Behavior of mesoporous activated carbon used as a remover in Congo red adsorption process

A_T

the equilibrium binding constant in Temkin model, L g⁻¹

b

constant in Langmuir isotherm model, L mg⁻¹

B

constant in Temkin model related to the heat of sorption, J mol⁻¹

C

thickness of the boundary layer

C_o

initial dye concentration, mg L⁻¹

C_e

equilibrium concentration of dye, mg L⁻¹

C_t

initial dye concentration at time t, mg L⁻¹

D_P

average pore diameter, nm

E

mean free energy, kJ mol⁻¹

FT-IR

Fourier transform infrared

K

equilibrium constant of adsorption, equal to q_m.b

k₁

rate constant in pseudo-first order model, min⁻¹

k₂

rate constant in pseudo-second order model, g mg⁻¹ min⁻¹

K_F

constant in Freundlich model, (mg g⁻¹) (L mg⁻¹)^1/n

k_id

rate constant in intra-particle diffusion model, mg g⁻¹ min^−0.5

m

dosage of adsorbent, g

N

number of data points

n_F

Freundlich power constant

q_D-R

maximum sorption capacity according to Dubinin–Radushkevich model, mg g⁻¹

q_e

adsorbed dye amount per gram of adsorbent at equilibrium, mg g⁻¹

q_e,cal

calculated amount of dye adsorbed per gram of adsorbent at equilibrium, mg g⁻¹

q_e,exp

experimental amount of dye adsorbed per gram of adsorbent at equilibrium, mg g⁻¹

q_m

monolayer adsorption capacity; mg g⁻¹

q_t

adsorbed dye amount per gram of adsorbent at time t, mg g⁻¹

R

the universal gas constant: 8.314 J mol⁻¹ K⁻¹

R²

correlation coefficient

R_L

separation factor or equilibrium parameter

SEM

scanning electron microscopy

t

time, min

T

temperature, K

V

solution volume, cm³

W

sorbent mass, g

XPS

X-ray photoelectron spectroscopy

α

initial retention rate constant in Elovich model, mg g⁻¹ min⁻¹

β

desorption rate constant in Elovich model, g mg⁻¹

ΔG^o

Gibbs free energy of adsorption, J mol⁻¹

ΔH^o

standard enthalpy of adsorption, kJ mol⁻¹

Δq (%)

normalized standard deviation

ΔS^o

standard entropy of adsorption, J mol⁻¹ K⁻¹

λ_max

maximum wavelength, nm

Contamination of ground and surface water by different organic pollutants has been a major environmental problem for a number of years. Dyes and pigments are widely used as the coloring organic agents. The total dye consumption in the textile industry worldwide is more than 10,000 tons per year and approximately 100 tons/year of dyes are discharged into waste streams by the textile industry. Dye effluents are esthetic pollutants that contain chemicals that exhibit toxic effect towards microbial populations and can be toxic and carcinogenic to organisms and mammals (Ahmad & Kumar 2010).

Several effective methodologies can be employed to remove dyes from wastewater. The most widely used advanced separation processes for dye removal are membrane filtration, photocatalysis, coagulation, precipitation, adsorption, flocculation and electrochemical techniques (Das et al. 2014; Nath et al. 2014; Dasgupta et al. 2015a, 2015b, 2016; Pettinato et al. 2015). Although these technologies show significant effectiveness, they remain economically poor. Among them, adsorption has been found to be superior to other techniques for dye wastewater treatment in terms of cost, simplicity of design, ease of operation and insensitivity to toxic substances (Garg et al. 2004; Singh et al. 2009). Most of the investigations are based on commercial and various other sources of activated carbons (ACs) and they have been found to be more effective than other adsorbents for dye removal. However, due to their high production costs, these materials tend to be more expensive than other adsorbents. This has led to a growing research interest in the production of ACs from renewable and cheaper precursors. The choice of precursor largely depends on its availability, cost, and purity, but the manufacturing process and intended applications of the product are also important considerations (Prahas et al. 2008; Bouhamed et al. 2012).

Congo red (CR), disodium 3,3′-((1,1′-biphenyl)-4,4′-diylbis(azo))bis(4-aminonaphthalene-1-sulphonate) is a benzidine-based anionic diazo dye prepared by coupling tetrazotized benzidine with two molecules of naphthionic acid. This anionic dye can be metabolized to benzidine, a known human carcinogen. CR is commonly used in the textile industry to give wool and silk a red color with yellow fluorescence. Effluent containing CR is largely produced from textiles, printing, dyeing, paper and plastic industries etc. The treatment of contaminated CR in wastewater is not straightforward, since the dye is generally present in sodium salt form giving it very good water solubility. Also, the high stability of its structure makes it difficult to biodegrade and photo-degrade (Vimonses et al. 2009). This dye is also toxic to animals and plants and thus its introduction to water streams is of potential health, environmental, and ecological concern (Ahmad & Kumar 2010). Therefore, it is very important to remove the remaining CR before discharge to clean water sources. Many ACs have been used for CR dye removal. But the adsorption capacity of most of these adsorbents is not large. So far, a study on CR dye adsorption using optimized AC prepared from tomato (Lycopersicon esculentum Mill.) waste (TW) by ZnCl₂ activation has not been reported in literature. Tomato is a very abundant and inexpensive material in Mediterranean countries. According to the records of the United Nations Food and Agriculture Organization, tomato is the most widely grown fresh vegetable product around the world with a production of 145.6 million tons. Turkey ranks fourth in the world with the production of 10 million tons of tomato (Sarısaçlı 2007).

The novelty of this study lies in the fact that it is the first report of CR adsorption capacity of an optimized AC prepared from TW (TAC) by ZnCl₂ activation. The purpose of this work is to examine the adsorptive property of TAC for the anionic textile dye CR removal from aqueous solutions. The ability of the adsorbent for CR removal was evaluated through both kinetic and equilibrium studies taking into account the major factors such as adsorbent dose, initial dye concentration, agitation time, ionic strength and temperature at natural pH. Furthermore, the TAC, before and after adsorption, was compared with scanning electron microscopy (SEM), attenuated total reflectance Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to analyze the corresponding adsorption mechanism.

TAC was prepared by ZnCl₂ (purchased from Sigma-Aldrich) activation using TW, collected from a tomato paste factory in the city of Adana, Turkey, as a precursor. Its preparation, effects of preparation parameters, some physicochemical characterizations, preparation yield and cost-estimation were discussed in our previous study (Sayğılı & Güzel 2016). Some of its physicochemical characteristics are given in Table 1.

The adsorbate, CR dye, was purchased from Sigma-Aldrich in Turkey. The general characteristics are given in Table 2. All the chemicals used throughout this study were of analytical-grade reagents.

Batch adsorption experiments were performed in a water bath shaker (Daihan-WSB-30) with temperature controlled. The pH of aqueous solutions was measured by a pH meter (Hanna pH meter 211). CR concentrations were determined by finding out the absorbance at the characteristic maximum wavelength using a UV-vis spectrophotometer (Perkin Elmer Lamda 25). A standard calibration graph was prepared by measuring the absorbance of different dye concentrations at maximum wavelength (λ_max = 497 nm) and unknown concentrations of dye before and after adsorption were determined form the calibration graph.

The surface physical morphologies of TAC before and after CR adsorption were identified by using SEM technique (JEOL JSM-6335F). FT-IR spectra were recorded with a Perkin Elmer Spectrum 100 spectrophotometer, in the range 650–4,000 cm⁻¹, using a resolution of 4 cm⁻¹ and 20 scans. The XPS measurements were performed using an XPS spectrometer (Thermo Scientific K-alpha) with an Al Kα monochromatized source.

All of the adsorption solutions in the experiment were prepared by dissolving CR dye into ultrapure water. The stock solution having 1,000 mg L⁻¹ concentration was prepared by dissolving the appropriate amount of CR dye and it was used in batch experiments by making the required dilution. Fifty millilitres of adsorbate solution was placed in a 100 mL Erlenmeyer flask at solution pH and room temperature (298 K) (Zahir et al. 2017). The flasks were agitated at 120 rpm for the required time in a shaker. In order to study the mechanism of adsorption of CR and determine the optimum adsorption conditions (Chahkandi 2017), the influences of various operating parameters such as adsorbent dosage (10–50 mg (50 mL)⁻¹), initial dye concentration (200–400 mg L⁻¹), contact time (5–480 min) and solution temperature (298–328 K) on adsorption at natural pH (6.95) level in aqueous solutions of CR dye were studied by varying the parameters under study and keeping other parameters constant. After each adsorption process, the samples were centrifuged (5,000 rpm, 10 min) for solid–liquid separation and the concentrations of dye in the solution before and after adsorption were analyzed at maximum absorbance wavelength of 497 nm by a UV-vis spectrophotometer. All the sorption experiments were repeated three times under similar conditions, and the values were averaged.

In this study, three kinetic models, namely, pseudo-first order (Lagergren 1898), pseudo-second order (Ho & Mckay 1999) and Elovich (Chien & Clayton 1980), were tested to find the best-fitted model for the experimental kinetic data. Besides, to elucidate the diffusion mechanism, the kinetic results were analyzed and fitted to the intra-particle diffusion model (Weber & Morris 1963). The linear forms of these models can be written as follows.

The isotherm data were then fitted to Langmuir (Langmuir 1918), Freundlich (Freundlich 1906), Dubinin–Radushkevich (D-R) (Dubinin & Radushkevich 1947) and Temkin (Temkin & Pyzhev 1940) isotherm models. These models can be expressed linearly as follows (Ahmed & Theydan 2012, 2013).

Desorption studies help to clarify the adsorption mechanism and recovery of the adsorbate and adsorbent. In this study, desorption studies were conducted in ethanol solution. For this, TAC was first loaded with dye by mixing 10 mg of sorbent with 50 mL of 100 mg L⁻¹ dye solution for 1 h at 298 K. The dye-loaded TAC was desorbed with 50 mL of ethanol solution (50%, V/V) for 1 h. Then, the dye-desorbed TAC was washed and used in five adsorption–desorption cycles to determine reusability of the TAC.

The effect of TAC dose on the amount of CR adsorbed was studied by varying the TAC dosages (10–50 mg (50 mL)⁻¹) in the test solution at fixed initial dye concentration (100 mg L⁻¹), temperature (298 K) and natural solution pH (6.95) value of CR dye for 1 h (figure not shown). The adsorption capacity decreased from 56.60 to 25.29 mg g⁻¹ as dose increased from 10 to 50 mg. This decrease is due to the reduction in total adsorption surface area available to CR dye, resulting from the overlapping or aggregation of adsorption sites with increasing amounts of TAC (Porkodi & Kumar 2007; Liu et al. 2014). Therefore, the 10 mg (50 mL)⁻¹ was chosen as the optimum dose and used for further adsorption experiments.

The effect of contact time on adsorption capacity of TAC for CR at different initial CR concentrations (200–400 mg L⁻¹) and temperature of 298 K is shown in Figure 1. As seen in this figure, the CR adsorption was noticeably fast at the initial stage, gradually became slow after 130 min and then reached equilibrium. This was because a large amount of vacant adsorption sites were available during the initial contact time and thereafter the available vacant sites decreased with contact time since more and more adsorption sites were occupied by the adsorbate. Moreover, the remaining vacant sites were much harder to be occupied because of the repulsive forces between the dye molecules adsorbed on the adsorbent surface and in the solution (Gao et al. 2015). The equilibrium was attained within 165, 190 and 230 min corresponding to CR initial concentrations of 200, 300 and 400 mg L⁻¹, respectively. Therefore, longer contact time was required to reach the equilibrium for the higher initial concentration since more dye molecules need to be adsorbed. According to the above results, 5 h was fixed as the contact time to guarantee the adsorption equilibrium in the subsequent adsorption studies.

The initial adsorbate concentration provides an important driving force to overcome all mass transfer resistances of dyes between the aqueous and solid phases (Ho et al. 2005). As also seen in Figure 1, the amount adsorbed increased from 156 to 236 mg g⁻¹ when the initial concentration was increased from 200 to 400 mg L⁻¹. The reason was that a higher initial concentration enhanced the driving force between the aqueous and solid phases and increased the number of collisions between dye ions and sorbent (Özer & Akkaya 2006). Moreover, it can be also found that longer time was required to reach the equilibrium at a higher initial concentration, since three stages were involved in an adsorption process. Firstly, the adsorbate molecules overcame the boundary layer effect, then passed through the boundary layer and reached the sorbent surface, and finally diffused into the internal pores of the adsorbent (Gao et al. 2015). It is clear that the removal of CR depends on the concentration of the dye.

The wastewater from textile-manufacturing or dye-producing industries contains various high salt concentrations which may significantly affect the performance of the adsorption process (Han et al. 2014). The influence of ionic strength on the adsorption capacity of CR onto TAC was analyzed in the sodium chloride solutions with concentrations ranging from 0.1 to 0.5 mol L⁻¹ at 298 K and natural solution pH value of CR (figure not shown). The adsorption capacity of CR increased from 39.10 to 144.52 mg g⁻¹ as the concentration of NaCl increased from 0 to 0.5 mol L⁻¹. This was due to the increase in the electrostatic interaction between CR and TAC caused by the increase in positive charges of the surface with increased ionic strength. Similar observations have been given by other researchers for CR sorption onto some adsorbents (Hu et al. 2010).

The influence of temperature was investigated by the addition of 10 mg of TAC to 50 mL of various initial concentrations prepared from stock solution of CR for 5 h at 298, 308, 318 and 328 K temperatures and pH 6.95. The results are shown in Figure 2. It is shown that there is an increase from 238 to 345 mg g⁻¹ in the adsorption capacity of CR as temperature increases from 298 to 328 K. The increase of the equilibrium adsorption with increase in temperature indicated that this process was endothermic. This may also be a result of an increase in the mobility of the CR dye ions as a result of the medium viscosity decrease with increasing temperature of the solution.

The experimental kinetic data in Figure 1 of CR adsorption onto TAC at different time intervals were examined using pseudo-first order, pseudo-second order, Elovich and intra-particle diffusion kinetic models (figure not shown). The kinetic parameters, R² and Δq(%) values determined from kinetic models used at different initial concentrations are presented in Table 3. The pseudo-first order kinetic model had the best fit, according to the high R² (>0.9949) and low Δq(%) (<0.50). In addition, the calculated adsorption capacity (q_e,cal) of the pseudo-first order model (143 mg g⁻¹) and experimental adsorption capacity (q_e,exp) (147 mg g⁻¹) have very similar values, when compared with the pseudo-second order and Elovich models. These results indicate that the adsorption of CR onto TAC obeys the pseudo-first order kinetics model.

The pseudo-first order, pseudo-second order and Elovich kinetic models cannot identify the diffusion mechanism. Therefore, the kinetic results were also analyzed by using the intra-particle diffusion kinetic model. According to the intra-particle diffusion model, a plot of q_t vs. t^0.5 should be linear if intra-particle diffusion is involved in the adsorption process, and if this line passes through the origin the intra-particle diffusion is the rate controlling step (Demiral & Gündüzoğlu 2010; Kumar et al. 2010).

The intra-particle diffusion plots drawn at different initial concentrations for CR adsorption onto TAC are multi-linear and there are three different portions, indicating the different stages in adsorption (not shown). The linear plots at each initial concentration did not pass through the origin, which indicates that the intra-particle diffusion was not only a rate controlling step. The k_id and C values calculated from the slope and intercept of the second region of the plots are given in Table 3. The k_id values increase with increase in the initial concentration. The results disclosed that the increase in dye concentration results in an increase in the driving force, which is an indication of the increase of the thickness of the boundary layer. The values of C were observed to be increasing with the initial concentration of CR, which indicates an increase in the thickness and the effect of the boundary layer (Güzel et al. 2015). The large C values (26–43 mg g⁻¹) of the kinetic study for CR dye confirm that the intra-particle diffusion was involved in the adsorption process, but was not the sole rate-limiting step. Similar observation was reported by other researchers previously (Khaled et al. 2009).

The experimental equilibrium data in Figure 2 are fitted with Freundlich, Langmuir, D-R and Temkin isotherm models (not shown). The isotherm constants, R_L and R² values are summarized in Table 4. In this table, comparison of the R² values shows that the Langmuir isotherm model fitted quite well with the experimental data with high R² values (>0.9913). As seen in Table 4, the constants q_m and b increased when the temperature is increased, indicating that the adsorption density was higher at higher temperatures. The maximum adsorption capacity increased from 312 to 434 mg g⁻¹ as the temperature increased from 298 to 328 K and the adsorption was an endothermic process. The K_F values showed the same trend as that of b in the Langmuir model. The R_L values were found to be between 0.166 and 0.150 at different solution temperatures and confirmed that the TAC is favorable for adsorption of CR dye under conditions studied. Magnitude of the exponent, 1/n, gives an indication of the favorability of adsorption (Yang & Qui 2010). The values of 1/n were found to be smaller than 1, indicating that CR is favorably adsorbed by TAC at all the temperatures studied. Furthermore, based on the analysis from the D-R model, the values of the mean adsorption energy (E) would give information about adsorption mechanisms, physical or chemical process. The adsorption behaviors were ascribed to physical adsorption when E was between 1.0 and 8.0 kJ mol⁻¹, but chemical adsorption when E was higher than 8.0 kJ mol⁻¹ (Zhang et al. 2012). The calculated E values presented in Table 5 were found to be in the range of 4.20–4.55 kJ mol⁻¹ over the range of temperatures studied, which shows that adsorption occurs physically. Also, the Temkin B isotherm constants at different temperatures in Table 4 show that the heat of adsorption increases when the temperature is increased, indicating an endothermic adsorption.

Thermodynamic parameters were calculated from Equations (13) and (14), respectively. Plotting the graph of ln K vs. 1/T from (Equation (14)) (not shown) yields a straight line (R² = 0.997) from which ΔH° and ΔS° were calculated from the slope and intercept, respectively. The values of ΔG^o were −1.39, −2.05, −2.42 and −2.75 kJ mol⁻¹ at 298, 308, 318 and 328 K, respectively. The negative ΔG^o values illustrated that the adsorption process was feasible and spontaneous. Also, the values of ΔG^o increased with increasing temperature, which indicated that adsorption was more beneficial at higher temperature. In addition, the values of ΔG^o in this study were within the ranges of −20 and 0 kJ mol⁻¹, which indicated that the mechanism of CR adsorption onto TAC was mainly a physical adsorption (Feng et al. 2011). The values of ΔH^o and ΔS^o for CR adsorption were determined as 11.92 kJ mol⁻¹ and 44.96 J mol⁻¹ K⁻¹, respectively. The positive ΔH^o is an indicator of the endothermic nature of the adsorption process and also its magnitude gives information on the type of adsorption, which can be either physical or chemical. The enthalpy of adsorption, ranging from 2.1 to 20.9 kJ mol⁻¹, corresponds to a physical adsorption (Anayurt et al. 2009). The positive ΔS^o value indicates an increase in randomness at the solid–solution interface.

SEM micrographs of TAC before and after CR dye adsorption are shown in Figure 3. As can be seen from this figure, the external surface of the TAC is full of cavities and the pores were of different sizes and shapes. It is clear that TAC appears to have a number of pores where there is a good possibility for dye to be trapped and adsorbed into these pores. SEM images showed a bright dark color on the surface (Figure 3(a)). The surface after adsorption was turned to light color (Figure 3(b)). This may be due to the adsorption of CR dye on the surface.

The FT-IR spectra of TAC before and after CR adsorption are shown in Figure 4. In the spectra of TAC in this figure, the broad band located in the region of 3,100–3,400 cm⁻¹ was related to O-H stretching vibrations. The band located at about 1,581.45 cm⁻¹ could be attributed to C=C vibration in aromatic rings (Lua & Yang 2005; Li et al. 2015). The band at 1,228.21 cm⁻¹ is due to the C-O stretching vibration of phenol group (Hameed & Daud 2008; Angın 2013). The FT-IR spectrum of TAC after CR adsorption in Figure 4 clearly shows some small peaks between 1,600 cm⁻¹ and 1,000 cm⁻¹, which may be attributed to the characteristic peaks of CR molecules. The peaks at 829.45 cm⁻¹ and 755.92 cm⁻¹ contribute to the out-of-plane bending vibration of =CH and C−H of benzene ring, respectively (Zhang et al. 2015). They are observed in the TAC after dye adsorption, which also demonstrates the existence of CR dye in the final sample.

XPS high-resolution spectra at the C1s, O1s and N1s regions of TAC with and without loaded dye are shown in Figure 5. As can be seen from Figure 5(a), the C1s spectrum of TAC was resolved into two individual component peaks, namely, (1) aliphatic or aromatic carbon group (CH_x, C–C/C=C; 284.68 eV); (2) alcohol and/or ether group (C-O; 285.91 eV). Figure 5(c) shows that the O1s region in the XPS spectrum of TAC exhibits two deconvoluted peaks at 531.11 and 532.8 eV, representing metal oxides and oxygen singly bonded to carbon in phenols and ethers groups, respectively (Burg et al. 2002; Biniak et al. 2013). Also, Figure 5(e) shows the high-resolution N1s spectrum of TAC. As can be seen from this figure, N1s XPS spectrum of TAC was fitted to two components: (1) free terminal amines (398.54 eV) and (2) carbon conjugated nitrogen (532.8 eV) (Marzorati et al. 2015). After exposure to CR, a new C1s peak was observed at 289.30 eV which is absent in the TAC sample; it corresponded with the characteristic carboxylic acid and/or ester group. Additionally, a drastic increase in relative content of elemental oxygen and nitrogen was observed for TAC (10.90% and 2.44%), in comparison to before adsorption (7.43% and 2.05%). Consequently, it may suggest that the interactions should take place between TAC and CR dye.

Advantages of these carbonaceous adsorbents include: ease of production, low cost, abundance of precursors, the use of green chemistry and reusability. Therefore, they may be efficient adsorbents for wastewater treatment. The comparison of maximum adsorption capacity of the TAC used in this work with various carbon adsorbents previously studied for the adsorption of CR are listed in Table 5. As can be seen, the CR adsorption capacity of TAC is higher than that of most other carbon adsorbents. Such comparison suggests that TAC may be an effective adsorbent for CR removal from contaminated water.

Cost is actually an important parameter for comparing the adsorbents. The overall cost of the adsorbent material is governed by several factors which include its availability (whether it is natural, industrial/agricultural/domestic wastes or by-products, or synthesized products), the processing required and reuse (Chowdhury et al. 2011).

TW is available in abundance and collected from a tomato paste factory at no cost; however, the handling charges for the collection and transportation will be involved. From the economic feasibility calculations made on our laboratory-scale production studies, the production yield was identified to be about 350 kg TAC per ton of TW (Sayğılı & Güzel 2016). According to our research from various sources via the internet, the cost of commercial ACs in the world market is in the range US$700–5,000 per ton based on the quality. The cost of AC production from TW from the calculations made on our three separate production studies was determined as about US$2,800 per ton. Accordingly, the cost of the maximum removal (435 mg CR g⁻¹ TAC) determined in this study is about US$28.0. Consequently, TAC is low cost and an excellent adsorbent for CR dye due to the cost and physicochemical properties.

The reusability of TAC was investigated by conducting the adsorption–desorption process for five cycles for CR dye, and the adsorption capacity in each cycle is presented in Figure 6. As clearly seen from this figure, the adsorption capacity of TAC decreased for each new cycle after desorption. The original adsorption capacity of TAC for CR dye is 90.49 mg g⁻¹. After five cycles, the adsorption capacities of sorbent drop down to 43.8 mg g⁻¹. The results showed that TAC could be regenerated and repeatedly used in wastewater treatment.

This study was focused on mesoporous structured activated carbon prepared from tomato paste waste, applied for effective and efficient CR adsorption removal in aqueous solution. The adsorption characteristics were examined at various contact times, solution temperatures, adsorbent dosages, and sodium chloride and initial dye concentrations. The amount of CR adsorbed onto the TAC increased with an increase in contact time, initial CR concentration, solution temperature and sodium chloride concentration, but decreased with increasing of adsorbent dosage. The kinetic data for the adsorption followed well the pseudo-first order kinetic model. The adsorption isotherm studies showed that the Langmuir adsorption isotherm model adequately described the adsorption of CR onto activated carbon and the maximum adsorption capacity was found to be 435 mg g⁻¹ at natural pH (6.97) of CR in aqueous solution and temperature of 328 K. The thermodynamic parameters ΔG^o, ΔH° and ΔS° showed spontaneous and endothermic adsorption. The adsorbate could be easy regenerated using ethanol, and excellent reusability was observed. Overall findings demonstrate that TAC formed by ZnCl₂ activation of TW, has great potential to be a low-cost and efficient adsorbent for CR removal in practical wastewater treatment applications.

The authors acknowledge the Scientific Research Fund of Dicle University for financial support (Project no. 12-ZEF-95).