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Abstract

In the present work, a new low-cost activated carbon was prepared from paper mill sludge in order to remove Cr(VI) ions from aqueous solution. The effects of adsorbent dosage, pH, contact time, metal ion concentrations, and temperature on adsorption efficiency were studied by experimental tests. The maximum equilibrium uptake of Cr(VI) by the adsorbent was 23.18 mg g⁻¹ at optimum pH = 4.0, contact time of 180 min, and temperature of 45℃. Analysis of equilibrium adsorption data in terms of several isotherm models revealed that Langmuir isotherm with respect to Freundlich isotherm indicates better agreement with the experimental data. The kinetics of Cr(VI) adsorption onto activated carbon was described with the pseudo-second-order model which indicates the dominance of chemisorption mechanism. Thermodynamic parameters indicated that the Cr(VI) adsorption onto adsorbent was feasible in nature, spontaneous, and endothermic.

Keywords

Cr(VI)paper mill sludge kinetics thermodynamics activated carbon

Introduction

Nowadays, the contamination of water resources has led to serious problems because of the indiscriminate disposal of heavy metals (Rozada et al., 2008). Chromium compounds are highly toxic contaminant. These compounds are generated from industrial plants such as leather tanning, textile dyeing, electroplating, and wood preservations (Bayazit and Kerkez, 2014). In the aqueous solutions, chromium exists in two stable oxidation states: Cr(III) and Cr(VI). The Cr(VI) compounds are 500 times more toxic than the trivalent one due to its high water solubility (Malkoc et al., 2006; Yang et al., 2015). According to United States Environmental Protection Agency, the most Cr(VI) level for discharging into potable water is 0.05 mg l⁻¹ and for inland surface waters is 0.1 mg l⁻¹. Therefore, it is essential for industries to reduce the Cr(VI) from their effluents to allowable level before discharging it into the environment (Muthukumaran and Beulah, 2011). Different treatment methods such as ion exchange, chemical coagulation, solvent extraction, electrochemical reduction, reverse osmosis, and adsorption have been employed for the removal of hexavalent chromium from industrial wastewaters (Baral et al., 2006; Cavaco et al., 2007; Hafez et al., 2002; Lakshmipathiraj et al., 2008; Song et al., 2004; Venkateswaran and Palanivelu, 2004). Among these methods, adsorption has been widely used due to its high efficiency and uncomplicated operation conditions (Babel and Kurniawan, 2004; Gueye et al., 2014). In recent years, elimination of Cr(VI) from water has been reported to be efficient by adsorbents derived from a wide variety of carbonaceous materials such as fertilizer industry waste material (Gupta et al., 2010), rice husk (Krishnani et al., 2008), coconut shell (Babel and Kurniawan, 2004), Ocimum americanum seed pods (Levankumar et al., 2009), olive oil industry waste (Malkoc et al., 2006), Eichhornia crassipes root (Giri et al., 2012), peanut shell (Al-Othman et al., 2012), distillery sludge (Selvaraj et al., 2003), walnut shell (Ghasemi et al., 2015), sago waste (Vennilamani et al., 2005), Aerobically Digested Activated Sludge (ADAS)-based activated carbon (Gorzin and Ghoreyshi, 2013), etc. Typical capacities of Cr(VI) adsorption by these adsorbents have been presented in Table 1. A recent problem in the paper industry is the generation of an extreme quantity of sludge during the paper production process and secondary treatment of wastewater. The conventional disposal options such as land filling, conversion to fertilizer, sea dumping, and incineration have been used to remove municipal and industrial sludge from environment. However, these methods are expensive and have significant limitations such as soil and air pollutions. Using paper mill sludge as a carbonaceous and lignocellulose raw material offers the dual benefits of reduction in the sludge volume and production of an effective adsorbent with lower cost than commercial activated carbons (Kacan, 2016; Khalili et al., 2000; Liu et al., 2010).

Table 1.

Adsorption capacity of different low-cost materials for Cr(VI) removal from aqueous systems.

Adsorbent	Maximum Cr(VI) concentration (mg l⁻¹)	pH	Adsorption capacity (mg g⁻¹)	Reference
Fertilizer industry waste	100	2	15.24	Gupta et al. (2010)
Walnut shell	100	2	40.83	Ghasemi et al. (2015)
Rice husk	100	2	13.1	Sugashini and Begum (2015)
Coconut shell charcoal	25	4	10.88	Babel and Kurniawan (2004)
Peanut shell	100	2	8.31	Al-Othman et al. (2012)
Sago waste	20	2	5.78	Vennilamani et al. (2005)
E. crassipes root biomass	100	4.5	36.34	Giri et al. (2012)
Olive oil industry waste	200	2	18.69	Malkoc et al. (2006)
Distillery sludge	25	3	5.7	Selvaraj et al. (2003)
O. americanum seed pods	200	1.5	83.33	Levankumar et al. (2009)
The ADAS-based activated carbon	250	2	70.15	Gorzin and Ghoreyshi (2013)
Paper mill sludge	100	4	23.18	This study

ADAS: Aerobically Digested Activated Sludge.

The literature provides few experimental data for preparation of adsorbent from paper mill sludge as a low-cost carbonaceous material. Herein, the prepared adsorbent was studied as a low-cost activated carbon for Cr(VI) removal from aqueous solutions. The isotherms, kinetics, and thermodynamics of Cr(VI) adsorption were studied to evaluate the effect of several parameters such as adsorbent dosage, pH, initial ion concentration, contact time, and temperature on the adsorption process. The Brunauer–Emmett–Teller (BET), Fourier transform infrared (FTIR), and SEM-energy dispersive X-ray (EDX) analyses were performed to assess the pores volume, chemical form, and surface morphology of the activated carbon, respectively. The equilibrium data have been analyzed using Langmuir and Freundlich isotherms. Moreover, the kinetics and thermodynamics of Cr(VI) adsorption were theoretically interpreted using different models.

Materials and methods

Materials

The paper mill sludge was collected from wastewater treatment plant of Mazandaran Wood and Paper Factory in Sari, Iran. All the primary reagents used in this study were of analytical grade purchased from Merck, Germany and used without further purification. A stock solution of 1000 mg l⁻¹ Cr(VI) ions was made by dissolving 2.8289 g of K₂Cr₂O₇ in distilled water. The working solutions of desired concentrations were made by suitable dilution of the stock solution with distilled water.

Synthesis of activated carbon

Proximate and ultimate analyses of the paper mill sludge are presented in Table 2. Raw materials were dried in an oven at 105℃ for 24 h and then the materials were crushed and sieved to desired particle size. For chemical activation, the crushed materials were soaked with 5 M ZnCl₂ solution in a ratio of 1:3 by mass and mixed for 8 h at 85℃ by magnetic stirrer. After chemical activation, materials were dried in an oven for 24 h at 110℃. Carbonization of the impregnated sludge was accomplished out in absence of air in a horizontal furnace at 650℃ for 1 h under N₂ gas flow of 0.4 l min⁻¹ at heating rate of 10℃ min⁻¹. At the end of carbonization, the samples were allowed to cool down to room temperature in N₂ gas flow. The products obtained were washed by 1 M HCl and then filtered and rinsed by warm distilled water several times until reached to neutral pH to remove residual inorganic matter. Finally, the raised products were dried in an oven at 105℃.

Table 2.

Proximate and ultimate analyses of the prepared activated carbon (results are expressed on a dry basis, except for the humidity).

	Ultimate analysis (wt. %)			Proximate analysis
Parameter	C	H	N	Humidity (%)	Ash
Value	19.07	2.7	3.68	53	33.1

Analysis

The textural characteristics of the prepared activated carbon from paper mill sludge including specific surface area, total pore volume, and pore size distribution were obtained from N₂ adsorption/desorption data at 77 K using surface area analyzer with BEL sorp-mini II, Japan model. The surface area of adsorbent was estimated by the BET equation. The total pore volume was calculated at relative pressure, p p₀⁻¹ of 0.99 and micropore volume was determined using the t-plot method. Mesopore volume can be calculated by deducting micropore volume from total pore volume (Gorzin and Ghoreyshi, 2013). The porous properties of the prepared activated carbon have been presented in Table 3 and the accuracy of values is based on the apparatus accuracy. The surface morphology of the developed activated carbon was visualized by using scanning electron microscopy with a VEGA II TESCAN, CZECH model. The specimens were coated with a thin layer of gold (Au) before scanning in the electron microscope. The qualitative elemental composition of the activated carbon was determined by EDX spectroscopy with a VEGA II TESCAN, CZECH model. FTIR spectra of the prepared activated carbon were recorded before and after Cr(VI) ion adsorption in the frequency range of 400–4000 cm⁻¹ by Shimadzu, FTIR1650 spectrophotometer, Japan.

Table 3.

Characteristic of produced activated carbon.

Parameter	Value
BET surface area (m² g⁻¹)	133.34
Total pore volume (cm³ g ⁻¹)	0.2525
Micropore volume (cm³ g ⁻¹)	0.1212
Mesoporosity (%)	52
Average pore diameter (nm)	7.6651

Batch adsorption procedure

The batch adsorption experiments were carried out to investigate the influence of major affecting parameters like adsorbent dosage, pH, initial Cr(VI) concentration, contact time, and temperature on the amount of adsorption capacity. For each experimental run, a specified weight of prepared activated carbon was added to 0.1 l of the Cr(VI) solution in a 0.25 l Erlenmeyer flask. The contents in the flask were shaken in incubator shaker with KS 4000i control, IKA model at 180 r/min. During the study, the adsorbent dosage was varied from 1.5 to 7 g l⁻¹, the pH from 2 to 6, the initial Cr(VI) concentration from 10 to 100 mg l⁻¹, the temperature from 25 to 45℃, and the contact time from 0 to 240 min. The pH of solution was adjusted using either 0.1 N HCl or 0.1 N NaOH solutions to the required value. After shaking, the solutions were filtered through a Whatman No. 42 filter paper.

Analysis of Cr(VI) ions

The residual concentration of Cr(VI) ions in the sample was determined by UV–visible spectrophotometer with 2100 SERIES, UNICO model using 1,5 diphenyl-carbazide in acidic solution. The absorbance of the purple colored complex was measured at 540 nm wavelength after 10 min (Association et al., 1915).

The Cr(VI) ion removed by adsorbent was determined according to

% R = \frac{C_{0} {- C}_{e}}{C_{0}} \times 100

(1)

where R is the percentage of Cr(VI) adsorbed by activated carbon and C₀ and C_e are the initial and equilibrium concentrations (mg l⁻¹), respectively.

The adsorption capacity of prepared activated carbon was calculated by the following equation

q_{t} = \frac{C_{0} V_{0} {- C}_{t} V_{t}}{m}

(2)

where q_t is the amount of Cr(VI) ion adsorbed per unit mass of adsorbent (mg g⁻¹), V₀ is the volume of the initial Cr(VI) solution and V_t is the volume of the Cr(VI) solution at time t (l), respectively, and m is the mass of prepared activated carbon (g).

Batch desorption procedure

In our previous work (Gorzin and Ghoreyshi, 2013), it was seen that the weakly bonded Cr(VI) ions were removed at acidic or neutral pH, while NaOH solution demonstrated a good desorption efficiency. Hence, in this study the reversibility of Cr(VI) ions adsorption on activated carbon was investigated using NaOH (0.5 M) as desorption agent. Desorption experiments were conducted using the following procedures. At first, 0.01 l of Cr(VI) solution with initial Cr(VI) concentration of 50 mg l⁻¹, pH 4.0, and 3.5 g l⁻¹ of the adsorbent was shaken at 45℃ in a 0.01 l centrifuge tube. After 180 min, solution was centrifuged and the residual Cr(VI) content of the supernatant solution was measured. For desorption purpose, NaOH solution (0.5 N) was added to adsorbent and shaken at 45℃ for 180 min. Finally, the solution was centrifuged and the residual concentration of Cr(VI) ions in the solution was determined. Adsorbent was washed with distilled water until pH reached to neutral. Then Cr(VI) concentration (50 mg l⁻¹, pH 4.0, and 3.5g l⁻¹ of adsorbent) was added to adsorbent for second adsorption–desorption cycle and shaken for 180 min at 45℃. Then, the NaOH solution (0.5N) was added to adsorbent. This procedure was repeated several times and after each cycle, the reused adsorbent was washed with distilled water. Sample was collected after 180 min to evaluate Cr(VI) recovery by the following equation

Metal recovery = \frac{Amount of metal ion desorbed}{Amount of metal ion adsorbed} \times 100

(3)

Results and discussion

Surface characterization

SEM image and EDX spectrum of synthesized activated carbon before and after adsorption of Cr(VI) ions have been shown in Figure 1(a) and (b), respectively. As can be seen in Figure 1(a), the surface of prepared activated carbon has an irregular surface containing pores with different size and shapes. As shown in Figure 1(b), some pores have been closed due to adsorbent surface converge by adsorbate. By comparing the EDX spectrum of Cr(VI) unloaded and loaded adsorbent, it can be concluded that Cr(VI) is adsorbed onto the activated carbon surface.

Figure 1.

SEM-EDX images of prepared adsorbent from paper mill sludge (a) before and (b) after adsorption of Cr(VI) ions. EDX: energy dispersive X-ray; SEM: scanning electron microscope.

In this work, the FTIR spectra were obtained in order to analyze the mechanism of the Cr(VI) adsorption and identify functional groups on the surface of prepared activated carbon. Figure 2 shows the FTIR spectra of the adsorbent before and after Cr(VI) adsorption. The FTIR spectra bands indicate three major absorption bands including carboxyl, hydroxyl, and amino groups. The differences in the band intensity can be attributed to interaction of Cr(VI) ions with functional groups on the adsorbent surface. The broad absorption band at about 3456 cm⁻¹ is attributed to the complexation between –OH groups which has been shifted to 3476 cm⁻¹ after Cr(VI) adsorption (Gorzin and Ghoreyshi, 2013). The next shift observed at 1618 to 1612 cm⁻¹ may be due to the complexation between Cr(VI) ions with carboxylic group (–C = O) (Levankumar et al., 2009). According to FTIR analysis reported in literature (Bansal et al., 2009; Giri et al., 2012), observed shift from 1004 to 1031 cm⁻¹ is likely due to the interaction of nitrogen from amino group with Cr(VI). Doke and Khan (2012) used treated waste newspaper as a low-cost adsorbent. They revealed that the most abundant functional groups found on the prepared activated carbon include aromatic (C–H), carboxylic acid (C–O, C = O, and O–H), carbonyl (C = O), alkane (C–H), and amine (N–H, C–N). On the other hand, since the paper mill sludge has a significant amount of phosphorus (shown in EDX analysis), so this peak may be referred to the aliphatic P–O stretching (Dehghani et al., 2016).

Figure 2.

FTIR spectra of the prepared adsorbent from paper mill sludge (a) without Cr(VI) adsorption and (b) with Cr(VI) adsorption. FTIR: Fourier transform infrared.

Adsorption results

Effect of adsorbent dosage

Adsorbent dose is a mainly important parameter in adsorption which determines the amount of removal as well as the economics of process. To examine the effect of adsorbent dosage on Cr(VI) adsorption, different amounts of activated carbon from 1.5 to 7 g l⁻¹ were used at temperature of 35℃ and contact time of 240 min for fixed initial Cr(VI) concentration of 50 mg l⁻¹ without pH adjustment. Experimental results in Figure 3 showed that the percentage of Cr(VI) removal increased from 47.68 to 76.32% when the adsorbent dosage increased from 1.5 to 7 g l⁻¹. At higher dosage of adsorbent, more adsorption sites are available for Cr(VI) ions. As shown in Figure 3, the adsorption capacity was decreased from 17.63 to 5.52 mgg⁻¹ when the adsorbent dosage increased from 1.5 to 7 g l⁻¹. The decrease in the Cr(VI) adsorption capacity with increase in adsorbent dosage is due to more active sites of adsorbent remained unsaturated during the Cr(VI) adsorption process (Radnia et al., 2012). It is evident that after adsorbent dosage of 3.5 g l⁻¹, extra increase in adsorbent dose did not appreciably change in Cr(VI) removal. Therefore, for all further experiments adsorbent dose was fixed at 3.5 g l⁻¹ solution.

Figure 3.

Effect of adsorbent dose on equilibrium adsorption capacity and removal efficiency (C₀ = 50 mg l⁻¹, T = 35℃).

Effect of pH

In this study, the effect of pH on adsorption capacity and removal efficiency of Cr(VI) ions by the prepared activated carbon was studied in the pH range of 2.0–6.0 using 0.35 g of adsorbent in 0.1 l of chromium solution with concentration of 50 mg l⁻¹ at 35℃. Figure 4 shows that the removal percentage of Cr(VI) increases from 68.86 to 84.72% while pH increases from 2 to 4. Then, the removal percentage decreases to 79.21% with more increase in pH up to 6.0. The experimental results indicated that the maximum removal percentage of Cr(VI) by the synthesized activated carbon occurred at pH 4.0, and more increase in pH did not illustrate any considerable improvement in Cr(VI) removal efficiency and adsorption capacity. Therefore, the pH value of 4.0 was selected as the optimal pH for more Cr(VI) adsorption experiments to achieve suitable removal efficiency and Cr(VI) uptake capacity. At low pH (2.0–4.0), removal efficiency was increased due to high protonated adsorbent surface. The high protonated adsorbent surface makes a strong electrostatic attraction between oxy-anion (CrO₄²⁻, Cr₂O₇², etc.) and positively charged activated carbon surface (Gupta et al., 2010). The decrease in removal percentage of Cr(VI) with increase in pH (4.0–6.0) may be due to increase in OH⁻ ions on the activated carbon surface which improves a repulsive force between negatively charged adsorbent surface and the oxy-anions of chromium (Selvaraj et al., 2003).

Figure 4.

Effect of initial pH on equilibrium adsorption capacity and removal efficiency (C₀ = 50 mg l ⁻¹, T = 35℃ and adsorbent dose = 3.5 g l ⁻¹).

Effect of contact time

The removal efficiency of Cr(VI) was investigated as a function of contact time at two initial concentrations, i.e. 50 and 100 mg l⁻¹ and optimum condition of pH (4.0) and adsorbent dosage (3.5 g l⁻¹). The experimental results were depicted in Figure 5. It is obvious that the removal percentage of Cr(VI) from aqueous solution increases with an increase in contact time and then decreases gradually till the equilibrium conditions after 180 min. At the initial period of adsorption, the rate of hexavalent chromium ions binding with activated carbon surface is great due to the availability of an abundance of adsorption sites, and then metal adsorption becomes slow. This is because of the diffusion of metal ions into the adsorbent pores by intraparticle diffusion (Kumar et al., 2008).

Figure 5.

Effect of contact time on removal efficiency (C₀ = 50 and 100 mg l⁻¹, T = 45℃ and adsorbent dose = 3.5 g l⁻¹).

Effect of initial Cr(VI) concentration

Chromium solutions with several initial concentrations in the range of 10–100 mg l⁻¹ at 45℃ and optimal pH 4.0 were equilibrated using 0.35 g adsorbent dose. Figure 6 indicates that the Cr(VI) ion equilibrium adsorption capacity increases from 2.82 to 23.18 mgg⁻¹ and removal efficiency decreases from 98.4 to 76.74% when its initial concentration increases from 10 to 100 mg l⁻¹. At higher initial concentrations, the active sites of prepared adsorbent would be surrounded with more Cr(VI) ions in the solution; hence the equilibrium adsorption capacity of activated carbon increases with increasing the Cr(VI) ion concentration which enhances the adsorption process (Ghorbani et al., 2008). In addition, the removal percentage decreases by an increase in metal initial concentration. At low initial concentrations, the ratio of initial number of chromium ions to the accessible active sites of adsorbent is low; therefore, the removal efficiency of Cr(VI) is higher and at higher concentrations, further residual Cr(VI) ions remain in the aqueous solution (Radnia et al., 2012).

Figure 6.

Effect of initial concentration of Cr(VI) on equilibrium adsorption capacity and removal efficiency (C₀ = 10–100 mg l⁻¹, T = 45℃, pH = 4, and adsorbent dose = 3.5 g l ⁻¹).

Effect of temperature

Temperature had an obvious effect on the extent of hexavalent chromium adsorbed onto activated carbon. To investigate the effect of temperature on the removal percentage of Cr(VI), experiments were carried out at different temperatures, i.e. 25, 35, and 45℃ using optimum adsorbent dose (3.5 g l⁻¹) with initial concentration of 100 mg l⁻¹ and pH equal to 4.0. The effect of temperature on Cr(VI) removal can be concluded from Figure 7. The removal percentage of Cr(VI) from aqueous solution was 65.08% at 25℃ which increased to 76.73% at 45℃. The increase in removal efficiency with the increase in temperature shows that the Cr(VI) adsorption process is endothermic in nature.

Figure 7.

Effect of temperature on removal efficiency (C₀ = 100 mg l ⁻¹, pH = 4, and adsorbent dose = 3.5 g l ⁻¹).

Adsorption isotherm

Adsorption isotherms were used to investigate the relationship between the amount of Cr(VI) ions adsorbed onto the activated carbon surface and the concentration of chromium ions in the aqueous phase at equilibrium state. The experimental adsorption data for Cr(VI) ions have been analyzed by Langmuir and Freundlich isotherms in nonlinear form which have been shown in Figure 8.

Figure 8.

(a) Langmuir and (b) Freundlich isotherm curves with experimental data at different temperatures (pH = 4, adsorbent dose = 3.5 g l⁻¹).

The Langmuir adsorption isotherm is based on the homogeneous surface by identical active sites and the monolayer adsorption of Cr(VI) ions onto the adsorbent surface which was described by Langmuir (1918)

q_{e} = \frac{q_{\max} {bC}_{e}}{{1 + bC}_{e}}

(4)

where q_e is the amount of metal ion adsorbed per unit weight of adsorbent at equilibrium (mg g⁻¹), q_max is the maximum adsorption capacity (mg g⁻¹), C_e is the equilibrium Cr(VI) concentration (mg l⁻¹), and b is the Langmuir equilibrium constant which is related to the bonding energy of metal ion adsorption (l mg⁻¹).

The Freundlich isotherm model is applied based on multilayer adsorption on heterogeneous surface and may be expressed as (Freundlich, 1906)

q_{e} {= k}_{F} C_{e}^{1 / n}

(5)

where k_F(mg g⁻¹(l mg⁻¹)^1/n) and n are the Freundlich constants referred to the adsorption capacity and intensity, respectively.

The model parameters along with nonlinear correlation coefficient (R²) are summarized in Table 4. It was observed from the correlation coefficient values that the Langmuir isotherm fitted the experimental data better than the Freundlich model which indicates the homogenous distribution of active sites on adsorbent surface.

Table 4.

Constants of Langmuir and Freundlich isotherms for Cr(VI) adsorption of the developed activated carbon.

	Langmuir constant				Freundlich constant
Temperature ℃	b (l mg⁻¹)	q_max (mg g⁻¹)	Relative error	R²	k_F (mg g⁻¹ (l mg⁻¹)^1/n)	n	Relative error	R²
25	0.0629	30.21	0.0311	0.9899	3.2469	1.85	0.0752	0.9893
35	0.1974	25.27	0.0038	0.9943	6.2724	2.58	0.1325	0.9656
45	0.3555	25.23	0.0161	0.9887	8.1187	2.83	0.1142	0.9673

Adsorption kinetics

Adsorption kinetics studies provide an understanding of adsorption rate and controlling mechanism of the process. In order to elucidate the adsorption kinetics of Cr(VI) ion by prepared adsorbent, the pseudo-first-order and the pseudo-second-order kinetics models were employed to test the experimental data.

Nonlinear forms of the pseudo-first-order (Singh and Tiwari, 1997) and the pseudo-second-order kinetic model (Ho and McKay, 1999) are represented by equations (6) and (7), respectively

q_{t} = q_{e} (1 - e^{- k_{1} t})

(6)

q_{t} = \frac{k_{2} q_{e}^{2}}{1 + k_{2} q_{e} t} t

(7)

where q_e and q_t (mg g⁻¹) indicate the adsorption capacities at equilibrium and at time t (min), respectively; k₁ (min⁻¹) is the pseudo-first-order rate constant; and k₂(g mg⁻¹min⁻¹) is the rate constant of pseudo-second-order equation. In order to determine the kinetic parameters involved in equations (6) and (7), Sigma Plot software version 12.0 was used to curve fitting and data analysis. Nonlinear model of pseudo-first-order and pseudo-second-order kinetics model for Cr(VI) adsorption onto prepared adsorbent were shown in Figure 9(a) and (b), respectively.

Figure 9.

(a) Pseudo-first-order and (b) pseudo-second-order kinetics models for Cr(VI) adsorption onto prepared adsorbent at different concentrations (pH = 4,T = 45℃, and adsorbent dose = 3.5 g l ⁻¹).

By comparing the R² and adsorption capacity values predicted in Table 5, the pseudo-first-order kinetic model is not appropriate for modeling the Cr(VI) adsorption onto prepared adsorbent. It is obvious from the results that the pseudo-second-order kinetic model (R²> 0.99) has been provided a better correlation of experimental data. This kinetic model suggests that the metal ion adsorption process is controlled by the chemisorption mechanism.

Table 5.

Pseudo-first-order and pseudo-second-order kinetic parameters of Cr(VI) adsorption onto adsorbent.

Initial concentration C₀ (mg l⁻¹)	Pseudo-first-order						Pseudo-second-order
Initial concentration C₀ (mg l⁻¹)	q_e,exp (mg g⁻¹)	q_e,cal (mg g⁻¹)	k₁ (min⁻¹)	Relative error	R²	q_e,cal (mg g⁻¹)	k₂ (g mg⁻¹ min⁻¹)	Relative error	R²
10	2.82	2.7	0.2914	0.0425	0.9927	2.75	0.03693	0.0248	0.9972
50	13.68	12.58	0.069	0.0804	0.9249	13.58	0.00082	0.0074	0.9790
100	23.18	19.74	0.1186	0.1484	0.9332	21.1	0.00092	0.0897	0.9755

In this study, in order to determine the diffusion mechanism, intraparticle diffusion model was also used. Such plots may present a multilinearity, indicating that two or more steps are taking place. The rate-determining step of adsorption reaction may be external film diffusion, intraparticle diffusion, or interaction. The first, sharper step is the external surface adsorption or instantaneous adsorption stage. The second step is the gradual adsorption stage, where intraparticle diffusion is rate controlled. The third step is the final equilibrium stage where intraparticle diffusion starts to slow down due to extremely low adsorbate concentrations in the solution. In fact the metal ions are slowly transported via intraparticle diffusion into the particles and are finally retained in micropores (Özacar and Şengil, 2004). This model is presented as

q_{t} = k_{id} t^{0.5}

(8)

where k_id is the intraparticle diffusion rate constant (mg g⁻¹min^−1/2). As can be seen in Figure 10, the external surface adsorption (stage 1) is absent because of fast completion. All plots have same general features, initial linear portion (stage 2) followed by second linear portion (stage 3).

Figure 10.

Intraparticle diffusion kinetic model for the adsorption of Cr(VI) ions on activated carbon.

The intraparticle rate constants as well as regression correlation coefficients (R²) have been presented in Table 6. As can be seen, the values of regression correlation coefficients (R²) are considerably smaller than those for pseudo-second-order kinetic model represented in Table 5. As a result, the intraparticle diffusion model cannot be a rate-limiting step.

Table 6.

Intraparticle diffusion kinetic model parameters for Cr(VI) ions adsorption by activated carbon.

C₀ (mg/l)	Initial concentration		Interparticle diffusion
C₀ (mg/l)	k_id1 (mg g⁻¹ min^1/2)	R²	k_id2 (mg g⁻¹ min^1/2)	R²
10	0.62	0.9205	0.01	0.9165
50	2.15	0.9603	0.34	0.8904
100	3.89	0.9507	0.43	0.9742

Adsorption thermodynamics

To investigate thermodynamic behavior of the Cr(VI) adsorption onto prepared activated carbon, thermodynamic parameters of Cr(VI) adsorption including Gibbs free energy change (ΔG⁰), enthalpy change (ΔH⁰), and entropy change (ΔS⁰) were determined using the following equations

{Δ G}^{0} = - RTln K_{eq}

(9)

where R (8.314J mol⁻¹ K⁻¹) is the universal gas constant, T (K) is the temperature, and K_eq is the adsorption equilibrium constant which is defined as follows

K_{eq} = \frac{C_{ads}}{C_{e}}

(10)

where C_ads and C_e (mg l⁻¹) are equilibrium concentrations of Cr(VI) on the prepared adsorbent and in the aqueous phase, respectively.

The values of ΔH⁰ (enthalpy change), ΔS⁰ (entropy change), and ΔG⁰ (Gibbs free energy change) can be estimated from the slope and intercept of plot

ln K_{eq}

versus T⁻¹, respectively, as shown in Figure 11

ln K_{eq} = - \frac{Δ H^{0}}{RT} + \frac{Δ S^{0}}{R}

(11)

{Δ G}^{o} {= Δ H}^{0} - T Δ S^{0}

(12)

The values of thermodynamic parameters for Cr(VI) adsorption onto prepared adsorbent have been presented in Table 7. The negative values of Gibbs free energy change confirm that the process is feasible and spontaneous. Decrease in values of ΔG⁰ with an increase in temperature suggests more Cr(VI) adsorption at higher temperature. According to data presented in Table 7, the positive value of ΔH⁰ confirms the endothermic nature of the Cr(VI) adsorption process. The magnitude of ΔH⁰ may give an idea about the type of adsorption process involved. According to literature (Khan et al., 2010; Uslu and Tanyol, 2006), if enthalpy change of adsorbent is higher than 40 kJ mol⁻¹, the process is chemisorption which includes strong electrostatic chemical bonding between chromium ions and adsorbent surface and when enthalpy change is less than 20 kJ mol⁻¹, it indicates the adsorption is physical in nature. In this study, the high value of ΔH⁰ (62.72 kJ mol⁻¹) reveals that the chemisorption is responsible for Cr(VI) adsorption on the prepared activated carbon. The positive value of entropy change (ΔS⁰) indicates the increase in irregularity and randomness at the solid–aqueous solution interface during the adsorption of Cr(VI) ions onto prepared adsorbent (Khalili et al., 2000).

Table 7.

Thermodynamic parameters for Cr(VI) ion adsorption onto the activated carbon at different temperatures.

Temperature	$K_{eq}$	ΔG (kJ mol⁻¹)	ΔS (kJ mol⁻¹ K⁻¹)	ΔH (kJ mol⁻¹)	E (kJ mol⁻¹)	R ²
298	3.8334	−3.385	0.218	62.72	46.7	0.9997
308	8.3025	−5.507
318	17.9193	−7.75

Figure 11.

Plot of ln K_eq versus 1000 T⁻¹ for adsorption of Cr(VI) ions onto the prepared activated carbon.

Moreover, in order to provide an idea concerning the type of metal ion adsorption, Arrhenius activation energy (E_a) was determined by using linear plot of Arrhenius equation as follows

\ln k_{2} = - \frac{E_{a}}{RT} + \ln k_{0}

(13)

where k₀ (g mg⁻¹min⁻¹) is frequency factor and k₂ (g mg⁻¹min⁻¹) is pseudo-second-order constant which was used as a function of temperature.

In physical adsorption, the activation energy is small due to the existence of weak forces between metal ions and adsorbent surface. In this case, the value of activation energy is commonly smaller than 4.184 kJ mol⁻¹. In the current study, the value of activation energy (E_a) (46.7 kJ mol⁻¹) is considerable and this event illustrates that chemisorption or reduction reaction has occurred (Duranoğlu et al., 2012; Dursun, 2006).

Desorption study

The reversibility of Cr(VI) ions adsorption onto prepared adsorbent was investigated using NaOH (0.5 M) as desorption agent. Adsorption–desorption efficiency of the adsorbent after three cycles has been presented in Table 8. The lower desorption efficiency in the first cycle can be attributed to the irreversible interactions between Cr(VI) ions and functional groups on the adsorbent surfaces, which become weaker due to successive contact with NaOH solution. After three cycles, desorption efficiency remained almost constant. Therefore, the results of adsorption–desorption experiments confirm that this adsorbent can be frequently used for the removal of Cr(VI) from water and wastewater.

Table 8.

Adsorption and desorption efficiency of Cr(VI) ions after three cycles (50 mg l⁻¹, pH 4.0, 45℃, and 3.5g l⁻¹ of the adsorbent).

Cycle	1	2	3
% adsorption	92.17	73	68.34
% desorption	62.56	70.27	75.37

Conclusions

A low-cost activated carbon was derived from paper mill sludge by chemical activation with ZnCl₂ which characterized and utilized for Cr(VI) removal from aqueous solution in a batch system. It was found that the adsorbent dosage, initial Cr(VI) concentration, pH of solution, temperature, and contact time influenced on the adsorption capacity of the prepared adsorbent significantly. The removal efficiency was increased with increase in temperature and decreased with increase in metal ion concentration. The maximum metal uptake capacity of prepared adsorbent was found to be 23.18 mg g⁻¹ at optimum condition of adsorbent dosage of 3.5 g l⁻¹, pH 4.0, initial Cr(VI) concentration of 100 mg l⁻¹, contact time of 180 min, and temperature of 45℃. The equilibrium adsorption capacity was tested by using Langmuir and Freundlich isotherm models. The Langmuir isotherm indicated better fit compared to Freundlich isotherm model. The Cr(VI) adsorption onto prepared activated carbon followed nonlinear form of pseudo-second-order kinetic model that consists of chemisorption. Moreover, high activation energy (E_a) indicated the chemical interaction among Cr(VI) ions and surface groups on prepared adsorbent. The negative value of ΔG⁰ confirmed the feasibility of prepared adsorbent and spontaneity of the Cr(VI) adsorption process. The positive value of ΔH⁰ and ΔS⁰ indicated endothermic and irreversibility of Cr(VI) adsorption. The results of the present study showed that the prepared adsorbent from paper mill sludge could be appraised as potential adsorbent in order to remove Cr(VI) ions from aqueous solutions.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Adsorption of Cr(VI) from aqueous solution by adsorbent prepared from paper mill sludge: Kinetics and thermodynamics studies

Abstract

Keywords

Introduction

Materials and methods

Materials

Synthesis of activated carbon

Analysis

Batch adsorption procedure

Analysis of Cr(VI) ions

Batch desorption procedure

Results and discussion

Surface characterization

Adsorption results

Effect of adsorbent dosage

Effect of pH

Effect of contact time

Effect of initial Cr(VI) concentration

Effect of temperature

Adsorption isotherm

Adsorption kinetics

Adsorption thermodynamics

Desorption study

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

Appendix

References