Application of Synthesized Vanadium–Titanium Oxide Nanocomposite to Eliminate Rhodamine-B Dye from Aqueous Medium

Abstract

In this study, a V@TiO₂ nanocomposite is examined for its ability to eliminate carcinogenic Rhodamine (Rh-B) dye from an aqueous medium. A simple ultrasonic method was used to produce the nanosorbent. In addition, V@TiO₂ was characterized using various techniques, including XRD, HRTEM, XPS, and FTIR. Batch mode studies were used to study the removal of Rh-B dye. In the presence of pH 9, the V@TiO₂ nanocomposite was able to remove Rh-B dye to its maximum extent. A correlation regression of 0.95 indicated that the Langmuir model was a better fit for dye adsorption. Moreover, the maximum adsorption capacity of the V@TiO₂ nanocomposite was determined to be 158.8 mg/g. According to the thermodynamic parameters, dye adsorption followed a pseudo-first-order model. Based on the results of the study, a V@TiO₂ nanocomposite can be reused for dye removal using ethanol.

1. Introduction

Industrial wastewater contains artificial pigments that constitute an environmental risk; thus, toxic dyes should permanently be removed from the aquatic system [1]. Dumping colored wastewater into receiving waterways has become a severe environmental issue worldwide [2]. The release of wool, rug, pulp, sheet, and textile effluents frequently tint the collecting waters for kilometers in the direction of the source [3]. The color is visually unappealing and inhibits the passage of light into the water, lowering the effectiveness of photosynthesis in aquatic plants and negatively affecting their development [4]. The extensive use of dyes in the fabric, publishing, latex, beauty product, plastic, and leather sectors produces a massive volume of colored wastewater [5,6,7]. Rhodamine B has a comprehensive application, encompassing textiles, food, laser technology, biomarkers, molecular probes, sensitizers, electrochemical-luminescence, and solar panels [8]. rhodamine B (Rh-B) dye is considered a skin-irritating agent, neurotoxic, chronically toxic to aquatic organisms, and carcinogenic to humans. [9]. Thus, purifying water bodies from Rhodamine B is an urgent task to protect the environment and human health. One of the most popular cationic water-soluble organic dyes is Rh-B [10]; it is toxic and lethal to aquatic environments [11]. The numerous procedures for decolorization are categorized as chemical, physical, and microbial techniques [12,13,14]. Physical treatments involve sorption, ozonation, and membrane processes [15]. While chemical procedures comprise oxidation, precipitation/co-precipitation, and photochemical processes, biological techniques include aerobic decomposition, bacterial destruction, and biosorption [16]. Even though these approaches are successful, they have several drawbacks, including increased chemical consumption and sludge formation, and they are expensive [17].

Among all the dye treatment procedures, adsorption is reported to be efficient and economical [18]. Activated carbon is an efficient adsorbent for removing dyes from industrial sewage effluents; nevertheless, its cost limits its usage [19]. Alongside traditional adsorbents, various economical nonconventional adsorbents have been demonstrated to eliminate dyes efficiently [20]. For the removal of dyes, investigations, including the analysis of practical and inexpensive adsorbents produced from available resources, are increasingly relevant [21]. Employing nanotechnology to clean contaminated environments has shown to be advantageous, saving a lot and lowering pollution levels to tolerable levels [22,23,24]. Metallic nanoparticles and nanocomposites are regarded as potential adsorbents with high dye removal capability due to their sensitivity, permeability, and recyclability within each known sorbent [25,26]. Lately, metal-based nanocomposites such as MgO/TeSe, (Y₂O₃)n–ZnO, GO-TiO₂, Fly-Ash@Fe₃O₄, and Fe₂O₃–TiO₂–graphene have been used to eliminate various dyes from water systems [27,28,29,30,31,32,33]. Several proposals for nanoparticle production have involved mechanical, physiochemical, and biological procedures [34,35,36]. Furthermore, nanostructured and composite materials can be employed to capture or destroy dye contamination in an aqueous environment [37,38,39]. V-TiO₂ was utilized as the base material to synthesize triple composites and used mostly as a photocatalyst [40,41,42].

This research aimed to establish a straightforward method for producing V-TiO₂ nanomaterial. The synthesized V-TiO₂ will be analyzed by physical means and introduced as a practical, environmentally friendly, and inexpensive composite to get rid of the coloring dyes from water.

2. Experimental Section

2.1. Fabrication of V@TiO₂ Sorbent

For the V-TiO₂ sorbent, 0.0338 moles of TiO₂ nanoparticles (Sigma Aldrich) were dispersed in 0.12 L isopropanol solvent using an ultrasonic bath for 20.0 min. Then, 0.00165 moles of V₂O₅ were added to the milky TiO₂ solution, and the mixture was sonicated for a further 40.0 min with vigorous stirring (550 rpm). After the mixtures were blended, they were heated for 20 h at 90 °C in an electric dry oven, and the resulting nanoparticles were ground before being calcined. The greenish-white powders were annealed for 2 h at 145 °C.

2.2. Sorbent Characteristics

The structural properties of the as-synthesized sample were investigated by X-ray diffraction (XRD) utilizing a Rigaku diffractometer operated with Cu K radiation (ʎ = 0.15406 nm) at 40 kV and 40 mA. Fourier transform infrared (FT-IR) spectra were acquired with the KBr pellet (400 to 4000 cm⁻¹) on a Thermo Scientific Nicolet 380 Fourier transform spectrometer. Using a JEM-2100 high-resolution transmission electron microscope (TEM), the microscopic characteristics of the materials were studied. X-ray photoelectron spectroscopy (XPS) was utilized to investigate surface chemistry using an RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Al K radiation ($hv=1486.6 \mathrm{eV}$).

2.3. Rh-B Sorption Studies

The batch test protocol was employed to investigate the Rh-B dye adsorption onto the V@TiO₂ nanocomposite. In 0.025 L small bottles, 5 to 100 ppm concentrations of Rh-B dye in distilled water were mixed with 10 mg of the nanocomposite. The mixture solution was constantly agitated for 20 h. Following the establishment of equilibrium with the aqueous phase, the nanocomposite was isolated by filtering, and Rh-B dye concentrations were measured using a UV-vis spectrophotometer. The amount of adsorbed Rh-B dye at any given time (in minutes) and adsorption equilibrium magnitudes Q_t and Q_e (mg/g) were calculated using the same equation (Equation (1)):

$$Q_t=V\left(C_0-C_t\right)/m$$

(1)

where V is the volume of the solution in liters; C₀, C_e, and C_t are the starting concentration, equilibrium concentration, and concentration of the Rh-B dye in solution, respectively, in milligrams per liter; and m is the mass of the adsorbent (g). The contact times data were used to investigate the Rh-B sorption via the pseudo-first-order, and pseudo-second-order models (PSFOM, PSSOM) as expressed in equations 2 and 3, respectively. Also, the sorption control mechanism was studied utilizing the liquid film and the intraparticle diffusion models (IDM and LDM) presented by Equations (4) and (5) [43]:

$$q_t=q_e\left(1-ex{p}^{-K_1·t} \right)$$

(2)

$$q_t=\frac{k_2·q_e^2·t}{1+k_2·q_e·t}$$

(3)

$$\ln \left(1-\mathrm{F}\right)=-{\mathrm{K}}_{\mathrm{LF}}\ast \mathrm{t}$$

(4)

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{K}}_{\mathrm{IP}}{\ast \mathrm{t}}^{\frac{1}{2}}+{\mathrm{C}}_{\mathrm{i}}$$

(5)

where q_e (mg g⁻¹) represents q_t at equilibrium and k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹) are the rate adsorption constants for the PSFO and PSSO models, which have been calculated from the slope and intercept values, respectively. The LFDM and IPDM constants were represented as K_IP (mg g⁻¹ min^−0.5) and K_LF (min⁻¹), and both were computed from their slope values. C_i (mg g⁻¹) is a boundary-layer-thickness factor [44].

2.4. pH Point of Zero Charges Experiment

In each flask, with a pH ranging from 1 to 12, ten milligrams of V@TiO₂ nanocomposite and ten milliliters of a 0.1 mole/L sodium chloride (NaCl) solution were added. A trace amount of hydrochloric acid or sodium hydroxide was incorporated into the solution in order to bring about a change in the pH. In order to achieve equilibrium, these bottles were left on a multi-stirrer at room temperature for exactly one hour; then, the pH levels of the solutions were measured. The point zero charges were found by comparing the initial and final pH values against the original pH graph.

3. Results and Discussions

3.1. XRD Analysis of V@TiO₂ Nanocomposite

X-ray diffraction (XRD) examination was conducted to verify the produced V@TiO₂ nanocomposite. Figure 1 depicts XRD data from generated samples pertaining to the structural properties of the V@TiO₂ nanocomposite. Peaks of phase obtained at $2theta$ = 27.51, 36.19, 44.19, and 54.42 are consistent with rutile phase (110), (101), (111), and (211), respectively. On the other hand, the appearance of diffraction peaks at 41.42° and 56.66° corresponds to orthorhombic phase planes (020) and (012) of V₂O₅, which is consistent with JCPDS Card No. 01-0359 [45]. By means of the Debye–Scherer equation [46], the average crystal size of the V@TiO₂ nanocomposite was determined to be 33.83 nm, with a d-spacing of 3.236 Å.

3.1.1. Morphological Observations

The TEM images of the V@TiO₂ sorbent at different magnifications confirmed the morphology and indicated the elemental distribution, as shown in Figure 2a–c. The semi-spherical form of the TiO₂ nanoparticles is seen clinging to the V₂O₅ nanomaterials, as shown in the TEM pictures. The elemental distribution of V, Ti, and O in the synthesized nanomaterials is characterized by means of EDX spectra (Figure 2d). The EDX analysis sheds light on the stoichiometry of oxygen, vanadium, and titanium, as well as the proportions of these elements and the actual integration of V₂O₅ into the TiO₂ lattice. The transmission electron microscope (TEM) gave an estimate of 30–100 nm for the typical particle size of the sorbent. The value, however, was comparable to the crystallite size determined by XRD.

3.1.2. XPS of V@TiO₂ Nanocomposite

Figure 3a–c depict O1s, Ti 2p, and V2p core-level spectra of the V₂O₅–TiO₂ samples. The O1s peak fit into two Gaussian peaks at 529.8 eV, which arose from the lattice O (Ti–O–Ti) and surface-absorbed hydroxyl groups of Ti–OH (Figure 3a) [47]. Due to the self-orbital coupling effect, the Ti 2p peak was subdivided into Ti 2p3/2 (458.3 eV) and Ti 2p1/2 (464.1 eV), as shown in Figure 3c. They were symmetrical with a normal Gaussian peak shape, indicating that Ti⁴⁺ was predominantly present in the undoped sample. Due to the influence of the O1s satellite peak, only the V 2p3/2 peak was displayed in the current experiment. The V doped in the TiO₂ nanoparticles (Figure 3d) consisted of two chemical states, V⁵⁺ and V⁴⁺, with energies of 514.9 and 527.33 eV, respectively. The literature [48] indicates that V dopants can exist in surficial VO²⁺, surficial V₂O₅ islands, interstitial, and replacement V ions. In light of the XRD and TEM analyses, as well as the fact that V doping can result in the O1s peak shifting, it is plausible to assume that the V ions existed in a substituted form and not as the V₂O₅ isolated phase, as suggested by the literature [49].

Usually, due to the similar radii of V⁴⁺ and Ti⁴⁺ species, V⁴⁺ ions can typically only integrate into the TiO₂ lattice by substituting Ti⁴⁺ ions. Therefore, the Ti-O-V bond, which results in oxygen vacancy and a high electron production capability, is formed by sharing the oxygen atoms of the V⁴⁺ ions in the TiO₂ lattice. Furthermore, the presence of more V⁴⁺ ions leads to the creation of more O₂ radicals, the most powerful oxidizing species in photocatalysis [50,51]. As a result, the existence of V⁴⁺ contributes significantly to the improvement of photocatalytic activity [52].

3.1.3. pHzc of V@TiO₂ Nanocomposite

The pH of the point of zero charge (pHzc) of the V@TiO₂ nanocomposite was evaluated using the pH drift method to completely comprehend the pH effect on the adsorptive removal of dye pollution from an aqueous medium. According to the obtained results, the pHZPC value of the V@TiO₂ nanocomposite was 4.2. At this pH (pHzc = 4.2), the surface charge on the V@TiO₂ nanocomposite is zero, whereas the surface charge is positive at pH values less than 4.2 and negative at pH values greater than the pH_ZPC (Figure 4). The influence of pH on Rh-B dye removal is a significant factor for the adsorption technique, as it modifies not only the active sites on the V@TiO₂ surface of the adsorbent that are capable of Rh-B dye bonding, but also the solubility of Rh-B dye in the aqueous solution.

3.1.4. Surface Characteristics of the V@TiO₂ Nanocomposite

From the surface property investigation, the isotherm and pore size distribution of the manufactured V@TiO₂ nanocomposite are depicted in Figure 5a. The resulting isotherm is of class IV with H1 hysteresis loops, which seem to be characteristic of nanostructured and mesoporous materials [53,54]. Figure 5b shows a BET surface area of 16 m²/g and a pore volume of 0.018 cc/g, along with a pore size distribution with a median of roughly 27.6 nm.

3.2. Adsorption of Rd-B by V@TiO₂ Sorbent

Figure 6a depicts the effect of contact duration on Rh-B dye sorption on V@TiO₂, with experimental q_t values of 138 mg g⁻¹ and adsorption equilibrium points of 4.0 h. It is worth mentioning that nearly 95% of these uptakes occurred during the first 60 min of interaction. In addition, the impact of the concentrations of Rh-B on sorption by V@TiO₂ was tested. Figure 6b illustrates that the q_t value increased proportionally up to 75 mg L⁻¹, after which the inflation occurred, indicating the suitability of a 1:4 sorbent solution ratio up to 75 mg L⁻¹. Typically, q_t values of 133.4 mg g⁻¹ from 100 mg L⁻¹ Rh-B solutions imply the applicability of the V@TiO₂ for treating industrial effluents with high pollutant concentrations. Additionally, the semi-complete Rh-B dye removal by V@TiO₂ from the 10 mg L⁻¹ solutions demonstrates its effectiveness in treating contaminated water resources. Furthermore, the decrease of qt values from the same concentrations as the temperature increased showed the exothermic nature of removing Rh-B dye by a V@TiO₂ sorbent.

Furthermore, the pH impact on Rh-B adsorption on V@TiO₂ was evaluated (Figure 6c). The obtained results show the suitability of mild alkaline media for removing Rh-B dye. The decrease in sorption capacity for V@TiO₂ in strongly acidic media is probably due to the protonation of oxygen atoms in the nanocomposite and/or turning part of the oxides into soluble salts. On the other hand, the hydroxyl groups in strong alkaline media may compete with Rh-B dye for sorbent sites and/or repulse it away from the V@TiO₂ surface [55,56].

3.2.1. Adsorption Kinetics

The linear regression plots of the PSFOM, PSSOM, IDM, and LDM for Rh-B dye sorption on V@TiO₂ are illustrated in Figure 7. The k₁, k₂, K_IDM, and K_LDM values gathered in

Table 1. The kinetic parameters for Rh-B adsorption on the V@TiO₂ nanocomposite from 50 mg L⁻¹ pollutant solution.

Adsorption Kinetic
Adsorption Rate Order
qe exp. (mg g−1)	PSFO			PSSO
	qe cal. (mg g−1)	R2	k1	qe cal. (mg g−1)	R2	k2
88.367	88.7	0.947	0.114	90.000	0.927	0.002
Adsorption mechanism
IPDM				LFDM
	KIP (mg g−1 min0.5)	C (mg g−1)	R2	KLF (min−1)	R2
Stage 1	3.4950	55.38	0.9959	0.015	0.956
Stage 2	0.3369	80.256	0.9306

were computed utilizing the extracted regression parameters (slope and intercept) [43,57]. The obtained results revealed that Rh-B sorption on V@TiO₂ fit the PSFOM model, which may explain their relatively short-time equilibrium. Further, the investigation of the rate-control step showed that the intraparticle diffusion step controlled the adsorption of Rh-B onto the V@TiO₂ surface. These findings indicate that Rh-B dye has a higher affinity toward the V@TiO₂ surface and imply fast pore-diffusion during the Rh-B dye removal by V@TiO₂ [58].

3.2.2. Adsorption Isotherms

The Langmuir and Freundlich models (LIM and FIM) were the most used isotherm models for describing adsorption processes. Both linearized models (Equations (6) and (7)) were utilized to analyze Rh-B sorption on synthesized V@TiO₂.

$$q_e=\left(\frac{K_l{q}_m{C}_e}{1+q_m{C}_e }\right)$$

(6)

$$q_e=K_F·C_e^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$n$}\right.}$$

(7)

In these equations, Ce (mg L⁻¹) is the equilibrium solution concentration, q_m is the computed maximum sorption capacity, 1/n is the Freundlich adsorption intensity, and KL and KF are the LIM and FIM constants, respectively [59]. The linear fits of LIM and FIM studies of Rh-B sorption on synthesized V@TiO₂ are shown in Figure 8. Better fitting to LIM was observed in the data presented in

Table 2. Removal parameters of the V@TiO₂ sorbent compared with the diverse Rh-B dye adsorbent nanomaterials.

Nanomaterials	Adsorption Capacity (mg g−1)	References
Halloysite HU	8.37	[63]
alpha alumina (α-Al2O3)	52.0	[64]
L-Asp capped Fe3O4 NPs	7.7	[65]
Magnetic ZnFe2O4	12.1	[66]
MWCNT-COOH	42.68	[67]
Humic acid functionalized MNPs	161.8	[68]
Sodium montmorillonite	42.19	[69]
Lignocellulose	82.34	[70]
NiO nanoparticles	111	[71]
NiZnAl-LDH nano-sheets	97.09	[72]
V@TiO2 nanocompostie	158.8	This paper

. The Rh-B sorption showed preferential sorption, as indicated by the 1/n value below unity, which was also consistent with its PFOM agreement [60,61,62].

As shown in Table 2, the performance of the V@TiO₂ nanocomposite in adsorbing Rh-B dye has been further evaluated and compared to that of other adsorbents that have been reported in the literature. First, the equilibrium time for the V@TiO₂ nanocomposite is shorter. These obtained results show that the Rh-B dye leaves the aqueous solution quickly. Further, the V@TiO₂ nanocomposite has a more significant adsorption capacity than the other nanostructures, i.e., 158.8 mg/g compared to 7–161 mg/g, as documented in Table 2.

3.2.3. Adsorption Thermodynamics

The thermodynamics of Rh-B removal by the V@TiO₂ were inspected. The slope and intercept extracted from the plot of Equation (8) (Figure 9) were utilized in computing the enthalpy and entropy (ΔS^o and ΔH^o). The Gibbs free energy (ΔG^o) was obtained by applying the ΔS^o and ΔH^o values in Equation (9). The ideal gas constant (R) was used as 0.0081345 kJ mol⁻¹ for calculating these parameters, and the findings are summarized in

Table 3. The isotherms and thermodynamic findings for the Rh-B sorption on the V@TiO₂ nanocomposite.

Adsorption Isotherms
Langmuir				Freundlich
R2	KL (L mg−1)	qm (mg g−1)		R2	Kf (L mg−1)	n−1 (a.u.)
0.952	0.056	158.8		0.880	0.536	0.065
Thermodynamic parameters
Fed conc. (mg L−1)	ΔHo (kJmol−1)	ΔSo (kJmol−1)	ΔGo (kJmol−1) 298 K	ΔGo (kJmol−1) 313 K		ΔGo (kJmol−1) 328 K
10	−28.450	−0.091	−1.439	−0.079		1.280
25	−34.026	−0.105	−2.714	−1.138		chem1.280
50	−18.856	−0.055	−2.335	−1.503		−0.672
75	−23.573	−0.073	−1.739	−0.640		0.460
100	−18.979	−0.063	−0.339	0.599		1.537

.

$$\ln K_c=\frac{\Delta H^{ o}}{RT}+\frac{\Delta S{ }^o}{R}$$

(8)

$$\Delta G^{ o}=\Delta H^{ o}-T \Delta S{ }^o$$

(9)

Table 3 shows that Rh-B sorption possessed negative ΔG^o values, indicating the sorption’s spontaneity and exothermic nature. Also, the negative ΔH^o values corroborate that V@TiO₂ removed the Rh-B dye via a physisorption process [73,74,75,76,77].

3.3. Rh-B Dye Adsorption Mechanism

Several variables, such as the functional group of adsorbents, the pH of a solution, the surface charge on the particles, the porosity, and the nature of the adsorbate, can affect the adsorption process. Rh-B dye was adsorbed on the V@TiO₂ nanocomposite surface in this study. Point zero charges (surface charge) and FTIR (functional groups) that existed on the V@TiO₂ sorbent could explain the likely mechanism of the adsorption process. Hydrogen bonding and electrostatic interactions might have contributed to the adsorption of the Rh-B dye (Figure 10a). In Figure 10a of the FTIR spectrum, the absorption peak at 2309 cm⁻¹ due to C–N stretching vibrations that appeared after the adsorption of Rh-B dye demonstrates its role in the adsorption of dye molecules. Due to the presence of C–H and C=C functional groups in the spectra of adsorbed Rh-B dye, the absorption band between 1539.58 and 1448 cm⁻¹ was observed. The narrow peak between 1000 and 900 cm⁻¹ is attributable to the C–H bond, which indicates their participation in the elimination of Rh-B dye. A prominent broad peak at 521 cm⁻¹, which shifted lower to 472 cm⁻¹, correlates to V@TiO₂ nanocomposite stretching; FTIR of the dye-loaded V@TiO₂ nanocomposite revealed a considerable drop in the peak, indicating its significance in the elimination of Rh-B dye from an aqueous solution.

The pHpzc study determined that the pHpzc exists at a pH of 4.2, above which the surface of V@TiO₂ is negatively charged, which may increase the sorption by withdrawing the positive N⁺–(CH₃)₂ group on Rh-B (Figure 10a); this hypothesis indicates high participation of electrostatic attractions in Rh-B removal by V@TiO₂. Furthermore, the adsorption of Rh-B dye on the surface of the V@TiO₂ sorbent is also the result of interactions involving H-H bonding and dipole-dipole interactions. In addition, the isothermal studies demonstrate that the multilayer adsorption of the Rh-B molecules is physical sorption; this result is in line with the ΔH^o values less than 80 kJmol⁻¹. Figure 10b depicts the subsequent adsorption of Rh-B dye molecules onto the V@TiO₂ sorbent. Rh-B dye is attracted to the nanocomposites via hydrogen bonds and electrostatic interactions.

3.4. V@TiO₂ Nanomaterials Regeneration

The regeneration of V@TiO₂ nanomaterials was studied by removing the Rh-B dye using absolute ethanol. During the regeneration process, ethanol was used as the desorption agent (Figure 11). The rapid discoloration of absolute ethanol filtrate confirmed Rh-B dye desorption from the V@TiO2 sorbent. The sorbent was re-immersed in a freshly produced Rh-B dye solution with the same concentration and volume (5 mg/L, 75 mL). A total of five repetitions of this step were performed. Figure 11 illustrates the effectiveness of Rh-B dye desorption by ethanol from the V@TiO₂ sorbent. The efficacy of the V@TiO₂ nanocomposite to remove dyes decreased with each desorption cycle.

4. Conclusions

Nanostructured V@TiO2 is synthesized from metal oxide using ultrasonic synthesis techniques. A study of the adsorption properties of Rh-B dye on V@TiO₂ was conducted. According to XRD, the V@TiO₂ nanomaterials had a crystal size of 33.83 nm. In aqueous media, the nanosorbent of V@TiO₂ was demonstrated to be capable of eliminating Rh-B dye molecules with a maximum adsorption capacity of 158.8 mg/g. Furthermore, the thermodynamic analysis showed that such reactions were highly spontaneous and endothermic. The Langmuir technique emphasized the adsorption monolayer. As a result, the adsorption kinetic could be fitted with a pseudo-first-order reaction model. Additionally, 100% ethanol was chosen as the best selective desorption eluent for reusing the adsorbent. Hence, the regeneration of V@TiO₂ sorbent is a cost-effective method for treating effluents from the textile industry.