Fe(II)-activated persulfate oxidation to degrade iopamidol in water: parameters optimization and degradation paths

Iodinated contrast media (ICM), which was widely used in medical imaging and was difficult to remove by conventional wastewater treatment methods, attained much attention due to its potential environmental impacts. Herein, iopamidol (IPM), one typical compound of ICM, was found to be rapidly degraded by ferrous activated persulfate oxidation (Fe(II)/PS) as compared with PS or Fe(II) alone. With a persulfate concentration of 1 mmol L−1, n(Fe(II))/n(PS) of 1:10, and a pH of 3.0, 78% IPM was degraded within 60 min, with a degradation rate of 0.1266 min−1. It was demonstrated that IPM degradation and deiodination were favored by a high temperature, while affected positively by acidic and neutral conditions. Radical quenching experiments and Electron Paramagnetic Resonace (EPR) spectra showed that the combined effects of SO4−· and ·OH contributed dominantly to degrade IPM, while the ·OH played an essential role during the degradation reaction. Through the Discrete Fourier Transform quantum chemical calculation, the possible reaction pathways for the oxidation of IPM by ·OH are as follows: IPM-TP651-TP667-TP541-TP557, IPM-TP651-TP525-TP557, IPM-TP705-TP631-TP661, and IPM-TP705-TP735. The obtained results showed that IPM could be degraded effectively by Fe(II)/PS system, giving a promising technique for IPM removal from water.


Scientific Reports
| (2020) 10:21548 | https://doi.org/ 10.1038/s41598-020-78468-y www.nature.com/scientificreports/ persulfate (PS) or peroxymonosulfate (PMS) based processes have the high rate constant and removal rate to deal with IPM among these processes as shown in Table 1. PS was used to generate sulfate radicals (SO 4 − ·) by either homogeneous or heterogeneous advanced oxidation process (AOPs). Being relatively stable at a room temperature, PS can be activated by several means, such as heat 12 , transition metals 14,15 , to generate SO 4-− ·. However, some PS based technology, such as PS/ZVA system 13 , PS/UV-A process 16 , have its own drawbacks, such as precise pH condition, and not eco-friendly, e.g. in the CuO/PMS system 14 . On the other hand, sometimes additional complexion agents were required that would increase the activation efficiency, such as PS/Fe(III)/GA system, but sidewise they may also increase the operational cost. Therefore, it is necessary to develop an economical technology that could degrade IPM under a neutral and mild conditions. Among these activators, transition metal Fe(II) shows high activity to initiate PS decomposition, as it could be injected directly in subsurface that promotes in-situ oxidation 24 . Especially, owning to its merits, e.g. non-toxic, environmental friendly and relatively low cost, Fe(II) is deemed as one of the most promising activators 25 . Zhu et al. established a kinetic model to describe the iohexol degradation in the Fe(II) activated PS system, and they found that the rate constant of iohexol reacting with sulfate radical was (1.83 ± 0.10) × 10 9 26 . Bu et al. proposed a mathematical model on Fe(II)-activated PS oxidation of atrazine (ATZ), and they observed that only small amount of ·OH were produced while SO 4 − · attributed a certain level to ATZ degradation in Fe(II)/PS system 27 . However, to the best of our knowledge, the investigation on IPM degradation by Fe(II)/PS are rarely reported, up to now. In the present study, Fe(II)/PS oxidation was employed to degrade IPM, and the influence of operational condition parameters on the IPM treatment efficiency was examined to obtain the optimized IPM removal condition parameters. Meanwhile, the IPM degradation pathway was also examined computationally. The obtained results will be very useful for IPM degradation and removal in water and wastewater.

Materials and methods
Experimental setup and design. All  ZnO-based materials and enzymes hybrid systems 12.9 Photocatalyst (SMA-Ce-ZnO-plus SBP) dosage = 1 g L −1 ; The experiment was performed in a 250 mL reagent bottle with a working volume of 100 mL, which was installed in an electro-thermal shaking chamber (Shanghai Bluepard Instrument, Co. Ltd., China). During the experiment, 10 μM IPM was first prepared with distilled water (Shanghai Hitech Instruments Co., LTD, China). Pre-determined amount of ferrous sulfate solution was added to the reagent bottle. The initial pH was adjusted with 0.1 M sulfate acid and 0.1 M sodium hydroxide solution, and its temperature was kept at a constant value throughout the reaction. Afterwards, a certain amount of PS (0.1 M) was added. With the progression of reaction, 0.5 mL mixture in the bottle was accurately sampled at selected time intervals (0, 5, 10, 30 and 60 min). Afterwards, the reaction was quenched intermediately by using sufficient volume of methanol, which is an effective scavenger to quench SO 4 − · and ·OH to stop oxidation reactions. Besides, Iodide ((I − ) and iodate (IO 3 − ) were analyzed after treatment.
The effect of Fe(II) activated PS oxidation process parameters on IPM degradation was investigated by comparison with a blank experiment. Three parallel experiments were carried out, respectively, by adding 1 mM PS, 0.1 mM Fe(II), and Fe(II)/PS, after that all of them were added and reacted at a pH of 3 at 25 °C.
Analytical procedures. The concentration of IPM in the solution was determined with an Alliance 2695 Series high-performance liquid chromatography (HPLC) system equipped with a C18 column (5 μm, 4.6 × 20 mm) at a UV wavelength of 242 nm. The column temperature was maintained at 35 °C, and the mobile phase was a mixture of methanol/water (60/40, v/v) with a flow rate of 1.0 mL min −1 . The injection volume was kept at 100 μL. Iodide (I-) and iodate (IO 3 − ) were determined by ion chromatography (IC, Thermo Dionex ICS-1500). The sample quenched by sodium sulfite were filtered by BOND ELUTE C18 to remove any organic matters, and then were injected into IC equipped with a IonPacAS9-HC column (250 mm × 4 mm). The mobile phase was 9 mM sodium carbonate and run at a flow of 1 mL min −1 .
Computational methods. The chemical calculations are performed in the framework of Discrete Fourier Transform (DFT) using Gaussian 09 package according to the previous report 28 . The computational accuracy, feasibility and economical computational time were considered when choosing the computational levels and basis sets. The geometrical parameters were optimized at B3LYP level with a standard of 6-31G* basis set. Regarding basis set, iodine was defined to fifth cycle element, making the calculations computationally intensive. The use of relativistic effective core potentials is necessary to make the calculations tractable while obtaining accurate theoretical single point energies. Specially, B3LYP /SDD was used for I atom, and the 6-31G* basis set was used for other elements.

Results and discussion
IPM degradation by Fe(II)/PS oxidation. The degradation efficiency of IPM by Fe(II) , PS and Fe(II)/ PS process, was carried out, and it was observed that IPM could be degrade with an obviously higher rate by Fe(II)/PS system, in contrast to Fe(II) or PS (Fig. 1a). As one kind of recalcitrant contaminant, IPM cannot be effectively degraded by sole PS or sole Fe(II), because both of them had a low oxidation ability without activation, and no more than 5% IPM was found to be removed. On the other hand, a rapid degradation of IPM in the combined system of Fe(II) and PS (Fe(II)/PS) was clearly observed, during which over 60% IPM could be degraded. These results revealed the effectiveness of Fe(II)/PS system for the removal of IPM in water. The IPM degradation profile by Fe(II)/PS system included apparently two stages, i.e. a fast IPM concentration decreasing www.nature.com/scientificreports/ stage followed by a slow rate, which could be well described by a model accounted for the two distinct kinetic regimes (Eq. 1) 29,30 with R 2 of 0.994, as follow: where C and C 0 is the substrate concentration at time t and 0, respectively. ρ and σ are characteristic kinetic constant, while 1/ρ corresponds to the initial reaction rate (i.e. at t = 0), and 1/σ corresponds to the maximum conversion rate that can be achieved at the end of the reaction. The first sharp IPM degradation was probably due to Fe(II), activating a large amount of radicals to oxidize IPM with a high efficiency (Eq. 2). Afterwards, the slow IPM degradation rate might be related with the low radical concentrations after the Fe(II) activation 31 .
As shown in Eqs. 3-5, the co-existence of Fe(II) and O 2 can lead to the generation of ·OH, which could oxidize organics effectively 32 . Meanwhile, the presence of O 2 in this system should not be ignored.
Within the IMP degradation in Fe(II)/PS system, the concentration of the produced products i.e. iodate (IO 3 − ) and iodide (I − ) were simultaneously determined (Fig. 1b), which was consistent with the IPM removal profiles. IO 3 − concentration in the bulk displayed a significant increasing trend during the reaction, while only a few amount of I − was detected. Because of the quick oxidation of I − into IO 3 − (Fig. 1b), the IO 3 − concentration in the bulk increased gradually from 0 to 7.3 μM, along with the continuous decreasing of IPM concentration, which indicates the occurrence of deiodination.

Optimization of reaction condition parameters in Fe(II)/PS system. Effects of initial PS and Fe(II)/
PS mole ratio. The effect of initial PS concentration (0.01, 0.05, 0.1, 0.5, 1.0 mM and 2.0 mM) on IPM degradation efficiency was carried out under a fixed Fe(II)/PS ratio, with initial pH of 3.0 at 25 °C. As the concentration of PS was increased from 0.01 mM to 1 mM, the IPM initial reaction rate was also increased from 0.0055 min −1 to 0.0903 min −1 (Fig. 2a), which was corresponded to the increased concentration of IO 3 − from 1.3 μM to 7.32 μM (Fig. 2b). However, with further increased concentration of PS over 2 mM resulted in a slight improvent of degradation rate constant, as well as a similar trend was observed in the profile of the change of IO 3 − concentration. The reason for this phenomenon might be due to the excess of PS which can acts as a scavenger of oxidative radicals in Fe(II)/PS system, as described in Eq. 6 33, 34 .
In Fe(II)/PS system, Fe(II) plays a role of an activator for the activation of PS to generate sulfate radicals, which is an important factor that controlled the overall oxidizing ability 35 . Under a constant PS dosage of 1 mM, as showed in Fig. 2c, the rate constant for degrading IPM was improved significantly, when the initial Fe(II)/ PS mole ratio increased from 1/50 to 2/1. The generated IO 3 − concentration was positively correlated with the Fe(II)/PS mole ratios, which reached to the maximum of 8.54 μM when the Fe(II)/PS mole ratio increased to 1/10 (Fig. 2d). However, this IO 3 − concentration began to decline once the Fe(II)/PS mole ratio increased up to 2/1, which revealed the presence of extra excessive Fe(II) preferentially reacted with SO 4 − · via Eq. 7. Once the SO 4 − · reached to a high concentration, it would be self-quenched (Eq. 8) 36 , to cause a decrease in the removal efficiency of IPM (Fig. 2d).

Effects of initial pH and reaction temperature.
In all Fe based AOPs, pH is an important factor affecting the effectiveness of activation of oxidants by Fe(II) and/or Fe(III) 37 . Herein, the IPM degradation rate by activated PS was investigated at pH 3.0, 5.0, 7.0, 9.0 and 11.0, respectively, by comparing its resulted IPM degradation efficiency and IO 3 − generation (Fig. 3). It was observed that the IPM initial reaction rate reached the maximum of 0.0903 min −1 with the initial pH of 3.0. With the increased pH value from 3.0 to 11.0, the rate constant decreased gradually from 0.0903 min −1 to 0.0493 min −1 , ,which was corresponds to the reduction of IO 3 − from 7.32 μM (pH = 3.0) to 3.89 μM (pH = 11.0) (Fig. 3a,b). It has been reported that PS is easily decomposed to produce SO 4 2− in an alkaline solution. As shown in Eq. 9, ·OH would be generated whose oxidation-reduction potential is lower than SO 4 −38 . However, sulfate and hydroxyl radicals were slightly influenced by the variation of the initial pH 38, 39 . Hence, the main reason was the formation of an Fe 2+ complex at pH > 4.0 that would be expected to hinder the  39 Hence, the oxidizing ability of the system was decreased, corresponding to a low IPM removal efficiency.
Besides, the variation of pH during the reaction was shown in Fig. S1, which displayed a slightly decreasing trend during IPM degradation reaction when the initial pH was 3.0. The pH level dropped significantly at the very beginning stage of the reaction, and then remained at a stable level after 5 min reaction, when the initial pH was kept above 3.0. This decreasing pH trend with the IPM degradation might be related closely to the continuous production of H + in the reaction (Eq. 10) 40 .
The impact of reaction temperature, i.e. 25, 30, 40 and 45 °C, on IPM degradation rate and subsequent formation of IO 3 − was also evaluated, under a PS dose of 1 mM, an n(Fe(II))/n(PS) ratio of 1:10 and an initial pH of 3.0. As shown in Fig. 3c, the IPM initial reaction rate was increased from 0.0903 min −1 to 0.1266 min −1 , when the temperature increased from 25 to 45 °C, during which the generated IO 3 − (Fig. 3d) increased from 7.32 to 10.27 μM. An elevated temperature promotes the degradation and deiodination of IPM, which was mainly caused by the generation of more sulfate radicals. At higher temperature condition, PS would be thermally activated to produce more SO 4 − ·. Moreover, the reaction of Fe(II) to activate PS is an endothermic reaction, and high reaction temperature is beneficial to this reaction, and thus to enhance the degradation and deiodination rates of IPM.
Moreover, the degradation of IPM in Fe(II)/PS system may be affected by inorganic ions and natural organic matter (NOM). Therefore, further studies on the environmental impacts would be paid more attention in future. For instance, Zhu et. al found that iohexol degradation would be promoted by the low concentration of chloride www.nature.com/scientificreports/ ion while inhibited by high concentration, and iohexol degradation was slightly inhibited by NOM 26 . In another study, Bu et. al found that ATZ degradation performed well at low initial ATZ and NOM concentrations 27 .

Identification of ROS.
Radical quenching experiments for the IPM degradation over the 3 samples under a pH of 3.0 were conducted to explore the catalytic mechanism during Fe(II)/PS system. Generally, it was thought that the active species SO 4 − · and ·OH were the possible main active oxidant species in Fe(II)/PS oxidation process 27,39 . During the reaction, methanol (MeOH) was employed as the scavengers of SO 4 − · and OH·, while t-butanol (TBA) was applied as the inhibitor of ·OH 41,42 . Figure 4a  was kept at 0, 50, and 500, respectively). Without any radical scavenger dosage, the IPM degradation efficiency was around 69%. In comparison, this removal efficiency decreased to 46% and 55%, when the MeOH doasges was 50 times and 500 times higher than PS concentration, respectively. Corresponding to the addition of TBA, the degradation efficiency of IPM was declined to 34% and 41%, respectively, suggesting that the ·OH was the dominant radical in the degradation of IPM.
The EPR spin-trap technique was also carried out to unveil the reactive species in Fe(II)/PS process for IPM degradation, where 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) was used as a spin-trapping agent to capture SO 4 − · and ·OH. As depicted in Fig. 4b, there was rather insignificant EPR signals with Fe(II) alone or PS alone, indicating rare absence of SO 4 − · and ·OH generation. Upon the PS and Fe(II) dosage to form Fe(II)/PS oxidation process, four characteristic peaks, including a N = a H = 14.9 G for DMPO-OH· adducts, and the characteristic peaks (a N = 13.9 G, a H = 10 G, a H = 1.48 G, a H = 0.78 G) for DMPO-SO 4 − · adduct were found in the EPR spectrum, whose intensity ratio was around 1:2:2:1 43 . In addition, the DMPO-SO 4 − · adduct showed weak signals than the DMPO-OH· adduct, which were in accordance with the radical trapping experiments. The main types of IPM and ·OH radical reactions are presented in Fig. 5. Accordingly, iodine on the IPM phenyl ring would firstly be removed to produce TP651 (Eq. 11). Secondly, at this position, a hydrogen atom would be replaced by a hydroxyl group to induce hydroxylation and to produce TP667 (Eq. 12). Afterwards, the iodine at another position on the benzene ring of TP667 was removed to produce TP541 (Eq. 13), and the hydroxylation

Scientific Reports
| (2020) 10:21548 | https://doi.org/10.1038/s41598-020-78468-y www.nature.com/scientificreports/ reaction occurred at the position where iodine was removed by TP541, where one hydrogen atom could be replaced by a hydroxyl group to form TP557 (Eq. 14). On the other hand, TP651 may also directly be removed iodine to produce TP525 (Eq. 15), whose benzene ring would then be hydroxylated to remove two iodines, and meanwhile the hydrogen atom could be replaced by a hydroxyl group to produce TP557 (Eq. 16). Therefore, it is reasonable to think that the IPM branch underwent a deacetylation reaction path with the action of ·OH.
In addition, there is another possible IPM degradation path, during which the acetyl group would be deacetylated to produce TP705 (Eq. 17). Then, the branch of TP705 would be cleaved under the action of ·OH to produce TP631 (Eq. 18), which afterwards could be oxidized by ·OH to produce TP661 (Eq. 19). Apart from the abovementioned paths, TP705 could also directly undergo amine oxidation path under the oxidation of ·OH, to produce TP735 (Eq. 20).

Conclusion
IPM, a typical non-ionic ICM compound, was effectively degraded by using sulfate activated PS oxidation (Fe(II)/ PS) system. The effect of PS concentration, Fe(II)/PS mole ratio, initial pH, and operation temperature, on the IPM removal rate, as well as on the loss of iodine, were comprehensively investigated. The optimum degradation condition parameters were as follows: the 10 mmol L −1 PS concentration, 1:10 n(Fe(II))/n(PS), 3.0 pH and 45 °C temperature. The PS concentration increased from 10 to 1 mM which could enhance the IPM degradation and deiodination rates. Compared to alkaline conditions, acidic and neutral conditions were favorable for IPM degradation. The combination of SO 4 − · and ·OH contributed to the effectiveness of IPM removal. Through DFT quantum chemical calculations, the possible reaction path of IPM degradation was mainly included hydroxylation, deiodination, deacetylation, and amine oxidation, and finally all of the iodine removed was transformed into IO 3 − . Herein, this IPM initiate reaction rate of 0.1266 min −1 is quite higher in contrast to those listed in Table 1, under the relatively mild reaction condition. Fe(II) /PS system for IPM degradation in this study showed an excellent performance, compared to anaerobic process, PS/Fe(III) or UV system. Meanwhile, Fe(II)/PS system had a high economic feasibility, as it could degrade IPM without any additional complexion agents or expensive catalysts dosage, such as gallic acid, Fe(VI) or zero-valent aluminum. Moreover, Fe(II)/PS system possessed the advantage of sustainable, which did not need any hazardous materials and could not result in any secondary pollution. Therefore, it is reasonable to realize that, the PS/Fe(II) system reported in this study provided a sustainable approach for the practical treatment of refractory pollutants, and further studies should be carried out to evaluate its environmental impacts and stability.

Data availability
All data or models generated or used during the study are available from the corresponding author by request.