Electrochemical and microbial decolourization of Congo Red dye-contaminated wastewater: experimental and computational studies

The aim of this study was to investigate the degradation of Congo Red (CR) dye in aqueous solution through two different processes: electrochemical oxidation (EO) and microbial degradation. In the electrochemical degradation experiment, several electrochemical parameters were examined to determine their influence on the degradation of CR dye. These parameters included the type of anode used, current density, supporting electrolytes, electrolyte concentrations and pH. Concurrently, microbial degradation was carried out using various indigenous isolates, namely Bacillus megaterium, Lactobacillus delbrueckii, Bacillus sphaericus, Pseudomonas sp., Bacillus lentus, Erwinia sp., Bacillus pumilus, Aspergillus flavus and Aspergillus niger. The effects of mineral salt and time on CR dye degradation were also investigated in the microbial degradation process. Additionally, density functional theory (DFT) computation was employed to analyse the degradation mechanism of CR dye. The results of the electrochemical degradation experiment indicated that the copper anode exhibited superior effectiveness in degrading CR dye compared to the graphite anode. Furthermore, the degradation rate of the dye increased as current density, electrolyte concentration and pH were elevated. In the microbial degradation process, the degradation of CR dye increased over time, and the presence of mineral salt enhanced the degradation rate. The DFT computations revealed that the degradation of the dye initiated at the azo chromophore, sulfonate molecule and ultimately the amide group.


Introduction
Dyes and textile industry contributes positively to the economy, but not without the environmental pollution caused by the discharge of untreated dye wastewater into the environment (Tebeje et al. 2022).It is estimated that 12-65 L of water are required to process one metre of fabric (Dey and Islam 2015), but around twenty per cent of the dye used in textile dyeing does not stick to the fibres (Singh et al. 2017), resulting in the generation of dye-bearing wastewater.Dye wastewater has negative impacts on the environment if discharged untreated.This includes pollution of marine habitats, which can be toxic to aquatic organisms and the food chain (Al-Tohamy et al. 2022).In recent years, conventional processes such as physical and chemical methods have been deployed for wastewater treatment, but these techniques can be complex to deploy and quite expensive, often having limited ability to degrade emerging contaminants (Aqeel et al. 2020).Therefore, there is a need for treatment technologies that are affordable, easy to deploy and efficient in pollutant removal.In this regard, electrochemical oxidation (EO) and microbial degradation processes have an encouraging potential for treating dye-bearing wastewater.Some microorganisms possess the ability to generate reactive oxygen species (ROS), such as hydrogen peroxide (H 2 O 2 ), hydroxyl radicals (•OH) and superoxide radicals (O 2 •−), through enzymatic activities (Nanda et al. 2010).These ROS species play a crucial role in the oxidative degradation of contaminants.The microorganisms serve as catalysts and electron acceptors, facilitating electron transfer and enzymatic reactions.Their cell membrane and cell wall provide sites for electron exchange and binding of contaminant molecules, while the microbial metabolism and enzymatic activities enable the breakdown and conversion of complex dye molecules into simpler and less toxic compounds.Understanding the fundamental electrochemical processes involved in microbial decolourization is crucial for optimizing treatment conditions and developing efficient strategies for the removal of dyes from wastewater.Thangaraj et al. (2021) reported microbial decolourization efficiency of 98% for both Reactive yellow 145 and Reactive red F3B dye by Enterobacter hormaechei SKB 16.Thoa et al. (2023) demonstrated that Aniline blue, Reactive black 5, Orange II and Crystal violet were independently decolorized by Fusarium oxysporum to achieve a decolourization efficiency of 97.06, 89.86, 91.38, and 86.67%, respectively.On the other hand, EO focuses on the oxidation of impurities by electroactive species generated in situ at the anode surface, in the presence of supporting electrolytes such as sodium chloride, potassium chloride and sodium sulphate (Sirés et al. 2014;Nidheesh et al. 2020).Chloride salts for instance can generate strongly oxidizing species like gaseous active chlorine, hypochlorous acid and hypochlorite ions, with the capacity to mineralize pollutants in the wastewater (El-Sayed et al. 2022).Nidheesh et al. (2020) using the EO method, reported a decolourization efficiency of 99.8% and chemical oxygen demand removal efficiency of 55% for mixed industrial wastewater composed of textile and chemical effluent, using aluminium electrodes and sodium chloride as supporting electrolytes.Chen et al. (2018a) conducted a study on the electrochemical decolourization of dyeing wastewaters contaminated with Reactive red 2, Acid red 66 and Direct red 80, with sodium sulphate as a supporting electrolyte.The results obtained on the decolourization rates for the respective dyes were greater than 90%.Samarghandi et al. (2020), studied the electrochemical degradation of Methylene blue using a graphite-doped PbO 2 anode and sodium sulphate electrolyte, and a decolourization efficiency of 96.2% was reported.
The sustained interests in the application of EO and microbial degradation for wastewater treatment necessitates deeper exploration and better understanding of effects of various process parameters and their interdependences.These include isolating the effects of precise electrochemical parameters like the nature of anode material, current density, nature of supporting electrolytes, concentration of supporting electrolytes and initial pH on degradation performance and efficiency.Understanding these parameters can optimize the treatment process and improve pollutant removal.While microbial degradation is known to be environmentally friendly and cost-effective, there are inherent challenges of low abundance and diversity.Hence, more efforts should be directed towards use of indigenous microorganisms for contaminant degradation.Identifying and isolating microorganisms that use contaminants of interest as nutrient source can provide some insights into their potential for use in wastewater treatment.Recent developments in computational modelling approaches for electronic structure modelling can significantly contribute to our understanding of contaminant degradation mechanisms.However, there are currently very few studies focused on predicting contaminant degradation profiles from electronic structure models.This study is focused on the electrochemical and microbial degradation of CR dye-contaminated wastewater.The effects of various electrochemical parameters such as nature of anode material, current density, nature of supporting electrolytes, concentration of supporting electrolytes and initial pH were investigated.Indigenous microorganisms were isolated from dye-soil mixture and their potential to degrade CR dye was investigated.Quantum chemical calculations within the framework of DFT were undertaken to provide valuable insights into the reactive sites of the CR dye molecule and mechanisms involved in the degradation process.By addressing the research gaps related to electrochemical parameters, indigenous microorganisms and reactive site identification, this study aims to contribute to the development of affordable and efficient treatment technologies for industrial dye wastewater.The findings will not only enhance our understanding of the degradation process but also offer practical insights for mitigating the environmental impact of textile industry wastewater discharge.

Materials for electrolysis of Congo Red dye
CR dye and other reagents including NaCl and KCl salts (supporting electrolytes), H 2 SO 4 and NaOH (pH adjusters) were analytical grade and were used as source without further purification.All the reagents were obtained from Sigma-Aldrich, Taiwan.

Electrochemical decolourization of Congo Red dye
Electrochemical degradation experiment of CR dye was accomplished in a 500 cm 3 capacity electrolytic cell.A graphite plate of dimensions 6 × 2.5 × 0.25 cm was used as cathode, while both graphite and copper electrodes of same dimensions were alternately used as anode.Both electrodes were connected to a direct current (DC) power supply, model MCH-K305D.In the experiments, CR dye was mixed with 10 mL of supporting electrolyte.Adjustment of pH was done by adding aqueous 1.0 M H 2 SO 4 or 0.1 M NaOH as needed.The experimental variables explored include nature of electrodes (copper and graphite), current density (27,53,80,107 and 133 mA/cm 2 ), nature of electrolyte (KCl and NaCl), electrolyte concertation (1.0, 0.5, 0.1, and 0.05 M) and solution pH (3.0, 7.0 and 9.0).For each experimental run, 10 mL of supporting electrolyte was added to the dye solution before starting electrolysis.Each experiment was carried out in triplicate and decolourization rates were monitored during the electrolysis process.Up to 4 mL of test solution was aspirated at 10-min intervals for up to 70 min and then sent for spectrophotometric analysis.

Materials for microbial degradation of Congo Red dye
4.5 kg of soil sample was collected at SOPS extension at the Federal University of Technology Owerri.The collecting shovel was inserted 30 cm below the earth crust.1.0 g/L of CR dye solution was mixed with the soil sample to contaminate it.The dye-soil mixture was allowed to stand for 21 days in order to identify and extract indigenous microbial strains capable of dye degradation.Three (3) separate culture media consisting of MacConkey agar (BioTech), sabouraud dextrose agar (BioTech), and nutrient agar (Lab) were prepared for the purpose of culturing and isolating indigenous microorganisms.

MacConkey agar (BioTech)
Fifty-two grams of MacConkey agar powder was dispersed in one litre of deionized water.Allowed to soak for 10 min before swirling to mix and sterilize by autoclaving at 121 °C for 15 min.After cooling to 47 °C, the solution was mixed and poured into Petri dishes.

Sabouraud dextrose agar (BioTech)
Sixty-five grams of the powder was dissolved in 1000 mL distilled water.The mixture was boiled to dissolve the medium completely at 121 °C for 15 min and cooled to 45-50 °C.Then it was properly mixed and dispersed.

Nutrient agar
Twenty-eight grams of nutrient agar powder was dissolved in 1.0 L of distilled water.Swirled to mix properly and dissolved completely.The solution was sterilized by autoclaving at 121 °C for 15 min, and the liquid poured into a petri dish and allowed to solidify.
The materials used were obtained from Aldrich.The media composition of the mineral salt (Table 1) was dissolved in 1.0 L of sterile water and autoclaved at 121 °C for 15 min.

Isolation and identification of Congo Red dye degrading microorganisms
1.0 g of the dye-soil mixture was diluted using tenfold serial dilution method of Cheesbrough (2004).An aliquot (100 μL) of 10 -5 , 10 -7 and 10 -9 dilutions was aseptically inoculated into the prepared sterile culture medium of MacConkey agar, sabouraud dextrose agar and nutrient agar using the spread plate method.The inoculated plates were incubated for 24 h at 37 °C, and identification of microbial strains present on the culture medium was done using gram staining, oxidase and catalase identification methods.Based on physical and cultural traits, nine variant colonies were identified and subcultured by streaking on a sterile nutrient agar plate to obtain pure culture for further use.

Microbial degradation of Congo Red dye
The nine identified isolates (CR1-CR9) were investigated for their independent ability to degrade CR dye.Screening experiments of the strains were conducted in a medium containing mineral salt, supplemented with CR dye at a concentration of 50 mg/L.Erlenmeyer conical flask was used as the medium, CR dye and mineral salt was aseptically poured to independent sterilized conical flask in the ratio of 2:1 (100 mL of dye: Table 1 The media composition of the mineral salt medium Mineral salt composition Amount (g/L) 50 mL of mineral salt, V/V) and 1:2 (50 mL of dye: 100 mL of mineral salt, V/V).Afterwards, each strain was independently introduced into the independent conical flask.An aliquot of 4.0 mL was aspirated aseptically every seven days for a period of 35 days and centrifuged at 4,000 revolution per-minute for 10 min using an LC-04R-N Centrifuge, HOSPIBRAND USA.The supernatant was collected for spectrophotometric analysis.

Analytical procedure
The maximum wavelength of CR dye, λmax = 500 nm was first determined through a wavelength scanning method using an ultraviolet-visible (UV-Vis) LI-722 spectrophotometer.
The LI-722 spectrophotometer was subsequently set at the λmax and the absorbance values of all aliquots taken during the electrochemical and microbial treatment experiments were recorded.The concentrations of the CR dye solution aspirated from the electrolysis cell at intervals were monitored using a Shimadzu UV-3600 Plus UV-Vis-NIR spectrophotometer, over a wavelength range of 200-500 nm.The absorbance of the dye solutions at λmax was specially monitored before and after experiments, as an indicator for CR dye degradation.
where A 0 is the initial absorbance of the CR dye solution before electrolysis and A t is the absorbance of CR dye at the given time t, after electrolysis.

Quantum chemical computations of Congo Red dye
The DMol3 program embedded in the BIOVIA Materials Studio Academic Research Suite (Product No: 5CB-LUR) was deployed for DFT electronic structure studies of the CR dye.CR structure was drawn using 3D atomistic document type of BIOVIA Materials Studio, afterwards the molecular structures were geometrically optimized using the COMPASS force field and smart minimization approach with high-convergence criteria.The computations were performed with the DMol3 program, and set in solvent mode, using the Hirshfeld population analysis and the Perdew-Wang local correlation, along with restricted spin polarization on the DND basis set.

Effect of the anode material on electrochemical degradation of Congo Red dye
To investigate the effects of the anode material on the degradation of the CR dye, a laboratory-scale electrolysis experiment was performed.Graphite and copper plates were independently used as anode materials in two separate electrochemical cells containing 30 mg/L of CR dye in 500 mL solution.In the first electrochemical cell, graphite plates were used as both the anode and cathode, whereas in the second electrochemical cell, a copper plate was used as the anode while graphite plate was deployed as the cathode (Fig. 1).
Figure 2 shows the result obtained when graphite and copper plates were used independently as anode materials.The electrolytic cell with copper anode achieved 97.32% decolourization of CR dye, while the graphite anode material achieved 87.49% percentage decolourization.These results clearly indicate that copper has better electrocatalytic activity and chemical stability than graphite, as the dissolution of graphite in the wastewater was noticeable during the experiment.Gutiérrez and Crespi (1999) reported that the oxidation rate is affected by the type of anode material.Similarly, Szpyrkowicz et al. (2005) claimed that the nature of anode material contributes to the kind of oxidizing species formed during electrochemical degradation.Copper performed better than graphite during the degradation of CR dye because copper has a higher catalytic activity and lower electrode potential compared to graphite.This means that copper is more likely to undergo oxidation and reduction reactions than graphite.On the other hand, graphite corrodes in the electrochemical cell because the acidic state of the electrochemical cell and the chloride salt used as an oxidant to break down the CR dye react with the graphite electrode.

Effect of current density on electrochemical degradation of Congo Red dye
In electrochemical degradation, the current density is responsible for the onset of electron flow in the electrochemical cell (Song et al. 2010).In this study, the effect of Fig. 1 Chemical structure of Congo Red dye current density on the decolourization of 30 mg/L of CR dye-contaminated wastewater was investigated.Five current densities-27, 53, 80, 107 and 133 mA/cm 2 at 25 V were applied independently to the electrochemical cells containing 500 mL of CR dye-contaminated wastewater.
The result presented in Fig. 3 shows degradation efficiency increased steadily with current density.This can be attributed to the fact that higher current densities favour the generation of more oxidants which oxidize and attack the dye compound, leading to the degradation of the dye (Ma et al. 2007;Gui et al. 2019).

Effect of electrolyte concentration on electrochemical degradation of Congo Red dye
Electrolytes are electrically conductive and contribute to electrochemical oxidation (Santos et al. 2020), and to increase the electrical conductivity of wastewater and generate hypochlorite ions, chloride salts are added to the wastewater (Morsi et al. 2011).In this study, the effects of different concentrations (1.0, 0.5, 0.1 and 0.05 M) of NaCl and KCl, on the decolourization of CR dye were investigated.Figure 4 shows the experimental result when NaCl is used as the supporting electrolyte, while Fig. 5 represents the KCl electrolyte reaction in CR dye solution.The result shows that CR dye decolourization rate increased with both concentration and time for the NaCl and KCl electrolytes.Using 1.0 M NaCl and KCl electrolyte, the percentage degradation was more than 80%.Interestingly, 0.5 M NaCl attains similar decolourization efficiency as 1.0 M NaCl, after about 60 min.The decolourization efficiency at higher electrolyte concentration may be related to the differences in ionic strength (Guenfoud et al. 2014;Thirugnanasambandham et al. 2015).This phenomenon is attributed to the presence of supporting electrolyte salts, which increase conductivity, decrease resistance and thus reduce the energy cost of electrolysis (Guenfoud et al. 2014).The Na + and K + from the supporting electrolyte (chloride salts) serve as electrical conductors in the wastewater.At the same time, however, the chloride salts also contribute significantly to the decolourisation of the dye, as the active chlorine has a strong oxidizing effect (Abdessamad et al. 2013), from which hypochlorite, a powerful oxidizing agent is generated.The oxidizing species attack the active sites of the CR dye compound like the chromophore group and lead to the degradation of the dye.

Effect of pH on electrochemical degradation of Congo Red dye
The importance of pH in the breakdown of micropollutants is well established (Periyasamy and Muthuchamy 2018).A favourable pH environment inhibits the decomposition of oxidants that are critical to the oxidative attack of wastewater pollutants (Zhu and Chen 2021).In this study, the influence of pH on the electrochemical degradation of CR dye was investigated by performing degradation experiments in acidic (pH 3.0), neutral (pH 7.0) and basic (pH 9.0) medium.The result presented in Fig. 6 show that pH 9.0 had the highest decolourization rate of 91.97%, followed by pH 7.0 (83.48%) and then pH 3 (70.61%).Hence, the optimal pH for CR dye degradation is attainable at pH 9.0, which implies that a basic pH medium optimally supports electrochemical degradation of CR dye.This might be attributed to the nature of oxidizing species present at different pH ranges.At acidic pH, the oxidizing species are usually hypochlorous, but as the pH of the solution becomes more neutral or alkaline, the oxidizing species present in the electrochemical cell may change.At neutral pH, the main oxidizing species is usually the hydroxyl radical (-OH).At higher pH values, hydroxyl radical (-OH), peroxyl radical (ROO-) and superoxide radical (O2-) occur as oxidizing species during electrochemical degradation.

Electrical energy consumption for the electrolysis of Congo Red dye
The main operational cost for electrochemical oxidation of CR dye is related to the use of electrical energy.To be able to distribute electrical energy for electrolysis of CR dye solution, a DC power source was connected to the electrolytic cell.Using Eq. 2, the precise electrical energy (kWh) distributed to the electrolytic cell can be calculated: where, I is current in ampere (A), v is cell voltage in volts (V), t is time in hours (hr), and Q is quantity/volume (dm 3 ) of the CR dye solution contained in the electrolytic cell.
(2) The applied voltage was constant at 25.0 V over a period of 70 min (1.17 h).
As shown in Table 2, the highest electrical energy consumption was when a current density of 133 mA/cm 2 was applied.Higher electrical energy consumption is linked to higher current density, which most likely results in higher decolourization efficiency.The least electrical energy consumption was at current density of 27 mA/cm 2 .As expected, it resulted in a lower dye decolourization efficiency, whereas current density of 133 mA/cm 2 attained more than 90% dye decolourization as seen in Fig. 3. dye to mineral salt ratio (V:V) -1:2 dye to mineral salt ratio (V:V) -2:1

Erwinia sp
Fig. 8 Effect of mineral salt concentration on Congo Red dye degradation

Isolation, screening and identification of microbial species
Indigenous microbial strains were isolated from dye-soil mixture.The isolates were identified to be seven bacterial species and two fungal species as shown in Tables 3 and 4, respectively.

Degradation of Congo Red dye by fungal and bacterial strains
The pure strains isolated from the dye-soil mixture were deployed for microbial degradation of CR dye wastewater in an anaerobic environment.The potential for microbial isolates to remove dye from aqueous medium containing mineral salt and CR dye was investigated, and the strains functioned independently without having to compete for nutrients.The obtained results (Fig. 7) show that most of the microbial strains-B.

Effect of mineral salt concentration on microbial degradation of Congo Red dye
Microbial strains have specific nutritional needs required for growth and synthesis of enzymes essential for dye degradation (Pandey et al. 2007).Mineral salt and glucose are some of the carbon sources used by microbial strains as nutrients.
Introducing mineral salt into wastewaters can influence the microbial strains responsible for dye degradation, such that the microbial strains utilize the energy from the mineral salt not only to degrade dye but also to grow, reproduce and populate.
In this work, the effect of mineral salt on the decolourization of CR dye-contaminated wastewater was investigated by varying the volumetric ratio of the dye to mineral salt.The initial volume-by-volume ratio of dye to mineral salt was 1:2 (50 mL of dye/100 mL of mineral salt).Afterwards, the volume-by-volume ratio for dye and mineral salt was changed to 2:1 (100 mL of dye/50 mL of mineral salt), and the effect of the change was investigated.
The result in Fig. 8 suggests that the 2:1 medium was optimal for the degradation activity of B. megaterium,L. delbrueckii,Pseudomonas sp.,B lentus,Erwinia sp.,B. pumilus,A. flavus and A. niger,resulting in percentages decolourization of 93.92,92.29,89.17,94.44,92.76,94.73,92.18and 92.47%, respectively.On the other hand, the 1:2 medium enhanced the degradation of CR dye by B. sphaericus.Ultraviolet-Visible (UV-Vis) study is a technique for detecting the presence of chromophore groups or conjugate bonds based on their light absorption properties.Figure 9 shows full wavelength scans of the degradation profile of CR dye at different time intervals, the decrease in absorbance peak provides evidence of CR dye degradation.This can be attributed to azo bond cleavage, one of the most active sites for oxidation (Sun et al. 2002).The cleavage of the chromophore group leads to the degradation of the dye and formation of intermediate oxidation products (Chen et al. 2018b).The disappearance of the absorption peak in Fig. 9 at a wavelength of 450 nm is associated with the degradation of the CR dye.For instance, Mora-Gomez et al. ( 2019) associated new absorption peak occurring between 200 and 220 nm, to the presence of short-chain carboxylic acids, which are oxidation byproducts.Similarly, in Fig. 9 new absorption peak is evident at wavelength of 250 nm indicating the formation of intermediates.

Kinetic evaluation of Congo Red dye degradation
To better determine the order of the electrochemical reaction of the CR dye, the absorbance (A) of CR dye was studied in time series: 0, 10, 20, 30, 40, 50, 60 and 70 min.Various kinetic models were considered to understand the kinetics of electrolysis of the dye including zero, first-and second-order kinetic models with first-order giving the best fit of the data obtained.Further plots of ln(A) versus time were made using different electrolyte concentrations, and the line of best fit was determined.Figure 10 shows the plot of ln(A) versus time data for CR dye degradation with a linear fit which can be approximated to a first-order reaction.

Computational studies on the decolourization initiation mechanism of Congo Red dye
The electronic properties of atoms and compounds can be calculated using DFT (Oguzie et al. 2020).In this study, DFT computations were carried out to model CR dye reactivity and identify the active sites through which oxidative (degradation) attack would be launched on the dye molecule.The computational procedure is as described elsewhere (Oguzie et al. 2021) using the DMol3 program, the Hirshfeld population analysis and the Perdew-Wang (PW) local correlation, along with restricted spin polarization on the DND basis set (Delley 1990(Delley , 2000)).The geometric optimized structure, the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and the Fukui functions are shown in Fig. 11a.The corresponding electronic structure parameters are listed in Table 5.In the electronic properties of CR dye, the LUMO area appears to spread out majorly on the sulfonate moiety (HO 3 S) and the quinone moiety of the CR dye compound, while the HOMO regions which have a high electron density are mostly saturated around the sulphonate and quinone moiety.The propensity of a species to release electrons is indicated by a high value of E HOMO , while the energy required to evict an electron from the outermost orbital also decreases as ΔE = E HOMO -E LUMO decreases.Chemical hardness or softness, which promotes close contact between the interacting species may be a significant determinant of the likelihood of electrophile/nucleophile interaction.
The reactive sites for CR dye degradation were evaluated using the Fukui indices for electrophilic (F−) and radical (F 0 ) attacks.The locations of the electrophilic Fukui functions which gauge the dye molecule's propensity to donate electrons coincide with the HOMO regions and represent the sites most vulnerable to attack by electron-seeking species.In contrast, the LUMO regions represent a molecule's propensity to accept electrons.Our computational results show that the atoms in CR dye that possess high index were mostly attacked by electrophiles in the following order: N ( 38 The intermediates formed during the oxidative attack of CR dye and their corresponding reaction energies, as shown in Fig. 11b, were proposed based on the DFT calculation of CR dye and a study by Zheng et al. (2021).Naphthylamine, naphthol and s-trans alkenes are successive degradation intermediates of CR dye.Naphthylamine, which appears to be an initial intermediate, is considered toxic and is a known human carcinogen.Naphthol can also be harmful to aquatic organisms and may affect human health through exposure.However, the s-trans alkenes, which appear to be a later degradation intermediate, are not directly toxic to humans or animals.

Conclusion
In this study, we investigated the effectiveness of indirect electrochemical oxidation and microbial remediation for the decolourization of CR dye in aqueous solutions.Our findings demonstrate the potential of both approaches for the treatment of CR dye-contaminated wastewater.
During the electrochemical degradation experiments, the presence of a copper anode significantly enhanced the removal of pollutants from the CR dye solution.The rate of pollutant removal was found to increase with higher current densities, electrolyte concentrations and pH levels.The degradation reaction followed a first-order kinetic model, indicating the potential for optimization and control of the process parameters.This electrochemical approach proved to be a relatively rapid method, achieving a high percentage of degradation within approximately 70 min.
In parallel, the microbial remediation experiments demonstrated that microbial degradation of CR dye is a viable and sustainable approach.The rate of microbial degradation increased over time, indicating the adaptation and growth of the microbial consortia to the dye-contaminated environment.Additionally, the presence of mineral salt in the system accelerated the degradation process.However, it is important to note that the microbial degradation process took days to achieve significant levels of degradation, suggesting that this approach may be more suitable for longterm, continuous treatment scenarios.Computational studies complemented our experimental findings by providing insights into the degradation mechanism of CR dye.The simulations revealed that the azo chromophore, sulfonate molecule and the amide group were the first to degrade.Understanding the degradation pathways at the molecular level can guide future research efforts and aid in the development of more efficient and selective degradation strategies for CR dye and potentially other synthetic dyes (Table 6).
In cases where rapid and efficient degradation of pollutants is required, especially in large industrial plants, electrochemical oxidation could be a suitable choice despite its higher cost.On the other hand, if the focus is on cost efficiency and environmentally friendly approaches, microbial degradation might be more suitable, especially in smaller plants or if the wastewater characteristics are well suited for microbial degradation processes.A combination of these methods, such as using microbial degradation to pre-treat wastewater prior to electrochemical oxidation, could provide a synergistic and efficient solution for the degradation of CR dye in industrial applications.In terms of suggestions for improvement, further investigations can be conducted to optimize the electrochemical process by exploring different electrode materials and configurations.Fine-tuning the operating parameters, such as current density, pH and electrolyte concentration, could lead to even higher degradation rates and energy efficiency.Additionally, exploring the use of advanced oxidation processes, such as electro-Fenton or photoelectrochemical methods, could enhance the degradation efficiency.For the microbial remediation approach, the identification and isolation of specific microbial strains or enzymes with enhanced dyedegrading capabilities could improve the degradation rate.
Investigating the synergistic effects of combining different microbial strains may lead to more efficient degradation and reduced treatment time.

Table 2
Calculated electrical energy for electrochemical degradation of Congo Red dye

Table 5
Quantum chemical calculations for Congo Red dye