Sulfate radical (SO4•−) based advanced oxidation processes (AOPs) from sulfite activation has recently received attention for abatement of microorganic pollutants in the aquatic environments. Trace level Co(II) has been demonstrated to be effective for promoting sulfite activation (simplified as the Co(II)/sulfite system) and the corresponding radical formation, yet this process is challenged by the limited valence inter-transformation of Co(II)/Co(III). In order to enhance this valence inter-transformation, a novel Co(II)/HPO42−/sulfite system is developed in this work, in which complexation of Co(II) with HPO42− can regulate the electronic structure of Co(II) and promote valence inter-transformation of Co(II)/Co(III) during the sulfite activation process. The Co(II)/HPO42−/sulfite system exhibits superior iohexol abatement performance under circumneutral conditions. For pH 8.0 and Co(II) dose of 1 μM, the iohexol abatement efficiency is as high as 98%, which is considerably higher than that of the Co(II)/sulfite system (50%). SO4•− is identified as the predominant reactive radical contributing to iohexol abatement. The presence of HPO42− broadens the pH adaptability of the Co(II)/sulfite system for iohexol abatement. In addition, the coexisting Cl− exerts an inhibitory effect on iohexol abatement while the other cations and anions shows negligible effect. The Co(II)/HPO42−/sulfite system displays good reusability and adaptability towards various organic pollutants. This study highlights the important role of complexation of Co(II) with HPO42− in sulfite activation and provide a feasible idea for abatement of the microorganic pollutants.
Research Article
Effect and mechanism of phosphate enhanced sulfite activation with cobalt ion for effective iohexol abatement
https://doi.org/10.21203/rs.3.rs-3331164/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 30 Nov, 2023
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Sulfite activation
phosphate
advanced oxidation processes
cobalt ions
complexation effect
Advanced oxidation processes (AOPs) typically based on OH• and SO4•− have exhibited effectiveness for the abatement of the microorganic contaminants in the aquatic environments (Chen et al. 2023; Wang et al. 2023; Hou et al. 2023). Compared with OH•, SO4•− has received the attention from the researchers due to its high redox potential (2.5–3.1 V), long halflives (t1/2=30–40 µs), high reaction rate with microorganic pollutants (106-109 M− 1s− 1) (Oh et al. 2016; Mei et al. 2019). The traditional precursors of SO4•− mainly include peroxydisulfate (PS) and peroxymonosulfate (PMS), yet their application is compromised by the secondary pollution potential and acute toxicity (Zhao et al. 2019a; Peng et al. 2022a, b; Saha et al. 2023). In contrast, due to the unique advantages of cost-effectiveness, low toxicity and environmental friendliness, industrial by-product sulfite has been recently considered as a promising precursor for SO4•− generation (Zhou et al. 2018; Chen et al. 2020). For instance, the LD50 (oral, rat) of Na2SO3 is 3560 mg/kg, which is more than 3-fold higher than that of K2S2O8 (802 mg/kg). Sulfite autoxidation process in the presence of dissolved oxygen can lead to the generation of SO4•− and other oxysulfur radicals including SO3•−, SO5•− and HO• accompanied by the oxidation of sulfite by dissolved oxygen to sulfate (Zhou et al. 2018). However, the sulfite autoxidation process exhibits a slow reaction rate and thus it is necessary to apply the activators for promoting this autoxidation process and subsequent abatement of organic pollutants.
Activating sulfite efficiently with transition metal ions (such as Fe(II), Fe(III), Cu(II), Co(II) and Mn(II)) has been widely studied for abatement of microorganic pollutant (Yuan et al. 2020; Xiao and Yu 2021). Among these ions, Co(II) has been demonstrated to possess superior performance in the sulfite activation compared to other transition metals under circumneutral conditions (Zhao et al. 2019b). The outermost electron arrangement of Co(II) corresponds to a configuration of 3d7, which can facilitate the generation of the electron holes via the electronic delocalization of sulfite, subsequently enhancing the formation of highly reactive species (Qiangwei et al. 2014). In a typical Co(II) activated sulfite process, sulfite molecule serves as the ligand to coordinate with Co(II) to form the CoSO3 complex, which is oxidized by dissolved oxygen to the CoSO3+ complexes and then decomposed into Co(II) and SO3•−. The Co(II)/Co(III) valence inter-transformation initiates free radical chain reactions in the presence of dissolved oxygen and sulfite, producing SO5•−, SO4•− and HO• for the efficient removal of microorganic pollutants (Zhou et al. 2018; Wu et al. 2022). However, the slow rate of Co(II)/Co(III) valence inter-transformation in the Co(II) activated sulfite process restricts the generation of highly active free radicals and the concomitant microorganic pollutants abatement.
Researchers have recently proposed to construct the activator with the reductive components in accelerating the Co(II)/Co(III) valence inter-transformation in the heterogeneous sulfite activation processes(Wu et al. 2022; Liu et al. 2023). For example, Wu et al. reported that the CoS/sulfite process exhibited efficient abatement performance with a sharp decline of iohexol within 0.5 min, due to superior electron transfer efficiency of CoS (Wu et al. 2022). Liu et al. demonstrated that the synergy between Mo, S, and Co components can promote the sulfite activation in the Co–MoS2/sulfite process (Liu et al. 2023). For the homogeneous sulfite activation, there are limited information reporting the enhancement of the Co(II)/Co(III) valence inter-transformation. In the AOPs, a feasible strategy to strengthen the metal valence inter-transformation is to introduce the complexing agents (Luo et al. 2014; Shao et al. 2023; Zhu et al. 2021). Appropriate ligands can stabilize the electronic and spatial structure of Co(II) and can maintain the catalytic activity in the AOPs owing to the formation of Co(II) complexes(Zhu et al. 2021; Shao et al. 2023). Inorganic ligands including HCO3−, HPO42− and Cl− are ubiquitous in water, which are possible candidates as appropriate ligands in AOPs. Our previous work has demonstrated that HCO3− can enhance the sulfite activation with trace concentration of Co(II) to produce the active components with abatement efficiency of iohexol as high as 92.0% in the first 5 min of reaction (Shao et al. 2023). HCO3− is also a typical radical scavenging agent, which can quench the reactive radicals. In comparison with HCO3−, HPO42− can complex with Co(II) without quenching effect, which has been reported to facilitate the electron transfer in the Co(III)/Co(II) valence inter-transformation in the AOPs. For instance, Zhu studied that the presence of HPO42− could elevate the catalytic activation of PMS with trace level of Co(II) (Zhu et al. 2021). Therefore, it can be inferred that HPO42− potentially show effectiveness in promoting the Co(II)/Co(III) valence inter-transformation in the sulfite activation at low concentrations of Co(II), exhibiting the application potential.
Iodinated contrast media is extensively employed in medical diagnostics, and its high detection frequency and concentration bring potential threat to water safety(Wang et al. 2019). Iohexol is a typical iodinated contrast media, which is detected in surface water with concentrations in the range from ng/L to µg/L (Sacher et al. 2001). Therefore, iohexol is chosen as the focal micro-pollutant in this research. The primary aims of this research encompass: (i) to validate the effectiveness for the iohexol abatement by the Co(II)/HPO42−/sulfite process; (ii) to distinguish the reactive radicals generated and their respective role in the iohexol abatement; (iii) to explore the underlying mechanism for sulfite activation with Co(II) in the presence of HPO42− via electrochemical analysis; (iv) to study the influence of various influencing factors including solution pH and the coexisting ions on iohexol abatement and evaluate the potential application prospects of Co(II)/HPO42−/sulfite system in the water treatment.
2.1 Chemicals
Sodium hydrogen phosphate (Na2HPO4, purity > 98%), sodium phosphate monobasic (NaH2PO4, purity > 98%), Anhydrous sodium sulfite (Na2SO3, purity > 97.0%), sodium hydroxide (NaOH, purity > 98%), sulfuric acid (H2SO4, purity > 98%), sodium chloride (NaCl, purity > 98%), ethylenediamineteraacetic acid (EDTA, purity > 99.5%), calcium nitrate (Ca(NO3)2, purity > 98%) and cobalt nitrate hexahydrate (CoNO3·6H2O, purity > 99.0%) were purchased from Sinopharm Chemical Reagent Co., China. Tert-butanol (TBA, purity > 99.5%), methanol (MeOH, purity > 99.9%), iohexol (purity > 99.5%) and 5,5′-dithiobis (2- nitrobenzoic acid) (purity > 99.0%) were supplied from Xiya Reagent Co., China. All the reaction solutions were prepared with 18.2 MΩ cm Milli-Q water. All the reagents employed in this work were at least analytical reagent grade and used without further purification.
2.2 Experimental procedure
Batch experiments were performed in the beaker at room temperature to investigate the degradation of iohexol. 100 mL reaction solution was firstly prepared, which consisted of iohexol (10 µM), followed by the addition of HPO42− and Co(II) stock solution. Then, concentrated sulfite solution was finally dosed to initiate the reaction. Samples were withdrawn at predetermined time intervals, which were quenched with excess MeOH (10 mM) for measuring the concentration of the residual iohexol in the reaction solution. Simultaneously, another sample for the measurement of sulfite concentration was terminated with 1 mL of EDTA, 2 mL of 5,5′-dithiobis (2- nitrobenzoic acid), 5 mL of Na2HPO4/NaH2PO4 buffer. All experiments were conducted in triplicate and the average values were calculated from the obtained results.
2.3 Analysis
High performance liquid chromatography (HPLC, Agilent 1290 Infinity) with Agilent C18 column (4.6 mm ×150 mm, 5 µm particle size) was employed to measure the residual concentrations of model contaminants including iohexol and the other selected pollutants. The mobile phase was 95% of ultrapure water and 5% of acetonitrile at the flow rate of 0.5 mL min− 1 and the detection wavelength of 245 nm via the Diode-Array Detection (DAD, Agilent 1290 Infinity II) diode array detector. The residual sulfite concentrations were measured according to a modified colorimetric method with 5,5′-dithiobis (2-nitrobenzoic acid), which was then detected by the UV-vis spectrophotometer (AOE Instruments V-1600) at the wavelength of 412 nm (Humphrey et al. 1970). The reactive oxysulfur species formed in the Co(II)/HPO42−/sulfite system was qualitatively measured by electron paramagnetic resonance (EPR, Bruker EMX-10/12 spectrometer). To examine the electron transfer efficiency, comparative experiments were conducted between the Co(II)/HPO42−/sulfite process and the Co(II)/sulfite process using electrochemical impedance spectroscopy (EIS, CH Instruments 760E) by controlling the frequencies in the range from 1000 kHz to 0.1 Hz and voltage amplitude of 0.01 V.
Enhancement factor (Ree) was used to identify the effect of HPO42− on the abatement of iohexol in the Co(II)/HPO42−/sulfite system (Chen et al. 2019b).
Where XA and XB represent the iohexol removal efficiency with or without HPO42−, respectively.
3.1 Validation of the effectiveness for the iohexol removal by the Co(II)/HPO42−/sulfite system
Figure 1(a) displays the iohexol abatement as a function of time in different comparative system. It can be seen that iohexol abatement in the sulfite process is negligible (< 5%). The abatement efficiency of iohexol approximates achieves 50% for the reaction of 10 min in the Co(II)/sulfite system. Especially, after adding HPO42− into the Co(II)/sulfite system, the abatement efficiency of iohexol is significantly elevated to 45% within merely 1 min of reaction and iohexol is completely degraded after 20 min. In addition, the sulfite decay tendency is in accordance with that of iohexol abatement (Fig. 1(b)), indicating that the formation of reactive ingredients corresponds to the sulfite decay process. The enhanced abatement of iohexol is due to that the synergy of HPO42− and Co(II) considerably promotes the sulfite activation and the corresponding formation of the active species.
Figure 1(c) further illustrates the influence of varying Co(II) and HPO42− concentration on iohexol abatement. By changing the concentration of HPO42−, the iohexol removal efficiency is observed to ascend from 58–88% when the concentration of Co(II) is ranging from 0.5 µM to 2 µM. However, when the concentration of HPO42− exceeds 2 mM, the degradation efficiency remains stable at around 88%. Increasing the concentration of Co(II) while keeping the concentration of HPO42− constant also results in a gradual stabilization of the degradation efficiency. This indicates that the strengthening effect on HPO42− comes from the complexation reaction with Co(II). What’s more, the degradation efficiency of iohexol can reach about 97% within the range of HPO42− concentration of 0.5 ~ 1.5 mM and Co(II) concentration of 0.75 ~ 2 µM, and adding more HPO42− or Co(II) at this range result in a decrease in the degradation efficiency. This is because when the concentration of Co(II) is constant, increasing the amount of HPO42− cannot promote the complexation reaction to enhance the catalytic efficiency of the system and may quench the reactive radicals, resulting in a decrease in the degradation efficiency of iohexol (Zhu et al. 2021). The enhancement effect of HPO42− (Ree) is the ratio of iohexol removal efficiency in the Co(II)/HPO42−/sulfite system and the Co(II)/sulfite system. As illustrated in Fig. 1(d)), the value of Ree rises from 1.05 to 1.7 as the concentration of HPO42− is increased to 1 mM with the concentration of Co(II) maintained at 1 µM. Thus, the inclusion of HPO42− significantly elevates the catalytic behavior of Co(II) in the process of sulfite activation, thereby facilitating the superior iohexol removal at trace concentration of Co(II).
As the precursor of SO4•−, sulfite plays a decisive role in the iohexol removal. As demonstrated in Fig. 1(e), the removal of iohexol is not positively correlated with the concentration of sulfite. When the dosage of sulfite is 250 µM, the abatement rate is 70% after 20 min of reaction. Upon escalating the sulfite concentration to 500 µM and 750 µM, the iohexol abatement rate surpasses 70% after 5 min of reaction with complete degradation after 20 min. However, further increase of sulfite dosage to 1000 µM leads to a decrease of the iohexol abatement rate. This highlights that a series of sequential reactions in the Co(II)/HPO42−/sulfite system transmute SO3•− into SO4•−, and thus within a particular range, increasing the concentration of sulfite accelerates the oxidative abatement of iohexol. Nevertheless, excessive sulfite might react with SO4•− and SO5•−, resulting in a competitive reaction with iohexol, thus inhibiting the oxidation of iohexol (Wang et al. 2019). As the sulfite concentration of 500 µM has been demonstrated for effective iohexol abatement, this dosage is used for the subsequent experiments.
According to a recent study, the presence of dissolved oxygen plays a crucial role in the transformation of sulfite into active free radicals (Ding et al. 2020). In order to evaluate the influence of dissolved oxygen, the removal of iohexol under aerobic and anaerobic conditions in the Co(II)/HPO42−/sulfite and Co(II)/sulfite system is conducted. As shown in Fig. 1(f), in the presence of dissolved oxygen, the abatement efficiency of the Co(II)/sulfite system and Co(II)/HPO42−/sulfite system could reach 50% and 98%, respectively. However, iohexol abatement for the above two processes is restrained to lower than 5% in the absence of dissolved oxygen (bubbling nitrogen), indicating that dissolve oxygen is involved in the transformation of CoSO3 and SO3•− to CoSO3+ and SO5•−. Therefore, the role of dissolved oxygen is negligible for iohexol abatement.
3.2 Identification of the reactive species
In a typical sulfite activation system, various reactive oxygen species could be generated, such as SO3•−, SO4•−, SO5•− and HO• (Zhou et al. 2018; Zhao et al. 2019b; Wu et al. 2022). As the reaction rate of SO3•− with dissolved oxygen can reach the diffusion-controlled reaction rate (~ 109 M− 1s− 1) and the oxidative ability of SO3•− is comparatively weak, the contribution of SO3•− to the abatement of iohexol can be ignored. The radical quenching agents including MeOH and TBA are used to identify the contribution of different radicals to the abatement of iohexol in this system. Previous studies have shown that TBA can effectively capture HO• without quenching effect for SO4•−, which is attributed to the fact that the reaction rate of TBA and HO• is as high as 6.0×108 M− 1s− 1, while the reaction rate of TBA and SO4•− is 8.0×105 M− 1s− 1 (Hu et al. 2020b). MeOH can effectively quench SO4•− and HO• due to the comparable reaction rates of 2.5×107 M− 1s− 1 and 9.7×108 M− 1s− 1, respectively (Chen et al. 2019a). However, the quenching effect of MeOH on SO5•− could be ignored.
To clarify the role of reactive radicals on iohexol removal in the Co(II)/HPO42−/sulfite system, radical quenching experiments are performed. According to Fig. 2(a), the abatement rate of Co(II)/sulfite system is 60% in the absence of the quenching agents. In comparison, the abatement efficiency of iohexol is about 50% in the presence of TBA, which is almost similar with the situation with no quenching agent. However, iohexol is almost not degraded in the presence of MeOH. As can be seen, HO• and SO5•− contribute little to the abatement of iohexol. SO4•− plays an important role in the abatement of iohexol, as evidenced by comparing the impact of MeOH and TBA on the efficiency of iohexol degradation in the Co(II)/sulfite system (Wang et al. 2016, 2019; Hu et al. 2020a). As shown in Fig. 2(b), a similar inhibitory effect of TBA and MeOH on iohexol abatement is observed in the Co(II)/HPO42−/sulfite system. It can also be observed that the inhibition effect of TBA is more obvious than that of the Co(II)/sulfite system because the abatement rate is reduced by 20% after the addition of TBA. This demonstrates that HPO42− enhances the Co(II)/sulfite system to produce more SO4•− and HO•.
Figure 2(c) compares the EPR signal in the single sulfite system, the Co(II)/HPO42−/sulfite system and the Co(II)/sulfite system. The single sulfite process exhibits no signal due to the negligible radical formation. In comparison, the Co(II)/sulfite system and Co(II)/HPO42−/sulfite system exhibit significant signals with the signal intensity of the Co(II)/HPO42−/sulfite system significantly stronger compared to that of the Co(II)/sulfite system. This signal with the hyperfine splitting constants of aN = 14.7 G and aH = 16.1 G can be attributed to the DMPO-SO3•− adducts (Huang et al. 2020). Despite the reaction between SO3•− and DMPO (kDMPO, SO3•− = 1.2×107 M− 1s− 1) exhibits lower reactivity than that with dissolved oxygen (kO2, SO3•− = 1.5×109 M− 1s− 1) (Ranguelova et al. 2010), a preferential production of DMPO-SO3•− can be attributed to the significantly higher concentration of DMPO compared to dissolved oxygen under the experimental conditions. This blocks the following chain reaction without the formation of SO3•−/SO4•− and HO•. Furthermore, the observed higher signal intensity in the Co(II)/HPO42−/sulfite system relative to the Co(II)/sulfite system is ascribed to the presence of HPO42−, which enhances sulfite activation with Co(II) and promotes the formation of SO3•−. Comparison of HO• generation in the Co(II)/sulfite system and Co(II)/HPO42−/sulfite system is depicted in Fig. 2(d). The HO• formation in Co(II)/sulfite system stabilize at 2.35 µM after 20 min of reaction. After the addition of HPO42−, the generation of HO• significantly is increased to 10.28 µM. This suggests that HPO42− plays an important role in promoting the generation of reactive radicals.
3.3 Contribution of complexation effect to the sulfite activation
Sulfite activation with Co(II) primarily is related to the generation of active components through a series of radical chain reactions, which is facilitated by the Co(II)/Co(III) valence inter-transformation. Electrochemical analysis is utilized to confirm the role of HPO42− in the enhancement of electron transfer in the Co(II)/HPO42−/sulfite system. As depicted in Fig. 3(a), the i-t response curve for the diverse conditions in the Co(II)/HPO42−/sulfite system is measured. A notable observation is that the amperometric current after dosing sulfite and Co(II) in the presence of HPO42− is significantly enhanced compared to the scenario in the absence of HPO42−, which infers that HPO42− plays a pivotal role in facilitating the electron transfer. The EIS Nyquist diagram is used to further explore the electron transfer in the Co(II)/HPO42−/sulfite system for the abatement of iohexol. EIS Nyquist plots for the Co(II)/sulfite and the Co(II)/HPO42−/sulfite system are presented in Fig. 3(b). Obviously, the Co(II)/HPO42−/sulfite system shows smaller semicircle diameter in the Nyquist plots than the Co(II)/sulfite system, which is similar with the trend of iohexol removal. According to the previous studies, the rate-limiting step in this process is the decomposition of the CoSO3+ complex, which serves as a crucial link between the cyclic inter-valence transformation of Co(II)/Co(III) and the chain reaction of the oxygen sulfur radical (Zhou et al. 2018; Zhao et al. 2019b; Wu et al. 2022). The complexation of Co(II) with HPO42− can effectively promote electron transfer process in the Co(II)/HPO42−/sulfite system.
The complexation of Co(II) with HPO42− is dependent on solution pH, which can influence the speciation of Co(II) and the concomitant the sulfite activation. Figure 4(a) portrays the abatement of iohexol in the Co(II)/sulfite system for the pH values ranging from 4.0 to 10.0. The concentration of iohexol exhibits negligible variation as the solution pH fluctuates from 4.0 to 6.0. And the iohexol abatement efficiency reaches about 30% for pH 7.0, which means the abatement efficiency of iohexol is significantly improved as pH increases. However, when the pH was increased to 8.0, 9.0 and 10.0, the abatement efficiency of iohexol exhibits no obvious difference, which is maintained at about 60%. Compared to the Co(II)/sulfite system, iohexol removal in the Co(II)/HPO42−/sulfite system is significantly enhanced (Fig. 3(b)). About 18% iohexol can be removed for pH 4.0 and the abatement efficiency can reach 80% for pH 7.0. As pH value rises to 8.0 and 9.0, the abatement efficiency reached almost 100%. The enhancing effect is more pronounced under neutral to weakly alkaline conditions, which might be owing to the speciation of Co(II) complexed by HPO42−.
The Co(II) species distribution curves under different pH conditions is conducted to confirm the enhancing effect of HPO42−. The determination of Co(II) speciation is conducted based on the hydrolysis constants (logK1 = -9.6, logK2 = -9.2, and logK3 = -12.7), as illustrated in Fig. 3(c-d) (Yüzer et al. 2008; Tan et al. 2011). In the absence of HPO42−, Co(II) is identified as the principal species at pH < 8.0. The contribution of CoOH+ complex to the activation of sulfite is significantly greater than that of Co(II) and Co(OH)2(aq) (Zhou et al. 2018). In the HPO42− solution, Co2+ prevails as the predominant species for acidic and neutral conditions. When the pH increases, the conversion of Co2+ to CoHPO4 occurs, which reaches in a maximum concentration at pH 8.5. At about pH 10.0, Co(OH)2(aq) becomes the dominant species in the solution with the reduction of iohexol degradation efficiency to 90%, due to a reduction of the Co(II) and CoHPO4 concentration. Therefore, it can be speculated that the complexation effect of HPO42− can significantly elevate the catalytic reactivity of Co(II) in the sulfite activation process, which is similar to the study that combination of the trace Co(II) and phosphate triggers efficient PMS activation (Zhu et al. 2021).
On the basis of the above analysis, the enhanced radical generation mechanism via complexation in the Co(II)/HPO42−/sulfite system is proposed. As shown in Fig. 5, Co(II) firstly complexes with HPO42− to form the CoHPO4 complex. The CoHPO4 complex then reacts with sulfite to form the CoHPO4SO32− complex. In the presence of dissolved oxygen, the CoHPO4SO32− complex is oxidized to the CoHPO4SO3− complex. The CoHPO4 complex undergoes internal electron transfer to form SO3•−. The presence of HPO42− facilitates the oxidation-reduction cycle of Co(II)/Co(III), followed by the free radical chain reactions: (1) in the presence of dissolved oxygen, SO3•− is converted to SO5•−, (2) SO5•− further reacts with sulfite to yield SO4•−, and SO4•− reacts with OH− to generate HO•. The generated SO4•− and HO• can achieve the effective iohexol removal.
After attack of iohexol by SO4•− and HO•, iohexol can be decomposed into a series of degradation intermediates. Table S1 and Fig. 6 present the intermediate products via UHPLC-QTOF/MS and the corresponding degradation pathways of iohexol, respectively. The results show that eight intermediate products detected, and the main processes involved in the degradation of iohexol are amide hydrolysis, hydrogen-abstraction, alcohol oxidation and decarboxylation, oxidation to carboxylates, amino oxidation, deiodination, and hydroxyl addition. These results are consistent with the findings of the previous reports (Hu et al. 2017; Wu et al. 2022; Shao et al. 2023). Specifically, the production of byproducts P747 and P705 is attributed to amide hydrolysis, which is caused by the electrophilic attack of SO4•− on either N or O atom in the amide group followed by hydrolysis (Hu et al. 2017). By further transformation, P705 can be converted to P735 and then P609 through amino oxidation and deiodination. Moreover, hydrogen abstraction and alcohol oxidation and decarboxylation lead to the conversion of iohexol to P788, which is then transformed to P785 and P805 by hydrogen abstraction and oxidation to carboxylates, respectively. Additionally, iohexol results in the formation of P711 by deiodination and hydroxyl addition.
3.4 Practical application potential evaluation of Co(II)/HPO42−/sulfite system
Since ions (Cl−, NO3−, Ca2+, HCO3− and Mg2+) and humic acid (HA) are commonly present in the aquatic environment, their influence on the iohexol abatement in the Co(II)/HPO42−/sulfite system is explored (Fig. 7). The iohexol removal efficiency is reduced by 30% when Cl− concentration escalates to 5 mM (Fig. 7(a)). According to the competitive kinetics, Cl− can react with SO4•− and HO• to form Cl• and Cl2•− (kCl−,SO4•− = 4.7 × 108 M− 1s− 1, kCl−,HO• =4.3×108 M− 1s− 1) (Eqs. (1)-(3)), exhibiting lower reactivity with iohexol (Zhou et al. 2019; Qi et al. 2021). As for the effect of NO3−, the negligible effect on the iohexol abatement is due to that the reaction rate of SO4•− with iohexol is much higher than that with NO3− (kNO3−,SO4•− = 2.1×100 M− 1s− 1, kiohexol, SO4•− =7.9×109 M− 1s− 1) (Fig. 7(b)) (Wu et al. 2022; Shao et al. 2022).
Cl−+ SO4•−→Cl• + SO42− (1)
Cl− + HO•→Cl• + OH− (2)
Cl• + Cl− →Cl2•− (3)
As depicted in Fig. 7(c), the variation in HCO3− concentration within the range of 0 to 5 mM exhibits a dual effect on the degradation of iohexol, initially promoting and subsequently inhibiting its degradation. Generally, HCO3− exerts a quenching effect on the reactive species including SO4•− and HO• owing to the high rate constants (kHCO3−, SO4•− = 2.8 × 106 M− 1s− 1, kHCO3−, HO• = 8.5 × 106 M− 1s− 1) (Zhou et al. 2019). Consequently, the generation of less reactive carbonate radicals (CO3•−) leads to a marked diminishment in the removal of organic compounds (Zhou et al. 2019). This quenching influence of HCO3− on the reactive radicals appears to contradict a previous study examining the effects of HCO3− (Wu et al. 2020). According to our previous study on the Co(II)/HCO3−/sulfite system, it is found that the addition of carbonate ions could enhance the degradation of iohexol. The addition of bicarbonate ions to the Co(II)/HPO42−/sulfite system is observed to have a promotional effect on the degradation of iohexol within the initial 10 min. As the reaction progresses, a slight inhibition of iohexol degradation is observed. It can be inferred that this inhibition might be attributed to the competition between phosphate ions and carbonate ions.
HA is a widely distributed natural organic matter found in the aquatic environments. It possesses many active substances, which can influence the AOPs. Figure 7(d) displays the impact of varying concentrations of HA (0, 0.1, 0.3 and 0.5 mg/L) on the abatement of iohexol in the Co(II)/HPO42−/sulfite system. The abatement efficiency of iohexol is enhanced in the presence of HA within the initial 10 min, which is primarily attributed to the complexing property of HA on Co(II). As shown in the Fig. 7(e), when the concentration of Ca2+ ranges from 0 to 5 mM, the degradation efficiency of iohexol is 97.0%, 96.0%, 93.1%, and 85.5%, respectively. This indicates that the increase of Ca2+ concentrations can result in the decreasing trend in the degradation efficiency of iohexol. This is because Ca2+ in the solution can form a CaSO3 complex with sulfite, reducing the activation efficiency of sulfite. Figure 7(f) suggests that the influence of Mg2+ on the degradation efficiency of iohexol. Similar as Ca2+, Mg2+ exhibits a weaker inhibition effect on the degradation of iohexol. This phenomenon is attributed to the lower complexation ability between Mg2+ and sulfite than that between Ca2+ and sulfite (Wu et al. 2022).
The reuse performance of the Co(II)/HPO42−/sulfite system is evaluated by a cycle experiment in which 10 µM iohexol is repeatedly added to the reaction solution for five times. As shown in Fig. 8(a), the abatement efficiency of iohexol decreases gradually from 95–80% in five cycles of experiment. The possible reason is that the intermediate product of iohexol abatement may inhibits the complexation between Co(II) and HPO42−. In order to prove the applicability of the Co(II)/HPO42−/sulfite for the abatement of the other organic pollutants in water environment, the degradation of five different pollutants including tetracycline, oxytetracycline, chrysomycin, penicillin, and bisphenol A is monitored. As can be seen from Fig. 8(b), all five pollutants exhibit significant abatement, with the efficiency of abatement ranging between 90% and 98%. These results indicate the promising application prospects of the Co(II)/HPO42−/sulfite system in the treatment of wastewater contaminated with iohexol, and also its potent abatement capacity for other organic pollutants.
In this study, based on the complexation of Co(II) and HPO42−, a novel advanced oxidation system of the Co(II)/HPO42−/sulfite system is proposed for efficient degradation of iohexol. SO4•− is identified to be the main species for iohexol abatement, in which process dissolved oxygen plays a decisive role. The complexation of CoHPO4 promotes electron transfer and accelerates the valence inter-transformation. The presence of coexisting ions (NO3−, Ca2+ and Mg2+) displays negligible influence on iohexol abatement whereas the presence of Cl− exerts significant inhibition due to its quenching effect on reactive radicals. The Co(II)/HPO42−/sulfite system demonstrates favorable reuse performance and oxidation effect on other microorganic pollutants in the water and wastewater treatment. In conclusion, the complexation effect between the transition metal ions and the ligands is expected to be a feasible method for the enhancement of sulfite activation for the degradation of organic pollutants.
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Consent to publish: Not applicable.
Authors Contributions: Yifan Song: Investigation, Data interpretation and Writing-original manuscript. Jiesheng Hong: Investigation. Shujing Shao: Visualization. Jiayi Wen: Visualization. Xiaodan Zhao: Writing – review & editing, Supervision, Funding acquisition.
Funding: This work was funded by the Natural Science Foundation of Xiamen (Grant No. 3502Z20227190) and Natural Science Foundation of Fujian Province (Grant No. 2022J05064).
Conflict of Interest: The authors declare that they have no conflict of interest.
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