2.1. Materials Banana peels (BPs) were collected from the Jahangirnagar University Campus area. Lead nitrate Pb(NO3)2 was applied as a stock solution for the experiment. 0.1 M Hydrochloric Acid (HCl) and 0.1 M Sodium Hydroxide (NaOH) were used for maintaining pH levels. Nitric acid (HNO3) was utilized for digestion purposes. For Fourier Transform Infrared Spectrophotometer (SHIMADZU IRPrestige-21), Dichloromethane (DCM) was used for cleaning purposes, and Potassium Bromide (KBr) was used for the analysis of the functional group. The metal ions were determined by Atomic Absorption Spectrometry (SHIMADZU AA-7000).
2.2. Methods 2.2.1. Preparation of Adsorbent The collected BPs were washed at least three times thoroughly with regular tap water and Milli Q water for removing impurities and other dust particles that may be clinging to the peels. The cleaned BPs cut into small pieces and dried in sunlight for 5 days. Later the peels were dried in an oven (Daihan Labtech, Model LDO-030E) for 72 hours at 100 °C for removing the moisture from banana peels properly. Dried peels were crushed via mortar and pestle and subsequently screened by a sieve to get uniform particle size of 1 mm. Dried and minced BPs surface functional groups were characterized by Fourier Transform Infrared Spectrophotometer (SHIMADZU IRPrestige-21). The prepared BPs was stored in a desiccator until further use.
2.2.2. Preparation of Lead (Pb) Solution Lead nitrate Pb(NO3)2 solution was prepared separately by dissolving suitable quantity of analytical grade lead nitrate (Sigma–Aldrich). For the preparation of 1000 ppm of the Pb stock solution, it is needed to add 1.6 g Pb(NO3)2 in oneliter deionized water. Through successive dilutions of the corresponding stock solutions, standard solutions of the desired concentrations were obtained. 50 ppm, 100 ppm, 150 ppm and 200 ppm working solutions were employed for the analysis. The solutions of different pH were prepared with adding Sodium Hydroxide (NaOH) and Hydrochloric Acid (HCl) in the stock solution to the expected solution. 2.2.3. Batch Adsorption Experiments The sorption study was carried out batch adsorption method to estimate the capacity of the adsorbent and analyzed the impact of six parameters. They were adsorbent dose, pH, Pb2+concentration, contact time, temperature, and agitation speed. For considering the consequence of a particular parameter, that parameter has been changed gradually holding the other five constants.
2.2.3.1. Optimal Dose of Adsorbent The optimal adsorbent dose was determined by varying the amount of 25, 45, 65 and 85 gL-1 BPs put into the separate beakers of 100 ppm of working solutions of pH 5 at the agitation speed of 100 rpm for 30 minutes at 25°C temperature. One 50 mL of lead solution was contained in control beakers without adding the adsorbent. Each mixture was agitated by shaking incubator (GFL - Gesellschaft für Labortechnik mbH D-30938 Burgwedel) and filtered by Whatman filter paper (grade - 42). Then concentrated nitric acid (HNO3) was added on the filtrates and heated them on a hotplate for digestion. After that, the mixtures filtered by a suction pump and finally the sample prepared for Atomic Absorption Spectroscopy (SHIMADZU AA-7000). The amount of adsorption at equilibrium, qe (mg/g) and the percent adsorption (%) was calculated as follows.:
2.2.3.2. Optimal pH for Adsorption The batch biosorption study was inspected at separate pH values to determine the optimum pH for Pb2+ uptake by banana peels. 100 ppm working solutions were adjusted to pH values of 3, 5, 6 and 8 (use buffer solution) using 0.1M Sodium Hydroxide (NaOH) and 0.1 M Hydrochloric Acid (HCl). Later fixed 45 g L-1 of BP adsorbent was added to each solution at the agitation speed of 100 rpm for 30 minutes at 25 °C. The control batch was handled similar but without BPs. Then the solution was filtered as well digested for AAS.
2.2.3.3. Optimal Initial Metal Ion Concentration Further adsorption investigation was carried out with the several initial concentrations of 50 ppm, 100 ppm, 150 ppm and 200 ppm Pb2+ solutions at optimum pH 5 with fixed dosage of BP adsorbent 45 g L-1 for 30 minutes at 100 rpm agitation speed and at the corresponding temperature 25°C. The supernatant in each beaker was filtered to separate the adsorbent from the solution and prepared for digestion.
2.2.3.4. Optimal Contact Time for Lead Adsorption 100 ppm working solution was determined as the optimum lead solution. The adsorption examination of Pb2+ by BP was observed at different time series (10 minutes, 20 minutes, 30 minutes and 40 minutes) among the fixed dosage of 45 g L-1 of BP adsorbent by shaken in the orbital shaker at 25 °C, at the agitation speed of 100 rpm. Similarly, the solution of each beaker was filtered and digested for AAS.
2.2.3.5. Optimal Temperature for Lead Removal The study was conducted out to define optimal temperature by varying the temperature at 25°C, 40°C and 50°C with the fixed dosage of BP adsorbent (45 g L-1 ) that included lead solution of 100 ppm. Beakers without BPs were set as the control batch. All the solutions of the beakers at pH 5, were agitated in the orbital shaker at 100 rpm for 30 minutes. The solution from each beaker was filtered to isolate the banana peel residues and set for digestion.
2.2.3.6. Optimal Agitation Speed for Adsorption The analysis was carried out fixed BP adsorbent dosage (45 g L-1 ) in 100 ppm working solution of lead at pH 5 to set optimal agitation speed by changing speeds at the level of 50 rpm, 100 rpm and 150 rpm for 30 minutes. The mixtures were agitated by orbital shaker at 25 °C. After 30 minutes, supernatants from beakers were filtered, digested and prepared for AAS.