1 Introduction

The potentially toxic trace metals are discharged into the natural environment by several industries such as steel production, mining, electroplating, transformation-coating, solder plates, etching, etc. [1, 2]. These metals can harm the flora and fauna of water bodies and all the aquatic systems. Cu (II) and Zn (II) are non-biodegradable metals, essential in biotic systems at low concentrations, and toxic at higher levels to animal and man health. Chemical treatments like ion exchange, precipitation, coagulation, reverse osmosis, membrane separation, and adsorption are the most common techniques with high affinity for removing metal ions from wastewater; [3, 4]. The most efficient one is the adsorption, especially using waste biomass materials (biosorption). Biosorption is an eco-friendly technique of low cost and readily biodegradable. There has been a great interest in employing agricultural wastes for HMs removal from industrial effluents [5]. Different biosorbents materials like Shea fruit shells, rice husk, oil palm waste potato peel, orange peel, banana peel, coconut husk, sugarcane bagasse, watermelon rind, coffee, tea, corn cob were employed for uptake the contaminants existing in wastewater [6,7,8]. Banana is consumed all over the world in massive amounts. Its peel is discarded as solid waste, representing 40% of the total banana weight. Due to its wide availability, banana peel is a promising biosorbent material for various pollutants, heavy metals, dyes, THMs, and phenolic compounds [9,10,11]. Banana peels have cellulose, hemicellulose, protein, and lignin. These substances contain carbonyl, carboxyl, hydroxyl, and amine groups which can adsorb various metal ions [12, 13] studied Cu (II) and Pb (II) uptake on BPs, and [14] analysed the equilibrium conditions using natural banana peel in the elimination of fluoride ions. Banana peels can be easily and inexpensively collected from local markets, chip manufacturing works, hotels, and homes. Banana peels are considered waste and emit greenhouse gases if disposed of under moist conditions [15, 16]. Thus, its use in treating toxic wastewater helps solve the waste disposal problem.

The objectives of this work include (1) improve the adsorption affinity of the banana peels (BPs) by chemical activation, (2) characterize the structure composition of activated biosorbent using FTIR and SEM–EDS, (3) enhance Cu(II) and Zn(II) adsorption under specific conditions such as pH of the solution, contact time, sorbent dose initial metal ion concentration and temperature, (4) assess the adsorption models of Cu (II) and Zn (II) via Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich, (5) evaluate the adsorption kinetic model e.g., pseudo first order, pseudo second order in addition to intraparticle diffusion mechanism, (6) explore the thermodynamics variables, including Enthalpy (ΔH°), Gibbs free energy (ΔG°), and Entropy (ΔS°), (7) test the application of activated BPs onto wastewater samples to evaluate its selectivity and capacity in the removal of different metal ions.

2 Materials and methods

2.1 Chemicals

Potassium Hydroxide (KOH), Copper Chloride (CuCl2.2H2O), and Zinc Nitrate Zn (NO3)2 (analytical grade of purity 95%), All the chemicals were purchased from Sigma Aldrich. Deionized water was used for the preparation of adsorbate.

2.2 Adsorbent preparation

Bananas were obtained from a local market in Egypt. The peels of banana were washed numerous times with hot deionized water to remove surface contaminants, and then the peels soaked in 50 ml KOH solution for 2 h. Then the filtered sample placed in an oven for 2 h. at 230 °C. Finally, the produced BPs was washed many times with deionized water to neutralize the pH value. The produced BPs was placed again in an oven at 80 °C overnight to dry and then ground into fine powders [15].

2.3 Characterization

FT-IR spectroscope is used to identify the molecular structure and chemical bonds present in the adsorbent material. This technique is carried out the measurement of the frequencies of infrared light that are absorbed by the sample, revealing information about the functional groups and molecular vibrations. SEM (Scanning electron microscope) was operated to analyze the morphological characteristics of the sorbent surface and its particle size. This technique involves bombarding the sample with a beam of electrons and detecting the resulting signals to generate a high-resolution image. SEM can provide data about the shape, size, and spreading of particles on the surface of the adsorbent material. By employing both FT-IR spectroscopy and SEM, a comprehensive description of the adsorbent material can be achieved, providing valuable insights into its chemical structure, surface properties, and potential applications. The FT-IR spectrum was recorded using a Bruker FT/IR Alpha-II instrument, which operates within the range of 400–4000 cm−1. The morphology and the fundamental structure of the adsorbent prepared were examined using a FESEM (Field Emission Scanning electron microscope -Zeiss SEM) with an Energy Dispersive X-ray (EDS).

2.4 Adsorbate preparation

A 2.683 g of Copper Chloride dihydrate [CuCl2.2H2O] was dissolved in a volumetric flask with 200 mL deionized water and subsequently top-up to a 1000-mL mark of the volumetric flask to obtained a stock solution of 1000 mg L−1 Cu (II). About 2.897 g of Zinc nitrate [Zn(NO3)2] was dissolved in 200 ml deionized water and subsequently top-up to a 1000-mL mark of the volumetric flask to obtained a stock solution of 1000 mg L−1 Cu (II). The bench solutions were prepared through serial dilution of the stock Cu (II) and Zn (II) solutions by the deionized water. A calibrated pH instrument, using buffers solution 4 and 7, was used to measure pH values. The pH of aqueous metal ion solutions was controlled by adding 1.0 M NaOH or 1.0 M HCl solutions. Cu (II) and Zn (II) concentrations were determined before and after adsorption using an inductively coupled plasma-optical emission spectrometer (ICP-OES; Optima 5300 DV, Perkin-Elmer, USA).

2.5 Equilibrium study

Batch adsorption experiments of copper and zinc were carried out to determine the adsorption capacity of Banana peels at different metal concentrations ranging from 2 to 50 mg L−1 and a fixed amount (0.50 g) of Banana peels to calculate the adsorption constant using different isotherms. 50 mL of different concentrations of copper (II) and zinc (II) solutions ranging from 2 to 50 mg L−1 was used. The Banana peels (0.5 g) were added to flasks and agitated at 25 °C and 400 rpm for 210 min for copper and zinc. The initial and final concentrations of the solutions were measured were determined by ICP-OES at the maximum adsorption wavelength and the adsorption capacities of the adsorbent were calculated. The effect of several factors: bioadsorbent dose (0.1–0.6 g), pH (3–7), Contact Time (30–210 min), Different concentrations of metal ions (2–50 mg/l), Temperatures (25–60 °C) were evaluated. After equilibrium was attained, the metal uptake capacity for each sample was calculated according to a mass balance on the metal ion using the Eq. 1 [17]:

$$\user2{\% metal removal} = \frac{{({\varvec{C}}_{{\varvec{o}}} {-}\user2{ C}_{{\varvec{e}}} )}}{{{\varvec{C}}_{{\varvec{o}}} }} \times 100$$

where, C0 and Ce are the initial and final metal ion concentrations, respectively. The adsorption capacity (Qe) was calculated by Eq. 2.

$${\varvec{Q}}_{{\varvec{e}}} \, = \,\frac{{\left( {{\varvec{C}}_{{\varvec{o}}} \,{-}\,{\varvec{C}}_{{\varvec{e}}} } \right)\, \times \,{\varvec{V}}}}{{\varvec{m}}}$$

V is the volume of the solution in (L), and m is the amount of the bioadsorbent in (g).

2.6 Adsorption isotherms

In this study, the Langmuir, Freundlich, Tempkin, and Dubinin-Radushkevich isotherms were used to describe the adsorption mechanism of Cu (II) and Zn (II) on activated BPs. The Langmuir model describes a homogeneinty of the adsorbent surface in which the adsorbent sites are energetically equivalent and thus can only hold one adsorbate species without any interactions between the adsorbate species [18].

Conversely, the isotherm that proposes non-ideal adsorption on heterogeneous surfaces is the Freundlich isotherm. Temkin isotherm considers the interactions of the surface of the bioadsorbent and the adsorbate molecules. Heat of adsorption released when molecules are adsorbed to the surface will decrease as more molecules are increased, due to the contact between the sorbent and adsorbate. Moreover, Dubinin–Radushkevich isotherm model (D-R) expresses the adsorbent surface heterogeneity and allows us to detect the process of adsorption, by calculating the surface energy [17]. Table 1 illustrates the equations used in calculating the adsorption isotherm models.

Table 1 The equations used in the adsorption isotherms

2.7 Adsorption kinetics

The kinetics models of the study were performed to recognize the equilibrium associations between adsorption and the contact time. The kinetic models like the pseudo-first-order (PFO), pseudo-second-order models (PSO), and the intraparticle diffusion model (IPD) were examined [18, 19]. Their equations are presented in Table 2.

Table 2 The equations included in calculating the adsorption kinetics

2.8 Thermodynamic study

Adsorption experiments were conducted under various states of temperatures (25–50 °C), initial concentration (5 mg L−1) of prepared metal ions (Cu (II) and Zn (II)) and fixed amount of BPs (0.5 g) adsorbent to analyze the spontaneity of the adsorption process. Thermodynamically parameters such as a variation in the entropy (ΔS˚), the enthalpy (ΔH˚), and the free energy (ΔG˚) were computed according to the next equations:

$$K_{d} \, = \,\frac{{C_{ads} }}{{C_{e} }}$$
$${\text{Ln}}\,{\text{K}}_{{\text{d}}} \, = \,\frac{{{\Delta S}^{ \circ } }}{R}\, - \,\frac{{{\Delta H}^{ \circ } }}{RT}$$
$${\Delta G}^{ \circ } \, = \, - \,RT\,LnK_{d}$$

where Kd is the constant of the distribution equilibrium, the temperature T is in (K), and the ideal gas constant R is (8.314 Jmol−1 K −1) [20, 21].

2.9 Application

The effectiveness of the BPs biosorbent in removing metals ions was evaluated using actual wastewater obtained from the Elwadi drain, Fayoum, Egypt,” under the optimal conditions (210 min shaking time, 0.5 g of sorbent, 500 rpm at 25 °C) without changing the actual pH value of drain. Then, the filtered sample was digested using conc. HNO3 on a hot plate at 180 °C. A 50 ml effluent without treatment was digested in the same method. The metals ions concentrations in filtered samples were measured before and after treatment by ICP-Plasma to assess the effectiveness of the BPs biosorbent.

3 Results and discussion

3.1 Adsorbent characterization

3.1.1 FTIR characterization

The FTIR analysis was conducted to explore the available functional groups on the surface of BPs (Fig. 1). The analysis revealed a broad peak at 3250.42 cm−1 indicating the presence of O–H hydroxyl group stretching of phenols and alcohols. The peak at 1585 cm−1 refers to the group of N–H scissoring of amino acids. Peaks at 1401.83 and 1304.81 cm−1 correspond to C-H group stretching of alkanes and O–H bending, respectively. The bands at 702.98–829.08 cm−1 are attributed to the group of N–H deformation of amines [18]. The spectra predicted the coexistence of –NH2, –OH, groups on BPs surface from cellulose and hemicellulose. The cation exchange may be the proposed mechanism of the adsorption [22].

Fig. 1
figure 1

FTIR spectra of activated BPs

3.1.2 SEM–EDS

Scanning Electron Microscopy (SEM) analysis was performed to examine the morphological construction of the sorbent surface. The SEM images revealed the presence of cavities with varying shapes and sizes on the surface of the activated BPs. Energy-Dispersive X-ray Spectroscopy (EDS) was then employed to determine the elemental composition of the activated BPs. The EDS analysis confirmed the presence of potassium and oxygen on the BPs surface, supporting the FTIR results that suggest a cation exchange mechanism for metal ion adsorption (Fig. 2). Additionally, the absence of other significant elements in the EDS data indicates the high purity of the activated BPs [11].

Fig. 2
figure 2

SEM–EDS Images of BPs

3.2 Equilibrium adsorption

3.2.1 Effect of adsorbent dose

When the adsorbent dose of BPs increased gradually from 0.1 to 0.5 g, the removal efficiency of Cu (II) increased from 33 to 60%. However, there was minimal improvement in the removal efficiency of Cu (II) between a dose of 0.5 and 0.6 g of the adsorbent (Fig. 3a), which is due to agglomeration of BPs particles at higher doses; thus, the available adsorption sites decreased. While, in the case of Zn (II) adsorption, the maximum removal percent was for an adsorbent dose of at least 0.4 g. The initial growth in the metal ions removal may be because of increasing the adsorbent surface area and, the available functional sets for the adsorption [23, 24].

Fig. 3
figure 3

a, and b Effect of adsorbent dose and pH on the removal of metal ion

3.2.2 Effect of pH

The dependence of Cu (II) and Zn (II) biosorption on the pH value of the aqueous medium is assessed and presented in Fig. 3a, b. The pH has an influence on the surface charge of the adsorbent and the chemical form of the metal ions. When the pH increased from 3 to 6, the removal efficiency increased from 30 to 40% for Cu (II) and increased from 42 to 58% for Zn (II). The lower removal percent at low pH is likely due to the competition of positive charge of hydrogen ions against positive charge of metal ions and the interaction with the active sites on the surface of PBs [25]. Figure 3b inferred that at pH 6 the BPs had the highest removal efficiency of Cu (II) and Zn (II) with 40 and 58% percentages, respectively. Our finding agreed with pervious research [22], which evaluated the impact of pH value from 2 to 8 on the biosorption of Cu (II) on the BPs surface and reported the highest removal % at pH value from 4 to 6 [10]. Stated the highest removal % of Zn (II) on BPs surface at pH 6.50, which agreed with our obtained results. Similarly [5], reported a pH of 5 as the optimum pH for Cd (II) adsorption on Shea fruit shell biomass.

3.2.3 Effect of contact time

Figure 4a illustrates the dependence of metal ions % removal on contact time. The removal efficiency increased gradually with rising time by the reason of many active sites on the BPs surface of adsorbent until it became constant at equilibrium. The equilibrium state likely occurs due to the saturation of active sites on the BPs surface and the repulsion forces between the adsorbed ions on the surface and those remaining in the aqueous solution [26]. The maximum removal percent of Cu (II) was 72% at 150 min, while Zn (II) achieved a higher removal of 80% at 180 min [27]. Reported 120 min as the equilibrium time in case of Cu (II) adsorption on the BPs surface.

Fig. 4
figure 4

a Effect of the contact time (min) and b the initial concentration of metal ion (mg/l) on the % removal of Cu (II) and Zn (II) on BPs

3.2.4 Effect of initial concentration

Figure 4b infers the removal percent of Cu (II) ion and Zn (II) ion on BPs at different initial concentrations. The removal percent of Cu (II) and Zn (II) decreased from (84 and 85%) to (60% and 64%); respectively, when the metal ion initial concentration raised from 2 to 50 mg L−1; this possibly due to the BPs surface overload at higher metal ion concentrations [28]. However, BPs adsorption capacity was 3.2 mg g-1 for Cu (II) metal ion and 2.8 mg g−1 for Zn (II) metal ion at the high initial concentration 50 mg L−1 (Fig. 5); this may predict a particle diffusion mechanism of metal ions on the BPs pores. This study reports a higher adsorption capacity for Cu(II) compared to previous research, which found a capacity of 2.433 mg g-1 for BPs [27] However, the observed adsorption capacity for BPs aligns with findings for Cr(VI) adsorption using another agricultural waste material [23] They reported an adsorption capacity of 2.355 mg g−1 for Cr(VI) under specific optimized conditions.

Fig. 5
figure 5

Adsorption capacity of Cu (II) and Zn (II) metal ions on BPs at different concentrations

3.3 Adsorption isotherms

Adsorption isotherm models offer valuable insights into the interaction mechanism between the molecules of adsorbate in the aqueous phase and the active sites that on the surface of the adsorbent under equilibrium states [18]. These models explain the surface properties and adsorption mechanism in sorption systems design. The isotherm models of Langmuir, Freundlich, Tempkin, and Dubinin–Radushkevich (D–R) were used to analyze the equilibrium results of Cu (II) and Zn (II) adsorption.

A plot of 1/Ce vs 1/Qe is presented in the Fig. 6. The Langmuir maximum sorption capacity (Table 3) was 4.51 for Cu (II) mg/g and 6.99 mg g−1 for Zn (II) respectively. R2 in Fig. 6 showed a good fit for Langmuir isotherm of Cu and Zn (0.96 and 0.95), respectively. The Langmuir separation factor (RL) describes the nature of adsorption; RL is 0.133 for Cu (II) and 0.216 for Zn (II). Thus, their adsorption process is favorable.

Fig. 6
figure 6

Langmuir adsorption isotherms for the removal of Cu(II) and Zn (II) on activated BPs at 25 °C

Table 3 Isotherm variables for Cu (II) and Zn (II) adsorption on activated BPs

The maximum adsorption capacity calculated from Freundlich isotherm for Cu (II) and Zn (II) was 1.377 and 0.771 (mg g−1) (L g −1)1/n, respectively (Fig. 7). 1/n value refers to the heterogeneity of the adsorbent surface [10]. 1/n values were estimated to be 0.624 and 0.665 for Cu (II) and Zn (II), respectively, suggesting a favorable adsorption process. As the n value (in Table 1) is more significant than 1 for Cu (II) and Zn (II) adsorption,

Fig. 7
figure 7

Freunlich adsorption isotherms for the removal of Cu(II) and Zn (II) on activated BPs at 25 °C

It suggests a physical adsorption process (weak intermolecular forces). Otherwise, n values are lower than one, inferred to chemical adsorption (more vital valence forces) [29]. High n values in the range of 2–10 indicate high adsorption intensity. On the other hand, n values less than 0.5 indicate low adsorption intensity. The Freundlich adsorption intensity in this study is moderate (n value is 1.5 of Cu (II), 1.6 of Zn (II)). Although Freundlich isotherm showed an excellent fitting for Cu adsorption, the Langmuir model fitted better with Cu equilibrium adsorption data, with a higher correlation coefficient (R2) and higher adsorption capacity calculated.

By comparing the applied adsorption models isotherms (Table 3 and Figs. 6, 7), the study gives a better fitness of the regression line of Langmuir isotherm than Freundlich. However, the Langmuir isotherm had a lower R2 but got a higher Zn (II) sorption capacity (Table 3). Thus, Cu (II) and Zn (II) metal ions adsorption on BPs followed the Langmuir model.

From the Tempkin adsorption isotherm, the heat adsorption of Cu (II) and Zn (II) metal ions were 3.047 and 3.229 J mol−1, respectively, suggesting the exothermic adsorption (Table 3). Tempkin constant (αT) describes the bioadsorbent affinity for the metal ions.

Cu (II) and Zn (II) had αT values of 0.625 and 0.723 (L g−1), respectively. The Tempkin isotherms illustrated an excellent fit of R2 estimated to be 0.93 for Cu (II) and 0.96 for Zn (II), respectively. High R2 values detected a superb interaction between the active sites on the adsorbent surface and the metal ions (Fig. 8).

Fig. 8
figure 8

Tempkin Isotherms of Cu (II) ion and Zn (II) ion Adsorption on BPs at 25 °C

The Dubinin–Radushkevich isotherm model was applied to calculate Qm and β in Fig. 9 from the intercept and slope value. E values of PBs bioadsorption for Cu (II) ion and Zn (II) ion was estimated to be 2.24 kJ mol−1. The E values less than 8 kJ mol−1 present a physisorption mechanism [30]. Therefore, the results suggest that the adsorption of Cu(II) and Zn(II) onto BPs likely occurs via physisorption.

Fig. 9
figure 9

D-R Isotherm of Cu (II) ion and Zn (II) ion adsorption on BPs at 25 °C

The maximum theoretical bioadsorption capacity estimated from D-R Isotherm was 1.86 and 2.1 mg g−1 for Cu (II) ion and Zn (II) ion, respectively. By comparing the investigated isotherm models, the results of Cu (II) and Zn (II) adsorption fitted better with Langmuir Isotherm (Table 3) with higher sorption capacity. The Langmuir model's suitability aligns with previous research demonstrating its effectiveness in describing the adsorption of various metal ions (Cd(II), Cr(VI), Co(II), and Cu(II)) onto BPs [4, 30, 31].

3.4 Adsorption kinetic studies

The kinetic models express the speed upon which adsorption occurs. It plays a major role in designing the batch adsorption experiments [17]. In this study, two kinetic models, Pseudo-First-Order (PFO) and Pseudo-Second-Order (PSO), were applied to analyze the Cu(II) and Zn(II) adsorption data (Figs. 10 and 11), the corresponding models parameters are presented in Table 6. R2 Values of Cu (II) obtained from PFO and PSO models were 0.86 and 0.84, respectively, for 5 mg L−1 initial metal concentration. The experimental adsorption capacity (qexp.) for Cu (II) showed a closer fit to the adsorption capacity (qcal.) calculated from the PSO than the PFO. However, values from the sums of squares error (% SSE) computed in the case of the PFO were less than that of the PSO model—a more excellent R2 value is in the PFO model. Thus, the PFO kinetic model explained well the Cu (II) adsorption confirmed by the superior fitness of the regression line in Fig. 10. On the other hand, the results of Zn (II) adsorption kinetics followed the PSO model as it evaluated higher R2 and lower % SSE in addition to the better closeness among the experimental adsorption capacity (Qe) and the calculated adsorption capacity (Qcal) (Table 4 and Fig. 11). It's important to note that neither the PFO nor the PSO model offers insights into the diffusion mechanisms or the rate-determining step of the adsorption process [20].

Fig. 10
figure 10

Kinetics plots for Cu (II) adsorption on activated BPs using a PFO and b PSO model

Fig. 11
figure 11

Kinetics plots for Zn (II) adsorption on activated BPs using a PFO and b PSO models

Table 4 Kinetics constant values of Cu (II)ion and Zn (II) adsorption on BPs

Chen et al. [32] stated that metal ions may penetrate the porous sites of BPs through the intraparticle diffusion mechanism. By plotting Qt against t0.5 (Fig. 12), the result of R2 is (0.94) for Cu (II) and (0.96) for Zn (II), respectively. High R2 values indicate that the diffusion process is a rate a limiting step. Also, because the trend line does not cross the origin, other mechanisms may include, e.g. boundary layer diffusion and liquid film diffusion mechanisms. Thus, The interaparticle diffusion (IPD) is not shown as the only variable that determines the rate [24].

Fig. 12
figure 12

IPD kinetic mechanism of for the adsorption of Cu (II) and Zn (II) ions on activated BPs at 25 °C

3.5 Thermodynamics

Table 5 summarizes the results of thermodynamic parameters for adsorbed Cu (II) and Zn (II) metal ions on the activated BPs. Kd values decreased from 7.33 to 1.22 for Cu (II) and from 11.5 to 1.94) for Zn (II) with rising temperature from 298 to 333 K, thus inferring the exothermic adsorption; This might be happening because of raised kinetic energy of some adsorbed metal ions that make them disassociate from the surface of adsorbent then back into the solution [4]. A plot of Ln Kd versus the reciprocal of temperature (1/T) of adsorbed Cu (II) and Zn (II) which allows us to calculate the thermodynamic constants (ΔHo and ΔSo). ΔGo values of Cu (II) and Zn (II) adsorption reduced as the temperature increased; however, ∆G° showed negative values at all temperatures that referred to a feasible, spontaneous process in addition to a good affinity of BPs adsorbent for Cu (II) ion and Zn (II) ion. The calculated ΔHo = −43.03 and −40.21 kJ/mol for Cu (II) and Zn (II) adsorption, respectively, inferring exothermic adsorption processes [33]. Stated that physisorption has ∆H° values of 2.1–20.9 kJ/mol, while, values exceeding 40 kJ/ mol may indicate chemisorption. Thus, the present results inferred Cu (II) and Zn (II) chemisorption on activated BPs. That might be attributed to the –NH2 and -OH groups on the BPs interacting with the metal ions [13].

Table 5 The variables of thermodynamics for adsorbed ions on activated BPs

The calculated entropy change (ΔS°) was −0.13 and −0.12 kJ mol−1 K−1 for Cu (II) and Zn (II) adsorption, respectively. These negative values suggest a decrease in randomness or increased order at the BPs surface during the adsorption process [34]. The BPs adsorption capacity was compared with other sorbents in previous studies are presented in Table 6 [5, 34,35,36,37,38].

Table 6 Results of Cu (II) and Zn (II) adsorption capacity on BPs are consistent with previous studies

3.6 Application

Several attempts were made to test the ability of BPs to remove metals ions from actual wastewater samples under optimal conditions. One sample from the Elwadi drain, discharged into the Rayan lakes in Fayoum, Cairo, was analyzed for the potentially toxic metals content by ICP-Plasma. El Wadi drain water contained Al, Ba, Cd, Cr, Cu, Fe, Pb, Mn, Mo, Ni, Se, and Sr in concentrations of 2.07, 0.08, 0.05, 0.05, 0.17, 0.98, 0.05, 0.20, 0.06, 0.09, 0.05, 0.61 mg/l; respectively. BPs was added to the wastewater sample at a rate of 0.5 g for each 50 ml, and shaking time was 200 min without changing the pH value of the drain water. Although the activated BPs removed only 8.33% Cu (II) where Zn (II) was absent into drainage water. BPs was also able to remove about 62% Cd (II), 6.52% Cr (II), 28.5% Fe (II), 95.83% Pb (II), 54.73% Mn (II), 14.29% Mo (II), 24.49% Se (II), 79.80% Sr (II) respectively, as shown in Table 7. Decreased removal percent of Cu (II) might be because of the influence of other interfering metals in the solution. Hence, BPs was efficacious adsorbent of harmfully metals: Cd, Pb, and Sr in addition to Mn. The choosiness of activated BPs towards the assorted metal ions followed the sequence: Pb > Sr > Cd > Mn > Fe > Se > Mo > Cu > Cr. This study revealed good sorbent BPs for treating wastewater from different metal ions.

Table 7 HMs concentrations (mg/l) of El-wadi drain before and after treatment using activated BPs

4 Conclusions

The present investigation is carried out to study the suitability of Banana peels bioadsorbent for the removal of heavy metals such as copper and zinc from the wastewater. The influence of process parameters such as pH, adsorbent dosage, temperature, contact time, and initial metal ion concentration were at moderate levels such that they can affect the removal efficiencies of the heavy metals was concerned. The optimum pH of the solution for Cu and Zn removal was found to be 6. Within the scope of the experimental investigation, the optimum temperature was found to be 25 °C. The optimum time for adsorption of zinc and copper was found to be 180 and 210 min, respectively. Initial metal ion concentration showed a negative effect on adsorption efficiency i.e. at lower levels the adsorption was higher. Kinetic studies of adsorption revealed that the adsorption process followed a pseudo-second-order kinetic model for both metals. The adsorption data were fitted to different isotherm model equations and the Langmuir model was found to be the best model for both metals i.e. Cu and Zn with R2 values of 0.96, and 0.95 respectively. Thermodynamic parameters of adsorption studies revealed that the adsorption of heavy metals using Banana peels is exothermic in nature; hence it can be concluded to be a physical adsorption phenomenon. In addition, BPs sorbent succeeded in the elimination of different metal ions from effluent water. The selectivity order of BPs towards the other metal ions followed the sequence: Pb > Sr > Cd > Mn > Fe > Se > Mo > Cu > Cr.