Electrocoagulation followed by sound agitation for removal of nitrogen and carbon-based pollutants from industrial wastewater

The world of textile industries demands varieties of synthetic dyes leading to its numerous intermediate products (Paździor et al. 2019). To cater for demand, the dye intermediates are synthesized and manufactured in substantial amount in India. Some regions of Gujarat have grabbed attention at global scale with reference to synthetic pigments and its toxic effluent (Chakraborty & Basu 2019). Dye intermediate manufacturing industries have a wide application in the field of textile, food, leather, rubber, etc. Due to these wide applications, a large amount of wastewater is produced from these industries and are discharged into water bodies with only partial treatment (Hayat et al. 2015; Guo et al. 2018). The harmful effects of dye intermediate effluents on aquatic environment were discussed in detail by the authors Pani et al. (2020).

The effluent includes a variety of hazardous contaminants such as high chemical oxygen demand (COD), ammoniacal nitrogen (AN) (NH₄⁺-N), high total organic carbon (TOC), high chloride content, high sulphates, high concentrations of heavy metals such as copper, nickel, chromium, and iron (Yaseen & Scholz 2019; Pani et al. 2020).

The alarming concentration of COD and AN in the discharged effluent endangers human health and damages aquatic community by exhausting the DO level in the receiving water bodies. Moreover, toxic algae such as Euglenophyta and Chrysophytaget stimulated by inorganic nitrogen pollution hampers the survival, growth and reproduction of aquatic organisms (Camargo & Alonso 2006; Dong et al. 2017; Soliman & Eldyasti 2018).

Several technologies are available for treatment of such industrial wastewaters. The commonly used include physical (Quan et al. 2009), chemical (Zhang et al. 2009; Huang et al. 2017) and biological methods (Phan et al. 2017; Miao et al. 2018; Zhou et al. 2018). Methods such as breakpoint chlorination, precipitation and ion exchange (chemical), ammonia stripping and membrane separation (physical); and nitrification, denitrification (biological) perform poorly in removing AN where high to very high concentration of COD is predominant (Pani et al. 2020).

Thus, few researchers have stressed on use of different advanced oxidation processes such as Fenton oxidation process (Xavier et al. 2015; Gonzalez-Merchan et al. 2016), electrochemical advanced oxidation processes (Candido & Gomes 2011; Zhang et al. 2018; Menon et al. 2021), electro-Fenton in combination with biological process (Li et al. 2018), ultrasonic irradiation (Ozturk & Bal 2015) and photo and electrochemical methods such as electrocoagulation (EC) (Kasmuri & Tarmizi 2018), etc. Electrochemical oxidation processes have seen widespread use over the past two decades and ultrasound technology has gained popularity in recent years as a means of treating wastewater (Hassani et al. 2022). The fundamental problem with the EC method is that a passive layer eventually forms on the surface of the electrode. By generating large concentrations of oxidants to remove the contaminants by generating high-pressure sites during the cavitation process, acoustic waves break the sediments generated (Moradi et al. 2021). By combining such methods, 99 and 62%, respectively, were determined to be the highest COD and colour removal efficacy for the anode-cathode combination of Fe/Fe (Prajapati 2021).

Further improvement in removal has been achieved by optimizing operating parameters using response surface methodology (RSM) technique. It uses regression analysis to predict the value of dependent variable from combination of independent variables (Huda et al. 2017). Various chemical, physical and biological processes for a variety of pollutants have been modelled and optimized using RSM (Asaithambi et al. 2016; Mehdipoor & Moosavirad 2020). A few investigators have applied it in Fenton and electro-Fenton process (Akkaya et al. 2019; Mohammadi et al. 2019a, 2019b).

Fenton oxidation process was effectively applied to remove AN in presence of high COD from dye intermediate wastewater (Pani et al. 2020). The removal efficiency of AN and COD were found to be 75.8% and 78.6% respectively at pH 3 and molar dosage of Fe²⁺ and H₂O₂ of 3:3 for 60 min reaction time. But, removal of 75.8% did not reduce their concentration to acceptable values, thus further work on electrochemical processes such as EF and EC were carried out (Menon et al. 2021).

EC process is one of the most economical technologies for the treatment of industrial wastewater (Syam-Babu et al. 2020). It involves dissolution of sacrificial anodes immersed in wastewater by using current.

The advantages of EC process include no addition of chemicals, easy maintenance, no harmful by-products, etc. (Priya & Jeyanthi 2019; Lu et al. 2022). According to recent studies, combined EC-sonication process is effective in eliminating the disadvantages of EC process like removing the passive film from electrode surface, breaking the flocs in the solution and reducing the particle size thus enhancing the sorption mechanism of EC (Lakshmi & Sivashanmugam 2013; He et al. 2016). According to the literatures reported earlier, only few studies are available in which such highly polluted wastewater is treated by EC-sonication process.

Considering the high removal efficiencies in EC and sonication processes as realized by various researchers for several types of wastewaters. The authors of the current study have explored the efficacy of batch EC process and EC followed by sonication process for the removal of AN and COD from dye intermediate manufacturing wastewater. The performance of EC batch process was evaluated by varying major operating parameters like pH, applied voltage and electrolysis time. The EC followed by sonication process was performed to study influence of sonication on the removal efficiency and to compare the performance of EC and EC followed by sonication process. The sonication process was performed on the optimum condition of EC batch process.

To quantify the influence of process parameters, an RSM was executed to predict the optimum values of major operating parameters on treatment efficiency by using MINITAB (version 19) statistical software and ANOVA.

The dye intermediate wastewater was collected in a 20 L container from a dye intermediate manufacturing industry in Ankleshwar, Gujarat, India. The analysis of all the parameters were carried out according to APHA and IS: 3025 methods. All chemicals and solvents used for analysis and experiment were of analytical grade and of Merck brand. The solution pH was analysed using Bench-top pH meter (HACH, sensION+). COD was analysed by closed reflux method and AN analysis was carried out using automatic nitrogen analyser (KELPLUS CLASSIC-DX). Dual DC power supply (Model: ALE100A) with power supply of 0 ± 30 V/5Amp was used for AN. Sonicator (Model: SM50US) operating at a frequency of 33 kHz and capacity of 5 L with UL power 100 W was used for the sonication process. Post treatment sludge analysis was done to study the morphology and shape of a sample, using scanning election microscope (SEM)/energy dispersive X-ray spectroscopy (EDS) analysis.

All parameters at an optimum condition. The physico-chemical characteristics of the wastewater are shown in Table 1.

The experimental setup of electrocoagulation comprised of a batch reactor with a working volume of 1,000 mL. The experiments were performed using iron electrodes (6*3*0.3 cm) with an inter electrode spacing of 4 cm. The ohmic drop in the reactor results from the ohmic resistance offered by the electrolyte and the spacing between the electrodes, the ohmic resistance is directly proportional to the spacing and inversely proportional to conductivity of the working solution. As the conductivity of the working solution is considerably very high the ohmic resistance offered by the spacing between the electrodes would be insignificant or minimum. Considering aforesaid fact, as well as based on the published articles, authors have considered the spacing between the electrodes as 4 cm in the current study. Electrolysis was conducted in the original conductivity of the dye intermediate wastewater without addition of supporting electrolyte using a digital DC power supply. The solution was mixed during the process with magnetic stirrer at set RPM for homogenisation of the process. The previous studies done by Vanlangendonck et al. (2005), Li & Liu (2009) and Mahvi et al. (2011) found that the optimum pH range for removal of ammonia is 5.5–10. Neutral pH is suggested for AN removal. In strongly alkaline conditions, the oxidation rate of AN decreases. Thus, the experiments were conducted at different pH ranging from 5 to 11, to study the effect of pH. The effectiveness of the process and the expense of the treatment are both influenced by the voltage, which is one of the key parameters in electrochemical methods of treatment. According to earlier study by Pani et al. (2020) and Menon et al. (2021), which used the same effluent, the ideal voltage range is from 1 to 4 V. Beyond 4 V, the power consumption will increase drastically, due to which, this study is only limited to maximum of 4 V. By creating alkaline conditions and changing the pH, metal hydroxides are produced, which increases the efficiency of removing contaminants (Shahedi et al. 2020). Thus, the experiments were conducted at potentiostat mode (constant voltage) varying the voltage from 0.5 to 4 V. The potentiostat mode consists of the application of a constant potential at the working electrode (WE) throughout the electrolysis time. During potentiostat mode current response value changes quite fast, when compared to the potential response during the galvanostatic mode of functioning.

Effect of electrolysis time was studied by varying the time between 0 and 120 min. At optimum conditions (acquired through experiment) of EC process, the sample was subjected to sonication for 60 min.

RSM applies combination of mathematical and statistical techniques for parameter optimization. The most important use of RSM is in those particular situations when several input parameters potentially influence the performance or quality characteristic of the process. Methodology is to fit a polynomial equation to the experimental data, which describes the behaviour of a data set with the objective of making statistical projections (Bezerra et al. 2008). A linear or square polynomial function is employed to describe the system and explore the experimental conditions, finally culminating into parameter optimization (Bezerra et al. 2008).

This section is divided into several subsections which explore the effect of pH, voltage and time on treatment in EC process. Then, treatment parameters are optimized which is followed by comparison between EC and EC-sonication process.

The performance of EC process is controlled by pH of the aqueous medium as the amount of Fe²⁺ and Fe³⁺ formation is influenced by pH. Precipitation of pollutants at lower pH is unfavourable due to production of mono and di-hydroxide complex of iron (Singh & Ramesh 2014; Yavuz & Ögütveren 2018). The speciation of various species of iron with respect to pH (Nidheesh & Singh 2017) is provided in Table 1s. According to Nidheesh & Singh 2017, the various forms of ferrous ion and ferric ion can exist in different pH conditions.

Higher pH (pH > 7) leads to the production of hydroxide ion resulting in more amount of Fe ion generation (Singh & Ramesh 2014). When pH value is increased above 10, due to high hydrogen evolution, the C/C₀ values of COD and AN were increased as shown in Figure 2(a) and 2(b). The oxidation reaction occurs at anode and the water reduction at cathodes which are represented in Equations (1)–(4). When iron electrode is used in EC process, Fe²⁺ is hydrolysed to give Fe(OH)₂ and on oxidation gives Fe³⁺ and then hydrolysed to Fe(OH)₃. The coagulants such as Fe(OH)⁺ and Fe(OH)₂⁺ are formed (Yavuz & Ögütveren 2018) which does not help in adsorption of pollutants.

The species like Fe²⁺, Fe(OH)²⁺, Fe(OH)₂+ and Fe(OH)⁻₃ exists in low pH range. The Fe(OH)₂ and Fe(OH)₃ are the coagulants that helps in COD and AN removal. As the main coagulants are present in low concentration, the C/C₀ values for COD and AN have high values leading to less removal efficiency than the neutral and basic pH (Figure 2(a) and 2(b)) as described elsewhere also (Garcia-Segura et al. 2017; Mamelkina et al. 2019).

At neutral pH, the species like Fe(OH)₃ exists as the principal coagulants in the solution. The pollutants are adsorbed effectively by these species and thehighest removal efficiency is achieved as found elsewhere also (Nidheesh & Singh 2017; Syam-Babu et al. 2020).

Amorphous flocs produced called sweep flocs have larger surface area which is useful for rapid adsorption of pollutants such as COD and AN and permit colloidal material to precipitate (Boudjema et al. 2014; El-Ashtoukhy et al. 2017). Fe⁺³ and OH⁻ ions produced at anode and cathode, respectively, react to form various monomeric species that finally transform into insoluble, stable, and amorphous Fe(OH)₃ through complex polymerization kinetics and the adsorption capacity of Fe flocs towards organic molecules increase in neutral pH and, consequently, the adsorption capacity of ferric flocs towards the organic molecule increases. The degradation of the organic contaminants from water occurs by surface complexation and electrostatic attraction that followed by coagulation (Daneshvar et al. 2006; El-Ashtoukhy et al. 2017).

The effect of major ions such as chloride and sulphate on electrocoagulation process has been analysed. In general, the presence of ions or electrolytes like chloride, sulphates increase the corrosion of metallic species in the anode regime by breaking down the passive anode layer formed by the insulating Ca²⁺ and Mg²⁺ ions present in the wastewater (Anantha-Singh & Ramesh 2014). The presence of high sulphate ions concentration was fairly unfavourable for power utilization and overall process efficacy. Whereas, the sulphate ions can act as an electrolyte that boost the current distribution in the reactor, which has been shown to improve the effectiveness of the process for organic industrial wastewaters treatment (Ghernaout & Ghernaout 2012).

In electrochemical conversion of ammonium to nitrogen gas, chloride plays an integral part. At different pH (5–11), formation of hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻) from chloride ions occurs. Regeneration of Cl⁻ ions takes place through breakpoint chlorination when hypochlorous acid (HOCl) reacts with NH³ or NH⁴⁺. The purpose of the Cl⁻ ions was to act as a catalyst, hence, after the reaction its concentration remains unchanged (Kabdaşlı et al. 2012).

Applied voltage is one of the strong factors affecting the removal of carbon and nitrogen-based pollutants in EC process. The production rate of metal hydroxide flocs increases with applied voltage due to which significant increase in pollutant removal is observed (Khandegar & Saroha 2013).

It can be seen that the minimum fraction remaining (C/C₀) was observed at 2 V. Afterwards, the process efficaciousness decreases with increasing applied voltage. Hence, the efficiency proportionate with current density to a certain extend achieving the optimum removal condition and then saturates to a constant value as reported elsewhere also Mollah et al. 2001; Deghles & Kurt 2016. The rigorous release of metal ions is initiated by dissolution of anodic material at higher current which is governed by Faraday's law. A series of insoluble metallic hydroxide species evolves through hydrolysis process. Hence, more iron ions are generated which successively react with OH⁻ to form iron-based hydroxides. N- species are not adsorbed by monomeric as well as polymeric species of iron such as Fe(OH)⁺, Fe(OH)²⁺, Fe(OH)₂⁺, Fe(H₂O)₅OH²⁺,Fe(H₂O)₄ (OH)₂⁺, Fe(H₂O)₈(OH)₂⁴⁺ which forms at different pH conditions as shown in Fig. 1s. Additionally, due to elevated current density, the hydrogen gas is produced in cathodic surface which enhances the upward flux resulting in more removal efficiency, similar findings have been reported elsewhere also (Izadi et al. 2018; Mohammadi et al. 2019a, 2019b). The production rate of coagulant Fe²⁺ or Fe³⁺, and the size and distribution of bubbles were strongly influenced by current density/applied voltage. By increasing current density above a certain range, the efficiency reduces as the rate of bubble formation and flocs increases. Also, the liberation of H₂ gas resulted in reduced adsorption on the surface of amorphous iron hydroxides (El-Ashtoukhy et al. 2017).

where,

• = Rated power in W

• t = Treatment time (h)

• V = Volume of the wastewater (L)

• M = Change in the concentration of the pollutant (gm/L)

The specific energy consumption was found to be 2.01 kWh/m³ for the COD removal under the EC process. The specific energy of sonication for COD and AN was calculated using Equation (7) and the values were found to be 0.1731 kWhr/kg and 3.8845 kWh/kg, respectively.

The cost of power consumption is driven by the runtime of the reactor (Sen et al. 2019). As the electrolysis time increases, the removal efficiency ascends to a certain extent, giving a significant yield and finally reaches to a constant value (Khandegar & Saroha 2013; Damaraju et al. 2017; Manikandan et al. 2018). As shown in Figures 2 and 3, this value was observed at 90 min experimentally, where the C/C₀ for COD is 0.244 and AN is 0.302 at the following conditions, i.e., voltage of 2 V, pH value of 7, electrode distance of 4 cm (emerging area = 18 cm²).

The release of coagulating species happens during dissolution of anodic material. With increase in electrolysis period, the production of the metal ion and the hydroxide flocs also increase. For a longer electrolysis time, there is an increase in generation of flocs and beyond the optimum electrolysis time, the flocs are not available; as a result, there is no further increase in removal efficiency, as found elsewhere (Aoudj et al. 2010; Manikandan et al. 2018).

An increase occurs in the concentration of ions and their hydroxide flocs occurs when the electrolysis period is increased. A change in colour of the dye solution from dark green to yellow brown indicates the increase in concentration of Fe(OH)₃. This formation of flocs due to spontaneous discharge of ferric ions further forms ferric hydroxide (brown flocs) (Ghernaout et al. 2008).

Sonication process generally works in the frequency 20 kHz–2 MHz. Application of sonication is found to be unproductive alone for removal or degradation of pollutants. Several studies reveal that on integrating sonication with other electrochemical process leads to good results in removal of pollutants (Gągol et al. 2018). Sonochemistry works through both the physical and chemical effect. The chemical effect through OH^. and physical effect through cavitation increases the treatment efficiency (Nidheesh et al. 2021).

The enhancement of removal efficiency in combined electrocoagulation and sonication process is due to generation of •OH, H• and HO₂• radicals by the ultrasound energy which are responsible for the degradation of organic pollutants (Kovatcheva & Parlapanski 1999; Chu et al. 2012; Raschitor et al. 2014; He et al. 2016; Al-Qodah et al. 2018; Menon et al. 2021.

• In this work, EC and EC-sonication combined was investigated for the removal of ammoniacal nitrogen and COD from dye intermediate wastewater.

• A set of experiment, by varying pH between 5 and 11, applied voltage from 0.5 to 4 V for an electrode area of 18 cm² and electrolysis time up to 120 min was performed. The minimum fraction remaining (C/C₀) were 0.244 and 0.302, respectively, for COD and AN, at electrolysis time of 90 min, pH 7 and applied voltage of 2 V.

• ANOVA and response surface methodology were used for estimating optimum conditions for achieving maximum removal efficiency.

• The optimal values of pH, time and voltage was ∼8.2, ∼90.9 min and ∼2.6 V, respectively, for AN and ∼7.6, ∼100.9 min and ∼2.6 V respectively for COD removal.

• In addition, experiments with combination of two processes: electrocoagulation and sonication, for the removal of AN and COD from dye intermediate wastewater was also performed.

• The removal efficiency was inferred using such experimental data only. The C/C₀ for COD and AN after EC-sonication process were 0.145 and 0.228, respectively, with a post EC-sonication time 60 min at a frequency of 33 kHz.

• The specific energy consumption for EC process was 2.01 kWh/m³ and for sonication process for COD and AN was 0.1731 kWh/kg and 3.8845 kWh/kg, respectively.

• The optimum cost of the process depends on the applied voltage, electrolysis time, cost of the reactor, cost of electrodes, mixing cost, etc. for the electrocoagulation process. The operating cost of the process is computed to be 0.711 USD/m³ of the wastewater treatment.

• It can be concluded that the application of both the processes, i.e., EC and sonication process, is effective for removing COD and AN from dye intermediate manufacturing wastewater. The sludge has been characterised and the metallic elements with the abundance of C > O > N > Cl > Cu > Fe > Al was observed.

• As the dye wastewater is highly concentrated with some traces of heavy metals, the recovery/treatment of heavy metals can be suggested. Further detailed characterization and disposal of sludge can be extended for future studies.

We would like to convey our gratification to Department of Science and Technology, Govt. of India for their support and financial assistance (Reference No- SR/WOS-A/ET-89/2017).

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

The authors declare there is no conflict.