Onion membrane: an efficient adsorbent for decoloring of wastewater
© Saber-Samandari and Heydaripour; licensee BioMed Central. 2015
Received: 9 October 2014
Accepted: 24 February 2015
Published: 13 March 2015
Recently, researchers have tried to design synthetic materials by replicating natural materials as an adsorbent for removing various types of environmental pollutants, which have reached to the risky levels in nature for many countries in the world. In this research, the potential of onion membrane obtained from intermediate of onion shells for adsorption of methylene blue (MB) as a model cationic dye was exhibited.
Before and after adsorption, the membrane was characterized by Fourier transform infrared spectroscopy (FTIR) and optical and scanning electron microscopy in order to prove its dye adsorption capability. The various experimental conditions affecting dye adsorption were explored to achieve maximum adsorption capacity.
The dye adsorption capacity of the membrane was found to be 1.055 g.g−1 with 84.45% efficiency after one hour and 1.202 g.g−1 with 96.20% efficiency after eight hours in contact with the dye solution (0.3 g.L−1). Moreover, the kinetic, thermodynamic and adsorption isotherm models were employed to described the MB adsorption processes. The results show that the data for adsorption of MB onto the membrane fitted well with the Freundlich isotherm and pseudo-second-order kinetic models. In addition, the MB adsorption from room temperature to ~50°C is spontaneous and thermodynamically favorable.
Evidently, the high efficiency and fast removal of methylene blue using onion membrane suggest the synthesis of polymer-based membranes with similar physical and chemical properties of onion membrane as a valuable and promising wastewater decoloring agents in water treatment.
Many industrial production activities (e.g. dye, cosmetic, plastics, food, textile, planting, and mining) result in water pollution since they produce pollutants such as water-coloring agents and toxic heavy metals that are extremely harmful to people and the environment even at low concentrations. The removal of these compounds from wastewaters before discharging them into the environment is of great importance, since many dyes and their degradation products are toxic and carcinogenic, posing a serious hazard to the environment. The conventional methods used to treat colored effluents include photocatalytic degradation, microbiological decomposition, electrochemical oxidation, membrane filtration, and adsorption techniques . Among these, adsorption is the most widely used method because of its efficiency, low cost, easy operation, simple design, less energy intensiveness, and non-toxicity. Recently, numerous approaches to develop adsorbents that are more effective have been studied . Among these, adsorbents containing natural and synthetic polymeric materials, industrial by-products, agricultural wastes and biomass were applied for removal of dyes from aqueous solution [3-6].
Dyes can be classified into cationic, anionic and nonionic dyes. Cationic dyes are basic dyes while the anionic dyes include direct, acid, and reactive dyes . However, cationic dyes are widely used in acrylic, wool, nylon, and silk dyeing, they considered as toxic colorants and can cause harmful effects such as allergic dermatitis, skin irritation, mutations and cancer . Cationic dyes carry a positive charge in their molecule, furthermore they are water soluble and yield colored cations in solution . Methylene blue, rhodamine B, and brilliant green are representative examples of cationic dyes . Methylene blue (MB) is an important basic dye and widely used in the textile industry. Acute exposure to MB may cause increased heart rate, shock, vomiting, cyanosis, jaundice, quadriplegia, heinz body formation, and tissue necrosis in humans .
The main aim of this study is to exhibit the ability of dried onion membrane for removal of MB from aqueous solution. However, many researchers have studied the dye and metal adsorption capacity of several biomasses such as rice, corn and coconut husks [12-14], papaya seeds , watermelon  and onion skin , but to the best of our knowledge, there is no study relating the adsorption properties of onion membrane obtained from intermediate of onion shells. In this study, the adsorption of MB was confirmed using FTIR and optical and electron microscopy. In addition, the effect of various factors such as contact time, initial dye concentration, pH, temperature and adsorbent dose on the adsorption rate was examined. Finally, the adsorption of MB was analyzed by employing the adsorption kinetic, isotherm models, and thermodynamics.
The onions were purchased from local markets in Famagusta (North Cyprus). Hydrochloric acid 37% (Merck), sodium hydroxide (Mediko Kimya), potassium chloride (Mediko Kimya), potassium hydrogen phthalate (Merck), sodium hydrogen carbonate (Aldrich), and potassium hydrogen phosphate (Merck) were used to prepare the buffer solutions with different pH values. Finally, MB (Aldrich) with a molecular formula of C16H18ClN3S was used without further purification.
The onions were peeled, chopped and then the membranes were removed from the leaves. 1 gram of onion membrane was obtained from approximately 250 g onion. The membranes (~4 g/kg of onion) were rinsed and washed with distilled water to remove impurities. Then, it was dried at 60°C in an oven for one day. After that, the dried adsorbent was kept in the desiccator to avoid moisture adsorption.
where W1 (g) and W2 (g) are the weights of the dried and swollen membranes, respectively.
where W is the mass of adsorbent in g, V is the volume of MB solution in L, and Ci and Ce are the initial and equilibrium concentrations in g.L−1, respectively.
In addition, the effect of variable conditions such as time, pH, adsorbent amount, and initial adsorbate concentration on the MB adsorption behavior of onion membranes was examined. For each case, one parameter was changed and analyzed and the other factors were kept constant. For instance, the influence of pH on adsorption was calculated by immersing 0.06 g of onion membrane in 250 mL of MB buffer solutions (0.3 g.L−1 concentration) with different pH values (3–11) and then shaken at room temperature (20°C) for eight hours (480 min).
Furthermore, the pH at point of zero charge (pHzpc), which shows the point that the acidic or basic functional groups do no contribute to the pH of the solution, was determined using the standard technique . For this purpose, 50 mL of 0.01 mol.L−1 NaCl solution was placed in a 250 ml flask. The pH of the solutions in each flask was adjusted from 3–11 by adding either sodium hydroxide or hydrochloric acid solutions (0.1 mol.L−1). Then, 0.06 g of onion membrane (15 × 15 mm2) was immersed in each solution at 20°C and was allowed to equilibrate in an isothermal shaker. After a contact time of 24 hours, the suspensions were filtered through filter paper and the final pH values of supernatant were measured again using a pH meter. Lastly, the final pH values were plotted against the initial pH values. The pH at which the curve crosses the line final pH = initial pH was taken as the pHzpc of the onion membrane .
Methods of characterization
A UV/VIS spectrophotometer (Lambda 25 UV/VIS, Perkin-Elmer, Llantrisant, UK) was used to determine the MB adsorption amounts by the onion membranes. The pH values of the solutions, which were used to investigate the effect of pH on adsorption, were checked by a pH meter (WTW InoLab, accuracy ± 0.1). To prove the adsorption of MB by the onion membranes, the FTIR (Perkin-Elmer, Llantrisant, UK) spectra of membranes before and after adsorption were observed in the range of 500–4000 cm−1. Images of the membranes were also taken with an optical microscope (MT9000 Polarizing Microscope, Meiji Techno Co. Ltd., Japan with Invenio 3S, Delta Pix Camera) and a scanning electron microscope (AIS2100, Seron Technology, Korea) operated at an acceleration voltage of 15 kV to study the change in surface morphology of the membrane after dye adsorption.
Results and discussion
Swelling properties of onion membrane
Dye adsorption properties of onion membrane
The FTIR spectrum of the onion membrane after adsorption of the dye was compared with the spectra of pure MB and dried onion membrane before adsorption in Figure 2i. The several peaks in the spectrum of dye-adsorbed membrane related to the MB and membrane are merged and slightly shifted. The peaks at 2,894 cm−1, 2,900 cm−1, and 1,744 cm−1 due to the C-H stretching in an aromatic methoxyl group and C = O stretching of carbonyl group of onion membrane, respectively, became stronger and were shifted to 2,916 cm−1, 2,850 cm−1, and 1,731 cm−1 in the dye adsorbed membrane . The sharp and strong peak at 1,592 cm−1 of MB due to the presence of C = C vibration and N-H bending was merged with a peak at 1,599 cm−1 of membrane and showed a broader peak at 1,598 cm−1 in the spectrum of the dye-adsorbed membrane . Like the two peaks at 1,443 cm−1 and 1,488 cm−1, which indicates C = N stretching, the peaks at 1,220 cm−1 and 1,250 cm−1 of C = C stretching in aromatic rings of MB are merged and form broad peaks at 1,416 cm−1 and 1,232 cm−1, respectively, in the spectrum of the dye-adsorbed membrane . Finally, the peaks corresponding to the C-O stretching of the onion membrane and the C-S bending of the MB rings appeared at 1,008 cm−1 and 954 cm−1, respectively . The FTIR results accompanied with the supportive results of the SEM and the optical microscope confirmed the adsorption of MB by the onion membrane.
Effect of time on adsorption
Comparison of kinetic models for adsorption of MB using onion membrane
Kinetic models and parameters
qe exp. (g.g−1)
qe cal. (g.g−1)
K2 × 10−4 (g.g−1.min−1)
qe cal. (g.g−1)
According to Equation 7, if the intra-particle diffusion is the main rate-controlling step, the plot of qt versus t0.5 should be linear and pass through the origin. However, the plot shown in Figure 4b did not pass through the origin and presented multilinearity, indicating the presence of two steps in the adsorption process. The intra-particle diffusion parameters for these steps are summarized in Table 1. The first linear segments can be attributed to the dye transfer from the solution onto the external surface or boundary layer of the onion membrane. The second step could be attributed to the final apparent equilibrium process, which reflects the intra-particle diffusion slowing down due to lowering the dye concentration in the solution . As seen from the data in Table 1, the intra-particle diffusion constants of the two linear segments are not similar and the first step comprises the bigger k3(1) value (0.1747 g.g−1.min-0.5) and the higher correlation coefficient 0.9414. This observation indicates that the adsorption of dye onto the onion membrane at the first section occurs more rapidly due to the availability of adsorption centers, then this is followed by the slow diffusion, which takes up to eight hours.
Effect of pH on adsorption
Effect of adsorbent amount on adsorption
Effect of the initial concentration of dye solution on adsorption
Isotherm of adsorption
Comparison of isotherm models for adsorption of MB using onion membrane
Isotherm models and parameters
qe cal. (g.g−1)
qe cal. (g.g−1)
Effect of temperature on adsorption
Thermodynamic parameters for adsorption of MB using onion membrane
The present study confirms the high potential of onion membranes with special physical and chemical characteristics for quick and efficient removal of MB from aqueous solutions. The amount of dye adsorbed varied with time, temperature, pH, adsorbent dosage, and initial dye concentration. The adsorption experiments indicated that onion membrane have a high MB adsorption capacity (1.9230 g.g−1) when 0.06 g of adsorbent was immersed in 250 ml of dye solution (0.9 g.L−1) with a pH of 7.1 at 20°C. The adsorption of MB by the onion membrane agreed with the pseudo-second-order model. Moreover, analysis of the equilibrium isotherms using the Langmuir and Freundlich isotherms showed that the Freundlich model fitted well with the experimental data. The thermodynamic studies suggested that the adsorption reaction was an exothermic and spontaneous process. Finally, the results suggest the synthesis of polymer-based membranes with similar physical and chemical properties of onion membrane as valuable and highly efficient adsorbents, which can be applied for dye removal in water treatment processes.
The authors thank Dr. Saeed Saber-Samandari of the Amirkabir University of Technology for his help with the SEM. The authors would like to acknowledge Mr. Ehsan Bahramzadeh (PhD student in Eastern Mediterranean University) for his help in preparing the manuscript.
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