Adsorptive Removal of Co and Ni by Peels of Banana from Aqueous Solution

The aims of the study were to investigate the effect of Banana peel on adsorption of Co and Ni by using flame atomic absorption spectroscopy (FAAS) for metal estimation. The effects of Co and Ni ions concentration, agitation time and temperature on adsorption of heavy metals onto Banana Peel was investigated. The experimental isotherm results were fitted using Langmuir and Freundlich equations. The Langmuir and Freundlich model agrees very well with experimental data. The maximum amounts of Co and Ni + adsorbed (qm), as evaluated by Langmuir isotherm, were 9.02 mg and 8.91 per gram of powder of Banana peel, respectively. Study concluded that Banana peels, a waste material, have good potential as an adsorbent to remove toxic metals like Co and Ni from water.


Introduction
Heavy metal pollution has become an environmental problem throughout the world because they can be accumulated into the food chain and caused serious problems, not only for ecosystems, but also for human health. The selective removal of industrial heavy metals from liquid waste is consequently the subject of considerable ecological and economic interest (Wan Ngah et al. 2002; Gode and Pehlivan, 2006). Heavy metal ions, aromatic compounds (including phenolic derivatives, and polycyclic aromatic compounds) and dyes are often found in the environment as a result of their wide industrial uses. Wastes containing soluble toxic heavy metals require concentration of the metals into a smaller volume followed by recovery and secure disposal. Heavy metals can be removed by adsorption on solid matrixes (Xu et al. 2008; Pacyna et al. 2001; Mas et al. 2010).
Banana plants belong to the family Musaceae. They are cultivated primarily for their fruit and to a lesser extent for the production of fiber and as ornamental plants. Banana plants are normally tall and sturdy. For some species, height can reach up to 8 m. each. Stem can produce a bunch of green bananas which when ripened turn yellow. Banana fruits grow in hanging clusters, with nearly 20 fruits to a hand (tier) and 3-20 hands to a cluster. The fruit averages 125 g, of which 25% is dry matter and the remaining is water. Banana is one of the largest consumed fruit in the world and useless peels, therefore, creates one of the major agro-waste problems. Preliminary investigations showed that several tons of banana peels are produced daily in market places and household garbage that create environmental nuisance. For this reason, banana peels have been tested as adsorbents for heavy metals from industrial wastewaters. It is therefore, essential to search agricultural by-products and to transform such materials to adsorbents. Nowadays, agricultural materials are receiving more and more attention as adsorbents for the removal of pollution from water (Anwar et al. 2010). A recent study by Memon et al., (2008) showed that banana waste material removed about 95% chromium ions from industrial effluent. Mas et al. (2010) showed that Removal of Methyl Red from Aqueous Solutions Using Modified Banana. Abbasi and Alikarami, (2012) showed that Removal of acetic acid from Aqueous Solutions Using Banana.
The goal of this study was to investigate the extent of removal of contaminant heavy-metal species (Co 2+ and Ni 2+ ) from aqueous Solution by Banana peel. Maximum adsorption capacity of adsorbent, adsorption intensity of the adsorbate on adsorbent surface and adsorption potentials of adsorbent were estimated by Langmuir and Freundlich isotherms.

Materials and Methods
Experiments were conducted with Banana peel. Peels were separated from the fruit gently, washed thoroughly and dried in sun light for 5 days and then in an oven at 70 °C. The dried Banana peels were then cut into small pieces, ground to a size of 200-400 μm and used in adsorption test.

Adsorption Experiments
Standard solution (1000 mg/ L) of each metal ion was prepared by dissolving required amount of nitrate salt of metal ion (Co(NO 3 ) 2 ·6H 2 O and Ni(NO 3 ) 2 ·6H 2 O obtained from Merck) into deionized water. The experimental solution of desired concentration was prepared by successive dilution of stock solution. In order to determine the effect of physicochemical parameters such as pH, adsorbent dose, contact time, initial metal ion concentration of solution and temperature. the adsorption experiments were performed by batch equilibrium method. The experiments were carried out in 150 ml of conical flasks by mixing a pre-weighed amount of adsorbent with 50 ml of metal ion solution. Initial pH of solutions was adjusted by 0.1 M NaOH or 0.1 M HNO 3 . All experiments were performed at room temperature and kept for stirring for given period of time.
There after the mixture was centrifuged and the initial and final metal ion concentrations were determined by Atomic Absorption Spectrophotometer. The % removal of metal ion and amount of metal ion adsorbed on Banana peel (q e ) was calculated by Eqs. (1) and (2), respectively: where C 0 and C e are the initial and final (equilibrium) concentrations of the metal ion in solution (M), V the solution volume (l) and m is the mass of Banana peel (g). All adsorption experiments were performed in triplicate and the mean values were used in data analysis.

Results and Discussions
The adsorption kinetics is influenced by various factors, which include initial heavy metals concentration, amount of adsorbent and time. The initial heavy metals concentration is one of the most important factors that determines the equilibrium concentration, but also determines the uptake rate of heavy metals and the kinetic character. The types of adsorbents for the removal of Ni (II) and Co(II) and their maximum adsorption capacities are shown in Table 1.

Effect of pH
To study the effect of pH on adsorption, experiments were carried out in the pH range 1-6 for Ni 2+ and Co 2+ . Fig.  1 shows that the removal of metal ions was increased with increasing initial pH of metal ion solution and maximum value was reached at pH 5.5 for Co 2+ and Ni 2+.  Fig. 2 shows that the adsorption of metal ions increased from 32 to 81% and 29 to 74% for Co 2+ and Ni 2+ , respectively, as Banana peel dose increased from 1 to 3 g/L. This is because at higher dose of adsorbent, due to increased surface area, more adsorption sites are available causing higher removal of Co 2+ and Ni 2+ (Abbasi and Alikarami., 2012) Further increase in adsorbent dose, did not cause any significant increase in removal of metal ion. This was due to the concentration of metal ions reached at equilibrium status between solid and solution phase.

Effect of Contact Time
Influence of contact time on adsorption of Co 2+ and Ni 2+ on Banana peel was investigated in the range of 1-70 min for the initial concentration of 10 mg/L for each metal ion (Fig. 3). Maximum rate of removal occurred within 3 min of contact time thereafter removal rate became slow and after 30 min of contact time no change was observed for Co 2+ (81%) and Ni 2+ (74%), which established that the system has reached the equilibrium point.

Effect of Equilibrium Metal Ion Concentration
The effect of initial concentrations of Co 2+ and Ni 2+ on the adsorption has been investigated at 303 K, Co 2+ and Ni 2+ adsorption capacities of Banana peel were given as a function of equilibrium concentration in Fig. 4. It was clear that Co 2+ and Ni 2+ adsorption capacities of Banana peel increased with increasing the equilibrium concentration. The adsorption capacity was in the decresing order of Co 2+ > and Ni 2+ . Two important physicochemical aspects for the evaluation of the adsorption process as a unit operation are the equilibrium of the adsorption and the kinetics. Equilibrium studies give the capacity of the adsorbent. The equilibrium relationships between the adsorbent and the adsorbate are described by the adsorption isotherms. The adsorption curves were applied to both the Langmuir and Freundlich equations. The Freundlich isotherm model, which assumes that the adsorption occurs on heterogeneous surfaces, is often expressed as; This equation is conveniently used in the following linear form: where K f is Freundlich isotherm constant (L/g) and n is Freundlich isotherm exponent. Values of K f and n were calculated from the intercept and slope of plots ln q e vs ln C e and a straight line indicates the confirmation of the Freundlich isotherm for adsorption (Fig. 5). The value of n should be greater than one confirming good adsorption of heavy metals onto peels of Banana. Langmuir isotherm, which assume that a monolayer of heavy metals is formed on a relatively regular adsorbent surface, using the partially protonated groups of the adsorbent. The Langmuir isotherm has been successfully applied to many real sorption processes and is expressed as follows: where q e is the amount adsorbed at equilibrium (mg/ g), C e the equilibrium concentration (mg/ L), b a constant related to the energy or net enthalpy of adsorption (L/ mg), and Q 0 the mass of adsorbed solute required to saturate a unit mass of adsorbent (mg/ g). Q 0 represents a practical limiting adsorption capacity when the surface is fully covered with heavy metals and allows the comparison of adsorption performance, particularly in the cases where the adsorbent did not reach its full saturation in experiments. The Langmuir equation can be described by the linearized form as follows: By plotting (C e /q e ) versus C e , Q 0 and b can be determined if a straight line is obtained (Fig. 6). The essential characteristics of Langmuir isotherm can be expressed in terms of a dimensionless constant, separation factor or equilibrium parameter, R L , which is defined by:

Effect of Temperature and Thermodynamic Parameters
The effect of temperature on the adsorption of heavy metals on Banana peel was investigated by conducting experiments for 30 mg/L of initial metals ion concentrations at 303, 313 and 323 K. It was observed that by increasing the temperature percentage removal of heavy metals increased. This showed that the adsorption process was endothermic in nature.
The thermodynamic parameters Gibb's free energy (ΔG°), enthalpy (ΔH) and entropy (ΔS°) were calculated using the following equations: Adsorptive Removal of Co 2+ and Ni 2+ by Peels of Banana from Aqueous Solution where m is the adsorbent dose (g/ L), C e is concentration of metals ion (mg/ L), q e is the amount of metals ion at equilibrium in unit mass of adsorbent (mg/ g), q e / C e is called the adsorption affinity. ΔH, ΔS° and ΔG° are change in enthalpy (kJ/ mol), entropy (J/ (mol K)) and free energy (kJ/ mol), respectively. R is the gas constant (8.314 J/mol K) and T is the temperature (K).
The values of ΔH and ΔS° were obtained from the slopes and intercepts of the Van't Hoff plots of ln (q e m/ C e ) vs. 1/ T, respectively, thereafter ΔG° values were determined from Eq. (9). The values of thermodynamic parameters are presented in Table 3. The results showed that the ΔG° values are negative and increased in their absolute values with temperature (Akmil et al., 2005) This result suggested that a high temperature is favored for the adsorption of heavy metals on Banana peel, indicated a spontaneous adsorption process. The values of heat of adsorption, ΔH is positive for metals ion, indicated that the adsorption process of heavy metals on Banana peel was endothermic. A positive ΔS suggested that heavy metals were not stable on the adsorption sites of Banana peel probably due to the increase in translational energy of metals ion.

Conclusion
The current study emphasizes on the ability of Banana peel to adsorb heavy metals from aqueous solutions. In batch mode studies the adsorption was dependent on initial metals ion concentration and agitation time. Although the adsorptive capacity of Banana peel is not high for heavy metals. Low cost of the material together with its adsorptive ability could offer a promising procedure for depollution of industrial wastewaters. The negative values of ΔG° suggested that the adsorption was spontaneous in nature. The positive value of ΔH and ΔS indicated endothermic adsorption process and increased randomness at surface-solution interface, respectively.