3.1 Characterization of CoFe-LDO
3.1.1 SEM analysis
SEM images of CoFe-LDO catalysts before and after catalytic degradation of amino acids were shown in Fig. 1 (a) and (b), respectively. SEM was carried out to investigate the morphology of the material surface. It could be seen that before the reaction, the catalyst CoFe-LDO was flaky, and its particle size was about 100 nm. It had sponge-like porous network structure, which was composed of interconnected nanoparticles. After the reaction, the flake part was destroyed to granular and the particle size was about 50nm. This phenomenon may lead to the reduction of the activity of the catalyst and influence the catalytic efficiency.
3.1.2 FTIR spectroscopy
The catalyst CoFe-LDO was characterized by FTIR in Fig. 1 (c) so as to test the structure of the sample surface. The absorption peak near 1633cm-1 is caused by the vibration of the H-OH group (Jing, et al., 2019), the absorption peak of 834cm-1 and 1383cm-1 was ascribed to CO32- (Zeng, et al., 2021). Compared with the FTIR spectra of CoFe-LDH in the literature (Li, et al., 2017), the absorption peak of CO32- is reduced and the peak was lower than the M-O absorption peak at 556cm-1, indicating that after calcination, the material was converted to CoFe-LDO.
3.1.3 XRD
The X ray diffraction pattern can be used to characterize the crystallinity, regularity of the catalyst from Fig. 1 (d). The samples had diffraction peaks at 2θ = 31.27°, 36.85°, 44.80°, 55.6°, 59.3° and 74.1°, which were in accordance with Co3O4 (JCPDS 43-1004) standard cards, and the diffraction peaks of 2θ at 33.1°, 39.2° and 43.5° matched the Fe2O3 (JCPDS 39-1346) crystal. The high intensity of the diffraction peak, which proved that the catalyst had good crystallinity, and there was no other diffraction peak, indicating the high purity of the catalyst.
3.1.4 XPS
The main purpose of the XPS analysis was to determine the chemical composition of the material surface. The XPS spectrum of the catalyst CoFe-LDO was displayed in Fig. 1 (e)-(g). According to the full pattern of Fig. 1 (g), there were three predominant elements of Co, Fe and O on the surface of the catalyst. The peak Co2p3/2 was located at 778.9eV, and the peak Co2p1/2 was located at 794.1eV in Fig. 1 (f), which was similar to the binding energy of standard map CoO and Co3O4 (standard value Co2p3/2 (780.3eV) and Co2p1/2 (795.9eV))(Jang, et al., 2013). Meanwhile, Co mainly existed in the form of Co3O4. The peak Fe2p3/2 was located at 709.3eV, and the peak Fe2p1/2 was located at 721.5eV, which was consistent with the binding energy of standard Fe2O3 (standard value Fe2p3/2 (711.3eV) and Fe2p1/2 (724.3eV)) as shown in Fig. 1 (e)(Yamashita and Hayes, 2008), consequently Fe was mainly in the form of Fe2O3.
3.1.5 Zeta potential
Zeta potential, as an important parameter for the particle stability in the reaction or colloidal suspension was tested. The catalyst was generally uniformly dispersed in the water, so that each catalyst particles had full contact and reaction in solution. As seen in Fig. 1 (h), the isoelectric point of CoFe-LDO was approximately 10. Therefore, the catalyst could be dispersed steadily in the solution with initial pH between 4.0 to 9.0.
3.2 Degradation efficiency
To evaluate the performance of degradation of histidine by CoFe-LDO/PMS system, several experiments including CoFe-LDO alone and PMS alone systems were conducted during 60 min by using 0.1 g/L CoFe-LDO, 1.5 mM PMS. As shown in Fig. 6, the concentration of histidine decreased from 10 mg/L to 3.9 mg/L during 60 min in the CoFe-LDO/PMS system. In contrast, almost no removal occurred in either Fe2O3、Co3O4、CoFe-LDO or PMS alone system. The sorting of the degradation effects of amino acids and DON in the composite system was CoFe-LDO/PMS >Fe3O4/PMS >Co3O4/PMS >Fe2O3/PMS. The result convincingly revealed that the combination of CoFe-LDO and PMS was conducive to the removal of histidine in aqueous solution and CoFe-LDO could be a favorable catalyst for PMS activation during the histidine removal process. Furthermore, the DON removal efficiency was slightly lower than that of histidine, which was explained by incomplete mineralization of amino acids into other organic nitrogen substances.
3.3 Influence of various factors
3.3.1 CoFe-LDO and PMS dosage
The initial conditions were as follows: the temperature was 25℃, the initial concentration of histidine was 10 mg/L, respectively. The results were shown in Fig. 3.
As seen in Fig. 3 (a), the removal of DON decreased with the increasing of the dosage of CoFe-LDO. The DON removal reached the highest when the dosage of catalyst CoFe-LDO was 0.04 g/L. This was because when the catalyst dosage increases to a certain extent, the catalyst was prone to agglomeration, resulting in the reduction of contact area with the target and the effective reaction site of the catalyst. These will slow down the reaction rate. Therefore, the optimum dosage of catalyst CoFe-LDO was 0.04 g/L.
The effect of PMS dosage on the histidine degradation was presented in Fig. 3 (b). Generally, with the increase of the concentration of PMS, the removal of DON showed the tendency to rise and then fall. It was observed that the increase of PMS dose from 0.2 mmol/L to 0.5 mmol/L could significantly enhance DON removal, the concentration of DON dropped from 1.65 mg/L to 0.78 mg/L; while with the PMS concentration further increased from 1.5 mmol/L to 2 mmol/L, the DON concentration increased to 1.2 mg/L.
This was because the active site of the catalyst was sufficient when the concentration of PMS was low, the increase of PMS concentration in the reaction system could produce more SO4−•, so as to improve the removal rate of DON. However, the active sites of COFE-LDO surface equivalent amount had been saturated when the PMS concentration was high, and then the excessive PMS could not be involved in the reaction to produce more SO4−• at this time. Similar inhibition appeared in the literature (Chen, et al., 2008).
3.3.2 Initial pH
It was reported that SO4−• based AOPs could work in a wide range pH compared with Fenton oxidation process. Thus, for purpose of evaluating the influence of different initial pH on DON removal, a series of experiments were carried out with pH ranging from 4.0 to 9.0. The effect of pH value on the histidine degradation was presented in Fig. 3 (c). It could be seen that DON removal increased first and then decreased with the increase of pH value. The removal of DON was up to 60% when pH value reached to 8. According to the zeta potential, the isoelectric point of CoFe-LDO was 10.0 and that of histidine was 7.59. Due to the surface properties of CoFe-LDO, net positive charge would be shown on the surface of CoFe-LDO when the solution pH was lower than 10. When the pH value was lower than 8, both the catalyst surface and histidine were positively charged, which reduced the probability of collision due to electrostatic repulsion. However, when the alkalinity of the solution was more than 10, the catalyst surface was negatively charged to repel negative ions produced by PMS dissolution, which led to the decreased removal of DON. Moreover, it was favorable for the formation of hydrogen bond between H+ and O which produced by the dissolution of PMS in the acidic environment (Cai, et al., 2020), thereby inhibiting the catalytic oxidation. In addition, the SO4−• would react with the OH- to form the •OH in the alkaline condition. The redox potential of hydroxyl radical was lower than that of sulfate radical, which affected the degradation. When the pH value was about 8, the catalyst surface was positively charged while histidine was negatively charged. The catalyst and oxidant collided due to electrostatic attraction and the degradation efficiency was greatly improved, which was consistent with the results shown in the Fig. 3 (c).
3.4 Stability and reusability of CoFe-LDO
In order to verify the stability and reutilization of CoFe-LDO catalyst, the leaching of Co and Fe was detected by inductively coupled plasma chromatography (ICP-OES), and the catalyst CoFe-LDO was investigated after four successive runs. It could be seen from table S2, the leaching of Co and Fe were 0.028 mg/L (<0.1 mg/L) and 0.015 mg/L (<0.3 mg/L) respectively, which were in accord with the limit concentration in the drinking water sanitation standard. It confirmed that CoFe-LDO was stable and reusable in the process of reaction. Moreover, the CoFe-LDO catalysts were collected and recovered for four times. As seen in Fig. 3 (d), the removal efficiency was 60%, 56%, 52% and 47% for the first, second, third and fourth time, respectively. There were two reasons for the reduction of DON removal efficiency: (1) a fraction of CoFe-LDO catalysts was lost with water in the process of catalytic and recovery washing, resulting in mass reduction; (2) according to the results of ICP-OES, it was possible that the amount of cobalt and iron ions decreased after 4 cycles of recycling, which made the proportion of homogeneous reactions in heterogeneous catalysis decrease, resulting in the reduction of DON efficiency.
3.5 Proposed mechanisms of CoFe-LDO/PMS system
3.5.1 Existence of free radicals
For purpose of further identifying the free radicals produced in the system, ESR was used to qualitatively detect the class of free radicals. Fig. 4 indicated that in the presence of DMPO, free radicals were not detected in separate catalytic materials. The typical peaks of 1:2:2:1 with low peak value were detected in the Fe2O3/PMS system, indicating that hydroxyl radicals were generated from the Fe2O3/PMS system. In the Co3O4/PMS reaction system, both OH• and SO4−• were detected at the same time, and the peak value was higher than that of Fe2O3/PMS system. In addition, DMPO-X was detected in CoFe-LDO/PMS, and the peak intensity ratio of ESR spectrum was 1:2:1:2:1:2:1. It was revealed that DMPO-X was produced by direct oxidation of single electron source (Wang, et al., 2015).
In order to explore the main free radicals in CoFe-LDO/PMS system, methanol (MeOH) and tert butyl alcohol (TBA) were used as quenching agents in this study. Methanol (MeOH) was an alcohol-containing α-H, which could quickly quench •OH and SO4−• by reacting with them. Tert-butyl alcohol (TBA) did not contain α-H, and it could only react rapidly with ·OH, while its reaction with SO4−• was much slower (Xie, et al., 2015). The result of the quenching of free radicals was shown in Fig. 5. The content of sulfate radical accounted for a high proportion in the total free radical. The removal efficiency reached 60% without quenching agent after 1 h. Under the conditions of 0.02 M TBA and 0.02 M MeOH respectively, the removal of DON decreased by 10% and 30%, respectively. The reduction of DON removal was substantial in the presence of MeOH compared with TBA, indicating that SO4−• as oxidizing species were mainly formed during the CoFe-LDO/PMS system.
3.5.2 Degradation pathway
The concentrations of total dissolved nitrogen (TDN), ammonium nitrogen (NH4+-N), nitrite nitrogen (NO3--N), and nitrate nitrogen (NO2--N) were determined to confirm the oxidation extent of DON in the different reaction time. The results were shown in Fig. 6.
It was shown that the main categories of nitrogen had different variation tendency due to the degradation pathway involved in the reaction. It was obvious that the concentration of DON decreased within 60 minutes, while the concentration of NH4+ increased and the content of TN and NO3- remained unchanged during the whole oxidation process. According to the variation value of DON and NH4+ (1.65 mg/L vs. 0.67 mg/L and 0.043 mg/L vs. 0.78 mg/L), it was found that the increase of the NH4+ was caused by the oxidation of histidine, and NH4+ was not converted to nitrogen at last. It was because that the reaction system was a multiphase catalytic oxidation reaction, which was the reaction of transition metal ions to activate persulfate to produce free radicals to oxidize the target. And there was no mutual transformation between NH4+ and NO3- during the reaction process.
According to the previous study (Chen, et al., 2019; Chen, et al., 2020), the mechanism of DON degradation in CoFe-LDO/PMS system could be described as follows:
Co2+ + HSO5- →Co3+ + SO4−• + OH-
Co3+ + HSO5- →Co2+ + SO5−• + H+
Fe3+ + HSO5- →Fe2+ + SO5−• + H+
Fe 2+ + HSO5- →Fe 3+ + SO4−• + OH-
Fe 2+ + Co3+ →Fe 3+ + Co2+
Fe 3+ + e- →Fe 2+ E0=0.77V
Co3+ + e- →Co2+ E0=1.81V
SO4−• + H2O → SO42- + •OH + H+
DON + SO4−• + •OH + SO5−• + H2O → NH4+ + OH- + Intermediate products
According to the reaction mechanism, it could be found that Co3+ and Co2+ were converted into each other through the reaction with persulfate, which was the same with Fe2+ and Fe3+. Co3+ was more likely to oxidize Fe2+ to Fe3+ in the system due to the stronger oxidation of Co3+ (E0=1.81V), which led to the change of Co3+ to Co2+. It might be difficult to form the effective cyclic transformation from Fe3+ to Fe2+ due to its low redox potential.
In addition, the degradation intermediates of histidine oxidation were identified by HPLC-MS. The results were shown in Fig. S1.
Fig. S1 investigated that the main mass-to-charge ratios (m/z) for mass spectrum of histidine after catalytic treatment are 154.0, 139.9, 112.9 and 222.0. The molecular formula of these substances can be tentatively determined according to the m/z and structure of amino acids (Table 1).
Table 1 Identification of histidine oxidation products
Amino acid
|
Monitoring ion
|
m/z
|
Molecular formula
|
Histidine
|
[M+Na]+
|
154.0
|
C6H9N3O2
|
[M+H]+
|
139.9
|
C6H9N3O
|
[M+H]+
|
112.9
|
C5H6N2O
|
[M+H]+
|
222.0
|
Co2+–Xaa complex
|
Referring to the molecular weight in Table 1, it is presumed that the transformation process follows the following rules. It can be seen from Fig. 15 that after the catalytic degradation of 60min by CoFe-LDO/PMS system, histidine degrades in two ways under the action of OH• and SO4−•. In the pathway (a), amino acid carboxyl groups are protonated, and Co2+ and amino acids react to form Co2+- Xaa complexes. The Co2+ in the Co2+ - Xaa complex was oxidized by HSO5- to Co3+ - Xaa complex, generating SO4−• and SO5−• at the same time. A hydrogen atom was extracted from the α-carbon of amino acid to form an amino acid radical centered on the carbon atom by SO4−•, which was generated near the amino acid of the complex. The free radical reacted with Co3+ in the complex and generated Co2+ and amino acid derivatives, which produced spontaneous hydrolysis of alpha ketoacids and NH4+. In the pathway (b), the anions were oxidized to carboxyl groups. Here, the alpha amino radical could be converted to alpha amino carbocation, thus forming protonated imines. In the end, aldehydes could be produced by hydrolysis.
3.6 N-DBPs formation and intermediate products
Fig. 8 revealed the formation of N-DBPs during chlorination of histidine oxidized by persulfate in the CoFe-LDO/PMS system. The concentration of DCAN decreased with the increasing of the reaction time. However, at the same time, the formation potential of DCAcAm increased first and then decreased, which was mainly due to the varying structure of R group.
The side chains of histidine contained imidazole ring, which had certain aromaticity and was prone to carry out electrophilic substitution reaction and ring opening reaction. There were two N atoms in imidazole heterocycle, which might be converted to DCAN by ring opening reaction. DCAN hydrolyzed to produce DCAcAm and the precursor of DCAcAm decreased. Finally, the reaction of DCAcAm formation was complete, and because of its own instability, hence it could be seen that the formation of DCAcAm during chlorination of histidine shows first increased and then decreased (Chu, et al., 2014; Yimeng, et al., 2017).
Moreover, it could be seen from Fig. S2 that the variation trend of intermediate products of the mass to charge ratio at different time accordance with histidine chlorination to DCAcAm. According to the result of Fig. 7 and Fig. S2, it was
predicted thatand the two intermediate products might be the source of increasing the concentration of histidine chloride to DCAcAm.