As porous crystal materials, metal-organic frameworks (MOFs) have attracted wide attention in the field of environmental remediation. In this study, a trivalent iron-tartaric acid metal-organic framework (T2-MOF) was successfully synthesized using the inexpensive raw materials ferric chloride (FeCl3.6H2O) and tartaric acid (C4H6O6). The physical and chemical properties of T2-MOF were studied by using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and Brunauer–Emmett–Teller. After that, T2-MOF was used as a catalyst for catalytic ozonation of succinonitrile. The results show that T2-MOF has obvious crystal characteristics and uniform structure. In addition, T2-MOF exhibits strong catalytic performance in ozonation of succinonitrile. The results indicate that the chemical oxygen demand (COD) removal rate is affected by various operating parameters including catalyst characteristics dosages and initial pH values. In the ozonation with 30 mg L−1 T2-MOF, the COD removal rate of 100 mg L−1 succinonitrile reached 73.1% (±4.6%) within 180 min, which was 67.3% (±4.4%) higher than that obtained in the process without catalyst. T2-MOF maintained strong catalytic performance with the pH range of 3.0–7.0. By monitoring the Fe2+ concentration at different reaction time, it was found that the homogeneous catalysis occurred simultaneously with the heterogeneous catalysis.
A metal-organic framework was synthesized using trivalent iron and tartaric acid.
This metal-organic framework, named T2-MOF, exhibited high activity for the catalytic ozonation of succinonitrile.
Orthogonal experiments established the priority of different influencing factors.
The dissociative Fe2+ and Fe3+ demonstrated that homogeneous and heterogeneous reactions occurred in the same system.
After five cycles of experimental use, T2-MOF showed strong reusability.
Acrylic fiber wastewater is recognized as a serious organic pollutant due to its high toxicity and persistence in the environment (Sun et al. 2015; Hu et al. 2016). Succinonitrile is the main refractory organic component of acrylic fiber wastewater. Therefore, the development of an effective wastewater treatment technology for succinonitrile would be very beneficial to the treatment of acrylic fiber wastewater.
In the last few decades, advanced oxidation processes (AOPs) have been regarded as effective techniques for treating wastewater containing refractory organic pollutants (Chong et al. 2010). In earlier reports, different oxidants in AOPs, such as hydrogen peroxide (H2O2), persulfate and ozone (O3), have been compared (Lian et al. 2017). H2O2 requires careful control of pH (2.5–4.0). Cl− inhibits the destruction of organic contaminants by persulfate-based AOPs (Huang et al. 2016). The above disadvantages make the use of H2O2 and persulfate difficult in industrialized applications. AOPs based on ozone have many attractive advantages, including low cost and lower residuals after treatment, which mean that AOPs based on ozone have broad application prospects.
At present, a more in-depth and feasible method is catalytic ozone oxidation (Mohapatra & Kirpalani 2019). There are usually two forms of catalytic ozone oxidation, homogeneous catalytic ozonation and heterogeneous catalytic ozonation. Homogeneous catalytic ozonation is fast and efficient, but it has the disadvantages of harsh reaction conditions, large catalyst dosage and secondary pollution. Heterogeneous catalytic ozonation has attracted the attention of many researchers due to the advantages of easy catalyst recovery and mild reaction conditions. In recent years, researchers have continuously developed new heterogeneous catalysts to improve the efficiency of ozone catalysis (Gayen et al. 2018).
The large number of catalytic sites in the heterogeneous catalyst can effectively improve the catalytic efficiency. Previous studies have shown that the greater the specific surface area of the heterogeneous catalyst, the more catalytic sites it contains (Gassie & Englehardt 2017). Therefore, increasing the specific surface area of the catalyst is an attractive development direction for heterogeneous catalytic ozonation.
In 1995, Yaghi et al. (1995) first reported a two-dimensional coordination compound synthesized from 1,3,5-benzenetricarboxylate (BTC) and cobaltous nitrate. The compound was named a metal-organic framework (MOF). Such materials are composed of transition metal ions containing vacant orbitals and organic bridge bonds formed through self-assembly (Zhou et al. 2012). MOFs exhibit diverse physical and chemical properties, such as a high surface area, controllable pore volume and thermal stability, etc. (Easun et al. 2017). Due to these unique properties, researchers have tried to synthesize various new MOFs for different fields (Hidalgo et al. 2018; Li et al. 2018). Tang & Wang (2018) used a CUS-MIL-100(Fe) catalytic H2O2 system to decompose sulfamethazine (SMT), and the degradation efficiency of SMT reached 100% after 100 min. Araya et al. (2017) used MIL-53(Fe) to photodegrade sulforhodamine B (SRB), and the removal rate of SRB was 96% after 120 min.
Although MOFs have enabled remarkable progress in the field of environmental remediation, there are also some disadvantages. Some reports about the application of MOFs in environmental remediation are shown in Table S1 in the Supplementary Information. MOFs exhibit diverse physical and chemical properties, such as a high surface area, controllable pore volume, thermal stability, etc. Due to these unique properties, MOFs have attracted extensive attention from researchers in wastewater treatment. However, the cost of the organic ligands (1,4-dicarboxybenzene, 1,3,5-benzenetricarboxylic acid, 2-methylimidazole etc.) used to synthesize these MOFs is high, which limits the application of these MOFs.
Most of the reported AOPs reactions have been carried out in the liquid phase. Moisture will destroy the structure of MOFs, causing them to irreversibly collapse, thereby reducing their specific surface area and pore volume. This structural collapse can reduce the catalytic performance of MOFs in AOPs (Guo et al. 2015). Thus, in order to improve the structural strength of MOFs, different researchers have proposed different solutions. Kim et al. (2001) noted that the large reticular crystal structure of MOFs was formed by the directional growth of metal-oxygen-carbon clusters with very stable chemical bonds, and these special clusters were defined as secondary building units (SBUs). Yaghi et al. (2003) suggested that a large number of hydroxyl or carboxyl groups could strengthen SBUs to increase the structural strength of MOFs. Each tartaric acid has two hydroxyl groups and two carboxyl groups, and is an ideal raw for the synthesis of MOFs. Moreover, it is widely present in higher plants and is low in cost.
In this work, a solvothermal method was employed to synthesize tartaric acid MOF (T2-MOF) precursors by the reaction of ferric chloride and tartaric acid with more carboxyl groups in a mixed solvent of N,N-dimethylformamide (DMF) at 105 °C for 24 hours. The factors affecting the degradation of succinonitrile by T2-MOF catalyzed ozone and reaction kinetics were studied.
MATERIALS AND METHODS
Ferric chloride, N,N-dimethylacetamide (DMA), sodium sulfate, sodium chloride, sodium hydroxide and tartaric acid were purchased from DAMAO (Tianjin, China). Mercury sulfate, silver sulfate, sulfuric acid, sodium carbonate and potassium dichromate were purchased from Kermel Chemical Reagents Company (Tianjin, China). All reagents were analytical grade. Succinonitrile (>99%) was purchased from J&K Scientific (Beijing, China).
Preparation of T2-MOF
In a typical procedure, 3.0 mmol FeCl3.6H2O and 2.5 mmol C4H6O6 were added slowly to 80 mL of DMA solution to prepared T2-MOF by a mild solvothermal reaction. The mixture was stirred at room temperature for 10 min, and then transferred into a Teflon-lined stainless-steel autoclave with a volume capacity of 150 mL. The autoclave was heated at 105 °C for 24 hours then allowed to cool naturally to room temperature. The prepared catalysts were filtered through common commercial qualitative filter paper. The pore size of the filter paper was 30–50 μm, and the material was cotton fiber. A yellow powder was obtained at the bottom of the autoclave. The yellow powder was filtered and washed four times, alternately using distilled water and acetone, followed by natural air drying 3 hours to obtain T2-MOF (Figure S1).
Characterization of T2-MOF
X-ray diffraction (XRD) with monochromatized Cu Kα radiation (λ = 0.15418 nm) was used to analyze the crystallinity of the as-synthesized T2-MOF (Model: Rigaku D/max-2200PC). Fourier transform infrared spectroscopic (FT-IR) was employed to investigate the chemical structure and identify the functional groups of the prepared T2-MOF (Model: IRTracer-100 FT-IR Spectrometric Analyzer). Scanning electron microscope (SEM) analysis was performed to observe the morphology of the T2-MOF (Model: Quanta 200). Energy-dispersive X-ray spectroscopy (EDS) equipped with an SEM was used to analyze the elemental composition of the T2-MOF. Surface electronic states of T2-MOF were analyzed with X-ray photoelectron spectroscopy (XPS) (Model: ESCALAB250Xi). The Brunauer–Emmett–Teller (BET) surface area and porous structure of T2-MOF were measured by N2 adsorption–desorption isotherms at 77 K (Model: ASAP 2020 PLUS HD88).
The ozonation of succinonitrile using T2-MOF as a catalyst was tested in a cylindrical organic glass reactor with a working volume of 5 L. A total of 150 mg T2-MOF was added. Ozone gas with a flux ranging from 20 mg L−1 to 25 mg L−1 was generated from a GUOLIN generator. The hydraulic retention time (HRT) was 180 min. At predetermined time intervals, the suspension was extracted and then centrifuged to obtain the supernatant for analysis.
The degradation effect of succinonitrile was indicated by COD. In the process, the trivalent iron on the surface of the T2-MOF can dissolve into solution. The dissolved trivalent ions can cause a homogeneous catalytic reaction and produce Fe2+. Therefore, the concentrations of Fe2+ and Fe3+ were determined by o-phenanthroline spectrophotometry at 510 nm (Model: 752N). The pH of the succinonitrile solution was adjusted by adding 0.1 mol L−1 H2SO4 and 0.1 mol L−1 NaOH and was determined by a pH meter (Model: PHS-3C).
Inorganic anion scavenging experiment
There are various reactive oxygen species (ROS) that play important roles in AOPs. As we know, many inorganic anions exist in wastewater, such as CO32−, Cl− and SO42−, and they may pose a threat to attaining the right catalytic property. The roles of sodium sulfate (Na2SO4) sodium chloride (NaCl) and sodium carbonate (Na2CO3) in the catalytic ozonation of succinonitrile system were studied because these inorganic anions may be scavengers for various ROS. Before every experiment, only one inorganic salt (10 mmol L−1) was added to the experimental system under initial reaction conditions.
Analysis of orthogonal experiments
Orthogonal experiments are a method for scientifically arranging and analyzing multifactor experiments using orthogonal tables. It is one of the most commonly used experimental design methods. The degradation of succinonitrile was mainly influenced by the initial pH, HRT and dosage of T2-MOF. To obtain the optimal reaction conditions and the order of importance of each factor, an orthogonal experiment was designed. Each factor had five levels, and the levels of the experimental factors are shown in Table S2. The orthogonal experimental design was L2556, as shown in Table S3. A detailed discussion of this section can be found in the Supplementary Information.
RESULTS AND DISCUSSION
Characterization of the T2-MOF
XRD was used to characterize the crystallographic structure of the T2-MOF and FeCl3.6H2O. As presented in Figure 1(a), the different diffraction peaks from T2-MOF and FeCl3.6H2O indicate that the crystal structure of T2-MOF was different from that of FeCl3.6H2O. Moreover, the strong diffraction peaks at 15.18°, 19.84° and 25.1° corresponding to the (200), (−111) and (020) reflections indicate that the T2-MOF had three preferred orientations (Zeng et al. 2015). These changes in crystal structure suggested that T2-MOF was synthesized.
The prepared T2-MOF was analyzed by FT-IR spectroscopy, and the result is shown in Figure 1(b). The peaks from 700 to 1,300 cm−1 could be assigned to C–H bending vibrations (Hermes et al. 2006). The characteristic absorption peak observed at 1,419 cm−1 was derived from the vibration of C–O bonds in the carboxylate groups of tartaric acid. The characteristic absorption peak at 3,422 cm−1 was attributed to polyhydroxyl groups in tartaric acid. Therefore, the combined results of XRD analysis and FT-IR spectroscopy clearly confirmed the formation of T2-MOF.
The morphology of T2-MOF sample was observed by SEM. As shown in Figure 2(a) and 2(b), many rod-like crystals are uniformly dispersed in the field of view. Further observation revealed that the crystal form of T2-MOF is distinct cubic structure (Figure 2(c)). The compositional analysis carried out by EDS measurements (Figure 2(d)) shows the presence of Fe, C and O elements in the T2-MOF sample. In addition, the EDS elemental mapping (Figure S2) further confirmed that these elements were evenly distributed in T2-MOF.
The detailed chemical components and electronic states of the T2-MOF were determined by XPS. The XPS survey spectrum and high-resolution XPS spectra of the T2-MOF are shown in Figure S3 and Figure 3, respectively. The XPS survey spectrum (Figure S3a) demonstrated that C, O, and Fe existed in the T2-MOF. The high-resolution XPS spectrum of C 1s (Figure S3b) could be fitted by two peaks at 284.9 eV and 288.7 eV, which were derived from single-bond carbon in tartaric acid and carboxylate groups, respectively (Ahimou et al. 2007). The peak at 288.7 eV was attributed to the double bond of carboxylate groups (Zhang et al. 2015). In the high-resolution XPS spectrum of O 1s (Figure 3(a)), each O 1s XPS peak is wide, showing that there was more than one electronic state. The binding energies of 531.6 eV and 532.2 eV are related to the hydroxyl groups of tartaric acid and single bonds between oxygen and carbon (C-O), respectively (Yu et al. 2019). The higher binding energy at 533.1 eV was attributed to the carboxyl groups of tartaric acid (Baldanza et al. 2016). Figure 3(b) shows the high-resolution XPS spectra of Fe 2p. The peaks at 711 eV and 713.1 eV are characteristic of Fe3+ in T2-MOF (Chandra et al. 2010). Overall, the XPS high-resolution spectra confirmed that the T2-MOF contained the expected components.
Specific surface area and porous structure of T2-MOF were measured by N2 adsorption–desorption isotherms at 77 K. Figure 4(a) is the N2 adsorption–desorption isotherms of T2-MOF displayed an intermediate mode between type I and type IV (Peng et al. 2012). The BET surface area and total pore volume of T2-MOF were calculated to be 24.9 m2 g−1 and 0.070 cm3 g−1, respectively. Calculated from the desorption branch of the nitrogen isotherm with the Barrett–Joyner–Halenda (BJH) analysis, the pore diameter of the T2-MOF was determined to be 14.23 nm (Figure 4(b)), while the illustration in figure S4 had shown the micropore size about 1.10 nm from the Horvath–Kawazoe method.
Catalytic activity of the T2-MOF
The COD removal rate was used to evaluate the degradation of succinonitrile in three ozonation systems: an ozone-alone system (OAS), trivalent iron-tartaric acid complex catalytic ozonation system (TCS) and T2-MOF catalytic ozonation system (MCS). The initial concentration of succinonitrile in the three catalytic ozonation systems was 100 mg L−1, and the gas phase ozone concentration was approximately 20–25 mg L−1. The pH was neutral. Figure S5a shows the variation in COD of succinonitrile with oxidation time in the various oxidation systems. In OAS, the decrease in COD associated with succinonitrile was 10.2 mg L−1 after 180 min; in combination with Figure 5(a), the COD removal rate was only 5.83%. In the other two oxidation systems involving catalysts, the COD reduction was significantly improved. Especially in MCS, the decrease in COD reduction was 134.5 mg L−1 (Figure S5a), and the corresponding COD removal rate reached 73.1% (Figure 5(a)). Moreover, the COD removal rate increased by 20.7% compared with that in TCS. These results indicate that T2-MOF is the most appropriate catalyst for catalyzing ozone to degrade succinonitrile in this study.
A comparison of data can highlight the improvement in catalyst performance. Despite the many papers we consulted, there are few reports on the treatment of succinonitrile by AOPs. We only found a master's thesis on the use of complexing agents to catalyze the degradation of succinonitrile by ozone. Zhong (2017) studied the effect of two catalysts for catalyzing ozone to degrade succinonitrile. In his study, the COD removal rates of succinonitrile catalyzed by Fe2+ ions alone and Fe2+/sodium pyrophosphate were 35.7% and 49.7%, respectively. Compared with the two catalysts reported by Zhong, T2-MOF increases the catalytic efficiency by 105% and 47.1%, respectively. This comparison shows that the catalytic performance of T2-MOF is excellent.
As the main component of acrylic fiber wastewater, succinonitrile is more difficult to degrade than acrylic fiber wastewater. Therefore, to improve the reliability of the comparison results, we compared two catalysts used in the treatment process similar to our work. An et al. (2011) studied the ozonation of acrylic fiber wastewater by synergized ultraviolet light. The results showed that the COD removal rate was only 25% under optimal conditions. In addition, Wei et al. (2013) studied the treatment of acrylic fiber wastewater by a Fenton process under acidic conditions. The results indicated that the COD removal rate was only 47% after 2 hours. As the main component of acrylic fiber wastewater, succinonitrile is the focus and the most challenging aspect of acrylic fiber wastewater treatment. In comparison with the above research, T2-MOF increased the catalytic efficiency by 2.9 and 1.6 times, respectively. This comparison shows that the performance of T2-MOF catalyzed ozone to degrade succinonitrile is excellent.
Figure 5(b) shows the pseudo-first-order reaction rate constants of different catalytic ozone systems. The pseudo-first-order reaction rate constants 3.62−4min−1, 4.34 × 10−3 min−1 and 7.75 × 10−3 min−1 correspond to OAS, TCS and MCS respectively. Based on the pseudo-first-order rate constant, the kinetic enhancement of succinonitrile degradation by T2-MOF is more significant than that obtained with other catalysts.
In order to understand the mechanism of T2-MOF catalytic ozonation of succinonitrile, water samples from different oxidation times were centrifuged, and the concentrations of Fe2+ and total iron ions in the supernatant were determined by o-phenanthroline spectrophotometry. As seen from Figure S6b, in the case where no other iron source was added except T2-MOF, Fe2+ and Fe3+ were detected in the oxidation system after 30 min. Moreover, the concentrations of Fe2+ and Fe3+ remained stable from 30 to 150 min. The low concentrations of Fe2+ and Fe3+ indicated that homogeneous catalysis occurred simultaneously with the heterogeneous catalysis by T2-MOF.
If the structure of the T2-MOF is unstable, Fe3+ will continuously detach from the interface of T2-MOF and enter the liquid phase. From the perspective of chemical reaction kinetics, the concentration of Fe2+ will increase with Fe3+ (Equations (11) and (12)). At the same time, the reaction of Equations (11) and (14) will be accelerated. However, as shown in Figure S6b, the concentrations of Fe2+ and Fe3+ in the ozonation system did not obviously increase after 30 min. In addition, the sum of the concentrations of Fe2+ and Fe3+ was much smaller than the concentration of T2-MOF. This result indicated that the structure of T2-MOF was stable, in contrast to expectations. The most important conclusion is that T2-MOF is an efficient catalyst for catalyzing ozone to degrade succinonitrile. The mechanism for the ozonation of succinonitrile catalyzed by T2-MOF is shown in Figure 6.
Effect of inorganic anions on catalytic ozonation
Reusability test of T2-MOF
Five successive cycles of degradation testing were carried out to evaluate the reusability of T2-MOF. As shown in Figure 8, the COD removal efficiency of succinonitrile decreases slightly in every run, but is still as high as 65.4% after the fifth cycle. Compared with the first cycle, the COD removal rate had only decreased by 8%, which indicated that T2-MOF had reusability.
In summary, T2-MOF was synthesized as a catalyst based on FeCl3.6H2O and C4H6O6, and had a uniform three-dimensional structure. The T2-MOF showed excellent catalytic efficiency for catalytic ozonation of succinonitrile. The catalytic activities were closely dependent on the various operating parameters, such as initial pH, HRT, and T2-MOF dosage. Under optimal conditions, the average COD removal rate of succinonitrile reached 73.1% within 180 min. This result was 12.6 times COD removal rate compared to using ozone alone. In addition, stable catalytic activity was maintained over a wide pH range (3.0–7.0). Moreover, stable concentrations of Fe2+ and Fe3+ were detected in the liquid phase of the reaction system, indicating that heterogeneous catalyzed ozonation and homogeneous catalytic ozonation occurred simultaneously in the reaction system, which was very beneficial for the degradation of succinonitrile. This study provides a new inexpensive material for wastewater treatment containing refractory organic pollutants.
The study was financially supported by PetroChina Innovation Foundation (No. 2016D-5007-0604), Heilongjiang Province Natural Science Fund (No. B2015012), Applied Technology Research and Development Project of Heilongjiang Province (No. GC13C305).
COMPLIANCE WITH ETHICAL STANDARDS
The authors declare that they have no competing interests.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.