Synthesis of Zirconium-Based MOF–Biochar Composites for Efficient Congo Red Removal from Industrial Wastewater

Abstract

Organic dye pollution in industrial wastewater is severe and difficult to degrade, posing a significant challenge to environmental management and water resource security. To meet the demand for the efficient elimination of Congo Red (CR) dye from industrial wastewater, this work prepared two zirconium-based metal–organic framework (MOF)–biochar composites, UIO-66@BY and UIO-67@BY, by in situ loading zirconium-based MOFs (UIO-66 and UIO-67) onto biochar (BY) via a solvothermal method. The composite material was comprehensively characterized using transmission electron microscopy (TEM), BET surface area analysis, and X-ray photoelectron spectroscopy (XPS). The adsorption results indicate that UIO-67@BY exhibits a significantly higher maximum adsorption capacity for CR dye compared to pristine biochar (BY), while UIO-66@BY also shows enhanced adsorption performance, but one that is slightly lower than that of UIO-67@BY. Further investigations reveal that the adsorption behavior conformed to a pseudo-second-order kinetic model and was well described by the Langmuir isotherm, suggesting that the adsorbent exhibited a homogeneous adsorption surface, and that chemical adsorption played a dominant role in the process. The primary mechanisms responsible for CR dye uptake by the composite include pore structure characteristics, coordination with functional groups, π–π stacking interactions, and electrostatic forces. The composite material developed herein provides an environmentally sustainable and economically efficient strategy for mitigating wastewater contamination.

1. Introduction

Rapid industrialization has driven economic growth but has also exacerbated global wastewater contamination problems. Among these, dye-containing effluents represent a major environmental concern due to their complex chemical composition, high toxicity, and strong resistance to biodegradation. Dyes are extensively applied in numerous sectors, such as textile manufacturing, leather processing, and paper production. Inadequate treatment or improper discharge of these pollutants poses a serious threat to aquatic ecosystems [1]. These dye compounds not only markedly diminish water clarity and interfere with photosynthesis in aquatic ecosystems, but also present serious risks to agriculture, fisheries, livestock, and human health because of their strong chemical stability, carcinogenic properties, and propensity to bioaccumulate in living organisms [2]. Congo Red, an anionic azo dye containing two azo linkages and a biphenylamine moiety, poses a severe environmental hazard even at trace concentrations. Exposure to this dye has been associated with adverse health effects, including skin sensitization, ocular irritation, and respiratory disorders [3]. Therefore, developing efficient and cost-effective methods for removing Congo Red has become a crucial research direction. Currently, various methods for removing dyes have been proposed, including adsorption, chemical precipitation, membrane filtration, and reverse osmosis. Among these, adsorption is widely used for dye wastewater treatment due to its simplicity, low energy consumption, and flexible design [4]. However, traditional adsorbents face challenges such as high cost, limited adsorption capacity, complicated regeneration processes, and potential risks of secondary pollution. For instance, although activated carbon possesses a high specific surface area, it suffers from non-uniform pore distribution and challenging regeneration processes. Resin-based adsorbents provide favorable adsorption efficiency but are costly and prone to fouling, while natural mineral adsorbents exhibit only limited adsorption capacity. Therefore, the urgent development of innovative and highly effective adsorbents is essential for advancing wastewater treatment technologies.

MOFs constitute a category of crystalline porous materials constructed through coordination interactions between metal ions and organic ligands. It is evident that these frameworks have recently attracted significant attention in the field of adsorption applications. The reasons for this attention are twofold: firstly, these materials possess exceptionally large specific surface areas; and secondly, they exhibit adjustability of their pore architecture, together with a high abundance of surface-active sites [5]. Notably, zirconium-based MOFs, exemplified by the UiO-66 (University of Oslo, zirconium-based MOF) series, demonstrate remarkable resistance to heat and acidic and basic media, as well as excellent water stability. This robustness is attributed to the strong coordination interactions between Zr⁴⁺ centers and carboxylate linkers, enabling these frameworks to preserve their structural integrity even under harsh wastewater treatment conditions [6]. UiO-66 is considered one of the most stable materials in the MOF family, with excellent heat resistance, chemical stability, high porosity, good biocompatibility, low toxicity, and strong acid resistance with moderate alkaline resistance [7]. Nevertheless, zirconium-based MOFs still encounter several obstacles in practical deployment, such as difficulties in recovering nanoscale particles, risks of secondary contamination, and substantial synthesis costs, all of which restrict their feasibility for large-scale utilization.

To address the aforementioned limitations of zirconium-based MOFs in practical applications, exploring composites with low-cost, high-stability materials has emerged as a critical direction to overcome their application bottlenecks. Biochar, a renewable porous carbon material derived from biomass pyrolysis, with its unique pore structure, excellent mechanical properties, and environmentally friendly characteristics, precisely provides an ideal solution to this challenge. Its unique pore structure provides an ideal substrate for MOF growth. The incorporation of biochar not only significantly enhances the mechanical strength and solid–liquid separation performance of composite materials but also reduces the overall material preparation cost by using agricultural and forestry waste as raw materials. As a widely available and nearly zero-cost biomass, the use of biochar offers substantial economic and environmental benefits [8]. Poplar trees, widely planted in central and northern China, produce fallen leaves and other waste during growth, which are often landfilled or burned, causing resource waste and environmental pollution. By using these low-value waste products to prepare biochar, the treatment cost of waste can be reduced, while providing a stable raw material source for composite material synthesis [9].

In this work, poplar leaves are employed as a biomass precursor for biochar production via pyrolysis, which is subsequently integrated with in situ solvothermal synthesis to fabricate a composite consisting of poplar leaf-derived biochar and zirconium-based MOFs. Benefiting from the carrier characteristics of biochar and the selective recognition capacity of MOFs, a hierarchical porous framework with abundant adsorption sites is engineered to enable efficient removal of Congo Red. This research not only introduces a promising material candidate for Congo Red remediation but also provides theoretical guidance for the development of “waste-to-waste” functional environmental materials.

2. Materials and Methods

2.1. Reagents and Materials

The poplar leaves were collected from Zhengzhou, Henan. The chemicals used in the experiment include zirconium tetrachloride (ZrCl₄), dimethylformamide (DMF), terephthalic acid (PTA), 4,4′-biphenyl dicarboxylic acid (BPDC), and Congo Red (CR) dye, all purchased from Shanghai Malkin Biotech Co., Ltd., Shanghai, China; potassium hydroxide (KOH), purchased from Taicang Lushi Reagents Co., Ltd. (Suzhou, China); and hydrochloric acid (HCl), purchased from Jintu Company (Zhengzhou, China). The reagents employed were of analytical grade, and all solutions were prepared using deionized water.

2.2. The Preparation of BY, UIO-66@BY, and UIO-67@BY

The leaves of the white poplar were initially pulverized and subsequently passed through a 100-mesh sieve. The obtained powder was then subjected to a series of rinses with deionized water, followed by oven-drying at 105 °C until a stable weight was achieved. Subsequently, 2 g of the dried powder was subjected to pyrolysis in a vacuum tube furnace at a temperature of 800 °C for a duration of 2 h. The heating rate was maintained at a constant rate of 10 °C per minute. Following a natural cooling process to ambient temperature, the pyrolyzed material underwent a second grinding and sieving process, utilizing a 100-mesh screen. This was followed by a thorough washing procedure with deionized water until the material reached a neutral state. Subsequently, an additional drying process was conducted at a temperature of 105 °C for a duration of 3 h. The resulting white poplar leaf biochar was designated as BY.

In accordance with the extant literature, UIO-66@BY was synthesized via the solvothermal method, with certain modifications [10]. In summary, 0.466 g of ZrCl₄ and 1 g of biochar derived from white poplar leaves were dissolved in 25 mL of DMF to prepare Solution A, which was magnetically stirred at room temperature for 2 h. Concurrently, 0.332 g of PTA was dissolved in 25 mL of DMF and sonicated for 30 min to ensure complete dissolution, forming Solution B. The two solutions were then combined and subjected to an additional 30 min of sonication. The resultant mixture was transferred into a high-pressure autoclave and subjected to a reaction at 120 °C for a period of 24 h. Subsequent to cooling to ambient temperature, the precipitate was isolated by means of centrifugation and washed on three occasions with DMF in order to remove residual PTA. The purified material was then subjected to vacuum-drying at a temperature of 70 °C for a duration of 12 h, a process which resulted in the formation of the UIO-66@BY composite.

The preparation of the UIO-67@BY material follows a similar procedure, except that in Solution B, the ligand is replaced with 0.484 g of BPDC.

2.3. Characterization Techniques

The surface area, pore volume, and pore size distribution of the samples were determined using a BET analyzer (BSD-PS2, Beijing, China). This was achieved by employing nitrogen adsorption–desorption isotherms as a means of analysis. The microstructure and surface features of the biochar, along with its elemental distribution, were examined via scanning electron microscopy (SEM, TESCAN MIRA LMS, Shanghai, China) coupled with elemental mapping. The crystallographic analysis was conducted using a high-resolution X-ray diffractometer (Bruker D8 Advance, Karlsruhe, Germany). The 2θ range scanned was from 10° to 80°, at a rate of 5°/min, with Cu Kα radiation. The characterization of chemical functional groups was accomplished through the utilization of Fourier-transform infrared (FTIR) spectroscopy (Nicolet iS20, Thermo Fisher Scientific, Waltham, MA, USA), employing KBr pellet preparation, encompassing a spectral range of 400–4000 cm⁻¹. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Waltham, MA, USA) was utilized in order to ascertain the elemental composition, valence states, and relative atomic content.

2.4. Batch Adsorption Experiment

Adsorption studies were conducted to examine the CR dye uptake performance of BY, UIO-66@BY, and UIO-67@BY. A stock solution of CR dye (1000 mg·L⁻¹) was prepared and diluted to the required concentrations, with 50 mL of solution used for each experiment. The influence of multiple variables—pH, adsorbent dosage, contact time, initial dye concentration, and temperature—on adsorption efficiency was systematically investigated. Solution pH was adjusted within the range of 2–12 using 0.1 M HCl and 0.1 M NaOH. Adsorbent loading varied between 0.1 g·L⁻¹ and 2 g·L⁻¹, contact times were set from 5 to 180 min, and initial CR concentrations were prepared at 100–200 mg·L⁻¹. Experiments were conducted at temperatures between 30 °C and 60 °C. After adsorption, solid–liquid separation was achieved using a 0.45 μm membrane filter. The residual dye concentration in the filtrate was quantified by UV–Vis spectrophotometry (UV-9000S) at 498 nm. The adsorption capacity (q_e, mg·g⁻¹) was determined according to the following equation:

$$q_e={\displaystyle \frac{(C_0-C\mathrm{e})}{m}}\times V$$

(1)

where C₀ and C_e represent the initial and equilibrium dye concentrations (g·L⁻¹), V is the solution volume (L), and m is the adsorbent mass (g). All experiments were performed in triplicate, and mean values were reported. The experimental adsorption data were analyzed using a range of isotherm and kinetic models. The mathematical expressions for all models employed are provided in the Supplementary Materials.

3. Results

3.1. Characterization of BY, UIO-66@BY, and UIO-67@BY

3.1.1. FTIR Analysis

The FTIR spectra of BY, UIO-66@BY, UIO-67@BY, and their Congo Red (CR) adsorption products are presented in Figure 1. For BY, the absorption band near 1050 cm⁻¹ is associated with the C–O stretching vibration, whereas the feature at approximately 3400 cm⁻¹ corresponds to the stretching mode of hydroxyl (O–H) groups [11]. Distinct differences are observed in the UIO-66@BY spectrum, which shows a sharp signal at 500 cm⁻¹, characteristic of Zr–O bond stretching, confirming the interaction between UIO-66 and the biochar surface [12]. Moreover, intense absorptions at 1550 cm⁻¹ and 1400 cm⁻¹ are assigned to the asymmetric and symmetric vibrations of the O–C=O moieties in the organic ligands, respectively, while a weaker peak at 1650 cm⁻¹ is attributed to the C=O stretching in carboxyl functionalities. After CR uptake, a noticeable decrease in the intensity of the carboxyl group band at 1650 cm⁻¹ indicates the participation of C=O groups in the adsorption mechanism. This reduction is ascribed to charge-transfer interactions between the electron-deficient aromatic rings of CR and oxygen-containing functional groups, weakening the C=O bond strength [13,14]. The persistent sharp Zr–O band suggests that the MOF remains structurally stable throughout the adsorption process, confirming the material’s chemical robustness [15].

3.1.2. SEM and EDS Analysis

The morphology and elemental composition of the BY, UIO-66@BY, and UIO-67@BY adsorbents were characterized by means of SEM and EDS, as illustrated in Figure 2. Figure 2a displays the SEM and EDS images of BY. It can be observed that the surface of BY exhibits numerous pores of varying sizes. The EDS analysis reveals that the main elements of BY are carbon (C, 91.49%) and oxygen (O), indicating the material’s high carbon purity. Figure 2b,c show the SEM images of BY loaded with UIO-66 and UIO-67, respectively. Both images demonstrate a uniform distribution of numerous nanoscale particles on the surface of the biochar framework. Some particles are embedded in the channel walls, while others are attached to the external surface of the framework. This observation suggests that the porous structure of the biochar provides anchoring points for the growth of UIO-66 and UIO-67, contributing to the material’s porosity and surface functionality [10]. Notably, these particles do not completely block the open channels of the biochar, implying that the loading process effectively preserves the material’s original porous structure, which is beneficial for the subsequent diffusion and mass transfer of guest molecules (e.g., gases and liquids). The EDS results show the presence of carbon (C), oxygen (O), and zirconium (Zr) elements, with the detection of Zr confirming the successful anchoring of UIO-66 and UIO-67 on the biochar surface. The distribution of Zr is uniform, with no significant aggregation observed [16]. Figure 2d,e illustrate the SEM images of the UIO-66@BY and UIO-67@BY adsorbents after CR adsorption. No significant particle detachment or structural collapse was observed on the material surface after CR adsorption, indicating that the composite system possesses good structural stability. Elemental quantification results show the presence of sulfur (S), which originates from the sulfonate groups (-SO₃⁻) in the Congo Red molecules. This confirms that CR has been effectively adsorbed and anchored on the surface of the material [17].

3.1.3. XRD Analysis

X-ray diffraction (XRD) was used to probe the crystal structure and phases of BY, UIO-66@BY, and UIO-67@BY (Figure 3). For BY, a strong reflection at 2θ = 26.7° is indexed to the (002) planes of graphitic carbon, and the signal at 29.4° corresponds to the (104) reflection of calcite (CaCO₃) [18]. The presence of CaCO₃ is plausibly due to the pyrolytic conversion of native calcium oxalate in poplar leaves. In the diffractogram of UIO-66@BY, the CaCO₃ features disappear, most likely because CaCO₃ dissolves under the mildly acidic conditions created by the carboxylate linkers during MOF synthesis. Instead, well-resolved reflections at 2θ = 7.3°, 8.5°, and 13.8° can be assigned to the (111), (200), and (222) planes of UIO-66, in agreement with literature values, confirming that UIO-66 is deposited on the biochar while retaining its crystallinity [19]. Signals at 5.7°, 6.6°, and 9.2° are attributable to the (111), (002), and (022) planes of UIO-67 [20]. Relative to UIO-66@BY, the peaks of UIO-67@BY shift slightly toward lower angles, which is consistent with differences in linker structure (e.g., terephthalic acid in UIO-66 vs. 4,4′-biphenyl dicarboxylate in UIO-67) that modify the lattice parameters.

3.1.4. BET Analysis

In order to investigate the pore characteristics of the adsorbents, nitrogen adsorption–desorption tests were conducted, and the pore size distribution (PSD) was determined using the Barrett–Joyner–Halenda (BJH) approach, as illustrated in Figure 4. Based on the IUPAC classification, BY exhibits a type IV adsorption–desorption isotherm accompanied by an H3 hysteresis loop, which indicates a predominantly mesoporous framework with a minor proportion of micropores. The dominant pore size is approximately 4 nm, contributing most to the pore volume and representing the principal pore type of this material. Similarly, UIO-66@BY and UIO-67@BY samples also demonstrate type IV isotherms with H3 hysteresis loops, suggesting that these MOF-modified adsorbents share a comparable pore architecture, mainly consisting of micro- and mesopores [21]. This hierarchical pore structure offers advantages for pollutant capture by improving surface accessibility and active site availability. As summarized in

Table 1. BY, UIO-66@BY, and UIO-67@BY. The specific surface areas and average pore sizes.

Sample	SBET (m2·g−1)	Average Pore Diameter (nm)
BY	22.93	7.38
UIO-66@BY	183.10	6.06
UIO-67@BY	135.21	5.27

, the BET surface area of pristine BY is 22.93 m²·g⁻¹, whereas UIO-66@BY and UIO-67@BY exhibit markedly higher surface areas of about 183.1 m²·g⁻¹ and 135.2 m²·g⁻¹, respectively. These findings confirm that the incorporation of MOFs effectively enlarges the material’s surface area, enhances the density of active adsorption sites, and significantly improves its adsorption efficiency toward target contaminants.

3.1.5. XPS Analysis

X-ray photoelectron spectroscopy (XPS) was utilized to analyze the elemental composition and surface chemical characteristics of BY, UIO-66@BY, and UIO-67@BY. The corresponding survey spectra for these materials are presented in Figure 5. The BC sample is mainly characterized by C1s and O1s signals, whereas UIO-66@BY and UIO-67@BY additionally exhibit distinct Zr3d peaks, confirming successful incorporation of the MOF structure. In the high-resolution C1s spectrum of BC (Figure 5b), two peaks are observed at 284.8 eV and 287.1 eV, corresponding to C–C and C–O bonds. For UIO-66@BY and UIO-67@BY (Figure 5d,g), additional peaks at 288.1 eV and 288.2 eV, respectively, are attributed to O–C=O functional groups. The O1s spectrum of BY (Figure 5c) reveals peaks at 532.1 eV and 532.5 eV associated with C–O and C–O–C bonds. In contrast, UIO-66@BY displays peaks at 531.9 eV and 532.0 eV, while UIO-67@BY shows peaks at 532.1 eV and 529.9 eV, corresponding to C–O and Zr–O bonds, respectively. Furthermore, the Zr3d spectrum of UIO-66@BY (Figure 5e) exhibits binding energies at 182.5 eV and 184.9 eV, assigned to Zr–O and Zr⁴⁺, confirming the formation of zirconium–oxygen clusters [22]. These findings collectively demonstrate the effective loading of UIO-67 onto the biochar support, as verified by XPS analysis.

3.2. BY, UIO-66@BY, and UIO-67@BY Study on Adsorption Performance

3.2.1. The Impact of pH

The adsorption behavior of three biochars—BY, UIO-66@BY, and UIO-67@BY—toward CR was systematically investigated across a wide pH range (2–12), revealing strong pH dependence. As shown in Figure 6a, under highly acidic conditions (pH = 2), UIO-67@BY achieved the highest adsorption efficiency (≈80%), followed by UIO-66@BY (≈70%) and BY (≈40%). This superior performance is attributed to the protonation of surface functional groups (e.g., –OH, –COOH), which introduces positive surface charges and enhances electrostatic attraction toward anionic CR species [23]. With increasing pH, a gradual reduction in adsorption efficiency was observed for all samples within the neutral-to-alkaline range (pH 6–12). At pH 12, UIO-67@BY maintained ~20% efficiency, while UIO-66@BY decreased to ~8%, and BY consistently showed <10% removal across pH 4–12, reflecting its limited adsorption capacity in these conditions. This decline is primarily attributed to functional group deprotonation, which elevates negative surface charge density and intensifies electrostatic repulsion against CR anions, thus reducing adsorption [24]. The particularly poor performance of BY at high pH levels is also linked to its scarcity of surface functional groups. Moreover, prior studies report that elevated pH destabilizes the crystalline framework of UiO-66, weakening hydrogen bonding, π–π interactions, and pore-filling effects [25]. Compared with pristine BY, both MOF-modified composites (UIO-66@BY and UIO-67@BY) exhibited enhanced pH adaptability, benefiting from the additional active sites and tunable surface wettability introduced by the MOF structures, which mitigates charge reversal effects. Among these, UIO-67@BY showed superior structural stability and adsorption performance over the entire pH range, indicating its strong potential as a robust CR adsorbent under varying aqueous environments.

3.2.2. The Impact of Adsorbent Dose

As shown in Figure 6b, the three biochar-derived adsorbents displayed a marked improvement in Congo Red (CR) removal efficiency with increasing adsorbent concentration. When the dosage was adjusted to 2.0 g·L⁻¹, UIO-67@BY exhibited the highest removal rate, of 98%, followed by UIO-66@BY at 90%, whereas unmodified BY achieved only 32%. These results indicate that the adsorption capability of the modified biochar rose almost proportionally with increasing dosage, and that UIO-67 functionalization significantly enhanced adsorption performance. Regarding maximum adsorption capacity, pristine BY reached 297.7 mg·g⁻¹ at 5 mg, while UIO-66@BY and UIO-67@BY achieved 822.15 mg·g⁻¹ and 1078.20 mg·g⁻¹, respectively, at 10 mg. Collectively, these data confirm that increasing adsorbent loading improves adsorption performance for all tested materials. UIO-67@BY showed particularly strong performance, maintaining high removal efficiency at low concentrations and approaching full CR removal at higher dosages. Such findings demonstrate its strong potential as an efficient adsorbent for real-world water treatment applications.

3.2.3. The Influence of Contact Time and Initial Concentration

As shown in Figure 6c, the adsorption capacity of the three adsorbents toward CR dye was evaluated over time intervals ranging from 0 to 180 min. All samples exhibited an upward adsorption trend with increasing contact time, though their adsorption kinetics and equilibrium times varied markedly. Within the first hour, adsorption efficiency increased rapidly: UIO-67@BY achieved 77.10%, UIO-66@BY reached 54.60%, while BY attained only 21.0%. After this stage, the rate of increase slowed, approaching equilibrium at 180 min. The rapid initial uptake was attributed to the high density of accessible active sites on the adsorbent surface. Once these sites became saturated, the incremental adsorption declined, indicating equilibrium adsorption had been reached [26]. The superior adsorption rate and equilibrium capacity of UIO-66@BY and UIO-67@BY were ascribed to the incorporation of MOF structures, which introduced abundant binding sites (e.g., Zr⁶⁺ nodes and carboxylate ligands). These features facilitated faster molecular diffusion and stronger CR binding, improving overall performance. Figure 6d indicates that the adsorption capacities of all tested materials increased as the initial CR concentration rose from 100 to 200 mg·L⁻¹, which can be attributed to stronger mass transfer driving forces at elevated contaminant levels. UIO-67@BY showed the most substantial increase, attaining a maximum adsorption capacity of 838.4 mg·g⁻¹ at 200 mg·L⁻¹. This exceptional performance underscores its improved concentration tolerance and adsorption potential compared to BY and UIO-66@BY, mainly due to the introduction of UIO-67, which improved hydrophilicity and pore accessibility of the biochar matrix [27]. Moreover, the ordered porous channels of the MOFs provided abundant adsorption sites, supporting sustained high removal efficiency even at elevated contaminant concentrations.

3.3. Study on Adsorption Kinetics

To elucidate the adsorption kinetics of CR, experimental results obtained for the three investigated adsorbents were fitted using three commonly employed kinetic models: the pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion (IPD) equations. The corresponding regression curves are depicted in Figure 7, while the derived kinetic constants and associated correlation coefficients are systematically summarized in

Table 2. BY, UIO-66@BY, and UIO-67@BY. Fit parameters for the adsorption kinetics model of CR.

		BY	UIO-66@BY	UIO-67@BY
Qe exp (mg∙g−1)		245.6	684.5	865.2
PFO	Qecal (mg·g−1)	215.898	619.258	806.283
	K1 (min−1)	0.185	0.073	0.059
	R2	0.572	0.822	0.869
PSO	Qecal (mg·g−1)	230.031	688.875	905.231
	K2 (min−1)	0.001	0.0001	0.00008
	R2	0.853	0.931	0.942
IPD 1st stage	C1 (mg·g−1)	129.533	129.995	206.809
	Ki (mg·g−1·min−1/2)	11.823	68.379	62.268
	R2	0.987	0.963	0.951
2nd stage	C2 (mg·g−1)	163.678	222.234	212.890
	Ki (mg·g−1·min−1/2)	6.009	43.673	63.752
	R2	0.982	0.984	0.840
3rd stage	C3 (mg·g−1)		577.725	634.691
	Ki (mg·g−1·min−1/2)		7.126	17.196
	R2		0.881	0.686

. Across all adsorbents, the PSO model exhibited consistently higher correlation coefficients than the PFO model, suggesting that chemisorption dominated the adsorption mechanisms [28]. Among the materials, UIO-67@BY demonstrated the highest adsorption capacity, achieving 855.55 mg·g⁻¹ at 180 min, which closely aligned with experimental observations. This confirms that chemical interactions primarily control its adsorption behavior. The outstanding performance of UIO-67@BY can be attributed to intrinsic MOF features, such as a large BET surface area and abundant active sites, which significantly improve both uptake rate and capacity [29]. In comparison, UIO-66@BY exhibited slightly reduced adsorption capacity, likely due to electronic effects from its terephthalic acid ligand, resulting in weaker CR interactions. The IPD model further indicated that the adsorption process occurs in three sequential stages. The first stage represents rapid surface adsorption due to numerous accessible surface sites, corresponding to the external film diffusion step. The second stage is characterized by intraparticle diffusion of CR molecules into the internal pore channels; in this stage, UIO-67@BY exhibited the highest slope (63.752 mg·g⁻¹), reflecting low intraparticle diffusion resistance. This behavior is linked to its well-developed porous network, where mesopores accelerate initial uptake, and micropores govern equilibrium adsorption [30]. The third stage reflects equilibrium, where all active sites are saturated. In addition, the fact that none of the fitted plots intersected the origin indicates that intraparticle diffusion alone cannot account for the rate-limiting process, suggesting that surface adsorption or film diffusion also play a significant role [31].

3.4. Study on Adsorption Isotherms

The adsorption behavior of the investigated materials was evaluated through Langmuir, Freundlich, and Temkin isotherm models, and the corresponding fitting results are illustrated in Figure 8. Across all three adsorbents, adsorption capacity increased as the initial CR concentration rose. Among the models applied, the Langmuir isotherm exhibited stronger correlation coefficients than the Freundlich model, implying that the adsorption mechanism aligns more closely with monolayer adsorption on a uniform surface. This observation supports chemisorption as the primary mechanism, involving chemical bond formation between adsorbent and adsorbate, rather than weak van der Waals forces typical of physisorption [32]. The maximum adsorption capacities of UIO-66@BY and UIO-67@BY were 668.27 mg·g⁻¹ and 838.40 mg·g⁻¹, respectively, both considerably higher than that of pristine BY, underscoring the enhanced adsorption performance of the composites. In addition, all materials displayed separation factor (RL) values below 1, confirming favorable adsorption behavior and strong adaptability toward CR removal [24]. The Temkin model fitting parameters are detailed in

Table 3. BY, UIO-66@BY, and UIO-67@BY. Fit parameters for the adsorption isotherm model of CR.

		BY	UIO-66@BY	UIO-67@BY
Langmuir	Qmax (mg·g−1)	489.632	690.257	940.701
	KL (L·mg−1)	0.0069	0.211	0.069
	R2	0.993	0.996	0.996
Freundlich	KF (g·mg−1·min−1)	16.356	488.464	343.158
	1/n	0.542	0.065	0.190
	R2	0.992	0.983	0.987
Temkin	A	0.053	69,870	2.93
	B	20.711	59.371	16.570
	R2	0.992	0.986	0.993

. Notably, the positive binding energy constant (BT) values for each adsorbent indicate that CR adsorption is exothermic. The relatively high BT values further substantiate chemisorption dominance over physisorption, reinforcing that chemical interactions govern the overall adsorption mechanism [33].

3.5. Thermodynamic Research

Figure 9 illustrates the van’t Hoff plot of ln Kc versus 1/T, revealing the thermodynamic characteristics of the adsorption process. The adsorption behavior of CR was systematically examined at three controlled temperatures (35 °C, 45 °C, and 55 °C), highlighting temperature as a critical thermodynamic parameter that modulates the molecular interactions and binding affinity between CR species and the adsorbent surfaces. Maximum adsorption capacities were achieved at 60 °C, with values of 312.6, 822.7, and 1009.8 mg·g⁻¹ for BY, UIO-66@BY, and UIO-67@BY, respectively. These results demonstrate that increased temperature enhances CR adsorption on the tested materials. The corresponding thermodynamic parameters, summarized in

Table 4. BY, UIO-66@BY, and UIO-67@BY. Thermodynamic fitting parameters for CR adsorption.

	ΔS° (Kj·mol−1)	ΔH° (kJ/mol)	ΔG° (Kj·Mol−1)
			30 °C	40 °C	50 °C	60 °C
BY	53.276	16.270	−0.069	−0.305	−0.800	−1.708
UIO-66@BY	50.582	11.448	−3.875	−4.397	−4.917	−5.385
UIO-67@BY	70.370	17.005	−4.289	−5.109	−5.705	−6.432

, show that for UIO-67@BY, the Gibbs free energy change (ΔG°) ranged from –4.29 to –6.43 Kj·mol⁻¹, while the enthalpy change (ΔH°) was determined to be 17.01 Kj mol⁻¹. The negative ΔG° values confirm that CR adsorption occurs spontaneously under the examined conditions. The positive ΔH° value indicates an endothermic adsorption process, suggesting that elevated temperature improves adsorption efficiency. Additionally, the positive entropy change (ΔS° > 0) implies increased molecular disorder and rearrangement at the solid–liquid interface during adsorption, reflecting enhanced molecular mobility at the interface [34,35]. Collectively, these thermodynamic findings establish that the adsorption of CR on UIO-67@BY and related adsorbents is both spontaneous and endothermic, with temperature serving as a key driver to boost adsorption capacity through entropic effects.

3.6. Study on Adsorption Mechanism

The proposed adsorption mechanism of Congo red onto the Zirconium-Based MOF–Biochar is comprehensively illustrated in Figure 10. The adsorption mechanisms of the three investigated materials can be elucidated by integrating material characterization results with adsorption model fitting. The carbonization of biomass and the incorporation of MOFs endow the composites with abundant micro- and mesopores, forming hierarchical pore architectures that provide efficient channels for mass transport. Furthermore, the homogeneous dispersion of MOFs within the carbonaceous pore networks introduces numerous active sites, enhancing adsorption performance via pore-filling effects that facilitate the capture of target molecules [36]. UiO-66, composed of zirconium oxide clusters coordinated with terephthalate ligands, forms a robust face-centered cubic lattice with relatively small and stable pores. The –COOH groups in the ligands engage in hydrogen bond networks, which significantly improve the capture of polar molecules. In contrast, UiO-67 employs bulkier imidazole-based linkers, leading to a three-dimensional mesoporous framework more favorable for the diffusion of large dye molecules [37,38]. In addition, Zr–OH moieties serve as Lewis acid sites capable of coordinating with oxygen- and nitrogen-containing pollutants, thus reinforcing chemisorption processes [39]. π–π stacking interactions are essential for the adsorption of CR molecules. Infrared spectroscopy confirmed the existence of C=C and C=O functional groups on the adsorbent surface, both of which provide high π-electron density. These unsaturated groups interact with the aromatic rings of CR molecules through π–π electron coupling, thereby improving adsorption performance [40]. Furthermore, pH variations significantly affect CR removal efficiency, highlighting the role of electrostatic interactions. Under acidic conditions, protonation of surface functional groups enhances electrostatic attraction between the adsorbent and negatively charged CR anions, ultimately leading to superior adsorption capacity [41]. Collectively, the adsorption of CR by MOF-doped biochar is governed by the synergistic contributions of four mechanisms: (i) pore-filling effects arising from the hierarchical porous structure, (ii) coordination interactions between Zr–OH acid sites and dye molecules, (iii) π–π stacking electron coupling between aromatic systems, and (iv) electrostatic interactions regulated by pH. This multi-mechanistic synergy accounts for the superior adsorption performance of the MOF–biochar composites.

4. Conclusions

In this study, a novel composite adsorbent was successfully synthesized by integrating zirconium-based metal–organic frameworks (UIO-66 and UIO-67) with poplar leaf-derived biochar (BY) via a solvothermal method. Structural characterizations (XRD, XPS, etc.) confirmed the stable anchoring of MOFs on the biochar surface, resulting in a significant increase in specific surface area and the creation of abundant active sites for dye removal. Adsorption experiments demonstrated that UIO-67@BY exhibited the highest adsorption capacity for CR, reaching 855.55 mg·g⁻¹, which was markedly superior to that of pristine BY (241.9 mg·g⁻¹). Kinetic and equilibrium studies demonstrated that the adsorption behavior was best described by a pseudo-second-order kinetic equation and fitted well with the Langmuir isotherm, suggesting that chemisorption on a uniform surface was the primary mechanism. Thermodynamic evaluation confirmed the process to be spontaneous and endothermic. The superior adsorption capacity of the composite was mainly attributed to the combined effects of its hierarchical pore network, surface-active functional groups, electrostatic interactions, and π–π stacking forces. However, this investigation did not include a systematic assessment of adsorbent recyclability and regeneration efficiency, which restricts a full appraisal of long-term performance and economic feasibility. Future research should therefore emphasize regeneration testing, structural integrity, and performance retention across multiple adsorption–desorption cycles to validate the practical applicability and sustainability. Overall, this work demonstrates an effective strategy for incorporating MOFs into biochar to design high-performance dye adsorption materials. In particular, the UIO-67@BY composite shows outstanding adsorption capacity and holds great promise as a scalable and sustainable material for environmental remediation.