Study of cerium adsorption in graphene oxide decorated with ZnO by X Ray Fluorescence

With the goal of recover strategic materials contained in spent electronic and electric devices; this work focuses on the recovery of Ce from Ce synthetic solutions, by using graphene oxides-based (GO) adsorbent materials. Graphene oxide is synthesized by the Hummers’ method and decorated with zinc oxide (ZnO-GO). After that, adsorption experiments at pH 2 and 6 were carried out using Ce(NO3)3 aqueous solutions with concentrations of 100-1000 mg/L. Compounds of GO and ZnO-GO with and without Ce were analysed by Raman where changes of GO structure were observed; by FTIR spectroscopy, after decoration, the reduction of COOH, C=O, C-OH and C-O groups were observed and after the adsorption the decrease of COOH and C-O groups as well as the presence of NO3 and CH2CH3 groups were detected. TEM micrographs show particles adsorbed in GO sheets, and XRF analysis helped to quantify cerium from solutions after adsorption. The ZnO-GO, obtained at pH 6, has higher Ce adsorption (3837.5 mgce/mgads) and highest percentage of Ce removal (43% of cerium); It concludes that ZnO improve the Ce adsorption, however, the adsorption mechanisms of Ce onto the graphene network are unknown, and Zn-Ce ion exchange was found depending on the pH.


Introduction
The need for strategic materials such as: Li, Si, Co, Ni, REEs, used for new technologies is increasing, however, the supply of some metals has been complicated due to geopolitical issues; In addition to this, sustainable extraction processes have been sought for a long time.For this reason, it is important to recover these materials that are contained in useless electronic and electrical devices.Nickel metal hydride batteries, for example, contain nickel (Ni), cobalt (Co), cerium (Ce), and lanthanum (La); These metals are considered strategic materials due to their high demand and shortage of supply.Rare earth elements (REEs), such as cerium and lanthanum, are necessary for the manufacture of electronic components, catalysts, permanent magnets and even cancer treatments, but its production is expensive since it requires complex separation processes.Over time, Ni, Co, Ce, and La end their useful life just like discarded batteries and can cause harm to the environment if not disposed of properly.However, through strategies such as Urban Mining, spent batteries can be considered an alternative source of REEs, which means that they can be reused; practicing the circular economy will reduce the environmental effects caused during mining extraction and will contribute to achieving of the Sustainable Development Goals.Urban mining uses hydrometallurgical processes to recover metals from scrap, these include leaching, electrochemical separation, and adsorption, for example.Several materials have been studied as adsorbents, but no search has been made for an adsorbent capable of forming a compound with the solute so that the compound has technological applications.On the other hand, in the search for a material that can adsorb metal ions from leachate solutions, it has been seen that, GO has a high surface area, oxygenated functional groups and negative surface charge that allow it to adsorb organic and inorganic species, it has been used to adsorb Cu, Co, Cd, Cr, Hg, Pb and Zn [1].To improve the efficiency of metal and dyes adsorption, graphene materials have been functionalized or decorated with organic molecules and metal oxides; for example, GO-Fe2O3 has been used to remove As [2].In other research, reduced graphene oxide (rGO) attached to ZnO nanorods (ZnO NR-rGO) were obtained by hydrothermal process which enabled uniform attachment of ZnO NRs along with their 1D axis and without restacking of rGO; the composites exhibited catalytic degradation efficiency under visible light irradiation and adsorptive behavior of metals and dyes [3].In the present work, the use of graphene oxide (GO) and ZnO-decorated GO (ZnO-GO) is tested as an alternative to recover Ce from stock solutions emulating the leachates from spent Ni-MH batteries.It promises to increase adsorption and catalytic behaviour for future applications of Ce-ZnO-GO in Hydrogen production.As first stage, to obtain ZnO-GO adsorbent materials, the Hummers´ method was employed; to decorate GO with ZnO, an innovator procedure with galvanized iron steel as source of Zn was applied; later, to adsorb Ce 3+ ions, 100 mg of GO or ZnO-GO were mixed with 50 ml of CeNO3 aqueous solutions at 5 concentrations (100, 200, 300, 500 and 1000 mgL).Finally, Raman, FTIR, XRF, and TEM analysis were developed to ensure the obtention of materials and to probe the adsorption of Ce 3+ .The preliminary results obtained by Raman and FTIR spectroscopy shown the obtention of GO and ZnO-GO and changes of GO structure were observed after adsorption.TEM micrographs show particles adsorbed in GO sheets and XRF analysis helped to quantify cerium from solutions after adsorption.The sample 6ZnO-GO has the higher Ce adsorption (3837.5 mgce/mgads) and highest percentage of Ce removal (43% of cerium); It concludes that ZnO improves the Ce adsorption in Ce aqueous solution however the adsorption mechanisms of Ce onto the graphene network are unknown, and Zn-Ce ion exchange were found depending on the pH.

Synthesis and modification of graphene oxide
Graphene oxide (GO) was prepared via modified Hummers´ method following the methodology of Guerrero-Contreras [4].ZnO-GO was prepared according to the procedure reported by Tolentino et al. [5].Briefly, an aqueous solution of 1 mg GO/mL was sonicated by 1 hour; the solution pH was 2. Next, 2 g of galvanized iron steel scrap strips (1 cm x 0.5 cm) was added.The temperature was increased to 80 °C while the solution was under a 40 sccm flow of Ar and stirring for 1 h.After cooling, the powders were recovered by centrifugation, washed with water and ethanol, and dried at 50 °C overnight.

Ce adsorption
Ce(NO3)3 solutions were prepared with different concentrations (100, 200, 300, 500, and 1000 mg/L).To adsorb Ce 3+ ions, 100 mg of GO were added to 50 mL of each solution.Before GO addition, pH was set to 2 or to 6 with 1 M HCl or 1 M NaOH respectively.The pH values were chosen according to the speciation diagram of Ce (III) in chloride media.The solutions were kept at room temperature while stirring for 1 hour.Afterwards GO was allowed to settle and centrifuged for 20 minutes and the solid was washed with 5 ml of deionized water; this process was carried out twice; The remaining solid was poured into a porcelain capsule, and dried in the oven at 50°C for 12 hours.Finally, it was grinded with an agate mortar and weighed.The same procedure was done with ZnO-GO.
To determine the Ce 3+ adsorption capacity of GO and ZnO-GO, X-ray Fluorescence (XRF) was used to quantify the amount of cerium present in the powders and in the remaining cerium solutions.As XRF do not detect elements lighter than Al, the method of internal standard was used by adding a constant amount (5 mg) of CuSO4 to the samples under analysis.Adsorption isotherms were prepared for each of the adsorption tests.The calibration curve was done using the five solutions with known Ce concentrations.After that, the concentration of analyte Ce in final solutions was determined.Later the value of Qe, sorption capacity (mg g −1 ), was calculated by subtracting the Ce value from the initial concentration,  0 , for each solution (the adsorbed solute mass) and dividing by the mass of used adsorbent (Eq.1).
Additionally, the properties of GO and ZnO-GO were characterized by infrared spectroscopy (FTIR), Raman spectroscopy and Transmission Electron Microscopy (TEM).Raman spectroscopy was carried out in a i-RamanPlus (BWTek) at 532 nm with a nominal power of 35 mW.The FTIR analysis was carried out in a Perkin Elmer Spectrum 1 instrument using an attenuated reflectance accessory (ATR) with a diamond pinhole.X ray fluorescence was performed in a MESA-50 EDXRF analyzer (Horiba) at 50 kV, 100 s of acquisition and no filter.TEM was done in a JEOL JEM-1010 microscope operating at 80 kV and equipped with a Gatan ORIUS camera.

Chemical characterization
Figure 1 presents the FTIR spectra of graphene oxide and ZnO-GO (Figure 1a) and those of the recovered materials after adsorption using GO at pH 2 (Figure 1b), ZnO-GO at pH 2 (Figure 1c) and ZnO-GO at pH 6 (Figure 1d).In the GO spectrum shown in Figure 1a, a broad band at 3203 cm -1 attributed to residual water intercalated between the graphene oxide sheets is observed.Also, stretching vibrations of carboxyl groups, COOH, at 1718 cm -1 ; carbonyl groups, C=O, at 1620 cm -1 , hidroxyl groups, C-OH at 1375 cm -1 , epoxy groups C-O at 1220 cm -1 , and alkoxy C-O vibrations at 1070 cm -1 are observed [2][6] [7].It has been reported that the -COOH and C=O groups are located at the edge of the sheet, while the -OH and epoxy C-O groups are located on the basal planes of the GO sheet [8].After ZnO decoration, characteristic peaks of GO decrease or disappear (Figure 1a, red line).This is ralated to the removal of oxygen functional groups [3].A band at 994 cm -1 is attributed to the asymmetric stretching vibrations of the Zn-O-C bond, due to the bridge-oxygen bonding between zinc oxide and graphene oxide [5]. Figure 1b shows the FTIR spectra of GO after the cerium absorption test in the Ce(NO3)3 solutions with increasing concentrations (100, 200, 300, 500 and 1000 mg/L).It can be observed that the bands related with carboxyl groups and epoxy groups reduce its intensity with respect to that of GO, suggesting Ce 3+ adsorption at these sites.Also, a band at 1330 cm -1 attributed to nitrate adsorption is observed.Methyl/methylene bands around 2800 cm -1 are more noticeable as the Ce concentration in the solution increases, suggesting breaking of C=C bonds in the graphene plane.Figure 2c) shows the FTIR spectra of ZnO-GO:Ce obtained at pH 2 and Figure 2d) shows the spectra of ZnO-GO:Ce obtained at pH 6. the spectra groups show bands related to the basic structure of reduced graphene oxide after decoration with ZnO, i.e. the C=C band and the Zn-O-C bonds, but the appearance of NO3 groups as well as the methyl/methylene bands, after adsorption tests is notable, particularly at pH 6, where a shoulder around 840 cm -1 that could be related with Ce-O bonds is observed as well as a noticeable increment in the CH2/CH3 bands which indicate that Ce 3+ could be oxidizing the C=C bonds.The results indicate that both GO and ZnO-GO are modified upon exposition to the Ce 3+ solution, and that pH is an important factor of Ce 3+ and NO3 -adsorption for the ZnO-GO composite.Figure 2 depicts the Raman spectra to pristine oxide (GO) and GO after Ce 3+ adsorption in the different Ce (NO3)3 solutions.In the spectra, the D and G bands appear at around 1344 cm −1 and 1585 cm −1 , respectively.These bands are linked to vibrations of the defect-activated ring breathing mode (A1g), known as the disorder band (D) and to vibrations of the sp2 bonded carbon mode (E2g), which are associated with the graphene structure (G) respectively.Another band called 2D is also identified, located at 2938 cm -1 .This 2D band is related to the exfoliation of the original material composed of multiple layers of GO to a few layers of rGO [9].The Raman spectra of graphene oxide (GO) after cerium adsorption, display the same bands, although changes in the D and G relation are apparent, which will be subject of a future work.In the spectra a band at 1047 cm -1 related with NO3 -and another at 450 cm -1 related with Ce-O bonds are observed, confirming the adsorption of Ce but also nitrate ions, which  Figure 3 presents transmission electron micrographs of pristine GO (Figure 3a), where wrinkled sheets are observed; even single sheets are noticeable.Figures 3 b-c) show up to seven stacked sheets of graphene oxide loaded with Ce, evident from small particles at the sheet plane.No nanoparticles were observed at the sheet edges, suggesting that the primary sites for Ce adsorption are epoxy or hydroxyl groups or even direct reduction of the graphene rings, and not carboxyl sites as the FTIR data suggested.

Adsorption capacity
Using the internal Cu standard, the total Ce removed from the solution was calculated to obtain the adsorption capacity at the different materials and conditions.The results of Ce adsorption using GO based materials are shown in Figure 5a) where the Ce concentration at equilibrium of the solutions is plotted against the adsorption capacity, Qe, and the higher Ce adsorption obtained was 3837.5 mgce/mgads for 6GOZn respectively.The Ce adsorption capacity and the concentration of Ce at equilibrium solution increased with increasing amounts of Ce in the solutions; this mean that the equilibrium has not been reached.As Gasser describes [10], with increasing contact time, REE adsorption increases, but at some point, it must remain constant to determine the maximum adsorption capacity; in that work the maximum concentration recovered of La was 409 mg/g using 5 g/l of La (III) solution, 2 h of contact time, 0.1 g/10 ml of adsorbent weight and pH 1 at 25 °C.For this reason, more studies will be carried out that evaluate contact time in our work.Also, the removal of Ce by GO absorbents as a function of the solute concentration was studied at constant temperature (25 ± 1 °C) by varying the Ce concentration from 100 to 1000 mg/L.The percentage of Ce removal was calculated with equation 2, where C0 and Ce are the initial and final concentration of Ce in the solution phase.
%  = (−)  100 Eq. 2 The highest value was 43% of cerium adsorbed by the ZnO-GO composite at pH 6. Manousi [11] who used GO to Recover REEs, report the pH 6 as optimum, obtaining recoveries ranged between 40 and 75%.This confirm that pH, which changes the metal speciation in solution and surface acidity of adsorbent, will affect the adsorption of metal ions.

Conclusions
GO and ZnO-GO were used to recover cerium from 100-1000 mg/L Ce(NO 3 ) 3 solutions at pH 2 or pH 6.These composites shown binding capacity for Ce, due to the increase of adsorption capacity compared with it of the pure graphene oxide.The results suggest different mechanisms for Ce adsorption: i) for GO at pH 2, direct chemisorption of Ce3+ onto the sheet plane; ii) ion exchange between Zn and Ce in the Zn-decorated GO at pH 2, and iii) direct reaction and C=C oxidation in the graphene sheet at pH 6.The results suggest a role of the NO3 -ion as an amount of it was identified in the FTIR and Raman spectra of the GO after Ce adsorption.Further chemical environment analysis is intended to clarify the adsorption mechanisms and nitrate ion role.The ZnO-GO composite was able to recover a maximum of 43 % of Ce from the solutions.These results indicate the feasibility of GO and ZnO-GO materials to separate of Ce or other rare earths from aqueous solutions.These achievements open the door to possible applications in environmental remediation and urban mining.

Figure 1 .
Figure 1.FTIR spectra of a) GO and GO:ZnO; b) GO and GO loaded with Ce at pH 2; c) ZnO-GO loaded with Ce at pH 2; d) ZnO-GO loaded with Ce at pH 6.
analysis.The band at ca 850 cm -1 is related with laser damage because high power was needed to observe the Ce-O and NO3 -bands.

Figure 2 .
Figure 2. Raman spectra of a) GO and GO after Ce adsorption at pH 2.

Figure 3 .
Figure 3. TEM micrographs of a) GO and b-c) GO loaded with Ce.

6 Figure 4 .
Figure 4. XRF spectra of Ce solutions after absorption test at a) pH 2 and b) pH 6

Figure 5 .
Figure 5. a) Adsorption capacity of GO and ZnO-GO; b) Efficiency of GO and ZnGO

Financing and acknowledgments
This work was financed by SIP-IPN 20230701 project and with the CONAHCYT 321595 Infrastructure Grant.Technical support from KSAA and CHN for sample preparation and adsorption tests is acknowledged.