Highly Efficient Fluoride Adsorption in Domestic Water with RGO/Ag Nanomaterials

Reduced graphene oxide loaded silver (RGO/Ag) nanomaterials as a new nanocomposite film were prepared by in-situ redox method. The

RGO/Ag nanomaterials were characterized by SEM, Raman spectra, XRD, UV-vis and TGA. The silver nanoparticles were well decorated and

dispersed on the RGO nanosheets. Moreover, RGO/Ag nanomaterials were used to remove fluoride ion in domestic water. The influence

factors including concentration of RGO/Ag, pH of water, the adsorption time and temperature on the removal rate of fluoride ion in domestic

water were discussed. The results show that the removal ratio of fluoride in domestic water reaches up to 90% at the condition of 0.1g/L oRGO/Ag, 60 C and 20 h of adsorption temperature and time. Our RGO/Ag nanomaterials have great potentials for water treatment toward

environmental remediation, especially for water treatment.

Keywords: Reduced graphene oxide; Silver nanoparticles; Fluoride; Water treatment

Received 30 November 2018, Accepted 9 February 2019

DOI: 10.30919/esee8c217

ES Energy & Environment

1* 1 2* 2 2 3*Chunling Lin, Zhiqiu Qiao, Jiaoxia Zhang, Jijun Tang, Zhuangzhuang Zhang and Zhanhu Guo

View Article Online

1School of chemistry and chemical engineering, Xi'an Shi'you

University, Shaanxi 710065, Xi'an, China2National Demonstration Center for Experimental Materials Science

and Engineering Education, Jiangsu University of Science and

Technology, Zhenjiang 212003, China3Integrated Composites Laboratory (ICL), Department of Chemcial &

Biomolecular Engineering, University of Tennessee, Knoxville TN

37996, USA

*E-mail: chunling405@xsyu.edu.cn; myzjx0359@163.com;

zguo10@utk.edu

RESEARCH PAPER

1. IntroductionFluorine is trace elements necessary for human body and adults under

normal circumstances which people every day can obtain from ordinary 1water and food. However, fluorine contamination is increasingly

serious in surface water and groundwater with the increase of

discharging We once collected 10 2from various industrial processes.

samples of rural drinking water resources in Northern Shaanxi and 3, 4found that the fluoride content in five samples are impermissibly high.

Due to the high solubility of fluorine in water, high fluoride content in

drinking water threatens the human environment. Drinking high fluorine 5water lead to fluorosis and dental fluorosis, even cause bone sclerosis

or osteoporosis, bone deformation, even paralysis and make people lose

labor ability. With increasing attention about clean environment and 2human health , technologies with high efficiency and low cost to

remove the fluorine content in drinking water and wastewater are in 6-8urgent demand.

In recent years, uoride as for the removal of fl the methods have 9-11involved in the use of cationic and anionic ion-exchange resins,

12,13 14,15 16,17chemical precipitation, flocculation, electrodialysis, membrane

separation, However, for defluoridation of 18, 19 20, 21adsorption, etc.

drinking water and wastewater, none of them can perfectly and

completely overcome the problems from the pollution treatment cost,

process complexity, and damage to the environment and removal

efficiency. For example, flocculation of the sludge sedimentation is slow 22,23and difficult in the chemical precipitation. Dehydration fluoride

wastewater sedimentation flocculants method commonly used 24aluminum salt. But the small amount of fluorine need use large

amount of coagulant dosage which will produce large amount of sludge, 25so the treatment cost also is high. For the ion-exchange resins,

efficiency is decreased in presence of other anion such as sulfate, 26phosphate and carbonate. In addition, the maintaining pH and

27regeneration of resin also are problems which maybe increase cost.

The membrane separation method may remove all the ions present in

water including some minerals which are essential for proper growth

and enhance acid of water. The process is expensive compared to other 26, 28options.

The adsorption with the simple operation, low processing cost,

and good effect has been widely used in environmental management.

Adsorption method is an especially effective way to deal with the low 29concentration of fluoride in wastewater. So it is the effective approach

to solve the global water resources shortage and deterioration of water 30environment from a long-term perspective . In adsorption techniques,

29, 31 32, 33the most usually used adsorbent are the activated alumina, zeolite, 34, 35activated carbon. But the adsorption capacity of these adsorption

materials is not enough high and it is difficult to adsorb the low

concentration fluoride. Therefore, it is extremely urgent to develop some

new adsorbents or modify existing adsorbents to satisfy the application

requirement.

Graphene as a newly emerging member of carbon materials has 2drawn much scientific attention since its discovery due to the sp -

hybridized single-atom-layer structure endowed with large surface area,

unique mechanical and electronic properties, excellent mobility of 36-39charge carriers and high thermal conductivity. So graphene exhibits

great promise for potential applications as adsorbent for water 40, 41treatment . Graphene with an adsorption fluoride capacity of up to

© Engineered Science Publisher LLC 2019 ES Energy Environ., 2019, 4, 27–33 | 27

ES Energy & EnvironmentResearch Paper

17.65 mg/g at initial concentration of 25 mg/L at 298 K is an excellent 29fluoride adsorbent. In addition, considering the outstandingantibacterial42,43properties of Ag, in this experiment, we reduced graphene oxide

(RGO) loaded Ag nanoparticles (Nps) by in-situ redox method and then

studied the influence of experimental parameters such as pH and

temperature on the fluoride adsorption properties. It has been found that

the removal fluoride ratio in water reaches up to 90% for 200mg/L

RGO/Ag adsorbent.

2. Materials and Experiment2.1Materials

44GO was prepared by Hummers method according previous reports .

Sodium fluoride (NaF) used in this study was obtained from Tianjin

Kemiou Chemical Reagent Co., Ltd. Silver nitrate (AgNO ) was 3

purchased from Sinopharm Chemical Reagent Co., Ltd. Sulfuric acid

(H SO ), hydrogen peroxide(H O ), anhydrous ethanol, and ammonia 2 4 2 2

water were produced by Xi'an Chemistry Reagent Company. Potassium

Permanganate (KMnO ) was provided by Tianjin hedong district red 4

crag reagent factory. Unless otherwise stated, all solvents were of

analytical grade.

2.2 Experimental method

2.2.1 Preparation of RGO/Ag

The stock solution of GO was prepared by transferring 10 mg GO

into100 mL deionized (DI) water and then dispersing the mixture using

a Sonic Dismembrator (Fisher Scientific, Model F550) at 25% power

for 30 min. The mixture was poured into a 250 mL round-bottom flask.

And then, 7 mL NaOH (1 M) and 4 mL AgNO (0.05 M) were added to 3

the above GO solution. 1mL CH O (37%) solution as a reduction agent 2

for GO and AgNO was slowly dropped in 15 min until the yellow 3

solution is appeared to obtain Ag NPs. At last, the crude products were owashed by alcohol for 3 times and kept drying at 80 C for 3 hours in

the oven to obtain the RGO/Ag nanomaterials. Fig. 1 displays the

preparation process of RGO/Ag nanomaterials.

Fig. 1 the synthesis schematic of RGO/Ag nanomaterials.

2.2.2 Fluorine ion Removal of water by RGO/Ag nanomaterials

Adding the above RGO/Ag into DI water and dispersing for 10 min by

ultrasonication obtain 100 mg/L - RGO/Ag solution. Adsorption of F by

RGO /Ag nanoparticles was tested by 100 mL mixing solutions -6 -containing100mg/L RGO/Ag and100 ~ 10 M of F in centrifuge tubes

on an oscillator. After overnight mixing, the samples were centrifuged at

a speed of 10,000 rpm for 30 min. The supernatant samples were

filtered through a 0.22 μm filter membrane and collected filtrate for -analysis of soluble Fconcentrations using a fluorine ion selective

electrode.

The removal ratio of fluorine ions adsorbed on RGO/Ag was

calculated according the following formula.

where R is the removal ratio of fluorine ions. C is the initial 0

concentration of fluorine ions in the solution (M). C presents the 1

equilibrium concentration of fluorine ions after adsorption and filtration

in the solution(M).

2.2.3 Characterization

The surface morphology and micro-structures of the RGO/Ag were

investigated by Scanning Electron Microscope (SEM, Apollo 300). The

surface morphology was obtained by scanning electron microscopy

(SEM, JEOL JSM7500F) equipped with a cold cathode UHV field

emission conica. X-ray diffraction (XRD) patterns were recorded on

Rigaku XRD-600 (Japan) with Cu Kα irradiation at 40 kV and 40 mA.

The scanning range is 0°~ 100° with 2°/ min. UV–vis spectra were

acquired on a PerkinElmer Lambda-850 spectrophotometer

(PerkinElmer Life and Analytical Sciences, Waltham, Mass, USA)

using a couple of 1cm optical micro-cuvettes (Fischer Scientific, USA)

with a sample volume of 0.1 mL. A 100-μL Hamilton syringe

(Bonaduz, Switzerland) was used to transfer samples solutions into the

micro-cuvettes. The Raman spectra were collected in high resolution

）（ 1-1 ×100%

C

C-CR

0

10=

© Engineered Science Publisher LLC 201928 | ES Energy Environ., 2019, 4, 27–33

mode with a dispersive Raman spectrometer (Thermo NicoletAlmega

XR) equipped with a CCD detector, optical microscope, digital camera,

and 780 nm laser line with a laser source power of 30 mW (50% power

was applied in theanalysis). Raman signal was excited using the 514.5

nm wavelength of an Ar ion laser operating at 20 mW. The sample's

heat stability were tested with a dynamic thermogravimetric method in

a range of 25 to 900 °C at a heating rate of 10 °C/min. A nitrogen gas −1flow at 60 mL min was used at below atmospheric pressure. The

RGO/Ag nanomaterials before and after absorption of fluorine ions

were characterized by fourier transform-infrared (FTIR) spectrometer -1(FTS2000) with KBr pellets. The scanning range was 400-4000 cm ,

-1the resolution was 1.5 cm , and the number of scans was 8 times. A

high sensitive monocrystalline fluorine electrode-based potentiometer.

The adsorption experiment was carried out to evalute the

adsorption mechanism and model by physisorption instrument (ASAP

2020) with 0.15g GO/Ag sample. The degassing temperature was set at

350 and the heating rate was 10 / min for 4 h. After degassing, °C °C

the sample tube is decreased to room temperature. Then the sample tube

was installed at the analysis station, and then the Dewar bottle

containing liquid nitrogen was placed and adsorbed for 6 h. A high

sensitive monocrystalline fluorine electrode-based potentiometer (PB-

10, SaiDuoLiSi Corp., German，June 2013) was used to determine

fluoride concentrations in water samples. Three replicates of each water

sample were tested. In order to depict the standard curve of fluorine ion,

2.210 g NaF was dissolved in deionized water and transferred into 1000

mL volumetric flask, then diluted to scale with deionized water, shaking -4 -5well. 10 -10 M fluorine standard solution was obtained by successive

-4 -5dilution method. Take 10 -10 M fluorine standard solution into 50 ml

volumetric flask and add 10 ml total ionic strength adjustment buffer

(TISAB) diluted with deionized water in the flask until the scale,

Fig. 2 The SEM images of GO (a) and RGO/Ag (b) EDS of GO (c) and RGO/Ag (d).

ES Energy & Environment Research Paper

shaking well. Then get 40 ml this solution into 50 ml beaker. The

fluoride electrode and the reference electrode are inserted, and reading

every half minute until the basic reading within the 1min (< 1 MV).

From low concentration to high concentration one by one The fluorine

content is calculated according to the formula (1-2).

where Va is the standard volume added (ml), C the concentration (M),

Vo the volume of analyzed sample (mlΔ), ΔE the increase of potential

(mV), and S the slope of the standard curve.

The TISAB was prepared as following: Adding 500 mL deionized

water to the 1000mL beaker, then adding 50mL glacial acetic acid, 12 g

sodium citrate, 58 g NaCl , putting the beaker in cold water bath,

adjusting the solution to PH 5.0 - 5.5 with ammonia water (14M),

comparing the pH test paper, Finally, it was diluted to 1 L with

deionized water.

3. Results and discussion3.1 RGO/Ag Nanostructures

Fig. 2 presents the morphology and structure of GO and RGO/Ag

nanomaterials. Fig. 2a. shows the 3 D porous network with a crumpled

sheet structure composed of randomly oriented graphene sheets. The

unique structure largely preventing the aggregation of graphene oxide

sheets endows a large surface area and great potential for further

functionalization and absorbent materials. Fig. 2b reveals Ag NPs are

uniformly encapsulated within the graphene oxide layers with high

density, implying an efficient load of Ag NPs onto graphene oxide

which avoids direct contact between Ag NPs. The EDS spectra also

© Engineered Science Publisher LLC 2019 ES Energy Environ., 2019, 4, 27–33 | 29

-C (F ) = V a *C

V (10 -1)0

Δ Es

exhibits the presence of the elements C, O for graphene oxide (Fig. 2c.)

and the elements C, O and Ag for RGO/Ag nanomaterials (Fig. 2d.).

The O content of RGO/Ag nanomaterials evidently decreases compared

with graphene oxide revealing the graphene oxide also partly reduced in

the preparation process of Ag Nps.

Fig. 3 shows the XRD curves of GO and RGO/Ag. GO has a osharp peak near 26 , which is the diffraction peak of graphite surface

(002). Because the structure of GO contains a lot of defects and oxygen 44 ogroups, the characteristic peak (001) appears at 2 θ about 12 . The

results show that the space arrangement of GO is not only regular on

graphene plane but also contains oxygen groups. The oxygen groups are

beneficial to be loaded Ag. After loaded Ag, a strong diffraction peak oappears near 2 θ about 38 , corresponding to the (111) crystal plane of

cubic phase Ag, which is the characteristic peak of Ag, while the peak

of oxygen group and graphite surface disappears obviously due to the

strong intensity of Ag.

Fig. 4 shows the UV absorption spectra of GO and RGO/Ag

nanomaterials in water. For GO, there is at 225 nm due absorption band

to tron of aromatic C=C. In addition, there the π-π* elec transition

appears a shoulder at 300 nm which is attributed to the n-π* tronelec

transitions of C=O. The two peaks are the characteristic absorption45, 46signals of GO. For the RGO/Ag nanomaterials, the peak of π-π*

elec transitiontron of aromatic C=C shifts from 225 nm to 250 nm

because the reduced GO tends to be flat (closer to the smooth level of

graphene) needing lower energy to transition. The signal at 375 nm is

correspond to the Ag band. The results imply the AgNO was reduced 3

to Ag, and the reduced GO was obtained at the same time.

Fig. 5 exhibits the Raman spectra of GO and RGO/Ag

Fig. 3 XRD of (a) GO and (b) GO/Ag . Fig. 4 UV-vis of GO (a) and RGO/Ag (b).

-1nanomaterials. GO occurs the D band peak at 1355 cm and G band -1peak at 1590 cm . D peak is caused by C-C disordered vibration and

3characterizes the carbon atom of the sp hybrid structure. The G band

peak is due to the stretching vibration of C-C and characterizes the 2carbon atom of structure sp hybrid structure. So, the peak intensity ratio

(I /I ) of D and G peak present structural regularity of carbon D G

nanomaterials. Fig. 5 shows the RGO/Ag nanomaterials have the

similar band, but a little red shift for D and G band peak. In addition,

the I /I (0.95) of RGO/Ag nanomaterials is higher than the GO (0.92) D G

which means that the Ag nanoparticles decrease the regularity degree of

RGO. However, the C/O ratio of GRO/Ag increase compared with GO

(see Fig. 5).

The oxygenic groups directly influence the stability of

nanomaterials at high temperature. Thermogravimetric analysis (TGA)

is a simple way to quantify the oxygenic groups of GO and RGO/Ag

nanomaterials. The GO has a 12 wt% weight loss near 115 °C,

evidently due to evaporation of water molecules held in the material

(see Fig 6.). The second significant weight loss (26 wt% loss at 235 °C)

is occurred from 180 to 235 °C of the GO. This is contributed to the

loss of CO, CO , and steam from the sample aroused by water 247, 48evaporation and decomposition of labile oxygenic groups . The third

weight loss (53 wt% loss at 1000 °C) is due to the decomposition of

carbon skeleton of graphene oxide. Interestingly, the RGO/Ag

nanomaterials lost a much smaller mass (5 wt % loss at 235 °C) over

the temperature span, even only 10 wt% weight losses over the total

testing temperature span. It is very likely due to a decreased amount of

oxygen functional groups in the RGO/Ag nanomaterials as would be

expected from the reduction process.

Fig. 5 Raman Spectra of GO and RGO/Ag. Fig. 6 The TGA curves of GO and RGO/Ag.

ES Energy & EnvironmentResearch Paper

© Engineered Science Publisher LLC 201930 | ES Energy Environ., 2019, 4, 27–33

3.2 The fluoride ion adsorption property

With the development of modern industrialization, water source is

getting worse and worse. But human requirement becomes higher and

higher for the healthy environment. The increasing attention has been

paid to the water treatment especially for the heavy metal and fluoride

ions which is highly toxic for people health in recent years. The

adsorption technique among all of the removal methods is the most

perhaps adopted method due to the low cost and convenience.

Nanomaterials show higher adsorption efficient than the corresponding

bulk materials owing to the nano-sized effects. So, the RGO/Ag as the

sorbents was used to treat the fluoride ion water. The influence of the

concentration of RGO/Ag, temperature, pH and time of adsorption on

the removal ratio were discussed. The concentration of RGO/Ag is oconsidered as variate from 0.04 to 0.2 g/L with the 60 C, 20 constant of

-5 h, and pH=7 and 10 M of the fluoride ion for the adsorption process

(see Fig.7a). With the concentration increasing of RGO/Ag, the removal

ratios also are evidently improved because the more RGO/Ag bring the

larger adsorption surface area to promote the faster electron transfer and

obtain greater adsorption capacity. But removal ratio holds around the

93.4% when the concentrations continually increase after 0.10 g/L

which imply the of fluoride ion adsorption and desorption performance

in water reach to the RGO/Ag adsorbentactivated balance for the . So,

the -50.10 g/L RGO/Ag is taken as the best content for the 10 M of the

fluoride ion in water considering the cost and removal efficiency. Then,

the solution pH is considered as variate from 4 to 10 with the constant

Fig. 7 The influence of concentration of GRO/Ag(a), pH(b), temperature(c) and time(d) on the removal ratio of fluorine ion.

of o -5 60 C, 20 h, 0.10 g/L of RGO/Ag and 10 M of the fluoride ion for

the adsorption process to determine the removal ratio for fluoride ion in

water (see Fig. 7b). As shown from the Fig. 7b, with the pH increasing

(up to pH=7), the removal ratios also are gradually increase then keep

the little amplitude around 98.7% from 7 to 10 for pH. The reason is

that the negative RGO/Ag maybe be neutralized in acid media which

decrease the removal fluoride ion efficiency. However, the increasing

negative in the water improve the dispersibility of negative RGO/Ag

due to charge rejection, and further promote the removal ratio. Usually,

the pH is about 7 for drink water, so 7 of pH is adopted the best pH

value. For the treating temperature, it can be found that the RGO/Ag oobtain the best removal ratio at 60 C from the Fig. 7c. About the

treating time, removal ratio of the fluoride ion gradually increases with

the prolonging treating time. However, when the adsorption time

reaches 18 hours, the fluoride removal ratio reaches up to 90% and

become stable after 20 h adsorption time. So we conclude that the best oabsorption is that the 0.10 g/L RGO/Ag, pH>7, 60 C and 20h

adsorption time, the fluoride removal ratio reaches up to 91%.

Fig. 8 (a) is distribution of pore size of GO/Ag. The pore size is

about 2-11 nm indicating that GO/Ag nanomaterials is a

micromesoporous material. Fig. 8 (b) is adsorption desorption curves of

GO/Ag. The first steep stage of the isotherm curve is under the N 2

relative partial pressure region (P/P0=0.1~0.2) represents the saturated

adsorption capacity of the monolayer. After a period of saturated

adsorption, with the increasing of pressure, both adsorption and

ES Energy & Environment Research Paper

© Engineered Science Publisher LLC 2019 ES Energy Environ., 2019, 4, 27–33 | 31

desorption have steep rise and fall under partial pressure region

(P/P0=0.4~0.8), which mean the multilayer adsorption. The adsorption

capacity increases sharply with the P/P0 increasing, which indicates that

the pore distribution of the sample is more uniform. Fig. 8(c) and (d)

were Langmuir and BET medels by the physical adsorption instrument

simulations, the adsorption behavior of the GO/Ag is the R2 = 0.9998

by fitting the BET equation, and the fitting Langmuir equation is

Fig. 8 (a)� distribution of pore size, (b) adsorption desorption,� (c) and (d) are Langmuir and BET models by the physical adsorption instrument simulations. For the adsorption behavior of the GO/Ag nanomaterials, the correlation coefficient (R2) is 0.9998 by fitting the BET equation, and the fitting Langmuir equation is obtained with R2 only 0.97187.

Fig. 9 (a) FTIR of RGO/Ag before (a) and after adsorption of F ions (b), (b) RGO/Ag adsorption mechanism for F ions.

obtained with R2 only 0.97187. The correlation of the fitted BET

equation is higher than that of the Langmuir equation, which is shown

that the adsorption behavior of the GO/Ag is in accordance with the

BET adsorption model. The BET model indicates that the GO/Ag is

mainly based on the multi-molecular layer adsorption.

Fig. 9(a) is the FTIR spectrum of RGO/Ag before and after -1absorption of fluoride ion. Absorption peak at 3423 cm shows -OH

ES Energy & EnvironmentResearch Paper

© Engineered Science Publisher LLC 201932 | ES Energy Environ., 2019, 4, 27–33

(a) (b)

stretching vibration, but the peak intensity become weaker after -1adsorption of fluoride ion. There are obvious peak at 1596 cm in the

two curves, which show that they all contain the aromatic skeleton of -1C=C stretching vibration. The peak both with at 1346 cm should be

assigned to the stretching vibrations of -C-O- indicating, oxidation of

RGO/Ag after fluoride ion adsorption has been enhanced. However, the

band positions are similar for RGO/Ag before and after absorption of

fluoride ion. Infrared spectroscopy before and after adsorption is that

RGO/Ag adsorbed F ions structure has not changed, that RGO/Ag

treatment of high the mechanism of fluoride content by ion exchange,

static electricity and so on. Such as Fig. 9(b) as shown fluorine ions are

strongly adsorbed between GO and silver by means of static molecules.

5. ConclusionRGO/Ag was prepared by oxidation-reduction reaction. The fluoride

removal for RGO/Ag nanomaterials in water displays the best oadsorption at 0.1 g/L of concentration, 60 C of temperature and 20 h of

adsorption time The F ions in solution are quickly adsorbed on .

RGO/Ag nanomaterials, and of multi-molecular layer adsorption

RGO/Ag is proceeded based on static electricity between RGO/Ag nanomaterials and F ions.

AcknowledgmentsWe gratefully acknowledge the supports from the project was supported

by the Special fund of Shaanxi Provincial Education Department

(16JK1612), National Natural Science Foundation of China

(No.51402132), Key research and development project of shaanxi

province in 2017 (2017GY-180) and Provincial College Students

Innovation and Entrepreneurship Program (201819018).

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