English

• 0.   Introduction

  The widespread use of tetracycline (TC) has made it a concerning environmental contaminant, primarily due to its role in fostering antibiotic resistance^[1-3]. Conventional porous materials like activated carbon suffer from limited adsorption capacity for TC, as their non-specific surfaces fail to effectively engage TC′s complex molecular structure, which contains both hydrophilic and hydrophobic functional groups and exhibits pH-dependent charge states^[4-7]. This results in weak affinity and low removal efficiency^[8]. To address this challenge, metal-organic frameworks (MOFs) have emerged as promising alternatives. Their structural and chemical versatility allows for precise pore engineering and functionalization, making them ideally suited for the targeted capture of such complex antibiotic molecules.

  MOFs are crystalline porous materials assembled from metal ions and organic ligands, exhibiting exceptional properties such as enormous specific surface areas, tunable pore structures, and rich surface chemistry^[9-24]. These characteristics underpin their revolutionary potential for TC adsorption, offering three key advantages over traditional adsorbents like activated carbon. Firstly, their ultrahigh porosity provides a vast number of adsorption sites, enabling exceptionally high capacities. They are often several to dozens of times higher than conventional materials. Secondly, their structural tailorability allows for precise design: pores can be sized to match TC molecules, and functional groups can be introduced to engage in specific interactions (e.g., hydrogen bonding, π-π stacking), ensuring high selectivity even in complex wastewater matrices^[25]. Finally, the adsorption process is typically based on reversible interactions, enabling straightforward regeneration through solvent washing or mild treatment. Many MOFs maintain high performance over numerous cycles, offering excellent long-term cost-effectiveness^[26]. Consequently, MOF-based adsorbents represent a highly promising solution for efficient TC removal.

  In this study, a nickel-based MOF with the formula {(NH₂(CH₃)₂)₂[Ni₃(O)(L)₃(NH(CH₃)₂)₃]}_n (Ni₃-MOF) was synthesized by reacting 4, 4′-biphenyldicarboxylic acid (H₂L) with Ni(NO₃)₂·6H₂O in an N, N-dimethylformamide (DMF) solution. A nano-adsorbent with a particle size of approximately 200 nm (denoted as Ni₃-MOF-N) was successfully prepared using a cell disruptor. The Ni₃-MOF-N is a highly efficient adsorbent for removing TC from wastewater.

  1.   Experimental

  1.1   Reagents and instruments

  All solvents and chemicals were commercial reagents and used without further purification. H₂L, Ni(NO₃)₂·6H₂O, anhydrous methanol, DMF, HCl, NaOH, sodium dodecylbenzenesulfonate, and TC were purchased from Saen Chemical Technology (Shanghai) Co., Ltd.

  The material Ni₃-MOF-N was characterized by a suite of analytical techniques. Elemental analysis (C, H, N) was conducted on a PerkinElmer 240 analyzer. FTIR spectra were recorded from KBr pellets using a Nicolet 5DX spectrometer. The power X-ray diffraction (PXRD) was performed using Rigaku′s D/max 2500 X-ray diffractometer with Cu Kα radiation (λ=0.156 04 nm); the tube voltage was 40 kV, the tube current was 150 mA; a graphite monochromator was used; and 2θ was 5° to 35°. The crystal structure was determined by a Bruker APEX-Ⅱ CCD. Thermogravimetric analysis (TGA) was performed on a PerkinElmer Pyris Diamond TG-DTA instrument. The surface area was determined from N₂ adsorption-desorption isotherms using the Brunauer-Emmett-Teller (BET) method (AUTO CHEM Ⅱ 2920). The morphology and composition of the samples were characterized using scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM-2100). For SEM observation, the samples were sputter-coated with gold and analyzed at an accelerating voltage of 20 kV. For TEM analysis, the specimens were prepared by drop-casting a dilute dispersion onto a carbon-coated copper grid. TEM imaging and energy dispersive X-ray spectroscopy (EDS) mapping were performed simultaneously in STEM mode at an accelerating voltage of 200 kV. The EDS maps were acquired using a silicon drift detector (SDD) with a typical collection time of 60 s per area. The ζ potential was measured with a Malvern Zetasizer (ZEN 3600). Solution pH was monitored using an Ohaus STARTER 3000 pH meter, and TC concentration was quantified at 357 nm using a Shimadzu UV-2550 spectrophotometer.

  1.2   Synthesis of Ni₃-MOF

  H₂L (0.121 0 g, 5 mmol), and Ni(NO₃)₂·6H₂O (0.030 0 g, 1 mmol) were placed into a 23 mL Teflon-lined stainless steel autoclave. Then, 10 mL of DMF and 1 mL of anhydrous methanol were added to the autoclave. After sealing the autoclave with its stainless steel shell, the reaction mixture was heated at 150 ℃ in a forced-air oven for 72 h. Upon cooling to room temperature, the autoclave was opened to afford blue block-shaped crystals. Yield: 92.33% (based on Ni(Ⅱ)). Anal. Calcd. for C₅₂H₆₁N₅Ni₃O₁₃(%): C, 54.78; H, 5.35; N, 6.14. Found(%): C, 54.71; H, 5.45; N, 6.21. IR (cm^-1): 3 613 w, 3 521 w, 3 422 s, 3 062 w, 2 931 w, 1 927 w, 1 658 w, 1 600 s, 1 547 s, 1 376 s, 1 173 m, 1 108 m, 1 003 w, 819 m, 760 s, 675 m, 533 w, 439 w.

  1.3   X-ray diffraction

  Diffraction data for Ni₃-MOF were collected on a Bruker SMART CCD diffractometer (Cu Kα radiation, λ=0.156 04 nm) in Φ-ω scan modes. The anisotropic displacement parameters were applied to all non- hydrogen atoms in full-matrix least-squares refinements based on F² were performed using SHELXL-2013. The structure was solved by direct methods and refined using the Olex2 program^[27]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The hydrogen atoms were placed at calculated positions and refined as riding atoms with isotropic displacement parameters. The disordered carbon atoms on dimethylamine and the aromatic rings of the L^2- in the structure of Ni₃-MOF were refined using appropriate techniques. Crystallographic crystal data and structure processing parameters for Ni₃-MOF are summarized in Table 1. Selected bond lengths and bond angles for Ni₃-MOF are listed in Table 2.

  Table 1

  

  Table 1.  Crystal data and structure parameters for Ni₃-MOF

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  Parameter	Ni3-MOF		Parameter	Ni3-MOF
  Empirical formula	C52H61N5Ni3O13		Dc / (Mg·m-3)	1.317
  Formula weight	1 140.13		μ / mm-1	1.742
  Temperature / K	293(2)		2θ / (°)	3.7-72.9
  Crystal system	Trigonal		Index ranges	-13 ≤ h≤ 14, -12 ≤ k ≤ 9, -29 ≤ l ≤ 28
  Space group	P3c1		Reflection collected	10 318
  a / nm	1.136 000(10)		Independent reflection	1 763
  b / nm	1.136 000(10)		Rint	0.056 7
  c / nm	2.372 30(4)		Rσ	0.035 5
  Volume / nm3	2.651 29(6)		Data, number of restraints, number of parameters	1 763, 263, 163
  F(000)	1 140.0		R indexes [I≥2σ(I)]	R1=0.042 3, wR2=0.118 2
  Crystal size / mm	0.13×0.12×0.10		Final R indexes (all data)	R1=0.046 2, wR2=0.123 3
  Z	2		(Δρ)max, (Δρ)min / (e·nm-3)	600, -430

  Table 2

  

  Table 2.  Selected bond lengths (nm) and bond angles (°) for Ni₃-MOF

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  Ni1—O1	0.199 35(5)	Ni1—O2ⅲ	0.208 8(4)	Ni1—O2Aⅲ	0.206 0(4)
  Ni1—O4ⅰ	0.205 8(4)	Ni1—N3	0.213 2(3)	Ni1—O2A	0.206 0(4)
  Ni1—O4ⅱ	0.205 8(4)	Ni1—O4Aⅰ	0.209 1(4)	Ni1—O4Aⅱ	0.209 1(4)
  Ni1—O2	0.208 8(4)
  O1—Ni1—O4ⅱ	90.39(12)	O4ⅰ—Ni1—O2	88.09(17)	O2ⅲ—Ni1—O4Aⅰ	73.38(17)
  O1—Ni1—O4ⅰ	90.39(12)	O4ⅱ—Ni1—N3	89.61(12)	O2—Ni1—O4Aⅱ	73.38(17)
  O1—Ni1—O2ⅲ	93.55(11)	O4ⅰ—Ni1—N3	89.61(12)	O2ⅲ—Ni1—O4Aⅱ	106.17(17)
  O1—Ni1—O2	93.55(11)	O4ⅱ—Ni1—O4Aⅰ	161.25(10)	O2—Ni1—O4Aⅰ	106.17(17)
  O1—Ni1—N3	180.0	O4ⅰ—Ni1—O4Aⅱ	161.25(10)	O4Aⅱ—Ni1—N3	86.49(11)
  O1—Ni1—O4Aⅱ	93.51(11)	O4ⅱ—Ni1—O4Aⅱ	18.60(10)	O4Aⅰ—Ni1—N3	86.49(11)
  O1—Ni1—O4Aⅰ	93.51(11)	O4ⅰ—Ni1—O4Aⅰ	18.60(10)	O4Aⅱ—Ni1—O4Aⅰ	173.0(2)
  O1—Ni1—O2Aⅲ	90.37(12)	O4ⅰ—Ni1—O2Aⅲ	110.22(18)	O2A—Ni1—N3	89.63(12)
  O1—Ni1—O2A	90.37(12)	O4ⅱ—Ni1—O2Aⅲ	69.78(18)	O2Aⅲ—Ni1—N3	89.63(12)
  O4ⅱ—Ni1—O4ⅰ	179.2(2)	O2—Ni1—O2ⅲ	172.9(2)	O2Aⅲ—Ni1—O4Aⅰ	91.85(17)
  O4ⅱ—Ni1—O2	91.86(18)	O2—Ni1—N3	86.45(11)	O2A—Ni1—O4Aⅰ	88.11(17)
  O4ⅰ—Ni1—O2ⅲ	91.86(18)	O2ⅲ—Ni1—N3	86.45(11)	O2A—Ni1—O2Aⅲ	179.3(2)
  O4ⅱ—Ni1—O2ⅲ	88.09(17)

  1.4   Synthesis of the nano-adsorbent Ni₃-MOF-N

  Ni₃-MOF-N (0.050 0 g) and sodium dodecylbenzenesulfonate (0.017 4 g) were added to a 250 mL beaker. Subsequently, 100 mL of deionized water and 25 mL of anhydrous methanol were added to the beaker. The resulting suspension was first subjected to ultrasonication for 5 min, followed by treatment using a cell disruptor for 15 min. The precipitate was then collected by centrifugation at 10 000 r·min^-1 and washed three times with 20 mL of deionized water and three times with 20 mL of anhydrous methanol, respectively. Finally, the washed precipitate was dried in a forced-air oven at 80 ℃ for 12 h to obtain the nano adsorbent Ni₃-MOF-N.

  1.5   Adsorption experiments

  The adsorption experiments were conducted by adding the adsorbent to 100 mL of a TC solution (ρ=40 mg·L^-1, prepared in deionized water). The solution pH was modulated with 0.01 mol·L^-1 NaOH or HCl where necessary. At specified time intervals (10, 20, 30, 40, and 50 min), 5 mL of the mixture was extracted, immediately filtered (0.45 μm), and diluted tenfold for analysis. The concentration of TC in the diluted filtrate was quantified by UV-Vis spectroscopy at 357 nm. Subsequently, the adsorption capacity was calculated based on the concentration difference using the formula:

  $ q=\left(\rho_0-\rho_t\right) V / m $

  where q is the adsorption capacity (mg·g^-1), ρ₀ is the initial mass concentration (40 mg·L^-1), ρ_t is the TC mass concentration at time t (mg·L^-1), V is the volume of the solution (0.1 L), and m is the mass of the adsorbent used (g).

  2.   Results and discussion

  2.1   Structure of Ni₃-MOF

  Single-crystal X-ray diffraction reveals that Ni₃-MOF crystallizes in the trigonal system with the space group P3c1 (Table 1). The crystal data are refined by PLATON software. The SQUEEZE calculation results show that a unit cell has 136 electrons. Since Z=2, the total number of electrons in the unsolved guest molecules and dimethylamine in the molecular formula is 68. The dimethylamine cation ([NH₂(CH₃)₂]⁺) is counted as 27 electrons. X-ray single crystal diffraction also shows the presence of an anionic framework [Ni₃(O)(L)₃(NH(CH₃)₂)₃]^2- in Ni₃-MOF (Fig.1a), which requires two counter cations. Therefore, the molecular formula of Ni₃-MOF is {(NH₂(CH₃)₂)₂[Ni₃(O)(L)₃(NH(CH₃)₂)₃]}_n, indicating an ionic MOF. The [Ni₃(O)(L)₃(NH(CH₃)₂)₃]^2- unit contains three Ni(Ⅱ) ions, three L^2- ligands, one coordinated O^2- ion, and three coordinated dimethylamine (Fig.1a). In Ni₃-MOF, each Ni(Ⅱ) ion is coordinated to five oxygen atoms (four from carboxyl groups in the L^2- ligand and one from an O^2- anion) and one nitrogen atom (originating from dimethylamine). Each L^2- adopts a μ₄-η¹∶η¹∶η¹∶η¹ bridging mode to connect four Ni(Ⅱ) ions, forming a 3D structure (Fig.1b). A coordinated O^2- is also present, which adopts a μ₃-η¹∶η¹∶η¹ mode to bridge three adjacent Ni(Ⅱ) ions (Fig.1a). The Ni—O bond lengths range from 0.199 35(5) to 0.209 0(4) nm, and the Ni—N bond length is 0.213 3(3) nm. The smallest and largest bond angles in the Ni₃-MOF structure are ∠O4^ⅱ—Ni1—O2A^ⅲ [69.78(18)°] and ∠O1—Ni1—N3 (180.0°), respectively. These bond lengths and angles are within the range of values reported in the literature^[28]. Notably, six L^2- ligands bridge six adjacent [Ni₃O] clusters to form a 3D framework with porous channels (Fig.1c). Within these 3D channels (1.6 nm×1.6 nm), the coordinated dimethylamine cations are located on the inner surface of the pores (Fig.1c), while some Ni(Ⅱ) ions are exposed on the external surface of the framework (Fig.1d). Therefore, it can be anticipated that Ni₃-MOF has the potential to adsorb pollutants from wastewater through mechanisms such as hydrogen bonding, pore filling, and surface complexation.

  Figure 1

  

  Figure 1.  (a) Anion framework of [Ni₃(O)(L)₃(NH(CH₃)₂)₃]^2- in Ni₃-MOF (H atoms are omitted for clarity); (b) μ₄-bridging mode of the L^2- connecting Ni(Ⅱ) centers; (c, d) 3D porous framework formed through the linkage of Ni(Ⅱ) centers by L^2-

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  2.2   Morphology and physicochemical properties of Ni₃-MOF-N

  Nanoscale MOF adsorbents generally exhibit superior adsorption performance compared to their bulk crystalline counterparts. This is because reducing MOFs from bulk crystals to the nanoscale involves not only a change in physical size, but also a comprehensive unlocking of their adsorption potential as porous materials, achieved by shortening diffusion pathways, increasing the accessibility of active sites, and enhancing mass transfer efficiency. As a result, they demonstrate superior performance in adsorbing pollutants from wastewater. Based on this, the present study employed a mechanical fragmentation method using a cell disruptor to treat Ni₃-MOF crystals, successfully obtaining the nanoscale adsorbent Ni₃-MOF-N (Fig.2a). SEM (Fig.2b-2d) and TEM (Fig.2e-2g) observations revealed that the particle size of Ni₃-MOF-N was approximately 200 nm. EDS mapping analysis (Fig.2h-2l) confirmed that the material is primarily composed of four elements: Ni, C, N, and O.

  Figure 2

  

  Figure 2.  (a) Powdered sample of Ni₃-MOF-N; (b-d) SEM images of Ni₃-MOF-N; (e-g) TEM images of Ni₃-MOF-N; (h-l) EDS elemental mapping of Ni, C, N, and O for Ni₃-MOF-N

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  PXRD patterns (Fig.3a) confirmed that Ni₃-MOF-N retains the crystalline structure of the bulk Ni₃-MOF. TGA (Fig.3b) showed that Ni₃-MOF-N lost the coordinated dimethylamine molecules from its structure in the temperature range of 30-430 ℃. When the temperature exceeded 430 ℃, the L^2- ligands in the framework began to decompose, ultimately leaving approximately 25.07% of NiO as the residue. The N₂ adsorption-desorption isotherm (Fig.3c) exhibited a typical type-Ⅱ shape, with a specific surface area, pore volume, and average pore size of 541 m²·g^-1, 0.022 61 cm³·g^-1, and 13.31 nm, respectively. The experimentally measured pore volume and pore size were both smaller than the theoretical values, which further provides indirect evidence for the presence of free counterions in the Ni₃-MOF structure. Furthermore, ζ potential, which significantly influences the adsorption capacity between the adsorbent and pollutants through electrostatic interactions, was measured for Ni₃-MOF-N under different pH conditions. The results (Fig.3d) indicated an isoelectric point at approximately pH=6.50. Thus, the surface of the material is positively charged at pH values below 6.50 and negatively charged at pH values above 6.50. It is generally acknowledged that adsorbents often exhibit optimal adsorption performance near their isoelectric point^[29].

  Figure 3

  

  Figure 3.  (a) PXRD patterns of Ni₃-MOF-N; (b) TGA curve of Ni₃-MOF-N; (c) N₂ adsorption-desorption isotherm of Ni₃-MOF-N; (d) ζ potential of Ni₃-MOF-N at different pH values

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  2.3   TC adsorption performance of Ni₃-MOF-N

  2.3.1   Effect of adsorbent dosage on TC adsorption

  The effect of Ni₃-MOF-N dosage on the adsorption of TC was systematically investigated at 25 ℃ and pH=7.00. The experimental results (Fig.4) indicate that when using a 100 mL 40 mg·L^-1 TC solution, the adsorption process reached equilibrium within 40 min. It is noteworthy that the adsorbent dosage significantly influenced the removal efficiency of TC. Specifically, when the dosages of Ni₃-MOF-N were 0.005 0, 0.010 0, 0.015 0, and 0.020 0 g, the corresponding TC removal efficiencies after 40 min (Fig 4a) were 31.72%, 67.60%, 85.01%, and 98.25%, respectively, with average adsorption rates of 0.317 1, 0.676 0, 0.850 1, and 0.982 5 mg·L^-1·min^-1, respectively. Under these conditions, the measured equilibrium adsorption capacities (Fig.4b and 4c) were 253.6, 270.4, 226.7, and 196.5 mg·g^-1, respectively. Kinetic analysis revealed that the adsorption behavior of TC onto Ni₃-MOF-N followed the pseudo-first-order kinetic model (R² > 0.992 5) (Fig.4d). The fitted theoretical adsorption rates under different dosages were 0.048 09, 0.057 06, 0.058 48, and 0.071 32 g·mg^-1·min^-1, respectively. These results are consistent with the experimental observation that the adsorption rate increased with higher adsorbent dosage. Meanwhile, the theoretical equilibrium adsorption capacities derived from the pseudo-first-order model were 227.2, 308.6, 246.8, and 207.2 mg·g^-1, respectively. Combining the experimental data and kinetic fitting results, Ni₃-MOF-N exhibited optimal adsorption performance toward TC at a dosage of 0.010 0 g. Therefore, for an initial TC concentration of 40 mg·L^-1, the optimal dosage of Ni₃-MOF-N is ρ=0.100 0 g·L^-1.

  Figure 4

  

  Figure 4.  Effect of Ni₃-MOF-N dosage on the adsorption of TC: (a) removal efficiency; (b) amount of TC adsorbed at different Ni₃-MOF-N dosages; (c) maximum equilibrium adsorption capacity at different Ni₃-MOF-N dosages; (d) pseudo-first-order kinetic model fitting for the adsorption of TC onto Ni₃-MOF-N

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  2.3.2   Effect of pH on TC adsorption

  Under experimental conditions of 25 ℃ and a fixed Ni₃-MOF-N dosage of 0.010 0 g, the effect of solution pH on its TC adsorption performance was systematically investigated. As shown in Fig.5, the solution pH exerted a pronounced influence on the adsorption behavior of TC by Ni₃-MOF-N. Under different pH conditions (5.00, 6.00, 6.50, 7.00, 8.00), the adsorption reactions reached equilibrium within 40 min (Fig.5a). At equilibrium, the removal efficiencies of TC were 45.45%, 56.79%, 81.09%, 67.60%, and 62.48%, respectively, with corresponding average adsorption rates of 0.444 5, 0.567 9, 0.810 9, 0.676 0, and 0.624 8 mg·L^-1·min^-1. The calculated actual equilibrium adsorption capacities (Fig.5b and 5c) were 177.8, 227.2, 324.4, 270.4, and 249.9 mg·g^-1, respectively. Adsorption kinetics analysis indicated that the process followed the pseudo-first-order kinetic model (R² > 0.984 4) under all pH conditions. The theoretical adsorption rates obtained from the model fitting were 0.042 48, 0.047 20, 0.062 52, 0.048 09, and 0.047 99 g·mg^-1·min^-1, and the theoretical maximum adsorption capacities were 211.8, 262.1, 348.3, 308.6, and 285.6 mg·g^-1, respectively. Notably, when the solution pH was 6.50 (the isoelectric point of Ni₃-MOF-N), both the experimentally measured and model-fitted adsorption capacities reached their maximum values, at 324.4 and 348.3 mg·g^-1, respectively. This phenomenon can be attributed to the coupling effect between the speciation distribution of TC and the surface charge of the adsorbent. Within the pH range of 3.30-7.70, TC primarily exists as an electrically neutral zwitterionic species. When pH < 6.50, the surface of Ni₃-MOF-N is positively charged, and a high concentration of H⁺ competes with TC for adsorption sites, leading to reduced adsorption capacity. Conversely, when pH > 6.50, the surface of Ni₃-MOF-N becomes negatively charged. However, as the pH increases beyond 7.70, TC gradually deprotonates into an anionic form (TC^-), and OH^- also competes for surface adsorption sites, resulting in a decline in adsorption performance.

  Figure 5

  

  Figure 5.  Effect of pH on the adsorption of TC: (a) removal efficiency; (b) amount of TC adsorbed at different pH values; (c) maximum equilibrium adsorption capacity at different pH values; (d) pseudo-first-order kinetic model fitting for the adsorption of TC onto Ni₃-MOF-N

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  2.3.3   Effect of temperature on TC adsorption

  Under the conditions of a Ni₃-MOF-N dosage of 0.010 0 g and a solution pH of 6.50, the effect of temperature (20, 25, 30, and 35 ℃) on its TC adsorption performance was systematically investigated. The experimental results (Fig.6) demonstrated that temperature significantly influenced the adsorption behavior of TC onto Ni₃-MOF-N. At 20 ℃, the adsorption process reached equilibrium within 40 min, with a TC removal efficiency of 89.54% (Fig.6a). Similarly, at 30 and 35 ℃, adsorption equilibrium was also achieved within 40 min, with removal efficiencies of 71.93% and 63.16%, respectively (Fig.6a). Notably, the adsorption capacity (in terms of removal efficiency) at 20 ℃ was higher than that at 25 ℃ (81.09%), while the adsorption performance at 30 and 35 ℃ was lower than that at 25 ℃. Overall, within the temperature range of 20-35 ℃, the adsorption capacity of Ni₃-MOF-N for TC decreased with increasing temperature. The corresponding average adsorption rates at 20, 25, 30, and 35 ℃ were 0.895 4, 0.810 9, 0.719 3, and 0.631 6 mg·L^-1·min^-1, respectively. This phenomenon can be attributed to the enhanced thermal motion of molecules in the solution at higher temperatures, which reduces the probability of effective contact and adsorption between TC and the active sites on the Ni₃-MOF-N surface. Consequently, as shown in Fig.6b, the maximum equilibrium adsorption capacities of Ni₃-MOF-N for TC at 20, 25, 30, and 35 ℃ were 358.2, 324.4, 287.7, and 252.6 mg·g^-1, respectively.

  Figure 6

  

  Figure 6.  Effect of temperature on the adsorption of TC: (a) removal efficiency; (b) amount of TC adsorbed at different temperatures; (c) pseudo-first-order kinetic model fitting for the adsorption of TC onto Ni₃-MOF-N; (d) thermodynamic analysis of TC adsorption on Ni₃-MOF-N

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  The adsorption kinetics analysis results (Fig.6c) indicate that the adsorption behavior of TC in wastewater by Ni₃-MOF-N at 20, 25, 30, and 35 ℃ conforms to the pseudo-first-order kinetic model (R² > 0.988 8). The theoretical maximum adsorption capacities obtained from the model fitting were 373.6, 348.3, 315.6, and 292.0 mg·g^-1, respectively. It is noteworthy that the fitted values were generally higher than the experimentally determined values. This discrepancy may arise from dynamic interfering factors in actual wastewater systems, such as concentration fluctuations, temperature variations, and fluid shear forces, which could cause the adsorption process to deviate from ideal kinetic assumptions. Nevertheless, both the experimentally determined values and the pseudo-first-order kinetic fitted values demonstrate that the adsorption capacity of Ni₃-MOF-N for TC is significantly higher than that of most MOF materials reported in the literature (Table 3). The adsorption thermodynamic data were fitted according to the Van′t Hoff equation^[21-22]. The thermodynamic analysis results (Fig.6d) indicate that the adsorption of TC from wastewater by Ni₃-MOF-N within the temperature range of 20-35 ℃ is a spontaneous exothermic process (ΔH < 0, ΔG < 0), which is consistent with the experimental observations (Fig.6a and 6b). Therefore, within the range of 20-35 ℃, 20 ℃ is identified as the optimal adsorption temperature. Under the optimal conditions (T=20 ℃, pH=6.50, Ni₃-MOF-N dosage: 0.010 0 g), the maximum equilibrium adsorption capacity of Ni₃-MOF-N for TC reached 358.2 mg·g^-1. This performance surpasses that of most MOF-based adsorbents reported in the literature (Table 3), highlighting the exceptional potential of Ni₃-MOF-N for TC removal applications. The high efficiency of Ni₃-MOF-N in adsorbing TC from wastewater can likely be attributed to two key factors. Firstly, the exposure of some Ni(Ⅱ) sites (Fig.1d) on the material′s surface may facilitate the adsorption of TC. Secondly, the large specific surface area (Fig.3c) of Ni₃-MOF-N and the presence of hydrogen atoms (Fig.3d) from dimethylamine molecules within its pores enable a synergistic adsorption mechanism. This allows Ni₃-MOF-N to effectively capture TC molecules through a combination of pore-filling effects, electrostatic attraction, hydrogen bonding, surface complexation, and π-π interactions.

  Table 3

  

  Table 3.  TC adsorption performance of some MOFs reported in the literature

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  MOF	Adsorption capacity / (mg·g-1)	Ref.
  MOF-5	2 325	[30]
  SBA-15@MOF-525	690.0	[31]
  CYCU-3	428.1	[32]
  PCN-333	303.3	[33]
  MIL-53(Fe)	298.0	[34]
  UiO-67	282.3	[35]
  NH2-MIL-53(Fe)	152.3	[36]
  Zr/Fe-MOFs/GO	74.00	[37]
  MIL-101(HCl)	44.76	[38]
  Ni3-MOF-N	358.2	This work

  2.4   Recycling performance

  The Ni₃-MOF-N suspension after TC adsorption was collected in a 1 L beaker and centrifuged at 10 000 r·min^-1 to obtain the precipitate. The precipitate was then washed three times using a HCl solution (pH=4.00) and anhydrous methanol. The washed samples were placed in a blast drying oven and thermally treated at 80 ℃ for 12 h. The dried samples were characterized by PXRD for phase analysis. The PXRD pattern (Fig.3a) showed that the diffraction peak positions of Ni₃-MOF-N after desorption were consistent with the theoretical simulation results, indicating that its crystal structure remained intact after the desorption process. This confirms the good structural restorability and regeneration capability of the material.

  Cyclic adsorption experiments for TC were conducted using Ni₃-MOF-N under the conditions of 20 ℃, pH 6.50, and an adsorbent dosage of 0.010 0 g. The results (Fig.7) demonstrated that the maximum equilibrium adsorption capacity in the first cycle was 357.9 mg·g^-1, with an adsorption efficiency of 99.92%. In the second cycle, the maximum equilibrium adsorption capacity was 356.5 mg·g^-1, with an efficiency of 99.53%. Even after five cycles, the material maintained an adsorption capacity of 354.8 mg·g^-1 and an efficiency of 99.05%. The recycling experimental results indicate that Ni₃-MOF-N exhibited excellent structural stability and reliable adsorption performance over multiple adsorption/desorption cycles. Therefore, this material shows great potential for practical applications in removing TC-based organic pollutants from water bodies.

  Figure 7

  

  Figure 7.  Cycling adsorption performance of the TC adsorption by Ni₃-MOF-N

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  3.   Conclusions

  A novel nickel-based MOF, {(NH₂(CH₃)₂)₂[Ni₃(O)(L)₃(NH(CH₃)₂)₃]}_n (Ni₃-MOF), was hydrothermally synthesized. Structural characterization revealed that it has a 3D framework with 1.6 nm×1.6 nm channels. The material Ni₃-MOF-N, prepared from Ni₃-MOF and composed of 200 nm granular particles, demonstrated outstanding performance in TC adsorption. Under optimal conditions (T=20 ℃, pH=6.50, dosage of 0.010 0 g), it achieved a high capacity of 358.2 mg·g^-1, outperforming most known MOF adsorbents. The process was spontaneous, exothermic, and followed pseudo-first-order kinetics. Notably, Ni₃-MOF-N exhibited excellent recyclability and stability, underscoring its promise for practical wastewater remediation.

  
  
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• 

  Figure 1  (a) Anion framework of [Ni₃(O)(L)₃(NH(CH₃)₂)₃]^2- in Ni₃-MOF (H atoms are omitted for clarity); (b) μ₄-bridging mode of the L^2- connecting Ni(Ⅱ) centers; (c, d) 3D porous framework formed through the linkage of Ni(Ⅱ) centers by L^2-

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  Figure 2  (a) Powdered sample of Ni₃-MOF-N; (b-d) SEM images of Ni₃-MOF-N; (e-g) TEM images of Ni₃-MOF-N; (h-l) EDS elemental mapping of Ni, C, N, and O for Ni₃-MOF-N

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  Figure 3  (a) PXRD patterns of Ni₃-MOF-N; (b) TGA curve of Ni₃-MOF-N; (c) N₂ adsorption-desorption isotherm of Ni₃-MOF-N; (d) ζ potential of Ni₃-MOF-N at different pH values

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  Figure 4  Effect of Ni₃-MOF-N dosage on the adsorption of TC: (a) removal efficiency; (b) amount of TC adsorbed at different Ni₃-MOF-N dosages; (c) maximum equilibrium adsorption capacity at different Ni₃-MOF-N dosages; (d) pseudo-first-order kinetic model fitting for the adsorption of TC onto Ni₃-MOF-N

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  Figure 5  Effect of pH on the adsorption of TC: (a) removal efficiency; (b) amount of TC adsorbed at different pH values; (c) maximum equilibrium adsorption capacity at different pH values; (d) pseudo-first-order kinetic model fitting for the adsorption of TC onto Ni₃-MOF-N

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  Figure 6  Effect of temperature on the adsorption of TC: (a) removal efficiency; (b) amount of TC adsorbed at different temperatures; (c) pseudo-first-order kinetic model fitting for the adsorption of TC onto Ni₃-MOF-N; (d) thermodynamic analysis of TC adsorption on Ni₃-MOF-N

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  Figure 7  Cycling adsorption performance of the TC adsorption by Ni₃-MOF-N

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  Table 1.  Crystal data and structure parameters for Ni₃-MOF

  Parameter	Ni3-MOF		Parameter	Ni3-MOF
  Empirical formula	C52H61N5Ni3O13		Dc / (Mg·m-3)	1.317
  Formula weight	1 140.13		μ / mm-1	1.742
  Temperature / K	293(2)		2θ / (°)	3.7-72.9
  Crystal system	Trigonal		Index ranges	-13 ≤ h≤ 14, -12 ≤ k ≤ 9, -29 ≤ l ≤ 28
  Space group	P3c1		Reflection collected	10 318
  a / nm	1.136 000(10)		Independent reflection	1 763
  b / nm	1.136 000(10)		Rint	0.056 7
  c / nm	2.372 30(4)		Rσ	0.035 5
  Volume / nm3	2.651 29(6)		Data, number of restraints, number of parameters	1 763, 263, 163
  F(000)	1 140.0		R indexes [I≥2σ(I)]	R1=0.042 3, wR2=0.118 2
  Crystal size / mm	0.13×0.12×0.10		Final R indexes (all data)	R1=0.046 2, wR2=0.123 3
  Z	2		(Δρ)max, (Δρ)min / (e·nm-3)	600, -430

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  Table 2.  Selected bond lengths (nm) and bond angles (°) for Ni₃-MOF

  Ni1—O1	0.199 35(5)	Ni1—O2ⅲ	0.208 8(4)	Ni1—O2Aⅲ	0.206 0(4)
  Ni1—O4ⅰ	0.205 8(4)	Ni1—N3	0.213 2(3)	Ni1—O2A	0.206 0(4)
  Ni1—O4ⅱ	0.205 8(4)	Ni1—O4Aⅰ	0.209 1(4)	Ni1—O4Aⅱ	0.209 1(4)
  Ni1—O2	0.208 8(4)
  O1—Ni1—O4ⅱ	90.39(12)	O4ⅰ—Ni1—O2	88.09(17)	O2ⅲ—Ni1—O4Aⅰ	73.38(17)
  O1—Ni1—O4ⅰ	90.39(12)	O4ⅱ—Ni1—N3	89.61(12)	O2—Ni1—O4Aⅱ	73.38(17)
  O1—Ni1—O2ⅲ	93.55(11)	O4ⅰ—Ni1—N3	89.61(12)	O2ⅲ—Ni1—O4Aⅱ	106.17(17)
  O1—Ni1—O2	93.55(11)	O4ⅱ—Ni1—O4Aⅰ	161.25(10)	O2—Ni1—O4Aⅰ	106.17(17)
  O1—Ni1—N3	180.0	O4ⅰ—Ni1—O4Aⅱ	161.25(10)	O4Aⅱ—Ni1—N3	86.49(11)
  O1—Ni1—O4Aⅱ	93.51(11)	O4ⅱ—Ni1—O4Aⅱ	18.60(10)	O4Aⅰ—Ni1—N3	86.49(11)
  O1—Ni1—O4Aⅰ	93.51(11)	O4ⅰ—Ni1—O4Aⅰ	18.60(10)	O4Aⅱ—Ni1—O4Aⅰ	173.0(2)
  O1—Ni1—O2Aⅲ	90.37(12)	O4ⅰ—Ni1—O2Aⅲ	110.22(18)	O2A—Ni1—N3	89.63(12)
  O1—Ni1—O2A	90.37(12)	O4ⅱ—Ni1—O2Aⅲ	69.78(18)	O2Aⅲ—Ni1—N3	89.63(12)
  O4ⅱ—Ni1—O4ⅰ	179.2(2)	O2—Ni1—O2ⅲ	172.9(2)	O2Aⅲ—Ni1—O4Aⅰ	91.85(17)
  O4ⅱ—Ni1—O2	91.86(18)	O2—Ni1—N3	86.45(11)	O2A—Ni1—O4Aⅰ	88.11(17)
  O4ⅰ—Ni1—O2ⅲ	91.86(18)	O2ⅲ—Ni1—N3	86.45(11)	O2A—Ni1—O2Aⅲ	179.3(2)
  O4ⅱ—Ni1—O2ⅲ	88.09(17)

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  Table 3.  TC adsorption performance of some MOFs reported in the literature

  MOF	Adsorption capacity / (mg·g-1)	Ref.
  MOF-5	2 325	[30]
  SBA-15@MOF-525	690.0	[31]
  CYCU-3	428.1	[32]
  PCN-333	303.3	[33]
  MIL-53(Fe)	298.0	[34]
  UiO-67	282.3	[35]
  NH2-MIL-53(Fe)	152.3	[36]
  Zr/Fe-MOFs/GO	74.00	[37]
  MIL-101(HCl)	44.76	[38]
  Ni3-MOF-N	358.2	This work

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