Advances in Applied Science Research Open Access

Keywords

Chemical coagulation; Electro fenton; COD; Pharmaceutical industry wastewater

Introduction

Pharmaceutical wastewater is one of the high polluted streams characterized by high concentrations of refractory organics and toxic pollutants [1]. The wastewater generated from pharmaceutical industries contains mainly agents, solvents of equipment washing and cleaning processes, and also reactants and catalysts used in manufacturing processes [2]. In many cases, effluents of these industries contain low biodegradability due to the presence of refractory organics in high concentrations. Therefore, the pollutant loads in terms of biological oxygen demand (BOD) may be negligible and chemical oxygen demand (COD) would be very higher than BOD [3]. On the other hand, many pharmaceutical compounds are only partially removed during biological treatment processes in sewage treatment plants and consequently release into surface waters [4,5]. So there are restrictions in the United States and Europe that require prospective Environmental risk assessments as part of the drug registration process [6]. Several different alternatives, including anaerobic–aerobic [7] membrane bioreactors [8,9] electrochemical oxidation [10] biochemical combined method [11] and advanced oxidation process (AOPS) [12-14]. In recent years, electrochemical methods have been used to treat highly toxic organic matter with low biodegradability. Electrochemical methods include electrical coagulation (EC), electrical oxidation and electrical photo-oxidation [15,16]. Electro-Fenton is a wellorganized method which has some advantages, such as less chemicals requirement, cost-effectiveness, simple equipment, low retention time, intensity in varying reaction conditions, rapid sedimentation of the electro-generated oxidants, less sludge production and high performance for degradation of refractory pollutants and color removal [17]. According to literatures the electrochemical oxidation has been successfully used to the treatment of textile dyeing wastewaters [18], tannery wastewater [19], and landfill leachate treatment [20], arsenic [21], olive oil mill wastewater [22] and petrochemical wastewater [23]. Various oxidation processes such as Fenton oxidation (FO), optical oxidation and electrical oxidation are used to increase the efficiency of the electrochemical method. In the electro Fenton method, Fenton oxidation and electrical coagulation are combined which has a high efficiency in the treatment of highly polluted wastewater [24]. One of the most important advantages of electro-Fenton method is the low sludge production and with high sedimentation potential compared to other common conventional coagulation methods. There are generally two different types of EF functions: 1) Electro Fenton (EF) system in which Fenton H₂O₂ and Fe²⁺ reagents are added to the reactor from the outside and stationary electrodes with high catalytic activity are used as anodes and 2) Which H₂O₂ is added from outside and Fe²⁺ is supplied by iron anodes [25]. Electro-Fenton oxidation is one of the advanced oxidation processes. The main purpose of this method is the generate hydroxyl radicals using a chemical reaction between H₂O₂ and Fe²⁺ (equation 2) [26,27]. The hydroxyl radical is one of the most reactive free radicals and can easily decompose organic matter (equation 3 and 4) [28]. In the EF method, Fe²⁺ ions are electrochemically produced by the iron anode (Equation 5-7), and then hydrogen peroxide added to the solution to provide the Fenton reaction conditions [29]. It also regenerates Fe²⁺ ions through equations 8 and 9 and increases process efficiency.

The electro-Fenton reactions are given below.

(Eq.1- at anode surface)

…… (Eq.2 - in substrate)

…….(Eq.3 - in substrate)

….. (Eq.4 - in substrate)

……. (Eq.5 - in substrate)

…… (Eq.6- in substrate)

…… (Eq.7- in substrate)

………. (Eq.8- in substrate)

………. (Eq.9 in substrate)

So the main aim of present study is the evaluation of efficiency of combined chemical coagulation by poly aluminium chloride (PAC) and electro-Fenton oxidation process in COD removal from pharmaceutical wastewater.

Materials and Methods

Wastewater sources and experiment procedure

The real pharmaceutical wastewater was obtained from a pharmaceutical industry effluent. Samples were collected from the equalization tank of treatment plant, prior to any treatment processes. Samples were stored in a refrigerator at 4°C before the experiments. Some main characteristics of the studied wastewater are given in Table 1. All chemicals were prepared of analytical grade and supplied by Merck and Poly aluminium chloride (PAC) was used as chemical coagulant. A Jar test set was used in order to determine the coagulant optimal dose. COD measurements were determined according to the Standard Methods for examination of water and wastewater (APHA Method 5220) and TS and VS (APHA Method 2540) [30]. An HACH COD reactor was used for digestion of the sample in COD vials. COD was spectrophotometrically determined by using a Jenwey 5350 spectrophotometer. The removal efficiency of COD was calculated using the equation as follow (Eq.10);

( E q . 1 0 )

Where: C0 is the initial COD of the raw leachate and Ct is the COD at the end of a predetermined settling time (t) after each running electrocoagulation.

Table 1 Characteristics of wastewater.

Experimental set-up

A batch electrochemical cell with operational capacity of 1000 ml was made of Plexiglas (Figure 1). Four electrodes with the dimensions of 2 × 20 × 200 mm were used as cathode and anode. Two electrodes (cathode and anode) installed in parallel form and the space between the electrodes was fixed 30 mm in all the experiments. A multi-output digital DC power supply (MICRO, PW-4053S) was used to establish optimum electrical condition. The current DC was kept constant at 10V, 20V and 30V. To keep homogenous mixing of the reactor content, magnetic stirring element by the speed of 200 rpm was used. In each run of the experiment the current density and electrolysis time were changed. The electrocoagulation experiments were performed for 60 min and settling time of 60 min. After 60 min settling, the COD and NH₄-N of the effluent were measured.

Figure 1: Schematic of electrocoagulation reactor.

Results and Discussion

Effect of PAC dosage on COD removal

Chemical coagulation by Jar test device was operated at controlled conditions e.g. room temperature, different concentrations of PAC, different pH values rapid mixing (100 rpm for 1 minute), flocculation mixing conditions (40 rpm for 20 minutes) and 1 hour settling time. All the experiments was carried out in pH=7 as optimal pH for chemical coagulation of PAC. Also comparing the removal efficiencies at different concentrations (25-50-75-100- 200-300 mg/L) showed a difference in COD removal efficiency. As shown in Figure 2 the COD removal percentage increased as the concentration of coagulant increases and the highest COD removal efficiency was achieved at 200 mg/L PAC. Also according to results by increasing PAC concentration from 25 to 200 mg/L, the COD removal efficiency increased to 49%, but the removal efficiency decreased at PAC concentrations higher than 200 mg/L, so that the COD removal efficiency decreased to 21% at 300 mg/L initial PAC concentration. Therefore 200 mg/L PAC was selected as the optimal dose. According to results the COD removal efficiency increases as the increasing in PAC concentration. So the COD removal efficiency increases with increasing the concentration of PAC to 200 mg/L. But the COD removal efficiency decreased at concentrations of higher than 200 mg/L that can be attributed to the re-stabilization of the clots formed in the sample and the accumulation of coagulant in substrate. As reported by previous studies when the amount of coagulant exceeds the optimum values, it increases the turbidity due to the presence of excess polymer in the effluent, which can reduce the removal efficiency of COD [31].

Figure 2: Effect of different concentrations of PAC on COD removal efficiency at pH 7.

Effect of pH and coagulant dose on turbidity removal efficiency

The turbidity removal efficiency at different concentrations of coagulant and at different pH values is shown in Figure 3. As can be seen, increasing in coagulant concentration, leads to an increase in turbidity removal percentage. So that at 200 mg/L PAC, the removal efficiency was 93.86% which is the highest turbidity removal in present study. But according to the results when the coagulant concentration increases from 200 to 300 mg/L, the efficiency dropped from 93.89% to 80%, which shows that the maximum effect of the coagulant dose is at 200 mg/L. Also high removal efficiency was achieved when pH value was adjusted at 7. As a result, 200 mg/L PAC and pH=7 determined as optimal coagulation condition.

Figure 3: Removal of turbidity at 200 mg/L concentrations of coagulant at different pH values.

The effect of initial H₂O₂ concentration on COD removal efficiency during the electro-Fenton process

As shown in Figure 4, the COD removal efficiency decrease by increasing in initial H₂O₂ concentration after 30 minute oxidation time. According to results increasing the initial H₂O₂ concentration from 100 mg/L to 4000 mg/L leads to a decrease in the efficiency of the electro-Fenton process in removing COD from 93.5% to 21% which indicates decrease in process performance at 4000 mg/L H₂O₂ concentration. The results show that by increasing the H₂O₂ concentration from 100 to 2000 mg/L, the decreasing trend is very slight and insignificant, but as the H₂O₂ concentration increased from 2000 to 4000 mg/L, the COD removal performance dropped sharply. Therefore, the optimal H₂O₂ concentration in the process was considered 100 mg/L. according to results presence of excess H₂O₂ leads to the formation of hydroxyl radicals (HO₂●) with less reactivity than OH● (Equations 11 and 12). On the other hand, the OH● radical is neutralized or repelled by Fe²⁺ (Equation 13). It should be noted that the oxidation potential of hydroxyl radicals (1.25 eV) is less than H₂O₂ (1.3eV).

…… (Eq.11)

…… (Eq.12)

….. (Eq.13)

Figure 4: Effect of hydrogen peroxide on the removal efficiency in the electro-Fenton process (voltage: 20 V, Oxidation time: 30 min and pH=3).

Therefore, reducing the access ability to active oxidizing species at high concentrations of H₂O₂ leads to a reduction in the removal of contaminants [30]. Excess H₂O₂ acts as an OH● receptor. Therefore, an increase in H₂O₂ concentration leads to decrease in hydroxyl radicals [2].

The effect of pH changes on the efficiency of the electro Fenton process

The effluent from the chemical coagulation and settling stage entered the electrofenton reactor. In order to evaluate the effect of pH changes, the initial pH of the oxidation medium was fixed in three ranges of 3, 7 and 10. Figure 5 shows the effect of pH changes on the efficiency of the electrofenton process. As can be seen, the removal efficiency decreases with increasing pH. The highest amount of COD removal at pH = 3 and the lowest COD removal efficiency at pH = 10 were 93.58% and 76%, respectively, which shows the inverse effect of increasing pH on process efficiency. According to literature all Fenton oxidation processes are generally more efficient in relatively acidic environments [27]. Increasing the initial pH of the oxidation medium from 3 to 10 reduces the COD removal efficiency due to the decrease in OH● oxidation potential. At pH values higher than 3, especially above 5, H₂O₂ converted to water and oxygen with a constant coefficient due to its instability. That the conversion factor is 2.3 × 10^-2 min^-1 and 7.4 × 10^-2 min^-1 at pH = 7 and pH = 10.5 respectively. At pH values above 7, the hydroxyl radical rapidly converted to O●-, which has much slower oxidation rate than OH●. At high pH values, ferrous ions precipitate as Fe(OH)₃ and inhibit the reaction between Fe²⁺ and H₂O₂. So the reproducibility of Fe²⁺, which is necessary for the continuation of the hydroxyl radical production process, get reduced. The absence of H+ when the pH is highly alkaline prevents the decomposition of H₂O₂. As a result, the production of hydroxyl radicals and consequently the decomposition efficiency of pollutants reduced significantly. Also, Fe(OH)₃ generated in high pH values accelerates the decomposition of H₂O₂ to O2 and H₂O, which reduces the production of ●OH radicals and ultimately reduces the efficiency of the electrofenton process [16].

Figure 5: Effect of pH on the removal efficiency in the electro- Fenton process (voltage: 20 V, Oxidation time: 30 min).

The effect of voltage changes on the efficiency of the electro Fenton process

The effect of voltage changes on the COD removal efficiency in the electrofenton process is shown in Figure 6. As can be seen the COD removal efficiency increased from 92 to 93.5 when the voltage increased from 10 to 20 V. However, the COD removal efficiency decreased by increasing the voltage to 30 V, According to the results, the efficiency of the electrofenton process decreases by increasing the voltage by more than 20 V. So 20 V value was determined as optimum voltage for electrofenton process. An important issue in the efficiency of the electrofenton process is that by increasing the voltage above the optimum value, energy consumption increases, which is not economically viable [32,33]. In general, voltage is a driving force for oxygen depletion that results in the production of hydrogen peroxide at the cathode.

Figure 6 Effect of voltage on removal efficiency in electro Fenton process (H2O2= 100 mg/L, Time= 30 min and pH =3).

High voltage values increase the amount of hydrogen peroxide produced. Hence, the number of highly active hydroxyl radicals increases. Also, with increasing voltage, the electrical production of Fe²⁺ ferrous ion increases and increases the efficiency of Fenton cycle reactions (Equation 14). According to the results of the study, the efficiency of the electro Fenton process was reduced at high voltages. This is because of the competitive reactions of the electrode in the electrolyte cell. At high voltages, oxygen depletion occurs at the anode surface (Equation 15) and hydrogen reduction occurs at the cathode (Equation 16). These reactions prevent the main oxidation reaction (Equation 17), which ultimately leads to a decrease in the oxidation efficiency of the electro Fenton.

…… (Eq. 14)

…… (Eq. 15)

…… (Eq. 16)

…… (Eq. 17)

The overall turbidity and COD removal efficiency in combined chemical coagulation and electro Fenton oxidation

The overall efficiency of the combined process of chemical coagulation and oxidation of electro-Fenton is given in Table 2.

Table 2 Overall efficiency of the combined process of chemical coagulation and electrofenton.

As can be seen, for PAC coagulation under optimal conditions the PAC concentration (200 mg/ l), pH=7 rapid agitation (100 rpm in 1 min), flocculation (40rmp in 20 min) and 1 hour settling time, 93.85% and 49% removal was observed for turbidity and COD respectively. Also the overall efficiency of the combined process Electro-chemical coagulation and in pH=3, voltage (20 V) reaction time (30 min) and H₂O₂ concentration (100 mg /L) 93.85% and 93.5% removal was observed for turbidity and COD respectively.

Conclusion

Since no harmful reagents are used in the electro Fenton process, it is considered as an environmentally friendly method for water and wastewater treatment. Present study showed that, with increasing oxidation time of more than 30 minutes, the removal efficiency did not significantly change due to the presence of resistant organic compounds in the sample. Also, the removal efficiency decreased due to the increase in the concentration of PAC in the amounts more than 200 mg/L (due to the destabilization of the clots), and also the increase of the H₂O₂ concentration in the amounts more than 2000 mg/L (due to the radical abduction of ●OH by the Excess H₂O₂) was observed. It was also observed that increasing the pH of the oxidation medium and increasing the voltage to higher than 20 volts reduce the removal efficiency of COD and turbidity. In this study, turbidity and COD values in the effluent from the oxidation reactor were 2 and 243 mg/L respectively, which are still far from environmental standards, but the ease and efficiency of the process, is acceptable. Overall, according to the results of the present study the electro Fenton process is an efficient and promising technology for water and wastewater treatment purposes.

Acknowledgments

The authors are grateful to the research center of the Lorestan University of Medical Science for providing financial support for this research.

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