Mature Musa acuminata (Enyeru variety) pseudo stems were collected on 15/03/2023 after harvesting fruits from them in Kikagati town council, Isingiro District, Uganda. The collected sheath material was washed with deionized water and thereafter cut into small pieces (2 × 2 cm) using a stainless-steel knife. They were dried at room temperature (25˚C) before grinding into fine powder. Biochar was prepared by heating the dry powder in an oven at 100˚C with limited oxygen.

Batch experiments were carried out by adding 1 g of the biochar into a 250 ml Erlenmeyer flask containing 50 ml of different aqueous solutions (5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 mg/l) of lead and cadmium nitrates at different conditions. The conditions varied were: biochar dose (0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4 and 1.6 g), pH (1, 2, 3, 4, 5, 6, 7, 8 and 9), contact time (15, 30, 45, 55, 60, 75, 90 105 and 120 minutes) and temperature (25˚C, 30˚C, 35˚C, 40˚C, 45, 50˚C, 55˚C, 60˚C and 65˚C). Agitation was done at 150 revolutions per minute (rpm) for one hour at 25˚C on a horizontal shaker for sorption equilibrium to occur.

After adsorption, the adsorbent was separated from the solution by filtration through a Whatman (0.45 μm) filter paper. The residual concentration of each metal in the equilibrium solutions (filtrates) was determined. Each treatment was done three times. The percentage removal of each heavy metal was calculated using Equation (1).

Adsorption ( % ) = C i − C e C i × 100 (1)

where C_i and C_e are the initial and residual concentrations of the heavy metal ion at equilibrium [13] .

The residual concentration of the two ions was determined using a flame atomic absorption spectrophotometer (Agilent 240AA, Agilent Technologies, Santa Clara, California, USA) at the respective 283.3 nm and 228.8 nm for Pb and Cd. The actual heavy metal concentrations were determined from calibration curves constructed from diluted working standards of 1000 ppm stock solutions of nitrate and chloride salts of the metals. The linearity of the calibration curves was checked, and these were within acceptable limits (r² > 0.9950 in all cases). Further, the quality of instrumental results was guaranteed through analysis of procedural blanks and spiked samples, whose recoveries (range: 97.9% to 101.5%) were analytically considered acceptable. Relative Standard Deviations of the experiments (analytical precision) ranged between 3.7% and 4.9%.

In adsorption studies, the equilibrium retention of the adsorbate onto the adsorbent is better described by mathematical models hinged on some assumptions relating to the coverage type, homogeneity, and heterogeneity of the solid surface, as well as the interaction between adsorbate species [14] . These models, which are otherwise termed adsorption isotherms, can explain the probable mechanism of adsorption. In this study, equilibrium data were analyzed using the Langmuir and Freundlich isotherms [15] .

Freundlich adsorption isotherm (model) works on the assumption that the adsorption of metal ions occurs on heterogeneous multilayer surfaces with different energy affinities. It is described by Equation (2), proposed by Freundlich in the first instance [16] .

Q e = K f C e 1 / n (2)

where K_f and n are Freundlich isotherm constants, which depend on the nature of the adsorbent at a given temperature.

Linearizing the equation, the Freundlich equation becomes (Equation (3))

log Q e = 1 n log C e + log K f (3)

where the intercepted logK_f gives the measure of adsorption capacity (distribution coefficient) and the slope ( 1 n ) gives the intensity of adsorption. The variable

Q_e is the amount of metal ion adsorbed at equilibrium (mg/g) and C_e is the concentration of the adsorbate at equilibrium (mg/g) in the solution [16] . In other words, the value of K_f increases with adsorption intensity and that of n is directly related to adsorption greaterlity i.e., 1 ≤ n ≤ 10 indicates that the adsorption process is favorable [17] .

A plot of lnQ_e against lnC_e for each metal under investigation was also made and analyzed independently. A plot that obeys Freundlich isotherm gives a straight line with slope ( 1 n ) [16] .

Langmuir model is based on the assumption that all adsorption sites are of homogenous surface on monolayer and independent of whether active sites are occupied or not. The linear Langmuir equation is given by Equation (4) [18] .

C e Q e = 1 Q max C e + 1 Q max K L (4)

where C_e = equilibrium metal ion concentration (mg/l), Q_e = amount of adsorbate adsorbed per unit mass of adsorbent (mg/g), Q_max = maximum adsorption capacity of adsorbent and K_L is the Langmuir constant.

A plot of C e Q e against C_e for each metal under investigation was made and analyzed independently. A plot that obeys Langmuir isotherm gives a straight line with slope 1 Q max Separation factor (R_L) was evaluated by using Equation (5) [14] .

R L = 1 1 + K L C o (5)

where K_L = Langmuir constant and C_o = original concentration. The value of R_L indicates whether adsorption is favorable or not and where, unfavorable (R_L > 1), linear (R_L = 1), favorable (0 < R_L < 1), or irreversible (R_L = 0) [19] .

Adsorption is a temperature-dependent process, and it is vital to perform thermodynamic studies to establish the feasibility and spontaneity of adsorption kinetics [20] . Thermodynamic study of the metal ions adsorption onto biochar from M. acuminata pseudo-stem biochar was performed at 293, 303, 308, 313, 318, and 323 K. The equilibrium constant K_c of adsorption was calculated using Equation (6).

K c = C a C e (6)

where K_c is the equilibrium sorption distribution coefficient, C_a is the amount of metal ions adsorbed on the biochar at a given temperature and C_e is the metal ion concentration in the solution at equilibrium.

The K_c values obtained were used to determine Gibb’s free energy change (∆G˚), enthalpy change (∆H˚), and entropy change (∆S˚) for adsorption of Pb²⁺ and Cd²⁺ ions on MAPS given by Equations (7) and (8).

Δ G ° = − R T log K c (7)

log K c = − Δ H ° R 1 T + Δ S ° R (8)

where ∆G˚ is the free energy change of adsorption, R is the universal gas constant (8.314 J∙mol⁻¹∙K⁻¹) and T is the absolute temperature in Kelvins. The K_c can be expressed in terms of the ∆G˚ and ∆S˚ as a function of temperature as given by the van Hoff’s reaction isotherm (Equation (8)) [21] .

A Freundlich linear equation was used to obtain sorption and desorption isotherms, and standard multiple linear regression analysis was used to find the best-fit isotherm. The plots of lnQ_e against lnC_e for Pb²⁺ and Cd²⁺ ions are depicted in Figure 6 and Figure 7, respectively. Each case obeyed Freundlich isotherm giving a straight line with slope 1/n, so n for Pb²⁺ was 0.309 (r² = 0.8869)

and for Cd²⁺ ions was 0.322 (r² = 0.8102) with calculated K_f values of 1.455 × 10⁻⁸ mg/g and 6.362 × 10⁻⁸ mg/g (Table 1). The model explains the values of n = 1, linear, n < 1, chemical adsorption process (n > 1), and physical adsorption process.

Similarly, the Langmuir linear equation was considered, and plots of C e Q e against C_e for Pb²⁺ and Cd²⁺ ions are given in Figure 8 and Figure 9. The plotted

graphs obeyed the Langmuir isotherm with a straight line whose slope was 1 Q max , so Q_max = 769.23 mg/g and 588.23 mg/g for Pb²⁺ and Cd²⁺ ions, respectively (Table 1). The straight line confirmed monolayer formation on active sites. From the present study, the coefficient values of r² = 0.9592 and 0.9726 for

Pb²⁺ and Cd²⁺ ions, respectively (Table 1) and this proved that the Langmuir isotherm model was the most suitable and favorable for the adsorption of the ions in the batch experiments using MAPS biochar as adsorbent material [14] . This was in agreement with the suggestion of the Langmuir isotherm model that adsorption occurred over homogenous surfaces on free active sites. The separation factor (R_L) was found to be 0.207 and 0.162 for Pb²⁺ and Cd²⁺ ions, suggesting that adsorption was favorable for Langmuir isotherm model for both metal ions.

The values of ∆H˚ and ∆S˚ for both metal ions were positive (Table 2), and were obtained from slopes and intercepts of the plot of logK_c against 1/T (Figure 10). The calculated thermodynamic values of ∆G˚ were negative, and this decreased further with an increase in temperature (Table 2).

This study recorded increased adsorption of both metal ions with an increase in contact time probably due to the availability of many vacant sites which provided a large surface area for more ions to get adsorbed onto the biochar active sites [15] [22] . Beyond 55 minutes and 60 minutes, there was no further increase in the adsorption for Pb²⁺ and Cd²⁺ ions due to the equilibrium set by the already adsorbed metal ions still in solution. Saturation of free active biochar sites and repulsion of molecules in the solid phase resulted in a percentage decrease in the adsorption of metal ions with contact time [23] [24] .

For solution pH, the initial increment in adsorption of the metal ions could be explained by the decrease in competition of metal ions due to the formation of soluble negative metal complexes on active sites of the biochar [25] . This may be due to an increase in the electrostatic binding capacity of metal ions with active sites of the biochar [20] . As pH increases, fewer protons are released which in turn increases charge density around the active sites of biochar and this increases attractive forces of metal ions towards the active sites of the biochar. The low adsorption percentages at low pH were due to protonation of hydronium ions and this creates repulsive forces between metal ions and free active sites of the biochar which hinders the metal ions binding capacity with functional groups on the biochar surface [26] .

In examining the effect of initial temperature, there was an initial increase in adsorption with an increase in temperature. This could be explained by an increase in the kinetic energy of the ions in the solution. There was more diffusion of metal ions into the abundant active sites existing on the biochar as temperature increased since diffusion is an endothermic process. Hence, an increase in solution temperature would result in the enlargement of biochar pore size due to activated diffusion triggering them to widen and create more surface for adsorption [20] . Apart from temperature as a factor, hydration ionic radius also affects the adsorption of metal ions on biochar i.e., a smaller hydration ionic radius leads to greater penetration into the pores of the biochar hence better adsorption. In this study, Pb²⁺ (0.401 nm) was adsorbed more than Cd²⁺ (0.426 nm), probably due to the weakening of adsorptive forces of attraction and the reduced number of active sites. This result is in agreement with a previous study using biochar of Ficus natalensis fruit which recorded maximum adsorption of 98.4% for Pb²⁺ at 45˚C adsorption [20] .

There was a noticeable increase in the removal of Pb²⁺ and Cd²⁺ ions with increasing biochar dosage until an optimum dosage of 1 g was reached (Figure 4). The highest adsorption was 80.0% and 77.9%, respectively. The gradual increase in adsorbed ions is because the increase in the amount of adsorbent dosage leads to increased probabilities of the ions getting into contact with the available active adsorption sites of the biochar (increase in surface area) [15] [27] . The decrease in adsorption beyond the adsorbent dosage of 1 mg/l could have been due to the shielding effect from precipitation of metal hydroxides and already adsorbed metal ions on active sites of the biochar or creation of excess unoccupied sites on the biochar [28] . A previous study also indicated the effects of dosage levels (1 g to 4 g) of biochar of rice husks to cause adsorption of 78.46% and 92.63% Pb²⁺ and Cd²⁺ ions [26] , which aligns with our present results.

The adsorption of Pb²⁺ and Cd²⁺ ions in this study was observed to decrease with an increment in the initial metal ion concentration (Figure 5). At low initial concentrations of metal ions, the increase in metal ion adsorption percentages was due to the availability of free active sites of biochar, which enabled greater interaction of the metal ions with the biochar. As the concentration increased, the lower adsorption percentages were caused by the saturation of the adsorption sites on the biochar [29] [30] [31] [32] . As expected, the percentage of Pb²⁺ ions adsorbed was higher than that of Cd²⁺, possibly due to differences in their ionic charge densities [20] .

From this study, results showed that it was more favorable for Pb²⁺ and Cd²⁺ ions under the Langmuir isotherm model than Freundlich because the r² values obtained were close to 1, and values of n were less than 1 which indicated that adsorption was chemical. This was in agreement with the results obtained by Sobh et al. [33] where adsorption of Pb²⁺ ions by Cymbopogon citratus was chemical since the value of n obtained was 0.76 less than one. High values of n show a uniform surface on the adsorbent material and low values of n show high adsorption and the presence of high-energy active sites on the adsorbent material as explained by the Freundlich isotherm model.

The values of ∆H˚ and ∆S˚ for both metal ions (Table 2) were positive, indicating the irreversibility adsorption of the metal ions onto MAPS biochar. That is, the process is endothermic and favorable [15] [21] . The negative values of ∆G˚ decreased with an increase in temperature. This indicated the feasibility of the adsorption process and affinity of the biochar for Pb²⁺ and Cd²⁺ ions from water solution.