In this study, Fe(III) and Cr(III) metal ion adsorption processes were carried out with three adsorbents in batch experiments and their adsorption performance was compared. These adsorbents were sesame stalk without pretreatment, bio-char derived from thermal decomposition of biomass, and activated carbon which was obtained from chemical activation of biomass. Scanning electron microscopy and Fourier transform–infrared techniques were used for characterization of adsorbents. The optimum conditions for the adsorption process were obtained by observing the influences of solution pH, adsorbent dosage, initial solution concentration, contact time and temperature. The optimum adsorption efficiencies were determined at pH 2.8 and pH 4.0 for Fe(III) and Cr(III) metal ion solutions, respectively. The experimental data were modelled by different isotherm models and the equilibriums were well described by the Langmuir adsorption isotherm model. The pseudo-first-order, pseudo-second-order kinetic, intra-particle diffusion and Elovich models were applied to analyze the kinetic data and to evaluate rate constants. The pseudo-second-order kinetic model gave a better fit than the others. The thermodynamic parameters, such as Gibbs free energy change ΔG°, standard enthalpy change ΔH° and standard entropy change ΔS° were evaluated. The thermodynamic study showed the adsorption was a spontaneous endothermic process.

adsorption, biosorption, heavy metal, isotherm, kinetic, thermodynamic

Industrial activities, which have made life more easy and comfortable, damage the environment. For instance, the presence of heavy metals in the surface/subsurface water and soils causes serious problems due to their toxicity (AL-Othman et al. 2012; Copello et al. 2012). Some heavy metal ions are the most toxic inorganic pollutants even if in low concentrations, and unlike organic pollutants, heavy metals are non-biodegradable in nature. In addition, their toxicity increases with accumulation in water and soils (Bradl 2004; Sarı et al. 2007). The most two common pollutant heavy metals are iron and chromium, which are widely used in coatings, alloys and pigments industries, among many others. Therefore, the wastewater of these industries must be treated to bring their heavy metal concentrations down to below the prescribed legal limit before discharge to natural resources (Liu et al. 2015).

Treatment processes for metal wastewater include chemical precipitation, membrane processes, solvent extraction and adsorption (Inyang et al. 2012). Among these methods, adsorption is attractive because of its flexible design, low cost and simple operation with high efficiency. In addition, since most of the adsorption processes are reversible, multiple use of the adsorbents is possible by suitable regeneration and desorption processes (Taha et al. 2011; Hua et al. 2012). Various materials have been investigated as adsorbents, including activated carbons, ion-exchange resins, zeolites, polymeric materials and chelating fibers. Due to the low adsorption capacities and long operation time of some adsorbents, many researchers have focused on production of new promising adsorbents. Many natural materials such as zeolites, clay or some industrial by-products and waste products are classified as low-cost adsorbents (Babel & Kurniawan 2003). Different types of adsorbent were developed such as modified clay (Vengris et al. 2001; Eren 2008), modified silica (Aguado et al. 2009) and activated carbon (Zaini et al. 2010; Yanagisawa et al. 2010). Biomass-based carbonaceous materials are useful for removal of pollutants from wastewater compared with commercial adsorbents.

The objective of this study is to investigate the potential utilizations of sesame stalk biomass for the removal of Cr(III) and Fe(III) heavy metal ions from aqueous solutions. Adsorption processes were carried out with three adsorbents and their removal performances were compared. One of these adsorbents was sesame stalk without pre-treatment; another was bio-char, derived from thermal decomposition of biomass in a closed system and the by-products of bio-oil production; and the last one was activated carbon which was obtained from chemical activation of biomass. Biomass and solid products were characterized by using scanning electron microscopy (SEM) and Fourier transform–infrared (FT-IR) techniques and evaluated as adsorbents for removal of heavy metal ions from aqueous solutions. The adsorption efficiencies of these adsorbents for heavy metal ions were investigated in different experimental conditions. The most important controlling parameters for the adsorption capacity – solution pH, adsorbent dosage, initial metal ion concentration, contact time and temperature – were studied to determine the optimum adsorption conditions. To describe the equilibrium isotherms the experimental data were analysed by the Langmuir, Freundlich, Dubinin–Radushkevich (D-R) and Temkin isotherm models. Experimental kinetic data were investigated using the pseudo-first-order, pseudo-second-order, intra-particle diffusion and Elovich adsorption kinetic equations to understand the adsorption mechanism. Thermodynamic parameters were calculated for predicting the adsorption nature.

Preparation of adsorbents

Sesame is a flowering plant that is used in the oil and food industry. Sesame seed has one of the highest oil contents of any seed but its stalk has no food value. Sesame stalk (SS) biomass was washed with distilled water and dried at room temperature and ground in a high-speed rotary cutting mill. Carbonization experiments were carried out using 100 mesh particle sizes. All chemicals used in this study were analytical grade and used without further purification. All reagents were supplied by Merck Chemicals (Turkey).

For production of bio-char (BC), about 20 g of biomass were carbonized. The carbonization experiments were performed in a fixed bed reactor from room temperature to 550 °C final temperature with a heating rate of 10°C/min under N2 flow of 100 cm3/min.

To produce activated carbon, sesame stalk was firstly impregnated with potassium hydroxide as chemical activation agent at 1:1 impregnation ratio. The impregnated samples were kept at room temperature for 24 h and then dried in an oven at 85 °C for 48 h. Then the samples were carbonized in a stainless steel fixed bed reactor at 700 °C under N2 flow of 100 cm3/min and at a heating rate of 10 °C/min. After being cooled, all the carbonized samples were washed several times with hot water until the pH became neutral and finally the samples were washed with cold water to remove residual chemicals. The washed samples were dried at 105 °C for 24 h to obtain the final activated carbon (AC).

Batch adsorption experiments

Adsorption studies were carried out to obtain equilibrium data by using the batch technique and were conducted by using 200 mL of metal ion solutions. The stock metal ion solutions of Cr(III) and Fe(III) were prepared by dissolving Cr(NO3)3.9H2O and Fe(NO3)3.9H2O, respectively. The stock solutions were then diluted to the required initial metal ion concentrations. The initial pH value of solutions was adjusted to the required value by addition of 0.1 M HCI or 0.1 M NaOH solutions. After the adsorption process the adsorbent was separated from the samples by filtering and the filtrate was analysed by an atomic absorption spectrometer (Varian Spectra A250 Plus model). The amount of heavy metal ion removed by the adsorbent, q, and the percent adsorption of the pollutant was calculated as follows, respectively:
formula
1
formula
2
where Ci and Ce are the initial and the equilibrium concentrations (mg/L), V is the volume of solution (L), and W is the weight of the adsorbent (g).

Mechanism of adsorption

Isotherms models are important for understanding the mechanism of an adsorption system and several models have been proposed to fit data for the removal of heavy metal ions. In this study, we have used Langmuir, Freundlich, D-R, Temkin, Jovanovic and Harkins–Jura models in order to investigate the mechanism of adsorption. Table 1 shows the equations of these equilibrium isotherms.

Table 1

Adsorption isotherm models and kinetic equations used in this work and their equations

 Linearized formParametersReference
Langmuir  qe (mg/g): the amount of metal ion adsorbed at equilibrium Annadurai et al. (2008)  
qm (mg/g): complete monolayer adsorption capacity 
Ce (mg/L): the equilibrium concentration 
KL (L/mg): the Langmuir adsorption constant 
Freundlich  n: the empirical parameter relating to the adsorption intensity, which varies with the heterogeneity of the material (dimensionless) Tang et al. (2013)  
KF ((mg/g)(L/mg)1/n): the Freundlich adsorption constant 
D-R  β (mol2/J2): the adsorption energy constant Matouq et al. (2015)  
ɛ: the Polanyi potential 
  R (8.314 J/(mol K)): the gas constant  
 T (K): the absolute temperature
E (J/mol): the mean free energy 		 
Temkin 
 		bT (J/mol): the Tempkin constant related to the heat of adsorption
KT (L/mg): the Temkin constant related to the equilibrium binding energy 		Allen et al. (2003)  
Jovanovic  Kj (L/g): Jovanovic isotherm constant
qmj (mg/g): the maximum adsorption capacity in the Jovanovic isotherm model 		Rangabhashiyam & Selvaraju (2015)  
Harkins–Jura  AH (g2/L) and BH (mg2/L): two parameters of the sorption equilibrium Sampranpiboon et al. (2014)  
Pseudo-first-order  qe (mg/g): the adsorption capacity at equilibrium Moussous et al. (2012)  
qt (mg/g): the adsorption capacity at time t 
t (min): contact time 
k1 (1/min): the rate constant of pseudo-first-order adsorption 
Pseudo-second-order  qt (mg/g): the adsorption capacity at time t Doğan et al. (2007)  
k2 (g/(mg min)): the rate constant of pseudo-second-order adsorption 
Intra-particle diffusion model  kP (mg/(g min1/2)) and c: the intra-particle diffusion rate constants Keskinkan et al. (2004)  
Elovich  qt (mg/g): the adsorption capacity at time t Wu et al. (2009)  
(mg/(g min)): initial sorption rate 
(g/mg): desorption constant 
Avrami  kAV and nAV: Avrami kinetic constants Cestari et al. (2006)  
Mass transfer  D: the fitting diameter Imaga & Abia (2015)  
KO: the mass transfer coefficient 
 Linearized formParametersReference
Langmuir  qe (mg/g): the amount of metal ion adsorbed at equilibrium Annadurai et al. (2008)  
qm (mg/g): complete monolayer adsorption capacity 
Ce (mg/L): the equilibrium concentration 
KL (L/mg): the Langmuir adsorption constant 
Freundlich  n: the empirical parameter relating to the adsorption intensity, which varies with the heterogeneity of the material (dimensionless) Tang et al. (2013)  
KF ((mg/g)(L/mg)1/n): the Freundlich adsorption constant 
D-R  β (mol2/J2): the adsorption energy constant Matouq et al. (2015)  
ɛ: the Polanyi potential 
  R (8.314 J/(mol K)): the gas constant  
 T (K): the absolute temperature
E (J/mol): the mean free energy 		 
Temkin 
 		bT (J/mol): the Tempkin constant related to the heat of adsorption
KT (L/mg): the Temkin constant related to the equilibrium binding energy 		Allen et al. (2003)  
Jovanovic  Kj (L/g): Jovanovic isotherm constant
qmj (mg/g): the maximum adsorption capacity in the Jovanovic isotherm model 		Rangabhashiyam & Selvaraju (2015)  
Harkins–Jura  AH (g2/L) and BH (mg2/L): two parameters of the sorption equilibrium Sampranpiboon et al. (2014)  
Pseudo-first-order  qe (mg/g): the adsorption capacity at equilibrium Moussous et al. (2012)  
qt (mg/g): the adsorption capacity at time t 
t (min): contact time 
k1 (1/min): the rate constant of pseudo-first-order adsorption 
Pseudo-second-order  qt (mg/g): the adsorption capacity at time t Doğan et al. (2007)  
k2 (g/(mg min)): the rate constant of pseudo-second-order adsorption 
Intra-particle diffusion model  kP (mg/(g min1/2)) and c: the intra-particle diffusion rate constants Keskinkan et al. (2004)  
Elovich  qt (mg/g): the adsorption capacity at time t Wu et al. (2009)  
(mg/(g min)): initial sorption rate 
(g/mg): desorption constant 
Avrami  kAV and nAV: Avrami kinetic constants Cestari et al. (2006)  
Mass transfer  D: the fitting diameter Imaga & Abia (2015)  
KO: the mass transfer coefficient 
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Adsorption kinetics study is important to determine the uptake rate of adsorbate (Ghasemi et al. 2012). The kinetic data were fitted to the pseudo-first-order, pseudo-second-order, Elovich, Avrami and mass transfer models. In addition to these models, to decide the rate-controlling step, the intra-particle diffusion model was applied to the adsorption kinetic data. In the solid–liquid sorption process, the sorption rate is controlled by several factors. Those factors are described as bulk diffusion, film diffusion, intra-particle diffusion in the solid phase and within the pores, and finally adsorption on the sites. Table 1 shows the equations of adsorption equilibrium kinetics models.

Adsorption thermodynamics

Basically, thermodynamics assumes that energy cannot be gained or lost and the entropy change is the driving force in an isolated system (Ding et al. 2012). Thermodynamic parameters including Gibbs free energy change ΔG° (kJ/mol), enthalpy change ΔH° (kJ/mol) and entropy change ΔS° (kJ/(mol K)) are the most important parameters for an adsorption system. Calculation of the value of ΔH°, ΔG° and ΔS° are given by (Foo & Hameed 2012):
formula
3
formula
4
formula
5
formula
6
where T is the temperature in Kelvin, qe is the amount of heavy metal ions adsorbed at equilibrium per mass unit of adsorbent and Ce is the equilibrium heavy metal ion concentration in solution.

Material characterizations

SEM images of SS, BC and AC were recorded by using a Zeiss EVO 50 scanning electron microscope. Samples were mounted on an aluminum stub using carbon bands and coated with a thin layer of gold–palladium in an argon atmosphere using an Agar sputter coater. Figure 1 shows the representative SEM images of the adsorbents. There are some observed differences between the surface topography of SS, BC and AC. A thick wall structure and a little porosity can be seen for SS. The surface of BC has holes and lots of big particles. The SEM image of AC shows that it has an irregular nature of carbon with lots of micropores. SEM micrographs of SS, BC and AC present the morphological characteristics available for heavy metal ion adsorption.
Figure 1
Figure 1. SEM micrograph of SS, BC and AC, from top to bottom, respectively.
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SEM micrograph of SS, BC and AC, from top to bottom, respectively.

Figure 1
Figure 1. SEM micrograph of SS, BC and AC, from top to bottom, respectively.
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SEM micrograph of SS, BC and AC, from top to bottom, respectively.

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The functional groups characterization was performed by using a Bruker tensor 27 FT-IR spectrometer in the scanning range of 4,000–400 cm−1 by using potassium bromide (KBr) pellets which were mixed with adsorbent samples at a ratio of 99:1. Figure 2 shows the FT-IR spectrum of the SS, BC and AC. As seen in Figure 2 the spectrum of SS shows a wide band located in the range of 3,600–3,300 cm−1 with a maximum at about 3,410 cm−1. This band is a result of O—H groups and adsorbed water. The band at 2,921 cm−1 is related to the C—H vibration. The band at 1,738 cm−1 is due to stretching of C=O groups (Liou 2010). The peaks in the 1,800–1,500 cm−1 region mostly result from C=O stretching vibrations of keto-carbonyl groups (1,738 cm−1 for the spectrum of SS) and C=C stretching vibrations of aromatic rings (1,663 and 1,545 cm−1 for the spectrum of BC and AC, respectively) (Unur 2013). The bands in the range of 1,450–1,350 cm−1 are ascribable to the deformation vibration of hydroxyl groups and in-plane vibrations of C—H in C=C—H structures for the spectrum of BC and AC. The bands between 1,260 and 1,000 cm−1 are appointed to the stretching vibration of C—O bonds, but it is difficult to assign each simple motion of specific functional groups or chemical bonds in this region for three samples (Yang et al. 2012).
Figure 2
Figure 2. FTIR spectrum of the SS, BC and AC.
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FTIR spectrum of the SS, BC and AC.

Figure 2
Figure 2. FTIR spectrum of the SS, BC and AC.
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FTIR spectrum of the SS, BC and AC.

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Adsorption equilibrium experiments

Effect of pH

The surface charge of adsorbent and degree of ionization are affected by the pH of solution (Memon et al. 2008). The effect of initial pH on the adsorption capacities of SS, BC and AC was investigated at 20°C solution temperature for 60 min contact time, using pH 2, 2.77, 3, 4, 5 and 2, 3, 4, 5, 6 values for Fe(III) and Cr(III) metal ion solutions, respectively. The pH values higher than 5.0 and 6.0 lead to Fe(III) and Cr(III) hydroxo complexes (Buerge & Hug 1997), respectively, or start their hydroxides precipitating from aqueous solutions (Iftikhar et al. 2009). The results are shown in Figure 3. The adsorption efficiencies and comparison of the optimum pH determined for Fe(III) and Cr(III) adsorption onto SS, BC, AC and other types of adsorbents are shown in Table 2. The low adsorption efficiencies at low pH are due to competition for binding sites with protons. The increasing effectiveness between pH 2.5 and 4 for both metal ions is likely due to decreasing proton concentration and the formation of metal hydroxide complexes (Lugo-Lugo et al. 2012).
Table 2

The adsorption efficiencies and comparison of the optimum pH determined for Fe(III) and Cr(III) adsorption onto SS, BC, AC and other types of adsorbents

pH Adsorption (%)
 
Fe(III)Cr(III)AdsorbentFe(III)Cr(III)Reference
2.7 4.0 SS 64.41 23.79 Present study 
BC 20.68 33.42 
AC 84.83 50.76 
3.0 5.0 Pretreated orange peel 94.50 76.60 Lugo-Lugo et al. (2012)  
3.0 5.0 Chitosan/attapulgite composite ≈95.00 ≈97.00 Zou et al. (2011)  
– 2.5 Bentonite clay 90.00 – Tahir & Naseem (2007)  
– 4.0 Coal fly ash porous pellets – ≈99.00 Papandreou et al. (2011)  
4.0 – Untreated activated carbon 52.5 – Üçer et al. (2006)  
– Tannic acid immobilized activated carbon 70.4 – 
– 2.0 Ozonized activated carbons – 99.37 Rivera-Utrilla & Sanchez-Polo (2003)  
– 4.0 – 61.16 
pH Adsorption (%)
 
Fe(III)Cr(III)AdsorbentFe(III)Cr(III)Reference
2.7 4.0 SS 64.41 23.79 Present study 
BC 20.68 33.42 
AC 84.83 50.76 
3.0 5.0 Pretreated orange peel 94.50 76.60 Lugo-Lugo et al. (2012)  
3.0 5.0 Chitosan/attapulgite composite ≈95.00 ≈97.00 Zou et al. (2011)  
– 2.5 Bentonite clay 90.00 – Tahir & Naseem (2007)  
– 4.0 Coal fly ash porous pellets – ≈99.00 Papandreou et al. (2011)  
4.0 – Untreated activated carbon 52.5 – Üçer et al. (2006)  
– Tannic acid immobilized activated carbon 70.4 – 
– 2.0 Ozonized activated carbons – 99.37 Rivera-Utrilla & Sanchez-Polo (2003)  
– 4.0 – 61.16 
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Figure 3
Figure 3. Effect of pH on Fe(III) and Cr(III) heavy metal ion sorption capacity.
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Effect of pH on Fe(III) and Cr(III) heavy metal ion sorption capacity.

Figure 3
Figure 3. Effect of pH on Fe(III) and Cr(III) heavy metal ion sorption capacity.
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Effect of pH on Fe(III) and Cr(III) heavy metal ion sorption capacity.

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Effect of adsorbent dosage

The amount of adsorbent plays an important role in any adsorption process (El-Naas et al. 2010). In order to determine the effect of adsorbent dosage, adsorbents in the range of 1–10 g/L were added to each heavy metal ion solution (300 mg/L) at 20°C temperature for 60 min contact time after adjusting optimum pH values. The results are given for adsorption efficiency (%) and the amount of removed per unit weight of biomass (qe) in Figure 4. When increasing the adsorbent dosage, the percentage removal efficiencies of both heavy metal ions increase. It may be due to the increasing of active sites at the higher dosage of adsorbent material. The effective surface area is decreased resulting in the conglomeration of exchanger particles (Rahmani et al. 2010). As can be seen in the figure, an increase in the adsorbent dosage for heavy metal ion solutions from 1.0 to 10.0 g/L resulted in a decrease of the qe for SS and AC, but increasing amount of BC dosage increased the qe for each heavy metal ions. After a certain dosage, the adsorption efficiency was not increased significantly. Therefore, the optimum amounts of SS, BC and AC for further Fe(III) heavy metal ion adsorption experiments were selected as 6 (56.1%), 8 (88.8%) and 5 g/L (94.6), respectively, and SS, BC and AC for further Cr(III) heavy metal ion adsorption experiments were selected as 5 (37.3%), 8 (62.0%) and 4 g/L (89.9%), respectively.
Figure 4
Figure 4. Effect of adsorbent dosage on Cr(III) and Fe(III) sorption capacity of SS, BC and AC, from top to bottom, respectively.
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Effect of adsorbent dosage on Cr(III) and Fe(III) sorption capacity of SS, BC and AC, from top to bottom, respectively.

Figure 4
Figure 4. Effect of adsorbent dosage on Cr(III) and Fe(III) sorption capacity of SS, BC and AC, from top to bottom, respectively.
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Effect of adsorbent dosage on Cr(III) and Fe(III) sorption capacity of SS, BC and AC, from top to bottom, respectively.

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Effect of initial metal ion concentration and contact time on temperature-dependent adsorption

The effect of initial metal ion concentration was determined within the range of 100–300 mg/L for Cr(III) and 100–600 mg/L for Fe(III) metal ion solutions at 20, 30, 40 and 50°C using optimum pH and adsorbent dosage. The temperature of the solutions was maintained by using a laboratory thermostatic shaking water bath. The effect of time on the adsorption process was studied to determine the equilibrium time within a range of 0–720 min; 1.5 mL samples of the heavy metal ion solutions were withdrawn at pre-set time intervals, then their concentrations were measured.

SS
The results are shown in Figures 5 and 6. The figures show that the adsorption capacity for the two heavy metal ions increases with increasing initial metal ion concentrations until the metals ion uptake value reached the state of equilibrium saturation. The increase of the adsorption capacity of SS can be due to the increase in the driving force of the concentration gradient with an increase in the initial metal ion concentration (Min et al. 2012). It was found that the adsorption capacity for the two heavy metal ions increased with increased contact time. However, the adsorption processes become saturated after 180 minutes for Fe(III) and Cr(III) metal ions and no more Cr(III) and Fe(III) were adsorbed. The adsorption capacity for heavy metal ions increased with the temperature increase, indicating the endothermic nature of the adsorption process.
Figure 5
Figure 5. Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of SS at different temperatures.
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Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of SS at different temperatures.

Figure 5
Figure 5. Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of SS at different temperatures.
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Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of SS at different temperatures.

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Figure 6
Figure 6. Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of SS at different temperatures.
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Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of SS at different temperatures.

Figure 6
Figure 6. Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of SS at different temperatures.
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Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of SS at different temperatures.

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BC
The results are shown in Figures 7 and 8. Adsorption capacity for the Cr(III) metal ions increases with increasing initial metal ion concentrations. Adsorption of Fe(III) heavy metal ions increases with the increasing initial concentration from 100 to 450 mg/L and then decreases with the increasing from 450 to 600 mg/L. It could be assignable to the high contact efficiency between the Fe(III) metal ions and the BC at 450 mg/L initial metal ion concentration. The experimental results showed that the adsorption capacity increased with increasing contact time and a large amount of metal ions was removed in the first 60 min. Equilibrium was reached in 180 minutes for Cr(III) and Fe(III) metal ions. Adsorption efficiency was increased as temperature increased.
Figure 7
Figure 7. Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of BC at different temperatures.
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Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of BC at different temperatures.

Figure 7
Figure 7. Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of BC at different temperatures.
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Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of BC at different temperatures.

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Figure 8
Figure 8. Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of BC at different temperatures.
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Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of BC at different temperatures.

Figure 8
Figure 8. Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of BC at different temperatures.
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Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of BC at different temperatures.

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AC
As seen in Figures 9 and 10, similar results were found for Fe(III) and Cr(III) heavy metal ion adsorption onto AC when compared with the results by adsorption onto BC. Adsorption capacity for the Cr(III) metal ions increases with increasing initial metal ion concentrations from 100 to 300 mg/L. Adsorption of Fe(III) heavy metal ion increases with the increase of initial concentration from 100 to 300 mg/L and then decreases with the increasing from 300 to 600 mg/L. Adsorption capacity increased with increasing contact time and equilibrium was reached in 90 and 180 minutes for Fe(III) and Cr(III) metal ions, respectively, indicating that the adsorption of Fe(III) ions was much faster than the adsorption of the Cr(III) ions. Similar to results of adsorption onto SS and BC, it was found that the adsorption capacity for the Fe(III) and Cr(III) metal ions increased with the temperature increase, indicating an endothermic nature of the adsorption process.
Figure 9
Figure 9. Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of AC at different temperatures.
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Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of AC at different temperatures.

Figure 9
Figure 9. Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of AC at different temperatures.
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Effect of initial metal ion concentration and time on Fe(III) adsorption capacity of AC at different temperatures.

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Figure 10
Figure 10. Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of AC at different temperatures.
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Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of AC at different temperatures.

Figure 10
Figure 10. Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of AC at different temperatures.
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Effect of initial metal ion concentration and time on Cr(III) adsorption capacity of AC at different temperatures.

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Theoretical modelling of experimental data

Adsorption isotherms

In this study, Langmuir, Freundlich, Dubinin–Radushkevich, Temkin, Jovanovic and Harkins–Jura isotherms were applied to describe the equilibrium between adsorbed metal ions and metal ions in solution. Table 3 shows the isotherm fitting (constant model values and correlation coefficient R2 values) for Fe(III) and Cr(III) adsorption onto SS, BC and AC at different temperatures. Calculated data showed that the Langmuir isotherm fitted best (regression coefficients of 0.925–0.999) for Cr(III) and Fe(III) adsorption onto SS, BC and AC at 20, 30, 40 and 50°C. This result may be due to the homogeneous distribution of active sites on the surface of the adsorbents. A comparison of the Langmuir adsorption isotherms showed that the obtained maximum qm values at same temperature were in the order BC > SS > AC (62.89 > 43.86 > 37.59 mg/g) and BC > AC > SS (43.48 > 19.61 > 11.43 mg/g) for adsorption of Fe(III) and Cr(III) metal ions, respectively. The maximum coverage follows the sequence Fe(III) > Cr(III) for all adsorbents. The values obtained are higher than obtained experimentally; however, the Langmuir model well fit the experimental data. The Jovanovic model keeps the same assumptions as those considered by the Langmuir isotherm, except that allowance is made in the former for the surface binding vibrations of an adsorbed species (Hadi et al. 2010). As can be seen in Table 3, maximum adsorption capacities for studied heavy metal ions based on the Jovanovic model were lower than the Langmuir maximum adsorption monolayer capacity. Additionally, R2 values for Jovanovic and Harkins–Jura isotherms were found to be the lowest in comparison to the Langmuir model used in the present study. As seen in Table 3, calculated data showed low Temkin constants values, bT, and low correlation coefficient values for the Temkin model. These low values indicate a weak interaction between metal ions and the adsorbents' surface, supporting the predomination of an ion-exchange mechanism (Sheha & Metwally 2007).

Table 3

Fitting parameters for the Langmuir, Freundlich, D-R, Temkin, Jovanovic and Harkins–Jura equations

 LangmuirFreundlichD-RTemkinJovanovicHarkins–Jura
  qmKLR2nKFR2qβER2BKTbTR2KjqmjR2AHBHR2
SS
20 °C Fe(III) 35.59 0.048 0.983 3.67 7.363 0.821 31.25 0.000050 100.00 0.669 6.70 0.588 363.47 0.789 −0.002 19.89 0.521 416 2.75 0.859 
Cr(III) 9.62 0.011 0.975 2.52 0.799 0.939 7.05 0.000500 31.62 0.913 2.23 0.095 1,092.57 0.743 −0.003 3.81 0.883 16 2.68 0.899 
30 °C Fe(III) 42.55 0.029 0.999 3.21 6.692 0.931 35.64 0.000060 91.29 0.884 8.24 0.370 305.63 0.957 −0.002 19.97 0.674 400 2.64 0.832 
Cr(III) 13.69 0.006 0.925 1.89 0.471 0.950 8.42 0.000600 28.87 0.920 3.33 0.052 757.34 0.740 −0.004 3.71 0.899 15 2.54 0.899 
40 °C Fe(III) 43.86 0.020 0.969 3.18 6.207 0.943 34.42 0.000070 84.52 0.879 8.24 0.303 315.63 0.943 −0.002 18.31 0.805 385 2.69 0.832 
Cr(III) 11.25 0.015 0.966 3.36 1.710 0.938 8.39 0.000200 50.00 0.711 2.22 0.214 1,174.42 0.780 −0.002 5.23 0.993 39 2.88 0.983 
50 °C Fe(III) 35.84 0.028 0.996 3.38 6.022 0.879 31.97 0.000100 70.73 0.984 6.86 0.371 391.51 0.928 −0.002 17.89 0.652 333 2.77 0.733 
Cr(III) 11.43 0.035 0.999 4.96 3.438 0.977 10.04 0.000100 70.71 0.932 1.79 1.343 1,502.67 0.969 −0.002 7.32 0.889 83 3.12 0.955 
BC
20 °C Fe(III) 44.84 0.203 0.999 6.84 19.788 0.888 42.17 0.0000010 707.11 0.972 4.62 54.433 526.87 0.942 −0.002 26.69 0.592 1,250 2.88 0.748 
Cr(III) 21.74 0.163 0.994 5.92 9.403 0.741 20.25 0.0000070 267.26 0.949 2.72 18.161 895.59 0.790 −0.003 14.11 0.572 303 2.69 0.641 
30 °C Fe(III) 50.51 0.124 0.994 6.37 20.526 0.926 44.54 0.0000010 845.15 0.913 5.26 38.879 479.19 0.964 −0.002 26.86 0.668 1,250 2.75 0.781 
Cr(III) 35.71 0.264 0.993 7.87 19.029 0.711 33.52 0.0000020 500.00 0.913 3.42 211.742 736.81 0.755 −0.004 23.92 0.577 1,111 2.89 0.617 
40 °C Fe(III) 62.89 0.060 0.963 4.59 17.574 0.956 48.76 0.0000010 707.11 0.791 8.08 4.895 321.97 0.926 −0.004 25.53 0.829 1,000 2.50 0.839 
Cr(III) 40.00 0.163 0.996 4.74 14.702 0.683 37.45 0.0000080 250.00 0.932 5.89 6.535 441.29 0.749 −0.005 24.27 0.482 714 2.43 0.554 
50 °C Fe(III) 65.36 0.071 0.983 4.38 17.784 0.959 52.78 0.0000020 500.00 0.885 8.67 4.414 309.62 0.969 −0.004 26.77 0.769 1,000 2.40 0.799 
Cr(III) 43.48 0.141 0.997 3.61 12.566 0.844 38.09 0.0000070 267.26 0.947 8.03 2.118 334.46 0.890 −0.006 24.27 0.541 555 2.06 0.727 
AC
20 °C Fe(III) 21.69 0.075 0.974 33.44 20.281 0.067 24.58 0.00000500 316.23 0.163 0.56 1.34 × 1016 4307.69 0.049 −0.001 22.91 0.004 2,500 6.75 0.112 
Cr(III) 17.54 0.033 0.986 3.51 3.517 0.747 15.69 0.00020000 50.00 0.963 3.43 0.478 709.75 0.784 −0.002 9.95 0.584 104 2.67 0.682 
30 °C Fe(III) 32.47 0.133 0.978 49.02 31.893 0.015 34.81 0.00000009 2,357.02 0.004 0.05 8.87 × 10248 5,4883.26 0.001 −0.001 33.43 0.003 333 5.00 0.083 
Cr(III) 19.05 0.042 0.957 4.05 4.837 0.414 17.84 0.00010000 70.71 0.596 3.13 1.349 803.78 0.418 −0.002 11.69 0.312 130 2.62 0.408 
40 °C Fe(III) 37.59 0.626 0.995 28.33 31.059 0.040 34.96 0.00000003 4,082.48 0.001 0.47 4.10 × 1023 5,508.64 0.006 −0.001 32.64 0.041 3,333 5.33 0.119 
Cr(III) 19.72 0.057 0.988 4.44 5.815 0.597 18.61 0.00009000 74.54 0.853 3.20 1.7692 812.32 0.622 −0.002 12.69 0.448 185 2.76 0.556 
50 °C Fe(III) 35.97 0.485 0.994 7.46 17.981 0.747 37.57 0.00000400 353.55 0.978 3.82 76.658 703.73 0.773 −0.001 26.04 0.391 1,111 3.00 0.655 
Cr(III) 19.61 0.114 0.998 5.99 8.215 0.725 18.92 0.00003000 129.09 0.988 2.60 9.430 1,032 0.746 −0.002 14.33 0.495 303 3.00 0.687 
 LangmuirFreundlichD-RTemkinJovanovicHarkins–Jura
  qmKLR2nKFR2qβER2BKTbTR2KjqmjR2AHBHR2
SS
20 °C Fe(III) 35.59 0.048 0.983 3.67 7.363 0.821 31.25 0.000050 100.00 0.669 6.70 0.588 363.47 0.789 −0.002 19.89 0.521 416 2.75 0.859 
Cr(III) 9.62 0.011 0.975 2.52 0.799 0.939 7.05 0.000500 31.62 0.913 2.23 0.095 1,092.57 0.743 −0.003 3.81 0.883 16 2.68 0.899 
30 °C Fe(III) 42.55 0.029 0.999 3.21 6.692 0.931 35.64 0.000060 91.29 0.884 8.24 0.370 305.63 0.957 −0.002 19.97 0.674 400 2.64 0.832 
Cr(III) 13.69 0.006 0.925 1.89 0.471 0.950 8.42 0.000600 28.87 0.920 3.33 0.052 757.34 0.740 −0.004 3.71 0.899 15 2.54 0.899 
40 °C Fe(III) 43.86 0.020 0.969 3.18 6.207 0.943 34.42 0.000070 84.52 0.879 8.24 0.303 315.63 0.943 −0.002 18.31 0.805 385 2.69 0.832 
Cr(III) 11.25 0.015 0.966 3.36 1.710 0.938 8.39 0.000200 50.00 0.711 2.22 0.214 1,174.42 0.780 −0.002 5.23 0.993 39 2.88 0.983 
50 °C Fe(III) 35.84 0.028 0.996 3.38 6.022 0.879 31.97 0.000100 70.73 0.984 6.86 0.371 391.51 0.928 −0.002 17.89 0.652 333 2.77 0.733 
Cr(III) 11.43 0.035 0.999 4.96 3.438 0.977 10.04 0.000100 70.71 0.932 1.79 1.343 1,502.67 0.969 −0.002 7.32 0.889 83 3.12 0.955 
BC
20 °C Fe(III) 44.84 0.203 0.999 6.84 19.788 0.888 42.17 0.0000010 707.11 0.972 4.62 54.433 526.87 0.942 −0.002 26.69 0.592 1,250 2.88 0.748 
Cr(III) 21.74 0.163 0.994 5.92 9.403 0.741 20.25 0.0000070 267.26 0.949 2.72 18.161 895.59 0.790 −0.003 14.11 0.572 303 2.69 0.641 
30 °C Fe(III) 50.51 0.124 0.994 6.37 20.526 0.926 44.54 0.0000010 845.15 0.913 5.26 38.879 479.19 0.964 −0.002 26.86 0.668 1,250 2.75 0.781 
Cr(III) 35.71 0.264 0.993 7.87 19.029 0.711 33.52 0.0000020 500.00 0.913 3.42 211.742 736.81 0.755 −0.004 23.92 0.577 1,111 2.89 0.617 
40 °C Fe(III) 62.89 0.060 0.963 4.59 17.574 0.956 48.76 0.0000010 707.11 0.791 8.08 4.895 321.97 0.926 −0.004 25.53 0.829 1,000 2.50 0.839 
Cr(III) 40.00 0.163 0.996 4.74 14.702 0.683 37.45 0.0000080 250.00 0.932 5.89 6.535 441.29 0.749 −0.005 24.27 0.482 714 2.43 0.554 
50 °C Fe(III) 65.36 0.071 0.983 4.38 17.784 0.959 52.78 0.0000020 500.00 0.885 8.67 4.414 309.62 0.969 −0.004 26.77 0.769 1,000 2.40 0.799 
Cr(III) 43.48 0.141 0.997 3.61 12.566 0.844 38.09 0.0000070 267.26 0.947 8.03 2.118 334.46 0.890 −0.006 24.27 0.541 555 2.06 0.727 
AC
20 °C Fe(III) 21.69 0.075 0.974 33.44 20.281 0.067 24.58 0.00000500 316.23 0.163 0.56 1.34 × 1016 4307.69 0.049 −0.001 22.91 0.004 2,500 6.75 0.112 
Cr(III) 17.54 0.033 0.986 3.51 3.517 0.747 15.69 0.00020000 50.00 0.963 3.43 0.478 709.75 0.784 −0.002 9.95 0.584 104 2.67 0.682 
30 °C Fe(III) 32.47 0.133 0.978 49.02 31.893 0.015 34.81 0.00000009 2,357.02 0.004 0.05 8.87 × 10248 5,4883.26 0.001 −0.001 33.43 0.003 333 5.00 0.083 
Cr(III) 19.05 0.042 0.957 4.05 4.837 0.414 17.84 0.00010000 70.71 0.596 3.13 1.349 803.78 0.418 −0.002 11.69 0.312 130 2.62 0.408 
40 °C Fe(III) 37.59 0.626 0.995 28.33 31.059 0.040 34.96 0.00000003 4,082.48 0.001 0.47 4.10 × 1023 5,508.64 0.006 −0.001 32.64 0.041 3,333 5.33 0.119 
Cr(III) 19.72 0.057 0.988 4.44 5.815 0.597 18.61 0.00009000 74.54 0.853 3.20 1.7692 812.32 0.622 −0.002 12.69 0.448 185 2.76 0.556 
50 °C Fe(III) 35.97 0.485 0.994 7.46 17.981 0.747 37.57 0.00000400 353.55 0.978 3.82 76.658 703.73 0.773 −0.001 26.04 0.391 1,111 3.00 0.655 
Cr(III) 19.61 0.114 0.998 5.99 8.215 0.725 18.92 0.00003000 129.09 0.988 2.60 9.430 1,032 0.746 −0.002 14.33 0.495 303 3.00 0.687 
View Large

Adsorption kinetics

The adsorption kinetic experiments were carried out at 20, 30, 40 and 50°C using optimum pH and adsorbent dosage on a shaker for the adsorption equilibrium time for each heavy metal ion solutions as mentioned in the effect of contact time. In order to investigate the mechanism of the sorption, kinetic models have been used to test the experimental data. The kinetic data were fitted into the pseudo-first-order, pseudo-second-order, intra-particle diffusion and Elovich models. In addition to traditional kinetic equations, Avrami and mass transfer kinetic models were used to find some new information on the adsorbent–adsorbate interaction. Table 4 shows the adsorption kinetic parameters of Fe(III) and Cr(III) metal ion solutions at different temperatures and initial metal ion concentration. The correlation coefficients for the pseudo-second-order kinetic model were higher than the pseudo-first-order, Elovich and Avrami models for the adsorption of Fe(III) and Cr(III) ions onto SS, BC and AC, indicating that the adsorption perfectly complies with the pseudo-second-order reaction. The calculated nAV and kAV Avrami constants are different from 20 to 50°C. Therefore, in this temperature range, the adsorptions of each heavy metal ion seem to present both temperature and contact time dependence in relation to the adsorption kinetic parameters, which agrees with the results outlined in the section on the effect of initial metal ion concentration and contact time. Also, the calculated theoretical qe values from the second-order kinetic model are consistent with the experimental qe values and the adsorption of Fe(III) and Cr(III) metal ions being controlled by the chemisorption process. Δqe values were calculated as follows:
formula
7
Table 4

Kinetic parameters obtained from pseudo-first-order, pseudo-second-order, Elovich, Avrami, intra-particle diffusion and mass transfer equations

   Pseudo-first-orderPseudo-second-orderElovichIntra-particle diffusionAvramiMass transfer
  qe(exp)k1qeR2k2qeR2βαR2kpR2nAVkAVR2DK0R2
SS
20 °C Fe(III) 34.64 0.037 22.48 0.936 0.003 36.36 0.997 0.215 58.493 0.946 1.5490 0.937 0.567 0.09 0.878 117.3 0.0029 0.752 
30 °C 35.07 0.031 16.23 0.972 0.005 36.10 0.999 0.253 210.909 0.983 1.2910 0.935 0.490 0.14 0.926 128.9 0.0023 0.727 
40 °C 31.89 0.036 22.48 0.880 0.006 32.68 0.998 0.283 220.655 0.977 1.1267 0.885 0.502 0.14 0.886 118.4 0.0022 0.653 
50 °C 29.08 0.025 22.48 0.931 0.012 29.24 0.999 0.535 7,0790.36 0.971 0.5969 0.883 0.342 0.48 0.892 122.5 0.0012 0.703 
20 °C Cr(III) 4.28 0.009 1.12 0.923 0.064 4.03 0.999 3.702 4,009.285 0.916 0.0917 0.942 0.216 0.45 0.905 19.4 0.0016 0.896 
30 °C 6.08 0.014 3.94 0.949 0.009 5.94 0.964 1.211 2.802 0.845 0.2930 0.948 0.404 0.03 0.822 17.8 0.0045 0.939 
40 °C 6.08 0.013 5.86 0.822 0.004 5.92 0.811 0.986 0.481 0.758 0.3675 0.888 0.548 0.01 0.795 10.0 0.0075 0.903 
50 °C 7.59 0.019 3.96 0.949 0.012 7.77 0.994 0.820 3.648 0.956 0.3967 0.904 0.518 0.06 0.950 26.3 0.0039 0.687 
BC
20 °C Fe(III) 19.50 0.031 2.34 0.984 0.038 19.65 0.999 1.719 1.5 × 1012 0.947 0.1939 0.939 0.304 1.95 0.862 90.5 0.0005 0.778 
30 °C 19.84 0.029 3.93 0.855 0.021 19.96 0.998 1.540 4.9 × 1010 0.808 0.2262 0.875 0.281 1.54 0.640 89.0 0.0007 0.857 
40 °C 19.52 0.014 1.35 0.855 0.059 19.61 0.999 1.885 3.2 × 1013 0.840 0.1600 0.689 0.204 11.78 0.922 91.2 0.0005 0.555 
50 °C 19.48 0.020 1.04 0.951 0.076 19.45 0.999 3.629 7.5 × 1027 0.972 0.0906 0.937 0.188 43.74 0.851 93.6 0.0003 0.822 
20 °C Cr(III) 11.48 0.026 3.65 0.938 0.020 11.63 0.998 1.203 4,728.684 0.961 0.2780 0.960 0.357 0.32 0.846 74.0 0.0015 0.822 
30 °C 12.01 0.016 1.38 0.882 0.020 12.05 0.998 3.312 1.8 × 1014 0.883 0.1020 0.911 0.190 0.89 0.726 87.9 0.0006 0.887 
40 °C 11.45 0.018 0.68 0.931 0.105 11.49 0.999 6.242 7.8 × 1027 0.916 0.0540 0.954 0.186 40.93 0.763 87.7 0.0003 0.893 
50 °C 11.64 0.012 0.68 0.944 0.109 11.63 0.999 5.379 1.1 × 1024 0.944 0.0600 0.886 0.149 127.80 0.889 88.4 0.0003 0.817 
AC
20 °C Fe(III) 20.74 0.058 5.23 0.971 0.027 21.14 0.999 0.567 3870.387 0.906 0.6796 0.787 0.935 0.39 0.981 87.1 0.0025 0.605 
30 °C 31.11 0.046 13.96 0.961 0.005 32.46 0.996 0.184 22.811 0.848 2.1049 0.744 0.611 0.11 0.896 95.1 0.0066 0.539 
40 °C 36.54 0.024 2.84 0.934 0.043 36.23 0.999 1.382 3.5 × 1019 0.959 0.3038 0.986 0.172 45.73 0.844 170.9 0.0008 0.976 
50 °C 35.06 0.024 10.82 0.972 0.009 34.24 0.999 0.293 811.393 0.992 1.3826 0.952 0.344 0.28 0.988 128.3 0.0038 0.871 
20 °C Cr(III) 13.78 0.037 13.79 0.963 0.002 16.64 0.977 0.279 1.428 0.928 1.1698 0.885 0.935 0.04 0.944 22.1 0.0071 0.629 
30 °C 14.07 0.017 13.56 0.928 0.001 16.13 0.906 0.325 1.048 0.884 1.0702 0.955 0.701 0.02 0.898 17.1 0.0084 0.804 
40 °C 16.34 0.027 12.69 0.932 0.004 17.39 0.980 0.399 8.196 0.893 0.8721 0.968 0.535 0.06 0.818 36.3 0.0041 0.874 
50 °C 18.99 0.015 26.26 0.987 0.002 21.55 0.984 0.215 2.143 0.929 1.5059 0.867 0.796 0.04 0.946 31.5 0.0067 0.621 
   Pseudo-first-orderPseudo-second-orderElovichIntra-particle diffusionAvramiMass transfer
  qe(exp)k1qeR2k2qeR2βαR2kpR2nAVkAVR2DK0R2
SS
20 °C Fe(III) 34.64 0.037 22.48 0.936 0.003 36.36 0.997 0.215 58.493 0.946 1.5490 0.937 0.567 0.09 0.878 117.3 0.0029 0.752 
30 °C 35.07 0.031 16.23 0.972 0.005 36.10 0.999 0.253 210.909 0.983 1.2910 0.935 0.490 0.14 0.926 128.9 0.0023 0.727 
40 °C 31.89 0.036 22.48 0.880 0.006 32.68 0.998 0.283 220.655 0.977 1.1267 0.885 0.502 0.14 0.886 118.4 0.0022 0.653 
50 °C 29.08 0.025 22.48 0.931 0.012 29.24 0.999 0.535 7,0790.36 0.971 0.5969 0.883 0.342 0.48 0.892 122.5 0.0012 0.703 
20 °C Cr(III) 4.28 0.009 1.12 0.923 0.064 4.03 0.999 3.702 4,009.285 0.916 0.0917 0.942 0.216 0.45 0.905 19.4 0.0016 0.896 
30 °C 6.08 0.014 3.94 0.949 0.009 5.94 0.964 1.211 2.802 0.845 0.2930 0.948 0.404 0.03 0.822 17.8 0.0045 0.939 
40 °C 6.08 0.013 5.86 0.822 0.004 5.92 0.811 0.986 0.481 0.758 0.3675 0.888 0.548 0.01 0.795 10.0 0.0075 0.903 
50 °C 7.59 0.019 3.96 0.949 0.012 7.77 0.994 0.820 3.648 0.956 0.3967 0.904 0.518 0.06 0.950 26.3 0.0039 0.687 
BC
20 °C Fe(III) 19.50 0.031 2.34 0.984 0.038 19.65 0.999 1.719 1.5 × 1012 0.947 0.1939 0.939 0.304 1.95 0.862 90.5 0.0005 0.778 
30 °C 19.84 0.029 3.93 0.855 0.021 19.96 0.998 1.540 4.9 × 1010 0.808 0.2262 0.875 0.281 1.54 0.640 89.0 0.0007 0.857 
40 °C 19.52 0.014 1.35 0.855 0.059 19.61 0.999 1.885 3.2 × 1013 0.840 0.1600 0.689 0.204 11.78 0.922 91.2 0.0005 0.555 
50 °C 19.48 0.020 1.04 0.951 0.076 19.45 0.999 3.629 7.5 × 1027 0.972 0.0906 0.937 0.188 43.74 0.851 93.6 0.0003 0.822 
20 °C Cr(III) 11.48 0.026 3.65 0.938 0.020 11.63 0.998 1.203 4,728.684 0.961 0.2780 0.960 0.357 0.32 0.846 74.0 0.0015 0.822 
30 °C 12.01 0.016 1.38 0.882 0.020 12.05 0.998 3.312 1.8 × 1014 0.883 0.1020 0.911 0.190 0.89 0.726 87.9 0.0006 0.887 
40 °C 11.45 0.018 0.68 0.931 0.105 11.49 0.999 6.242 7.8 × 1027 0.916 0.0540 0.954 0.186 40.93 0.763 87.7 0.0003 0.893 
50 °C 11.64 0.012 0.68 0.944 0.109 11.63 0.999 5.379 1.1 × 1024 0.944 0.0600 0.886 0.149 127.80 0.889 88.4 0.0003 0.817 
AC
20 °C Fe(III) 20.74 0.058 5.23 0.971 0.027 21.14 0.999 0.567 3870.387 0.906 0.6796 0.787 0.935 0.39 0.981 87.1 0.0025 0.605 
30 °C 31.11 0.046 13.96 0.961 0.005 32.46 0.996 0.184 22.811 0.848 2.1049 0.744 0.611 0.11 0.896 95.1 0.0066 0.539 
40 °C 36.54 0.024 2.84 0.934 0.043 36.23 0.999 1.382 3.5 × 1019 0.959 0.3038 0.986 0.172 45.73 0.844 170.9 0.0008 0.976 
50 °C 35.06 0.024 10.82 0.972 0.009 34.24 0.999 0.293 811.393 0.992 1.3826 0.952 0.344 0.28 0.988 128.3 0.0038 0.871 
20 °C Cr(III) 13.78 0.037 13.79 0.963 0.002 16.64 0.977 0.279 1.428 0.928 1.1698 0.885 0.935 0.04 0.944 22.1 0.0071 0.629 
30 °C 14.07 0.017 13.56 0.928 0.001 16.13 0.906 0.325 1.048 0.884 1.0702 0.955 0.701 0.02 0.898 17.1 0.0084 0.804 
40 °C 16.34 0.027 12.69 0.932 0.004 17.39 0.980 0.399 8.196 0.893 0.8721 0.968 0.535 0.06 0.818 36.3 0.0041 0.874 
50 °C 18.99 0.015 26.26 0.987 0.002 21.55 0.984 0.215 2.143 0.929 1.5059 0.867 0.796 0.04 0.946 31.5 0.0067 0.621 
View Large
Calculated average Δqe values are 0.92, 0.09 and 0.72 mg/g for the adsorption of Fe(III) onto SS, BC and AC, respectively, and 0.18, 0.06 and 2.13 mg/g for the adsorption of Cr(III) onto SS, BC and AC, respectively.

The first-order, pseudo-second-order, Elovich and Avrami models cannot identify the diffusion mechanism. There are two main mechanisms of mass transfer: diffusion and mass transport by convection (Imaga & Abia 2015). As can be seen in Table 4, the low R2 values suggest that the mass transfer model does not support the adsorption of the heavy metal ions using the SS, BC and AC. The intra-particle diffusion model has been used to determine the diffusion mechanism because in many cases intra-particle diffusion is the possible rate-limiting step. If the adsorption process is in accordance with the intra-particle diffusion model, then the plot of uptake, qt, versus t1/2 should be linear. Also, when the plot passes through the origin then the only rate-limiting process is intra-particle diffusion. If the plots do not pass through the origin, the intra-particle diffusion is not the only rate-limiting step. This indicates that some other mechanisms may be also involved, all of which may be operating simultaneously (Kilic et al. 2011; Li et al. 2012;). The obtained plots were not linear over all the time range and also the plots did not pass through the origin. It indicates that intra-particle diffusion was not the only rate-limiting process and other kinetic models may taking place simultaneously.

Adsorption thermodynamics

In order to determine the nature of the adsorption process, thermodynamic studies were performed. The parameters at various temperatures are presented in Table 5. The negative values of ΔG° at various temperatures implied that the adsorption process occurred spontaneously and the process was feasible. The positive ΔH° values confirm the endothermic nature of the adsorption of Fe(III) and Cr(III) metal ions. The positive ΔS° values suggest that the degrees of freedom increase at the solid–liquid interface. According to the calculated positive values of ΔS°, during the adsorption process, the coordinated water molecules were displaced by both Fe(III) and Cr(III) metal ion species, resulting in increased randomness in the adsorbent–metal ions system (Baraka et al. 2007).

Table 5

The thermodynamic parameters of the adsorption of Fe(III) and Cr(III) metal ions onto SS, BC and AC

SS
 Fe(III)
Cr(III)
T (°C)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)
20 −13.67 51.54 1.42 −9.88 115.71 24.31 
30 −14.21 −10.32 
40 −14.73 −11.87 
50 −15.21 −13.26 
BC
 Fe(III)Cr(III)
T (°C)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)
20 −14.81 91.62 12.06 −14.62 173.31 35.98 
30 −15.74 −16.57 
40 −16.43 −18.78 
50 −17.64 −19.61 
AC
  Fe(III)Cr(III)
T (°C)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)
20 −9.09 93.92 18.11 −12.96 128.74 25.08 
30 −10.70 −13.53 
40 −11.64 −14.97 
50 −11.85 −16.83 
SS
 Fe(III)
Cr(III)
T (°C)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)
20 −13.67 51.54 1.42 −9.88 115.71 24.31 
30 −14.21 −10.32 
40 −14.73 −11.87 
50 −15.21 −13.26 
BC
 Fe(III)Cr(III)
T (°C)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)
20 −14.81 91.62 12.06 −14.62 173.31 35.98 
30 −15.74 −16.57 
40 −16.43 −18.78 
50 −17.64 −19.61 
AC
  Fe(III)Cr(III)
T (°C)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)ΔG° (kJ/mol)ΔS° (J/molK)ΔH° (kJ/mol)
20 −9.09 93.92 18.11 −12.96 128.74 25.08 
30 −10.70 −13.53 
40 −11.64 −14.97 
50 −11.85 −16.83 
View Large

Comparison of adsorption capacities with various adsorbents

The comparison of maximum adsorption capacity of SS, BC and AC with various adsorbents in literature is represented in Table 6. As seen from Table 6, the adsorption capacity of SS, BC and AC for Fe(III) and Cr(III) metal ions is comparable with other adsorbents. The adsorption capacity depends mainly on characteristics of the adsorbent.

Table 6

Comparison of maximum adsorption capacities by adjustment to the Langmuir model qm (mg/g) obtained for Fe(III) and Cr(III) adsorption onto SS, BC, AC and other types of adsorbents

 Maximum adsorption capacity qm (mg/g) 
AdsorbentFe(III)Cr(III)Reference
SS 43.86 13.69 Present study 
BC 65.36 43.48  
AC 37.59 19.72  
Cork powder – 6.30 Machado et al. (2002)  
Coal fly ash porous pellets – 22.94 Papandreou et al. (2011)  
Raw clinoptilolite 98.00 – Öztaş et al. (2008)  
Chitosan 90.09 – Ngah et al. (2005)  
Chitosan/attapulgite composites 62.50 65.36 Zou et al. (2011)  
 Maximum adsorption capacity qm (mg/g) 
AdsorbentFe(III)Cr(III)Reference
SS 43.86 13.69 Present study 
BC 65.36 43.48  
AC 37.59 19.72  
Cork powder – 6.30 Machado et al. (2002)  
Coal fly ash porous pellets – 22.94 Papandreou et al. (2011)  
Raw clinoptilolite 98.00 – Öztaş et al. (2008)  
Chitosan 90.09 – Ngah et al. (2005)  
Chitosan/attapulgite composites 62.50 65.36 Zou et al. (2011)  
View Large

This study highlighted the potential of SS as an efficient raw precursor for the adsorption processes. The effects of pH, adsorbent dosage, initial metal ion concentration, contact time and the solution temperature on the adsorption process were determined. The optimum values and adsorption efficiencies were determined at pH 2.8 and pH 4.0 for Fe(III) and Cr(III) metal ion solutions, respectively. The Langmuir, Freundlich, D-R and Temkin isotherms were used to describe the experimental sorption data. The kinetic data were fitted into the pseudo-first-order, pseudo-second-order, intra-particle diffusion and Elovich models. In all cases, the equilibriums were well described by the Langmuir adsorption isotherm model. Despite the qm values obtained from Langmuir isotherms being higher than obtained experimentally, it can be seen in Figures 5–10 that the adsorption capacity of the three adsorbents was in the order of AC, BC and SS, from biggest to smallest. The pseudo-second-order kinetic model better described the sorption kinetics with high correlation coefficients for adsorption of each metal ion onto all adsorbents. Carbonization of SS led to increase in the pore size. Therefore, adsorption efficiency of BC was higher than SS for each metal ion solution. In addition to this, chemical activation with KOH caused enhancement of the adsorption capacity compared to SS and BC for the adsorption of Fe(III) and Cr(III). Based on all of these results, SS can be effectively used as an alternative adsorbent raw precursor for the adsorption of Fe(III) and Cr(III) metal ions from aqueous solutions. The material commends itself because it is relatively cheap and available.

The authors would like to thank Anadolu University Scientific Research Council for the financial support of this work through the project 1202F032.

Aguado
J.
Arsuaga
J.
Arencibia
A.
Lindo
M.
Gascn
V.
2009
Aqueous heavy metals removal by adsorption on amine-functionalized mesoporous silica
.
Journal of Hazardous Materials
163
,
213
–
221
.
Google Scholar
Crossref
Search ADS
PubMed
 
Allen
S. J.
Gan
Q.
Matthews
R.
Johnson
P. A.
2003
Comparison of optimised isotherm models for basic dye adsorption by kuzdu
.
Bioresource Technology
88
,
143
–
152
.
Google Scholar
Crossref
Search ADS
PubMed
 
Babel
S.
Kurniawan
T. A.
2003
Low-cost adsorbents for heavy metals uptake from contaminated water: a review
.
Journal of Hazardous Materials
B97
,
219
–
243
.
Google Scholar
Crossref
Search ADS
 
Bradl
H. B.
2004
Adsorption of heavy metal ions on soils and soils constituents
.
Journal of Colloid and Interface Science
277
(
1
),
1
–
18
.
Google Scholar
Crossref
Search ADS
PubMed
 
Buerge
I. J.
Hug
S. J.
1997
Kinetics and pH dependence of chromium(VI) reduction by iron(II)
.
Environmental Science and Technology
31
(
5
),
1426
–
1432
.
Google Scholar
Crossref
Search ADS
 
Cestari
A. R.
Vieira
E. F. S.
Vieira
G. S.
Almeida
L. E.
2006
The removal of anionic dyes from aqueous solutions in the presence of anionic surfactant using aminopropylsilica – a kinetic study
.
Journal of Hazardous Materials
137
,
133
–
141
.
Google Scholar
Crossref
Search ADS
 
Copello
G. J.
Diaz
L. E.
Dall'Ortoa
V. C.
2012
Adsorption of Cd(II) and Pb(II) onto a one step-synthesized polyampholyte: kinetics and equilibrium studies
.
Journal of Hazardous Materials
217–218
,
374
–
381
.
Google Scholar
Crossref
Search ADS
PubMed
 
Ding
L.
Wu
C.
Deng
H.
Zhang
X.
2012
Adsorptive characteristics of phosphate from aqueous solutions by MIEX resin
.
Journal of Colloid and Interface Science
376
,
224
–
232
.
Google Scholar
Crossref
Search ADS
PubMed
 
El-Naas
M. H.
Al-Zuhair
S.
Alhaija
M. A.
2010
Removal of phenol from petroleum refinery wastewater through adsorption on date-pit activated carbon
.
Chemical Engineering Journal
162
,
997
–
1005
.
Google Scholar
Crossref
Search ADS
 
Eren
E.
2008
Removal of copper ions by modified Unye clay
.
Journal of Hazardous Materials
159
,
235
–
244
.
Google Scholar
Crossref
Search ADS
PubMed
 
Ghasemi
Z.
Seif
A.
Ahmadi
T. S.
Zargar
B.
Rashidi
F.
Rouzbahani
G. M.
2012
Thermodynamic and kinetic studies for the adsorption of Hg(II) by nano-TiO2 from aqueous solution
.
Advanced Powder Technology
23
,
148
–
156
.
Google Scholar
Crossref
Search ADS
 
Hua
M.
Zhang
S.
Pan
B.
Zhang
W.
Lv
L.
Zhang
Q.
2012
Heavy metal removal from water/wastewater by nanosized metal oxides: a review
.
Journal of Hazardous Materials
211–212
,
317
–
331
.
Google Scholar
Crossref
Search ADS
PubMed
 
Iftikhar
A. R.
Bhatti
H. N.
Hanif
M. A.
Nadeem
R.
2009
Kinetic and thermodynamic aspects of Cu(II) and Cr(III) removal from aqueous solutions using rose waste biomass
.
Journal of Hazardous Materials
161
(
2–3
),
941
–
947
.
Google Scholar
Crossref
Search ADS
PubMed
 
Imaga
C. C.
Abia
A. A.
2015
Adsorption kinetics and mechanisms of Ni2+ sorption using carbonized and modified sorghum (Sorghum bicolor) hull of two pore sizes (150 μm and 250 μm): a comparative study
.
International Journal of Chemical Studies
2
(
5
),
59
–
68
.
Google Scholar
 
Inyang
M.
Gao
B.
Yao
Y.
Xue
Y.
Zimmerman
A. R.
Pullammanappallil
P.
Cao
X.
2012
Removal of heavy metals from aqueous solution by biochars derived from anaerobically digested biomass
.
Bioresource Technology
110
,
50
–
56
.
Google Scholar
Crossref
Search ADS
PubMed
 
Keskinkan
O.
Goksu
M. Z. L.
Basibuyuk
M.
Forster
C. F.
2004
Heavy metal adsorption properties of a submerged aquatic plant (Ceratophyllum demersum)
.
Bioresource Technology
92
(
2
),
197
–
200
.
Google Scholar
Crossref
Search ADS
PubMed
 
Li
W.
Tang
Y.
Zeng
Y.
Tong
Z.
Liang
D.
Cui
W.
2012
Adsorption behavior of Cr(VI) ions on tannin-immobilized activated clay
.
Chemical Engineering Journal
193–194
,
88
–
95
.
Google Scholar
 
Lugo-Lugo
V.
Barrera-Diaz
C.
Ureña-Núñez
F.
Bilyeu
B.
Linares-Hernández
I.
2012
Biosorption of Cr(III) and Fe(III) in single and binary systems onto pretreated orange peel
.
Journal of Environmental Management
112
,
120
–
127
.
Google Scholar
Crossref
Search ADS
PubMed
 
Machado
R.
Carvalho
J. R.
Correia
M. J. N.
2002
Removal of trivalent chromium(III) from solution by biosorption in cork powder
.
Journal of Chemical Technology & Biotechnology
77
(
12
),
1340
–
1348
.
Google Scholar
Crossref
Search ADS
 
Matouq
M.
Jildeh
N.
Qtaishat
M.
Hindiyeh
M.
Al Syouf
M. Q.
2015
The adsorption kinetics and modeling for heavy metals removal from wastewater by Moringa pods
.
Journal of Environmental Chemical Engineering
3
(
2
),
775
–
784
.
Google Scholar
Crossref
Search ADS
 
Memon
J. R.
Memon
S. Q.
Bhanger
M. I.
Memon
G. Z.
El-Turki
A.
Allen
G. C.
2008
Characterization of banana peel by scanning electron microscopy and FT-IR spectroscopy and its use for cadmium removal
.
Colloids and Surfaces B: Biointerfaces
66
(
2
),
260
–
265
.
Google Scholar
Crossref
Search ADS
 
Moussous
S.
Selatnia
A.
Merati
A.
Junter
G. A.
2012
Batch cadmium(II) biosorption by an industrial residue of macrofungal biomass (Clitopilus scyphoides)
.
Chemical Engineering Journal
197
,
261
–
271
.
Google Scholar
Crossref
Search ADS
 
Öztaş
N. A.
Karabakan
A.
Topal
Ö.
2008
Removal of Fe(III) ion from aqueous solution by adsorption on raw and treated clinoptilolite samples
.
Microporous and Mesoporous Materials
111
,
200
–
205
.
Google Scholar
Crossref
Search ADS
 
Papandreou
A. D.
Stournaras
C. J.
Panias
D.
Paspaliaris
I.
2011
Adsorption of Pb(II), Zn(II) and Cr(III) on coal fly ash porous pellets
.
Minerals Engineering
24
,
1495
–
1501
.
Google Scholar
Crossref
Search ADS
 
Rahmani
A.
Mousavi
H. Z.
Fazli
M.
2010
Effect of nanostructure alumina on adsorption of heavy metals
.
Desalination
253
,
94
–
100
.
Google Scholar
Crossref
Search ADS
 
Sampranpiboon
P.
Charnkeitkong
P.
Feng
X.
2014
Equilibrium isotherm models for adsorption of zinc(II) ion from aqueous solution on pulp waste
.
WSEAS Transactions on Environment and Development
10
,
35
–
47
.
Google Scholar
 
Sarı
A.
Tuzen
M.
Uluözlü
Ö. D.
Soylak
M.
2007
Biosorption of Pb(II) and Ni(II) from aqueous solution by lichen (Cladonia furcata) biomass
.
Biochemical Engineering Journal
37
(
2
),
151
–
158
.
Google Scholar
Crossref
Search ADS
 
Sheha
R. R.
Metwally
E.
2007
Equilibrium isotherm modeling of cesium adsorption onto magnetic materials
.
Journal of Hazardous Materials
143
,
354
–
361
.
Google Scholar
Crossref
Search ADS
PubMed
 
Tahir
S. S.
Naseem
R.
2007
Removal of Cr(III) from tannery wastewater by adsorption onto bentonite clay
.
Separation and Purification Technology
53
,
312
–
321
.
Google Scholar
Crossref
Search ADS
 
Tang
Y.
Chen
L.
Wei
X.
Yao
Q.
Li
T.
2013
Removal of lead ions from aqueous solution by the dried aquatic plant, Lemna perpusilla Torr
.
Journal of Hazardous Materials
244–245
,
603
–
612
.
Google Scholar
Crossref
Search ADS
PubMed
 
Üçer
A.
Uyanık
A.
Aygün
Ş. F.
2006
Adsorption of Cu(II), Cd(II), Zn(II), Mn(II) and Fe(III) ions by tannic acid immobilised activated carbon
.
Separation and Purification Technology
47
,
113
–
118
.
Google Scholar
Crossref
Search ADS
 
Vengris
T.
Binkiene
R.
Sveikauskaite
A.
2001
Nickel, copper and zinc removal from wastewater by a modified clay sorbent
.
Applied Clay Science
18
,
183
–
190
.
Google Scholar
Crossref
Search ADS
 
Zaini
M. A. A.
Amano
Y.
Machida
M.
2010
Adsorption of heavy metals onto activated carbons derived from polyacrylonitrile fiber
.
Journal of Hazardous Materials
180
,
552
–
560
.
Google Scholar
Crossref
Search ADS
PubMed
 
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