Enhanced chromium (VI) removal using activated carbon modified by zero valent iron and silver bimetallic nanoparticles
© Kakavandi et al.; licensee BioMed Central Ltd. 2014
Received: 28 January 2014
Accepted: 11 August 2014
Published: 21 August 2014
Recently, adsorption process has been introduced as a favorable and effective technique for the removal of metal ions from aqueous solutions. In the present study, bimetallic nanoparticles consisting of zero valent iron and silver were loaded on the activated carbon powder for the preparation of a new adsorbent (PAC-Feo/Ag). The above adsorbent was characterized by using XRD, SEM and TEM techniqes. Experimental data were exploited for kinetic, equilibrium and thermodynamic evaluations related to the adsorption processes. The Cr(VI) adsorption process was found to be favorable at pH 3 and it reached equilibrium state within 60 min. The stirring rate did not have a significant effect on the adsorption efficiency. Furthermore, the monolayer adsorption capacity of Cr(VI) based on the Langmuir model was measured to be 100 mg/g. The experimental equilibrium data were fitted to the Freundlich adsorption and pseudo second-order models. According to the thermodynamic study, the adsorption process was spontaneous and endothermic in nature, indicating the adsorption capacity increases with increasing the temperature. The results also revealed that the synthesized composite can be potentially applied as a magnetic adsorbent to remove Cr(VI) contaminants from aqueous solutions.
Along with industries’ development, the contaminants originating from their activities have significantly increased. Heavy metals are the main pollutants end up to the environment by these activities. High toxicity of these metals causes serious problems to the ecosystem even at low concentrations . Chromium (Cr) is one of the most dangerous heavy metals that has many applications in the metal cleaning and plating baths, painting, tannery and fertilizer industries . Cr mainly exists in two stable oxidation states, Cr(VI) and Cr (III). Cr(VI) form is more toxic to living things than Cr(III) due to its carcinogenicity, toxicity and high aqueous solubility ,. When the concentration of this metal reaches 0.1 mg/g of the body weight, it can be heavily lethal. Therefore, The United States Environmental Protection Agency (US-EPA) and World Health Organization (WHO) have set the maximum allowable concentration (MAC) for Cr at 0.1 and 0.05 ppm in drinking water, respectively ,.
The different methods namely, membrane filtration, electrochemical precipitation, ion exchange, adsorption, reduction of Cr(VI) to Cr(III), reverse osmosis, evaporation, chelating, solvent extraction, electrolysis and cyanide treatment were all employed for the removal of Cr(VI) from water and wastewater ,,. Most of these methods have some drawbacks such as low efficiency, high demand for energy, high cost, requiring special chemicals, and the problems related to the disposal of sludge ,. While; the adsorption process due to its ease of operation, flexibility in design, low cost and high efficiency, has been effectively applied to removal of heavy metals including Cr(VI) .
In previous studies, various adsorbents such as granular activated carbon (GAC), powder activated carbon (PAC), mineral cartridge, biological and agricultural waste have been used for the removal of Cr (VI) –. Amongst these adsorbents, PAC due to its high porosity, large surface area and high efficiency has gained more interests than the others. In a comparative study by Jung et al., they compared the removal of Cr(VI) using PAC, chitosan, and single/multi-wall carbon nanotubes and found out that the maximum adsorption capacity of PAC (46.9 mg/g) was the highest within the studied adsorbents .
However, the main problem concerning PAC lies within its reusability and separation of it from aqueous solution. Thus, establishing the optimal conditions to facilitate the separation of PAC from the solution after the adsorption process seems to be essential. A way to achieve this purpose is to induce the magnetic properties into an adsorbent followed by the use of a magnet for physical separation. This so-called method has been widely used for the last few years due to its simplicity and high-speed ,. Lv et al.  used nano Zero Valent Iron (nZVI)-Fe3O4 nanocomposite as an adsorbent for the removal of Cr(VI) and they demonstrated that 96.4% of Cr(VI) could be removed within 2 h under the conditions of pH 8.0 and initial Cr(IV) concentration of 20 mg/L. They also reported that the experimental data were fitted best to the pseudo second-order kinetic and Langmuir and Freundlich isotherm models .
Herein, we used silver nanoparticles, due to their high electrochemical potential (E0 = 0.8), to enhance the catalytic ability of nZVI . Having high specific area -at nanometer scale-, they could be used as a unique adsorbent for removal of pollutants ,. Based on the above-mentioned findings, some researchers loaded Ag nanopartices on the adsorbents such as activated carbon and multiwall carbon nanotubes for the removal of dyes and heavy metals from the aqueous solutions ,.
So far, the removal of Cr(VI) using PAC-Feo/Ag as an adsorbent has not been reported in the literature. This prompted us to combine the advantages of activated carbon and Feo/Ag bimetallic nanoparticles for the preparation of magnetic composite PAC-Feo/Ag as a new adsorbent for the removal of Cr(VI) from aqueous solutions.
Material and method
The resulting bimetallic nanoparticles were separated by a magnet and immediately washed many times with water and finally dried under N2-purged for 2 h.
A stock solution of Cr(VI) (1000 mg/L) was prepared by dissolving the required amount of potassium dichromate (K2Cr2O7) in water and further diluted to prepare the solutions in the concentration range of 4–100 mg/L. The residual concentration of Cr(VI) was measured using a UV–VIS spectrophotometer (7400CE CECIL) at 540 nm by diphenylcarbazine method.
Characterization of the synthesized adsorbent
The micro image, surface morphology, size and distribution of Feo/Ag were analyzed by scanning electron microscopy (SEM, edxS360, Mv2300). The crystalline structure of the bimetallic nanoparticles coated on PAC was investigated by X-ray diffraction (XRD, Quantachrome, NOVA2000) using Cu-kα radiation and λ = 1.54Å 40kVp and 30 mA. The dimension and shape of the adsorbent was determined by transmission electron microscopy (TEM, PHILIPS, EM 208 S) with 100 keV.
Batch adsorption experiments
All experiments were carried out under a batch condition using 100 ml Erlenmeyer flasks, each containing 50 ml of 4 mg/L Cr(VI) and a certain amounts of the adsorbent. The effect of pH in the range of 3–9 on the adsorption efficiency was studied under the following condition: contact time of 120 min and the stirring rate of 200 rpm. Herein, the pH of solution was adjusted using 0.1 M HCl or/and 0.1 M NaOH. The optimal contact time was then established under the condition of 0.3 g/L adsorbent, 4 mg/L Cr(VI) and room temperature. The Erlenmeyers were stirred in the range of 50, 100, 200, 300 and 400 rpm to determine the optimal agitation speed.
Where C0 and Ce are initial and equilibrium concentration of Cr(VI) (mg/L), respectively. V is the volume of the aqueous phase (L) and m is the mass of PAC-Feo/Ag (g).
The linear equations and parameters regarding Cr(VI) adsorption onto PAC-Fe o /Ag
KL and qm
KF and n
ln(qe − qt) = ln qe − k1t
qe and k1
qe and K2
kL (L/mg) is the empirical constant related to energy and qm (mg/g) represents the maximum adsorption capacity. kF and n are the Freundlich constants related to the adsorption capacity and intensity, respectively. The qm and kL parameters are calculated from the slope and intercept of the Ce/qe plot versus Ce, respectively. The Freundlich isotherm parameters (kF and n) are also calculated from the slope and intercept of the lnCe plot versus lnqe, respectively.
Where, Co is the initial concentration of Cr(VI). The adsorption will be favorable if RL lies within 0 and 1. For RL > 1, the adsorption is unfavorable; for RL = 1 and 0, the adsorption is linear and irreversible, respectively .
Kinetics of adsorption
Chemical kinetics deals with the experimental conditions influencing the rate of a chemical reaction. Herein, two kinetic models including the pseudo first-order and pseudo second-order models were applied for the modeling of the adsorption process of Cr(VI) onto PAC-Feo/Ag. The linear equations of the mentioned models along with respective parameters are given in Table 1.
K1 (1/min) and k2 (g/(mg.min)) are the constant rate of the pseudo first-order and pseudo second-order models, respectively. The parameters related to the mentioned kinetic models can be obtained from the plots of ln(qe-qt) and qt/t against t.
Thermodynamics of adsorption
Where qe is the amount of Cr(VI) adsorbed at equilibrium (mg/g) and Ce is the equilibrium concentration of Cr(VI) in solution (mg/L). R (8.314 J/mol K) is the universal gas constant and T (°K) is the solution temperature. The parameters of ∆Ho and ∆So can be obtained from the intercept and slope of the van’t Hoff plot (lnkd versus 1/T), respectively.
Results and discussion
The shape of Feo/Ag bimetallic nanoparticles was analyzed by using TEM micrographs with 100 keV (Figure 3(b)). It can be deducted that the synthesized absorbent structure was polygon with irregular shape. Figure 3(a, insert) reflects the synthesized composite has a high magnetic sensitivity in the presence of an external magnetic field. Finally, it can be concluded that PAC-Feo/Ag can be potentially applied as a magnetic adsorbent the for removal of Cr(VI) contaminants from aqueous solutions and, subsequently, the secondary pollution could be avoided.
Effect of solution pH
In addition; Since Feo particles could be easily oxidized to Fe2+ by Cr(VI) at pH < 6, they can promote the adsorption of Cr(VI). Therefore, it is concluded that the reduction process (i.e., the reduction of Cr(VI) to Cr(III)) at acidic condition promotes the efficiency of Cr(VI) removal, which was also suggested by other reports in the literature ,.
Since the maximum Cr(VI) adsorption (91.95%) was obtained at pH 3, this pH was selected as the optimum. This result is in good agreement with the previous studies ,. In a further related studies, pH 3 was also reported as the optimal pH for the removal of Cr(VI) once nZVI–Fe3O4 nanocomposites, active carbon and saw dust adsorbents were employed ,,.
Effect of contact time
Figure 4(b) illustrates the effect of contact time on the Cr(VI) adsorption at the following condition: 0.3 g/l solution of the adsorbent, optimal pH (pH = 3.0 ± 0.1) and the contact time of 120 min. As indicated in Figure 4(b), the Cr(VI) adsorption efficiency was increased sharply up to 60 min and then it reached the equilibrium state right after 60 min. The sharp increase in the adsorption efficiency may be due to the existence of enormous vacant active sites in the adsorbent surface. However, by raising the contact time the availability of Cr(VI) ions to the active sites on the adsorbent surface is limited, which makes the adsorption efficiency reduce . In a similar study, this phenomenon was investigated using different adsorbents ,. In a further related study, Tang et al. reported that the adsorption of Cr(VI) on nano-carbonate hydroxyl apatite reached the equilibrium state at 90 min at different concentrations of Cr(VI) . Since 90 min is more than the optimal time obtained in the present study, it can be noted that the PAC-Feo/Ag has higher adsorption rate than nano-carbonate hydroxyl apatite.
Effect of agitation speed
Effect of adsorbent dosage
Jung et al.  reported that with an increase in the dosage of various adsorbents, the Cr(VI) removal was enhanced . However, a decrease in the adsorption capacity with an increase in the adsorbent dosage is probably due to instauration of the active sites on the adsorbent surface during the adsorption process. This phenomenon can also be due to the aggregation resulting from high adsorbate concentrations, leading to the decrease in the active surface area of the adsorbent .
Effect of Cr different concentrations
Figure 6(b) shows the effect of different concentrations of Cr(VI) (4, 10, 25, 50 and 100 mg/L) on the efficiency of adsorption process. By increasing the initial Cr(VI) concentration from 4 to 100 mg/L, the percentage of adsorption decreased from 95.17 to 44.85%. The limit of active sites on the surface of adsorbent seems to be the main reason for the above-mentioned result ,. Figure 6(b) also indicates that increasing the initial concentration of Cr(VI) has a positive impact on the adsorption capacity. This phenomenon may be attributed to the rise in the concentration gradient, which is similar to the findings by Cho and Luo ,.
The parameters regarding the adsorption isotherm models for Cr(VI) adsorption on PAC-Fe o /Ag
Maximum adsorption capacities (q m ) of Cr(VI) on PAC-Fe o /Ag and the other adsorbents documented in the literature
Single-wall carbon nanotubes
Powdered activated carbon
Multi-wall carbon nanotubes
Kinetics of adsorption
The parameters regarding the adsorption kinetic models of Cr(VI) on PAC-Fe o /Ag
The analysis of data from the pseudo second-order equation suggests that the adsorption of Cr(VI) onto PAC-Feo/Ag is controlled by chemisorptions ,. In addition, Table 4 also indicates that the adsorption capacity (qe,cal) calculated from the pseudo second-order model is well suited to the experimental data (qe,exp). Therefore, it can be concluded that the kinetics of Cr(VI) adsorption on PAC-Feo/Ag fits best to the pseudo second-order model, which is in agreement with the previous reports on Cr(VI) adsorption ,,. This result also confirms that adsorption rather than reduction is more likely to be the predominant mechanism (i.e., the rate-limiting step of the process) .
Thermodynamics of adsorption
The values of thermodynamic parameters of Cr(VI) adsorption on PAC-Fe o /Ag
In the present study, the synthesized bimetallic nano composite (PAC-Feo/Ag) was used as an adsorbent for the removal of Cr(VI) from the aqueous solutions. The results illustrated that the synthesized adsorbent showed a high efficiency in adsorption of Cr(VI). The optimum conditions for the adsorption process obtained at acidic pH (pH = 3), the contact time of 60 min and the temperature of 50°C. Moreover, the equilibrium and kinetic studies indicated that the Cr(VI) adsorption followed the Freundlich isotherm and pseudo second-order kinetic models. The values regarding the thermodynamic parameters also implied that the adsorption of Cr(VI) was spontaneous and endothermic in nature. Due to favorable performance of PAC-Feo/Ag in the removal of Cr(VI) and its feasible separation from the aqueous solutions, it can be used as an efficient adsorbent in the treatment of water and wastewater with no need of further filtering and centrifugation, etc., and also it could be used as an alternative to activated carbon.
This study was done by financial support of Tehran University of Medical Sciences and Iranian Nano Technology Initiative Council.
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