Performance evaluation of photolytic and electrochemical oxidation processes for enhanced degradation of food dyes laden wastewater

Today, the entire world faces outrageous issues of safe drinking and sterile water for the consumption of human beings because of groundwater adulteration and industrial effluents (Rasheed et al. 2017; Gnanasekaran et al. 2019; Liu et al. 2019; Shetti et al. 2019). The wastewater originating from such industries contains suspended solids, organic matter, and an immense amount of dyes that have an undesirable impact on living life forms (Ali et al. 2018; Jager et al. 2018; Tiwari & Ram 2019). The presence of a little concentration of dyes in wastewater can influence the light levels in photosynthesis in amphibian plants (Ali et al. 2019; Bilal et al. 2018a, 2018b, 2018c; Salazar et al. 2018). The strong color of dyes is due to the azo bond and chromophore groups, which are stable to environmental factors (Veisi et al. 2019; Zukawa et al. 2019; Ali et al. 2020).

For electrochemical degradation of dyes, diverse anode materials like platinum cathode, boron-doped diamond (BDD), and titanium-based dimensional stable anode (DSA) terminals have been utilized (Vasconcelos et al. 2019). Owing to its great chemical stability, high rates of removal in the presence of Cl⁻ ions, minimal effort, and environmental compatibility, titanium ruthenium oxide electrodes were applied in this work for Allura red and erythrosine dye degradation. Both the photolytic and electrochemical degradation methods were compared for erythrosine and Allura red dyes. In addition, the effect of various critical factors such as pH, current density, and the concentration of electrolyte was also studied.

The reagents and chemicals utilized during the trial were of scientific standard. Erythrosine (Red No. 3) and Allura (Red No. 40) dyes were acquired from Sigma Aldrich, USA. Sodium chloride (NaCl), sodium sulfate (Na₂SO₄), and sodium nitrate (NaNO₃) were obtained from Merck, Germany. All the necessary solutions were prepared using deionized water. The equipment used was DAZHENG DC Power Supply (model-PS-303D), double-beam UV–Vis spectrophotometer (model UV-1602 Biomedical sciences), and a hotplate stirrer (model DAIHAN LABTECH CO. LTD). UV light (254 nm) and titanium ruthenium oxide anode were provided by De Nora Brazil.

The photolysis of the dye solution was carried in a Pyrex glass container comprising 400 mL of dye wastewater. The UV light (254 nm) was immersed in the middle of the reactor and the wastewater solution was stirred using a magnetic stirrer. During the photolytic assay, 1 mL aliquot was withdrawn at a predetermined time interval (I h) and analyzed using a spectrophotometer. The decolorization was estimated by a reduction in absorbance intensity at ʎ_max of the dyes. The effect of temperature (30–40 °C), time (1–6 h) and pH (3–12) on the photolysis was also investigated. The reaction pH was maintained using droplets of sulfuric acid (H₂SO₄) or ammonium hydroxide (NH₄OH).

For the prediction of the relationship between the dependent variable (experimental results) and independent variables (experimental variables), empirical modeling techniques such as response surface methodology (RSM) is often utilized (Muhamad et al. 2018). Based on the statistically designed regression model of experimental data, RSM not only predicts the future response but also gives the opportunity to optimize it. Several types of design are available in RSM modeling, including central composite, Box–Behnken, user-defined, and historical data designs. In this study, historical data design was selected to evaluate the approximate function between dependent and independent variables in which design points are defined on the basis of existing experimental data without any limitation of the number of factors and levels compared to other RSM model designs (Jeirani et al. 2013; Muhamad et al. 2018).

The ʎ_max for both the dyes was determined with a wavelength scan range from 350–650 nm through a UV–Vis spectrophotometer. Allura red dye showed maximum absorption at 505 nm (Figure 3(a)), whereas the highest absorbance was recorded at 530 nm in the case of erythrosine dye (Figure 3(b)).

Irradiation duration exerts a noteworthy impact on the discoloration of dyes. The discoloration rate of photolysis increased by increasing the irradiation time. As the contact time of UV irradiation with dye increases, the degradation of dye increases, breaking the azo bonds and hence, results in high discoloration of dye. It can be observed from Figure 4(a) that the absorbance of Allura red dye (200 ppm) was reduced, resulting in 63.29% decolorization after exposure to 6 h UV illumination. Similarly, a substantial decrease in absorbance was also noticed for erythrosine dye (50 ppm) concomitant with 90.84% degradation after 6 h of UV radiation (Figure 4(b)). Hence, it was inferred that the degradation rate of dyes increased progressively with an increase in light span. Additionally, it is also revealed that the degree of dye degradation increases by extended interaction time of dye with UV light (Boucenna et al. 2019). The photolysis showed that the erythrosine dye is more stable than Allura red dye. Due to this reason, the concentration of Allura red (200 ppm) and erythrosine dye (50 ppm) was used in the experiments carried out for photolysis and electrochemical degradation.

Figure 5(a) demonstrates that 35.24% decolorization of Allura red was achieved at pH 5.0. With an increase in solution pH to 6.0, the photolysis of dye was enhanced to 55% and maximum removal of dye up to 95.3% occurred at pH 12 after 1 h exposure to UV light. Consequently, it is presumed that an increase in pH prompts an increase in discoloration ability. For erythrosine dye, 90.84% decolorization was achieved at pH 6.0 after 6 h of irradiation time, and a further increase in pH values gave a diminished color removal (Figure 5(b)) (Wu et al. 2019). The photolysis of both the dyes increased when the pH of the solution was changed from highly acidic to slightly acid or alkaline. With an increase in pH, the time of photolysis was decreased. However, when in aqueous alkaline solution in the presence of UV irradiation the highly reactive hydroxyl radical is formed due to reactions with oxygen, which results in the degradation of Allura red dye. Without the presence of oxygen, the degradation of dye was prevented, suggesting that the presence of oxygen is important for the production of reactive radical species. Thus, the photolysis of Allura red dye in basic media could be preceded via direct reactions of the dye with highly reactive radical species formed in the presence of UV irradiation, as already reported in the literature (Soltani & Entezari 2013).

The effect of temperature during the erythrosine and Allura red dyes discoloration was examined with a temperature range varying from 30 to 40 °C for 1 h. A high degradation rate of 95.69% was achieved at 35 °C as depicted in Figure 6(a). Photolysis of Allura red dye increased by enhancing the temperature to 35 °C and diminished gradually with further enhancement in temperature up to 40 °C. A similar inclination occurred for 4-chloro-2-aminophenol utilizing joined procedures like hydrodynamic cavitation, UV photolysis, and ozone at temperature (30–38 °C), where the highest discoloration was accomplished at 35 °C (Wang et al. 2019). During the photolytic discoloration of erythrosine dye, the effective results were attained at 30 °C and diminished subsequently with a temperature rise after 6 h (Figure 6(b)). It implies that there is not much impact of temperature on photolytic removal of color and a rise in temperature from 32 to 40 °C poses no significant impact on the photolytic discoloration of methylene blue dyes. It means that UV light and oxygen has a major role in the photolysis of these dyes, which produces reactive radicals necessary for the process. The photolysis will stop in the absence of any of them (Soltani & Entezari 2013). However, temperature has a small role in photolysis and followed the same trend as reported earlier by Soltani & Entezari (2013). The photolysis of methylene blue dye, by increasing the temperature to 40 ºC , has no effect on dye degradation. In an inert environment (Ar atmosphere) the photolysis was completely stopped and under solar irradiation at the air atmosphere completed within 90 min (Soltani & Entezari 2013).

UV spectra represents a high degradation of Allura red dye (95.69%) after 1 h treatment at pH 12 and 35 °C (Figure 7(a)). On the other hand, the greatest dye discoloration of 90.84% was achieved for erythrosine dye under the optimized conditions of pH 6.0 and 30 °C after 6 h exposure to UV light (Figure 7(b)). In a recent report, Soltani & Entezari (2013) observed the complete photolysis of methylene blue dye within 30 min at pH 12 and 32 °C.

The main purpose of the electrochemical method is to direct an electric flow through electrodes, bringing about various compound responses. By using a reducing agent, a conventional strategy is replaced by an inventive cathodic electron exchange (electrons are utilized, rather than synthetic compounds). An electrochemical cell is used to carry out the oxidation process. The dyeing mechanical assembly is coupled to the electrochemical cell and the dye bath is adequately diminished through an electrochemical procedure. The elements that influence electrochemical implementation include current density, structure geometry, type, number and dispersing of electrodes utilized, pH, temperature, and nature of the electrolyte, etc.

Electrochemical degradation of Allura red dye was carried out at the current flow of 2–7 mA cm⁻² with different parameters remaining constant (Figure 8(a)). The degradation rate increases by increasing the current density. The ideal current density of 5 mA cm⁻² was chosen for greater coloring removal and most minimal costs of vitality, in light of the a further increment in current density did not have any critical impact on dye discoloration. According to previously reported literature, the discoloration of reactive blue 19 dye at titanium-based DSA anode expanded directly with expanding ebb and flow densities (7.22–36.10 mA cm⁻²) and with NaCl as an electrolyte (Baddouh et al. 2018). Therefore, the extraordinary current flow of 2–12 mA cm⁻² was used to contemplate the proficiency of erythrosine dye's electrochemical removal with 0.1 mol L⁻¹ NaCl at pH 6. The degradation efficiency noticeably increases with expanding current density in the range of 2–12 mA cm⁻² (Figure 8(b)). The electrochemical decontamination of between 1 and 100% was achieved at 8 mA cm⁻² after 10 min of electrolysis. With an increase in current density, the release of Cl/HClO was enhanced in NaCl electrolyte solution, leading to increased dye elimination (Jager et al. 2018). Hence, the ideal current flow for this situation was set as 8 mA cm⁻² to reduce costs of energy and high dye removal. Siedlecka et al. (2018) investigated the removal of acid black 210 at BDD anode and found that discoloration efficiency of dye increased with increases in the ebb and flow densities from 25 to 100 mA cm⁻².

The influence of pH was evaluated on dye degradation with different variables being consistent, as shown in Figure 9. For Allura red, high degradation efficiency (99.60%) was observed at pH within 5 min electrolysis treatment. The electrochemical discoloration rate decreased with an increment in the pH of the mixture, as shown in Figure 9(a). The Allura red dye demonstrates greater discoloration in the acidic medium compared to the basic solution (3.86%) at pH 12, leading to a decrease in Cl₂/Cl⁻ production. Along these lines, further analysis was done at pH 5.0. Sajid et al. (2018) achieved 70% discoloration of wastewater at pH 4 after 120 min of electrolysis. The impact of various pH on erythrosine dye's electrochemical degradation is shown in Figure 9(b). A degradation of 78.02% occurred after 6 min at pH 3.0, which increased to 99.61% at pH 6.0. The effectiveness of erythrosine discoloration was reduced at higher pH, which demonstrates that the removal of erythrosine was higher using the acidic dye solution. In this way, the pH 6.0 was chosen as an ideal pH for analysis. In acidic dye solution, the Cl⁻ is present as HClO in the mixture and has a higher oxidation potential (1.49 V) than OCl⁻ (0.94 V) (García-Espinoza et al. 2018) due to the fact that in basic medium, the OCl⁻ is present, which produces low degradation at higher pH.

It is obvious that NaCl was observed as the best electrolyte for both Allura red and erythrosine dyes based on its greater removal efficiency. Referring to the previous reports, the pesticides are completely degraded at 0.1 mol L⁻¹ of NaCl within 60 min of electrolysis process (Zhu et al. 2018).

The impact of NaCl concentrations on Allura red dye degradation was examined under a consistent current flow of 5 mA cm⁻¹ after 5 min of electrolysis, and responses are presented in Figure 11(a). With the increase in NaCl concentration, the removal efficiency of dye also increases. The greatest discoloration (99.60%) was achieved with 0.1 mol L⁻¹ NaCl, and therefore the same concentration of NaCl was utilized throughout the test work. Figure 11(b) portrays the impact of various concentrations of NaCl on the electrochemical discoloration effectiveness of erythrosine dye. The lowest (18.51%) and the highest (99.61%) discoloration was recorded at 0.02 and 0.1 mol L⁻¹ of NaCl after 10 min under the consistent current flow of 8 mA cm⁻². Thus, further analysis was done with 0.1 mol L⁻¹ of NaCl electrolyte. A comparison was made with a previous report where the almost complete discoloration (95.1%) of tartrazine E102 dye was noted utilizing carbon/lead dioxide anodes after 10 min with NaCl (Hamad et al. 2018). The electrochemical degradation pattern of Allura red dye in NaCl using a BDD electrode (Thiam et al. 2015) gave a benzenic derivative due to breaking of (–N = N–) of the dye molecule. The hydroxylation, demethylation, and carboxylation along with the chlorination reactions occurred, resulting in primary aromatic compounds. The chloro-derivatives are formed due to active chlorine. These compounds were removed as the electrolysis proceeded. Jain et al. (2005) proposed the electrochemical degradation pathway of erythrosine dye. After the electrolysis of erythrosine dye, the chemical oxygen demand value decreased significantly compared to the original dye solution.

From the above discussion, a brief comparison of electrochemical and photolytic degradation procedures can be deduced. In the case of photolytic degradation, up to 90% of degradation was obtained at the cost of high-energy consumption. While in the case of electrochemical degradation, 100% results were obtained with low cost, cheap equipment, more feasibility, and more accuracy. Hence, the latter was found to be the most effective method for the degradation of both tested dyes.

To test the adequacy and validity of the second-order model, ANOVA was performed as shown in Table 4. The high F-value of the models (i.e. 88.2 for Allura red and 15.4 for erythrosine) are higher than for Fisher Table (2.424 for a 95% coefficient level), demonstrating that the model is fit to the experimental results (Zhang et al. 2018). Moreover, linear, self-interaction, and two-way interactions among the variables are significant as in most cases p < 0.05. A good presentation of coefficient of determination for Allura red (R²: 0.9584) implies that 95.84% of variables are explained by the model, whereas only 4.16% of variables could not be explained, and 89.03% in case of erythrosine dye (Figure S1, Supplementary Information).

The interaction effects of time, pH, NaCl concentration, and current density on removal efficiency of Allura red is presented in Figure S2(a)–S2(e), which shows the variation in removal rate when two parameters are varied while the remaining two parameters hold at mean values according to Table 3. It is evident that in all cases, a higher removal rate is achieved at lower pH values in the tested range, while the other parameters were varied between their low to upper range. Similar criteria were set to evaluate the interaction effect of various variables on the degradation efficiency of erythrosine dye as shown in Figure S3(a)–S3(e), which shows that accelerated removal rates are achieved at low pH values, higher NaCl concentrations, and elevated current densities.

Figure S4 presents the Pareto chart of standardized effect for a percentage of Allura red removal, which shows that all four input variables have a significant influence on the decay efficiency of Allura red. Among all four operating parameters, pH (70.04%) and electrolysis time (11.60%) are the main factors on the Allura red degradation efficiency, while the contribution of individual factors for erythrosine degradation was found in the order: NaCl > pH > time > current density (Figure S5).

The optimized conditions for Allura red and erythrosine dye removal were determined by the response optimizer. The aim was set as ‘maximize’ to obtain the maximum degradation efficacy of Allura red and determine the optimum operating conditions. Many solutions/operating conditions are available to achieve maximized degradation efficiency of Allura red and erythrosine dye (Tables S3 and S4), and one representative solution for Allura red degradation is presented in Figure S6, which shows that the highest Allura red degradation efficiency (100%) was achieved at 1.95 min, pH 5.05, NaCl concentration of 0.1 mol/L, and applied current of 7 mA. However, many solutions are available that have the desired removal efficiency where operating parameters vary between their upper and lower limits to achieve the same target. It is interesting to see that >95% decay rate is only achieved when pH varies between 4 and 5, which is in accordance with the Pareto chart indicating that pH is the most influential parameter (Table S4). In Table S4, the selection criteria for the best operating condition should be based on minimum energy consumption together with accelerated and desired removal rates.

In this study, the photolytic and electrochemical degradation of erythrosine and Allura red dyes have been evaluated. Critical parameters that affect these processes, such as pH, temperature, supporting electrolytes, NaCl concentration, and current density, were optimized for maximum discoloration efficiency. The following conclusions can be drawn from the study:

• The photolytic process was slow and degraded 95.69% Allura red in 1 h and took 6 h to degrade erythrosine up to 90.84%. Solution pH and temperature has an influence on the rate of degradation.

• Photolytic degradation mainly involves the breaking of bonds due to the absorption of UV radiation and hydroxyl radical produced in an aqueous medium.

• Electrochemical degradation using Ti/Ru0.3Ti0.7O₂ anode was fast and degraded erythrosine and Allura red dye in 10 and 5 min, respectively.

• The electrochemical degradation of Allura red and erythrosine was strongly influenced by pH and NaCl concentration and moderately affected by current density.

• The electrochemical process mainly involves indirect oxidation of dyes in the bulk of solution, and the reactive chlorine species, such as Cl₂, HClO, ClO⁻, are generated on the anode surface and oxidize the dye molecules in solution.

• The high removal efficiency and fast degradation rate demonstrate the electrochemical process as an alternative to treat wastewater containing food dyes.

All listed authors are grateful to their representative departments and universities for the financial support and analytical services used in this study.

The authors declare that they have no conflict of interest.

The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.182.