Coupled removal of Rhodamine B and Cu²⁺ by activating persulfate using micron zero-valent iron

Dye wastewater, a common type of pollutant, is continuously generated by industries such as those producing rubber, paper, and textiles, which results in environmental and ecological problems all over the world (Ferreira et al. 2016; Bilal et al. 2017). What is more, Cu²⁺ is commonly found in wastewater as well as in the food chain, which can create severe renal, central, and hepatic nervous system damage and mucosal irritation (Doong et al. 2012). Nevertheless, most developed technologies aim to remove single contaminants in water; hence, it is universally acknowledged that methods to remove dyes and Cu²⁺ from water adequately are necessary.

As is well known, in removing dyes from water, many methods have been reported including coagulation, oxidation, biological processes, membrane filtration, electrochemical degradation, and adsorption (Morshedi et al. 2013; Fernandez et al. 2014; Lin et al. 2015; Yang et al. 2016; Wei et al. 2017). Among them, advanced oxidation processes (AOPs) are favorable in dye water pollution control because of their superior performance and efficiency (Sharma et al. 2015). Nowadays, persulfate (PS) has drawn much attention from researchers due to its high oxidation potential (E⁰ = 2.01 V) and higher stability and solubility at room temperature (Liang et al. 2004; Zhou et al. 2013). Vicente et al. (2011) reported that the non-selective and high redox potential sulfate radical (·SO₄⁻, E⁰ = 2.6 V) by the activation of PS is similar to ·OH. In addition to dye removal, several technologies, including adsorption, ion exchange, chemical precipitation, and electrolysis, have been developed to remove Cu²⁺ from wastewater (Yueming et al. 2008; Doong et al. 2012).

In this work, RhB and Cu²⁺ were selected as the targets. Our study mainly focuses on: (1) comparing the ZVI/PS system and ZVI/PS/Cu²⁺ system; (2) investigating the removal of Cu²⁺ and/or RhB in the ZVI/PS/Cu²⁺ system under different conditions; (3) identifying the radical species in the ZVI/PS system; (4) analyzing the mechanism of the simultaneous removal of Cu²⁺ and RhB.

All reagents and chemicals were of analytical reagent grade and were used as received, without further purification. The water used for the preparation of the reagents and reaction system was purified by a Milli-Q (18.25 MΩ·cm) system. The solution pH was adjusted by H₂SO₄ and NaOH solution. Micron scale zero-valent iron powder (ZVI, ≥99.9%), sodium persulfate (PS, ≥99%), 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO, ≥97%), potassium iodide ( ≥99.0%), and neocuproine hemihydrate (NP, ≥98%) were purchased from Sigma-Aldrich Co., Ltd (Shanghai, China). Rhodamine B (RhB, ≥99%), sulfuric acid (H₂SO₄, analytical grade), sodium hydroxide (NaOH, analytical grade), L-ascorbic acid, tert-butyl alcohol, ethanol, sodium bicarbonate, potassium iodide, and 1,10-phenanthroline were obtained from Kelong Chemical Co., Ltd (Chengdu, China).

Twenty-five mg of ZVI was immersed in a beaker (250 mL) with a total solution volume of 250 mL of ultrapure water containing 20 mg/L RhB (and/or 0.02 mmol/L Cu²⁺, and/or 1.0 mmol/L PS). The mixture was stirred at a constant rate (500 r/min) for 60 min under 25 ± 1 °C condition, then the solution was filtered by a micro PES membrane (pore size: 0.22 μm). The initial reaction time was defined at the moment when ZVI and PS were added. The samples were withdrawn from the reaction solution and the residual samples' concentration in the filtered solution was determined using UV-visible spectrophotometer. The experiments were conducted in air condition. Control experiments were conducted and excluded the interaction between the filter and RhB. All experiments were conducted twice.

The pH values of solutions were measured by a pH-meter (FiveEasy Plus, Mettler-Toledo, Shanghai). A UV-visible spectrophotometer (UV-1100, Shanghai Mapada Instrument Co., Ltd) was used for analyzing the dye concentrations by measuring the maximum absorbance at a wavelength of 554 nm for RhB. After filtration with a cellulose ester membrane of 0.45 μm pore size and freeze vacuum drying in turn, the solid samples of residual ZVI after the reaction (reaction time 60 min) were prepared. The morphologies and elemental compositions of the ZVI samples before and after oxidation were investigated by a field emission scanning electron microscope (SEM, FEI Inspect F50(FSEM)) equipped with an energy dispersive X-ray spectrometer (EDX, FEI Inspect F50 (FSEM)). X-ray diffraction (XRD, Bruker D8 ADVANCE A25X, Germany) and X-ray photoelectron spectroscopy (XPS, Escalab 250Xi) were also characterized. Electron paramagnetic resonance (EPR) was performed to detect ·OH and ·SO₄⁻, with DMPO as the spin-trapping agents. The ESR spectrum was measured under the following conditions: a center field of 3,505.40 G, a sweep width of 150.00 G, a microwave frequency of 9.84 GHz, a microwave attenuator of 20.00 dB, a microwave power of 2.00 mW, and a sweep time of 10.53 seconds. A multi N/C 3100 analyzer (Analytikjena) was employed to analyze the total organic carbon (TOC).

The concentration of PS was tested by the iodometric titration method using a UV-vis spectrometer (Mapada UV-1100). 0.1 g sodium bicarbonate, 0.5 g potassium iodide, and 4.75 mL Milli-Q water were mixed to create a stock solution, then 0.25 mL of the filtered PS sample was added to stock solution. After 20 min, the concentration of PS was detected by a UV-vis spectrometer at 352 nm (Zhou et al. 2017).

The total concentration of dissolved copper (dissolved iron) was determined using a UV-vis spectrometer (Mapada UV-1100). The 2 mL filtered sample, 0.8 mL 0.1 M L-ascorbic acid, 0.4 mL 10 mM NP (0.8 mL 10 mM 1,10-phenanthroline), and 6.8 mL (6.4 mL) of Milli-Q water were mixed to create the Cu⁺–NP ([Fe(phen)₃]²⁺) complex solution. The concentration of dissolved copper (dissolved iron) was detected by a UV-vis spectrometer at 454 nm (510 nm) (Zhou et al. 2017).

To investigate the morphologies and structures of ZVI before and after reaction in the ZVI/PS system and ZVI/PS/Cu²⁺ system, the spectroscopy SEM-EDS were characterized. As shown in Figure 2(a) and 2(b), it can be seen that after the corrosion of PS, the residual ZVI was rougher than the pristine ZVI powder. Figure 2(c) shows that there are a great deal of spheroidal particles in the ZVI surface areas after reaction of the ZVI/PS/Cu²⁺ system, which is attributed to the generated zero-valent copper (ZVC) (Zhou et al. 2017). The EDS result also showed that the ZVI surface contained a large amount of copper, approximately 90%, which indicated that metallic copper was generated and integrated into the ZVI. The EDS results are consistent with the analysis obtained from the SEM analysis.

XRD patterns for the three samples are shown in Figure 3. The characteristic peaks of the pristine ZVI and ZVI after ZVI/PS system and ZVI/PS/Cu²⁺ system reaction powder at 44.7°, 65.3°, and 82.5° matched well with the standard patterns of Fe⁰ (PDF#06-0696) (Li et al. 2015). The strongest diffraction peaks of the ZVI after ZVI/PS/Cu²⁺ system reaction at 43.3°, 50.4°, and 74.1° also matched well with the standard pattern of Cu⁰ (JCPDS-PDF#04-0836) (Zhou et al. 2017). Figures 3 and 4(d) indicated that most of the Cu²⁺ species were converted into CuO and Cu⁰ after reaction.

XPS was used to further investigate the detailed chemical states and the chemical composition of the ZVI powder before and after reaction in the ZVI/PS system and the ZVI/PS/Cu²⁺ system. The atomic percentages of Fe, O, and Cu elements on the surface of three composites are shown in Table 1 and the XPS survey of the three composites is shown in Figure 4(a), and mainly indicates the presence of Fe 2p, O 1s and Cu 2p signals. Figure 4(b) presents the core level of Fe 2p spectras which can be decomposed into two peaks at 724.1 and 710.6 eV, belonging to 2p1/2 and 2p3/2 binding energies of Fe²⁺ and Fe³⁺ on the ZVI surface (Du et al. 2016; Lü et al. 2019). As demonstrated in Figure 4(c), the O 1 s peaks at 532.3, 531.1, and 529.5 eV correspond to H₂O or other oxidized iron, OH⁻ and O⁻, respectively (Wu et al. 2013). With respect to the areas of the spectra, the amount of Fe 2p and O⁻ increased after reaction in the ZVI/PS system, and we can conclude that the original ZVI was oxidized in the open air, and more ZVI was oxidized to oxidized iron after the reaction. Conversely, the amount of Fe 2p and O⁻ or other oxidized iron decreased under the reaction of the ZVI/PS/Cu²⁺ system, and this decrease resulted in generating Cu⁰ and CuO. The results in Figure 4(d) indicated that Cu⁰ and CuO were detected on ZVI surface. To be specific, the Cu 2p peaks at 954.0 and 934.0 eV represent the binding energy of Cu 2p3/2 and Cu 2p1/2 of Cu⁰, and the other peaks located at 962.6, 955.2, 942.5 eV, and 935.0 eV are attributed to CuO on the surface of ZVI powders (Zhou et al. 2017). Thereby, it is further confirmed that the Cu²⁺ was removed together with removing RhB.

The increased ZVI dosage from 20 to 160 mg/L on the removal of RhB and Cu²⁺ in the ZVI/Cu²⁺ system and ZVI/PS/Cu²⁺ system were investigated. In the ZVI/Cu²⁺ system, a higher dosage of ZVI was beneficial in removing Cu²⁺ (Figure 5(c)), but not RhB (Figure 5(a)), which suggests that without PS, the removal rates of RhB were negligible. However, ZVI loading posed significant impacts on the removal of Cu²⁺ and RhB in the ZVI/PS/Cu²⁺ system (Figure 5(b) and 5(c)). The curves between logarithms of C/C₀ (RhB) and time were plotted in the different ZVI dosage systems, and showed good linearity between C/C₀ and time (R² = 0.9976, 0.9713, 0.9378, 0.9951, and 0.9871 for the ZVI dosage of 20, 40, 80, 120, and 160 mg/L, respectively). The slopes of C/C₀ against time were estimated to be 0.040, 0.089, 0.289, 0.301, and 0.322, respectively, which can be taken as the orders of reactivity in the RhB removal rates. Further, the higher removal of Cu²⁺ reached 81.0% in the ZVI/PS/Cu²⁺ system.

Figure 6(a) indicates that the reaction between ZVI-Cu²⁺ and RhB without PS could be neglected, which is consistent with the analysis obtained from Figure 6(a). As shown in Figure 6(b), it is observed that the removal of RhB could also be enhanced slightly when the concentration of Cu²⁺ increased, but the removal rate constants were all higher than those in the ZVI/PS system (data not shown). Figure 6(c) revealed the slight variation in Cu²⁺ degradation rate with raising the initial concentration of Cu²⁺. On the other hand, Figures 5(c) and 6(c) show a slight decrease in degradation efficiency of Cu²⁺ in the ZVI/PS/Cu²⁺ system compared to the ZVI/Cu²⁺ system. However, these results also clearly demonstrate that the simultaneous removal of RhB and Cu²⁺ can be successfully achieved in the ZVI/PS/Cu²⁺ system.

There is no doubt that the solution pH plays a significant role in the oxidation process and pH is important for the lifetime and reactivity of ZVI in water. The effect of initial pH on the removal of RhB by the ZVI/PS system and the simultaneous removal of RhB and Cu²⁺ in the ZVI/PS/Cu²⁺ system are shown in Figure 7. Figure 7(a) and 7(c) show that the lowest initial pH can effectively enhance the RhB removal rate and accelerate the process to reach a steady-state condition within a shorter reaction time. When pH reaches as high as neutral, the RhB removal efficiency drops drastically. Moreover, it is noted that in the same pH conditions, the RhB removal and its removal rate constants in the ZVI/PS/Cu²⁺ system were far superior to that in the ZVI/PS system. As we know, the released Fe²⁺ during the corrosion of ZVI is the main effective iron species to activate PS to produce ·OH and ·SO4⁻ in the ZVI/PS system and the Fe²⁺ can be released in low pH acidic conditions (Equation (6)). However, Cu²⁺ can accelerate the corrosion of ZVI to release Fe²⁺ under higher pH acidic conditions because of ion exchange (Equation (5)) in the ZVI/PS/Cu²⁺ system, which showed a wider pH working range. The Cu²⁺ removal rates also showed a slight change when the pH increases from acidic to neutral conditions.

It is universally acknowledged that ·SO4⁻ and ·OH can be produced via catalyzing PS by ZVI. In this study, the effects of radical scavengers, including ter-butanol and ethanol, were used to investigate the presence of ·OH and ·SO4⁻. Ter-butanol as a scavenger for ·OH exhibited little effect on RhB degradation in the ZVI/PS system and ZVI/PS/Cu²⁺ system. As shown in Figure 8(a) and 8(b), the removal of RhB was significantly inhibited in the presence of ethanol, which is a scavenger for both ·OH and ·SO4⁻. It is confirmed that both ·OH and ·SO4⁻ existed in the ZVI/PS system and ZVI/PS/Cu²⁺ system, indicating that the ·OH and ·SO4⁻ was likely generated in the reactions between PS and ZVI/Cu²⁺.

More specifically, EPR spectroscopy, with the spin-trapping reagent of DMPO, was conducted to directly detect the generation of ·OH and ·SO4⁻. In our study, as shown in Figure 8(c), ·OH and ·SO4⁻ were quickly produced in the first 2 min.

The effectiveness of the oxidative process might be affected by the composition of the raw water, and the effects of water matrices on the RhB and Cu²⁺ removal by the ZVI/PS/Cu²⁺ system are illustrated in Figure 10. The water samples were collected at Sichuan University. Table 2 displays the characteristics of five water samples. Apparently, the oxidative efficiency of RhB in deionized water (DW, 95.9%, within 10 min reaction) was much higher than in running water (RW, 92.9%, within 30 min reaction), Jiang'an River water (JAW, 88.5%, within 30 min reaction), Mingyuan Lake water (MYW, 74.7%, within 45 min reaction), and landscape water (LW, 76.1%, within 45 min reaction). As shown in Figure 10(b), it is notable that the TOC removal efficiency of RhB in DW (36.4%) was much higher than those in RW (13.3%), JAW (6.4%), MYW (18.8%), and LW (10.8%). Interestingly, an opposite effect on the Cu²⁺ removal rate was found in the water matrices. Nearly 95.8, 82.4, 60.1, and 92.5% of Cu²⁺ removal rates were found at the water matrices of RW, JAW, MYW, and LW, respectively, which were all higher than that in DW (60.1%).

There may be some reasons for that, on the one hand, the removal of RhB and TOC were inhibited in actual water which was probably because of the higher pH and the natural dissolved organic matters and inorganic species in actual water, especially the high TOC levels. The main constituents of TOC are involved in hydrophobic, transphilic, and hydrophilic DOCs (Vigneswaran 2006; Oh et al. 2017; Hu et al. 2018), where they can compete with RhB for reactive radicals. On the other hand, because the oxidative efficiency of RhB was much lower, for the same amount of ZVI, there was more ZVI to remove Cu²⁺.

All in all, for real water bodies, all water samples showed a decrease in the degradation efficiency of RhB but a distinctly increased effect on Cu²⁺ removal. Nevertheless, the present ZVI/PS/Cu²⁺ system showed great potential to get rid of the RhB and Cu²⁺ wastewater.

In our study, we have demonstrated the simultaneous removal of RhB and Cu²⁺ in the ZVI/PS/Cu²⁺ system under acidic conditions. It was clearly shown that Cu²⁺ could enhance the removal of RhB in the ZVI/PS/Cu²⁺ system. A high reactivity towards the coupled removal of RhB and Cu²⁺ was successfully achieved in the ZVI/PS/Cu²⁺ system. Furthermore, the RhB removal increased with higher ZVI and lower pH, but had little impact on the Cu²⁺ concentration. Investigation of the mechanism revealed that the main free radicals produced from the ZVI/PS/Cu²⁺ system were ·SO₄⁻ and ·OH. What is more, it showed that the ZVI/PS/Cu²⁺ system had a wider pH working range and had great potential to clean up heavy metal and organic compounds in running water and other realistic conditions based on the AOPs. Most importantly, the study confirmed the promise of the system for the simultaneous removal of heavy metal and organic compounds, which can be harnessed for the practical treatment of complicated water matrices.

This research did not receive any specific funding. We are grateful for the characterization examination test provided by ‘Ceshigo’ (www.ceshigo.com).