Algae inactivation and algal metabolic pollutant removal of Chlorella and Duneliella salina from seawater by the Fe-ACF electro Fenton-like process has been studied. The experiments were conducted at neutral condition of pH 8.3 (raw water) and 6.2 to break through the limit of strong acidic conditions. Experimental results indicated that the Fe-ACF electro Fenton-like process has a good effect for algae inactivation and for pollutant removal in neutral conditions, and pH has no obvious effect for the inactivation of algae. At optimum conditions, the inactivation efficiency for algae could reach up to 98%. A pH of 8.3 is more favorable for pollutant removal. Under neutral pH conditions, the coupling effect of indirect oxidation of •OH, direct oxidation of free chlorine and coagulation and adsorption of ferric and ferrous hydroxy complexes in the electro Fenton-like process was achieved, and the coupling effect promotes the inactivation of algae and the removal of metabolic pollutants.
Algae metabolites, such as algae toxins, cause serious water pollution (Huang et al. 2008; Laabir et al. 2012; Vichi et al. 2012). The objective of our study was to reflect the mechanism of inactivation of algae and pollutant removal by an electro Fenton-like process. In the study, Chlorella and Dunaliella salina were selected as the representative algae. To break through the limitation of strong acidic conditions, the Fe-ACF electro Fenton-like process was conducted at a neutral pH range.
MATERIALS AND METHODS
Electro Fenton-like setup and characterization
The electro Fenton-like setup used here is shown in Figure 1. Batch mode experiments were conducted in an undivided glass cell under vigorous stirring performed by a magnetic stirrer with 1,000 mL of solution, equipped with an air pump, a DC power supply and a conductivity/pH meter. The applied potential and temperature were optimized in the range of 1–6 V and room temperature, respectively.
The anode was a sheet of iron sized 10cm × 10cm × 0.2 cm, and the cathode was an activated carbon fiber sized 10 cm × 10 cm × 0.2 cm, and the distance between anode and cathode was 5 cm. Compressed air was fed to the cathode by an air pump at 250 mL/min for 5 min before the power was on, and was maintained during the process of electrolysis.
Chlorella and Dunaliella salina were cultured in seawater.
The pH was an important factor in water. Although pH less than 5 would be favored for the production of •OH in the electro-Fenton process, pH less than 5 or greater than 9 would affect the viability of algae in water. In the study, the pH of the water was selected at 8.3 (initial pH of raw water) and 6.2 (adjusted by addition of solution of 0.1 mol/L sulfuric acid and sodium hydroxide). The two different pH values were near neutral and were in the common pH range of natural water, and the pH does not affect the viability of the algae.
The concentration of the algae was analyzed by microscope counting and optical density of OD685 synchronously. The method of microscope counting to express the content of algae in water is simple and intuitive, but there is a risk of human error. Optical density of OD685 can be used to present the density of algae, but it lacks accuracy. The concentration of pollution was analyzed by UV254 (absorbance at 254 nm) (Ng et al. 2012; Gerrity et al. 2012; Yu et al. 2014; Li et al. 2015a, b) and CODMn (potassium permanganate index) (Wu et al. 1987; Pei et al. 2007; Khorsandi et al. 2014; Jiao et al. 2015; Li et al. 2015a, b). The CODMn concentration analysis referred to standard methods (EPAC 2002). Free residue chlorine, Cl−, Fe2+ and Fe3+ were analysed by chemical methods. Hydrogen radicals were trapped with dimethyl sulfoxide and quantified by high performance liquid chromatography (HPLC) (Tai et al. 2004). The evaluation was carried out with a reversed-phase Nucleodur C18 Column (250 mm × 4.6 mm, 5 μm) and UV-visible detector (Agilent 1200 DAD) at 275 nm. The mobile phase was methanol/water (55:45%), with a flow rate of 1 mL/min and temperature of 23 °C. Both solvents were previously filtered through a 0.45 μm polytetrafluoroethylene (PTFE) filter and degassed ultrasonically. The pH was measured using a pH meter.
Laboratory-scale experiments were carried out with DC conditions power supply. The volume of water was 1,000 mL. The pH was adjusted with a solution of 0.1 mol/L sulfuric acid and sodium hydroxide. The cell voltage varied from 1–6 V, the current density applied ranged from 0.5–5 mA/cm2. At certain time intervals, samples of 20 mL volume were taken from the solution and filtered through 0.20 μm Nylon filters and analyzed for OD685, COD, and UV254.
RESULTS AND DISCUSSION
Inactivation effects for Chlorella and Dunaliella salina
The inactivation effects for Chlorella and Dunaliella salina are shown in Figures 2 and 3. The inactivation efficiency was calculated by Equation (4), and the number of algae was expressed by microscopic counting.
The results in Figures 2 and 3 show that the inactivation rate for algae increased with the increase of voltage. From the inactivation for Chlorella at a pH of 8.3 in Figure 2(a), it could be seen that the inactivation efficiency increased to 99.4% in 25 min at a voltage of 6 V. In Figure 2(b) at a pH of 6.2, the inactivation efficiency reached 97.9% in 25 min at a voltage of 3 V. These results showed that the ACF-Fe electro Fenton-like process has high inactivation efficiency for Chlorella under neutral pH conditions and the inactivation effect at pH 6.2 was a little better.
Figure 3 shows the inactivation effect for Dunaliella salina. The results showed that the inactivation effect for Dunaliella salina increased with the increase of voltage and electrolysis time.
At a natural pH of 8.3, the inactivation efficiency reached to 46.3% in 25 min at a voltage of 1 V, and increased to 97.2% at a voltage of 6 V. At a pH of 6.2, the inactivation efficiency was 66.7% in 25 min at a voltage of 1 V, and increased to 97.5% at a voltage of 6 V. These results showed that the Fe-ACF electro Fenton-like process was effective for Dunaliella salina inactivation.
The results in Figures 4 and 5 show that the optical density of OD685 reduced with increasing voltage during the electrolysis process, and there was no obvious difference between pH 8.3 and 6.2. The results of optical density were consistent with that of microscope counting. The methods of optical density and microscope counting for algae analysis were actually feasible.
Removal efficiency for UV254
UV254 was used to express the relative contents of organic matters that have conjugated π-ponds and so on. Usually, UV254 is used to represent the relative organic pollution degree of water. The removal efficiency for UV254 in the process was shown in Figures 6 (Chlorella) and 7 (Dunaliella salina).
From Figures 6 and 7, it can be seen that the Fe-ACF electro Fenton-like process has some effect on removal for UV254. In Figure 6, when pH was 8.3, the removal effect for UV254 increased with the increase of voltage. When the voltage was 1 V, the removal efficiency for UV254 was small. But when the voltage was 6 V, the UV254 decreased from 0.324 to 0.001 in 25 min, and the removal efficiency was 99.7%. At pH 6.2, the removal efficiency for UV254 was smaller than that at pH of 8.3, and the removal efficiency was 53.4% at 6 V in 25 min. These results showed that a pH of 8.3 was advantageous for the removal for UV254.
Removal efficiency for CODMn
CODMn is used to express the relative content of substances that can be oxidized in water. In the study, the concentration of CODMn was used to present the contaminants of algae metabolites. Figures 8 and 9 show the removal results for CODMn.
It can be seen that CODMn decreased in some extents during the electro Fenton-like process, and the removal effects increased with the increase of operational voltage.
For Chlorella, at a pH of 8.3, the removal efficiency for CODMn increased from 75.7% at 1 V in 25 min to 98.5% at 6 V in 25 min. At a pH of 6.2, the CODMn removal efficiency increased from 22.7% at 1 V in 25 min to 65.8% at 6 V in 25 min.
For Dunaliella salina, which was the same as Chlorella, the removal efficiency for CODMn could increase up to 81.3% and 82.8% at a pH of 8.3 and 6.2.
CODMn expresses the relative content of organic matter in water. The results of CODMn removal showed that the Fe-ACF electro Fenton-like process has higher removal capacity for organic pollutants produced by algae.
The ECCOD of the above trials calculated from Equation (12) vs. the percentage of CODMn removal during the electro Fenton-like treatments was analyzed. In the process, the voltage was between 1–6 V, I was 0.1–1 A, and t was about 0.5 h. Then the EC was calculated, which was 0.1–0.3/(kWh(g COD−1)).
Mechanism analysis for algae inactivation and pollutants removal
As one of advanced oxidation process, electro-Fenton technology has been used widely in organic pollutants degradation in water. In this study, the electrolysis was conducted under neutral conditions to break through the limit of strong acidic pH condition. The study results showed that the Fe-ACF electro Fenton-like process has a high efficiency for algae inactivation and pollution removal.
Felix-Navarro et al. (2011) have shown that in the process of electrolysis, there has been coexistence of indirect oxidation of •OH and direct oxidation of other oxidants, and Bedoui et al. (2009) have shown the multi function of direct oxidation, indirect oxidation of •OH and other oxidation of and so on.
In this study, iron and ACF were used as the anode and cathode, respectively, in the electro Fenton-like system in seawater with a high concentration of Cl− at neutral pH conditions. Although under neutral conditions, the production of •OH is not very high, but the iron anode would be oxidized to Fe2+ and Fe3+ and then form ferric and ferrous hydrous compounds through hydrolysis, which would help the removal of algae by coagulation. On the other hand, because of the high reactivity of Cl−, large amounts of Cl− in seawater would be oxidized to free chlorine, which would take a role in the inactivation of algae and pollutant removal. In order to clarify the mechanism of algae inactivation and pollutant removal, the intermediate products of •OH, free chlorine and Fe2+ and Fe3+ were analyzed.
Analysis of •OH
The •OH concentration was analyzed by HPLC. In the process of electrolysis, the change of •OH concentration is shown in Figure 10.
The •OH was produced in the process of electrolysis. At a pH of 8.3, the concentration of •OH increased with the increase of operation voltage from 1 V to 4 V, while it decreased at 5 V and 6 V, and the concentration of •OH increased at the beginning and then decreased. At a pH of 6.2, the concentration of •OH did not increase to the same as that at pH 8.3. At a voltage of 1 V, 2 V and 6 V, the concentration of •OH was high relatively, but lower at 3 V, 4 V and 5 V. Compared with that of pH 8.3, the concentration of •OH was higher at pH 6.2, which shows that weak acid conditions are favored for the production of •OH.
In the Fe-ACF electro Fenton-like system, Fe2+ and H2O2 would be produced in the anode and cathode respectively, and then react with each other to produce •OH. When the operational voltage was low, the electrolysis in the two poles increased with the increase of voltage, and the production of •OH increased. But, when the voltage was high enough, the concentration polarization of the electrodes increased, thus the intermediate products and the reaction between Fe2+ and H2O2 decreased, and then the production of •OH decreased.
From Figure 10, it can be seen that the curves of ·OH concentration change almost oppositely under pH of 6.2 and 8.3. In this study, experiments were conducted on seawater with a high concentration of Cl−. Under alkaline conditions of pH 8.3, the small amount of •OH produced would be consumed by Cl−, and so on (seen in Equations (5)–(9)), so the curves of •OH concentration change almost oppositely under pH of 6.2 and 8.3.
Analysis results of free chlorine
During the process of electrolysis, Cl− in water would be oxidized to chlorine and hypochlorite, which would take roles in the oxidation in pollutant removal from water. The production and change of free chlorine was analyzed to clarify the role of free chlorine in algae inactivation and pollutant removal. The change result of free chlorine is shown in Figure 11, and the change of Cl− is shown in Figure 12.
From the results in Figure 11, it can be seen that there was free chlorine generated in the process of electrolysis, and the concentration of free chlorine increased with the increase of operational voltage to some extent, but it decreased when the voltage increased to a higher value. At the same time, the concentration change trend of free chlorine at the studied pH values is essentially the same.
The change of Cl− concentration in Figure 12 showed that the concentration of Cl− decreased in the process of electrolysis, which was confirmed with the change trend of free chlorine.
In the electro Fenton-like process, there were effects of chlorine or hypochloric acid, and the effects of free chlorine did not vary with pH values.
The occurrence of chlorate and perchlorate is undesirable and becomes the main drawback in application of electrochemical technology.
Analysis results of Fe2+ and Fe3+
In the electrolysis, the Fe anode would be oxidized to Fe2+, which would react with H2O2, generated in the cathode, to produce •OH. In the process of electrolysis, some Fe2+ produced in the anode was unable to react with H2O2 and would be oxidized to Fe3+, which would reduce the generation rate of •OH, and in neutral pH condition, Fe3+ would form ferric hydroxide, etc. with the effects of coagulation and adsorption, and which would agglomerate algae cells and adsorb pollutants in water, and contribute to the removal for algae and pollution. Figures 13 and 14 show the results of the concentration change of Fe2+ and Fe3+ in the process.
At low voltage of 1 V and 2 V, there was no Fe2+ accumulation in 25 min. But with the increase of voltage, there was more and more Fe2+ accumulation. This is because at low voltage, the amount of Fe2+ generated in the anode was small, and the Fe2+ would be consumed by H2O2 quickly, so there was no Fe2+ accumulation. With the increase of voltage, more and more Fe2+ was produced in the anode, but due to limitations such as mass transfer rate, etc., the increasing Fe2+ cannot react with H2O2 in the cathode, and results in the accumulation of Fe2+. But, conversely, Fe2+ is not stable, and finally it would be consumed by O2. So, the total concentration of Fe2+ was low.
The change of Fe3+ concentration was the same as that of Fe2+, which showed that with the accumulation of Fe2+, more and more Fe3+ accumulated. In neutral conditions, Fe3+ would hydrolyze to ferric hydroxide gradually, which would affect agglomeration and adsorption on removal of algae and some pollutants.
Mechanism of the Fe-ACF electro Fenton-like process
The analysis results showed that •OH, Fe2+, Fe3+ and free chlorine coexist with certain regularity in the electro Fenton-like process.
The production of •OH increased with the increase of voltage. When the voltage increased to a higher level, the production of •OH decreased. At a certain voltage, the production of •OH increased to the highest point, then declined gradually. A pH of 6.2 is favorable for the production of •OH.
The production of free chlorine has no obvious differences at the two pH values.
At low voltage, there was no accumulation of Fe2+. However, with the increase of voltage, more and more of Fe2+ was accumulated in the solution.
In the process of electrolysis, there was an accumulation of Fe3+, and the production of Fe3+ increased with the increase of voltage.
In the process of the Fe-ACF electro Fenton-like process, Fe2+ and H2O2 were produced in the anode and cathode, respectively, and then reacted to each other to produce •OH.
During a shorter period of electrolysis, the Fe2+ generated in the anode reacted with H2O2 in the cathode to produce •OH, so there was no accumulation of Fe2+, and the production of •OH increased. During a longer period of electrolysis, more and more Fe2+ was accumulated in the solution, and the concentration of Fe3+ increased. In the study condition of neutral pH, Fe3+ would gradually hydrolyze into hydroxide complexes.
To sum up, it could be concluded that in the process of algae and pollution removal by an electro Fenton-like process, there were effects of indirect oxidation of •OH, coagulation and adsorption of ferric and ferrous hydroxide complexes, and direct oxidation of free chlorine, etc. The synergistic coupling effect of these factors implements the removal of algae and organic pollution.
Under optimum experimental conditions, the Fe-ACF electro Fenton-like system could break through the limit of the single constraint of advanced oxidation of •OH at acidic conditions. The coupling effects of different factors achieved higher removal efficiency for pollution.
This study has focused on inactivation of Chlorella and Dunaliella salina using an Fe-ACF electro Fenton-like process, and the mechanism for algae inactivation and pollution removal was clarified.
The process has a high effect on inactivation for Chlorella and Dunaliella salina. The inactivation efficiency for Chlorella and Dunaliella salina reached nearly 98%.
The process could be conducted at a neutral pH of 8.3 (raw water) and 6.2, which break the restraint of strong acidic conditions. The pH has no obvious effect on the inactivation of algae and the removal efficiency of algal pollution.
Synergistic coupled effects of the oxidation of •OH, coagulation and adsorption of iron hydroxide complexes, and oxidation of free chlorine, etc. in the Fe-ACF electro Fenton-like process were achieved at neutral pH conditions.
The authors are grateful for the support of the Dalian Construction, Science and Technology Planning Project of 2014 and the Central Gold Project of Sea Area, China, 2012 (Environmental)-Vegetation Repair and Reconstruction of Shuangtaizi Estuary Wetland of Liaoning province.