Mechanistic and Kinetic Evaluation of Hexavalent Chromium Adsorption Using Polyaniline‐Functionalized Ti 3 C 2 T x MXene Composite

Abstract

In the past few decades, different efforts have been made toward chromium (VI) removal from wastewater. In this study, a Ti₃C₂T_x/PANI composite was synthesized via in situ polymerization from (PANI) and Mxene (Ti₃C₂T_x). The composite was characterized using XRD, FTIR, SEM–EDS, XPS, and Raman spectroscopy. FTIR spectra revealed characteristic functional groups of both Ti₃C₂T_x and PANI, with significant shifts and intensity changes upon Cr(VI) adsorption. EDS and XPS also showed the presence of Cr after adsorption. Raman spectroscopy further verified PANI polymerization and Cr(VI) adsorption. The adsorption performance for Cr(VI) is markedly influenced by the pH level of the solution, with optimal adsorption occurring at an acidic pH. The experimental findings showed good agreement with the Langmuir–Freundlich and Khan isotherm models, while the pseudo‐second‐order and MESO kinetic models best describe the adsorption process. The Ti₃C₂T_x/PANI composite exhibits a high adsorption capacity of 342.5 mg g⁻¹ and good stability, maintaining its adsorption efficiency over seven cycles. The adsorption mechanism involves electrostatic attraction, surface complexation, and physical adsorption. Notably, the developed Ti₃C₂T_x/PANI composite exhibits a considerably higher Cr(VI) adsorption capacity than most previously reported MXene (Ti₃C₂T_x) and polyaniline (PANI)‐based adsorbents. This superior performance is attributed to the synergistic interaction between the conductive polymer PANI and the high surface reactivity of Ti₃C₂T_x layers, which enhances active site accessibility and electron transfer during adsorption. The findings demonstrate the composite’s potential as an efficient and robust adsorbent for Cr(VI) adsorption.

Keywords

adsorption characterization Cr(VI)kinetic studies nanocomposite wastewater

1. Introduction

The expansion of global populations and the pace of industrialization are primarily responsible for the escalation in water contamination levels [1]. Various studies have highlighted the detrimental impacts of water pollution emerging from the direct release of industrial effluents from industries such as textile, food, dye, and paint. These effluents are laden with a mix of different substances, posing a risk to living things, including humans, at a low level [2]. Such pollutants not only harm aquatic organisms but also lead to the degradation of aquatic ecosystems by escalating biological oxygen demand. Widely used in numerous industrial processes, chromium becomes a significant pollutant when discharged into water bodies, potentially leading to Cr(VI) contamination [3]. This contamination poses severe environmental and health risks [4]. To combat these challenges, research has explored various nanoparticles known for their pollutant adsorption capabilities, thanks to their expansive surface area and porosity. Among these are carbon materials [5], zeolites [6], metal oxides ferrous oxide (Fe₃O₂), manganese dioxide (MnO₂), zinc oxide (ZnO), titanium dioxide (TiO₂) [7–10], biochar [11], graphene and its derivatives [12], organometallic frameworks [13], carbon nanotubes (CNTs) [14], conductive polymers polyaniline (PANI), polypyrrole (Ppy), polythiophene (PTh) [15–17], and MXenes [18]. Researchers have shown that these materials, with their unique surface properties and affinity for hexavalent chromium ions, are particularly effective in removing chromium from wastewater. As a type of a two‐dimensional inorganic compound with thin layers of carbonitrides, transition metal nitrides or carbides. MXenes show promise for use in water treatment because they have a lot of surface area, are good at conducting electricity, and are hydrophilic [19–21]. They are identified by the formula M_n+1X_nT_x, where T is the surface termination group (Cl, OH, and F), and X is the number of terminations [22–24]. They are synthesized by etching MAX phases (M_n+1AX_n), where M is the transition metal, A is the group IIIA/IVA element, and X is a C and/or N atom [25]. MXenes are known for having excellent characteristics such as high mechanical strength, excellent electrical conductivity, and broad hydrophilicity [26]. PANI is known for its nontoxicity, ease of synthesis, environmental stability, high conductivity, and cost‐effectiveness, making it suitable for various applications, including sensors, batteries, and anticorrosion coatings. Because it has many functional groups, a large surface area, and a variety of adsorption abilities, PANI is a good candidate for reducing agents. This is because it can be in a variety of oxidation states. However, the challenge of inadequate regeneration and recycling capabilities in bulk PANI necessitates the development of PANI‐based materials that are efficient in adsorbing Cr(VI) and capable of regeneration and reuse [27, 28]. Consequently, material composites combining MXenes and conductive polymers have emerged as potent solutions for water purification, leveraging their unique physicochemical properties [29]. This interest in composite materials stems from their innovative designs and potential to address water contamination effectively [30].

In this study, the Ti₃C₂T_x/PANI nanocomposite was synthesized via an intercalation of PANI and Mxene and then used for Cr(VI) absorption from the water environment. Characterization, synthesis, and batch adsorption investigations of the materials are all part of the systematic inquiry that makes up this experimental approach. The structural and compositional properties of the combined material have been elucidated through in‐depth physicochemical studies using techniques such as X‐ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy–energy‐dispersive spectroscopy (SEM–EDS), X‐ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Although various MXene‐ (Ti₃C₂T_x) and PANI‐based composites have been explored for heavy metal removal, most suffer from limited adsorption capacity, poor interfacial compatibility, or inadequate regeneration stability. In this study, a Ti₃C₂T_x/PANI composite was designed through a controlled in situ polymerization, ensuring strong interfacial interaction and enhanced electron transport. This significantly improves active site accessibility and redox reactivity toward Cr(VI) ions. The composite exhibits a notably higher adsorption capacity (342.5 mg g⁻¹) and better reusability compared to many previously reported Ti₃C₂T_x‐ and PANI‐based adsorbents.

2. Materials and Methods

Titanium aluminum carbide (Ti₃AlC₂ 99% purity) was used as the MAX phase precursor; dimethyl sulfoxide (DMSO 99.9%), potassium dichromate (K₂Cr₂O₇ 99.99%), hydrogen fluoride HF (99.98%), aniline (99.5%), APS (99.9%), and hydrogen chloride (HCl 35%) were purchased from Sigma‐Aldrich. All the chemicals for this study were analytical research grade.

Mxene (Ti₃C₂T_x) for this work was synthesized with a few modifications from a previous study. Typically, we added 1 g of Ti₃AlC₂ powders, pinch by pinch, to a 60 mL HF (35%) in a 100 mL polypropylene (PP) beaker, stirred continuously for 48 h, and centrifuged at approximately 8000 rpm for 10 min. The HF etching process was performed in a well‐ventilated fume hood using appropriate personal protective equipment including acid‐resistant gloves, face shield, and lab coat. HF waste was disposed following standard laboratory safety protocols. Then, the powder (Ti₃C₂T_x) underwent freeze‐drying for 24 h. The Ti₃C₂T_x product obtained through centrifugation was dispersed and subjected to ultrasonic treatment in DMSO to produce individual sheets of Ti₃C₂T_x that are not agglomerated. Finally, we used a freeze dryer to dry the Ti₃C₂T_x powders for 24 h [31]. Figure 1 depicts the exfoliation process of Mxene (Ti₃C₂T_x).

Figure 1

Schematics of the Ti₃C₂T_x synthesis: (a) Ti₃AlC₂ MAX phase; (b) exfoliated Ti₃AlC₂ MAX by HF; and (c) delaminated Ti₃C₂T_x MXene by DMSO.

The composite is produced by the in situ polymerization process, as previously reported [32]. Typically, 0.2 g of Ti₃C₂T_x powder was evenly mixed with a solution of 30 mL of 1 M HCl. After that, the mixture endured ultrasonic dispersion for 1 h in order to achieve a homogeneous solution. Subsequently, a volume of 100 μL of distilled aniline (ANI) was introduced into the previously indicated solution and subsequently dispersed with the application of ultrasonication for 1 hour. Afterward, ammonium persulfate solution (0.335 g APS in 30 mL of 1 M HCl) was prepared. Then, the solution was added to the Ti₃C₂T_x suspension Figure 2. The mixture was immersed in a container of ice and stirred vigorously at a temperature ranging from 5°C to 7°C for 6 h. Lastly, the Ti₃C₂T_x/PANI composite powder was obtained by freeze‐drying.

Figure 2

In situ polymerization of the Ti₃C₂T_x/PANI composite.

Mxene (Ti₃C₂T_x), Ti₃C₂Tx/PANI, and Cr(VI)@Ti₃C₂Tx/PANI were characterized by using XRD (Bruker D8 Advance x‐ray) with Cu Kα radiation (λ = 1.541 A°) to study the crystalline nature of each material. Functional groups of materials were studied using ATR mode FTIR (Bruker Tensor II) at a 4 cm⁻¹ resolution in the wavenumber range of 4000–450 cm⁻¹ region. The electronic structure of elements for a comprehensive understanding of their chemical surroundings and their bonding characteristics were examined by XPS (PHI 5000 VersaProbe III). The morphology was studied by using SEM–EDX (Zeiss ULTRA 55 FESEM). Raman spectroscopy (STR‐300; Make‐Seki Technotron cropy, Japan) for phonon vibrations both before and after adsorption studies was used.

The synthesized Ti₃C₂T_x/PANI composite was employed for the adsorption of Cr(VI) from simulated wastewater at room temperature. Adsorption experiments were conducted using the batch adsorption method in Erlenmeyer flasks. A 10 mM Cr(VI) stock solution was prepared in a 1000 mL flask. To investigate the effect of initial ion concentration, varying concentrations (10–50 ppm) were mixed with 50 mg of Ti₃C₂T_x/PANI and agitated in a shaking incubator at 200 rpm for 2 h. Similar experiments were performed for pH and adsorbent dosage optimization studies. After each experiment, the solutions were filtered using Whatman No. 1 filter paper. The adsorption capacity was determined by using equation (1). All experiments were conducted at room temperature

\begin{matrix} q = (\frac{C_{O} - C}{m}) V \end{matrix}

where C and C_O are the final and initial concentrations of Cr (VI) solution, respectively, V is the volume, and m represents the mass of the T₃C₂T_x/PANI composite.

To comprehensively evaluate the adsorption mechanism and avoid bias toward a single theoretical framework, the experimental data were fitted to multiple isotherm and kinetic models. The fitting of the selected models was evaluated using R². Five adsorption models (Avrami, intraparticle diffusion (IPD), pseudo‐first‐order (PFO), pseudo‐second‐order (PSO), mixed first‐ and second‐order (MFSO) were used to study the adsorption kinetics. Nine isotherm models were used to study the experimental data (Redlich–Peterson, Langmuir, Langmuir–Freundlich, Freundlich, Fritz–Schlunder, Khan, Sips, Baudu, and Toth).

3. Results and Discussion

3.1. Material Characterization

The peaks shown in Figure 3 (a) for pristine Ti₃C₂T_x correspond to the crystallographic planes (002), (004), (006), (0010), (0012), and (110), with peak positions at 9.0, 18.3, 27.8, 35.9, 41.7, and 60.5, respectively. These peaks correspond to the distinctive features of Ti₃C₂T_x MXene, which aligns with previous research findings. There is an observed phenomenon that shows a wider peak and lower intensity [31]. The diffraction pattern of Ti₃C₂T_x/PANI is also depicted in Figure 3(b). The diffraction pattern of Ti₃C₂T_x/PANI exhibits a distinct weak peak at 25.2°, corresponding to the crystal faces (200) of PANI. This confirms the successful synthesis of the Ti₃C₂T_x/PANI composite, with aniline being polymerized onto the Ti₃C₂T_x surface. The outcome of our work aligns with the findings of the prior study [33]. The polymerization of aniline on Ti₃C₂T_x resulted in the disappearance of diffraction peaks at 9.0 and 18.3, which can be attributed to the amorphous nature of PANI, as depicted in Figure 3(b). The XRD diffraction pattern was also collected after the adsorption of Cr(VI), as shown in Figure 3(c). It divulges that the peak related to PANI is disappeared, which may be due to Cr(VI) adsorption.

Figure 3

XRD of (a) Ti₃C₂T_x, (b) Ti₃C₂T_x/PANI, and (c) Ti₃C₂T_x/PANI after Cr(VI) adsorption.

The FTIR spectra of Ti₃C₂T_x are shown in Figure 4(a). The peaks seen at 882 and 618 cm⁻¹ are likely a result of the deformation vibration of the Ti–O–Ti linkage and Ti–O bonding, respectively [34]. The peak at 1102 cm⁻¹ corresponds to the stretching vibration of the C–Cl bond. The spectra depicted in Figure 4(b) reveal that the peak observed at 1581 cm⁻¹ corresponds to the C–C stretching of quinoid rings, whereas the peak at 1484 cm⁻¹ corresponds to the C=C stretching of benzenoid rings in the Ti₃C₂Tx/PANI composite. The peak detected at 1147 cm⁻¹ corresponds to the vibration mode of N = Q = N, which is an electronic band associated with the quinoid ring. This finding is consistent with previous reports [35]. The band observed at 1296 cm⁻¹ represent the C−N stretching [36]. The FTIR spectra analysis performed after the adsorption of Cr(VI) (Figure 4(c)) shows some changes on the intensity and the position of peaks. The intensity of the benzoid groups of the Ti₃C₂T_x/PANI has increased after Cr(VI) adsorption, as PANI loses electron from the quinoid group to the Cr(VI) and reduces it to Cr(III) [37]. The prominent peak of Ti₃C₂T_x/PANI at 1147 cm⁻¹ undergoes a shift to 1149 cm⁻¹ with reduced intensity following the adsorption of Cr(VI). A new peak emerges at 1310 cm⁻¹ upon adsorption, which was absent in the Ti₃C₂T_x/PANI composite. The observed alterations in the intensity and locations of peaks provide evidence that the presence of functional groups on our composite facilitates the formation of complexes with Cr(VI) [38].

Figure 4

FTIR spectra of (a) Ti₃C₂T_x, (b) Ti₃C₂T_x/PANI, and (c) Ti₃C₂T_x/PANI after Cr(VI) adsorption, where B and Q rings are benzenoid and quinoid rings.

The morphology and surface elemental composition of Ti₃C₂T_x/PANI and Cr(VI)@Ti₃C₂T_x/PANI are shown in Figure 5; the composite shows a porous structure [39]. The EDS analysis of the Ti₃C₂T_x/PANI composite revealed the presence of titanium (Ti), carbon (C), oxygen (O), fluorine (F), and nitrogen (N) in the material, confirming the successful incorporation of PANI onto the Ti₃C₂T_x surface. Specifically, the detection of nitrogen (N) is a clear indicator that PANI, which contains nitrogen in its structure, is chemically bound to the Ti₃C₂T_x surface, as shown in Figure 4(a). The presence of these elements is consistent with the formation of the Ti₃C₂T_x/PANI composite, with the fluorine (F) and oxygen (O) likely originating from the surface functional groups on Ti₃C₂T_x and carbon (C) from both Ti₃C₂T_x and PANI. After the adsorption of Cr(VI), the EDS spectra of the Ti₃C₂T_x/PANI composite (Figure 5(b)) revealed the appearance of chromium alongside the previously detected elements, demonstrating that Cr(VI) was successfully adsorbed onto the composite. The incorporation of Cr was observed in the form of Cr(VI), further confirming that the Ti₃C₂T_x/PANI composite material actively engages in the adsorption process and binds Cr ions onto its surface.

Figure 5

SEM–EDS of (a) Ti₃C₂Tx/PANI and (b) Cr(VI)@Ti₃C₂T_x/PANI.

Additionally, analyzing the XPS spectrum allows for the identification and characterization of different species and their corresponding chemical states of Ti₃C₂T_x, Ti₃C₂T_x/PANI, and also after Cr adsorption. Figure 6 depicts the XPS scan spectrum for Ti₃C₂T_x, Ti₃C₂T_x/PANI, and after Cr(VI) adsorption, which shows that these materials are made from (Ti, C, O, and F), (Ti, C, O, F, and N), and (Ti, C, O, F, N, and Cr), respectively. The XPS of Ti 2p, Ti 2p_1/2, and Ti 2s_1/2 in Figure 6(b) can be fitted with binding energies at 460.13, 514.13, and 561.49 eV, respectively. The F 1s p_3/2 peak centers at 686.03 eV, while the peaks at 286.88 and 533.05 correspond to C 1s and O, respectively. The peak centered at 401.13 eV, which corresponds to N 1s (Figure 6(b)), appeared after in situ polymerization with PANI, which indicates that Ti₃C₂T_x/PANI is formed successfully. The XPS survey scan of Cr@Ti₃C₂T_x/PANI is given in Figure 6(c), and the peak found at 580 eV is due to Cr (VI) [40]. We also used high‐resolution XPS to identify the oxidation number of Cr species present in Ti₃C₂Tx/PANI. An optimal match was achieved by decomposing the Cr 2p spectra into three distinct peaks by deconvolution in Figure 6(d). Cr (III) 2 p_3/2 and 2 p_1/2 are responsible for the peaks observed at 577.5 and 586.7 eV, respectively. On the other hand, the peaks at 588.2 eV are characteristic of Cr(VI) 2 p_1/2 [41, 42]. The extent of Cr(VI) to Cr(III) reduction was not quantitively determined in this study.

Figure 6

XPS of (a) Ti₃C₂T_x, (b) Ti₃C₂T_x/PANI, (c) Cr@Ti₃C₂T_x/PANI, and (d) deconvoluted Cr.

(b)

(c)

(d)

From the Raman spectrum, the peaks found at 206 and 711 cm⁻¹ in Figure 7(a) correspond to the A_1g mode, which represents the out‐of‐plane vibrations of Ti and C, respectively. The peaks observed at 279, 409, and 616 cm⁻¹ are attributed to E_g in the plane (shear) vibration of Ti, C, and terminal group atoms, respectively [43]. The peaks observed at 1162 and 1192 cm⁻¹ correspond to the C–H stretching vibration of the B ring; 1492 cm⁻¹ is due to the C–N stretching of the Q ring; and 1593 cm⁻¹ corresponds to the C–C stretching of the B ring in Ti₃C₂T_x/PANI, which indicates that PANI has undergone polymerization on the surface of Ti₃C₂T_x [44]. A peak is around 580 cm⁻¹, confirming the adsorption of Cr(IV) by the nanocomposite (Figure 7(b)). Additionally, the adsorption of Cr(VI) resulted in a reduction in intensity and a displacement in the peak position, as shown in Figure 7(b). Ti₃C₂Tx/PANI after adsorption exhibited a small peak about 911 cm⁻¹ as shown in Figure 7(b), which is the distinctive peak associated with Cr(VI) [45, 46]. “B” is benzenoid, and “Q” is quinoid ring.

Figure 7

Raman spectra of (a) Ti₃C₂T_x/PANI and (b) Ti₃C₂T_x/PANI after Cr(VI) adsorption.

3.2. Optimization Study

pH significantly influences the adsorption process [47]. In the case of Cr(VI), different anionic forms exist, including Cr₂O₇ ²⁻, HCr₂O₇ ⁻, CrO₄ ²⁻, etc. Their form depends on their pH in aqueous solutions [48]. To see how pH changes the adsorption process, we did several studies with the same temperature, initial ion concentration (10 mg/L), and Ti₃C₂T_x/PANI mixture dosage (10 mg/L). We found a hexavalent chromium ion in the form of H₂CrO₄ at pH values less than 2 [49]. As pH increases to 2–9, it is predominantly detected in the form of Cr₂O₇ ²⁻ and HCrO₄ ⁻. Beyond the pH of 7, chromium (VI) transforms into the CrO₄ ²⁻ form [50]. We determined point of zero charge (PZc) to further understand the behavior of the Ti₃C₂Tx/PANI composite, and found it to be 6.46, as shown in Figure 8(a). These findings support the notion that these composite surfaces exhibit positive charges at pH values below pHPZc. The existence of Cr(VI) in different forms, like HCrO₄ ⁻ and Cr₂O₇ ²⁻, has its own impact on their adsorbed capacity. Notably, a decrease in adsorption efficiency was seen at pH > 2 due to the existence of HCrO₄ ⁻. The strong competition between H₂CrO₄ and the proton for an adsorption site is the primary cause of this decrease. Acidic solutions also result in functional groups being protonated, which leads to strong electrostatic interaction between the Ti₃C₂T_x/PANI nanocomposite and Cr₂O₇ ²⁻ ions. Conversely, at higher pH levels (between 4.0 and 9.0), the dominant form of chromium is CrO₄ [2–51]. At these levels, an excessive presence of the hydroxide ion (OH⁻) competes with the CrO₄ ²⁻ ion for adsorption. It is found that the adsorption efficiency decreases as the pH increases [52]. The maximum removal efficiency obtained at pH = 2, but it significantly decreased as pH increased Figure 8(b).

Figure 8

Optimization studies of (a) pHPZc, (b) pH, (c) dosage, and (d) initial concentration.

(b)

(c)

(d)

Additionally, we conducted a study to evaluate the impact of the Ti₃C₂T_x/PANI dosage on the removal of hexavalent chromium ions from wastewater, using adsorbent dosages ranging from 10 to 50 mg/L as depicted in Figure 8(c). An increase in the dose of Ti₃C₂T_x/PANI resulted in an increase in the removal efficiency of chromium adsorption. Elevated composite concentrations result in higher doses and an increase in the total surface area [53]. The increased surface area of Ti₃C₂T_x/PANI increases the removal efficiency. Ti₃C₂T_x/PANI has adsorption sites that promote the surface binding of the Cr(VI) ion. At the same time, adsorption capacity decreased because of the presence of excessive adsorption sites, which are left unsaturated.

Effect of the initial ion concentration was studied at pH = 2 for a time of 480 min and a dosage of 10 mg/L at ambient temperature. Figure 8(d) displays the findings of an empirical study examining the impact of varying concentrations of initial ions (10–50 mg/L). As the starting concentration of Cr(VI) went up, the removal efficiency increased from 66.6 mg g⁻¹ to 228.56 mg/L. The adsorption capacity reached its maximum before the concentration of Cr(VI) exceeded 50 mg/L. At the same time, the percentage removal decreased due to surface saturation [54]. Due to higher concentration of Cr(VI), and saturated adsorbents, the removal efficiency decreased [55].

3.3. Kinetic Studies

To investigate the adsorption kinetics, PFO, PSO, MFSO, IPD, and Avrami models were studied as shown in Figure 9 for hexavalent chromium adsorption at five initial concentrations over time at pH = 2 and 0.15 g/L of dosage. As the initial concentrations of hexavalent chromium increased, the maximum adsorption capacity also increased. The PSO and MESO models are found to be better to describe the Cr(VI)@Ti₃C₂T_x/PANI system. For PSO, the experimental values and correlation coefficients (R²) are as high as 0.990, 0.995, 0.984, 0.985, and 0.976, and for MFSO, they are 0.991, 0.995, 0.984, 0.985, and 0.976. These values correspond to Cr(VI) initial concentrations of 10, 20, 30, 40, and 50 mg/L, respectively. Followed by PFO and MFSO, the Avrami model had a better R² = 0.974, 0.982, 0.948, 0.954, and 0.942 for Cr(VI) initial concentrations of 10, 20, 30, 40, and 50 mg/L, respectively, as shown in Table 1. However, all the values of R² are greater than 0.90; IPD model is not suitable because the estimated adsorption capacities do not align with the experimental data. Therefore, chemisorption is the mechanism by which Cr(VI) is adsorbed onto Ti₃C₂T_x/PANI.

Figure 9

Fitting of the experimental data with kinetic model results of Cr(VI) adsorption onto Ti₃C₂T_x/PANI at initial concentrations (a) 10, (b) 20, (c) 30, (d) 40, and (e) 50 mg/L.

(b)

(c)

(d)

(e)

Table 1

Variables used in the kinetic models by using the Ti₃C₂T_x/PANI nanocomposite.

Model	Parameter	C_o (mg/L)
Model	Parameter	10	20	30	40	50
	q_exp (mg g⁻¹)	66.6	109.7	150.3	190.9	228.6
PFO q_t = q_e (1 − exp(−k₁t))	k₁ (min⁻¹)	0.017	0.023	0.021	0.019	0.018
	q_e (mg g⁻¹)	64.21	104.53	140.32	178.12	216.98
	R² [−]	0.973	0.982	0.948	0.954	0.942
PSO q_t = $(k_{2} q_{e}^{2} t) / (1 + k_{2} q_{e} t)$	k₂ [g mg⁻¹ min]	0.0003	0.0003	0.0002	0.0001	0.0001
	q_e (mg g⁻¹)	73.64	116.95	158.42	202.64	249.06
	R² [−]	0.990	0.995	0.984	0.985	0.976
MFSO q_t = q_e (1 − exp(−kt)/1 − f₂exp (−kt))	K [mg g⁻¹ min⁻¹]	0.0005	0.0008	0.0003	0.0003	0.0003
	q_e (mg g⁻¹)	72.72	115.37	157.55	201.21	246.94
	f₂ [−]	0.974	0.971	0.989	0.989	0.987
	R² [−]	0.991	0.995	0.984	0.985	0.976
Avrami q_t = q_e [1 − exp(−k_av t) ^nav]	q_e (mg g⁻¹)	64.21	104.53	140.32	178.12	216.98
	k_av (min⁻¹)	0.175	0.203	0.196	0.189	0.178
	n_av [−]	0.097	0.113	0.109	0.105	0.099
	R² [−]	0.974	0.982	0.948	0.954	0.942
IPD q_t = k_ip $\sqrt{t}$ + c_ip	k_ip [mg g⁻¹ min^1/2]	2.91	4.49	6.22	7.99	9.91
	c_ip (mg g⁻¹)	11.97	26.24	32.13991	37.81	40.71
	R² [−]	0.916	0.857	0.903	0.907	0.920

Note: where q_t and q_e are the adsorption capacity at time t and equilibrium (mg g⁻¹), respectively. The IPD, PFO, PSO, and Avrami models’ constants are c_ip (mg g⁻¹), k₁ (min⁻¹), k₂ (g mg⁻¹ min⁻¹), k (mg g ⁻¹min⁻¹), and k_av (min⁻¹), respectively. The MFSO and IPD coefficients are f₂ (−) and k_ip (mg g⁻¹ min^−1(1/2), and the Avrami component is n_av (−).

3.4. Modeling of Adsorption Isotherms

The isotherms of adsorption are essential for studying the process and affinity of Ti₃C₂T_x/PANI toward hexavalent chromium ions. Nine models were used to accurately match the experimental results of the Ti₃C₂T_x/PANI nanocomposite’s adsorption of the hexavalent chromium ion at a temperature of 298 K. These results are represented as shown in Figure 10 and Table 2.

Figure 10

Different isotherm models for Cr(VI) adsorption by Ti₃C₂T_x/PANI at a temperature of 298 K.

Table 2

The variables in the adsorption isotherm model for Cr(VI) by Ti₃C₂T_x/PANI.

Model	Parameter	Value	Model	Parameter	Value
Langmuir q_e = q_max(K_L Ce)/(1 + K_L Ce )	q_max	342.5	Freundlich q_e = K_f $C_{e}^{1 / n}$	1/n	0.251
	K_L	0.117		K_F	101.4
	R ²	0.868		R ²	0.912
Langmuir–Freundlich q_e = $(q_{\max} {(K_{L F} C_{e})}^{M L F}) / (1 + {(K_{L F} C_{e})}^{M L F})$	q _MLF	13579	Sips q_e = $(q_{\max} K_{s} {(C_{e})}^{1 / n s}) / (1 + K_{s} {(C_{e})}^{1 / n s})$	q_max	32071
	K _LF	3.4E‐09		K_s	0.003
	M _LF	0.250		1/n	0.250
	R ²	0.911		R ²	0.911
Redlich–Peterson q_e = $(K_{R} C_{e}) / ({1 + a}_{R} C_{e}^{β R})$	K_R	5.6E06	Toth q_e = $(K_{T} C_{e}) / {(a_{T} + C_{e}^{Z})}^{1 / Z}$	K_t	0.033
	a_R	76958		a_T	0.006
	β	0.611		z	0.002
	R ²	0.904		R ²	0.751
Khan q_e = $(q_{\max} b_{K} C_{e}) / {({1 + b}_{K} C_{e})}^{a K}$	q_max	5.94	Fritz–Schlunder q_e = $(q_{\max} K_{1} C_{e}^{m 1}) / (1 + K_{2} C_{e}^{m 2})$	q_mFSS	290.0
	b_K	79138		K₁	0.350
	a_K	0.748		K₂	0.002
	R ²	0.912		m₁	0.251
Baudu q_e = $(q_{\max} b o C_{e}^{1 + x + y}) / (1 + b o C_{e}^{1 + x})$	q_m	342.5		m₂	0.002
	b₀	0.117		R ²	0.912
	x	0
	y	0
	R ²	0.868

The variables in the equation are as follows: q_e represents the adsorption capacity at equilibrium (mg g⁻¹). C_e stands for the concentration of adsorbate (Cr(VI)), and q_max is the maximum adsorption capacity (mg g⁻¹). K_L (L mg⁻¹) is the Langmuir isotherm. K_LF is the equilibrium constant for heterogeneous solids. K_F (L mg⁻¹) is the Freundlich isotherm coefficient, and n is a constant. There are several numbers that describe different types of things in a system: m₁, m₂, K₁, and K₂; k_s is the Sips isotherm constant; ns is the Sips isotherm model exponent (also called the heterogeneity factor); K_R and a_R are the Redlich–Peterson constants; β_R is an exponent that ranges from 0 to 1; K_T (mg g⁻¹) is a constant; a_T (mg/L) is the Toth constant; and z is the degree of heterogeneity of the adsorption systems.

The tendency of Ti₃C₂Tx/PANI to bind hexavalent chromium depends a lot on how much of it is present at the start (Figures 8(a), 8(b), 8(c), 8(d)). The Ti₃C₂T_x/PANI nanocomposite can adsorb 66.6, 109.7, 150.3, 190.9, and 228.6 mg g⁻¹ of Cr(VI) ions when the levels of Cr(VI) ions are 10, 20, 30, 40, and 50 mg/L, respectively. With increasing concentrations, the removal efficiencies decrease by 99.9%, 82.27%, 75.14%, 71.59%, and 68.57%, respectively. The fitted nonlinear Freundlich and Redlich–Peterson models have correlation coefficients (R²) of 0.912 and 0.904, respectively. The resulting values surpass the 0.868 values produced by the Langmuir and Baudu models. The results suggest that the adsorption system of Cr(VI)@Ti₃C₂Tx/PANI aligns more closely with the Freundlich model. Additionally, the Freundlich model’s coefficient (1/n) has a value of 0.251, a value below 1. This suggests that the process of adsorption is favorable. This agrees with the kinetic adsorption results. Although the correlation coefficients of the Langmuir–Freundlich, Khan, Sips, and Fritz–Schlunder models are high (0.911, 0.912, 0.911, and 0.912, respectively), the predicted data according to these models did not match the experimental one. Also, the Toth model did not explain the Cr(VI)@Ti₃C₂Tx/PANI adsorption system. The calculated data are very different from the experimental data, and the R² value is only 0.751.

4. Comparison Studies

The data in Table 3 show the adsorption capacity of the Ti₃C₂T_x/PANI composite adsorbent (this work) compared to other adsorbents from the literature, using Ti₃C₂T_x‐ and PANI‐based composites for Cr(VI). The capacity for adsorption of the adsorbents is determined by various elements, such as pH, time, temperature, dosage, initial ion concentration, etc.

Table 3

Comparative analysis of the adsorption capacity for chromium (VI) by PANI‐ and Ti₃C₂T_x‐based composites based.

Adsorbent	pH	Initial ion con. (mg/L)	Adsorption capacity (mg g⁻¹)	Dose (g/L)	Temperature (K)	Ref.
Ti₃C₂T_x@GMA/PANI		1000	303.3		298	[56]
CTS‐EDTA@Ti₃C₂‐OH	2	45	124.76	1.0	298	[57]
Ti₃C₂@IMIZ	2	19	119.5	0.01	298	[58]
PANI‐PVA	2	5	33.01	2	298	[59]
Ti₃C₂T_x	3	100	104	0.1	303	[60]
PANI@MIBWS	4	500	162.02	1.0	308	[61]
PANI‐Ppy‐PA	3	20	222.716	0.2	298	[16]
PANI/MIL100(Fe)	2	50	72.37	1.0	298	[62]
PANI‐PP filter	6	5	113.8	0.08	298	[63]
ZnO@PANI NRs	2	50	310.47	0.13	318	[64]
CuO/PANI	6	25	295.74	25	298	[57]
Ti ₃ C ₂ T _x /PANI	2	10	342.5	0.1	298	This work

Note: The bold values are the maximum adsorption capacity found in the current work.

5. Reusability Study

The reusability of an adsorbent is a key factor in achieving a cost‐effective and sustainable adsorption process. To evaluate the recycling potential of the Ti₃C₂T_x/PANI composite, sequential batch adsorption–desorption experiments were performed. In each cycle, 10 mg of the Ti₃C₂T_x/PANI composite was mixed with a 10 mg L⁻¹ of hexavalent chromium solution and stirred for 8 h. The composite was reused for seven consecutive cycles, as illustrated in Figure 11. The first cycle exhibited the good recovery efficiency (88.94%) in an acidic medium, indicating the composite’s excellent capability to adsorb and remove Cr(VI) from wastewater. The adsorbent stability tends to decrease with increasing contact time or repeated use due to the physical or chemical deterioration of the adsorbent surface or structure [65].

Figure 11

Recyclability studies.

6. Adsorption Mechanism

The FTIR, EDS, and XPS spectra confirmed that Cr species is adsorbed onto the surface of the Ti₃C₂T_x/PANI composite. The prominent N = Q = N peak at 1147 cm⁻¹ of FTIR as depicted in Figure 4 indicates the presence of the quinoid ring on the Ti₃C₂T_x/PANI surface. After Cr(VI) uptake, the FTIR peak’s intensity red‐shifted to 1149 cm⁻¹, indicating an interaction between the composite and Cr(VI) [22]. In addition, the EDS analysis of the Ti₃C₂T_x/PANI composite after adsorption showed the presence of Cr (Figure 5(b)). Besides, the XPS spectrum of Cr 2p of the Ti₃C₂T_x/PANI after Cr(VI) sorption clearly revealed the presence of Cr(VI) at 580 eV and the optimal match was achieved by decomposing the Cr 2p spectra into two distinct peaks, with Cr (III) 2p_3/2 and 2 p_1/2 responsible for 577.5 and 586.7 eV, respectively (Figure 6(d)). Based on FTIR, EDS, and XPS results, the Cr(VI) adsorption process by the Ti₃C₂T_x/PANI composite takes place by (i) electrostatic force between the negatively charged Cr(VI) and positively charged H⁺ or ‐NH₂ species, generated from acid solutions and PANI oxidation, (ii) surface complexation/chelation of Cr(VI) and ‐OH, =O, and N–H functional groups, and (iii) physical adsorption of Cr(VI) on the composite surface [58]. The adsorption mechanism is shown in Figure 12.

Figure 12

Adsorption mechanism of Cr (VI) by the Ti₃C₂T_x/PANI composite.

7. Conclusion

Using an in situ polymerization method, a Ti₃C₂T_x/PANI nanocomposite was made so that it could effectively remove Cr(VI) from simulated wastewater. The Ti₃C₂T_x/PANI composite was thoroughly studied using SEM–EDX, XRD, FTIR, Raman spectroscopy, and XPS methods. The material exhibited a significant Cr(VI) removal capability, influenced by parameters such as adsorbent dosage, initial Cr(VI) concentration, and pH. Notably, the interaction time and dosage required in this study were considerably lower than those reported previously for Cr(VI) removal. Under typical environmental conditions, the maximum adsorption capacity reached 342.5 mg g⁻¹ at 2 pH.

The composite also showed excellent regeneration and reuse potential, making it a strong candidate for practical Cr(VI) remediation in aqueous systems. The Ti₃C₂T_x/PANI composite achieves efficient Cr(VI) removal through a combination of electrostatic attraction, surface complexation, and physical adsorption mechanisms. Overall, the higher adsorption capacity and reusability of the Ti₃C₂T_x/PANI composite demonstrate its novelty and efficiency compared to previously reported Cr(VI) adsorbents using Ti₃C₂T_x and PANI, mainly due to the synergistic interaction between the Ti₃C₂T_x layers and the conductive polymer PANI. Furthermore, the solution‐based, low‐temperature synthesis process makes the material amenable to scale‐up, and its rapid adsorption kinetics support potential integration into fixed‐bed adsorption columns, packed‐bed reactors, or point‐of‐use treatment cartridges. The material can be regenerated using mild acidic solutions, enabling multiple adsorption–desorption cycles with minimal performance loss. These features position the Ti₃C₂T_x/PANI composite as a practical and scalable adsorbent for Cr(VI) removal in real wastewater management systems.

Data Availability Statement

Data are available on request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Author Contributions

Hailemariam Assefa: conceptualization, methodology, data curation, and writing–original draft.

Sathiesh Kumar Subramaniam: visualization, investigation, supervision, editing, and revision of manuscript.

Funding

The author(s) received no specific funding for this work.

Footnotes

Acknowledgments

The authors would like to thank the Faculty of Material Science and Engineering, Jimma University, and the Department of Materials Engineering, Indian Institute of Science, Bangalore.

ORCID

Hailemariam Assefa

Sathiesh Kumar Subramaniam

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