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Effective removal of mercury(II) from aqueous solutions by chemically modified graphene oxide nanosheets
⁎Corresponding author. mselshal@vcu.edu (M. Samy El-Shall)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Abstract
Improved graphene oxide (IGO) is chemically modified with chloroacetic acid and ethylene diamine to form carboxylated improved graphene oxide (IGO-COOH), aminated improved graphene oxide (IGO-NH2), and imino-diacetic acid improved graphene oxide (Imino-IGO). These novel solid phase adsorbents are utilized to adsorb mercury ions from aqueous solutions. The IGO, IGO-COOH, IGO-NH2, and Imino-IGO adsorbents are fully characterized by FT-IR, UV–Vis, XPS, XRD, SEM, TEM, and Raman spectroscopy. Batch adsorption experiments are conducted to evaluate the adsorption of Hg(II) ions by IGO, IGO-COOH, and Imino-IGO under different conditions. The effects of pH, adsorbent dose, temperature, contact time, and initial concentrations on the removal of Hg(II) ions are studied. The results reveal that, at pH 5, the maximum adsorption capacity for the removal of mercury (mg Hg/g adsorbent) follows the order: Imino-IGO (230.0) > IGO-COOH (122.0) > IGO (24.0 mg/g), in which mercury ions’ complexation is highly dependent on the concentrations of (NH), and (OH) containing functional groups in the adsorbent.
The IGO-COOH, and Imino-IGO adsorbents show 100% removal of Hg(II) at concentrations as low as 10 ppm. The equilibrium isotherms for evaluating the mechanism of adsorption process show good fits to the Langmuir model. The Imino-IGO could retain more than 93.0% of its original adsorption capacity after six adsorption-desorption cycles. All data confirmed that the Imino-IGO is a promising material to extract Hg (II) from wastewater.
Keywords
Graphene oxide
Chemically modified graphene oxide
Wastewater
Adsorption
Mercury removal
1 Introduction
Water contaminations by many pollutants especially heavy metals are creating major risks to public health and the environment. Heavy metal ions such as Hg (II) have attracted special attention due to their high toxicity and long half-life which increase their potential for bio accumulation and for serious harm to the environment (Wang et al., 2012; He et al., 2013; Schwarzenbach et al., 2006; Dave et al., 2010). Mercury can be adsorbed into human bodies through skin, lungs, and gastrointestinal tract resulting in several acute and chronic disorders such as emphysema, hypertension, nervous system damage, cancer, and testicular atrophy (Zhao et al., 2011; Jiang et al., 2015; Xiao et al., 2016). As a heavy metal, mercury is widely charged into aquatic environments from industrial processes, including metal processing, leather tanning, metal polishing, electroplating, paint manufacturing, metal plating, mining, ceramics, battery manufacturing, oil refining, plating, electrical, fertilizers, waste disposal, and rubber processing (Choi et al., 2016; Boening, 2000). The removal of mercury ions from the environment is one of the biggest challenges today because ions can quickly spread to large areas due to high mobility (Henriques et al., 2016).
Various processes and methods have been developed for the removal of Hg (II) ions from aqueous solutions, such as chemical oxidation or reduction (Jiang et al., 2015), ion exchange, electrochemical treatment (Dabrowski et al., 2004), flotation membranes (Bodagh et al., 2013), desalination, chemical precipitation (Kyzas et al., 2014), membrane filteration (Charerntanyarak, 1999), extraction (Agboola et al., 2016), and adsorption for the removal of heavy metals from wastewater (Erturk et al., 2007). Most of these methods have some limitations and disadvantages when used for wastewater treatment (Erturk et al., 2007). For example, precipitation is inefficient because it produces large quantities of sludge that require careful management, and desalination and electrochemical treatments require relatively high operating costs (Jiang et al., 2015). Among different approaches, adsorption is the most extensively used method due to its simplicity, flexibility, insensitivity to toxic substances, and high efficiency in large scale applications (Volesky, 2001; Di Natale et al., 2006; Li et al., 2011; Di Natale et al., 2011; Xiang et al., 2017). The main disadvantage of the adsorption technique is the high cost of efficient adsorbents, which increases the cost of wastewater treatment. Therefore, it is important to find efficient and cost-effective adsorbents for the removal of heavy metals from wastewater. Traditional adsorbents such as activated carbon, clay, chitosan, magnetic sorbents (Pinheiro et al., 2014), resins (Di Natale et al., 2011), silica (Qu et al., 2004; Kabiri et al., 2015), zeolites, metal oxides, and alginate suffer from poor adsorption sites, low selectivity and poor regeneration which limit their practical applications real processes. Therefore, it would be valuable to enhance the performance of traditional adsorbents by introducing specific functional groups through chemical modification (Theron et al., 2008).
Among the new 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 (Kabiri et al., 2015; Zare-Dorabei et al., 2015; Shih et al., 2014; Gao et al., 2011; Zhu et al., 2013). The oxygen-containing functional groups in GO such as hydroxyl and epoxy groups on the basic plane and carboxyl groups at the edges, are responsible for improving the hydrophilicity of GO promoting the interfacial interaction with the adsorbates (metal ions and ionic dyes). These functional groups are capable of donating their lone pair of electrons to the vacant orbitals of the transition metal ions and thus forming metal complexes (Zare-Dorabei et al., 2015; Gao et al., 2013; Tadjarodi et al., 2016).However, such groups cannot provide strong interactions with heavy metals such as Hg(II) according to the theory of hard and soft acids and bases (HASAB), but they can provide a unique potential for the chemical modification of GO (Alfarra et al., 2004; Yang et al., 2012). Therefore, functionalization of GO with molecules containing strong chelating groups such as nitrogen and thiol can significantly enhance the removal efficiency of GO-based materials (Tadjarodi et al., 2016; Ziaei et al., 2014; Kumar and Jiang, 2015; Ding et al., 2016).
The present work aims to develop a multi-functional GO adsorbent for high efficiency removal of Hg(II) from aqueous solutions. The major objective of this work is to develop and characterize chemically modified Improved GO (IGO) nanosheets continuing imino groups (Imino-IGO) and evaluate their potential for the Hg(II) removal from aqueous solutions through detailed studies of the isotherms and thermodynamics of their adsorption in comparison with the parent IGO nanosheets and those containing extra carboxylic groups (IGO-COOH).
2 Experimental section
2.1 Materials
Graphite powder of high purity 99.5% was used. The oxidizing mixture was concentrated 98% H2SO4, 99% H3PO and 99% KMnO4. Other chemicals used: 30% H2O2, 99% SOCl2, 99% Tri ethylamine, 99% ethylene diamine, 99% chloro-acetic acid (ClCH2COOH), mercuric chloride (HgCl2), were commercially available and were used as received without further purification. All reagents were bought from sigma Aldrich. Deionized (DI) water was used in all experiments.
2.2 Preparation of improved graphene oxide IGO
IGO was prepared according to the method of Reference (Marcano et al., 2010). A 9:1 mixture of concentrated H2SO4/H3PO4 (540:60 mL) was added to a mixture of graphite flakes (4.5 g) and KMnO4 (27.0 g) and maintained below 30 °C using an ice bath. The reaction was then heated to 50 °C and stirred for 12 h. The reaction was cooled to room temperature and poured onto ice (600 mL) containing 30% H2O2 (4.5 mL). The mixture was then centrifuged and the remaining solid material was washed in succession with 200 mL of water, 200 mL of 30% HNO3, and 200 mL of 2% ethanol. The IGO was then vacuum dried overnight at 60 °C.
2.3 Preparation of acetic acid functionalized IGO (IGO-COOH)
The prepared IGO (0.5 g) was dispersed in 500 mL DI water for 1 h to give a clear solution. 10 g (Cl-CH2COOH) and 12 g NaOH were added to the IGO solution and sonicated for 3 h, then the mixture the pH of the solution was adjusted to 6.5 using HNO3. The product was centrifuged, washed with DI water several times and dried in an oven at 70 °C for 12 h.
2.4 Preparation of acyl-chloride functionalized IGO (IGO-Cl)
0.5 g of IGO-COOH was well dispersed in 10 mL anhydrous DMF by sonication for 1 h and then was treated with SOCl2 (75 mL) at 80 °C for 3 days. The product was separated by centrifugation, washed with anhydrous DMF and dried under vacuum.
2.5 Preparation of ethylene diamine functionalized IGO (IGO-NH2)
A dispersion of IGO-Cl (0.5 g) in 15 mL anhydrous DMF (75 mL) ethylene diamine and (2 mL) Et3N were placed in a around bottom flask and refluxed at 80 °C for 48 h. After the reaction, the solution was cooled to room temperature, filtered, and washed with ethanol/DI water (1:1) for several times and then dried in an oven at 70 °C for 10 h.
2.6 Preparation of imino-diacetic acid functionalized IGO (Imino-IGO)
In a typical reaction, 0.2 g of IGO-NH2 was dispersed in 300 mL carbonate buffer (pH 9.5–10) and 11.24 g ClCH2COOH was added and sonicated for 15 min. The pH maintained at (pH = 9.5–10) during the reaction. The mixture was stirred and refluxed for 10 h at 70 °C. The product was filtered and washed with DI water several times and dried in an oven at 70 °C.
2.7 Characterization
The IGO and functionalized IGO were characterized by FT-IR spectroscopy using the Nicolet-Nexus 670 FTIR Spectrometer (4 cm−1 resolution and 32 scan), Diamond Attenuated Total Reflectance (DATR), X- ray diffraction using an X’Pert Philips Materials Research Diffractometer, 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.
2.8 Removal of Hg (II) ions using batch method
The Adsorption of Hg(II) ions was studied in batch experiments using a series of 20 mL glass vials containing 10 mL of Hg(II) ions solution at the desired initial concentration, pH, and agitation time. The residual concentration of Hg(II) ions was measured by using Inductively Coupled Plasma Atomic Emission (ICP-OES) where the samples were acidified with 2% HNO3 prior for analysis. The amount of mercury adsorbed per unit mass of adsorbent (IGO, IGO-COOH and Imino-IGO) and the percentage of removal were calculated as follows (Pourbeyram, 2016).
2.9 Regeneration and recycling studies
For the desorption experiments, Hg-loaded adsorbent was collected from the suspension by centrifugation and was slightly washed with DI water and dried at 60 °C. Then, certain amount of adsorbent loaded with Hg(II), 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 stirred at 25 °C for 5 h. The final concentration of Hg(II) ions in the eluent was determined by ICP-AES. After desorption, the recovered adsorbent was treated with 0.005 M NaOH to restore the pH of the surface to pH 5 then washed with deionized water several times and then dried in at 80 °C. and subjected to the next five adsorption- desorption with the same procedure described above.
3 Results and discussion
The design strategy of the current adsorbent systems is based on the introduction of oxygen and nitrogen chelating groups within chemically functionalized IGO as shown in Scheme 1. The IGO was produced by oxidation of graphite using H3PO4/H2SO4/KMnO4 according the reported method (Marcano et al., 2010). IGO was first modified with chloroacetic acid to increase the negative charge and the number of carboxyl groups on the surface by converting hydroxyl groups to O-CH2COOH followed by reaction with SOCl2 to convert the oxygen-containing groups (hydroxyl, carboxyl) into acyl chloride groups which can react with ethylene diamine. The final step is the incorporation of the imino-diacetic acid chelating groups by the reaction of the unreacted amino group in ethylene diamine with chloroacetic acid in basic medium. Since imino diacetic functional groups could supply NH functional groups, the adsorption capacity for the removal of mercury is expected to increase significantly.
Fig. 1(a) displays the UV–Vis absorption spectra of the IGO, IGO-COOH, and Imino-IGO nanosheets dispersed in water. The three spectra show two major bands: a maximum between 208 and 240 nm, which can be related to the π-π* absorption of aromatic C⚌C bonds; and a shoulder at 295 nm assigned to n-π* transitions of the C⚌O, C—O, and C-N bonds (Hassan et al., 2009; Abdelsayed et al., 2010). It is clear that the IGO-COOH sample shows a strong absorption peak at 240 nm indicating the presence of a large number of conjugated C⚌C and C⚌O groups. The red shift of the π-π* transition of the aromatic C⚌C bond to 248 nm in IGO-COOH indicates the partial reduction of IGO and the restoration of some of the C⚌C bonds in the IGO-COOH sheets (Khanra et al., 2012). This is also consistent with the change in color from brown of the IGO solution to black in the IGO-COOH solution.
The surface functional groups of the prepared samples IGO, IGO-COOH, IGO-Cl, IGO-NH2, and Imino-IGO were confirmed by FT-IR spectra as shown in Fig. 1(b). The spectrum of IGO shows broad absorption peaks at 3350 and 1735 cm−1 related to the O—H and C⚌O stretching vibrations in the carboxylic acid groups at the IGO surface (Hassan et al., 2009; Abdelsayed et al., 2010; Tang et al., 2016). The spectrum also shows weak characteristic bands at 1200, 1050, and 830 cm−1 assigned to the C—OH stretching, symmetric, and asymmetric stretching vibrations in the epoxy groups, respectively. The spectrum of the IGO-COOH depicts the same absorption peaks of IGO but with higher intensity confirming the increase in the number of carboxylic groups on the surface of IGO. Following the addition of thionyl chloride, the spectrum of IGO-Cl shows a band at 1690 cm−1 assigned to C⚌O stretching of the Cl—CO group. Also, the peaks at 670, 1134, 1330 cm−1 correspond to the stretching vibrations of the C—Cl group. Following the addition of ethylene diamine, the spectrum of IGO-NH2 shows peaks around 1650 and 1500 cm−1 attributed to the C⚌O stretching vibration of NHCO (amide) group and the bending vibration of N—H in the NH2 group, respectively. The peaks at 3254, and 1060 cm−1 are assigned to the N—H and C—N stretching vibrations of the C—NH2 group, respectively. Finally, the Imino-IGO spectrum shows a strong peak at 3350 cm−1, due to the stretching vibration of O—H in the carboxylic acid group. The peaks in the range of 1550–1350 cm−1 are attributed to asymmetric and symmetric COO− stretches. The FT-IR spectra of the intermediate and final products shown in Fig. 1(b) confirm the successful grafting of desired functional groups on the surface of improved graphene oxide.
Fig. 2(a) shows the Raman spectra of the IGO, COOH-IGO and Imino-IGO sheets with the two characteristic G and D bands. 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 IGO and COOH-IGO with a slight blue shift in Imino-IGO (1598 cm−1) which could be due to charge transfer from the imino functional groups to the IGO sheets. The intensity ratio of the D-band to the G-band is usually used as a measure of the degree of disorder and defects in the graphitic structures. The ID/IG in the COOH-IGO (1.07 at excitation wavelength 532 nm) is slightly higher than that of IGO (0.95 at excitation wave length 532 nm) indicating a small increase in the disordered structures. The appearance of a new band at 1465 cm−1 in COOH-IGO, which shifts to 1465 cm−1 in Imino-IGO, could be due to the covalent interactions of the imino-diacetic acid with the IGO sheets (Zedan et al., 2013).
The X-ray Diffraction patterns (XRD) of IGO, COOH-IGO, and Imino-IGO are shown in Fig. 2(b). The sharp peak of IGO at 2θ = 10.55° corresponding to an interlayer distance of 0.801 nm is attributed to the oxygen-containing groups on the IGO sheets (Hassan et al., 2009; Abdelsayed et al., 2010). This peak is shifted to 2θ = 11.50° with an interlayer spacing of 0.75 nm in IGO-COOH which may be due to the distortion of the crystal structure of IGO with the reaction of chloroacetic acid in basic medium. The XRD pattern of Imino-IGO shows a new sharp peak at 2θ = 6.8° corresponding to a large interlayer distance between the sheets due to the presence of the large functional groups –O-CH2CONHCH2CH2N(CH2COOH)2 on the surface of the Imino-IGO sheets. The disappearance of the peak at 2θ = 10.55° provides evidence for the reduction of IGO. The broad peak at 2θ = 24.5° with an interlayer spacing of 0.35 nm may suggest the presence of stacked Imino-IGO graphene like platelets (Namvari and Namazi, 2014).
Figs. 3a and 3b display the TEM and SEM images, respectively, of the IGO and the chemically modified IGO sheets. The TEM images indicate that the IGO sheets have smooth surfaces and appear to be thicker than the thin and slightly crumpled chemically modified sheets (IGO-COOH and Imino-IGO) which appear to have rougher and more wrinkled surfaces probably because of the covalent attachments of the oxygen and nitrogen containing functional groups. The SEM images displayed in Fig. 3b show the graphene-like flake shape morphology with more leaf-like veins especially in the Imino-IGO sheets.
The XPS C1s and O1s spectra of IGO before the adsorption of Hg(II) are shown in Fig. 4(a) and (b), respectively. The C1s and O1s spectra after the adsorption of Hg(II) are shown in Fig. 4(c) and (d), respectively, and the Hg 4f spectrum of IGO after the adsorption of Hg(II) is shown in Fig. 4(e). The corresponding XPS data for IGO-COOH and Imino-IGO before and after the adsorption of Hg(II) are shown in Figs. S1 and S2 (Supporting Information).
XPS data for IGO (Fig. S1) and Imino-IGO (Fig. S2); removal data from 10 ppm Hg(II) solution (Fig. S3), effect of temperature on the removal capacity as a function of the Hg(II) ion’s concentration (Fig. S4), Langmuir and Frundlich adsorption plots (Fig. S5), van’t Hoff plots (Fig. S6), Table S1 for the effect of temperature on the maximum adsorption capacity, and Tables S2–S4 for the desorption data. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.arabjc.2018.06.018.
The XPS C1s and O1s spectra of IGO before the adsorption of Hg(II) are shown in Fig. 4(a) and (b), respectively. The C1s and O1s spectra after the adsorption of Hg(II) are shown in Fig. 4(c) and (d), respectively, and the Hg 4f spectrum of IGO after the adsorption of Hg(II) is shown in Fig. 4(e). The corresponding XPS data for IGO-COOH and Imino-IGO before and after the adsorption of Hg(II) are shown in Figs. S1 and S2 (Supporting Information).
Supplementary Information
Supplementary InformationDeconvolution of the C1s spectrum shown in Fig. 4(a) identifies three components at 284.7 eV (non-oxygenated ring carbon), 286.2 eV (C in C—O bonds) and 288.3 eV (carbonyl carbon C⚌O) (Abdelsayed et al., 2010; Sreeprasad et al., 2011). The O1s peak at 531.1 eV shown in Fig. 4(b) is assigned to the oxygen bound to carbon. (Gomez De Arco et al., 2010) After the Hg (II) adsorption, it is clear that the relative intensities of both C—O and C⚌O peaks are more pronounced as shown in Fig. 4 (c) due to the interaction of Hg(II) with the oxygenated carbon. Moreover, the appearance of a strong shoulder at 533.5 eV in the O1s spectrum shown in Fig. 4(d) confirms that charge transfer from oxygen to Hg(II) has occured following the adsorption of Hg(II). Similar observations are found in the O1s spectra of IGO-COOH and Imino-IGO following the adsorption of Hg(II) as shown in Figs. S1 and S2 (Supporting Information). The XPS data also confirms a significant decrease in the intensity of the C1s peak of the C—N bond in Imino-IGO following the adsorption of Hg(II) (Fig. S2(d), Supporting Information) which is consistent with charge transfer from the C—N bond in Imino-IGO to Hg(II). The presence of Hg(II) in all the IGO, IGO-COOH and Imino-IGO samples after the adsorption experiments is clearly evident by the two distinguished peaks for the 4f7/2 and 4f5/2 electrons of Hg(II) at binding energies of 100.5 eV and 104.6 eV, respectively with a spin-orbital splitting of 4.1 eV as shown in Fig. 4(e) for IGO (Kumar and Jiang, 2015; Chandra and Kim, 2011).
Deconvolution of the C1s spectrum shown in Fig. 4(a) identifies three components at 284.7 eV (non-oxygenated ring carbon), 286.2 eV (C in C—O bonds) and 288.3 eV (carbonyl carbon C⚌O) (Abdelsayed et al., 2010; Sreeprasad et al., 2011). The O1s peak at 531.1 eV shown in Fig. 4(b) is assigned to the oxygen bound to carbon. (Gomez De Arco et al., 2010) After the Hg (II) adsorption, it is clear that the relative intensities of both C—O and C⚌O peaks are more pronounced as shown in Fig. 4 (c) due to the interaction of Hg(II) with the oxygenated carbon. Moreover, the appearance of a strong shoulder at 533.5 eV in the O1s spectrum shown in Fig. 4(d) confirms that charge transfer from oxygen to Hg(II) has occured following the adsorption of Hg(II). Similar observations are found in the O1s spectra of IGO-COOH and Imino-IGO following the adsorption of Hg(II) as shown in Figs. S1 and S2 (Supporting Information). The XPS data also confirms a significant decrease in the intensity of the C1s peak of the C—N bond in Imino-IGO following the adsorption of Hg(II) (Fig. S2(d), Supporting Information) which is consistent with charge transfer from the C—N bond in Imino-IGO to Hg(II). The presence of Hg(II) in all the IGO, IGO-COOH and Imino-IGO samples after the adsorption experiments is clearly evident by the two distinguished peaks for the 4f7/2 and 4f5/2 electrons of Hg(II) at binding energies of 100.5 eV and 104.6 eV, respectively with a spin-orbital splitting of 4.1 eV as shown in Fig. 4(e) for IGO (Kumar and Jiang, 2015; Chandra and Kim, 2011).
The XPS results also show that after the adsorption of Hg(II) on the Imino-IGO sheets, the intensity of the O1s peak corresponding to O⚌C—O— was reduced significantly and a new peak corresponding to O⚌C—O…Hg appeared as shown in Fig. S2 (Supporting Information). Based on the changes in the XPS of the C1s and O1s of the Imino-IGO following the adsorption of Hg(II), it can be concluded that the mechanism of removal of the Hg ions depends mainly on the complexation with the carboxylic groups in the imino-diacetic acid IGO surface as shown in Scheme 2.
The XPS results also show that after the adsorption of Hg(II) on the Imino-IGO sheets, the intensity of the O1s peak corresponding to O⚌C—O— was reduced significantly and a new peak corresponding to O⚌C—O…Hg appeared as shown in Fig. S2 (Supporting Information). Based on the changes in the XPS of the C1s and O1s of the Imino-IGO following the adsorption of Hg(II), it can be concluded that the mechanism of removal of the Hg ions depends mainly on the complexation with the carboxylic groups in the imino-diacetic acid IGO surface as shown in Scheme 2.
Various metal-binding mechanisms are thought to be involved in the removal process including ion exchange (release of H+), surface adsorption, chemisorption and adsorption-complexation (Imino group). Metal-Binding mechanisms may be assumed to involve the following three steps: migration of Hg(II) from bulk of the solution to the surface of the adsorbent; diffusion of Hg(II) through the boundary layer to the surface of the adsorbent; and adsorption of Hg(II) at the active sites (COOH, O⚌C—NH, NH2) on the surface of different adsorbents with release of protons.
3.1 Adsorption parameters of the Hg(II) ions by the chemically modified IGO sheets
The effect of the initial pH on the removal of Hg(II) using IGO, IGO-COOH, and Imino-IGO nanosheets is shown in Fig. 5(a). The adsorption capacity increased when the pH increased and maximum adsorption of Hg(II) ions occurred at pH 5. When the pH increases from pH 1 to pH 6, the amount of Hg (II) ions adsorbed at equilibrium (qe) increases from 0 to 22.4 mg/g, from 2.8 to 41.60 mg/g, and from 18.80 to 49.2 mg/g for IGO, IGO-COOH and Imino-IGO, respectively. The effect of pH is expected because of the presence of various oxygen containing functional groups such as carboxyl groups and hydroxyl groups in IGO and IGO-COOH, and amide groups in Imino-IGO. The electrostatic and ionic interaction between the Hg(II) ions and different adsorbents at different pH values could play a vital role in the adsorption process. The decrease in the removal efficiency of Hg(II) ions at low pH values can be attributed to competition between Hg(II) ions and hydrogen ions for the adsorption sites of different adsorbents. The hydrogen ion is a strong competitor for adsorption due to its small size. At high pH values, the repulsion forces became weaker and the mercury ions may be transported to the surface of the adsorbent due to the electrostatic attraction between the negatively charged surface and the Hg(II) ions (Wang et al., 2010). The high maximum adsorption at pH 5 is explained by the decrease of the solubility of mercury due to extensive hydrolysis (the percentage of HgClOH and Hg(OH)2 species increases) (Carrott et al., 1998).
The effect of the initial concentration on the removal of Hg(II) ions using IGO, IGO-COOH, Imino-IGO at the optimum pH = 5 and at room temperature, is shown in Fig. 5(b). The results show that the equilibrium sorption capacities of the sorbents increase with increasing the initial Hg (II) ion concentration. This is because the higher the initial Hg(II) ion concentration, the higher the driving force of the concentration gradient at solid-liquid interface which causes an increase of the amount of Hg(II) ions adsorbed on the adsorbent. When the initial concentration of Hg(II) ions increases from 10.0 to 250.0 mg/L for IGO, from 50.0 to 400.0 mg/L, and from 50.0 to 600.0 mg/L for Imino-IGO at 25 °C, the amount of Hg(II) ions adsorbed at equilibrium (qe) increases from 8.6 to 24.0 mg/g, from 40.7 to 122.0 mg/g, and from 49.2 to 230.0 mg/g for IGO, IGO-COOH, Imino-IGO, respectively. The removal efficiency of mercury at the highest concentrations of 250.0, 400 and 600 mg/L decreases from 86.0% to 9.2%, from 81.5% to 30.3%, and from 98.4% to 38.3% for IGO, IGO-COOH and Imino-IGO, respectively. This decrease is probably related to the blockage of the hydroxyl and carboxyl groups at higher concentrations of Hg(II) ions. As expected, the adsorption efficiency increased with the initial Hg(II) ion concentration as shown in Fig. 5(b). With more Hg(II) ions present in solution, a larger fraction of the active sites is involved in the adsorption process and the adsorption efficiency reaches a plateau indicating saturation of the available binding sites on the adsorbent. It should be noted that at very low concentrations of Hg(II) such as 10 ppm, a 100% removal can be achieved by both the IGO-COOH and Imino-IGO adsorbents as shown in Fig. S3 (Supporting Information).
The effect of the initial concentration on the removal of Hg(II) ions using IGO, IGO-COOH, Imino-IGO at the optimum pH = 5 and at room temperature, is shown in Fig. 5(b). The results show that the equilibrium sorption capacities of the sorbents increase with increasing the initial Hg (II) ion concentration. This is because the higher the initial Hg(II) ion concentration, the higher the driving force of the concentration gradient at solid-liquid interface which causes an increase of the amount of Hg(II) ions adsorbed on the adsorbent. When the initial concentration of Hg(II) ions increases from 10.0 to 250.0 mg/L for IGO, from 50.0 to 400.0 mg/L, and from 50.0 to 600.0 mg/L for Imino-IGO at 25 °C, the amount of Hg(II) ions adsorbed at equilibrium (qe) increases from 8.6 to 24.0 mg/g, from 40.7 to 122.0 mg/g, and from 49.2 to 230.0 mg/g for IGO, IGO-COOH, Imino-IGO, respectively. The removal efficiency of mercury at the highest concentrations of 250.0, 400 and 600 mg/L decreases from 86.0% to 9.2%, from 81.5% to 30.3%, and from 98.4% to 38.3% for IGO, IGO-COOH and Imino-IGO, respectively. This decrease is probably related to the blockage of the hydroxyl and carboxyl groups at higher concentrations of Hg(II) ions. As expected, the adsorption efficiency increased with the initial Hg(II) ion concentration as shown in Fig. 5(b). With more Hg(II) ions present in solution, a larger fraction of the active sites is involved in the adsorption process and the adsorption efficiency reaches a plateau indicating saturation of the available binding sites on the adsorbent. It should be noted that at very low concentrations of Hg(II) such as 10 ppm, a 100% removal can be achieved by both the IGO-COOH and Imino-IGO adsorbents as shown in Fig. S3 (Supporting Information).
Fig. 6(a) illustrates the effect of contact time on the adsorption capacity of IGO, IGO-COOH, and Imino-IGO for the Hg(II) ions with concentrations of 50, 300 and 600 ppm at pH 5. It is clear that an initial rapid increase in the removal efficiency occurs where a large fraction of the total amount of Hg(II) ions is removed within 5 min reaching more than 43.0%, 57.0%, and 69.0% of the maximum adsorption capacity for IGO, IGO-COOH and Imino-IGO, respectively. This means that a large number of vacant adsorption sites on the adsorbent’ surface is available at this stage. Thereafter, the adsorption rate becomes slower at the adsorption equilibrium and the maximum removal of Hg (II) ions occur within 30 min, 60 min, and 120 min for IGO, IGO-COOH, Imino-IGO, respectively. At these times, the amount of mercury being adsorbed onto the adsorbent is in equilibrium with the amount desorbed from the adsorbent.
The dependence of the removal of Hg(II) ions on the dosage of IGO, IGO-COOH and Imino-IGO is shown in Fig. 6(b). The increase in the sorbent’s dose with the percent of Hg(II) removal is almost linear for the Imino-IGO nanosheets. When the sorbent’s dose increases from 0.01 to 0.035 g, the percent of mercury removal using IGO, IGO-COOH and Imino-IGO increases from 48.0% to 65.8%, from 39.8% to 54.60%, and from 38.3% to 87.8%, respectively. This can be related to the availability of more sorption sites and the increased in the total sorbent surface area with increasing the dosage. However, at higher dosages the sorbents’ particles aggregate leading to a decrease in the total surface area of the adsorbent and consequently a decrease in the removal capacity. This behavior is observed for IGO and IGO-COOH when the dosage is increased to 0.035 g where the percent of Hg(II) removal levels off at 65.8% and 54.60% for initial Hg(II) ion’s concentrations of 50 and 300 ppm, respectively. Interestingly, Imino-IOG at the same dosage of 0.035 g and an initial Hg(II) ion’s concentration of 600 ppm, the percent removal does not reach a plateau indicating no saturation of the available binding sites.
Fig. S4 and Table S1 (Supporting Information) illustrates the temperature effect on the adsorption capacity of Hg(II) ions using the IGO, IGO-COO and Imino-IGO adsorbents. The results indicate that the maximum adsorption capacity of mercury using IGO, IGO-COOH, and Imino-IGO increases from 24.0 to 36.5 mg/g, from 122.0 to 145.0 mg/g, and from 230.0 to 315.0 mg/g, respectively, when the temperature is increased from 298 K to 318 K. This effect is attributed to the increase in the kinetic energy and mobility of Hg(II) ions in solution with temperature. The above results clearly establish that maximum adsorption capacity of Hg(II) from water by the chemically modified GO adsorbents increases in the order of Imino-IGO > IGO-COOH > IGO under all experimental conditions studied such as pH, Hg(II) ion’s concentration, temperature, and sorbent dose.
Fig. S4 and Table S1 (Supporting Information) illustrates the temperature effect on the adsorption capacity of Hg(II) ions using the IGO, IGO-COO and Imino-IGO adsorbents. The results indicate that the maximum adsorption capacity of mercury using IGO, IGO-COOH, and Imino-IGO increases from 24.0 to 36.5 mg/g, from 122.0 to 145.0 mg/g, and from 230.0 to 315.0 mg/g, respectively, when the temperature is increased from 298 K to 318 K. This effect is attributed to the increase in the kinetic energy and mobility of Hg(II) ions in solution with temperature. The above results clearly establish that maximum adsorption capacity of Hg(II) from water by the chemically modified GO adsorbents increases in the order of Imino-IGO > IGO-COOH > IGO under all experimental conditions studied such as pH, Hg(II) ion’s concentration, temperature, and sorbent dose.
3.2 Adsorption isotherms
The batch adsorption isotherms for Hg(II) ions by IGO, IGO-COOH, and Imino-IGO determined based on the Langmuir and Freundlich models are displayed in Fig. S5(a) and (b) (Supporting Information). The Langmuir model assumes homogeneity of adsorption surface with all the adsorption sites having equal adsorption affinity (Eq. (3), while the Freundlich model would suggest heterogeneity of the adsorption surface sites (Eq. (4)) (Pourbeyram, 2016).
The batch adsorption isotherms for Hg(II) ions by IGO, IGO-COOH, and Imino-IGO determined based on the Langmuir and Freundlich models are displayed in Fig. S5(a) and (b) (Supporting Information). The Langmuir model assumes homogeneity of adsorption surface with all the adsorption sites having equal adsorption affinity (Eq. (3), while the Freundlich model would suggest heterogeneity of the adsorption surface sites (Eq. (4)) (Pourbeyram, 2016).
| Adsorbent | Langmuir parameters | Freundlich parameters | ||||||
|---|---|---|---|---|---|---|---|---|
| R2 | b (L/mg) | Qmax, Calc (mg/g) | QExp (mg/g) | RL | R2 | Kf | 1/n | |
| IGO | 0.984 | 0.220 | 23.050 | 24.0 | 0.018 | 0.759 | 10.402 | 0.160 |
| IGO-COOH | 0.996 | 0.081 | 128.205 | 122.0 | 0.030 | 0.768 | 26.869 | 0.302 |
| Imino-IGO | 0.992 | 0.039 | 247.520 | 230.0 | 0.040 | 0.945 | 48.375 | 0.279 |
The plots shown in Fig. 7(a) and (b) clearly indicate that the adsorption behaviors of IGO, IGO-COOH, and Imino-IGO can be well represented by the Langmuir rather than the Freundlich model. The maximum monolayer adsorption capacity (Qmax) calculated from the Langmuir’s model for IGO, IGO-COOH, and Imino-IGO at 25 °C (23.05, 128.21, and 247.52 mg/g, respectively) compare very well with the experimentally determined values of 24.0, 122.0 and 230.0, mg/g, respectively as shown in Table 1. Moreover, insight into the nature of the Hg(II) adsorption isotherm can be obtained by calculating the Langmuir RL parameter defined by Eq. (5) (Pavan et al., 2008).
3.3 Estimation of thermodynamic parameters
The enthalpy change ΔH°, the free energy change ΔG°, and the entropy change ΔS° for the adsorption of Hg(II) onto the IGO, IGO-COOH and Imino-IGO adsorbents are calculated from the van't Hoff plots (Fig. S6, Supporting Information) of temperature dependence of the adsorption coefficient (Kd) defined by Eq. (6), where ρ = 1000 g/L makes a dimensionless Kd (Milonjić, 2007).
The enthalpy change ΔH°, the free energy change ΔG°, and the entropy change ΔS° for the adsorption of Hg(II) onto the IGO, IGO-COOH and Imino-IGO adsorbents are calculated from the van't Hoff plots (Fig. S6, Supporting Information) of temperature dependence of the adsorption coefficient (Kd) defined by Eq. (6), where ρ = 1000 g/L makes a dimensionless Kd (Milonjić, 2007).
The thermodynamic parameters (ΔG°, ΔH° and ΔS°), given in Table 2, confirm the spontaneous nature of the sorption process of Hg(II) by the IGO, IGO-COOH, and Imino-IGO adsorbents. The positive value of ΔH° confirms the endothermic nature of the sorption process consistent with the increase in the maximum adsorption capacity with increasing temperature. The positive ΔS° values indicate an increase of the disorder at the solid/solute interface during the adsorption process resulting from the liberation of water molecules solvating the metal ion to the solution.
| Adsorbent | Mercury concentration (mg/L) | ΔH° (kJ/mol) | ΔS° (kJ/mol K) | −ΔG° (kJ/mol) |
|---|---|---|---|---|
| IGO | 100.00 | 24.22 | 0.21 | 38.36 |
| IGO-COOH | 100.00 | 66.22 | 0.29 | 20.20 |
| Imino-COOH | 100.00 | 67.45 | 0.29 | 26.67 |
3.4 Regeneration and recycling studies
The desorption studies of Hg(II) from the IGO, IGO-COOH and Imino-IGO adsorbents show that the Hg(II) ions could be quantitatively desorbed with recovery above 99.0%.using 0.01 M EDTA or 1 M HCl. The results are given in Tables S2–S4 (Supporting Information). The regenerated adsorbent using HCl as desorbing agent is further treated with NaOH to restore the negative charge of carboxylic group of Imino-IGO then washed several times with deionized water. In order to determine the reuse ability of the composites, the successive adsorption-desorption process was carried out for six times at 300 ppm. Fig. 7 shows that after six adsorption-desorption cycle, in spite of slight decline, over 93.0% regeneration efficiency is still retained. These results confirmed that Imino-IGO has sufficient chemical stability over several adsorption-desorption cycles.
The desorption studies of Hg(II) from the IGO, IGO-COOH and Imino-IGO adsorbents show that the Hg(II) ions could be quantitatively desorbed with recovery above 99.0%.using 0.01 M EDTA or 1 M HCl. The results are given in Tables S2–S4 (Supporting Information). The regenerated adsorbent using HCl as desorbing agent is further treated with NaOH to restore the negative charge of carboxylic group of Imino-IGO then washed several times with deionized water. In order to determine the reuse ability of the composites, the successive adsorption-desorption process was carried out for six times at 300 ppm. Fig. 7 shows that after six adsorption-desorption cycle, in spite of slight decline, over 93.0% regeneration efficiency is still retained. These results confirmed that Imino-IGO has sufficient chemical stability over several adsorption-desorption cycles.
3.5 Comparison of the adsorption capacities of different adsorbents toward Hg(II)
Table 3 lists comparisons of the maximum adsorption capacities of IGO, IGO-COOH, and Imino-IGO adsorbents prepared in this study with various adsorbents previously used for the adsorption of Hg(II) (Choi et al., 2016; Tadjarodi et al., 2016; Luo et al., 2016; Stoycheva et al., 2016). The comparisons indicate that the chemically modified GO-based adsorbents IGO, IGO-COOH and Imino-IGO exhibit higher adsorption capacity than that of most of the other adsorbents reported in the literature, suggesting that these adsorbents are excellent candidates for commercial applications involving the effective removal of Hg(II) from aqueous solutions.
| Adsorbents | Adsorption capacity (mg/g) | Ref. |
|---|---|---|
| 1-Acylthiosemicarbazide-modified activated carbon | 67.8 | Choi et al. (2016) |
| Silica gel with alkynyl terminated monolayer | 174.3 | |
| A novel thymine-functionalized MIL-101 | 51.7 | Luo et al. (2016) |
| Magnetic Fe3O4@SiO2 nanoparticles | 78.3 | Tadjarodi et al. (2016) |
| Magnetic b-cyclodextrin bead and GO | 88.4 | |
| Silica-3-chloropropyltriethoxysilane | 48.1 | |
| Xanthate functionalized magnetic GO | 118.0 | |
| GO/L-cysteine | 79.3 | |
| Thiol-functionalized magnetite/GO | 289.9 | |
| Graphene oxide/chitosan composite | 187.0 | Stoycheva et al. (2016) |
| Multi-walled carbon nanotubes | 89.0 | |
| Improved Graphene Oxide (IGO) | 24.0 | This work |
| Carboxylate IGO (IGO-COOH) | 122.0 | |
| Imino-diacetic acid IGO (Imino-IGO) | 230.0 | |
4 Conclusions
For the removal of mercury from water or wastewater, we have developed chemically modified graphene oxide nanosheets IGO, IGO-COOH and Imino-IGO via simple chemical reactions with chloroacetic acid and ethylene diamine. The results of batch experiments show that the pH of the solution, contact time and initial concentration significantly affect the adsorption amount of Hg(II). The maximum sorption capacity is obtained at pH 5.0. The adsorption equilibrium follows the Langmuir isotherm and the calculated maximum adsorption capacities of 26.5, 123.2 and 235.8 mg/g agree well with the experimentally determined values of 24.0, 122.0 and 23.0 for the IGO, IGO-COOH and Imino-IGO nanosheets, respectively. The efficient adsorption of these materials and low cost of the reagents used in chemical modifications of graphene oxide provide economic feasibility for the commercial applications of these materials for the effective removal of mercury ions from aqueous solutions or wastewater.
Acknowledgments
This work was supported by the US 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.
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