3.1 Effect of pH on decolorization
In chemical processes, the solution pH is an important element affecting the removal of organic compounds efficiency. The effect of solution pH on the degradation rate of SY was investigated on the process performance (SY concentration: 50 mg/L, H2O2: 1 mM, PS 1 mM and ZVI: 50 mg/L). The results are illustrated in Fig. 2. As can be seen here, the highest removal efficacy (99.71%) was achieved at pH = 3 in the H2O2/ZVI/PS process. So that the dye removal efficiency decreased significantly with increasing pH. Previous studies have indicated that the solution pH has a considerable effect on the stability of H2O2, OH• concentration and the iron species in the solution (Malakootian et al. 2020). Generally, the acidic condition causes the fast disappearance of Fe0 and the release of more Fe2+ in accordance with Eq. (7), resulting in the production of more free radicals (SO4●− and OH●) for the better dye removal performance that can be generated according to Eqs. (5,8) (Ike et al. 2018). In contrast, under alkaline conditions, Fe2+ and Fe3+ are almost insoluble that causes the iron ions rapidly in solid and colloidal (FeOH+) form and affects their ability to activate PS Eq. (9) (Li et al. 2013), thereby significantly slow down the dye degradation. In addition, S2O82− decomposition at acidic pH occurred through acid-catalyzed reactions (Eqs. (10 and 11)). On the other hand, ●OH can be generated through SO4●− reaction with H2O (Eq. 12) or HO− (Eq. 13) at alkaline pH, resulting in less SO4●− available for decolorization (Anipsitakis & Dionysiou 2004).
Fe0 + H+ → Fe2+ + H2 (7)
Fe2++ H2O2 →Fe3+ + OH−+ OH• (8)
Fe2+ + H2O→ FeOH+ +H+ (9)
S2O82−+ H+→HS2O8− (10)
2H2O2 →2H2O + O2 (11)
SO4●− + H2O→SO42− + OH• +H+ (12)
SO4●− + HO−→SO42− + OH• (13)
3.2 Effect of oxidants concentration
In Advanced oxidation processes (AOPs), the concentration of oxidants directly affects the amounts of generating free radicals. In addition, choosing optimal dosage of oxidants can obviously decrease the process costs (Li et al. 2017). The effect of the H2O2 and PS concentration in the range of 0.25–2 mM were investigated on the H2O2/ZVI/PS process efficiency for decolorization. As shown in Fig. 3a, the increasing H2O2 dosage enhances the process efficiency for decolorization, due to the fact that H2O2 is the source of hydroxyl radicals. So that the obvious increase of H2O2 dosage increases the concentration of reactive hydroxyl radicals (•OH), and then decolorization. Nonetheless, the dye removal rate decreased with further increase in the H2O2 concentration from 1mM to 1.5 mM, and even with the further increase in H2O2 concentration up to 2 mM. The reduced oxidation efficiency at higher H2O2 values can be attributed to the: (i) the reaction of excess ferrous iron with excess H2O2, (ii) scavenging effect of •OH by H2O2, therefore, one or more of side reactions occur under such conditions (Eqs. (14)–(18)), which results in the formation of oxidants like HOO and O2−, which has much lower oxidation potential compared with active hydroxyl radicals (•OH) (de Souza Santos et al. 2015). In H2O2/ZVI/PS process, more persulfate concentration leads to more contact between ZVI and PS. Therefore, more sulfate radicals were generated in aqueous solution. Nonetheless, as shown in Fig. 3b, as the PS dosage increased, the dye removal decreased, and even with the further increase in PS concentration, the efficiency of decolorization decreased, which could be attributed to the: (i) reaction between S2O82− and SO4−• (Eq. (19)), (ii) the recombination between two SO4−• (Eq. 20) (Wu et al. 2019). Thus, it can be concluded though entering a high amount of PS into the system is theoretically associated with a high decolorization rate. But the gain effect is limited and the quantity of PS added is just useful up to a defined concentration, after which it has negative effects. Therefore, PS concentration of 1 mM is selected as the optimum concentration for the decolorization.
Fe0 + H2O2 → Fe2+ + 2O− (14)
Fe2+ + H2O2 → Fe3+ + •OH (15)
H2O2 + •OH→ HO2/ O2−• + H2O (16)
Fe3+ + HO2 → Fe2+ + O2 + H+ (17)
Fe2+ + HO2 → Fe3+ +HO2− (18)
SO4−• + S2O82− → SO42− + S2O8− + H+ (19)
SO4−• + SO4− •→ S2O82− (20)
3.3 Effect of ZVI dose
In order to evaluate the effect of ZVI dose on the H2O2/ZVI/PS process for the dye removal, experiments were carried out at various catalyst concentrations in the range between 25 mg/L and 150 m/L, under constant conditions (SY concentration: 50 mg/L, solution pH: 3, H2O2 dosage: 1 mM, PS: 1 mM and ZVI: 50 mg/L. As shown in Fig. 4, the increasing concentration of ZVI enhanced the process efficiency for decolorization. Nonetheless, the further increase in ZVI concentration diminished the process performance. This result is due to the fact that with increasing ZVI dose the number of surface active sites also increased. This phenomenon enhanced H2O2 and PS decomposition and the production of hydroxyl and sulfate radicals in accordance with Eqs. (14, 15, and 21), which consequently improved the decolorization efficiency. However, excessive dosage of ZVI causes particle aggregation and the scavenging of •OH and SO4●− radicals through the undesirable reaction leads to a decrease in dye removal efficiency (Eqs. 23, and 22) (Babuponnusami & Muthukumar 2012; Zha et al. 2014).
Fe0 + S2O82− → Fe2+ + 2SO42− (21)
Fe2+ + •OH→ Fe3+ +OH− (22)
Fe0 + SO4−• → Fe3+ + SO42− (23)
3.4 The kinetic model and synergistic effect
The performance of decolorization by different processes include ZVI, PS, H2O2, ZVI/ H2O2, ZVI /PS, H2O2/ PS, and H2O2/ZVI/PS was evaluated under identical experimental conditions (solution pH 3, initial SY concentration 50 mg/L, ZVI 50 mg/L, H2O2 1 mM, PS 1 mM, and reaction time 30 min ) (Fig. 5 (a)). It can be seen that the treatment of H2O2 or PS alone had a negligible effect on the decolorization process, which may due to related to the absence of generation of oxidizing radicals (i.e., OH and SO4). Furthermore, With Fe0 composite only, about 15% removal was observed mainly due to surface adsorption. The results further proved synergistic effect existed in the binary PS/H2O2, Fe0/PS, and Fe0/H2O2 systems, In the presence of both PS and H2O2, the decolorization reached 45% after 30 min reaction. The low efficiency of PS/H2O2 system may be explained by the short treatment time, which with adding catalyst could be enhanced significantly (Wang & Xu 2012). In PS/Fe0 system plenty of H + ion released from hydrolysis of persulfate and resulting in the decrease of dissolved pH (Eqs. (24) and (25)), which could the accelerate Fe0 corrosion continuously. And then the released iron corrosion products could activate persulfate to produce SO4●− (Zhao et al. 2010). On other hand released iron corrosion products can in Fe0/H2O2 systems, catalyze the Fenton reaction in the presence of H2O2 for the generation of oxidizing radicals (i.e., •OH and SO4●−). Also, H2O2 can continuously the corrosion rate of Fe0 accelerate and prepare the fresh Fe0 from passivation (Desai et al. 2016; Guo et al. 2016). Since, the Fenton or Fenton-like reaction usually under the acidic condition is performed, therefore in the presence of PS can be found the strong synergistic effect between Fe0 and H2O2 through hydrolysis of persulfate and release H + ion under the acidic condition (Matzek & Carter 2016). According to the results, the highest removal efficiency (100%) was obtained by Fe0 /PS/H2O2 process after 30 min reaction by synergistic effects between persulfate and H2O2, and adding catalyst, which can be attributed to the activation of H2O2 with Fe2+/Fe3+ in the presence of Fe species (Eqs. (16, 17, 26, and 27)) and reaction between HO2− with PS that results in the production of superoxide radicals (Eq. (28)) (Wang & Xu 2012; Tang et al. 2016). In this regard, similar results also found by LI, Jun et al. for the degradation of PNP (p-nitrophenol). In the mentioned study, 99.9% after 6 min treatment obtained that there is a strong synergistic effect in the Fe0/H2O2/persulfate process (Li et al. 2017). On the other hand, In order to determine the speed of chemical reaction, the kinetic of first-order for decolorization was calculated using Eq (ln C0/Ct = kt). (39–41). Where C0 and Ct represent the initial and residual dye concentration (mg/L), t is the reaction time (min), and K the corresponding rate constant (h− 1). According to Table 1, the first-order kinetic model was for all processes with constant rate (R2) of more than 0.9. Figure 5 (b) displays the linear forms of first-order model that according to the results, the rate constants (0.1919 min− 1) of H2O2/ZVI/PS process much more than that of other systems.
H2O + S2O82− →2HSO4− + 1/2O2 (24)
HSO4− →SO42− + H+ (25)
HO2· →O2·−+ H+ (26)
Fe3+ + H2O2 →Fe2+ + HO2·+ H+ (27)
HO2− + S2O82− →SO42− + SO4·− + H+ + O2·− (28)
Table 1 The rate constant, R-squared of kinetic models.
First-order (Ln C0/Ct = k1t)
|
|
R2
|
kobs(min− 1)
|
ZVI
|
0.971
|
0.0073
|
PS
|
0.976
|
0.0100
|
H2O2
|
0.972
|
0.0115
|
H2O2/ PS
|
0.990
|
0.0194
|
ZVI/PS
|
0.952
|
0.0459
|
ZVI/ H2O2
|
0.978
|
0.0661
|
PS/ZVI/ H2O2
|
0.902
|
0.1991
|
3.5 Effect of anions
Anions in aquatic environments are commonly considered to be radical scavengers, which could play a detrimental role in the performance of AOPs (Cui et al. 2016). Thus, in the present study, the effect of various anions, including Cl−, NO3−, NO2−, and HCO3−, was investigated on the proposed process at 5 mM of matrix species under predetermined conditions (solution pH of 3, initial SY concentration of 50 mg/L, ZVI of 50 mg/L, H2O2 of 1 mM, PS of 1 mM, and reaction time of 30 min) (Fig. 6). As shown in Fig. 4, the decolorization effect is considerably inhibited by NO2, while the scavenging effects by NO3−, Cl−, and HCO3− were negligible on decolorization. This scavenging effect in the presence of different anions can be attributed to direct reaction of the scavengers with highly oxidant radicals (•OH and SO4●−) and production form radicals with less oxidant power according to Eqs. (29–36) (Fan et al. 2015; Wu et al. 2019).
Cl− + SO4• − → SO42−+Cl• (29)
NO3− + SO4• −→SO42− + NO3• (30)
NO2− + SO4• −→ NO2 + SO42− (31)
HCO3− + SO4• −→SO42− + HCO3• (32)
Cl− + •OH → HO− + Cl• (33)
NO3− + •OH → HO− + NO3• (34)
NO2− + •OH → NO2 •+ OH− (35)
HCO3− + •OH → H2O + CO3•− (36)
3.6 Scavenger tests
In this study, the radical scavenging experiments were conducted to differentiate the contributions of SO4·− and ∙OH for decolorization with methanol (EtOH) and tert-butyl alcohol (TBA). EtOH is a quencher of both ∙OH and SO4·− (kEtOH, ·OH= (1.8–2.8) × 109M−1s− 1, kEtOH, SO4·− = (1.6–7.7) × 107M−1s− 1). TBA is a strong quencher of ∙OH, and has 418–1900 times higher rate of reaction with than with SO4·− (kTBA, ·OH= (3.8–7.6) ×108M−1s− 1, kTBA, SO4·− = (4.0–9.1) × 105M−1s− 1) (Qi et al. 2018). As shown in Fig. 7, without a quenching reagent, the complete decolorization was achieved within 30 min in the ZVI/PS/HP system. With the addition of 300 mM EeOH, the decolorization was decreased from 100–65% in 30 min. When TBA was introduced in the PS/ZVI/H2O2 system, the decolorization efficiency decreased from 100% to79% in 30 min, and hence the EtOH showed a stronger inhibitory effect than that of TBA. Therefore, both the SO4·− and ∙OH had important contributions to decolorization, while •OH was relatively more dominant than SO4·− for the decolorization in the PS-Z/nZVI system. The prominent contribution of hydroxyl radicals can be due to the reaction between SO4 and water (Eq. (37)) or the reaction between ZVI and PS (Eq. (38)). Similar findings have been also reported by Yuan et al., 2014 (Yuan et al. 2014).
SO4− + H2O → SO42− + OH + H+ (37)
S2O82− + Fe0 + H2O → 2SO42− + •OH + Fe2+ + H+ (38)
3.7 Reusability and stability of ZVI particles
The stability and reusability of the catalyst as important factors can reduce the operational cost of such processes in large scale. For this reason, in the present study, the stability of the proposed catalyst was evaluated in five successive cycles under constant conditions (solution pH: 3, initial SY concentration: 50 mg/L, ZVI: 50 mg/L, H2O2: 1 mM, PS: 1 mM, and reaction time: 30 min). After every run, the catalyst was separated by an external magnet, washed three times with deionized water and dried at 80°C for 1 h, afterward, the dried catalyst was used for the next run of the experiments. (Fig. 8). As can be seen here, the catalytic activity of ZVI was constant in five consequent uses. The decolorization efficiency for the fifth run was as high as 80%. The decrease in ZVI efficiency can be attributed to the changes of the catalyst surface and deactivation of the catalyst by intermediates formed on the surface. Also, Fe2+ leaching can result in a decrease in active sites on the ZVI surface (Hussain et al. 2017; Kim et al. 2018). According to the results, it can be stated that the ZVI as a promising catalyst offers a high stability.
3.8 Mineralization
The aim of the degradation in the advanced oxidation technology is not only the pollutant degradation, but also the organic pollutant mineralization to CO2 and H2O completely (Yan et al. 2017). In addition, total organic carbon (TOC) directly reflected the change of organic matter content in aqueous solution. In this regard, the mineralization rate of the dye was determined through measuring the TOC concentration of the samples taken from the reactor at regular time intervals. Figure 10 presents dye degradation rate at solution pH 3, initial SY concentration 50 mg/L, ZVI 50 mg/L, H2O2 1 mM, and PS 1 mM. As can be seen in Fig. 9, the decolorization efficiency reached 100% after 30 min, while the mineralization of dye was 65% at that time. This difference in mineralization and removal rate can be attributed to the formation of intermediate compounds. Furthermore, comparing the degradation efficiency showed that a longer contact time is required to achieve the desired TOC degradation efficiency.