Macroporous Resin-Based La-N Co-Doped TiO₂ Composites for Efficient Removal of Environmental Pollutants in Water via Integrating Adsorption and Photocatalysis

Abstract

Integrating photocatalysis with adsorption represents an efficient approach to improving the removal performance of organic contaminants from aqueous environments. To address the issues of severe charge recombination and poor adsorption activity in TiO₂ photocatalysts during the photocatalytic degradation of organic pollutants. In this study, we used macroporous resin as a carrier and prepared La/N-doped TiO₂/macroporous resin composite materials (La/N/TiO₂-MAR) via a hydrothermal-assisted sol–gel method. The results show that the composite material has a spherical morphology. N can be doped into the TiO₂ crystal, while La³⁺ remains on the surface of TiO₂ without entering the crystal lattice. La/N/TiO₂-MAR demonstrates a higher specific surface area and enhanced light absorption capacity, which facilitates both adsorption and photocatalytic degradation. At the La³⁺ doping concentration of 0.05 M, La_0.05/N/TiO₂-MAR demonstrates optimal photocatalytic degradation performance, achieving an 85.36% removal rate of Rhodamine B after 240 min of visible-light exposure.

1. Introduction

The contamination of water bodies by organic compounds has emerged as a worldwide environmental concern [1,2]. Especially since the 1950s, the development of the chemical industry has led to a rapid increase in both the variety and quantity of synthetic organic compounds, with over ten thousand types identified to date [3]. Synthetic organic pollutants are infiltrating water bodies via multiple routes and persistently accumulating, posing a significant hazard to both human and environmental well-being [4]. The development of efficient and low-cost approaches for purifying water contaminated with organic compounds has been a major focus of research. semiconductor materials with excellent photocatalytic performance have been extensively studied to address various energy and environmental challenges. These materials have been found to possess many advantages, such as high catalytic activity, mild reaction conditions, operational simplicity, and the absence of secondary pollution [5,6,7]. As a result, they have attracted significant attention in the field of photocatalytic degradation of organic pollutants, particularly in aqueous environments.

To date, numerous photocatalysts with remarkable catalytic activity, including TiO₂, ZnO, ZrO₂, Fe₂O₃, and CdS, have been widely investigated [8]. Among these catalysts, TiO₂ has emerged as a more attractive choice due to its high stability, low cost, non-toxicity, and strong oxidation ability [9]. The effectiveness of TiO₂ in photocatalysis is notably constrained due to its limited visible-light absorption and the quick recombination of photogenerated charge carriers.

The addition of specific metals that aid in separating electrons from holes can significantly lower the recombination rate of charge carriers [10]. Lanthanum (La), the most renowned rare earth metal, possesses the [Xe] 5d¹6s² electronic configuration, which can improve the performance of semiconductor materials [11]. Due to the larger ionic radius of La³⁺ (1.170 Å) compared to Ti⁴⁺ (0.745 Å), La is likely to disperse on the TiO₂ surface in the form of La₂O₃, increasing the density of surface oxygen vacancies [12,13]. In addition, titanium ions may partially substitute for lanthanum sites in the form of oxides, resulting in charge imbalance within the lattice, which can be compensated by the surface adsorption of OH⁻ [14]. The introduction of La into the TiO₂ lattice not only refines particle size and increases surface area but also significantly boosts its thermal stability [15].

Studies have shown that nitrogen doping can substitute oxygen atoms with nitrogen, forming hybridized N-2p and O-2p orbitals, thereby reducing the band gap and enhancing the photocatalytic activity of TiO₂-based materials [16,17]. La and N co-doped TiO₂ photocatalysts have been widely reported to exhibit enhanced visible-light responsiveness due to synergistic effects. The catalysts demonstrated remarkable ability in degrading organic compounds such as Rhodamine B [18] and phenanthrene [19] through photocatalysis. To overcome its low adsorption capability, TiO₂ is frequently supported on carriers such as activated carbon or carbon cloth, which improves the adsorption properties of catalyst [20]. Macroporous adsorption resin (MAR) is a polymer-based adsorption and separation material characterized by a macroporous structure and the absence of ion-exchange groups. Compared with other commonly used catalyst supports, macroporous adsorption resins offer numerous advantages such as high adsorption capacity, rapid adsorption rate, long service life, and low cost, and have been widely applied in wastewater treatment and the chemical industry [21]. However, studies on TiO₂ photocatalysts supported by macroporous adsorption resins remain limited.

In this study, a La/N co-doped TiO₂ photocatalyst anchored on macroporous adsorption resin (La/N–TiO₂–MAR) was successfully synthesized via a sol–gel method combined with hydrothermal treatment. The macroporous adsorption resin (MAR) served not only as a structural support but also provided a high-surface-area framework with interconnected macropores, which facilitated the dispersion of TiO₂ nanoparticles and enhanced mass transport. The co-doping of La³⁺ and N, along with the MAR support, effectively improved both the adsorption and photocatalytic performance of the composite. The photocatalytic activity of the La/N co-doped resin-based TiO₂ samples was assessed by comparing the degradation efficiency of Rhodamine B under light and dark conditions. Additionally, the effects of several key factors—including solution pH, coexisting ions, reaction kinetics, and catalyst reusability—were systematically investigated. This study highlights the synergistic advantages of rare earth/non-metal co-doping and macroporous support integration for enhanced visible-light-driven photocatalysis.

2. Results and Discussion

2.1. Characterizations

The SEM images of the modified macroporous resin-based TiO₂ photocatalysts are presented in Figure 1. The composites exhibit a uniformly spherical morphology across all samples. The spherical morphology is attributed to the macroporous resin. Importantly, the spherical framework of the macroporous resin remains intact post-modification, with no evidence of structural collapse. This suggests that the macroporous resin is structurally stable and thus well-suited for use as a catalyst support. As the La³⁺ concentration increased, a greater number of nanoparticles became attached to the spherical surfaces (Figure 1a–c). High-magnification SEM images revealed pronounced surface roughness in the modified samples, characterized by visible undulations and numerous protrusions of varying sizes (Figure 1d–f). These features, with irregular flake-like or flocculent morphologies, were distributed over the resin surface, forming basin-like structures with raised peripheries and sunken central regions. The presence of this structure is beneficial for maximizing light absorption, ultimately boosting the photocatalytic degradation efficiency.

The XRD patterns of various modified macroporous resin-based TiO₂ photocatalysts under visible light are presented in Figure 2a. The XRD patterns exhibit distinct diffraction peaks at 2θ values of 25.2°, 38.1°, 47.8°, 54.8°, 62.8°, and 70.3°, which correspond to the (101), (004), (200), (211), and (204) crystal planes of anatase TiO₂, as indexed by the JCPDS card No. 21-1272. These results indicate that the synthesized photocatalyst predominantly possesses an anatase-phase TiO₂ crystal structure [22,23]. The presence of sharp and well-defined diffraction peaks indicates the formation of a highly crystalline anatase phase. No distinct diffraction peaks associated with La³⁺-based oxides were observed, likely due to the low doping concentration of La³⁺ [24]. Initially, the TiO₂ diffraction peaks appear relatively sharp and narrow; however, as the La³⁺ doping level increases, the peaks gradually broaden and their intensities decrease. This behavior suggests a reduction in crystallite size and an increase in the specific surface area of the material. Given that the ionic radius of La³⁺ (0.115 nm) is significantly larger than that of Ti⁴⁺ (0.0605 nm), La³⁺ ions are unlikely to substitute directly into the TiO₂ lattice [25]. Instead, La³⁺ species are presumed to exist as highly dispersed surface oxide layers on the TiO₂ nanoparticles, which are typically undetectable by XRD due to their amorphous or monolayer nature. Nevertheless, partial substitution of Ti⁴⁺ by La³⁺ can induce lattice distortion and expansion, which may introduce defect states that function as photon traps or charge carrier recombination centers—thereby influencing the photocatalytic activity of the material. For La³⁺-doped TiO₂, La³⁺ ions are generally present as a surface monolayer of oxide species on the TiO₂ nanoparticles. Due to their high dispersion and low crystallinity, these surface oxides are typically undetectable by XRD analysis. Nevertheless, when La³⁺ is introduced into nanostructured TiO₂, it may partially substitute Ti⁴⁺ in the lattice, leading to lattice distortion and expansion. These structural modifications can introduce defect states that serve as photon traps or charge carrier capture centers, influencing the photocatalytic performance of the material [26]. Figure S1 shows that N, O, Ti, and La elements are uniformly distributed on the surface of the macroporous resin in La_0.05/N/TiO₂-MAR, further confirming the XRD results.

XPS analysis was further conducted to provide detailed insights into the chemical states and elemental composition present on the surface of the samples. High-resolution C 1s XPS spectra of the composite samples were recorded (Figure S2). The C 1s spectra were deconvoluted into three distinct peaks centered at approximately 284.6, 285.8, and 289.2 eV. The peak at 284.6 eV corresponds to C–C or C=C bonds, typically attributed to adventitious carbon or aromatic structures remaining from the organic framework. The peak at 285.8 eV is ascribed to C–O bonds, indicating partial oxygenation of carbon species, while the peak at 289.2 eV is assigned to C=O groups, suggesting the presence of oxidized carbon moieties. These results provide direct evidence of the presence and chemical state of carbon in the composite and confirm the successful integration of MAR-derived organic components into the TiO₂ structure. As shown in Figure 2b, the XPS analysis reveals two characteristic Ti 2p peaks located at binding energies of 465.3 eV (Ti 2p_1/2) and 459.3 eV (Ti 2p_3/2), indicating that the titanium in the sample exists predominantly in the Ti⁴⁺ oxidation state. These values are consistent with the reported binding energies of Ti⁴⁺ in TiO₂, confirming the chemical state of Ti in the synthesized material. Compared to TiO₂-MAR, the Ti 2p_1/2 peak of N/TiO₂-MAR and La_0.05/N/TiO₂-MAR shifts to lower binding energies, which can be attributed to the modification of the electronic structure caused by nitrogen and lanthanum incorporation into the TiO₂ lattice [15]. The shift in the Ti-related peaks toward higher binding energies suggests a decrease in the electron density around Ti ions, confirming that Ti⁴⁺ is the only oxidation state present on the catalyst surface [27]. As shown in Figure 2c, the O 1s spectrum presents a dominant peak at ~530.1 eV, attributed to lattice oxygen (Ti-O-Ti), and a shoulder at ~531.8 eV, corresponding to surface hydroxyl (-OH) groups and adsorbed water. Additionally, a weaker peak at ~533.0 eV is assigned to C-O bonding, consistent with the presence of organic functionalities retained from the MAR support. These functional groups may enhance dye adsorption via hydrogen bonding or electrostatic interactions [28]. Figure 2d presents the high-resolution N 1s XPS spectra of N/TiO₂-MAR and La_0.05/N-TiO₂-MAR. For both samples, the N 1s signal can be deconvoluted into two distinct peaks. The first peak, located at approximately 398.5 eV, corresponds to substitutional nitrogen forming O-Ti-N bonds, where nitrogen atoms replace lattice oxygen in the TiO₂ structure [29]. This type of doping is known to introduce localized energy states above the valence band, effectively narrowing the bandgap and enhancing visible-light absorption capability. The second peak, typically centered around 401.7 eV, is associated with interstitial nitrogen species or surface-adsorbed nitrogen-containing groups (e.g., NO_x), which may also contribute to light harvesting and charge separation, though to a lesser extent. These results confirm the successful incorporation of nitrogen into the TiO₂ lattice in both substitutional and interstitial forms, which is consistent with the observed red shift in the UV–vis spectra. Finally, in Figure 2e, the characteristic peaks at 837.3 eV and 854.2 eV correspond to the La 3d_5/2 and La 3d_3/2 orbitals, respectively [30]. Based on the XRD results, the La element does not integrate into the TiO₂ lattice, but instead resides at interstitial sites, forming Ti-O-La bonds.

The FTIR spectra (Figure 2f and Figure S3) reveal characteristic broad absorption peaks corresponding to hydroxyl stretching vibrations at 3450.6 cm⁻¹ and bending vibrations of adsorbed water at 1636.6 cm⁻¹ on the composite surface [31]. The distinct peak observed at 1408.5 cm⁻¹ is assigned to the stretching vibrations of N-O groups introduced via nitrogen doping [32]. Notably, this peak is absent in the FTIR spectrum of MAR/TiO₂, confirming the successful incorporation of N into the MAR/TiO₂ composite. In addition to the characteristic -OH and N-O signals, a moderate absorption band is observed at approximately 1100 cm⁻¹, which is attributed to C-O stretching vibrations originating from ether or ester groups in the macroporous adsorption resin (MAR) framework [33]. This signal confirms the partial retention of organic functional groups from the resin precursor after TiO₂ incorporation. These oxygen-containing groups may enhance interfacial compatibility and facilitate π-π or hydrogen bonding interactions with dye molecules during photocatalysis. This conclusion is corroborated by the high-resolution C 1s XPS spectra (Figure S2), which clearly show the C–O component at ~285.8 eV. Moreover, a noticeable absorption band appears around 660 cm⁻¹, which is attributed to the bending vibration of Ti-O-Ti bonds within the TiO₂ lattice. This peak is commonly observed in anatase-type TiO₂ structures and is consistent across all samples, suggesting that the fundamental TiO₂ framework remains intact after doping and modification. The persistence of this vibrational feature also supports the preservation of the Ti-O network, despite the incorporation of dopants such as La and N.

As shown in Figure 3a, the UV–vis diffuse reflectance spectra (DRS) of TiO₂-MAR and its doped derivatives exhibit strong absorption in the UV region (λ < 350 nm), primarily attributed to charge transfer (CT) transitions from the O 2p valence band to the Ti 3d conduction band [33]. Notably, TiO₂-MAR also shows measurable absorption in the visible range (400–700 nm), indicating partial visible-light activity. Upon N doping, the absorption edge exhibits a red shift, accompanied by enhanced absorption intensity in the visible-light region, suggesting improved light-harvesting capability [34,35]. Furthermore, the co-doping of La³⁺ and N further increases absorption in the visible region, likely due to the synergistic effects of La-induced defect levels and N-induced narrowing of the band gap. These enhancements are confirmed by Tauc plot analysis (Figure S4), where the estimated band gaps decrease progressively from 4.21 eV (TiO₂-MAR) to 3.18 eV (La_0.05/N-TiO₂-MAR), substantiating the improved optical properties resulting from co-doping. Such spectral shifts and enhanced visible-light absorption are expected to contribute significantly to the superior photocatalytic performance of the composite materials under visible-light irradiation [22]. The observed improvement is likely due to the synergistic interaction between La³⁺ and nitrogen, which introduces oxygen vacancies and defect states, alters the electronic structure, and facilitates enhanced light absorption. These modifications are highly favorable for improving visible-light-driven photocatalytic activity [22,34,35].

As shown in Figure 3b, the N₂ adsorption–desorption isotherms of all catalyst samples exhibit type IV behavior with pronounced H3-type hysteresis loops, indicating the coexistence of microporous and mesoporous structures. This hierarchical pore structure facilitates increased surface area and improved diffusion of reactant molecules, thereby enhancing their photocatalytic performance. In addition, all samples were synthesized using macroporous adsorption resin (MAR) as a structural support. MAR was introduced during the sol–gel process and acts as a porous organic framework that further contributes to the overall hierarchical porosity of the composites. The interconnected macropores provided by MAR offer abundant adsorption sites for organic pollutants and serve as diffusion channels, promoting the accessibility of active sites and facilitating charge migration during photocatalysis. Among the series, the specific surface area of La_0.01/N/TiO₂-MAR was measured to be 468.88 m²/g, which is higher than that of both TiO₂-MAR and N/TiO₂-MAR. This increase is attributed to the random dispersion of a low concentration of La-based rare earth oxides on the catalyst surface, along with the structural contribution of MAR. However, as the La³⁺ doping concentration increases, both the specific surface area and pore volume show a decreasing trend (Figure 3b and

Table 1. Parameters of physical adsorption.

Photocatalyst	Specific Surface Area (m2/g)	Pore Size (nm)	Total Pore Volume (m3/g)	Comments
TiO2/MAR	425.19	9.12	0.36903	Error ± 2% (BET analysis)
N/TiO2/MAR	415.37	6.72	0.26753	Error ± 2% (BET analysis)
La0.01/N/TiO2/MAR	468.88	9.40	0.42903	Error ± 3% (BET analysis)
La0.05/N/TiO2/MAR	322.72	11.67	0.46488	Error ± 3% (BET analysis)
La0.1/N/TiO2/MAR	452.90	8.016	0.38931	Error ± 2% (BET analysis)

). Although a larger surface area and pore volume are generally advantageous for improving adsorption performance, the interfacial adsorption and subsequent photocatalytic degradation of pollutant molecules are more significantly influenced by surface oxygen vacancies and charge distribution. These factors enhance charge separation and increase the density of reactive sites, which are essential for achieving high photocatalytic efficiency [36].

2.2. Adsorption Performances

The photocatalytic removal of Rhodamine B by macroporous resin-based TiO₂ is significantly influenced by the adsorption process. Adsorption isotherms and kinetic experiments were carried out on a range of catalysts to better understand their contribution to pollutant removal. Based on the equilibrium adsorption capacity ($q_e$) and the equilibrium concentration ($C_e$) at varying initial concentrations of Rhodamine B, adsorption isotherms were constructed for TiO₂-MAR, N/TiO₂-MAR, and La³⁺-doped N/TiO₂ composites. According to Figure 4a, as temperature increased, the adsorption capacity dropped, indicating an exothermic adsorption process for Rhodamine B. Hence, cooler conditions are more favorable [37]. With continued increases in Rhodamine B concentration, the adsorption capacity gradually approached saturation, particularly for materials with relatively low specific surface areas. A more rapid attainment of adsorption saturation reflects weaker adsorption affinity and a lower saturation capacity. Among the materials studied, La_0.05/N/TiO₂-MAR demonstrated the highest saturation adsorption capacity, suggesting superior adsorption performance.

To further analyze the adsorption behavior, the obtained experimental results were interpreted using both Langmuir and Freundlich isotherm equations [38]. The linearized equation for fitting the Langmuir isotherm model is presented as follows [39]:

$${\displaystyle \frac{C_e}{q_e}}={\displaystyle \frac{1}{q_{max}{K}_L}}+{\displaystyle \frac{C_e}{q_{max}}}$$

(1)

$C_e$ denotes the equilibrium concentration of the adsorbate (mg/L), while $q_e$ refers to the equilibrium adsorption amount per unit mass of the adsorbent (mg/g). $q_{max}$ indicates the theoretical maximum adsorption capacity (mg/g), and $K_L$ is the Langmuir constant (L/mg), reflecting the affinity between the adsorbent surface and the adsorbate molecules.

The separation factor ($R_L$), also referred to as the equilibrium constant within the Langmuir isotherm model, serves as a key indicator for assessing the favorability of the adsorption process. It is mathematically expressed as [40]

$$R_L={\displaystyle \frac{1}{1+K_L{C}_0}}$$

(2)

where $K_L$ is the Langmuir adsorption constant (L/mg) and $C_0$ represents the initial concentration of the adsorbate in solution (mg/L).

In contrast, the Freundlich model describes heterogeneous surface adsorption and multilayer formation. Its linear form is expressed as [41]:

$${\mathit{log}q}_e={\mathit{log}K}_F+{\displaystyle \frac{1}{n}}{\mathit{log}C}_e$$

(3)

$q_e$ denotes the adsorption capacity at equilibrium (mg/g), and $C_e$ is the equilibrium concentration of the adsorbate in solution (mg/L). The parameters $K_F$ and ${\displaystyle \frac{1}{n}}$ are Freundlich constants, representing the adsorption capacity and adsorption intensity, respectively.

The findings summarized in

Table 2. Adsorption isotherm model fitting parameters (Langmuir and Freundlich) for Rhodamine B on different La³⁺-modified resin materials.

Catalyst Samples	Temperature (K)	Langmuir Model			Freundlich Model
		qmax (mg/g)	KL (L/mg)	R2	KF (mg/g)	1/n	R2
TiO2/MAR	288	15.439	0.039	0.9513	0.2431	0.484	0.9701
	298	15.659	0.058	0.9781	0.5360	0.413	0.9795
	308	16.165	0.082	0.9818	0.7005	0.340	0.9724
N/TiO2/MAR	288	22.276	0.129	0.9855	0.7503	0.440	0.9648
	298	22.727	0.168	0.9778	0.8021	0.418	0.9571
	308	23.142	0.240	0.9773	0.8712	0.365	0.9814
La0.05/N/TiO2/MAR	288	37.495	0.115	0.9327	0.8834	0.478	0.9541
	298	38.328	0.154	0.9424	0.9276	0.454	0.9352
	308	38.431	0.221	0.9515	0.9779	0.403	0.9172

indicate a stronger correlation between the adsorption behavior of Rhodamine B on La_0.05/N/TiO₂-MAR and the Langmuir isotherm, implying that the adsorption likely takes place as a monolayer on a consistent and uniform surface. This result confirms that the adsorption of Rhodamine B on the surfaces of TiO₂-MAR, N/TiO₂-MAR, and La_0.05/N/TiO₂-MAR primarily follows a monolayer adsorption mechanism [42]. Based on the Langmuir isotherm model at 308 K, the estimated maximum adsorption capacities ($q_{max}$) for TiO₂-MAR, N/TiO₂-MAR, and La_0.05/N/TiO₂-MAR were 16.165, 23.142, and 38.413 mg/g, respectively, indicating a significant enhancement in adsorption performance with nitrogen and lanthanum co-modification. The close agreement between the theoretical and experimental results (Figure 4a) confirms the reliability of the Langmuir model in capturing the adsorption characteristics of the prepared composites, suggesting monolayer adsorption on a homogeneous surface.

To analyze the adsorption kinetics of Rhodamine B on TiO₂-MAR, N/TiO₂-MAR, and La_0.05/N/TiO₂-MAR, both pseudo-first-order and pseudo-second-order kinetic models were applied. The Lagergren pseudo-first-order model is formulated as follows [43]:

$$In\left(q_e-q_t\right)=In{q}_e-k_{1}t$$

(4)

Here, $q_e$ and $q_t$ (mg/g) denote the adsorption capacities at equilibrium and at a given time $\left(t\right)$, respectively, while $K_1$ (1/min) corresponds to the pseudo-first-order rate constant.

The expression for the pseudo-second-order adsorption kinetic model is given as

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_e}}$$

(5)

The adsorption capacities at equilibrium and at any specific time are denoted by $q_e$ and $q_t$ (mg/g), respectively, while $k_2$ (g·mg/g/min) denotes the rate constant for the pseudo-second-order kinetic model.

As shown in Figure 4b and

Table 3. Kinetic modeling of Rhodamine B degradation over La³⁺-modified resin materials: comparison of pseudo-first-order and pseudo-second-order models.

Catalyst Samples	qe.exp (mg/g)	Pseudo-First-Order			Pseudo-Second-Order
		qe (mg/g)	k1 (l/min)	R2	qe (mg/g)	k2 (g/mg/min)	h	R2
TiO2/MAR	5.715	6.397	6.49 × 10−3	0.9788	5.892	4.51 × 10−3	0.1564	0.9951
N/TiO2/MAR	13.776	10.541	6.93 × 10−3	0.9800	14.334	1.16 × 10−3	0.3313	0.9885
La0.05/N/TiO2/MAR	20.225	12.518	7.55 × 10−3	0.9670	21.303	9.47 × 10−4	0.4301	0.9823

, comparison of pseudo-first-order and pseudo-second-order kinetic models indicates that the pseudo-second-order model better fits the experimental data, exhibiting higher correlation coefficients (R²) and calculated equilibrium adsorption capacities ($q_e$) closely matching the experimental values. This suggests that the adsorption of Rhodamine B on the modified macroporous resin-supported TiO₂ materials is primarily controlled by chemisorption. The initial adsorption rate (h) analysis reveals that the La-doped sample (La_0.05/N/TiO₂/MAR) exhibits the fastest adsorption rate, indicating that La doping effectively enhances the adsorption kinetics [44].

2.3. Photocatalytic Performances

The adsorption and photocatalytic activities of various macroporous resin-based TiO₂ composites were evaluated through the degradation of Rhodamine B. As shown in Figure 4c, Rhodamine B exhibited minimal direct photodegradation under light irradiation. In contrast, the photocatalytic degradation of Rhodamine B by the composites was significantly more effective than its removal through dark adsorption. La/N/TiO₂-MAR demonstrated superior photocatalytic performance in degrading Rhodamine B under visible light, with a degradation efficiency of 73.4% after 4 h, outperforming both the TiO₂-MAR and N/TiO₂-MAR samples. These findings suggest that the removal of Rhodamine B by the modified macroporous resin-supported TiO₂ photocatalysts results from a synergistic effect between adsorption and photocatalytic degradation.

As shown in Figure 5, the influence of different concentrations of coexisting ions (Cu²⁺, Cd²⁺, Zn²⁺, NO₃⁻, Cr³⁺, and SO₄²⁻) on the photocatalytic degradation of Rhodamine B using TiO₂/MAR, N/TiO₂/MAR, and La_0.05/N/TiO₂/MAR was systematically investigated. The results indicate that coexisting ions can inhibit the photocatalytic process to varying degrees, depending on their type and concentration. Among them, NO₃⁻ and SO₄²⁻ showed the strongest inhibitory effects at high concentrations, likely due to competitive adsorption and interference with active sites. Interestingly, at low concentrations (e.g., 0.1 mM), some ions, such as NO₃⁻, Zn²⁺, and Cu²⁺, exhibited a slight enhancement in degradation efficiency. This may be attributed to improved ionic conductivity or the generation of reactive oxygen species such as •OH from the photolysis of nitrate [21]. However, as the concentration increased, the ions began to compete more effectively with Rhodamine B for adsorption sites, leading to a noticeable decline in photocatalytic efficiency. These results highlight the dual role of coexisting ions: trace levels may promote photocatalytic activity via indirect mechanisms, while elevated levels hinder pollutant removal through surface site occupation. Moreover, the photocatalytic degradation efficiency was also found to be strongly influenced by the initial Rhodamine B concentration, which affects the equilibrium between pollutant molecules and available active sites on the photocatalyst surface.

The effect of catalyst dosage on the degradation efficiency of Rhodamine B is illustrated in Figure 6a. An increase in catalyst dosage positively influences the degradation of Rhodamine B. However, when the dosage exceeds a certain threshold, the dye concentration no longer decreases significantly and tends to level off. When the catalyst dosage is below 0.1 g/L, it directly influences the rate of the photocatalytic reaction. At a catalyst dosage of 0.1 g/L, the removal rates of Rhodamine B after 4 h of visible-light exposure are 57.06%, 72.46%, and 85.36% for TiO₂/MAR, N/TiO₂/MAR, and La_0.05/N/TiO₂/MAR, respectively. This can be attributed to the higher catalyst dosage providing more active sites for photocatalytic reactions, promoting the process. When the catalyst dosage is further increased, the degradation rate of Rhodamine B by La_0.05/N/TiO₂/MAR remains at a high level after 4 h, without significant improvement, indicating that the photocatalytic degradation reaction is less influenced by the catalyst dosage at this point. Therefore, the optimal catalyst dosage should be 0.1 g.

Figure 6b illustrates how pH influences the adsorption–photocatalytic performance of TiO₂/MAR, N/TiO₂/MAR, and La_0.05/N/TiO₂/MAR. The pH value of the solution has a limited impact on the adsorption–photocatalytic degradation performance of TiO₂/MAR, N/TiO₂/MAR, and La_0.05/N/TiO₂/MAR toward Rhodamine B. However, slightly acidic or alkaline conditions can enhance the efficiency of pollutant removal. This can be attributed to the fact that changes in pH affect the dissociation degree and charge state of the solution. Under acidic conditions, the high concentration of H⁺ ions result in a positively charged TiO₂ surface, which facilitates the adsorption of Rhodamine B molecules containing negatively charged sulfonic acid groups [45,46]. Consequently, the mass transfer rate of adsorption is enhanced, improving the pollutant removal efficiency. Moreover, both acidic and alkaline conditions facilitate the interaction between photogenerated charge carriers and the catalyst surface, promoting the formation of hydroxyl radicals and enhancing the photocatalytic degradation activity.

As shown in Figure 6c, the initial degradation efficiency of TiO₂/MAR, N/TiO₂/MAR, and La_0.05/N/TiO₂/MAR increases with rising Rhodamine B concentration, reaching maximum values of 81.76%, 91.26%, and 96.63%, respectively, at a concentration of 30 mg/L after 240 min of visible-light irradiation. However, despite the higher absolute degradation efficiency, the overall degradation rate decreases with increasing initial pollutant concentration. This phenomenon can be attributed to the limited number of active sites on the catalyst surface. At higher Rhodamine B concentrations, a greater number of dye molecules compete for adsorption onto the photocatalyst, thereby reducing the availability of surface sites required for •OH generation and subsequent degradation reactions. Additionally, increased Rhodamine B concentration leads to reduced solution transparency, which limits light penetration and diminishes the catalyst’s light-harvesting efficiency. These combined effects result in a slower photocatalytic reaction rate despite the higher absolute removal percentage.

The stability of the catalyst was evaluated through multiple cycles of photocatalytic degradation tests. As shown in Figure 6d, in the first cycle, the degradation rates of Rhodamine B by TiO₂/MAR, N/TiO₂/MAR, and La_0.05/N/TiO₂/MAR were 57.06%, 72.46%, and 85.36%, respectively. After three cycles, the rates decreased to 42.03%, 61.79%, and 70.14%, and a further decline was observed in the fifth cycle. This performance drop may be attributed to the partial retention of Rhodamine B or its intermediates on the photocatalyst surface, which blocks the active sites and hinders further reaction. Additionally, repeated irradiation may cause minor aggregation of TiO₂ nanoparticles or reduction in surface hydroxyl content, thereby affecting light absorption and reactive species generation. Nevertheless, La/N co-doped TiO₂-MAR maintained over 50% of its initial efficiency after three cycles, indicating acceptable reusability for practical applications.

To elucidate the dominant reactive species in the photocatalytic process, three typical radical scavengers were employed: tert-butanol (•OH scavenger), potassium iodide (KI, h⁺ scavenger), and p-benzoquinone (PBQ, •O²⁻ scavenger) [47]. Each scavenger was added individually to the Rhodamine B degradation system at appropriate concentrations, and the corresponding changes in photocatalytic efficiency were measured. As shown in Figure 6e, all three scavengers resulted in a moderate suppression of degradation efficiency, indicating the involvement of multiple reactive oxygen species in the reaction mechanism. While KI led to the greatest reduction among the three, the decrease was not drastic, suggesting that holes (h⁺) may contribute significantly but do not solely dominate the process. The inhibitory effects of PBQ and tert-butanol further imply that superoxide (•O₂⁻) and hydroxyl radicals (•OH) are also actively involved. These findings support the view that the degradation of Rhodamine B is governed by a synergistic mechanism involving multiple reactive species. As shown in Figure 6f, the TOC content in the solution decreased gradually during the initial stage of the photocatalytic reaction, but began to decrease sharply after 30 min, indicating the onset of significant mineralization at that point. A comparative analysis of the TOC degradation curves revealed consistent trends across all three materials. After reaching adsorption saturation, significant mineralization reactions commenced after 4 h of photocatalytic treatment, resulting in the formation of simple inorganic products such as carbon dioxide and water.

3. Experimental Section

3.1. Materials

Tetrabutyl titanate (TBT), lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O, 99%), and Polyethylene glycol 400 were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China) Urea (CH₄N₂O), ammonium hydroxide, and macroporous adsorbent resin (MAR) were obtained from Sinopharm Chemical Regent Co., Ltd. (Shanghai, China). Rhodamine B was obtained from Shanghai Macklin (Shanghai, China). All chemicals were employed in their as-received state without undergoing any supplementary purification processes.

3.2. Catalyst Preparation

A specific amount of La(NO₃)₃·6H₂O was dissolved in a mixture of 30 mL anhydrous ethanol and 12 mL tetrabutyl titanate under vigorous stirring. Then, 1.0 mL of concentrated hydrochloric acid and 10 mL of 1% PEG-400 (dissolved in ethanol) were added dropwise to form a stable sol. Subsequently, 3.0 g of pretreated macroporous adsorption resin (MAR) and 2.4 g of urea (dissolved in 3.0 mL ultrapure water) were added sequentially while maintaining continuous stirring. The resulting mixture was ultrasonicated for 60 min to ensure uniform dispersion, aged at room temperature for 24 h, and dried at 105 °C. The dried gel was then redispersed in water, and the pH was adjusted using dilute ammonia to promote gelation. The final sol was subjected to hydrothermal treatment at 200 °C for 12 h in a PTFE-lined autoclave. Afterward, the solid product was dried and calcined to obtain the La_x/N/TiO₂-MAR composite photocatalyst.

3.3. Photocatalytic Experiment

Photocatalytic reactions were carried out using a Xe-JY 500 photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China), which was fitted with a 500 W xenon light source and maintained at a stable temperature by a circulating water-cooling unit. To maintain the reaction temperature at 25 °C, continuous water circulation was employed throughout the irradiation process. Each quartz reaction tube contained a magnetic stir bar to ensure uniform dispersion of the catalyst in the solution. Rhodamine B (RhB) was used as the target pollutant at an initial concentration of 50 mg/L, and the catalyst dosage was fixed at 2 g/L. To evaluate the photocatalytic degradation performance under various conditions, aliquots were withdrawn at 30 min intervals, filtered through a 0.45 μm membrane, and analyzed. Adsorption experiments conducted in the dark under identical conditions served as controls.

3.4. Characterization Methods

The microstructures of various samples were examined and characterized using a field-emission scanning electron microscope (FE-SEM, XL-30ESEM, Philips GmbH, Hamburg, Germany). Energy-dispersive X-ray spectroscopy (EDX) was conducted at an accelerating voltage of 10 kV to determine the elemental composition and distribution on the sample surfaces. X-ray diffraction (XRD) analysis was performed on a Bruker (Billerica, MA, USA) D8 ADVANCE diffractometer with Cu Kα radiation to investigate the crystalline phases of the materials. X-ray photoelectron spectroscopy (XPS, PHI1600, PerkinElmer Inc., Waltham, MA, USA) was used to analyze the surface elemental composition and chemical states. FTIR spectra were recorded using an instrument from Nicolet Instrument Corporation, Madison, WI, USA, to identify surface functional groups. The specific surface area, average pore volume, and pore diameter were measured by nitrogen adsorption–desorption isotherms using a BET analyzer Micromeritics ASAP 2020 (Micromeritics Instrument Corp., Norcross, GA, USA). UV–visible diffuse reflectance spectra (UV–vis DRS) were collected using a Shimadzu (Tokyo, Japan) UV-2550 spectrophotometer in the range of 210–700 nm with BaSO₄ as the reference to evaluate the optical absorption properties of the materials.

4. Conclusions

Overall, we used a hydrothermal-assisted sol–gel method to prepare La³⁺- and N-doped TiO₂/mesoporous resin composite materials (La/N-TiO₂/MAR). The La/N/TiO₂/MAR composite material presents a regular spherical morphology, with TiO₂ particles uniformly distributed on the surface of the mesoporous resin. The co-doping of La and N notably improves the light absorption capability of TiO₂. La³⁺ and N co-doping endowed the material with a larger specific surface area and enhanced light absorption capacity. Consequently, when Rhodamine B was used as the substrate, La/N-TiO₂/MAR exhibited the best photocatalytic degradation performance, achieving a removal rate of 85.36% after 240 min of irradiation. The Langmuir and pseudo-second-order kinetic models exhibit good agreement with the adsorption isotherm and kinetic data of La_0.05/N/TiO₂/MAR, respectively. The photocatalytic performance is influenced by various factors, including solution pH, coexisting ions, initial substrate concentration, and catalyst dosage. Despite potential inhibition under certain conditions, the composite material exhibits excellent efficiency in the removal of Rhodamine B. Radical scavenging experiments show that the excellent photocatalytic degradation performance of the La_0.05/N/TiO₂/MAR system can be attributed to the synergistic effects of reactive species including h⁺, •O²⁻, and •OH.