Two-Dimensional Titanium Carbides (Ti3C2Tx) Functionalized by Poly(m-phenylenediamine) for Efficient Adsorption and Reduction of Hexavalent Chromium.

Titanium carbides (MXenes) are promising multifunctional materials. However, the negative surface charge and layer-by-layer restacking of MXenes severely restrict their application in the field of anionic pollutants, including in hexavalent chromium (Cr(VI)). Herein, Ti3C2Tx MXenes was functionalized through in situ polymerization and intercalation of poly(m-phenylenediamine) (PmPD), then Ti3C2Tx/PmPD composites were obtained. Delightedly, Ti3C2Tx/PmPD composites exhibited positive surface charge, expanded interlayer spacing, and enhanced hydrophobicity. Furthermore, the specific surface area of Ti3C2Tx/PmPD composite was five and 23 times that of Ti3C2Tx and PmPD, respectively. These advantages endowed Ti3C2Tx/PmPD composite with an excellent adsorption capacity of Cr(VI) (540.47 mg g-1), which was superior to PmPD (384.73 mg g-1), Ti3C2Tx MXene (137.45 mg g-1), and the reported MXene-based adsorbents. The Cr(VI) removal mechanism mainly involved electrostatic adsorption, reduction, and chelation interaction. This study developed a simple functionalization strategy, which would greatly explore the potential of MXenes in the field of anionic pollutants.

1. Introduction

Hexavalent chromium (Cr(VI)) pollution poses a serious crisis to human beings and the ecosystem, due to its high mobility, toxicity, and potential carcinogenicity [1,2]. It is extremely urgent to treat Cr(VI) contamination. In contrast, trivalent chromium (Cr(III)) usually has low levels of toxicity, is immobile, and even is an essential micronutrient for organisms [3,4,5]. At present, adsorption remains an effective method for Cr(VI) remediation [6], which involves the conversion from toxic Cr(VI) to mild Cr(III) in the adsorption process [7,8]. Various adsorbents have been developed, such as biochar [9,10], the metal–organic framework [11,12], nanoscale zero-valent iron [13,14], graphene oxide [15,16], and organic polymer [17,18]. Unfortunately, current adsorbents generally suffer from unsatisfactory removal capacity, a low adsorption rate, and weak reduction capacity. Therefore, the development of novel adsorbents with an outstanding performance is still a paramount challenge.

Transition metal carbides (MXenes) are novel two-dimensional (2D) materials, which were first reported by Yury Gogotsi in 2011 [19,20,21]. Their unique physicochemical properties (such as a layered structure, high hydrophilic surface, and excellent electrical conductivity) endow MXenes with promising advantages in electromagnetic interference shielding [22], energy storage fields [23], and conducting thin films [24]. In recent years, MXenes have received increasing attention in the field of environment owning to their large amounts of surface negative terminations (such as -O, -OH, and -F) [25,26,27]. These negative terminations render MXenes with favorable removal capacity for cationic pollutants.

However, MXenes still face great challenges in the remediation of anionic pollutants due to the charge repulsion between MXenes and anionic pollutants [28]. Moreover, due to the hydrogen bonding between the surface functional groups, MXenes are always reassembled as tightly as graphene and other 2D materials in practical applications [29,30], which would inevitably decrease the mass transfer efficiency and availability of MXenes [31]. Regulating surface charge and interlayer spacing are feasible approaches to fully explore the potential of MXenes in the field of Cr(VI) remediation. However, there is still no relevant report on this at present.

As a kind of conjugated polymer, poly(m-phenylenediamine) (PmPD) has been widely employed to functionalize matrix materials because of its simple synthesis and abundant amino groups [32,33,34]. In this research, Ti3C2Tx (X = OH, O or F) was selected as the representative of the MXenes family, and the regulation of surface charge and expansion of interlayer spacing of Ti3C2Tx were successfully achieved by in situ polymerization and intercalation of PmPD. Accordingly, a novel MXenes/poly(m-phenylenediamine) (Ti3C2Tx/PmPD) composite was obtained and utilized to adsorb Cr(VI) from aqueous solution. Finally, the preparation mechanism and adsorption mechanism were investigated in detail.

2. Material and Methods

2.1. Materials

Ti3AlC2 MAX powder (400 mesh) was purchased from 11 Technology Co., Ltd. m-Phenylenediamine (99.5%) was purchased from Aladdin Reagent. All other reagents were of an analytical grade and were purchased from Sinopharm Chemical Reagent.

2.2. Preparation of Ti3C2Tx, Ti3C2Tx/PmPD, PmPD

Ti3C2Tx was synthesized by etching and delaminating Ti3AlC2 MAX powder through the typical minimally intensive layer delamination (MILD) approach [20]. Ti3C2Tx solution with different concentrations was obtained by dissolving Ti3C2Tx in deionized (DI) water. The detail process of Ti3C2Tx synthesis was described in the supporting information.

To synthesis Ti3C2Tx/PmPD, 1 g mPD monomer was firstly dissolved in 30 mL DI water, and added to a certain concentration of 100 mL Ti3C2Tx dispersion. The above mixture was continuously sonicated and stirred for 30 minutes. After that, Na2S2O8 solution (20 mL, 0.11 g mL−1) was slowly added to the above solution, and the reaction was maintained for 4 h, at −4 °C by ice bath. Ti3C2Tx/PmPD composites with different mass ratios of mPD to Ti3C2Tx were prepared in turn by changing the concentration of Ti3C2Tx. The obtained composites were labeled Ti3C2Tx/PmPD-X (mass ratios X = 2/1, 5/1, 10/1). The final sediment was centrifuged, rinsed with amount DI water, and dried under vacuum (−55 °C, 12 h). Finally, Ti3C2Tx/PmPD-X composites were obtained.

2.3. Characterization

The morphology and structure of as-obtained composites were characterized by scanning electron microscope (SEM, FEI Nova NanoSEM 230, FEI company, Hillsboro, OR, USA), scanning transmission electron microscope (STEM-EDS, JEM-2100F, Japan Electronics Co., Ltd. (JEOL), Tokyo, Japan), atomic force microscope (AFM, NanoMan VS, Bruker, Germany), X-ray powder diffraction patterns (XRD, D/max 2550 VB + XX diffractometer, Rigaku International Corp, Tokyo, Japan), X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, Thermo Scientific, Waltham, MA, USA), Raman scattering spectra (532 nm, Renishaw inVia, Renishaw, London, England), and Fourier transformed infrared spectra (FT-IR, Nicolet IS10, Thermo Scientific, Waltham, MA, USA). The contact angles were measured using a Date Physics JY-82C goniometer (Dingsheng testing machine testing equipment Co., Ltd, Jinan, China). Zeta potentials were recorded using a Malvern Nano-ZS Zetasizer. The N2 adsorption-deposition isotherms were measured by bjbuilder KUBO-X1000 (Beijing Builder Electronic Technology Co., Ltd., Beijing, China).

2.4. Batch Experiments

Potassium dichromate (K2Cr2O7) was dissolved in DI water to obtain aqueous solutions with different Cr (VI) concentrations. The obtained composites (10 mg) were put into 100 mL polyethylene bottle with 20 mL Cr(VI) solution, then the mixture was shaken at 30 °C for 12 h at 180 rpm speed. UV−vis spectrophotometer (540 nm) was utilized to detect the residual Cr(VI) concentration. All the experimental data were the average values of three measurements, whose relative error was less than 5%.

3. Results and Discussion

3.1. Material Characterization

The structure and morphology of as-obtained composites were studied by using TEM, SEM, XRD, AFM, and XPS technologies. As can be seen from Figure 1a–c, Ti3C2Tx MXene exhibited 2D ultrathin morphology with a small average thickness of ~4 nm. The disappearance of the peak at 39° and the shift of the (002) peak to 6.04° also indicated the formation of Ti3C2Tx nanosheets (Figure 1d) [35,36]. As shown in Figure 1e–g, Ti3C2Tx/PmPD-X composites showed a 2D dispersed and wrinkled morphology with a thickness ranging from ~15 nm to ~70 nm. In addition, taking Ti3C2Tx/PmPD-5/1 as an example, Ti3C2Tx/PmPD-X composites owned a thin and uniform shape (Figure 1i). The homogeneous dispersion of C, Ti, and N elements also revealed the homogeneous polymerization of PmPD on surface Ti3C2Tx nanosheets (Figure 1j).

Figure 1

SEM images of (a) Ti3C2Tx, (e) Ti3C2Tx/PmPD-2/1, (f) Ti3C2Tx/PmPD-5/1 and (g) Ti3C2Tx/PmPD-10/1; TEM images of (b) Ti3C2Tx and (i) Ti3C2Tx/PmPD-5/1; (c) AFM image of Ti3C2Tx; (d) XRD patterns of Ti3AlC2, Ti3C2Tx, Ti3C2Tx/PmPD-X and PmPD; (h) XPS survey of Ti3AlC2, Ti3C2Tx, PmPD, Ti3C2Tx/mPD and Ti3C2Tx/PmPD-X; (j) STEM-EDS mapping of Ti3C2Tx/PmPD-5/1; (k) Water contact angle measurements of Ti3C2Tx (right) and Ti3C2Tx/PmPD-5/1 (left). (l) N2 adsorption−desorption isotherms of Ti3C2Tx, PmPD and Ti3C2Tx/PmPD-X.

As can be seen from Figure 1d, the (002) peak of Ti3C2Tx/PmPD-2/1 shifted to a smaller 2θ angle (5.54°) compared to that of Ti3C2Tx (6.04°). The decreased 2θ angle indicated a significant expansion of the interlayer spacing of Ti3C2Tx/PmPD-2/1 from 14.6 to 15.9 Å. Furthermore, the (002) peak of Ti3C2Tx/PmPD-5/1 shifted to 5.02° (17.6 Å interlayer spacing), which increased about 3.0 Å compared to that of original Ti3C2Tx. In addition, with the further increase of mass ratio of mPD to Ti3C2Tx, the (002) peak of Ti3C2Tx/PmPD-10/1 would shift to the minimum 2θ angle, revealing that the interlayer spacing of Ti3C2Tx nanosheets were further expanded. The enlargement of interlayer spacing may be ascribed to the intercalation of PmPD in the polymerization process as well as the barrier effect of PmPD layer. These results revealed that the interlayer spacing of PmPD/Ti3C2Tx composites could be regulated from 14.6 to 17.6 Å, or even greater, by adjusting the mass ratio. It was noteworthy that the expansion of interlayer spacing would facilitate exposing the active sites and developing the adsorption potential of Ti3C2Tx MXene.

XPS spectrum were shown in Figure 1h. The presence of N element on intermediate Ti3C2Tx/mPD indicated that mPD monomers were enriched on the Ti3C2Tx surface through electrostatic interaction and hydrogen bonding [37]. The enrichment of mPD was beneficial to the uniform polymerization of PmPD. The peak of N element was gradually enhanced with the increase of mass ratio of mPD to Ti3C2Tx, which corresponded to the increase of composites thickness. In contrast, pure PmPD was homogeneously polymerized in solution and formed spherical shape with a diameter of 200–2000 nm (Figure S1). In contrast, no spherical morphology was observed in Ti3C2Tx/PmPD composites, which further indicated the uniform polymerization of mPD on Ti3C2Tx surface.

In the contact angle experiments (Figure 1k right), the water droplet was immediately absorbed by Ti3C2Tx within ~1 s, which indicated that Ti3C2Tx had high hydrophilicity. It is commonly known that high hydrophilicity of Ti3C2Tx provides a good dispersion, which makes the adsorbent difficult to separate after treating pollutants [38]. In contrast, the contact angle of Ti3C2Tx/PmPD increased to ~59° (Figure 1k left), which meant that the hydrophobicity of Ti3C2Tx/PmPD was improved. The improvement of hydrophobicity was helpful to enhance the separation and recycling ability of Ti3C2Tx/PmPD. N2 adsorption-desorption isotherms and calculated parameters were displayed in Figure 1l and Table 1, respectively. The specific surface areas of Ti3C2Tx and PmPD were 10.42 and 2.44 m2 g−1, respectively. The unfavorable specific surface area was probably caused by serious restacking [30]. Nevertheless, the specific surface areas of Ti3C2Tx/PmPD-X were far beyond that of Ti3C2Tx and PmPD. In addition, the specific surface area of Ti3C2Tx/PmPD-5/1 was five and 23 times that of Ti3C2Tx and PmPD, respectively. The improvement of specific surface area may be due to the expansion of Ti3C2Tx interlayer spacing and the inhibition of the stacking degree, which was consistent with the XRD results.

Table 1

The specific surface area, pore volume, and average pore diameter parameters of Table 2. PmPD and Ti3C2Tx.

Composites	SBET (m2 g−1)	Pore Volume (cm−3 g−1)	Average Pore Diameter (nm)
Ti3C2Tx	10.42	0.14	27.25
Ti3C2Tx/PmPD-2/1	43.74	0.17	7.86
Ti3C2Tx/PmPD-5/1	55.93	0.18	6.34
Ti3C2Tx/PmPD-10/1	38.99	0.11	5.40
PmPD	2.44	0.029	23.54

Raman spectra were recorded to further study the structure characteristics of as-obtained composites. As can be seen from Figure 2a, the modes of Ti3C2Tx at 200 and 718 cm−1 belonged to the vibrations of Ti and C, respectively. Moreover, the modes at 283, 375, and 618 cm−1 belonged to the vibrations of Ti [39]. With the enrichment of mPD, the main peaks of PmPD at 607, 1356 (quinoid imine), and 1567 cm−1 (benzenoid amine) appeared on intermediate Ti3C2Tx/mPD [40]. After in situ polymerization of PmPD, the Ti3C2Tx peaks of Ti3C2Tx/PmPD absolutely disappeared. The Raman spectrogram of Ti3C2Tx/PmPD had two strong peaks similar to that of PmPD at ~1355 and ~1558 cm−1 [41], indicating the strong interaction between PmPD and Ti3C2Tx.

Figure 2

(a) Raman spectra of Ti3C2Tx, PmPD, and Ti3C2Tx/PmPD. High-resolution spectra of Ti 2p (b) and C 1s (c). (d) Zeta potentials of Ti3C2Tx/PmPD and Ti3C2Tx.

The chemical composition of Ti3C2Tx/PmPD were further investigated by XPS technology, and the high-resolution spectra of Ti 2p and C 1s are shown in Figure 2b,c, respectively. Furthermore, the peaks of Ti 2p3/2 at 455.0, 456.2, 457.5, and 458.8 eV originated from Ti-C, Ti(II), Ti(III), and Ti-O bonds, respectively. The peaks of Ti 2p1/2 at 461.2, 462.2, 463.0, and 464.3 eV were attributed to Ti-C, Ti(II), Ti(III), and Ti-O bonds, respectively [39,42]. The appearance of Ti(III) revealed that Ti3C2Tx was partially oxidized in the process of polymerization. The seven components centered of C 1s core level at 281.8, 282.4, 284.2, 284.8, 285.6, 286.3, and 289.3 eV were attributed to C-Ti, Ti-C-O, C=C, C-C, C-N, C-O, as well as O=C-O bonds, respectively [43,44].

Zeta potentials were recorded to study the surface charge property of Ti3C2Tx before and after functionalization. As seen in Figure 2d, Ti3C2Tx had a negative surface charge at a wide range of pH because of its terminating functional groups. However, the Ti3C2Tx/PmPD showed a strongly positive surface charge when the pH value was less than 5. The strongly positive surface probably originated from protonated amino groups (-N+=) formed by attracting H+. Therefore, the conversion of Ti3C2Tx surface charge from negative to positive verified the successful modification. Ti3C2Tx/PmPD with positive surface charge would show improved potential in removing anionic Cr(VI).

The preparation mechanism of Ti3C2Tx/PmPD is illustrated in Figure 3. Firstly, Ti3C2Tx nanosheets were prepared from Ti3AlC2 MAX through the MILD method. In the process of functionalization, mPD monomers were attracted to the surface and interlayers of Ti3C2Tx nanosheets by electrostatic interaction and hydrogen bonding [37], These interactions induced the formation of intermediate Ti3C2Tx/PmPD, namely Ti3C2Tx/mPD. After that, mPD monomers were gradually polymerized on the surface and interlayers of Ti3C2Tx when adding oxidant, where Ti3C2Tx nanosheets served as templates or substrates. Finally, Ti3C2Tx/PmPD composite with a positive surface charge and expanded interlayer spacing was obtained.

Figure 3

Preparation mechanism of Ti3C2Tx/PmPD.

3.2. Adsorption Experiments

Adsorption performance of as-obtained composites was firstly investigated, as seen from Figure S2. The adsorption performance of Ti3C2Tx/PmPD-X was higher than that of PmPD and Ti3C2Tx. Moreover, Ti3C2Tx/PmPD-5/1 owned the maximum removal capacity. The remarkable advantage of Ti3C2Tx/PmPD-X probably originated to the synergistic effects. Therefore, Ti3C2Tx/PmPD-5/1 was chosen as the representative of Ti3C2Tx/PmPD in the following experiments.

3.2.1. Effect of pH

The effect of pH on the removal performance of composites was investigated, as shown in Figure 4a. When decreasing pH value, the removal efficiency of Cr(VI) showed an upward trend. pH = 2 was the optimal condition, when Cr(VI) ions mainly existed in the forms of HCrO4− (93.03 %) and Cr2O72− (6.42%) (Figure 4a inset) [45]. Low pH facilitated the formation of a strongly positive surface charge, and thus further enhanced the removal capacity of composites. In the next experiments, the optimal pH value was set to 2.

Figure 4

(a) Effects of pH (insets, the speciation diagram of Cr(VI) simulated by Visual MINTEQ); (b) Isotherms adsorption fitting; (c) Effect of adsorption time; (d) Pseudo-first-order kinetic model and pseudo-second-order kinetic model fitting.

3.2.2. Adsorption Isotherms

The adsorption isotherms were systematically investigated at different initial Cr(VI) concentrations. As seen in Figure 4b, with the increase of initial concentration, the adsorption capacity of PmPD, Ti3C2Tx and Ti3C2Tx/PmPD increased, and finally reached saturation. The isotherms data were fitted by Langmuir, Freundlich, and Redlich–Peterson models to investigate the adsorption process and potential [46,47].

Figure 4b and Table S1 showed the isotherm parameters of the fitting models. The fitting coefficient of Redlich−Peterson model was higher than that of Langmuir and Freundlich model. Therefore, the adsorption behavior of Cr(VI) on Ti3C2Tx/PmPD was appropriately simulated by the Redlich−Peterson model, indicating the hybrid adsorption process. The maximum theoretical adsorption capacity of Ti3C2Tx/PmPD reached 540.47 mg g−1, exceed that of Ti3C2Tx (137.45 mg g−1) and pure PmPD (384.73 mg g−1). The excellent adsorption capacity of Ti3C2Tx/PmPD also overstepped that of the reported MXene-based composites and other typical adsorbents, as can be seen from Table 2, indicating the brilliant application prospects of Ti3C2Tx/PmPD.

Table 2

Comparison of removal performance of as-obtained Ti3C2Tx/PmPD, MXene-based composites, and other typical adsorbents.

Adsorbents	Qm (mg g−1)	pH	References
PDMDAAC	95.2	2	[48]
carbon nano-onions	23.5	3	[49]
Biochar	45.88	2	[50]
Fe@GA beads	33.9	3	[14]
nZVIRS700-Pd	117.1	3	[13]
Modified MXene	225	6	[51]
MXene	250	2	[52]
Ti3C2Tx/PmPD	540.47	2	this work

3.2.3. Adsorption Kinetics

The effects of contact time on the removal performance of composites were also studied, and the initial Cr(VI) concentration was 100 mg L−1. As seen from Figure 4c, Ti3C2Tx/PmPD exhibited favorable removal performance than PmPD and Ti3C2Tx. The removal efficiency of Ti3C2Tx/PmPD reached 90% within 10 minutes. Moreover, the final removal efficiency was close to 100% within 120 minutes, thereby indicating the excellent adsorption rate. The kinetic data of Ti3C2Tx/PmPD, PmPD, and Ti3C2Tx were fitted by pseudo-first-order and pseudo-second-order adsorption models [50,53]. According to the fitting results (Figure 4d and Table S2), the pseudo-second-order model was suitable for describing the adsorption process, indicating that the adsorption process was mainly involved chemical adsorption.

3.2.4. Adsorption Mechanism

To better understand the improved removal performance of Ti3C2Tx/PmPD, the adsorption mechanism was investigated in detail. FT-IR spectra of Ti3C2Tx/PmPD before and after treating Cr(VI) is shown in Figure 5a. The benzenoid amine peak (~1508 cm−1) of Ti3C2Tx/PmPD-Cr(VI) was obviously decreased, and the quinoid imine peak (~1620 cm−1) was relatively enhanced, which implied that the oxidation state of Ti3C2Tx/PmPD was improved after treating Cr(VI) [54].

Figure 5

(a) FT-IR and (b) XPS survey spectra of Ti3C2Tx/PmPD-Cr(VI) and Ti3C2Tx/PmPD, respectively. XPS high-resolution of (c) Cr2p and (d) N1s.

Furthermore, the chemical compositions of Ti3C2Tx/PmPD-Cr(VI) and Ti3C2Tx/PmPD were determined by XPS to further illustrate the adsorption mechanism. As seen from Figure 5b, there were two peaks of Cr2p on Ti3C2Tx/PmPD-Cr(VI), and the high-resolution spectrum of Cr2p was displayed in Figure 5c. The contributions at ~587.7 and ~577.6 eV originated from Cr(VI), while the contributions at ~586.4 and ~576.6 eV originated from Cr(III) [55]. The appearance of large amounts of Cr(III) (~51.6%) suggested that there was a redox reaction between Cr(VI) and Ti3C2Tx/PmPD. As seen from Figure 5d, N1s peak of Ti3C2Tx/PmPD were split into protonated quinoid imine at ~400.49 eV (19.3%), benzenoid amine at ~399.55 eV (62.1%), and quinoid imine at ~398.77 eV (18.6%), respectively [56]. After the treatment of Cr(VI), the percentage of benzenoid amine of Ti3C2Tx/PmPD-Cr(VI) deceased from 62.1% to 46.9%, and the percentage of the quinoid imine increased from 19.3% to 42.3%. The results implied that there was a conversion of oxidation state from benzenoid amine to quinoid imine resulted from the oxidation of Cr(VI). Moreover, -N+= also occurred by doping positive Cr(III). It was noted that large percentage of benzenoid amines still existed after the treatment of Cr(VI). Hence, in the next adsorption cycle, Cr (VI) would also be converted into Cr (III) by existing benzenoid amines.

Herein, the adsorption mechanism of Ti3C2Tx/PmPD could be reasonably deduced, as shown in Figure 6. Firstly, anionic Cr(VI) was adsorbed onto Ti3C2Tx/PmPD composite. Then, about 51.6% of Cr(VI) were converted to Cr(III) by benzenoid amine. At the same time, benzenoid amine was oxidized to quinoid imine by using Cr(VI). After that, Cr(III) was still adsorbed onto protonated quinoid imine of Ti3C2Tx/PmPD composite through chelation. Hence, the adsorption process involved adsorption, reduction, and chelation interaction.

Figure 6

Cr (VI) adsorption mechanism of Ti3C2Tx/PmPD.

3.2.5. Regeneration

The recycling ability of Ti3C2Tx/PmPD was evaluated through adsorption-desorption experiments. After adsorption of Cr(VI), Ti3C2Tx/PmPD was filtrated, rinsed with DI water, and then treated by NaOH solution (0.5 mol L−1) for the next cycle. As seen from Figure 7, the Cr(VI) removal efficiency still remained at ~90% after five recycle rounds with the initial Cr(VI) concentration of 100 ppm, revealing the favorable recycling performance of Ti3C2Tx/PmPD.

Figure 7

Regeneration of Ti3C2Tx/PmPD.

4. Conclusions

This research developed a simple strategy to functionalize MXenes for efficient removal of Cr(VI). With the aid of PmPD, the surface charge of Ti3C2Tx/PmPD was successfully converted from negative to positive. Furthermore, the interlayer spacing of Ti3C2Tx/PmPD was enlarged from 14.6 to 17.6 Å, and the specific surface area of Ti3C2Tx/PmPD was increased from 10.42 to 55.93 m2 g−1. These improvements indicated that the layer-by-layer restacking was successfully restrain. The maximum Cr(VI) adsorption of Ti3C2Tx/PmPD was 540.47 mg g−1, which was superior to pure PmPD (384.73 mg g−1), Ti3C2Tx (137.45 mg g−1), and the reported MXene-based adsorbents. The excellent performance is attributed to the synergistic effects of Ti3C2Tx MXene and PmPD. The Cr(VI) adsorption mechanism mainly involved reduction, chelation, and electrostatic interaction. This study indicates that the strategy of in situ polymerization and intercalation was feasible and effective, which provides guidance for enhancing the performance of MXenes in the field of anionic pollutants.