Controllable synthesis of iron-introduced cobalt-iron bimetallic MOFs for rapid removal of tetracycline hydrochloride in water

For sake of further enhancing the catalytic performance of Co-MOFs and reducing th leaching of Co 2+ , Fe-doped Co-MOFs was prepared by one-step solvothermal method. The Fe x /Co 1 − x -MOFs with the best catalytic activity (Fe 0.25 Co 0.75 -MOFs) was obtained by changing the doping ratio of Fe 3+ . Under the action of 0.1g/L Fe 0.25 Co 0.75 -MOFs, pH = 7 and 0.2g/L PMS, 98.4% TC can be e�ciently degraded only within 30 min, Moreover, the degradation e�ciency increased with the increase of the catalyst and PMS. The effects of initial pH on tetracycline hydrochloride degradation was discussed, showing that Fe 0.25 Co 0.75 -MOFs can degrade tetracycline hydrochloride with excellent stability. Furthermore, the Fe 0.25 Co 0.75 -MOFs exhibits good reusability and stability in cycling experiments. Ultimately, free radical quenching experiments show that both SO 4− • and •OH participated in the degradation of tetracycline hydrochloride, and SO 4− • plays a major role. Based on some intermediates identi�ed by lc-mg, possible degradation pathways are proposed.


Introduction
In the past few decades, plenty of private products and drugs have been extensively developed and used [1] .Among them, Tetracycline (TC) is a typical antibiotic that has been often used to promotion of animal growth and treatment of bacterial virus infection [2,3] .Unfortunately, huge amounts of TC is discharged into the natural environment via biological metabolism and ultimately brings serious pollution burden to different aquatic systems, exempli gratia, freshwater, agricultural runoffs, wastewater, and solid waters [4][5][6][7] .Owing to its stable chemical structure, various conventional wastewater treatment process have been reported, such as adsorption [8] , biodegradation [9] and membrane separation [10] .Nevertheless, these processes often suffer from the disadvantages of slow processes, secondary contamination, high costs and unsatisfactory removal.Therefore, It is urgent to nd a low-cost and e cient method to remove TC from aqueous solution.
Advanced oxidation processes ( AOPs ) have been favored in recent years due to their practicality and effectiveness in removing complex organic compounds [11] .Three oxidants, peroxymonosulfate (PMS), hydrogen peroxide (H 2 O 2 )and peroxydisulfate (PDS), are often used in AOPs.Noteworthily, because the structure of PDS is symmetrical and stable, it is hard to be activated than PMS [12] .And compared with hydroxyl radicals (•OH), which is produced by activating the traditional Fenton oxidant H 2 O 2 , sulfate radicals (SO 4 − •) obtained by catalyzing PMS have several signi cant advantages, including excellent selectivity, longlife of theactive radicals and broadly applicable pH range [13][14][15][16] .Besides, PMS can also generate radicals by heat or UV activation, but the high energy-consumption limits their application in practical wastewater treatment [17,18] .
Heterogeneous catalysts composed of transition metals(Fe, Co, Mn and Cu) are generally regarded as superior PMS activators due to their unique properties such as high e ciency, low energy consumption, good stability, and recyclability [19][20][21][22] .After activated by transition metals, the radicals generated by PMS exhibit e cient decontamination for organic pollutants in wastewater.Among the transition metals, it was reported that cobalt ions(Co 2+ ) had the best catalytic ability [23] .Although the homogeneous homogeneous Co 2+ can activate PMS to effectively purify pollutants, the carcinogenic Co 2+ threatens human health [24] .
Some studies have shown that the synthesis of cobalt-containing heterogeneous catalysts can not only promote the removal of pollutants but also reduce cobalt ion leaching [25,26] .
Metal-organic frameworks (MOFs), composed of organic linkers and metal centers, have attracted much attention in catalytic applications on account of their diverse functions, tunable structures, large surface areas, and high chemical resistance [27][28][29][30] .Currently, several monometallic MOFs like ZIF-67, ZIF-8, MIL-53(Fe) and MIL-53(Al), have been explored as PMS-activated catalysts for the treatment of organic pollutants in water [1,26,31−32] .However, the catalytic activity of these non-homogeneous catalysts still needs to be improved.Several studies have demonstrated that the introduction of secondary active metal centers in MOFs can enhance their catalytic activity and stability [33] .Due to its superior redox ability, iron is a satisfactory choice as a second active metal to increase the active site of the catalyst, enhancing the catalytic activity.S1.

Degradation experiments
All experiments were performed in foil-wrapped 100mL beakers placed on a magnetic stirrer.Frist,5mg asprepared sample and 10mg PMS were dispersed in 50 mL TC solutions at a concentration of 20 mg/L to initiate oxidation reaction at room temperature.Subsequently, at the speci ed time, an appropriate amount of the solution was collected, ltered through a 0.45um lter membrane and quenched with 0.5ml of methanol.The ultraviolet spectrophotometer (752 N, INESA, Shanghai) was used to measure the residual concentration of TC at the wavelength of 357 nm.This experiment uses HCl or NaOH to adjust the pH environment required.In addition, a pseudo-rst-order kinetic pattern( ln(C 0 /C t ) = k obs t) was used to re ect the degradation rate of TC, k obs (min − 1 ) is the pseudo-rst-order rate constant.

Charaterization
As shown in Fig. 1a, FTIR spectroscopy was utilized to analyze the functional groups in MOFs.The tensile vibration absorption peaks of carbon-oxygen and carbon-carbon double bonds appear at 1544 cm − 1 and 1593 cm − 1 [34] .More notably, the intensive peak at 1374 cm − 1 are ascribed to the symmetric vibration of -COOH, implying that H 2 BPDC exists in the catalyst [35] .The absorption peak caused by the bending vibration of C-H in the catalyst is at 769 cm − 1[36] .Morever, a new peak at 582cm − 1 corresponds to the stretching vibration of Fe-O, indicating the successful introduction of iron ions [37] .
The lattice structure of the prepared MOFs can be explained by an X-ray diffractometer (Fig. 1b and Fig. S1).
The obvious diffraction peaks of Co-MOFs can be found at 2θ = 6.3°, 12.5°, 14.1°, 15.4° and 18.8°.Noteworthly, compared with Co-MOFs, the characteristic diffraction peaks of Co/Fe-MOFs at 2θ = 15.4° and 18.8° are shifted to the left (Fig. S1), indicating the incorporation of heteroatoms larger than the host atomic radius in Co-MOFs and con rming that the Co/Fe-MOFs are not a simple mixture of two monometallic MOFs.Besides, the characteristic diffraction peaks of Fe/Co-MOFs at 2θ = 14.1° disappears(Fig.S1).These features all verify the successful synthesis of Co/Fe-MOFs.The speci c morphology of the different samples can be clearly observed through the SEM image.As shown in Fig. S2a, Co-MOFs show uneven strip images with an average diameter of 5um.Compared to the monometallic MOFs samples, The Co 1 − x Fe x -MOFs sample did not show obvious morphological changes(Fig.S2b-d), indicating that the introduction of iron did not change its structure.

Catalytic performance of catalysts
In this work, the adsorption of TC in the rst 30 min and the degradation trend of TC after adding PMS were studied to evaluate the effect of different bimetallic Co 1 − x Fe x -MOFs on the removal of tetracycline hydrochloride.Noted from Fig. 2a, the adsorption of tetracycline hydrochloride by several catalysts is less than 6%, showing that the adsorption capacity is limited and can be neglected.Meanwhile, it can also be observed that the Fe x Co 1−x -MOFs can rapidly degrade TC after the addition of PMS.Fe 0.25 Co 0.75 -MOFs has the best removal effect on tetracycline hydrochloride, reaching over 80% in only 10 minutes, which was 15% higher than that of Co-MOFs (about 65%) and 60% than that of Fe-MOFs in the same time.After 30 min, removal rates of Fe-MOFs and Co-MOFs were 31.6% and 77.9%, respectively at the same conditions.It proved the excellent degradation of Fe 0.25 Co 0.75 -MOFs, which can be used for fast removal TC in water.
Hence, Fe 0.25 Co 0.75 -MOFs (simpli ed as Co/Fe-MOFs) was selected as the catalyst for the following research experiments on degradation performance.The enhanced catalytic performance of bimetallic Co/Fe-MOFs is mainly due to the synergistic effect of cobalt and iron pecies in the catalyst structure, promoting the activation of PMS to form more SO 4 − • radicals to degrade TC [38] .
PDS and PMS were compared to probe into oxidants' in uence on degradation, as shown in Fig. 4b.PDS alone has almost no effect on TC, even in the presence of Co/Fe-MOFs, only 6.9% of the TC was removed after 30 minutes.In contrast, with PMS alone, about 20% TC was removed after 30 min.Besides, the removal e ciency of TC was signi cantly improved under the condition of adding Co/Fe-MOFs into the PMS solution, reaching 87.1% within 30 min, It suggested that PMS was e ciently activated by Co/Fe-MOFs for degradation of TC.These properties just indicate that Co/Fe-MOFs is more likely to activate PMS, thereby increasing the TC degradation.Futhermore, approximately 4% of TC was adsorbed by Co/Fe-MOFs within 30 min, futher proving the limited adsorption capacity of individual catalysts.The analysis of the pseudo-rstorder kinetic constants for different systems also reveals the above pattern(Fig.S3a).In particular, the kinetic constants of the Co/Fe-MOFs/PMS system are as high as 0.0714(min − 1 ).Therefore, PMS was chosen as the strong oxidizing agent to carry on the next step of degradation research.

Effect of concentration of PMS and Co/Fe-MOFs
The effects of PMS concentration are shown in Fig. 3a and Fig. S3b.The k obs from 0.03439 to 0.2434 indicates that the catalytic performance was signi cantly improved by increasing the PMS concentration from 0.05 to 0.3 g / L. However, when the PMS addition was less than 0.1 g/L, the degradation performance was not ideal.Moreover, some studies have demonstrated that a higher dosage of PMS may also cause a decline of removal rate because of the self-quenching reaction of excess PMS with SO 4 − • [39] .Nevertheless, this circumstance did not appear in the dose range of PMS we studied, demonstrating that the system of Co/Fe-MOFs/PMS can conquers the competitive effect of PMS.From a practical point of view, it wouldn't be an a thrifty strategy that the degradation experiments is carried out through further increasing the PMS dose (> 0.2g/L) considering the exorbitant price of PMS.Therefore, 0.2 g/L PMS was selected for catalytic degradation.It can be clearly seen from Fig. 3b and Fig. S3c that with the increase of catalyst amount, the degradation rate of TC increases.The above phenomenon can be explained by the fact that the expansion of the catalyst can provide more active sites to generate more active radicals.However, it is worth noting that between 100 and 150 mg/L of catalyst, the nal removal e ciency of TC is the same.Thus, 100 mg/L of Co/Fe-MOFs is chosen for the following catalytic degradation .

Effect of initial pH
To study the effect of pH on the degradation of TC, the initial pH of the reaction solution was adjusted to vary from 2 to 10. Figure 4 demonstrates that the initial pH has effect on TC degradation.As the initial pH values increased from 2.0 to 7.0, the removal e ciency of TC increased obviously.As shown in Fig. 4a, the degradation e ciency of TC is only 50.9% and 82.5% under the extremely acidic conditions of pH 2 and 3 within 30 min, while more than 94% of tetracycline hydrochloride is removed with pH of 5-10 in same reaction time.The lower removal e ciency of TC under hyperacidic conditions may be due to excess H + interfering with HSO 5 − , and degrading PMS to SO 4 2− instead of SO 4 − • [13] .In conclusion, the results show that Co/Fe-MOFs demonstrates satisfactory catalytic activity for PMS in a wide pH scale of pH 5-10.

Effect of co-existing ions
Abundant inorganic ions exist in natural water, and it is practically signi cant to probe their effects on the degradation of TC.Thus, the effect of inorganic anion concentrations (Cl − , SO 4 2− , NO 3 − and H 2 PO 4 − ) on the TC removal e ciency was investigated.As shown in Fig. S5a and b, SO 4 2 − and H 2 PO 4 − had no prominent in uence on the expulsion of TC in the Co/Fe-MOFs/PMS system.The Cl − exerted obvious inhibitory effects on the degradation of TC(Fig.S5c).Meanwhile, although a small amount of nitrate hardly affected the degradation of TC,when the nitrate concentration was increased to 10 mM, the nal removal rate of TC decreased by about 5% (Fig. S5d).The NO 3 − and Cl − react directly with SO 4 − • or •OH to form radicals with less oxidant power in the Co/Fe-MOFs/PMS system, resulting in the reduction of TC removal e ciency(Eqs.1-6) [40][41][42] .Overall, the Co/Fe-MOFs/PMS system's anti-interference ability to inorganic anions is quite impressive.

Investigation into the possible mechanism
To explore the mechanism, methanol and TBA were chosen as quenchers of sulfate radicals and hydroxyl radicals, respectively, and ethanol acts as a free radical scavenger to quench SO 4 − • and HO•.As shown in Fig. 5a, When there is no quencher, 98.4% of TC could be removed.Whereas, when 100 mM methanol and TBA were introduced into the reaction system, the degradation rate of TC declined to 67.6% and 95.5%.
Moreover, the degradation e ciency of TC also decreased to 65.6% under ethanol quenching.Obviously, The scavenging effect of methanol on free radicals was greater than that of TBA, which indicates SO 4 − • are the main reactive species in the degradation process of TC.
Figure 5b clearly recorded XPS full-range scan spectra of the Co/Fe-MOFs before and after reaction, which can also indicating successful co-doping of iron and cobalt in Co/Fe-MOFs.For the nascent catalyst (Fig. 5c), two peaks at 782.4 eV and 781.2 eV are ascribed to Co2p3/2, implying the presence of Co 3+ and Co 2+ in the catalyst.For the reacted catalyst (Fig. 5d), the ve peaks are displaced minutely to 782.2eV and 781.0eV for Co2p3/2, and 797.1eV for Co2p1/2 [42,43] .Furthermore, Fe 2+ (710.1eV and 723.2eV),Fe 3+ (711.3.eV,713.5eV and 725.2eV) and the satellite peaks (717.8eV and 731.7eV) can be found at the Fe 2p spectrum of the fresh catalyst [44,45] .For the reacted catalyst, the peak (Fig. 5d) at 710.5eV, 711.6eV, 713.5eV are ascribed to Fe2p3/2, representing the state of Fe 2+ , Fe 3+ and Fe 3+ , respectively.Meanwhile, the ratio of Fe 2+ /Fe 3+ increased from 0.3168 to 0.4925 after the degradation.And the ratio of Co 3+ /Co 2+ decreased from 0.5219 to 0.3655 after the degradation.The above analysis results not only con rmed that both iron and cobalt active sites were involved in the activation of PMS, but also indicated that there was a continuous cycle between Fe 3 + / Fe 2 + and Co 3 + / Co 2 + .In addition, the catalyst can accelerate the catalytic degradation process by electron transfer between the two metals, thereby promoting the generation of ROS, thus promoting the generation of ROS.EIS Nyquist plots (Fig. S4) can further verify that the introduction of iron de nitely accelerates the electron transfer rate on the catalyst surface.
Metal valence change and free radical conversion process can be speculated through the above analysis.
First of all, the Co 2 + and Fe 2+ in catalyst is utilized to activate the PMS (Eq.7-8), which can generate SO 4 − •with the ability to oxidize pollutants [46] .Then, the HSO 5 − can effectively reduced Co 3 + to Co 2+ , accomplishing a cycle conversion between Co( ) and Co( ) (Eq. 9) [47] .The Fe 3+ could be effectively reduced to Fe 2+ through the reaction with HSO 5 − , which also achieves a cycle conversion of two valence states of iron (Eq.10).Meanwhile, some •OH could also be obtained by the reaction of SO 4 − • with water (Eq.11) [19] .

Degradation pathway of TC
The intermediate products were identi ed by LC-MS, and the corresponding mass spectrometry was shown in Fig. S6.Based on these results, the detected intermediates and 11 possible molecular structures including m/z = 437, 409, 393, 365, 327, 304, 319, 123, 74 and 60, were shown in Fig. 6.Depending on these probable structures, two possible TC degradation pathways are proposed.TC can be degraded to P 1 through dealkylation reaction and hydroxylation reaction in pathway [48,49] .As the reaction proceeds, the intermediate P 2 might be generated through the hydrogenation reaction and the attack on CONH 2 [50] .In addition, P 2 opened the loop to produce P 3 with the further attacks of •OH and SO 4 − •.For the second degradation pathway, P 6 and P 7 are the dehydration product of TC and the ring-opening product of P6, respectively [51] .With the continue attack of •OH and SO 4 − •, the pathway intermediates P 3 , P 4 and P 8 gradually breaks down into some smaller molecules, such as P 5 , P 9 , P 10 and P 11 .Eventually, these small molecule intermediates would be mineralized into H 2 O and CO 2 .

Stability and practicality of Co/Fe-MOFs
In order to study the stability of its long-term use and practicality of degrading other pollutants, four cycles of TC degradation experiments were carried out, resulting in 98.4%, 97.8%, 96.4% and 93.1% removal of TC, respectively(Fig.7a), indicating that Co/Fe-MOFs still exhibits good activity after repeated use.The rst-level dynamics (Fig. 7b) of the simulated cyclic test also follows the above-mentioned trends.Notably, the XRD and IR spectra of the Co/Fe-MOFs after four consecutive uses did not change signi cantly, which further proved the stability of the Co/Fe-MOFs(Fig.S7).In addition, three pollutants (DOX, RhB and MB) that could be detected in the wastewater except for TC were selected as degradation models and it was found that the removal rate of each pollutant reached more than 95% within 30 minutes(Fig.S8a).The kinetic constants of the various pollutants can be seen in Fig. S8b, demonstrating the difference in their degradation rates.

Figure 2 The
Figure 2

Figure 3 Effect
Figure 3

Figure 5 (
Figure 5 Therefore, inspired by this, a series of Fe/Co-MOFs with different Fe/Co contents were synthesized.The obtained catalyst was subsequently used for activating PMS to eliminate TC under different conditions.

Table 1
The comparison of catalytic performance between the catalyst used in this work and other catalysts capability, despite it is affected by inorganic anions (Cl − >SO 4 2− > NO 3 − >H 2 PO 4 − ) to varying degrees.Additionally, Co/Fe-MOFs can maintain satisfactory activity and good stability in recycling.The potential mechanism of Co/Fe-MOFs activating PMS was proposed in accordance with.Additionally, two possible degradation pathways of TC were proposed in the Fe/Co-MOFs/PMS system via several oxidation intermediates detected by LC-MS.Considering the excellent characteristics of Co/Fe-MOFs catalyst in the above experiments, it has a broad practical application prospect in water treatment.
In summary, the Co/Fe-MOFs with different metal ratios were prepared and activated PMS to degrade TC.Co/Fe-MOFs exhibited more excellent catalytic performance than pure single metal MOFs.Moreover, 98.4% of TC was nally removed under certain conditions (Fe/Co molar ratio, initial pH, PMS and Fe/Co-MOFs dosage were 1/3, 7, 0.2g/L and 0.10 g/L, respectively).Overall, the Co/Fe-MOFs/PMS system has strong anti-interference Declarations Figures Page 14/18