Adsorption of Nickel(II) Ions from Aqueous Solution using Banana Peel and Coconut Shell

Adsorption; ANOVA; Banana peel; Coconut shell; Nickel (II) ions

Coconut shell and banana peel are low cost adsorbents usually regarded as agricultural waste. Various studies have analyzed their adsorbing characteristics using different adsorbates and conditions. In various capacities, adsorption depends on the characteristics of the individual adsorbent, the extent of surface modification, and the initial concentration of the adsorbate, temperature, pH, adsorbent dosage and size, among other variables. Annadurai et al. (2002) worked on the adsorption of heavy metals from water using banana and orange peel. At 30 ^oC, adsorption capacity decreased in the order Pb²⁺> Ni²⁺ >Zn^{2 +}>Cu²⁺ >Co²⁺ for both adsorbents, and rose with increasing pH. Song et al. (2013) described the removal of Pb²⁺ ions from aqueous solution using coconut shell, activated with KOH. The Freundlich isotherm described the adsorption data, while kinetics indicated a pseudo-second order kinetic model. In addition, Olayinka et al. (2009) investigated the removal of Cr (VI) and Ni (II) from industrial waste effluents using adsorption. The adsorption mechanism was found to fit the pseudo second order after evaluation with pseudo first order and second order kinetics. Abbasi et al. (2013) carried out a study on the adsorptive removal of Co²⁺ and Ni²⁺ by banana peel from aqueous solution. The maximum amounts of Co²⁺ and Ni²⁺ adsorbed (q_m), from the Langmuir isotherm were 9.02 and 8.91 mg per gram of banana peel, respectively.

Abbas et al., (2013) also studied the potentiality of banana peel to remove cyanide ion pollutant from waste water using an adsorption process by simulating a synthetic aquatic solution; maximum removal efficiency was 95.65% for cyanide ion removal. Okafor et al. (2012) explored the adsorption capacity of coconut shell for the removal of Pb²⁺, Cu²⁺, Cd²⁺ and As³⁺ from aqueous solutions. Adsorption capacity followed the trend Pb²⁺> Cu²⁺> Cd²⁺> As³⁺ and the kinetic treatment gave a pseudo second order type, while the Freundlich adsorption isotherm best described the adsorption. Soco and Kalembkiewicz (2013) worked on the adsorption of Ni²⁺ and Cu²⁺ ions from aqueous solution using coal fly ash. Optimum conditions of adsorption of Cu and Ni ions in the systems were established and the coefficient of adsorption was obtained using the Freundlich and Langmuir equations. Divakaran et al. (2012) also studied the adsorption of Ni (II) ions and Cr (VI) ions by chitin and chitosan when both ions were present together. The adsorption of Cr (VI) ions was much lower than that of Ni (II) ions.

Garba et al. (2016) reported on the ideal conditions for the adsorption of Ni (II) and Cd (II) ions onto Modified Plantain Peel (MPP) from aqueous solution. The Langmuir model and pseudo second order kinetics best described the two adsorption processes. The factors, effects and mechanisms of the adsorption of Hg (II), Cd (II) and Ni (II) on charged liposomes was reported by Gong et al. (2018). Attention was paid to the effect of pH, ionic strength and particle size of the liposomes on sorption. The mutual effects between graphene oxide and Ni (II) ions with regard to their adsorption and co-adsorption on two minerals (goethite and hematite) in aqueous phase have been reported (Sheng et al., 2018). A pseudo second order kinetic model with chemisorption was reported by Rao and Khan (2017) in the adsorption of Ni (II) on alkali-treated pineapple residue in batch and column studies. The influence of metal ion concentration and pH was investigated by Pino et al. (2006) in the biosorption of cadmium by green coconut shell powder. In another useful account of agricultural waste, banana peel particles have been effectively used as a replacement for asbestos in brake pad manufacture (Idris et al., 2015).

This present work is focused on the separate removal of nickel (II) ions from an aqueous solution using coconut shell and banana peel, to study the comparative effect of various parameters such as pH, contact time, adsorbent dose, adsorbate dose, particle size and temperature. Further investigations of this present work include fitting the adsorption process with suitable isotherms such as those of Langmuir, Freundlich, Temkin and Dubinin-Radushkevich, and to establish the kinetics of the adsorption process, as well as to optimize the adsorption process and statistically correlate and justify the importance of the process parameters.

?2.1.    Materials

Banana peel (Musa Acuminata) was collected from the local market in Bariga, Lagos, while coconut shell (Cocos nucifera L.) was obtained from Badagry, Lagos, Nigeria.

2.1.1.   Adsorbent preparation

The banana peel was prepared by adopting the method of Abbasi et al., (2013) and Taimur et al. (2012). It was washed thoroughly with distilled water to remove dust and soil, dried in sunlight for 5 days and kept in an oven at 70 ^oC. The dried peel was then cut into small pieces, after which it was ground. The coconut shell was prepared based on the method used by Ayub and Khorasgani (2014) and Tharannum et al. (2015). After collection it was sun dried for 2 days, crushed with a hammer mill, sieved and pre-treated with 0.1 M NaOH for 3 hours, then washed with distilled water to remove dust, soil and NaOH traces. The adsorbent was sieved and dried again at 50 ^oC in an oven. It was then stored in desiccators for use.

2.1.2.   The adsorbate

Nickel(II) nitrate hexahydrate was used as the adsorbate and was obtained from Finlab, Ikorodu Road, Lagos. It was prepared according to the method adopted by Gonen and Serin (2012). Ni(II) ions were prepared by diluting 1000 mg/L of Ni(NO₃)₂.6H₂O stock solution with distilled water to a desired concentration range of between 10 and 200 mg/L.

2.1.3.   Apparatus and reagents used

The following apparatus and reagents were used: analytical grade nitric acid (HNO₃); analytical grade sodium hydroxide (NaOH); analytical grade nickel(II) nitrate hexahydrate salt [Ni (NO₃)₂.6H₂O]; a mesh sieve B.S.S (200); Buck atomic absorption spectrophotometer (AAS); hammer mill; electric water bath; electric oven; electric water bath; graduated beakers and cylinders; Whatman filter paper; electronic weighing balance; electronic pH meter; furnace; funnels; stopwatch and distilled water.

2.2.    Experimental Procedure

The initial pH of the solutions was adjusted by 0.1 M NaOH or 0.1 M HNO₃. All the experiments were carried out at room temperature, and the initial and final metal ion concentrations were determined using the AAS. All runs were conducted three times, with averages taken. The % removal of the Ni(II) ion and the amount of (q_e) adsorbed on the coconut shell and banana peel were calculated using Equations 1 and 2, respectively:

Removal (%) =

3.1.    Effect of Adsorbent Dose on Adsorption

From Figure 1, it can be seen that the percentage removal of each adsorbent rose with increasing adsorbent dosage, but tended to remain constant at about 1.3 g. This may be due to overlapping of the adsorption sites as a result of overcrowding of the adsorbent particles. A similar explanation was given by Suresha and Deepa (2014) for the biosorption of Ni(II) ions from aqueous solution using Araucaria cookie leaves. 

Figure1 Effect of varying adsorbent doses on adsorption

3.2.    Effect of pH on Adsorption

Figure 2 shows that increments in pH resulted in a high percentage removal, and remained constant for pH values of about 6 to 8. This is due to the fact that pH has a significant effect on adsorption because it affects the solubility of metal ions, the concentration of counter ions on the functional group of the adsorbent, and the degree of ionization of the adsorbate during the reaction. Sorption does not seem to occur in highly acidic and alkaline conditions because hydrogen ions and hydroxyl ions compete for active sites on the adsorbent surface, as reported by Shah et al. (2016).

Figure 2 Effect of varying pH on adsorption

3.3.    Effect of Contact Time on Adsorption

It is evident from Figure 3 that an increase in contact time favors only adsorption using banana peel as the adsorbent. Decreasing the contact time between the solution and coconut shell would improve the percentage removal. It seems that banana peel has more capacity than coconut shell for adsorption. Increases in time are expected to enhance sorption until saturation at equilibrium. From the observed phenomenon, the optimized maximum uptake of banana peel compared to coconut shell seems to be higher, as reported separately by Annadurai et al., (2002) and Aziz et al. (2005).

Figure 3 Effect of contact time on adsorption

3.4.    Effect of Temperature on Adsorption

As shown in Figure 4, with an increase in temperature the percentage removal decreases as a result of the desorption taking place; that is, the nickel ions do not attach themselves properly to the adsorbent as the temperature is increased over time. Naturally, temperature increases the kinetic energy of the ions and molecules in solution, which has an adverse effect on the adsorption. Similar accounts were given by Matouq et al. (2015) for the sorption of nickel onto moringa pods, and by Hannachi et al. (2014) for the adsorption of nitrate ions onto a membrane.

Figure 4 Effect of temperature on adsorption

3.5.    Effect of Adsorbate Concentration on Adsorption

Increasing the adsorbate concentration slightly reduced the percentage removal for banana, while it appeared constant for coconut shell, as shown in Figure 5. Increasing adsorbate concentration presented a situation whereby more ions were provided to be adsorbed, without increasing the adsorbent quantity. This led to supersaturation of the active sites available for adsorption. Matouq et al. (2015) and Khelifi et al. (2016) similarly reported that as the initial concentration for metal ions increases, the removal percentage decreases for a fixed adsorbent dose and contact time. This behavior was also explained by the limited active site on the adsorbent surface.

Figure5 Effect of adsorbate concentration

3.6.   Effect of Particle Size on Adsorption

As shown in Figure 6, the percentage removal by adsorption fell with increased particle size. This could be explained from the fact that as the particle size increased, the active available surface area decreased, which hindered adsorption for both adsorbents. The same parametric observation was made by Hossain et al. (2012).

Figure 6 Percentage Removal with Varying Particle Size

It has been shown that banana peel and coconut shell are both good adsorbents for the retrieval of Ni (II) ions with consideration of the six parameters, but that banana peel has a higher percentage removal for most of the factors. The ideal pH for obtaining the maximum amount of Ni (II) ion uptake by the adsorbents was 8, so this can be considered to be the optimum dosage in specific conditions. A very good comparative percentage, similar to existing works, was also obtained in this study. From the experimental approach design, the highest percentage removal for banana peel suggested an adsorbent dose of 4.5 g, contact time of 120 mins and temperature of 25^oC, while the conditions for the maximum Ni (II) ion uptake using coconut shell are 4.5 g adsorbent dose, 30 min contact time and temperature of 25^oC. The percentage removal of Ni (II) ions was found to decrease with increasing temperature for both adsorbents, which indicated the exothermic nature of the process. The adsorption operation was in close agreement with the Langmuir isotherm for both adsorbents, indicating a monolayer adsorption process. The constants obtained from the Langmiur and Freundlich adsorption isotherms had similar values to those of Chaudhari (2009). The statistical test with ANOVA and Bonferroni-Holm Posthoc parametric significance test gave good insight into the significance and interdependencies of variables, or the parameters needed for improved and more focused future operations. Lastly, banana peel and coconut shell are effective adsorbents for the removal of Nickel(II) ions from their aqueous solution, with the highest removal for coconut shell obtained at 75.9%, and at 77.8% for banana peel, at 20 ^oC.These adsorbents are cost effective and could be considered for the treatment of heavy metals present in industrial wastewater.

The supporting services and useful advice during theexperimental stage of this work provided by the Chemical and PetroleumEngineering Department, as well as the Central Research Laboratory, of theUniversity of Lagos, Akoka, Yaba, Lagos are greatly appreciated.

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