Adsorption is one of the most favorable techniques applied for water treatment, mainly because it is suitable for low concentration of metal ions and cost effective [ 8,9,10]. Adsorption is a physico-chemical process where the substance called adsorbate accumulates at the interface of solid called adsorbent [ 9,11]. Activated carbon is commonly used as an adsorbent for water treatment because it has porous structure and large surface area [ 11,12,13]. In spite of its great physical properties, the use of activated carbon is restricted to its high cost [ 11,12,13]. Therefore, alternative adsorbents derived from low-cost materials should be introduced to minimize the operating cost particularly in developing countries.

The leaves biomass of GPL and CPL were collected within Gombe metropolis, Gombe State. All chemicals used were of analytical grade. Stock solution (1000mg/L) was prepared from which the experiment was conducted by dissolving the salts Pb (NO₃)₂ and CdCl₂ in distilled water, HCl and NaOH solution were for pH adjustments. From the stock solution prepared, initial concentration of each metal was prepared and concentrations were analyzed using Atomic Adsorption Spectrophotometer (AAS).

The leaf biomass were washed with ordinary water thoroughly and with distilled water in order to remove impurities and debris present, after which the samples were allowed to dry for about 8 days to completely remove moisture content. These dried samples were then grounded to a fine powder and then sieve with a micrometer sieve of 150μm. The sieved powder was stored and used for batch experiments.

FT-IR was used to give a qualitative analysis of the main chemical functional groups present on the biomass and responsible for the adsorption processes which was adopted by Wassila et al. [ 17]. The analyses were carried out using Perkin Elmer Spectrophotometer with model number 10.03.09.

The Scanning electron microscopy (SEM) was implemented to examine the physical structure change using SEM model Phenom ProX, by phenom World Einhoven, Netherland. Samples were placed on double adhesive which was on a sample stub, coated with a sputter coater by quorum technologies model Q150R, with 5nm of gold. Thereafter it was taken to the chamber of the SEM machine where it was viewed via NaVCaM for focusing and little adjustment, it was then transferred to SEM mode where the focused and brightness contrasting was automatically adjusted.

XRD was used for the characterization crystalline materials and also provides information on structural elucidation, phases, preferred crystal orientations (texture), and other structural parameters, such as average grain size, strain, and crystal defects. The analyzed materials were finely grounded, homogenized, and average bulk compositions were determined. The powdered samples were then prepared using the sample preparation block and compressed in the flat sample holder to create a flat, smooth surface that waslater mounted on the sample stage in the XRD cabinet. The samples were analyzed using the reflection transmission spinner stage using the Theta-Theta settings. The 2θ starting position is 4 degrees and ends at 75 degrees with a θ step of 0.026261 at 8.67 seconds per step.

Batch adsorption experiments were carried out by varying contact time, initial concentration, pH, temperature and adsorbent dose. Batch adsorption experiment process carried out in contact with the aqueous solution containing Pb²⁺ and Cd²⁺ in a 250ml conical flask. The flask was agitated in an orbital shaker at a constant speed of 150 rpm. The adsorption process was conducted under the following conditions; initial concentration (50-1000mg/L), contact time (15-60mins), adsorbent dose (0.2-1g), pH (2-10) and temperature (25-60 ^oc) which were then varied to study their effects on metal ion.

After equilibrium was reached, each sample was filtered using Whatman 41 filter paper and analyzed using Atomic Adsorption Spectrophotometer (AAS). Percentage removal and amount of metal ion adsorbed at equilibrium were calculated using the formulas below;

Percentage Removal: E% = (Ci – Ce)/Ci x 100

Amount Adsorbed: qe = (Co – Ce) v/w

Where; Ci = the initial concentration of the metal solution (mg/L),

Ce = the equilibrium concentration of the metal solution (mg/L),

C₀ = the initial concentration of the sorbate.

W = the mass of adsorbent (g)

V = the volume of the solution (L)

Isotherms curves in expressing the difference in amount of gas adsorbed by adsorbent with pressure at constant temperature, most commonly used isotherms are Freundlich and Langmuir Isotherm. The Freundlich isotherm model deals with adsorption at multilayer heterogeneous surfaces [ 10] while the Langmuir isotherm model assumes all sites on adsorbents have equal energy [ 12,18]. Linear form of Freundlich isotherm model is expressed as:

Where K_F (mg/g) and n are the Freundlich constants related to adsorption capacity and intensity, respectively. A linear plot of q_e against log C_e will give K_F and n values while that of Langmuir Isotherm can be written in the following linear form

Where Ce (mg/l) is the concentration of adsorbent at equilibrium, K_L (mg/l) is the Langmuir constant related to the adsorption energy and qm (mg/g) is the adsorption capacity. The adsorption capacity can be correlated with the variation of the surface area and porosity of the adsorbent. Higher surface area and pore volume resulted in higher adsorption capacity [ 19]. The essential characteristics of the Langmuir isotherm can be expressed by a dimensionless constant called the equilibrium parameter, R_L [ 20].

Is the measure of adsorption with respect to time at constant pressure and concentration.

Pseudo First order equation of Lagergren [ 21] is given as follows:

Where qt and qe are the amounts adsorbed at time t and equilibrium (mg/g), respectively, and k1 is the pseudo first-order rate constant for the adsorption process (1/min). The linear graph of ln (qe − qt) vs. t shows the applicability of first order kinetic.

For Pseudo Second Order Kinetics the chemisorption kinetic rate equation is expressed as follows:

Where k₂ is the equilibrium rate constant of pseudo-second order equation (g/mg min). The linearity of t / qt vs t suggests the best fitted with pseudo-second order kinetic [ 22].

The free energy of adsorption (∆G°) can be related with Langmuir adsorption constant [ 23] by the following equation:

Thus, a plot of lnK_L versus 1/T should be a straight line. ∆H° and ∆S° values were obtained from the slope and intercept of this plot, respectively.

The FT-IR spectra of GPL and CPL adsorbent are shown inFigure figure 1 and 2 respectively.Figure 1, FT-IR spectra shows abroad bend at 3444.06cm^-1 corresponding to O-H stretch, due to the adsorbed water and the presence of hydroxyl group (OH) of alcohol, carboxylic acids and phenols, widely found in lignocellulosic materials [ 24]. 2923 cm^-1 is attributed to C-H stretch, stretch at 1633.74cm^-1 corresponding to C=C group. Peak at 1456.98 cm^-1 is attributed to C-C band. The band at 1032.94 cm^-1 is assigned to C-O stretch [ 25].

FT-IR spectrums ofFigure 2 for CPL shows a strong and broad peak which were originated from O-H stretch is at 3444 cm^-1. Other bends are 2924 cm^-1 and 1634 cm^-1 which were found to be at C-H and C=C stretches respectively. Similar work was carried out by Navid et al. [ 26] where the results were in agreement with the current study.

SEM analysis results inFigure 3 show the surface morphology of GPL with scattered layer, porous surface and some particles have irregular sharp which makes GPL suitable for adsorption process. Similar work has been done by Himanshu and Vashi [ 27]. While for CPL inFigure 4, the surface morphology appears to have a rough surface with irregular sharps.

Figures 5 and 6 below displayed XRD spectra of GPL and CPL, respectively. The XRD specturm, of GPL shows pattern at 2θ = 14.96, average crystal size was calculated to be 35.54nm while for CPL inFigure 6 shows adsorbent peak at 22.48 from which average crystal size was calculated to be 35.76nm.

The effect of adsorbent dose of GPL and CPL were investigated from the range of 0.2 – 1g where initial concentration (50 – 1000mg/L) and other parameters were kept constant, as shown in Figures 7 and 8, percentage removal of Pb²⁺ for GPL and Cd²⁺ for GPL and CPL increases as adsorbent dose also increase until it reaches optimum condition at 1g. While for Pb²⁺ CPL which equilibrium was attended at 0.8g. This could be due to the increased adsorbent surface area, pore size and volume, and the availability of vacant sites. The higher adsorbent dosage gave rise to higher removal of metal ions providing an important driving force to overcome all mass transfer resistance of the metal ions between the aqueous and solid phase [ 28]. Optimum dose reached will be used during the experiment.

The effect of initial metal concentration (50 – 1000 mg/L) is shown on Figures 9-10 below, where adsorbent dose is at 1g while other parameters were kept constant. From the results obtained percentage removal increases from 89.78 to 99.48 as initial concentration also increases. At low initial concentrations (50 mg/l) most of the metal ions have been removed from aqueous solution. High adsorption at low initial metal concentrations can be attributed to availability of vacant sites for metal binding [ 10,29,30].which suggests that increase in adsorbate concentration increase number of available molecules per binding site.

Effect of pH on adsorption capacity of GPL and CPL on Pb²⁺ and Cd²⁺ were investigated with pH range of 2 – 10 while the contact time and other parameters were kept constant. The results are shown in figures 11 and 12 below indicate that the amount of metal ion adsorbed for Pb²⁺ (GPL) and Cd²⁺ (GPL and CPL) increases until it reaches equilibrium at pH 7 and for Pb²⁺ (CPL) which reaches it optimum level at pH 8. This observation can be explained by the fact that the concentration of H₃O⁺ ion was high at low pH. The scenario will cause completion between H₃O⁺ and metal ions for active sites on the surface of biosorbents [ 29].

The effect of contact time on the percentage removal of Pb2+ and Cd2+ from aqueous solution is shown in Figures 13 and 14 below. Percentage removal increases as time tend to increase until it reaches equilibrium at 60mins, increase in time increases, and the number of sites adsorbed to the metal ions, which becomes difficult for the metal ions to search for the very few remaining sites.

Effect of temperature was studied by conducting the experiment at different temperatures stating from 25 to 60⁰ C while other parameters were at equilibrium. As illustrated in Figures 15 and 16 below, percentage removal increases as temperature also increases at 55⁰C which is the optimum level before decreasing. This may be due to the endothermic nature of the adsorption; also the increase of metal ions could be as a result of increase in mobility of the metal ions acquired in the system [ 28].

The experimental data were analyzed using Langmuir and Freundlich as the two most commonly used isotherm models.Table 1 and 2 give the overall summaries of both Langmuir and Freundlich models. For Pb²⁺ (GPL) with high R² values of (0.9116) which show that the experimental data fits well with Freundlich model. The value of K_F obtained was 7.3029 while that of n was 1.2494. This indicates that the value of n was favorable as it lies between 1 and 10. While, Pb²⁺ (CPL) obeys Langmuir model with R² value at 0.9855 and K_L value of -0.0368.

For Cd²⁺ (GPL and CPL), the values of R² (0.9945 and 0.9948) indicated that Langmuir have better fitted model than Freundlich isotherm. The Langmuir model obtained the adsorbent has a limit number of binding sites with all sites having binding power and the adsorption of each site is non-aligned of each other. As a result, monolayer formation of large ion or molecule between heavy metal ions and the adsorption substrate is the dominant coverage in the untreated control sample [ 31].

Tables 3 and 4 below show a summary of both kinetic models. The adsorption kinetics of each heavy metal ion determine under precise time condition was performed. The kind of adsorption process depends on physio-chemical characteristic of the adsorbent and system status like temperature [ 31]. The linear graph of pseudo-first order was plotted from ln (qe-qt) against t (mins) and the graph of pseudo-second order was plotted from t/qt against t (min). From the results obtained, pseudo-second order correlated well with the experimental data better than pseudo-first order. The R² from pseudo-second order were much higher than that of pseudo first order and also the theoretical and experimental values are in a good correlation with each other. By contrast, in the pseudo second order model, the calculated qe(cal) values are very close to qe (exp), and the R² values converge to 1, indicating the validity of the pseudo second order [ 32], With this we can say that pseudo second order is best fitted for the adsorption.

Thermodynamic parameters are properly assessed; they provide in-depth information regarding the inherent energy and structural changes after adsorption [ 33]. ΔG is the change in free Gibbs energy (kJmol^−1), R is the universal gas constant (8.314 Jmol^−1K^−1), Ke is the thermodynamic equilibrium constant and T is the absolute temperature (K).

The values of ΔH and ΔS were determined from the slope and the intercept from the plot of lnKe against 1/T (K^−1) in Tables 5 and 6 below which show the summary. For Pb²⁺ (GPL), ΔG value is negative which implies that the processes was spontaneous, a positive ΔH indicated that adsorption is endothermic while a positive ΔS indicated an increased randomness at solid-liquid interface. A similar study was reported by Ahmaruzzaman and Gupta [ 34] for the adsorption of heavy metals by rice husk ash, and the positive entropy ΔS° confirms the increased randomness at the solid liquid interface [ 17]. Pb²⁺ (CPL), ΔG was negative while ΔS and ΔH were also negative. Negative value in ΔH and ΔS means that both ΔH and ΔS are exothermic. Cd²⁺ (GPL), ΔG was positive indicating that the adsorption took place, but non spontaneous at lower temperature and ΔH and ΔS were both positive as well. Cd²⁺ (CPL), adsorption showed that ΔG is negative, while negative ΔH indicated that the adsorption was exothermic. Positive value of ΔS showed that affinity of the adsorbent for the solid-liquid interface [ 28].

This study provides a simple, economical and effective adsorption process for removal of adsorbent from wastewater and environment. Instrumental analysis of FTIR, SEM AND XRD results showed promising signs as they were in agreement with most literatures, various percentage removals were obtained for Pb²⁺ and Cd²⁺ using (GPL and CPL) at optimum conditions. The equilibrium data fitted well with Langmuir isotherm model for Pb²⁺ (CLP) with R² value 0.9855, Cd²⁺ for (GPL and CPL) with R² values (0.9945 and 0.9948) while Pb²⁺ (GPL) with correlation coefficient at 0.9116 best fitted well with Freundlich isotherm model. Pseudo second order best fitted well than that of the first order kinetics in this study. The thermodynamic study shows that ΔG is negative in most cases expect for Cd²⁺ (GPL) were ΔG is positive. Whereas ΔH and ΔS are positive in some cases showing an endothermic and spontaneous adsorption processes respectively, as well as negative in some. Based on this study, GPL and CPL could be used as a natural adsorbent to remove Pb²⁺ and Cd²⁺ from wastewater and environment due to their high removal efficiencies.