Solar photo-assisted electrochemical peroxidation process for the sustainable treatment of tannery industry wastewater

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.

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.

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.

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.

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.

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).

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].

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.

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.

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 2,³–4. 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.

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.

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 2,³–4 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.

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 2,³–4. 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.

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.

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.

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.