Decontamination of Ni(II) and Co(II) ions from Aqueous Medium using the Cola lepidota Pericarp

In this study, the batch adsorption of Ni(II) and Co(II) ions was investigated by using the pericarp of Cola lepidota as the adsorbent. The pulverized sample was modified using sodium hydroxide. In addition, the effects of pH, particle size, contact time, temperature, and initial concentration on adsorption were investigated. Results revealed that maximum adsorption of metal ions are observed at pH of 6, a contact time of 60 min, and a temperature of 70 oC. In addition, modification was to considerably impact the effectiveness of the adsorbent. The percentage removal of the metal ions on the modified (MCLR) and unmodified adsorbents (UCLR) followed increasing order of Co(II) ion on UCLR < Ni(II) ion on UCLR < Co(II) ion on the NaOH-modified adsorbent < Ni(II) ion on the NaOH-modified adsorbent. Equilibrium data were analyzed by the Langmuir and Freundlich isotherm models, and kinetic studies were carried out using pseudo-first-order and pseudo-second-order models. The adsorption data were well fitted to the Freundlich isotherm model equation, with correlation coefficient (R) values greater than 0.950, and the pseudo-second-order kinetic model, with correlation coefficient (R) values greater than 0.990. The monolayer adsorption capacities for modified C. lepidota residue and unmodified C. lepidota residue were 5.890 mg/g and 5.627 mg/g, respectively.


Introduction
Population increase and urbanization are typically accompanied by a rapid increase in energy utilization and waste generation from industrial and domestic sources. If these wastes are not treated appropriately, they contribute to the contamination of the ecosystem. In areas in which marginal adherence to stringent environmental protection regulations is observed, humans do not comply with waste management practices.
Industries discharge untreated or insufficiently treated sewage (effluents) into the surroundings. As a result of human negligence, aquatic surroundings are contaminated, making them unfit or hazardous for consumption.
The disposal of wastewater introduces pollutants to surroundings, which mostly exceeds the permissible sanitary level [1]. A previous study has revealed that some of these pollutants include aromatic hydrocarbons, synthetic materials, and heavy metals. Phenols and aromatic compounds are known to lead to unpleasant in water, and some heavy metals such as lead contribute to stunted growth and low intelligence quotient in children, mental retardation, and kidney problems [2].
As a result of pollution, industries are placed under strict regulations to check or mitigate the release of hazardous substances into the surrounding to a minimal level. These regulations have encouraged the development of antiseptic, energy-efficient, holistic, and intrinsically safe methods for the treatment of waste before discharge.
According to Sharma et al. [3], the global awareness on environmental deterioration has received immense attention. Nevertheless, various treatment techniques must be selected on the basis of the properties of wastewater Over the years, researchers have made appreciable efforts to develop remediation procedures for polluted water bodies. Denis et al. [4], have reported that some undesirable metals and organic pollutants can be removed from aqueous surroundings by ion exchange, electrodeposition, reverse osmosis, and adsorption. Adsorption, which involves the use of cost-effective adsorbents that can be easily used and regenerated, is very interesting topic. Numerous materials such as carbonized maize tassels, banana pitch, and cassava waste have been investigated for the removal of metal ions and organic pollutants from an aqueous solution [5].

Figure 1. Cola lepidota Fruit
As an underutilized fruit, monkey kola is dominant in South-Eastern Nigeria. It is a member of the family Sterculiaceae and genus Cola. It has three species: red (Cola laleritia), white (Cola pachycarpa), and yellow (Cola lepidota) [6]. Figure 1 shows the types of monkey kola used.
Several physicochemical and phytochemical analyses of different species of monkey kola fruit have been carried out, and the result revealed that monkey kola fruit is highly nutritious, with medicinal properties, and it can be used to produce different beverages [7,8].
Several agro-wastes have been used for the dislodgement of metals from aquatic surroundings, but marginal or no information has been reported on the use of C. lepidota pericarp as an adsorbent for decontamination.
Hence, to examine the metal binding affinity for remediation, the possibility of using the modified and unmodified forms of a pulverized C. lepidota pericarp residue is investigated.

Methods
Collection of sample and Preparation. Fruits of C. lepidota were purchased from Igwuruta and Oil Mill markets in Port Harcourt, River state, Nigeria. The fruits were sorted and washed with deionized (DI) water to remove undesirable particles attached to the pericarp. The pericarp was peeled off and dried in the oven at 40 ºC for 5 days. The dried C. lepidota pericarp was pulverized to a powder and sieved through a 150 µm sieving mesh.
Extraction Procedure. First, an average mass of 50 g of the 150-µm pulverized C. lepidota pericarp was added in the thimble of Soxhlet extractor, and 750 mL of acetone was added in a 1000-mL round bottom flask. The Soxhlet extraction set-up was completed with a condenser attached to the thimble, which was fixed on the round-bottom flask and then placed in a water bath. The Hose pipes were connected to the inlet and outlet of the condenser to ensure a steady, continuous flow of water through the condenser for the condensation of acetone vapor. Extraction was continued until the acetone in the thimble became colorless, indicative of the process completion. An average time of 5 h was used for complete sample extraction. The residue was removed from the thimble, dried, and weighed, and the fresh sample was loaded; this process was repeated for 1000 g of the sample. The residue (raffinate) collected from each extraction was collected together, dried at 50 ºC for 12 h, and stored in a corked container. A part of the dried residue was crushed and sieved through a 150µm sieving mesh and referred to as the unmodified C. lepidota pericarp (UCLR).

Modification of Residue.
The residue was modified using sodium hydroxide. First, 100 mL of 0.1 M NaOH was added in a round-bottom flask containing 10 g of the residue. Second, the flask was placed on a stirrer and heated at 60 ºC for 5 h. The modified adsorbent was allowed to cool and washed with DI water to remove residual reactants present in the adsorbent. The washed adsorbent was dried at 50 ºC. Then, the resultant product of the Na-modified C. lepidota pericarp was referred to as MCLR.
Preparation of Metal Ion Solution. First, 0.283 g of a NiCl 2 6H 2 O sample was added into a 250-mL beaker, and 100 mL of DI water was added to dissolve the salt. Second, the dissolved Ni(II) ion was transferred to a 1000-mL volumetric flask, and the solution diluted up to the mark to prepare a 59.5 mg/L Ni(II) ion stock solution. In addition, 0.2826 g of CoCl 2 .6H 2 O was dissolved in a 250-mL beaker, and 100 mL of DI water was added to dissolve the salt. The dissolved cobalt(II) chloride salt was transferred into a 1000-mL volumetric flask, and the solution was diluted up to the mark to prepare a 59.5 mg/L Co(II) ion stock solution. The working solutions were prepared using the serial dilution formula in equation 1.
where C 1 and C 2 represent the concentrations of the stock and working solutions and V 1 and V 2 represent the volumes of stock and working solutions, respectively. HCl. First, 0.5 g each of UCLR and MCLR was added into a flask and the flask was appropriately corked and agitated by using a mechanical shaker, followed by standing for 48 h [9]. After 48 h, the mixtures were subjected to filtration using a filter paper. The pH of the filtrates was recorded and various changes in the pH value between the initial (pH to ) and final (pH t1 ) were noted. Plots of the changes between the initial and final pH against the initial pH were recorded. The points on the pH-axis were recorded as the zero-point charge [10].
Gas Chromatography -Mass Spectrophotometry (GC -MS) Characterization. The C. lepidota pericarp extract and particularly the residue were subjected to digestion using a 3:1 mixture of HCl and HNO 3 prior to GC -MS analysis.
Adsorption Tests. Adsorption tests were performed at various times, concentrations, and temperatures. The resultant mixtures were filtered using an 18-cm filter paper. The filtrates were collected, and metal-ion concentrations were analyzed by atomic absorption spectrophotometry (AAS).
The percentage removal of Ni(II) and Co(II) ions and the adsorption amount (q e ) in mg/g were calculated by equations 2 and 3, respectively.
Percentage metal-ion removal = where C o is the initial metal-ion concentration (mg/L), C e is metal-ion concentration (mg/L) at any time t, V is the volume (L), and M is the mass (g) of the adsorbent. Effect of Temperature. At a constant pH and time, 0.5 g of UCLR and MCLR sorbents was weighed into separate 150-mL beakers containing 50 mL of 4.25 mg/L each of Ni(II) and Co(II) ions. The solutions were heated in a water bath at 29, 40, 50, 60, and 70 ºC for 1 h, and the filtrates were analyzed to quantify the equilibrium metal ions by AAS, and equations 2 and 3 were utilized to estimate the percentage removal and adsorption amount, respectively. Thermodynamic parameters were determined by using equations 4 and 5.
Where ∆Gº represents the standard free-energy change, ∆S o represents the standard entropy change, ∆H o represents the standard enthalpy change, and Kc represents the equilibrium adsorption constant. The constant, Kc was generated by the relationship proposed by Tran et al. [11] as shown in equation 6.
The plot of In K c against 1/T should exhibit a linear plot, and ∆S o , ∆H o , and ∆G o can be estimated from the slope and intercept.
Effect of Time. First, 0.5 g of UCLR and MCLR with a particle size of 150 µm and a constant metal concentration of 59.5 mg/L were used. Samples were removed at 5, 10, 20, 30, 40, 50, 60, and 70 min. Supernatants were analyzed according to the abovementioned procedures. Kinetic studies were performed using the linearized form of the pseudo-first and secondorder models as shown in equations 7 and 8, respectively.
where q e and q t are the amounts adsorbed at equilibrium and at time t 9mg/g), respectively. k 1 is the rate constant, and k 1 and q e values are obtained from the slope of the plot shown in equation 7.
where k 2 is the second-order rate constant (g/mg/min). A plot of t/q t against t afforded a straight-line graph with a slope equal to 1/q e . Hence, k 2 and q e are calculated from the slope and intercept, respectively.

Effect of Concentration.
The effect of concentration on adsorption was investigated at a pH of 6. In this test, 0.5 g of UCLR and MCLR (150 µm) were first weighed into different plastic bottles, and Ni(II) and Co(II) ion solutions with concentrations of 4.25, 8.50, 17.0, 25.5, 42.5, and 59.5 mg/L were added into the bottles. Second, mixtures were agitated for 1h using a JP Selecta vibrator at 29 ºC. Next, the mixture was filtered, and the metal-ion concentration of the filtrate determined by AAS. The percentage removal and adsorption amount were estimated from equations 2 and 3, respectively. The obtained equilibrium data were analyzed by Langmuir and Freundlich linearized adsorption models as shown in equations 9 and 10, respectively C e q e = 1 k L q m + C e q m (9) where C e is the equilibrium liquid-phase concentration, q e is the amount of heavy metal ions (mg/g) adsorbed at equilibrium, q m (mg/g) is the monolayer adsorption capacity, and K L is the Langmuir constant. The q m and k L values were determined from the intercept and slope of the line obtained by plotting C e /q e against C e , respectively.
Predicting whether adsorption by the C. lepidota adsorbent is favorable or unfavorable for Langmuir-type adsorption, the isotherm shape was categorized by a variable R L , which is a dimensionless constant and a separation factor defined in equation 10 as follows: For R L > 1, it is unfavorable. For R L = 1, adsorption is linear, and for 0 < R L < 1, adsorption is favorable [12].
log q e = log K F + 1 n log C ୣ where K F and n are constants obtained from the slope and intercept of a plot of log q e against C e .
Effect of Adsorbent Size. Adsorbent with particle sizes of 250, 425, 150, and 65 µm were used to quantify the effect of adsorbent size on the adsorption amount. A 70 mg/L metal solution was used to perform the test following the above-mentioned procedures. Each procedure was repeated for the different adsorbent sizes.

Results and Discussion
Zero-Point Charge (pH zpc ). From the plots of MCLR and UCLR shown in Figure 2, the zero-point charge of UCLR is observed at a pH of approximately 5.0, while that of MCLR is observed at a pH of 7.0. The adsorption of Ni(II) and Co(II) ions was considerably favored at pH higher than pH pzc due to a large amount of negative charge on the adsorbent surface.
The adsorbent surface exhibits neither a positive nor a negative charge at the zero-p oint charge. Hence, at the zero-point charge, the sorbent is said to be electrically neutral [9].
GC-MS Analysis. The results of the investigation carried out on the extract and residue of the C. lepidota pericarp are as follows (Figure 3).  Tables 1 and 2 show their corresponding compositions.
Effect of pH. Figure 5 shows the effect of pH on the adsorption amount. Equilibrium data obtained from the adsorption tests revealed that the process is dependent on pH.
According to previously reported studies [13,14], at low pH, strong competition is clearly observed between the metal ions and net positive charge on the adsorbent surface, which invariably decreases the adsorption rate. However, the maximum adsorption for the modified C. lepidota resin was observed at a pH of 6.0, and the maximum adsorption for unmodified C. lepidota resin was observed at a pH of 5.0. Subsequently, at a pH greater than 6.0, Ni(II) and Co(II) adsorption gradually decreased. The reduction in adsorption was possibly related to the saturation of pores. Furthermore, the uniformity of the adsorption amount at a pH greater than 6.0 revealed the formation of Ni(OH) 2 and Co(OH) 2 precipitates. The solubility product constant (K sp ) for these metal hydroxides at a pH of 6.0 on the adsorbents were as follows: MCLR Ni(OH) 2 (1.02 × 10 −5 mol 3 L −3 ), MCLR Co(OH) 2 (9.72 × 10 −10 mol 3 L −3 , UCLR Ni(OH) 2 (8.90 × 10 −10 mol 3 L −3 and UCLR Co(OH) 2 (8.86 × 10 −10 mol 3 L −3 ).

Effect of Temperature.
Results revealed that adsorption decreases with increasing temperature (Figure 6). The optimum temperature for this process was 312.5 K.
Thermodynamics. Table 3 summarizes the thermodynamics parameters, and Figure 7 shows the plot of In K versus 1/T. The thermodynamic parameters constitute parts of the pre-requisites for characterizing adsorption [15][16]. The values obtained revealed that the adsorption of Ni(II) and Co(II) ions on MCLR and UCLR is exothermic. The negative value for the entropy change revealed that the adsorption of Ni(II) and Co(II) ions on MCLR and UCLR occurs via an associative mechanism. In other words, adsorption leads to order via the formation of an activated complex between the adsorbate and adsorbent. The negative ∆G values revealed the feasibility and spontaneity of the process.
Results revealed that significant changes do not occur in the internal structure of the adsorbent during adsorption.

Effect of Time.
The concentration of heavy metals in the solution decreases with increasing time [17]. Figure  8 shows the plots of the effect of contact time on the adsorption of Ni(II) and Co(II) ions. The optimum percentage removal values for Ni(II) and Co(II) ions on MCLR were 97.01% and 96.94%, respectively, and adsorption was completed in 60 min. The corresponding values were 96.56% and 95.64% on UCLR in 60 min.       Adsorption Kinetics. Kinetic models were applied to determine the mechanism of adsorption of Ni(II) and Co(II) ions on UCLR and MCLR [18]. To holistically comprehend this mechanism, the generated equilibrium data were used to investigate the pseudo-first-order and pseudo-second-order kinetics. Figures 9 and 10 show the experimental plots. Notably, with increasing time, the adsorption of Ni(II) and Co(II) ions on the adsorbent increased.
Netai et al. [19] have reported that the increased adsorption is probably related to the increased number of empty sites on the adsorbent. However, with the increase in time to greater than 60 min, the percentage removal decreased, literally indicating that all of the available adsorbent sites are already occupied. Notably, the adsorption amount increased to 5.5202 mg/g and 5.2513 mg/g for Ni(II) and Co(II) ions, corresponding to 97.01% and 96.94% removal percentages, respectively. At greater than 60 min, the adsorption amount decreased. Greater than 90% adsorption within 60 min corresponded to the initial accumulation of the metal ion on the C. lepidota pericarp surface, thereby using the large surface area as the binding sites are increasingly occupied; hence, adsorption consequently slowed down [20].
Experimental data were analyzed by using pseudo-firstorder and pseudo-second-order model equations. Table  5 summarizes the parameters of the pseudo-first-order kinetics, with correlation coefficients (R 2 ) of 0.669 for Ni on MCLR, 0.449 for Co on MCLR, 0.729 for Ni on UCLR, and 0.778 for Co onto UCLR. Table 6 summarizes the parameters of the pseudo-second-order, with correlation coefficient (R 2 ) of 0.998 for Ni on MCLR, 0.996 for Co on MCLR, 0.990 for Ni on UCLR, and 0.980 for Co on UCLR. The result revealed good agreement between the equilibrium data and pseudosecond-order kinetic model at different times. The pseudo-second-order rate constant revealed that the adsorption of Ni(II) ion is more rapid than Co(II) ion. This mechanism showed that adsorption involves valence forces via the sharing of electrons between the adsorbent and adsorbate in the rate-limiting step [15]. The removal of Ni(II) and Co(II) ions by the adsorbent followed chemical adsorption. However, from the adsorption tests, the uptake of Ni(II) and Co(II) ions by the adsorbent clearly involved two steps. The first step was more rapid in the first 30 min, and the second step was slower, which was the rate-determining step, as evident from the kinetic evaluation. Figure 11 shows the plot of the effect of concentration on the adsorption efficiency.

Effect of Adsorbent Particle Size on Adsorption
Amount. Figure 14 shows the effect of adsorbent particle sizes on the percentage removal of Ni(II) and Co(II) ions. With the increase in the particle size from 65 µm to 425 µm, the percentage removal of Ni(II) and Co(II) ions decreased. This is indicative of the fact that the lower the particle size, the higher the percentage of heavy metal ions removed [21].

Conclusion
In this study, the pulverized pericarp of Cola lepidota was used to investigate its adsorption efficiency for the removal of Ni(II) and Co(II) ions from an aqueous medium. The GC -MS analysis of the milled C. lepidota pericarp extract and residue revealed that the sample is structurally composed of diesters and dicarboxylic acids. The result obtained at various particle sizes indicated that the adsorbent exhibits a high percentage metal-ion removal at reduced particle size. The modified and unmodified C. lepidota adsorbents exhibited immense potential as an adsorbent for the removal of Ni(II) and Co(II) ions from an aqueous medium. In addition, the batch adsorption system revealed that the maximum adsorption of Ni(II) ion is observed at a pH of 6.0, while the maximum adsorption of Co(II) ion is observed at a pH of 5.0. Equilibrium data showed reasonable agreement with the pseudosecond-order kinetics, with a correlation coefficient (R 2 ) greater than 0.990 for the adsorption of Ni(II) and Co(II) ions on UCLR and MCLR. Moreso, equilibrium data analysis revealed that the adsorption follows the Freundlich isotherm model, with a correlation coefficient (R 2 ) greater than 0.950 for the adsorption of Ni(II) and Co(II) ions on UCLR and MCLR, indicative of multilayer adsorption.