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

Flue gas desulfurization (FGD) wastewater is generated when wet scrubbers wash dirty exhaust streams in coal-fired 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⁻, SO₄²⁻, F⁻ and NO₃⁻, and many kinds of heavy metals like Ni, Pb, Cu and Hg, which makes the wastewater treatment complex and difficult. Among these, high concentrations of fluoride are very toxic and generally appear in coal-fired power plant effluents. It is also an essential trace element for organisms, but it can present perniciousness at concentrations above the tolerance level (>1.5 mg/L) (Muthu Prabhu & Meenakshi 2014). It causes some adverse effects such as teeth deterioration and dental caries in slightly contaminated drinking water, and osteoporosis and serious problems for organs in seriously polluted water (Palahouane et al. 2015). FGD wastewater also contains many kinds and large amounts of heavy metals like Ni, Pb, Cu and Hg. These heavy metals are non-biodegradable carcinogens and have a bioaccumulation effect. As the most common heavy metal element in the Earth's crust, nickel is frequently involved in industrial production, especially in coal combustion which generates vast amounts of FGD effluent containing Ni. In addition, nickel ions have stronger chelation with organics, like nucleic acids, than other heavy metals (Gupta 1998). Therefore, discharge of FGD wastewater into natural water without proper treatment would endanger public health, threaten the survival of indigenous aquatic biota and even have a fatal effect (Kabuk et al. 2014). Recently, the Ni ion discharge standard in many industries was raised to 0.1 mg/L in China (DB 44/1597-2015).

Conventional technologies applied to remove fluoride and heavy metals from FGD wastewater include chemical precipitation, filtration and ion exchange (Guan et al. 2009; Khosa et al. 2013; Muthu Prabhu & Meenakshi 2014). However, chemical precipitation requires the addition of a large amount of chemicals such as lime, polyacrylamide, 2,4,6-trimercaptotriazine, trisodium salt, nonahydrate (TMT-15), which may produce secondary pollutants and a large amount of sludge (Guan et al. 2009). Expensive membrane filtration is often limited by the high concentration of suspended solids (SS). Ion exchange shows excellent performance in the laboratory, but it incurs high operation costs for an extremely complex FGD wastewater treatment. These methods are insufficient to meet elevated environment standards and achieve economic feasibility.

Electrocoagulation (EC) technology generated coagulants, created by an in situ electro-dissolution metal anode, have attracted much attention in the last two decades. Due to its fast reaction, reasonable cost and simple operation, EC has been employed successfully to treat fluoride-containing wastewater (Zhao et al. 2010), organic wastewater (Kuokkanen et al. 2015), and heavy metal wastewater (As (Balasubramanian & Madhavan 2001), Cr (Arroyo et al. 2009), Ni (Beyazit 2014)). However, to our knowledge, FGD wastewater treated by EC has not been reported in the literature. Nevertheless, many efforts have been made to develop the EC method in treating fluoride and nickel ions. Sandoval et al. (2014) found that aluminum as the sacrificial electrode was efficient for fluoride removal. Similar results have been obtained in many studies (Drouiche et al. 2009; Palahouane et al. 2015). Comparing the iron and aluminum anode, Bazrafshan et al. (2012) demonstrated higher efficiency for the aluminum electrode for the removal of fluoride. This is as a result of the precipitate formation of aluminum fluoride hydroxide complexes (Al_nF_m(OH)_3n−m). 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. 2013). 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. 2015). 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 2009). 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.

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. All chemicals used in the experiments were analytical grade and purchased from Sigma. The graphite, iron (≥99.9%) and aluminum (≥99.9%) plates were arranged from Shenzhen Quanfu Metal Co. Ltd, China.

Before the experiments were carried out, the iron and aluminum plates were scrubbed with sandpaper to clean the surface passivation layer and washed with ultrapure water, then all electrodes were soaked in 0.25 M H₂SO₄ solution for 10 min and rinsed with ultrapure water, before being finally dried in the vacuum drying oven and placed in a desiccator to cool down.

An electrolytic cell with a volume of 1,000 mL wastewater was used at room temperature (25 ± 0.8 °C) for EC in batch experiments. In order to optimize the EC parameters, the different anode electrode types, like Fe/C/Fe, Al/C/Al and Fe/C/Al, different electrode distances from 5–40 mm, different current densities from 1.88–6.25 mA/cm² and different pH values from 4–10 were tested. The pH was adjusted to a desirable value by using 0.1 M HCl or NaOH solution. The samples were taken from the electrolytic cell at a given time and stored at 4 °C. All experiments were carried out three times and the average values are reported here.

The electrical conductivity and pH were measured by conductivity meter (DDSJ-308A, Rex Electric Chemical, China) and pH meter (sensION+ Ph1, HACH, USA), respectively. Chemical oxygen demand (COD), sulfate and chloride of the samples were tested by standard methods (Greenberg 2005). In detail, the COD values were calculated by the dichromate method and sulfate was measured by the gravimetric method. Concentration of chloride was analyzed using the silver nitrate titration method. An expandable ion analyzer (EA 940, Orion, USA) with fluoride ion selective electrode (PF-1-01, Rex Electric Chemical, China) was adopted for the quantitative analysis of fluoride (Palahouane et al. 2015). All samples for heavy metal analysis were filtered through a 0.45 μm glass fiber filter on site and measured by an Inductively Coupled Plasma-Atomic Emission Spectrometer (PerkinElmer, USA). SS of samples were analyzed using a turbidimeter (2100N, HACH, USA).

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, Na₂SO₄, KI or NaClO₄ into low electrical conductivity industrial wastewater and drinking water to limit the voltage drop (Sahu et al. 2014). They have reported that a solution with low conductivity increases the electrical consumption and decreases the life span of machine. However, as shown in Table 1, FGD wastewater with a high conductivity of 20,723–21,309 μS/cm met the working condition of EC without additional chemicals. The presence of chloride ions of 5,400–6,432 mg/L could also increase the dissolution of the anode metal due to pitting corrosion to improve the removal efficiency of the Ni and fluoride from FGD wastewater (Yetilmezsoy et al. 2009).

It is clear that a small space between electrodes will lead to a small IR-drop when the other parameters are fixed in the equation. Nevertheless, as small inter-electrodes distance as possible may not be optimal because short-circuit faults happen and the difficult dispersion of concentrated iron or aluminum ion or precipitates beside the electrode plates would reduce the pollutants removal efficiency. However, the result that energy consumption became stable after 30 mm seems difficult to explain directly by Equation (4). It might be explained by the high electrical conductivity k_aq playing a key role beyond a certain range of inter-electrode distance (d) in small volume of solution (Attour et al. 2014).

It can be seen from Equation (3) that the amount of dissolved iron and aluminum is proportional to the applied current and reaction time. Hence the achievement of a certain amount of dissolved metal can be obtained through increased time under a small applied current or increased applied current under a short time.

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) = 4.57 × 10⁻³³ and K_sp(Fe(OH)3) = 4.0 × 10⁻³⁸, 25°C). The results showed that the effect of initial pH on the pollutants removal from FGD wastewater was not neglectable.

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/cm² 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 (≤0.2 mg/L, 88%) and Cu (≤0.1 mg/L, 98%). These indicated iron and aluminum hydroxide species generated from Fe/C/Al EC had excellent absorption performance for heavy metals in the optimal condition. The effluent concentration of Fe and Al was 1.22–0.73 mg/L and 0.53–0.22 mg/L, respectively, which met China's industrial wastewater emission standards.

It is generally believed that EC is the interaction of three main mechanisms including electrochemical coagulation, electro-oxidation and electro-flotation. Electrochemical coagulation, relying on the dissolution of the Al or Fe anode, is the most important influence for fluoride and nickel removal. According to the literature (Sahu et al. 2014; Sandoval et al. 2014), the equations involved in electrochemical coagulation of Al and Fe are listed below.

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/cm² 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.

The research was financially supported by grants from National Natural Science Foundation of China (21677052), Major Science and Technology Program for the Industry-Academia-Research Collaborative Innovation (201605122301117, 201604010043), Guangdong Province Science and Technology Project (2016B090918104, 2013B090200016, 2015B020215007, 2015B020235009, 2016B020240005), Joint Fund of Guangdong Province (U1401235), State Key Laboratory of Pulp and Paper Engineering (2016C03), Electric Power Research Institute of Guangdong Grid Co. (No. KGD2013-0501) and Zhanjiang of Guangdong Energy Co. (ZY-KJ-YX-2016X085F). Shinian Liu and Xiaokun Ye contributed equally to this paper.