Our findings proved that micron-scale zero-valent iron (mZVI) particles with pre-magnetization combined with peroxymonosulfate (PMS) can markedly enhance the removal of acid orange 7 (AO7). Investigation into the mechanism showed that PMS accelerated the corrosion of ZVI to release Fe2+ under acidic conditions, and the in-situ generated Fe2+ further activated PMS to produce SO4•− and •OH, resulting in AO7 removal. Further, the Lorentz force strengthened the convection in the solution and the field gradient force tended to move Fe2+ from a higher to a lower field gradient at the pre-magnetized ZVI (Pre-ZVI) particle surfaces, thus indicating that pre-magnetization promoted the corrosion of ZVI to release Fe2+, which resulted in the enhancement of PMS activation. Nano-scale ZVI (nZVI) was more effective than mZVI in activating PMS to degrade AO7, but the pre-magnetization effect on mZVI was better than on nZVI. AO7 removal increased with higher ZVI and PMS dosage, lower AO7 concentration, and acidic conditions (pH = 2, 3). This study helps to understand the reactive radicals-based oxidation process with application of pre-magnetized ZVI in activating PMS.
Over the last few decades, the increasing demand for dyes has led to a high pollutant potential in the textile industry. Acid orange 7 (AO7), a typical azo dye, is widely used as a coloring agent in a variety of products such as leather, pharmaceuticals and paper. AO7 is known to be toxic, non-biodegradable, and potentially carcinogenic (Xiao et al. 2014; Zheng et al. 2016). Many physical and chemical methods have been applied to dispose of the wastewater but they still suffer from many disadvantages. Physical methods only transfer the pollutants instead of destroying them, and chemical processes usually demand high-impact solutions to degrade pollutants (Gomathi Devi et al. 2012; Xiao et al. 2014). Advanced oxidation processes (AOPs) have gained wide attention as efficient methods for degrading persistent organic contaminants in wastewater with the generation of reactive radicals (Gogate & Pandit 2004). The sulfate radical (SO4•−) is a strong oxidant with a redox potential of 2.5–3.1 V, which is comparable to the hydroxyl radical (•OH) with a redox potential of 1.8–2.7 V (Guan et al. 2011). The oxidation of compounds with carbon-carbon double bonds and benzene rings is much easier with SO4•− than •OH, which means it is highly efficient for the mineralization of organic pollutants (Anipsitakis & Dionysiou 2003, 2004), and tends to be generated by activating persulfates such as peroxymonosulfate (PMS, HSO5−) and peroxydisulfate (PDS, S2O82−) (Qi et al. 2016). Normally, the activation of PMS can be achieved by transition metals (Wang & Wang 2018), ultraviolet light (Sharma et al. 2015), ultrasound (Liu et al. 2017), and electrolysis (Lin et al. 2014).
Moreover, the reactivity of ZVI is strongly enhanced by some pretreatments, such as sonication and acid washing (Ren et al. 2017). The addition of a weak magnetic field (WMF) to the ZVI system can enhance the degradation of Orange G and Se(IV), for example (Liang et al. 2014a; Xiong et al. 2014). However, it might be impractical and expensive to use a device to generate a WMF in situ. The fact that ZVI is a ferro-magnetic material suggests that it can retain magnetic memory after being exposed to a magnetic field. Some studies have investigated the effect of pre-magnetization on ZVI systems (Li et al. 2015a; Ren et al. 2018) or ZVI/PS systems (Li et al. 2017), and proved that pre-magnetization can enhance the activity of ZVI particles and accelerate dissolution, so that more Fe2+ can be accumulated and play an activator role. Therefore, it is reasonable to combine the pre-magnetized ZVI (Pre-ZVI) particles with PMS and speculate that a Pre-ZVI/PMS system would be more effective and convenient for removing contaminants.
Consequently, the aims of this study were to investigate the removal of AO7 by activating PMS using Pre-ZVI, and mainly focused on: (I) analysis of the mechanism of the Pre-mZVI/PMS system; (II) comparison of the nZVI/PMS and mZVI/PMS systems; (III) the influence of pH, AO7 concentration, and PMS dosage; (IV) the application of Pre-mZVI/PMS system in real waters.
MATERIALS AND METHODS
Chemicals and materials
All chemicals were analytical grade and used as received without any further purification. Nano-scale zero-valent iron powder (nZVI, ≥99.9%), micron-scale zero-valent iron powder (mZVI, ≥99.9%), 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO, ≥97%) and acid orange 7 (AO7, ≥98%) were purchased from Aladdin Industrial Corporation. Peroxymonosulfate (PMS, KHSO5•0.5KHSO4•0.5 K2SO4, ≥99.5%) was purchased from Sigma-Aldrich. Acetic acid, sodium acetate, 1,10-phenanthroline, hydroxylamine hydrochloride, sodium hypochlorite, sulfuric acid, hydrochloric acid, sodium hydroxide, sodium bicarbonate, potassium iodide and sodium nitrite were all supplied by Chengdu Kelong Chemical Reagent Factory.
All the experiments were carried out in 500 mL beakers with a total solution volume of 500 mL of ultra-pure water under constant mechanical stirring (500 rpm) in a 20 ± 1 °C water bath. The initial pH value was adjusted with 1 M H2SO4 and 0.5 M NaOH, and each run was initiated by adding the desired dosages of AO7, PMS and ZVI. After filtration through a 0.22 um PTFE membrane, the concentrations of AO7 and total dissolved iron (TDI) were analyzed at pre-scheduled intervals. All the experiments were carried out in duplicate. The pre-magnetization method of the ZVI was as follows: the desired amount of ZVI powder was withdrawn from the reagent bottle and placed directly on the surface of a magnet (188.86 mT) for 1 min. After use in the reactions, the residual ZVI particles were withdrawn, vacuum filtered through a 0.22 um PTFE membrane, and dried in a vacuum dryer, for further characterization.
TDI was detected after complexation with 1, 10-phenanthroline, and the concentrations of TDI as well as AO7 were measured with a visible spectrophotometer (Mapada V-1200) at wavelengths of 510 nm and 484 nm, respectively. The concentration of PMS was also analyzed by the spectrophotometric method, modified from that suggested by Liang et al. (Liang et al. 2008). The pH value was monitored by a pH meter (FiveEasy Plus, Mettler Toledo, Shanghai). The magnetic field intensity was determined with a teslameter (TD8620, Changsha Tunkia Co., Ltd). A multi N/C 3100 analyzer (Analytikjena) was employed to analyze the total organic carbon (TOC). ZVI powders before and after reaction were characterized by scanning electron microscopy (SEM, JSM-7500F (JEOL, Japan)) and X-ray diffraction (XRD, AXIS Ultra DLD (Kratos, UK)). The particle sizes of mZVI and nZVI were monitored by Particle Sizing Systems (780SIS and Z3000 (PSSNICOMP, USA)). X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermofisher) and Brunauer-Emmett-Teller (BET, Autosorb iQ, Quantachrome) was used to characterize the chemical composition and specific surface of mZVI and nZVI, respectively. Electron spin resonance (ESR, Bruker EMX plus) experiments were conducted to distinguish reactive radicals with DMPO as the spin-trapping agent, and the specific characteristic method is explained in Text S1 (available with the online version of this paper).
RESULTS AND DISCUSSION
Enhancement of AO7 degradation by Pre-mZVI by activating PMS
As can be seen in Figure 1(a), low AO7 removal was exhibited in the PMS system, the mZVI system and the Pre-mZVI system, whose AO7 degrading rates were 0.16%, 5.49% and 3.67%, respectively. However, the AO7 removal efficiency improved to 34.7% when mZVI was combined with PMS. A significant enhancement of the AO7 degradation rate to 90.78% was seen with the addition of Pre-mZVI in the Pre-mZVI/PMS system.
As shown in Figure 1(b), no TOC removal was detected in the PMS system, and the TOC removal rates in the mZVI and the Pre-mZVI systems were 15.1% and 16.2%, respectively. The addition of PMS enhanced the mineralization of AO7, and the TOC removal rates were 29.7% and 35.8% in the mZVI/PMS and the Pre-mZVI/PMS systems, respectively.
Comparing the Pre-mZVI/PMS system with the mZVI/PMS system, the AO7 degradation increased by 56% in 30 min and the TOC removal increased by 6.1% in 3 hours, meanwhile, the reaction rate constants rose from 0.0145 to 0.0747 min−1 (Figure S1, available with the online version of this paper).
These results showed that pre-magnetization of mZVI helped to enhance the removal and strengthening of the mineralization of AO7.
Discussion of mechanism
Enhancement of Fe2+ release by pre-magnetization
Previous studies have revealed that a variety of contaminants were removed by ZVI but the removal efficiency was low (Li et al. 2015a). As shown in Figure 1(c), the TDI were very close in the mZVI system (5.31 mg/L) and the Pre-mZVI system (4.92 mg/L), and the TDI was 7.78 mg/L in the mZVI/PMS system, up by 46.5% compared with the mZVI system. The amount of TDI was 18.31 mg/L in the Pre-mZVI/PMS system, double that in the mZVI/PMS system.
Corresponding to the enhancement of TDI accumulation, the consumption of PMS was accelerated in the pre-magnetization process. Figure 1(d) shows that 79.4% of PMS was consumed in the Pre-mZVI/PMS system at 30 min, compared to only 44.1% in the mZVI/PMS system.
Characterization of mZVI powders before and after reaction
As shown in Figure 2, the pristine mZVI powder has a smooth surface without any corrosion. However, there is a lot of pitting and the roughness obviously increases on the surface of mZVI. It can be observed that superficial pitting is smaller and more intensive and the surface is rougher on Pre-mZVI than on mZVI. In addition, there are a few areas of crevice corrosion, which result in large specific surface areas and more active adsorption sites on Pre-mZVI (Graca et al. 2018). Thus, the residual Pre-mZVI powder's surface condition reflected the enhancement of Fe2+ generation.
As shown in Figure 3(a), the characteristic peaks of the pristine mZVI powder at 44.7°, 65.3°, and 82.5° match well with the standard patterns of Fe0 (PDF#06-0696) (Li et al. 2015c), and no other peaks were seen. The spectra obtained from the residual mZVI powders in both the mZVI/PMS system and the Pre-mZVI/PMS system are completely consistent with the patterns of the pristine mZVI powder, which reveals that no significant structural change occurs on the mZVI surface in both systems.
Identification of reactive radicals
Impact of pre-magnetization on enhancement of AO7 degradation
ZVI has ferro-magnetism and magnetic memory after exposure to a magnetic field (Xiong et al. 2014; Li et al. 2015b), and the induced magnetic field around ZVI particles lead to an inhomogeneous flux density distribution (Sueptitz et al. 2010; Zhang et al. 2019). There are the Lorentz and field gradient forces in an inhomogeneous magnetic field. The Lorentz force slightly drives micro-convection, which enhanced the cathodic hydrogen ion reduction, leading to the free corrosion potential (Sueptitz et al. 2010). The field gradient force would move ferrous ions along the field gradient, resulting in speeding up the corrosion of ZVI (Sueptitz et al. 2011). All the theories accounted for the observations in our experiments that the pre-magnetization accelerated ZVI corrosion enhanced the PMS decomposition and AO7 removal.
Comparison of nZVI/PMS and mZVI/PMS systems at different Fe0 dosage
As shown in Figure 4, the AO7 removal rates were 9.4%, 17.6%, 44.7%, 62.6%, and 81.9% under five mZVI dosages (20 mg/L, 40 mg/L, 80 mg/L, 120 mg/L, 160 mg/L) in the mZVI/PMS system, showing that AO7 removal increased with increasing mZVI dosage. In the Pre-mZVI/PMS system the AO7 degradation efficiencies were 17.2%, 30.3%, 93.8%, 83.7%, and 97.5%, respectively, which indicated that AO7 removal improved greatly after pre-magnetization. It is worth noting that 93.8% AO7 was removed at 80 mg/L of mZVI in the Pre-mZVI/PMS system, more than double compared to the mZVI/PMS system of 44.7%. At the same time, the TDI was also enhanced with Pre-magnetization and increasing ZVI dosage.
By comparison, nZVI gave a remarkable performance. The AO7 was almost completely removed in the nZVI/PMS and Pre-nZVI systems with nZVI dosage ranging from 40 to 160 mg/L. When the nZVI dosage was 20 mg/L, nZVI showed better catalytic ability than Pre-nZVI owing to nZVI's magnetic interaction and high surface energy, so that nZVI tended to undergo self-aggregation in micro-scale particles, which reduced its specific surface area (SSA) and limited its removal effects (Jiang et al. 2018). The kobs values were higher after pre-magnetization, which suggested that the magnetic field had the same impact on nZVI as mZVI.
Two observations can be made from Figure 4, as follows. (1) The nZVI/PMS system showed better performance on AO7 removal rate, kobs values and TDI than the mZVI/PMS system. This can be explained as nZVI has a smaller particle size (58.3 nm) and higher SSA (6.242 m2/g) than mZVI (5.90 μm, 1.008 m2/g), as shown in Figures S2 and S3 (available online). (2) There were higher AO7 degradation and TDI after pre-magnetization, which was because the ZVI retained a magnetic field after pre-magnetization, and the Lorentz force and the field gradient force accelerated the corrosion of ZVI, then abundant Fe2+ activated PMS more and faster to degrade AO7 (Liang et al. 2014a; Li et al. 2015a; Ren et al. 2017).
Influence of initial pH on degradation
The observations indicated that the mZVI and Pre-mZVI favored acidic conditions rather than neutral or alkaline conditions. The essential reason for that might be related to the formation of iron hydroxides, which can be proved by XPS spectra showing the existence of Fe2O3 and FeO (Figure S4, available online). The solubility product constant of ferric hydroxide is Ksp (Fe(OH)3) = 4 × 10−38. For example, the maximum Fe3+ concentration is 4 × 10−11 mol/L in water solution at pH = 5.0.
In addition, the H+ concentration will also decrease with increasing pH value, thus the iron oxides on the surface of mZVI powders cannot be dissolved effectively, which inhibits the corrosion of mZVI to release Fe2+ for further activation of PMS.
Influence of initial AO7 concentration on degradation
To investigate whether the concentrations of the target pollutant have an effect on catalyst activity, an experiment with different AO7 initial concentrations was carried out and the results are shown in Figure 6. The AO7 removal decreased from 74.9% to 14.7% with AO7 concentrations ranging from 20 to 200 μM in the mZVI/PMS system, indicating that pollutant removal was reduced with increasing pollutant concentration. After pre-magnetization, AO7 removal decreased from 98.3% to 49.1%. Taking AO7 of 100 μM as an example, the degradation efficiency of 91.8% in the Pre-mZVI/PMS was almost triple that of 34.9% in the mZVI/PMS system, which demonstrated that pre-magnetization had a positive effect on AO7 removal in the mZVI/PMS system at all concentrations of pollutant.
The lower degradation of AO7 at higher AO7 initial concentrations might be associated with the formation of a passive film on mZVI (Liang et al. 2014b). The build-up of the passive film would prevent the release of Fe2+, with the activating rate of PMS being reduced accordingly. In the mZVI/PMS system, the TDI detected showed a tendency to reduce with increasing initial AO7 dosages, which was in accordance with the passive film forming on the surface of mZVI.
However, in the Pre-mZVI/PMS system, the AO7 removal rates were greatly improved and the TDI amounts increased dramatically. This was due to pre-magnetization speeding up the corrosion of mZVI and the formation of SO4•−, so that more AO7 was eliminated and more Fe2+ was accumulated.
Influence of initial PMS dosage on degradation
The influence of initial PMS dosage on AO7 degradation was investigated, as shown in Figure 7. There was an increase in AO7 degradation when the dosage of PMS increased from 0.01 mM to 0.2 mM, but a decrease when the dosage was further increased to 2 mM. The AO7 removal efficiency was enhanced slightly in the Pre-mZVI/PMS system below the PMS dosage of 0.2 mM, and the TDI showed a small increase. With PMS dosage increasing over 0.2 mM, there was a great enhancement in AO7 degradation and TDI accumulation after pre-magnetization. Taking a PMS of 1.0 mM as an instance, the AO7 degradation efficiency was 44.9% and the TDI was 7.78 mg/L in the mZVI/PMS system, but in the Pre-mZVI/PMS system, these values were increased to 90.8% and 18.3 mg/L.
PMS is the source of SO4•− and •OH in the mZVI/PMS and the Pre-mZVI/PMS systems. Sufficient quantities of PMS were activated to produce SO4•− and •OH to oxidize the pollutants at low concentrations, although there was excess Fe2+ in solution. Hence, AO7 degradation efficiencies were close in the two systems when the PMS concentration was below 0.2 mM.
Degradation of AO7 in real waters
The degradation efficiencies of AO7 in the mZVI/PMS system and the Pre-mZVI/PMS system were compared in real waters (pure water, tap water, Jiangan river water, and Mingyuan lake water). As shown in Figure 8, the degradation efficiency of AO7 in pure water was 81.92% at 30 min, while in tap water, Jiangan river water and Mingyuan lake water it was 53.25%, 51.32% and 56.55%, respectively. The inhibition of AO7 removal in these real waters might be related to the consumption of reactive radicals by anions such as HCO3−/CO32− and H2PO4− (Díez-Mato et al. 2014; Hu et al. 2018) and other dissolved organic matter.
However, pre-magnetization can also enhance AO7 removal in these real waters. The AO7 degradation efficiencies were 97.48%, 97.98%, 98.81% and 97.53% in the Pre-mZVI/PMS system with the solvents being pure water, tap water, Jiangan river water and Mingyuan lake water, respectively. Hence, the Pre-mZVI/PMS system showed a great potential to degrade pollutants in real waters.
The Pre-mZVI/PMS system gave rise to a significant improvement in the degradation of AO7. Both PMS and pre-magnetization can accelerate ZVI to generate Fe2+. The in-situ generated Fe2+ can further activate PMS to produce reactive radicals; ESR spectra confirmed that SO4•− and •OH were the dominant reactive species. The reaction rate constants of the AO7 degradation in the Pre-mZVI/PMS system were 1.69–4.96 times those in the mZVI/PMS system. Moreover, the reaction rates of AO7 degradation in the nZVI/PMS system were 4–35 times higher than those in the mZVI/PMS system with ZVI dosage ranging from 20 to 160 mg/L. A higher AO7 removal was obtained in acidic conditions, with pH ranging from 2 to 3. Based on the great enhancement of AO7 removal in the Pre-mZVI/PMS system in real waters, applying it to degrade refractory pollutants is proven to be a promising method of wastewater treatment.