b Sino-German Centre for Water and Health Research, Sichuan University, Chengdu 610065, China;
c Institute of Environmental Engineering, RWTH Aachen University, Aachen, Germany
p-Nitrophenol (PNP) is a common and prominent pollutant that exhibits high toxicity, carcinogenicity, bioaccumulation, and normally let out in the aquatic system from the industries such as synthetic dyes, petrochemical, plasticizers, pesticides and pharmaceutical . If PNP wastewater is directly discharged into the environment, it will cause serious environmental problems that endanger public health and welfare due to its wide presence, toxicity, and slow rate of degradation. Therefore, it is necessary to develop a potential technology for the treatment of toxic and refractory PNP contained wastewater.
Over the past decades, zero-valent iron (ZVI or Fe0) applied to treat toxic and refractory pollutants in wastewater or groundwater has drawn great attention and its inspiring removal efficiencies have been documented [2-4]. Moreover, studies have also proven sulfate radical (SO4·-)-advanced oxidation processes (AOPs) to be effective in oxidizing toxic and refractory organic contaminants, and the generated of SO4·- is a strong oxidant with the potential of 2.6 V. Persulfate (PS) is a common source of SO4·- through activation strategies including heat , light [6-8], electrochemical , transition metal ions  and so on. Fe0 as an alternative activator has been employed to activate PS by releasing Fe2+ stably and continuously . Fe0 can not only release Fe2+ in aerobic/ anaerobic condition, but also release Fe2+ by the reaction with PS (Eq. (1)). Moreover, Fe0 can also recycle Fe3+ to Fe2+ (Eq. (2)) during the reaction process. However, the reactivity of Fe0 is susceptible to surface passivation and narrow working pH . In order to enhance the performance of Fe0, microscale Fe/Cu (mFe/Cu) prepared by plating Cu on mFe0 particles has been attempted to overcome shortcomings of Fe0, since planted Cu can facilitate iron corrosion due to the high potential between Fe and Cu (0.78 V) and hence enhance remarkably Fe0 reactivity . The performance of mFe/Cu-air system and mFe/Cu-PS system has been investigated in our previous studies [14, 15]. However, its oxidative reactivity coupled with the aerobic condition and PS for treatment pollutants in water has not been studied in detail.
Therefore, to combine the benefits of ·OH and SO4·- generated during Fenton-like and PS activated processes by mFe/Cu particles, the mFe/Cu-air-PS system was proposed for treatment of PNP in this work. Firstly, it is important to obtain the best performance for mFe/Cu-air-PS system through optimizing its operational parameters. Meanwhile, the superiority of mFe/Cu-air-PS system would be evaluated through control systems (i.e., mFe/Cu, air, PS, mFe/Cu-air, mFe/Cu-PS, air-PS and mFe-air-PS). Furthermore, the degradation pathway of PNP in mFe/Cu-air-PS system was proposed by analysis of COD, TOC, UV–vis spectra and HPLC chromatography.
Through analysis COD removal efficiency and pH variation of effluent in mFe/Cu-air-PS system, the key operating parameters (i.e., aeration rate, TMLCu, mFe/Cu dosage, PS total dosage, the feeding times of PS (N) and initial pH) were optimized. The optimal conditions (i.e., TMLCu of 0.110 g Cu/g Fe, mFe/Cu dosage of 15 g/L, PS total dosage of 15 mmol/L, N of 5, initial pH 5.4) were obtained by the single-factor experiments (in the Supporting information).
To comparative study the superiority of mFe/Cu-air-PS system, seven control systems including mFe/Cu, air, PS, mFe/Cu-air, mFe/ Cu-PS, air-PS and mFe-air-PS were set up under same conditions within 60 min process. The performance of mFe/Cu-air-PS system and control systems were investigated comparatively through analyzing the COD removal efficiency, effluent pH, total iron ions concentration (i.e., Fe2+, Fe3+ and flocculent precipitate) and UV–vis spectra in different reaction systems.
As shown in Fig. 1a, the COD removal efficiency of 71.0%, 60.8%, 50.8%, 42.5%, 11.2%, 0%, 0%, 0% were obtained by mFe/Cu-air-PS, mFe-air-PS, mFe/Cu-PS, mFe/Cu-air, mFe/Cu, PS-air, PS, air systems, respectively. It can be concluded that PNP was rarely oxidized by PS due to the limited oxidation capability of PS (Eθ = 2.0 V). In addition, it also could be found that PNP cannot be removed by air blowing. Moreover, the COD removal obtained by mFe/Cu-air-PS system was remarkably higher than those of seven control experiments during 60 min treatment. It further confirms the capacity of oxidative degradation of PNP by mFe/Cu-air-PS system was superior to other seven control systems. Meanwhile, the COD removal efficiencies of mFe/Cu-air-PS, mFe-air-PS, mFe/Cu-PS, mFe/Cu-air and mFe/Cu system were about 8.0% at 10 min treatment, indicating that oxidization ability for PNP by these systems was poor before adding PS. However, it was obvious different when added PS after 10 min reaction. For instance, the uptrend of mFe/Cu system was slowest in these systems, and the uptrend of mFe/Cu-air system had clearly increased after 20 min, but the uptrends of mFe/Cu-PS, mFe-air-PS and mFe/Cu-air-PS systems rose significantly after 10 min. In addition, the uptrends of mFe/Cu-air-PS system was higher the mFe/Cu-air and mFe/Cu-PS system. The phenomena demonstrate that oxidization ability for PNP by mFe/Cu-based system was elevated obviously by PS or aeration.
|Fig. 1. COD removal efficiency (a), effluent pH (b), total iron ion concentration (c) obtained by the different system during 60 min treatment ([PNP] = 500 mg/L, mFe/Cu dosage = 15 g/L, mFe dosage = 15 g/L, PS dosage = 15 mmol/L, aeration rate = 1.0 L/min, initial solution pH 5.4).|
Fig. 1b shows the effluent pH of different systems during the 60 min treatment. It is clear that the effluent pH of mFe/Cu-air-PS, mFe-air-PS, mFe/Cu-PS, mFe/Cu-air, mFe/Cu systems dramatically rose up to 9.5 at 10 min. However, the pH values of those systems with PS sharply dropped after 10 min and then mildly declined to less than 7.0 in the following process, while that of those two systems without PS continued to creep up and eventually exceeded 10.5 after 10 min.Inaddition, Fig. 1b also shows that the effluent pH of PS-air, PS system decreased mildly from 5.4 to about 3.4 during the 60 min treatment. The phenomenon could be explained from two aspects: (ⅰ) The processes of iron corrosion and Fenton-like reactions could consume H+ ions and cause the increase of solution pH; (ⅱ) The decomposition ofPS could releaseH+ ions and cause a decrease of pH values when PS was added reaction solution.
Meanwhile, when PS was added, there was evident different between mFe/Cu-air-PS and mFe/Cu-PS system that the effluent pH of the former was lower than that of the latter in the whole process, and finally it reached 5.2 and 6.2 after 60 min, respectively. The result supports the previous section of effect of aeration rate that the effluent pH declined with an increase of air aeration rate in mFe/Cu-air-PS system. However, in the systems without PS, the effluent pH of mFe/Cu system was higher than that of mFe/Cu-air system at first 25 min and no longer increased and stayed around 10.5, while that of mFe/Cu-air system rose continuously and exceeded mFe/Cu system after 25 min and eventually maintained about 11.1 after 60 min treatment. The results could be explained from two aspects: (ⅰ) Oxidizing capacity of PNP by mFe/Cu-air system was stronger than mFe/Cu system, which benefited to generation of small molecule organicacids that lead to the decrease of pH, and hence its effluent pH waslower than that of mFe/Cu system atfirst 25min; (ⅱ) Air aeration in mFe/Cu-air system would accelerate iron corrosion and Fentonlike reactions which would consume acid in solution and generate iron hydroxides, and hence cause higher effluent pH after 25 min. In conclusion, adding PS in reaction solution has an obvious impact on effluent pH.
Fig. 1c shows the variation of total iron ions (i.e., Fe2+, Fe3+ and flocculent precipitate) concentration in reaction solution during 60 min treatment process by the different systems. It presented that the all total iron ions concentrations of mFe/Cu-air-PS system were much higher than mFe/Cu-air, mFe/Cu, mFe-air-PS and mFe/ Cu-PS system during 60 min treatment. In addition, it is clear that total iron ion concentration was lower than 335.0 μg/mL at first 10 min in each system, and it increased remarkably faster after 10 min for mFe/Cu-air-PS, mFe-air-PS and mFe/Cu-PS system, and eventually rose up to 2051.0, 1800.9 and 1622.6 μg/mL, respectively. It also could be found that the total iron ion concentration in mFe/Cu and mFe/Cu-PS system was lower than its air aeration condition within the whole reaction process. The above results suggest that both of adding PS and air aeration could significantly promote iron corrosion and releasing of iron ions , however the contribution by PS was greater than by air aeration because the decomposition of PS could release H+ ions in solution and the standard redox potential for S2O82-/SO42- (2.01 V) is higher than that of O2/H2O (1.23 V). Meanwhile, throughout the whole process, the total iron concentration in mFe/Cu-air-PS system was more than in mFe/Cu-PS system. It indicates that the intensity of iron corrosion was stronger and the availability of dissolved Fe2+ was more efficient in mFe/Cu-air-PS system since Fenton-like reactions happened under aeration. The dissolved Fe2+ could facilitate the generation of amount of ·OH and SO4·- by Fenton-like reaction and activation PS reaction, which result in higher reaction reactivity of mFe/Cu-air-PS system. It further demonstrates aeration condition could enhance the capacity of oxidative degradation pollutants by mFe/Cu-PS system.
The UV–vis spectra from 190 nm to 500 nm of the influent and effluent of mFe/Cu-air-PS, mFe/Cu-air, mFe/Cu-PS and mFe/Cu system are provided in the Supporting information. The results further suggested that toxic and refractory PNP in aqueous solution could be decomposed effectively and transformed into lower toxicity intermediates by mFe/Cu-air-PS system.
The previous studies show that main intermediates during PNP degradation processes include reductive product of p-aminophenol (PAP), oxidative products of hydroquinone (HQ), p-benzoquinone (BQ) and some small molecular organics (e.g., fumanic acid, arylic acid) [17, 18]. In order to investigate degradation pathway of PNP in mFe/Cu-air-PS system, including target pollutant of PNP, three intermediates (PAP, HQ and BQ) were detected by HPLC analysis during reaction process. The concentration of PNP, PAP, HQ and BQ were also quantified by HPLC. And molar balance profile over the duration of process was obtained by summing these four species (PNP, PAP, HQ, BQ). Fig. 2a shows that there were 1.570 mmol/L of residual PNP, 1.891 mmol/L, 0.046 mmol/L and 0.015 mmol/L of generation of PAP, BQ and HQ at 10 min treatment by mFe/Cu-air-PS system. The results demonstrate that in the first 10 min reaction period, reduction reaction is the main reaction pathway in this system due to no PS addition. Furthermore, it also could be found from Fig. 2a that nearly 95.17% PNP vanished, but PAP increased and peaked at 2.19 mmol/L at 20 min when adding PS for the first time, which indicates PS could improve the performance of mFe/Cu-air-PS system for PNP degradation through facilitating iron corrosion. In addition, HQ increased slightly up to 0.07 mmol/L, but BQ nearly decomposed and the molar balance obviously dropped at 20 min treatment process. Based on previous analysis of UV–vis spectra, once the PS was added into the solution, PS could be decomposed to generate amount of SO4·-. Thus, PS could enhance the degradation efficiency of PNP via oxidation reaction. And then, PNP was entirely decomposed, PAP and the molar balance descended, but HQ gradually rose and BQ fluctuated with further adding PS after 20 min. Finally, the molar balance, PAP and HQ of the effluent achieved 1.236, 0.580 and 0.655 mmol/L, respectively after 60 min treatment, indicating oxidative degradation PNP was further promoted by feeding PS in mFe/Cu-air-PS system. In a word, reduction action and oxidation action both facilitate the PNP degradation in mFe/Cu-air-PS system.
|Fig. 2. Comparison of (a–c) molar concentration profiles of PNP and the intermediates (PAP, HQ, BQ) with treatment time and (d) UV–vis spectra of the effluent participated with MeOH or TBA in mFe/Cu-air-PS system.|
In literature, methanol (MeOH) has a similar reaction rate with SO4·- (1.6–7.7 ×107 L mol-1 s-1) and ·OH (1.2–2.8 × 109 L mol-1 s-1), but tert-butyl alcohol (TBA) react with SO4·- (4.9–9.1 ×105 L mol-1 s-1) much slower about 1000 times than with ·OH (3.8– 7.6 ×108 L mol-1 s-1) . Therefore, these two radical scavengers with concentration of 100 mmol/L were applied to quenching tests in order to explore contribution for PNP oxidative degradation through SO4·- of PS activation reaction and ·OH of Fenton-like reaction in mFe/Cu-air-PS system. It could be observed from Figs. b and c that the evolution profiles of these species in mFe/ Cu-air-PS systems with MeOH and TBA were similar to that without scavengers, but their concentrations and molar balance were lower than that without scavengers. In addition, it also could be found from Fig. 2d that the UV–vis spectral curves with the added scavengers were higher than that without the scavengers. The results indicate that these scavengers could restrained PNP degradation during reaction process. Compared the system added MeOH and TBA in mFe/Cu-air-PS system, the concentration of residual PNP, PAP, HQ and BQ in the existence of MeOH was similar to that in the existence of TBA, as well as the intensities of UV–vis spectra peaked at 317 nm and 227 nm. However, the concentrations of PAP and molar balance of the former (1.311 and 1.747 mmol/L) were higher than the latter (0.679 and 1.607 mmol/L) after 60 min treatment, while those of HQ and BQ (0.436 and < 0.001 mmol/L) were lower that the latter (0.926 and 0.003 mmol/L). Furthermore, the UV–vis spectrum peaked at 227 nm of the former was higher the latter after 60 min treatment. The results suggest that ·OH and SO4·- both facilitate the degradation of pollutants, and SO4·- act as the main oxidant.
Furthermore, as shown in Fig. S3 (Supporting information), the removal efficiencies of COD and TOC both increased as the reaction progress, and reached 71.0%, 65.8% after 60 min treatment, respectively. The results suggest that aeration and feeding of PS in batches could obviously improve the performance of mFe/Cu to degradation of PNP, and the intermediates also could be further mineralized. As above described, thus the main degradation pathway of PNP in mFe/Cu-air-PS is proposed and presented in Scheme 1. Specifically, at the early stage of the reaction (10 min), PNP was mainly reduced to PAP through directly accepting electron from Fe0 or by indirectly [H] formed on the active sites of Cu, combined slight oxidative degradation of PNP into BQ and further ring opening compounds e.g., small molecular organics) by ·OH formed through Fenton-like reactions. Furthermore, the oxidative products of BQ and HQ largely formed because the added PS could be activated to generate SO4·-, making the oxidation reaction predominant. Meanwhile, a part of PNP was further reduced to PAP due to little PS in the reaction solution at 10 min. Oxidation reaction proceed with the PS further added into solution. In other words, when PS was further added in reaction solution, the oxidation by SO4·- and ·OH instead of reduction of PNP plays a predominant role, thus PAP was further oxidized to HQ and BQ. Finally, their benzene rings were broken to small molecular organics, and further mineralized to CO2 and H2O.
|Scheme 1. Proposed possible reaction pathway for the degradation of PNP by mFe/Cu-air-PS system.|
In this study, a system of mFe/Cu-air-PS was conducted to treat PNP in aqueous solution. Firstly, the optimal operating parameters (aeration rate of 1.0 L/min, TMLCu of 0.110 g Cu/g Fe, mFe/Cu dosage of 15 g/L, PS total dosage of 15 mmol/L, feeding times of PS of 5, initial pH 5.4 (unadjusted)) were obtained by the batch experiments. Moreover, compared with the control experiments (i.e., mFe/Cu, air, PS, mFe/Cu-air, mFe/Cu –PS, air-PS and mFe-air-PS), mFe/Cu-air-PS system exerted superior performance for degradation of PNP, and higher COD and TOC removal efficiencies (71.0%, 65.8%) after 60 min treatment due to the strong synergistic effect between mFe/Cu, air and PS. In addition, this reinforcement by feeding of PS in batches was greater than by aeration based on control systems and radical quenching tests results. Furthermore, the degradation pathway of PNP in mFe/Cu-air-PS process was proposed through formed intermediates measured by HPLC, which reveals that both oxidation and reduction effects facilitate the PNP degradation. As a result, toxic and refractory PNP in aqueous solution could be degraded into intermediates with lower toxicity products and could be further mineralized into CO2 and H2O. Therefore, the mFe/Cu-air-PS system can be considered as an effective, robust and feasible treatment method for PNP in aqueous solution.Acknowledgments
The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (No. 51878423) and China Postdoctoral Science Foundation (No. 2018M631077).Appendix A. Supplementary data
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.01.025.
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