Single and competitive adsorption of Cd(II) and Pb(II) ions from aqueous solutions onto industrial chili seeds (Capsicum annuum) waste Academic research paper on "Chemical sciences"

Accepted Manuscript

Single and competitive adsorption of Cd(II) and Pb(II) ions from aqueous solutions onto industrial chili seeds (Capsicum annuum) waste

Nahum A. Medellin-Castillo, Erika Padilla-Ortega, María C. Regules-Martínez, Roberto Leyva-Ramos, Raúl Ocampo-Pérez, Candy Carranza-Alvarez

PII: S2468-2039(16)30170-4

DOI: 10.1016/j.serj.2017.01.004

Reference: SERJ 71

To appear in: Sustainable Environment Research

Received Date: 1 October 2016 Revised Date: 25 November 2016 Accepted Date: 19 January 2017

Please cite this article as: Medellin-Castillo NA, Padilla-Ortega E, Regules-Martínez MC, Leyva-Ramos R, Ocampo-Pérez R, Carranza-Alvarez C, Single and competitive adsorption of Cd(II) and Pb(II) ions from aqueous solutions onto industrial chili seeds (Capsicum annuum) waste, Sustainable Environment Research (2017), doi: 10.1016/j.serj.2017.01.004.

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Received 1 October 2016

Received in revised form 25 November 2016

Accepted 19 January 2017

Single and competitive adsorption of Cd(II) and Pb(II) ions from aqueous solutions onto industrial chili seeds (Capsicum annuum) waste

Nahum A. Medellin-Castillo a b*, Erika Padilla-Ortega c, María C. Regules-Martínez b, Roberto Leyva-Ramos d, Raúl Ocampo-Pérez d, Candy Carranza-Alvarez e

a Graduate Studies and Research Center, Faculty of Engineering, Autonomous University of

San Luis Potosi, San Luis Potosi 78290, Mexico b Graduate Multidisciplinary Program in Environmental Sciences, Autonomous University of San Luis Potosi, San Luis Potosi 78210, Mexico c CINVESTAV-Queretaro, Queretaro 76230, Mexico d Center for Graduate and Research Studies, Faculty of Chemical Sciences, Autonomous

University of San Luis Potosi, San Luis Potosi 78260, Mexico e Multidisciplinary Academic Unit in the Huasteca Region, Autonomous University of San

Luis Potosi, Ciudad Valles 79060, Mexico

Keywords: Adsorption; Heavy Metals; Isotherms; Chili Seeds; Competitive Adsorption Cd(II)-Pb(II); Water

Corresponding author:

E-mail address: nahumanca@hotmail. com

ABSTRACT

In this work, the single and binary adsorption of Cd(II) and Pb(II) onto industrial chili seeds (CS) (Capsicum annuum) from aqueous solutions was investigated as a possible low-cost biosorbent for the removal of toxic heavy metals from aqueous solutions. The dependence of the adsorption capacity of CS on the solution pH and temperature, and the presence of competitive metal were also studied in detail. The adsorption equilibrium experiments of Cd(II) and Pb(II) on CS were conducted in a batch adsorber. The Freundlich and Langmuir isotherm models were fitted to the single adsorption equilibrium data and the latter provided a better fit. Moreover, it was found that the adsorption capacity of CS towards Cd(II) and Pb(II) ions was greatly increased by increasing the solution pH. The effect of the

pH was attributed to the electrostatic interaction between the negatively charged CS surface

and the Cd and Pb cations in the aqueous solution. The adsorption capacity was slightly increased by raising the temperature because the adsorption of Cd(II) or Pb(II) ions on CS was an endothermic process. The experimental binary adsorption data were satisfactorily interpreted using the modified Langmuir multicomponent isotherm and the competitive adsorption of Cd(II)-Pb(II) on CS revealed that the affinity of Pb(II) for CS was more than 5 times higher than that of Cd(II). 1. Introduction

Water pollution is one of the main environmental concerns, affecting human health, water resources, and ecosystems. Particularly, heavy metal contamination is one of the most pressing environmental problems nowadays. Various industries continue producing and discharging waste with different heavy metals into the environment, such as mining and smelting of metals, energy and fuel production, iron and steel, electroplating, electrolysis, electro-osmosis, leatherworking, photography, electric appliance manufacturing, metal surface treating, aerospace and atomic energy installations [1-3].

Heavy metals in raw water sources and wastewater represent acute toxicity to aquatic, animal and human life [4,5], even at very low concentrations [3]. The major toxic heavy metals are Hg(I), Cd(II), Pb(II), Cr(IV), Fe(III) and Cu(II) [6].

The adsorption process is considered as one of the most promising methods for the removal of toxic metals and other pollutants from aqueous solutions due to its simplicity, flexibility and elevated efficiency in industrial applications [7,8]. Lately, there has been considerable interest in the use of industrial and agriculture residues or by-products as materials to remove toxic metals from aqueous solution by adsorption since they are cheap and have high efficiency for adsorption of these pollutants [9,10]. The use of different organic or biological residues in the treatment of industrial wastewater or contaminated water is known as biosorption, and the adsorbents are known as biosorbents [10]. Agricultural waste materials contained hemicellulose, lignin, lipids, proteins, simple sugars, hydrocarbons, starches, ash and many more compounds, which have very diverse functional groups that can bind in the metal cations from solution [6,11]. Several biosorbents have been tested for the removal of heavy metals such as carrot residues, cork biomass, pine and eucalyptus bark, apple waste, sunflower stalks, tea waste, plum seeds, sugarcane bagasse, corncob, coconut husk, tree sawdust and peanut shell [9,10,12-16].

In Mexico, the annual average production volume of chili is 2.2 Mt and chili seeds (CS) represent the 3 wt% of this production, therefore they can be considered as a potential biosorbent due to its low cost and abundance since they are generally a by-product from chili processing companies. Chili is widely cultivated and used for nutritional and condimental purposes. Their seeds are separated from the pods and discarded after eating or processing [17]. Although, several studies have been reported on the application of a great variety of biosorbents for the removal of heavy metals from aqueous solutions, few works have been reported about the use of CS as a biosorbent [9,17-19]. Moreover, no study has been

investigated the individual and binary biosorption of heavy metals on industrial CS from aqueous solutions.

Therefore, the main aim of this work was to study the adsorption mechanism of Cd(II) and Pb(II) in single and binary solutions onto CS, as well as, the relationship between the adsorption capacity and the physicochemical properties of this material. The adsorption mechanism will be elucidated by obtaining the single and binary adsorption equilibrium of Cd(II) and Pb(II) from aqueous solution onto CS. The possible utilization of CS as an alternative biosorbent for the removal of Cd(II) and Pb(II) was investigated in detail.

2. Materials and methods

2.1. CS

Industrial CS (Capsicum annuum) waste used throughout this study were supplied by the company CONDIMEX, located in San Luis Potosi, Mexico. The CS were separated and extensively washed with deionized water and dried in an oven at 80 °C for 24 h. Afterwards, the biosorbent was crushed, sieved to an average particle diameter of 1.1 mm, and stored in plastic bottles for further use.

2.2. Characterization of CS

The textural properties of CS (surface area, pore volume and mean pore diameter) were determined using a surface area and porosimeter analyzer (Micromeritics, ASAP 2020), and the surface area was calculated by the N2-BET method. The concentrations of acidic and basic sites were determined using the acid-base titration method proposed by Boehm [20].

The surface charge and point zero charge (pHpzc) of CS were determined by a titration method [21], and the moisture and ash contents of CS were quantified by standard gravimetric methods [22]. The lignin was quantitatively assessed by the method proposed by Runkel and

Wilke [23], and the total content of cellulose and hemicellulose was quantified as holocellulose by the method suggested by Wise [24]. The Raman spectroscopy analysis of CS was recorded at room temperature using a Micro-Raman laser spectrometer (Thermo Scientific, laser DXR 780 nm), with a scanning range between 50-3500 cm-1 and at a laser power of 24 mW.

The morphology of CS particles was observed by a scanning electron microscope (FIB/SEM, FEI-Helios Nanolab 600). The microscope was equipped with an energy dispersive detector EDAX for microanalysis. The thermogravimetric analysis of samples was carried out using a thermogravimetric analyzer (TGA, Thermal Advantage, model TGA Q500).

2.3 Determination of the concentrations of Cd(II) and Pb(II) in aqueous solution

The concentrations of Cd(II) and Pb(II) in aqueous solution were determined by using an atomic absorption spectrophotometer (Varian, model SpectrAA-20), and the concentration of a metal in a water sample was estimated from a calibration curve prepared with standard solutions of the metal.

2.4 Adsorption equilibrium data

The experimental adsorption equilibrium data for the single adsorption of Cd(II) and Pb(II) on CS were obtained in a batch adsorber, which was a 50 mL conical vial. A volume of 50 mL of a solution having a known initial concentration of Cd(II) or Pb(II) with predetermined pH was added to the batch adsorber, and a sample of 10 mL was taken to corroborate the initial concentration of the heavy metal. Then, a mass of 0.2 g was poured into the adsorber, and the latter was partially immersed in a thermostatic water bath, and the metal solution and CS were left in contact until equilibrium was reached, which took 5 d.

Throughout the adsorption experiment, the solution pH was measured periodically with a pH-meter and was kept constant by adding few drops of 0.1 or 0.01 N HNO3 and NaOH solutions. The mass of Cd(II) or Pb(II) adsorbed on CS was calculated by a mass balance that can be mathematically expressed as:

q = V<C„ - Ce> << („

where C0 (mg L-1) and Ce (mg L-1) are the initial and equilibrium concentrations of metal, respectively, m (g) is the mass of CS, qe (mg g-1) is the mass of metal adsorbed per unit mass of CS and V (L) is the volume of the metal solution in the adsorber.

The binary adsorption equilibrium data of Cd(II)-Pb(II) on CS were obtained by a similar procedure as described above. In this case, the CS was contacted with aqueous solutions with known initial concentrations of Cd(II) and Pb(II) at pH = 5.0 and T = 25 °C.

3. Results and discussion

3.1. Physicochemical and textural properties and chemical composition of chili seeds

The main components of the CS, the pHPZC and the concentrations of acidic and basic sites are given in Table 1. The results corroborated that the CS are mainly composed of holocellulose and lignin.

The surface charge distribution of CS is depicted in Fig. 1 with the pHPZC = 4.26. As seen in this figure, the surface charge is positive at pH < pHPZC, neutral at pH = pHPZC and negative at pH > pHPZC. Similar values of pHPZC have been reported for other biosorbents, but no value of pHpzc was found for CS [16,25,26].

As shown in Table 1, the concentration of total acid sites was 7.2 times greater than that of basic sites, indicating that the nature of the CS surface was acidic. Additionally, the concentration of acidic sites of CS decreased in the following order: Phenolic > Lactonic > Carboxylic. The predominance of phenolic sites over lactonic and carboxylic sites could be

attributed to that the lignin in CS contained these functional groups. The presence of the acidic sites favors the adsorption of metal cations since they can accept or donate protons depending on the solution pH [26]. It is important to mention that the quantification of the

concentrations of acidic sites from CS has not been reported elsewhere.

The surface area of the CS was 0.19 m g- , indicating that this material is nonporous. Besides, the surface area of CS has not been reported in the available literature. The pore volume and average pore diameter were not given in this work because the experimental error involved in their calculation was significant.

3.2. SEM, Energy Dispersive X-Ray Spectroscopy (EDX), RAMAN and TGA analyses of the CS

SEM, EDX, Raman and TGA analyses were used to characterize the surface of CS and identify the possible metal-adsorbent interactions. These analyses were conducted using CS without Cd(II) and Pb(II) and CS loaded with Cd(II) or Pb(II), which were designated as CS-Cd and CS-Pb, respectively.

The SEM images in Fig. 2 shows the morphology of the CS surface, and a netted pattern is noticed at the CS surface that is formed by a network of troughs and ridges (Figs. 2a-2c). This reticulated formation is typical of the seed coat [27]. Additionally, some channels and cavities are also observed at the surface of CS.

Likewise, the chemical microanalyses of the CS, CS-Cd, and CS-Pb surfaces were performed by an EDX attachment of the SEM. The EDX spectrum of CS is shown in Fig. 2a and revealed that the CS particles were mainly composed of C, O, P, Mg, Ca and K due to the reason that CS is primarily constituted by holocellulose, lignin, and impurities. The EDX spectra of the CS-Cd and CS-Pb exhibited in Figs. 2b and 2c, corroborating the presence of Cd and Pb on the surface of CS due to biosorption [6].

The Raman spectra of CS, CS-Cd and CS-Pb samples were obtained to provide qualitative information about the characteristic functional groups, existing in the CS structure as shown in Fig. 3. The Raman spectrum of CS showed the typical bands corresponding to cellulose and lignin, principal components of CS. The bands observed at 1094, 1121 and 1330 cm-1 were attributed to v-CCO, v-CC and v-HCO of cellulose, respectively. Moreover, lignin can be identified with the bands at 1148 cm-1 ( 8-CCH), 1186 cm-1 (8-COH), 1270 cm" 1 (v-CO), 1330 cm-1 (v-OH), 1375 cm-1 (8-OHphenolic), 1459 cm-1 (V-CH2), 1657 cm-1 (v-C=C) and 1738 (v-C=O) cm-1; even more, the band assigned to the aromatic skeletal vibration of lignin at 1601 cm-1 can be noted [28-30].

The CS-Cd spectrum did not exhibit the bands corresponding to bending and stretching mode of -OH (1330 and 1375 cm-1) present in the CS spectrum, and the band v-CCO from cellulose showed a displacement from 1094 to 1082 cm-1. In addition, the band at 1441 cm-1 due to the alteration of aromatic skeletal vibration combined with C-H in-plane deformation of lignin. Similarly, the Raman spectrum of CS-Pb showed a displacement of the bands corresponding to aliphatic and phenolic -OH; these bands can be found at 1338 and 1378 cm-1. These results indicate that hydroxyl groups from lignin are involved in the adsorption of Cd(II) and Pb(II). Additionally, the changes in the intensity of the absorbance peak at 1657

cm-1 in CS-Cd and CS-Pb indicated that the adsorption of Cd(II) or Pb(II) was also due to the

p-cation interaction between the aromatic rings of the lignin and Cd or Pb cations in solution. The p-cation interactions are important in the adsorption of heavy metals from aqueous solution onto the aromatic rings of the graphene layers of activated carbon [16,31].

The mass-loss (TG) and derivative mass-loss (DTG) curves of the CS, CS-Cd and CS-Pb samples are depicted in Figs. 4a and 4b, respectively. Figure 4a shows that the percentages of mass-loss curves of the CS, CS-Cd, and CS-Pb were very similar in the temperature range of 60-600 °C, the mass-loss percentage were 79, 76 and 75% for CS, CS-Cd, and CS-Pb

samples, respectively, and all the samples presented the highest mass loss percentage in the temperature range between 200 and 500 °C. Besides, the shapes of the TG curves of the CS-Cd and CS-Pb samples were similar to that of CS, indicating that metal ions adsorbed on the surface of CS did not affect the thermal degradation pathways of the main constituents of CS.

By analyzing the DTG curves (Fig. 4b), it was observed that the CS sample exhibited three peaks at 284, 350 and 406 °C, which could be related to the combined effect of the thermal decomposition of hemicellulose, lignin and cellulose [32]. The overlapping of these thermal decomposition stages made it difficult to identify the percentage of mass loss in the TG curves. The CS sample presented a peak at 284 °C that might be due the decomposition of the hemicellulose, lignin or cellulose; this peak was shifted to 293 and 300 °C for the CS-Cd and CS-Pb samples, respectively. The peak displacement suggested that the Cd(II) and Pb(II) adsorbed on CS increased the temperature of decomposition of CS. Similar results have been reported for the TGA analysis of raw and agave bagasse saturated with Cd(II), Pb(II) and Zn(II) ions [32].

3.3. Adsorption isotherms

The Freundlich and Langmuir isotherms were fitted to the experimental single adsorption equilibrium data of Cd(II) or Pb(II) onto CS. These isotherm models can be represented by the subsequent mathematical relationships:

q = kiQ1'"1

q = qmiKiCi

qi 1+KC

where Ci (meq L-1) is the concentration of metal i in aqueous solution at equilibrium, i is the metal and qi (meq g-1) is the amount of metal i adsorbed per mass of chili seeds. The parameters K (L meq-1) and qmi (meq g-1) are the Langmuir constants for the metal i related to

the energy of adsorption and maximum adsorption capacity, respectively. The parameters ki (L1/n meq1-1/n g-1) and ni are the Freundlich constants for the metal i related to the adsorption capacity and intensity, respectively.

The isotherm constants were estimated by a least-squares method based on an optimization algorithm, and their values are shown in Table 2. Besides, the average absolute percentage deviation, %Dev, was computed from the following equation as also given in Table 2:

%Dev = - Y qexp qcal N 1=f qexp

X100 % (4)

where N is the number of experimental data points, qexp (meq g-1) is the experimental amount of Cd(II) or Pb(II) adsorbed, and qcal (meq g-1) is the amount of Cd(II) or Pb(II) adsorbed predicted with the adsorption isotherm model.

The experimental adsorption equilibrium data were satisfactorily interpreted by both isotherm models since the average percentage deviations were less than 12 and 15% for the Langmuir and Freundlich isotherms, respectively. As shown in Table 2, the Langmuir isotherm better fitted the adsorption equilibrium data since the %Dev for the Langmuir isotherm was lower than that for the Freundlich isotherm in 6 out of the 11 isotherm cases (Table 2). Consequently, the Langmuir isotherm was chosen to represent the adsorption equilibrium data of Cd(II) and Pb(II) on CS.

3.4. Effect of solution pH on the adsorption isotherm

The effect of the solution pH on the capacity of CS for adsorbing Cd(II) or Pb(II) was investigated by determining the adsorption isotherms at solution pH values of 2.0, 3.0, 5.0 and 7.0 and T = 25 °C, and results are graphed in Figs. 5a and 5b, respectively. In the case of Pb(II), no adsorption experiments were conducted at pH > 5.0 since the Pb(II) started to

precipitate at pH > 5.0. As shown in these figures, the capacity of CS for adsorbing Cd(II) and Pb(II) was significantly dependent on the solution pH since the capacity was raised by increasing the solution pH. The maximum adsorption capacity of CS towards Cd(II) was 0.08, 0.12, 0.18 and 0.23 meq g-1 at pH values of 2.0, 3.0, 5.0 and 7.0, respectively. In other words, the adsorption capacity increased about 2.9 times by raising the solution pH from 2.0 to 7.0. Similar behavior was observed for the adsorption of Pb(II) on CS. The maximum capacity of CS for adsorbing Pb(II) was 0.072, 0.096 and 0.21 meq g-1 at pH values of 2.0, 3.0 and 5.0, respectively. In this case, the adsorption capacity augmented about 3.0 times by increasing the solution pH from 2.0 to 5.0.

The effect of pH on the adsorption capacity can be explained by considering the

electrostatic interactions between the surface charge of CS and the metal cations in the

2+ 2 +

solution. The metals were adsorbed on CS as the Cd [16] and Pb2+ [26] species because these species were predominant at pH < 7 and pH < 5, respectively (see Figs. 6a and 6b). The pHPZC of CS was 4.26 (Table 1). At pH values lower than pHPZC, the surface of CS was mainly charged positively so that the electrostatic repulsion disfavored the adsorption of Cd(II) or Pb(II) on CS. On the other hand, at pH values higher than pHPZC, adsorption of Cd(II) or Pb(II) was favored because of the electrostatic attraction between the metal cations in solution and the negatively charged surface of CS. It is important to point out that the Cd(II) or Pb(II) was adsorbed on CS at pH below pHPZC, even though the metal cations were repelled from the surface of CS. Salazar-Rabago and Leyva-Ramos [26] reported that one of

the adsorption mechanisms of Pb(II) onto White Pine sawdust is ion exchange since the H+

cations were transferred from the surface of the sawdust to the solution, and the Pb+2 moved

in the opposite direction. In this work, it was noticed that the solution pH was reduced while

the Cd and Pb2+ cations were adsorbed on CS. Hence, ion exchange could also take place during the adsorption as well as the electrostatic attractions and p-cation interactions.

3.5. Effect of temperature on the adsorption isotherm

The effect of the solution temperature on the capacity of CS for adsorbing Cd(II) or Pb(II) was analyzed by determining the adsorption isotherms at the temperatures of 15, 25 and 35 °C and pH = 5.0. As seen in Figs. 7a and 7b, the adsorption capacity of CS towards metal cations was slightly enhanced with the temperature. The maximum adsorption capacity of CS towards Cd(II) increased 1.1 and 1.1 times, and that towards Pb(II) increased 1.2 and 1.5 times when the temperature was raised from 15 to 25 °C and 25 to 35 °C, correspondingly. These results indicated that the adsorption of Cd(II) or Pb(II) was endothermic.

The heat of adsorption of Cd(II) and Pb(II) onto CS was estimated from the relationship between the Langmuir constant Ki and the temperature [9,15]. This relationship is known as the van't Hoff equation, which is shown below in its linear form:

AH«. : 1

Ln Ki =--+ Ln Ki0 (5)

where AHAd,i is the heat of biosorption of metal i, J mol-1; T is the temperature of the solution, K; R is the ideal gas law constant, 8.31 J mol-1 K-1; Ki is the Langmuir isotherm constant of metal i, L mol-1; and Ki0 is the pre-exponential factor of metal i, L mol-1. The experimental values of Ki are listed in Table 2. The values Ln Ki vs. 1/T are plotted in Fig. 8, exhibiting a lineal behavior so that the effect of temperature on Ki was interpreted using Eq. (5). The values of AHAd,i for Cd(II) and Pb(II) were estimated to be 19 and 40 kJ mol-1, respectively, indicating that the biosorption of both metals on CS was an endothermic process.

3.6. Binary adsorption of Cd(II) and Pb(II)

The experimental binary adsorption equilibrium data of Cd(II) and Pb(II) on CS at pH = 5 and T = 25 °C were interpreted with the Modified Langmuir Multicomponent Isotherm (MLMI) with an interaction factor n [34]. The MLMI model is based on the Langmuir

isotherm and uses the single metal Langmuir isotherm parameters. The MLMI can be represented by the following mathematical relationship:

= qnyKi (Q/ n) qi 1+jKj

where h and hj are interaction factors. These factors modify the Langmuir isotherm model to consider competitive effects and are characteristic of each adsorbate-adsorbent system.

The interaction factors of the multicomponent isotherms were estimated using the Micromath Scientist software, which is based on the optimization algorithm of LevenbergMarquardt. The following objective function was minimized:

i=2 Nm / Np

Least squares= Z Z (qi,j,exp - qi,j,calf i=1 j=1

where Nm is the number of experimental data, qi,j,exp is the experimental uptake of metal i corresponding to data number j (meq g-1), and qi,j,cal is the uptake of metal i predicted with the MLMI model and corresponding to data number j (meq g-1). The average percentage deviation was estimated as follows:

2 i=1 Nm j=1

qi,j,exp qi,j,cal

x100 %

The values of the interaction factors and the average percentage deviation are shown in Table 3. The MLMI model adjusted reasonably well the experimental data since the average percentage deviation was 17.8%. This model has been considered as the best option for the multicomponent adsorption models based on the Langmuir isotherm for modeling the binary adsorption of heavy metals from aqueous solutions onto natural clays because it uses the single metal Langmuir isotherm parameters [34]. The interaction factor values allowed to infer that CS had a greater affinity for Pb(II) than for Cd(II) because the value of nCd (2.42) was five times higher than that of ^pb (0.44).

The binary adsorption equilibrium data of Cd(II)-Pb(II) on CS and the adsorption surface predicted by the MLMI model are shown in Figs. 9a and 9b. The effect of the presence of Pb(II) on the uptake of Cd(II) can be seen in Fig. 9a, and in the concentration of Pb(II) ranging from 0.0 to 3.0 meq L-1, the uptake of Cd(II) diminished drastically while increasing the concentration of Pb(II) at equilibrium.

The dependence of the uptake of Pb(II) on the concentration of Cd(II) at equilibrium is depicted in Fig. 9b. The uptake of Pb(II) was slightly affected by the competition of Cd(II). The results of the competitive adsorption of Cd(II)-Pb(II) have demonstrated that the Pb(II) ions presented a higher affinity for the cationic sites of the CS than for Cd(II) ions. For a metal concentration at equilibrium of 4.0 meq L-1, the selectivity ratio, S = qPb(ii)/qCd(ii), was calculated to be 1.2 when the uptakes of Pb(II) and Cd(II) were predicted from the single adsorption isotherms. However, the S was 6.25 when the uptakes of Pb(II) and Cd(II) were predicted from the MLMI model. This competitive selectivity value is quite similar to the ratio of the interaction factors (nCd/^Pb = 5.5). Hence, the Pb(II) exhibited strong antagonism in the competitive adsorption of Cd(II), whereas the Cd(II) did not significantly affect the competitive adsorption of Pb(II) or did not present antagonism. The affinity in the adsorption capacities of the CS towards the metals may be ascribed to ionic radius of the metal cations, Pb (1.19 A) > Cd (0.97 A) and Pauling electronegativity, Pb (2.33) > Cd (1.69).

The Raman spectrum of a CS sample loaded with both Cd and Pb, designated as CS-

Cd/Pb, was recorded to find out if the Cd and Pb cations competed for the same acidic sites during the competitive adsorption of both metals. The spectrum of CS-Cd/Pb is plotted in Fig. 3 and shows a displacement and alteration of the principal bands assigning to vibration of -OH and lignin. Besides, a strong band appears at 1424 cm-1 due to aromatic skeletal vibrations of lignin. These results confirm that both metals competed for the same sites during the binary adsorption.

4. Conclusions

The results of this work revealed that CS is nonporous material and the nature of its surface is acidic with a pHPZC value of 4.26. EDX analysis corroborated the presence of Cd and Pb on the surface CS after adsorption. The Raman spectrum showed that hydroxyl groups

present in lignin were involved in the adsorption of Cd(II) and Pb(II) on CS and p-cation

interactions between the aromatic rings of lignin and Cd and Pb cations in solution occurred during adsorption.

The single metal adsorption capacity of CS was highly dependent on the solution pH because of the electrostatic interactions, and an ion exchange mechanism may also take place at pH values higher than pHPZC. The positive value of DHAd,i confirmed the endothermic nature of the adsorption process.

The binary adsorption equilibrium data of Cd(II)-Pb(II) on CS were interpreted reasonably well by MLMI model. In the competitive adsorption of Cd(II) and Pb(II) on CS, Pb(II) exhibited strong antagonism in the competitive adsorption of Cd(II), whereas Cd(II) did not affect the competitive adsorption of Pb(II), but both metals competed for the same acidic sites. Acknowledgements

The authors are grateful to company CONDIMEX, located in San Luis Potosi, Mexico and G.J. Labrada-Delgado (IPICyT) for their technical assistance.

This work was funded by Fondo de Apoyo a la Investigación (FAI)-Universidad Autonoma de San Luis Potosi (UASLP), Consejo Nacional de Ciencia y Tecnologia-CONACyT, Mexico, and the federal program PROMEP through grants Nos.: FAI-2016, Inmersión a la Ciencia 2016, CB-2012-02-182779 and CB-2013-01 (221757), and PROMEP/103.5/12/3953, respectively.

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Physicochemical properties and chemical composition of industrial chilli seeds (CS)

Acidic sites (meq g-1) Lactonic sites (meq g-1) Carboxylic sites (meq g-1) Phenolic sites (meq g-1) Basic sites (meq g-1) pHpzc Moisture (%) Ash (%) Lignin (%) Holocellulose (%)

1.87 0.28 0.17 1.42 0.26 4.26 5.22 0.32 27.2 68.3

Values of the parameters for the Langmuir and Freundlich adsorption isotherms

T (°C) Freundlich Langmuir

Metal pH ki (L1/nmeq1"1/ng-1) 1/n %Dev R2 Ki qm (L meq-1) (meq g-1) %Dev R2

Cd 15 5 0.114 0.227 3.01 0.983 4.374 0.165 7.90 0.934

25 2 0.017 0.806 13.7 0.875 0.090 0.078 12.0 0.892

3 0.034 0.653 9.11 0.921 0.156 0.117 5.73 0.956

5 0.126 0.240 4.55 0.968 5.701 0.177 7.72 0.936

7 0.169 0.223 9.06 0.922 10.13 0.228 8.36 0.929

35 5 0.150 0.196 5.24 0.961 7.319 0.197 5.79 0.955

Pb 15 5 0.117 0.385 6.45 0.948 2.945 0.179 7.53 0.937

25 2 0.021 0.916 14.7 0.865 0.044 0.072 12.0 0.892

3 0.050 0.411 10.4 0.908 1.414 0.096 9.95 0.913

5 0.162 0.299 7.28 0.940 5.983 0.213 11.1 0.901

35 5 0.263 0.279 10.9 0.903 8.782 0.328 7.77 0.935

Parameters of the modified Langmuir multicomponent isotherm (MLMI) for the binary adsorption of Cd(II)-Pb(II) on CS at T = 25 °C and pH = 5

Interaction Factors %D R2

hcd = 2.42 17.8 0.834

hPb = 0.44

5 I I I I I I I I I I I I I I I I I I I I I I I I 11 I 11 I I I I I I I I I I I I I I I I I I I I I

(S -5 -

-15 I-

2 3 4 5 6 7 8 9 10 11 12

Solution pH

Fig. 1. Surface charge distribution of the CS.

(a) c TJ

/ \ nf C

J * t^

r ' 7 - ^ ^

Utvvy \

del HV 11 i.sg spot DualBSD 25 OO kV 500x| 5 4 WD 1 10 4 mm v>\>V<;y V ^kk r - - — ¿J ■ 100 pm ■ Blanco (IPICYT-LINAN)

MgK PK

0.70 1.40 2.10 2.80 3.S0 1.20

■M1H ir y ^.yVEj

det HV I mag I spot I WD I pressure I

DualBSD I 25 OO kV:50C x| 5 7 I 9 9 mm | 70 Pa

^'•^Ci t

100 pm ■ Pb-400 (1PIC YT-LI NAN)

0.70 1.40 2.10 2.80 3.50 4.20 4.90

I AIK I

2.00 4.00

Fig. 2. SEM micrographs and EDX spectra of the CS samples (a) CS, (b) CS-Cd, and (c) CS-Pb.

Raman shift, cm-1

Fig. 3. Raman spectra for CS, CS-Cd, CS-Pb, and CS-Cd/Pb samples.

100 90 80 70 60 50 40 30 20 10 0.9

\\ - v ■

% -CS :

re ....................................CSCd "

% ---CSPb j

s \ ^ •

100 200 300 400 500 600 Temperature, °C

Fig. 4. Thermogravimetric analysis curves of the CS, CS-Cd, and CS-Pb samples (a) Mass-loss (TG), (b) Derivative weight (DTG) curves.

¡? 0.20

13 ■p

¡3 0.15

3 0.10

f 0.05

(a) / * ♦ ♦ !

i ..,<•—* -

1 / :(/ i; J ¡- „ - - ~

. • pH J^-V^...........■ ph = 2.0 ♦ = 3.0 A pH = 5.0 ; pH = 7.0 •

0 1 2 3 4 5 6 7 Concentration of Cd(II) at equilibrium, meq L-1

Concentration of Pb(II) at equilibrium, meq L-1

Fig. 5. Effect of the solution pH upon the adsorption isotherm on chili seeds at 25 °C (a) Uptake of Cd(II) adsorbed and (b) Uptake of Pb(II) adsorbed. The lines represent the Langmuir isotherm.

Fig. 6. Speciation diagram for (a) Cd(II) and (b) Pb(II).

ST 0.20

"S3 0.15

m 13 c3

g 0.10

2 0.05

0 1 2 3 4 5 6 7 Concentration of Cd(II) at equilibrium, meq L-1

0.35 0.30

„ 0.25

% 0.20 ^ 0.15

% 0.05 0.00

(b) 1 | 1 1 1 1 | 1 1 1 1 ■

. --■---

' / ♦

A.................................

. 1 ■ 1 ...................... ' 1 .....,..................."" 1' ♦ • :

■ 1 • 1 / __— •

i / Z • T = 15 °C ■

iL/ ♦ T = 25 °C ■

1 . . . . 1 . . . ■ . 1 . . . T 1 = 35 °C •

0.0 0.5 1.0 1.5 2.0 2.5 3.0 Concentration of Pb(II) at equilibrium, meq L-1

Fig. 7. Effect of the solution temperature upon the adsorption isotherm on chili seeds at pH 5 (a) Cd(II) adsorbed and (b) Pb(II) adsorbed. The lines represent the Langmuir isotherm.

Fig. 8. Effect of temperature on the Langmuir isotherm parameter Ki.

Fig. 9. Binary adsorption isotherms of Cd(n)-Pb(n) on CS. The adsorption surfaces are predicted with the MLMI model (a) Uptake of Cd(II) and (b) Uptake of Pb(II) at pH = 5.0 and T = 25 °C.