A electrocoagulation (EC)/peanut shell (PS) adsorption coupling technique was studied for the removal of malachite green (MG) in our present work. The addition of an appropriate PS dosage (5 g/L) resulted in remarkable increase in the removal efficiency of MG at lower current density and shorter operating time compared with the conventional EC process. The effect of current density, pH of MG solution, dosage of PS and initial concentration of MG were also investigated. The maximum removal efficiency of MG was 98% under optimum conditions in 5 min. And it was 23% higher than that in EC process. Furthermore, the unit energy demand (UED) and the unit electrode material demand (UEMD) were calculated and discussed. The results demonstrated that the EC/PS adsorption coupling method achieved a reduction of 94% UED and UEMD compared with EC process.
Textile and tannery industries are some of the most contaminated in developed countries with 7 × 105 tons of dyestuffs produced annually (Asghar et al. 2015; Suárez-Escobar et al. 2015). Dyes are normally very large aromatic molecules consisting of many linked rings (Zodi et al. 2013). Dye molecules present a real threat to the environment and human health because of their structural stability and complexity (Njoku et al. 2014). The disposal of dye-containing water is currently a major problem from a global viewpoint (Ahmad et al. 2014). Various physical, physicochemical, and biological techniques were used to remove dyes from wastewater. Some of these processes include adsorption (Reddy et al. 2012; Dotto et al. 2015), coagulation–flocculation (Saitoh et al. 2014), filtration (Kajekar et al. 2015), electrocoagulation (EC) (Chafi et al. 2011), advanced oxidation (ozonation, Fenton's reagents, and UV radiation/hydrogen peroxide) (Bellebia et al. 2009), ion-exchange (Wu et al. 2008), biological treatment (Rodrigues et al. 2014) and air flotation (Liang et al. 2014).
The EC technique is considered to be a potentially effective method for wastewater treatment in past decades (Brillas & Martínez-Huitle 2015). EC is a process that involves creating flocs of metallic hydroxides within the effluent to be cleaned via electrodissolution of soluble anodes, usually aluminum or iron (Ricordel & Djelal 2014), and in a simultaneous reaction, hydroxyl ions and hydrogen gas are produced on the surface of the cathode, which leads to the production of various ferrous, ferric or Al (III) hydroxide, and polyhydroxy species, depending on the pH of the electrolyte (Kobya et al. 2011). The proposed mechanism for the generation of coagulation by using Al as an anode is presented below (de Carvalho et al. 2015a):
The advantages of EC are a compact treatment facility, less sludge production, minimal requirement of chemicals and the possibility of complete automation (Hu et al. 2016). However, it is easy to form an impermeable oxide film on the cathode, which results in higher energy consumption and lower efficiency (Avsar et al. 2007; Secula et al. 2013). Hence, great efforts have been made to improve EC progress. Many studies showed that granular active carbon/EC coupling technique was more efficient and faster compared to conventional EC progress (Narayanan & Ganesan 2009; Secula et al. 2012a). As with active carbon, peanut shell (PS) is also a high surface area porous material with exceptional adsorptive properties. PS adsorption as an effective method to treat wastewater has been reported in various literature, such as removing methylene blue, brilliant cresyl blue, neutral red (Gong et al. 2005), heavy metals (Xu & Liu 2008; Witek-Krowiak et al. 2011) and organic compounds from aqueous solution. However, coupling the EC technique with PS adsorption was seldom reported before. PS is an effective and low-cost adsorbent and therefore ideal for application in developing countries.
In this work, we investigated the removal of malachite green (MG) from aqueous solution by EC/PS adsorption coupling method. PS was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and pHpzc. The effects of current density, dosage of PS, initial MG concentration and pH on the removal efficiency were discussed. The unit energy demand (UED) and the unit electrode material demand (UEMD) were also determined and discussed.
MATERIALS AND METHODS
Peanuts were purchased at a local market (Changchun, China) and were manually peeled immediately. PS was ground by a grinding mill and sieved to obtain a particle size range of 80 mesh. The powders were washed with distilled water for three times to remove the suspended solids and water-soluble materials. Then they were dried at 60 °C for 12 h. The dried PS were stored in plastic bottle for further use.
The Na2SO4 was purchased from Xilong Chemical Co., Ltd (Shantou, China).
The H2SO4, HCl and NaOH were obtained from Beijing chemical works (Beijing, China).
EC/PS adsorption coupling experiments
For every experimental run, 400 mL of MG solution was placed into the reactor. The electrodes were connected to a digital DC power supply to obtain a constant current. The current density and electrode space were set to a desired value and then a certain quantity of PS was added to the solution. The pH was adjusted by adding 0.1 M HCl or 0.1 M NaOH and measured using a PHS-3E pH meter (Shanghai INESA and Scientific Instrument Co., Ltd, China). The experiments were performed at room temperature and the process was carried out for 1 h.
Unit energy and electrode material consumption
Energy consumption is one of the vital parameters for the feasibility of the electrochemical process with a focus on EC, which deserves special attention from researchers in the area.
SEM measurements and energy dispersive X-ray (EDX) were performed by a field emission scanning electron microscope (Hitachi, S-4800) at an operating voltage of 3 kV and at an operating voltage of 20 kV, respectively. FTIR spectrums were obtained from a FT-IR spectrophotometer (Nicolet NEXUS470, Nicolet Co., Ltd, USA) with KBr disks. The point of zero charge (pHpzc) of PS was determined by batch equilibration method in which nine different solutions were prepared having pH values ranging from 2 to 10. At first, 20 cm3 of distilled water was kept in several beakers and their pH values were adjusted by adding varying amounts of 0.1 mol/L of NaOH or HNO3 solution. Then a portion of PS sample (50 mg) was added into beakers and kept for equilibration at room temperature. After 24 h, the final pH was measured and plotted as a function of initial pH values. The final pH value in the curve where a common plateau is obtained is the pHpzc of the PS (Reddy et al. 2016).
RESULT AND DISCUSSION
Characterization of PS
pHpzc of PS
Effect of current density
Effect of PS dosage
Effect of initial pH
Effect of initial MG concentration
UED and UEMD analysis
UEMD represents the consumption of electrode material in relation to the mass unity of removed dye as defined by Equation (3). Figure 11(b) shows the UMED of EC process. The values varied from 0.06 to 0.45 g/g in EC process while it was 0.02 g/g in EC/coupling technique. Same as UED, UMED reduced 94% in EC/PS adsorption coupling system compared with that in EC system.
From our present study, it was found that the removal of MG by the EC/PS adsorption coupling method is a feasible process. The addition of PS as adsorbent resulted in remarkable increase in the removal efficiency of MG at lower current density and operating time than the traditional EC process. The optimal conditions for the EC/PS coupling technique were as follows: current density was 2 mA/cm2, dosage of PS was 5 g/L, initial MG concentration was 50 mg/L with an initial pH of solutions around neutral. Under optimal conditions, the optimal removal efficiency of MG was found to be 98% in 5 min, which was 23% higher than that in the EC process in 60 min. The energy consumption and unit material energy consumption were 94% lower in the coupling process than that in the traditional EC process. Compared with similar state-of-the-art materials and method to treat MG, such as spent tea leaves activated carbon adsorption (Akar et al. 2013), almond shell adsorption (Ozdes et al. 2010) and poultry feather adsorption process (Beak et al. 2010), the EC/PS adsorption coupling method is the most efficient and cost-optimal method. Thus, we could conclude that the EC/PS adsorption coupling technique for MG removal is considerably cheaper, and is suitable for developed and developing countries.
This work was supported by the National Natural Science Foundation of China (No. 51308252), Jilin Province Science and Technology Development Plans (No. 20130101091JC) and the analysis and Changchun Technology Innovation Fund (No. 2009086).