 0.50, 0.24, and 0.38 ngmL1. At pH 4.0, the commonly coexisting ions did not interfere with the adsorption and the adsorbent gave a preconcentration factor of 75. A composite, prepared by functionalizing dopamine polymer onto the surface of GO, exhibited high adsorption capacity (145.39 mgg1) for uranium (VI), in aqueous medium [38]. The combined effect of high surface area (of GO) and multifunctional groups (of dopamine) contributed to the enhanced adsorption capacity. The adsorption of uranium (VI) was found to follow Langmuir adsorption isotherm and pseudo-second-order kinetics. A coprecipitation method was adopted for synthesizing a Zn(II)-imprinted polymer with GOemagnetic chitosan nanocomposite as supporting materials [28]. The optimized experimental procedure displayed a linear dynamic range of 0.5e5.0 mgL1, a detection limit of 0.09 mgL1, and a quantification limit of 0.3 mgL1. The sorbent exhibited a maximum adsorption capacity (for Zn(II)) of 71.4 mgg1. An electrochemical sensor, containing nanocomposite iron oxide (Fe2O3)/graphene as one of the electrodes, exhibited improved catalytic activity and high sensitivity during the determination of trace Zn(II), Cd(II), and Pb(II) ions [32]. This nanocomposite electrode was synthesized by a solventless thermal decomposition method. The linear range exhibited by this electrode was 1e100 mg L1 and the detection limits were found to be 0.11 mg L1, 0.08 mg L1, and 0.07 mg L1 for Zn(II), Cd(II), and Pb(II), respectively. For tenfold repetition, the %RSD was 1.68%, 0.92%, and 1.69% for Zn(II), Cd(II), and Pb(II),

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respectively. A microwave irradiation method was employed for synthesizing a composite comprising triethylenetetramine-modified GO and chitosan [46]. At pH 2, the maximum adsorption capacity (of 219.5 mg g1) for Cr(VI) was observed and was found to increase with increasing temperature. The process of adsorption was found to fit pseudo-second-order kinetic and Langmuir isotherm models. Another composite was synthesized by chemically bonding GO to magnetic cyclodextrinechitosan [47]. The high surface area, abundant hydroxyl and amino groups, as well as the magnetic property of Fe3O4 collectively facilitated the adsorption of Cr(VI). At low pH, higher removal efficiency was observed. The adsorption process was befitting to the Langmuir isotherm. The adsorption efficiency exhibited high dependency on the concentration of surface charge as well as the surface area. An electrode was modified with N-doped graphene for the simultaneous determination of Cd2þ, Pb2þ, Cu2þ, and Hg2þ through differential pulse stripping voltammetry [48]. The structure and electronic properties, of the N-doped graphene electrode, played a vital role in imparting an improved catalytic activity during the selective detection of the metal ions. The limits of detection for Cd2þ- Hg2þ and Pb2þ-Cu2þ were 0.05 and 0.005 mM, respectively. A nanocomposite film prepared by the successive electrodeposition of exfoliated GO followed by in situ plating of bismuth film rendered a modified glassy carbon electrode for the determination of trace Zn2þ, Cd2þ, and Pb2þ ions [49]. The linear calibration curve for these metal ions was in the range of 1e100 mg L1. The detection limits for Zn2þ, Cd2þ, and Pb2þ ions were found to be 1.8 mg L1, 0.18 mg L1, and 0.11 mg L1, respectively.

3. Magnetic GO The GO sheets may be further modified by incorporating (or anchoring) metal and/or metal oxides into its structure so as to impart magnetic property. Thus a magnetic GO (mGO) was prepared by a one-step coprecipitation method and subsequently employed for the sorption of gold ions through dispersive solid-phase microextraction [50]. The elution of the retained gold ions was done with 0.5 M thiourea in 0.1 M HCl. The linear range was found to be 0.02e100.0 mgL1, while the limit of detection was observed at 4 ngL1 with an enrichment factor of 500. At a concentration of 20 mg L1, the relative standard deviations of 3.2% and 4.7% were observed for intraday and interday analysis, respectively. The increase in pH from 2.0 to 6.0 increased the sorption of Eu(III) on mGO, which attains its maximum value at pH ˃ 7.0 [51]. The independency on ionic strength suggested the main role of innersphere surface complexation in the sorption process. The maximum adsorption capacity was found to be 70.15 mg g1 at pH 4.5 and 298 K. The exothermic adsorption process was found to occur through monodentate and multidentate inner-sphere surface complexation at higher side of the pH range.

3.1 Functionalized mGO The adsorption of Cd(II), onto a sulfanilic acid-laden mGO, was dependent strongly on pH and was influenced by the ionic strength of the background electrolytes [52]. At pH 6, the divalent cationic electrolyte had more influence than the monovalent cationic electrolyte. The presence of Cl and NO3  restricted the adsorption of cadmium by virtue of their ability

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to form complexes with Cd(II) ions. At 0.01 mol L1 Na3PO4, the removal of Cd(II) was low at pH < 5, whereas it got enhanced beyond pH 5. It was found that the adsorption mechanism was closer to Freundlich’s isotherm model. The affinity of cysteine for Cd(II) imparted selectivity to mGO nanosheets after its functionalization with cysteine [53]. Adsorption occurred satisfactorily at the pH range 5e9 and the adsorption capacities were in the range of 24.39e30.30 mgg1 at varying temperatures (298e328 K) with 5 min contact time. The endothermic adsorption process was well represented by the Langmuir isotherm model and pseudo-second-order kinetic model. A polymeric adsorbent was synthesized for the removal of Cr(VI) from aqueous solution [54]. It was prepared by grafting mGO-ethylenediamine derivative on b-cyclodextrin. The adsorption process was well described by Langmuir’s isotherm and pseudo-second-order kinetic models. The spontaneous endothermic adsorption of Cr (VI), onto the adsorbent, was influenced by pH and ionic strength of the solution. The same polymeric adsorbent was used for the adsorption of Cu (II), which displayed strong dependence on the pH (of solution), ionic strength, electrolytes (background), and citric acid [55]. An enhanced adsorption, of Cu (II), was observed at 0e0.1 M NaNO3 at pH below 8. The removal of Cu (II) was slightly improved in the presence of LiNO3, NaNO3, KNO3, NaCl, and NaClO4, and citric acid. The rate of adsorption, of Cu (II), was well described by pseudo-second-order, Freundlich isotherm, Temkin isotherm, and film diffusion models. The SPE of Cd2þ was done with a magnetic adsorbent constituting rGO functionalized with hydroxypropyl-b-cyclodextrin [52]. A linear range of 0.50e100.0 mg L1 and a detection limit of 0.23 mg L1 was observed for water samples, while a linear range of 0.050e5.0 mg g1 with detection limit of 0.015 mg g1 was found for food samples. The recoveries, from spiked samples, were found to be 87.5%e102.4% with enrichment factors of 38e41. The reusability of the adsorbent was found to be 50-fold. The incorporation of 2-pyridinecarboxaldehyde thiosemicarbazone group into mGO rendered an adsorbent for Hg(II) [56]. The enrichment factor was found to be 193 and the limit of detection was achieved at 0.0079 mg L1. A nanosorbent, comprising of mGO modified with 2-mercaptobenzothiazole groups, could be regenerated by the application of external magnetic field [57]. The adsorption of Cd(II), Cu(II), and Pb(II) followed a linear range of 0.3e80 ng mL1, 0.4e100 ng mL1, and 1e140 ngmL1, with detection limits of 0.19, 0.24, and 0.35 ng mL1, respectively.

3.2 Nanocomposite mGO A nanocomposite was prepared by simultaneous oxidation polymerization of pyrrole and thiophene onto the surface of a composite, containing GO sheets and magnetic nanoparticles [58]. The magnetic SPE of Cu(II), Pb(II), Zn(II), Cr(III), and Cd(II) rendered a maximum sorption capacity of 201, 230, 125, 98, and 80 mg g-1, respectively. The detection limit was observed to be in the range of 0.15e0.65 mg L-1. A one-step reaction method was followed for the synthesis of a magnetic composite which comprised of GO, diethylenetriamine, and Fe3O4 nanoparticles [59]. The so-formed composite adsorbent could remove Cr(VI) within 40 s of contact time with a maximum adsorption capacity of 123.4 mg g1. The adsorption of Cr(VI) was found to be a spontaneous endothermic process that was dependent on the pH. Adsorption kinetics followed a pseudo-second-order model as well as the Langmuir adsorption isotherm. The Cr(VI) was reduced to Cr(III) prior to its adsorption onto the composite. An mGO-polyimide nanocomposite was prepared for the preconcentration and

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soild-phase extraction of Pb2þ ions from mollusk and fish [60]. The linear curve was obtained for the concentration range of 0.8e400 mgL1. The limit of detection and quantification was observed to be 0.25 mgL1 and 0.80 mgL1, respectively. A preconcentration factor (with standard deviation of 7.3%) was found to be 141. The magnetic SPE of Pb(II), Cd(II), Cu(II), Ni(II), and Co(II) was done with magnetic allylamineemodified GO-poly(vinyl acetate-co-divinylbenzene), which rendered a preconcentration factor of 40 [61]. The detection limit was observed at 0.37e2.39 mgL1 with a relative standard deviation of ˂ 3.1%. The pseudosecond-order kinetic model was followed by the adsorption process of mGO for Sr(II) and Cs(I) [62]. The adsorption was found to increase from pH 2.0 to 6.0 and reached the maximum at pH ˃ 6.0. At 293 K and pH 4.0, the maximum adsorption capacity was found to be 14.706 and 9.259 mg g-1 for Sr(II) and Cs(I), respectively. The exothermic adsorption process occurred mainly through cation exchange and inner-sphere surface complexation. A coprecipitation method [63] was adopted for the synthesis of a composite constituting cucurbituril [6], GO, and Fe3O4, which was subsequently employed for U(VI) removal. The cucurbituril [6] was attached to the GO nanosheets through hydrogen bonding during the precipitation of Fe3O4 nanoparticles onto the latter. Two types of nanocomposites were prepared by coprecipitating salts of nickel and iron in association with GO [64]. The two composites, namely paramagnetic GO nickel ferrite and superparamagnetic rGO nickel ferrite, had porous surfaces, and the average particle sizes were 41.41 and 32.16 nm, respectively. An endothermic adsorption of U(VI) and Th(IV), which increased with increasing temperature (293e333 K), was exhibited by the adsorbents. The adsorbent could be recycled for 5 times after use without any loss in the adsorption capacity. The nanocomposite, comprising of magnetic chitosan and graphene quantum dots, was employed for the preconcentration of Cu(II). The linear range was observed at 0.05e1500 mg L1 and the detection limit was found to be 0.015 mg L1 [65].

4. Reduced GO rGO is generally obtained by chemical or electrochemical reduction, whereby the oxygencontaining functional groups (at the bases and edges) get removed. In chemical reduction, the commonly used reducing agents include NaBH4, N2H4, C6H4(OH)2, and C6H8O6 (ascorbic acid).

4.1 Derivatives of rGO An electrochemical sensor containing a modified electrode, comprising rGOeNHe carboimidazole, was prepared for detecting heavy metal ion, including Hg2þ and Pb2þ [66]. This modified electrode showed stronger complexing capacity than rGO because of carboimidazole which constitutes amino groups. The detection limits for Hg2þ and Pb2þ were 0.2 and 3.0 nM, respectively. In another study, a film of GO was drop-pasted on graphitereinforced carbon and employed for the electrochemical detection of Pb(II) ions [67]. The GO would undergo irreversible reduction at a range of 0.7 to 1.6 V versus Ag/AgCl (3M) in acidic medium and thus enhances the sensitivity of Pb(II) detection (at nanomolar level). In 1 M HCl, the linear range was found to be at 3e15 nM with a detection limit of

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0.5 nM. In another work, gold nanodendrites were deposited on ionic liquid-rGO for the determination of iron electrochemically [68]. The latter provided a support with ambient surface area and facilitated the electrochemical reduction of iron. Subsequently, the composite was incorporated onto the electrode surface containing Nafion as a cation exchange polymer. The linear range was observed at 0.30e100 mmolL1and exhibited a detection limit of 35 nmolL1. A nanocomposite was synthesized by depositing Au nanoparticles (20 nm) onto rGO in ethylene glycol medium through a one-step solvothermal method [69]. The detection limit (3s) was found to be 0.25 mg L-1 for Hg2þ. A glassy carbon electrode was electrocoated with a film of composite, containing rGO and Au nanoparticles, for the determination of As(III) in aqueous 0.20 M HCl [70]. The limit of detection was found to be 2.7 nM and the linear range was observed at 0.01e5.0 mM. The nanoparticles of zerovalent iron was incorporated onto rGO to give an enhanced specific surface area of 117.97 m2g1 [71]. The adsorption capacity for Cd(II) was found to be 425.72 mg g1, which could be attained within 50 min of contact time. Another nanocomposite was prepared, by integrating ZnO nanorod into rGO, through a facile template-free hydrothermal route [72]. When rGO was present by 7.5 wt% (in association with ZnO nanorods), the efficiency for the removal of Cu(II) and Co(II) ions increased fourfold. The adsorption was found to follow pseudo-second-order kinetics. A clean and simple one-pot synthesis of a nanocomposite was carried out by dispersing Au nanoparticles onto the surface of rGO through UV irradiation [73]. An electrode, composed of this nanocomposite, could detect As(III) at concentration as low as 0.3e20 ppb with the detection limit of 0.1 ppb.

4.2 Magnetic-rGO A composite was prepared by modifying magnetic- rGO with ionic liquid-for the dispersive solid-phase microextraction of Hg(II) [74]. The Hg(II) species was electrostatically adsorbed through complex formation with the ionic liquid that constituted the nanocomposite. A linear response was observed in the range of 0.08e10 ng mL1 and the limit of detection was found to be 0.01 ng mL1. The intraday and interday precisions were found to be 3.4% and 4.5%, respectively. A nanosorbent was synthesized through a hydrothermal method whereby a nanocomposite, comprising a magnetic rGO coupled to thioglycolic acide capped CdTe quantum dots, was formed that was capable of extraction and preconcentration of Ce3þ (from aqueous solutions) [75]. A linear range of 0.1e511.0 mgL1 and detection limit of 0.1 mgL1 (Ce3þ), with an enrichment factor of 125, was observed for this nanosorbent. The precisions for this procedure was obtained as 3.6% (RSD for n ¼ 5) and 9.1% (RSD for n ¼ 5) for single-sorbent repeatability and sorbent-to-sorbent reproducibility, respectively.

5. Future prospectives GO derivatives have a promising role in the field of removal and determination of metal ions from various environmental samples. GO derivatives may be fabricated so that enhanced sensitivity and selectivity, as well as multifunctionalities, could be achieved. These may subsequently be applied to tools pertaining to electronic, electrochemical, and optical sensing techniques that could detect biological and chemical molecules, as well as

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differentiate stem cells. Moreover, the existing uncertainty in reproducibility and uniformity of structures (of GO derivatives) may be minimized by adopting more mature and smarter techniques. The unique feature of the florescent GO and rGO in quenching the fluorescence of targets, adsorbed onto their surface, may be very promising for the detection of DNA or other biomolecules.

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