Removal of Cr(VI) from an Aqueous Solution via a Metal Organic Framework (Ce-MOF-808)

Abstract

Hexavalent chromium (Cr(VI)) is a carcinogenic and highly mobile pollutant in aquatic environments. In this study, three cerium-based metal–organic frameworks (Ce-UiO-66, Ce-UiO-66-NO₂, and Ce-MOF-808) were synthesized and evaluated for their ability to remove Cr(VI) from aqueous solutions. Among the frameworks studied, Ce-MOF-808 exhibited the highest adsorption capacity and was selected for detailed investigation. To elucidate its structure and adsorption behavior, Ce-MOF-808 was characterized using XRD, FT-IR, SEM-EDS, TG-DTA, XPS, and Zeta potential analyses. The zeta potential results showed that the adsorbent surface remained positively charged in the pH range of 2.8–8.6, enabling electrostatic attraction toward anionic chromate species. XPS further revealed valence transitions between Ce³⁺/Ce⁴⁺ and Cr(VI)/Cr(III), demonstrating the occurrence of partial redox transformation during adsorption. Batch experiments showed that the adsorption was strongly pH-dependent and favored acidic conditions (pH 2). The kinetics followed the pseudo-second-order model, whereas the isotherm data were better described by the Langmuir model, yielding a maximum adsorption capacity of 42.74 mg/g. Thermodynamic analysis indicated a spontaneous and exothermic process. Moreover, Ce-MOF-808 maintained high Cr(VI) uptake in real water samples, demonstrating its environmental applicability. Overall, Ce-MOF-808 is a promising redox-active adsorbent for efficient Cr(VI) removal in water treatment applications.

1. Introduction

Hexavalent chromium (Cr(VI)) is regarded as a highly hazardous and persistent heavy metal contaminant discharged from electroplating, leather tanning, pigment manufacturing, and metallurgical industries [1,2,3]. Due to its strong oxidizing ability, high solubility, and resistance to natural attenuation, Cr(VI) can remain in aquatic environments for prolonged periods and readily penetrate biological systems through ingestion, dermal contact, or trophic transfer, resulting in carcinogenic, mutagenic, and teratogenic effects [4]. The World Health Organization (WHO) classifies Cr(VI) as a Group I carcinogen and restricts its concentration in drinking water to below 0.05 mg/L [5], emphasizing the need for reliable and effective removal strategies to mitigate Cr(VI) pollution in aquatic systems.

In aqueous environments, Cr(VI) predominantly exists in the form of chromate (CrO₄²⁻) and hydrogen chromate (HCrO₄⁻), whose distribution is strongly governed by pH conditions [6,7]. Under acidic conditions, HCrO₄⁻ is the dominant species, whereas CrO₄²⁻ is more prevalent under alkaline conditions. These oxyanionic species exhibit high mobility and resistance to natural degradation, making Cr(VI) significantly more challenging to remove than trivalent chromium (Cr(III)), which is less toxic and more easily precipitated or immobilized through hydrolysis or ligand complexation [8]. Therefore, developing adsorbents that can effectively capture Cr(VI) in its anionic form under acidic conditions is crucial for wastewater treatment.

A variety of physicochemical and biological methods have been employed to remove Cr(VI) from aqueous systems, including membrane separation, electrochemical reduction, bioremediation, and adsorption [9,10,11]. Among these options, adsorption is considered particularly attractive because it combines operational simplicity with a relatively low cost and mild processing conditions. In addition, adsorption is compatible with continuous-flow treatment configurations and typically does not generate large amounts of secondary sludge, facilitating downstream handling and reducing the risk of secondary contamination [12].

The interactions responsible for Cr(VI) adsorption may involve electrostatic attraction, ion exchange, surface complexation, or, in some systems, redox reactions occurring between active sites on the adsorbent and the oxyanionic chromium species [13,14]. Nevertheless, adsorption efficiency is strongly dependent on the physicochemical properties of the adsorbent as well as environmental parameters including pH, ionic strength, along with presence of competing anions [15,16,17].

Despite extensive research efforts, many conventional absorbents such as activated carbon, natural clays, and biomass-derived porous carbons exhibit limited affinity toward anionic Cr(VI) species, particularly under acidic conditions. Castro-Castro et al. [18] reported that organo-bentonite achieved a peak adsorption capacity of merely 10.04 mg/g for Cr(VI), while Gorzin et al. [19] demonstrated that sludge-derived activated carbon exhibited a capacity of merely 23.18 mg/g. These values fall below the level required for practical implementation, especially considering that Cr(VI)-containing industrial effluents are commonly discharged at low pH levels.

Conventional adsorbents’ reduced performance in acidic media is often attributed to protonation of surface functional groups and the consequent decline in electrostatic attraction toward negatively charged chromate species [20]. In addition, the absence of redox-active components in these materials limits their ability to transform Cr(VI) into less toxic Cr(III), leaving the captured chromium species in their high-toxicity state. These limitations indicate the necessity of developing adsorbents with both strong affinity in acidic conditions and additional functionality that can contribute to Cr(VI) detoxification.

Metal–organic frameworks (MOFs), composed of metal ions or clusters coordinated with organic linkers, have emerged as a versatile class of porous materials owing to their exceptionally large surface area, ordered pore channels, and tunable architectures [21,22,23]. Unlike conventional porous solids, whose pore geometry and surface chemistry are difficult to precisely control, MOFs can be rationally designed by selecting suitable metal nodes or functional ligands or applying post-synthetic modification to introduce desired binding sites or functionalities [24,25]. Recently, the properties and applications of MOFs have become increasingly diversified. Originating from the pioneering works of Kitagawa et al. [26], MOFs have been further extended to adsorption-driven applications, including gas storage, catalysis, and water purification [27,28].

As shown in previous studies, various MOFs have been employed for the adsorption and removal of aqueous pollutants. For example, Jung et al. [29] used MIL-53 to remove aromatic compounds, and Fan et al. [30] employed Zr-MOF for the removal of antibiotics. However, many conventional MOFs suffer from poor thermal and acid stability in aqueous environments, compromising their removal efficiency [31].

Recent years have witnessed increasing interest in MOF-based adsorbents for Cr(VI) removal; however, existing studies have revealed several critical limitations. ZIF-8 and its composites exhibit poor performance under low pH conditions due to their limited acid stability, resulting in a maximum Cr(VI) uptake generally below 5 mg/g [32]. Similarly, UiO-66-NH₂ shows moderate adsorption capability for Cr(VI), but its mechanism relies heavily on electrostatic attraction between protonated -NH groups and chromate species. Then, the material also experiences reduced structural integrity and diminished adsorption efficiency under acidic conditions [33]. In addition, amine-functionalized MOFs such as M-9, N-9, and their derivatives increase Cr(VI) uptake by introducing more positively charged sites, yet their adsorption mechanism remains dominated by electrostatic interactions. Consequently, their performance can decline markedly in the presence of competing anions such as PO₄³⁻, HCO₃⁻, and Cl⁻ commonly found in real water matrices [34]. These observations indicate that many existing MOFs still suffer from insufficient acid stability, single-mode electrostatic adsorption mechanisms, and strong sensitivity to competing ions. Therefore, developing MOF materials with enhanced structural robustness under acidic conditions, strong affinity toward chromate species, and redox-active sites capable of reducing Cr(VI) to Cr(III) is essential for achieving efficient Cr(VI) remediation.

Therefore, it is critical to develop MOF materials with chemical robustness and high efficiency for the removal of chromate and other oxyanionic contaminants under acidic conditions.

Cerium-based frameworks (Ce-MOFs), numbering among the various MOF families, are attracting increasing attention due to the coexistence of Ce³⁺/Ce⁴⁺ redox pairs and their strong coordination interactions with polycarboxylate ligands [35,36,37]. The reversible redox nature of cerium enables Ce-MOFs not only to adsorb Cr(VI) but also to partially convert it into the less toxic Cr(III) species during the adsorption process. Unlike most adsorbents that rely exclusively on electrostatic attraction or surface complexation, Ce-MOFs provide a dual-function pathway that combines physical uptake with in situ redox transformation. This functional feature is particularly relevant in acidic systems where the adsorption performance of conventional materials is substantially diminished.

In addition to their redox reactivity, Ce-MOFs retain the inherent advantages of MOF structures, including high porosity and chemical tunability, allowing enhanced accessibility of active sites and potential adaptation to various wastewater environments [38]. These characteristics make Ce-MOFs promising candidates for addressing the challenges associated with Cr(VI) removal in acidic aqueous systems.

In this work, Ce-MOF-808 was synthesized and selected as the target material due to its redox-active Ce³⁺/Ce⁴⁺ sites and promising performance in preliminary screening. Systematic characterization and adsorption evaluation were subsequently conducted to assess its feasibility as a sorbent for treating chromium-contaminated wastewater.

2. Experimental Section

2.1. Materials and Apparatus

All chemicals used in this study were of analytical grade and used without further purification. Cerium ammonium nitrate, 1,3,5-benzenetricarboxylic acid (H₃BTC) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Potassium dichromate, N,N-dimethylformamide (DMF), nitric acid, formic acid, and acetone were obtained from Kanto Chemical Co., Inc. (Tokyo, Japan). Throughout the entire experiment, water (>18.2 MΩ) treated with an ultra-pure water system (RFU 424TA, Advantech Aquarius, Irvine, CA, USA) was used. In addition, a water bath incubator (BT100, Yamato Kagaku Co., Ltd., Tokyo, Japan), a vacuum-drying oven (DP33, Yamato Kagaku Co., Ltd., Tokyo, Japan), and a pH meter (HORIBA F-72, Tokyo, Japan) were used. Furthermore, scanning electron microscopy ((SEM) JEOL, Tokyo, Japan: JCM-6000 with JED-2300), ion sputtering (JFC-1100E, JEOL, Tokyo, Japan), Fourier transform infrared spectroscopy (FT-IR4200, Jasco, Tokyo, Japan), X-ray diffraction (XRD; D2 Phaser, Bruker, Billerica, MA, USA), and inductively coupled plasma atomic emission spectrometry (ICP-AES, ICPE-9820, Shimadzu, Kyoto, Japan) were applied.

2.2. Synthesis of Adsorbent (Ce-MOF-808)

Ce-MOF-808 was synthesized according to the procedure reported in Farrag [39]. Trimesic acid (H₃BTC, 224 mg) was dissolved in 12 mL of N,N-dimethylformamide (DMF) under continuous stirring at room temperature. Subsequently, 6 mL of 0.533 mol/L ammonium cerium(IV) nitrate ((NH₄)₂[Ce(NO₃)₆]) solution and 2.57 mL of formic acid, serving as a modulator, were added sequentially. The resulting mixture was heated in an oil bath at 100·°C for 15 min. After cooling to room temperature, the solid product was collected by centrifugation and washed with DMF to remove unreacted species. The solid was then subjected to three additional DMF washing cycles followed by four acetone washing cycles to extract residual DMF and other guest molecules. The resulting material was dried at 80 °C for 12 h. The obtained powder was designated as Ce-MOF-808. The synthesis process is illustrated in Figure 1.

2.3. Adsorption Experiment

Many studies have shown that the maximum (or equilibrium) adsorption capacity is commonly used to evaluate and express the adsorption performance of an adsorbent. For data analysis, kinetic models and adsorption isotherm models (equations) are used to interpret the data and determine the degree of adsorption. The adsorption capacity of Cr(VI) at equilibrium was calculated using Equation (1):

$$q_e=\left(C_i-C_e\right)\times V/m$$

(1)

Here, $q_e$ is the adsorption capacity at equilibrium ($\mathrm{m}\mathrm{g}/\mathrm{g}$), $C_i$ and $C_e$ are the initial and equilibrium Cr(VI) concentrations ($\mathrm{m}\mathrm{g}/\mathrm{L}$), $V$ is the volume of the solution ($\mathrm{L}$), and $m$ is the mass of the adsorbent (g) [40,41,42].

2.4. Adsorption Isotherms

Adsorption isotherms were employed to describe the equilibrium interaction between Cr(VI) and the adsorbent surface. In this study, the equilibrium data were fitted using the Langmuir and Freundlich isotherm models.

The Langmuir isotherm assumes there is monolayer adsorption on a homogeneous surface without interaction between adsorbed molecules [43]. It is expressed as

$${\displaystyle \frac{C_e}{q_e}}={\displaystyle \frac{C_e}{q_{max}}}+{\displaystyle \frac{1}{K_L{q}_{max}}}$$

(2)

where $C_e$ (mg L⁻¹) denotes the equilibrium concentration and $q_e$ (mg g⁻¹) represents the adsorption capacity of Cr(VI), respectively; $q_{max}$ (mg g⁻¹) is the theoretical maximum adsorption capacity; and $K_L$ (L mg⁻¹) is the Langmuir adsorption constant. A linear plot of $C_e/q_e$ versus $C_e$ yields a slope of $1/q_{\mathrm{m}\mathrm{a}\mathrm{x}}$ and an intercept of $1/\left(K_L{q}_{\mathrm{m}\mathrm{a}\mathrm{x}}\right)$.

The Gibbs free energy of adsorption ($\Delta G_{\mathrm{a}\mathrm{d}\mathrm{s}}$) can be obtained from the Langmuir constant using Equation (3):

$$\mathrm{n}∆G_{ads}=-RT\ln K_L$$

(3)

where $R$ denotes the universal gas constant (J·mol⁻¹·K⁻¹), $T$ represents the absolute temperature (K), and $K_L$ is the equilibrium constant at temperature $T$. The equilibrium constant may also be obtained from adsorption equilibrium data according to Equation (4):

$$K_L=q_e/C_e$$

(4)

The dimensionless Hall separation factor ($R_L$), defined in Equation (5), was used to assess the favorability of adsorption:

$$R_L=\left({\displaystyle \frac{1}{1+K_L{C}_0}}\right)$$

(5)

where $C_0$ (mg L⁻¹) represents the initial Cr(VI) concentration. Adsorption is considered unfavorable when $R_L>1$, linear when $R_L=1$, favorable when $0<R_L<1$, and irreversible when $R_L=0$ [44].

The Freundlich isotherm assumes there is multilayer adsorption on a heterogeneous surface without saturation and is expressed as

$$\ln q_e=\ln K_F+\left(1/n\right)\ln C_e$$

(6)

where $K_F$ is the Freundlich adsorption constant and $1/n$ is the adsorption strength. The value of $1/n$ reflects adsorption favorability, with $1/n=0$ indicating irreversible adsorption, $0<1/n<1$ indicating favorable adsorption, and $1/n>1$ indicating unfavorable adsorption [45].

2.5. Kinetic Model

Kinetic models are important for understanding the mechanism and pathway of a reaction [46,47]. The general rate law of a chemical reaction is given in Equation (7)

$$\begin{array}{cccc}& r=k\left[A{]}^x\right[B{]}^y& & \end{array}$$

(7)

where $\left[A\right]$ and $\left[B\right]$ are the reactant concentrations, $x$ and $y$ represent the reaction orders determined experimentally, and $k$ is the rate constant.

In this study, the adsorption kinetics were fitted using the pseudo-first-order and pseudo-second-order models. The pseudo-first-order model is expressed as follows:

$$\begin{array}{cccc}& \mathrm{l}\mathrm{n}(q_e-q_t)=\mathrm{l}\mathrm{n}{q}_e-k_{1}t& & \end{array}$$

(8)

where $q_e$ and $q_t$ (mg g⁻¹) are the adsorption capacities at equilibrium and at time $t$, respectively, and $k_1$ (h⁻¹) is the rate constant.

The pseudo-second-order model is given by the following expression:

$$\begin{array}{cccc}& {\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_e}}& & \end{array}$$

(9)

where $k_2$ is the pseudo-second-order rate constant, and $q_e$ and $q_t$ retain the same definitions as above [48].

2.6. Adsorption Thermodynamics

Thermodynamic analysis was conducted to evaluate the spontaneity and energetic characteristics of the adsorption process [49]. The Gibbs free energy change ($\Delta G°$) is commonly used to determine whether the adsorption process is spontaneous. The corresponding relationship between $\Delta G°$ and the equilibrium constant is given by the following formula:

$$\begin{array}{cccc}& \Delta G°=-RT\mathrm{l}\mathrm{n}{K}_d& & \end{array}$$

(10)

where $R$ is the universal gas constant and $T$ is the absolute temperature. The values of enthalpy change ($\Delta H°$) and entropy change ($\Delta S°$) can be obtained from the Van’t Hoff equation:

$$\begin{array}{cccc}& \mathrm{l}\mathrm{n}{K}_d={\displaystyle \frac{\Delta S°}{R}}-{\displaystyle \frac{\Delta H°}{RT}}& & \end{array}$$

(11)

The Gibbs free energy can also be calculated using the following thermodynamic relation:

$$\begin{array}{cccc}& \Delta G°=\Delta H°-T\Delta S°& & \end{array}$$

(12)

These thermodynamic parameters were calculated using the equilibrium constants derived from the isotherm models [50,51].

2.7. Adsorption Experiments Using Real Water Samples

Two water samples were collected from different public parks in Niigata Prefecture (Japan). The water samples were taken in May 2025 from two lagoons with distinct surrounding environments: Sakata Lagoon (sampling point S1: 37.8151619° N, 138.8773966° E), located near a natural ecological observation park that supports an abundance of wild birds such as ducks and egrets, and Uwasekigata Lagoon (sampling point S2: 37.7893063° N, 138.8644047° E), which is adjacent to farmland where local residents cultivate vegetables.

The collected water was passed through a filter prior to use in order to remove suspended particulates, and no pH adjustment was performed. The adsorption tests for the real water samples were conducted under the same conditions as those used in the laboratory batch experiments (adsorbent dosage, 0.01 g; solution volume, 30 mL; initial Cr(VI) concentration, 50 mg/L; temperature, 25 °C; contact time, 120 min). The concentrations of coexisting anions (Cl⁻, NO₃⁻, and SO₄²⁻) in both samples were determined using ion chromatography (Thermo Scientific Dionex ICS-1100, Waltham, MA, USA) prior to the adsorption tests.

2.8. Characterization

The morphologies of the samples were examined using scanning electron microscopy (SEM) under high-vacuum conditions at an accelerating voltage of 15 kV after gold sputter-coating. The functional groups were characterized using Fourier transform infrared spectroscopy (FT-IR) within the spectral region of 600–3900 cm⁻¹. The surface elements and chemical states before and after adsorption were determined using X-ray photoelectron spectroscopy (XPS) in the binding energy range of 0–1350 eV. The crystalline structure was characterized by X-ray diffraction (XRD) with Cu–Kα radiation covering a 2θ span of 5–50°. In addition, the surface charge of the adsorbent at different pH values was evaluated using zeta potential measurements.

3. Results and Discussion

3.1. Characterization of Ce-MOF-808

3.1.1. SEM-EDS Analysis

The surface morphology of Ce-MOF-808 before and after Cr(VI) adsorption was examined using SEM (Figure 2a). The pristine material exhibited aggregated particles with relatively uniform sizes and smooth surfaces. After adsorption, the surface became rougher, and the particles appeared more agglomerated, with less defined edges.

Because Ce-MOF-808 exhibits poor electrical conductivity, obtaining high-quality SEM images, especially after Cr(VI) adsorption, was technically challenging. Therefore, the clearest and most representative images were selected to illustrate the morphological change, even though the magnifications differ.

Subsequently, EDS analysis was performed to verify the presence of chromium on the adsorbent (Figure 2b). Elemental mapping showed the coexistence of Ce and Cr on the surface, and the EDS spectrum confirmed the presence of Cr with a weight percentage of 3.65%, indicating successful adsorption of Cr onto Ce-MOF-808.

3.1.2. XRD Analysis

The XRD patterns of Ce-MOF-808 before and after Cr(VI) adsorption, together with the simulated reference pattern, are shown in Figure 3. The pristine Ce-MOF-808 exhibited diffraction peaks that were highly consistent with the simulated standard pattern, confirming the successful formation of the target framework via the hydrothermal method [52]. The characteristic reflections corresponding to the (111), (311), and (222) planes were clearly observed, although the (111) peak appeared partially truncated due to the instrument’s starting angle at approximately 5°.

On the other hand, the diffraction peaks of Ce-MOF-808 after Cr(VI) adsorption were markedly weakened or nearly absent, indicating a significant decrease in crystallinity. Only a broad and weak low-angle feature remained, which cannot be unambiguously assigned to a specific crystal plane. This feature is more likely associated with substantial peak broadening and increased background scattering caused by structural distortion or partial amorphization rather than a well-defined crystalline reflection.

Such degradation of long-range order is consistent with the strong interaction between chromium species and the Ce-O clusters, which can disrupt the framework and lead to the collapse or amorphization of local structural domains. This interpretation is further supported by the SEM observations showing particle agglomeration and surface roughening after adsorption.

3.1.3. XPS Analysis

The XPS survey spectra (Figure 4a) show the change in the elemental composition of Ce-MOF-808 after Cr(VI) adsorption. In Ce-MOF-808, the coexistence of Ce³⁺ and Ce⁴⁺ originates from the Ce⁴⁺ precursor (ammonium cerium(IV) nitrate) and the intrinsic mixed-valence stability of Ce clusters. During framework formation, a fraction of Ce⁴⁺ can be partially reduced to Ce³⁺ through local charge-balancing interactions with organic linkers, resulting in a thermodynamically favored mixed-valence state. Such Ce³⁺/Ce⁴⁺ coexistence is a static structural feature of Ce-based MOFs, and it stabilizes the framework while providing potential redox-active centers. The high-resolution Cr 2p spectrum through narrow scan (Figure 4b) indicates the simultaneous presence of Cr(VI) and Cr(III), confirming that a fraction of the adsorbed Cr(VI) was reduced during the adsorption process. Meanwhile, the Ce 3d spectrum (Figure 4c) shows an increase in the Ce⁴⁺ fraction from 58.6% to 60.8% after adsorption, indicating the participation of Ce³⁺ in the redox process. These observations corroborate the redox interaction described by

Ce³⁺ + Cr⁶⁺→Ce⁴⁺ + Cr³⁺

and demonstrate that Ce-MOF-808 removes Cr(VI) through a redox conversion mechanism, providing a dual-function pathway for both the capture and detoxification of chromate species.

3.1.4. FT-IR Spectra

The FT-IR spectra of Ce-MOF-808 before and after Cr(VI) adsorption are presented in Figure 5. For the pristine material, the broad band near 3440 cm⁻¹ may be attributed to O-H stretching, while the peak at ~1650 cm⁻¹ is likely associated with the C=O vibration of the organic linker [53]. The bands in the 1400–1600 cm⁻¹ range could be assigned to Ce-O-C coordination, and the absorption band at 750–800 cm⁻¹ may correspond to Ce-O vibrations of the metal cluster.

After Cr(VI) adsorption, the O-H band intensity decreased, and a novel band appeared near 950 cm⁻¹, which could be attributed to Cr-O vibrations. These spectral changes suggest that surface hydroxyl groups and Ce-O clusters may participate in the adsorption process through surface interactions, while the appearance of the Cr-O band indicates that chemisorption is likely involved [54].

Although Cr(VI) mainly exists as the anionic species HCrO₄^- under the acidic conditions used in this study, trace amounts of neutral H₂CrO₄ may form under strongly acidic environments. While the proportion of neutral species is small and its contribution is expected to be minor, it may still interact with Ce-MOF-808 through surface complexation pathways such as hydrogen bonding or weak Ce-O-Cr coordination. Direct identification of Ce-O-Cr coordination by FT-IR is relatively difficult because its characteristic vibrations often overlap with the intrinsic Ce-O and Cr-O absorptions of the framework. Nevertheless, the observed weakening of the O-H stretching band and the emergence of Cr-O vibrations provide indirect evidence that surface complexation, in addition to electrostatic attraction, contributes to the overall adsorption process.

3.1.5. TG-DTA Analysis

The thermal behavior of Ce-MOF-808 before and after Cr(VI) adsorption was evaluated using TG-DTA (Figure 6a,b). For the pristine Ce-MOF-808, significant mass loss accompanied by an exothermic peak was observed at 369.4 °C, suggesting that the core MOF structure had begun to degrade [53]. After Cr(VI) adsorption, the exothermic peak shifted to 415.9 °C, and the corresponding decomposition temperature increased accordingly. This shift suggests that the incorporation of Cr(VI) enhanced the thermal stability of the framework, likely due to the interaction between Cr(VI) species and the Ce-MOF-808 structure.

3.1.6. Zeta Potential Analysis

The surface charge of Ce-MOF-808 at different pH values was evaluated using zeta potential measurements (Figure 7). The material exhibited a positive zeta potential within the pH range of 2.82–8.60, while the value became negative at higher pH levels. The positive surface potential under acidic to near-neutral conditions indicates that electrostatic attraction is favorable for the adsorption of negatively charged chromate species in this pH region.

3.1.7. Mechanism of Cr(VI) Removal by Ce-MOF-808

The possible mechanism underlying the removal of Cr(VI) by Ce-MOF-808 can be interpreted as the combined contribution of electrostatic attraction, inner-sphere surface complexation, and Ce³⁺/Ce⁴⁺-mediated redox transformation.

At the initial stage, the adsorption process is predominantly governed by electrostatic forces. Under acidic conditions, Cr(VI) mainly exists as HCrO₄⁻, whereas Ce-MOF-808 exhibits a positively charged surface over a wide pH range (2.82–8.60). This charge disparity facilitates the accumulation of Cr(VI) species in proximity to the Ce₆ clusters, thereby promoting subsequent chemical interactions.

Following electrostatic pre-concentration, inner-sphere complexation becomes the dominant mechanism governing the chemical immobilization of chromium species. The active sites responsible for this process are the coordinatively unsaturated Ce₄⁺ centers, typically manifested as surface ≡Ce-OH groups. The diminished O-H stretching intensity and the emergence of Cr-O vibrational bands in the FT-IR spectra indicate that Cr(VI) undergoes a ligand-exchange process, whereby HCrO₄⁻ replaces terminal hydroxyl groups to form Cr-O-Ce linkages. This inner-sphere coordination mode provides a more stable binding configuration than outer-sphere interactions, thereby contributing to the strong chemisorption of chromium species.

Simultaneously, redox transformation is evident from the XPS analysis. The appearance of Cr(III) signals alongside an increased Ce⁴⁺ fraction suggests that part of the adsorbed Cr(VI) is reduced to Cr(III) during the process. This reduction is attributed to the electron-donating capability of Ce³⁺ within the Ce₆ cluster. The presence of the Ce³⁺/Ce⁴⁺ redox couple thus endows Ce-MOF-808 with the ability not only to capture but also to detoxify Cr(VI) through an intrinsic electron-transfer mechanism.

Overall, these results support the synergistic mechanism in which (i) electrostatic attraction facilitates the proximity of Cr(VI) to the active metal centers, (ii) inner-sphere complexation driven by ligand exchange anchors Cr species directly onto Ce⁴⁺ sites, and (iii) Ce³⁺-mediated redox conversion ensures the transformation of toxic Cr(VI) into the more stable Cr(III) (Figure 8). This multi-step pathway highlights the structural and redox advantages of Ce-MOF-808 in the remediation of Cr(VI) contaminated water.

3.2. Adsorption Experiments

3.2.1. Effect of Adsorbent Dosage

The effect of adsorbent dosage on Cr(VI) removal was investigated by adding different amounts of Ce-MOF-808 (0.01–0.05 g) to 30 mL of a Cr(VI) solution (50 mg/L) under identical conditions (25 °C, 12 h, pH unadjusted). As presented in Figure 9, the adsorption capacity increased as the dosage rose to 0.03 g, a result that is likely due to the increased availability of active binding sites. However, further increasing the dosage led to a decrease in adsorption capacity, likely due to the insufficient quantity of Cr(VI) ions relative to the excess active surface, resulting in underutilization of the additional adsorbent.

3.2.2. Effect of pH Value

The effect of the initial pH on Cr(VI) adsorption was examined in the range of 2–12 under fixed conditions (0.01 g Ce-MOF-808, 30 mL of 50 mg/L Cr(VI), 12 h, 25 °C). As shown in Figure 10, the adsorption capacity was strongly dependent on pH and reached its maximum at pH 2. The enhanced uptake at acidic pH levels is consistent with the positive surface charge of Ce-MOF-808 indicated by the zeta potential (positive in the measured range of pH 2.82–8.60), favoring electrostatic attraction toward protonated chromate species (e.g., HCrO₄⁻). With an increasing pH, the capacity declined, likely due to electrostatic repulsion and competition from OH⁻ in alkaline media.

3.2.3. Effect of Contact Time

The effect of contact time on Cr(VI) adsorption was investigated in the range of 5–360 min under the optimized conditions (0.01 g adsorbent, 30 mL of 50 mg/L Cr(VI), pH 3, 25 °C). As illustrated in Figure 11, the adsorption capacity rose rapidly in the initial stage, a result that can be attributed to the abundance of available active sites. The adsorption approached equilibrium at around 180 min, and further prolongation of contact time did not result in a significant increase. These results were used to support the subsequent kinetic model fitting.

3.2.4. Adsorption Kinetics Study

The adsorption kinetics were investigated to clarify the time-dependent behavior of the uptake of Cr(VI) onto Ce-MOF-808. The experimental data were fitted using the pseudo-first-order and pseudo-second-order kinetic models. Linear plots of ln(q_e − q_t) versus t and t/qₜ versus t are shown in Figure 12a and Figure 12b, respectively, and the corresponding kinetic parameters are summarized in

Table 1. Kinetic parameters of the adsorption of Cr(VI) onto Ce-MOF-808.

		Pseudo-First-Order Model			Pseudo-Second-Order Model
Target	qexp (mg/g)	qe (mg/g)	k1 (min−1)	R2	qe (mg/g)	k2 (g/mg min−1)	R2
Cr(VI)	43.23	8.478	0.0135	0.683	43.29	0.0065	0.999

.

As presented in Table 1, the pseudo-second-order model exhibited a substantially higher correlation coefficient (R² = 0.9996) than the pseudo-first-order model (R² = 0.6826), indicating that the former better describes the adsorption process [55]. This finding suggests that chemisorption may be the dominant rate-controlling mechanism, a notion consistent with the partial reduction of Cr(VI) observed in the XPS analysis.

In addition, the good agreement between the experimental and calculated q_e values further supports that the pseudo-second-order model captures the intrinsic adsorption behavior of the system. The physical meaning of the PSO model implies that the rate-limiting step is associated with surface chemical interactions, particularly inner-sphere complexation and the Ce³⁺/Ce⁴⁺ redox-coupled process. Such a mechanism is consistent with the involvement of ≡Ce-OH groups and Ce-O clusters revealed by FT-IR, as well as the electron-transfer evidence confirmed by XPS.

3.2.5. Adsorption Isotherms

Adsorption isotherms were used to evaluate the equilibrium behavior of the adsorption of Cr(VI) onto Ce-MOF-808. The experimental data obtained under optimal adsorption conditions (pH 2, 25 °C, 12 h, 0.01 g adsorbent) with initial Cr(VI) concentrations ranging from 25 to 150 mg/L were fitted using the Langmuir (Figure 13a) and Freundlich (Figure 13b) isotherm models. The corresponding fitting parameters and correlation coefficients are listed in

Table 2. Isotherm parameters of the adsorption of Cr(VI) onto Ce-MOF-808.

Target	T (°C)	Langmuir Isotherm				Freundlich Isotherm
		qmax (mg/g)	RL	KL	R2	KF (mg/g)	1/n	R2
Cr(VI)	25	42.74	0.0224	0.58	0.992	32.01	0.0614	0.957

.

Both models were able to describe the adsorption process; however, the Langmuir model exhibited a higher correlation coefficient (R² = 0.9924) than the Freundlich model (R² = 0.9566), indicating that the adsorption of Cr(VI) onto Ce-MOF-808 is more consistent with monolayer adsorption on a homogeneous surface. The maximum adsorption capacity (q_max) obtained from the Langmuir model was 42.74 mg/g.

These results suggest that active sites on Ce-MOF-808 are energetically uniform during adsorption and that adsorption proceeds preferentially through surface saturation rather than multilayer accumulation [56].

3.2.6. Thermodynamic Analysis

The thermodynamic parameters of the adsorption of Cr(VI) onto Ce-MOF-808 were calculated using the van’t Hoff equation based on the equilibrium distribution coefficient (K_d) at different temperatures. A linear fit of ln K_d versus 1/T (Figure 14) yielded a slope of −518.1 and an intercept of −1.51, corresponding to an enthalpy change (ΔH°) of −4.31 kJ/mol and an entropy change (ΔS°) of −12.56 J/mol·K. The calculated Gibbs-free-energy change (ΔG°), summarized in

Table 3. Thermodynamic parameters of the adsorption of Cr(VI) onto Ce-MOF-808.

Sample	T (K)	∆H° (KJ/mol)	∆S° (J/mol)	∆G° (KJ/mol)
Cr(VI)	288	−4.31	−12.59	−0.68
	298	-	-	−0.55
	308	-	-	−0.43
	318	-	-	−0.30

, ranged from −0.68 to −0.30 kJ/mol in the temperature range of 288–318 K. All ΔG° values were negative, reflecting the spontaneous nature of the adsorption process. The gradual decrease in the absolute magnitude of ΔG° with an increasing temperature is consistent with an exothermic process [57]. The van’t Hoff plot exhibited a moderate correlation (R² = 0.76), which limits the quantitative reliability of the derived thermodynamic parameters. The moderate R² value is likely due to the narrow temperature window and limited variation in K_d, and therefore the obtained thermodynamic parameters are regarded as qualitative indicators rather than precise energetic constants.

3.3. Adsorption Performance in Actual Water Samples

As presented in Figure 15, the Cr(VI) adsorption capacities obtained from the actual lagoon water samples (S1 and S2) in Niigata Prefecture were comparable to, or even slightly higher than, the values predicted by the Langmuir isotherm.

Table 4. Concentration of Cl⁻, NO₃⁻, and SO₄²⁻ in actual water samples (S1 and S2).

Sample	${\mathrm{C}\mathrm{l}}^{-}$ (mg/g)	${{\mathrm{N}\mathrm{O}}_3}^{-}$ (mg/g)	${{\mathrm{S}\mathrm{O}}_4}^{2-}$ (mg/g)
$S_1$	5.02	4.7	7.84
$S_2$	13.94	0.98	7.57

shows that the concentrations of major anions (Cl⁻, NO₃⁻, and SO₄²⁻) varied between the two sampling sites, reflecting the differences in their surrounding environments and possible anthropogenic influences.

Nevertheless, the effective Cr(VI) uptake observed in both the lagoon water samples indicates that Ce-MOF-808 maintains high adsorption efficiency even under complex ionic conditions, demonstrating its applicability in realistic environmental water matrices.

3.4. Comparison with Other Adsorbents

To further evaluate the adsorption performance of Ce-MOF-808, its maximum adsorption capacity for Cr(VI) was compared with the capacities of previously reported adsorbents, including activated carbon-based composites, MOF-derived hybrids, and biopolymer-modified materials (

Table 5. Comparison of adsorption properties of various adsorbents. AC* denotes activated carbon.

Target	Adsorbent	qmax (mg/g)	Reference
Cr(VI)	Sludge (AC*)	23.18 (exp)	[19]
	Fe3O4-BC-ZIF-8	125 (L)	[58]
	Organoclays	10.04 (L)	[18]
	Walnut Shells (AC*)	38.46 (L)	[59]
	PANI@MOF	369.5 (exp)	[60]
	Eucalyptus bark (AC*)	10 (exp)	[61]
	Chitosan-MnO2	61.56 (L)	[62]
	Ce-MOF-808	42.74 (L)	This study

). Although several materials, such as PANI@MOF and Fe₃O₄-BC-ZIF-8, exhibit higher maximum adsorption capacities, these systems generally require more elaborate synthesis routes or rely on structures with limited aqueous stability. In contrast, Ce-MOF-808 provides a competitive adsorption capacity of 42.74 mg/g; simultaneously, it is simple to prepare and chemically robust and offers redox-active centers. These combined characteristics suggest that Ce-MOF-808 is a technically feasible and practically relevant candidate for removing Cr(VI) from water.

4. Conclusions

In this study, Ce-MOF-808 was successfully synthesized and applied for the removal of Cr(VI) from aqueous solutions. This superior performance of Ce-MOF-808 can be attributed to its Ce-O clusters and mixed Ce³⁺/Ce⁴⁺ redox pairs, which facilitate both electrostatic interaction and redox transformation during Cr(VI) uptake. Batch experiments showed that the adsorption of Cr(VI) by Ce-MOF-808 was strongly dependent on pH and favored acidic conditions. The adsorption reached equilibrium within 240 min and was well described by the pseudo-second-order kinetic model. The equilibrium data fitted the Langmuir isotherm with a maximum adsorption capacity of 42.74 mg/g. Thermodynamic analysis demonstrated that the process was spontaneous and exothermic within the examined temperature range. Characterization results confirmed that surface functional groups and Ce³⁺/Ce⁴⁺ redox pairs were involved in adsorption, and XPS analysis verified the partial reduction of Cr(VI) to Cr(III). The positive surface charge of Ce-MOF-808 under acidic conditions favors the adsorption of anionic chromate species. Moreover, Ce-MOF-808 maintained high removal efficiency in actual water samples, demonstrating good environmental applicability. Overall, the combination of a favorable adsorption capacity, redox-active sites, and satisfactory performance in complex aqueous matrices suggests that Ce-MOF-808 can be a promising candidate for practical Cr(VI) remediation in water treatment applications.

Although Ce-MOF-808 exhibited excellent Cr(VI) removal performance in its first use, the post-adsorption PXRD pattern revealed a substantial loss of long-range crystallinity. This structural degradation suggests that maintaining framework integrity during repeated cycles may be challenging. Therefore, improving the structural robustness and evaluating regeneration strategies for Ce-MOF-808 will be important directions in our future work.