Algal extracts based biogenic synthesis of reduced graphene oxides (rGO) with enhanced heavy metals adsorption capability

Accepted Manuscript Title: Algal Extracts Based Biogenic Synthesis of Reduced Graphene Oxides (rGO) with Enhanced Heavy Metals Adsorption Capability Authors: Shahbaz Ahmad, Aftab Ahmad, Sikandar Khan, Shujaat Ahmad, Idrees Khan, Shah Zada, Pengcheng Fu PII: DOI: Reference:

S1226-086X(18)30542-2 https://doi.org/10.1016/j.jiec.2018.12.009 JIEC 4302

To appear in: Received date: Revised date: Accepted date:

4 September 2018 22 November 2018 4 December 2018

Please cite this article as: Shahbaz Ahmad, Aftab Ahmad, Sikandar Khan, Shujaat Ahmad, Idrees Khan, Shah Zada, Pengcheng Fu, Algal Extracts Based Biogenic Synthesis of Reduced Graphene Oxides (rGO) with Enhanced Heavy Metals Adsorption Capability, Journal of Industrial and Engineering Chemistry https://doi.org/10.1016/j.jiec.2018.12.009 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.

Algal Extracts Based Biogenic Synthesis of Reduced Graphene Oxides (rGO) with Enhanced Heavy Metals Adsorption Capability Shahbaz Ahmad a, Aftab Ahmad a, Sikandar Khan a,*, Shujaat Ahmad b, Idrees Khan c, Shah

SC RI PT

Zada a, Pengcheng Fu a, d, * a

College of Life Science and Technology, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Chaoyang District, Beijing 100029, China. b

Department of Pharmacy, Shaheed Benazir Bhutto University, Sheringal, KPK, Pakistan.

c

Department of Chemistry, Bacha Khan University, Charsadda, KPK, Pakistan.

d

N

U

State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, 570228, China.

A

Author and Co-authors’ details:

[email protected]

2. Aftab Ahmad

[email protected]

3. Sikandar Khan *

[email protected]

4. Shujaat Ahmad

[email protected]

6. Shah Zada

D

EP

7. Pengcheng Fu *

TE

5. Idrees Khan

M

1. Shahbaz Ahmad

[email protected] [email protected] [email protected]

Sikandar Khan, [email protected], +86-131-2697-9600

A

CC

* Corresponding Author: Pengcheng Fu, [email protected], +86-10-6443-8058

Graphical abstract 1

SC RI PT

Abstract

Efficient reduction of GO was performed, using cellular extracts of three algal strains. The rGO

U

were characterized by SEM, TEM, UV-Visible spectroscopy, XRD, FTIR, Raman spectroscopy,

N

Zeta potential and redox potentials. The rGO were then used as decontaminating agents for heavy

A

metals (Cu and Pb), in waste water. GO reduced via JSC-1 have removed up to 93% Cu and 82%

M

Pb (adsorption capacity of 93 mg.g-1 and 82 mg.g-1, respectively), by 211-9a have removed 74% (74 mg.g-1) Cu and 89% (89 mg.g-1) Pb, while by 211-11n have adsorbed 91% (91 mg.g-1) Cu and

TE

D

95% Pb (95 mg.g-1), within 30 min.

CC

EP

Keywords: Green reduction; reduced graphene oxide; microalgae; heavy metal; adsorption

A

1. Introduction

Graphene is one atom thick planer sheet of carbon atoms (sp2-bonded) which are packed

densely in a honeycomb crystal lattice with carbon atoms arranged in hexagonal lattice [1, 2]. It exhibits excellent electrical, mechanical, optical and thermal properties which are stable under ambient conditions [3]. Graphene has a high surface area (2630 m2g-1) which is greater than that 2

of carbon nanotubes and graphite [4]. Due to these properties, graphene has found a variety of applications in the fields of nano-electronic devices, energy storage, transparent conductors, batteries and environmental protection [5, 6]. Graphene oxide (GO) and its reduced form (rGO)

SC RI PT

are characterized as graphene family materials (GFMs) and have various potential applications in industry [7]. GO is a highly oxidized and chemically modified form of graphene, bearing negative surface charges containing epoxide, carboxylic acid, carbonyl and hydroxyl functional groups [8]. GO is effectively used for adsorption of heavy metal ions and dyes owing to their excellent water dispersion and bio-compatibility. The GO’s high surface reactivity contributes to various

U

medicinal and biological applications, while it is also responsible for the toxic effects [9, 10].

N

Bearing oxygenated functional groups, GO is highly hydrophilic to yield a stable dispersion in

A

water with high surface negative charge [11].On the other hand, GO can form insulating materials

M

due to its disrupted sp2-bonding system, which can be re-established by converting GO to rGO

D

[12].

TE

A variety of electrochemical, thermal, photochemical and chemical reduction methods have been reported for the reduction of GO to rGO [13]. Among these methods, chemical reduction

EP

of GO offers the advantages of graphene production on industrial scale [14]. Several reducing agents such as sodium borohydride, hydrazine and its derivatives, hydroquinone, and lithium

CC

aluminum hydride have been used for the conversion of GO to rGO [15]. Chemically reduced GO

A

shows irreversible agglomeration due to the strong Vander Waals interactions between the successive graphene sheets, which can be prevented by surface modification. As the reducing agents are highly toxic even when existing in minute quantities in rGO, they cannot be eliminated by biological methods [16]. Likewise, hydrazine and its derivatives are unstable and highly toxic, that thus need to be handled with great care [17, 18]. Therefore, alternative natural reagents have 3

attracted an increasing attention for GO to be reduced to rGO in bulk for large-scale conversion [19]. Various natural reagents were utilized for the GO reduction such as Terminalia chebula seeds extract [20], plant leaves extracts [17], green tea solution [14], Salvadora persica L. Root

SC RI PT

(Miswak) Extract [21], crude polysaccharide [22], Lycium barbarum extract [23], and pomegranate juice [11]. Apart of these phytochemical extracts, different bacteria have also been used for the reduction of GO [24].

Several adsorbents have been synthesized in the past in order to find out a sustainable and cost effective way of pollutants removal from the environment [8, 25-30]. In this study, algal

U

extracts were used for the reduction of GO into rGO which were then used for the adsorption of

N

Pb and Cu from the aqueous medium. Three algal strains, Scenedesmus vacuolatus, Chloroidium

A

saccharophilum and Leptolyngbya JSC-1 were cultured and their biomass/extracts were collected,

M

purified and finally used to convert GO to rGO. Algae being bionanofactories of various

D

biomolecules are potential materials for the green synthesis of nanoparticles [31]. Algae contain

TE

various valuable macromolecules such as carbohydrates, pigments, vitamins, lipids and proteins which have wide applications in green synthesis and chemistry [32]. Algal extracts have been

EP

utilized for the biogenic synthesis of nanoparticles, due to their natural availability and high efficacy. The functional groups and enzymes available in the algal cells act as reducing agents

CC

which can be used to combine with the metal ions to form metal oxide nanoparticles under ambient

A

conditions [33, 34]. To our knowledge, this was the first ever utilization of algal extracts as a green reducing agent for efficient reduction of GO. 2. Experimental 2.1 Chemicals and Materials

4

Sodium nitrate, Sodium carbonate, Citric acid, Ethylene Diamine Tetraacetic acid disodium (EDTA-Na2), Calcium chloride hydrate (CaCl2.H2O), Magnesium sulfate heptahydrate (MgSO4.7H2O), Boric acid, Manganese (II) chloride tetra-hydrate, Copper sulfate pentahydrate, molybdate

dihydrate

(Na2MoO4.2H2O),

Cobalt

(II)

nitrate

hexahydrate

SC RI PT

di-Sodium

(CO(NO3)2.6H2O), Zinc sulfate heptahydrate (ZnSO4.7H2O), potassium phosphate dibasic (K2HPO4), (NH4)FeC12H10O14, graphite flakes, Sulfuric Acid, sulfate (2SO4), potassium permanganate, hydrogen peroxide, Hydrochloric acid were purchased from Sigma Aldrich Co. and Sino pharm chemical Reagents Co. Ltd, China. These analytical grade chemicals were used

U

without further purification. Three microalgal strains including a siderophilic cyanobacterium

N

Leptolyngbya JSC-1, Scenedesmus vacuolatus (211-11n) and Chlorella saccharophilum (211-9a)

D

2.2.1 Growth of Algae

M

2.2 Preparation of Green Algal Extract

A

were used in this study.

TE

Algal strain Leptolyngbya JSC-1 was provided by Igor I. Brown, Beijing University of

EP

Chemical Technology, Scenedesmus vacuolatus (211-11n) and Chloroidium saccharophilum (211-9a) were gifted by Professor Prezemyslaw Malec and Dr. Jan Burczyk from Jagiellonian

CC

University, Krakow, Poland. They were first grown in agar slants, and then cultivated in fresh BG11 medium. After inoculation, JSC-1 was incubated at 45 oC while the other two strains were

A

incubated at 30 oC and illuminated at 25-40 (μE m-2s-1). Initial seed culture was carried out in 250 ml sterile flasks having 100 ml BG-11 medium. It was progressively sub-cultured, first in 500 ml flasks and then in larger 1000 ml flasks in order to get high biomass. 2.2.2 Preparation of Extracts 5

After growing for 14 days, the cultures of Scenedesmus vacuolatus, Chloroidium saccharophilum, and Leptolyngbya JSC-1 were centrifuged at 6000 rpm for 10 min at the ambient temperature. The supernatant was discarded and the pellets were collected and washed several

SC RI PT

times with the de-ionized water. The pellets were put into the freeze dryer (Bench top lab vacuum freeze dryer (LGJ-10-1, NANBEI) China) for 24 h. Five grams of pellets (algal biomass) was homogenized, dissolved in 100 ml of de-ionized (DI) water and kept at 60 oC for 30 min. Following this, the resultant algal suspensions were magnetically stirred at 250 rpm for 3 h at the same temperature. It was then filtered using sterilized Whattman filter paper and the filtrate was

U

collected and kept at 4 oC for further use.

A

N

2.3 Synthesis of Graphene Oxide from Graphite

M

Graphene oxide was synthesized from graphite flakes by Hummer's procedure with little modification [35, 36]. Sodium nitrate (1.5 g) and graphite flakes (2 g) were stirred for 1 h in a

D

concentrated solution of H2SO4 (68 ml, 98%) and preserved at ≤ 5 oC in an ice bath. Then, 9 g of

TE

KMnO4 were added to the solution while stirring at ambient temperature for 5 days. The obtained solution was heated for 2 h at 90 oC, with diluted H2SO4 (30 ml, 3%) added during continuous

EP

stirring. Next, H2O2 (30% 5ml) was added during continuous stirring at ambient temperature for 2

CC

h. The resultant bright yellow color suspension was then centrifuged at 6000 rpm for 15 min. The pellet was then washed with dilute H2SO4 (25 ml, 5%), HCl (400 ml, 3%) and H2O2 (10 ml, 0.5%)

A

three times with DI water to obtain the neutral suspension. It was dried for 5 days for further use. 2.4 Reduction of Graphene Oxide by Algal Extracts GO (100 mg) was dissolved in 100 ml of DI water under ultra-sonication for 1 h. It was added with algal extracts (100 ml) and kept at 95 °C for 24 h in a thermostatic bath. The three algal 6

extracts Scenedesmus vacuolatus (211-11n), Chloroidium saccharophilum (211-9a), and Leptolyngbya JSC-1 were labeled as N-RGO, A-RGO, and J-RGO, respectively. Ethanol and DI water were used to wash the resultant suspension of rGO three times in order to remove all the

SC RI PT

impurities. 2.5 Characterization

GO and rGO were characterized via double beam UV-Vis spectrophotometer, Fourier Transformed Infrared Spectroscopy, Raman spectroscopy, X-ray Diffraction, Zeta potential,

U

Redox potential, SEM, and TEM in order to confirm the reduction [6, 37-43].

N

2.6 Adsorption of Heavy Metals by rGO

A

The adsorption of toxic metal ions, Pb and Cu, by algal extracts based reduced GO was

M

investigated in batch experiments using glass vials containing 10 ml of Pb and Cu ions solutions

D

separately at neutral pH~7. Initial concentration used was 100mg/L and agitation time was set as

TE

(15, 30, 60, 90, 180, 360, 540 min) at 30 0C. Atomic absorption spectroscopy was used to monitor the residual concentration of both Pb and Cu ions. The samples were treated with acid (2% HNO3)

EP

before final analysis. The amount of Pb and Cu adsorbed per unit mass of rGO (adsorbent) and the

A

CC

removal rate were calculated using the formula as follows;

𝐸𝑥𝑡𝑟𝑎𝑐𝑡𝑖𝑜𝑛 % =

𝐶0 − 𝐶𝑒 𝑋 100 … … … … … … … . (1) 𝐶0

While the adsorption capacity of absorbent was calculated by using the following equation [44].

𝑞𝑒 =

(𝐶0 − 𝐶𝑒 )𝑉 … … … … … … …. 𝑚 7

(2)

Where C0 and Ce are the initial concentration and equilibrium of Pb and Cu ions (mg.L-1), respectively, qe is the capacity of adsorption (mg.g-1), V represents the volume of Pb and Cu ions solution (L), and m is the mass of adsorbent (g) [45-47].

SC RI PT

3. Results and Discussion 3.1 Morphological study 3.1.1 SEM and TEM

The morphological study is very important to evaluate topography, crystallographic

U

observation, shape, composition, and size of materials [48]. Fig. 1 shows the SEM (JEOL JEM-

N

3100) images of GO and rGO (reduced by J-RGO, A-RGO, and N-RGO). Fig. 1A shows the

A

SEM image of GO in which GO displays a sheet-like ultrathin and flexible structure with ridged

M

morphology full of wrinkles and folded regions. It can be ascribed to a significant amount of

D

hydroxyl, epoxy, and carboxyl (such as oxygen-containing functional groups) on the surface of

TE

GO [49]. They are covalently attached with the carbon atoms, disrupting the extended sp2 conjugated system of the graphene sheet, in the honeycomb-lattice shape. The GO also shows a

EP

shrunken structure with multi-folds, which may be due to some electrostatic interaction of

CC

functional groups on the GO surface. In comparison, Fig. 1B, 1C, and 1D show the SEM images of rGO, reduced by J-RGO, A-RGO and N-RGO respectively. The structure of the rGO is super-

A

thin and display a smooth surface with minimum folding, which is due to the existence of new organic functional groups on the surface, after successful reduction of oxygen groups by algal extracts on the surface. The SEM results verified that the algal extract of all strains are capable of graphene reduction with wrinkle structures and larger slices [50]. These figures show a clear difference between GO and biogenic reduced GO, while the chemically reduced GO constitute 8

aggregated and thin corrugated sheets which results in rough structure and there is a high resemblance between GO and rGO [51]. Fig. 1

SC RI PT

Investigation about the phase and microstructure of the GO and rGO has been further carried out using TEM (FEI-Tecnai G2 20). The TEM images of the GO show fewer wrinkles and thick flattened nano-sheet like surfaces, as shown in Fig. 2A. This was due to the organic functional groups and electrostatic interaction of oxides on the surface that provided an extra thickness to the graphene sheets. Fig. 2B, 2C, and 2D are the TEM images of rGO reduced by J-RGO, A-RGO and

U

N-RGO, respectively. These images show that the algal strains substantially affect the surface

N

morphology of graphene oxide by neutralizing or counterbalancing the oxygen-containing

A

functional groups on its surface to reduce the functionalization effect that was responsible for the

M

thickness of nano-sheets. It has been reported that the inherent nature of rGO, ascribed to its 2D

D

structure which could be considered thermodynamically stable due to rippling effect [52]. Such

Fig. 2

EP

Cu in this study).

TE

type of morphology is considered to be very effective for the adsorption of heavy metals (Pb and

CC

3.2 UV-Visible spectroscopy

A

Fig. 3 is the UV-Visible spectra (Shimadzu 2450, Spectrophotometer) of GO and rGO

reduced by different algal extracts. The spectra of GO shows two maximum absorbance peaks at 227 nm and at 297 nm that could be attributed to the π→π* transition of C─C aromatic bonds and n→π* transition of C═O bond, respectively [28, 53], while the absorbance peak for rGO (J-RGO, A-RGO, and N-RGO) were red-shifted to 287 nm, 288 nm and 287 nm respectively, and were 9

assigned to the π→π* transition of the aromatic C─C bond [54, 55].This result indicates the reduction and restoration of electronic conjunction in the J-RGO, A-RGO and N-RGO sheets. Fig. 3

SC RI PT

3.3 Fourier Transformed Infrared Spectroscopy

The FTIR spectra of the samples were measured between 4000 and 500 cm−1 [56]. Fig. 4 shows the FTIR spectra (Nicolet 6700 FT-IR spectrometer) of GO, J-RGO, A-RGO and N-RGO, in which the peaks at 3413 and 1395 cm-1 indicate the vibrational bands of O─H, due to stretching, 1622

U

cm-1 for C═C stretching, 1720 cm-1 for C═O stretching (ketone group) and 1066 cm-1 indicates

N

C─O stretching [57, 58]. It can be seen that the intensities of the GO peaks decrease after treatment

A

with algal extracts, indicating the reduction of oxygen functionalities and the synthesis of a reduced

M

form of GO. Based on FTIR spectral analysis, it could be concluded that biochemicals in the

D

aqueous algal extracts have a strong potential of reducing GO into rGO, which could be used as

Fig. 4

EP

graphene oxide.

TE

an economical and renewable source of natural products for large-scale production of reduced

CC

3.4 X-Ray Diffraction (XRD) The phase structures of GO and rGO were investigated by XRD (Powder X-ray-D8 advanced

A

diff ractometer, Burker) and shown in Fig. 5. The (002) peak appears at 2θ=11.7o is typical of graphene oxide. Here, the intensity indicates the formation of GO nano-sheets from pristine graphite. Generally, the 2θ value of (002) peak seems to be similar for both GO and rGO, while the peak (001) in reduced graphene oxide varies by 20-30o depending on the different microalgal

10

reducing agents applied to graphene oxide. The broad reduction peak shown 2θ=26.8 (001) in the XRD patterns is assigned to reduced graphene oxide with algal strain N-RGO, as this strain has considerably reduced the graphene oxide. The A-RGO strain of algae shows a partial reduction of

SC RI PT

graphene oxide at 2θ= 28.2 (001) with very broad peak intensity. Similarly, the algae strain J-RGO shows an excellent reduction of graphene oxide with an impeccable peak (001) at 2θ=20.6 and confirming the virtually complete reduction of graphene oxide [59]. These findings are completely consistent with the XRD analysis of the available literature investigating the green reduction approaches [60, 61].

N

U

Fig. 5

A

3.5 Raman Spectroscopy

M

Fig. 6 are the Raman Spectroscopic (Horiba Lab RAM ARAMIS) results of GO, J-RGO, A-RGO, and N-RGO with two main bands, the G and D bands at 1593 cm−1 and 1336 cm−1, respectively.

D

The G band of J-RGO, A-RGO, and N-RGO are shifted to appear at 1590 cm−1, 1590 cm−1 and

TE

1599 cm−1, while the D band of them are 1342 cm−1, 1337cm−1 and 1347 cm−1, respectively. Based

EP

on the observation, we assume that the sp2 degeneration has resulted to extensive oxidation for the sheet defects. It should be noted that the G band in J-RGO, A-RGO and N-RGO is slightly

CC

narrower and shifted to 1592 cm−1 and 1599 cm−1. The D band is also shifted to 1342 cm−1, 1337 cm−1, and 1347 cm−1. A comparison of the Raman spectra of both GO and J-RGO, A-RGO, N-

A

RGO depicted that the G bands of J-RGO, A-RGO, and N-RGO are shifted by 3 cm−1, 3 cm−1 and 6 cm−1 from 1593 to 1590, 1590 and 1599 cm−1, respectively, whereas a slight shift was observed in the D band from 1336 to 1342, 1337 and 1347 cm−1, respectively. The shifts in the bands of JRGO, A-RGO and N-RGO after reduction towards the ideal positions of the G band (1590, 1590

11

and 1599 cm−1) and D band (1342, 1337 and 1347 cm−1) of graphene is a clear indication towards the restoration of the sp2 characters of J-RGO, A-RGO and N-RGO, and correspond to a high degree of reduction.

SC RI PT

Fig. 6 3.6 Zeta Potential

In order to determine the surface electric charge of GO and rGO, zeta potential (Malvern Zetasizer Nano-ZS) measurement was carried out. Fig. 7 represents the zeta potential of GO and rGO. Fig.

U

7a and 7b shows that GO and J-RGO exhibited the zeta potential of -43.2 mV and -25.8 mV,

N

respectively. This decrease in the zeta potential of rGO is due to a decrease in oxygen-containing

A

functional groups on the surface of rGO and loss of negative charge of J-RGO compared to GO,

M

illustrates the excellent reduction reaction method with the algal strain treatment. Fig. 7c and 7d represent the zeta potential of A-RGO and N-RGO with an even greater reduction in zeta potential,

D

showing -23.5 mV and -17.1 mV, respectively. This significant decrease in the zeta potential is

TE

attributed to the efficient reduction of GO to rGO by A-RGO and N-RGO. These results are in

EP

accordance with the synthesis of rGO utilizing non-aromatic amino acids for graphene oxide

CC

reduction reported in the literature [62]. Fig. 7

A

3.7 Redox potential study Redox potential exhibits the electron transfer inside a cell and can be detected both optically as well as electrochemically. The redox potential for graphene oxide solution and J-RGO, A-RGO and N-RGO were measured by using various experimental factors and presented in Table 1. These

12

results indicate the high reducing capabilities of the samples. The actual redox potential and the calculated values were found to be closely related to each other. The more negative values correspond to the stronger reducing environment while the positive redox potential indicates that

SC RI PT

the reduction of graphene oxide by certain bioactive components carried out spontaneously [63, 64]. Table 1 3.8 Adsorption of Pb and Cu by reduced graphene oxide

U

The rGO were utilized for the adsorption of toxic heavy metals, such as lead (Pb) and copper (Cu)

N

from aqueous media. Fig. 8A displays the removal efficiency of rGO (by J-RGO) for Pb and Cu

A

at different time intervals with a given initial concentration of 100 mgL-1. The results revealed that

M

about 93% Cu and 82% Pb were removed within initial 30 min of exposure. As shown in Fig. 8, removal may further increase with increasing the exposure time. Within the maximum exposure

D

time of 360 min, Cu and Pb removal percentage of J-RGO were 96% and 94%, with adsorption

TE

capacity (qe) of 96 mg.g-1 and 94 mg.g-1, respectively. The A-RGO also significantly removed the

EP

mentioned metals and it was observed that 74% Cu and 89% Pb within 30 min were removed from the media, which later on increased to 98% and 95% by increasing adsorption time to 360 min

CC

(Fig. 8B). Similarly, the N-RGO at the start removed Pb faster than Cu, and it was found that within 30 min the N-RGO removed 91% Cu and 95% Pb (Fig. 8C). When the adsorption time was

A

further increased, no additional Pb was removed and total Pb removed remain constant, while Cu removal increased to 98% by increasing adsorption time to 360 min. These results revealed that 30 min is the optimum time for metals adsorption by the as reduced GO. This efficient removal is attributed to the uniform and thin flattened sheets of rGO that provided sufficient surface area for

13

the adsorption of cationic heavy metals. Thus, the algal extract based rGO can be considered as an effective adsorbent for afore mentioned metals. A comparison was made among our current synthesized bio-adsorbent and some other adsorbents reported in the literature, which reflect the

SC RI PT

superior efficiency of this novel algal extract based rGO in metal ions adsorption (Table 2). Fig. 8 Table 2 4. Conclusion

U

The green reduction is an effective, low cost, sustainable and environmentally friendly approach

N

for reducing GO to rGO. The algal extracts were used for the first time in this study and were

A

found to be highly effective in the reduction of GO. Among the various extracts, the 211-11n = N-

M

RGO was found to be more effective in GO reduction, as the rGO obtained as a result of treatment

D

with this extract has lowest zeta potential. Furthermore, the rGO were utilized as efficient

TE

adsorbents and decontaminating agents for the removal of Cu and Pb from aqueous media. It was found that JSC1 = J-RGO removed about 93 % Cu and 82% Pb, 211-9a = A-RGO removed 74%

EP

Cu and 89% Pb while 211-11n = N-RGO removed 91% Cu and 95% Pb within 30 min. It was

CC

concluded that 211-11n based reduced GO most efficiently removed the heavy metals, as compared to the other rGO within the same adsorption time.

A

Acknowledgments This study was funded by China Scholarship Council (CSC) through a scholarship to S.A. (No.2015GXZ239), and by Fundamental Research Funds for the Central Universities in China (No.YS0417).

14

References [1] T. Kuila, S. Bose, A.K. Mishra, P. Khanra, N.H. Kim, J.H. Lee, Prog. Mater. Sci. 57 (2012) 1061-1105.

[3] S. Pei, H.-M. Cheng, Carbon 50 (2012) 3210-3228. [4] K. Saeed, I. Khan, Turk. J. Chem. 41 (2017) 391-398.

SC RI PT

[2] O.C. Compton, S.T. Nguyen, small 6 (2010) 711-723.

[5] S. Du, P. Zhang, R. Zhang, Q. Lu, L. Liu, X. Bao, H. Liu, Chemosphere 164 (2016) 499-507. [6] S. Rajendran, M.M. Khan, F. Gracia, J. Qin, V.K. Gupta, S. Arumainathan, Sci. Rep. 6 (2016)

U

31641.

N

[7] J. Zhao, X. Cao, Z. Wang, Y. Dai, B. Xing, Water research 111 (2017) 18-27.

A

[8] D. Robati, B. Mirza, M. Rajabi, O. Moradi, I. Tyagi, S. Agarwal, V. Gupta, Chem. Eng. J. 284

M

(2016) 687-697.

D

[9] P. Nogueira, D. Nakabayashi, V. Zucolotto, Aquat. Toxicol. 166 (2015) 29-35.

TE

[10] F.S. Awad, K.M. AbouZeid, W.M.A. El-Maaty, A.M. El-Wakil, M.S. El-Shall, ACS Appl. Mater. Inter. 9 (2017) 34230-34242.

EP

[11] F. Tavakoli, M. Salavati-Niasari, F. Mohandes, Mater. Res. Bull. 63 (2015) 51-57. [12] C. Singh, M.A. Ali, G. Sumana, ACS Sustain. Chem. Eng. 4 (2016) 871-880.

CC

[13] M. Khan, A.H. Al-Marri, M. Khan, N. Mohri, S.F. Adil, A. Al-Warthan, M.R.H. Siddiqui,

A

H.Z. Alkhathlan, R. Berger, W. Tremel, RSC Adv. 4 (2014) 24119-24125. [14] Y. Wang, Z. Shi, J. Yin, ACS Appl. Mater. Inter. 3 (2011) 1127-1133. [15] C. Li, Z. Zhuang, X. Jin, Z. Chen, Appl. Surf. Sci. 422 (2017) 469-474. [16] S. B Maddinedi, B. K Mandal, Curr. Nanosci. 12 (2016) 94-102. [17] G. Lee, B.S. Kim, Biotechnol. Progr. 30 (2014) 463-469. 15

[18] T.A. Saleh, V.K. Gupta, Adv. Colloid Interfac. 211 (2014) 93-101. [19] D. Hou, Q. Liu, H. Cheng, K. Li, D. Wang, H. Zhang, Mater. Chem. Phys. 183 (2016) 76-82. [20] S.B. Maddinedi, B.K. Mandal, R. Vankayala, P. Kalluru, S.R. Pamanji, Spectrochim. Acta.

SC RI PT

Part. A: 145 (2015) 117-124. [21] M. Khan, A.H. Al-Marri, M. Khan, M.R. Shaik, N. Mohri, S.F. Adil, M. Kuniyil, H.Z. Alkhathlan, A. Al-Warthan, W. Tremel, Nanoscale Res. Lett. 10 (2015) 281.

[22] A. Dasgupta, J. Sarkar, M. Ghosh, A. Bhattacharya, A. Mukherjee, D. Chattopadhyay, K. Acharya, PloS one 12 (2017) e0171607.

U

[23] D. Hou, Q. Liu, H. Cheng, H. Zhang, S. Wang, J. Solid State Chem. 246 (2017) 351-356.

N

[24] M. Agharkar, S. Kochrekar, S. Hidouri, M.A. Azeez, Mater. Res. Bull. 59 (2014) 323-328.

A

[25] A. Mittal, J. Mittal, A. Malviya, V. Gupta, J. Colloid Interf. Sci. 344 (2010) 497-507.

M

[26] V.K. Gupta, R. Jain, A. Nayak, S. Agarwal, M. Shrivastava, Mater. Sci. Eng. 31 (2011) 10621067.

D

[27] R. Saravanan, M.M. Khan, V.K. Gupta, E. Mosquera, F. Gracia, V. Narayanan, A. Stephen,

TE

RSC Adv. 5 (2015) 34645-34651.

EP

[28] R. Saravanan, M.M. Khan, V.K. Gupta, E. Mosquera, F. Gracia, V. Narayanan, A. Stephen, J. Colloid Interf. Sci. 452 (2015) 126-133.

CC

[29] V. Gupta, I. Tyagi, S. Agarwal, R. Singh, M. Chaudhary, A. Harit, S. Kushwaha, Global J. Environ. Sci. Manag. 2 (2016) 1-10.

A

[30] R. Saravanan, V. Gupta, E. Mosquera, F. Gracia, J. Mol. Liq. 198 (2014) 409-412. [31] M. Ramakrishna, D.R. Babu, R.M. Gengan, S. Chandra, G.N. Rao, J. Nanostruct. Chem. 6 (2016) 1-13.

16

[32] K. Miazek, W. Iwanek, C. Remacle, A. Richel, D. Goffin Int. J. Mol. Sci. 16 (2015) 2392923969. [33] K.S. Siddiqi, A. Husen, Nanoscale Res. Lett. 11 (2016) 363.

SC RI PT

[34] M. Ahmaruzzaman, V.K. Gupta, Ind. Eng. Chem. Res. 50 (2011) 13589-13613. [35] R. Bhaskar, H. Joshi, A.K. Sharma, A.K. Singh, ACS Appl. Mater. Inter. 9 (2017) 2223-2231. [36] M.L. Yola, V.K. Gupta, T. Eren, A.E. Şen, N. Atar, Electrochim. Acta. 120 (2014) 204-211. [37] T.A. Saleh, V.K. Gupta, J. Colloid Interf. Sci. 371 (2012) 101-106.

[38] R. Saravanan, E. Sacari, F. Gracia, M.M. Khan, E. Mosquera, V.K. Gupta, J. Mol. Liq. 221

U

(2016) 1029-1033.

N

[39] M. Devaraj, R. Saravanan, R. Deivasigamani, V.K. Gupta, F. Gracia, S. Jayadevan, J. Mol.

A

Liq. 221 (2016) 930-941.

M

[40] R. Saravanan, S. Karthikeyan, V. Gupta, G. Sekaran, V. Narayanan, A. Stephen, Mater. Sci. Eng. 33 (2013) 91-98.

D

[41] R. Saravanan, E. Thirumal, V. Gupta, V. Narayanan, A. Stephen, J. Mol. Liq. 177 (2013) 394-

TE

401.

93.

EP

[42] R. Saravanan, V. Gupta, T. Prakash, V. Narayanan, A. Stephen, J J. Mol. Liq. 178 (2013) 88-

CC

[43] M. Ghaedi, S. Hajjati, Z. Mahmudi, I. Tyagi, S. Agarwal, A. Maity, V. Gupta, Chem. Eng. J. 268 (2015) 28-37.

A

[44] N. Mohammadi, H. Khani, V.K. Gupta, E. Amereh, S. Agarwal, J. Colloid Interf. Sci. 362 (2011) 457-462. [45] F. Wu, G. Zhang, Ying yong sheng tai xue bao= J. Appl. Ecol. 13 (2002) 1595-1599.

17

[46] C.S.E.H. Network, S.J. Balk, C.F. Bearer, I. Clemente, Training Manual on Pediatric Environmental Health: Putting it Into Practice, Children's Environmental Health Network 1999. [47] A. Asfaram, M. Ghaedi, S. Agarwal, I. Tyagi, V.K. Gupta, RSC Adv. 5 (2015) 18438-18450.

SC RI PT

[48] V.K. Gupta, N. Atar, M.L. Yola, Z. Üstündağ, L. Uzun, Water Res. 48 (2014) 210-217. [49] R. Saravanan, V. Gupta, V. Narayanan, A. Stephen, J. Taiwan Inst. Chem. E. 45 (2014) 19101917. [50] K. Kim, H. Choi, Water Sci. Technol. 38 (1998) 95-101.

[51] N.C. Joshi, R. Sharma, A. Singh, J. Chem. Chemi. Sci. 7 714-724.

U

[52] M.N. Vara Prasad, H.M. de Oliveira Freitas, Electron. J. Biotechn. 6 (2003) 285-321.

N

[53] L. Thöni, L. Yurukova, A. Bergamini, I. Ilyin, D. Matthaei, Atmos. Environ. 45 (2011) 1899-

A

1912.

M

[54] F. Solomon, Environ. Commun. 25 (2009) 25-28.

[55] R. Saravanan, N. Karthikeyan, V. Gupta, E. Thirumal, P. Thangadurai, V. Narayanan, A.

D

Stephen, Mater. Sci. Eng. 33 (2013) 2235-2244.

TE

[56] T.A. Saleh, V.K. Gupta, Sep. Purif. Technol. 89 (2012) 245-251.

1261-1268.

EP

[57] V.K. Gupta, I. Ali, T.A. Saleh, M. Siddiqui, S. Agarwal, Environ. Sci. Pollut. R. 20 (2013)

CC

[58] R. Saravanan, V.K. Gupta, V. Narayanan, A. Stephen, J. Mol. Liq. 181 (2013) 133-141. [59] M. Nourbakhsh, Y. Sag, D. Özer, Z. Aksu, T. Kutsal, A. Caglar, Process Biochem. 29 (1994)

A

1-5.

[60] N.C. Joshi, J. Life Sci. Bioinform. Pharm. Chem. Sci. 4 (2018) 59. [61] M.M. Aslam, I. Hassan, M. Malik, A. Matin, Electron. J. Environ. Agric. Food. Chem. 3 (2004) 658-664.

18

[62] K. Trivunac, S. Stevanovic, Chemosphere 64 (2006) 486-491. [63] J.F.W.E.C.o.F.A. Meeting, J.F.W.E.C.o.F. Additives, W.H. Organization, Evaluation of certain food additives and contaminants: sixty-first report of the Joint FAO/WHO Expert

SC RI PT

Committee on Food Additives, World Health Organization 2004. [64] T. Bakalár, M. Búgel, L. Gajdošová, Acta Montan. Slovaca 14 (2009) 250.

[65] W. Shen, S. Chen, S. Shi, X. Li, X. Zhang, W. Hu, H. Wang, Carbohyd. Polym. 75 (2009) 110-114.

[66] S. Chen, Y. Zou, Z. Yan, W. Shen, S. Shi, X. Zhang, H. Wang, J. Hazard. Mater. 161 (2009)

U

1355-1359.

N

[67] L. Min, Y.-m. Zhang, X.-h. Guan, X.-h. Xu, T.-t. Gao, T. Nonferr. Metal. Soc. 24 (2014)

A

1912-1917.

A

CC

EP

TE

D

M

[68] M.-W. Wan, C.-C. Kan, B.D. Rogel, M.L.P. Dalida, Carbohyd. Polym. 80 (2010) 891-899.

19

Figure Captions

CC

EP

TE

D

M

A

N

U

SC RI PT

Fig. 1 SEM images of GO and rGO. (A) GO (B) J-RGO (C) A-RGO and (D) N-RGO.

A

Fig. 2 TEM images of GO and rGO. (A) GO (B) J-RGO (C) A-RGO and (D) N-RGO.

20

SC RI PT U N A M D

TE

Fig. 3 UV–vis spectra of GO and rGO in the experimental medium. N-RGO= 211-11n, A-RGO=

A

CC

EP

211-9a and J-RGO= JSC-1.

21

SC RI PT U N

A

Fig. 4 Fourier transformed infrared spectra of Graphene oxide (GO) and microalgal extracts

A

CC

EP

TE

D

M

based reduced graphene oxide (rGO).

22

Fig. 5 The inserted portrait disclosed only the XRD pattern of GO, whereas, the other XRD patterns refer to the rGO with algal strains of J-RGO (black color), A-RGO (red color) and N-

D

M

A

N

U

SC RI PT

RGO (blue color spectrum). N-RGO= 211-11n, A-RGO= 211-9a and J-RGO= JSC-1.

TE

Fig. 6 The Ramen shift spectra of GO, along with algal extract based rGO e.g. J-RGO (red color),

A

CC

JSC-1.

EP

A-RGO (blue color) and N-RGO (pink color). N-RGO= 211-11n, A-RGO= 211-9a and J-RGO=

23

SC RI PT U N

A

Fig. 7 Zeta potential, Zeta Deviation and Conductivity measurements of (A) Graphene oxide, algal

M

extracts based reduced graphene oxide (B) J-RGO= JSC-1, (C) A-RGO= 211-9a, and (D) N-RGO=

A

CC

EP

TE

D

211-11n.

24

Fig. 8 Effect of different exposure times on the removal of Pb and Cu by rGO. (A) J-RGO (B)

A

CC

EP

TE

D

M

A

N

U

SC RI PT

A-RGO and (C) N-RGO; initial concentration (100 mg/L)

25

Tables Table 1. Measurements of the Redox potential of Graphene oxide (GO) and algal extracts based

SC RI PT

reduced graphene oxide (rGO). N-RGO= 211-11n, A-RGO= 211-9a and J-RGO= JSC1.

Eo(V) ±0.005

Reactions

Reduction half-reaction of GO

1 o E red

+0.405

Reduction half reaction of JSC-1 aqueous extracts

2 o E red

Oxidation half reaction of JSC-1aqueous extracts

1 o E ox

-0.127a hdh +0.127

Overall redox reaction (1Eored+1Eoox)

Eoredox

+0.532b

Eoredox (Mixture)

+0.473

N

Redox potential of mixture (GO + JSC-1 extract)

U

Details

A

Reduction half reaction of 211-9a aqueous extracts Overall redox reaction (1Eored+2Eoox)

M

Oxidation half reaction of 211-9a aqueous extracts

3 o E red

0.128

2 o E ox

-0.128

Eoredox

+0.277b

Eoredox (Mixture)

+0.221

Reduction half reaction of 211-11n aqueous extracts

4 o E red

-0.014a

Oxidation half reaction of 211-11n aqueous extracts

3 o E ox

+0.014

Overall redox reaction (1Eored+3Eoox)

Eoredox

+0.419b

Redox potential of mixture (GO + 211-11n extract

Eoredox (Mixture)

+0.365

EP

TE

D

Redox potential of mixture (GO + 211-9a extract)

Note: a Negative values indicate high reducing environment, b Positive value shows spontaneity

A

CC

of the reaction.

26

Table 2. Comparison among the maximum adsorption capacity of algal extracts based rGO and adsorbents reported earlier towards Cu and Pb ions.

SC RI PT

Adsorption efficiency (mg.g-1)

Adsorbent

Reference

Pb

Reduced GO (N-rGO)

91

95

Current Study

Reduced GO (A-rGO)

74

89

Current Study

Reduced GO (J-rGO)

93

82

Current Study

Carboxymethylated-BC

20.35

Amino-BC

35.83

[65] [66]

55.65

[67]

8.18

12.32

[68]

M

65.53

A

CC

EP

TE

D

Chitosan-coated sand

87.41

N

63.09

A

Diethylenetriamine-BC

U

Cu

27