Nanoscale zero-valent iron-impregnated agricultural waste as an effective biosorbent for the removal of heavy metal ions from wastewater
© Kumar et al. 2016
Received: 16 December 2015
Accepted: 3 March 2016
Published: 15 March 2016
A novel, nanoscale zero-valent iron-impregnated cashew nut shell (NZVI-CNS) was synthesized towards the removal of Ni(II) ions from aqueous solution using impregnation procedure. The factors affecting Ni(II) ion adsorption in a batch mode were studied including the initial metal ion concentration, solution pH, temperature, adsorbent dosage, and contact time. The adsorption isotherm and kinetics could be described well with Freundlich and pseudo first-order, respectively. The maximum monolayer adsorption capacity for the removal of Ni(II) ions was found to be 70.05 mg/g. The calculated thermodynamic parameters showed that the removal of Ni(II) ions by the NZVI-CNS was spontaneous, feasible, and exothermic in nature. The amount of adsorbent needed to treat the known volume of the effluent was calculated by using single-stage batch adsorber design. The experimental results specifies that the NZVI-CNS have a high adsorption capacity for the removal of Ni(II) ions from aqueous solution.
In recent decades, industrialization and urbanization have grown exponentially to meet the human needs. As a result, the environmental impact has been increasing dramatically due to the direct discharge of toxic effluent into the water bodies. The toxic substances are mostly hazardous to human health which includes heavy metals, pharmaceuticals, pesticides, and dyes. Among which, heavy metals received a special attention by many researchers because they are persistent nature due to lack of biodegradability (Martins et al. 2013; Singh and Das 2013; Gong et al. 2013). In recent years, these heavy metals play a vital role in most of the industries. Particularly, nickel is widely used in diverse metal products and processes. Few of their applications are in industries such as electroplating, batteries manufacturing, mining, metal finishing, porcelain enameling, and paint formulations. The toxicity of nickel ions will cause severe health issues such as dermatitis, allergies, renal disturbances, hepatitis, infertility, lung cancer, stomatitis, gingivitis, insomnia, nauseas, and different poisoning degrees to the kidney and cardiovascular system (Paulino et al. 2007; Fu et al. 2015; Jeon and Cha 2015). Hence, the effluents with nickel ions should be treated before letting them into the receiving water bodies. According to the US EPA, the permissible limit of nickel in wastewater effluent is 2 mg/L for short-term effluent reuse and 0.2 mg/L for long-term effluent reuse (El-Sadaawy and Abdelwahab 2014; U.S EPA 2004). According to the Bureau of Indian Standards, the maximum permissible limit for Ni(II) ions in drinking water has been fixed at 0.02 mg/L (BIS 1994). Therefore, many technologies such as chemical precipitation (Purkayastha et al. 2014), ion exchange, electrochemical technologies (Fu and Wang 2011), adsorption (Bilal et al. 2013), and membrane filtration process such as microfiltration (Tashvigh et al. 2015), reverse osmosis (Wei et al. 2014), ultrafiltration (Tanhaei et al. 2014), nanofiltration (Alzahrani and Mohammad 2014), and electrodialysis (Dermentzis 2010) have been employed for both removal and recovery of Ni(II) ion from aqueous environment. Most of these technologies are expensive and incompatibility to remove the trace level of heavy metal ions. Among these technologies, many researchers focused on adsorption technology which offers simplicity, technical feasibility, economical viability, and social acceptability and also got high removal efficiency of heavy metal from aqueous solution. On the other hand, the main advantage of this process is reversible, because the adsorbents used for metal ions removal can be regenerated by employing appropriate desorption process (Barakat 2011; Zhou et al. 2015; Mahmoud et al. 2015). The commercial adsorbent such as activated carbons (Rajkumar et al. 2014), polymeric materials (Anitha et al. 2015), clays (Lee and Tiwari 2012), biosorbents (Kumar et al. 2015), and other adsorbents have been commonly used for this adsorption process. Many adsorbents usually have lower adsorption capacity, higher cost, longer equilibrium time, poor regeneration abilities, and separation problems (Prabu et al. 2015, 2016). Hence, adsorbents qualifying the aforementioned problems were synthesized and employed for removal of metal ions from aqueous environment.
Nanoscale zero-valent iron (NZVI) is a promising technology because many researchers focused on this process and implemented in the treatment of hazardous contaminants from wastewater. This increase in interest on the use of NZVI is owing to its higher surface area, lower cost, non-toxic, and higher reactivity (Uzum et al. 2009; Boparai et al. 2011; Arshadi et al. 2014; Prabu et al. 2016). The removal mechanism is directional transfer of electrons from NZVI to the contaminants and then the contaminant is transformed into non-toxic or less-toxic species. In addition, it also degrades and oxidize the organic compounds in the presence of dissolved oxygen (DO) (Fu et al. 2014; Prabu et al. 2015, 2016). Mostly, direct of use of NZVI is restricted due to its lack of stability, easy aggregation, and facing difficulties in separating NZVI from treated effluents. To mitigate these issues, NZVI is impregnated with supporting material such as cashew nut shell (CNS) with the help of sonication operation. This does not only provide better operation but enhances the reduction ability of NZVI (Fu et al. 2014; Prabu et al. 2016). Finally, the efficient removal of nickel ions in aqueous environment can be achieved by utilizing the synthesized nanoscale zero-valent iron-impregnated cashew nut shell (NZVI-CNS). In this research, the batch adsorption studies for the removal of Ni(II) ions were investigated by using the several parameters such as initial Ni(II) concentration, solution pH, temperature, adsorbent dose, and contact time. The adsorption isotherm, kinetics, design, and thermodynamic studies have been discussed to explain the adsorption process.
Chemicals and equipment
The chemical nickel(II) sulfate hexahydrate [NiSO4·6H2O] was used to prepare the stock solution of Ni(II) ions. The nickel salt was taken in a measured amount and diluted in double distilled water. To make the desired Ni(II) ions concentration solution (25–150 mg/L), the stock solution was further diluted with distilled water. The pH of the different ion concentration solution was measured with a pH meter (Elico Limited, India) and was adjusted using 0.1 N NaOH and 0.1 N HCL solutions. The concentration of Ni(II) ions onto the ions solution before and after adsorption process was measured with an atomic adsorption spectrophotometer (AAS, SL 176 Model, Elico Limited, Chennai, India).
Preparation of the adsorbent
The CNS system was completely washed using distilled water followed by a wash with methanol to prevent the corrosion formation. At last, the CNS system was dried and stored in an oxygen-free nitrogen environment. This prepared material was abbreviated as NZVI-CNS (NZVI-impregnated CNS). During the synthesis of ZVI, the particles of boron were formed. These particles are toxic in nature, so special care must be needed for separating the boron particles.
Batch adsorption studies
where C o (mg/L) and C e (mg/L) are the initial and equilibrium concentrations of Ni(II) ion solution, respectively.
Adsorption equilibrium experiments
The adsorption isotherm studies were performed in a series of 100 mL Erlenmeyer flasks by adding 0.2 g of adsorbent material (NZVI-CNS) into 100 mL of the Ni(II) ion solution with the various initial Ni(II) ion concentrations in the range of 25 to 150 mg/L at an optimum condition, and then, the mixtures were kept in a temperature-controlled shaking incubator until its equilibrium time. Once the equilibrium time is reached, the flasks were withdrawn from the incubation shaker and the adsorbent material NZVI-CNS was centrifuged to separate the supernatant and the spent adsorbent. The concentration of Ni(II) ion in the supernatant was examined by using atomic adsorption spectrophotometer (AAS) (SL 176 Model, Elico Limited, Chennai, India), and the values are substituted in the following formula to know the amount of Ni(II) ion adsorbed onto the NZVI-CNS at equilibrium, q e (mg/g).
where C o and C e are the concentrations of Ni(II) ions in the solution initially and at equilibrium (mg/g), respectively, V is the volume of the Ni(II) ion solution (L), and m is the mass of the adsorbent (g).
The two-parameter and the three-parameter isotherm models were used to explain the Ni(II) ions adsorption towards the selected adsorbent. The different parameters, R 2, and the error values are analyzed by the MATLAB R2009a software.
where q e is the adsorption capacity at equilibrium (mg/g), q m is the maximum monolayer adsorption capacity (mg/g), C e is the concentration of the metal ions at equilibrium (mg/L), and K L is the Langmuir equilibrium constant (L/mg).
where C e is the concentration of the metal ions at equilibrium (mg/L), K F is the Freundlich constant [(mg/g)(L/mg)(1/n)] related to the bonding energy, and n is a measure of deviation from the linearity of adsorption (g/L). The significance of “n” is as follows: n = 1 (linear), n > 1 (physical process), and n < 1 (chemical process).
where C e is the concentration of metal ions at equilibrium (mg/L), β RP is the exponent which lies between 0 and 1, K RP is the Redlich-Peterson isotherm constant (L/g), α RP is the Redlich-Peterson isotherm constant (L/mg)1/β RP. The significance of β is as given as follows: β = 1 (Langmuir adsorption isotherm model is a preferable adsorption isotherm model) and β = 0 (Freundlich adsorption isotherm model is a preferable adsorption isotherm model).
where K s is the Sips model isotherm constant (L/g) βS, α s is the Sips model constant (L/g)1/βS, and β S is often regarded as the heterogeneity factor, with values close to 1 indicating a homogeneous binding site and values greater than 1 indicating a heterogeneous adsorption system.
Adsorption kinetic experiments
where q t is the amount of Ni(II) ions adsorbed by the adsorbent at any time (mg/g), C t is the concentration of Ni(II) ions at particular time (mg/L), V is the volume of the metal ion solution (L), and m is the mass of the adsorbent (g).
The pseudo first-order, pseudo second-order, and Elovich kinetic models are used to determine the rate of the adsorption process.
where q t is the adsorption capacity at any time (mg/g), q e is the equilibrium adsorption capacity (mg/g), k 1 is the pseudo first-order kinetic rate constant (min−1), and t is the time (min).
where k 2 is the pseudo second-order kinetic rate constant (g/mg min) and t is the time (min).
where β E is the desorption constant related to the activation energy of chemisorption (g/mg) and α E is the initial adsorption rate (mg/(g min)).
Adsorption thermodynamic study
where C Ae is the amount of Ni(II) ions adsorbed onto the adsorbent per liter of solution at equilibrium (mg/L), C e is the equilibrium concentration in solution (mg/L), T is the temperature (K), R is the gas constant (8.314 J/mol/K), and K c is the equilibrium constant. The values of ∆S o and ∆H o were calculated from the slope and the intercept of the plot of log K c versus 1/T.
Results and discussions
Influence of initial Ni(II) ion concentration on the adsorption of Ni(II) ions and adsorption isotherms
Isotherm constants of the two-parameter and three-parameter models for the removal of Ni(II) ions by NZVI-CNS
q m (mg/g)
K L (L/mg)
K F [(mg/g)(L/mg)(1/n))]
K RP (L/g)
α RP (L/mg)(1/βRP)
K S (L/g) βs
α s (L/mg)(βs)
Influence of solution pH on the adsorption of Ni(II) ions
Influence of temperature on the adsorption of Ni(II) ions and thermodynamic studies
Thermodynamic parameters for the adsorption of Ni(II) ions onto NZVI-CNS
Concentration of Ni(II) ion solution (mg/L)
∆G o (kJ/mol)
Influence of adsorbent dosage on the adsorption of Ni(II) ions
Influence of contact time on the adsorption of Ni(II) ions and kinetic studies
Adsorption kinetic data for the removal of Ni(II) ions by NZVI-CNS
Adsorption kinetic model
q e (mg/g)
q e, exp (mg/g)
k 1 (min−1)
q e, cal (mg/g)
k 2 (g/mg min)
Elovich kinetic model
α E mg/(g min)
β E (g/mg)
Design of a single-stage batch adsorber
In summary, we reported that the NZVI-CNS adsorbent material was successfully synthesized through the impregnated method which have been effectively used for the removal of Ni(II) ions from aqueous solution. The adsorption was studied kinetically using diverse adsorption kinetic models such as pseudo first-order, pseudo second-order, and Elovich kinetic models. The results showed that the adsorption process followed the pseudo first-order model based on higher correlation coefficient with low error values. The adsorption of Ni(II) ions onto NZVI-CNS was influenced by several operating parameters such as initial Ni(II) ion concentration, solution pH, contact time, temperature, and adsorbent loading. The maximum monolayer adsorption capacity for the removal of Ni(II) ions was found to be 70.05 mg/g at an optimum condition. The equilibrium isotherm has been analyzed using different isotherm models such as Langmuir, Freundlich, Redlich-Peterson, and Sips model. Among these, the Freundlich isotherm model was identified as a well-suitable adsorption isotherm model based on the high coefficient of determination values with low error values. The adsorption thermodynamics illustrated that the electrostatic interaction plays an important role in the adsorption process. The values of thermodynamic parameters such as Gibbs free energy, entropy, and enthalpy indicated that the adsorption process was spontaneous, feasible, and exothermic in nature. A batch adsorber was designed to determine the amount of adsorbent dose needed for the treatment of a known volume of the desired Ni(II) ion concentration. Based on the experimental studies, it can be concluded that the NZVI-CNS materials has more potential for the removal of Ni(II) ions, and correspondingly for many other heavy metal ions from aqueous solution, with the advantages of being low cost, recyclable, efficient, and economically friendly.
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