Simultaneous removal of Ni(II) and ﬂ uoride from a real ﬂ ue gas desulfurization wastewater by electrocoagulation using Fe/C/Al electrode

Large amounts of anions and heavy metals coexist in ﬂ ue gas desulfurization (FGD) wastewater originating from coal- ﬁ red power plants, which cause serious environmental pollution. Electrocoagulation (EC) with Fe/C/Al hybrid electrodes was investigated for the separation of ﬂ uoride and nickel ions from a FGD wastewater. The study mainly focused on the technology parameters including anode electrode type, time, inter-electrode distance (5 – 40 mm), current density (1.88 – 6.25 mA/cm 2 ) and initial pH (4 – 10). The results showed that favorable nickel and ﬂ uoride removal were obtained by increasing the time and current density, but this led to an increase in energy consumption. Eighty-six percent of ﬂ uoride and 98% of Ni(II) were removed by conducting the Fe/C/Al EC with a current density of 5.00 mA/cm 2 and inter-electrode distance of 5 mm at pH 4 for 25 min and energy consumption was 1.33 kWh/m 3 . Concomitant pollutants also achieved excellent treatment ef ﬁ ciency. The Hg, Mn, Pb, Cd, Cu, SS and chemical oxygen demand were reduced by 90%, 89%, 92%, 88%, 98%, 99.9% and 89%, respectively, which met stringent environmental regulations.


INTRODUCTION
Flue gas desulfurization (FGD) wastewater is generated when wet scrubbers wash dirty exhaust streams in coalfired power plants. During this process various hazardous substances are stripped off and go into liquid phase. FGD wastewater requires special attention mainly due to the combination of a high concentration of anions, such as Cl À , would endanger public health, threaten the survival of indigenous aquatic biota and even have a fatal effect (Kabuk et al. ). Recently, the Ni ion discharge standard in many industries was raised to 0.1 mg/L in China (DB /-).
Conventional technologies applied to remove fluoride and heavy metals from FGD wastewater include chemical precipitation, filtration and ion exchange (Guan et  For the high removal efficiency of Ni ions, some controversies between iron and aluminum as the working electrode have developed; it is widely accepted that the iron hydroxide complexes have higher flocculation than aluminum for removal of most heavy metals (Khosa et al. ). However, some studies indicated that a satisfactory Ni treatment efficiency can be achieved for an aluminum electrode rather than an iron one ( Jagati et al. ). This may be related to aeration, target pollutant properties or coexisting substances, and so on. For example, Fe(III) hydroxide complex presented in aeration exhibited stronger coacervation than Fe(II) hydroxide (without aeration) for metals (Martinez-Huitle & Brillas ). Hence, more exploration needs to be carried out in order to determine optimum EC treatment conditions for FGD wastewater.
In this study, the treatment of real FGD wastewater by EC with the combination of iron and aluminum sacrificial electrode was investigated. Fluoride and nickel in the FGD wastewater are selected as the main target pollutants.
Optimization of various parameters such as electrode material, electrode distance, initial pH and current density for fluoride and nickel removal efficiency were explored.
Finally, we also assessed the effectiveness of the Fe/C/Al EC method for the removal of other concomitant substances in FGD wastewater and energy consumption.

Chemical and samples
The samples of real FGD wastewater were collected from a coal-fired power plant located in Zhanjiang city, China.
Some physicochemical characteristics of raw wastewater used in this experiments are listed in Table 1

Analytical methods
The electrical conductivity and pH were measured by con-

Calculations
The pollutants removal was determined in terms of removal efficiency defined as: where C 0 and C are concentrations of pollutants (Ni 2þ , F À , Hg 2þ , Cu 2þ , and so on) in the original FGD wastewater and treated one at given time (t), respectively. The amount of energy consumption for the EC treatment is a very important industrial parameter, which can be calculated as follows (Ardhan et al. ): in which E is the electric energy consumption (kWh/m 3 ), U is the voltage across the circuit (V), I is the current (A), t EC is the EC time (min) and V is the volume of given wastewater (m 3 ).
According to Faraday's law, the theoretically dissolved mass of aluminum or iron from the sacrificial electrode during the EC process can be calculated by the following where m is the amount of the dissolved anode material (g), I is the applied current (A), t EC is the reaction time of the EC process (s), M r is the specific molecular weight of the anode metal (g/mol), z is the number of electrons involved in the

RESULTS AND DISCUSSION
EC has vast advantages compared with the conventional coagulation process, but the effective utilization of it depends highly on the conductive capability of the solution.
For this reason, many researchers have added an electrolyte like NaCl, Na 2 SO 4 , KI or NaClO 4 into low electrical conductivity industrial wastewater and drinking water to limit water with current density of 5.00 mA/cm 2 at pH ¼ 5 for 25 min. Figure 3 shows that the removal efficiency of fluoride and Ni increased from 32% to 78% and 70% to 97%, respectively, when the electrode distance decreased from 40 mm to 5 mm. However, the energy consumption of the EC system increased rapidly from 0.94 to 1.53 kWh/m 3 with the increasing of electrode distance from 5 mm to 30 mm and then became stable after 30 mm. Obviously, Figure 2 | Effects of electrode type on fluoride (a) and Ni (b) removal efficiency. Conditions: inter-electrode distance of 5 mm, current density of 5.00 mA/cm 2 , pH ¼ 5.
the optimal inter-electrode distance parameter was 5 mm.
The phenomenon can be explained by the following equation (

Effect of current density
Applied current (current density) is a very important parameter in the EC system because it determines the yield of coagulants and electric energy consumption. To study the effect of current density for pollutants removal from FGD wastewater, batch experiments were carried out with the current density varying from 1.88 to 6.25 mA/cm 2 and electrode distance of 5 mm at initial pH ¼ 5. Figure 4(a) shows that the fluoride removal efficiency increased with the increase of applied current density. The highest defluorination efficiency of 84% was obtained with a current density of 6.25 mA/cm 2 at 25 min, but defluorination efficiency of only 69% was achieved with the current density of 1.88 mA/cm 2 at 30 min. In the same conditions, Ni removal efficiency shows a significant difference between different applied currents in Figure 4(b). It only needed 15 min to reach the highest removal efficiency of 96% at 6.25 mA/cm 2 , whereas  respectively. It was also found that at 30 min the energy consumption grew exponentially with the current density varying from 1.88 to 6.25 mA/cm 2 .
It can be seen from Equation (3)  Ni removal efficiency decreased firstly from 92% to 49% when initial pH was varied from 4 to 8 and then increased slowly to 61% when the initial pH was further increased to 10 through 10 min EC treatment ( Figure 5(b)). This is because the dissolution of iron and aluminum were accelerated at acidic pH and the precipitates of Ni(OH) 2 were generated by the reaction between OH À and nickel (Equation (15)) at alkaline pH. The optimal pH 4 obtained was consistent between Ni and fluoride.
Similar removal tendencies of fluoride and nickel were found after 25 min. All of the EC processes achieved a high fluoride and Ni removal efficiency and the maximal removal efficiency of fluoride and Ni were 85.8% and 98.1% at pH 4, respectively. Besides, since the system initial pH was 5.6-6.2 and after the EC process increased to 6.7-7.3, Al(OH) 3 and Fe(OH) 3 were able to be deposited according to their solubility product (K sp(Al(OH)3) ¼ 4.57 × 10 À33 and K sp(Fe(OH)3) ¼ 4.0 × 10 À38 , 25 W C). The results showed that the effect of initial pH on the pollutants removal from FGD wastewater was not neglectable.

Removal efficiency of concomitant pollutants
Significant removal of fluoride and Ni(II) were obtained after the optimization, but the removal efficiency for the concomitant pollutants in the FGD wastewater was not clear. Hence, the removal of concomitant pollutants was tested. Table 1 describes the removal efficiency of pollutants from FGD wastewater by EC using Fe/C/Al electrode combination with the current density of 5.00 mA/cm 2 and inter-electrode distance of 5 mm at pH 4 for 25 min. The results confirmed that the EC treatment system can effectively achieve a broad spectrum of pollutants removal. Remarkable efficiency for toxic heavy metals was obtained such as Hg ( 0.03 mg/L, 88%), Mn ( 0.4 mg/L, 89%), Pb ( 0.08 mg/L, 92%), Cd 2Cl À ! Cl 2 þ 2e À (5) These experiment results demonstrated the feasibility of the EC technology in treating real FGD wastewater. Another advantage for FGD wastewater with the EC process was better energy consumption due to the high electrical conductivity in the raw solution without additional electrolyte. In this condition, the energy consumption was 1.34 kWh/m 3 , which was much less than in some other studies, such as 3.9 kWh/m 3 for As-containing groundwater (Flores et al.

Removal mechanism
It is generally believed that EC is the interaction of three Anode: Cathode: Solution: for aluminum, and for iron, In the EC system, the rapid dissolution of the submerged Al and Fe electrode in situ happened in an energized state, which transferred the Al and Fe ions into the solution. Then, Fe 2þ and Al 3þ produced the precipitates and flocs through Equations (11), (15) and (16). The precipitates and dissolved metal ions in the solution play an important role in the treatment of pollutants.
According to the study, the fluoride removal was attributed to adsorption of Al(OH) 3 particles (Equation (12)) and coprecipitation of Al 3þ (Equation (13) (14), (18) and (19)). From this, it can be seen that the Fe/C/Al have a satisfactory performance for fluoride removal in the EC system. Therefore, to simultaneously remove F À and Ni 2þ , the Fe/C/Al electrodes combination was chosen in the EC system for further FGD wastewater treatment. However, the reasonable disposal of heavy metal-containing sludge generated from the EC process should be considered, such as the utilization of lime or cement for stabilization/solidification or usage as raw material for production of ceramics (Butt et al. ; Verbinnen et al. ).

CONCLUSIONS
This paper investigated the use of Fe/C/Al hybrid electrodes in an EC system for FGD wastewater treatment which allowed for simultaneous removal of Ni(II), fluoride and most toxic pollutants to improve the effluent quality. The effect of parameters like anode material type, time, inter-electrode distance, current density and initial pH on the removal of fluoride and nickel was evaluated. Results showed that the optimum removal of fluoride and Ni achieved was 85.8% and 98.1%, respectively, in current density of 5.00 mA/cm 2 and inter-electrode distance of 5 mm with pH 4 for 25 min. Meanwhile, excellent removal efficiency was also observed for some concomitant pollutants of Hg, Mn, Pb, Cd, Cu, SS and COD and reasonable energy consumption was used. In conclusion, the EC process with the Fe/C/Al electrode type is an effective method for removal of fluoride, nickel and most pollutants in FGD wastewater to meet the stringent environmental regulations.