Photocatalytic degradation of methyl orange and methyl violet dyes by UV/Cu²⁺/PDS process Open Access

Water is one of the most significant components of the planet Earth. Water covers approximately 71% of the earth's surface (El-Regal & Satheesh 2023). To survive, all plants and animals require water in their bodies. Water is used for plenty of purposes, including drinking, bathing, washing, cooking, cleaning, and plantation, and in industries for various industrial practices and the production of various commercial products (Inyinbor Adejumoke et al. 2018). Different anthropogenic activities play a major role in water contamination. Among these, industries play a major role by releasing massive quantities of hazardous materials, including dyes, antibiotics, and pesticides, either directly or indirectly into the aquatic environment and polluting it (Kuchangi et al. 2023). Textile industries contribute significantly to water pollution due to the release of various dyes into aquatic systems (Sidabutar et al. 2017). According to the literature, 10–20% of dyes are lost to wastewater due to inefficient operation in the dyeing process. The dye-contaminated wastewater inhibits the reoxygenation capacity of water, cuts off sunlight and disrupts biological activity in aquatic life (Zaharia et al. 2009).

The most intriguing of these dyes is methyl orange (MO, C₁₄H₁₄N₃NaO₃S), an azo-anionic dye soluble in water. MO turns red in acidic and yellow in basic media. Because of its clear and distinct color variation at different pH media, MO is used as an indicator in the titration process (Al-Mamun et al. 2021). Another most important dye is methyl violet (MV, C₂₄H₂₈N₃Cl), a cationic dye that belongs to the aromatic organic compounds family. It gives yellow color at low pH (0.15) and changes to violet as the pH increases. As MV is a mutagen and mitotic poison, there are concerns about the environmental impact because of its release into the freshwater body (Thekkedath et al. 2022). These dyes (MO and MV) are persistent in nature, non-biodegradable and highly soluble in water; therefore, their removal from the aquatic system is of high concern (Shah 2023). Due to their persistent and non-biodegradable nature, these dyes cannot be easily eliminated from aquatic bodies by traditional wastewater treatment procedures.

Different types of efficient advanced oxidation processes (AOPs) have been widely investigated, such as Fenton processes, UV-photolysis-driven processes, ozonation and sulfate radical-based AOPs (SR-AOPs), for the removal of dyes from wastewater (Liu et al. 2023b; Khan et al. 2024). The UV-based photocatalytic processes are also well documented for the production of hydrogen energy as a clean energy (Orak & Yüksel 2021; Orak & Yüksel 2022a, 2022b; Keskin et al. 2024). In addition, various oxidants such as peroxymonosulfate and peroxydisulfate (PDS) were broadly accepted in recent years in both research and application for the remediation of organic pollutants in wastewater (Liu et al. 2023a). Activation of these oxidants through alkaline medium, UV, heat and transition metals can produce strong sulfate radicals (⁠⁠) that cause contaminants degradation in aqueous media (Zhang et al. 2014). Among the said processes, SR-AOP is the most efficient way the treatment of dye-contaminated wastewater.

Chakma et al. (2017) used sono-hybrid techniques (US/Fe²⁺/UVC) for the activation of persulfate (PS). The efficiency of the system was determined in terms of azorubine degradation in aqueous media. The results revealed that US/UVC/PS is the best technique for the degradation of azorubine in water. The incorporation of Fe²⁺ in the US/UVC/PS system decreased the efficiency of the processes because of the scavenging of by Fe²⁺ ions in aqueous media. Bougdour et al. (2020) used the PDS/Fe(II)/UV technique for the elimination of RY17, RR120 and RB19 dyes in synthetic wastewater (SW) and real water samples. Under optimum conditions ([PDS] = 1 mM, T = 25 °C), 96.1, 99.2 and 100% degradation were observed for RY17, RR120 and RB19, respectively, in 2 h of reaction time. Moreover, about 80% mineralization of the mixture of these three dyes (RY17, RR120 and RB19) was observed at a treatment time of 2 h in an SW sample. In addition, the real waste sample showed 66% degradation of the selected contaminants under the same conditions.

The present study is aimed at the degradation of MO and MV dyes in aqueous media using a UV/Cu²⁺/PDS system. The influences of different operational factors such as initial contaminant concentration, initial amount of catalyst and oxidant and initial solution pH toward contaminants degradation were also examined. The efficiency of the UV/Cu²⁺/PDS system was also determined in different water systems including distilled water (DW), SW and industrial wastewater (IW) samples. Furthermore, scavenger experiments were also carried out to determine the active species responsible for the degradation of the selected contaminants.

MO (C₁₄H₁₄N₃NaO₃S, 99%), MV (C₂₄H₂₈N₃Cl, 84%), sodium hydroxide (NaOH), perchloric acid (HClO₄), methanol (CH₃OH, 99.85%) and isopropyl alcohol (CH₃)₂CHOH, 99.5%) were provided by Sigma Aldrich. Tertiary butanol (C₄H₁₀O, 99.65%), copper (II) chloride (CuCl_2.2H₂O, 98%) and sodium PDS (Na₂S₂O_8, 98%) were purchased from Scharlau Spain.

A 150 mL Petri dish was used for performing the degradation experiments. The Petri dish was placed under the UV light source, ensuring that the entire solution was exposed to UV radiation. The magnetic stirrer was set up inside the reaction mixture in a Petri dish to maintain uniform mixing during the degradation process. The photocatalytic setup for the degradation of the MO and MV dyes was carried out under a 15 W UV lamp, placed in a wooden chamber.

The analysis of degradation byproducts (DPs) of MO and MV was done using gas chromatography coupled mass spectrometry (GC-MS) QP 2010 plus (Shimadzu, Japan) installed with a DB-5MS column (30m × 0.25mm × 0.25m). The DPs of MO and MV were analyzed for the m/z values in the range of 40–800. The analytical conditions of GC were injection temperature = 240 °C and oven temperature was first set at 40 °C and then was increased to 280 °C at the ramping rate of 4 °C/min. For GC-MS analysis, the DPs of MO and MV were extracted in methanol by the solvent extraction technique. The details of the GC-MS procedure are documented in our earlier publication (Zohaib et al. 2024).

Control experiments were conducted without Cu²⁺ ions and/or PDS to evaluate the contribution of each component to the degradation process. Optimization experiments were performed by varying parameters such as reaction time, initial concentration of Cu²⁺ ions and PDS to determine the optimal condition for efficient degradation. Moreover, the degradation of MO and MV was also performed in distilled, SW and IW samples to determine the efficiency of the UV/Cu²⁺/PDS system.

Sodium peroxydisulfate or simply sodium persulfate (NaS₂O₈) was the active source utilized for the production of sulfate radicals (⁠⁠). When exposed to light, PDS acts as an oxidizing agent that leads to the formation of highly reactive sulfate radicals (⁠⁠) (Equation (2)) (Nidheesh et al. 2022). The generated sulfate radicals attack the dye molecules (MO and MV) and cause their elimination from aqueous media. The removal of electron-rich functional groups from the dye molecule is frequently involved in this process, which results in the formation of simpler, colorless compounds (Ismail & Sakai 2022).

These results are in agreement with the study conducted by Badi et al. (2019) where the degradation performance was reduced for the degradation of dimethyl phthalate when the PS concentration was further increased from the optimum amount. The results of Pasalari et al. (2022) are also inconsistent with these results where the photocatalytic degradation of 2,4-dinitrophenol was significantly reduced when PS concentration was increased to 100 mg L⁻¹.

Once the optimum concentration of PDS (1.5 mM) was achieved, then the next experiment was carried out using various initial concentrations of Cu²⁺ ions in combination with PDS (1.5 mM). The concentration of PDS 1.5 mM was taken constantly while the concentration of Cu²⁺ varied from 0.5 to 3 mM.

However, further increasing the initial concentration of Cu²⁺ beyond the optimum value (2 mM for MO and 1.5 mM for MV) showed a slight negative impact on the degradation of these contaminants. This could be associated with the limited PDS concentration at higher Cu²⁺ doses. These results are in agreement with the conclusions of Wang et al. (2020). Pasalari et al. (2022) also observed identical results where the excessive concentration of Cu²⁺ (25 mg L⁻¹) in the degradation performance of 2,4-dinitrophenol was reduced by the UV/PS/Cu²⁺ process. However, it is recommended to perform more research studies to explore exactly the role of Cu²⁺ at optimum concentrations.

The results of Figure 6 indicate that the highest percent degradation of MO by the UV/Cu²⁺/PDS system was achieved at a pH of 6.0. The free radical distribution in aqueous solution by PS activated system is such that at pH = 2–7 (acidic and neutral media) the main reactive species is ⁠, at pH = 8–10 both ^•OH and co-exists while at pH >10, OH is the dominant species (Li et al. 2014; Naz et al. 2024). Thus, with the rise in pH, gradually converts to OH. Similarly, MO showed a higher percent photocatalytic degradation at pH = 5.6 by the UV/Cu²⁺/PDS process. It also gives an indication that is important for the effective photocatalytic degradation of MO and MV by the UV/Cu²⁺/PDS process.

The photocatalytic efficiency of the UV/Cu²⁺/PDS system toward MO and MV degradation was also determined in synthetic and IW samples besides the DW. The SW sample was prepared in the laboratory by the addition of various types of salts in aqueous media containing MO and MV dyes. The IW sample was collected from the Coca Cola plant. Prior to the treatment, the sample was contaminated with the known concentration of MO and MV dyes.

The respective solutions of MO and MV dyes after treatment in the UV/Cu²⁺/PDS system were analyzed by GC-MS. The GC-MS analysis revealed that various DPs results from the degradation of MO and MV dyes. The structures of these DPs were obtained from their chemical formulae and m/z values.

The photocatalytic degradation of MO and MV dyes by the UV/Cu²⁺/PDS system showed promising results. The results indicated that the photocatalytic degradation of MO (10 ppm) and MV (25 ppm) was optimized at Cu²⁺ and PDS having concentrations of 2 and 1.5 mM, respectively. The maximum photocatalytic degradation of MV was achieved at pH = 3.0, while for MO, it was 6.0. The scavenger studies indicated that is the dominant species involved in the photocatalytic degradation of MO and MV. The results from different water types (DW, IW, SW) showed that despite the quality of water, the UV/Cu²⁺/PDS process is the best environmentally friendly and effective treatment technology for the degradation of MO and MV dyes in water.

The authors acknowledge the Higher Education Commission (HEC) of Pakistan for providing financial support to this work through project No. 17212.

The individual contributions of all the authors to this paper can be summarized as follows: QK contributed to conceptualization, data curation, formal analysis, Investigation, methodology, and writing the original draft. AA contributed to formal analysis, investigation, validation, and editing. MS contributed to project administration, supervision, and funding acquisition. IG contributed to validation and editing. FG contributed to revising the manuscript. FR contributed in revising and editing the manuscript. MZ contributed to revising the manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.