Moringa oleifera Lam . Seeds as a Natural Solid Adsorbent for Removal of AgI in Aqueous Solutions

Este trabalho descreve o potencial de sorção das sementes de Moringa oleifera para descontaminação de Ag em soluções aquosas. Espectroscopia de infravermelho foi usada para elucidação de possíveis grupos funcionais responsáveis pela adsorção de Ag. Estudos de adsorção foram feitos em batelada usando soluções padrão de Ag como função da massa do adsorvente, tempo de extração, tamanho da partícula e pH. A Ag foi quantificada antes e após os experimentos de remoção usando espectrometria de absorção atômica por chama. Adicionalmente, considerandose os estudos de adsorção e isotermas de adsorção aplicadas ao modelo de Langmuir foi possível verificar que as sementes de M. oleifera apresentam alta capacidade de adsorção. As condições ótimas foram: 2,0g de adsorvente com tamanhos de partícula de 75-500 μm, 100 mL de solução 25,0 mg L Ag, tempo de extração de 20 min e pH 6,5. Os resultados mostram que as sementes de Moringa oleifera podem ser usadas para remoção de Ag em soluções aquosas.


Introduction
Industrial processes have historically been an important factor of environmental degradation.The disposal of industrial wastes, mainly containing metal ions in water sources represents a problem of great concern not only in relation to biota in the receiving environment, but also to humans.Silver is included among the main pollutants because of its high toxicity. 1,2Therefore methods to minimize pollution caused by this metal are attractive.
Conventional techniques used for removing metal ions include filtration, precipitation, flocculation, ion exchange resins and reverse osmosis. 3][6] On the other hand, the biosorption process offers several advantages such as low cost of the biosorbent, high efficiency, minimization of chemical and/or biological sludge and regeneration of biosorbent. 72][13] However, biosorption of metal ions onto microorganisms is affected by several factors such as pH, metal concentration, biomass loading, temperature and other parameters.
Lignocellulosic materials such as cane bagasse, onion skins, peanut skins, rice husks, and so on, have been used for metal ions adsorption due to the characteristics of the functional groups present in these materials. 14ecently, natural adsorbents have been proposed for removing metal ions due to their good adsorption capacity. 1 In this context, the Moringa oleifera seeds present themselves as an alternative material for this propose. 15,16. oleifera seeds have been used for treating turbid water due to their flocculating properties.A flocculating protein from M. oleifera Lam.seeds was isolated by Gassenschmidt et al. (1995). 17The molecular protein mass was determined at about 6.5 kDa, the isoelectric point was above pH 10. Flocculation activity could be explained by the patch charge mechanism due to low molecular weight and high charge density.
The present work describes the sorption potential of M. oleifera seeds for decontamination of Ag I in aqueous solutions.Variables such as particle size, pH, extraction time and sorbent mass were evaluated.The proposed adsorbent was applied to effluent samples.

Sorbent preparation
The M. oleifera seeds were obtained from trees which were cultivated in the city of Uberlandia (Minas Gerais, Brazil) and collected from September-November 2007.The seeds were separated from the pods, crushed in a household blender (Black & Decker, São Paulo, Brazil) and strained through 500, 180 and 75 μm sieves.
The functional groups present in M. oleifera were characterized by Fourier transform infrared (FT-IR) spectrometer (Shimadzu, IRPrestige-21, Tokyo, Japan).The spectral range varied between 4000 to 500 cm -1 , with twenty eight scans in a resolution of 4 cm -1 .

Sorption studies
Removal studies using standard Ag I solutions were carried out in triplicate.The seeds were shaken with 50 of 25 mg L -1 Ag I for 20 min using a magnetic agitator and then the suspension was filtered using filter paper (Whatman 42,28 μm).Ag I was quantified before and after the removal experiments.
In order to obtain the optimum condition for the percentage of Ag I removal, the following variables were studied: particle size of the sorbent (500, 180 and 75 μm), pH (2.0, 4.0, 6.0 and 8.0), extraction time (5, 20, 35 and 50 min) and sorbent mass (0.1, 0.2, 0.3, 0.4, 1.0, 2.0, 3.0 and 4.0 g).The efficiency of the removal was evaluated from the Qe (uptake of metal per unit weight of sorbent) obtained by the equation 1: 16 (1) where Ci (mg L -1 ) is the Ag I initial concentration, Cf (mg L -1 ) is the Ag I final or equilibrium concentration, V (L) is the volume of the test solution and m (g) is the sorbent mass.

Isotherm adsorption
The experiments were carried out at a temperature range of between 25 and 28 o C, using 50.0 mg of non-shelled M. oleifera seeds (500 μm) and 50 mL of Ag I solution in a concentration of 5 to 100 mg L -1 .The pH of the mixture was adjusted at 6.5 using 0.5 mol L -1 NaOH and extraction time was 20 min.The mixture was filtrated and the Ag I was quantified using flame atomic absorption spectrometry.

Ag I determination
Standard solutions of Ag I were prepared from 1000 mg L -1 Ag I (Carlo Erba, Val de Reuil, France).The Ag I was quantified using an atomic absorption spectrometer (SpectrAA-220 Varian, Victoria, Australia).Instrumental conditions such as wavelength of 328.3 nm, slit of 0.1 nm, current lamp of 4.0 mA, air flow rate of 13.5 L min -1 and acetylene flow rate of 2.0 L min -1 were used.

Studies of interferent ions on the removal of Ag I
The removal of Ag I in the presence of Pb II , Cu II , Cd II and Co II , in various proportions, denominated by concomitant ions, was evaluated using non-shelled M. oleifera seeds.The experimental conditions were: particle size of 500 μm, extraction time of 20 min, sorbent mass of 2.0 g and pH of 6.5.

Application of the proposed method
The proposed sorbent was applied to residue obtained from chemistry classes, residue tested for chemical oxygen demand in this institution; film developing solution and residue from graphic material, obtained from local businesses.These effluents contained Ag I , Pb II , Cu II , Cd II and Co II ions.Samples were filtered through filter paper and the pH adjusted at 6.5, using 0.5 mol L -1 NaOH.One hundred milliliters samples were shaken with 2.0 g of non-shelled Vol.21, No. 9, 2010 M. oleifera seeds for 20 min Ag I was quantified using FAAS before and after removal experiments.

Biosorbent characterization
The main functional groups present in the M. oleifera seeds were characterized by infrared material analysis.The broadband centered at 3420 cm -1 can be attributed to O-H stretching of the connection in this protein, fatty acids, carbohydrates and the lignin units.Due to the seed high protein content there is also a relevant contribution to this region because of N-H stretching in the bondage of amides. 18The peaks that appear in 2923 and 2852 cm -1 correspond respectively to the asymmetric and symmetric stretching of the connection of the C-H in CH 2 group.Due to the high intensity of these bands it is possible to assign them to the predominantly lipid component of the seed that is present in high proportion similar to the proportion of the protein. 19In the 1800 to 1500 cm -1 region there are a number of overlapping bands between 1750 and 1630 cm -1 .This set can be attributed to the C=O connection stretching.Owing to the heterogeneous type of seed, the carbonyl group may be linked to different neighborhoods as part of the fatty acid portion of lipid and protein portion of the amides.The carbonyl that appears due to the lipid component in 1740 and 1715 cm -1 , which can be observed in the spectrum as a small peak or shoulder part of the main band at 1658 cm -1 , was assigned to the carbonyl amides in the protein portion.The peak at 1587 cm -1 can be attributed to the stretching of the C-N linkage and the deformation of the N-H linking proteins present in the seed jug.Literature reports that M. oleifera Lam.seeds present a protein mass composition of 29.36%. 20Figure 1 presents the seeds infrared spectrum.

Studies of sorption variables
Figure 2 shows Qe values in the function of sorbent mass for non-shelled and shelled seeds and husk using 5.0 mg L -1 Ag I .It was observed that husks showed more efficiency for Ag I removal than non-shelled and shelled seeds for quantities of 50 mg of sorbent or less.However, in quantities of sorbent above 50 mg, differences were not observed between non-shelled and shelled seeds and husk.Significant differences were not observed for the test-F with a 95% assurance.Therefore, non-shelled seeds were used for further studies.
The following variables were studied: particle size of sorbent, pH, time of extraction, sorbent mass, metal concentration and volume of Ag I solution.
Qe values in function of the adsorbent particle size (500, 180 and 75 μm) were obtained (Figure 3).A decrease in particle size showed a favorable effect on metal sorption justified by an increase in the adsorbent surface area.However, particle sizes of 180 and 75 μm were not used because the filtration step was very slow.Moreover, Qe values were similar, i.e., 0.289 and 0.301 mg g -1 for ≤ 500 μm and ≤ 75 μm, respectively.
The effect of pH range on Ag I adsorption was studied from 2.0 to 8.0 (Figure 4).The conditions were: 500 μm particle size, 20 min extraction time, 4.0 g adsorbent mass and 50 mL of 25.0 mg L -1 Ag I .Optimum values of Qe were obtained in pH > 4.0; above this Qe values were found constant.
Biosorbent materials primarily contain weak acidic and basic functional groups.This is in accordance to the acidbase equilibria theory, in the 2.5-5 pH range, the binding of heavy metal cations is primarily determined by the  state of dissociation of the weak acidic groups.Carboxyl groups (-COOH) are the important groups for metal uptake by biological materials. 16The ionic states of cell wall functional groups can be used to explain the pH dependence of biosorption.Low pH conditions allow hydrogen and hydronium ions to compete with metal binding sites on the biomass, causing a poor uptake.At higher pH values, 5-7, there is a lower number of competing hydrogen ions and more ligands are exposed with negative charges, resulting in greater sorption.Whereas at a pH > 7, it occurs the solution precipitation, leading to a reduction sorption capacity. 16hese results are in accordance with literature.Therefore, the 6.5 pH value was then chosen for further studies.
The effect of extraction time on Ag I sorption was studied in the 5 to 50 min range.Figure 5 shows that the amount of silver ions retained by the adsorbent material increased considerably for up to 20 min and was constant after that.The time of 20 min was chosen due to the good results presented in the removal (Qe = 0.302 mg g -1 ) and the shortest time for sorption process.
Figure 6 shows the removal studies made using 25 mg L -1 Ag I and non-shelled M. oleifera seeds from 0.1 to 4.0 g.The results show that the uptake of quantity of adsorbed metal per unit weight of sorbent decreases suddenly until 1.0 g of M. oleifera, indicating an increase in the Ag I removal.In the 1.0 to 4.0 g interval, the Qe values remained constant probably due to the equilibrium reached.

Isotherm adsorption
One of the important physicochemical aspects for the sorption process evaluation is the equilibrium of sorption.The isotherm curve obtained indicates favorable adsorption process and highlights the strong tendency of the process for monolayer formation.Therefore, the extent of sorption was adequate to the Langmuir model.The Langmuir equation assumes that adsorption is limited to the monolayer and a maximum adsorption indicates saturation of this monolayer.The experimental results obtained were plotted (correlation coefficient of 0.99352) in Langmuir isotherm linear equation from 5 to 100 mg L -1 Ag I (Figure 7).The plot of Ce/Qe versus Ce gives a straight line of slope 1/Qmax and intercept 1/Qmaxb.The magnitude with Langmuir Qmax and b constants show adsorption capacity of 23.13 mg of Ag I per gram of non-shelled M. oleifera seeds and adsorption energy of 0.1586 L mg -1 , respectively.

Comparison of the present study with other adsorbents
The present study showed that Moringa oleifera seeds demonstrated good removal capacities for Ag I (23.13 mg g -1 ) as compared to the reports with other adsorbents (Table 1).

Studies of interferent ions in the removing of Ag I
Removing of Ag I in the presence of Pb II , Cu II , Cd II and Co II ions was studied in the following conditions: 500 μm particle size, 20 min extraction time, 2.0 g adsorbent mass, 6.5 pH and 100 mL Ag I volume.The results are shown in Table 2.The influence of interferents was verified by comparing the percentages of removal of a solution containing only Ag I and a solution containing Ag I ions and interferent ion.Removal percentage was calculated based on analytical signals obtained before and after the removal procedure and the tests performed in triplicate.
In this study, a specie is considered as interferent for the removal of Ag I in the proposed procedure, when the difference between the values of removal of the solution containing only ions Ag I and those containing the possible interfering ions is more than 10%. 25hus, the interference factor, IF, is defined by equation 2: where A′ is the value of the percentage of removal on the solution of Ag I in the presence of possible interference, and A is the percentage of removal on the solution of Ag I in the absence of possible interference.In this case, IF = 1.00 means that there is no interference, while factors greater than 1.10 or smaller than 0.90 indicate an increase or a decrease of analytical signal due to interference.
Table 3 shows the results of removal of Ag I in real samples.The values obtained satisfactory confirm the excellent potential for removal of ions Ag I using M. oleifera Lam seeds.The methodology is simple, low cost and can therefore be used in remediation techniques.

Conclusions
In this work non-shelled M. oleifera seeds were used as sorbent for removing Ag I in aqueous solutions.The optimum conditions for removing Ag I were: 2.0 g of adsorbent with 75-500 μm particle size, 20 min extraction time and 6.5 pH.The adsorption capacity was 23.13 mg of Ag I per gram of M. oleifera seeds.The proposed method has advantages, such as, the low cost of sorbent, high efficiency and the minimization of chemical sludge.The sorbent is an alternative material for chemical remediation and is economical and environmental friendly.

Figure 2 .
Figure 2. Capacity of sorption of the Moringa oleifera seeds in the Ag I removal.Experimental conditions: Extraction time = 5 min; pH = 6.5;Volume of Ag I = 25.0 mL; Concentration of Ag I = 5.0 mg L -1 . non-shelled seeds,  husk,  shelled seeds.

Figure 4 .
Figure 4. Ag I removal in the function of the pH.Experimental conditions: particle size = 500 μm; Extraction time = 20 min; Moringa oleifera mass = 4.0 g.; Volume of Ag I = 50.0mL; Concentration of Ag I = 25.0 mg L -1 .

Figure 7
Figure7shows the isotherm sorption of Ag I on non-shelled M. oleifera seeds.

Table 2 .
Effect of interferent ions on the removal of Ag I .Experimental conditions: Particle size = 500 μm, extraction time = 20 min, mass of adsorbent = 2.0 g, pH 6.5

Table 3 .
Results of the removal of Ag I ions in the samples * ND = no detected.