Two-Dimensional Ti3C2Tx MXene Nanosheets for Efficient Copper

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Two-Dimensional Ti3C2Tx MXene Nanosheets for Efficient Copper Removal from Water Asif Shahzad, Kashif Rasool, Waheed Miran, Mohsin Nawaz, Jiseon Jang, Khaled Mahmoud, and Dae Sung Lee ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02695 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 29, 2017

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ACS Sustainable Chemistry & Engineering

Two-Dimensional Ti3C2Tx MXene Nanosheets for Efficient Copper Removal from Water Asif Shahzad1, Kashif Rasool2, Waheed Miran1, Mohsin Nawaz1, Jiseon Jang1, Khaled A. Mahmoud2,*, Dae Sung Lee1,*

1

Department of Environmental Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea

2

Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University (HBKU), Qatar Foundation, P.O. Box 5824, Doha, Qatar.

*To whom all correspondence should be addressed. Tel.: +82-53-953-7286, Fax: +82-53-950-6579 E-mail: [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT: The performance of two-dimensional (2D) Ti3C2Tx MXene nanosheets in the adsorption and copper removal from aqueous media was investigated. Delaminated (DL)-Ti3C2Tx exhibited excellent Cu removal ability due to their large specific surface area, hydrophilicity, and unique surface functional properties. SEM–EDS, TEM, BET, XPS and XRD analyses used to analyze the structural changes in Ti3C2Tx MXene and its interaction with Cu ions. Oxygenated moieties in the layered structure of MXene facilitated reductive adsorption of Cu2+ forming Cu2O and CuO species. DL-Ti3C2Tx exhibited a higher and faster Cu uptake compared to multilayer (ML)Ti3C2Tx. The maximum experimental adsorption capacity (Qexp.max) was 78.45 mg g-1 and 80% of the total content of metal ions was adsorbed within one minute. A pseudo-second-order kinetic model and the Freundlich adsorption isotherm accurately describe the equilibrium time and maximum Cu uptake onto the adsorbent material, respectively. Thermodynamic analysis revealed that the adsorption process was endothermic. The adsorption capacity (Qe) of DLTi3C2Tx was 2.7 times higher than that of a commercially available activated carbon. The present results illustrate the promising potential of 2D MXene nanosheets for the removal of toxic metals from water.

Keywords: Ti3C2Tx MXene; adsorption; metal carbide; isotherms; heavy metals


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INTRODUCTION Heavy metals are the main cause of environmental pollution due to their non-degradable and persistent nature in the environment; these elements are considered highly hazardous even at low concentrations.1–3 Copper (Cu), a potentially toxic heavy metal, must be removed from wastewater and drinking water, because short- and long-term acute exposure to it causes food poisoning, gastrointestinal illnesses, nausea, copper homeostasis, and liver toxicity to both human and aquatic life.4 The allowable limits of Cu2+ is 1.5 mg L-1 in drinking water and 1.3 mg L-1 in industrial waste according to World Health Organization (WHO) and United State Environmental Protection Agency (EPA), respectively. Therefore, Cu presence in the water beyond the prescribed limit should be addressed and removed. A number of techniques, e.g., ion exchange, chemical precipitation, membrane filtration, flocculation and/or coagulation, and adsorption are being used for the removal of Cu and other heavy metals from water and wastewater.5,6 Among those available technologies, adsorption is a simple and economical technique, as it does not require further treatment after copper ions are removed from the matrix. In recent years, the engineering of micro- and nanomaterials and their application in Cu adsorption has been widely discussed. These materials emerged as efficient adsorbents for Cu and other heavy metals in wastewater treatment due to their unique and high adsorption capacities.7,8 On the other hand, there has been continuous efforts aimed at developing better and more easily available adsorbent materials. MXenes are a new family of 2D transition metal carbide nanosheets analogous to graphene.9,10 Several transition metal carbides have been produced by selective etching of certain elements from hexagonal MAX phases via aqueous and non-aqueous methods. MAX phases (Mn+1AXn; n = 1, 2, or 3),11 are ternary carbides and nitrides composed of early transition metals (M), group


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IIIA and IVA elements (A), and a carbon and/or nitrogen component (X).12 Owing to their unique structural, electrical, and chemical properties, they are used in many applications such as energy storage, chemical sensors, catalysts, and supercapacitors.13–15 Due to the easy availability, hydrophilic behavior, and tunable chemistry of MXene, its use in applications for environmental pollution remediation such as heavy metal adsorption has recently been explored:16 for instance, both adsorption/photodegradation of dyes and adsorption of heavy metals on MXene have been reported.17,18 Some theoretical investigations of the removal of pollutants by MXenes based on density functional theory (DFT) and other ab initio methods have also been reported.17,19,20 According to DFT calculations, fully Cu- or Pb-adsorbed MXenes are quite stable and no dissociation was observed at room temperature for saturated MXenes.21 However, results indicated that Ti3C2Tx nanosheets may suffer from surface oxidation in aqueous solution and formation of TiO2 crystals over time.21 Alkalization-intercalated (Alk-)MXene has been previously used to remove lead (Pb2+) from water,18 whereas the bactericidal properties of Ti3C2Tx have recently been proven.22 In this study, the adsorption behavior of Cu on delaminated (DL)-MXene nanosheets was investigated and compared with that on multilayer (ML)-MXene and activated carbon. The influence of co-existing heavy metals (Pb, Cd, and Cr, present in an electroplating wastewater) on adsorption of Cu was also investigated. Thermodynamics, reaction kinetics, and isotherm modeling studies were carried out for the better understanding of Cu removal. DL-Ti3C2Tx nanosheets were produced from ML-Ti3C2Tx by ultra-sonication; a number of characteristic analyses were used to study the structural changes in Ti3C2Tx MXene and its interaction with Cu ions. The DL-Ti3C2Tx samples produced in this work exhibited excellent performance in the adsorption of Cu from aqueous solutions.


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EXPERIMENTAL SECTION Preparation of Delaminated (DL)-Ti3C2Tx. ML-MXene was obtained from Drexel University; details of its synthesis have been reported elsewhere.23 Ultrasonication was used to delaminate ML-Ti3C2Tx into single- and/or few layer- thick nanosheets of DL-Ti3C2Tx, as reported in a previous work.22 For this purpose, 100 mg of ML-Ti3C2Tx ground powder was dispersed in 60 mL of degassed and deionized water (DI). Probe ultrasonication was carried out under nitrogen gas flow at 60% amplitude with pulse rate of 3 s (on) and 1 s (off), for 30 min. After sonication, the dispersion was centrifuged at 3500 rpm for 30 min and the supernatant was collected. The resulting DL-Ti3C2Tx nanosheet suspension was stored at 277 K until further use. The concentration of DL-Ti3C2Tx in the supernatant was measured by freeze-drying 1 mL of suspension.

Characterization of Ti3C2Tx. To understand the internal structure, morphology, surface charge, and composition of Ti3C2Tx before and after adsorption of Cu, SEM–EDS, TEM, BET, and XRD analyses were conducted. The XRD patterns of the Ti3C2Tx nanomaterial before and after adsorption were recorded on a Rigaku D/MAX 2500PC powder XRD instrument with Cu Kα radiation (λ = 0.15418 nm) operating at 40 kV and 200 mA, in the scanning range of 2–80°. Field-emission scanning electron microscopy (FE-SEM) images coupled with SEM–EDS data were obtained using a HITACHI S-4800 (Japan) scanning electron microscope. The structural changes in Ti3C2Tx were examined by TEM using a Titan G2 ChemiSTEM Cs Probe (Netherlands) microscope with an acceleration voltage of 200 kV. The surface area and pore volume of the samples were calculated by N2 adsorption at 77 K using a Micromeritics ASAP2020 surface analyzer. The Ti3C2Tx MXene samples were degassed for 8 h at 573 K to remove


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any moisture or adsorbed contaminants that may have been present on their surface. The BET surface area was analyzed using the manufacturer’s software to apply the BET equation to the adsorption data. Zeta potential was analyzed using a Zetasizer (Nanotrac Wave II, Microtrac, USA). The zeta potential of DL-Ti3C2Tx MXene was measured at initial concentration of 20 mg/L of DL-Ti3C2Tx at different pH values (1.5-10.5). The measurement principle of the zeta potential was based on Dynamic Light Scattering in a 180° Heterodyne – backscatter arrangement. A part of the laser beam is added to the scattered light. The measurements were conducted at room temperature (25°C) and data was analyzed using Microtrac Flex Analysis software (according to Smoluchowski principle).

Adsorption Experiments. A stock solution of Cu (1000 mg L-1) was prepared by dissolving a specific amount of Cu(NO3)2·3H2O in DI water at room temperature (RT). Standardized HNO3 or NaOH was used to control the pH of the Cu solution. For the adsorption studies, a 0.5 g L-1 Ti3C2Tx solution was added to 10 mL (25 mg L-1) of Cu solution with an initial pH of 5.0 at 298 K in a crystal tube and agitated at 200 rpm in a shaking incubator. Sampling was carried out at specific time intervals. The samples were then filtered with a 0.22 µm hydrophilic polyvinylidene fluoride (PVDF) syringe filter and the filtrate was analyzed by inductively coupled plasma–optical emission spectroscopy (ICP-OES, Perkin-Elmer, 213.594 nm wavelength) in order to determine the residual Cu concentration. Further experimental detail is given in the supplementary information (SI)

RESULTS AND DISCUSSION


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Surface Characterization of Ti3C2Tx and Cu-Adsorbed Ti3C2Tx. DL-Ti3C2Tx was prepared by exfoliation of ML-Ti3C2Tx nanosheets by ultrasonication as detailed in the experimental section and used for studying the adsorption of Cu. Morphological changes in DL-Ti3C2Tx before and after Cu adsorption were evaluated by SEM and TEM, as shown in Fig. 1, which highlights the significant difference in the surface morphology of DL-Ti3C2Tx before and after adsorption. SEM micrographs show the sheet-like structure of DL-Ti3C2Tx aggregates before adsorption (Fig. 1A). After Cu2+ adsorption, the sheets were restacked with increased surface roughness (Fig. 1B). The BET specific surface areas of ML-Ti3C2Tx and DL-Ti3C2Tx were 0.5 and 67.66 m2 g-1, respectively. The large difference in specific surface area confirmed the successful delamination of MXene into one or few sheets and increasing the exposed surface area which is essential for significantly improving the metal adsorption capacity. Additionally, EDS analysis confirmed the presence of Cu in the elemental composition of the DL-Ti3C2Tx nanosheets after adsorption of Cu2+, indicating that the metal ion is adsorbed onto the surface of Ti3C2Tx (Fig. S1). Elemental mapping also confirmed the presence of surface terminal groups (O, OH, and F) on Ti3C2Tx MXene. The TEM images further illustrated the structural changes that occurred in DL-Ti3C2Tx upon Cu adsorption. Figure 1C shows the sheet-like structure of Ti3C2Tx with a lateral size around 500 nm, averaged over several sheets, and a random spatial distribution of the nanosheets. The selected area electron diffraction pattern shows that the 2D DL-Ti3C2Tx nanosheets retain the hexagonal structure of the DL-Ti3C2Tx phase. In contrast, after Cu adsorption granular particles of 40 nm on average and with uniform shape were observed together with adsorbed Cu, indicating the formation of TiO2 crystals on the surface (Fig. 1D). TEM analysis exposed the growing of TiO2 crystals in an oxygenated environment as reported in earlier studies.24 The TEM


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elemental mapping analysis (Fig. S2) shows the presence of Ti, O, C, and Cu in 52, 25, 6, and 15 wt. % fractions, respectively.

Figure 1. SEM (A&B) and TEM images (C&D) of Ti3C2Tx MXene nanosheets before and after Cu adsorption (yellow arrows indicating TiO2 nanoparticles).

XPS analysis was used to confirm the changes in the Ti oxidation state of Cu-Ti3C2Tx, with respect to pristine Ti3C2Tx. The formation of rutile TiO2/Ti3C2 heterojunctions was confirmed by the XPS data. As shown in Fig. 2A, for pristine Ti3C2Tx, the Ti 2p3/2 core level peaks centered at


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455.7 and 459 eV can be assigned to Ti–X from substoichiometric TiCx (x < 1) or titanium oxycarbides and Ti4+ ions (TiO2), respectively.25,26 After Cu adsorption in the CuTi3C2Tx structure (Fig. 2B), the intensity of the TiO2 peak increases while that of the other species decreases, indicating surface oxidation of Ti3C2 to TiO2 crystals and a change in the oxidation state from Ti3+ to Ti4+, as also indicated by the peak shift from 455.5 to 458.7 eV. The C 1s peak at 281.7, corresponding to C–Ti bonds, has disappeared from the CuTi3C2Tx spectrum, confirming the surface oxidation of Ti3C2, whereas the peak at 284.8 eV corresponding to graphitic C–C/C–H has shifted from 284.8 to 285.2 eV (Figs. S3B and S3C).25 The Ti–OH peak at 531.3 eV denoting the presence of −OH groups on the surface of Ti3C2 has been replaced by a large band at 530.6 eV indicative of TiO2 formation. The Cu 2p peaks at 933.2 and 953 eV in Fig. 2C could be assigned to CuO and Cu2O, confirming the reduction of Cu2+ to Cu+ at the surface of MXene.27 XRD patterns of DL-Ti3C2Tx before and after adsorption are presented in Fig. 2D. Before adsorption of Cu2+, freeze-dried and ground DL-Ti3C2Tx showed the presence of water and/or Li ions between the Ti3C2Tx nanosheets, as indicated by the c lattice parameter of the basal–plane reflection (001) indicative of interlayer spacing around 25 Å.22 A large intense peak at 5.9º is characteristic of the (002) basal plan of DL-Ti3C2Tx (Fig. 2D-a). After Cu2+ adsorption, the DLTi3C2Tx nanosheets were covered with Cu2+ ions. The peak around 5.8º from the Cu2+intercalated DL-Ti3C2Tx is sharper than that of the pristine DL-Ti3C2Tx and the (002) peak shifts to lower angles (inset, Fig. 2D), corresponding to an increase of the c-lattice parameter from 27.1 Å for pristine Ti3C2Tx to 33.4 Å for Cu2+-intercalated DL-Ti3C2Tx,27,28 indicating that Cu2+ intercalation leads to volume expansion in the MXene structure and increases the size and homogeneity of the layered domains.29


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The peak at 25º indicates the presence of rutile TiO2 crystals, most likely as a result of Ti3C2Tx oxidation after the adsorption of Cu ions.27 Some layer stacking were also observed in the XRD pattern between 20° and 40°, which might be due to the good periodicity between Ti3C2Tx layers.22 The XRD analysis of the oven-dried (at 333

K) DL-Ti3C2Tx sample after adsorption

(Fig. 2D-b) revealed the presence of Cu ions in the form of Cu2O and CuO (which can be assigned to the 2θ angles of 33.94, 35.86, 37.68, 41.66, and 47.49°, according to JCPDS card No. 85–1326).

Figure 2. (A) Ti 2p XPS spectra of pristine Ti3C2Tx, (B) Ti 2p XPS spectra of Cu-Ti3C2Tx, and (C) Cu 2p XPS spectra of Cu-Ti3C2Tx; (D) XRD patterns of Ti3C2Tx (Da) and Cu-Ti3C2Tx (Db). The Cu peaks are marked by asterisks.


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Copper Adsorption. Effect of pH on Cu adsorption. Water/wastewater contaminated with Cu is considered a serious hazard for the health of living organisms; removal of Cu from water thus represents a task of great importance. A number of batch experiments were conducted to understand the influence of different parameters on the adsorption of Cu onto Ti3C2Tx nanosheets. In the first batch, the effect of pH on the adsorption was investigated (Fig. 3A). The adsorption of Cu onto DL-Ti3C2Tx was found to be pH-dependent: in particular, the nanosheets showed low adsorption of Cu at lower pH, where the surface charge of the adsorbent is reduced, potentially leading to a competition between H+ and metal ions (M2+) for gaining access to the surface sites. This is due to the possible protonation of the hydroxyl terminal groups on the surfaces of Ti3C2Tx nanosheets, which makes them positively charged in high acidic media as predicted by zeta potential measurement (Fig. S4). The point of zero charge (PZC) was measured at pH 2.7. After which, DL-Ti3C2Tx surface becomes negatively charged and z-potential value reached -38 at pH 9. When the pH was increased from 2 to 5, the adsorption capacity increased and reached to removal capacity of 39.24 mg g-1, corresponding to a removal efficiency of 79%. Around pH 5, the lower surface charge could enhance the electrostatic attraction between positively charged metal ions and the surface of the nanosheets.4,20 At higher pHs of 5.5 and 6, adsorption capacity decreased to 32.76 and 19.48 mg g-1, respectively. Moreover, according to the zeta potential analysis results of DL-Ti3C2Tx, at low pH (< 2.5) the surface of nanosheets acquired positive charge and with the increase in pH the increased negative charges were observed. Therefore, above pH 2.7 (PZC) the adsorbent surface was negatively charged and available for positively charged metal ions (Cu2+) attachment. However, above pH 5, the adsorption process was not conceivable due to the precipitation of Cu2+ into Cu(OH)2 in aqueous


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phase depending on temperature and concentration of Cu2+ solution.30,31 Therefore, pH 5 was used as optimal pH for the subsequent Cu removal studies.

Figure 3. (A) Effect of pH on adsorption, (B) effect of adsorbate concentration, (C) comparison of Cu adsorption onto ML-Ti3C2Tx and DL-Ti3C2Tx, and (D) the effect of temperature on adsorption.

Effect of contact time and adsorbent concentration. Figure 3B depicts the effect of contact time on the adsorption of Cu at 0.5 g L-1 DL-Ti3C2Tx concentrations and initial Cu concentration of 25 mg L-1, 298 K temperature and pH 5.0. About 80% total removal was achieved within 60 s as compared with (51%) for ML-Ti3C2Tx at the same concentration, and there was no significant


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removal observed after 3 min. The hydrophilicity, large specific surface area and high density of surface functional groups (O, OH, and F) make Ti3C2Tx MXene a time-efficient and effective adsorbent for Cu adsorption. The initial high removal rate could be attributed to the large number of vacant binding sites available on the Ti3C2Tx for the adsorption of metal ion. Upon saturation of Ti3C2Tx surface, the metal ion uptake rate began to decrease, and finally an apparent equilibrium was reached within 3 min. It was found that the Cu adsorption process was much faster for DL-Ti3C2Tx when compared with ML-Ti3C2Tx. A similar fast adsorption had been previously reported for chromium and lead on Ti3C2Tx MXene.18,20 Ti3C2Tx showed an absolute removal capacity (Qe) of Cu about 40.85 mg g-1, i.e., 2.7 times higher than that of activated carbon (15.07 mg g-1). A number of earlier studies reported very low maximum adsorption capacity of AC for Cu2+ as compare to Ti3C2Tx.32 Moreover, after adsorption the activated carbon forms a sludge that is difficult to process in order to desorb and regenerate the adsorbent. Also, the removal efficiency of Cu by MXene nanosheets was close or higher than that of 2D graphene oxide (GO) and graphene-based adsorbents.33,34 Further functionalization and modification of Ti3C2Tx could lead to more effective removal of Cu and of other toxic heavy metals. The effect of the adsorbent dose was investigated at DL-Ti3C2Tx concentrations ranging between 0.1 and 1.5 g L-1 at an initial Cu concentration of 25 mg L-1, 298 K temperature and pH 5.0. The adsorption of Cu2+ increased sharply from 48% to 94% with increasing adsorbent dose from 0.1 to 1.0 g L-1. After which, the adsorption rate reached saturation with slight increase to 97% at 1.5 g L-1 Ti3C2Tx (Figure 3C).

Effect of temperature and adsorption thermodynamics. Cu2+ adsorption on Ti3C2Tx, was evaluated at different temperature of 298, 308, and 318 K, at an initial Cu concentration of 25 mg


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L-1, and pH 5. It can be depicted from Figure 3D that increasing the temperature in the given temperature range resulted in a slight increase in Cu2+ adsorption on the Ti3C2Tx nanosheets. The removal capacities were 39.24, 40.43, and 43.34 mg g-1 at 298, 308, and 318 K, respectively. A similar behavior was observed for Pb2+ adsorption by Alk-MXene.18 A linear plot of the standard Gibbs free energy change (∆G) vs. temperature was used to obtain the values of the free standard enthalpy and entropy changes (∆H and ∆S, respectively). The ∆G decreased from -22.04 to 22.05 with increasing temperature from 298 to 318 K, which indicates the endothermic nature of Cu2+ adsorption on DL-Ti3C2Tx (Table 1). The higher positive value of ∆H (22.85 kJ mol-1) suggests that the adsorption of metal ions onto DL-Ti3C2Tx was an endothermic process. Furthermore, the positive ∆S value (0.151 kJ mol-1) indicates a higher degree of disorder at the Cu/Ti3C2Tx interface.

Table 1. Adsorption thermodynamic parameters of Cu adsorption onto DL-Ti3C2Tx at 298, 308, and 318 K. Parameters

Temperature (K)

∆G (kJ mol-1)

298

-22.04

308

-23.16

318

-25.05

∆H (kJ mol-1)

∆S (kJ mol-1)

22.85

0.152

Adsorbent kinetics and isotherms. As described in Figure 4A, the linearized Lagergren pseudofirst-order and pseudo-second-order models were applied to analyze the adsorption kinetics of Cu2+ onto Ti3C2Tx. The pseudo-second-order model was found to provide a very accurate


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description of the adsorption process (Figure 4A). The second-order regression higher coefficient value (R2 = 0.999) was remarkably higher than the first-order one (R2 = 0.240) at 298 K (Fig. S5). At the three different temperatures considered, the equilibrium adsorption capacity (Qe) estimated by the pseudo-second-order model was very close to the experimental value (Table 1S). Moreover, according to the pseudo-second-order rate constant values, the adsorption process was rate-limiting.

A 20

B

2nd order fit, R2 = 1

16

90 72

Qe (mg g-1)

t / Qt

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


12 8 4

54 36

Experimental Langmuir, R2=0.900 Freundlich, R2=0.962 Tempkin, R2=0.878

18

0

0 0

120

240

360 t (sec)

480

600

720

0

50

100

150 200 Ce ( mg L-1)

250

300

Figure 4. (A) Pseudo-second-order kinetic graph and (B) adsorption isotherm of Cu.

The diffusion mechanism of adsorption process was anticipated using the Weber-Morris intraparticle diffusion model as follows: ‫ݍ‬௧ = ݇௜ ‫ݐ‬

ଵൗ ଶ

+‫ܥ‬

(1)

where qt (mg g-1) is adsorption capacity at time t, ki is intraparticle diffusion rate constant (mg g-1 min-1), and C (mg g-1) is intercept. The kinetic data were used to examine the presence or absence of intraparticle diffusion. The plot of the qt against time t1/2 was drawn over entire time interval for Cu2+ onto DL-Ti3C2Tx and ML-Ti3C2Tx. The intra particle diffusion rate constants were calculated from the slope of the multi-linear plots at different temperatures (Table S2). The


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plots of both adsorbents represented multi-linearity, suggesting that more than one mode of adsorption occurred in the uptake Cu2+ onto the adsorbents as reported previously.35 In the DLTi3C2Tx and ML-Ti3C2Tx, three and two linear portions were identified, respectively. In the case of DL-Ti3C2Tx, the initial linear portion might be due to external surface adsorption and a higher metal uptake rate was noticed. The intermediate linear portion refers to gradual adsorption due to intraparticle diffusion. Thereafter, the final linear portion corresponds to very stable and comparatively slow uptake as the process approached the equilibrium stage. On the other hand, the plot of ML-Ti3C2Tx was linear in initial portion and then slowly reached to equilibrium in later portion. As described earlier there was comparatively a slow adsorption process which yields low adsorption efficiency (Figure 3C). The Freundlich isotherm was most suitable to describe the adsorption of Cu onto Ti3C2Tx as compared with Langmuir and Temkin. In Figure 4B, the corresponding isotherm curve closely reproduced the experimental data, with an R2 value of 0.962, higher than that of the Langmuir (R2 = 0.900) and Temkin (R2 = 0.88) isotherm models. The Freundlich model assumes that the interactions at solid–liquid interfaces involve the homogenous adsorption of the adsorbate on the heterogeneous and multilayer surface of the adsorbent. Furthermore, the n > 1 value in Table 2 shows the good fit of the Freundlich isotherm model for Cu2+ adsorption onto Ti3C2Tx. it is assumed that the heterogeneous surface of Ti3C2Tx nanosheets contains terminal/functional groups such as O that offer stable binding sites for Cu2+.17 The maximum experimental adsorption capacity (Qexp.max) was 78.45 mg g-1.

Ti3C2Tx application in simulated electroplating wastewater. DL-Ti3C2Tx MXene was applied to a simulated wastewater effluent from an electroplating plant in USA reported by Tingyi et. al.36


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The concentrations of four heavy metal ions (Cu2+, Pb2+, Cd2+, and Cr3+) in the simulated wastewater and after 5 hr contact time with the adsorbent are given in Table S3. The removal efficiency of Cu2+, Pb2+, Cd2+, and Cr3+ were 98.29, 90.04, 52.90, and 73.64%, respectively. The experimental results suggest that common cations in wastewater do not significantly affect the binding of Cu2+ on the DL-Ti3C2Tx MXene.

Table 2. Adsorption isotherm parameters for Cu adsorption onto DL-Ti3C2Tx at 298 K. Adsorption Isotherm

Parameters

Values

K ( L mg-1)

0.034

Langmuir

R2

0.900 -1

Qmax (mg g )

86.53

KF (mg g-1)

20.29

Freundlich

0.962 n

4.05

BT

7.03

Temkin

0.88 AT

100.51

Adsorption Mechanism. A possible interpretation for Cu adsorption mechanism on the surface of MXene nanosheets may involve the contribution of the surface functional groups. MXene nanosheets surfaces contain terminal groups such as O, OH, and F, which could act as available sites for capturing cationic metal ions.37 Cu adsorption can also be influenced by the hydrophilic nature of MXene and its negatively charged surface functionalities. Several functional groups were previously identified on the Ti3C2Tx surface,37 including C–Ti–Ox, C–Ti–(OH)x, and C–Ti–


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Fx. GO is another member of the family of 2D materials, with the same nanosheets structure as Ti3C2Tx. Cu adsorption on the 2D GO nanosheets was explained by a–layer mechanism, where Cu undergoes an ion exchange reaction with the carboxylic (–COOH) and hydroxyl (–OH) surface functional groups,38 Similarly, Cu adsorption on DL-Ti3C2Tx could proceed through an ion exchange reaction between positively charged Cu ions and negatively charged terminal groups (–O and –OH) on the Ti3C2Tx surface. Ti–O and Ti–OH have a strong adsorption affinity for metallic ions. Therefore, Cu could form CuO, Cu2O, and/or Cu(OH) in rapid reactions with Ti3C2–Ox and Ti3C2–(OH)x and settle down at the bottom of reaction tube. The SEM images and XRD patterns of DL-Ti3C2Tx before and after the adsorption of Cu provided further insight into the adsorption mechanism. The XPS spectra and XRD analysis confirmed the presence of Copper on Ti3C2Tx in a form of Cu+/Cu2 mixture after adsorption. As described above from XPS interpretation, the shift of the Ti 2p in Cu-Ti3C2Tx has indicated the formation of Ti−O−Cu bonding. Similar shifts concurrently observed for Cu 2p peaks due to reduction of Cu2+ to Cu+ is confirming the possible strong interaction between Ti−O and Cu2+ ions followed by reduction to Cu2+ and formation of Cu2O. This strong affinity between the Cu2+ ions and Ti3C2Tx MXene may result from inner-sphere complex formation onto the surface of Ti3C2Tx MXene.18 Moreover, XRD peaks at 2θ angles of 34°, 38.8°, 41.6°, and 44.8° can be assigned to CuO and Cu2O, whereas the XPS peaks at 933.2 and 953 eV in the Cu 2p region are indicative of the formation of Cu2O and CuO,39 This was also confirmed by the partial oxidation of the Ti3C2Tx flakes and the formation of rutile (TiO2) nanoparticles. Recent experimental and computational study on a similar system suggested that MXenes with different surface functional groups possess extremely high Pb adsorption capacities that are much higher than other 2D materials.21 Another study correlated the sorption behavior of alkalized Ti3C2Tx to surfaces intercalation to the


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hydroxyl groups in activated Ti sites, where metal ion is exchanged by forming hexagonal potential trap.18 In comparison with DL-Ti3C2Tx, the adsorption process in ML-Ti3C2Tx was slow, with a comparatively small amount of Cu adsorbed on the surface. This result might be due to the layer stacking in ML-Ti3C2Tx which ultimately reduces the available surface area leading to a lower amount of available sites for adsorption. Moreover, in the case of ML-Ti3C2Tx the adsorption equilibrium was achieved in ~45 min, whereas DL-Ti3C2Tx reached equilibrium in just 3 min.

Adsorbent Regeneration Study. Once the adsorption equilibrium was achieved, the DLTi3C2Tx adsorbent was recovered by centrifugation and desorbed in an acidic mixture of nitric acid and calcium nitrate for 5 h. The adsorption capacity of Cu onto the regenerated DL-Ti3C2Tx adsorbent was 80% in first cycle but decreased to 47 and 30% in the second and third cycle, respectively (Figure S6). This sharp decline in adsorption capacity in adsorption-desorption cycles can be explained by the partial oxidation of Ti3C2Tx into TiO2 nanoparticles during adsorption as described earlier from XPS analysis, and there might be the incomplete desorption of initially adsorbed Cu2+ ion. Therefore, it may be difficult to completely recover the adsorption capacity of Cu2+ after regeneration of Ti3C2Tx adsorbent since the adsorption/removal of Cu2+ by DL-Ti3C2Tx involves an irreversible reductive reaction. It is clear from our finding that DLTi3C2Tx MXene nanosheets are capable of simultaneously reduces Cu2+ to Cu+, rapidly and effectively adsorbs and removes the reduced metal ions from water. However, at a preliminary level, the Cu removal efficiency exhibited by the regenerated adsorbent appears quite satisfactory.


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In this work, we explored the heavy metal adsorption capability of 2D Ti3C2Tx MXene nanosheets. Ti3C2Tx exhibits outstanding aqueous phase adsorption of Cu. Characterization and morphological analyses confirmed the superior Cu adsorption capability of DL-Ti3C2Tx over ML-Ti3C2Tx in terms of removal efficiency and time to reach the adsorption equilibrium. SEM– EDS, TEM, XPS, and XRD analysis confirmed the reductive adsorption of Cu on the surface of Ti3C2Tx resulting in surface oxidation, as indicated by the formation of TiO2 rutile nanoparticles.

CONCLUSIONS Owing to the high specific surface area, dispersibility, and decoration with hydrophilic terminal groups, the experimentally obtained maximum adsorption capacity (Qexp.max) of DL-Ti3C2Tx for Cu was 78.45 mg g-1. The XRD analysis showed that Cu strongly reacts with the oxygenated surface functional groups of Ti3C2Tx, forming Cu2O and CuO species. Additionally, DL-Ti3C2Tx was proven to be a time-efficient adsorbent for heavy metal adsorption as the corresponding equilibrium state was achieved within 3 min. A pseudo-second-order kinetic model and the Freundlich adsorption isotherm were found to provide an accurate description of the equilibrium time and maximum metal ion uptake onto the adsorbent material, respectively. A thermodynamic analysis showed that the adsorption reaction was spontaneous and exothermic. The regenerated DL-Ti3C2Tx presented excellent adsorption efficiency in three cycles of adsorption, with a significant adsorption capacity in the adsorption-elution cycles. Moreover, a higher Cu removal rate was achieved by DL-Ti3C2Tx than by commercially available activated carbon. Therefore, the hydroxyl terminated Ti-surface in DL-Ti3C2Tx reveals unique adsorption behavior towards Cu. Successful synthesis of DL-Ti3C2Tx and the corresponding high adsorptive removal of heavy metals open up new ways to promote its use to resolve other environmental issues.


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ASSOCIATED CONTENT Supporting Information: Adsorption Experiment detail, EDX spectra of DL-Ti3C2Tx and DLTi3C2Tx-Cu, HAADF-STEM image of DL-Ti3C2Tx-Cu, zeta-potential measurement, Complete XPS spectra for DL-Ti3C2Tx, Linearized Lagergren pseudo-first order graph, and Recyclability graph of DL-Ti3C2Tx,

ACKNOWLEDGEMENT This work was supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education (ME) and National Research Foundation (NRF) of Korea (NRF-2014H1C1A1066929). This study was also supported by grants NRF2013R1A1A4A01008000 and NRF-2009-0093819 through the ME and NRF of Korea and by the NRF grant by the Korea government (MSIP) (NRF-2015M2A7A1000194). This work was also made possible by NPRP grants # 8-286-2-118 from the Qatar National Research Fund (A Member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. The authors are grateful to Prof. Yury Gogotsi of Drexel University for providing the MXene samples.

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TOC Art

Synopsis: Strong and hydrophilic nanosheets with outstanding sorption properties for heavy metals removal in wastewater, a new material for green, clean and sustainable environmental remediation.


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Two-Dimensional Ti3C2Tx MXene Nanosheets for Efficient Copper

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