Micro zero-valent iron enabled persulfate activation for treating Rhodamine B-containing wastewater: Degradation performance, mechanism and column experiment

doi:10.21203/rs.3.rs-2125077/v1

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Micro zero-valent iron enabled persulfate activation for treating Rhodamine B-containing wastewater: Degradation performance, mechanism and column experiment

https://doi.org/10.21203/rs.3.rs-2125077/v1

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Organic dyes cause serious contamination of aquatic environments and pose adverse effects to human health. Therefore, it is of great concern to find an efficient and cost-effective way to treat wastewater containing organic dyes. In this study, commercial micro zero-valent iron (mZVI) was applied to activate persulfate sodium (PS) for degrading Rhodamine B (RhB). The results revealed that the combined application of mZVI and PS exhibited optimum degradation of RhB compared with the individual mZVI and PS systems. Moreover, the RhB could be removed efficiently within a wide initial concentration of 5-100 ppm and a broad pH range of 3–9. Furthermore, Electron spin resonance (ESR) and trapping experiments confirmed that, in the mZVI/PS system, both the •SO₄⁻ and •OH radicals were produced, while •SO₄⁻ radicals played the predominant role in the degradation of RhB. Remarkably, a long-term column experiment using mZVI as the packing media was performed. After discontinuous operation for seven days, the removal efficiency of the running RhB was still up to 65%. The result displayed the feasibility of mZVI/PS system for treating wastewater containing real organic dye. This study broadens the knowledge of mZVI-activated persulfate-based advanced oxidation processes, and provides a promising method for controlling water pollution.

Micro zero-valent iron

RhB degradation

Reactive oxygen species

Degradation mechanism

Column experiment

Organic dye is widely used in printing, textile and leather industries (Kishor et al., 2021; Krishna Priya et al., 2016; Shanker et al., 2017). Due to its dark color and bio-refractory degradation behavior, detrimental effects are observed on the environment. For example, as a typical organic dye, the water-soluble Rhodamine B (RhB) is not only harmful to aquatic organisms, but also to humans and animals (Burhanuz Zaman et al., 2019; La et al., 2021). Therefore, it is necessary to develop an effective approach for treating wastewater containing organic dyes.

In the last decade, advance oxidation processes (AOPs) have widely been used as an effective technology to remove organic dyes from aquatic environment because of their superior degradation and mineralization efficiencies. Among various AOPs, persulfate (PS) activation technologies are an emerging class of methods to treat organic pollution because of the high oxidant property and the stability of PS at room temperature. Additionally, the method can be applied to considerable distances in the subsurface more easily compared to H₂O₂ (Bajagain et al., 2018). Persulfate itself is a non-selective oxidant because of its high redox potential (E^ꝋ) of 2.01 V, and can be regarded as a strong oxidant to directly degrade organic contaminants (Liang et al., 2007; Zhu et al., 2016). As is well-known, persulfate can be activated using heat, a base, UV irradiation, electricity and use of transition metals, especially iron-bearing materials, all of which lead to the cleavage of O-O bond to form sulfate radical (•SO₄⁻, E^ꝋ=2.5-3.1V), which is an even stronger oxidizing agent than persulfate (Furman et al., 2010; Xia et al., 2017a). Meanwhile, •SO₄⁻ could react with water to produce its derived hydroxyl radicals (•OH, E^ꝋ=2.7-2.8V), which was also responsible for the oxidation of contaminants(Jia et al., 2016; Shi et al., 2017; Sun et al., 2012; Wang et al., 2015). Using these PS-activated methods, chemical oxidation of organic pollutants for water purification has been demonstrated in numerous studies (Lei et al., 2015; Li et al., 2017; Xia et al., 2017b). For example, Hu et al. studied the degradation behavior of iohexol using heat-activated persulfate method, and found that the iohexol could be effectively removed with a removal efficiency of more than 85% (Hu et al., 2020). Po KeungWong et al. reported that phenol could be degraded efficiently using the magnetic pyrrhotite/PS system, whereas the electron paramagnetic resonance spectra indicated that acidic conditions favored the generation of both •SO₄⁻ and •OH species, however only the •OH signal survived under alkaline conditions during the oxidative degradation of phenol (Xia et al., 2017b).

In the past decades, as one of the most important iron-based materials, zero-valent iron (ZVI) has widely been used in AOPs for wastewater purification due to its high efficiency and environment-friendly nature(Li et al., 2021; Li and Liu, 2021; Segura et al., 2016). However, the need of large quantities of nanoscale ZVI and high costs limited the applications of ZVI in remedying aquatic pollution. Although many studies have focused on the synthesis of modified nZVI to further improve the activity of nZVI, excessive processes and chemicals led to the increase in cost. By contrast, micro zero-valent iron (mZVI) was cost-effective and easily available. The application of mZVI for the activation of PS might be a promising method for organic dyes in real environmental remediation process. Therefore, it is important to provide theoretical basis for water restoration through the evaluation of mZVI/PS technology.

In this study, commercial mZVI was used as an activator for activating PS to produce reactive oxygen species for the degradation of RhB. The effects of RhB’s concentration and initial pH value on the degradation of organic dye were evaluated. To elucidate the mechanism of RhB’s degradation in the mZVI/PS system, the ESR and trapping experiments were carried out to explore the role of reactive oxygen species (ROSs), and the variation in mZVI during the degradation of RhB was examined carefully. Besides, the reusability of mZVI and column experiment were performed to evaluate the feasibility of mZVI/PS system for treating real-world organic dye-containing wastewater.

Chemicals and materials

Micro zero-valent iron and sodium persulfate (Na₂S₂O₈) were purchased from National Medicines Corp., Ltd., China. Rhodamine B (RhB), ethanol (EtOH), and tert-butanol (TBA) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Ultrapure water used throughout the experiments was obtained from a commercial purification system. All other reagents were also of analytical grade and used as received.

Experimental procedure

RhB degradation experiments were carried out in 100 mL round-bottomed flask by adding 40 mL RhB solutions. The RhB solutions with different initial concentrations were obtained from the dilution of stock RhB solution (1000 mg/L). The influence of initial pH was investigated within the pH range of 3–11. The initial pH value was adjusted using sulfuric acid and sodium hydroxide. The PS powder (10 mg) and mZVI (40 mg) were added into the solutions to initiate the reaction. The mixtures were transferred into an incubator shaker and maintained at a constant speed of 200 rpm at 25 ºC. After a pre-determined time, 0.9 mL of sample solution was separated from the adsorbents using a nylon syringe filter (0.22 µm) to determine the concentration of RhB. 0.1 mL ethanol was added in advance to quench the reaction mixture.

The column experiment was performed for seven days with discontinuous operation. The equipment was operated during daytime, and the operation was suspended at night. The concentration of the influent RhB was set at 2.0 mg/L, and the PS concentration in the mixture was 125 mg/L. The liquid sample was collected from the outlet of the column for analyzing RhB concentration.

Analytic methods

The powder X-ray diffraction (XRD) patterns were collected on a Rigaku MiniFlex 600 X-ray diffractometer at 40 kV and 40 mA using Cu Kα radiation (k = 1.54178 Å). A mastersizer laser diffraction particle size analyzer (Malvern Zetasizer Nano As, United Kingdom) was used to determine the size distribution of ZVI particles. The morphologies were studied using a scanning electron microscope (SEM, Zeiss Sigma 300, Germany). Moreover, X-ray photoelectron spectroscopy (XPS) data were collected using a Thermo Scientific K-Alpha X-ray photoelectron spectroscopy.

The concentration of RhB was measured using a UV-Vis 2500 spectrophotometer (Shimadzu, Japan) at a wavelength of 554 nm. The signals of free radicals generated during the activation of PS were detected using an electron spin resonance (ESR) spectrometer (Bruker E500, Germany) at room temperature.

As shown in Fig. 1a, the XRD pattern showed two distinct peaks at 44.7° and 65.1°, indicating that the iron powder was consistent with the zero-valent iron (Fe⁰) (Jin et al., 2018). Furthermore, the grain size of zero-valent iron was characterized using laser particle analyzer. The result revealed that approximately 50% of the obtained ZVI had a diameter of less than 418.1 µm, whereas 90% of them were below 590.1 µm (Fig. 1b), confirming that ZVI possessed the micro-sized size distribution.

Rhb Degradation In The Mzvi/ps System

Characterization

First, the degradation of RhB (5 mg/L) was analyzed under different conditions without pH adjustment. It was found that only 10% of RhB was degraded by the addition of mZVI alone. This was due to the possible adsorption and the molecular oxygen activation of mZVI. When only PS was added to the system, the degradation efficiency of RhB was 25% within 60 min. The low degradation efficiency was ascribed to the limited oxidation capability of PS. However, RhB was nearly completely removed by the combined addition of mZVI and PS within 10 min (Fig. 2a). Although accurate rate constant could not be obtained because of the rapid reaction in the mZVI /PS system, the high reaction rate for the oxidation process was obvious. Considering the pH value would affect the degradation efficiency of RhB, the pH value was checked in the three systems. As shown in Fig. 2b, with only the addition of mZVI, the pH value decreased slightly from 7.7 to 7.4. For the PS system, the pH value decreased rapidly from 7.7 to 3.4 within 5 min and remained unchanged at this level, which was ascribed to the unavoidable hydrolysis of PS, resulting in the generation of H⁺. Similarly, in the mZVI/PS system, the solution pH value fell sharply from 7.7 to 3.2 in 10 min. The decrease in pH value was due to the exchange of proton between the Fe/Fe²⁺ and PS (Eqs. 1–2), as well as the reaction between •SO₄⁻ and H₂O (Eq. 3). To check how the pH value affected the degradation of RhB, the initial pH was maintained at the value of 3.2 in the individual mZVI and PS systems. It was found that, within one hour, the RhB degradation efficiencies were 14% and 31% (Fig. 2c), respectively, which were similar to the values obtained without the adjustment of pH. These results ruled out the contribution of lower pH value to a better RhB degradation performance by the combined application of mZVI and PS.

Fe⁰ + S₂O₈²⁻ → Fe²⁺+•SO₄⁻ +SO₄²⁻ (1)

Fe²⁺+ S₂O₈²⁻+ 3H₂O → Fe(OH)₃+•SO₄⁻ +SO₄²⁻+ 3H⁺ (2)

•SO₄⁻+H₂O →H⁺+SO₄⁻+•OH (3)

Effects of initial concentration and pH value on the degradation of RhB in the mZVI/PS system

The degradation performance was also evaluations for different initial RhB concentrations that was varied within the range of 5–200 mg/L. As shown in Fig. 3a, when the initial concentration of RhB was varied through values of 5 mg/L, 10 mg/L and 20 mg/L, the RhB could be completely degraded within 20 min. When the RhB concentration was further increased to 40–200 mg/L, the degradation efficiency gradually decreased. However, when the RhB concentration was 200 mg/L, the degradation efficiency still reached 50%, demonstrating the outstanding degradation performance by using the mZVI as the PS activator. Subsequently, the effect of initial pH value on the degradation of organic dye was also investigated. As displayed in Fig. 3b, when the pH values were at 3 and 5, the RhB (5 mg/L) was removed immediately, and no obvious difference was found compared to the degradation performance in the mZVI/PS system without pH adjustment. This could be ascribed to the lower pH value due to the addition of mZVI and PS. When the pH values were adjusted to 9 and 11, the RhB removal rate was obviously inhibited, and the degradation efficiency decreased to 90% and 30% within 1 h, respectively. The higher the pH value, the more suppressed the dissolution of Fe²⁺. Consequently, the PS activation process to degrade RhB was restricted. Nonetheless, these results revealed that RhB could be removed efficiently within a broad range of pH (3–9), indicating that the mZVI-activated PS process might be a feasible method for treating wastewater containing organic dyes.

Reactive oxygen species generation

To the best of our knowledge, the degradation of organic contaminant was mainly attributed to the generation of reactive oxygen species (ROSs) in the persulfate activation process, such as •SO₄⁻ and •OH (Ismail et al., 2017). In order to confirm this point, ESR technique was employed to monitor the generation of ROSs in the mZVI/PS system at room temperature. Strong four-line ESR signals with the relative intensities of 1:2:2:1 in the mZVI/PS system were observed, which indicate a typical pattern of •OH adducted to DMPO (Fig. 4a) (Fan et al., 2022). Besides, DMPO-•SO₄⁻ adduct with 1:1:1:1:1:1 sextet peak was also detected in the system, indicating the production of •SO₄⁻ when PS was activated by mZVI. Moreover, in order to explore the probable contribution of ROSs in the PS decomposition process, trapping experiments were performed by adding different scavengers. Both •OH and •SO₄⁻ could be trapped by ethanol (kSO₄•- = 1.2–2.8 × 10⁹ and k•OH = 1.6–7.7 × 10⁹ M^− 1s^− 1) (Ding et al., 2017). As shown in Fig. 4b, when ethanol was added, the degradation efficiency of RhB decreased to 30% in the mZVI/PS system. Due to the significant difference of reaction rate constants between TBA and •OH/•SO₄⁻ (kSO₄•-= 4.0–9.1 × 10⁵ M^− 1s^− 1 and k•OH = 3.8–7.6 × 10⁸ M^− 1s^− 1), TBA was chosen to explore the role of ROSs in the degradation of RhB (Ding et al., 2017). Due to the presence of TBA, the degradation rate decreased to 30%. Furthermore, TBA mainly reacted with •SO₄⁻, indicating that •SO₄⁻ was the dominant ROS for the degradation of RhB in the mZVI/PS system. From the above-mentioned pH change experiments (Fig. 2b), when the pH value of the solution immediately decreased to acidic, the •SO₄⁻ could react with water to produce •OH. However, the rate constant for this reaction, which was less than 2 ×10^− 3 s^− 1, was much lower than the •SO₄⁻ trapped by the organics contaminants (Mogharbel et al., 2019), indicating that •SO₄⁻ was the primary ROS for RhB’s degradation.

Mechanisms of RhB degradation in ZVI/PS system

In order to elucidate the possible mechanism of RhB’s degradation in the mZVI/PS system, the variation in mZVI after RhB degradation was examined. The XRD patterns revealed that the residual solid could maintain the ZVI structure (Fig. S1). However, the SEM images revealed that the agglomerated nano-particles were produced on the surface of mZVI after the removal of RhB (Fig. 5a and 5b), which might be attributed to the formation of ferric (hydro)oxide because of the oxidation of mZVI. On the basis of XRD results, the produced ferric (hydro)oxide might be amorphous. Furthermore, high-resolution XPS analysis was performed to observe the surface of mZVI. As shown in Fig. 5c, two distinct bands could be observed from the high-resolution Fe2p XPS spectrum, which corresponded to the Fe2p_1/2 and Fe2p_3/2 spin-orbit splitting, respectively. Through the calculation of integral areas, the molar ratio of Fe(II)/Fe(III) on the pristine mZVI was found to be 1.8:1. In contrast, this molar ratio dropped to 1.3:1 after the degradation of RhB (Fig. 5d), confirming that the surface of mZVI was oxidized and that Fe(III) (hydro)oxide was produced in the degradation process. For the O1s spectra, two peaks were located at 531.3 and 529.7 eV, which corresponded to hydroxyl groups (-OH) and oxide-oxygen (O²⁻), respectively (Fig. 5e). As per some previous reports, the surface -OH group might be attributed to the formation of iron (hydro)oxide, while O²⁻ was suggested to be Fe–O (Sleiman et al., 2017; Yang et al., 2019). In this study, the content of -OH group on the surface of mZVI increased from 66–79% after the degradation of RhB (Fig. 5f), suggesting the possible formation of iron (hydro)oxide on the surface of mZVI in the oxidation process. This phenomenon was in accordance with some previous reports(Al-Shamsi and Thomson, 2013; Huang et al., 2018; Kang et al., 2021).

On the basis of the degradation performance of RhB, ROSs detection, and morphological investigation of mZVI before and after the organic treatment, the plausible mechanisms of RhB’s degradation in mZVI/PS system were proposed (Scheme 1). When PS was added, the surface Fe(II) covered the mZVI, and free Fe(II) ions released through the proton dissolution and Eq. 1 routes, both of them could directly activate PS for further decomposition. Although Fe(II) and Fe(III) (hydro)oxides shell inhibited the direct reduction of PS (by Fe⁰), the (hydro)oxide shell exhibited the property of n-type semiconductor, which enabled the electron transfer from iron core to the outer shell (Wang et al., 2020), after which, the electron might participate in the activation of PS. After the decomposition of PS to produce •SO₄⁻ and SO₄⁻, a part of •SO₄⁻ reacted with H₂O to produce •OH radicals, which were also responsible for the organic degradation. However, •SO₄⁻ was the main ROS to achieve the discoloration process in the mZVI/PS system.

Reusability of mZVI

After the reaction, the used mZVI was collected using a magnet, and washed with pure water and ethanol. The mZVI was reused for the degradation of RhB by re-introducing PS in each run. It was found that the RhB’s degradation efficiency gradually decreased during the reuse cycles. After five runs, the degradation efficiency of RhB decreased to 50% in 1 h (Fig. 6). Understandably, the deactivation was derived from the unavoidable oxidation of mZVI.

Column Experiment

A column experiment was carried out to simulate the permeable reactive barriers (PRBs) technology to treat the running polluted water using the mZVI as the reactive medium. The experimental equipment used for the experiment is displayed in Fig. 7a. Cotton, river sand and mZVI were packed in the acrylic column as the PRB reactor. Figure 7b shows the schematic of the process and various of its parameters. Prior to feeding the RhB, the solid PS was added into the solution and the mixture was pumped into the packed column at 2.0 mL/min. The plots of RhB’s degradation efficiency against the effluent solution are shown in Fig. 7c. When 12.5 L of RhB’s solution (2 ppm) was pumped and flowed through the column, the average removal efficiency was maintained at approximately 90%. After the injection volume was increased to 20 L (corresponding 7 days with discontinuous operation), the removal efficiency was still 65%. These results demonstrated that the mZVI-activated PS would be an operable method for real organics-containing groundwater treatment. During the column experiment, it was found that more and more yellow iron (hydro)oxide was generated and attached to the column wall with the elapse of time. Besides, loose oxidation products were scattered at the bottom of the packed column (Fig. S2a and S2b). The XRD pattern revealed that the iron sludge mainly consisted of Goethite (α-FeOOH) (Fig. S3).

In this work, commercial mZVI was used to activate PS to treat RhB containing wastewater. In comparison with the individual mZVI and PS systems, the combined mZVI/PS system could degrade RhB more efficiently within a broad pH range. The Fe(0), surface Fe(II), as well as the free Fe(II), derived from the mZVI, could decompose PS to produce •SO₄⁻ and •OH, whereas •SO₄⁻ was the main ROS to attack RhB for its oxidative discoloration. More importantly, the column experiment revealed that mZVI might be a promising reactive medium for the PRB technology to treat the real wastewater. These findings broaden the insights of mZVI-activated persulfate-based advanced oxidation processes, and also propose a promising method for treating organic wastewater.

Author contribution

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Xiaobing Wang, Jiaying Hu, Qiyao Liu, Lina Jiang, Qingqing Huang and Yue Hu. The first draft of the manuscript was written by Xiaobing Wang and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

This work was supported by the Guangdong Basic and Applied Basic Research Foundation (2022A1515010368), the Characteristic Innovation Projects Guangdong Provincial Department of Education (Natural Science Category, 2018KTSCX208 and 2021KQNCX084), the Projects of Science and Technology of Shaoguan City (200811094530800), the Shaoguan University Research Project (SZ2020KJ04 and SY2020KJ04), the Student Science and Technology Innovation Cultivating Program of Guangdong Province (pdjh2021b0457), and the Student Innovation and Entrepreneurship Training Program of China (S202110576033).

Data availability

All data generated or analyzed during this study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and Consent to participate Not applicable.

Consent for publication Not applicable.

Competing interests The authors declare no competing interests.

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Scheme 1 is available in Supplementary Files section.

Scheme1.png
Scheme 1 Plausible mechanisms of RhB degradation in mZVI/PS system
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Micro zero-valent iron enabled persulfate activation for treating Rhodamine B-containing wastewater: Degradation performance, mechanism and column experiment

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Abstract

Figures

Introduction

Experimental

Chemicals and materials

Experimental procedure

Analytic methods

Results And Discussion

Rhb Degradation In The Mzvi/ps System

Characterization

Reactive oxygen species generation

Mechanisms of RhB degradation in ZVI/PS system

Reusability of mZVI

Column Experiment

Conclusions

Declarations

References

Scheme

Supplementary Files

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