Two-Dimensional Ti3C2Tx MXene Nanosheets for Efficient Copper

Research Article pubs.acs.org/journal/ascecg

Two-Dimensional Ti3C2Tx MXene Nanosheets for Efficient Copper Removal from Water Asif Shahzad,† Kashif Rasool,‡ Waheed Miran,† Mohsin Nawaz,† Jiseon Jang,† Khaled A. Mahmoud,*,‡ and Dae Sung Lee*,† †

Department of Environmental Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University (HBKU), Qatar Foundation, P.O. Box 5824, Doha, Qatar

‡

S Supporting Information *

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, because of their large specific surface area, hydrophilicity, and unique surface functional properties. Scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM−EDS), transmission electron microscopy (TEM), Brunauer−Emmett−Teller (BET), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) analyses were performed 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 1 min. 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 DL-Ti3C2Tx 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, because of their nondegradable and persistent nature in the environment; these elements are considered highly hazardous, even at low concentrations.1−3 Copper (Cu), which is 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 the World Health Organisation (WHO) and the U.S. Environmental Protection Agency (EPA), respectively. Therefore, the presence of Cu in the water beyond the prescribed limit should be addressed and removed. Several 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, because it does not require further treatment after Cu ions are removed from the matrix. In recent years, the engineering of micromaterials and nanomaterials and their application in Cu © 2017 American Chemical Society

adsorption have been widely discussed. These materials emerged as efficient adsorbents for Cu and other heavy metals in wastewater treatment, because of 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 two-dimensional (2D) transition-metal carbide nanosheets that are analogous to graphene.9,10 Several transition-metal carbides have been produced via the selective etching of certain elements from hexagonal MAX phases via aqueous and nonaqueous methods. MAX phases (Mn+1AXn; n = 1, 2, or 3),11 are ternary carbides and nitrides composed of early transition metals (M), group IIIA and IVA elements (A), and a carbon and/or nitrogen component (X).12 Because of their unique structural, electrical, and chemical properties, they are used in many applications, such as energy storage, chemical sensors, catalysts, and supercapacitors.13−15 Because of the easy availability, hydrophilic behavior, and tunable chemistry of MXene, its use in Received: August 6, 2017 Revised: October 18, 2017 Published: October 27, 2017 11481

DOI: 10.1021/acssuschemeng.7b02695 ACS Sustainable Chem. Eng. 2017, 5, 11481−11488

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (A, B) SEM and (C, D) TEM images of Ti3C2Tx MXene nanosheets before and after Cu adsorption (yellow arrows indicate TiO2 nanoparticles). 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 a nitrogen gas flow at 60% amplitude with a 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, scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM−EDS), transmission electron microscopy (TEM), Brunauer−Emmett−Teller (BET), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (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 an SEM system (Model S-4800, Hitachi, Japan). The structural changes in Ti3C2Tx were examined by TEM, using a Titan G2 ChemiSTEM Cs Probe (The 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 ASAP-2020 surface analyzer. The Ti3C2Tx MXene samples were degassed for 8 h at 573 K to remove 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 DLTi3C2Tx MXene was measured at an 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 (DLS) in a 180° Heterodyne backscatter arrangement. Part of the laser beam is added to the scattered light. The measurements were conducted at room temperature (25 °C), and data were analyzed using

applications for environmental pollution remediation such as heavy-metal adsorption has recently been explored:16 for instance, both the adsorption/photodegradation of dyes and the adsorption of heavy metals on MXene have been reported.17,18 Some theoretical investigations regarding 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 Pbadsorbed 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 the 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 coexisting heavy metals (Pb, Cd, and Cr, present in an electroplating wastewater) on the 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 ultrasonication; many 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 MLTi3C2Tx into single-layer and/or few-layer- thick nanosheets of DL11482


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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 (spectrum (a)) and Cu−Ti3C2Tx (spectrum (b)). The Cu peaks are marked by asterisks.

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. In addition, 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 (Figure S1 in the SI). 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 sheetlike structure of Ti3C2Tx with a lateral size of ∼500 nm, averaged over several sheets, and a random spatial distribution of the nanosheets. The selected area electron diffraction (SAED) pattern shows that the two-dimensional (2D) DL-Ti3C2Tx nanosheets retain the hexagonal structure of the DL-Ti3C2Tx phase. In contrast, after Cu adsorption, granular particles with an average size of 40 nm and uniform shape were observed together with adsorbed Cu, indicating the formation of TiO2 crystals on the surface (Figure 1D). TEM analysis exposed the growth of TiO2 crystals in an oxygenated environment, as reported in earlier studies.24 The TEM elemental mapping analysis (Figure S2 in the SI) shows the presence of Ti, O, C, and Cu in fractions of 52, 25, 6, and 15 wt %, respectively. 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

Microtrac Flex Analysis software (according to the 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 deionized (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 poly(vinylidene fluoride) (PVDF) syringe filter and the filtrate was analyzed by inductively coupled plasma−optical emission spectroscopy (ICP-OES) (PerkinElmer, wavelength of λ = 213.594 nm) in order to determine the residual Cu concentration. Further experimental detail is given in the Supporting Information (SI).

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RESULTS AND DISCUSSION Surface Characterization of Ti3C2Tx and Cu-Adsorbed Ti3C2Tx. DL-Ti3C2Tx was prepared by exfoliation of MLTi3C2Tx 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 Figure 1, which highlights the significant difference in the surface morphology of DL-Ti3C2Tx before and after adsorption. SEM micrographs show the sheetlike structure of DL-Ti3C2Tx aggregates before adsorption (Figure 1A). After Cu 2+ adsorption, the sheets were restacked with increased surface roughness (Figure 1B). The BET specific surface areas of ML11483


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Figure 3. (A) Effect of pH on adsorption, (B) effect of adsorbate concentration, (C) comparison of Cu adsorption onto ML-Ti3C2Tx and DLTi3C2Tx, and (D) the effect of temperature on adsorption.

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 The peak at 25° 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 also was 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 (spectrum (b) in Figure 2D) 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 File Card No. 85-1326). Copper Adsorption. Effect of pH on Cu Adsorption. Water/wastewater contaminated with Cu is considered a serious hazard for the health of living organisms; therefore, the removal of Cu from water represents a task of great importance. Many 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 (Figure 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 (see Figure S4 in the SI). The point of zero charge (PZC) was measured at pH 2.7, after which point the DL-Ti3C2Tx surface becomes negatively charged and the zeta potential value reached −38 at

was confirmed by the XPS data. As shown in Figure 2A, for pristine Ti3C2Tx, the Ti 2p3/2 core level peaks centered at 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 Cu−Ti3C2Tx structure (Figure 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 eV to 458.7 eV. The C 1s peak at 281.7 eV, which corresponds to C−Ti bonds, has disappeared from the Cu−Ti3C2Tx spectrum, confirming the surface oxidation of Ti3C2, whereas the peak at 284.8 eV, which corresponds to graphitic C−C/C−H, has shifted from 284.8 eV to 285.2 eV (Figures S3B and S3C in the SI).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, which is indicative of TiO2 formation. The Cu 2p peaks at 933.2 and 953 eV in Figure 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 Figure 2D. Before the adsorption of Cu2+, freezedried and ground DL-Ti3C2Tx showed the presence of water and/or Li ions between the Ti3C2Tx nanosheets, as indicated by lattice parameter c of the (001) basal-plane reflection, which is indicative of an interlayer spacing of ∼25 Å.22 A large intense peak at 5.9° is characteristic of the (002) basal plane of DLTi3C2Tx (spectrum (a) in Figure 2D). After Cu2+ adsorption, the DL-Ti3C2Tx nanosheets were covered with Cu2+ ions. The peak at ∼5.8° from the Cu2+-intercalated DL-Ti3C2Tx is sharper than that of the pristine DL-Ti3C2Tx and the (002) peak shifts to lower angles (see the inset in Figure 2D), corresponding to an increase in lattice parameter c, from 27.1 Å for pristine 11484


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respectively. A similar behavior was observed for Pb 2+ adsorption by Alk-MXene.18 A linear plot of the standard Gibbs free-energy change (ΔG) versus temperature was used to obtain the values of the free standard enthalpy and entropy changes (ΔH and ΔS, respectively). The ΔG value decreased from −22.04 kJ mol−1 to −25.05 kJ mol−1 with increasing temperature from 298 K to 318 K, which indicates the endothermic nature of Cu2+ adsorption on DL-Ti3C2Tx (see Table 1). The higher positive value of ΔH (22.85 kJ mol−1)

pH 9. When the pH was increased from 2 to 5, the adsorption capacity increased and reached a removal capacity of 39.24 mg g−1, corresponding to a removal efficiency of 79%. At 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 pH values 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 (2.7 (PZC), the adsorbent surface was negatively charged and available for positively charged metal ions (Cu2+) attachment. However, at pH >5, the adsorption process was not conceivable, because of the precipitation of Cu2+ into Cu(OH)2 in the aqueous phase, depending on the temperature and concentration of the Cu2+ solution.30,31 Therefore, pH 5 was used as optimal pH for the subsequent Cu removal studies. Effect of Contact Time and Adsorbent Concentration. Figure 3B depicts the effect of contact time on the adsorption of Cu at DL-Ti3C2Tx concentrations of 0.5 g L−1 and an initial Cu concentration of 25 mg L−1, temperature = 298 K, and pH 5.0. Approximately 80% total removal was achieved within 60 s, compared to that observed for ML-Ti3C2Tx at the same concentration (51%), and there was no significant 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 the 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 of ∼40.85 mg g−1, i.e., 2.7 times higher than that of activated carbon (15.07 mg g−1). Several earlier studies reported very low maximum adsorption capacities of AC for Cu2+, compared 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 DLTi3C2Tx concentrations ranging between 0.1 and 1.5 g L−1 at an initial Cu concentration of 25 mg L−1, a temperature of 298 K, and pH 5.0. The adsorption of Cu2+ increased sharply, from 48% to 94%, as the adsorbent dose increased from 0.1 g L−1 to 1.0 g L−1. After which, the adsorption rate reached saturation with a slight increase to 97% at 1.5 g L−1 Ti3C2Tx (Figure 3C). Effect of Temperature and Adsorption Thermodynamics. The Cu2+ adsorption on Ti3C2Tx was evaluated at different temperatures of 298, 308, and 318 K, at an initial Cu concentration of 25 mg 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 Cu 2+ 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,

Table 1. Adsorption Thermodynamic Parameters of Cu Adsorption onto DL-Ti3C2Tx at 298, 308, and 318 K Parameters −1

temperature (K)

ΔG (kJ mol )

ΔH (kJ mol−1)

ΔS (kJ mol−1)

298 308 318

−22.04 −23.16 −25.05

22.85

0.152

suggests that the adsorption of metal ions onto DL-Ti3C2Tx was an endothermic process. Furthermore, the positive ΔS value (0.152 kJ mol−1) indicates a higher degree of disorder at the Cu/Ti3C2Tx interface. Adsorbent Kinetics and Isotherms. As described in Figure 4A, the linearized Lagergren pseudo-first-order and pseudosecond-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 description of the adsorption process (Figure 4A). The second-order regression coefficient value (R2 = 0.999) was remarkably higher than the first-order one (R2 = 0.240) at 298 K (Figure S5 in the SI). 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 in the SI). Moreover, according to the pseudo-second-order rate constant values, the adsorption process was rate-limiting. The diffusion mechanism of adsorption process was anticipated using the Weber−Morris intraparticle diffusion model as follows: qt = kit 1/2 + C

(1)

−1

where qt (mg g ) is adsorption capacity at time t, ki is the intraparticle diffusion rate constant (mg g−1 min−1), and C (mg g−1) is the 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 the entire time interval for Cu2+ onto DL-Ti3C2Tx and ML-Ti3C2Tx. The intraparticle diffusion rate constants were calculated from the slope of the multilinear plots at different temperatures (see Table S2 in the SI). The plots of both adsorbents represented multilinearity, suggesting that more than one mode of adsorption occurred in the Cu2+ uptake onto the adsorbents, as reported previously.35 In the DL-Ti3C2Tx and ML-Ti3C2Tx, three and two linear portions were identified, respectively. In the case of DLTi3C2Tx, the initial linear portion might be due to external surface adsorption and a higher metal uptake rate was noted. The intermediate linear portion refers to a 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 the initial portion and then slowly reached equilibrium in a later portion. As 11485


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

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−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 at the bottom of the 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 were concurrently observed for Cu 2p peaks, because of the reduction of Cu2+ to Cu+, thus confirming the possible strong interaction between Ti−O and Cu2+ ions, followed by a reduction to Cu2+ and the 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 hydroxyl groups in activated Ti sites, where metal ion is exchanged by forming a 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

described earlier, there was a comparatively slow adsorption process, which yields low adsorption efficiency (Figure 3C). The Freundlich isotherm was most suitable to describe the adsorption of Cu onto Ti3C2Tx, compared with Langmuir and Temkin. In Figure 4B, the corresponding isotherm curve closely reproduced the experimental data, with an R2 value of 0.962, which is 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 homogeneous 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 Table 2. Adsorption Isotherm Parameters for Cu Adsorption onto DL-Ti3C2Tx at 298 K adsorption isotherm

parameter

value

R2

Langmuir

K (L mg−1) Qmax (mg g−1)

0.034 86.53

0.900

Freundlich

KF (mg g−1) n

20.29 4.05

0.962

Temkin

BT AT

7.03 100.51

0.88

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 the United States, as reported by Tingyi et al.36 The concentrations of four heavy metal ions (Cu2+, Pb2+, Cd2+, and Cr3+) in the simulated wastewater and after 5 h of contact time with the adsorbent are given in Table S3 in the SI. The removal efficiency values of Cu2+, Pb2+, Cd2+, and Cr3+ were 98.29%, 90.04%, 52.90%, and 73.64%, respectively. The experimental results suggested that common cations in wastewater do not significantly affect the binding of Cu2+ on the DL-Ti3C2Tx MXene. Adsorption Mechanism. A possible interpretation for the 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 11486



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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 within ∼45 min, whereas DL-Ti3C2Tx reached equilibrium within just 3 min. Adsorbent Regeneration Study. Once the adsorption equilibrium was achieved, the DL-Ti3C2Tx 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 the first cycle but decreased to 47% and 30% in the second and third cycle, respectively (see Figure S6 in the SI). This sharp decline in adsorption capacity in the 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 an incomplete desorption of the initially adsorbed Cu2+ ion. Therefore, it may be difficult to completely recover the adsorption capacity of Cu2+ after regeneration of the Ti3C2Tx adsorbent, since the adsorption/removal of Cu2+ by DLTi3C2Tx involves an irreversible reductive reaction. It is clear from our findings that DL-Ti3C2Tx MXene nanosheets are capable of simultaneously reducing Cu2+ to Cu+, which results in the rapid and effective adsorption and removal of the reduced metal ions from water. However, at a preliminary level, the Cu removal efficiency exhibited by the regenerated adsorbent appears quite satisfactory. 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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02695. Adsorption experiment details, EDX spectra of DLTi3C2Tx and DL-Ti3C2Tx-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 DLTi3C2Tx (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K. A. Mahmoud). *Tel.: +82-53-953-7286. Fax: +82-53-950-6579. E-mail: [email protected] (D. S. Lee). ORCID

Khaled A. Mahmoud: 0000-0003-1246-4067 Dae Sung Lee: 0000-0003-3579-0076 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 (No. NRF-2014H1C1A1066929). This study was also supported by Grant Nos. NRF2013R1A1A4A01008000 and NRF-2009-0093819 through the ME and NRF of Korea and by the NRF grant by the Korea government (MSIP) (No. NRF-2015M2A7A1000194). This work was also made possible by NPRP Grant Nos. 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 (Drexel University) for providing the MXene samples.

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CONCLUSIONS Because of the high specific surface area, dispersibility, and decoration with hydrophilic terminal groups, the experimentally obtained maximum adsorption capacity (Qexp,max) of DLTi3C2Tx 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. In addition, 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-secondorder 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 endothermic. 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 DLTi3C2Tx than by commercially available activated carbon. Therefore, the hydroxyl-terminated Ti surface in DL-Ti3C2Tx reveals unique adsorption behavior toward 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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Two-Dimensional Ti3C2Tx MXene Nanosheets for Efficient Copper

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