Efficient Removal of Heavy Metals from Polluted Water with High

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Efficient Removal of Heavy Metals from Polluted Water with High Selectivity for Mercury(II) by 2-Imino-4Thiobiuret Partially Reduced Graphene Oxide (IT-PRGO) Fathi Awad, Khaled M AbouZeid, Weam M Abou El-Maaty, Ahmed M El-Wakil, and M. Samy El-Shall ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10021 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

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ACS Applied Materials & Interfaces

Efficient Removal of Heavy Metals from Polluted Water with High Selectivity for Mercury(II) by 2-Imino-4-Thiobiuret Partially Reduced Graphene Oxide (IT-PRGO) Fathi S. Awad†‡, Khaled M. AbouZeid†, Weam M. Abou El-Maaty‡, Ahmad M. El-Wakil‡, and M. Samy El-Shall†* †

Department of Chemistry, Virginia Commonwealth University, Richmond, VA 23284, USA ‡

Chemistry Department, Faculty of Science, Mansoura University, Mansoura 35516, Egypt

Abstract: A novel chelating adsorbent, based on the chemical modification of graphene oxide by functionalization amidinothiourea to form 2-imino-4-thiobiuret-partially reduced graphene oxide (ITPRGO), is used for the effective extraction of the toxic metal ions Hg(II), Cu(II), Pb(II), Cr(VI), and As(V) from wastewater. FTIR and Raman spectroscopy, XRD and XPS confirm the successful incorporation of the imidinothiourea groups within the partially reduced GO nanosheets through nucleophilic substitution reactions with the acyl chloride groups in the chemically modified GO. The ITPRGO adsorbent shows exceptional selectivity for the extraction of Hg(II) with a capacity of 624 mg/g, placing it among the top of carbon-based materials known for the high capacity of Hg(II) removal from aqueous solutions. The maximum sorption capacities for As(V), Cu(II), Cr(VI), and Pb(II) are 19.0, 37.0, 63.0, and 101.5 mg/g, respectively. The IT-PRGO displays a 100% removal of Hg(II) at concentrations up to 100 ppm with 90%, 95% and 100% removal within 15 min, 30 min and 90 min, respectively at 50 ppm concentration. In a mixture of six heavy metal ions containing 10 ppm of each ion, the IT-PRGO shows a removal of 3% Zn(II), 4% Ni(II), 9% Cd(II), 21% Cu(II), 63% Pb(II), and 100% Hg(II). A monolayer adsorption behavior is suggested based on the excellent agreement of the experimental sorption isotherms with the Langmuir model. The sorption kinetics can be fitted well to a pseudo-secondorder kinetic model which suggests a chemisorption mechanism via the imidinothiourea groups grafted on the reduced graphene oxide nanosheets. Desorption studies demonstrate that the IT-PRGO is easily regenerated with the desorption of the metal ions Hg(II), Cu(II), Pb(II), Cr(VI), and As(V) reaching 96%, 100%, 100%, 96% and 100%, respectively from their maximum sorption capacities using different eluents. The IT-PRGO is proposed as a top performing remediation adsorbent for the extraction of heavy metals from waste and polluted water.

KEYWORDS: Partially reduced graphene oxide, Heavy metal ions, Amidinothiourea, Wastewater, Adsorption, Removal of Hg(II), Removal of Pb(II), Removal of Cr(VI).

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INTRODUCTION There are growing public health and environmental concerns regarding the condition of

clean water all around the world.1,2 Water contaminations by many pollutants, especially heavy metals, pose many public health and environmental concerns as reported in the list of hazardous substances compiled by the US EPA.3 Metal ions such as Hg(II), Cu(II), Pb(II), As(V), and Cr(VI), have high toxicity and non-biodegradable properties.4,5 The accumulation of these heavy metals in the human body can lead to several severe and chronic disorders such as impairment of pulmonary function, renal damage, emphysema, hemoptysis, hyper-tension, chest pain, skeletal malformation in fetuses, tremors, and impaired cognitive skills.6,7 Thus, there is a critical need to extract these toxic metal ions from polluted and wastewater. Various processes and techniques have been developed for the extraction of heavy metals from polluted water such as chemical precipitation, coagulation, flotation, reverse osmosis, electrochemical methods, membrane filtration, ion exchange, irradiation, and adsorption.8-10 Adsorption by chelating resins is proven to be the most effective method for the extraction of metal ions from polluted and wastewater. Several types of materials such as zeolites11, clay12, mesoporous carbon13, polymers14, metal-organic frameworks (MOFs)15,16, and covalent organic frameworks (COFs)17,18 have been utilized for the possible removal of toxic ions. However, most of these materials are characterized by either low efficiency or complicated post synthesis modifications with long processing time and prohibitive cost making their practical use for wastewater treatment less likely. Therefore, it is important to find more cost-effective and efficient adsorbents for the extraction of heavy metals from wastewater. The key challenge is the design of high surface area adsorbents with accessible chelating sites characterized by high affinity for capturing and retaining toxic metal ions from polluted water. Among the carbon-based adsorbents, graphene oxide (GO) has attracted increased attention as an efficient adsorbent of dyes and heavy metal ions due to its good dispersion in water, biocompatibility, and relatively easy and cost effective preparation methods.19-24 GO has different oxygen-containing functional groups on its surface such as hydroxyl, epoxide, carbonyl and carboxylic acid, which provide the possibility of covalent modifications with strong chelating groups that have high tendency to coordinate with the metal ions in order to develop superior water treatment agents.19,20 Recently, there have been efforts to chemically functionalize


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GO with amines and thiols for the removal of environmental pollutants from contaminated water.25,26 For example, thiol-functionalized magnetite/GO27, CoFe2O4 reduced GO28, polypyrrole-reduced GO composite29, chitosan-reduced GO composite30, GO modified with 2pyridinecarboxaldehde thiosemicarbazone25, and Zr-P modified GO23 have been utilized as new adsorbents for the extraction of heavy metals from polluted water. In the present work, we introduce the novel adsorbent 2-Imino-4-Thiobiuret Partially Reduced Graphene Oxide (IT-PRGO) for the effective extraction of heavy metal ions from water with a remarkable high selectivity for Hg(II). This particular modification of GO has never been reported before. The design strategy of the IT-PRGO was motivated by the introduction of the amidoxime (AO) functional group within the GO surface due to its known strong affinity for mercury resulting in the formation of strong complexes with high-binding interactions with the Hg(II) ions.31,32 The general procedure for the preparation of IT-PRGO is shown in Scheme 1. The method involves the activation of the carboxylic groups of GO by SOCl2 followed by a nucleophilic substitution reaction with amidinothiourea in the presence of tetra butyl ammonium bromide as a catalyst. This simple design strategy results in the formation of IT-PRGO where GO is partially reduced to form PRGO nanosheets containing the strongly chelating AO functional groups.

Scheme 1. General procedure for the preparation of 2-Imino-4-Thiobiuret Partially Reduced Graphene Oxide (IT-PRGO).


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The major objectives of this work are to develop the IT-PRGO nanosheets containing the amidinothiourea groups as effective chelating agents for Hg(II) and other toxic metal ions such as Cu(II), Pb(II), Cr(VI) and As(V), and to provide detailed studies of the kinetics of the adsorption isotherms in addition to the demonstration of their facile regeneration for the costeffective large scale applications for the extraction of heavy metal ions from polluted water with a superior selectivity for Hg(II). 2. EXPERIMENTAL SECTION 2.1. Materials. All reagents used in this work were analytical grade, used without further purification, and purchased from Sigma Aldrich. Graphite powder (99.999 %), Phosphoric acid (99 %), Sulfuric acid (99%), Potassium permanganate (99 %), Hydrogen peroxide (30%), Thionyl chloride (99%), 2-Imino-4thiobiuret (99%), and Tetra butyl ammonium bromide (99%). Stock solutions of various concentration of each metal ion prepared from HgCl2, CuCl2.2H2O, Pb(NO3)2, K2Cr2O7, KH2AsO4, Ni(NO3)2.6H2O, Cd(NO3)2 and Zn(NO3)2.6H2O were used as sources for the Hg(II), Cu(II), Pb(II), Cr(VI), As(V), Ni(II), Cd(II) and Zn(II) ions, respectively. 2.2. Synthesis of 2-Imino-4-Thiobiuret-Reduced Graphene Oxide (IT-PRGO). GO was prepared by the improved Hummer method33 where a 1: 9 volume mixture of concentrated H3PO4 :H2SO4 (60:540 mL) was used with graphite flakes (4.5 g) and KMnO4 (27.0 g), and the mixture was maintained below 30 °C using an ice bath. The reaction was then stirred and heated for 12 h at 50 °C. The mixture was cooled to room temperature and poured onto ice (600 mL) with 30% H2O2 (4.5 mL). The mixture was then centrifuged, and the solid material was washed several times with 200 ml of 30% HNO3, 200 mL of water, and 200 ml of ethanol (2%). The final product was vacuum dried overnight at 60 °C, resulting in 7.5 g of product. To increase the number of carboxylic groups on the graphene oxide surface, 0.5 g of GO was well dispersed in 500 ml DI water for 60 min to give a clear solution.10 g Cl-CH2COOH and 12 g NaOH were added to the GO solution and sonicated for 3 h. The suspension was then neutralized to pH 6.5 using HNO3. The product was separated by centrifugation, washed several times with DI water and dried in oven at 70 °C for 12 h. For the preparation of acyl chloride graphene oxide, 0.5 g of GO-COOH was dispersed well in 10 ml anhydrous N,N-dimethyl-form amide (DMF) by sonication for 1 h and was then treated with thionyl chloride (SOCl2) (75 mL) at 80 °C for 72 h. The final material was separated by centrifugation, washed with anhydrous DMF and dried under vacuum. For the preparation of IT-PRGO, a suspension of GO-COCl (1 g) in anhydrous DMF (20 mL) was added to a solution of 2-imino-4thiobiuret (2.3 g) and tetrabutyl-ammonium bromide (0.24 g) in 10 mL of DMF, and the mixture was stirred at 25 °C for 120 min and then refluxed at 70 °C overnight. Finally, the product IT-PRGO was


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separated using filter paper, washed with distilled ethanol then water and dried for 24 h at 65°C under vacuum. 2.3. Characterization. The GO and IT-PRGO were characterized by X-ray diffraction using an X’Pert Philips Materials Research Diffractometer, FT-IR spectroscopy using the Nicolet-Nexus 670 FTIR Spectrometer (4 cm-1 resolution and 32 scan), Diamond Attenuated Total Reflectance (DATR), X-ray Photoelectron Spectroscopy (XPS) using the Thermo Fisher ESCAlab 250, SEM using the Hitachi SU-70 FE-SEM, TEM using the Jeol JEM-1230 microscope, and Raman spectroscopy using the Thermo Scientific DXR Smart Raman with 532 nm excitation. 2.4. Extraction of Heavy Metal Ions. The Adsorption of toxic metal ions, e.g. Hg(II), was studied in batch experiments using a series of 20 mL glass vials containing 10 mL of Hg(II) ions solution at the desired pH, initial concentration, and agitation time. Inductively Coupled Plasma Optical Emission (ICPOES) was used to measure the residual concentration of Hg(II) ions where the samples were acidified with 2% HNO3 prior for analysis. The amount of mercury adsorbed per unit mass of adsorbent and the percentage of removal were calculated as follows.

Extraction % = q =

(C − C ) × 100 (1) C

(C − C ) V (2) m

Where Ce and Co are the equilibrium and initial and concentration Hg (II) ions (mg/L), respectively, qe is the adsorption capacity (mg/g), V is the volume of the solution of Hg (II) ions (L), and m is the mass of adsorbent (g). 2.5. Adsorption kinetics. The time required to reach equilibrium adsorption of each heavy metal ion on the IT-PRGO was determined by measuring the adsorption capacity as a function of time. Adsorption kinetics were conducted by adding 5 mg of adsorbent to 20 mL glass vials containing 5 ml of x mg/L of each toxic metal ion solution at room temperature. The vials were then stirred and the solutions were filtered at different time intervals (5, 15, 30, 45, 60, 90, 120, 180, 240, 360. 420 min) using filter papers. ICP-OES was used to measure the equilibrium concentration of the heavy metals in the supernatant. The amount of metal ions adsorbed onto the IT-PRGO at time t, qt (mg/g) was determined using Eq.2. The effect of adsorbent dose on the extraction of the heavy metal ions Hg(II), Cu(II), Pb(II), Cr(VI), and As(V) was studied using different amounts of IT-PRGO (5, 10, 15, 20, 30, 35 mg) in a 20 ml glass vial containing 5 ml of each heavy metal ion at definite concentration. The effect of pH was determined by adjusting the solution pH (1-8) using a few drops of 0.01 M NaOH and 0.01 M HCl solutions. The batch


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adsorption studies were conducted under continuous magnetic stirring for 4 h at 25 °C. After each adsorption experiment, the IT-PRGO sample was removed and the residual concentrations of the metal ions were determined by ICP-OES. 2.6. Desorption Studies. In these experiments, metal-loaded adsorbent was collected from the suspension by centrifuging, washed with DI water and dried at 60 °C. Then, a certain amount of the adsorbent loaded was placed in a series of glass vials containing 10 mL of different eluents (0.2 - 1.0 M HCl, 0.01 EDTA) and the mixture was agitated at 25 °C for 5 hours. The final concentration of metal ions in the eluent was determined by ICP-OES. The desorption efficiency were then calculated as:

Desorption efficiency =

(C − C )V X 100 (3) q m

Where Co is the initial concentration of heavy metal ions in the eluent (mg/L), Ce is the equilibrium concentration (mg/L) after desorption, qe is the adsorption capacity in the removal test, Ve is the volume of the eluent (L) and m is the mass of the IT-PRGO adsorbent. 2.7. Selectivity Studies. The selectivity of the IT-PRGO towards Hg(II) was investigated by adding 5 mg of the resin to a glass vial containing 5 mL of a mixture of the metal ions Hg(II), Cu(II), Pb(II), Ni(II), Zn(II), Cd(II), and stirred for 4 h and then filtered. The concentration of each metal ion in the filtrate was determined by ICP-OES.

3. RESULTS AND DISCUSSION 3.1. Characterization of 2-Imino-4-Thiobiuret-Partially Reduced Graphene Oxide (ITPRGO) The UV−Vis spectrum of GO, displayed in Figure 1(A), shows two characteristic peaks: a shoulder at 295 nm corresponding to n-π* transitions of C=O, C=S, C-O, C-S, and C-N bonds and a maximum peak around 229 nm, which can be assigned to π-π* transitions of C=C bonds and.34,35 The disappearance of the 295 nm peak and the red shift of the π-π* transition of the aromatic C=C bond to278 nm in the spectrum of IT-PRGO indicate the partial reduction of GO and the restoration of some of the C=C bonds in the PRGO sheets.34,35 Figure 1(B) displays the XRD patterns of GO and IT-PRGO. The sharp peak of GO at 2θ = 10.6° indicating an interlayer distance of 0.80 nm is due to the presence of the oxygen functional groups on the surface of the GO sheets which lead to larger separation between the layers as compared to graphite.19,34 The XRD pattern of the IT-PRGO exhibits a weak broad


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diffraction peak at a small diffraction angle of 5.9° suggesting an increased interlayer spacing of 1.49 nm due to the chemical grafting of 2-imino-4-thiobiuret onto the GO sheets which could result in larger spacing between the exfoliated layers due to the bulky size of the IT groups. The disappearance of the peak at 2θ = 10.6° could also provide evidence for the partial reduction of GO.19,34 On the other hand, the broad peak at 24.5° with interlayer spacing 0.35 nm may be due to the restacking of IT-PRGO sheets. It appears that after the removal of water, the majority of the IT-PRGO nanosheets restack resulting in the diffraction peak at 2θ = 24.5° and only a small fraction stays exfoliated showing a diffraction peak at 2θ = 5.9°. The Raman spectra of IT-PRGO and GO are displayed in Figure 1(C) and they show the two characteristic peaks (D and G bands) of graphene-based materials. The G band is associated with the stretching vibration of the conjugated C=C groups and it appears at almost the same frequency of 1592 cm-1 in GO and IT-PRGO.19,34 The D band is related to the disorder in the graphitic structure, and the degree of disorder and extent of defects in the graphitic structures are typically determined by the intensity ratio of the D-band to the G-band (ID/IG). The ID/IG ratio of GO (0.94) increases after the chemical modification with the 2-imino-4-thiobiuret to 1.08, suggesting an increase in the degree of disorder and number of defects in the partially reduced GO sheets of the IT-PRGO.19,34,36 In addition, a new Raman peak at 506 cm-1 is observed in the spectrum of IT-PRGO and is assigned to the C-S stretching thus providing evidence for the incorporation of amidinothiourea groups into the PRGO sheets.37

Figure 1. (A) UV-Vis, (B) XRD patterns and (C) Raman spectra of GO and IT-PRGO.


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Figures 2(A) and 2(B) display SEM images of GO and IT-PRGO, respectively, and the corresponding TEM images are shown in Figs. 2 (C) and 2(D), respectively. Clearly, the images of GO (A and C) show a layered structure with a smooth surface due to the interactions between oxygen-containing functional groups. The wrinkled nanosheets (B and D) are observed after functionalization of GO due to the grafting of the 2-imino-4-thiobiuret ligand into the surface and edges of the GO nanosheets.19,34 Additional SEM images with EDX analysis of the ITPRGO nanosheets before and after the adsorption of Hg(II) are displayed in Figures S1 and S2 of the Supporting Information. The EDX analysis (Figure S2) clearly shows the presence N, Cl and S in addition to C and O in the IT-PRGO before the Hg(II) adsorption. Following the Hg(II) adsorption, the EDX shows clearly the presence of Hg, and the SEM images (Figure S1) show high concentrations of Hg nanoparticles adsorbed within the IT-PRGO nanosheets.

Figure 2. SEM (A, B) and TEM images (C, D) of GO (A and C) and IT-PRGO (B and D).


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The surface functional groups of GO and chemically modified GO are identified using FTIR spectroscopy. The FTIR spectrum of GO, shown in Figure 3, exhibits absorption peaks corresponding to the stretching of hydroxyl (OH), carbonyl (C=O), epoxy (C-O), and aromatic (C=C) at wavenumbers of 3350, 1735, 1213, and 1613 cm-1, respectively.28 As shown in Figure 3, significant changes in the spectrum of GO occur upon modification with chloroacetic acid and thionyl chloride to form (GO-Cl). The intensity of the C=O stretching of the COOH group increases which confirms the increase in the number of surface COOH groups by converting hydroxyl groups to O-CH2COOH in GO-Cl. While the stretching of the hydroxyl groups at 3350 cm-1 in GO nearly disappears in GO-Cl, the intensity of the peak at 1753 cm-1, assigned to – C=O/-COOH on GO, is redshifted to 1680 cm-1 due to to the C=O stretching vibrations of the ClCO group in GO-Cl. Also, the new bands appearing at 670, 1134, 1330 cm−1 can be attributed to the stretching vibrations of the C-Cl groups. Comparing the spectrum of GO-Cl with that of ITPRGO, the stretching vibration bands of the C–Cl groups in GO-Cl disappear and new bands near 1350 cm−1 and 1442 cm−1 corresponding to C–S vibrations, and at 1035 and 1164 cm-1 characteristic of –C=S vibrations31 are clearly observed in the IT-PRGO spectrum. Also, the bands observed at 1542 cm-1 and 1680 and in the IT-PRGO spectrum can be attributed to the bending vibrations of N-H in the NH2 group, and the C=O stretching vibration of the NHCO (amide), respectively. Furthermore, the absorption peaks at 3071 and 3241 cm-1 in the IT-PRGO spectrum are attributed to the C-N stretching vibration of the amide group, and the N-H stretching vibration of the NH2 group, respectively.36


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Figure 3. FTIR spectra of GO, GO-COOH, GO-Cl and IT-PRGO.

The chemical attachment of the 2-imino-4-thiobiuret ligand onto the surface of the GO nanosheets is also evident by the XPS data shown in Figure 4. The survey scans of GO and ITPRGO, displayed in Figs. 4(A) and 4(B), respectively, show a significant decrease in the intensity of the O1s peak and an increase in the intensity of the C1s peak in the IT-PRGO spectrum with new peaks corresponding to N1s and S2p photoelectrons clearly observed in the IT-PRGO survey scan. These observations are consistent with the covalent attachment of the 2imino-4-thiobiuret to the GO nanosheets through chemical reactions between the amine and/or thiol groups of the IT and the oxygen functional groups and the C-Cl groups of GO-Cl. Detailed analyses of the XPS spectra of the IT-PRGO provide further confirmation for the presence of the C-O, O=C-N C=O, C-N, C=N, C-S, C-S-C, and C=S groups within the IT-PRGO nanosheets as described below.


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Deconvolution of the C1s spectrum of GO represented in Fig. 4(C) identifies three components at 288.3 eV (carbonyl carbon C=O), 284.7 eV (non-oxygenated ring carbon), and 286.2 eV(C in C-O bonds)

34,39,40

However, the C1s spectrum of IT-PRGO (Fig.4(D)) is

deconvoluted to five peaks with binding energies of 284.7 eV (non-oxygenated carbon), 285.4 eV ( C in C-N bonds), 287.9 eV (C in C=O), 286.2 eV (C in C-O or C-S bonds), and 289.1 eV ( C in O=C-N or C=N).39,41,42 The presence of the amidoxime groups in IT-PRGO is clearly confirmed by the peaks at 285.4 eV (C in C-N bond), 289.1 eV (C in C=N or O=C-N bonds), and 286.2eV (C in C-S bonds). The chemical shift between the carboxyl and amide groups is due to the smaller electronegativity of nitrogen as compared to oxygen.43,44 The successful incorporation of the amidoxime groups in IT-PRGO is also evident by the N1s and S2p spectra shown in Figs. 4 (E) and 4(F), respectively. The N1s spectrum in Fig. 4(E) can be deconvoluted into three peaks at binding energies of 401.7 eV (N in N-H bonds), 400.2 eV (N in O=C-N bonds), and 399.2eV (N in C-N-C or C=N bonds).

45

Similarly, the S2p

spectrum shown in Fig.4 (F) is fitted to five peaks at binding energies of 162.0 eV (S in S-H bonds), 163.5 eV (S in C-S-C bonds), 164.5 eV (S in C=S bonds), 168.0 eV, and 169.4 eV (S in C-SOX bonds).46 Therefore, the C1s, N1s and S2p XPS spectra of IT-PRGO provide clear evidence for the presence of O=C-N, C-S-C, and C-S, and C-N covalent bonds within the surface of the IT-PRGO nanosheets.


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Figure 4. XPS survey spectra of GO (A) and IT-PRGO (B), and XPS high resolution spectra of C1s in GO (C), C1s in IT-PRGO (D), N1s in IT-PRGO (E), and S2p in IT-PRGO (F).

The combination of the above results particularly the FTIR, Raman and XPS spectra provides strong evidence for the formation of IT-PRGO by nucleophilic substitution reactions involving -NH2 or -SH groups from the 2-imino-4-thiobiuret and the Cl-CO or the C-Cl groups


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of the chlorinated GO (GO-Cl), and therefore two possible structures of the IT-PRGO can be suggested as shown in Scheme 2.

Scheme 2. Two possible pathways for the nucleophilic substitution reactions between 2-imino-4thiobiuret (amidinothiourea) and acyl chloride functionalized graphene oxide (GO-Cl).

3.2. Adsorption Capacity of Heavy Metal Ions on IT-PRGO Since the pH of the solution is expected to affect the adsorption capacity of metal ions from aqueous solutions through deprotonation and protonation of the functional groups on the adsorption surface, a series of batch equilibrium measurements are conducted to investigate the effect of pH on the adsorption of Hg(II), Cu(II), Pb(II), Cr(VI) and As(V) ions by IT-PRGO. Figure 5(A) shows the uptake plots of the metal ions onto the IT-PRGO surface at different pH values. The maximum sorption capacity is observed at pH 5.0 - 5.5 for Cu(II), Pb(II), and Hg(II) ions and at pH 1.0 – 3.0 for As(V) and Cr(VI). The predominant species of the As(V), and Cr(VI) ions are H2AsO4-, and HCrO4-, respectively47 and in acidic solutions, protonation of the NH2 groups on the surface of IT-PRGO will result in strong electrostatic attraction between the protonated amino groups of the adsorbent and the anionic species of the adsorbate.28,47,48 On the other hand, the adsorption capacity of Hg(II),Cu(II), and Pb(II) increases with increasing the pH of the solution, since at acidic conditions the transformation of NH2 into NH3+ results in a few – NH2 sites on the IT-PRGO surface available to coordinate with these metal ions. When the pH increases, protonation of the –NH2 groups is reduced and the surface of the adsorbent will have


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more –NH2 groups to coordinate with the Hg(II), Cu(II), Pb(II) ions. These ions start to precipitate as hydroxides at pH > 6. Therefore, the pH = 5.0 - 5.5 is selected for further investigation of the removal of the metal ions in order to avoid metal precipitation conditions.22

Figure 5. (A) Dependence of the IT-PRGO adsorption capacity of Hg(II), Pb(II), Cr(VI), Cu(II), and As(V) ions on the pH of the solution [Conditions: Co = 500 mg/L (Hg(II)), 50 mg/L Pb(II), 50 mg/L Cr(VI) , 50 mg/L Cu(II), 25 mg/L (As(V); T= 273 K; Adsorbent dose = 0.005 g/ 5 ml]. (B) Effect of initial concentration on the removal of Hg(II), Pb(II), Cr(VI), Cu(II), and As(V) on ITPRGO [Conditions: Co = 25-1100 mg/L Hg(II), 10-500 mg/L Pb(II), 10-300 mg/L Cr(VI), 10-250 mg/L Cu(II), 5-150 mg/L As(V); pH = 5 Hg(II), 5.5 Pb(II) and Cu(II), 3 Cr(VI), 2.5 As(V); T= 273 K; Adsorbent dose = 0.005 g/5 ml].

The effect of initial concentrations of the metal ions on the adsorption capacity of Pb(II), Hg(II), Cr(VI), Cu(II), and As(V) onto the IT-PRGO adsorbent is shown in Figure 5(B). It is clear that that the amount of metal ions adsorbed on the IT-PRGO increases by increasing the initial concentrations of the heavy metals owing to the higher driving force of the concentration gradient at the solid-liquid interface until it reaches the state of equilibrium saturation. The results show that the efficiency of the Hg(II) removal (at pH 5) is 100% for initial concentrations up to 100 ppm and a maximum adsorption capacity of 622 mg/g could be achieved from a


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starting value as high as 900 ppm. For Pb(II) (at pH 5.5) the removal efficiencies are 98% and 95% for initial concentrations of 10 and 50 ppm, respectively and the maximum adsorption capacity is 100 mg/g from a starting value as high as 400 ppm. For Cr(VI) (at pH 3), Cu(II) (at pH 5.5) and As(V) (at pH 2.5), the removal efficiencies from a starting concentration of 10 ppm are 92%, 85% and 60%, respectively. The maximum adsorption capacities for As(V), Cu(II), and Cr(VI) are 19.0, 33.0, and 61.5 mg/g, from initial concentrations of 100, 150, and 250 ppm, respectively. These results demonstrate strong potential of IT-PRGO as an effective adsorbent for the removal of heavy metals from water. The adsorption ability of IT-PRGO toward Pb(II) and Hg(II) is much higher than for the other ions studied. The high adsorption capacity of ITPRGO for the removal of Hg(II) is attributed to the sulfur low-lying empty 3d orbitals which can interact strongly with the Hg(II) ions.49 Figures 6(A) and 6(B) illustrates the effect of contact time on the extraction of Pb(II), and Hg(II) with initial concentrations of 25 and 50 ppm, respectively at pH 5 by the IT-PRGO nanosheets. After only 30 min contact time, the concentrations of these ions decrease from the starting values by 80% and 68%, respectively. The IT-PRGO displays excellent removal efficiency for Hg(II) reaching 100% from the initial concentration of 50 ppm within 60 min contact time as shown in Fig. 6(A). For Pb(II) the removal efficiency is 98% from the initial concentration of 25 ppm within 75 min contact time as described in Fig. 6(B). The IT-PRGO adsorbent also displays excellent performance at very high concentrations of heavy metal ions as shown in Fig. 6(C). The equilibrium contact time for the maximum removal of As(V), Cr(VI), Cu(II), Pb(II), and Hg(II) at initial concentrations 150, 250, 250, 250, 500, and 1000 ppm, respectively can be reached within 120 min contact time. Even at longer contact time up to 8 h, the amounts of metal ions adsorbed remain constant and therefore the agitation time can be optimized at 120 min for all investigated heavy metal ions.


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Figure 6. Dependence of the % removal of metal ions on the contact time for (A) Hg(II) 50 ppm; (B) Pb(II) 25 ppm; (C). Effect of contact time on the removal of Hg(II), Pb(II), Cr(VI), Cu(II), and As(V)) on IT-PRGO [Conditions: Co = 1000 mg/L Hg(II), 500 mg/L Pb(II), 250 mg/L Cr(VI), 250 mg/L Cu(II), 150 mg/L As(V); pH = 5 Hg(II), 5.5 Pb(II) and Cu(II), 3 Cr(VI), 2.5 As(V); Adsorbent dose = 0.005 g/5 ml; T= 273 K).

The effect of the adsorbent dose on the adsorption efficiency of Hg(II), Cu(II), Pb(II), As(V), and Cr(VI) and is presented in Figure S3 (Supporting Information). The results show that the removal efficiency onto the IT-PRGO increases rapidly from 62.0 % to 96.3 % for Hg(II), from 38.8 % to 98.3 % for Pb(II), from 14.8% to 44% for Cr(VI), from 24.4 to 60.5 for Cu(II), and from 18.9 to 86.1 for As(V) with increasing the dosage of IT-PRGO from 1 g/L to 3.5 g/L. This is explained by increasing the availability of the active sites at higher dosage of IT-PRGO. 3.3. Adsorption Isotherms The Langmuir isotherm model, presented by Eq. (4), based on a monolayer adsorption on a homogenous surface is used to fit the adsorption measurements. C 1 C = & (4) Q b q Q Where Ce is the equilibrium concentration of adsorbate (mg L-1), b is a constant related to the energy of adsorption (Lmg-1),

qe is the amount adsorbed per unit mass of adsorbent at


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equilibrium (mg g-1), b (Lmg-1) is the Langmuir constant, and Q is the Langmuir monolayer adsorption capacity (mg g-1).49,51 The parameter RL, defined by Eq. 5, is used to predict the shape of the isotherm to be either linear (RL = 1), irreversible (RL= 0), favorable (0 < RL< 1) or unfavorable (RL> 1) 50,51 The values of Q are calculated from the slope of the linear plots of Ce/qe versus Ce, and b can be can be obtained from the intercept and slope of the plots shown in Figure 7(A) for the studied heavy metal ions. R) =

1 (5) 1 & bc

Figure 7 (A). Langmuir isotherm model for the adsorption of Hg(II), Pb(II), Cr(VI), Cu(II), and As(V) ions on IT-PRGO. (B). Pseudo second-order kinetic model for the adsorption Hg(II), Cu(II), Pb(II), Cr(VI) and As(V) ions on IT-PRGO.

The calculated parameters for the Langmuir model are summarized in Table 1. The adsorption data of the five studied metal ions fit well with the Langmuir model as indicated by the high values of the correlation coefficients (R2) in the range of 0.957-0.998. All the RL values are between 0 and 0.1 indicating favorable adsorption.50,51 In addition, the maximum adsorption capacities (Qmax) for the five metals at 25 °C are calculated to be 657.9, 37.9, 102.2, 66.2, 20.8 mg/g for Hg(II), Cu(II), Pb(II), Cr(VI), and As(V) ions, respectively in excellent agreement with the practical values as shown in Table 1. This reveals that the adsorption of the metal ions on IT-


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PRGO is homogenous with all the active sites have the same attraction to the adsorbates leading to a monolayer adsorption.52 Table 1. Parameters of the Langmuir isotherms for the adsorption of Hg(II), Pb(II), Cr(VI), Cu(II), and As(V) ions on IT-PRGO. Metal ion

Hg(II) Pb(II) Cr(VI) Cu(II) As(V)

Langmuir parameters R2

b ( L/mg)

Qmax, fitted

Qexp

RL

0.957 0.998 0.994 0.991 0.989

0.028 0.118 0.067 0.079 0.082

657.9 102.2 66.3 37.9 20.8

624.0 101.5 63.0 37.0 19.0

0.032 0.017 0.048 0.048 0.075

3.4. Adsorption Kinetics on the IT-PRGO Surface The second-order kinetic model proposed for understanding the mechanism of adsorption is expressed as:53 t 1 t = & (6) q+ K-qq Where k2 (g mol-1 min-1) is the second-order rate constant of adsorption, qe and qt are the adsorbed amount (mg g-1) at equilibrium and at time t (min), respectively. Values of k2 and qe are calculated from the intercept and slope of the plots of t/qt versus t shown in Figure 7(b), and the calculated kinetic parameters are given in Table 2. The correlation coefficients (R2) of pseudosecond order kinetic model (R2 >> 0.99) are very high indicating that the adsorption kinetics of the studied ions can be well described by the pseudo-second order model, as shown in Table 2. Furthermore, the experimental value of qe agree very well with the calculated values using the pseudo-second-order kinetic model as shown in Table 2. The second-order model assumes a bimolecular interaction between the adsorbate and adsorbent where sharing and exchange of electrons is involved consistent with the presence of the strong chelating amidinothiourea groups within the IT-PRGO adsorbent.53


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Table 2. Kinetic parameters for adsorption of Hg(II), Cu(II), Pb(II), Cr(VI), and As(V) on IT-PRGO. Metal Ion Hg(II) Pb(II) Cu(II) Cr(VI) As(V)

qe exp. mg.g−1 624.0 101.5 37.0 63.0 19.0

qe cal. mg.g−1 625.0 105.3 38.5 62.9 19.6

k2 g. mol-1 .min−1 0.0003 0.0006 0.0061 0.0007 0.0076

R2 0.9998 0.9975 0.9994 0.9987 0.9985

3.5. Proposed Mechanism for the Extraction of Heavy Metal Ions by IT-PRGO To gain more insight into the adsorption mechanism of metal ions on the IT-PRGO surface, XPS spectral analyses of the IT-PRGO adsorbent before and after the removal of metal ions were conducted as shown in Figure S4 and S5 (Supporting Information). The survey scan of the IT-PRGO obtained after the Hg(II) adsorption clearly shows the presence of the 100.9 eV and 104.8 eV peaks corresponding to the Hg4f7/2 and Hg4f5/2 electrons of Hg(II), respectively with a spin-orbital splitting of 4.1 eV as shown in Figure S4.29,54 In fact, the high-resolution spectrum of the Hg4f7/2 and Hg4f5/2 electrons measured following the Hg(II) adsorption ions on IT-PRGO shows shift to higher binding energies compared to free Hg(II) ions, as shown in Figure 8(A), thus suggesting the formation Hg(II) complexes with the S and N atoms in the ITPRGO adsorbent.47,56 This is confirmed by the changes observed in the high-resolution S2p spectra of IT-PRGO before and after the Hg(II) adsorption as shown in Figs. 8(B) and 8(C), respectively. The de-convoluted peaks of the S2p spectrum at 169.0 eV and 170.4 eV shown in Fig. 8(B) can be assigned to S2p electrons in C-SOx groups. These peaks decrease greatly in intensity following the Hg(II) adsorption and new de-convoluted peaks at 162.8 eV and 164.0 eV appear in the spectrum after the adsorption of Hg(II) (Fig. 8(C)) and could be assigned to Hg4f7/2 and Hg4f5/2 electrons in C-S...Hg and C=S...Hg complexes, respectively.29,54 Fig. 8(D) shows the XPS spectrum of Cr2p electron in the IT-PRGO nanosheets after adsorption with Cr(VI). The spectrum displays of two broad peaks assigned to the Cr2p3/2 and Cr2p1/2 at electron binding energies of 577.0 eV and 587.1 eV, respectively. These values are close to the binding energies of Cr(III) in Cr2O3 at 577.1 eV and 586.6 eV.57 However, deconvolution of the two broad peaks of the Cr2p1/2 and Cr2p3/2 electrons shown in Fig. 8(D) yields also two higher binding energy components at 578.4 and 588.9 eV which are nearly


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similar to the binding energies of Cr(VI) in CrO3.56 The presence of both Cr(VI) and Cr(III) species on the IT-PRGO surface is intriguing and suggests that the Cr(VI) removal process involves partial reduction of the adsorbed Cr(VI) to Cr(III) species by the electron rich amidinothiourea groups incorporated on the surface of the IT-PRGO adsorbent. To understand the reduction of Cr(VI) to Cr(III) on the surface of IT-PRGO, the N1s spectrum is measured before and after Cr(VI) adsorption as shown in Figs. 8 (F) and 8(F), respectively. Deconvolution of the N1s spectrum shows three peaks at 399.1, 400.8 and 402.2 eV. The 402.2 and 400.8 eV components can be attributed to amino (=NH2+) and ammonium (NH3+) species in good agreements with the literature values.57,58 The maximum sorption capacity of Cr(VI) on IT-PRGO is observed at pH 3 as shown in Fig. 5(A). At pH 3, large amount of H+ present in the solution could lead to the protonation of amino groups of the 2-imino-4-thiobiuret in IT-PRGO. This suggests that the adsorption of Cr(VI) is initiated by electrostatic interaction between the HCrO4− ions and the positive ammonium species on the surface of IT-PRGO followed by the reduction of Cr(VI) to Cr(III) where the proton of the surface ammonium group is transferred to the aqueous solution. The newly generated amino species with the Lewis basicity of the nitrogen atom acts to retain the positive chromium (III) species on the surface of the IT-PRGO.57


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Figure 8. XPS spectra of the IT-PRGO nanosheets for the binding energies of the N1s electron before (A) and after (B) the Hg(II) adsorption, the S2p electron before (C) and after (D) the Hg(II) adsorption, the Hg4f electron after (E) the Hg(II) adsorption, and the Cr2p electron after (F) the Cr(VI) adsorption.

The XPS spectra of IT-PRGO before and after the Hg(II) adsorption indicate that Hg(II) ions have strong chemical interactions with the nitrogen and sulfur atoms in the IT-PRGO adsorbent. These results suggest that the adsorption mechanism occurs through chelation processes between the Hg(II) ions and IT-PRGO as shown in Scheme 3.

Scheme 3. Suggested structures of chelating compounds of Hg(II) ions with IT-PRGO.

3.6. Selectivity for the Extraction of Hg(II) Ions by IT-PRGO


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The selectivity of the IT-PRGO for Hg(II) was conducted in Hg(II), Zn(II), Pb(II), Cd(II), Ni(II), and Cu(II) multiple metal ions system. Figure 9 and Table S1 display that the removal efficiency for Hg(II) ions was larger than other metal ions. Hence, the IT-PRGO adsorbent has a superior selectively for Hg(II) ions from the mixed metal ions solution, and can be used to extract Hg(II) ions among contaminated water.

Figure 9. (A) Metal ions removal on IT-PRGO from a six mixed metal ions solution (C0 = 10 mg/L, adsorbent dose = 1 g/L, pH = 5.0, T = 298 K,). (B) (C0 = 250 mg/L, pH = 5.0, adsorbent dose = 1 g/L, T = 298 K)

3.7. Desorption Studies from the Surface of IT-PRGO For economically feasible wastewater treatments with adsorption, the regeneration of the adsorbent is one of the most important challenges that need be addressed by a practical and simple approach. Tables S2, S3, S4, and S5 (Supporting Information) show that the results of different eluents used for the recovery of Pb(II), Hg(II), Cr(VI), Cu(II), and As(V)from the metal loaded IT-PRGO. The results indicate that Hg(II) could be easily regenerated with 5 ml of (6 % thiourea + 2 M HNO3) with recovery of more than 95.0%. The desorption of Pb(II) and Cu(II) increases from 50.9% to 100 % and from 66.6 to 100%, respectively, when the concentration of nitric acid increases from 0.5 mol/L to 1.5 mol/L. The desorption of As(V) and Cr(VI) increases from from 41.0 % to 100%, and 47.3% to 95.9%, when the concentration of NaOH increases from 0.2 mol/L to 1.0 mol/L. These results demonstrate that IT-PRGO has the potential for high efficiency as well as low cost effectiveness for the commercial applications of waste water treatment.


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3.8. Comparison of IT-PRGO with other Adsorbents To demonstrate the remarkable efficiency of IT-PRGO for the Hg(II) removal, Table 3 compares the values of qmax for the Hg(II) removal by IT-PROG and other adsorbents developed in the literature.25,27,59 -62 It is clear that IT-PRGO records one of the highest qmax values for the removal of Hg(II) ions from water. The Hg(II) removal capacity of IT-PRGO (624 mg/g) is even higher than most of the benchmark highly porous materials including MOF Zr-DMBD (197 mg/g)63, COF-LZU8 (236 mg/g)64, porous carbon (518 mg/g)65, and mesoporous silica (600 mg/g).66 It should be noted that the performance of IT-PRGO is comparable to those of the imine-linked and thiol-linked COFs which are considered the best porous materials for the Hg(II) removal from aqueous solutions.67,68 However, the complicated post synthesis modifications and the high cost associated with the synthesis and activation of these materials make the IT-PRGO a more realistic candidate for waste water treatment applications.

Table 3. Adsorption capacities of various adsorbents for Hg(II) ions. Adsorbent IT-PRGO GO-2-pyridinecarboxaldehyde thiosemicarbazone Layered double hydroxide with MoS42− KMS-1 KMS-2 GO-thiol-functionalized magnetite Mercarptosuccinic acid-LDH

qe max (mg/g) 624 555

Reference This work 25

500 377 297 163 161

59 60 61 27 62

The IT-PRGO also shows superior removal capacity of Pb(II) ions (101.5 mg/g) compared to other GO based materials such as chitosan-GO (99 mg/g)69 and magnetic chitosanGO (79 mg/g)70, and comparable performance to the recently reported Hg-Al layered double hydroxide-PRGO (116.2 mg/g).71 For the removal of Cr(VI) ions, the IT-PRGO (63 mg/g) performs better than the Fe3O4-GO (54 mg/g)72 and the CTAB modified GO (21.6 mg/g)10 adsorbents. Similarly, for the extraction of Cu(II) ions, IT-PRGO shows a removal capacity of 34 mg/g as compared to 18.3 mg/g and 3.3 mg/g for the Fe3O4-GO74 and MWCNT adsorbents, respectively.74


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4. Conclusions and Outlook A novel chelating adsorbent IT-PRGO for the adsorptive extraction of heavy metal Hg(II), Cu(II), Pb(II), Cr(VI), and As(V) ions from water was developed by grafting a simple and cost effective 2-imino-4-thiobiuret ligand on the surface of partially reduced graphene oxide nanosheets. The IT-PRGO exhibits performance in both capacity and selectivity superior to most of the reported materials used for the extraction of toxic metal ions from water. It displays a 100% removal of Hg(II) at concentrations up to 100 ppm and the adsorption is exceptionally rapid showing 90%, 95%, and 100% removal by 15 min, 30 min, and 90 min, respectively at 50 ppm concentration.. The maximum adsorption capacity of 624 mg/g is achieved from a starting

concentration as high as 900 ppm Hg(II) ions. For Cu(II), and Cr(VI), and Pb(II) at a concentration of 10 ppm, the removal efficiency is 85%, 92% and 98%, respectively. In a mixture of six heavy metal ions containing 10 ppm of each ion, the IT-PRGO shows a 4% Zn(II), 6% Cd(II), 7% Ni(II), 22% Cu(II), 69% Pb(II), and 100% Hg(II). This remarkable efficiency and selectivity is

attributed to the combination of the amidoxime functional groups with the large surface area of the partially reduced graphene oxide nanosheets. Sorption isotherms of all the ions studied agree with the Langmuir model suggesting a monolayer adsorption. The sorption kinetics can be fitted well to a pseudo-second-order kinetic model which suggests a chemisorption mechanism via the imidinothiourea groups grafted on the reduced graphene oxide nanosheets. Desorption studies demonstrate that the ITPRGO is easily regenerated with the desorption of the heavy metal ions Hg(II), Cu(II), Pb(II), Cr(VI), and As(V) reaching 96%, 100%, 100%, 96% and 100%, respectively from their maximum sorption capacities using different eluents. The IT-PRGO is proposed as a top performing remediation adsorbent for

the extraction of toxic metal ions from polluted water.

ASSOCIATED CONTENT Supporting Information SEM images (Figure S1) with EDX analysis (Figure S2) of the IT-PRGO nanosheets before and after the adsorption of Hg(II), the effect of the adsorbent dose on the adsorption efficiency of Cr(VI), Hg(II), Cu(II), Pb(II), and As(V) (Figure S3), XPS survey spectra of IT-PRGO after adsorption of Hg(II) and Cr(VI) (Figures S4 and S5). Adsorption capacities of IT-PRGO in a mixture of Cu(II), Hg(II), Zn(II), Pb(II), Cd(II), and Ni(II) ions (Table S1), and the desorption


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results for the recovery Hg(II), Cu(II), Pb(II), Cr(VI), and As(V) ions from the metal loaded ITPRGO adsorbent (Tables S2 to S5). The supporting Information is available free of charge on the ACS Publication website at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author * M. Samy El-Shall: Fax: 804-828-8599; Tel: 804-828-2753; E-mail: [email protected]. ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE-1463989). We gratefully acknowledge the financial support by the Egyptian Ministry of Higher Education and Scientific Research for the Joint Supervision PhD Fellowship for FSA. REFERENCES 1. Fu, Z.; Guo, W.; Dang, Z.; Hu, Q.; Wu, F.; Febg, C.; Zhao, X.; Meng, W.; Xing, B.; Giesy, J. P. Refocusing on Nonpriority Toxic Metals in the Aquatic Environment in China. Environ. Sci. Technol. 2017, 51, 3117-3118. 2. Ruj, B., Water quality and corrosivity of ground water of northwestern part of Bankura District, West Bengal. Journal of Environment and Pollution 2001, 8 (4), 329-332. 3. ATSDR, C., CERCLA Priority List of Hazardous Substances. Agency for Toxic Substances and Disease Registry 2007. 4. Krabbenhoft, D. P.; Sunderland, E. M., Global change and mercury. Science 2013, 341 (6153), 1457-1458. 5. Schwarzenbach, R. P.; Escher, B. I.; Fenner, K.; Hofstetter, T. B.; Johnson, C. A.; Von Gunten, U.; Wehrli, B., The challenge of micropollutants in aquatic systems. Science 2006, 313 (5790), 1072-1077. 6. McNutt, M., Mercury and health. Science 2013, 341 (6153), 1430-1430. 7. Kim, K.-H.; Kabir, E.; Jahan, S. A., A review on the distribution of Hg in the environment and its human health impacts. Journal of hazardous materials 2016, 306, 376-385. 8. Crittenden, J. C.; Howe, K. J.; Hand, D. W.; Tchobanoglous, G.; Trussell, R. R., Principles of Water Treatment. John Wiley & Sons, Incorporated: 2012. 9. Bodagh, A.; Khoshdast, H.; Sharafi, H.; Shahbani Zahiri, H.; Akbari Noghabi, K., Removal of cadmium (II) from aqueous solution by ion flotation using rhamnolipid biosurfactant as an ion collector. Industrial & Engineering Chemistry Research 2013, 52 (10), 3910-3917. 10. Wu, Y.; Luo, H.; Wang, H.; Wang, C.; Zhang, J.; Zhang, Z., Adsorption of hexavalent chromium from aqueous solutions by graphene modified with cetyltrimethylammonium bromide. Journal of Colloid and Interface Science 2013, 394, 183-191.


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Efficient Removal of Heavy Metals from Polluted Water with High

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