New effective 3-aminopropyltrimethoxysilane functionalized magnetic sporopollenin-based silica coated graphene oxide adsorbent for removal of Pb(II) from aqueous environment

Journal of Environmental Management 253 (2020) 109658

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Research article

New effective 3-aminopropyltrimethoxysilane functionalized magnetic sporopollenin-based silica coated graphene oxide adsorbent for removal of Pb(II) from aqueous environment Abdulaziz Mohd Hassan a, b, Wan Aini Wan Ibrahim a, c, *, Mohd Bakri Bakar a, Mohd Marsin Sanagi a, Zetty Azalea Sutirman a, Hamid Rashidi Nodeh d, e, Mohd Akmali Mokhter a a

Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia Department of Pure and Applied Chemistry, Faculty of Science, Kebbi State University of Science and Technology Aliero, Nigeria c Centre for Sustainable Nanomaterials, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia, 81310, UTM Johor Bahru, Johor, Malaysia d Department of Chemistry, Faculty of Science, University of Tehran, Tehran, Iran e Department of Food Science and Technology, Faculty of Food Industry and Agriculture, Standard Research Institute (SRI), Karaj, Iran b

A R T I C L E I N F O

A B S T R A C T

Keywords: Magnetic sporopollenin Graphene oxide Silica Adsorption Lead Wastewater

A new effective adsorbent, 3-aminopropyltrimethoxysilane functionalized magnetic sporopollenin (MSp@SiO2NH2) based silica-coated graphene oxide (GO), (GO@SiO2-MSp@SiO2NH2) was successfully syn­ thesized and applied for the first time in the removal of hazardous Pb(II) ions from aqueous solution. The properties of the composite were characterized using Fourier-transform infrared spectroscopy (FTIR), ther­ mogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX) and vibrating-sample magnetometery (VSM). Evaluation of GO@SiO2-MSp@SiO2NH2 adsorption performance at optimum conditions revealed that the adsorbent has a maximum adsorption capacity of 323.5 mg/g for Pb(II) using 50–200 mg/L initial Pb(II) ions concentrations. Initial and final concentrations of Pb(II) ions in aqueous solution were analyzed using graphite furnace atomic absorption spectroscopy (GF-ASS). The adsorption behavior of Pb(II) ions onto GO@SiO2-MSp@SiO2NH2 was studied using Langmuir, Freundlich and Temkin isotherms models. The values of coefficient of determination showed that the adsorption best fitted the Langmuir model (R2 ¼ 0.9994). Kinetic studies suggested that the adsorption of Pb(II) ion followed a pseudosecond-order rate model (R2 ¼ 1.00) and thermodynamic studies revealed that the adsorption process is endo­ thermic and spontaneous. The effect of co-existing ions on Pb(II) ion adsorption were also studied and found to have considerable effects only at higher matrix concentration. The adsorbent can be reused up to ten times and retain its good adsorption capacity. In addition, GO@SiO2-MSp@SiO2NH2 showed great potential for Pb(II) removal from industrial wastewater samples.

1. Introduction Environmental pollution arisen from industrial waste discharge as a consequence of industrialization process is a serious problem that needs to be controlled. Industrial waste effluents originating from mining operations, fertilizer industry, metal plating, tanneries and textile in­ dustries, usually contains toxic heavy metals in their effluents (Bailey et al., 1999; Spiro and Stigliani, 1996). Disposal of these effluents into water bodies may cause damage to the aquatic environment. These

heavy metals can also lead to severe physiological and health effects if exposed to human even in small concentrations (Rengaraj et al., 2004). For instance, lead (Pb) is one of the most hazardous and toxic heavy metal (Gracia and Snodgrass, 2007). Exposure to Pb(II)-contaminated environment can cause severe health problems including cancer, ner­ vous system dysfunction, gastrointestinal tract diseases, hemo-toxic ef­ fects, impaired hemoglobin formation and function, causing anemia, interferences in the metabolism of calcium and vitamin D (Patrick, 2006; Papanikolaou et al., 2005). Considering the hazardous effects of Pb(II)

* Corresponding author. Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia. E-mail address: [email protected] (W.A. Wan Ibrahim). https://doi.org/10.1016/j.jenvman.2019.109658 Received 2 May 2019; Received in revised form 10 September 2019; Accepted 28 September 2019 Available online 25 October 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.

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exposure, the World Health Organization (WHO) and the U.S. Envi­ ronmental Protection Agency (EPA) have established an action level of 0.005 mg/L (World Health Organization, 2011) and 0.015 mg/L, respectively for Pb(II) in drinking water (Environmental Protection Agency, 2016). Heavy metals are permitted to be discharged only in a very low concentration in wastewater to prevent public streams and water resources from becoming contaminated. Therefore, the polluting industries must adequately treat their effluents prior to discharge, in order to conform to environmental regulations and standards. Conventional methods for removing metal ions from wastewater include chemical precipitation (Espinoza et al., 2012), electrochemical treatment (Rajkumar and Palanivelu, 2004), membrane separation (Caetano et al., 1995) and ion exchange (Hamoda and Fawzi, 2004). However, adsorption is considered more effective alternative feasible method because of its low cost and high efficiency (Yang et al., 2009; Imamoglu and Tekir, 2008). Therefore, the search for new natural and modified materials as adsorbents that are cost effective with good adsorption capacities has been intensified. Carbon-based materials such as carbon nanomaterials (CNM) (Ruparelia et al., 2008), multiwall carbon nanotubes (MWCNTs) (Tawabini et al., 2010), activated carbon (AC) (Pyrzynska and Bystrzejewski, 2010) and graphene oxide (GO) (Tan et al., 2015) have been studied as adsorbents for removal of metal ions from aqueous environments. GO is a carbon-based material derived from graphite oxidization. It has high specific surface areas (theoretical value of 2620 m2/g) (Zhao et al., 2011) and is water soluble with low conductivity (Park et al., 2009). The oxidation of graphite to GO introduces abundant oxygen functional groups (OH, COOH, CO) on GO surface that can be used as anchoring sites for metal ion complexation as well as addition of desired surface functional groups with high affinity for desired metal ion in improving its adsorption performance (Seenivasan et al., 2015). Recently, GO was functionalized with amidinothiourea to produce 2-imino-4-thiobiuret partially reduced GO (IT-PRGO). The IT-PRGO was used as adsorbent for the removal of Pb(II) ions from wastewater with maximum adsorption capacity of 101.5 mg/g (Fathi et al., 2017). However, it is difficult to separate GO materials from aqueous solution after adsorption due to its low typical lateral size (1 μm) and thickness (1 nm) of the GO sheets (Zhou et al., 2017a; Xiang et al., 2015). Mag­ netic separation provides a promising technique for the separation of GO materials in aqueous solution because it is less tedious, highly efficient and required less time compared to filtration and centrifugation (Qin et al., 2014; Zhou et al., 2017b; Huang and Yuan, 2016). Magnetic Fe3O4 nanoparticles on GO (Fe3O4/GO) prepared in situ in one step process was applied as adsorbent to remove Pb(II) from water and obtained a maximum adsorption capacity (qmax) of 86.2 mg/g (Nodeh et al., 2016). Recently, magnetic GO grafted polymaleicamide dendrimer (GO/Fe3O4-g-PMAAM) was synthesized as adsorbent for removal of Pb(II) from aqueous solution and attained qmax of 181.4 mg/g (Ma et al., 2017). Carbon gel-supported Fe (III)-graphene disks (Fe-G/RF-C) synthesized via sol-gel polymerization of resorcinol-formaldehyde (RF), followed by carbonization of the result­ ing polymeric mass, was used as an adsorptive material for removal of Pb(II) from aqueous solution. Fe(III)-GO were in situ doped in the gel at the onset of the polymerization process, which qmax of the synthesized Fe-G/RF-C for Pb(II) was 172 mg/g at optimum conditions (Mishra et al., 2017). Furthermore, Fe3O4 nanoparticles incorporated with other materials were also reported as adsorbents for the removal of Pb(II) from aqueous solution. The qmax value of magnetic functionalized MCM-48 mesoporous silica with amine and melamine-based dendrimer amines (MDA-magMCM-48) (Anbiaa et al., 2015), polyethylenimine magnetic carboxy-methyl chitosan (MCMCPEI) (Wang et al., 2017), sulfonated magnetic nanoparticle (Fe3O4–SO3H-MNPs) (Chen et al., 2017), chito­ san Schiff’s base/magnetic nanoparticles (CSB@Fe3O4) (Weijiang et al., 2017) and Fe3O4/Chitosan nanoparticles (Fe3O4/CS NPs) (Fana et al., 2017) were 127.2, 124.0, 108.9, 83.3 and 79.2 mg/g, respectively. Another material of interest in this study, sporopollenin (Sp), has

also been modified with magnetic nanoparticles (Fe3O4-Sp) and applied as adsorbent for the removal of Pb(II) with qmax of 22.7 mg/g (Sener et al., 2016). Sp is a bio-macromolecule obtained naturally from Lyco­ podium clavatum and exhibits a molecular structure that is stable to attack by mineral acids and alkalis. The chemical nature of Sp is not completely understood, however it is believed to consist of an aliphatic chain with aromatic groups and abundant hydroxyl group on its network (Gezici et al., 2006; Kamboh and Yilmaz, 2013). The natural adsorptive characteristics of Sp can be enhanced through functionalizing the main network with functional groups that are suitable to a particular adsorption process (Gezici et al., 2006). Recently, 1-(2-hydroxyethyl) piperazine-magnetic nanoparticles-sporopollenin (MNPs-Sp-HEP) was synthesized via functionalization of Sp spores with 1-(2-hydroxyethyl) piperazine and applied as adsorbent for the removal of Pb(II) and As(III) from aqueous solution. This adsorbent showed higher selectivity for As(III). The adsorbent qmax of 13.36 and 69.85 mg/g was attained for Pb(II) and As(III), respectively in aqueous media (Ahmad et al., 2017). Meanwhile, chelating resins based on carboxylated epi-chlorohydrine-Sp (CEP–Sp) and bis-diaminoethylglyoxim-Sp (b-DAEG–Sp) were used as adsorptive materials to remove chromium from aqueous solution with qmax of 133.33 and 1.23 mmol/g for CEP–Sp and b-DAEG–Sp, respectively (Gode and Pehlivan, 2007). To the best of our knowledge, there is no study reported on 3-amino­ propyltrimethoxysilane functionalized magnetic-Sp grafted onto GO for removal of heavy metals or other contaminants from aqueous solution. The quest to harness the full potential of Sp-based materials for the removal of Pb(II), led us to synthesize for the first time, 3-aminopropyl­ trimethoxysilane (APTS) functionalized magnetic-Sp (MSp@SiO2-NH2) and then grafted onto silica-coated GO (GO@SiO2) to form a new hybrid adsorbent (GO@SiO2-MSp@SiO2-NH2) with improved efficiency for removal of Pb(II) from aqueous media using batch adsorption technique. The introduction of APTS coating on MSp in this study, not only protects the Fe3O4 on MSp from being oxidized in acidic solution (Zhang et al., 2013a) but also provide additional functional groups (-NH2) which can serve as active sites for Pb(II) sorption (Zhang et al., 2013b). 2. Materials and method 2.1. Materials Lycopodium clavatum spores (Sp) with 20 μm particle size and graphite with particle size of 44 μm were obtained from Fluka Chemicals (Shanghai, China). All other chemicals such as FeCl3. 6H2O, FeCl2. 4H2O, KMnO4, HNO3 (65%), H2SO4 (97%), 5 M HCl, H2O2 (30%), toluene, 3-aminopropyltrimethoxysilane (APTS), conc. HCl, NaOH and ammonia used in this work were obtained from Shanghai Chemical Reagent Corporation (PR China). All the reagents were of analytical grade. 2.2. Synthesis of magnetic sporopollenin Magnetic sporopollenin (MSp) was prepared as follows: 13.32 g FeCl3.6H2O, 19.88 g FeCl2.4H2O, 5 mL 5 M HCl, 40 mL milliQ water and 5 mL ethanol were mixed in a 100 mL flask and refluxed at 40 � C until complete dissolution of the salts. Then, 2 g Sp was dispersed in 60 mL of this solution and stirred for 2 h at room temperature. The Sp suspension was filtered and quickly washed with milliQ water on the filter. The filtrate was immediately transferred into 40 mL ammonia solution (1 M). After 2 h stirring at room temperature, the prepared MSp was collected by using an external magnet placed on the wall of the beaker and thoroughly washed with milliQ water and dried under vacuum (Kamboh and Yilmaz, 2013). 2.3. Synthesis of graphene oxide Graphene oxide (GO) was synthesized using the modified 2

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Hummers’s method (Hummers and Offeman, 1958). It was obtained from graphite via oxidation using concentrated acids. Graphite was powdered and dried at 80 � C in an oven and then sieved through a 71 μm sieve. 2 g of the powdered graphite was then mixed with a solution of 30 mL H2SO4 (97%) and 20 mL HNO3 (65%), and stirred for 24 h. KMnO4 (3.0 g) was gradually added into the mixture and stirred at 50 � C for 20 h. The mixture was cooled and poured over ice (300 g), followed by a gradual addition of 3 mL H2O2 (30%) until the solution turned yellow. The resulting yellow solution was diluted using 800 mL distilled water and allowed to precipitate overnight at ambient temperature. The solid product (GO) obtained was decanted and washed using milliQ water until neutral pH.

vacuum dried (48 h) for further analysis (Gode and Pehlivan, 2007). Fig. 1 shows the proposed synthetic pathway for GO@SiO2-MSp@SiO2NH2. 2.5. Instrumentation The synthesized GO@SiO2MSp@SiO2NH2 was characterized using Fourier transform infrared spectrometry (FTIR), thermogravimetric analysis (TGA), field emission-scanning electron microscopy (FESEM), energy dispersive X-Ray analysis (EDX) and vibrating sample magne­ tometry (VSM). A TM 400 FTIR spectrometer from PerkinElmer (Wal­ tham, MA, USA) was used for FT-IR measurements using KBr pellet technique. All spectra were recorded in transmission mode from 400 cm 1 to 4000 cm 1. A JSM-6701 F electron microscope from JEOL (Tokyo, Japan) was used for FESEM analysis. A JED-2300 EDX from JEOL (Tokyo, Japan) was employed for the elemental analysis of the synthesized adsorbents. A TGA 8000 thermal analyzer from PerkinElmer (Waltham, MA, USA) was applied for thermal analysis of the adsorbents. Samples were heated in a nitrogen atmosphere at a flow rate of 100 mL/ min with a heating rate of 10 � C/min. The magnetic properties of the materials were studied using a VSM 7400 vibrating sample magnetom­ eter from Lake Shore Cryotronics (Ohio, USA), at an applied maximum field of 15,000 G at room temperature. Oxidation and reduction poten­ tial (ORP) portable meter MW500 (Milwaukee, USA) was used for ORP measurement. HI9813-5 Portable Electric conductivity (EC) meter (Woonsocket, USA) was used for salinity measurement.

2.4. Synthesis of 3-aminopropyltrimethoxysilane functionalized magnetic-Sp grafted onto silica coated GO (GO@SiO2-MSp@SiO2NH2) The immobilization of the 3-chloropropyltrimethoxysilane (CPTS) onto GO was performed as follows: 10 g of GO was dissolved in 100 mL toluene followed by addition of 10 mL CPTS. The mixture was refluxed for 72 h and conditioned under vacuum to obtain GO@SiO2Cl. 10 g of Fe3O4-Sp (MSp) was dissolved in 100 mL toluene, then 25 mL 3-amino­ propyltrimethoxysilane (APTS) was added and the solution was refluxed for 72 h. The product (MSp@SiO2NH2) was separated from solution with an external magnet, washed with n-hexane and conditioned under vacuum. Finally, 5 g GO@SiO2Cl and 5 g MSp@SiO2NH2 were further dissolved in 100 mL toluene and refluxed for 24 h to produce GO@SiO2MSp@SiO2NH2, collected and washed with n-hexane (150 mL) before

Fig. 1. Proposed synthetic pathway of GO@SiO2-MSp@SiO2NH2 3

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2.6. Adsorption experiment

3. Results and discussion

Adsorption experiments of Pb(II) ions onto the prepared GO@SiO2MSp@SiO2NH2 adsorbent were conducted using batch method. The ef­ fect of solution pH, contact time, temperature and adsorbent dose on Pb (II) ions adsorption were studied. For adsorption equilibrium experi­ ment, a fixed adsorbent dose (20 mg) was weighed into a 100 mL conical flasks containing 50 mL Pb(II) ions of different initial concentrations (50–200 mg/L). The pH of the solutions was set to pH 6.0 and shaken until equilibrium condition at 30 min. The adsorbent was separated from solution using external magnet. The residual concentration of Pb(II) ions in the solution was analyzed using a PerkinElmer 3110 graphite furnace AAS (GF-AAS). The percentage removal (% R) of Pb(II) was calculated using Eq. (1);

3.1. Characterization

%R ¼

Ci

Ce Ci

� 100

The FTIR spectrum for the synthesized material was compared with that of pure Sp (Fig. 2a). IR bands assigned to Sp were 3400 cm 1, 29252850 cm 1, 1705 cm 1 and 1450 cm 1 corresponding to O–H stretching –O vibrations, C–H stretching vibrations of saturated carbons, C– stretching and C–H bending vibrations, respectively (Gode and Pehli­ van, 2007). The materials presented similar set of bands for Sp morphology as the based material. However, after the addition of Fe3O4 and subsequent functionalization of Sp with APTMS to produce the in­ termediate MSp@SiO2NH2 (Fig. 2b), the FTIR spectrum changed due to the disappearance of C–H stretching, C–H bending, C–O stretching peaks at 2854 cm 1, 1450 cm 1, 1300 cm 1 and appearance of new peaks of N–H, Si–O–Si, and Fe–O at 1656 cm 1, 1080 cm 1 and 578 cm 1, respectively. Similarly, the FT-IR spectra of the synthesized GO@SiO2-MSp@SiO2NH2 (Fig. 2c), showed characteristic peaks at 3400 cm 1 and 3200 cm 1 which were assigned to O–H stretching fre­ quency and adsorbed water molecules, respectively (Seenivasan et al., 2015). Peaks at 2928 cm 1 and 2854 cm 1 were attributed to C–H – O stretching vibrations and 1656 cm 1 to stretching, 1715 cm 1 to C– N–H bending vibrations (Kamboh and Yilmaz, 2013). In addition, peaks at 1113 cm 1 and 1060 cm 1 were assigned to Si–O–Si stretching, while peaks at 644 cm 1 and 578 cm 1 were attributed to Fe–O bending vi­ brations (Kamboh and Yilmaz, 2013; Ahmad et al., 2017; Gode and Pehlivan, 2007). It was observed that, the intensity of Fe–O peaks at 644 cm 1 and 578 cm 1 decreased while Si–O peaks at 1113 cm 1 and 1060 cm 1 increased on the final product (Fig. 2c) due to contribution of more SiO2 from the addition of GO@SiO2Cl in the synthesis process. Similar phe­ nomenon of magnetism decreases with increasing SiO2 layer, due to the covering of Fe–O nanoparticles by SiO2 was also observed and confirmed by Deng et al. (2005). The peak at 3200 cm 1 on the final product is due to adsorbed water molecules on GO. Furthermore, there was disap­ pearance and reduction in the intensity of peaks assigned for C–H at 2854 cm 1 and 2926 cm 1 after GO@SiO2Cl was immobilized. Thermal properties of GO@SiO2-MSp@SiO2NH2, MSp@SiO2NH2 and Sp were investigated using TGA (Fig. 3). In the first step of thermal degradation of the materials, weight loss occurred from 50 � C to 200 � C, which could be assigned to removal of volatile oxygen containing functional groups such as CO, OH, COOH, and H2O vapors from the sample caused by the destruction of the oxygenated functional groups (Xu et al., 2008a). Compared to Sp and MSp@SiO2NH2, GO@SiO2-MS­ p@-SiO2NH2 showed abrupt and 10% more weight loss due to the additional oxygen containing functional groups of GO contained in the intermediate GO@SiO2Cl. The weight loss in the range from 200 to 500 � C could be assigned to decomposition of silica and aminopropyl groups grafted onto silica surface (SiO2NH2) (Wang et al., 2010). However, Sp showed no weight loss in this temperature range due to the absence of SiO2NH2. Weight loss between 500 � C and 800 � C shows combustion of carbon skeleton while inorganic residues are left behind at 1000 � C (Ren et al., 2013). Overall, GO@SiO2-MSp@SiO2NH2 appeared to be more thermally stable with 40% weight loss at 1000 � C. FESEM images for the morphologies of Sp, MSp, MSp@SiO2NH2 and GO@SiO2-MSp@SiO2NH2 are presented in Fig. 4. Open and uniform pores structure can be seen for pure Sp (Fig. 4a). The image for MSp reveals that the magnetic nanoparticles are predominantly localized inside the open pores of Sp and on its pore walls (Fig. 4b). Following the addition of APTS, the surface cavity and open pores structure of Sp were filled completely (Fig. 4c) with foreign materials presumably SiO2NH2. However, Fe3O4 and SiO2NH2 nanoparticles were appropriately dispersed on Sp surface as shown in Fig. 4b and 4c, respectively. In Fig. 4d, graphene-sheet like material was observed covered with MSp@SiO2NH2, suggesting successful grafting of the Sp-derivative onto GO.

(1)

where, Ci (mg/L) is initial concentration and Ce (mg/L) is final con­ centration (residual concentration in aqueous sample). The adsorption capacity, qe of the magnetic adsorbent was calculated using Eq. (2) (Kamboh and Yilmaz, 2013). � Ci Cf V (2) qe ¼ m where, qe is the adsorption capacity in (mg/g), V is volume of solution (mL), m is mass of adsorbent (mg), Ci is initial concentration of metal ion (mg/L) and Cf (mg/L) final concentration of the metal ion. The method detection limit (MDL) and recovery data for Pb(II) analysis are provided in supplementary material (Table S1). 2.7. Regeneration studies 0.5 M EDTA and 0.1 M HCl were used for desorption studies. The used adsorbent was thoroughly washed with deionized water, dried at room temperature, and then transferred to a flask containing 10 mL EDTA (0.5 M) and 5 mL HCl (0.1 M) solution. The solution was stirred for 90 min at 298 K. After desorption, the adsorbent was removed from solution with the help of an external magnet and the adsorbent was washed thoroughly using 100 mL milliQ water until a neutral pH was obtained and dried at 90 � C for 24 h. The residual Pb(II) concentration in the solution was determined by using GF-AAS (Gode and Pehlivan, 2007). The adsorption and desorption processes were repeated till a significant change in the performance of the adsorbent was observed. 2.8. Application of GO@SiO2-MSp@SiO2NH2 for treatment of real samples In order to evaluate the applicability of GO@SiO2-MSp@SiO2NH2 to remove Pb(II) from real samples, three industrial wastewater samples were obtained from (1). Chalawa tanneries, Northwest Nigeria (2). Petrochemical company in Southern Nigeria and (3). Wastewater of industrial location along Skudai river tributary at Kulai, Johor, Malaysia. Suspended particles were removed from the wastewater samples, and GO@SiO2-MSp@SiO2NH2 was applied without further pretreatment of the wastewater. However, prior to GF-AAS analysis, the wastewater samples were separately digested using conc. HNO3 to avoid interferences. The initial concentrations of Pb(II) ion in the wastewater samples were 4.9 mg/L, 1.7 mg/L and 1.6 mg/L for tannery effluent, petrochemical effluent and Kulai wastewater, respectively. The Pb(II) concentrations were spiked at 100 mg/L in each of the samples analyzed, then treated with 20 mg GO@SiO2-MSp@SiO2NH2.

4

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Fig. 2. FTIR spectra of (a) sporopollenin, Sp, (b) MSp@SiO2NH2 and (c) GO@SiO2-MSp@SiO2NH2

Fig. 5 illustrates the magnetic hysteresis loop of MSp, MSp@SiO2NH2 and GO@SiO2-MSp@SiO2NH2. This result suggested that the sample is super paramagnetic and the saturation magnetization values for MSp, MSp@SiO2NH2 and GO@SiO2-MSp@SiO2NH2 were found to be 58, 50 and 30 emu/g, respectively. The decline in magnetization value can be ascribed to the surface modification of MSp by SiO2NH2 and GO@SiO2. Although, the saturation magnetization value of the adsorbent is lower compared to MSp, but the magnetic property of the adsorbent remained high enough to achieve magnetic separation in short time (within 70s) and facilitated reusability (Ren et al., 2013). Similar declined in satu­ ration magnetization due to surface modification of materials were re­ ported by Abd-Ali et al. (2016) and Liu et al. (2017). 3.2. Effect of pH

Fig. 3. TGA plot for GO@SiO2MSp@SiO2NH2

(a)

Sp,

(b)

MSp@SiO2NH2

and

The pH of point of zero charge (pHzpc) for GO-based materials are approximately 3.4–6.3 (Wu et al., 2014; Yan et al., 2015). This suggest that, the adsorbent will attract Pb(II) ions at a relatively higher pH so­ lution. The amino groups on the GO@SiO2-MSp@SiO2NH2 are easily þ protonated to NHþ 3 in acidic solution, and the resulting –NH3 groups exhibited strong electrostatic repulsion with Pb(II) ions, thus decreased the percentage removal at lower pH values. Meanwhile, increase in pH reduced the competition between Hþ and Pb(II) ions for attachment onto the adsorbent oxygenated active sites, such as COOH and OH (Fialova et al., 2014). In basic solution, the functional groups are deprotonated to COO and O providing the surface charges needed for electrostatic interactions that is favorable for the adsorption of Pb(II) ion. Fig. S2a, shows increased removal of Pb(II) at higher pH values (Xu et al., 2008b), but Pb(II) precipitate as Pb(OH)2 at pH values of 7.0 and above (Espinoza et al., 2012). Therefore, pH 6 is selected as the optimum pH for further studies to avoid potential interference of Pb(OH)2 pre­ cipitate (Kumar et al., 2014).

(c)

The elemental analysis (EDX) of the prepared GO@SiO2MSp@SiO2NH2 adsorbent further proves the presence of C, N, O, Fe and Si (Fig. S1). Table 1 gives the EDX elemental percentage composition results of the GO@SiO2-MSp@SiO2NH2 based on percentage of atoms of the respective elements present. Fig. S1a shows the EDX spectra of Sp consisting of only the spectral lines of C and O. Fig. S1b (MSp) shows the presence of Fe in addition to C and O, confirming the presence of magnetic nanoparticles (Fe3O4) on the modified-Sp. In Fig. S1c (MSp@SiO2NH2) the presence of C, N, O, Fe and Si was observed as expected which confirmed the successful grafting of APTS functional­ ized MSp. The EDX of GO@SiO2-MSp@SiO2-NH2 (Fig. S1d) displayed the existence of those elements with slightly higher percentage of atoms of C, O and Si. However, the reduction in atom percentage for N and Fe compared to that of MSp@SiO2NH2 could presumably due to the grafting of GO@SiO2 onto MSp@SiO2NH2.

3.3. Effect of contact time Equilibrium time is another important parameter in heavy metals treatment process. The effect of the contact time on the adsorption of Pb (II) ion onto GO@SiO2-MSp@SiO2NH2 was investigated using series of adsorption experiments for contact time within the range of 10–90 min 5

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Fig. 4. FESEM images of (a) Sporopollenin, Sp, (b) Magnetic sporopollenin, MSp, (c) MSp@SiO2NH2 and (d) GO@SiO2-MSp@SiO2NH2. Mag. ¼ 5.00 k. Table 1 Elemental composition of GO@SiO2-MSp@SiO2NH2. Sp MSp MSp@SiO2NH2 GO@SiO2-MSp@SiO2-NH2

%C

%O

% Si

%N

% Fe

79 46 43 46

21 37 32 34

–

– – 8 7

– 17 14 8

-

3 5

and the results are presented in Fig. S2b. Evidently, the adsorption rate of Pb(II) ions increases as the contact time is increased from 10 min to 30 min and remains constant at 30 min to 90 min. Therefore, equilib­ rium for Pb(II) is attained after shaking for 30 min. However, it can be seen that the removal rate of Pb(II) was rapid within the first 10 min, wherein about 80% of Pb(II) ion was adsorbed then increased slowly and steadily until adsorption equilibrium is attained at 30 min. The initial fast removal of Pb(II) was due to the high number of active sites on the surface of the adsorbent available for Pb(II) ion. As the contact time was increased, the available active sites gradually decreased and the driving force weakened, resulting in the slow removal rate. Similar findings of fast adsorption of metal ions onto the adsorbtion the first few minutes were also observed by other researchers (Wang et al., 2013; Wu et al., 2013).

Fig. 5. VSM plot for (a) Magnetic sporopollenin, MSp, (b) MSp@SiO2NH2 and (c) GO@SiO2-MSp@ SiO2NH2

time. The percentage removal of Pb(II) increased with an increase in temperature, 48%, 53%, 70% and 80% at 20, 25, 30 and 35 � C, respectively (Fig. S3). This observed trend is possibly due to the increase in diffusion and decrease in viscosity of the solution when the temper­ ature is increased (Kumar et al., 2014). The increase in percentage removal of Pb(II) with increasing temperature indicate that the nature of

3.4. Thermodynamic study Thermodynamic experiments were performed to evaluate the influ­ ence of temperature on the adsorption process of 100 mg/L Pb(II) in 50 mL aqueous solution, pH 6, 15 mg adsorbent dose and 30 min contact 6

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adsorption process is endothermic. As the temperature increases, the vibrational frequencies of the respective molecule also increase, thereby resulting in increase in effective collisions between the metal ions and the adsorbent active sites. Thermodynamic equilibrium constant, KC is defined by Eq. (3). Thermodynamic parameters i.e., enthalpy changes ðΔHÞ and entropy changes ðΔSÞ were calculated according to Van’t Hoff Eq. (4) and Gibbs free energy ðΔGÞ (Eq. (5)). KC ¼

qe Ce

of Pb(II) at 50, 100, 150 and 200 mg/L solutions (Fig. 6b). The experi­ mental adsorption capacity (qe) increased gradually up to value of 323 mg/g at 200 mg/L where equilibrium was attained. 3.7. Adsorption studies Sorption isotherm studies are important in understanding the mechanism of adsorption process and provide information on the ad­ sorbent’s maximum adsorption capacity (qm) (Li et al., 2013). Langmuir, Freundlich and Temkin models were applied to understand the in­ teractions between Pb(II) and GO@SiO2-MSp@SiO2NH2. The Langmuir model is based on the assumption of monolayer coverage of adsorbate on a homogeneous adsorbent surface. Adsorption takes place via chemisorption at specified homogeneous sites within the adsorbent. Langmuir equation is described in Eq. (6) (Ramirez et al., 2009).

(3)

where, qe is adsorption capacity in mg/g calculated using Eq. (2) and Ce is final concentration of analyte (mg/L) after adsorption. ln KC ¼ ΔG ¼

ΔH=RT þ ΔS=R

(4)

RT ln KC

(5)

Ce Ce 1 ¼ þ qe qm qm kL

where T is temperature (K) and R is the universal gas constant (8.3145 J mol 1 K 1). The plot of ln KC verses 1/T (K) gives a straight line (Fig. S4) with good coefficient of determination (R2 ¼ 0.995). The negative value of slope indicates the endothermic nature of adsorption process. The values of ΔH and ΔS were obtained from the slope and intercept of the straight line, respectively (Gode and Pehlivan, 2007; Hou et al., 2015). Table 2 shows values of ΔH (kJ mol 1) and ΔS (J mol 1 K 1) at optimum temperature of 308 K. The positive values of ΔH indicate endothermic adsorption of Pb(II) onto GO@SiO2-MSp@SiO2NH2 and the positive values of ΔS demon­ strate the increased randomness at the solid–solution interface during sorption and the affinity of GO@SiO2-MSp@SiO2NH2 towards Pb(II) species. The negative value of ΔG confirms that the adsorption was feasible and spontaneous (Gubbuk, 2011). Furthermore, the value of ΔH related to sorption energy can indicate the sorption mechanism involved, either chemisorption or physisorption. Van der Waals in­ teractions and hydrogen bonding are associated with energies of 4–8 kJ mol 1 and 8–40 kJ mol 1 respectively, while the enthalpy requirement for chemisorption starts from 40 kJ mol 1, known as the transition energy boundary (Gubbuk, 2011). Therefore, it can be concluded that, the adsorption of Pb(II) ions onto GO@SiO2-MS­ p@-SiO2NH2 is controlled by chemisorption.

where, qm is the maximum adsorption capacity (mg/g), qe is the experimental adsorption capacity, kL is the Langmuir constant (L mg 1) and Ce is the residual concentration of metal ions in the solution after adsorption. The adsorption capacity was calculated by plotting Ce =qe versus Ce (Fig. 6c). Here, qm and kL were calculated from slope (1=qm ) and intercept (1=qm kL ) of the straight line, respectively (Feng et al., 2012). Heterogeneous surface adsorption is explained by Freundlich model and the adsorption mechanism is controlled by physisorption (i.e. Van der Waals interaction), followed by multilayer adsorption. Freundlich equation is presented in Eq. (7) (Luo et al., 2014). 1 Log qe ¼ Log KF þ Log Ce n

qe ¼

In order to examine the effect of adsorbent dose on the Pb(II) removal, adsorption experiments were set up with various amounts of GO@SiO2-MSp@SiO2NH2 from 1 to 20 mg. Fig. 6a shows the effect of adsorbent dosage for the adsorption efficiency of Pb(II) onto GO@SiO2MSp@SiO2NH2. It was observed that the removal of Pb(II) increased rapidly from 4% to 100% with increasing adsorbent dose from 1 to 20 mg. This is probably due to increase in the number of available active sites as the adsorbent dose increases until equilibrium was attained at 20 mg dose. Thus, an adsorbent dosage of 20 mg was selected for further studies.

The adsorption behavior of Pb(II) on GO@SiO2-MSp@SiO2NH2 was studied in relation to concentration using four different concentrations Table 2 Thermodynamic parameters for the adsorption of Pb(II) onto GO@SiO2MSp@SiO2NH2

99.901

)

ΔS (J mol 0.349

1

K

1

)

-ΔG (kJ mol

)

298 K

303 K

308 K

1.975

3.435

5.131

6.627

(8)

3.8. Kinetic studies

1

293 K

RT RT ln Ce þ ln A b b

where, B ¼ ðRT =bÞ is constant related to heat of sorption (J⋅mol 1), A is Temkin isotherm constant (L⋅g 1), R is Universal gas constant (8.314 J⋅mol 1 K 1) and T is temperature (K). The Temkin linearity was obtained from plots of (qe versus ln Ce). The results are presented in Table 3. The values of b and A are given by the slopes and intercepts of the straight lines, respectively (Inyinbor et al., 2016). Table 3 presents the calculated parameters values of Langmuir, Freundlich and Temkin equations. By comparing the coefficient of determination (R2) values for the three isotherms, it suggests that adsorption of Pb(II) onto GO@SiO2-MSp@SiO2NH2 can be best described by Langmuir isotherm since it exhibited higher R2 (0.9994) and a comparable maximum adsorption capacity (qm) of 323.5 mg/g with experimental equilibrium capacity (qe) of 323 mg/g. Therefore, Langmuir model demonstrates that the adsorption sites on GO@SiO2MSp@SiO2NH2 has good affinity for Pb(II) and the adsorption is monolayer.

3.6. Effect of concentration

1

(7)

where, KF is the adsorption capacity (mg g 1) and Freundlich constant and n is the heterogeneity factor that represents the bond distribution. The Freundlich linearity was obtained by plotting Log qe versus Log Ce then KF and n were calculated from intercept (Log qe ) and slope (1=n), respectively. Temkin isotherm assumes heterogeneous surface energy and nonuniform distribution of sorption heat on adsorbents (Luk et al., 2017; Atar et al., 2012). The Temkin equation is given by Eq. (8);

3.5. Effect of adsorbent dosage

ΔH (kJ mol

(6)

The adsorption kinetics was studied using pseudo-first-order and pseudo-second order rate models under the same set of experimental conditions for optimum contact time studies. The linearity of pseudo7

A.M. Hassan et al.

Journal of Environmental Management 253 (2020) 109658

Fig. 6. Effect of (a) adsorbent dosage on the adsorption of Pb(II) onto GO@SiO2-MSp@SiO2NH2. (Conditions: 100 mg/L Pb(II) in 50 mL aqueous solution pH 6, 30 min contact time at 35 � C) and (b) initial concentration on the adsorption capacity, qe of GO@SiO2-MSp@SiO2NH2 for Pb(II); Plot of (c) Langmuir linearity and (d) pseudo 2nd order linearity for adsorption of Pb(II) onto GO@SiO2- MSp@SiO2NH2

pseudo-second-order rate is higher and closer to unity than that for pseudo-first-order rate, suggesting that the adsorption of Pb(II) on to GO@SiO2-MSp@SiO2NH2 is controlled by chemisorption, since pseudosecond-order rate is based on the assumption that the rate limiting step may be chemical adsorption or chemisorption (Ren et al., 2013; Monier et al., 2010).

Table 3 Sorption and kinetic studies values for adsorption of Pb(II) onto GO@SiO2MSp@SiO2NH2. Model

constant

value

Langmuir

qm (mg/g) R2 KL (mg) qe (mg/g) n KF R2 A b R2 k1 (mg/g min) R2 qe (mg/g) k2 R2 qe (mg/g)

323.5 0.9994 0.9677 323 8.10 192 0.5729 1995 95 0.6671 0.003 0.4267 174 0.004 0.9999 170

Freundlich Temkin Pseudo-first- order Pseudo -second-order

3.9. Adsorption efficiency of as prepared adsorbents The comparison of Pb(II) removal efficiencies of as prepared Fe3O4, Sp, GO, MSp, MSp@SiO2NH2 and GO@SiO2-MSp@SiO2NH2 are pre­ sented in Fig. S5. The efficiency reached up to 100% on the composite GO@SiO2-MSp@SiO2NH2 and decreased down to 10% for Fe3O4. This might be due to the limited active sites on Fe3O4. Although Sp and GO have more active sites compared to Fe3O4 but they are less soluble and less dispersive than MSp in aqueous media. Due to the synergistic effect of SiO2 and Fe3O4 in MSp@SiO2NH2, it tends to be more soluble and dispersive than MSp in aqueous solution as a result Pb(II) ions interacts more effectively with the active sites on MSp@SiO2NH2. In addition to being more dispersive, the presence of amine groups provides additional active sites for binding with Pb(II) ions. On the other hand, the presence of SiO2 prevented GO sheets from aggregation and also improves its dispersity and solubility in aqueous solution, given Pb(II) ions more access to the binding sites on the GO@SiO2 sheets. Thus, combining the two intermediate gives robust and improved composite (GO@SiO2MSp@SiO2NH2) with high efficiency for lead adsorption. Table 4 lists the maximum lead adsorption capacities of some re­ ported adsorbents. Comparing their respective Pb(II) adsorption ca­ pacities with the synthesized GO@SiO2-MSp@SiO2NH2, it can be seen that the latter has a much higher adsorption capacity than the GO-based and other reported adsorbents. Good thermal stability, quick perfor­ mance, ease of separation and reusability are added advantages of GO@SiO2-MSp@SiO2NH2 over various adsorbent.

first-order and pseudo-second-order rate models are given in Eq. (9) and Eq. (10), respectively. ln ðqe

qt Þ ¼ ln qe

t 1 t ¼ þ qt k2 q2e qe

k1 t

(9) (10)

where qe is the adsorption capacity (mg/g) and qt is the equilibrium capacity (mg/g) at time t. k1 and k2 are the pseudo-first-order and pseudo-second-order rate constants, respectively. The values of qe and k1 were calculated from slope ( k1 ) and intercepts (ln qe ) obtained from the linearity of pseudo-first-order rate by plotting lnðqe qt Þ versus k2 and qe were calculated from intercept 1=ðk2 q2e Þ and slope (1= qe ) from the linearity of pseudo-second-order rate by plotting ðt= qt Þ versus t (Fig. 6d) (Akoz et al., 2012). The values of pseudo-first- order and pseudo-second-order rates models for the adsorption of Pb(II) ion onto GO@SiO2-MSp@SiO2NH2 are presented in Table 3. The coefficient of determination (R2) value for

3.10. Regeneration result GO@SiO2-MSp@SiO2NH2 can be regenerated continuously for ten cycles and still retain good adsorption capacity after the tenth cycles 8

A.M. Hassan et al.

Journal of Environmental Management 253 (2020) 109658

Kulai wastewater, non-saline (Rusydi, 2018). This results suggested that GO@SiO2-MSp@SiO2NH2, could effectively be used to remove Pb(II) from slightly saline to non-saline oxidizing waters.

Table 4 Maximum Pb(II) adsorption capacities of some reported adsorbents Adsorbent

Adsorption Capacity (mg/g)

pH

Reference

GO@SiO2-MSp@SiO2NH2 Chitosan – methyl acrylate -diethylenetriamine Graphene oxide functionalized with dithiocarbamate Graphene oxide (GO)

323.5 239.2

6 –

132

5.3

This work Zhang et al. (2017) Gao et al. (2017)

120

–

Poly (-acrylamide) grafted on modified Fe3O4 Synthetic mineral

158.7

6

268

3

Poly (methacrylamide) grafted crosslinked chitosan Silica-cyanopropyl magnetic GO (MGO/SiO2–CN) Tinospora cordifolia (batch adsorption) Tinospora cordifolia (Column adsorption) Modified polypyrrole films

250

4

111.11

5

20.83

4

Raghubanshi et al. (2017) Moradi et al. (2017) Chen and Shi (2017) Sutirman et al. (2016) Gabris et al. (2018) Sao et al. (2017)

63.77

4

Sao et al. (2017)

64

6

Sall et al. (2018)

3.13. Mechanism of Pb(II) adsorption onto GO@SiO2-MSp@SiO2NH2 pH, sorption and kinetic studies in Section 3.2, 3.7 and 3.8 respec­ tively, suggested that, the adsorption of Pb(II) onto GO@SiO2MSp@SiO2NH2 is controlled by chemisorption. This could involve different mechanisms due to the presences of hydroxyl, carboxyl and amine functional groups on the adsorbent. Coordination bond, ion ex­ change and electrostatic attraction are possible mechanisms for the adsorption of Pb(II) onto the adsorbent (Li et al., 2011; Kumar et al., 2014). Essentially, Pb(II) breaks the hydrogen bond of the hydroxyl, carboxyl and amine groups on the adsorbent for ion exchange with Hþ (Yuvaraja et al., 2019; Kumar et al., 2014). Furthermore, the oxygen and nitrogen atoms of hydroxyl and amine groups of the adsorbent provide lone pair of electrons for bonding with Pb(II). These lone pairs of elec­ trons forms coordination bond with Pb(II) during the adsorption process (Yuvaraja et al., 2019; Luo et al., 2014; Heidari et al., 2009; Kumar et al., 2014). The adsorption mechanism is demonstrated by the following reactions:

(Fig. S6). The adsorption capacity of GO@SiO2-MSp@SiO2NH2 decreased from 323 mg/g to 319 mg/g after three cycles (1.2% decre­ ment) suggesting that the composite has good regeneration ability. The adsorption capacity decreased down to 295 mg/g at the tenth cycle with. 8.6% decrement, indicating good regeneration ability even after 10th adsorption-desorption cycles. The decline in adsorption capacity is probably due to weight loss (1.8 mg) of the adsorbent after regeneration.

GO ðGO GO ðGO

3.11. Effect of co-existing ions Wastewater usually contains matrix of cations and anions in which metals in the form of sulfates, acetates and chlorides, co-exist [(Nguyen et al., 2013). To study the effect of co-existing ions on Pb(II) adsorption, five varying matrix concentrations were used ranging from 1 to 20 mg/L consisting of the following co-existing ions: Naþ, Kþ, Mg2þ, Cl , CO23 , Cu2þ Ca2þ, Zn2þ and SO24 in 100 mL aqueous solution added to 50 mL 100 mg/L Pb(II). The percentage removal of Pb(II) decreased from 100% to 73% as the concentrations of co-existing ions increases (Fig. S7) due to competition for the limited active sites on the adsorbent (Nguyen et al., 2013) and the formation of stable complexes (Ghodbane et al., 2008). These results suggested that Pb(II) could be effectively removed using GO@SiO2-MSp@SiO2NH2 despite the presence of competing co-existing ions up to 10 mg/L co-existing ions. Furthermore, the selectivity for Pb(II) demonstrated by the adsorbent over the various co-existing cations was probably due to the higher electronegativity value of Pb(II) (2.33), which made Pb(II) more susceptible to reduction than the corresponding co-existing ions that are more electropositive and easily oxidized. Thus, GO@SiO2-MSp@SiO2NH2 preferentially adsorbed Pb(II) (Ni et al., 2019; Hur et al., 2015; Ping et al., 2016).

COOH þ Pb2þ →GO COOHÞ2 þPb2þ → GO OH þ Pb2þ →GO

OH ​ þ ​ Pb2þ →Sp

O

ðSp

OHÞ2 þPb2þ → Sp

O

Sp

NH2 þHþ →Sp

� 2

Pb2þ þ2Hþ

Pb2þ þHþ �

O

Sp

NH2 þPb2þ →Sp

COO

O

OHÞ2 þPb2þ → GO

Sp

Pb2þ þHþ

COO

2

Pb2þ þ2Hþ Pb2þ þHþ

� 2

Pb2þ þ2Hþ

NH2 Pb2þ NHþ 3

4. Conclusion Chemically modified Sp was grafted onto silica coated GO to suc­ cessfully synthesize the chelating material, GO@SiO2-MSp@SiO2NH2 as adsorbent for excellent Pb(II) removal from aqueous samples. At opti­ mum conditions, the adsorbent showed excellent adsorption capacity for Pb(II) in water, followed the Langmuir adsorption model and can be reused several times (up to 10 times with 8.6% decrement in adsorption capacity). The adsorption process followed pseudo second-order rate kinetics, suggesting chemical adsorption involving valence forces through sharing or exchange of electrons and complexation between the Pb(II) and GO@SiO2-MSp@SiO2NH2. The calculated thermodynamic parameters indicated that, the adsorption of Pb(II) onto GO@SiO2MSp@SiO2NH2 was endothermic and spontaneous. The synthesized adsorbent is practical and efficient for the treatment of Pb(II) contami­ nated water with advantages of high adsorption capacity, ease of sepa­ ration, high thermal stability, ease of regeneration and good reusability, environmental friendly and its raw materials are naturally available in large quantities. Thus, this system can serve as a sustainable and cost effective means of Pb(II) decontamination in aquatic environments.

3.12. Real sample results Table S2, S3 and S4 presents the heavy metals composition of tan­ nery effluent, petrochemical effluent and Kulai wastewater, before and after treatment, respectively. GO@SiO2-MSp@SiO2NH2 showed selec­ tivity for Pb(II) ions in the wastewater samples, but higher concentra­ tions of co-existing ions could affect its removal due to competition for limited active sites. However, 89%, 97% and 91% removal of Pb(II) was achieved for tannery effluent, petrochemical effluent and Kulai waste­ water, respectively. The wastewaters have salinity (electrical conduc­ tivity, EC) values of 703, 418 and 452 μS/cm for tannery effluent, petrochemical effluent and Kulai wastewater, respectively. The tannery effluent could be classified as slightly saline while the petrochemical and

Acknowledgement This work was supported by Universiti Teknologi Malaysia and the Ministry of Education (MOE), Malaysia under the Fundamental Research Grant Scheme (FRGS) vote number R.J130000.7826.4F735 and Universiti Teknologi Malaysia under the Research University Grant 9

A.M. Hassan et al.

Journal of Environmental Management 253 (2020) 109658

(RUG) vote number Q.J130000.2526.18H86.

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