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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.31 No.6 pp.345-352

Ni Nanoparticle-Graphene Oxide Composites for Speedy and Efficient Removal of Cr(VI) from Wastewater

Wan-Xia Wang1, Dong-Lin Zhao1, Chang-Nian Wu1, Yan Chen1, Won-Chun Oh2
1Key Laboratory of and Functional Molecule Design and Interface Process, Anhui Jianzhu University, Hefei 230601, PR China
2Department of Advanced Materials Science & Engineering, Hanseo University, Seosan 31962, Republic of Korea
Corresponding author E-Mail : (D.-L. Zhao, Anhui Jianzhu Univ.) (W.-C. Oh, Hanseo Univ.)
May 8, 2021 May 27, 2021 June 1, 2021


In this study, Ni nanoparticle supported by graphene oxide (GO) (Ni-GO) is successfully synthesized through hydrothermal synthesis and calcination, and Cr(VI) is extracted from aqueous solution. The morphology and structure of Ni- GO composites are characterized by scanning electron microscopy (SEM), trans mission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). High-resolution transmission electron microscopy (HRTEM) and XRD confirms the high dispersion of Ni nanoparticle after support by GO. Loading Ni on GO can obviously enhance the stability of Ni-GO composites. It can be calculated from TGA that the mass percentage of Ni is about 60.67%. The effects of initial pH and reaction time on Cr(VI) removal ability of Ni-GO are investigated. The results indicate that the removal efficiency of Cr(VI) is greater than that of bared GO. Ni-GO shows fast removal capacity for Cr(VI) (<25 min) with high removal efficiency. Dynamic experiments show that the removal process conforms to the quasi-second order model of adsorption, which indicates that the rate control step of the removal process is chemical adsorption. The removal capacity increases with the increase of temperature, indicating that the reaction of Cr(VI) on Ni-GO composites is endothermic and spontaneous. Combined with tests and characterization, the mechanism of Cr(VI) removal by rapidly adsorption on the surface of Ni-GO and reduction by Ni nanoparticle is investigated. The above results show that Ni-GO can be used as a potential remediation agent for Cr(VI)-contaminated groundwater.


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    With the rapid expansion of modern urban industry, a large amount of waste water with an appropriate concentration of Cr(VI) has been produced.1) Cr(VI) is one of the most poisonous heavy metals in the world and abundant in the earth's crust.2) As an important metal, it has been widely used in automotive components, video cassettes, the production of stainless steel and other fields. But it also has disadvantages. Excessive Cr(VI) can do great harm to the body and the environment.3) However, the virulence of chromium has a bearing on its chemical valence.4) The distribution of chromium in aqueous solutions as hexavalent and trivalent.5) Cr(VI) is a mutagenic and carcinogenic pollutant, which is frequently often found in waste water, possibly stemming from leather tanning, pigment production, electroplating and metal plating, etc.6) Cr(VI) has highly toxic effects on human beings, animals and plants. However, trivalent chromium is notoxic and considered to be an essential nutrient in the diet to maintain the metabolism of useful lipids, glucose and protein.7,8) Therefore, the investigation of hexavalent chromium to trivalent chromium is a vital topic in the field of inorganic pollutant removal.9) Studies in recent years have shown that hexavalent chromium can be retuned to trivalent chromium using various reductive agents, such as organic compounds, Fe3O4, Fe(0) and so on.10)

    For the last several years, graphene has fascinated comprehensive research interest owning to its mechanical, optical properties, battery, and extraordinary thermal.11,12) Because of its special structural characteristics, for instance, larger specific surface area and stronger interaction between hydrogen and carbon atoms, graphene has been widespread used in adsorption experiments.13) Proper modification of graphene oxide (GO) can not only reduces its polymerization, but also increase its dispersion, thus improving its ability to absorb pollutants.14,15) Graphene is covered with a variety of metal and metal oxide nanoparticles, forming a unique set of nanocomposites.16,17) These structures not only have the excellent properties of graphene, but also enhance the properties of metal nanoparticles. Metal nanoparticles are widely used in catalysis, adsorption, batteries, supercapacitors and other fields.18) Ni nanoparticles on microcrystalline cellulose was investigated towards reduction of 4-nitrophenol to 4- aminophenol in water at room temperature.19) So metalgraphene nanocomposites are a new research topic at present. In the light of literature research findings, there were few studies on Cr(VI) removal by Ni-GO composites.

    The main purpose of this work was to coat Ni on GO and studied its removal performance for Cr(VI) in aqueous solution. The effect of Ni-GO on Cr(VI) adsorptionreduction in wastewater was investigated. This study mainly discusses the effect of Ni-GO on the adsorption and reduction of Cr(VI) in wastewater. The influences of temperature, kinetics and pH of Cr(VI) removal were studied. The probable mechanism of Cr(VI) removal by Ni-GO composites was investigated by XPS analysis.

    2. Materials and Methods

    2.1 Materials

    Chemical analytic reagents, including graphite powder (30 μm 99.85%, Shanghai Colloid Chemical Plant, China), Ni(OAc)2·4H2O purchased from Sinopharm Chemical Reagent Co., Ltd. Urea [CO(NH2)2], potassium permanganate (99.8 %, GR), K2Cr2O7 (99.8 %, GR), H2SO4 (98 %, AR), H3PO4 (98 %, AR), H2O2 (30 %, AR) and Ethylene glycol were brought by Shanghai Macklin Biochemical Co.,Ltd. Ethanol was bought by Tian in Fuyu Fine Chemical Co.,Ltd. Hydrazine hydrate 80 % was bought by Tianjin ZhiYuan Reagent Co., Ltd. Cr(VI) was prepared by dissolving an appropriate amount of K2Cr2O7 in deionized water. The chemical agents used in this work were all analytical reagents and were not futher decontamination. All experiments used deionized water.

    2.2 Preparation of Ni-GO

    GO was prepared by an improved Hummers' method.20,21) About 100 mg GO was dispersed in 30 mL deionized water and prepared into a mixed solution by ultrasonic vibration at room temperature for 2 h. 0.002 mol of Ni(OAc)2·4H2O and 0.4 g of urea were added to 50 mL of ethylene glycol and 25 mL of deionized water. Subsequently, 6 mL hydrazine hydrate 80% and GO dispersion solution were added to the mixed solution under vigorous stirring. The mixture was then transferred to stainless steel autoclave and heated at 180 °C for 24 h. The material was cleaned several times with deionized water and ethanol, and then dried at 60 °C. Finally, the product was put into a tubular furnace and heated at 500 °C for 3 hours under the protection of nitrogen gas at a heating rate of 2.5 °C·min−1. Compared to other methods, this method has the advantages of low cost and easy operation.

    2.3 Sample characterization

    The crystal morphology of the compound material was determined by a Cu-KA X-ray diffraction analyzer (Bruker D8 X-ray diffraction analyzer, Germany). The morphology was obtained by (SEM, JEOL JSM-7500F + EDX Oxford X-MAX-20, Japan) and TEM (JEM-2100Plus, Japan). Determination of Fourier transform infrared (FT-IR) spectrum by the FTIR-1500 (China). XPS (PHI-5300, UK) were used to characterize the surface binding energy and element information (C, H, Ni, O and Cr) in Ni-GO before and after removal, respectively. The bronol-Emmett teller (BET, Autosorb-iQ, USA) method was used to calculate the specific surface area. The aperture distribution of Ni-GO was calculated by density functional theory. The Raman spectrometer (invia UK) measured the Raman spectrum. TGA was recorded by a Mettler TGA/DSC3+ (Switzerland) under nitrogen atmosphere (heating rate: 10 °C·min−1, temperature ranges: 40-700 °C). The concentration Cr(VI) was measured by ultraviolet-visible spectrophotometer (T6 new century, China).

    2.4 Batch experiments

    The effects of pH and contact time on Cr(VI) removal were investigated in a 180 rpm and 298K oscillatory water bath thermostatic oscillator. Typically, Cr(VI) solution of a certain concentration and volume was prepared in the centrifuge tube, and the Ni-GO adsorbent was uniformly dispersed and transferred to the centrifuge tube through the tube. A small amount of 1M acid (HCl) or alkali (NaOH) solution was dropped into the tube, and the pH value was adjusted to 2.0-10.0. Then, it was transferred to a thermostatic water bath oscillator and oscillated for 12 h. The supernatant was taken to determine the content of Cr(VI). The content of Cr(VI) was determined by ultraviolet-visible spectrophotometer. Removal capacity (qt, mg·g−1) was calculated according to Eq. (1). For adsorption kinetics, the mixed solution was stirred at a certain temperature and rotation speed, and about 3.5 mL solution was extracted from the stirred mixture on a regular basis and quickly separated with a magnet. The removal capacity of Cr(VI) (qt, mg·g-1) was calculated according to Eq(1).22)

    q t = ( C 0  - C 1 )  × V m

    Where C0 and Ct denote the initial concentration and time t of Cr(VI) in solution, respectively (mg·L-1). qt (mg·g-1) is the removal capacity of Cr(VI) at time t, V(L) is the volume of initial solution, and m(g) is the mass of adsorbent. Each removal test was repeated several times.

    3. Results and Discussion

    3.1 Characterization results

    The typical morphology of GO and Ni-GO can be clearly seen in Fig. 1. GO was synthesized from Hummer natural oxygen fossil toner. Fig. 1(C, D) showed TEM images of the nanosheet structure of Ni-GO. TEM of Ni- GO showed that nano Ni had low agglomeration rate and uniform distribution on graphene. Consistent with the SEM result [Fig. 1(B)], the nano-Ni is uniformly loaded on GO, indicating that Ni-GO effectively inhibits the aggregation of nano-Ni. In the Ni-GO HRTEM image [Fig. 1(E)], the lattice spacing of 0.196 nm measured corresponds to the (111) plane of Ni0, showing the formation of nickel nanoparticle. The results show that nickel can be uniformly loaded onto graphene. The EDS of Ni-GO is presented in Fig. 1F, the distribution of Ni, C, O and N on the material surface was determined.

    Fig. 2(A shows the XRD patterns of GO and Ni-GO. The pea)k at 2θ = 9.6º represents the crystal surface of (100), which is consistent with the literature report.23) The results showed that the Ni-GO composites had Ni0 and graphene diffraction peaks. The strong peak and peak at 44.3º, 51.6º and 76.1º indicate the (111), (200) and (220) crystal plane of Ni0, respectively. The lattice spacing is 0.2 nm at 2θ = 44.3º calculated by the Bragg equation, which is consistent with the result of HRTEM. Characteristic diffraction peak at 2θ = 25.5º is observed in Ni-GO composites, indicating GO has been successfully reduced. In addition, the inter-layer distance of GO is calculated as 0.345 nm by the Bragg equation, marginally larger than that of the primary graphite, which is 0.34 nm.23) Due to the introduction of Ni, many faultiness and vacancies are formed on the thin plate, leading to a large distance between the layers.

    The FT-IR results of Ni-GO is presented in Fig. 2(B). The characteristic broadband appeared near 3,385 cm−1, which was in accordance with the stretching vibration before and after O-H groups in Ni-GO. The existence of the O-H group may be owning to the presence of bound water in Ni-GO. The peak value at 2,359 cm−1 corresponds to the unreduced C=C group in graphene. In GO strong peak of C=O was obeseved at 1,618 cm−1, which was of low intensity in Ni-GO composite. The strong and sharp adsorption band at 1,404 and 582 cm−1 is C-H bond stretching vibration. Furthermore, the peak value at 1,049 cm−1 is the stretching vibration of C-O-C bond. It is shown that the reduction of GO with ethylenediamine is successful and the result is also confirmed by XRD analysis.

    Raman spectroscopy is a tool for measuring the disorder degree of Ni-GO composites. Raman spectrum of GO and Ni-GO samples in shown in Fig. 2(C), the Raman spectra of GO and Ni-Go show the characteristic D (1,325 cm−1) and G (1,580 cm−1) peaks.24) The D peak indicates the structure defect or disordered structure of GO, and the G peak is related to the amount of sp2 carbon.25) The ID/IG ratio represents the relative disorder caused by structural defects. Due to the addition of Ni, the ID/IG ratio of the composite increased, and the ID/IG ratios of GO and Ni-GO were 0.868 and 1.123 respectively, indicating that the defect density of Ni-GO is much higher than that of GO. This further demonstrates the successful reduction of GO.

    In addition, the thermal stability of the Ni-GO composites material was explored. As shown in Fig. 2(D), the total weight loss of the Ni-GO composites before 700 °C is about 11 wt%. Further analysis showed an insignificant mass loss of 2.02 wt% at 170 °C, which may be caused by evaporation of the water in the material. The weightlessness of Ni-GO occurs at 200 ~ 400 °C, which may be due to the decrease of unstable oxygencontainning groups. The weight loss of Ni-GO above 400 °C may be related to oxidation of Ni.26) GO starts a sharp loss of quality between 160 °C and 200 °C temperature, which may be due to the decomposition of oxygen containing functional groups. When the temperature exceeds 200 °C, GO thermal decomposition weightlessness is due to carbon skeleton.27) It can be calculated from Fig. 2(D) that the mass percentage of Ni was about 60.67 %. The results show that the thermal stability of GO was improved due to the introduction of Ni. The BET results show that Ni-GO has larger specific surface area (127.617 m2·g−1) (Fig. 2E) and pore volume (0.2107 cm3·g−1) [Fig. 2(F)].

    3.2 Removal of Cr(VI) by Ni-GO

    3.2.1 Effects of initial pH and different adsorbent

    Fig. 3(A) shows the influence of initial pH value (2-10) of Cr(VI) removal. The results showed that the removal capacity of Cr(VI) increased with the decrease of pH value. Therefore, the removal of Cr (VI) is favorable at a lower pH value. The maximum removal capacity was reached at pH 2.0. Similar studies can be found in several articles.28-30) Fig. 3(B) shows the removal influence of different adsorbents. Under the same condition, The removal capacity of GO to Cr(VI) is about 20 mg·g−1, while the removal capacity of Ni-GO is about 95 mg·g−1 [Fig. 3(B)]. It can be seen that the Ni-GO composites material greatly improves the removal capacity. The result showed that the main mechanism of removing Cr(VI) by Ni-GO was reduction. It might follow the following reaction:30)

    Cr 2 O 7 2-  + 14H +  + 3Ni 2Cr 3+ + 3Ni 2+ + 7H 2 O

    The reduction rate decreases with the increase of pH. Thus, the reduction reaction is easier to perform at lower pH. The initial pH of the dichromate solution is 6. But when added HCl, the pH of the solution gone down to 2. In this case, hydrochloric acid maintained the pH of the reaction mixture, and Ni acts as a reducing agented to reduce Cr(VI) to Cr(III).

    3.2.2 Effects of time and solution temperature

    Fig. 3(C) shows the kinetics of Cr(VI) removal by the Ni-GO. The removal of Cr(VI) is first and foremost rapid, because there are many sites in the initial phase. Finally, the equilibrium was reached after 30 min of Cr(VI) removal, and the maximum removal rate of Ni- GO was about 97.09 mg·g−1 at 298 K, while the removal capacity reached 129.87 mg·g−1 at 308 K. Apparently, the removal efficiency of Cr(VI) by Ni-GO adsorbent was better with the increase of temperature.

    The influence of contact time on adsorption kinetics was studied to evaluate its potential application value [Fig. 3(B)], pseudo-first-order and pseudo-second-order models were used to study the experimental data.31,32) The linear form of the pseudo-first-order dynamic model is given as :

    ln (q e  - q t ) = ln q e - kt

    The form of pseudo - second - order dynamic model is given as :

    t q t  =  1 K 2 q e 2  +  t q e

    Where t stands for reaction time (min) and k1 (min-1) and k2 (g·mg−1·min−1) are rate constant of the pseudofirst- order and pseudo-second-order, respectively. qe (mg·g−1) is adsorption amount of Cr(VI) at equilibrium while qt (mg·g−1) is removal capacities at t time. Table 1 shows the simulation parameters of the two models.

    Compared with the pseudo-first-order model (R2 = 0.976, qe = 59.55 mg·g−1), the pseudo-second-order dynamic model had higher R-squared (R2 = 0.996) and higher removal capacity of theoretical equilibrium (qe = 97.09 mg·g−1), which is near to the test data. It shows that the pseudo-second-order kinetics model is more consistent with Cr (VI) removal by Ni-GO. Therefore, The removal of Cr(VI) on Ni-GO is a chemical reaction rather than ordinary mass transport.33)

    As can be seen from Fig. 3(C), The removal capacity of Cr(VI) increases with the increase of temperature from 298 K to 308 K. It may be that at higher temperature, the mass transfer rate of Ni nanoparticle in the solution is accelerlated, which is conducive to the removal of Cr(VI). Activation energy (Ea, kJ·mol−1) is the minimum energy required to go from a reactant molecule to an active molecule in a chemical reaction, and the adsorption type is determined by Arrhenius equation.34)

    ln K 2  = - Ea RT  + lnk

    Where K2 (g·mg−1·h−1) represents the reaction rate constant, R (8.314 J·mol−1·K−1) represents the molar gas constant, and T (K) represents the temperature of the solution. The Ea was calculated as 28.85 kJ·mol−1 and the result showed that Ni-GO composites of Cr (VI) removal was dominated by chemical absorption effect.3)

    3.3 XPS analysis

    The composition and chemical valence of Ni-GO were further studied with XPS. From the spectra measured before and after the Ni-GO removal of Cr [Fig. 4(A)], the binding energies of C1s, N1s, O1s and Ni2p in the Ni-GO before removal were 285.0 eV, 400.0 eV, 532.0 eV and 856.0 eV, respectively. After reaction, the binding energy of Cr2p in Ni-GO was 579.0 eV. The presence of Cr(VI) indicates that Cr(VI) adsorbed on the Ni-GO surface. In Fig. 4(B), the XPS spectrum of O1s has three peaks of 530.2 eV, 531.8 eV and 533.4 eV, corresponding to anion oxygen (O2−), hydroxide (OH-) and adsorbed water (H2O) respectively.35) After the reaction of Cr (VI), the strength of H2O in the Ni-GO composites was higher than that of OH, and the peak of the O2− disappeared, indicating that the Ni-GO composites had a low degree of oxidation and good stability after the reaction. Fig. 4(C) shows the Ni 2p spectra in Ni-GO before and after Cr(VI) removal. The peaks at 854.7 and 862.3 eV correspond to Ni0,36) the peak at 856.8 eV corresponds to NiO and the peak at 874.0 eV corresponds to Ni2+.37) The peak is detected at about 880.5 eV due to the frequent occurrence of multiple level splitting in nickel oxide systems.38,39) After reaction, the peak value of Ni0 decreases, some even disappear, and a new peak value of Ni2+ appears, indicating that Ni0 participates in the reduction reaction,40) which is consistent with the above conjecture. As shown in Fig. 4(D), it can be seen that Cr(VI) is adsorbed on the surface of Ni-GO. The characteristic peaks at 576.7 and 580.7 eV refer to Cr 2p3/2, indicating that chromium ions exist in the form of Cr(VI). The characteristic peak of Cr 2p1/2 is 588.1 eV, indicating the presence of chromium ions in the form of Cr(III).33) Through XPS spectra analysis, it can be concluded that all Cr(III) is partially Cr(VI) reduced.

    4. Conclusions

    Ni-GO was prepared by hydrothermal method and calcination way. XRD and HRTEM analysis results showed that Ni-GO synthesis was successful and the Ni nanoparticle was stably loaded on the GO surface. In addition, the effects of different influencing factors on Cr(VI) removal were studied by dynamic experiments, which also provided a new idea for the experimental design of Cr(VI) removal in the future. The removal capacity of Ni-GO to Cr(VI) increased with the increase of temperature. The kinetic study showed that the pseudosecond- order model was suited for the removal of Cr(VI) under different conditions, and the removal of Cr(VI) on Ni-GO composites material was chemical adsorption. The removal mechanism of Cr(VI) by Ni-GO showed that the loading of Ni was conducive to the reaction between Ni and Cr(VI), and Cr(VI) was rapidly reduced to Cr(III). Ni-GO composites have the advantage of high speed. In addition, the effect of Ni nanoparticle loading on Cr (VI) removal of Ni-GO composites needs to be further studied and discussed in the follow-up work.


    Financial support from the National Natural Science Foundation of China (21876001) is acknowledged.



    TEM image (A) of GO; SEM image (B) and TEM image (C) of Ni-GO; HRTEM images (D, E) of Ni-GO; EDS (F) of Ni-GO.


    XRD patterns for GO and Ni-GO (A); FT-IR spectra of Ni-GO and GO (B); Raman spectra of GO and Ni-GO (C); TGA of GO and Ni-GO (D); Nitrogen adsorption-desorption istherms (E) and pore size distributions (F) of Ni-GO.


    Effect of initial pH (A) of Cr(VI) removal on Ni-GO composites; Effect of different adsorbent (B) and temperature (C) of Cr(VI) removal on Ni-GO composites.


    XPS spectra of Ni-GO before and after reaction (A); High-resolution scan of O 1s (B)? Ni 2p (C) before and after reaction; Highresolution scan of Cr 2p after reaction (D).


    Kinetic parameters for the removal of Cr(VI) on Ni-GO.


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