Banana Stalks were first sundried, cut into small pieces and washed with distilled water to remove dirt particles. BS pieces were then dried in the oven at 100˚C for 24 hours to constant weight before being grounded and screened to 300 - 425 mm mesh particle size. It was then stored as Raw Banana Stalk (RBS). RBS of 200 g was soaked with 1 M H₃PO₄ in ratio 1:1, kept in the oven for 24 hours at 80˚C for activation and stored as Acid Activated Banana Stalk (AABS). Similarly, Base Activated Banana Stalk (BABS) was prepared by soaking another 200 g of the RBS in 0.1 M KOH for 24 hours at 80˚C in an oven and stored as BABS. The Proximate analysis of the biosorbents was carried out to know the percentage compositions of its constituents. Fourier Transform Infrared (FTIR) spectroscopic analysis was performed on RBS, AABS and BABS using FTIR spectroscope (FTIR-2000, Perkin-Elmer). The spectra were measured from 4000 to 400 cm⁻¹.

Simulated wastewater samples containing Pb(II) was prepared from Pb(NO₃)₂ of 1000 mg・L⁻¹ stock solutions. Reagents used were of analytical grade and deionized water was used in solution preparation. Other concentrations (20 - 100 mg/L) were obtained from this stock solution by serial dilution. Fresh dilutions were used for each experiment. The concentration of Pb(II) in simulated wastewater was analysed by Atomic Absorption Spectrophotometer (model PyeUnicam SP-9 Cambridge, UK).

Batch equilibrium tests were carried out on the adsorption of Pb(II) on RBS. The effect of initial metal ion concentration, contact time, temperature, solution pH and adsorbent dose were investigated. Sample solutions were withdrawn at time interval and equilibrium to determine residual concentrations. Solutions were filtered prior to analysis in order to minimise the interference of the RBS. For equilibrium studies, the experiment was carried out for 360 minutes to ensure that equilibrium was reached. The linear Beer-Lambert relationship between absorbance and concentration with the calibration curve was established by plotting the graph of absorbance versus concentration of the lead solution. The concentration of lead solution before and after adsorption was determined using an Atomic Absorption Spectrophotometer (model PyeUnicam SP-9 Cambridge, UK). This experiment was done in turns for AABS and BABS.

The adsorbed phase concentration (, in mg/g) at time (t) was calculated using Equation (1)

where, and are the initial and the final lead concentration (mg/L), respectively; is the water sample volume (L); and m is the mass of adsorbent used (g).

The biosorption at equilibrium, (mg/g), was calculated according to Equation (2)

where, and are the initial and the final (equilibrium) lead concentration (mg/L) respectively; is the water sample volume (L); and m is the mass of adsorbent used (g). The percentage of lead ion removal was calculated using Equation (3):

RBS of 1 g was measured into a 125 ml Erlenmeyer flask with 100 ml of 100 mg/l of Pb(II) solution with pH of 7 monitored using a pH meter (model CD70 WPA) and controlled with 0.1 M NaOH and 0.1 M HCl. The flask was agitated at 30˚C at a constant 125 rpm using the Thermostatic water bath shaker, the sorption experiment carried out for sufficient time of 360 minutes. The supernatant was filtered using Watman Filter Paper and Lead(II) concentration was analysed using Atomic Absorption Spectrophotometer (AAS). The effect of contact time (30, 60, 90, 120, 150, 180, 210, 240, 300, 360 minutes), biosorbent dose (0.2, 0.4, 0.6, 0.8, 1.0, 1.5 and 2.0 g), initial metal ion concentration (20, 40, 60, 80, 100 and 120 mg/L), pH (4, 6 and 8) and temperature (30, 40 and 50˚C) were evaluated during the study. During the experiment, the particular parameter considered was varied while the other four were kept constant variously for RBS, BABS and AABS biosorbents.

The result of proximate analysis carried out on the biosorbent samples are as presented in Table 1. It revealed that the AABS, BABS and RBS are rich in crude fibre of 67.69%, 62.07% and 58.47%, respectively. This rich crude fibre comprise of lignin, cellulose and hemicellulose content. Lignin is a compound rich with functional groups, such as carbonyl, ether and hydroxyl [21] [41] and has been used for metals adsorption by some researchers [40] [49] . The implication of this is that the presence of the active site presupposes that banana stalks are good biosorbents. The FTIR spectra characteristics for the three biosorbents are as shown on Table 2 while main spectra are on Figure 1. The analysis revealed that the biosorbents are rich in hydroxyl, carboxyl and the phenolic groups which indicates that RBS, AABS and BABS are viable for the adsorption of Pb(II) onto the biosorbents.

The rate of biosorption is a function of the initial concentration and contact time of the sorbate and these makes them important factors for effective biosorption. Figure 2 shows the effect of contact time on Pb(II) adsorption onto RBS, BABS and AABS. It was found that the adsorption of Pb(II) by the three banana stalks increased with contact time until equilibrium was reached at around 180 minutes. The biosorption process was rapid initially

due to increased availability of active binding site on the biosorbent surface and this is controlled by diffusion process from the bulk to the surface. However, during the last stages, biosorption is likely to be attachment- controlled process due to reduction in the available active sites. AABS has 96.00% removal at equilibrium, followed by BABS (66.90%) while RBS had the least percentage removal and adsorptive capacity even beyond 180 minutes of 63.9% and 6.39 mg・g⁻¹, respectively. Figure 3 reveals that, at lower concentrations of 20 - 40 mg・L⁻¹, biosorption was completed at about 60 minutes while at higher concentration, the biosorption process became constant at 180 mins as observed in Figures 3(a)-3(c), respectively. At lower concentrations, all metal ions present in the solution interacted with binding sites and then facilitated about 70.1% on RBS (Figure 3(a)) 74.6% on BABS (Figure 3(b)) with 93% Pb(II) removal on AABS (Figure 3(c)). At higher concentrations between 80 - 120 mg・L⁻¹, more Pb(II) is left unabsorbed in the solution due to the saturation of binding sites. This may be due to the increase in the number of ions competing for available binding sites on the three Banana Stalks considered in this study. This result is similar to other researcher findings [18] [50] .

Varying adsorbent dosage of 0.2 to 2.0 g were used to test for the effect of adsorbent dose on the biosorption process keeping the initial lead concentration at 100 mg・L⁻¹, while all other parameters were kept constant. It was observed as shown on Figure 4 that, when the adsorbent dose was increased from 0.2 g to 2.0 g, the percentage adsorption generally increased from 30.2% to 97.3%, but the amount adsorbed per unit mass of adsorbent decreased considerably. The increase in the adsorption percentage or decrease in unit adsorption with increased dose of the biosorbent is due to the increase in active sites on the adsorbent and thus making easier penetration of the metal ions to the adsorption sites which is in agreement with previous researcher [33] . When the adsorbent added is beyond 0.6 g, the decrease in lead ion adsorption is not very prominent which is perhaps due to the formation of adsorbent agglomerates reducing available surface area and blocking some of the adsorption sites. Maximum adsorption capacity of lead ions was observed with adsorbent dose of 0.6 g with adsorption capacity of 15.8 mg・g⁻¹ and thereafter, a slow increase in the percentage removal was seen reaching a constant value with respect to adsorbent dosage.

Surface charges on the adsorbent and degree of ionization of the sorbate are affected by the pH of the solution and this makes pH and important factor in adsorptive studies. Figure 5 reveals that, the adsorptive capacity was best at pH 8 with 89% removal of the lead ion. This is because biosorption is low at strong acidic medium (pH 2 - 4) because the surface charge on the biosorbents is positive, thus, Pb(II) is not favourable attached because of electrostatic repulsion between the RBS, BABS and AABS surfaces and the positively charged Pb(II). At higher pH, uptake increased because more metal binding sites could be exposed and carried negative charges, with

subsequent attraction of the positively charged metal ions with the biosorbent surface and this agreed with Bello et al. [40] on the adsorptive features of banana stalk-based activated carbon for malachite green dye removal. Experiments were carried out with the pH values of up to 8 due to the fact that metal precipitation appeared at higher pH values and interfered with the accumulation or biosorbent deterioration which was in agreement with the result obtained by Puranik and Paknikar [51] .

The percentage Pb(II) removal decreased with increased temperature for AABS as shown on Figure 6 while BABS and RBS followed a reverse trend. AABS highest percentage removal was at 30˚C (82.4%) and declined with increasing temperature. The reverse in the trend of result obtained for AABS compared with the other two may be due to the fact that at increased temperature, AABS surface for adsorption has acquire the maximum energy for adsorption while RBS and BABS had not. A similar result, in which adsorptive capacity is inversely proportional to temperature was obtained by Paresh et al. [52] using a biosorbent. Lower temperature of 30˚C also favoured the Biosorption of Lead(II) and Nickel(II) using Pigeon peas hulls waste by Ramana et al. [53] .

Thermodynamic parameters; standard free energy, standard enthalpy and standard entropy were calculated using the following equation:

where is the Langmuir constant (L/mol), is the temperature in and is the gas constant (kJ・mol⁻¹・K⁻¹). A plot of against gave linear plot from which and values were obtained from the slope and intercept respectively. From the pseudo-second-order rate constant, (Table 3), the activation energy for the adsorption of Pb(II) onto AABS could be determined using equation

where is the biosorption rate constant, A is the Arrhenius constant, is the activation energy (kJ・mol⁻¹). By plotting versus, can be obtained from the slope of the linear plot and is presented in Table 6. The negative free energy values at various temperature as shown on Table 6 indicates that the adsorption of Pb(II) unto AABS is spontaneous and thermodynamically favoured. In addition, the range of values between 0 to −20 kJ・mol⁻¹ further confirms that the adsorption process is a physisorption process. The positive value of obtained shows that the process is endothermic in nature and it range of values (2.1 - 20.9 kJ・mol⁻¹) implies that the process is physically controlled. The values of is positive indicating that there was randomness at the solid-liquid interface during the adsorption of the Pb(II) onto AABS.