Highly efficient scavenging of Cr(VI) by two-dimensional titanium carbide nanosheets: kinetics, isotherms and thermodynamics analysis

Chromium contamination arises from several industries, such as leather tanning, stainless steels, pigments, textiles (Xu et al. 2011; Hoang et al. 2020), and so on. The trivalent form and hexavalent form of chromium are the usual valence states, of which hexavalent chromium is more toxic than trivalent chromium and is the main cause of pollution. Chromate is limited to be absorbed by aquifer minerals due to it having negative charges; as a result, chromate can move more easily in groundwater and is absorbed in the human body by bioaccumulation (Hong et al. 2008). It seriously influences human beings' health, owing to its carcinogenicity and destructive effects on the respiratory system (Wise et al. 2006; Anandkumar & Mandal 2011). According to the World Health Organization (WHO) requirements, the maximum concentration of Cr(VI) in drinking water should not exceed 0.05 mg/L (Miretzky & Cirelli 2010). Currently, the methods for the removal of Cr(VI) include chemical reduction (Park et al. 2005), adsorption (Karthikeyan & Meenakshi 2019), oxidation (Ouejhani et al. 2008), photocatalysis (Mao et al. 2020; Preethi et al. 2020), reverse osmosis (Das et al. 2006) and ion exchange (Xing et al. 2007). Among these treatment methods, the adsorption technique gets a lot of attention because of its simplicity, environmental friendliness and high efficiency. Adsorption materials, such as activated carbon (Chen et al. 2020), metal organic frameworks (MOF) (Ha et al. 2016; Alqadami et al. 2018), humic acid-iron-pillared bentonite (Xu et al. 2020), zeolite (Li et al. 2021) and cross-linked cyclodextrin polymer (CDP) (Wang et al. 2021) have been studied extensively. However, these adsorption materials are fabricated by multiple-step, complex procedures, or have less functional groups on the surface. As a result, their low efficiency of adsorption, highly cost and stability issues restrict the practical application for the Cr(VI) removal. Therefore, it is important to develop an economic and effective method to solve these practical issues.

In recent years, MXene has actively appeared on many areas owing to its remarkable abilities, such as hydrophilicity, thermostability, flexibility and electroconductibility (Du et al. 2019). MAX is the precursor of MXenes, where M is an early transition metal (e.g., Nb or Ti). A is an IVA/IIIA element (e.g., Al, Si, Sn and so on), and X is carbon or nitrogen. MXenes can be achieved by etching the A levels, and the chemical formular is M_n+1X_nT_x. T_x is a surface functional group (e.g., -F, -OH or -O). As an adsorbent, MXene exhibits high adsorption capacity and adsorption efficiency, due to its special structure, enormous surface area and plenty of functional groups on the surface (Li et al. 2019). For example, Jun et al. reported that MXene had high capacity for dyes adsorption (∼140 mg/g) (Jun et al. 2020a). They also used MXene to remove Ba²⁺ and Sr²⁺, and the adsorption capacity could reach 180 mg/g and 225 mg/g, respectively (Jun et al. 2019a). Zhang et al. reported that carboxyl functionalized MXene nanosheets showed superior adsorption capacity for Eu(III) and U(VI) ions, which can reach 97.1 mg/g and 344.8 mg/g (Zhang et al. 2020). Nevertheless, to our knowledge, there are few comprehensive systematic studies for the Cr(VI) removal by Ti₃C₂T_x MXene.

In this paper, Ti₃C₂T_x MXene was used to remove Cr(VI) and the adsorption behavior of Cr(VI) on Ti₃C₂T_x MXene was investigated systematically. The chemical compositions, lattice structure, micromorphology and surface functional groups of Ti₃C₂T_x MXene were characterized by SEM-EDS, XRD, HRTEM, FI-TR and XPS in detail. The effects of solution pH, contacting time, reaction temperature, initial concentration and adsobent dosage for the Cr(VI) removal by Ti₃C₂T_x MXene were investigated in detail to evaluate the adsorption performance. In addition, the adsorption kinetics and thermodynamic data were also investigated to comprehend the mechanism of adsorption behaviors. Notably, Ti₃C₂T_x MXene exhibited excellent adsorption performance for the Cr(VI) removal, and it had a good directive significance for eliminating Cr(VI) in the water environment.

Ti₃C₂T_x MXene was synthesized by etching Al from the Ti₃AlC₂ using lithium fluoride (LiF) and hydrochloric acid (HCl) (Alhabeb et al. 2017; Ding et al. 2017). In brief, the etchant was prepared by adding a certain amount of LiF and HCl with continuous stirring for 5 min. Then, the Ti₃AlC₂ powder was gradually added into the above etchant with 24 h stirring, and the mixture was washed with deionized water to remove reaction products and residual acid until pH 6–7 was achieved. Subsequently, the obtained powders were sonicated and vacuum-dried at room temperature for 24 h to produce the final product.

Fourier transform infrared spectra (FT-IR) were recorded on a NEXUS spectrometer to estimate the surface functional groups. Transmission electron microsopy (HRTEM, JEM-2100F) and scanning electron microscopy (SEM, JSM-6360LV) were used to observe the morphology and structure of Ti₃C₂T_x MXene, respectively. X-ray diffraction (XRD, Empyrean, Cu Kα radiation) was used to characterize the phase structure of Ti₃C₂T_x MXene. AUV-vis spectrophotometer was used to measure the residual concentrations of Cr(VI). An energy-dispersive spectrometer (EDS) was used to characterize the elements existing on the Ti₃C₂T_x after adsorption. X-ray photoelectron spectra (XPS) was recorded on a Thermo Scientific K-Alpha to analyze the material composition and binding energies.

The surface morphology, lattice structure and functional groups of Ti₃C₂T_x MXene before and after Cr(VI) adsorption were analyzed by HRTEM, SEM, XRD, FTIR and SEM-EDS, as shown in Figure 1, Figure S1 (Supporting Information) and Figure 2. Figure 1(a) and 1(c) and Figure S1 present the Ti₃C₂T_x MXene SEM and HRTEM image before adsorption. It can be seen that Ti₃C₂T_x MXene possessed a multi-nanolamellar stacking structure with a few layers like grapheme (Avila et al. 2014). After adsorbing Cr(VI), there was black shadows existing on the surface of Ti₃C₂T_x MXene (Figure 1(b) and 1(d)), which indicated that Cr(VI) had been successfully adsorbed on the Ti₃C₂T_x MXene. Figure 1(e) presented the XRD of Ti₃AlC₂, Ti₃C₂T_x before and after Cr(VI) adsorption. The characteristic peaks of Ti₃AlC₂ at 9.58°, 19.14°, 34.01° and 38.98° were the (002), (004), (101) and (104) plane (Tran et al. 2021). After being etched by HCl-LiF, the Ti₃AlC₂ characteristic peak of Al layers at 39° disappeared, showing that Ti₃C₂T_x MXene was successfully prepared. Meanwhile, the Ti₃C₂T_x MXene of (002) plane was shifted down to 8.84°, the d-spacing of Ti₃C₂T_x MXene became larger than Ti₃AlC₂ (Mu et al. 2018). However, the characteristic peaks were basically the same after adsorbing Cr(VI), which indicated the structure of Ti₃C₂T_x MXene was stable during the adsorption process. Figure 1(f) showed the FTIR of Ti₃C₂T_x MXene before and after Cr(VI) adsorption. It can be seen that the distinctive peaks at 3,436, 1,631 and 805 cm⁻¹ were corresponding to the –OH stretching vibrations of Ti₃C₂T_x MXene, intimating that there were water moieties and –OH group existing on the surface of the adsorbent (Shahzad et al. 2019). The vibration band at 547 cm⁻¹ presented the Ti–O stretching bands (Karthikeyan et al. 2021a, 2021b), and the adsorption peak at 1629 and 1400 cm⁻¹ belonged to the C = O and C–O stretching vibration of the carboxyl group (Dong et al. 2019). After Cr(VI) adsorption, the peak at 3,436 cm⁻¹ was shifted to 3,430 cm⁻¹, indicating the hydrogen bonds were formed between the Ti₃C₂T_x MXene and Cr(VI). Similarly, the Ti–O, C = O and –OH peaks area of Ti₃C₂T_x MXene tended to a lower peak area, revealing that there was an interaction between the Ti₃C₂T_x MXene material and Cr(VI). Figure 2 showed the SEM-EDS of Ti₃C₂T_x MXene after Cr(VI) adsorption. Obviously, the elements C, Ti, F, O and Cr were detected, and they were distributed uniformly. These results exhibited that Cr(VI) was successfully adsorbed onto Ti₃C₂T_x MXene with surface interactions, which were consistent with the HRTEM result in Figure 1(d).

Figure 3 presented the XPS spectra of Ti₃C₂T_x before and after Cr(VI) adsorption. From Figure 3(a), it can be seen that the elements of Ti, C, O, F were detected before reaction. After Cr(VI) adsorption, the additional peaks of Cr at ∼578 eV and ∼588 eV were detected, indicating that Cr(VI) had been successfully adsorbed onto Ti₃C₂T_x MXene. The results were consistent with TEM and SEM-EDS in Figure 1. In order to acquire more information about the elemental chemical states before and after Cr(VI) adsorption, high resolution XPS spectra of Ti2p, C1s, O1s, F1s and Cr2p were analyzed. For the spectra of Ti2p (Figure 3(b)), the peaks were located at 454.7 eV, 455.7 eV, 457.4 eV, 461.1 eV, 462.3 eV and 465.4 eV respectively. The peaks at 454.7 eV and 461.1 eV were associated with Ti − C, while the peaks centered at 455.7 eV, 457.4 eV, 462.3 eV and 465.4 eV can be assigned as Ti²⁺ oxide, Ti ions with reduced charge state (Ti_xO_y), Ti³⁺ oxide and Ti − O − F (Peng et al. 2016; Ding et al. 2017; Elumalai et al. 2020). The high-resolution O1s spectrum (Figure 3(c)) can be appropriately fitted to four peaks at 529.5 eV, 530.0 eV, 531.7 eV and 533.2 eV. The peaks located at 529.5 and 530.0 eV were ascribed to surface adsorbed O species and Ti–O–Ti, and the peak centered at 531.5 and 533.2 eV were attributed to Ti–OH and C–OH species (Peng et al. 2016). The C1s spectra of Ti₃C₂T_x (Figure 3(d)) was fitted by four peaks at 281.7 eV, 282.1 eV, 284.7 eV and 286.2 eV, corresponding to C − Ti, C − Ti-O, C − C and C − O, respectively (Rakhi et al. 2015; Peng et al. 2016). Binding energies of F1s (Figure 3(e)) at 685.2 and 686.5 eV were ascribed to F − Ti and F − C, respectively (Ding et al. 2017). Notably, the peaks of Ti2p, C1s, O1s, F1s were obviously shifted to some extent after adsorption, reflecting the strong binding affinity and interaction between Cr(VI) and Ti₃C₂T_x MXene. Similarly, the spectra of Cr 2p (Figure 3(f)) can be divided into three peaks at around 577.2 eV, 578.5 eV, 587.4 eV and 588.1 eV. The peaks centered at 577.2 and 588.1 eV were attributed to the peak centered at 531.5 and 533.2 eV were attributed to Cr(VI), and the peaks located at 578.5 and 587.4 eV were ascribed to Cr(III) and Cr₂O₃, respectively (Wu et al. 2020; Karthikeyan et al. 2021a, 2021b). These results implied Cr(VI) was moderately converted to Cr(III), because of an additional feature of Ti₃C₂T_x. And similar findings have also been reported (Karthikeyan et al. 2021a, 2021b).

The solution pH is a critical condition for Cr(VI) removal since it can affect the physicochemical properties of both the adsorbate and adsorbent (Karthikeyan et al. 2021a, 2021b). Therefore, the Cr(VI) adsorption on Ti₃C₂T_x MXene at different pH values was investigated, to figure out the influence of pH on adsorption. Normally, the Cr(VI) ions exist in several forms in aqueous solution varied with different pH. When the pH value is between 2.0 and 6.0, the main form of Cr(VI) is HCrO₄⁻ and Cr₂O₇²⁻. The CrO₄²⁻ is predominant while the pH value is greater than 6.0, whereas H₂CrO₄ mainly exists at pH <1.0. The pH value was ranged from 2.12 to 9.06 using a 0.1 mol/L HCl/NaOH aqueous solution. As shown in Figure 4(a), the adsorption of Cr(VI) on Ti₃C₂T_x MXene decreased persistently with the increase of pH value. At pH = 2.12, the adsorption capacity of Cr(VI) on Ti₃C₂T_x MXene was 110.25 mg/g. However, it was only 21.86 mg/g when pH = 9.06 under the similar conditions. The reason for this phenomenon was that the surface charge was affected by solution pH and the adsorption of Cr(VI) was electrostatic in nature (Jun et al. 2020b). Under acidic conditions, the functional groups on Ti₃C₂T_x MXene would be protonated to generate positive charge sites, leading to stronger electrostatic attraction to negatively charged Cr(VI) ions. However, the degree of protonation decreased and the OH⁻ increased with the increase of pH value. The competition between OH⁻ ions and negatively charged Cr(VI) ions resulted in the decrease of adsorption capacity. In previous investigations, Saravanan et al. explored the influence of pH on the adsorption of Cr(VI) on magnetic nanoparticles coated with mixed fungal biomass (Saravanan et al. 2021). Feng et al. discussed the impact of pH on MXene/PEI functionalized sodium alginate aerogel for Cr(VI) removal (Feng et al. 2021). Tangtubtim et al. studied the influence of pH on Cr(VI) removal with polyethyleneimine-carbamate linked pineapple leaf fiber (Tangtubtim & Saikrasun 2019). And similar findings have also been reported by them.

Figure 4(b) shows the influence of sorbent dosage on Cr(VI) removal by Ti₃C₂T_x MXene. It can be seen that the adsorption capacity was decreasing gradually with adding sorbent dosage. However, the tendency of the removal efficiency was totally opposite. When the sorbent dosage was 0.05 g/L, the adsorption capacity and the removal efficiency of Cr(VI) on Ti₃C₂T_x MXene were 138.42 mg/g and 23.01%, respectively. However, the adsorption capacity reduced to 103.86 mg/g and the removal efficiency of Cr(VI) increased to 100% when the sorbent dosage was 0.30 g/L. The reason for this phenomenon was the removal ratio and the amount of unoccupied adsorption sites. As the dosage of sorbent increased, the Cr(VI) diffusion rate and the reactive sites of adsorption onto Ti₃C₂T_x MXene were reduced at high dosage. Jun et al. and Li et al. explored the effects of adsorbent dose on lead by GO and MOF, and Ni(II) by porous hexagonal boron nitride, respectively (Jun et al. 2019b; Li et al. 2020), and similar findings have been reported by them.

In conclusion, we comprehensively studied and characterized Ti₃C₂T_x by SEM, TEM, XRD, FI-TR, SEM-EDS and XPS. The effect of pH, sorbent dosage, contact time, initial concentration and temperature were also studied systematically. The results exhibited that the adsorption of Cr(VI) on Ti₃C₂T_x was in favor of the low pH. At pH = 2.0, the adsorption capacity for Cr(VI) removal by Ti₃C₂T_x can reach 169.8 mg/g. The adsorption kinetics revealed that the pseudo-second-order kinetic was more appropriate than the pseudo-first-order model, indicating that chemisorption was more dominant than physisorption. As for thermodynamics analysis, the adsorption of Cr(VI) on Ti₃C₂T_x was better fitted with the Freundlich model, implying the Cr(VI) removal by Ti₃C₂T_x was multilayer adsorption. Furthermore, the adsorption of Cr(VI) on Ti₃C₂T_x was a spontaneous endothermic process based on the Gibbs free energy change (⁠⁠), standard enthalpy change(⁠⁠) and standard entropy change (⁠⁠). In summary, Ti₃C₂T_x MXene could be an outstanding adsorbent in the removal of heavy metals in aqueous solutions.

We gratefully acknowledge the funding from Scientific Research Fund (no. 20175003, 2015LG1004) in Shaoxing University. We also sincerely thank the young and middle aged academic cadres from Shaoxing University.

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