Hydrogen Peroxide/Zero Valent Iron/Persulfate Approach for Dye Degradation: Key Operating Parameter and Synergistic Effect

Azo dyes due to the presence of benzene ring, toxicity, mutagenicity, carcinogenicity and low biodegradability have become a major problem in the aqueous environment. In this study, Zero Valent Iron (ZVI) was employed as a catalyst to activate persulfate (PS) and hydrogen peroxide (H 2 O 2 ) for removal of Sunset Yellow (SY) from aqueous solutions using integrated H 2 O 2 /ZVI/PS process. The effects of operational parameters (solution pH, H 2 O 2 concentration, PS concentration, and ZVI dose) were studied on SY removal. According to the results, about 100% eciency was obtained by the H 2 O 2 /ZVI/PS for dye removal at: pH = 3, ZVI 50 mg/L, 1 mM H 2 O 2 concentration, 1 mM PS concentration, and 30 min reaction time. The kinetic study implied that the H 2 O 2 /ZVI/PS process followed the rst-order kinetic model. The total organic carbon (TOC) test showed that about 65% of mineralization was achieved after 30 min. Moreover, the ZVI particles showed a suitable eciency after ve cycles, and hence, it can be used as an eco-friendly, cost effective and reusable catalyst for the treatment of wastewaters contaminated with such dyes.


Introduction
Azo dyes are about 70% of the synthetic dyes used in the textile industries and known as environmentally persistent compound (Maroudas et al. 2020). These compounds have an azo group (-N = N-) and aromatic rings in their structure, which can cause environmental problems (Matyszczak et al. 2020).
Hence, various methods have been used for wastewater treatment containing colored compounds.
Advanced oxidation processes such as ozonation, UV/O 3 , UV/H 2 O 2 , Fe 2+ /H 2 O 2 , and UV/O 3 / H 2 O 2 has received remarkable attention for the removal of azo dyes (Chang et al. 2006). Over the last few decades, chemical oxidants such as ozone, hydrogen peroxide (HP) and persulfate (PS) through the production of hydroxyl radicals (OH • ) and sulfate radicals (SO 4 •− ) has been applied for removing dyes from large volumes of wastewater (Khan et al. 2013; Wordofa et al. 2017;Tariq & Khan 2020). Hydrogen peroxide (H 2 O 2 ) with oxidation potential 2.07 V is a common oxidizing agent used in the oxidation of organic compounds (Hou et al. 2012). In recent years, persulfate as a strong oxidizing agent with a redox potential of 2.01 V has been introduced for degradation of persistent pollutants. Moreover, persulfate due to its characteristics such as cheapness, non-selective oxidation, and high stability of the produced radical under various conditions and high solubility compared to other oxidants makes it an attractive option for the oxidation of organic contaminants (Oh et al. 2009). Organic matter degradation by PS is achieved at slow kinetics at room temperature. Therefore, it is necessary to activate PS for accelerating the process. The PS can be activated by heat (Eq. 1), UV light (Eq. 2) and transition metals (Me 2+ ) according to Eq. 3. The activated PS produces sulfate radical (SO 4 − ) with highly reactive oxygen species ( E0 = 2/07 V), which can convert the contaminant compounds into harmless products (Oh et al. 2011).
Among metals, the iron is a most usage related which requires high value, high volume production of sludge and consumption of SO 4 • radicals at high concentrations of the main problems is this activator (Oh et al. 2010

Experimental Procedures
In the present study, all experiments were conducted in a batch reactor at atmospheric temperature and pressure containing 50 mL of sunset yellow dye, which includes different concentration of zero valent iron particles (25 to 150 mg/L), hydrogen peroxide (0.25 to 2 mM) and persulfate (0.25 to 2 mM). Initial pH of dye solution was adjusted using 1 N sodium hydroxide and sulfuric acid solutions using a digital pH meter (EUTECH-pH 1500). Afterward, in order to investigate the effects of operating parameters on the H 2 O 2 /ZVI/PS process, some experiments were performed to determine the optimal conditions for SY removal. The zero valent iron particles was collected from the solution using a magnet.

Analytical Method
In order to determine the concentration of residual dye in the aqueous phase, liquid samples were withdrawn periodically (in six 5-min intervals) and then read by using an UV − vis, UV/Vis spectrophotometer (DR-5000, HACH, USA). The characteristic wavelength for this compound is 482 nm.
Then the removal e ciency of dye (%) was obtained in accordance with Eq. (6): Residual dye (%) = (C 0 -C t )/C 0 *100 (6) Where, C 0 is the initial dye concentration, and C t is the dye concentration at times. TOC was measured using a Shimadzu VCHS/CSN Japan analyzer by oxidative combustion at 680°C using an infrared detector (Feizi et  determine the speed of chemical reaction, the kinetic of rst-order for decolorization was calculated using Eq (ln C 0 /C t = kt). (39-41). Where C 0 and C t 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 rstorder kinetic model was for all processes with constant rate (R 2 ) of more than 0.9. Figure 5 ( 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/H 2 O 2 system, the decolorization e ciency decreased from 100% to79% in 30 min, and hence the EtOH showed a stronger inhibitory effect than that of TBA.

Mineralization
The aim of the degradation in the advanced oxidation technology is not only the pollutant degradation, but also the organic pollutant mineralization to CO 2 and H 2 O completely (Yan et al. 2017). In addition, total organic carbon (TOC) directly re ected 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, H 2 O 2 1 mM, and PS 1 mM. As can be seen in Fig. 9 Data Availability Data availability is not applicable.
Author's Contributions Mehdi Ahmadi contributed to the conception and design of the study. Rozhan Feizi conducted material preparation, data collection, and analysis. The rst draft of the manuscript was written by Rozhan Feizi, and all authors reviewed early drafts of the manuscript. All authors read and approved the nal manuscript.

Funding
This work was supported by the Ahvaz Jundishapur University of Medical Sciences Grant No.

Figure 1
The structural formula of Sunset Yellow