Isotherms and kinetics studies of biosorption nickel (II) and chromium (VI) from aqueous solution by dried activated sludge
Mohammad Malakootian1, Seyed Kamal Ghadiri2, Nader Yousefi2, Ali Fatehizadeh3
1 Environmental Health Research Center and Department of Environmental Health Engineering, School of Public Health, Kerman University of Medical Sciences, Kerman, Iran
2 Department of Environmental Health Engineering, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran
3 Department of Environmental Health Engineering, School of Public Health, Isfahan University of Medical Sciences, Isfahan, Iran
|Date of Web Publication||28-Mar-2012|
Kerman University of Medical Sciences, School of Public Health, Haftbagh-Alavi Highway, Kerman
© 2012 Malakootian et al; This is an open-access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Aims: The aim of this study is the recovery of municipal refuse and using it as a biosorbent for Nickel (II) and Chromium (VI) (Ni 2+ an d Cr 6+ )removal from aqueous solutions.
Materials and Methods: Activated sludge was obtained from the wastewater treatment plant in a dairy industry. All experiments were performed in the batch system and effective parameters such as the pH, adsorbent dosage, and the initial concentration and contact time of Ni 2+ and Cr 6+ were investigated. The adsorption isotherms and kinetics were evaluated to describe the metal uptake and dynamic reactions.
Results: The results of this study showed that with an increasing adsorbent dose and contact time, Ni 2+ and Cr 6+ removal efficiency increased. The maximum adsorption of Ni 2+ and Cr 6+ was obtained in pH 6 and 2, respectively. Meanwhile, with increasing Ni 2+ and Cr 6+ concentration, the removal efficiency decreased. The results best fitted the Langmuir isotherm and the maximum absorption capacity of Ni 2+ and Cr 6+ onto dry activated sludge (DAS) were 2.17 and 2.23 mg/g, respectively. Analysis of the adsorption kinetics showed that the intraparticle diffusion kinetic had been good and presented Ni 2+ and Cr 6+ uptake onto DAS, and the intraparticle diffusion rate constant of Ni 2+ and Cr 6+ were 0.044 and 0.042 (mg/g min 0.5 ), respectively.
Conclusions: According to the results, dry activated sludge is suggested as a low cost and available adsorbent for removing Ni 2+ and Cr 6+ from aqueous solutions.
Keywords: Adsorption isotherms, adsorption kinetics, biosorbent, heavy metals
|How to cite this article:|
Malakootian M, Ghadiri SK, Yousefi N, Fatehizadeh A. Isotherms and kinetics studies of biosorption nickel (II) and chromium (VI) from aqueous solution by dried activated sludge. Int J Env Health Eng 2012;1:2
|How to cite this URL:|
Malakootian M, Ghadiri SK, Yousefi N, Fatehizadeh A. Isotherms and kinetics studies of biosorption nickel (II) and chromium (VI) from aqueous solution by dried activated sludge. Int J Env Health Eng [serial online] 2012 [cited 2013 Jun 19];1:2. Available from: http://www.ijehe.org/text.asp?2012/1/1/2/94386
| Introduction|| |
Today, water body pollution to organic and inorganic matters such as heavy metals and chlorinated organic compounds is one of the most significant problems. , Pollutants can lead to changes in the physical, chemical, and biological quality in water reservoirs.  Heavy metals such as cobalt, nickel, and chromium have been identified in mines, plating, tannery, electronics, petrochemical, and melting industry wastewaters. ,,,, Chromium and nickel metals are a cause of major concern in developing countries, due to their excessive usage and their nondegradabile nature.  Chromium(VI) is largely soluble in water and carcinogenic. ,, Nickel(II) is a more toxic and carcinogenic metal than nickel(IV). ,, According to the standard, the industrial effluent permissible discharge level of Cr(VI) and Ni(II) into a water body is 0.1 and 3.0 mg/L, respectively.  In recent years, different methods such as ion exchange, reverse osmosis and chemical precipitation, , electrochemical treatment and adsorption, , solvent extraction, membrane processes, and electrodialysis , have been used for heavy metals removal. The disadvantages of these methods are: The high investment costs, lack of usage in a small industry, operation problems, high energy consumption, and sludge production. ,, At present, studies on the biosorption mechanisms of heavy metal removal have increased due to their low cost, high removal efficiency, non-use of chemicals and nutrients, biosorbent regeneration, and the recovery of heavy metals. , The aim of present study is the recovery of municipal refuse, to investigate the capacity of dried sludge to adsorb the heavy metals Ni (II) and Cr (VI) from aqueous solutions. The results provide a useful insight into the metal uptake by dried activated sludge at a low cost, and it is a natural adsorbent.
| Materials and Methods|| |
Preparation of dried activated sludge
Activated sludge was collected as slurry from the sludge return line of the wastewater treatment facility in a dairy industry (Tehran, Iran). The performance characteristics of the sludge used in the experiment are shown in [Table 1]. Activated sludge was dried at 105°C for 12 hours in a hot air oven to reach a constant weight and then sieved through a 70 - 60 standard mesh (0.21 - 0.25 mm). Following this, the dried activated sludge (DAS) was loaded with H + in a solution of 0.1 M HCl (DAS concentration of 50 g/L) for 30 minutes, under slow stirring (HANA-HI 190 model). Subsequently, the DAS was washed with deionized water to remove the excess hydrogen ions. Finally the DAS was again dried at 105°C for 12 hours.
A stock solution of Ni 2+ and Cr 6+ were prepared by dissolving Ni (NO 3 ) 2 .6H 2 O and K 2 Cr 2 O 6 , in double distilled water. Before performing the experiment, the Ni 2+ and Cr 6+ concentration in the stock solution was measured. All the chemicals in this study were of extra pure or analytical grade. Ni 2+ and Cr 6+ concentrations in solution were measured using VARIAN 240 Atomic absorption. All the adsorption experiments were carried out at room temperature (25 ± 2°C). The removal efficiency and Ni 2+ and Cr 6+ absorption rate with DAS were calculated as depicted in equations 1 and 2, respectively.
Batch biosorption experiments
Effect of pH
The effect of pH on the adsorption of Ni 2+ and Cr 6 on DAS was carried out using 100 ml of the metal ion solution containing a concentration of 5 mg/L. 0.1 N HCl or 0.1 N NaOH was used to adjust the pH in the range of 2 to 7 (2, 3, 4, 5, 6, and 7) and 0.5 g of DAS was added to each container. After 240 minutes, the remaining concentrations of Ni 2+ and Cr 6 in the solution were determined. In all stages, the suspensions were stirred using an agitation speed of 100 rpm.
Effect of contact time
The effect of contact time was performed by shaking 2 g DAS in 100 ml solution with 5, 10, and 20 mg/L Ni 2+ and Cr 6+ concentration and a constant pH (6 ± 0.2). Next, the remaining concentrations of Ni 2+ and Cr 6+ were determined at predetermined time intervals (15 minutes) up to 240 minutes. At this stage, for any time was considered one container in order to fix the absorbent/solution ratio.
Effect of Ni 2+ and Cr 6+ initial concentration and adsorbent dose
This stage was conducted using 0.5, 1, and 2 g DAS in 100 ml solution of Ni 2+ and Cr 6+ with initial concentrations of 2 to 35 mg/L (2, 5, 8, 11, 14, 17, 20, 23, 26, 29, 32, 35 mg/L). The remaining amounts of Ni 2+ and Cr 6+ were analyzed after 180 minutes (equilibrium time).
| Results|| |
Effect of pH on Ni 2+ and Cr 6+ removal
[Figure 1] illustrates the results of the pH effect on Ni 2+ and Cr 6+ removal. Maximum Ni 2+ and Cr 6+ removal was obtained with pH 6 and 2, respectively. According to [Figure 1], by increasing pH from 2 to 6, Ni 2+ removal efficiency increased from 31 to 49% then dropped after 6.
|Figure 1: Effect of pH on Ni2+ and Crsup>6+ removal (initial concentration 5 mg/L, 5 g/L DAS, 240 minutes contact time)|
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Effect of contact time on Ni 2+ and Cr 6+ absorption
[Figure 2] and [Figure 3] present the effect of contact time on Ni 2+ and Cr 6+ adsorption. According to the results, Ni 2+ and Cr 6+ adsorption increased with an increase in contact time, and 180 minutes was the equilibrium time.
|Figure 2: Effect of contact time on Ni2+ adsorption (initial concentration 5, 10, and 20 mg/L, 20 g/L DAS, pH: 6)|
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|Figure 3: Effect of contact time on Cr6+ adsorption (initial concentration 5, 10, and 20 mg/L, 20 g/L DAS, pH: 6)|
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Effect of the Ni 2+ and Cr 6+ initial concentration and the DAS dose on the adsorption rate
The adsorption capacity (mg/g) ascended with increasing the metal initial concentrations and decreasing the adsorption sites to less than the number of input metal ions [Figure 4] and [Figure 5]. In addition, when the DAS dose increased, the removal efficiency increased, but the adsorption capacity decreased. The maximum absorption capacity and removal efficiency were obtained at the Ni 2+ and Cr 6+ initial concentrations of 35 mg/L and 2 mg/L, respectively.
|Figure 4: Effect of Ni2+ initial concentration and DAS dose on adsorption rate (Ni2+ initial concentration 2-35 mg/L, 5, 10, 20 g/L DAS and pH: 6)|
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|Figure 5: Effect of Cr6+ initial concentration and DAS dose on adsorption rate (Cr6+ initial concentration 2-35 mg/L, 5, 10, 20 g/L DAS and pH: 6)|
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| Discussion|| |
Effect of pH on Ni 2+ and Cr 6+ removal
The pH of the solutions in the biosorption process should be below 7, as metal ions precipitated at a pH above 7.  According to the previous studies, the adsorption capacities for heavy metals was strongly dependent on the pH of the solution. , At low pH, the dominant species of Cr 6+ included HCrO 4- , Cr 2 O 7 2- , Cr 3 O 10 2 , and Cr 4 O 13 2- , which could be adsorbed primarily because of the electrostatical nature.  At very low pH, the adsorbent surface was coated with hydromium ions, which increased Cr 6+ that interfered with the binding sites on the dried activated sludge. By increasing the pH, the surface charge on DAS became negative and Cr 6+ absorption was reduced. The results of this study for Cr 6+ removal was significant with pomegranate husk carbon (pH: 1),  wheat bran (pH: 2),  activated carbon (pH: 1),  Aspergillus Niger (pH: 2),  and marine algal mass (pH: 2). 
According to [Figure 1], by increasing the pH, the removal efficiency increases and then drops after 6. This can be attributed to the ion exchange. The results of this study are consistent with Aspergillus Niger (pH: 5),  Tithonia diversifolia (pH: 5),  Chlorella vulgaris (pH: 5),  and activated sludge (pH: 4.5)  on Ni 2+ removal.
Effect of contact time on Ni 2+ and Cr 6+ absorption
The results of this study showed that as the time increased, the removal efficiency and adsorption capacity increased. The rate of Ni 2+ and Cr 6+ removal was initially rapid and then decreased gradually to reach an equilibrium, which at that time was not observed to be a significant increase in the removal rate. Perhaps the initial rapid sorption was due to the participation of specific functional groups and active surface sites.  The association between the contact time and Ni 2+ and Cr 6 removal efficiency was statistically significant (P valve<0.001).
Effect of Ni 2+ and Cr 6+ initial concentration and the DAS dose on the adsorption rate
In a batch system, the initial concentrations provide an important driving force to overcome all mass transfer resistances of the metal between the aqueous and solid phases. On the other hand, the adsorption capacity (mg/g) ascends with increasing the metal initial concentrations and decreases the adsorption sites to less than the number of input metal ions [Figure 4] and [Figure 5]. In addition, when the DAS dose is increased, the removal efficiency increases, but the adsorption capacity decreases.
The equilibrium of sorption is one of the significant physicochemical aspects for the evaluation of the sorption process as an operation unit.  The correlation of the isotherm information with the theoretical or empirical equation is appropriated for practical operation. In this study, the conventional isotherms in water and wastewater have been analyzed. [Table 2] and [Table 3] show the isotherm equation and results of the isotherm calculations, respectively.
The most appropriate isotherm to describe the results is the Langmuir isotherm. The maximum adsorption capacities for Ni 2+ and Cr 6+ adsorption onto DAS are 2.17 and 2.23 (mg/g), respectively.
According to [Table 3], in the Freundlich isotherm model, K f and n indicate the Freundlich isotherm constants and adsorption intensity, respectively. The value of these parameters for Ni 2+ adsorption is higher than Cr 6+ adsorption. In the Langmuir isotherm model, Q m depicts the required amount of metal ions for the monolayer formation. The Q m in Cr 6+ adsorption is higher than in Ni 2+ adsorption, which shows that higher amounts of Cr 6+ are needed to form a monolayer. On the other hand, the absorption capacity for Cr 6+ removal is higher than that of Ni 2+ removal. Also, the K L constant represents the adsorption energy. K L in Ni 2+ adsorption is more than in Cr 6+ adsorption, so Ni 2+ is adsorbed with higher adsorption energy than Cr 6+ . The Langmuir and Freundlich isotherms represent monolayer adsorption and the BET isotherm reveals double-layer adsorption. The Langmuir isotherm best fits the experimental data, so Ni 2+ and Cr 6+ adsorption is monolayered.
The equilibrium factor (R L ), derived from the Langmuir isotherm, is calculated according to equation 3: ,
R L can be interpreted according to [Table 4].
The computed R L are 0.63, 0.38 and 0.18 for Ni 2+ adsorption and 0.65, 0.42 and 0.21 for Cr 6+ adsorption onto 5, 10 and 20 (g/L) DAS, respectively.
According to [Table 4], results showed that adsorption of Ni 2+ and Cr 6+ onto DAS is favorable. Other researches have also confirmed that heavy metal adsorption onto a biosorbent is favorable. 
Kinetic models are very useful to determine mechanisms such as mass transfer and chemical reaction. Some models have been used in batch reactors to explain the transport of species inside the adsorbents. Among them are the homogeneous surface diffusion, pore diffusion, and heterogeneous diffusion models. , [Table 5] and [Table 6] show the kinetic equation and results of the parameter calculation, respectively.
|Table 6 Parameters obtained from kinetic models with different Ni2+ and Cr6+ initial concentrations|
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[Table 6] shows that the intraparticle diffusion kinetic model is the most appropriate model for data expression. The maximum rate constants for Ni 2+ and Cr 6+ adsorption onto DAS are 0.044 and 0.042 (mg/g min 0.5 ), respectively.
The k 2 and K dif are rate constants in the second order and intraparticle diffusion kinetic models, respectively. According to [Table 5], the k 2 and K dif for Ni 2+ adsorption are higher than those for Cr 6+ . The calculated q cal for Cr 6+ adsorption is more than for Ni 2+ , and it is a confirmed isotherm calculation. The α and h parameters in the second order and the Elovich kinetic models define the initial absorption rate. These values for Cr 6+ adsorption are more than for Ni 2+ . The ß in the Elovich model is the desorption constant, which is greater for Ni 2+ adsorption.
Therefore, according to the results presented in [Table 7], activated sludge can be considered as a low-cost adsorbent and is relatively efficient for Ni 2+ and Cr 6+ removal from aqueous solutions.
|Table 7: Comparison between various adsorbents used for Ni2+ and Cr6+ on the basis of adsorption capacity|
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| Acknowledgements|| |
The authors would like to thank Environmental Health Research Committee of Kerman University of Medical Sciences for approving this research. Financial support for this research was provided by Vice-Chancellery for Research and Technology of Kerman University of Medical Sciences.
C Thickness of the boundary layer(mg/g)
C 0 Initial concentration (mg/L)
C e Equilibrium concentration in solution (mg/L)
C s Saturation concentration in solution(mmol/L)
C t Equilibrium concentration in solution at time t (mg/L)
h Initial sorption rate (mg/g minute)
k 1 Pseudo first-order rate constant (1/minute)
k 2 Pseudo second-order rate constant (mg/g minute)
K B BET constant
K dif Intraparticle diffusion rate constant (mg/g minute 0.5 )
K f Freundlich isotherm constants (L/g)
K L Langmuir isotherm constants (L/mg)
n Adsorption intensity
q e Equilibrium adsorbent concentration on adsorbent (mg/g)
q e cal Calculated values of q e (mg/g)
Qm Maximum monolayer capacity (mg/g)
qt Adsorbed metal concentration at time t (mg/g)
R 2 Correlation coefficients
RL Dimensional separation factor
α Initial adsorption rate (mg/g min)
β Desorption constant (g/mg)
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]