Effect of Anions and Cations on Tartrazine Removal by the Zero-Valent Iron/Peroxymonosulfate Process: Efficiency and Major Radicals

Abstract

Zero-valent iron/peroxymonosulfate (Fe⁰/PMS) has been considered as a promising approach for wastewater treatment. Anions and cations are widely present in wastewater and have significant effects on the performance of the Fe⁰/PMS system for wastewater treatment. Thus, in the present study, tartrazine was selected as the target model; SO₄²⁻, NO₃⁻, HCO₃⁻, and Cl⁻ were selected as representative anions and Ca²⁺, Cu²⁺, Mg²⁺, and Mn²⁺ were chosen as representative cations. The effect of these anions and cations on tartrazine removal and major radicals in the Fe⁰/PMS were systematically investigated. The presence of a certain concentration of SO₄²⁻ and Cl⁻ had positive, NO₃⁻ had negative, and HCO₃⁻ had negligible effects on tartrazine removal in the Fe⁰/PMS system. SO₄²⁻ and HCO₃⁻ had a small effect on the contribution proportion of reduction, SO₄^•− and ^•OH; a certain concentration of Cl⁻ could enhance the contribution proportion of ^•OH; and NO₃⁻ would decrease the contribution proportion of SO₄^•− and ^•OH. A certain concentration of each of Ca²⁺, Cu²⁺, Mg²⁺, and Mn²⁺ could enhance the tartrazine removal in the Fe⁰/PMS system. Ca²⁺, Cu²⁺, and Mg²⁺ had no effect of the contribution of reduction, SO₄^•− and ^•OH, while a certain concentration of Mn²⁺ could enhance the contribution proportion of SO₄^•−. These results can provide some references for the Fe⁰/PMS system to treat actual wastewater containing anions and cations.

1. Introduction

Recently, with the rapid development of the economy, water pollution has had a great adverse impact on human health and ecosystems [1]. The pollutants of water mainly include antibiotic pollution, phenol pollution, and dye pollution, among others [2,3,4]. Advanced oxidation processes (AOPs) have been declared as efficient techniques for wastewater treatment owing to the production of reactive species such as sulfate radical (SO₄^•−) and hydroxyl radical (^•OH) [5,6,7,8,9,10].

Peroxymonosulfate (PMS) has been seen as the emerging oxidant because PMS’s O-O bond length is short, which is more easily broken [11]. PMS can be catalyzed by metal ions, ultraviolet, ultrasound, and carbon-based materials producing SO₄^•−, ^•OH, and other reactive species [12,13,14,15,16]. Among these catalytic methods, metal ions can be conducted simply and occurred in conventional environments. Among various mental ions (Co²⁺, Mn²⁺, Fe²⁺, and so on), Fe²⁺ is non-toxic, abundant, and cheap. Reactive species can be produced in the Fe²⁺/PMS system via Equations (1)–(3) [17,18,19,20]. However, the Fe²⁺/PMS process’s efficiency is limited by the low recycle efficiency of Fe²⁺.

$$F{e}^{2+}+HS{O}_5^{-}\to F{e}^{3+}+S{O}_4^{•-}+O{H}^{-}$$

(1)

$$F{e}^{2+}+HS{O}_5^{-}\to F{e}^{3+}+S{O}_4^{2-}+{}^{•}OH$$

(2)

$$S{O}_4^{•-}+H_{2}O\to S{O}_4^{2-}+{}^{•}OH+H^{+}$$

(3)

Nowadays, zero-valent iron (Fe⁰) can be used to substitute Fe²⁺ because Fe²⁺ can be generated from the corrosion of Fe⁰ via Equation (4) [21]. Fe⁰ can also activate PMS directly via Equation (5), producing reactive species and Fe²⁺. Meanwhile, the produced Fe³⁺ via Equation (1) could be reduced to Fe²⁺ via Equation (6), which can further increase the efficiency [22]. Moreover, Fe⁰ loses electrons and undergoes a reduction reaction, which degrades pollutants [23].

$$F{e}^0+O_2+2H^{+}\to F{e}^{2+}+H_2{O}_2$$

(4)

$$2F{e}^0+3HS{O}_5^{-}\to 2F{e}^{2+}+2S{O}_4^{•-}+3O{H}^{-}+S{O}_4^{2-}$$

(5)

$$2F{e}^{3+}+F{e}^0\to 3F{e}^{2+}$$

(6)

Besides, anions and cations such as SO₄²⁻, HCO₃⁻, Cl⁻, Ca²⁺, and Mg²⁺ are generally present in wastewater. These ions have a significantly effect on AOP’s efficiency for pollutants’ removal. Ions can enhance the corrosion rate of Fe⁰, but radicals could react with ions, forming less reactive species [24]. Therefore, the effect of both anions and cations on the Fe⁰/PMS process’s efficiency for pollutants’ removal and the role of various radicals (SO₄^•−, ^•OH, and so on) have not been systematically investigated and need further investigation. It is possible that these results might help to promote the application of the Fe⁰/PMS process.

In the present study, tartrazine was chosen as the model pollutant; SO₄²⁻, NO₃⁻, Cl⁻, and HCO₃⁻ were chosen as anion representatives; and Ca²⁺, Cu²⁺, Mg²⁺, and Mn²⁺ were chosen as anion representatives. Then, the effect of these anions and cations on efficiency of Fe⁰/PMS process for tartrazine removal was investigated. Meanwhile, the effect of these anions and cations on the role of SO₄^•− and ^•OH and reduction in the Fe⁰/PMS system for tartrazine removal was also systematically studied.

2. Materials and Methods

2.1. Chemicals

Fe⁰ powder was bought from Shanghai Jinshan smelter (Shanghai, China). Tartrazine was obtained from Shanghai Macklin Biochemical Technology Co., Ltd., (Shanghai, China). Sodium sulfate (Na₂SO₄), sodium chloride (NaCl), sodium nitrate (NaNO₃), and sodium bicarbonate (NaHCO₃) were supplied by Aladdin China Company (Shanghai, China). Tert-butyl alcohol (TBA) was bought from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China).

2.2. Batch Experiments

All degradation experiments were performed in a 500 mL system using glass beakers with a 50 mg/L tartrazine solution under a constant stirring rate of 350 rpm. After adding 4 mM PMS and various ions, the initial pH would be adjusted to 3 using 1 M/0.1 M NaOH and HCl solutions. Then, 0.8 g/L Fe⁰ was added into the working solution and stirred with a mechanical stirrer to trigger the degradation reaction. Samples (1 mL) withdrawn at scheduled time intervals by a pipette were immediately quenched by rapidly adding 0.5 mL MeOH, and filtered through 0.22 µm membrane filters for further analysis. Quenching experiments were performed in order to distinguish the reactive species by adding 500 mM TBA into the solution before initiating the reaction.

2.3. Analytical Methods

Tartrazine concentration was detected by a spectrophotometer (VI-1501, Tianjin Gangdong Sci & Tech Development Co., Ltd., Tianjin, China) at 428 nm.

Tartrazine removal efficiency (η/%) and value of k were calculated by Equations (7) and (8), respectively.

$$\eta =\left(1-\frac{C_t}{C_0}\right)\ast 100\%$$

(7)

$$k=ln\frac{C_t}{C_0}$$

(8)

where C₀ is the initial tartrazine concentration and C_t is the tartrazine concentration at reaction time t.

3. Results and Discussion

3.1. Effect of SO₄²⁻ on Tartrazine Removal and Major Radicals

It can be seen from Figure 1a,b that the removal efficiency of tartrazine within 60 min by the Fe⁰/PMS process was 43.1% and the value of k was 0.0084 min⁻¹ in the absence of any ion. The tartrazine removal efficiency and value of k were 56.5%, 0.0146 min⁻¹; 64.7%, 0.0206 min⁻¹; 64.1%, 0.0195 min⁻¹; 57.5%, 0.0159 min⁻¹; and 53.5%, 0.0132 min⁻¹ in the presence of 1, 2, 5, 10, and 20 mM SO₄²⁻, respectively. The removal efficiency and value k increased with a low concentration of SO₄²⁻, but decreased with a large concentration of SO₄²⁻. To maintain the local charge balance, Fe²⁺ would move together with negatively charged anions, thus the presence of SO₄²⁻ could also enhance the Fe⁰ corrosion rate, thereby enhancing the Fe²⁺ generation rate [24]. From this point, the tartrazine removal efficiency and value of k would increase in the presence of SO₄²⁻. However, the large concentration of SO₄²⁻ induced the decrease in the tartrazine removal efficiency and value of k, which might be because the oxidation reduction potential (ORP) of SO₄^•−/SO₄²⁻ would decrease in the presence of SO₄²⁻, thereby significantly decreasing the PMS activation efficiency [25]. The results were consistent with the results using Fe⁰/PMS treating Rhodamine B [25].

Figure 1. (a) Tartrazine removal efficiency by the Fe⁰/PMS process in the presence of different SO₄²⁻ concentrations; (b) value of k in the presence of different SO₄²⁻ concentrations; (c–e) tartrazine removal under different inhibitors in the presence of 0, 5, and 20 mM SO₄²⁻; and (f) contributions of SO₄^•− and ^•OH and reduction for tartrazine removal. Condition: tartrazine 50 mg/L, Fe⁰ 0.8 g/L, pH 3, PMS 4 mM, TBA 500 mM.

Tartrazine could be removed by reduction and oxidized by SO₄^•− and ^•OH in the Fe⁰/PMS system. Therefore, the contributions of the reduction of SO₄^•− and ^•OH for tartrazine removal in the Fe⁰/PMS system in the absence and presence of SO₄²⁻ were calculated by Equations (9)–(11), and the results are shown in Figure 1c–f. k_reduction, k_•OH, and k_SO4•− for tartrazine removal in the Fe⁰/PMS system were all enhanced in the presence of SO₄²⁻, suggesting that the presence of SO₄²⁻ could increase the generation of SO₄^•− and ^•OH. Reduction and oxidized by SO₄^•− and ^•OH exhibited 17.8%, 23.1%, and 59.1% contributions for tartrazine removal in the Fe⁰/PMS system in the absence of ion. Further, reduction and oxidization by SO₄^•− and ^•OH exhibited 15.0%, 20.1%, and 64.9% contribution for tartrazine removal in the Fe⁰/PMS system in the presence of 5 mM SO₄²⁻. In the presence of 20 mM SO₄²⁻, reduction and oxidization by SO₄^•− and ^•OH exhibited 15.4%, 29.6%, and 55.0% contribution for tartrazine removal in the Fe⁰/PMS system. Radical scavenging experiments suggested that the presence of SO₄²⁻ had a negligible effect on the contribution role of reduction and oxidization by SO₄^•− and ^•OH in removing tartrazine, which might be because SO₄²⁻ would increase the number of electrons lost from Fe⁰ and enhanced the reduction reaction. The increased Fe⁰ corrosion rate would also enhance the Fe²⁺ production rate, thereby enhancing PMS activation for SO₄^•− and ^•OH production.

$$k\left(\mathrm{reduction}\right)=k\left(F{e}^0/TBA\right)$$

(9)

$$k\left({}^{•}\mathrm{OH}\right)=k\left(F{e}^0/PMS\right)-k\left(F{e}^0/PMS/TBA\right)$$

(10)

$$k\left(S{O}_4^{•-}\right)=k\left(F{e}^0/PMS/TBA\right)-k\left(F{e}^0/TBA\right)$$

(11)

3.2. Effect of NO₃⁻ on Tartrazine Removal and Major Radicals

As shown in Figure 2a,b, the tartrazine removal efficiency and value of k were obviously decreased in the presence of NO₃⁻ in the Fe⁰/PMS system and the results were not in accordance with the results of SO₄²⁻. The tartrazine removal efficiency and value of k were only 26.4%, 0.0053 min⁻¹; 23.3%, 0.0045 min⁻¹; 20.3%, 0.0041 min⁻¹; 19.3%, 0.0037 min⁻¹; and 16.5%, 0.0032 min⁻¹ in the presence of 1, 2, 5, 10, and 20 mM NO₃⁻, respectively. NO₃⁻, like SO₄²⁻, is also an anion that can improve the corrosion of Fe⁰. The tartrazine removal efficiency and value of k were obviously decreased in the presence of NO₃⁻ in the Fe⁰/PMS system, which might because of the following reasons: (1) Fe⁰ could directly react with NO₃⁻ to form iron oxides and affected the formation of Fe²⁺ via Equation (12) [26]; (2) NO₃⁻ could adsorb on Fe⁰ surface while iron powder eroded and many cavities were produced, which could also affect the Fe²⁺ generation [26]; and (3) SO₄^•− and ^•OH could react with NO₃⁻ to form less reactive species via Equations (13) and (14) [27]. According to literature reports, the effect of NO₃⁻ on the pollutant treatment efficiency of the PMS-based system was not obvious because of reason (3) [27]. Therefore, the effect of NO₃⁻ on the Fe⁰ corrosion rate was the main reason for the decrease in the treatment efficiency in the Fe⁰/PMS system.

$$10F{e}^0+6N{O}_3^{-}+3H_{2}O\to 5F{e}_2{O}_3+6O{H}^{-}+3N_2$$

(12)

$$S{O}_4^{•-}+N{O}_3^{-}\to S{O}_4^{2-}+N{O}_3^{•}$$

(13)

$${}^{•}OH+N{O}_3^{-}\to O{H}^{-}+N{O}_3^{•}$$

(14)

Figure 2. (a) Tartrazine removal efficiency by the Fe⁰/PMS process in the presence of different NO₃⁻ concentrations; (b) value of k in the presence of different NO₃⁻ concentrations; (c,d) tartrazine removal under different inhibitors in the presence of 5 and 20 mM NO₃⁻; and (e) contributions of SO₄^•− and ^•OH and reduction to tartrazine removal. Condition: tartrazine 50 mg/L, Fe⁰ 0.8 g/L, pH 3, PMS 4 mM, TBA 500 mM.

Radical scavenging experiments in the presence of NO₃⁻ in the Fe⁰/PMS system were also performed. As shown in Figure 2c–e, reduction of SO₄^•− and ^•OH also all contributed to tartrazine removal in the presence of NO₃⁻ in the Fe⁰/PMS system. k_reduction, k_•OH, and k_SO4•− for tartrazine removal in the Fe⁰/PMS system were all decreased in the presence of NO₃⁻, suggesting Fe²⁺ released by the reduction reaction was related to the PMS activation efficiency. Reduction and oxidized by SO₄^•− and ^•OH exhibited 26.2, 35.2%; 24.7%, 33.5%; and 49.1%, 31.3% contribution for tartrazine removal the Fe⁰/PMS system in the presence of 5 and 20 mM NO₃⁻. Oxidation contribution for tartrazine removal decreased in the presence of NO₃⁻, which might be because the presence of NO₃⁻ inhibited the generation of Fe²⁺.

3.3. Effect of Cl⁻ on Tartrazine Removal and Major Radicals

As shown in Figure 3a,b, the tartrazine removal efficiency and value of k were significantly increased in the presence of Cl⁻ in the Fe⁰/PMS system. The tartrazine removal efficiency and value of k increased from to 43.1%, 0.0084 min⁻¹ to 75.4%, 0.024 min⁻¹; 76.1%, 0.025 min⁻¹; 77.2%, 0.026 min⁻¹; 81.4%, 0.029 min⁻¹; and 87.0%, 0.035 min⁻¹ in the presence of 1, 2, 5, 10, and 20 mM Cl⁻, respectively. Although Cl⁻ could react with SO₄^•− and ^•OH to generate less reactive radicals via Equations (15) and (16) [27], the presence of Cl⁻ could significantly enhance the Fe²⁺ generation, and then improved the production of SO₄^•− and ^•OH. Likewise, according to the literature reports, the reaction between Cl⁻ and SO₄^•− and ^•OH had no obvious effect on pollutants’ removal in PMS-based systems [27]. Furthermore, the results also demonstrated that the positive effects of Cl⁻ on tartrazine removal in the Fe⁰/PMS system were significantly stronger than the side effects.

Figure 3. (a) Tartrazine removal efficiency by the Fe⁰/PMS process in the presence of different Cl⁻ concentrations; (b) value of k in the presence of different Cl⁻ concentrations; (c,d) tartrazine removal under different inhibitors in the presence of 5 and 20 mM Cl⁻; and (e) contributions of SO₄^•− and ^•OH and reduction for tartrazine removal. Condition: tartrazine 50 mg/L, Fe⁰ 0.8 g/L, pH 3, PMS 4 mM, TBA 500 mM.

According to the radical scavenging experiment results in Figure 3c–e, k_reduction, k_•OH, and k_SO4•− for tartrazine removal in the Fe⁰/PMS system were all enhanced with the increase in the concentration of Cl⁻. Reduction and oxidization by SO₄^•− and ^•OH exhibited 34.6%, 17.1%; 11.6%, 11.4%; and 53.8%, 71.6% contribution to tartrazine removal in the Fe⁰/PMS system in the presence of 5 and 20 mM Cl⁻. The proportion of ^•OH contribution was significantly enhanced in the presence of Cl⁻, which might be because the oxidizing ability of ^•OH was stronger than that of SO₄^•− and the enhanced Fe²⁺ generation could improve both SO₄^•− and ^•OH, thus the contribution of ^•OH would be significantly enhanced.

$$S{O}_4^{•-}+C{l}^{-}\to S{O}_4^{2-}+C{l}^{•}$$

(15)

$${}^{•}OH+C{l}^{-}\to O{H}^{-}+C{l}^{•}$$

(16)

3.4. Effect of HCO₃⁻ on Tartrazine Removal and Major Radicals

As depicted in Figure 4a,b, the tartrazine removal efficiency and value of k were almost unchanged in the presence of HCO₃⁻ in the Fe⁰/PMS system. HCO₃⁻ was also able to enhance the Fe²⁺ generation via Equation (17) [28]. Meanwhile, HCO₃⁻ might inhibit the release of Fe²⁺ from Fe⁰ owing to the generation of an insulating film and iron compounds on the Fe⁰ surface. Meanwhile, HCO₃⁻ could react with SO₄^•− and ^•OH to generate less reactive radicals via Equations (18) and (19). Moreover, HCO₃⁻ could consume both H⁺ and OH⁻ by Equations (20)–(21), and thus maintain a stable pH during the reaction, which further inhibited Fe⁰ corrosion and PMS activation [28]. In general, HCO₃⁻ had a small effect on the removal of pollutants by the Fe⁰/PMS system. According to the literature reports, HCO₃⁻ had a significant effect on pollutants’ removal in PMS-based systems [27], thus the promotion of Fe⁰ corrosion by HCO₃⁻ had a very positive effect on pollutants’ removal efficiency in the Fe⁰/PMS system. According to the radical scavenging experiment results in Figure 4c–e, HCO₃⁻ also had a small effect on k_reduction, k_•OH, and k_SO4•− and the contribution proportion of reduction of SO₄^•− and ^•OH for tartrazine removal in the Fe⁰/PMS system.

$$F{e}^0+2HC{O}_3^{-}\to 2C{O}_3^{2-}+F{e}^{2+}+H_2$$

(17)

$$S{O}_4^{•-}+HC{O}_3^{-}\to S{O}_4^{2-}+HC{O}_3^{•}$$

(18)

$${}^{•}OH+HC{O}_3^{-}\to O{H}^{-}+HC{O}_3^{•}$$

(19)

$$H^{+}+HC{O}_3^{-}\to H_{2}O+C{O}_2$$

(20)

$$O{H}^{-}+HC{O}_3^{-}\to H_{2}O+C{O}_3^{2-}$$

(21)

Figure 4. (a) Tartrazine removal efficiency by the Fe⁰/PMS process in the presence of different HCO₃⁻ concentrations; (b) value of k in the presence of different HCO₃⁻ concentrations; (c,d) tartrazine removal under different inhibitors in the presence of 5 and 20 mM HCO₃⁻; and (e) contributions of SO₄^•− and ^•OH and reduction for tartrazine removal. Condition: tartrazine 50 mg/L, Fe⁰ 0.8 g/L, pH 3, PMS 4 mM, TBA 500 mM.

3.5. Effect of Ca²⁺ on Tartrazine Removal and Major Radicals

The effect of Ca²⁺ on tartrazine removal and major radicals in Fe⁰/PMS was also investigated and the results are shown in Figure 5a–e. Ca²⁺ had no obvious effect on tartrazine removal and value of k, and tartrazine removal and value of k could be enhanced from 43.1% and 0.0084 min⁻¹ to 49.8% and 0.0114 min⁻¹, respectively, while Ca²⁺ increased from 0 to 50 mg/L. According to the previous studies, Ca²⁺ could not efficiently activate PMS for pollutants’ removal. The adsorbance of Ca²⁺ on the Fe⁰′ surface was conducive to enhancing the PMS activation, and Ca²⁺′ bridging of negatively charged pollutants also favored degradation, possibly because of the enrichment of the catalyst by the substrate. Thus, a certain amount of Ca²⁺ could enhance the tartrazine removal in the Fe⁰/PMS system. Meanwhile, 50 mg/L Ca²⁺ also could enhance k_reduction, k_•OH, and k_SO4•−, while it had no obvious effect on the contribution proportion of reduction of SO₄^•− and ^•OH for tartrazine removal in the Fe⁰/PMS system.

Figure 5. (a) Tartrazine removal efficiency by the Fe⁰/PMS process in the presence of different Ca²⁺ concentrations; (b) value of k in the presence of different Ca²⁺ concentrations; (c,d) tartrazine removal under different inhibitors in the presence of 5 and 20 mM Ca²⁺; and (e) contributions of SO₄^•− and ^•OH and reduction for tartrazine removal. Condition: tartrazine 50 mg/L, Fe⁰ 0.8 g/L, pH 3, PMS 4 mM, TBA 500 mM.

3.6. Effect of Cu²⁺ on Tartrazine Removal and Major Radicals

As shown in Figure 6a,b, the tartrazine removal and value of k increased from 43.1%, 0.0084 min⁻¹ to 49.2%, 0.010 min⁻¹, 60.6%, 0.015 min⁻¹; 64.3%, 0.016 min⁻¹; 66.9%, 0.018 min⁻¹; and 70.8%, 0.020 min⁻¹, respectively, while the Cu²⁺ concentration increased from 0 to 1, 5, 10, 20, and 50 mg/L in the Fe⁰/PMS system. The tartrazine removal efficiency increased with the addition of Cu²⁺, which might be mainly because PMS could be also activated by Cu²⁺ and Cu²⁺ could also promote the corrosion of Fe⁰. Meanwhile, Cu²⁺ could also inhibit SO₄^•− and ^•OH via Equations (22) and (23) [25]. In general, the presence of Cu²⁺ could enhance the tartrazine removal in the Fe⁰/PMS system. It could found from Figure 6c–e that k_reduction, k_•OH, and k_SO4•− were all enhanced in the presence of Cu²⁺. Reduction and oxidization by SO₄^•− and ^•OH exhibited 12.4%, 21.4%; 29.7%, 38.4%; and 57.9%, 40.2% contributions for tartrazine removal in the Fe⁰/PMS system in the presence of 10 and 50 mg/L Cu²⁺. Cu²⁺ had no obvious effect on the contribution of reduction of SO₄^•− and ^•OH on tartrazine removal, which might be because Cu²⁺ could affect not only the PMS activation, but also the corrosion of Fe⁰.

$$S{O}_4^{•-}+C{u}^{2+}\to S{O}_4^{2-}+C{u}^{3+}$$

(22)

$${}^{•}OH+C{u}^{2+}\to O{H}^{-}+C{u}^{3+}$$

(23)

Figure 6. (a) Tartrazine removal efficiency by the Fe⁰/PMS process in the presence of different Cu²⁺ concentrations; (b) value of k in the presence of different Cu²⁺ concentrations; (c,d) tartrazine removal under different inhibitors in the presence of 5 and 20 mM Cu²⁺; and (e) contributions of SO₄^•− and ^•OH and reduction for tartrazine removal. Condition: tartrazine 50 mg/L, Fe⁰ 0.8 g/L, pH 3, PMS 4 mM, TBA 500 mM.

3.7. Effect of Mg²⁺ on Tartrazine Removal and Major Radicals

As shown in Figure 7a,b, tartrazine removal efficiency increased with the increase in Mg²⁺ concentration, and tartrazine removal and the value of k were 41.0%, 0.0086 min⁻¹; 43.4%, 0.0091 min⁻¹; 52.1%, 0.012 min⁻¹; 77.1%, 0.029 min⁻¹; and 68.4%, 0.020 min⁻¹ in the presence of 1, 5, 10, 20, and 50 mg/L Mg²⁺ in the Fe⁰/PMS system, respectively. Mg²⁺ was similar to Cu²⁺, which could activate PMS and enhance the corrosion of Fe⁰, and thus could improve the Fe⁰/PMS system for tartrazine removal. The removal efficiency decreased at 50 mg/L Mg²⁺ in the Fe⁰/PMS system, which might be because of the reaction between Mg²⁺ and SO₄^•− and ^•OH via Equations (24) and (25) [25]. According to the radical scavenging experiment results from Figure 7c–e, k_reduction, k_•OH, and k_SO4•− were all enhanced in the presence of Mg²⁺, which also confirmed that the presence of Mg²⁺ could enhance the corrosion of Fe⁰ and PMS activation. Moreover, Mg²⁺ also had no obvious effect of the contribution of reduction of SO₄^•− and ^•OH.

$$S{O}_4^{•-}+M{g}^{2+}\to S{O}_4^{2-}+M{g}^{3+}$$

(24)

$${}^{•}OH+M{g}^{2+}\to O{H}^{-}+M{g}^{3+}$$

(25)

Figure 7. (a) Tartrazine removal efficiency by the Fe⁰/PMS process in the presence of different Mg²⁺ concentrations; (b) value of k in the presence of different Mg²⁺ concentrations; (c,d) tartrazine removal under different inhibitors in the presence of 5 and 20 mM Mg²⁺; and (e) contributions of SO₄^•− and ^•OH and reduction for tartrazine removal. Condition: tartrazine 50 mg/L, Fe⁰ 0.8 g/L, pH 3, PMS 4 mM, TBA 500 mM.

3.8. Effect of Mn²⁺ on Tartrazine Removal and Major Radicals

It could found from Figure 8a,b that Mn²⁺ could also significantly enhance the Fe⁰/PMS system for tartrazine removal. Tartrazine removal and value of k increased from 43.1%, 0.0084 min⁻¹ to 46.1%, 0.0099 min⁻¹; 47.6%, 0.010 min⁻¹; 53.1%, 0.013 min⁻¹; 58.4%, 0.015 min⁻¹; and 61.2%, 0.019 min⁻¹, while Mn²⁺ concentration increased from 0 to 1, 5, 10, 20, and 50 mg/L in the Fe⁰/PMS system, respectively. As previous literatures reported that Mn²⁺ was a good transition metal for PMS activation via Equations (26) and (27), it thus induced the increase in tartrazine removal efficiency in the Fe⁰/PMS system [29]. Meanwhile, the effect of Mn²⁺ on the Fe⁰/PMS system improvement for tartrazine removal was lower than that of Mg²⁺, which might be because Mn³⁺ and Mn²⁺ could react with Fe²⁺ via Equations (28) and (29), thus affecting its performance [29]. As depicted in Figure 8c–e, both k_SO4•− and the contribution of SO₄^•− were significantly enhanced in the presence of Mn²⁺. The results also indicate that Mn²⁺ might affect the activation ability of Fe²⁺ and become the dominant activated metal.

$$M{n}^{2+}+HS{O}_5^{-}\to M{n}^{3+}+S{O}_4^{•-}+O{H}^{-}$$

(26)

$$M{n}^{3+}+HS{O}_5^{-}\to M{n}^{4+}+S{O}_4^{•-}+O{H}^{-}$$

(27)

$$M{n}^{3+}+F{e}^{2+}\to F{e}^{3+}+M{n}^{2+}$$

(28)

$$M{n}^{4+}+F{e}^{2+}\to F{e}^{3+}+M{n}^{3+}$$

(29)

Figure 8. (a) Tartrazine removal efficiency by the Fe⁰/PMS process in the presence of different Mn²⁺ concentrations; (b) value of k in the presence of different Mn²⁺ concentrations; (c,d) tartrazine removal under different inhibitors in the presence of 5 and 20 mM Mn²⁺; and (e) contributions of SO₄^•− and ^•OH and reduction for tartrazine removal. Condition: tartrazine 50 mg/L, Fe⁰ 0.8 g/L, pH 3, PMS 4 mM, TBA 500 mM.

4. Conclusions

In this study, the effect of anions (SO₄²⁻, NO₃⁻, HCO₃⁻, and Cl⁻) and cations (Ca²⁺, Cu²⁺, Mg²⁺, and Mn²⁺) on tartrazine removal and major radicals in Fe⁰/PMS were systematically investigated. SO₄²⁻, Cl⁻ Ca²⁺, Cu²⁺, Mg²⁺, and Mn²⁺ could enhance tartrazine removal; NO₃⁻ could inhibit tartrazine removal; and HCO₃⁻ had no obvious effect on tartrazine removal. SO₄²⁻, HCO₃⁻, Ca²⁺, Cu²⁺, and Mg²⁺ had little effect on the contribution proportion of reduction of SO₄^•− and ^•OH; NO₃⁻ would decrease the contribution proportion of SO₄^•− and ^•OH, a certain concentration Cl⁻ could enhance the contribution proportion of ^•OH; and a certain concentration of Mn²⁺ could enhance the contribution proportion of SO₄^•−. These dates can provide some references for the Fe⁰/PMS system to treat actual wastewater containing anions and cations.