Characterization of Cu-MOF@Co-MOFThe morphology of the prepared materials was characterized by SEM. Cu-MOF exhibited an octahedral structure shown in Fig. 2a. Figure S2 showed that Co-MOF was a dodecahedral morphology. Both the Cu-MOF and Co-MOF had uniform particle size and excellent dispersion. Figure 2b–d listed SEM images of Cu-MOF@Co-MOF composites. It was found that Co-MOF was dispersed and wrapped on the surface of Cu-MOF. With the increase of Co(NO3)2·6H2O dosage, the content of Co-MOF attached to the surface of Cu-MOF gradually increase. The Cu-MOF@Co-MOF-5 synthesized in the presence of 5 mmol Co(NO3)2·6H2O showed a uniform coating layer. When the amount of Co(NO3)2·6H2O continues to increase to 7 mmol, Co-MOF could not grow completely on the surface of Cu-MOF, leaving a lot of residual agglomerated Co-MOF around Cu-MOF@Co-MOF-7. To prove the crystal structure of these MOFs materials, XRD experiments were performed. The characteristic diffraction peaks of prepared Cu-MOF were located at 2θ = 6.69°, 9.48°, 11.60°, 13.41°, which were consistent with the simulated Cu-MOF (Fig. 3a). The characteristic peaks of Co-MOF at 7.33°, 10.38°, 12.70°, 18.01°, represented Co-MOF (001), (002), (112) and (222) peak of Co-MOF, respectively, confirming the successful synthesis of Co-MOF12. There was almost no characteristic peak of Co-MOF in the XRD pattern of Cu-MOF@Co-MOF-3.75, which might be because the Co content was too small to successfully wrap on the surface of Cu-MOF. The characteristic peaks of Cu-MOF and Cu-MOF could be clearly seen in the XRD patterns of Cu-MOF@Co-MOF-5 and Cu-MOF@Co-MOF-7, which confirmed the successful synthesis of the composite bimetal-organic framework.Figure 2SEM images of (a) Cu-MOF; (b) Co-MOF@Co-MOF-3.75; (c) Co-MOF@Co-MOF-5; (d) Co-MOF@Co-MOF-7.Figure 3Characterization results of (a) XRD patterns; (b) FTIR spectra; (c) XPS spectra; and (d) N2 adsorption–desorption isotherms.The composition of surface functional groups was characterized by FT-IR. Figure 3b showed the FTIR spectra of Cu-MOF, Co-MOF, Co-MOF@Co-MOF-5, and Cu-MOF@Co-MOF-7. The bands 1444 cm−1 and 1640 cm−1 represented the –O–C–O– group of Cu-MOF, and the bands of 1373 cm−1 and 1568 cm−1 were the C=C stretching vibration of the organic ligand13. The bands near 729 cm−1 belonged to the stretching vibration of Cu–O, indicating that the coordination between the oxygen atom and the Cu atom14. The main absorption peak of the FT-IR spectrum of Co-MOF was related to 2-methylimidazole, and the peak at 600–1500 cm−1 indicated the stretching and bending modes of the imidazole ring. The peaks at 2927 and 3131 cm−1 belonged to the stretching patterns of the aromatic ring of 2-MIm and the C–H bond of the aliphatic chain, respectively. The peak at 1582 cm−1 was caused by the stretching pattern of the C=N bond in 2-MIm15. The appearance of three new absorption bands at 3433, 2920, and 2850 cm−1 in Cu-MOF@Co-MOF-5, which could be attributed to N–H bond bending vibrations of tertiary amines and C–H stretching vibrations, respectively, confirmed the fixation of Co-MOF on the Cu-MOF surface during Cu-MOF@Co-MOF-5 synthesis. In addition, compared with Cu-MOF, the FT-IR spectrum of Cu-MOF@Co-MOF-5 showed a redshift at the characteristic peaks of 1620, 1435, 1369, and 727 cm−1, which further confirmed the successful recombination of Cu-MOF and Co-MOF. All the unique absorption bands of Co-MOF and Cu-MOF were present in the structure of Cu-MOF@Co-MOF-5 and Cu-MOF@Co-MOF-7 with a reduction in the spectral intensity due to the recombination process.Bimetallic MOFs with different Cu-Co ratios were studied by XPS analysis. From the XPS spectrum in Fig. 3c, it could be seen that the characteristic peaks of Cu2p, Co2p and C1s coexisted in the spectrum, and the intensity of the characteristic peaks of XPS of Co increased significantly with the increase of Co content, which further confirmed the successful synthesis of Cu-Co bimetallic MOFs. The specific surface area was calculated using the nitrogen adsorption–desorption isotherm of the Cu-MOF@Co-MOF-5 sample combined with the Brunauer-Emmet-Teller method (Fig. 3d). The BET surface areas, pore size and pore volume of the Cu-MOF@Co-MOF-5 were 581.93 m2/g, 2.23 nm and 0.324 cm3/g, respectively, indicating the large specific surface area and mesoporous structure of the material. The above characterization results confirmed the successful synthesis of MOF-on MOF structure, and revealed various functional groups in and on the surface of Cu-MOF@Co-MOF-5 composites, which contributed to the adsorption of TCs pollutants. Therefore, Cu-MOF@Co-MOF was used to refer to Cu-MOF@Co-MOF-5 in the following description.Adsorption experimentIn order to investigate the adsorption performance of Cu-MOF@Co-MOF materials on TCs, the variables such as the initial concentration, pH value, TC concentration, adsorption time and coexisting ions during the adsorption of MOF materials on TCs were studied and optimized.Effect of initial concentration on the adsorption of TCsThe effect of the initial concentration on the removal efficiency of TCs was studied at a dosage of 10 mg Cu-MOF@Co-MOF and a contact time of 5 and 15 min. The experiments were tested at initial concentrations of TC and DOX of 10–450 mg/L. As exhibited in Fig. S3a, the adsorption capacity of Cu-MOF@Co-MOF increased with the increase of TC and DOX loading, which might be due to the increasing number of collisions between the Cu-MOF@Co-MOF and TCs molecules, which led to a greater driving force for the diffusion of TCs molecules with higher concentration from solution to the adsorption site. When the concentrations of TC and DOX were increased to 240 and 400 mg/L, respectively, the adsorption capacities of the two TCs reached equilibrium.Effect of adsorption time and temperatureThe high adsorption rate can greatly reduce the cost of water treatment by shortening the water treatment time, which reflects the application potential of a new type of adsorbent. To test the effect of adsorption time on removal efficiency, adsorption experiments from 1 to 25 min were performed at 25 °C. The initial adsorption rate of Cu-MOF@Co-MOF for TC and DOX was very fast, which was attributed to the abundant adsorption sites and strong affinity provided by the Cu-MOF@Co-MOF adsorbent. Then, as more and more adsorption sites were occupied by TCs molecules, the adsorption and removal rate enhanced slowly with the increase of contact time. Finally, the adsorption processes of TC and DOX tended to equilibrium at 5 and 15 min, respectively, and the maximum removal rate reached 95% (Fig. S3b).Temperature affects the cavitation phenomenon and solubility of the analyte, which in turn affects the mass transfer process and also a major factor affecting the adsorption removal efficiency of TCs. The effect of temperature was investigated at 25 ℃, 35 ℃ and 45 ℃, and the adsorption capacity decreased with the increase of temperature. In contrast, TCs exhibited the best removal performance at 25 °C.Effect of incipient solution pHThe determining factors of pH value affecting adsorption capacity are surface functional groups and TCs molecular structure, as it will affect the degree of ionization of the Cu-MOF@Co-MOF and the form of the molecule. The effect of Cu-MOF@Co-MOF on adsorption in pH range of 2.0–11.0 was investigated and shown in Fig. S3c. Although the surface of Cu-MOF exhibits a negative charge16, the Co-MOF attached to its surface in most cases has a positive surface charge, evidenced by the zeta potential values in the PH range of 2–1017. Therefore, when pH was lower than 5, Cu-MOF@Co-MOF surface was positively charged, and TC and DC molecules were converted into TC+ and DOX+ ions, respectively. The repulsive force between TCs+ molecules and the adsorbent surface limited the adsorption capacity to some extent. When the pH was between 5 and 9, the TC and DOX molecule were neutral, and the adsorption process mainly relied on non-electrostatic attraction. At pH above 10, Cu-MOF@Co-MOF became negatively charged, and the electrostatic repulsion between TCs and Cu-MOF@Co-MOF surface again resulted in a decrease of adsorption capacity. However, even at pH below 5 and above 9, the maximum adsorption capacity was not less than 420 mg/g, which indicated that the Cu-MOF@Co-MOF can efficiently remove TC and DOX from environmental pollution systems over a wide pH range.Effect of coexisting anions and ionic strength interferenceIn addition to tetracycline pollutants, various environmental waters and industrial wastewater usually contain various toxic compounds such as salt acid, alkali, and metal ions, which will greatly reduce the removal performance of the adsorbents. In theory, when there is a repulsive electrostatic interaction between the pollutant and the adsorbent, the increase of ionic strength or salt concentration will increase the adsorption capacity of the adsorbent. If the electrostatic interaction is mutually attractive, the effect of increasing the salt concentration on the adsorption capacity will be weakened18. Therefore, solutions with different ionic strength were obtained by adjusting NaCl, KCl, CaCl2 and MgCl2, and based on this, the effect of ionic strength on the adsorption capacity of antibiotics was investigated. Under the same conditions, when 10 mol/L salt solution was added to TCs solution, the adsorption capacity of TC and DOX decreased by 48.9 and 20%, respectively, indicating that the addition of salt ions would reduce the removal rate of TCs.Adsorption mechanism studyIsotherm adsorptionAdsorption isotherms are a class of correlation curves used to describe the equilibrium distribution relationship between adsorbate and adsorbent material in a solution at a certain temperature. The potential adsorption capacity was evaluated using the Langmuir, Freundlich, Temkin and Dubinbin-Radushkevich (D-R) isotherm models and the adsorption mechanism was further determined19,20,21.$$\frac{{{\text{C}}_{{\text{e}}} }}{{{\text{q}}_{{\text{e}}} }} = \frac{1}{{{\text{q}}_{{\text{m}}} {\text{K}}_{{\text{L}}} }} + \frac{{{\text{C}}_{{\text{e}}} }}{{{\text{q}}_{{\text{m}}} }}$$
(4)
$${\text{ lnq}}_{{\text{e}}} = \ln {\text{K}}_{{\text{F}}} + \frac{1}{{\text{n}}}{\text{ln C}}_{{\text{e}}}$$
(5)
$${\text{q}}_{{\text{e}}} = {\text{BlnK}}_{{\text{T}}} + {\text{BlnC}}_{{\text{e}}}$$
(6)
$$\ln {\text{q}}_{{\text{e}}} = \ln {\text{q}}_{{\text{m}}} – {\text{k}}_{{{\text{DR}}}} \,\varepsilon^{2} { }\left( {\varepsilon = {\text{RTln}}\left( {1 + \frac{1}{{{\text{C}}_{{\text{e}}} }}} \right)} \right)$$
(7)
$${\text{E}} = \left( {2{\text{K}}_{{{\text{DR}}}} } \right)^{ – 1/2}$$
(8)
qm is the maximum adsorption capacity (mg/g); Ce stands for equilibrium concentration (mg/L), KL represents the Langmuir constant (L/mg), 1/n and KF are heterogeneity factor and Freundlich constant, respectively, B is the constant related to the heat of adsorption, KT represents the equilibrium binding constant at the maximum binding energy (L/mg), ε is the Polanyi potential (KJ/mol), KDR is the D-R isotherm constant (mol2/KJ2), E (KJ/mol) is the amount of free energy change of 1 mol TCs from solution to surface, which is beneficial to estimate the type of adsorption reaction, which can be obtained from kad.The adsorption isotherm can not only describe the adsorption capacity of the adsorbent, but also reveal the molecular layer characteristics when the analyte and the adsorbent reach adsorption equilibrium. The fitting curves and corresponding parameters of four isothermal models were fitted to the experimental data at different concentrations, and the results were exhibited in Fig. 4 and Table 1. The Langmuir model is originally used for gas adsorption on solid surfaces, assuming that adsorption is mainly a uniform monolayer chemical adsorption process. Usually there is no spatial resistance and lateral force between adsorbents and adsorbate is a single layer covered on the surface of the adsorbent. When each adsorption site on the adsorbent surface adsorbs a molecule, its adsorption capacity reaches the maximum value, and the whole adsorption process is in a dynamic equilibrium state. Moreover, the Langmuir model can also evaluate the potential maximum adsorption capacity of the adsorbent. Here, the Langmuir isotherm model was used to determine the maximum saturated adsorption capacity, adsorption mechanism and adsorption driving force. The fitting curves corresponding to the Langmuir model in Fig. 4a showed that the correlation coefficients (R2) between TC and DOX reached 0.999, which was higher than that of the other three models. The maximum adsorption capacity (qm) of TC and DOX was 434.78 and 476.19 mg/g, respectively, calculated by Langmuir isothermal. Moreover, the adsorption data obtained by the experiment were also closer to the Langmuir isotherm model, indicating that the adsorption of TC and DOX occurred at the homogeneous adsorption site.Figure 4Isotherms fitting curves of TCs on Cu-MOF@Co-MOF: (a) Langmuir model and (b) Freundlich model.Table 1 Adsorption isothermal models and its correlation fitting coefficients.The Freundlich isotherm model is used to represent the multilayer adsorption characteristics and equilibrium data of heterogeneous surfaces under non-ideal conditions. The adsorption amount is the sum of adsorption at all sites, which can explain the experimental results in a wider range. For the process of adsorption of TCs by Cu-MOF@Co-MOF, the correlation coefficient (R2) of the fitting curve of the Freundlich model ranged from 0.629 to 0.94 (Fig. 4b). n is a parameter that can be used to represent the adsorption process, which can be physical (n > 1), chemical (n < 1), or a linear (n = 1) process. The n of this model was about 5, indicating that there was a partial physisorption process. In addition, 1/n is an important indicator of surface heterogeneity. As the surface of the material becomes more heterogeneous, 1/n will be closer to zero. The 1/n value of this model was 0.05–0.08, indicating a certain degree of heterogeneity on the surface of Cu-MOF@Co-MOF22.The Temkin adsorption isothermal model is a real model, which assumes that the adsorption heat of molecules in the adsorption layer decreases linearly with the increase of coverage, and also takes into account the influence of the interaction between the adsorbate and the adsorbent. The parameters of the Temkin model in this work were the slope and intercept of the curve between qe and lnCe. The Temkin fitting model in Fig. S4a,b showed that Temkin also had a relatively good fitting results, indicating that with the increase of the number of layers covered by TCs molecules, the binding force decreased linearly, so the adsorption process between TCs and Cu-MOF@Co-MOF was dominated by uniformly distributed binding force23.The D-R isotherm model, proposed in 1947 by Dubinbin and Radushkevich, is an empirical formula based mainly on pore filling theory that can be used to illustrate the effects of the porous structure of adsorbents. This model is more suitable for describing the actual adsorption situation and characteristics. The D-R isothermal adsorption model is used to describe whether the adsorption process is physical or chemical adsorption, which is mainly related to the type of adsorption reaction. The mean adsorption free energy (E) calculated from D-R isothermal model constant kad can be used to evaluate the mechanism of adsorption and confirm the adsorption reaction type, such as physical adsorption (1.0 kJ/mol < E < 8.0 kJ/mol), ion exchange and electrostatic attraction (8.0 kJ/mol < E < 16.0 kJ/mol), and chemisorption processes (E > 16.0 kJ/mol). The E values of TC and DOX shown in Fig. S4c,d and Table 1 were 0.79 and 2.5 kJ/mol, respectively, which were lower than 8 kJ/mol, indicating that the adsorption of Cu-MOF@Co-MOF to the two TCs was mainly dominated by physical adsorption24,25.The above adsorption isotherms verified that the adsorption process of Cu-MOF@Co-MOF to TCs contained both chemical and physical adsorption, while the heterogeneous adsorption on the surface of the Cu-MOF@Co-MOF comprised monolayer and multilayer adsorption.Kinetics studyThe adsorption kinetics were studied by measuring the adsorption equilibrium time and adsorption rate to elucidate the adsorption process. Adsorption kinetic models are commonly used to describe rapid adsorption processes, and their fitting data are used to analyze the rate-limiting steps and adsorption mechanisms. In order to further explore the adsorption mechanism, the adsorption kinetics of TCs were investigated using Cu-MOF@Co-MOF as adsorbent. Some widely used dynamic adsorption models such as pseudo-first-order, pseudo-second-order, Elovich, liquid film diffusion and intraparticle diffusion models26,27,28 were used for fitting, shown in eqn. as follows.$${\text{ln}}\left( {{\text{q}}_{{\text{e}}} – {\text{q}}_{{\text{t}}} } \right) = {\text{lnq}}_{{\text{e}}} – {\text{k}}_{1} {\text{t}}$$
(9)
$$\frac{{\text{t}}}{{{\text{q}}_{{\text{t}}} }} = \frac{1}{{{\text{k}}_{2} {\text{q}}_{{\text{e}}}^{2} }} + \frac{{\text{t}}}{{{\text{q}}_{{\text{e}}} }}$$
(10)
$${\text{q}}_{{\text{t}}} = \frac{1}{\beta }\ln \left( {\alpha \beta } \right) + \frac{1}{\beta }{\text{ln}}\left( {\text{t}} \right)$$
(11)
$$\ln \left( {1 – {\text{F}}} \right) = {\text{C}}_{1} – {\text{k}}_{{{\text{fd}}}} {\text{ t}}$$
(12)
$${\text{F}} = \frac{{{\text{q}}_{{\text{t}}} }}{{{\text{q}}_{{\text{e}}} }}$$
(13)
$${\text{q}}_{{\text{t}}} = {\text{k}}_{{{\text{id}}}} \cdot {\text{t}}^{0.5} + {\text{C}}_{2}$$
(14)
where k1 is the rate constant of the pseudo-first-order adsorption (min−1), qe is the adsorption capacity at equilibrium (mg/g), qt is the adsorption amount at time (mg/g), k2 (g/mg/min) is the rate constant of the pseudo-second-order equation, α is the initial adsorption rate of the reaction (mg/g/min), β is the rate constant related to surface coverage and chemical adsorption energy (g/mg), Kid is the intraparticle diffusion rate constant (mg/g/min0.5), C1 and C2 is the constant related to thickness and boundary layer (mg/g), Kfd is the liquid film diffusion constant (min−1).The fitting curves for several kinetic models were shown in Fig. 5, and the corresponding parameters were summarized in Table 2. The adsorption equilibrium of the TCs basically reached within 15 min, and the adsorption rate was relatively fast in the first 5 min, and then slowed down. The fitting of the pseudo-first-order model indicates that the adsorption is dominated by physical adsorption, while the fitting of the pseudo-second-order kinetic model means that chemical adsorption is the main rate control step in the adsorption process. As shown in Fig. 5a,b and Table 2, the equilibrium adsorption capacity of TCs calculated by the pseudo-second-order model was relatively close to the experimental equilibrium adsorption capacity. The correlation coefficient (R2) of the pseudo-second-order model of the two TCs (≥ 0.995) was higher than that of other models, which confirmed that the pseudo-second-order model was more suitable for understanding the adsorption mechanism, indicating that the adsorption process was mainly controlled by chemisorption.Figure 5The kinetic models fitting curves of the Cu-MOF@Co-MOF for TCs: (a) Pseudo-first-order model; (b) Pseudo-second-order model; (c) Elovich model; and (d) Liquid film diffusion model.Table 2 Adsorption kinetic models and its correlation fitting coefficients.The Elovich model is based on the assumption that the heat of adsorption on the adsorbent surface decreases linearly with the increase of its surface coverage. This model is often used to investigate the existence of chemisorption processes on the non-uniform surface of adsorbents during water treatment. The fitting curve of the model generally presents two stages, which are the stage of rapid surface adsorption and slow diffusion speed, respectively. For Elovich kinetic model, the calculated values for the model-related parameters such as α (adsorption rate) and β (desorption rate) were shown in Table 2. The α-values of TC and DOX were 4.62 × 109 and 9.11 × 104 mg/g/min, respectively, confirming the faster absorption of TC. The β-values of TC and DOX were 0.0061 and 0.0059 g/mg, respectively, indicating that TC also had a faster desorption rate. The linear correlation coefficients of the fitting curves were 0.965 and 0.978 for TC and DOX, respectively, which verified the good fitting of the Elovich kinetic model (Fig. 5c).The C value of the liquid film diffusion model reflected the external surface or instantaneous adsorption (Fig. 5d). The small C values shown in Table 2 indicated that the liquid film diffusion was not dominant during TCs adsorption29. Intraparticle diffusion models is mainly applied to evaluate the diffusion mechanism during adsorption kinetics. For the adsorption kinetic model, the initial adsorption usually occurs on the outer surface of the adsorbent, and when all the surface adsorption sites reach saturation, the internal adsorption and intra-particle diffusion processes may occur. As shown in Fig. S5, the intraparticle diffusion model was used to fit the adsorption kinetic data, and two stages of adsorption of TCs in Cu-MOF@Co-MOF were obtained. The two stages were fast adsorption and tardiness adsorption, respectively. The first stage of rapid adsorption was external diffusion and the abundant adsorption sites on Cu-MOF@Co-MOF greatly promoted this rapid adsorption. In the second stage, the tardiness adsorption process was mainly dominated by intra-particle diffusion. The fitting data of the intraparticle diffusion models were linear, and the fitting curve did not cross the origin, indicating that intraparticle diffusion models had a significant effect on the adsorption of two TCs on Cu-MOF@Co-MOF, but it was not the only step rate-determing step.Thermodynamics studyThe determination of thermodynamic parameters is important to evaluate whether the adsorption process is exothermic or endothermic, which can reveal the law of adsorption process change with temperature. Thermodynamic studies were carried out by investigating the adsorption process at different temperatures (298 K, 308 K, 318 K). Effective factors that need to be quantified in thermodynamic studies include Gibbs free energy (ΔG°), entropy change (ΔS°), and enthalpy change (ΔH°), which are calculated as follows27:$${\text{K}}_{{\text{c}}} = \frac{{{\text{q}}_{{\text{e}}} }}{{{\text{c}}_{{\text{e}}} }}$$
(15)
$$\ln {\text{K}}_{{\text{c}}} = \frac{{\Delta {\text{S}}^{^\circ } }}{{\text{R}}} – \frac{{\Delta {\text{H}}^{^\circ } }}{{{\text{RT}}}}$$
(16)
$$\Delta {\text{G}}^{^\circ } = \Delta {\text{H}}^{^\circ } – {\text{T}}\Delta {\text{S}}^{^\circ }$$
(17)
where, R (8.3145 J/mol/K) stands for the ideal gas constant, T (K) is the absolute temperature during adsorption, Kc represents the thermodynamic equilibrium constant. The Ce and qe are the equilibrium concentration and adsorption capacity of TCs, respectively. The slope and intercept are calculated by lnKc-1/T fitting to obtain ΔH° and ΔS°.With the increase of temperature, the viscosity of the solution and the thickness of the boundary layer around the adsorbent decreases, which is conducive to accelerating the diffusion of the mass transfer process. If the adsorption process is mainly physical adsorption, the adsorption capacity will decrease when the temperature increased. The thermodynamic parameters were obtained by the slope and intercept of van’t Hoff equations and the relevant thermodynamic results were depicted in Fig. S6 and Table 3. It could be seen in Table 3 that the adsorption capacity decreased with the increase of temperature, and the Gibbs free energy (ΔG°) at all temperatures was negative, confirming that the adsorption process of TC and DOX on the Cu-MOF@Co-MOF could be carried out spontaneously. The value of enthalpy change (ΔH°) of the TC and DOX was − 14.507 and − 8.548 kJ/mol, respectively, indicating that the adsorption process of TC and DOX was an exothermic reaction. In addition, the calculated negative entropy change value (ΔS°) of TC and DOX indicated a decrease in irregularity between the adsorbent and solution, which led to a decrease in the removal process of TC and DOX30. In summary, the process of TC adsorption and removal is a spontaneous, entropy-decreasing and exothermic reactions.Table 3 Thermodynamic parameters of TC and DOX adsorption onto Cu-MOF@Co-MOF at different temperatures.Possible adsorption mechanismThe study of the adsorption mechanism is of great benefit to the development of novel high-performance adsorbents, and can effectively promote the application of existing adsorbents. The Cu-MOF@Co-MOF before and after adsorption of TCs was characterized by XRD, FTIR, BET and XPS, and the adsorption mechanism was further discussed. As shown in Fig. 6a, compared with Cu-MOF@Co-MOF, the spectra of Cu-MOF@Co-MOF-TC and Cu-MOF@Co-MOF-DOX showed obvious TC and DOX fingerprint peaks (1600–750 cm−1), respectively, indicating that TCs molecules were successfully adsorbed. Moreover, the stretching vibration peak of the N–H bond at 3433 cm−1 shifted significantly after the adsorption of TCs, and all of them moved to the lower wavenumber to 3430 and 3420 cm−1, indicating that hydrogen bonding was involved in the adsorption process. Besides, the transition of C=C aromatic stretching from 1613 to 1618 cm−1 demonstrated Cu-MOF@Co-MOF could adsorb TCs by π-π interaction. The XRD spectra of Cu-MOF@Co-MOF before and after adsorption were shown in Fig. 6b. After TCs adsorption, the intensity of all the XRD characteristic peaks was greatly reduced, which might be due to the formation of coordination bonds between Co and Cu of Cu-MOF@Co-MOF and N and O atoms of TC molecules.Figure 6(a) FTIR spectra; (b) XRD spectra; and (c–f) XPS spectra: (c) XPS Survey, (d) Cu2p, (e) Co2p, (f) C1s of Cu-MOF@Co-MOF before and after adsorption.The adsorption mechanism was further confirmed by the XPS spectroscopy of Cu-MOF@Co-MOF before and after adsorption of TCs. As can be seen in Fig. 6c, all the spectra showed obvious characteristic peaks around 284.68 eV, 780.98 and 932.58 eV, representing C1 s, Co2p and Cu2p, respectively. After TCs adsorption, the height of characteristic peaks of each element was slightly different, mainly reflected in that the O1s peak was significantly stronger and the C1s, Co2p and Cu2p peak was weaker, which proved that TCs was successfully adsorbed on Cu-MOF@Co-MOF. The strength of O1s increased significantly after TCs adsorption, indicating that hydrogen bond interaction was involved in the adsorption process31. It was noteworthy that after TCs adsorption, the XPS spectra of Co2p and Cu2p peaks shown in the Fig. 6d,e exhibited a slight shift, indicating charge transfer between Cu-MOF@Co-MOF and TCs, which could benefit from the strong surface complexation between the Cu and Co unsaturated sites of the adsorbent and the oxygen (nitrogen) elements of TCs32. Cu and Co acquired electrons from the amino and hydroxyl groups of TCs and form Cu(Co)–O–C and Cu(Co)–N–C bonds via coordination bonds. Compared with pure Cu-MOF@Co-MOF, the binding energy of C1s peaks (C–C, C–O and C–O bonds) in Cu-MOF@Co-MOF–TC and Cu-MOF@Co-MOF–DOX was lower, and the XPS spectra showed obvious redshift, which might be caused by the π–π conjugated interaction between TC molecules and the benzene ring of Cu-MOF@Co-MOF (Fig. 6f)5.The pore filling effect also occupied a large proportion in the adsorption process of TCs. The large specific surface area and abundant pore structure of Cu-MOF@Co-MOF can promote the adsorption of TCs22. We compared the BET area, pore volume and pore size of Cu-MOF@Co-MOF before and after TC adsorption to further confirm this adsorption mechanism. Before adsorption, the pore size of Cu-MOF@Co-MOF was mainly about 2.23 nm, which was larger than that of TCs (1.27 nm), indicating that TCs could shuttle to the inside of the pores. As shown in Table 4, the pore size of Cu-MOF@Co-MOF increased to 13.08–17.47 nm and the pore volume decreased by 0.16–0.17 nm after the adsorption of TCs, which confirmed that the TCs entered the adsorption site of Cu-MOF@Co-MOF through the mesoporous channel and filled the inside of the hole. Furthermore, the surface area of Cu-MOF@Co-MOF decreased significantly from 558.25 to 30–45 m2/g after adsorption, verifying that pore filling played a key role in this process.Table 4 Pore character of Cu-MOF@Co-MOF before and after adsorption.From the characterization spectra before and after adsorption combined with the adsorption model data, it could be concluded that the adsorption between TCs and Cu-MOF@Co-MOF was enhanced by the synergistic effect of hydrogen bonding, π–π interaction, coordination, electrostatic interaction and pore filling effect due to the structure of multiple phenolic hydroxyl, carbonyl and amino functional groups, abundant conjugated benzene rings of TCs and the unique superior properties of MOFs (Fig. 7).Figure 7The adsorption mechanism of TCs on Cu-MOF@Co-MOF.Comparison of adsorption property with that of other adsorbentsThe adsorption properties of TCs were investigated based on Cu-MOF, Co-MOF and Cu-MOF@Co-MOF adsorbents, respectively. As shown in Fig. S6, the adsorption capacity of Cu-MOF for TC and DOX was 81.69 and 142.70 mg/g, respectively, while the adsorption capacity of Co-MOF was higher than that of Cu-MOF (282.44 and 229.39 mg/g, respectively), which was much lower than that of Cu-MOF@Co-MOF for TCs under the same conditions. Furthermore, thanks to the high specific surface area of Cu-MOF@Co-MOF and the synergistic effect of multi-target adsorption in the bimetallic center, the Cu-MOF@Co-MOF exhibited superior removal properties for TCs, demonstrating the great advantages of Cu-Co bimetallic MOFs in the field of TCs removal.In addition, the Cu-MOF@Co-MOF adsorbent used in this work was also compared with other reported adsorbents to further evaluate its application prospects. Table 5 provided a comparison of Cu-MOF@Co-MOF with other adsorbents reported recently for the removal of TC and DOX. It could be seen that compared with most of the adsorption materials reported in 2023, the adsorption time of the prepared Cu-MOF@Co-MOF composites for TC and DOX was greatly shortened, and the adsorption capacity was significantly higher than that of the existing adsorption capacity, indicating that Cu-MOF@Co-MOF composites had great application potential in the removal of TCs in polluted water bodies.Table 5 Comparison of the adsorption of TC and DOX by Cu-MOF@Co-MOF and other reported adsorbents.TCs removal from actual waterSince the composition of actual wastewater is more complex than that of deionized water, the removal efficiency of adsorbents for simulated wastewater is often different from that of actual wastewater under the same conditions. Therefore, it is valuable to investigate its adsorption properties in actual wastewater in order to investigate the practical application potential of Cu-MOF@Co-MOF. In this work, river water and tap water were selected as actual samples for adsorption experiments. Under the same conditions, the removal rates of TC and DOX from the two actual water samples by Cu-MOF@Co-MOF were 85.49–87.35% and 98.78–99.03%, respectively. The adsorption capacity was lower than that of deionized water, but the satisfactory removal effect was still maintained. The results showed that Cu-MOF@Co-MOF could effectively remove TC and DOX from actual wastewater. It will be a promising direction to construct MOF-on MOF structure to achieve multi-target synergistic, low-cost, rapid and efficient removal of pollutants.
Cu–Co bimetallic organic framework as effective adsorbents for enhanced adsorptive removal of tetracycline antibiotics
