Subscriber access provided by UNIV OF DURHAM
Article
Graphene Oxide Decorated with Cerium(IV) Oxide in Determination of Ultra-trace Metal Ions and Speciation of Selenium Anna Baranik, Rafal Sitko, Anna Gagor, Ignasi Queralt, Eva Margui, and Beata Zawisza Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00137 • Publication Date (Web): 18 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
Graphene Oxide Decorated with Cerium(IV) Oxide in Determination of Ultra-trace Metal Ions and Speciation of Selenium
Anna Baranika, Rafal Sitkoa, Anna Gagorb, Ignasi Queraltc, Eva Marguíd, Beata Zawiszaa*
a
University of Silesia, Institute of Chemistry, Szkolna 9, 40-006 Katowice, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland c Institute of Environmental Assessment and Water Research, Dep. of Geosciences, IDAEA-CSIC, Jordi Girona St., 18-26, 08034 Barcelona, Spain d Department of Chemistry, University of Girona, Faculty of Sciences, C/M.Aurèlia Campmany, 69, Girona, Spain
b
ABSTRACT: Graphene oxide decorated with cerium (IV) oxide (GO/CeO2) was synthesized and applied in adsorption of several metal ions such as As(III), As(V), Se(IV), Cu(II) and Pb(II) from aqueous samples. The important feature of GO/CeO2 nanocomposite is also its selectivity towards selenite in the presence of selenate. The structure of GO/CeO2 has been proven by microscopic and spectroscopic techniques. The maximum adsorption capacities of GO/CeO2 calculated by Langmuir model towards arsenic, selenium, copper and lead ions are between 6 and 30 mg g-1. An interesting feature of this adsorbent is its excellent dispersibility in water. Thus, GO/CeO2 nanocomposite is ideal for fast and simple determination of heavy metal ions using dispersive micro-solid phase extraction (DMSPE). Moreover, coupling DMSPE with energy-dispersive X-ray fluorescence spectrometry (EDXRF) is extremely beneficial because it allows direct analysis of adsorbent. Thus, the analyte elution step, as needed in many analytical techniques, was obviated. The influence of sample volume and the sorption time as well as the influence of foreign ions and humic acid on the recovery of determined elements are discussed in the paper. The results showed that developed methodology provided low limits of detection (0.07−0.17 µg/L) and good precision (RSD< 4%). The GO/CeO2 nanocomposite was applied to analysis of real water samples and certified reference materials (CRM) groundwater (BCR®-610) and Pig Kidney (ERM®-BB186).
The pollution of the environment with heavy metal ions is known to be dangerous for flora and fauna as well as human health. Generally, most metals are non-biodegradable and highly toxic. Some of them are also considered as carcinogenic.1 Therefore, the determination of metals in environmental samples is a matter of great importance. Arsenic, selenium, copper and lead ions are some of the most dangerous. The determination of both total metal and also different species of selected metals is very important. That being so, interests in the determination of different species of arsenic (As(III) and As(V)) is caused by the fact that toxic effects of arsenic are connected with its oxidation states and chemical forms. The determination of arsenic in the different oxidation states provides a new insight into the linkage between exposure, metabolism and toxicity. It is necessary to determine lower concentrations of arsenic to recognize its accumulation and
toxicity to organisms. The International Agency for Research on Cancer and United States Environmental Protection Agency (US EPA) have designated arsenic as a group A ‘known’ human carcinogen. Whereas, selenium is an essential micronutrient to people and animals at low concentrations, but highly toxic at elevated concentrations. It causes hair loss, damage to the peripheral nervous system, fatigue, irritability and damage to liver and kidney as well as the nervous and circulatory systems. Se(IV) is more toxic than the Se(VI). Copper, on the one hand, is an essential substance to human life, but on the other hand, in high doses it can cause anemia, liver and kidney damage as well as stomach and intestinal irritation. Lead explicits toxic in small amounts. Typical symptoms of lead poisoning include abdominal pain, headaches, convulsions, anemia, chronic nephritis, brain damage and central nervous system disorders.2 According to US EPA regulations for
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
save drinking water3 the maximum concentrations of 1300, 15, 10 and 50 ng mL-1 cannot be exceed for Cu(II), Pb(II), As(III), and Se(IV), respectively. Bearing all that in mind, the development of analytical methods that allow determining not only the total concentration of an element at low levels but also differentiate chemical forms and oxidation states is urgent. Nowadays, laboratories offer several sensitive and selective analytical techniques for the determination of wide range of elements, i.e. flame and electrothermal atomic absorption spectrometry (FAAS and ETAAS), inductively coupled plasma optical emission spectrometry (ICPOES), and inductively coupled plasma mass spectrometry (ICPMS), as well as total reflection X-ray fluorescence spectrometry (TXRF)4. Nevertheless, the direct determination of ultra-trace amounts of metal ions with most spectroscopic techniques is limited, and the use of preconcentration methods is necessary. Solid phase extraction (SPE) is considered as one of the best approaches for the preconcentration/separation of many metals.5 SPE methods offer rapid phase separation and preconcetration of trace elements as well as the possibility of combination with different detection techniques. In SPE, different sorbents, including graphene and graphene oxide are used. The large surface area of graphene and graphene oxide and their hexagonal arrays of carbon atoms are ideal for strong interactions with other molecules. The adsorption capacities of GO are much higher than those of any of the sorbents currently described in literature. In order to improve GO selectivity or sorption capacity various modification were applied, i.e. by silica,6,7 amine groups,8,9 thiol groups and Fe3O4 nanoparticles10,11 are known. These sorbents are characterized by high maximum sorption capacities from 6.0 mg g-1 6 to 210 mg g-1 9 towards different divalent metal ions such as Cu(II), Ni(II), Co(II), Zn(II), Cd(II) and Pb(II). Most sorbents based on graphene oxide mentioned above have found an application in determination of cationic species of elements. Meanwhile, the determination of anionic species of metals as well as speciation analysis are of prime importance. In the scientific literature there are papers describing the sorption/removal of selected forms of elements using modified nanomaterials. Selected ions of chromium, arsenic and selenium were adsorbed by multiwalled carbon nanotubes (MWCNTs) modified by (3-aminopropyl) triethoxysilane (APTES),12 the three-dimensional graphene oxide foam/Fe3O4 nanocomposite (GOF/Fe3O4),13 GO modified hydrated zirconium oxide nanoparticles (GO-ZrO(OH)2)14 and magnetic MWCNTs.15 The GO modified (3-mercaptopropyl)trimethoxysilane (MPTMS) beside speciation analysis of As(III)/As(V) was also used for the determination of Co(II), Ni(II), Cu(II), Cd(II) and Pb(II).16 The GO-ZrO(OH)2,14 magnetic MWCNTs15 and GO-SH16 are highly selective towards As(III) and As(V); Se (IV) and As(III), respectively. GOF/Fe3O4 is suitable for sorption of Cr(VI),13 As(III) and As(V)17 and Sb(III).18 Most of studies mentioned above have focused on removal of toxic elements from samples not their determination. In modern analytical chemistry, attention is paid to both the synthesis of new sorbents, as well as the development of precise, sensitive and selective analytical methods that allow trace elements to be determined. The use of GO for this purpose is of particular interest. However, the development of functionalized GO is recommended particularly in the context of anionic species adsorption. Functionalization of GO may further enhance the selectivity of SPE. The novel GO/CeO2 nanocomposite was developed and applied in dispersive micro-solid phase extraction (DMSPE) in order to determine ultra-trace amounts of arsenic, selenium, copper and lead ions. The developed method prevents the most common problems associated with the use of nanomaterials in the classical SPE, such as high back-pressure in the SPE cartridge caused by very small particles and an escape of very small particles from the cartridge. The aim of the research was to show that GO/CeO2 nanocomposite can be used as effective sorbent allowing not only
simultaneous determination of both cationic as well as anionic species of selected metals, but also selenium speciation. This newly synthesized sorbent was used as a solid phase in DMSPE for the preconcentration of analytes prior to their determination by energy dispersive X-ray fluorescence spectrometry (EDXRF). The great advantage of GO/CeO2 and DMSPE/EDXRF combination results from the fact that no coincidences are observed in EDXRF spectra, hence the direct determination of analytes on the sorbent is possible. Consequently, elution of analytes from the sorbent is avoided completely. Thus, the developed method can be considered as eco-friendly and consistent with the principles of green analytical chemistry. EXPERIMANTAL SECTION Reagents and solutions. Arsenic stock solution (1 mg mL-1 of As(V)), selenium stock solution (1 mg mL-1 of Se(IV)), copper stock solution (1 mg mL-1 of Cu(II)) and lead stock solution (1 mg mL-1 of Pb(II)) purchased from Merck (Darmstadt, Germany); humic acid was acquired from Sigma-Aldrich (St. Louis, USA); graphite powder (purity 99.9995% and pore size 325 µm) purchased from Alfa Aeras (Karlsruhe, Germany); nitric acid (65%, Suprapur), sulfuric acid (98%, p.a.), chloric acid (35-38% p.a.), ammonium hydroxide solution (25%, p.a.), hydrogen peroxide (30%, p.a.), potassium permanganate (p.a.), sodium nitrate (p.a.), cerium (III) nitrate hexahydrate (p.a.), sodium hydroxide (p.a.), potassium nitrate (p.a.), calcium nitrate tetrahydrate (p.a.), magnesium nitrate hexahydrate (p.a.), iron (III) nitrate nanohydrate (p.a.), aluminium nitrate nanohydrate (p.a.), buffer solution (pH 4.00 and pH 8.00) were acquired from Avantor Performance Materials Poland S.A. (Gliwice, Poland). Standard solutions were diluted with high purity water obtained from Milli-Q system (Millipore, Molsheim, France). Filters (with a pore size 0.45 µm) purchased from Merck. Standard Reference Material: groundwater [BCR®-610] and Pig Kidney [ERM®-BB186] were acquired from Institute for Reference Material and Measurement. Instruments. SEM micrographs were analysed using FEI Nova NanoSEM 230 microscope. X-ray powder diffraction data (XRD) (PANalytical, The Netherlands) were collected on X'Pert PRO Xray diffractometer with PIXcel ultrafast line detector and Soller slits for Cu Kα radiation. The measurements were performed in Bragg-Brentano geometry. The Raman spectra were measured at room temperature using RenishawInVia Raman spectrometer (Renishaw, United Kingdom) equipped with confocal DM 2500 Leica optical microscope, a thermoelectrically cooled Ren Cam CCD detector and a diode laser operating at 830 nm. Mappings by micro-EDXRF were performed by using a benchtop small-spot EDXRF spectrometer (XDV-SDD model, Helmut Fischer GmbH, Germany). It consists of a microfocus tungsten anode X-ray tube, operating at fixed voltages of 10, 30, or 50 kV using a current in the range of 0.1 to 1 mA (max power of 50 W) and a SDD semiconductor detector (Peltier cooling at -50 ºC; with 145 eV FWHM at Mn Kα line). The spectrometer is equipped with five primary filters (nickel 10, molybdenum 70, aluminium 500, aluminium 1000, and titanium 300 microns), which can be used to reduce the spectral background and eliminate undesirable tungsten tube signals. Spectral data from EDXRF analysis has been evaluated using the WinFTM™ software, version 6.35, linked to the instrumentation.19 Inductively coupled plasma optical emission spectroscopy (ICP-OES) (Spectro Analytical Instruments GmbH, Germany) measurements were performed using a SpectroBlue FMS16a spectrometer with inductively coupled plasma (ICP) excitation (Spectro Analytical Instruments) and a charge coupled device detector. The following operation parameters were used for measurements: plasma power – 1.45 kW; coolant gas – Ar, 12 L min-1; auxiliary gas – Ar, 1 L min-1; nebulizer gas – Ar, 1 L min-1; nebulizer pressure – 3.2 bar; nebulizer-cross-flow type; sample uptake rate – 2 mL min-1; wavelength – 193.759 nm, 196.090 nm,
ACS Paragon Plus Environment
Page 2 of 9
Page 3 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry 324.754 nm and 220.353 nm for As, Se, Cu and Pb, respectively. Energy-dispersive X-ray fluorescence spectrometry (EDXRF) measurements were performed using Epsilon 3 (PANalytical, Almelo, The Netherlands) with a Rh target X-ray tube with a 50 µm Be window and max. power of 9 W. The spectrometer is equipped with a thermoelectrically cooled silicon drift detector (SDD) with 8 µm Be window and resolution of 135 eV at 5.9 keV. The spectrometer is equipped with spinner and five primary filters that can be selected to improve measuring conditions for determined elements. Synthesis of GO/CeO2. GO was synthesized by improved Hummers’ method.20 The method is described in Supporting Information (S-3). In order to obtain GO/CeO2, 1 g of GO was dispersed in 20 mL of water and sonificated for 1 h. Then, 20 mL of cerium (III) nitrate hexahydrate (0.035 g mL-1) solution was added slowly into the solution. The reaction mixture was stirred for 1 h. Then, 60 mL of sodium hydroxide (0.5%) solution was pipetted into the mixture until the pH value achieved 9. At the end the solid product was dried at 70 °C and next it was heated in air atmosphere at 450 °C for 20 minutes in order to oxidize Ce(OH)3 particles to CeO2 nanoparticles. Finally, a gray product of GO/CeO2 was obtained.21 Batch adsorption of As(V), Se(IV), Cu(II) and Pb(II) on GO/CeO2. The batch adsorption experiments were carried out with 1 mg of GO/CeO2 and 25 mL of metal aqueous solutions with the desired concentration and pH. The pH values of the suspensions were adjusted with nitric acid and ammonia solutions. Then, the suspensions were stirred for 1.5 h to achieve the adsorption equilibrium. The suspensions were filtered through a 0.45 µm membrane filter. The amounts of selected metal ions adsorbed on the GO/CeO2 surface (mg g-1) were calculated from the difference between the initial concentration C0 (mg L-1) in aqueous solution and the equilibrium concentration Ce (mg L-1) determined in solution after filtration. Element concentrations were determined by ICP-OES. The relationship can be described using the formula: qe = (C0 – Ce)V/madsorbent, where V is the volume of the suspension, and madsorbent is the mass of GO/CeO2. The recovery is expressed as follows: Recovery (%) = ((C0 – Ce)/C0. ) x 100 Preconcentration procedure based on dispersive solid microphase extraction (DMSPE). The developed DMSPE procedure was as follows: 1 mg of GO/CeO2 was added to 25 mL of the analyzed solution. The pH values of the suspensions were adjusted with nitric acid and ammonia solutions. Then, the suspensions were stirred for 5 minutes. After that, the sample was passed through membrane filter using filtration assembly of 5 mm diameter. After drying, adsorbed metal ions on GO/CeO2 surface were determined directly by EDXRF. RESULTS AND DISSCUSION Characterization of GO/CeO2. GO/CeO2 was characterized by SEM, EDXRF, XRPD and Raman spectroscopy (see Figure 1). As it is shown in the SEM image displayed in Figure 1a nanometric crystallites of CeO2 can be clearly seen on the surface of the GO nanosheets. EDXRF spectra of graphite, GO and GO/CeO2 (Figure 1b) also suggest the successful modification of GO with nanoparticles of CeO2. EDXRF spectrum of GO/CeO2 shows Ce Kα, Ce Lβ peaks and even small peak of Ce Lγ. These peaks of Ce are not observed in the spectra of graphite and GO. Rh Kα and Rh Kβ peaks present in EDXRF spectra arise from the X-ray tube. Figure 1c presents the x-ray powder diffraction (XRPD) patterns of GO and GO/CeO2. The diffraction patterns of GO/CeO2 are dominated by the cerianite phase. The indexed peaks come from cubic Fm-3m phase of CeO2 (cerianite, 28709ICSD).22 The broad peak that match the spacing between the graphene oxide sheets in GO (at ~13°) is not present on the X-ray
patterns of GO/CeO2. Possibly, due to the lack of the long range order in the stacking direction of GO phase. The mean size of the cerianite crystallites, calculated from the Scherer formula,23 is equal to 9 nm. The unit cell parameters for nano-CeO2 in GO/CeO2 are equal to 5.4150(2) Å. Figure 1d shows Raman spectra for GO/CeO2 powder. The main features of the spectrum are two prominent bands which correspond to so-called G and D modes.24 The G peak at 1578 cm-1 is related to E2g vibrational mode of ordered in-plane sp2 carbons and is characteristic for all sp2-hybridized carbon structures. The structural defects and impurities manifest as D and D’ peaks, linked to the breathing modes of carbon rings. In the infinite graphite and graphene layers the D peaks are not active. They appear with the reduction of the size of the crystallites and are always present in the nano-sized carbon structures.25 The intensity and the widths of the G and D peaks carry information about the ‘amount’ of disorder in the sample. The broader the bands and the higher the ID/IG intensity ratio the higher the disorder is.26 In the Raman spectra of GO/CeO2 the D band, with maximum at 1354 cm-1, is intense and broad. The G band is broaden and highly asymmetric. After deconvolution, which is presented in the inset of the Figure 1d, G and D’ bands may be extracted from this asymmetric peak. The high intensity ratio ID/IG =0.9 for GO/CeO2 is due to the increased defect density upon functionalization of the graphitic basal planes. The presence of the D and D’ bands in the Raman spectrum of GO/CeO2 indicate a large number of the edge carbons in the sample that may be related to the small size of the GO sheets.
Figure 1. SEM image (100000x) (a), EDXRF spectra of graphite, GO and GO/CeO2 (b), XRPD patterns for GO and GO/CeO2 (c), Raman spectra for GO/CeO2. The inset presents deconvolution of G and D’ modes. Excitation with 830 nm (d). Effect of pH. The acidity of solution plays a very important role in the sorption process because it influences on both the adsorbent surface charge and the metal species present in solution. The analytes can be adsorbed via active sites on the surface of CeO2 nanoparticles and also via oxygen functional groups of GO that are not occupied by CeO2 nanoparticles. The pH effect on As(III), As(V), Se(IV), Se(VI), Cu(II), Pb(II), Mn(II), Co(II), Ni(II), Zn(II), Cd(II) and Bi(III) recoveries was investigated in the range from 1 to 9. Mn(II), Co(II), Ni(II), Zn(II), Cd(II) and Bi(III) are quantitatively preconcentrated on the adsorbent at pH> 7. Under these conditions, the metals become to precipitate as hydroxides. Therefore, they were not considered in further studies. Obtained results for As(III), As(V), Se(IV), Se(VI), Cu(II) and Pb(II) are presented in the Figure 2. As it can be seen in Fig 2, Cu(II) and Pb(II) recoveries increased quickly at pH 4-6 for Cu(II) and at pH 3-5 for Pb(II). The recoveries ca. 100% were maintained at pH 6.0-9.0 and pH 5.0-9.0 for Cu(II) and Pb(II), respectively. In acidic solution, the positive charge is generated on the surface of GO/CeO2 due to the protonation of active sites. Therefore, the
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
electrostatic attraction between adsorbent and cations is not possible. In neutral or basic solution, due to the dissociation of functional groups the surface charge of GO/CeO2 is negative and the electrostatic interactions between the metal cations and GO/CeO2 nanosheets become stronger. Moreover, besides the electrostatic interactions, the mechanism of chelation is also possible. The Pb(II) and Cu(II) ions can be chelated by the neighboring carboxyl and hydroxyl groups remaining on the GO surface. Another phenomena can be observed in case of oxyanions. In acidic solution, the electrostatic attraction between oxyanions and protonated function groups of GO/CeO2 adsorbent is possible. At pH > 4, the oxyanions cannot be adsorbed via oxygen functional groups of GO because of their negative charge.27 However, the oxyanions can be adsorbed on CeO2 nanoparticles by the surface complexation mechanism (Figure 3) which can result in insoluble precipitate.28
Figure 2. The effect of pH on the recovery of selected metal ions on GO/CeO2 (Experimental conditions: mGO/CeO2 = 1 mg, Canalytes = 250 ng mL-1, V = 25 mL, tstirring = 5 min, the error bars correspond to one standard deviation, n = 3). According to Figure 2, As(V) as well as As(III) were adsorbed on GO/CeO2. At pH < 3, As(V) ions were adsorbed on GO/CeO2 nanosheets with recovery ranging between 70 to 90%. However, the maximum sorption recovery close to 100% was achieved at the pH 4-5 and pH 4-6 for As(V) and As(III), respectively. The recovery of As(V) at pH > 5 decreased to 60%, whereas the recovery of As(III) was constant up to pH = 9. Obtained results showed also that only Se(IV) is adsorbed on the nanosheets of GO/CeO2 from the solutions containing Se(IV) and Se(VI).
Figure 3. The adsorption mechanism for As(V) (a), Se(IV) (b), Cu(II) and Pb(II) (c) ions on the surface of GO/CeO2. From pH 2 to 5, the Se(IV) ions were adsorbed on GO/CeO2 with the recovery of 100%. Above the pH 5, the recovery decreased to 70%. Since, Se(VI) is not practically adsorbed on the nanosheet of GO/CeO2 in whole range of pH. Summarizing, in the subsequent study, pHs 3 and 6 were chosen for the sorption of Se(IV), and Cu(II), respectively, and pH 5 for As(V), As(III) and Pb(II). Adsorption process. The adsorption process of metal ions on GO/CeO2 was simulated using Langmuir29,30 and Freundlich31
models (S-3). Isotherm parameters were obtained by fitting the adsorption equilibrium data to the isotherm models and are presented in Table S-1. Obtained results indicate that the adsorption isotherms are fitted better by the Langmuir model as it can be also appreciated from the experimental adsorption isotherms displayed in Figure S-1. This fact suggests, that the sorption was controlled by chemical adsorption based on the reaction between metal ions and the ≡Ce-OH and/or ≡Ce-OH2+ groups located on the adsorbent surface of the GO/CeO2 nanosheet. The maximum adsorption capacities calculated by Langmuir model for As(V), Se(IV), Cu(II) and Pb(II) were 5.8, 10.7, 25.4 and 30.0 mg g-1, respectively. These results can be considered as very good features for the sorbent in SPE of trace metal ions. Effect of sample volume and contact time. The contact time can significantly influence the adsorption percentage particularly for higher sample volumes and fixed mass of GO/CeO2. Thus, the effect of contact time (5-120 minutes) was investigated for fixed GO/CeO2 mass (1 mg) and various solution volumes (10-500 mL). The obtained results (Figure 4) displayed that, the adsorption of As(V) and Se(IV) on the GO/CeO2 nanosheets was immediately achieved even for the sample volume of 500 mL, that is, 0.002 mg of GO/CeO2 per 1 mL of liquid sample. The adsorption of Cu(II) and Pb(II) on GO/CeO2 was achieved instantly for the sample volume of 100 mL, that is, 0.01 mg of the nanocomposite per 1 mL of liquid sample. A decrease in the recovery values (
