Tannery wastewater is a complex mixture of organic and inorganic components from various processes with high concentrations of Cr, BOD, COD, TDS, strong colour and pH. The goal of the present work was to determine the optimal solar photo-assisted electrochemical peroxidation process (SPECP) experimental conditions for the treatment of tannery wastewater. Experiments were conducted in a bench-scale stirred tank SPECP reactor of 5 L with iron plates as anode and cathode in a contact effective area of 218 cm2. In the stirred tank reactor at optimal conditions, the SPECP yielded 97% COD, 98% of colour and 92% of chromium (III) removal after 60 min at 15 mA/cm2. SPECP improved the biodegradability (BOD5/COD) of tannery wastewater from 0.4 to 0.6 in 15 min. These results showed that wastewater from tannery industries could be treated up to the level of the minimal national standards of India for waste disposal; COD=90 mg/L, BOD=30 mg/L and chromium (III)=1.2 mg/L at a treatment time of 60 min. The operating cost of the best economic condition with maximum degradation was $8.2/m3.

  • SPECP using iron electrodes was very effective for the treatment of tannery wastewater.

  • Effect of process parameters was investigated with CCD design.

  • The closeness of experimental & CCD predicated shows great model roundness.

  • 97% of COD, 96% of BOD, 98% of colour and 92% of Cr (III) removal efficiency was obtained.

  • Less energy consumption and higher degradation efficiency provided SPECP as a cost-effective approach.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Tanneries are known to generate widely varying as well as high strengths of wastewater which are largely characterized as pollution-intensive industrial complexes. In general, the presence of chemicals in tannery wastewater is mostly derivatives of tannins. Characterized by their complex nature of structures, tannins are comprised of different types of substances consisting of formaldehyde, melamine-based syntans, acrylic resins and naphthalene. Tannins BOD5/COD ratio is lower than other complexes. However, it is worth noting that none of these complexes showed a BOD5/COD ratio greater than 0.4, showing a very low biodegradability for each of them (Selvabharathi et al. 2016). Several treatment methods were developed to remove organic matters in that the activated sludge process is a commonly used method in India (CLRI, 2012). However, this method seems to have lower removal efficiency due to the presence of complex organics and pollutant concentrations with lower biodegradability of organics existing in tannery wastewater, which signifies a major technological challenge to maintain a safer environment while (Natarajan & Manivasagan 2018) discharging the wastewater.

Due to the implementation of stringent regulations for industrial wastewater discharge to avoid environmental pollution, research has been enforced to develop a new technique and to improve the efficiency of available advanced techniques. The Fenton process is identified as an effective treatment method by many researchers for decreasing obstinate impurities in water. However, the conventional Fenton process has issues such as the economic factor associated with H2O2 usage, consumption of catalyst and disposal of sludge produced during processes which limit the scale-up operation Fenton process. For commercial application, the Fenton reaction is enhanced by using solar energy and electrochemical technology which develop a synergistic reaction. After the primary treatment of tannery wastewater, the biodegradability was 0.40. This signifies the presence of non-biodegradable substances in the wastewater thus leading to further biological treatment. Even after the completion of full treatment, the treated water did not satisfy the minimal national standards of India for discharge (MINAS 2016). Therefore, it is very important to effectively remove the toxicity of the wastewater discharging from the tannery industry to avoid environmental pollution. In order to meet increasingly stringent discharge limits and to protect the environment and economy, the removal of pollutants from tannery wastewaters has become necessary. Therefore, researchers have focused on the development of emerging technologies for complementing and/or even replacing the existing lower-efficiency processes. Electrochemical Advanced Oxidation Processes (EAOP) are advanced methods to clear away organic pollutants from tannery wastewater; however, the usage of chemicals and their biodegradable nature would be the major concern of this EAOP process usage. The EAOPs highly recommend preventing and remediating environmental pollution, especially focusing on water streams (Sires et al. 2014, Brillas et al. 2009). From the literature review, the vast majority of the EAOPs utilize a combination of oxidants and light (O3/H2O2/UV), reagents and illumination (Fe2+/H2O2; UV/TiO2). The use of electrochemical principles for treating wastewater in turn oxidizes and produces OH inside the wastewater. In recent times, EAOPs have been used as an emerging technique and along with this, the addition of solar energy to EAOPs would be a powerful technology in order to remove complex organic and inorganic pollutants (Galve et al. 2021). Regardless, solar-based illumination causes a reduction of most of the environmental effects. The combined process of EAOP and solar energy produces hydroxyl radicals which help to remove strong powerful oxidizing agents. The reaction between the OH and organic compound R is summarized in Equations (1)–(4). EAOPs, iron-based procedures and photo activity could be used for oxidative and reductive procedures and are suitable to decrease the organic compounds, while Fe2+/H2O2 is an appropriate technique to produce OH radicals (Marinho et al. 2018):
formula
(1)
formula
(2)
formula
(3)
formula
(4)
OH is an oxidizing agent used by EAOPs to drive contaminants decomposition in wastewater. OH can be created at the electrode, in the direct electro-oxidation methods or in bulk solution and leads to degradation of the pollutants in industry wastewater (Devi et al. 2018). The ECP process can be combined with illumination by UVA light or solar energy and is called UVA Photo Electro Chemical Peroxidation (PECP) or Solar Photo-Assisted Electrochemical Peroxidation (SPECP) processes (Li et al. 2018). Solar energy is the cleanest of all existing energy resources:
formula
(5)
formula
(6)

The SPECP process is environmentally friendly and the utilization of sunlight could be beneficial to initiate the photo-Fenton process electrochemically, thereby decreasing the consumption of electric current. In the SPECP method, OH can be attained in different ways. Solar energy illumination could improve the adequacy of numerous oxidants delivered in the vicinity of the electrode surface, either by direct electron transfers on the electrode surface or by the activity of hydroxyl radicals. The photo activation produces H2O2 or Fe2+ in the electrochemical reaction (Jain et al. 2018, Abdessalem et al. 2010). Electro generation of H2O2 occurs at the cathode surface as in Equation (5). Electro-generated H2O2 and external addition of Fe2+ enhance the production of OH radicals and this Fenton reaction is given in Equation (6). Moreover, the photolysis of Fe(OH)2+ and the particular Fe3+ species at pH 3.5 (approximately.) produces extra Fe2+ and delivers more OH and the mechanism is given in Equation (5). The synergistic reaction of oxidants, such as solar, Fenton, hydrogen peroxide and electrode, are prompting the creation of OH and deterioration of hydrogen peroxide into hydroxyl radicals. Because of the low solubility of iron species, it can be hinged with peroxidation and various procedures can be adapted such as electrochemical peroxidation and the solar photo-Fenton process (Brillas 2020). The SPECP process has been applied to treat various wastewaters and the operational efficiency of this process is listed in Table 1. Very limited literature is available on the possible application of the EAOP process to remove pollutants from industrial wastewater with the determination of electrical energy consumption performance analysis (pollutant removal efficiency) of SPECP process for tannery wastewater not been reported (Mirra et al. 2020). Hence an attempt has been initiated to study the SPECP process on colour and COD removal efficiency with electrical energy consumption from the tannery wastewater.

Table 1

Operational parameters and removal efficiencies of various EAOP process from various wastewater

Type of wastewaterInitial COD (mg/L)Operational Parameters
Removal (%)References
pHH2O2Current densityTime
Landfill leachate 5,500 300 mg/L 0.30 A/dm2 240 min 97 Asaithambi et al. (2020)  
Dyes wastewater 7,500 0.50 MM 66.7 mA/cm2 360 min 85 Florenza et al. (2014)  
Industrial wastewater 8,000 3.3 g/L 90 mA/cm2 240 min 65 Salmerón et al. (2020)  
Ponceau SS dye 300 0.150 mM 33.3 mA/cm2 360 min 85 Santos et al. (2020)  
Textile dye wastewater CSA, PANI-WO 3-Rgo – 130 mg/L 1.54 mA/cm2 300 min 60 Hosseini et al. (2020)  
Tannery lime wastewater 7,475 – – 25 mA/cm2 240 min 92 Selvaraj et al. (2020)  
Sunflower oil refinery wastewater 6,500 6.5 – 11.56 400 min 90 Sharma et al. (2020)  
Sugar beet industry process wastewater 13,500 5.3 21 mL/L 48.5 400 65 Sharma et al. (2020)  
Canola-oil refinery effluent 6,500 150 mg/L 13.66 7 h 85 Sharma et al. (2019)  
Tannery wastewater 3,000 3 0.015 g/L 15 mA/cm2 60 min 97 Present study 
Type of wastewaterInitial COD (mg/L)Operational Parameters
Removal (%)References
pHH2O2Current densityTime
Landfill leachate 5,500 300 mg/L 0.30 A/dm2 240 min 97 Asaithambi et al. (2020)  
Dyes wastewater 7,500 0.50 MM 66.7 mA/cm2 360 min 85 Florenza et al. (2014)  
Industrial wastewater 8,000 3.3 g/L 90 mA/cm2 240 min 65 Salmerón et al. (2020)  
Ponceau SS dye 300 0.150 mM 33.3 mA/cm2 360 min 85 Santos et al. (2020)  
Textile dye wastewater CSA, PANI-WO 3-Rgo – 130 mg/L 1.54 mA/cm2 300 min 60 Hosseini et al. (2020)  
Tannery lime wastewater 7,475 – – 25 mA/cm2 240 min 92 Selvaraj et al. (2020)  
Sunflower oil refinery wastewater 6,500 6.5 – 11.56 400 min 90 Sharma et al. (2020)  
Sugar beet industry process wastewater 13,500 5.3 21 mL/L 48.5 400 65 Sharma et al. (2020)  
Canola-oil refinery effluent 6,500 150 mg/L 13.66 7 h 85 Sharma et al. (2019)  
Tannery wastewater 3,000 3 0.015 g/L 15 mA/cm2 60 min 97 Present study 

The influence of the various operational parameters such as pH, current density, H2O2 dosage and Fe2+ dosage to maximize the colour and COD removal efficiency with minimum electrical energy consumption using the SPECP process by using response surface methodology (RSM) was investigated. In addition, to study the reaction kinetics and the synergetic effect on the rate of degradation for the SPECP processes and cost analysis were evaluated.

Collection and characterization of tannery wastewater

Tannery industry wastewater WAS collected from a small tannery industry near Dindigul, Tamil Nadu, India. The sample was collected after the plain sedimentation process and it was stored in a laboratory at 4 °C. All samples were analyzed to determine the physicochemical characteristics of tannery wastewater as per standards methods (APHA 2012). The physicochemical characteristics of tannery wastewater were pH = 7.5, COD = 2,500 mg/L, colour = dark brown, BOD = 985 mg/L, total solids = 15,250 mg/L, chromium (III) = 16 mg/L and electrical conductivity = 17,840 μs/cm.

Chemical reagents

Ferrous sulphate heptahydrate [FeSO4·7H2O] reagent grade and hydrogen peroxide (H2O2) (30%, w/v) were used to perform the experiment while sulphuric acid (H2SO4) and sodium hydroxide (NaOH) were used for pH adjustment. The COD analysis involved the usage of sodium thiosulphate (Na2SO3), potassium dichromate (K2Cr2O7), mercuric sulphate (HgSO4) and ferrous ammonium sulphate [Fe (NH4)2(SO4)·6 H2O]. Sodium sulphate (Na2SO3) acted as a quencher for the reaction; all other reagents were procured from Merck (India) and commercial TiO2 (P25) was purchased from Degussa (Germany). P25 has specifications of 30 nm of an elementary particle size, 50 m2/g of Brunauer–Emmett–Teller (BET) specific surface area and crystalline mode comprises of 80% anatase and 20% rutile. Deionized water was utilized for various concentrations of solution preparation, and all the investigations were carried out at room temperature.

SPECP reactor and procedures

All the experimentations were carried out on Anna University Campus, Thoothukudi, India (8°44′N, 77°44′E). Experiments were conducted in a bench-scale stirred tank SPECP reactor of 5 L with iron plates as anode and cathode in a contact effective area of 218 cm2. A photographic view of the EAOP experimental reactor is shown in Figure 1. The electrodes were cleaned between the successive runs using emery paper (No.P320) and further washed using H2SO4 solution (5% v/v) in order to reduce the effects on successive experiments. The reactor was engraved with grooves for keeping a fixed distance of 6 cm between the electrodes.
Figure 1

Photograph of SPECP reactor.

Figure 1

Photograph of SPECP reactor.

Close modal

The SPECP experiments in this study were carried out using iron plates as anode and cathode. To provide the desired current, the electrodes were connected to a DC power supply (0–30 V, 0–5 A) for the process. The reactor was exposed to solar energy and the necessary power supply was acquired from a solar panel for the SPECP process. Thus, an electrolytic cell was built based on iron plates, and the SPCEP reaction was conducted by the H2O2 concentrations added, as well as the iron ion dissolved by its own deterioration due to the electrochemical process. The initial pH of the solution was regulated to the desired values using concentrated sulphuric acid or sodium hydroxide. In order to activate Fenton's reaction, a pre-decided quantity of ferrous sulphate heptahydrate and hydrogen peroxide were added into the reactor during each run, before the electrical current was turned on. Samples of 5 mL volume were taken out every 10 min and immediately the pH was modified by using NaOH and sodium sulphite, which quenches the generation of OH thereby stopping the degradation process and thus allowing the remaining iron to precipitate. The samples were given 30 min settling time to bring about coagulation and the supernatant was then taken for quality measurements (APHA 2012).

Central composite rotational design

RSM is a statistical mathematical model employed to optimize the process variables in recent times of wastewater treatment studies and it is applied for the design of experiments and optimization using Stat-Ease Design Expert® version 8.0.7.1. Optimization of process variables is quite challenging and also time consuming and it is limited to multivariable systems and is expensive. The RSM model was used as an effective tool for optimization of process independent variables to obtain the desired response and it can be used to study the combined effect of individual process variables on the response (Davarnejad & Nasiri 2017). The following second-order mode (Equation (7)) attains the correlations between the responses and the independent variables:
formula
(7)
where Y is the response, β0 is a constant coefficient; βj, βij and βjj are the coefficients for the linear, quadratic and interaction effects respectively. Xi and Xj are the coded levels for the independent variables and k is the number of independent variables.

CCD was designed using the rotatable experimental plan for an electrochemical peroxidation process. For four variables (n = 4) and their factor levels (low (–), middle (0) and high (+)), the total number of experiments was analyzed by the expression: 2n (24 = 16: factor points) + 2n (2 × 4 = 8: axial points) +6 (centre points: six replications) which amounts to a total of 30 experiments (Thirugnanasambandham & Sivakumar, 2014). It was implemented to establish the association among the SPECP process responses (COD, colour removals and electrical energy consumption) with the most important variables, i.e. factors pH, Fe2+ dosage, H2O2 dosage and current density. The second-order model in Equation (7) accomplishes the relationships between the independent factors and the responses. The range and levels of CCD were executed to examine the effects of four independent operational variable conditions: pH (2–6), Fe2+ dosage (0–0.005 g/L), H2O2 dosage (0–0.02 g/L) and current density (10–30 mA/cm2) on the responses Y1, Y2 and Y3 [% of colour removal (Y1), % of COD removal (Y2) and electrical energy consumption (Y3), respectively].

Analysis

Removal efficiency

The colour and COD removal efficiency was calculated using Equations (8) and (9):
formula
(8)
where A and At are the absorbances of tannery wastewater before and after the treatment process at the corresponding wave length (λmax = 465 nm):
formula
(9)
where COD and CODt are the Chemical Oxygen Demand (COD) of tannery wastewater before and after the treatment process.

Electrical energy consumption

The performance of EAOP is estimated based on the electrical energy consumption. The energy consumption in kWh/L during COD removal in time t was calculated using Equation (10):
formula
(10)
where electrical energy consumption is in kWh/L, V is cell voltage in volt (v), I is current in ampere (A), t is time (h) and Vs is the volume of solution (L).

The combination of solar photo-Fenton and electrochemical peroxidation process is called the SPECP process. The SPECP process conducted with real industrial wastewaters is limited, and if considering its combination with a solar light source, the number of researches published is rare. This fact has slowed down the development of light-assisted electrochemical systems at the industrial level. Moreover, it would be strongly recommended to use, whenever possible, a renewable resource such as solar energy as a light source instead of artificial lamps, particularly in a tropical country like India. The main objective was to study the performance of solar-assisted electrochemical processes by treating tannery wastewaters in order to demonstrate that a higher concentration of photoactive radiation implies a higher performance of the system and the requirement of lower electrical energy consumption to achieve target treatment objectives.

The CCD design of the SPECP process optimization includes the response of the statistically important combinations, estimation of various coefficients and fitting data to the response. To analyse the influence of operational parameters on the colour and COD removal with electrical energy consumption of tannery wastewater, pH, Fe2+ concentration, H2O2 concentration and current density were chosen as independent variables and analysis of variance (ANOVA) was conducted.

Statistical experimental design and modelling using RSM

The accuracy of the polynomial model was explicated by the factor of determination (R2). After conducting the preliminary runs, the range of the parameters was fixed. In this study, four independent variables, namely pH (2–6), Fe2+ (0–0.005 g/L), H2O2 (0–0.02 g/L) and current density (10–30 mA/cm2) were studied. A set of 30 experiments was designed to enhance the condition for the colour and COD removal efficiencies of tannery wastewater by the SPECP process. The following regression Equations (11)–(13) present the CCD model of COD and colour and EEC:
formula
(11)
formula
(12)
formula
(13)
where A, B, C and D are pH, Fe2+, H2O2 and current density, respectively. It is seen in Equations (11)–(13) that in the case of COD removal, the independent variables B, D and the interaction variables AB, AC, AD are significant model terms. Other model terms are not significant, as the probability value is larger than 0.05. The degradation efficiency increases with increasing Fe2+, as can be inferred by the positive linear coefficient. However, based on the negative coefficients, an inhibitory effect is observed with the values of pH, H2O2 and current density. For colour removal, the test variables, i.e. B, D, AB, AD, BC, BD, are the important model terms and the remaining insignificant model terms were eliminated. Similarly, for colour removal the test variables, i.e. B, BC, AC, CD, are the important model terms and the remaining insignificant model terms were eliminated. It was examined that the highest COD and colour removal was obtained for the optimum conditions at pH = 3, Fe2+ = 0.002 g/L, H2O2 = 0.015 g/L and current density = 15 mA/cm2. The experimental design matrix by CCRD (SPECP) is listed in Table 2.
Table 2

Experimental deign matrix by CCD (SPECP)

RunFactor 1Factor 2Factor 3Factor 4Responses
PhFe2+ (g/L)H2O2 (g/L)Current density (mA/cm2)Colour removal (%)COD removal (%)EEC (kWh/L)
0.025 0.01 20 95.95 95.95 0.0142 
30 59.19 60.66 0.0113 
0.025 20 97.15 92.8 0.0149 
0.05 10 60.67 58.45 0.0142 
0.025 0.01 20 96.07 95.44 0.0145 
0.025 0.01 20 97.03 95.34 0.0143 
0.025 0.01 20 96.04 95.57 0.0134 
0.05 0.02 30 75.83 76.83 0.0146 
0.025 0.01 30 91.42 85.04 0.0163 
10 0.05 30 80.06 79.32 0.0146 
11 0.05 30 59.37 57.7 0.0175 
12 0.02 30 60.66 68.34 0.0176 
13 0.02 10 48.53 50.12 0.0189 
14 0.025 0.02 20 97.49 94.41 0.0158 
15 0.025 0.01 20 96.8 96.27 0.0134 
16 0.025 0.01 10 84.77 75.82 0.015 
17 10 57.31 53.72 0.0152 
18 0.05 0.02 10 57.7 55.88 0.0098 
19 0.025 0.01 20 70.65 83.26 0.0055 
20 0.025 0.01 20 96.95 96.54 0.0142 
21 30 60.01 58.51 0.0162 
22 0.025 0.01 20 73.94 91.48 0.0047 
23 10 44.03 41.62 0.0082 
24 0.05 0.02 30 55.93 53.17 0.0093 
25 0.05 0.02 10 64.56 62.29 0.0106 
26 0.02 30 61.7 62.11 0.0159 
27 0.02 10 61.57 59.85 0.0195 
28 0.01 20 82.61 82.92 0.0179 
29 0.05 0.01 20 91.82 87.1 0.0147 
30 0.05 10 65.37 65.55 0.0084 
RunFactor 1Factor 2Factor 3Factor 4Responses
PhFe2+ (g/L)H2O2 (g/L)Current density (mA/cm2)Colour removal (%)COD removal (%)EEC (kWh/L)
0.025 0.01 20 95.95 95.95 0.0142 
30 59.19 60.66 0.0113 
0.025 20 97.15 92.8 0.0149 
0.05 10 60.67 58.45 0.0142 
0.025 0.01 20 96.07 95.44 0.0145 
0.025 0.01 20 97.03 95.34 0.0143 
0.025 0.01 20 96.04 95.57 0.0134 
0.05 0.02 30 75.83 76.83 0.0146 
0.025 0.01 30 91.42 85.04 0.0163 
10 0.05 30 80.06 79.32 0.0146 
11 0.05 30 59.37 57.7 0.0175 
12 0.02 30 60.66 68.34 0.0176 
13 0.02 10 48.53 50.12 0.0189 
14 0.025 0.02 20 97.49 94.41 0.0158 
15 0.025 0.01 20 96.8 96.27 0.0134 
16 0.025 0.01 10 84.77 75.82 0.015 
17 10 57.31 53.72 0.0152 
18 0.05 0.02 10 57.7 55.88 0.0098 
19 0.025 0.01 20 70.65 83.26 0.0055 
20 0.025 0.01 20 96.95 96.54 0.0142 
21 30 60.01 58.51 0.0162 
22 0.025 0.01 20 73.94 91.48 0.0047 
23 10 44.03 41.62 0.0082 
24 0.05 0.02 30 55.93 53.17 0.0093 
25 0.05 0.02 10 64.56 62.29 0.0106 
26 0.02 30 61.7 62.11 0.0159 
27 0.02 10 61.57 59.85 0.0195 
28 0.01 20 82.61 82.92 0.0179 
29 0.05 0.01 20 91.82 87.1 0.0147 
30 0.05 10 65.37 65.55 0.0084 

ANOVA analysis and adequacy of the quadratic models

The adequacy of the developed regression models is evaluated using ANOVA. It produces statistics such as F-, p- and R2-values for comparing the models. ANOVA was carried out and the results are shown in Table 3. The goodness of fit for the model is repeated by the examined coefficient (R). When R2 is close to 1, the model can predicate the response and high correlation coefficients R2>0.9986, R2 adjusted>0.9973 and R2 predicted >0.9945 for all responses as shown in Table 2 (Gholikandi & Kazemirad 2018). Since the prob>F-values from the ANOVA were less than 0.05, the model was deliberated as statistically important. All developed models have values of Prob>F<0.0001. The Fischer's F-statistics values (F values) were 773.27 for colour removal, 839.29 for COD removal and 63.10 for electrical energy consumption. The larger F values and smaller value of p for all responses revealed that the regression model might clarify most of the variation in the responses.

Table 3

ANOVA results for the quadratic models

S. No.SourceColour removal (%)COD removal (%)EEC (kWh/L)
Standard deviation 0.90 0.86 0.0006645 
Mean 74.71 59.90 0.014 
R-squared 0.9986 0.9987 0.9833 
Adj R-squared 0.9973 0.9975 0.9677 
Pred R-squared 0.9945 0.9934 0.9320 
PRESS 48.49 57.93 0.00002696 
Coefficient of variance (CV%) 1.21 1.16 4.86 
Adequate precision 85.240 88.074 30.77 
F-Value 773.27 839.29 63.10 
10 Prob>F <0.0001 <0.0001 <0.0001 
S. No.SourceColour removal (%)COD removal (%)EEC (kWh/L)
Standard deviation 0.90 0.86 0.0006645 
Mean 74.71 59.90 0.014 
R-squared 0.9986 0.9987 0.9833 
Adj R-squared 0.9973 0.9975 0.9677 
Pred R-squared 0.9945 0.9934 0.9320 
PRESS 48.49 57.93 0.00002696 
Coefficient of variance (CV%) 1.21 1.16 4.86 
Adequate precision 85.240 88.074 30.77 
F-Value 773.27 839.29 63.10 
10 Prob>F <0.0001 <0.0001 <0.0001 

From the 3D surface plots, two variables varying within the investigational ranges and one factor kept at constant were strategized and utilized to evaluate the interaction effect between the variables and response for SPECP process treatment of tannery wastewater. Figures 2,34 represent the 3D response surface plot and the percentage effects of each factor for 30 experimental runs. This also proved that the RSM methodology was applicable for optimizing the operational parameters of degradation of tannery wastewater treatment.
Figure 2

Three-dimensional contour plots illustrating the colour removal on SPECP Process: (a) Fe2+ and pH, (b) H2O2 and pH, (c) current density and pH, (d) H2O2 and Fe2+, (e) current density and Fe2+, (f) current density and H2O2.

Figure 2

Three-dimensional contour plots illustrating the colour removal on SPECP Process: (a) Fe2+ and pH, (b) H2O2 and pH, (c) current density and pH, (d) H2O2 and Fe2+, (e) current density and Fe2+, (f) current density and H2O2.

Close modal
Figure 3

Three-dimensional contour plots illustrating the COD removal on SPECP Process: (a) Fe2+ and pH, (b) H2O2 and pH, (c) current density and pH, (d) H2O2 and Fe2+, (e) current density and Fe2+, (f) current density and H2O2.

Figure 3

Three-dimensional contour plots illustrating the COD removal on SPECP Process: (a) Fe2+ and pH, (b) H2O2 and pH, (c) current density and pH, (d) H2O2 and Fe2+, (e) current density and Fe2+, (f) current density and H2O2.

Close modal
Figure 4

Three-dimensional contour plots illustrating the EEC on SPECP Process: (a) Fe2+ and pH, (b) H2O2 and pH, (c) current density and pH, (d) H2O2 and Fe2+, (e) current density and Fe2+, (f) current density and H2O2.

Figure 4

Three-dimensional contour plots illustrating the EEC on SPECP Process: (a) Fe2+ and pH, (b) H2O2 and pH, (c) current density and pH, (d) H2O2 and Fe2+, (e) current density and Fe2+, (f) current density and H2O2.

Close modal

Interactive effect of pH on COD removal

The performance of the SPECP process for the degradation and decolourization of tannery wastewater was examined at pH 2–6. At initial pH of 2–6 at Fe2+ = 0.002 g/L, H2O2 = 0.015 g/L, current density = 15 mA/cm2 and reaction time of 60 min. The results were obtained from the 3D response surface plots as shown in Figures 2,34. According to the 3D response surface plot, pH is the influential parameter for the colour, COD removal and EEC and the acquired outcome demonstrates that the pH value influences the generation of OH and ferrous ions concentrations. The rising in the removal efficiencies at the range of 2–6 was due to the occurrence of Fenton's reaction where the OH reacts with organic pollutants and degrades them (Ozyonar & Karagozoglu 2015; Suarez-Escobar et al. 2016). It can be clearly seen from the figures that the effective process in the acidic range and the oxidation capability of OH was high and it helps to remove the pollutants (Steter et al. 2018). Therefore, the significant generation of free radicals at acidic conditions improved the oxidation of organics such that the highest removal efficiency of 96% COD and 98% colour with the electrical energy consumption of 0.0014 kWh/L was achieved at pH = 3 after a treatment time of 60 min.

For pH 4, 5 and 6, the concentration of COD was found to decrease from the initial value of 2,400 mg/L to 1,350, 1,050 and 840 mg/L respectively after a treatment time of 60 min. The attained outcome demonstrates that the pH value influences the generation of hydroxyl radicals and ferrous ions concentrations (Gholikandi & Kazemirad 2018). It was noticed that an increase in the pH beyond 3 caused a decrease in removal efficiency owing to the development of Fe(OH)+, which results in decreased OH production due to a lesser reaction rate (Moussavi et al. 2014). Similarly, the production of OH was inhibited due to the conversion of H2O2 into ineffective H3O2+ (Equation (14)):
formula
(14)

In pH values lower than 3, the presence of Fe-complexes such as [Fe(H2O)6]2+ react slowly with hydrogen peroxide. The effects showed that the pH influences the degradation significantly and the highest colour and COD removal were noticed at pH 3. Moreover, in the presence of high H+ concentrations, hydrogen peroxide would decompose and convert into the stable species (H3O2+) which reacts with Fe2+ slower than hydrogen peroxide (Gholikandi & Kazemirad 2018). Similar results were found in previous studies and the optimum pH was 2–4 in SPECP (Gozzi et al. 2016). Pekey (2016) employed ECP to remove colour and TOC from co-complex dye and achieved the highest colour and TOC at optimum pH 3.

Moreover, at high pH (>7) the iron ions are generated as ferric ions in a colloidal form which promotes the production of ferric hydroxo complexes. In addition, ferric ions could form Fe(OH)3 which will not react with H2O2 and consequently the removal efficiency decreased. From the results and observations, it seems that at pH values higher than 6, the COD removal concentration decreased to 840 mg/L in 60 min. As discussed above, high efficiency was at pH 3 and, such findings agree with the results of previous research (Kokkali 2011; Pekey 2016). The highest colour and COD removal efficiency with minimal electrical energy consumption was obtained as 98 and 96%, respectively at pH 3 with 0.014 kWh/L of energy consumption and a reaction time of 60 min. The degradation profile at different pH concentrations and kinetics is represented in Figure 5.
Figure 5

Effect of pH on colour, COD removal and EEC.

Figure 5

Effect of pH on colour, COD removal and EEC.

Close modal

Interactive effect of current density on COD removal

The performance of the SPECP process for the degradation and decolourization of tannery wastewater was investigated at a current density of 10–30 mA/cm2, pH = 3, Fe2+ = 0.002 g/L, H2O2 = 0.015 g/L and reaction time of 60 min. The results were obtained from the 3D response surface plots as shown in Figures 2,34. From the 3D response surface plot, the current density is the influential parameter for the colour, COD removal and EEC. The acquired outcome demonstrates that current density influences the generation of hydroxyl radicals and ferrous ions concentrations. In the SPECP process, the current density is one of the most important parameters for controlling the reaction rate within the SPECP reactor since its value proves to be decisive in determining operational cost and process efficiency, and it affects the H2O2 production, Fe2+ reduction and OH production. As stated in the literature, the current density is a vital parameter of upgrading the SPECP process achieved since it demands high electrical energy (Asaithambi et al. 2020). When the more current density is delivered to the electrochemical reactor, more Fe2+ ions are dissolved from the sacrificial iron anodes. The current density leads to faster Fe2+ regeneration (Equation (15)) and increases the efficiency of Fenton chain reactions:
formula
(15)
Since a suitable amount of Fe2+ in the reactor propagates to the Fenton's reaction, this is the factor that will have the largest impact on the efficiency of the SPECP process. The amount of COD, colour and chromium reduction increased with reaction time and was directly proportional to the current density. As shown in Figure 6, when the current density was raised from 10 to 20 mA/cm2, colour, COD and colour removal efficiency increased from 61 to 81% and 68 to 83%, respectively, and minimum EEC was found to decrease from 0.014 to 0.013 kWh/L with the reaction time of 60 min. When current density increases from 20 to 30 mA/cm2, it shows that the COD and colour removal efficiency decreased and EEC increased which may be owing to the oxygen discharge on the anode (Equation (16)) and the discharge of H+ on the cathode (Equation (17)) (Nidheesh & Gandhimathi 2012). The Fenton reactions are significantly inhibited beyond a current density of 20 mA/cm2:
formula
(16)
formula
(17)
Figure 6

Effect of current density on colour, COD removal and EEC.

Figure 6

Effect of current density on colour, COD removal and EEC.

Close modal

Additionally, current density plays a critical role in the SPECP process because it influences the production of H2O2, the reduction of Fe2+, and the production of OH. It is one of the most critical parameters for controlling the reaction rate within the SPECP reactor because its value is crucial in determining the operational expenses and the validity of the system. Importantly, the SPECP procedure uses the applied current density to generate oxidized iron (Fe2+) at the anode, which also stimulates oxygen reduction at the cathode to generate H2O2 (Jegadeesan et al. 2021). This intensification in removal efficiency can be explained as an increase in electrical current between the iron electrodes increases the dissolution of anode and metal species is formed. The current density is the dynamic force for the decrease of oxygen leading to the production of H2O2 at the cathode. The current density is related to H2O2, and accordingly raises the quantity of OH formed which directly depends on the current density supplied to the reactor (Zhang et al. 2006). Figures 2,34 depict the 3D response for the 30 runs derived using CCD. Approximately 10–20% of reactive yellow 160 azo dyes pollutant removal performance using EAOPs in a previous study was reported using UV irradiation from tannery wastewater. As a result, the optimal current density of 15 mA/cm2 for wastewater was found to be suitable for the SPECP process. The degradation profile at different current density concentration and kinetics is described in Figure 6.

Interactive effect of H2O2 on COD removal

The performance of the SPECP process for the degradation and decolourization of tannery wastewater was explored at H2O2 dosage of 0–0.02 g/L, pH = 3, Fe2+ = 0.002 g/L, current density = 15 mA/cm2 and reaction time of 60 min. The results were obtained from the 3D response surface plots as shown in Figures 2,34. According to the 3D response surface plot, the H2O2 dosage is the important parameter for the colour, COD removal and electrical energy consumption. The acquired outcome demonstrates that H2O2 dosage influences the generation of hydroxyl radicals and ferrous ions concentrations.

Controlling the reaction rate within the SPECP reactor creates an impact on determining operational cost and process efficiency, and it affects the OH production. The influence of H2O2 concentration on the degradation of tannery wastewater was studied in the range of 0–0.02 g/L. The effects attained after the treatment time of 60 min are shown in the figures and it can be seen that an increase in H2O2 concentration from 0 to 0.015 g/L increased the colour and COD removal, along with the corresponding electrical energy consumption for each reaction (Krysa et al. 2018). The highest degradation of 96% colour and COD removal of 98% were found with 0.015 g/L of H2O2. The degradation profile at different H2O2 concentrations and kinetics is shown in Figure 7. This increase in removal efficiency could be due to the Fe2+ ions which react with H2O2 to generate a higher quantity of hydroxyl radicals. However, when H2O2 concentration increased, the optimal concentration, quantity of degradation and COD removal decreased. Therefore, 0.015 g/L of H2O2 concentration was considered to be the optimal value for colour and COD removal.
Figure 7

Effect of H2O2 on colour, COD removal and EEC.

Figure 7

Effect of H2O2 on colour, COD removal and EEC.

Close modal

Interactive effect of spiked Fe2+ on COD removal

The performance of the SPECP process for the degradation and decolourization of tannery wastewater was investigated at Fe2+ 0–0.005 g/L, pH = 3, H2O2 = 0.015 g/L, current density = 15 mA/cm2 and reaction time of 60 min. The results were obtained from the 3D response surface plots as shown in Figures 2,34. The varying Fe2+ concentration was in the range of 0–0.005 g/L and the results show the effect in the SPECP process at 60 min of treatment: 99 and 96% of colour removal were achieved at 0.0025 and 0.003 g/L dosage of Fe2+, respectively. This degradation in decolourization can be associated with the associated loss of OH formed from the Fenton's reaction (Equation (18)) and encouraged by a photolytic reaction due to the rise in the rate of the following reaction with increasing amounts of Fe2+:
formula
(18)
Experiments were carried out at different Fe2+ dosages (0–0.005 g/L) to explore the impact of Fe2+ concentration. The results are shown in Figure 8. Compared to synthetic wastewater, real wastewater requires a higher concentration of Fe2+ to start the Fenton reaction, which will damage large molecules (Selvabharathi et al. 2019). If the concentration is kept low, adding ferrous salts to achieve metal coagulation may be enough to entirely remove the organic contaminants. Figure 8 shows that the elimination effectiveness was found to be very low when the Fe2+ dose was not present. This could be due to the lower production of ferrous ions as the hydrogen peroxide's oxidizing strength was insufficient to annihilate large molecules. The removal of contaminants by SPECP using an iron electrode is accelerated by the presence of Fe2+ (El-Taweel et al. 2015). By increasing the concentration of Fe2+ from 0 to 0.002 g/L, it was possible to raise the COD removal effectiveness from 40 to 86% and the colour reduction from 36 to 92%. This is because the combination of Fe2+ and H2O2 causes a rise in the generation of hydroxyl radicals. By absorbing a proton from the organic compounds it attacked, this OH transformed into oxidized, highly reactive organic radicals. However, the concentration of COD and colour removal was negatively impacted when Fe2+ was greater than 0.0025 g/L. The reactor may experience Fe2+ dissolution in the anode and Fe2+ regeneration in the cathode when iron electrode is used as both an anode and a cathode. Fe2+ dosage over the optimal level of >0.025 mg/L causes OH produced in SPECP reactions to be scavenged. Additionally, the use of OH produced when Fe3+ oxidizes themselves into Fe2+ and the suspected solids retained by the compounds led to the precipitation of metal hydroxides, which is how iron sludge accumulates dissolved pollutants (Pekey 2016; Pantazopoulou et al. 2017). When initial Fe2+ concentrations were raised from 0 to 1 mM, Wang et al. (2014) noticed an increase in the rate of TOC decomposition by the E-Fenton process. It was reported that the ferrous ions in the electrolyte solution, when present in excess, consume the hydroxyl radicals and affect the extent of degradation.
Figure 8

Effect of Fe2+ on colour, COD removal and EEC.

Figure 8

Effect of Fe2+ on colour, COD removal and EEC.

Close modal

It was also observed that the process did not produce too much sludge; the accumulated sludge produced at optimum conditions of reaction was found to be 12.4 mL/L. The sludge production was proportional to the current density and contact time for the ECP process. To better clarify this behaviour, the colour disappeared more quickly with an Fe2+ dosage of 0.002 mg/L. The decolourization rate dropped gradually when increasing the Fe2+concentration from 0.003 to 0.005 g/L (Wang et al. 2010). The drop in decolourization efficiency when the wastewater concentration rises from 0.003 to 0.005 mg/L Fe2+ points to the development of Fe(III) complexes (Apaydin et al. 2009; Khac et al. 2018). External addition of Fe2+ concentration accelerates the Fenton reaction and increases process efficiency to its optimum value, and further increases in Fe2+ concentration have a significant impact on process efficiency by converting effective hydroxyl radicals into iron complexes by the excess Fe2+. The degradation profile at different Fe2+ (Nidheesh et al. 2018) concentration and kinetics is shown in Figure 8.

Response optimization and validation of the experimental design

The experimental values for removal of colour, COD removals and corresponding energy consumption are tabulated in Table 4. The corresponding experimental values for the degradation of tannery wastewater for the combined process under optimum conditions were found to be 96 and 98% for COD and colour removal with minimum electrical energy consumption 0.014 kWh/L, respectively, and were attained at pH = 3, Fe2+ = 0.002 g/L, H2O2 = 0.015 g/L, current density = 15 mA/cm2 at a reaction time of 60 min for the SPECP process, being in good agreement with the values predicted using CCD.

Table 4

Comparison between experimental and predicted values for COD removal using RSM

S. No.ParameterspHFe2+ (g/L)H2O2 (g/L)Current density (mA/cm2)Colour removal (%)COD removal (%)EEC (kWh/L)
Predicted value 0.002 0.015 15 97 95 0.014 
Experimental value 0.002 0.015 15 98 96 0.014 
S. No.ParameterspHFe2+ (g/L)H2O2 (g/L)Current density (mA/cm2)Colour removal (%)COD removal (%)EEC (kWh/L)
Predicted value 0.002 0.015 15 97 95 0.014 
Experimental value 0.002 0.015 15 98 96 0.014 

Influence of contact time

The efficacy of the SPECP process was carried out under optimal conditions. The COD removal was 98% at the reaction time of 60 min, as shown in Figure 9, and the treatment of tannery wastewater by the SPECP process improved BOD5/COD from 0.4 to 0.6 in 15 min. Therefore, for the removal of the organic compound of wastewater combined SPECP can be efficient. The SPECP process was validated to be an efficient technique for the treatment of tannery industry wastewater, allowing permissible discharge limits.
Figure 9

Effect of contact time.

Figure 9

Effect of contact time.

Close modal

Kinetics studies on the SPECP process

The kinetics of the SPECP process, solar photo-Fenton and electrochemical peroxidation for the removal of COD are evaluated. It is significant to verify the coefficients of kinetic in a reaction to attain the accurate design of a reactor (Gilpavas et al. 2019). For practical applications, the kinetic reaction under different conditions is required. These results revealed that COD and colour removals of tannery wastewater follow the below-mentioned order: SPECP>solar photo-Fenton>electrochemical peroxidation process. The rate constant, k (min–1) was selected as the basic kinetic parameter to understand the reaction mechanisms of the reactor (Coppath Hamza et al. 2017; Selvakumar et al. 2019) and therefore enables the determination of the photocatalytic activity (see Table 5). The experimental data appear to fit the linear kinetic equation, and consequently, the degradation efficiency follows the first-order kinetics as shown in Figure 9. The order of rate constants was SPECP (0.0684 min–1) > solar photo-Fenton (0.0420 min–1)>electrochemical peroxidation (0.0423 min–1). An important development of the degradation efficiency was noticed during the application of the coupled solar photo-Fenton and electrochemical peroxidation processes. Equation (19) was used to evaluate the synergetic influence on the degree of degradation (Sivagami et al. 2016):
formula
(19)
Table 5

Kinetics rate constant at various conditions on COD removal

Degradation processk (min–1)Linear coefficient (R2)COD removal (%)Time (min)
Solar photo-assisted electrochemical peroxidation 0.0684 0.9939 96 60 
Solar photo-Fenton 0.0420 0.9794 82 300 
Electrochemical peroxidation 0.0423 0.9894 82 300 
Degradation processk (min–1)Linear coefficient (R2)COD removal (%)Time (min)
Solar photo-assisted electrochemical peroxidation 0.0684 0.9939 96 60 
Solar photo-Fenton 0.0420 0.9794 82 300 
Electrochemical peroxidation 0.0423 0.9894 82 300 
While developing the combined process, apart from the efficacy of the process, some of the issues such as design, construction, operation and maintenance of coupled advanced oxidation processes are challenging. However, less capital and operational costs were reachable by combining the various technologies. A combination of EAOP is required to improve the degradation degree which is not possible by a single process (Gutierrez-Mata et al.2017). The synergetic effect of the SPECP process was 1.2, created on the reaction rate constant. Thus, the combined system of EAOP improves the degradation and biodegradability in a short time. Equation (20) defines the performance of the incorporation of coupled EAOP treatment processes by the biodegradability improvement:
formula
(20)
The amount of additional BOD formed by the coupled process is shown in Equation (20). The synergetic biodegradability improvement influence of this SPECP process is created on the BOD5/COD ratio as shown in Figure 10. The BOD5/COD ratio was 0.6, 0.65 and 0.82 for ECP, SPF and SPECP processes, respectively, for 1-h treatment. It was observed that the coupled process is more effective than the single processes with the positive synergic effect indicated by SI>1, while SI<1 denotes a negative effect (Mechaa et al. 2016).
Figure 10

Effect of biodegradability assessment.

Figure 10

Effect of biodegradability assessment.

Close modal

Removal of chromium (III) by the SPECP process

The removal of chromium (III) experiments were carried out using the SPECP process at optimum conditions, i.e. pH = 3, Fe2+ dosage = 0.002 g/L, H2O2 dosage = 0.015 g/L and current density = 15 mA/cm2. The high SPECP process efficiency is obtained due to the strong dislocation via catalytic ion solution provided by iron plates and further addition of H2O2 in the reaction, created in situ by electrochemical hydrolysis leading to generating more amounts of OH. as the foremost oxidant agent (Marinho et al. 2019), reacting with chromium (III) and its conversion to Cr(OH)3 as a by-product (Golbaz et al. 2013; Kuppusamy et al. 2017). Figure 11 shows the removal efficiency of chromium (III) by the SPECP process. The chromium (III) concentration decreased from 16 to 1.2 mg/L at 120 min of reaction time. The MINAS discharge standards were attained by this process at a treatment time of 60 min. The SPECP methods proved to be an efficient alternative for the treatment of tannery industry wastewater allowing permissible discharge limits to be obtained. The application of a combined method, namely AOP and EAOP, for removing chromium (III) is economically feasible, able to attain the 95% of chromium (III) removal and decreases environmental issues compared to other treatment processes.
Figure 11

Effect of contact time for chromium (III) removal by solar photo-assisted electrochemical peroxidation process.

Figure 11

Effect of contact time for chromium (III) removal by solar photo-assisted electrochemical peroxidation process.

Close modal

A bench-scale SPECP reactor with iron plates as anode and cathode was successfully used for the treatment of tannery wastewater. The RSM model combined with CCD was established to optimize pH, Fe2+, H2O2 concentration and current density of the SPECP process. The results demonstrated that pH, Fe2+ concentration and current density positively affected colour and COD removal efficiency in the process. Taking into consideration the discharge limits, contribution to low energy consumption and cost-effective treatment, the optimum concentration of Fe2+ dosage was selected as 0.002 g/L. As the current density increased, an evident increase was seen on COD removal efficiency but 15 mA/cm2 was chosen to minimize the energy consumption. In the SPECP process, the overall degradation process (k = 0.0684 min−1) occurs due to the contributions of the solar photo-Fenton process (k = 0.0420 min−1) and the electrochemical peroxidation process (k = 0.0423 min−1). The synergetic effect between both processes was 1.2 based on the first-order rate constants for COD removal. SPECP improved the biodegradability (BOD5/COD) of tannery wastewater from 0.4 to 0.6 in 15 min, suggesting that the wastewater is able to transfer to the conventional treatment process, i.e. a biological treatment process, for further cost effectiveness. The obtained SPECP process optimum condition is as follows: a current density of 15 mA/cm2, initial pH = 3, H2O2 = 0.015 g/L, Fe2+ = 0.002 g/L. The best attained COD removal efficiency was 97%. The initial COD concentration of 2,500 mg/L was reduced to 90 mg/L; chromium concentration was reduced from 16 to 1.2 mg/L after the treatment time of 60 min. Since the allowable COD concentration for tannery wastewater according to minimal national standards of India for waste disposal is 250 mg/L, this treatment will be a complete treatment for providing direct dischargeable wastewater. From these results, it can be concluded that SPECP can eventually represent a suitable alternative process for the treatment of tannery wastewater.

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

The authors declare there is no conflict.

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