Accepted Manuscript Title: Amino siloxane oligomer-linked graphene oxide as an efficient adsorbent for removal of Pb() from wastewater Author: Shenglian Luo Xiangli Xu Guiyin Zhou Chengbin Liu Yanhong Tang Yutang Liu PII: DOI: Reference:

S0304-3894(14)00248-9 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.03.062 HAZMAT 15835

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

28-12-2013 27-3-2014 28-3-2014

Please cite this article as: S. Luo, X. Xu, G. Zhou, C. Liu, Y. Tang, Y. Liu, Amino siloxane oligomer-linked graphene oxide as an efficient adsorbent for removal of Pb(Pi) from wastewater, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.03.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Amino siloxane oligomer-linked graphene oxide as an efficient adsorbent for removal of Pb(Π) from wastewater

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Shenglian Luoa,*, Xiangli Xua, Guiyin Zhoua, Chengbin Liua,*, Yanhong Tangb, Yutang

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a

cr

Liua

State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University,

b

an

Changsha 410082, P. R. China

Colleage of Materials Science and Engineering, Hunan University, Changsha 410082, P.

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d

M

R. China

* Corresponding authors. Tel. /Fax: +86 731 88823805. E-mail address: [email protected] (S. Luo); [email protected] (C. Liu)

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Abstract A high performance sorbent, oligomer-linked graphene oxide (GO) composite, was prepared through simple cross-linking reactions between GO sheets and poly3-

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aminopropyltriethoxysilane (PAS) oligomers as crosslinking agents. The three-dimensional

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PAS oligomers prevented GO sheets from aggregation, provided foriegn molecules with easier access, and introduced a large mount of amino functional groups. The morphology,

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structure and property of the PAS-GO composite were determined by scanning electron microscope (SEM), transmission electron microscope (TEM), Fourie transform infrared

an

(FTIR), X-ray diffractometer (XRD), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The adsorption performance of PAS-GO was in

Pb(Π)

removing

ions

M

investigated

from

water.

Compared

to

3-

aminopropyltriethoxysilane functionalized GO (AS-GO) which was prepared by the direct

d

reaction between 3-aminopropyltriethoxysilane and GO, PAS-GO exhibited much higher

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adsorptivity toward Pb(Π) with the maximum adsorption capacity of 312.5 mg/g at 303 K

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and furthermore the maximum adsorption capacity increased with increasing temperature. The adsorption could be conducted in a wide pH range of 4.0-7.0. Importantly, PAS-GO had a priority tendency to adsorb Pb, Cu and Fe from a mixed solution of metal ions, especially from a practical industrial effluent.

Keywords: Graphene oxide; oligomer; Pb(II) removal; adsorption capacity; selectivity

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1. Introduction Lead (Pb), a kind of common heavy metal, widely exists in air, soil and water [1]. Lead is not biodegradable and tends to accumulate in living organisms, causing various diseases

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and disorders such as anemia, reproductive, genotoxic, carcinogenic, and neurological

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effects, especially to children [2-8]. The World Health Organization recommends that the permissible limit of lead for potable water is 0.01 mg/L [9]. Since lead is widely used in

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mining and metallurgical engineering, battery manufacturing processes and traditional gasoline, the method to separate it from environment should be seriously studied.

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Researchers and scientists around the world have indeed paid attention in solving the heavy metal pollution in environment recently. There are several methods, such as membrane

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filtration [10], coagulation [11], adsorption [12], ion exchange [13, 14], precipitation and bio-sorption [15, 16], available for removal of lead from wastewater. Among these methods,

d

sorption technique has been widely used as it is simple, economical, and efficient [17].

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Some sorbents, such as clay minerals, oxides, and carbon materials, have been extensively

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used to remove lead from aqueous solutions. However, these materials suffer from either low sorption capacities or high cost [18-21]. Therefore, widely available adsorbents with high adsorption capacity should be developed to remove lead from wastewater before discharge.

Graphene oxide (GO) prepared through Hummers method [22] contains a certain amount

of oxygen-containing functional groups on its surface, such as -COOH, -C=O, and -OH. These groups are available for removing heavy metals from wastewater through the chelation with metal ions. Considering the chelation of oxygen-containing functional groups and the large surface area of graphene oxide sheet, graphene oxide-based materials used as sorbents to remove arsenic [23, 24], methylene blue [25], 4-chloro-2-nitrophenol [26],

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Page 3 of 48

aniline [27], cationic red XGRL [28], and heavy metals [17, 29-30] have received great concern recently. However, the limited amount of functional groups on GO is not enough to effectively remove heavy metals from wastwater, and also the oxygen-containing functional

containing functional groups can exhibit stronger coordination ability.

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groups can not provide strong coordination with heavy metals. In contrast, nitrogen-

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Meanwhile, 3-aminopropyltriethoxysilane (AS) is well-known for forming stable chelates

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with metal ions. Therefore, it is a smart way to introduce AS onto the surface of GO. Several reports have involved the functionalization of GO with AS [31-34]. However, the

an

functionalized GO sheets with AS small molecules are easily scattered in water, resulting in recycling difficulty. Furthermore, the amount of functional groups in the materials is still

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limited. Even so, aminopropyltriethoxysilane modified GO materials have been not used as adsorbents for removing heavy metals from wastewater.

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In this study, poly3-aminopropyltriethoxysilane (PAS) oligomer was used to modify GO

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for the first time. PAS not only possessed much more amino groups than AS, but also was a

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multi-arm polymer molecule. Many GO sheets could be linked into a tridimensional network with the multi-arm PAS bridges (Scheme 1). Because of more functional groups in PAS than in AS, PAS-GO exhibited much higher adsorption capacity toward Pb(Π) than AS-GO. The morphologies, structures, and properties of the resultant materials were investigated. The adsorption mechanism was also elucidated.

2. Materials and methods 2.1. Materials Graphene

oxide

was

prepared

using

a

Hummer’s

method

[22],

3-

aminopropyltriethoxysilane (AS) and other chemicals were of analytical grade in the

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Page 4 of 48

experiments. All solutions were prepared using deionized water. The initial Pb(Π) solution was prepared by dissolving Pb(NO3)2 in deionized water. 2.2. Methods

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2.2.1. Preparation of PAS-GO composite

The graphite oxide was exfoliated in deionized water by ultra-sonication for 3 hours to

cr

obtain a homogeneous 0.3 mg/mL GO dispersion. One milliliter of AS was dropwise added

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into 250 mL deionized water with constantly stirring at 316 K for 4 h to get PAS oligomer. The PAS oligomer aqueous solution was poured into the graphene oxide dispersion, and

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then the mixture was stirred until the tawny solution became clear accompanying the formation of brown flock. The brown flock was separated by centrifugation, washed several

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times with ethanol and deionized water, and dried by cryodesiccation for 24 h to get PASGO composite. As a comparison, AS-GO was prepared by the direct reaction of AS with

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2.2.2. Characterization

te

composite.

d

GO through a similar procedure. Scheme 1 is schematic of the formation of PAS-GO

The morphologies of the samples were characterized by field emission scanning electron

microscope (FESEM, Model S-4800) and transmission electron microscope (TEM, JEM 3010). The spacing structures of the samples were determined by an X-ray diffractometer with Cu-Kα radiation (XRD, M21X, MAC Science Ltd., Japan). The X-ray photoelectron spectroscopy (XPS) measurements were made on Thermo Fisher Scientific ESCALAB 250Xi. The Fourie transform infrared (FTIR) spectroscopy measurements were carried out on Fourier transform infrared Nicolet 5700 spectrophotometer (American) in KBr pellet at room temperature. The Raman spectra were acquired from Labram-010 Laser Raman spectrometer. The thermogravimetric analysis (TGA) curves were collected with

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Page 5 of 48

(TG/DTA7300) from room temperature to 800 oC with a heating rate of 10 oC/min and a nitrogen flow rate of 50 mL/min. PAS oligomer was analyzed by liquid chromatographyion trap mass spectrometry (LCQ-Advantage Thermo Finnigan). The zeta potentials of the

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samples in different pH solutions were analyzed by Zetasizer 3000 HSA (Malvern Zetasizer). The concentrations of metals in solution were measured by an atomic absorption

cr

spectrometer (Hitachi Z-2000, Japan). Surface area measurement was performed by

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Brunauer-Emmett-Teller (BET) method at liquid nitrogen temperature using conventional gas adsorption apparatus (Belsorp-Mini II).

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2.2.3. Pb(Π) adsorption experiments

The batch experiments of Pb(Π) sorption on adsorbents were carried out by adding 10 mg

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of adsorbent to 10 mL Pb(Π) solution at different temperature (293 K, 303 K, and 313 K). The initial Pb(Π) concentration varied from 10 to 400 mg/L, and the pH value of solution

d

was adjusted by adding 0.01 mol/L HNO3. The effect of pH on Pb(Π) adsorption was

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investigated in different pH (2.0, 3.0, 4.0, 5.0, and 6.0) solutions. The relatively superior pH

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condition for Pb(Π) adsorption on PAS-GO was greater than 4.0. The effect of treatment time was tested by the similar procedure mentioned above. The solution was filtered immediately when the reaction time ranged from 0 to 24 h, and the residual Pb(Π) in the filtrate was analyzed for adsorption kinetics analysis. All the adsorption experiments were carried out in duplicate. The adsorbed amount of

Pb(Π) per unit mass of adsorbent was evaluated by using the mass balance equation:

qt 

(C0  Ct )V m

(1)

In Eq. 1, qt (mg/g) is the adsorbed amount per gram of adsorbent at time t (min), C0 is the initial concentration of Pb(Π) in solution (mg/L), Ct is the concentration of Pb(Π) at

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Page 6 of 48

adsorption time t (mg/L), m is the mass of adsorbent (g), and V (L) is the initial volume of Pb(Π) solution. 2.2.4. Selectivity of heavy metal ions.

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The selective removal of metal ions for PAS-GO was tested through a 7-hour adsorption process at 303 K and pH 4.9. The solution used in this experiment contained six metal ions

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including Pb(Π), Ni(Π), Cu(Π), Mn(Π), Zn(Π) and Cd(Π), and the initial concentration of

from 1 g/L to 8 g/L.

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2.2.5. Application of PAS-GO to industrial effluent

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each kind of metal ion was around 50 mg/L. The dosage of PAS-GO gradually increased

PAS-GO was used to remove the metal ions in a practical industrial effluent from

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Shuikoushan smelting plant in Hengyang, Hunan province, China. The effluent was filtered by a 0.45 μm millipore membrane filter for measuring the chemical oxygen demand (COD)

d

and the suspended substance (SS). The concentrations of metal ions in the practical effluent

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were measured by an atomic absorption spectrometer (Hitachi Z-2000, Japan).

3. Results and Discussions 3.1 Characterization

The molecular weights of AS and PAS were determined by liquid chromatography-ion

trap mass spectrometry (LC-ITMS) (Fig. 1). The m/z 221.9 peak in Fig. 1a referred to the characteristic molecular ion fragment of AS while the m/z 490.7 and 476.9 fragment ion peaks in Fig. 1b were corresponding to AS trimers after hydrolytic condensation of AS. The SEM images show that the graphene oxide (GO) from lyophilization was fully exfoliated (Fig. 2b) while the GO separated from traditional drying method [22] piled up seriously (Fig. 2a). PAS-GO showed an integral structure (Fig. 2c) while AS-GO was

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Page 7 of 48

loosened (Fig. 2d), due to the fact that multi-arm PAS chain could bind numerous GO sheets to constitute an undifferentiated whole while single AS molecule only could bind few or even one GO sheet (Scheme 1). Obviously, PAS-GO structure possessed better stability.

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The BET surface area of freeze-dried PAS-GO, AS-GO and GO were 53.7 m2/g, 39.0 m2/g and 26.0 m2/g, respectively. The TEM images of PAS-GO before (Fig. 2e) and after (Fig.

cr

2d) adsorbing Pb(Π) demonstrate that a large amount of Pb(Π) were strongly adsorbed onto

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the surface of PAS-GO adsorbent.

Compared with the FTIR spectrum of GO, the FTIR spectra of both PAS-GO and AS-GO

an

show two new peaks at 3430 cm-1 for -NH2 stretching vibration and at 2927 cm-1 associated with the methylene stretching vibration bond in AS molecules (Fig. 3a) [35]. Although there

M

is no apparent difference in the FTIR spectra of PAS-GO and AS-GO, the intensity of -NH2

of -NH2 groups in PAS-GO.

d

stretching vibration in PAS-GO is much higher than that in AS-GO, meaning higher content

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In the Raman spectra (Fig. 3b), all samples show the characteristic peaks of GO at 1590

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cm-1 (G band) related to the vibration of sp2 carbon atoms and at 1326 cm-1 (D band) associated with the vibration of sp3 carbon atoms [36, 37]. Additionally, there is a new peak at 1120 cm-1 in both PAS-GO and AS-GO, which is totally different from GO. It might be caused by the antisymmetric stretching vibration of Si-O-Si. The thermal stabilities of GO, AS-GO and PAS-GO were measured by thermogravimetric analysis (Fig. 3c). Obviously, both AS-GO and PAS-GO showed much higher thermal stability than GO. At about 170 oC, all samples abruptly lost part of mass due to the decomposition of the labile oxygen or nitrogen-containing functional groups. Subsquently, GO started to sharply lose mass over 200 oC while PAS-GO and AS-GO only smoothly lose mass. The weight loss at higher

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Page 8 of 48

temperature should be due to the pyrolysis of carbon skeleton of GO. The results indicate that the introduction of PAS or AS could effectively increase the thermal stability of GO. For XRD chanraterization analysis, the acute diffraction peak at 2θ=10.03º in GO

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indicates the existence of layered structure while there is no obvious diffraction peak in the patternings of both PAS-GO and AS-GO, showing that the GO sheets in PAS-GO and AS-

cr

GO were effectively exfoliated.

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X-ray photoelectron spectroscopic (XPS) measurements were also performed to further explore the surface chemical changes associated with the reaction shown in Scheme 1. The

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full XPS spectra for GO, AS-GO and PAS-GO are shown in Fig. 4a. The appearances of Si2p (101 ev), Si2s (152 ev), and N1s (400 ev) peaks in the XPS surveys of both AS-GO

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and PAS-GO demonstrate the successful introduction of AS or PAS. Fig. 4b, c and d reproduce the high-resolution C1s spectra for GO, AS-GO and PAS-GO, respectively.

d

Compared with GO, the ratios of the C-O peak intensity in AS-GO and PAS-GO

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significantly decreased, especially in PAS-GO, indicating that the oxygen-containing

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groups on GO involved the cross-linking reaction accompanying the formation of C-N bond in PAS-GO (Fig 4c).

3.2 Adsorption study

3.2.1 Comparison of adsorbents

Fig. 5 shows the Pb (II) adsorption capacities of PAS-GO, AS-GO and GO adsorbents. It

is found that the maximum Pb(II) adsorption capacities of PAS-GO, AS-GO and GO at 303 K were 312.5 mg/g, 119.05 mg/g and 204.08 mg/g, respectively, calculated by the slopes of Langumir adsorption isotherm. The highest adsorption capacity of PAS-GO should be ascribed to the factors: 1) PAS chians provided more functional groups availble for binding metal ions and 2) multi-arm PAS oligomer chains prevented GO sheets from aggregation, giving metal ions easier access to binding sites. The lower adsorption capacity of AS-GO

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Page 9 of 48

than GO might be due to the reasons: 1) AS consumed part of the original functional groups on the surface of GO but did not introduce more functional groups and 2) AS probably prevented the functional groups from contacting freely with metal ions. Unless otherwise

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specified, only PAS-GO was investigated in detail. Table 1 also lists the maximum Pb(II) adsorption capacities (qmax) on PAS-GO and some commonly used adsorbents. Although a

cr

direct comparison between PAS-GO and other sorbents is difficult because of the different

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experimental conditions, the much higher qmax of PAS-GO than those of the reported sorbents reveals that PAS-GO is a better Pb(II) sorbent.

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3.2.2 Effect of pH.

It is well known that the initial pH influences not only the surface charges and states of

M

the functional groups on the surface of adsorbents but also the species of metal ions in solution [38, 39]. Different optimum pH values of adsorption have been noted for different

d

metals [40]. The adsorption experiments on PAS-GO were conducted in different pH range

te

from 2.0 to 6.0, and the results are presented in Fig. 6. The initial Pb(II) concentration was

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100 mg/L, the sorbent dose was 1 g/L, the experiment temperature was 303 K and the contact time was 7 h which guaranteed the processes under equilibrium adsorption. It is found that at relatively low pH values of 2.0 and 3.0, the nitrogen-containing groups in PAS-GO were prone to protonation, resulting in positvely charged surfaces where positively charged metal ions were repeled because of electrostatic repulsion. When increasing the pH to 4.0, the effect of protonation became weak and the adsorption effeciency reached nearly 100% much higher than those at low pH values of 2.0 and 3.0 (11% at pH 2.0 and 27% at pH 3.0). When further increasing pH to 6.0, the adsorption effeciency still kept nearly 100%. In fact, the pH of practical Pb-containing wastewater is below 7.0 because Pb ions in basic

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Page 10 of 48

soultion will precipitate. So PAS-GO adsorbent can be used at a wide pH range of about 4.0-7.0 for treating wastewater containing Pb ions. 3.2.3 Adsorption kinetics.

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The adsorption behavior of Pb(II) on PAS-GO in relation to contact time was carried out by varying the contact time from 0 to 24 h in each concentration of Pb(II) solution. The

cr

dose of PAS-GO absorbent was 1.0 g/L and the initial Pb(II) concentration ranged from 10

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to 130 mg/L. As shown in Fig. 7, lower initial concentration brought about shorter adsorption equilibrium time. Furthermore, higher concentration helped to increase the

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equilibrium capacity.

Various adsorption kinetic models have been used in the literatures to describe the

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adsorption mechanism. In the pseudo first-order kinetic model, the rate constant of Pb(II) adsorption is distinctived as [41]:

ln (qe - qt )  ln (qe ) - k1t

d

(2)

te

where qe and qt are the amounts of adsorbate adsorbed (mg/g) at equilibrium and at any

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instant of time t (min), respectively, and k1 is the rate constant of pseudo first-order adsorption (min-1).

The Elovich model is presented by the following equation [28, 42]:

qt 

1 1 ln( )  ln(t )  

(3)

where the Elovich coefficients, α and β, represent the initial adsorption rate (mg/g·min) and the desorption coefficient (g/mg) respectively. The rate constants of intra-partical diffusion (ki) at the stage i are determined by [43]:

qt  ki t

1

2

 Ci

(4)

where Ci is the intercept at stage i.

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Page 11 of 48

After being fitted by several models, the parameter values for each system are calculated from the linear least square method and the correlation coefficients are presented in Table 2. The results show the adsorption kinetics of Pb(II) on PAS-GO could be well-described by

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pseudo second-order kinetic model. The pseudo second-order kinetic model can be expressed as:

cr

t t 1   2 qt k 2 qe qe

us

(5)

where k2 is the constant of the pseudo second-order sorption (g/(mg·min)). As shown in Fig.

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8, a plot of t/qt against t demonstrated the correlation coefficient (R2) values for pseudo second-order model were nearly 1.0 in 10, 30, 70 and 130 mg/L of Pb(II) in the presence of

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1.0 mg/L PAS-GO when the contact time was 7 h. 3.2.4 Adsorption isotherms.

d

Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherms were tried to

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describe the adsorption behavior of Pb(II) on PAS-GO. Langmuir isotherm is often

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applicable to homogeneous adsorption surface with all the adsorption sites having equal adsorbate affinity (Fig. 9a). The equation can be described as [44]:

Ce C 1   e qe qmax k L qmax

(6)

where qe and Ce are the adsorption capacity (mg/g) and the equilibrium concentration of the adsorbate (mg/L), while qmax and KL represent the maximum adsorption capacity of adsorbents (mg/g) and the Langmuir adsorption constant (L/mg), respectively. The values of qmax and KL are calculated from the slope and intercept of the linear plot of Ce/qe against Ce .

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Freundlich isotherm model assumes heterogeneity of adsorption surface (Fig. 9b). The formula is expressed as follows [44]: 1

qe  K F C e n

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(7)

where qe and Ce are the amount of adsorbed Pb(II) (mg/g) and Pb(II) concentration at

cr

equilibrium, respectively. KF and n are the Freundlich isotherm constants. KF value indicates the adsorption capacity and n is related to the energetic heterogeneity (average

us

energy of sites). KF and n can be obtained from the intercept and the slope of the linear plot

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of ln(qe) versus ln(Ce).

The Temkin isotherm model is given by the following equation [45]: (8)

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qe  B ln AT  B ln Ce

where AT is Temkin isotherm equilibrium binding constant (L/g) and B is the constant

d

related to heat of sorption(J/mol).

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Dubinin-Radushkevich isotherm is generally applied to express the adsorption mechanism with a Gaussian energy distribution onto a heterogeneous surface. The model is

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represented by the following equation [46, 47]:

ln qe  ln(qs )  ( K ad  2 )

  RT ln[1 

1 ] Ce

(9)

(10)

where qe and qs are the amount of adsorbate in the adsorbent at equilibrium (mg/g) and the theoretical isotherm saturation capacity (mg/g), Kad is the Dubinin-Radushkevich isotherm constant (mol2/KJ2), ε is the Dubinin-Radushkevich isotherm constant, and R, T and Ce represent the gas constant (8.314 J/mol K), absolute temperature (K) and adsorbate equilibrium concentration (mg/L), respectively.

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Table 3 reveals that the adsorption of Pb(II) on PAS-GO fitted well to Langmuir model with a correlation coefficients R2 values of 0.9996, 0.9989 and 0.9874 at 293 K, 303 K and 313 K, respectively. For comparison, the resulting correlation coefficients of the other four

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models at the three temperatures were lower than those of Langmuir model. The better fitting of Langmuir isotherm model indicated the homogeneous adsorption surface with all

cr

the adsorption sites having equal adsorption affinity. The maximum adsorption capacity

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qmax can be calculated by the slopes of Langumir adsorption isotherm. By increasing the temperature from 293 K to 313 K, qmax increased from 200.0 mg/g to 344.8 mg/g. This

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phenomenon indicates the adsorption process in the study would be an endothermic process. Increasing the temperature is helpful for adsorption of Pb(II).

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3.2.5 Adsorption thermodynamic study

The thermodynamic parameters (ΔH0, ΔG0, and ΔS0) for Pb(II) sorption on PAS-GO are

d

used to determine the adsorption nature. Thermodynamic parameters can be calculated from

te

the variation of the thermodynamic equilibrium constant K0 with the change in temperature

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[48]. K0 is defined as follows:

K0 

as v C  s s ae ve C e

(11)

where αs is the activity of adsorbed metal ions, αe is the activity of metal ions in solution at equilibrium, Cs is the amount of metal ions adsorbed by per mass of PAS-GO (mg/g), Ce is the adsorption equilibrium concentration (mg/L), νs and νe are the activity coefficient of the adsorbed metal ions and ions in solution, which can be regarded as zero in the sorption system. K0 can be obtained by plotting ln (Cs/Ce) versus Ce and extrapolating Ce to zero. The standard free energy change (ΔG0) can be calculated from the following equation:

 G 0   RT ln K 0

(12)

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Page 14 of 48

where R is the universal gas constant (8.314 J/mol·K) and T is the temperature in Kelvin. ΔG0 = -10256.43 J/mol at 293 K, -11800.46 J/mol at 303 K, and -13001.5 J/mol at 313 K. The negative ΔG0 values also indicates the spontaneous process of Pb(II) sorption under the

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conditions applied. The decrease of ΔG0 with increase temperature indicates more efficient sorption at high temperature.

cr

The standard enthalpy change (ΔH0) and the standard entropy change (ΔS0) are calculated

S 0 H 0  R RT

(13)

an

ln K 0 

us

from the following equation [17]:

The thermodynamic parameters are calculated from the plot of lnK0 versus 1/T using Eq.

M

13. The thermodynamic parameters of Pb(II) sorption on PAS-GO are ΔH0 = 30036.82 J/mol and ΔS0 = 137.63 J/mol·K. The positive value of ΔH0 for Pb(II) sorption indicates

d

that Pb(II) sorption on PAS-GO is a endothermic process while the positive ΔS0 value of

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Pb(II) means the spontaneous with high affinity. The data are presented in Table 4. 3.3. Adsorption mechanism

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Different sorption mechanisms could be involved in adsorption process such as

electrostatic interaction, chelation and complexation [49, 50]. As shown in Fig. 10, the zeta potential of GO was negative while the zeta potential of PAS-GO was positive in the pH range from 2.0 to 6.0, which might attribute to the introduction of amine groups on the surface of PAS-GO where the amine groups were protonated. Obviously, the positive zeta potential of PAS-GO sorbent repulsed the positively charged metal ions through electrostatic force. So the electrostatic interactions were adverse rather than stimulative to the sorption. It has been proved that metal ions could be strongly bound to electron-rich groups such as hydroxyl or amino groups by forming coordination bonds with oxygen or nitrogen atoms

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[51, 52]. Fig. 3a shows the FTIR spectra of PAS-GO before and after Pb(II) adsorption. After adsorbing Pb(II), the absorbance peak of amine groups (stretching vibration) obviously shifted from 3430 cm-1 to 3069 cm-1 and the absorbance peaks of C-O stretching

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of carboxyl groups and the bending vibration bands of hydroxyl groups at 1123 cm-1 were displaced to 1103 cm-1. This behavior reflects the interaction between oxygen and nitrogen

cr

atoms and lead ions [53]. XPS analysis were also applied to characterize the changes of

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functional groups before and after adsorption. The peak of -NH2 shifted from 401.16 eV to 401.52 eV after adsorbing Pb(II) (Fig. 11), further demonstrating the formation of the

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coordination bond of N-Pb(II). The lone pair of electrons in nitrogen atoms of amine groups were shared with metal ions, leading to the decrease of electron densities of nitrogen atoms

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and the increase of binding energy [39, 54]. Similar results of Pb(II) bound to amine or

3.4. Selective adsorption test

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oxygen-containing groups by chelation or coordination have been reported [39, 55-57].

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To examine the selectivity of heavy metal ions on PAS-GO, different sorbent dosages

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were used to deal with a mixed solution of metal ions containing Pb(Π), Cu(Π), Ni(Π), Zn(Π), Mn(Π), and Cd(Π). As shown in Table 5, the removal percentage of different metal ions increased with the sorbent dosage more or less. When the dose of PAS-GO increased to 8 mg/L, the removal percentages of Pb(Π) and Cu(Π) reached to over 97% while other metals were removed little. According to the adsorption mechanism mentioned above, metal ions provided empty orbits for the lone pair of electrons in nitrogen and oxygen atoms. Because of the difference of chelation ability of different metal species, preferential adsorption might happen in a competition environment. This might be the reason why Pb(Π) and Cu(Π) were preferentially removed. Several researches had the similar phenomenon [57-59]. So PAS-GO could be utilized to treat Pb(Π) or Cu(Π)-containing wastewater.

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3.5. Dealing with practical industrial effluent To investigate the metal adsorption capability of PAS-GO in practical wastewater, smelter industry effluent was used as feed solution. The chemical oxygen demand (COD)

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of the raw effluent of Shuikoushan smelting plant was 75 mg/L and the suspended

cr

substance (SS) of the influent was 110 mg/L. All the water used in this practical experiment was the supernate of the industry wastewater standing for 24 h. There were more than ten

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metals mainly including Pb, Cu, Cr, Ni, Fe, Cd, Mn and Zn with the corresponding concentrations of 3.866, 10.296, 0.042, 0.285, 12.65, 5.02, 15.9 and 112 mg/L, respectively

an

(Table 6). The initial pH value of the wasterwater was 2.88, and the pH was adjusted to 3.8 before treatment with PAS-GO. When putting to use low dosage sorbent of 1g/L, almost

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total Cr, 55% Fe, and 33% Pb were removed while other metals nearly remained unchanged. This indicates the strong coordination abilities of Cr, Fe, and Pb to the

d

functional groups of PAS-GO. In particularly, Fe was preferentially removed due to its

te

3d64s2 subshell which offered empty orbital and strongly coordinated with amino groups

Ac ce p

[60]. When further increasing the sorbent dosage to 3 g/L, Pb and Fe were preferentially removed to lower concentrations whereas other metals did not obviously change. Subsequently, when the dose of sorbent increased to 5 g/L, about 93.4% Pb and 87.3% Cu were removed. It is also found that the metals of Ni, Cd, Mn and Zn were removed slightly. In consequence, PAS-GO could be used as an effective adsorbent to treat practical wasterwater containing Pb, Cu and Fe.

4. Conclusions An efficient adsorbent, PAS-GO, was fabricated by a simple cross-linking reaction between graphene oxide and oligomeric poly3-aminopropyltriethoxysilane. PAS-GO adsorbent exhibited high adsorptivity toward Pb(Π) with the maximum adsorption capacity

1

Page 17 of 48

of 312.5 mg/g at 303K. Importantly, PAS-GO has a priority tendency to adsorb Pb, Cu and Fe from a mixed solution of metal ions, especially from practical industrial effluent. This study shows that using oligomers as crosslinkers to fabricate functional meshworks is an

ip t

effective strategy for the development of high performance sorbents for the removal of

cr

heavy metal ions from wastewater.

us

Acknowledgements

This work was supported by Hunan Provincial Natural Science Foundation of China

an

(14JJ1015), the National Natural Science Foundation of China (51178173, 51202065, 51238002, 51272099, 51378187), and Program for Innovation Research Team in University

d te

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Legends for Schemes, Figures and Tables: Scheme 1. Schematic illustration of PAS-GO preparation and application. Fig. 1. LC-ITMS spectra of (a) AS and (b) PAS.

ip t

Fig. 2. SEM images of (a) GO by drying at 40 oC, (b) GO by freeze-drying, (c) PAS-GO, and (d) AS-GO; TEM images of (e) PAS-GO and (f) PAS-GO after Pb(II) adsorption.

cr

Fig. 3. (a) FTIR spectra of GO, AS-GO, and PAS-GO before and after Pb(II) adsorption, (b)

us

Raman spectra of GO, PAS-GO, and AS-GO, (c) TGA curves of GO, PAS-GO, and AS-GO, and (d) XRD of GO, PAS-GO, and AS-GO.

for (b) GO, (c) PAS-GO, and (d) AS-GO.

an

Fig. 4. (a) XPS spectra of GO, PAS-GO and AS-GO; high-resolution XPS spectra of C1s

M

Fig. 5. Langmuir isotherm model for Pb(II) adsorption on PAS-GO, AS-GO, and GO at 303 K.

d

Fig. 6. Effect of pH values on the adsorption capacity of Pb(II). Experiment conditions: the

te

initial Pb(II) concentration was 100 mg/L, the adsorbent dose was 1 g/L, the temperature

Ac ce p

was 303 K, and the contact time was 7 h. Fig. 7. Time profile for Pb(II) removal with PAS-GO. The concentration of PAS-GO adsorbent was 1.0 g/L and the initial Pb(II) concentration ranged from 10 to 130 mg/L. Fig. 8. Removal kinetics of 10, 30, 70 and 130 mg/L of Pb(II) in the presence of 1.0 g/L PAS-GO.

Fig. 9. (a) Langmuir and (b) Freundlich isotherm model for Pb(II) adsorption on PAS-GO at 293 K, 303 K and 313 K. Fig. 10. Zeta potentials of GO and PAS-GO as a function of solution pH. Fig. 11. XPS spectra of N1s in PAS-GO (a) before and (b) after Pb(II) adsorption. Table 1. Maximum Pb(II) adsorption capacities of different sorbents.

2

Page 27 of 48

Table 2. Calculated kinetic parameters for pseudo first-order, second-order kinetic models, Elovich, intra-particle diffusion kinetics models for Pb(II) adsorption using PAS-GO as an adsorbent.

ip t

Table 3. Parameters of Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm for Pb(II) adsorption.

cr

Table 4. Thermodynamic parameters for Pb(II) adsorption on PAS-GO.

us

Table 5. Selectivities of heavy metal ions.

Table 6. Characteristics of effluents collected from smeltery and concentrations of metals

Ac ce p

te

d

M

an

before and after adsorption by different sorbent dosage.

2

Page 28 of 48

NH2

NH2

OH COOH

NH2 NH2

+

NH2

NH2

Heavy metals

HO O

Adsorption

OH

cr

NH2 NH2

COOH

NH2

us

NH2 NH 2

GO

PAS-GO

Ac ce p

te

d

M

an

PAS

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Scheme 1

2

Page 29 of 48

Ac ce p

te

d

M

an

us

cr

ip t

Fig. 1

3

Page 30 of 48

Fig. 2

b

cr

ip t

a

c

M

an

us

d

f

Ac ce p

te

d

e

3

Page 31 of 48

Ac ce p

te

d

M

an

us

cr

ip t

Fig. 3

3

Page 32 of 48

Fig. 4

80000

C1s

ip t

50000 40000 30000

C=O (288.63)

20000 10000

AS-GO

0

200

400

600

800

-10000 282

1000

Binding Energy/eV

283

80000 70000

PAS-GO-C

50000

C=O 287.8

C-N 285.63

20000 10000

284

285

286

287

288

289

290

C=C/C-H 284.4

C-O 286.28

AS-GO-C

50000 40000 30000

C=O 287.7

20000 10000

ed

283

(d)

60000

0 282

286

M

40000 30000

285

an

60000

C-O 286.15

C=C/C-H 284.25

Counts (s)

(c)

284

Binding Energy (E) (eV)

80000 70000

GO-C

us

0

PAS-GO

N1s

Si2p Si2s

C in graphite (284.64)

60000

C-O (286.68)

cr

N1s

Si2p Si2s

(b)

70000

GO

Counts (s)

Couns (s)

90000

O1s

(a)

Counts(s)

287

288

289

0 282

283

284

285

286

287

288

289

290

Binding Energy (E) (eV)

ce pt

Binding Energy (E) (eV)

290

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Page 33 of 48

33

Fig. 5

3.0

AS-GO

1.5

ip t

2.0

GO

cr

1.0

PAS-GO

0.5 0.0 0

50

100

150

200

us

Ce/qe (g/L)

2.5

250

350

ce pt

ed

M

an

Ce (mg/L)

300

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Page 34 of 48

34

100

ip t

80

cr

60

us

40 20

an

Adsorption efficiency %

Fig. 6

0 2

3

4

5

6

ce pt

ed

M

pH

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Page 35 of 48

35

Fig. 7

140

300min

120

ip t

130mg/L 70mg/L 30mg/L 10mg/L

187min

cr

80

119min

40 20 0

us

60

52min 0

200

400

600

an

qt (mg/g)

100

800 1000 1200 1400 1600

ce pt

ed

M

t (min)

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Page 36 of 48

36

Fig. 8

180

-1 t/qt (min.g.mg )

140 120

ip t

130 mg/L 70 mg/L 30 mgL 10 mg/L

160

100

cr

80

us

60 40

0 0

200

400

600

an

20

800 1000 1200 1400 1600

ce pt

ed

M

t (min)

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Page 37 of 48

37

Fig. 9

1.6

(a)

(b)

5.6

1.2

5.2

293K

313K

lnqe

1.0 0.8

303K

0.6 0.4

313K

303K

293K

4.8 4.4

ip t

1.4

Ce/qe (g/L)

4.0

0.2

0

50

100

150

200

250

300

0

1

2

3

lnCe

4

5

ce pt

ed

M

an

us

Ce (mg/g)

cr

3.6 0.0

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Page 38 of 48

38

Fig. 10

GO PAS-GO

40

ip t

30 20

cr

10 0

us

-10 -20 -30 -40 2

3

4

an

Zeta potential (mv)

50

5

6

7

ce pt

ed

M

pH

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Page 39 of 48

39

Fig. 11

20000

9000

401.16 -NH2

16000

PAS-GO-Pb-N 401.52 -NH2

8000

14000 12000

7000

398.92 -N-

ip t

18000

(b)

PAS-GO-N

398.85 -N-

Couns(s)

(a)

Couns(s)

6000

396

398

400

402

404

5000 396

Binding Energy/eV

398

cr

10000 400

402

404

ce pt

ed

M

an

us

Binding Energy/eV

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Page 40 of 48

40

Table 1. Maximum Pb(II) adsorption capacities of different sorbents. pH

Reference

Rice husk

11.4

7.0

[61]

Porous NiFe2O4adsorbent (PNA)

48.98

5.0

[62]

Cellulose-manganese oxide hybrid material

80.1

5.0±0.2

[63]

Polyhedral oligomeric silsesquioxanes (POSS) incorporated with Fe3O4

90.9

Rice husk ash

91.74

a

EGTA -modified chitosan

101.4

PAS-GO

312.5

cr

ip t

Maximum Pb(II) adsorption capacity (mg/g)

an

us

Around 7.0

[64]

5.0

[65]

4.0

[66]

4.0~5.0

This work

te

d

M

EGTA: ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid.

Ac ce p

a

Sorbent

4

Page 41 of 48

Table 2. Calculated kinetic parameters for pseudo first-order, second-order kinetic models, Elovich, intra-particle diffusion kinetics models for Pb(II) adsorption using PAS-GO as an

Pb(II) concentration (mg/L)

ip t

adsorbent. 8.8

32

75

8.7 0.0352 0.9647

30.5 0.0323 0.951

71.9 0.013 0.9883

qe (mg/g) k2 (g/mg·min) R2 Elovich model

8.8 0.0104 0.9994

31.3 0.0014 0.9977

73.5 0.0004 0.997

135.1 0.0001 0.9937

α (mg/g·min)

0.5994

2.6542

-

β (g/mg) R2 Intra-particle diffusion model

0.2693 0.9674

1.4587 0.0986

0.9306

0.0495 0.9604

-

3.5392

5.4348

7.2384

-2.4175

-5.9878

-5.9702

-2.4929

0.9725

0.9232

0.997

0.9775

us

qe (mg/g) k1 (min-1) R2

C

M

d

Ac ce p

R

2

1.4426

te

ki (mg/g·min0.5)

an

Pseudo-second-order model

128 0.009 0.9531

cr

Pseudo-first-order model

140

4

Page 42 of 48

Table 3. Parameters of

Langmuir, Freundlich, Temkin and Dubinin-Radushkevich

isotherm for Pb(II) adsorption.

Langmuir Qmax (mg/g)

200.0

312.5

KL (L/mg)

0.2392

0.2759

0.9996

0.9989

53.4956

49.2939

n

42.1022 2

1.23581

0.9313

0.9307 1.2778 102.05 0.9922

178.0

268.9

270.9

0.3692

0.6038

0.7413

0.9106

0.9585

0.9333

Ac ce p

te

R

42.1022

1.5959 95.659 0.9902

M

2

Kad (mol /KJ ) 2

9.1072 26.853 0.9476

d

2

0.8310

0.9874

1.31917

an

R Temkin AT (L/g) B (J/mol) R2 Dubinin-Radushkevich qs (mg/g)

0.1480

us

R Freundlich KF (mg1-n.Ln/g)

344.8

cr

2

313 K

ip t

293 K

Temperature (pH 5.0) 303 K

Isotherm

4

Page 43 of 48

Table 4. Thermodynamic parameters for Pb(II) adsorption on PAS-GO. Temperature (K)

Thermodynamic 293

303

313

K0

4.0282

4.682

4.9938

ΔG0 (J/mol)

-10256.43

-11800.46

-13001.50

ΔH0 (J/mol)

30036.82

30036.82

30036.82

ΔS0 (J/mol·K)

137.63

137.63

ip t

constant

Ac ce p

te

d

M

an

us

cr

137.63

4

Page 44 of 48

Table 5. Selectivities of heavy metal ions a.

Cu

Ni

Zn

Mn

Cd

24.43

33.10

0.53

4.51

5.88

2

41.15

52.89

0.56

5.64

5.88

4

77.79

87.84

0.56

9.26

9.62

8

97.19

97.85

0.91

14.92

9.80

14.66

6.41 9.66

cr

11.89

The initial concentration of each metal ion is 50 mg/L, the pH of the solution is 4.9 and the temperature

us

a

Pb

ip t

The removal percentage of heavy metals on PAS-GO/(%) Sorbent dosage (g/L) 1

Ac ce p

te

d

M

an

is 303 K.

4

Page 45 of 48

Table 6. Characteristics of effluents collected from smeltery and concentrations of metals before and after adsorption by different sorbent dosage a

Cu

Cr

Ni

Fe

Cd

3.866

10.296

0.042

0.285

12.65

1

2.586

9.86