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Research Article | Open Access
Eco-Friendly Photocatalyst Derived from Egg Shell Waste for Dye Degradation
This study is focused on removal of dyes in water bodies using calcined egg shell powder obtained from waste egg shells as a new material for photocatalytic dye removal. The photocatalytic activity of calcined egg shell powder (CESP) was compared with the raw egg shell powder (RESP) under light and dark conditions. The results reveal that CESP has significantly a higher dye degradation capability of 80% compared to the RESP which is 20% under the same condition. Furthermore, under light conditions, CESP has shown nearly 50% increase in dye degradation compared to the same material in the dark. The kinetics of dye degradation follows pseudo-second-order kinetics suggesting the chemisorption process and the Freundlich adsorption isotherm is best fitted (R2 value is 0.96 for the linear fit) with the dye adsorption process. The application of CESP in industry is studied with a textile acid dye Lanasyn Rez F5B, and the results reveal it follows pseudo-second-order kinetics in dye removal.
Colour is a major parameter considered when defining the quality of water. Colour of water is often changed due to dissolved or suspended solids. Naturally, colour of water can be changed due to tannings from decomposing leaves or mud on a rainy day. However, water bodies are polluted because of the release of improperly treated industrial waste containing various dyes and pigments. These could prevent penetration of sunlight into the bottom of rivers and lakes causing a decrease in dissolved oxygen content and threat to aquatic plants and animals in threat. Furthermore, dyes and degraded by-products of dyes are toxic and carcinogenic [1, 2].
The removal methods available for textile dyes can be classified as physical methods, biological methods, and chemical methods . Physical methods often consider adsorption of a dye on to a suitable sorbent. Here, activated carbon is the most efficient in removing variety of dyes . However, it has limitation in application due to the high cost and material loss during regeneration . Consequently, there are some low-cost sorbents introduced with biological origin as alternatives [4–8]. Apart from physical methods, biological treatments include use of algae or bacteria to metabolize decomposition of dyes. [9, 10]. Conversely, most of the dyes are designed to give a long-lasting colour to the textile. Therefore, degradation of these dyes requires advanced chemical oxidation methods. Some of the most studied and used oxidants are ozone  and hydrogen peroxide in the presence of UV light. The Fenton process is also used in degradation of textile dyes . All these methods utilize highly reactive radical species of oxygen or hydroxyl to breakdown the dye [12–14]. However, these reactive oxygen species are short lived; hence, industrial application of hydrogen peroxide is limited. There are novel methods emerging in situ generation of hydrogen peroxide/hydroxyl radical for more viable use [12, 15]. Metal oxides such as titanium oxide [16–18] and zinc oxide [19, 20] are widely studied and used in degradation of dye materials photocatalytically. The inherent problems which limit the use of TiO2 is the band gap which is ∼3.2 eV and requires UV light to operate as a photocatalyst . Studies are extended to include composites of TiO2 or ZnO with other metal oxides aiming higher degradation efficiency and photocatalytic activity using visible light [21–25].
There is constant search for novel photocatalysts other than traditional TiO2- or ZnO-based catalysts. In recent years, there have been number of studies conducted on calcium-based photocatalysts such as calcium hydroxyapatite  and calcium antimony oxide hydroxide  for dye degradation. In addition, calcium hydroxide is also used as a catalyst for transesterification in biodiesel production . Although there are many alternatives for calcium sources, egg shells can be utilized as it is available in abundant  and often considered as a waste material which is discarded without further use. The current trends to use egg shell waste are in agriculture as a fertilizer or as animal feed . The anatomy of egg shell indicates the presence of pores which facilitate air exchange for the growing embryo . Therefore, this material serves as an effective surface for dye adsorption processes. There have been studies conducted for the use of egg shell powder and egg shell membrane for dye removal [32–38]. Furthermore, egg shell powder has been used to develop several composite materials for dye removal incorporating TiO2, polymer, and clay materials [39–42]. There is a significant enhancement in dye removal activity observed for egg shell powder composites compared to egg shell powder alone [39–42].
However, the dye removal capability of egg shell powder which is thermally treated above the thermal decomposition temperature (830°C) is not studied so far. Also, the photocatalytic activity is not well established for the calcined egg shell powder (CESP). Therefore, the main focus of this paper is to evaluate the dye removal efficiency of CESP as an adsorbent and as a photocatalyst. The term calcination is defined as the heating of a substance to high temperature allowing oxidation and release of any volatile substances . In many papers, calcined egg shell powder indicates well purified and dried egg shell powder (100–120°C) . However, in in this paper, the “calcined egg shell powder (CESP) denotes egg shell powder heated at 900°C which is above the decomposition temperature of calcium carbonate.
Dye removal efficiency and photocatalytic dye degradation of the two materials were compared using methylene blue as the model dye. The dye adsorption processes are studied with the Langmuir and Freundlich isotherms, and rates were evaluated as first-order and pseudo-second-order kinetics. Furthermore, dye removal capacity of textile acid dye Lanasyn F5B was studied as an application of the CESP in industrial dye removal.
2. Materials and Methods
2.1. Materials and Chemicals
Egg shells were obtained from the university cafeteria. Methylene blue (Merck limited, India) and industrial dye (Lanasyn Rez F5B) (Riddhi Siddhi Trading Co. Maharashtra, India) were purchased and used as received.
2.2. Calcined Egg Shell Powder Preparation
Egg shells were collected from the university cafeteria and cleaned with distilled water thoroughly to remove all the unnecessary materials adhered to the egg shells. Then, egg shells were dried at 120°C in the oven for 24 hours. Dried egg shells were ground and calcined in a muffle furnace (HD-230 “PAD,” Forns Hobersal SL, Barcelona) under static air conditions at 900°C to produce CESP for 3 hrs.
2.3. Calcined Egg Shell Powder Characterization
Characterization of CESP and RESP was done by Fourier-transform infrared (FTIR, Brucker, Alpha) and X-ray diffraction (Rigaku Ultima IV) (S1.Tables 1–4 for supporting information regarding experimental conditions).
2.4. Photocatalytic Experiments
The photocatalytic activity of CESP and RESP was compared by evaluating dye removal efficiencies of solutions with dye and egg shell powder under the dark and light conditions. The removal efficiency of calcined egg shell powder and raw egg shell powder was calculated by taking 0.50 g of each powder dispersed in MB solutions (50 ml, 10 mg/l) in a glass stoppered bottle. Then, for each type of powder, one solution was kept under light condition, while the other solution was kept under dark condition for 2 hrs. Light condition was provided by 23 W of fluorescent bulb inside a light box. Solutions were centrifuged, and absorbance was measured using the UV-visible spectrophotometer (GENESYS, Thermo Scientific) every 30 minutes. Each experiment was conducted in triplicate. Removal efficiency was calculated by using equation (1). Removal efficacies were repeated for CESP under light conditions in triplicate and in the dark for two hours and 9 hrs. RESP study was repeated for 2 hrs and 9 hrs duration (S2.Tables 1 and 2 and S3.Tables 1 and 2 and S3.Figures 1 and 2 for supporting information regarding experimental conditions):where is the removal efficiency, C0 is the initial MB concentration (mg/l), and Cf is the MB concentration at equilibrium (mg/l).
2.5. Adsorption Experiments
Adsorption studies were conducted for CESP (0.50 g) placed in a glass stoppered bottle with 50 ml solutions of MB aqueous solutions (4 mg/l, 6 mg/l, 8 mg/l, 10 mg/l, 12 mg/l, 14 mg/l, 16 mg/l, 18 mg/l, and 20 mg/l). These solutions were studied under the light conditions. Then, absorbance measurement of each solution was obtained after 28 hrs. Amount of dye adsorbed and degraded at the equilibrium (qe) was calculated by equation (2) for Langmuir and Freundlich isotherms. A control experiment was conducted in parallel without the CESP. Experiment was repeated twice:where qe is the amount of MB adsorbed at the equilibrium (mg/g), C0 is the initial MB concentration (mg/l), Ce is the MB concentration at equilibrium (mg/l), V is the volume of the solution (L), and W is the mass of the dry adsorbent (g) (S4.Figures 1–4 for supporting information regarding repeated trials for isotherms fitting):
2.6. Kinetic Studies
CESP (0.50 g) was dispersed in MB solution (10 mg/l, 50 ml). One solution was used as the control without exposing to light by covering it with an aluminium foil. All samples were kept under the 23 W fluorescent bulb for 2 hrs inside the light box. Absorbance of each sample was measured at 15-minute intervals. The amount of MB adsorbed from the solutions at various time intervals was calculated by equation (3). Same procedure was followed for RESP. The experiment was conducted in triplicate:where qt is the amount of MB adsorption at time t (mg/g), C0 is the initial MB concentration (mg/l), Ct is the concentration of MB at any time t (mg/l), V is the volume of the solution (L), and W is the dry mass of the adsorbent (g).
2.7. Removal Efficiency of Industrial Dye
The removal efficiency of industrial dye was studied using 50 mg/l, 60 mg/l, 70 mg/l, 80 mg/l, 90 mg/l, and 100 mg/l of industrial dye solutions. Maximum absorbance wavelength of the industrial dye was 530 nm. Adsorption isotherms were developed using 1.5 g of CESP dispersed in 25 ml of 50 mg/l, 60 mg/l, 70 mg/l, 80 mg/l, 90 mg/l, and 100 mg/l dye solutions. Then, absorbance of these solutions was measured after keeping them under the light condition for 28 hours to calculate . Kinetic experiments were conducted for 25 ml of 100 mg/l industrial dye solutions with 1.5 g of CESP. These solutions were kept under light for 2 hours, and absorbance of samples was measured for every 15 minutes.
3. Results and Discussion
3.1. Sample Characterization
According to the data of XRD illustrated in Figures 1(a) and 1(b), crystalline calcite in RESP and calcium hydroxide in CESP are present. Therefore, it is clear that calcium carbonate in raw egg shell powder has been transformed into the calcium oxide at 900°C. But due to exposure to the atmosphere, it is converted to calcium hydroxide. The presence of narrower peaks indicates that CESP and RESP are crystalline. The size of the crystallite is calculated using Debye–Scherrer’s formula  considering crystals spherical with cubic symmetry (K = 0.94) for Cu-Kα wavelength (1.5406 Å) for peak between 2Θ values 30 deg. to 60 deg.
Accordingly, the average crystallite size of RESP is about 30–45 nm and that of CESP is about 15–23 nm.
Tsai et al  have characterized RESP with FTIR spectroscopy. The FTIR spectrum of RESP (Figure 2) indicates presence of organic matter corresponding to C-H vibration peaks at 2969 cm−1, 2875 cm−1, and 2527 cm−1. These FTIR bands are absent in egg shell powder calcined at 900°C, indicating decay of organic matter. The peak at 1416 cm−1, 879 cm−1, and 705 cm−1 is related to the carbonate anion [45, 46]. Furthermore, the band at 515 cm−1 for CESP indicate the OH band and for the calcium oxide band. According to the XRD data and FTIR data, further confirmation is provided for the transformation of calcium oxide to calcium hydroxide.
3.2. Comparison of Removal Efficiencies and Photocatalytic Activity
The removal efficiencies of MB were studied with raw egg shell powder and calcined egg shell powder recording UV-Visible spectra every 30 minutes under dark conditions. The UV-visible spectrum of MB clearly indicates a decrease of the band at 665 nm as MB is adsorbed into egg shell powder, as illustrated in Figures 3(a) and 3(b).
Figures 3(a) and 3(b) represent the absorbance curves of MB due to RESP and CESP, respectively. MB concentration of both solutions has been reduced gradually within 2 hours. But absorbance curves of MB solution with CESP indicate blue shift of the band at around 560 nm after 75 minutes. But this is absent in the solution of MB-RESP, and only reducing absorbance curve is observed. If only adsorption is the process which was proceeding in the experiment, reducing curves must be generated without any movement of curves. This suggests another reaction is the reason for the shifting of this curves.
Egg shell is designed by nature to give protection to the embryo from harmful attacks of bacteria other factors. A thin layer of the protein matrix is bonded to the egg shell containing calcite. The egg shell has many pores to allow exchange of gaseous products and water for healthy growth of the embryo . Therefore, decrease in MB concentration in solution observed in raw egg shell powder and calcined egg shell powder under dark could be due to physical adsorption, which is in accordance with the work of Tsai and coworkers [32, 45]. Furthermore, the work of El-Kemary and coworkers on TiO2/egg shell nanocrystals reported increasing removal efficiency when the calcination temperature is increased from 400°C to 900°C for the acid red nylon 57 dye . Therefore, the enhanced dye degradation of CESP compared to RESP is in agreement with the above data.
The photocatalytic activity was studied for RESP and for CESP. Figures 4(a) and 4(b) indicate the absorbance curve of MB solutions with RESP and CESP in the presence of visible light. Figure 4(b) clearly indicates a degradation reaction proceeding in the solution with CESP. In the presence of light, even at the beginning, the absorbance curve has been blue shifted. But these changes are not observed in the solution with the RESP. Only deducing curves can be observed. The solution with CESP indicates reduction of MB concentration, and bands of the absorbance curves after 105 minutes have been changed significantly, shifting the maximum absorbance towards 615 nm. However, Figure 3(b) (dark condition) indicates small shifting of curves with comparison to Figure 4(b) (light condition). Studies of Yang et al. on photocatalytic degradation of MB with the rice husk/TiO2/SiO2 catalyst observed rapid degradation of MB in the presence of visible light .
CESP mainly consists of Ca(OH)2 which can give basic solution when dissolved in water. Hence, it can be presumed that the reason for the alteration of adsorption bands could be due to the basicity. In order to clarify this fact, absorbance curves of MB solution were measured with different pH values. Figure 5 interprets the absorbance curves of MB at different pH values. The pH value of solutions is adjusted with using HCl and NaOH solutions. According to Figure 5, it is clear that pH alone is not the agent for shift in UV-visible bands in MB solution. However, the pH of the solution is an important factor which decides the surface potential of the material. In a study where TiO2 is used as the photocatalyst, it is observed that the dye adsorption and degradation are enhanced in a higher pH value. This is due to the availability of the anionic surface (TiO− in this case) to bond with the -C-S+=C- group in MB to initiate degradation of MB producing the sulfone group . Therefore, possibly a similar phenomenon can be suggested for the CESP.
Removal efficiency of MB was calculated for both RESP and for CESP. Figures 6(a) and 6(b) compare the removal efficiencies of MB under dark and light conditions for both types of egg shell powders. In the presence of light, removal efficiency of CESP is higher than RESP. The reported efficiency for CESP after 2 hours is 82%, while that of RESP is 21%. Comparatively, CESP has higher removal efficiency than RESP under light conditions. Furthermore, when CESP considered, the removal efficiency in the presence of light (82%) is much higher than that in the dark (44%). These figures clearly interpreted that removal efficiency of MB in the light condition is higher compared to the removal efficiency in the dark condition. The calculated removal efficiencies under dark and light conditions are given in S2.Tables 1 and 2 in the supporting information, respectively.
The removal efficiencies were calculated with standard deviation between 10 and 5%. This suggests a possible degradation pathway involving photocatalytic activity. The semiconductor materials absorb photons from the UV radiation and promote electrons in the valence band to the conduction band, creating a hole in the valence band. These excited electrons can react with dye or oxygen to produce the superoxide anion. At the same time, holes in the valence band can also react with water molecules to produce hydroxyl radical. Consequently, the presence of reactive oxygen species leads to degradation of dye . RESP consists of mainly CaCO3, and CESP has Ca(OH)2. The reported band gap of these materials is 5.07 eV  and 5.7 eV [49, 50], respectively. These band gaps suggest photocatalytic activity under visible light conditions is hardly possible. However, studies of Zang et al. suggest observed band gap for Ca(OH)2 is further reduced to 3.2 eV subtracting energy gap between bottom of the conduction band to the Fermi level (2.5 eV) from the band gap of conduction and valence (5.7 eV) . This allows excitation wavelength around 400 nm; thus, catalyst could be active at the margin of visible radiation.
When photo is excited, MB is converted to cationic dye radical which degrades the by-products. The absorption bands at 665 nm and 615 nm of MB correspond to monomer and dimer units [51, 52]. The decrease in band at 665 nm in the presence of light with calcined egg shell powder suggests rapid degradation of monomer units. In addition, the blue shift observed is due to further degradation of phenothiazine . Furthermore, the study using CESP was continued for more than 9 hrs, and the results reveal over 90% efficiency of MB removal under light conditions end of first 180 min, while it was only 65% under dark conditions. The recorded UV-visible spectra for long exposure study under light and dark conditions are given under supporting information S3.Figures 1 and 2. The efficiencies and maximum amount removed (qmax) are given in S3.Tables 1 and 2, respectively.
3.3. Kinetic Experiments for CESP and RESP
The kinetic behaviour of the dye degradation was studied with pseudo-first-order and pseudo-second-order kinetics to evaluate the photocatalytic effect. The first-order and second-order kinetic models are given in equations (4) and (5).
First-order kinetic model equation :where qt and q1 are the amount of dye adsorbed at time t and at equilibrium (mg/g) and k1 is the first-order kinetic rate constant.
Second-order kinetic model :where q2 is the maximum adsorption capacity, qt is the adsorbed amount at time t (mg·g−1), and k2 is the equilibrium rate constant for pseudo-second-order kinetic (g mg−1·min−1).
Kinetic modes for MB on to the CESP are shown in Figures 7(a) and 7(b). When correlation coefficient values of pseudo-first-order and pseudo-second-order are compared, the R2 value of pseudo-second-order is higher than the pseudo-first-order value. Therefore, pseudo-second-order kinetic is the best fitted kinetic model for MB on to the CESP. The kinetic study was conducted for the RESP in the same way, and RESP also follows pseudo-second-order kinetic model which is in agreement with Tsai and coworkers [32, 45] for MB adsorption on to egg shell powder. Since both CESP and RESP indicate pseudo-second-order kinetics, it is possible that the rate limiting step could be bimolecular and chemisorption governs the adsorption process  According to the results, CESP has higher adsorption capacity (11.025 mg·g−1) compared to the RESP (0.23 mg·g−1). Table 1 provides comparison of kinetic study for both types of powders.
Initial MB concentration = 10 mg/l; amount of CESP = 0.5 g.
3.4. Adsorption Isotherms for Calcined Egg Shell Powder
Langmuir isotherm:where qe is the equilibrium concentration on the calcium oxide (mg/g), Ce is the equilibrium concentration in the solution (mg/l), qmax is the monolayer adsorption capacity of the calcined egg shell powder, and KL is the Langmuir constant (l/mg).
Freundlich isotherm:where qe is the equilibrium concentration on the calcium oxide (mg/g), Ce is the equilibrium concentration in the solution (mg/l), and Kf (l/g) and n are Freundlich constants.
According to Figures 8(a) and 8(b), Freundlich isotherm has a high R2 value than the Langmuir isotherm. Therefore, the Freundlich isotherm is fitted better than the Langmuir isotherm. When Freundlich constant n gets some value other than 1, it gives a parabolic shape for the Freundlich isotherm. The value of n observed for calcined egg shell powder is one, suggesting a linear adsorption of the dye. These linear Freundlich graphs are vital in the industrial applications because linear adsorption provides direct applications for water treatment industries via direct calculations. Linear adsorption Freundlich graph can be used for calculation of dosage of adsorbent that should be added for maximum removal of the dye from the solution. Furthermore, the Freundlich adsorption isotherm suggests monolayer adsorption of the dye on the surface of calcined egg shell powder, thus facilitating the photocatalytic activity. Tsai and coworkers have studied the adsorption isotherm models for raw egg shell powder with MB. They have observed that the Freundlich isotherms fit more than Langmuir isotherms similar to the results obtained here [32, 45].
3.5. Removal of Industrial Dye Using Calcined Egg Shell Powder
Lanasyn F5B is an acid dye widely used in textile industry. The UV-visible spectra of the industrial dye and removal efficiencies at different time periods are given in S5.Figure 1 and S5.Table 1. The removal this dye using CESP was studied constructing adsorption isotherms and kinetic studies. According to Figure 9, the industrial dye does not a follow linear adsorption with Langmuir isotherm or Freundlich isotherm. The Freundlich adsorption isotherm constant (KF) for the Lanasyn F5B dye is 0.058 Lg1, and the Freundlich adsorption isotherm constant (n) is calculated as 0.985. The linear fit for the Freundlich adsorption isotherm is 0.6017 for R2 suggesting the multilayer adsorption process may proceed with CESP. The industrial dye can be removed with more than 70% efficiency photocatalytically by CESP (S5.Figure 2).
Kinetic studies on this dye removal follow pseudo-second-order kinetics, as illustrated in Figure 9(d), which is comparable to the pseudo-second-order kinetics observed for MB adsorption with CESP. Table 2 provides a summary of adsorption isotherm data and results of kinetic experiments conducted for industrial dye using CESP.
Initial MB concentration = 100 mg/l; amount of CESP = 1.5 g.
3.6. Comparison of Use of Egg Shell Powder for Removal of Dye
Egg powder has been used to remove several well-known dyes. The studies have been conducted to evaluate dye removal capacity considering egg shell powder, egg shell membrane, and mixture of both components. In addition, there are several composite materials developed utilizing egg shell powder. Table 3 provides a comparison of performance of egg shell power in dye removal. In general, egg shell powder has a higher affinity for the anionic dyes compared to the cationic dyes. The MB adsorption on to CESP in this study is higher than the previously reported value for egg shell powders. Furthermore, recent studies on composite materials developed with egg shell powder indicate better removal capacity.
The photocatalytic ability of CaO was compared with TiO2 by the work of Madhusudhana and coworkers for Violet GL2B azo dye and found that CaO is a more efficient catalyst compared to TiO2 at high pH values. . In another study, indigo carmine dye degradation was studied with CaO . Table 3 provides a summary of utilizing egg shell-based materials for dye removal applications.
Egg shells are waste materials often without any reuse. Therefore, this study focused on how waste egg shell can be converted to a photocatalyst to remove toxic dye material. The two materials, raw egg shell powder (RESP) and calcined egg shell powder (CESP), have been compared for the dye removal efficiency under light and dark conditions. The enhanced dye degradation efficiency is observed with CESP in the presence of light compared to RESP suggesting it is a photocatalytic effect. The pseudo-second-order kinetic studies conducted for the RESP and CESP suggest MB is chemisorbed on the material. The equilibrium amount of dye adsorbed on the CESP is higher compared to RESP. The adsorption process best fitted with the Freundlich isotherm, suggesting monolayer of MB is adsorbed on to RESP and CESP. Furthermore, the CESP can be used to remove industrial dye Lanasyn F5B with 1.17 mg/g equilibrium dye adsorption capacity. In conclusion, the CESP has enhanced performance for dye degradation compared to RESP, highlighting its potential use as a novel and eco-friendly photocatalyst for dye degradation.
Data relevant for XRD studies and efficiency calculation are included in the supporting information. If further data related to analysis are required, the corresponding author can provide upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest.
The authors thank Department Science and Technology; Department of Animal Science, Uva Wellassa University; and Instrument Center, Faculty of Applied Sciences, University of Sri Jayewardenepura, for providing facilities for this study. Dr. Thusitha Etampawala is acknowledged for the discussions regarding XRD studies. The authors also thank Uva Wellassa University for the funding under grant UWU/R/G/2015/18.
S1.Table 1: measurement conditions—raw egg shell powder (RESP). S1.Table 2: qualitative analysis results—raw egg shell powder and peak list (RESP). S1.Table 3: measurement conditions of calcined egg shell (CESP). S1.Table 4: qualitative analysis results of calcined egg shell and peak list (CESP). S2.Table 1: removal efficiency of RESP and CESP in dark condition. S2.Table 2: removal efficiency of RESP and CESP in light condition. S3.Figure 1: long exposure of MB on to the CESP in the light condition. S3.Figure 2: long exposure of MB on to CESP in dark condition. S3.Table 1: data of absorbance, equilibrium concentration, and equilibrium concentration of dye on the CESP and removal efficiency (dark). S3.Table 2: removal efficiency data of long exposure of MB with CESP powder (light). S4.Figure 1: adsorption isotherm trial for CESP. S5.Figure 1: UV-visible spectra of industrial dye. S5.Table 1: removal efficiency of industrial dye with CESP. S5.Figure 2: removal efficiency of industrial dye with CESP. (Supplementary Materials)
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