Keywords

1 Introduction

Water is an essential natural resource for sustaining life and environment that we have always thought to be available in abundance and free gift of nature. Over the past few decades, the ever-growing population, urbanization, industrialization, and unskilled utilization of water resources have led to degradation of water quality and reduction in per capita availability in various developing countries. Iron is one of the most common elements in nature as it represents about 5% of the earth’s crust (Hashim et al. 2017), and it can be found in freshwaters at a concentration of 0.5–50 mg/L. In addition to the natural occurrence of iron, many industries, such as mining and steel industries, contribute to the occurrence of iron in the water. However, iron represents an essential element for human health, where the daily intake of iron is recommended to be between 10 and 50 mg depending on the person’s gender, age, physiological status, and the bioavailability of iron. Based on these considerations, the World Health Organization (WHO) limits their own concentration in drinking water to 0.3 mg/L (Vasudevas et al. 2009). Electrocoagulation has been suggested as an alternative to chemical coagulation in the treatment of waters and wastewaters (Balasubramanian et al. 2009; Chaturvedi and Dave 2012). In this technology, metal cations are released into the water by dissolving metal electrodes. Electrochemistry, coagulation, and flotation are identified as the key elements in the electrocoagulation process (Kobya et al. 2003; Al-Qodah et al. 2017).

The electrocoagulation (EC) has been considered as a suitable process to remove iron in drinking water treatment because it lowers the amount of sludge and also provides some significant advantages such as quite compact and easy operation, no chemical additives needed, and high flow rates (Fu and Wang 2011). In this study, Taguchi method was implemented to investigate the effect of different parameters (viz. initial concentrations of heavy metal, current intensity, conductivity of solution, inter-electrode distance, and initial pH of solution) affecting EC process in the removal of iron from wastewater. The experiments were conducted at four different levels with five process parameters.

2 Materials and Methods

2.1 Materials

All the reagents used were analytical grade and were used without further modification. Iron sulfate heptahydrate (FeSO4, 7H2O) was procured from Merck, India. Sodium hydroxide (NaOH), sodium chloride (NaCl), and sulfuric acid (H2SO4) were also procured from Merck, India. Double distilled water was used to prepare the various solutions of iron. Aluminum sheets (~98% purity) for electrode fabrication were procured from local market.

2.2 Experimental Matrix Designing by Taguchi Method

Influential parameters affecting EC process were studied from various literatures. Parameters like the initial concentration of iron, current intensity, inter-electrode distance, conductivity, and initial pH were chosen. Taguchi method was implemented to analyze the effects of these parameters and to obtain the regression equation (Tir et al. 2015; Irdemezet al. 2006). In this work, orthogonal array OA16 (45) with five different parameters (P) of four different levels (L) were implemented. The level of each of the chosen parameters is shown in Table 1. N is the number of runs required for the orthogonal array, which is recognized using Eq. (1):

Table 1 Experimental parameters and levels for the removal of iron using electrocoagulation
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$$N=\left(L-1\right)P+1$$
(1)

There are 16 experiments (L16) to be carried out as per the Taguchi methodology and are shown in Table 2 (Karmakar et al. 2018). Minitab-17 software was utilized to implement Taguchi method for obtaining the optimized parameter values, the effectiveness of the parameters, and the regression equation. Each experiment was repeated at least three times to obtain the experimental removal efficiency (Martinez-villafane and Montero-Ocampo 2010). To evaluate the effect of parameters and to obtain their optimized value a response is chosen. In this study, the percentage removal of iron is chosen as the response of the Taguchi method, and experimental data were analyzed using the signal-to-noise (S/N) ratio. Since better the removal of iron from the solution, the “larger is better” characteristic has been chosen.

Table 2 L16 experimental matrix from Taguchi method
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$$\frac{S}{N}= -10{\mathrm{log}}_{10}\left(\frac{1}{n}\sum_{i=1}^{n}\frac{1}{{y}_{i}^{2}}\right)$$
(2)

where S/N is the sound to noise ratio, n is the number of repetitions for one experimental combination, and \({y}_{i}\) is the performance value of the ith experiment (Asghari et al. 2012).

2.3 Experimental Setup and Procedure

A 5-L beaker was used for the electrocoagulation setup (Fig. 1). A 3-L of iron solutions of different concentrations were made for each of the experiments. Aluminum plates were used as electrodes with dimensions of 100 mm × 30 mm × 1.5 mm. A perplex rod was used to support the anode and cathode. The inter-electrode distance varied from 8 to 14 mm. A DC power supply (Make: Aplab L6410 S) was connected to maintain a steady flow of current. A constant speed was maintained in the EC setup using a magnetic stirrer (Make: Remi 5 MLH Plus). The experiments were conducted at ambient temperature (25 ± 1 °C). The pH was varied for the experiments using 0.1 M solution of H2SO4 and NaOH. NaCl was used as the electrolyte. Each experiment had a fixed time span of 40 min. Samples were withdrawn at the end of the experiment and filtered through filter paper (Whatman Cat No 1001-110) and electrodes were cleaned by dilute HCl after each experiment.

Fig. 1
figure 1

Schematic diagram of electrocoagulation setup: (1) digital DC power, (2) magnetic bar stirrer, (3) multimeter meter, (4) anode, (5) cathode, (6) magnetic bar, (7) temperature probe, and (8) pH meter

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The filtered sample was analyzed using ICP–OES spectrometry (Make: Perkin Elmer Optima 8000). The percentage removal of iron was calculated using the following equation:

$$\text{Removal} \,{\text{efficiency}}= \frac{{C}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{f}}}{{C}_{\mathrm{i}}}\times 100$$
(3)

where Ci and Cf are the initial and final concentrations of iron solutions in mg L−1, respectively (Hashim et al. 2019). Conductivity and pH were measured by multimeter meter (Make: EUTECH Instruments PC2700).

2.4 Analysis of Variance

Electrocoagulation of iron solution was performed in accordance with the set of parametric conditions acquired by experimental matrix design using L16 orthogonal array methodology. The removal efficiency of iron using monopolar electrocoagulation process was analyzed statistically for an assessment of the importance of the model selected for optimization and the effects of separate process parameters on the response by means of ANOVA studies. ANOVA is a dynamic technique used to inspect the importance of a discrete parameter and selected the optimization model for the establishment of a mathematical model equation. It emphasizes on the analysis of the variance around the mean of the performance features and is accomplished by assessing the Fischer’s test value (F-value). The influence of any parameter is elucidated by its F-value and the corresponding sum of squares (Shah et al. 2017). Higher F-value and sum of squares of any parameter specify its comparative importance in the procedure of the response. Contrariwise, the extents of acquired values of these tests are entirely owed to response signals and are assured by a p-value. The p-value simplifies the probability of attaining an F-value of this order due to noise values below 0.05 or 5% confirms the significance of the specific process parameter (Pundir et al. 2018).

3 Results and Discussion

3.1 Analysis of Iron Removal and Determination of Condition

The electrocoagulation process of iron solution was performed with a set of parametric conditions obtained by experimental matrix design using L16 orthogonal array approach. The percentage removal efficiency of iron was as a response and is shown in Table 2. In order to determine the effective parameters and their significance levels on removal efficiency, a statistical analysis of variance (ANOVA) was performed. The results of ANOVA are given in Table 3. The obtained adjusted sum of squares for the model was 479.299 and total degree of freedom (DF) was obtained to be 15. The F-value for initial concentration of iron was 71.26 which is the most significant but conductivity and pH were 0.03 and 0.62, which were least significant parameters. The F-value of the other parameters, current intensity and inter-electrode distance were found out to 30.32 and 33.97, respectively. These values can be assured from the P-value. Except conductivity and initial pH, other three parameters play a significant role in the removal of iron using electrocoagulation. Table 4 shows the priority of the parameters affecting in EC. From Delta-Rank, we can see that initial concentration of iron is the most significant parameter followed by electrode distance, current intensity, initial pH, and conductivity (initial concentration > inter-electrode distance > current intensity > initial pH > conductivity).

Table 3 Analysis of variance (ANOVA) of the parameters
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Table 4 Delta-rank of various parameters
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3.2 Effect of Various Parameters

To investigate the effect of various parameters like initial concentration of iron, current intensity, pH, inter-electrode distance, and conductivity on the removal efficiency of iron by electrocoagulation process, L16 experiments were carried at a fixed time.

3.2.1 Effect of Initial Concentration of Iron

Effect of initial concentration of iron is one of the most significant factors affecting the electrocoagulation process. Under reaction conditions of reaction temperature = 30 °C, reaction time = 40 min, monopolar connection of aluminum electrodes and agitation speed = 400 rpm, maximum removal efficiency of iron was obtained at 15 ppm of initial iron concentration, as demonstrated in Fig. 2. Figure 2 reveals that with an increase in initial concentration of iron the removal efficiency decreases. This is due to the fact that the time of each electrocoagulation was limited to 40 min and as the amount of iron increased the S/N ratio decreased. The maximum removal was observed at 15 ppm (S/N ratio: 37.33) and the least was at 30 ppm (S/N ratio: 36.03). At lower concentration (up to 15 mg/L), available aluminum hydroxide complexes are adequate to remove the Fe(II) molecules within 40 min with appropriate oxidizing environment. The rate of generation of aluminum hydroxide complexes is not sufficient to remove high Fe(II) concentration (>15 mg/L) within 40 min of operation. Therefore, longer residence time is required for electrocoagulation process of high Fe(II) concentration.

Fig. 2
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Removal efficiency of iron versus initial concentration of iron by monopolar electrocoagulation process

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3.2.2 Effect of Current Intensity

The increase in current intensity increases the removal efficiency of iron. The amount of coagulants formed in electrocoagulation process depends greatly on the current passed and the time. Choice of electrode material is also an important factor affecting the cell voltage (different oxidation potential for different electrode materials) and the separation attained. In our work, aluminum was carefully chosen as the electrode material because of its cheapness, ready availability, nonharmfulness and it requires comparatively less oxidation potential. Here a sorption coagulation mechanism follows resulting in the creation of loose aggregates. As time progresses, further aluminum cation accumulation results in amorphous aluminum hydroxide precipitation that stimulates pollutant aggregation through a sweep coagulation followed by precipitation mechanism. During the final stages, coagulated aggregates act together with bubbles and float to the surface or settle to the bottom of the reactor. The size of the H2 bubbles formed and the coagulant production rate is adjusted by varying the current intensity that also determines the floc growth in the system. Thus high efficiency of electrocoagulation is obtained using high current intensity. From Fig. 3, it can be seen that an increase in current intensity increases the S/N ratio (i.e., removal efficiency of iron). At 1.25 A, maximum S/N ratio was observed at 37.12 and at 0.5 A, the least removal of iron was determined at S/N ratio of 36.18.

Fig. 3
figure 3

Removal efficiency of iron versus current intensity (A) by monopolar electrocoagulation process

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3.2.3 Effect of Inter-electrode Distance

The effective electrode area along with the inter-electrode distance between the cathode and the anode has a significant role in the electrocoagulation process. Increase in inter-electrode distance decreases the S/N ratio and it is shown in Fig. 4. This is due to the fact that as the applied voltage is constant as the inter-electrode distance increases the resistance for the applied current also increases leading to poor coagulation formation. The maximum S/N ratio was observed at 8 mm (S/N ratio: 37.02) and the least of 35.99 at 14 mm. The electrode setup plays a significant role in the effective surface area and also the inter-electrode distance. The equation that governs the variation in the voltage drop (ηIR) is:

$$\eta IR = I \cdot \frac{d}{s*k}$$
(4)

where, I = current (A), d = distance between two electrode (m), S = active anode surface (m2), k = specific conductivity (103 mS/m) (Ghosh et al. 2008). From the above equation, it can be inferred that at constant anodic surface area and conductivity of the solution, voltage drop increases with the increase in inter-electrode distance. The resistance between the electrodes and the distance between them are directly related. Thus, with the increase in the inter-electrode distance the electric current decreases and to achieve the required current intensity, the voltage had to be increased.

3.2.4 Effect of pH

The solution pH plays a significant role in the autocatalytic disappearance of aqueous Fe(II) with the incentive of iron removal in slightly basic (pH > 7) range. Electrocoagulation is believed to be a satisfactory technology due to the formation of more OH− ions in the electrolysis of water. In electrocoagulation where Al electrode is used, it has been witnessed that at somewhat basic ambiance Al(OH)3 precipitation occurs and the sweep-flock mechanism dominates (Fig. 5).

Fig. 4
figure 4

Removal efficiency of iron versus inter-electrode distance by monopolar electrocoagulation process

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From Fig. 5, we can evaluate that initial pH has not much effect on the removal of iron from the solution. The S/N ratio lies from 36.56 to 36.73. However, maximum removal was obtained at pH 6 with S/N ratio of 36.73.

Fig. 5
figure 5

Removal efficiency of iron versus initial pH by monopolar electrocoagulation process

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3.2.5 Effect of Conductivity

In this study, conductivity was maintained using sodium chloride (NaCl) salt. Conductivity increases the mobility of the ions present in the electrocoagulation process. Addition of salt increases the conductivity of the solution, which was directly influenced the cell voltage, energy consumption, and current efficiency in the electrolytic cell. The use of NaCl was also accompanied by the production of chloride ions that reduces the effects of other anions, such as bicarbonate and sulfate which may lead to the precipitation of Ca2+ leading to the high-ohmic resistance of the electrochemical cell. Hence no significant effect is noticed with varied conductivity. The S/N ratio varies from 36.60 to 36.71 (Fig. 6).

Fig. 6
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Removal efficiency of iron versus conductivity by monopolar electrocoagulation process

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4 Conclusion

From the above study, we can interpret that electrocoagulation is a suitable process for iron removal from wastewater. The regression equation obtained from the Taguchi method has an R2 value of 93.16. The optimized conditions were found to be 15 ppm initial concentration, 1.25 A current intensity, 8 mm inter-electrode distance with conductivity at 360 µS, and pH of solution at 5. From Delta-Rank, we can see that initial concentration of iron is the most significant parameter followed by electrode distance, current intensity, initial pH, and conductivity (initial concentration > electrode distance > current intensity > initial pH > conductivity).