Potential use of activated carbon derived from Persea species under alkaline conditions for removing cationic dye from wastewaters

Peer review under responsibility of University of Bahrain.

Abstract

The use of Persea americana has been studied as an alternative source of activated carbon for the removal of dyes from wastewater. Chemical activation using phosphoric acid was employed for the preparation of the activated carbon (C-PAN). The BET surface area and the total pore volumes were found to be 1593 m²/g and 1.053 cm³/g, respectively. This study investigates the effect of some parameters like, dye concentration, adsorbent dose, contact time and pH for the best comprehension of the adsorption manner. Adsorption kinetic follows pseudo-second order kinetic model. Langmuir and Freundlich isotherms models were used to analyze the adsorption equilibrium data and the best fits to the experimental data were provided by Langmuir model. Maximum adsorption capacity is equal to 400 mg/g of Basic Yellow 28 onto the activated carbon derived from Persea americana. Thermodynamic parameters, such as standard Gibbs free energy (ΔG⁰), standard enthalpy (ΔH⁰) and standard entropy (ΔS⁰) has been calculated. The adsorption process was found to be spontaneous and exothermic process.

Keywords:

• Activated carbon
• Adsorption
• Basic Yellow 28
• Kinetics
• Persea americana species
• Thermodynamic studies

1 Introduction

Among the different pollutants of aquatic ecosystems, dyes are a large and important group of industrial chemicals for which world production in 1978 was estimated at 640,000 tons of which about 20–30% are wasted in industrial effluents during the textile dyeing and finishing processes (Clarke and Anliker, 1980). Moreover, most of these dyes can cause allergy, dermatitis, skin irritation and also provoke cancer and mutation in humans (Clarke and Anliker, 1980; Baughman and Perenich, 1988). Also, the presence of very small amounts of dyes in water less than one ppm for some dyes is highly visible, very difficult to biodegrade, extremely difficult to eliminate in natural aquatic environments and undesirable (Banat et al., 1996; Robinson et al., 2001). Currently, much attention has been paid to the removal of dyes from industrial wastewater (Gupta et al., 2013, 2012a; Jain et al., 2003; Mittal et al., 2010a, 2009a).

Many treatment processes have been applied for the removal of dyes from wastewater such as: Fenton process (Behnajady et al., 2007), photocatalytic degradation (Gupta et al., 2012b, 2011; Saleh and Gupta, 2012; Saravanan et al., 2016, 2015a,b, 2014), sono-chemical degradation (Abbasi and Asi, 2008), photo-Fenton processes (Garcia-Montano et al., 2007), chemical coagulation/flocculation, ozonation, cloud point extraction, oxidation, nano-filtration, chemical precipitation, ion exchange, reverse osmosis and ultra-filtration (Lorenc-Grabowsk and Gryglewic, 2007; Malik and Saha, 2003; Malik and Sanyal, 2004; Banat et al., 1996; Tawfik et al., 2012). Adsorption techniques have gained favor due to their simplicity in operation, cost-effectiveness and efficiency in the removal of pollutants too stable for conventional methods (Mittal et al., 2010b, 2009b; Gupta and Nayak, 2012; Gupta et al., 2015a,b, 2012b; Saleh and Gupta, 2014). More recently, new researches on the effect of the ultrasound on the adsorption–desorption process have been reported (Asfaram et al., 2017; Alipanahpour Dil et al., 2017; Ghaedi et al., 2015; Mazaheri et al., 2016). Almost all the work related to adsorption techniques for color removal from industrial effluents was based on studies using natural and abundant materials. Activated carbon (Regti et al., 2016a,b, 2017b; Gupta et al., 1997, 2013; Saleh and Gupta, 2014) is still the most popular and widely used adsorbent. Activated carbon can be prepared using a variety of chemical (Attia et al., 2008) and physical (Heibati et al., 2015) activation methods and in some cases using a combination of both types of methods (Albero et al., 2009). Chemical activation is the process where the carbon precursor is firstly treated with aqueous solutions of dehydrating agents such as phosphoric acid, zinc chloride, sulfuric acid, and potassium hydroxide. Afterwards, the carbon material is dried at 373–393 K to eliminate the water traces followed by its heating between 673 K and 1073 K under nitrogen atmosphere (Faria et al., 2008). The physical activation consists of a thermal treatment of previously carbonized material with suitable oxidizing gases, such as air at temperatures comprised between 623 K and 823 K by using steam and/or carbon dioxide (Heibati et al., 2015).

We have already used the potential efficiency of animal bone meal material (El Haddad et al., 2012, 2013a,b), calcined mussel shells (El Haddad et al., 2014a,b), calcined eggs shells (Slimani et al., 2014) and activated carbon derived from Medlar nuts (Regti et al., 2017a) as new low cost adsorbents to remove anionic and cationic dyes from aqueous solutions. For ongoing our program research for the decolourization solutions, we have used activated carbon derived from natural and low cost materials (Persea americana Nuts). Other studies have been performed using activated carbon prepared from agricultural wastes for the removal of dyes from aqueous solution (Gupta et al., 2015a,b).

In this current study, the removal of Basic Yellow 28 dye from aqueous solutions onto activated carbon derived from Persea americana nuts was investigated. The choice of this agricultural wastes returns to the increased consumption of avocado fruit. In this fact, the effect of different parameters such as pH, contact time, adsorbent dose, initial dye concentration and temperature were investigated. The removal rate kinetics, thermodynamics and isotherms for Basic Yellow 28 adsorption onto treated Persea americana were also studied.

2 Materials and methods

2.1 Materials

Persea americana Nuts (PAN) used for the preparation of the activated carbon, was collected, washed with distilled water and dried at room temperature for several dyes. The dried material was then milled and separated by manually shaking stainless steel mesh screens with the opening of standard 0.45 mm. an appropriate weight of PAN was immersed in 25 mL of concentrated phosphoric acid with a mass ratio (1:4) for 12 h and then dried in an oven for 24 h at 105 °C. The PAN was placed in a sealed ceramic oven and pyrolysed at 500 °C and remained constant for 2 h. The resulting activated carbon (C-PAN) was filtered and rinsed with distilled water until the washings were free of excess acid. The final product of C-PAN was dried at 105 °C for 24 h, crushed to get the particles size of 200–300 µm and kept in desiccators for further study.

Adsorption–desorption isotherms of nitrogen at −196 °C were measure with an automatic gas sorption analyzer (NOVA-1000, Quanta Chrome Corporation, USA) in order to determine surface areas and total pore volumes.

The Characterization of C-PAN was achieved by FT-IR spectroscopy and X-ray powder diffraction measurements. FT-IR spectra (4000–450 cm⁻¹ range) were recorded with a Nicolet 5700 FT-IR spectrometer (Thermo Corp, USA) on samples prepared as KBr pellets. The polycrystalline sample of adsorbent was lightly ground in an agate mortar, pestle and filled into 0.5 mm borosilicate capillary prior to being mounted and aligned on an Empyrean PANalytical powder diffractometer (B.V Company) using Cu K_α radiation (λ = 1.54056 Å). Three repeated measurements were collected at room temperature in the 10° < 2θ < 60° range with a step size of 0.01°. Scanning Electronic Microscopy (SEM) images were obtained with (HITACHI-S4100, JAPAN) equipment operated at 20 kV.

De-ionized water was used throughout the experiments for solutions preparation. The adsorption studies for evaluation of the C-PAN adsorbent for the removal of the Basic Yellow 28 dye from aqueous solutions were carried out in triplicate using the batch contact adsorption method. Basic Yellow 28 dye used in this study abbreviated as BY28 was purchased from Sigma–Aldrich (Germany). Chemical structure of BY28 is shown in Fig. 1.

Figure 1 Chemical structure of Basic Yellow 28.

2.2 Adsorption experiments

For the adsorption experiments, fixed amounts of adsorbents (0.2 g/L–1.0 g/L) were placed in a 100 mL glass Erlenmeyer flask containing 50 mL of dye solution at various concentrations (40–100 mg/L), which were stirred during a suitable time (5–65 min) from 303 to 333 K. The pH of the dye solutions ranged from 2 to 12 was adjusted by 0.1 M HCl or 0.1 M NaOH to investigate the effect of pH on adsorption processes. Subsequently, in order to separate the adsorbents from the aqueous solutions, the samples were centrifuged at 3600 rpm for 10 min, and aliquots of 1–10 mL of the supernatant were taken. At predetermined time, the residual dye concentration in the reaction mixture was analyzed by centrifuging the reaction mixture and then measuring the absorbance by UV–Visible spectroscopy of the supernatant at the maximum absorbance wavelength of the sample at 436 nm (Zermane et al., 2010).

The amount of equilibrium adsorption q_e (mg/g) was calculated using the following expression:(1) where C_e (mg/L) is the liquid concentration of dye at equilibrium, C₀ (mg/L) is the initial concentration of the dye in solution. V is the volume of the solution (L) and W is the mass of biosorbent (g). The BY28 removal percentage (%) can be calculated as follows:(2) where C₀ is the initial dye concentration and C_e (mg/L) is the concentrations of dye at equilibrium.

3 Results and discussion

3.1 Characterization of C-PAN adsorbent

In order to investigate the surface characteristic of C-PAN adsorbent, FT-IR and XRD spectrums were recorded. As shown in FT-IR spectrum in Fig. 2, the frequencies of the absorption bands of C-PAN are 876, 1074, 1149, 1563, 2916 and 3415 cm⁻¹. The absorption band at 3415 cm⁻¹ is attributed to the hydroxyl groups (O—H) vibration, bands at 2916 and 1563 cm⁻¹ are assigned for P=O group, while bands at 1149 and 1074 cm⁻¹ are attributed for P—O group and finally band at 876 cm⁻¹ is attributed for P—H group (Liu et al., 2012 Wang et al., 2011). These function groups are due to the present of H₃PO₄ acid as an activation agent in the preparation of C-PAN.

Figure 2 FT-IR spectrum of C-PAN adsorbent.

Fig. 3 shows an X-ray powder diffraction pattern of C-PAN. An amorphous peak with the equivalent Bragg angle at 2θ = 24.6 was recorded, together with other peaks recorded at 2θ = 17.5°, 31.2° and 47.5°. Thus, as presented in Fig. 3, a wide and high intensity peak between 20 and 30° centered at 25° indicates the amorphous nature of C-PAN adsorbent. Furthermore, XRD data gave clear evidence to the high purity of the amorphous C-PAN.

Figure 3 XRD spectrum of C-PAN adsorbent.

The surface morphology of the C-PAN adsorbent was examined. Fig. 4 shows the SEM image indicating that the surface contains many pores of different size and different shape could be observed, revealing the potential adsorption power and suggesting that dyes were adsorbed on the mesopore or microspore. The BET surface area and the total pore volumes of the obtained Persea americana activated carbon were found to be 1593 m²/g and 1.053 cm³/g, respectively.

Figure 4 SEM image of C-PAN adsorbent.

3.2 Effect of C-PAN dosage on the adsorption capacity

The effect of the C-PAN dosage on the uptake (mg/g) is shown in Fig. 5. The adsorbent dosage was varied from 0.2 g/L to 1 g/L using V = 50 mL of BY 28 (100 mg/L) at ambient temperature. The adsorption capacities increases with increasing time and the response becomes constant for all amounts studied. The maximum uptake (q = 325 mg/g) was observed with the dosage of 0.2 g/L. The capacity of adsorption decreased slightly with the higher dosage of the adsorbent. The reason may be the interparticle interaction, such as aggregation, resulting from high adsorbent dose, aggregation would lead to a decrease in the total surface area of the adsorbent and on an increase in diffusional path length, while, at equilibrium, the adsorption efficiency (%) increases from 65% to 98% by increasing the C-PAN dose from 0.2 g/L to 1 g/L due to the increase in the number of sites available for dye adsorption, this is in agreement with our previous observations on BY28 dye removal by calcined bones biosorbent (El Haddad et al., 2013a,b).

Figure 5 Effect of C-PAN adsorbent dosage and contact time on adsorption capacity of BY 28 from aqueous solution. Initial dye concentration: 100 mg/L; V = 50 mL and temperature: 25 °C.

3.3 Effect of contact time and BY28 dye concentration

In order to achieve the effect of contact time and initial dye concentration (40–100 mg/L) with 0.2 g/L of C-PAN and temperature fixed at 25 °C on the adsorption capacity of BY28 by C-PAN, different experiments were realized. Fig. 6 depicts these results. It was observed that the adsorption capacity of BY28 is increased with increasing the contact time of all initial dye concentrations. Furthermore, the adsorption capacity dye is increased with the increase of initial dye concentration. Therefore, at higher initial dye concentration, the number of molecules competing for the available sites on the surface of C-PAN was high, hence, resulting in higher basic yellow adsorption capacity.

Figure 6 Effect of initial dye concentration and contact time on the removal of BY 28 from aqueous solution. C-PAN: 0.2 g/L; V = 50 mL and temperature: 25 °C.

Through these results, it appears that for the first 15 min, the adsorption capacity uptake was rapid then it proceeds at a slower adsorption rate and finally it attains saturation at 30 min. The obtained removal curves were single, smooth and continuous, indicating monolayer coverage of dye on the surface of adsorbent. Analogue results are shown in many works (Mittal et al., 2013; El Haddad et al., 2012).

3.4 Effect of pH on the removal of BY28 dye onto C-PAN

Fig. 7 shows the variation of dye removal versus pH. In this fact, to study the effect of initial pH of aqueous dye solution, we have used a concentration of BY28 dye at 100 mg/L and 0.4 g/L of C-PAN adsorbent, keeping the temperature at 25 °C and at different pH values in the range 2–12. The amount of dye adsorbed onto C-PAN was found to be constant for all pH values being studied. The experiments carried out at different pH show that there was no significant change in the percent removal dye over the entire pH range. This indicates the strong affinity of the dye to C-PAN and that either H⁺ or OH⁻ ions could influence the dye adsorption capacity. Whatever the pH of the dye solution, the dye removal % is constant and equal to 98%. The constant removal efficiency of BY28 dye by C-PAN adsorbent indicates that the ion exchange between the moieties of BY28 and C-PAN was not the only one mechanism for dye removal in this system. Other type of interactions may involve too. In this fact, the C-PAN can also interact with BY28 dye molecules by hydrogen bonding and van der Waals interactions. A similar result was obtained and previously achieved.

Figure 7 Effect of pH on the removal of BY 28 from aqueous solution onto C-PAN. Initial dye concentration: 100 mg/L, m = 0.4 g/L, V = 50 mL and temperature: 25 °C.

3.5 Thermodynamic study

Thermodynamic data reflect the feasibility and favorability of the adsorption process. Parameters such as free energy change (ΔG⁰), enthalpy change (ΔH⁰) and entropy change (ΔS⁰) can be determined by the change of equilibrium temperature. The free energy change of the adsorption reaction is given by (Smith and Van Ness, 1987; Khormaei et al., 2007):(3) where ΔG⁰ is the free energy change (kJ/mol), R is the universal gas constant (8.314 J/mol K), T is the absolute temperature (K) and K_C states the equilibrium constants (q_e/C_e). The values of ΔH⁰ and ΔS⁰ can be calculated from the following equation:(4) where K_C is plotted against 1/T, a straight line with the slope (−ΔH⁰/R) and intercept (ΔS⁰/R) are given. The calculated thermodynamic parameters are presented in Table 1.

Table 1 Thermodynamic data for the adsorption of BY 28 onto C-PAN.

Temperature (K)	ΔG^0 (kJ/mol)	ΔH^0 (kJ/mol)	ΔS^0 (J/mol K)
303	0.367	−3.806	−15.375
313	0.442
323	0.499
333	0.570

The positive values of ΔG⁰ (0.367 kJ/mol, 0.442 kJ/mol, 0.499 kJ/mol and 0.570 kJ/mol) at all temperatures (303 K, 313 K, 323 K and 333 K respectively) confirmed the thermodynamic feasibility. The increasing temperature from 303 to 333 K causes increasing of values of ΔG⁰ indicating a decrease in feasibility of adsorption at higher temperatures.

Values of ΔH⁰ and ΔS⁰ were obtained as −3.81 kJ/mol and −15.38 J/mol K, respectively. The negative value of ΔH⁰ shows that the adsorption is exothermic in nature. The negative value of ΔS⁰ suggests that the degree of randomness state at the solid/solution interface increased during the BY28 dye onto C-PAN.

3.6 Adsorption isotherms

Adsorption isotherms are basic requirements for the design of adsorption systems. It can express the relationship between the amounts of adsorbate (BY28) by unit mass of adsorbent (C-PAN) at a constant temperature. Herein, we analyzed our experimental data by Langmuir and Freundlich isotherms models. The best-fitting model was evaluated using the correlation coefficient. In this case, Langmuir and Freundlich isotherm models are applied for giving the experimental data during the isothermal adsorption studies. The linear form of Langmuir isotherm is expressed as (Safa and Bhatti, 2011):(5) where C_e (mg/L) is the equilibrium concentration of BY28 dye, q_e (mg/g) is the amount of BY28 adsorbed par unit mass of adsorbent. K_L (L/mg) and q_m (mg/g) are Langmuir constants related to rate of adsorption and adsorption capacity, respectively. A straight line with slope of 1/q_m and intercept of 1/q_mK_L is obtained when C_e/q_e is plotted against C_e. Table 2 shows the values of these parameters. The essential characteristics of Langmuir equation can be expressed in terms of dimensionless separation factor, R_L defined as:(6) where C₀ is the initial concentration of BY28 dye, the R_L value implies whether the adsorption is:

Table 2 Adsorption isotherm constants for removal of BY 28 onto C-PAN adsorbent at ambient temperature.

BY 28 (mg/L)	Langmuir isotherm model				Freundlich isotherm model
	RL	qm (mg/g)	KL	r^2	n	Kf	r^2
40	0.099	400	0.227	0.997	3.745	128.82	0.952
60	0.068
80	0.052
100	0.042

Unfavorable: R_L > 1

Linear: R_L = 1

Favorable: 0 < R_L < 1

Irreversible: R_L = 0.

As depicted in Table 2, R_L values versus the initial dye concentration at ambient temperature are calculated. It was observed that all the R_L values obtained were comprised between 0 and 1, showing that the adsorption of BY28 onto C-PAN was favorable. The R_L values decrease upon increasing the initial dye concentration, which indicates that the adsorption was more favorable at higher BY28 concentration. The maximum adsorption capacity was calculated (400 mg/g) under ambient temperature.

The linear form of Freundlich isotherm is expressed as (Deniz and Karaman, 2011):(7) where K_f and n are Freundlich isotherm constants. K_f (mg/g) (L/g) is an indication of the adsorption capacity, while 1/n is the adsorption intensity. The plot of log (q_e) versus log (C_e) that should give a straight line with a slope of (1/n) and intercept of log (K_f) and all data are shown in Table 2. In this case, the value found for n was superior to 1, which proves that the adsorption of BY28 onto C-PAN is favorable. The best fit of experimental data was obtained with the Langmuir model with r² value 0,997. This result suggests the monolayer and homogenous adsorption.

3.7 Adsorption kinetic

The adsorption kinetics gives the idea about mechanism of adsorption, from which efficiency of process estimated. In order to investigate the adsorption process of BY28 onto C-PAN adsorbent, the data obtained from adsorption kinetic experiments were simulated pseudo-first order and pseudo-second order models. The pseudo-first order equation is generally represented as follows (Ozcan et al., 2007):(8) where, q_e is the amount of dye adsorbed at equilibrium (mg/g), q_t is the amount of dye adsorbed at time t (mg/g), k₁ is the first-order rate constant (min⁻¹) and t is time (min).

The values of k₁, q_e calculated from the equation and the correlation coefficient (r²) values of fitting the pseudo-first order rate model at different concentrations are presented in Table 3. The linearity plots of log (q_e–q_t) versus time at different initial dye concentrations (Fig. 8) suggested that the process of dye adsorption did not follow the pseudo-first order rate kinetics. Also from Table 3, it is indicated that the values of the correlation coefficients are not high for the different dye concentrations. Furthermore, the estimated values of q_e calculated from the equation q_e (cal) differ substantially from those measured experimentally q_e (exp). That gives confirmation that the adsorption process of BY28 onto C-PAN did not obey the pseudo-first order model.

Figure 8 Pseudo-first order plots for different initial dye concentrations removal using C-PAN adsorbent. C-PAN adsorbent: 0.2 g/L; temperature 25 °C.

Table 3 Pseudo-first order and pseudo-second order kinetic parameters for BY 28 removal using C-PAN adsorbent.

Concentration of BY 28 (mg/L)	Pseudo-first order				Pseudo-second order
	k1 (min^−1)	qe (cal) (mg/g)	qe (exp) (mg/g)	r^2	k2 × 10^−4 (g/mg.min)	qe (cal) (mg/g)	r^2
40	0.096	119.67	179.27	0.926	17.8	181.81	0.999
60	0.080	166.34	253.70	0.924	9.93	263.15	0.999
80	0.046	151.00	291.42	0.974	6.12	285.71	0.998
100	0.050	188.36	325.97	0.993	4.42	344.82	0.997

The pseudo-second order equation is generally represented as follows (Moussavi and Mahmoudi, 2009):(9) where, k₂ is the pseudo-second order rate constant (g/mg.min). A plot of t/q_t and t should give a linear relationship if the adsorption follows pseudo-second order model. To understand the applicability of the model, a linear plot of t/q_t versus time under different dye concentrations were plotted in Fig. 9. The values of k₂, q_e and correlation coefficients (r²) were calculated from the plot and are given in Table 3. The q_e (cal) determined from the model along with correlation coefficients indicated that q_e (cal) is very close to q_e (exp) and correlation coefficient is also greater than 0.99. As a matter of consequence, the system BY28-C-PAN could be well described by the pseudo-second order model.

Figure 9 Pseudo-second order plots for different initial dye concentrations removal using C-PAN adsorbent. C-PAN adsorbent: 0.2 g/L; temperature 25 °C.

3.8 Comparison with other derived activated carbon

The value of maximum adsorption capacity q_m (mg/g) is of importance to identify which adsorbent shows the highest adsorption capacity and is useful in scale-up considerations. Some studies have been conducted using various types of activated carbon derived from many materials for removing dyes from aqueous solutions. Table 4 depicts a comparison of the adsorption capacity of C-PAN with that reported for other adsorbents. It can be seen from the Table 4 that the C-PAN adsorbent show a comparable adsorption capacity with the respect to other adsorbents, revealing that the C-PAN is suitable for the removal basic dye from aqueous solutions.

Table 4 Comparison of maximum monolayer adsorption capacities of C-PAN with those of various AC adsorbents.

AC adsorbent	qm (mg/g)	Reference
Rambutan peel	215.05	Njoku et al. (2014)
Stricta algae-based	526.00	Attouti et al. (2013)
Pomelo skin	501.10	Foo and Hameed (2011)
Pomegranate peel	370.86	Mohd et al. (2014)
Persea americana	400.00	This study

3.9 Regeneration of C-PAN

The economic aspect and environmental of the use of adsorbent materials, makes important the reuse of biosorbents, (Fig. 10) displays the difference in adsorption capacity(mg/g) of first and five cycles, the adsorption capacity was reduced from 235 to 37.3 mg/g, its due to the reduction in the specific surface of this material. Therefore, C-PAN shows excellent adsorption performance and regeneration, and its use can be extended to environmental applications for wastewater treatment.

Figure 10 Effect of regeneration cycles on the adsorption capacity of BY28 onto C-PAN. C-PAN: 0.4 g/L; V = 50 mL, BY 28 (100 mg/L) and temperature: 25 °C.

4 Conclusion

The results of this study show that C-PAN obtained from the nuts of Persea americana can be successfully used as a low cost and eco-friendly adsorbent for the removal of basic yellow 28(BY28) from aqueous solution and The following conclusions have been drawn from the above investigations:

–	The adsorption process was influenced by a number of factors such as adsorbent dose, pH, contact time, and the initial BY28 concentration.

–

The uptake (mg/g) was observed to increase with increasing initial dye concentration and decrease with increasing adsorbent dose. The adsorption parameters for the Langmuir and Freundlich isotherms were determined and the equilibrium data were best described by the Langmuir isotherm model with acceptable r2, which indicates that a homogeneous adsorption takes place between BY28 dye and C-PAN.

–	The pseudo-second order equation was best described for the kinetics of the C-PAN adsorption system due to its high r2. In addition, the theoretical qe generated by the pseudo-second order equation is in good agreement with the experimental qe value.

–	Thermodynamic studies indicate that the adsorption process is exothermic and spontaneous in nature.

Notes

Peer review under responsibility of University of Bahrain.