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

Chunling Lin ^1,*, ZhiqiuQiao¹, Jiaoxia Zhang ^2*, Jijun Tang², Zhuangzhuang Zhang and Zhanhu Guo^3,*

 

¹School of chemistry and chemical engineering, Xi'an shi‘you University, Shaanxi 710065, Xi'an, China

²National Demonstration Center for Experimental Materials Science and Engineering Education, Jiangsu University of Science and Technology, Zhenjiang 212003, China

³Integrated 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

 

ABSTRACT:

Reduced graphene oxide loaded silver (RGO/Ag) nanomaterials as a new nanocomposite film were prepared byin-situ redox method.The RGO/Ag nanomaterialswere 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 removefluoride 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 werediscussed. The results show that the removal ratio of fluoride in domestic water reaches up to 90% at the condition of 0.1g/L RGO/Ag, 60^oC and 20hof adsorption temperature and time.So the RGO/Ag nanomaterials are potentialcandidates in water treatment.

Table of Content

Reduced graphene oxide loaded silver (RGO/Ag) nanocomposite film was used to remove fluoride ions from domestic water

 

 

 

Keywords: Reduced grapheneoxide;Silver nanoparticles; Fluoride;Water treatment

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1. Introduction

Fluorine is trace elements necessary for human body and adults under normal circumstances which people every day can obtain from ordinary water and food.¹ However, fluorine contamination is increasingly serious in surface water and groundwater with the increase of discharging from various industrial processes.² We once collected 10 samples of rural drinking water resources in Northern Shaanxi and found that the fluoride content in five samples are impermissibly high.^3,4 Due to the high solubility of fluorine in water, high fluoride content in drinking water threatens the human environment. Drinking high fluorine water lead to fluorosis and dental fluorosis,⁵ even cause bone sclerosis or osteoporosis, bone deformation, even paralysis and make people lose labor ability. With increasing attention about clean environment and human health², technologies with high efficiency and low cost to remove the fluorine content in drinking water and wastewater are in urgent demand.^6-8

    In recent years,as for the removal of fluoride the methods have involved in the use of cationic and anionic ion-exchange resins,^9-11 chemical precipitation,^12,13 flocculation,^14,15 electrodialysis,^16,17 membrane separation,^18,19adsorption,^20,21 etc. However, for defluoridation of 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 and difficult in the chemical precipitation.^22,23 Dehydration fluoride wastewater sedimentation flocculants method commonly used aluminum salt.²⁴ But the small amount of fluorine need use large amount of coagulant dosage which will produce large amount of sludge, so the treatment cost also is high.²⁵ For the ion-exchange resins, efficiency is decreased in presence of other anion such as sulfate, phosphate and carbonate.²⁶ In addition, the maintaining pH and regeneration of resin also are problems which maybe increase cost.²⁷ 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 options.^26,28

    The adsorption with the simple operation, low processing cost, and good effect has been widely used in environmental management. Especially Adsorption method is an effective way to deal with the low concentration of fluoride in wastewater.²⁹ So it is the effective approach to solve the global water resources shortage and deterioration of water environment from a long-term perspective.³⁰ In adsorption techniques, the most usually used adsorbent are the activated alumina,^29,31 zeolite,^32,33 activated carbon.^34,35 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 drawn much scientific attention since its discovery due to the sp²-hybridized single-atom-layer structure endowed with large surface area, unique mechanical and electronic properties, excellent mobility of charge carriers and high thermal conductivity.^36-39 So graphene exhibits great promise for potential applications as adsorbent for water treatment^40,41. Graphene with an adsorption fluoride capacity of up to 17.65 mg/g at initial concentration of 25 mg/L at 298 K is an excellent fluoride adsorbent.²⁹ In addition, considering the outstanding antibacterial properties of Ag,^42,43 in this experiment, we obtained reduced graphene oxide (RGO) loaded Ag nanoparticles (Nps) by in-situ redox method and then studied the influence of experimental parameters such aspH 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 Experiment

2.1 Materials

GO was prepared by Hummersmethodaccordingpreviousreports.⁴⁴ Sodium fluoride (NaF) used in this study was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. Silver nitrate (AgNO₃) was purchased from Sinopharm Chemical Reagent Co., Ltd. Sulfuric acid (H₂SO₄), hydrogen peroxide(H₂O₂), anhydrous ethanol, and ammonia water were produced by Xi’an Chemistry Reagent Company.Potassium Permanganate (KMnO₄) was provided by Tianjin hedong district red 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 waspoured into a 250 mL round-bottom flask. And then, 7 mL NaOH(1 M) and 4mLAgNO₃ (0.05 M) were added to the aboveGO solution. 1mL CH₂O (37%) solution as a reduction agent for GO and AgNO₃ was slowly dropped in 15 min until the yellow solution is appeared to obtain Ag NPs. At last, the crude products were washed by alcohol for 3 times and kept drying at 80 ^oC for 3 hours in the ovento obtain the RGO/Ag nanomaterials. Fig.1 displays the preparation process of RGO/Ag nanomaterials.

2.2.2 Fluorine ionsRemoval of water by RGO/Ag nanomaterials

Where R is the removal ratio of fluorine ions. C₀ is the initial concentration of fluorine ions in the solution (M). C₁ presents the 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) patternswere recorded on Rigaku XRD-600 (Japan) with Cu Kαirradiation 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 0.1mL. 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 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 lasersource power of 30 mW (50% power was applied in theanalysis). Raman signal was excited using the 514.5nm 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 flow at 60mLmin⁻¹ was used at below atmospheric pressure. TheRGO/Ag nanomaterials before and after absorption of fluorine ions were characterized by fourier transform-infrared (FTIR) spectrometer (FTS2000) with KBr pellets.The scanning range was 400-4000 cm^-1, the resolution was 1.5 cm^-1, 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 °C and the heating rate was 10 °C / min for 4 h. After degassing, 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 1000mL volumetric flask, then diluted to scale with deionized water, shaking well. 10^-4-10^-5Mfluorine standard solution was obtained by successive dilution method.  Take 10^-4-10^-5 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 flaskuntil the scale, shaking well. Then get 40 ml this solution into 50 ml beaker. Thefluoride 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 electrode slope (mV).

The TISAB was prepared as following: Adding 500mL deionized water to the 1000mL beaker, then adding 50mL glacial acetic acid, 12g sodium citrate, 58g 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 1L with deionized water.

3. Results and discussion

3.1 RGO/Ag Nanostructures

Fig. 2presents the morphology and structure of GO and RGO/Ag nanomaterials. Fig 2a. shows the 3D 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 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. 2 The SEM images of GO (a) and RGO/Ag (b) EDS of GO (c) and RGO/Ag (d).

Fig. 3 shows the XRD curves of GO and RGO/Ag. GO has a very sharp peak near 26^o, which is the diffraction peak of graphite surface (002). Because the structure of GO contains a lot of defects and oxygen groups, the characteristic peak (001)⁴⁴appears at 2 θ about 12^o. 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 appears near 2 θ about 38 ^o, 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. 3 XRD of (a) GOand (b) GO/Ag.

Fig. 4 shows the UV absorption spectra of GO and RGO/Ag nanomaterials in water. For GO, there is absorption band at 225nm due to the π-π* electron transition of aromatic C=C. In addition, there appears a shoulder at 300nm which is attributed to the n-π* electrontransitions of C=O. The two absorption peaks are the characteristic signals of GO^{45, 46}. For the RGO/Ag nanomaterials, the peak of π-π* electron transition of aromatic C=C shifts from 225nm to 250nm because the reduced GO tends to be flat (closer to the smooth level of graphene) needing lower energy to transition. The signalat 375nm is correspond to the Ag band. The results imply the AgNO₃ was reduced to Ag, and the reduced GO was obtained at the same time.

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

Fig. 5 exhibits the Raman spectra of GO and RGO/Ag nanomaterials. GO occurs the D band peak at 1355 cm^-1 and G band peak at 1590cm^-1. D peak is caused by C-C disordered vibration and characterizes the carbon atom of the sp³ hybrid structure. The G band peak is due to the stretching vibration of C-C and characterizes the carbon atom of structure sp² hybrid structure. So, the peak intensity ratio (I_D/I_G) of D and G peak present structural regularity of carbon 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_D/I_G (0.95) of RGO/Ag nanomaterials is higher than the GO (0.92) 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).

Fig. 5 Raman Spectra of GO and RGO/Ag.

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, CO₂, and steam from the sample aroused by water evaporation and decomposition of labile oxygenic groups^{47, 48}. 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. 6 The TGA curves of GO and RGO/Ag.

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 considered as variate from 0.04 to 0.2 g/L with the constant of 60 ^oC, 20 h, and pH=7 and 10^-5 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 adsorption and desorption performance of fluoride ion in water reach to the activated balance for the RGO/Ag adsorbent. So, the 0.10 g/L RGO/Ag is taken as the best content for the 10^-5M of the fluoride ion in water considering the cost and removal efficiency. Then, the solution pH is considered as variate from 40 to 10 with the constant of 60 ^oC, 20 h, 0.10 g/L of RGO/Ag and 10^-5 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 obtain the best removal ratio at 60 ^oC from the Fig. 7c. About the treating time, removal ratio of the fluoride ion gradually increases with the prolonging treating time. However, when the adsorptiontime reaches 18 hours, the fluoride removal ratio reaches up to 90% and become stable after 20h adsorption time. So we conclude that the best absorption is that the 0.10 g/L RGO/Ag, pH>7, 60 ^oC and 20h adsorption time, the fluoride removal ratio reaches up to 91%

 

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

Fig. 8 (a) is distribution of pore size of GO/Ag. The pore size is about 2-11 nm indicating that GO/Agnanomaterials is a micromesoporous material. Fig. 8 (b) is adsorption desorption curves of GO/Ag. The first steep stage of the isotherm curveis under the N₂ 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 desorption have steep rise and fall under partial pressure region (P/P0=0.4~0.8), which mean the multilayer adsorptio. The adsorption capacity increases sharply with theP/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 instrumentsimulations, the adsorption behavior of the GO/Ag is the R2 = 0.9998 by fitting the BET equation, and the fitting Langmuir equation is 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. 8 (a) distribution of pore size, (b) adsorption desorption, (c) Langmuir and (d) BET simulated diagram of RGO/Ag.

Fig. 9 (A) FTIR of RGO/Ag before (a) and after adsorption of Fions(b), (B) RGO/Agadsorption mechanism for Fions.

Fig. 9(A)is the FTIR spectrum of RGO/Ag before and afterabsorption of fluoride ion.Absorption peak at 3423 cm^-1shows -OH stretching vibration, but the peak intensitybecome weaker after absorption of fluoride ion. There are obvious peak at 1596cm^-1 in the two curves, which show that they all contain the aromatic skeleton of C=C stretching vibration. The peak both with at 1346cm^-1 should be assigned to the stretching vibrationsof -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 afterabsorption 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. Conclusion

RGO/Ag was prepared by oxidation-reduction reaction. The fluoride removal for RGO/Agnanomaterials in water displays the best adsorption at 0.1 g/L of concentration, 60^oC of temperature and 20h of adsorption time. The Fions in solution was quickly adsorbed on RGO/Ag, multi-molecular layer adsorptionof RGO/Ag is proceeded based on static electricity between RGO/Ag nanomaterials and Fions.

Acknowledgments

We 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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