Lead (II) Remotion in Solution Using Lemon Peel (Citrus limonum) Modified with Citric Acid

Lead remotion was evaluated in a synthetic solution with 100 μg L of Pb (II), using the lemon peel as a biosorbent. The adsorption capacity of the lemon peel at different pH values and particle sizes was studied.A maximum adsorption capacity of 19.556 mg g was achieved with 0.5 g of biosorbent dose at pH 6, being able to remove up to 97.78% with the unmodified biomass and 93.83% after the cross-linking process of the adsorbent material.The adsorption kinetics is based on a Pseudo Second Order model for the biomass of lemon pre-treated with citric acid (R = 0.9586) and not pre-treated (R = 0.9408). It is concluded that lemon peel is a good precursor of lead adsorbent in aqueous solution. Keyword Biosorption, adsorption kinetics, Freundlich isotherm, lead.

material and the ion and the number of loads and activity of the binding groups [26], [27].Among the vegetable residues recently used for adsorption of Pb (II) are African palm pre-treated with citric acid, olive pits modified with HNO 3 , H 2 SO 4 and NaOH, red seaweed, orange peel and tuna, dried or crushed orange peel, with and without cross-linking treatment (with CaCl 2 ) .In this context, the main objective of this work was to demonstrate the use of pre-treated lemon waste materials with citric acid as a source of biomass to remove Pb (II) from industrial wastewater [28]- [31].

A. Preparation of the bioadsorbent
The biomass was collected in the best possible condition to prevent its rapid decomposition, washed with abundant distilled water to eliminate tannins, reducing sugar resins and other impurities, which may intervene in the adsorption process.Then, it was dried in an oven at 90 °C for 24 h. The size was then reduced by a roller mill for 20 min. Sorting was carried out in a sieve shaker by selecting the sizes: 0.355 mm, 0.5 mm and 1 mm [28].

B. Adsorbent characterization
Once the bioadsorbent material had been prepared, the functional groups in the lemon peel were identified and the FTIR (Fourier Transformed Infrared Spectra) analysis was carried out [20].

C. Modification of lemon peel with Citric Acid
Once the lemon peel was conditioned, the modification was carried out with citric acid, for this purpose 40 g of biomass was mixed with 200 mL of a 0.6M citric acid solution. The mixture was left in agitation for 2 h at a temperature of 60°C, after this time, the biomass was washed with abundant deionized water, then dried in an oven for 24 h at 55°C [32].

D. Preparation of synthetic wastewater
A Shaking incubator IN-666 was used to perform the adsorption tests, which previously contained an Erlenmeyer with 0.5 g of biomass and a Pb (II) solution at 100 µg L -1 , which was prepared by adding 0.1 g of Pb (NO 3 ) 2 to one L of deionized water [31].

E. Bioadsorption tests
The adsorption tests were carried out at a temperature of 25°C and 150 rpm for 2 h, the pH values were 2, 4 and 6, which were controlled by the addition of 0.1N HCl and 0.5 % NaOH p v -1 [33].Final concentration analysis was performed by atomic absorption spectroscopy at 283.3 nm through mass balance [34]. After these tests the adsorption capacity was calculated using: (1) Where, q e is the adsorption capacity in equilibrium (mg g -1 ), C o and C f are the initial concentrations and equilibrium (mg L -1 ) of Pb (II) in the solution, V is the volume (L) of solution taken and M is the mass (g) of adsorbent used [34].

F. Adsorption kinetics
In order to determine the Pb (II) remotion kinetics, batch-type experiments were carried out at 150rpm, by contacting different initial metal concentrations (25,50,75, 100 mg L -1 ) for the unmodified lemon peel pretreated with citric acid.The experiments were performed at pH 3, which was adjusted by adding drops of NaOH or HCl 0.1 N and 0.5 % w v -1 , respectively; and a ratio of 2 g L -1 . The experiments were placed in 1000 mL beakers and shaken with a magnetic stirrer for 4 h [28], [31].

III. RESULTS AND DISCUSSIONS
The chemical-proximal analysis of the natural lemon peel showed a high carbon content of 38.48%, followed by cellulose 18.49%, lignin 7.22% and hemicellulose 6.07%. In addition, it presented low ash content (3.68%). Fig. 1 shows an FTIR analysis of the lemon peel to determine the functional groups that favour the lead adsorption process, as well as the lemon peel pre-treated with citric acid.    results of the FTIR analysis for lemon peel pre-treated with citric acid after the lead adsorption, a C-O stretch band is observed in 1000cm -1 , as well as in 1600cm -1 corresponding to the vibrations by stretching of the carboxylic ionic groups -COO.In addition, the 3700cm -1 band also shows vibrations, demonstrating that a high concentration of hydroxyl groups favors the removal of metals, since they allow chelation between the methyl ion to be treated and biomass [36]. The pH handled in the heavy metal solution influences the biosorption process due to the influence on the main surface sites and the nature of the metal. Figure 3 shows that at higher pH values (pH=6) the concentration of hydroxyl is increased, causing changes in the surface of the adsorbent and thus increasing the removal capacity of the metal ion, since the surface could be protonated favouring the adsorption of lead ions which favours the adsorption of lead in its anionic form,which is due to the influence of pH on surface electrostatic interactions between biomass and metal chemistry [37]. The influence of the particle sizes 0.355, 0.5 and 1 mm evaluated in this paper are shown in Fig. 3. It is observed that for an intermediate particle size of 0.5 mm the best metal removal rate was obtained with 97%; however, the three particle sizes tested are suitable for removal of the Lead (II) ion with lemon peel.Although, the decrease in adsorption capacity as the particle size decreases may be due to agglomeration of the particles in the pores of the biomaterial [36]. Figure 4 shows that the model that best fits the lead data in the untreated lemon peel treated with citric acid is Pseudo Second Order with an R2 of 0.9586 and 0.9408, respectively.According to this, the ions are adsorbed in two active biomass sites, which in this case would be the hydroxyl and carboxyl functional groups, making a chemisorption. Removal percentages were obtained after 310 minutes of 97.78% and 93.83% contact with the untreated and pre-treated lemon, respectively, with better results with natural biomass were obtained [34], [37].

IV. CONCLUSION
According to the results obtained in this investigation, it was found that the lemon peel presents a great capacity of adsorption of Pb (II) ions in synthetic water, since it was able to remove up to 97.78% obtaining a maximum adsorption capacity of 19,556 mg/g with 0.5 g biosorbent dose, however once modified the adsorption capacity was reduced to 93.83%.The best conditions under which the Pb (II) ion adsorption process was performed were achieved using pH of 6 and 0.5 mm particle size.The fitting of the experimental data obtained for the different selected models indicates that the Pseudo Second Order model is the mathematical model that best describes the adsorption kinetics of Pb (II) for the residual biomass of lemon residual untreated and pre-treated with citric acid.On the other hand, the isothermal model that best describes the results obtained was the one proposed by Freundlich, which proved that the adsorption process is controlled by chemical reaction.