Adsorption Behavior of Pb(II), Cd(II), and Zn(II) onto Agave Bagasse, Characterization, and Mechanism.

Biosorption is an alternative procedure to remove metal ions from aqueous media using agricultural waste. In this work, the adsorption capacity and removal efficiency of agave bagasse (AB) toward Pb(II), Cd(II), and Zn(II) were analyzed. Parameters such as equilibrium pH, particle size, AB dosage, time, and initial metal ion concentration were discussed. The results showed that pH 5.5, 0.4 g (<250 μm), and only 15 min of contact assured conditions for maximum adsorption capacity. The kinetic studies were fitted to the pseudo-second-order model, whereas the isotherms showed good agreement with the Langmuir model. AB has a higher affinity for Pb(II) over Cd(II) and Zn(II), and the maximum adsorption capacities were 93.14, 28.50, and 24.66 mg g-1, respectively. The results of the characterization evidenced two adsorption mechanisms. Scanning electron microscopy and X-ray diffraction displayed adsorption via the ion exchange mechanism by releasing Ca(II). The 13C cross-polarization mode with magic-angle spinning nuclear magnetic resonance analysis demonstrated a complexation mechanism by cellulose, hemicellulose, and lignin groups with Pb(II) and Cd(II), whereas the complexation is mainly observed by cellulose groups for Zn(II). AB is a good alternative for the removal of metals without prior thermal or chemical treatment, with rapid kinetics, suitable adsorption capacity, and high removal efficiency contributing to waste management.

Introduction

Water pollution has been increasing because of population growth, anthropogenic activities, urbanization, industrialization, and excessive use of water. The treatment of industrial effluents or wastewater is complex due to the variety of pollutants such as the heavy metals, Pb, Cd, Zn, Cr, Hg, and Ni in different oxidation states, besides other organic compounds that are commonly used in the stages of the productive processes. The heavy metals cause several health problems in humans, animals, and plants, and their elimination is imperative. Several methods for their removal have been proposed, such as chemical precipitation, ion exchange, membrane filtration, and adsorption.[1−3] Nevertheless, adsorption has advantages over other methods in terms of effectiveness and feasible regeneration.[4] Different sorbents have been developed and evaluated for the removal of heavy metals, including minerals such as aluminosilicates or vermiculites and activated carbon.[5] The latter has limited use because of its high cost.

Other kinds of sorbents with a relatively recent application are the residues from food or agriculture industries. These wastes are considered as biosorbents,[6,7] and so far, studies show that longer adsorption times[8−10] and preliminary chemical treatments are required,[3,11−13] which undoubtedly affect their further application. Notwithstanding, the feasibility of the adsorption process has been reported in a continuous system besides at a pilot scale.[14,15]

In Mexico, thousands of tons of agave bagasse (AB) are discarded annually due to the high production of tequila spirit or mezcal, and it is close to 40% of the total processed agave.[16] AB is a porous material mainly composed of cellulose, hemicellulose, and lignin.[17] It has been proposed for different applications such as animal feed[18] or reinforcement for composite materials.[19] Because of its active sites, AB is capable to adsorb heavy metals.[12,20] However, the adsorption kinetics and isotherms in a wide range of concentration as well as the role of AB compounds (cellulose, hemicellulose, and lignin) in the adsorption of metal ions have not been evaluated.

The present research is focused on the behavior of AB as a biosorbent for the heavy metals Pb(II), Cd(II), and Zn(II), evaluating different parameters in the system such as pH, particle size, biosorbent dosage, time, and metal ion concentration. The adsorption capacity of the metal ions from single and binary mixtures was discussed. In addition, the characterization of AB before and after the adsorption process was performed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and solid-state 13C cross-polarization mode with magic-angle spinning nuclear magnetic resonance (CP/MAS NMR) to evidence the influence of cellulose, hemicellulose, and lignin in metal ion adsorption.

Results and Discussion

Effect of pH

In Figure , the adsorption capacities and removal efficiencies are presented, which were calculated using the equations described in the Experimental Section. It is possible to see in Figure a that the adsorption capacity (qM(II)) increases with equilibrium pH and reaches a maximum around pH 5.5. After this, it remained without significant changes for all metal ions. In Figure a, it is notable that AB has more affinity for Pb(II) than Cd(II) and Zn(II). For instance, at pH 5.5, qPb(II) was 11.28 mg g–1, whereas Cd(II) and Zn(II) exhibited similar behavior, obtaining 4.06 and 4.15 mg g–1, respectively. The removal efficiency showed the same trend as that of the adsorption capacity (Figure b). At pH 5.5, 96.8% Pb(II), 41% Cd(II), and 40% Zn(II) were removed.

Figure 1

Influence of pH on (a) adsorption capacity of AB and (b) removal efficiency. C0 = 100 mg L–1, wAB = 0.4 g (>250 μm), and t = 30 min.

These results can be explained by the biosorbent surface charge as it is affected by pH. The point of zero charge (pHPZC) of AB was evaluated, and the result is shown in Figure .

Figure 2

Point of zero charge for AB.

pHPZC of 4.7 indicates that at pH < 4.7, the ΔpH values (and the biosorbent surface) are positive, whereas at pH > 4.7, the surface is negative. pHPZC is indicated by the vertical dashed line in Figures a and 2.

Cellulose, hemicellulose, and lignin are the main compounds of AB, and they contain functional groups such as hydroxyl −OH and carboxyl −COOH. The pKa value for the hydroxyl group is in the range of 9.5–13.[21] Therefore, the negative charges found on the biosorbent surface between 4.7 and this range (9.5–13) of pH should be predominantly because of the carboxylate groups, whereas those below should be because of the phenolate groups. As the present study involves metal ions, the regions more basic than pH 7–8 was avoided, aiming to minimize the formation of metallic hydroxocomplexes and other related complexes. Therefore, the phenolate groups do not contribute to the surface charge in the pH region studied in the paper.

The carboxylic groups must be deprotonated to generate negative charges on the surface and H+ in aqueous media and permit the adsorption of different metal ions. This is carried out from pH 1.7 (pKa: 1.7–4.7[21]), thus promoting a rise in the metal ion adsorption by a complexation mechanism that explains the moderate adsorption capacities at pH < pHPZC.

The results are in agreement with the pH range reported for cucumber peel,[22,23] rapeseed biomass, and bamboo, presenting the higher adsorption capacity at pH between 5 and 6.

Effect of Particle Size

Figure shows the variation of qM(II) as a function of particle size. It can be observed that the adsorption capacity increases when the particle size range diminishes from >250 to 250–180 μm for Cd(II) and Zn(II). This is related to the milling process that exposes the active sites in AB inducing more contact area during the adsorption experiments. However, a greater reduction in particle size does not represent a higher adsorption capacity, obtaining 9.6 and 9.2 mg g–1 for Cd(II) and Zn(II), respectively. For Pb(II), the adsorption capacity remains constant at 10.9 mg g–1 for all the particle size ranges analyzed. It means that the adsorption of Pb(II) is independent of this parameter.

Figure 3

Adsorption capacity of AB as a function of particle size range. C0 = 100 mg L–1, wAB = 0.4 g, t = 30 min, and pH = 5.5.

The removal efficiencies are presented in Table . Particle sizes <250 μm exhibited a very good performance, removing around 99% of Pb(II), 91% of Cd(II), and 80% of Zn(II). The results show that with smaller particle size, the contact area increases, improving adsorption, unlike the results shown in Figure where only 41 and 40% were reached for Cd (II) and Zn (II), respectively. Therefore, particle sizes lower than 250 μm were selected for further experiments to assure better conditions for the metal ion adsorption capacity.

Table 1

Removal Efficiency (%) for Pb(II), Cd(II), and Zn(II) at Different Particle Size Rangesa

	removal efficiency (%)
particle size range (μm)	Pb(II)	Cd(II)	Zn(II)
>250	96.81 ± 0.17	41.09 ± 1.15	37.14 ± 1.57
250–180	99.21 ± 0.14	91.78 ± 0.23	78.34 ± 1.62
180–150	99.21 ± 0.09	91.80 ± 0.13	80.07 ± 0.81
<150	99.40 ± 0.28	90.94 ± 0.47	80.79 ± 1.60

C0 = 100 mg L–1, wAB = 0.4 g, t = 30 min, and pH 5.5.

Influence of Dosage AB

The adsorption capacity and removal efficiency as a function of AB dosage are displayed in Figure for Pb(II), Cd(II), and Zn(II). The results indicate that the adsorption capacity increases significantly at lower AB amounts. qM(II) reaches 42.6, 27.27, and 24.2 mg g–1 for Pb(II), Cd(II), and Zn(II), respectively, with 0.1 g of AB (Figure a). The lower adsorption capacity and little difference for the three metal ions are observed with 0.6 g AB. The diminution in adsorption capacity is attributed to the increase of active sites when the AB mass is augmented, and then a higher concentration of metal ions is required to allow the saturation of the biosorbent.

Figure 4

Effect of AB dosage on (a) adsorption capacity and (b) removal efficiency. C0 = 100 mg L–1, t = 30 min, pH = 5.5, and AB particle size <250 μm.

In Figure b, the difference in removal efficiency is distinguished when the AB dosage is modified. For Cd(II) and Zn(II), the removal efficiency increases with the AB mass. Cd(II) showed better results from 0.3 g with approximately 95%, whereas Zn(II) required 0.4 g of AB to reach 83%. For Pb(II), it can be observed that the removal efficiency is very similar around 99.4% for all amounts of AB. These results demonstrate the high adsorption capacities of AB and the preference for Pb(II) compared to Cd(II) and Zn(II).

Based on the above results, the amount of 0.4 g of AB was selected as the best condition for the removal efficiency with acceptable adsorption capacities for all metal ions, and it was kept constant for kinetic studies and isotherms.

Adsorption Kinetics

Figure a shows the adsorption capacity of Pb(II), Cd(II), and Zn(II) onto AB as a function of time.

Figure 5

(a) Effect of time on the adsorption capacity of AB. (b) Pseudo-second-order kinetic model according to eq . C0 = 100 mg L–1, wAB = 0.4 g (<250 μm), and pH = 5.5.

The plot displays that qM(II) for all metal ions remained constant from the first 5 min of contact with AB. The adsorption capacities are 11.11 ± 0.22, 10.04 ± 0.20, and 9.60 ± 0.17 mg g –1 corresponding to Pb(II), Cd(II), and Zn(II). The removal efficiencies calculated from Figure a are 99, 93, and 82% for Pb(II), Cd(II), and Zn(II), respectively, in only 5 min. Then, the adsorption of metal ions onto AB rapidly proceeded with high removal efficiencies.

In general, the biosorbents have been identified as materials that need long times of contact to reach the adsorption equilibrium with moderate efficiency. For example, Taro (Colocasiaesculenta (L.) Schott) adsorbs Pb(II) in 120 min,[24]Posidonia oceanica fibers uptake Pb, Cd, and Zn in 80 min,[25] whereas 60 min is necessary with cucumber peel for Pb(II) and Cd(II).[22,23] Therefore, the results found in this work demonstrate that AB reaches the adsorption equilibrium in a very short time as compared to other biosorbents. The following experiments were set up at 15 min to assure the equilibrium at different conditions.

Adsorption kinetics for metal ions onto AB have not been previously reported; the experimental results were fitted to pseudo-first- and pseudo-second-order models. The pseudo-first-order or Lagergren equation[26] is expressed bywhere q and qe are the adsorption capacity at time t and at equilibrium (mg g–1), respectively, and k1 is the pseudo-first-order rate constant. According to eq , the plot of ln(qe – q) against t gives a straight line, where the slope matches with k1.

The linear form of the pseudo-second-order kinetic model is[26]where k2 is the pseudo-second-order rate constant, and it is obtained from the plot t q–1 against t.

The accuracy of models was verified by the standard error (SE) and was computed by the following equation[27]where qexp and qcalc represent the experimental and predicted adsorption capacity by the model, respectively, and N is the number of experiments.

According to eq , an acceptable fitting to experimental data is not observed (correlation coefficient, R2 < 0.93) unlike that of the pseudo-second-order equation. Figure b presents the fitting according to eq . The adsorption of metal ions is described by the pseudo-second-order model with remarkable agreement to the correlation coefficient (R2) higher than 0.99.

The results of adsorption kinetics from both models are resumed in Table . The k2 values for Pb(II), Cd(II), and Zn(II) were 1.0602, 1.2910, and 0.8969 g mg–1 min–1, respectively. The SEs for the three metal ions were lower than 0.05, which corroborates a very good agreement between the experimental and predicted values of adsorption capacity. The solid lines in Figure a correspond to the predicted adsorption capacity based on the obtained parameters of the pseudo-second-order model. This model is representative of the adsorption behavior of divalent metal ions such as Pb(II), Cd(II), and Zn(II).[28]

Table 2

Calculated Parameters for Kinetic Models

	Pb(II)	Cd(II)	Zn(II)
qe exp (mg g–1)	11.11 ± 0.22	10.04 ± 0.20	9.60 ± 0.17
Pseudo-First-Order or Lagergren Equation
qe (mg g–1)	0.0720	0.0578	0.0933
slope	–0.0112	–0.0111	–0.0157
intercept	–2.6308	–2.8508	–2.3722
R2	0.7876	0.9265	0.8147
k1 (min–1)	0.0112	0.0111	0.0157
SE	12.1255	10.9493	10.4309
Pseudo-Second-Order Model
qe (mg g–1)	11.1247	10.0397	9.5956
slope	0.0899	0.0996	0.1042
intercept	0.0076	0.0077	0.0121
R2	1.0000	1.0000	1.0000
k2 (g mg–1 min–1)	1.0602	1.2910	0.8969
SE	0.0493	0.0446	0.0457

Adsorption Isotherms

Figure a presents the adsorption isotherms for each metal ion in an interval of the initial concentration of 10–1000 mg L–1. At low concentrations, it is notable that qM(II) increases with the equilibrium concentration. In this case, the mass-transfer resistance between solid and liquid phases is diminished through driving force by concentration effect enhancing adsorption capacity.[29] However, at higher concentrations, the adsorption capacity achieves a plateau that matches with the maximum adsorption capacity (qmax,exp), as the active sites are occupied, inducing the biosorbent saturation. For Pb(II), qmax,exp was 93.14 ± 3.38 mg g–1 at 560 mg L–1 as the equilibrium concentration. However, when the concentration is higher, it is observed that the adsorption capacity is lower than the maximum adsorption capacity (indicated with an arrow in Figure a). This phenomenon is attributed to the high amount of Pb(II) on the surface promoting the desorption of metal ions from the saturated active sites.[30] In the case of Cd(II) and Zn(II), the adsorption behavior is similar, and the plateau is rapidly achieved at 28.50 ± 3.72 and 24.66 ± 2.97 mg g –1, respectively. An extended comparison with other references is presented in Table , and it is noticed that the majority of biosorbents do not have the high adsorption capacity exhibited by AB in this work in a very short time. Comparable results have been reported using mango or cucumber peel after 60 min.[22,23,31,32] In this work, the experiments only required 15 min for similar adsorption capacities. The results reported by other authors for AB without treatment indicate that the adsorption capacity for Pb(II), Cd(II), and Zn(II) are 35.6, 13.27, and 7.84 mg g–1, respectively.[12] These values are lower than those obtained in this work owing to different experimental conditions used by the authors, such as narrow concentration interval, higher particle size, and lower pH.

Figure 6

(a) Adsorption isotherms. (b) Experimental data fitted to the Langmuir model. C0 = 10–1000 mg L–1, wAB = 0.4 g (<250 μm), t = 15 min, and pH = 5.5.

Table 3

Adsorption Capacities Reported in the Literature for Pb(II), Cd(II), and Zn(II) and the Results Obtained in This Work

biosorbent	time (min)	pH	concentration interval (mg L–1)	qPb(II) (mg g–1)	qCd(II) (mg g–1)	qZn(II) (mg g–1)
AB (this work)	15	5.5	10–1000	93.14 ± 3.38	28.50 ± 3.72	24.66 ± 2.97
AB[12]		5	1–100	35.6	13.27	7.84
Agave Americana fibers[33]	30–60	5		40	12.5
cucumber peel[23]	60	5	20–300		112.3
cucumber peel[22]	60	5	20–350	133.6
Posidonia oceanica fibers[25]	80	6–8	30–250	48.33	30.22	37.90
mango peel[32]	60	5	10–600	99.05	68.92
mango peel[31]	60	5–6	10–500			28.21
rapeseed biomass[8]	180	5.2	5–250	21.29
peanut shell[9]	180	5.5	100–350	39
rice straw[34]	180	5	25–350		13.9
taro[24]	120	6	10–500	291.56

The Langmuir and Freundlich equations were considered to represent the experimental systems in terms of equilibrium concentration, Ce, and adsorption capacity, qe.[35,36] The first one in linear form is defined bywhere qmax is the saturated monolayer adsorption capacity and KL is the Langmuir constant. According to eq , the plot of Ce·qe–1 against Ce gives a straight line whose slope is equivalent to qmax–1, and from the intercept, it is possible to calculate the KL value.

Freundlich isotherm describes heterogeneous systems by the following equationwhere n is the adsorption intensity and KF is the Freundlich isotherm constant. The logarithmic graph of qe versus Ce gives a straight line, where the slope is related to n–1 value and the intercept corresponds to log KF. The experimental data were analyzed considering both models.

The results of the fitted data such as slope, intercept of the straight line, correlation coefficient, and the obtained parameters of eqs and 5 are given in Table .

Table 4

Calculated Parameters for the Isotherms of Pb(II), Cd(II), and Zn(II) onto AB Using the Langmuir and Freundlich Models

	Pb(II)	Cd(II)	Zn(II)
qmax,exp (mg g–1)	93.14 ± 3.38	28.50 ± 3.72	24.66 ± 2.97
Langmuir Equation
slope	0.0107	0.0345	0.0412
intercept	0.1431	0.4966	0.9541
R2	0.9954	0.9926	0.9943
qmax (mg g–1)	93.4517	28.9809	24.2557
KL (L mg–1)	0.0748	0.0695	0.0432
SE	5.2988	2.5188	2.0904
Freundlich Equation
slope	0.5241	0.3585	0.3834
intercept	0.8315	0.5650	0.3816
R2	0.9250	0.9412	0.9521
n	1.9081	2.7897	2.6081
KF (L mg–1)	6.7850	3.6726	2.4078
SE	29.7646	4.1407	3.3146

In Figure b, it is possible to see that the experimental data for Pb(II), Cd(II), and Zn(II) have a very good agreement with the Langmuir model (eq ). The correlation coefficient is higher than 0.99, which indicates that this model is suitable to predict the adsorption capacity of AB for the three metal ions. The Langmuir constant for Pb(II), Cd(II), and Zn(II) is 0.0748, 0.0695, and 0.0432 L mg–1, respectively. Then, the adsorption onto AB is brought out by the monolayer formation to reach the saturation of the biosorbent, and this is associated with a complex mechanism.[37] Despite the higher constant obtained for Pb(II), SE is also elevated. This is attributed to the wide concentration interval evaluated. At higher concentrations, the adsorbed Pb(II) onto AB raises and significant interactions take place. Thus, deviations from the Langmuir model were observed, increasing the SE.

With regard to the calculated parameters for the Freundlich model (Table ), the correlation coefficients do not exceed the value of 0.95, indicating an acceptable fitting. A high n value suggests a stronger interaction of the biosorbent with the heavy metal.[38] In this context, Pb(II) results in an n value equal to 1.90, which is higher for Cd(II) and Zn(II), that is, 2.79 and 2.60, respectively. This shows the stronger interaction of AB with Cd(II) and Zn(II) than that with Pb(II) in contrast to the experimental results. It is associated with the low correlation coefficient that implies differences between the experimental data and the model.

Although the experimental data satisfactorily fitted with both models, for the Freundlich model, the correlation coefficients are lower than that of the Langmuir equation (Table ), and the main deviations are for the higher concentrations. AB is not a homogeneous biosorbent, but the adsorption data fitted well to the Langmuir model, and it is related to the small particle size used in the adsorption experiments. In consequence, the active sites of AB are available homogeneously to adsorb metal ions.

Adsorption Behavior in Binary Mixtures

The results discussed above correspond to the adsorption behavior of Pb(II), Cd(II), and Zn(II) onto AB from single solutions without considering the interaction with other ions.

Figure includes the adsorption capacities of Pb(II), Cd(II), and Zn(II) in binary mixtures and their comparison with single solutions. The adsorption capacity of Pb(II) is improved in the presence of Cd(II) from 10.5 to 13.41 mg g–1, whereas the obtained adsorption capacity is 8.28 mg g–1 for the mixture Pb(II) + Zn(II) (Figure a). In contrast, slight changes are observed in the adsorption capacity of Cd(II) when Pb(II) or Zn(II) is included in the solution (Figure b). In the case of Zn(II), a diminution in adsorption capacity is remarkable when Pb(II) or Cd(II) is present (Figure c).

Figure 7

Comparison of the adsorption capacities of (a) Pb(II), (b) Cd(II), and (c) Zn(II) onto AB from single and binary mixtures. C0 = 100 mg L–1, wAB = 0.4 g (<250 μm), t = 15 min, and pH = 5.5.

The competition effect in the adsorption capacity in binary mixtures was calculated by the following relationshipwhere qem and qe0 are the adsorption capacities of the metal ion in a mixture (superscript m = Pb + Cd, Pb + Zn, or Zn + Cd) and single solution (superscript 0), respectively.[39] Thus, when r is higher than unity, the adsorption of the metal ion improves in the mixture or a synergistic effect is observed. In contrast, a value smaller than 1 implies a diminished adsorption capacity or an antagonistic effect of metal ions, and finally r equal to 1 indicates that there is no interaction between two metal ions. The calculated values of r using eq are condensed in Table .

Table 5

Relationship between the Adsorption Capacities from Single and Binary Solutions According to eq a

metal ion	qePb+Cd/qe0	qePb+Zn/qe0	qeCd+Zn/qe0
Pb(II)	1.19	0.74
Cd(II)	1.07		1.05
Zn(II)		0.63	0.75

C0 = 100 mg L–1, wAB = 0.4 g (<250 μm), t = 15 min, pH = 5.5.

According to that described above, the synergistic effect is found for Pb(II) in the presence of Cd(II), obtaining a ratio of 1.19, whereas in the presence of Zn(II), it diminished considerably (r = 0.74). There is no significant competence for the active sites on AB by the other ions over Cd(II) as r is very close to 1. In the case of Zn(II), the presence of Pb(II) or Cd(II) induces an antagonistic effect, obtaining an r value of 0.63 and 0.75, respectively. It is reported in the literature[40] that the hydrated ion radius of Pb (401 pm) which is smaller than that of Cd and Zn (426 and 430 pm, respectively), along with its higher covalent-bonding character, promotes a highly favorable interaction with ligands such as the carboxyl groups of AB, inhibiting the adsorption of other metal ions. This explains the diminution in the adsorption capacity of Zn(II). Similar effects have been found in the adsorption of Pb and Ni mixtures onto coconut shell and water hyacinth[28] or Pb and Cd mixtures onto cucumber peel.[23]

Characterization of Biosorbent and Adsorption Mechanisms

Scanning Electron Microscopy

Figure shows the scanning electron micrographs for AB and the biosorbent after metal ion adsorption at a maximum adsorption capacity. The AB particles shown in Figure a present several ducts in the transversal section, whereas the longitudinal section has a rough surface which promotes a large area of contact to perform the adsorption process. The morphology of AB after the adsorption process is not modified (Figure c,e,g). A SEM/energy-dispersive spectrometry (EDS) analysis was carried out in the samples. For AB, the presence of calcium besides carbon and oxygen is notable (Figure b). The peaks of copper are due to the metallization treatment of the samples to make them conductive on the surface.

Figure 8

Micrographs and SEM/EDS of (a,b) AB, (c,d) AB–Pb, (e,f) AB–Cd, and (g,h) AB–Zn at maximum adsorption capacity.

The calcium percent in AB corresponds to 2.3 wt %, whereas after the adsorption process, values of 0.77, 0.94, and 0.86 wt % were obtained for AB–Pb, AB–Cd, and AB–Zn, respectively (Figure d,f,h). The variation in the calcium percent indicates an ionic exchange mechanism in which calcium is exchanged by the metal ions during the adsorption process. The SEM/EDS mapping of micrographs are included in Figure for AB–Pb, AB–Cd, and AB–Zn. The different elements in the samples were identified by color: carbon, red; oxygen, green; calcium, cyan; and Pb, Cd, or Zn in blue. It is observed that heavy metals are homogeneously distributed in AB, confirming the adsorption of metal ions onto AB, as predicted by the Langmuir model.

Figure 9

SEM/EDS mapping of (a,b) AB–Pb, (c,d) AB–Cd, (e,f) AB–Zn.

X-ray Diffraction

Figure exhibits the XRD patterns of AB and those acquired after the adsorption process at maximum capacity. The peaks observed in the AB patterns at 14.7, 15, and 20° are associated with cellulose.[41] The lignocellulosic component gives a peak at 24.14°,[42] whereas peaks at 29.88, 37.97, and 45.63° were indexed to calcium oxalate monohydrate or whewellite, CaC2O4·H2O (COD 9000763). Similar results were reported by other authors.[43] No peaks were related to lignin because it is an amorphous macromolecule.

Figure 10

XRD patterns for AB, AB–Pb, AB–Cd, and AB–Zn at maximum adsorption capacity.

The difference between the XRD patterns of AB and those after the adsorption of metal ions, AB–Pb, AB–Cd, and AB–Zn, is notable. The pattern of AB–Pb displays a lower intensity of peaks associated with cellulose in comparison with those of AB–Cd and AB–Zn. The metal ions adsorbed on AB induce a variation in the cellulose structure by linking through functional groups, which reflect a change in the peak intensity. The van der Waals halo (broad peak at 15–20°) is also modified. This is attributed to the presence of metal ions on the polymeric structure, reducing the interaction by van der Waals forces, as is mainly observed for Pb(II). The foregoing results confirm that adsorption is carried out by a complexation mechanism through the cellulose groups. Nevertheless, the effect of other compounds such as hemicellulose or lignin on AB must be analyzed by another technique.

With reference to CaC2O4·H2O, all peaks show a decrease in intensity because of the cationic exchange of Ca(II), as has been proven by SEM/EDS (Figure ). This effect indicates that Cd(II) and Zn(II) are mainly adsorbed onto AB by a cationic exchange mechanism. For Pb(II), the adsorption mechanism is also associated to complexation with the carboxyl and hydroxyl groups of cellulose. From the XRD analysis, it is possible to explain that AB has more affinity for Pb(II) because its adsorption is performed by two mechanisms, whereas the Cd(II) and Zn(II) adsorption depends mostly on calcium exchange. It was reported that the adsorption of heavy metals onto sugar beet pulp was brought out by complexation and cationic exchange mechanism.[44]

Solid-State 13C CP/MAS NMR

In order to elucidate the role of AB compounds in the complexation mechanism during the adsorption, further characterization by solid-state 13C CP/MAS NMR was performed. Figure shows the obtained spectrum of AB. The peaks were identified at the carbon atoms of cellulose, lignin, and hemicellulose with the initials C, L, and H, respectively.

Figure 11

Solid-state 13C CP/MAS NMR spectra for AB, AB−Pb, AB−Cd, and AB−Zn at maximum adsorption capacity.

In the case of cellulose, the peak at 104.4 ppm is related to C1 according to the structure in Figure . The signal at 73.5 ppm is attributed to C2 and C3, whereas C5 gives a peak at 71.8 ppm.[45] Unlike XRD, NMR is useful to differentiate the crystalline and amorphous cellulose in AB.[46] Thus, peaks at 88.2 and 64.3 ppm are designed for C4 and C6 of ordered cellulose and linked by hydrogen bonds (crystalline),[41] as identified by XRD (Figure ). Signals at 82.4 and 62.1 ppm correspond to C4′ and C6′ in amorphous cellulose (indicated by quotation marks).[41,45,47]

The hemicellulose gives signals at 104.6, 78.3, 77.0, 75.6, 74.1, 65.7, and 62.1 ppm.[44] At these chemical shifts are observed only the cellulose peaks that overlap with those of hemicellulose because of their similar structure. In addition, the spectrum shows peaks at 173.3 and 20.6 ppm that are characteristic of −COOH and −CH3 in the acetyl groups of hemicellulose.[47]

The region between 120 and 160 ppm is reported as characteristic of carbon in the aromatic ring of lignin. At 153.3 and 150.8 ppm are displayed the peaks for L3 and L5, whereas the signal of the chemical shift at 121.5 ppm corresponds to L4. The methoxy groups −OCH3 are identified by the peak at 55.3 ppm.[47−49] The α, β, and γ carbons in lignin must give signals in the region 60–90 ppm,[49] which clearly are overlapped by the cellulose peaks. CH3 in the acetyl groups in lignin can be assigned also at 20.6 ppm beside hemicellulose, whereas 24.9–32.3 ppm is designated to the α and β carbons in the methylene groups.[41] Finally, the carboxyl groups in lignin give a signal in 167.6–168.3 ppm.

In Figure are also shown the spectra for AB–Pb, AB–Cd, and AB–Zn at maximum adsorption capacity conditions. For better comparison, the AB spectrum was included as a dashed line. After metal ion adsorption, several cellulose peaks in the spectra are shifted to a lower field mainly for C2,3 bonded to the −OH group and C4′. This observation confirms that the metal ion adsorption is performed by the complexation with cellulose by H+ exchange of the hydroxyl group. The modification of the chemical structure of cellulose is perceptible as a split peak assigned to C2,3, mostly evident to Pb(II) and Zn(II) corresponding to the cellulose-free and to the cellulose-linked metal ion. Thus, a lower intensity in this peak is notable. With respect to C4 and C4′, the spectrum AB–Pb does not show any modification in these signals. However, the AB–Cd spectrum presents a change in C4′, whereas AB–Zn is modified in both C4 and C4′. This indicates that only Cd(II) and Zn(II) have interaction with C4 in cellulose, and Cd(II) has a preference by the crystalline over amorphous cellulose. In contrast, Pb(II) and Cd(II) adsorption modified the structure of cellulose in C6 and C6′ unlike Zn(II). The observed changes in the spectra indicate that the polymeric structure of cellulose is altered, reducing their crystallinity. This observation agrees with the results obtained by XRD where a decrease in van der Waals halo is noted.

On the other hand, it is remarkable that a shoulder appears around 101 ppm related to C1 of hemicellulose.[47] In this case, the chemical environment of cellulose was modified with the metal ion adsorption showing the signal of hemicellulose. Novel peaks are observed after Pb(II) and Cd(II) adsorption at 175.9 and 175.1 ppm, respectively. These peaks are attributed to the −COO–M(II) group bonded to the metal ion in hemicellulose, whereas the peak around 173 ppm represents the −COOH groups.

The spectra show evident changes in L4, methoxyl, and carboxyl groups, which are associated with the complexation of metal ions. For Pb(II) and Zn(II), a novel peak was found that can be assigned to the α carbon in lignin.[41]Table presents the assignment of spectra.

Table 6

Assigned Peaks in the Solid-State 13C CP/MAS NMR Spectra for AB, AB–Pb, AB–Cd, and AB–Zn. C: Cellulose; H: Hemicellulose; L: Lignin

	chemical shift, (ppm)
carbon atom	AB	AB–Pb	AB–Cd	AB–Zn
C1	104.4	104.5	104.4	104.6
C2, C3	73.5	74.8	73.8	73.9
C4 (crystalline)	88.2	88.2	88.1	88.3
C4′ (amorphous)	82.4	83.2	83.0	83.1
C5	71.8	71.8	71.6	71.8
C6 (crystalline)	64.3	64.3	64.3	64.5
C6′ (amorphous)	62.1	61.9	61.9	62.0
H–COOH acetyl group	173.3	173.0	173.3	173.6
H–COO–M(II) acetyl group		175.9	175.1
H–CH3 acetyl group	20.6	20.5	20.5	20.7
H–C1 (shoulder)		101.6	101.2	101.0
L3	153.3	152.9		152.6
L5	150.8	150.6	151.6	151.2
L4	121.5
L-α		68.8		68.2
L-OCH3 methoxyl group	55.3	55.2	54.9	55.3
acetyl group	20.6	20.5	20.5	20.7
L-α,β acetyl group	24.9–32.3	29.6	29.6	29.8
L-carboxyl group	167.6–168.3	167.6–168.4	167.5–168.4	167.8–168.5

The solid-state 13C CP/MAS NMR spectra provide important evidence to the complexation mechanism for the metal ion adsorption onto AB. Then, Pb(II) is adsorbed by the cellulose, hemicellulose, and lignin groups. This explains the high affinity of AB for Pb(II). For the adsorption of Cd(II), the three compounds of AB participate in the adsorption process in a lesser grade than Pb(II). Zn(II) is adsorbed by the amorphous cellulose with a scarce contribution by the hemicellulose and lignin groups, which is in agreement with the low adsorption capacity of AB.

Conclusions

The adsorption behavior of AB has been investigated to remove Pb(II), Cd(II), and Zn(II), and the results proved AB to be a potent biosorbent. The adsorption capacity is highly influenced by pH, particle size, dosage, time, and metal ion concentration. The kinetic studies demonstrated that adsorption rapidly proceeds with high removal efficiencies, that is, 99, 93, and 82% for Pb(II), Cd(II), and Zn(II), respectively, in only 5 min and follows the pseudo-second-order equation. The isotherms show a Langmuir behavior with the maximum adsorption capacity of 93.14, 28.50, and 24.66 mg g–1 in the case of Pb(II), Cd(II), and Zn(II), respectively. In binary mixtures, the presence of other ions affect the adsorption capacity of Cd(II) and Zn(II). Finally, better affinity for Pb(II) over Cd(II) and Zn(II) was evidenced from characterization by SEM/EDS and XRD. The results reveal a combined adsorption mechanism by complexation with cellulose functional groups and calcium exchange for Pb(II); meanwhile, Cd(II) and Zn(II) are mainly adsorbed onto AB by ionic exchange.

Experimental Section

Materials

The reagents used in this study were of analytical grade, without further purification. The aqueous solutions of metal ions were prepared by dissolving appropriate amounts of Pb(NO3)2, Cd(NO3)2, and ZnSO4 (J.T. Baker, US) in distilled water to obtain the desired concentration. The solutions were kept at ambient temperature in polyethylene flask until their utilization.

AB was obtained from the State of Jalisco (Mexico) as a residue of tequila spirit production. The content of cellulose, lignin, and hemicellulose in the same AB was reported elsewhere[50] and corresponds to 42 ± 2, 15 ± 1, and 20 ± 1%, respectively. AB was dried in a furnace at 80 °C for 2 h to remove the moisture and to avoid further degradation. Prior to adsorption experiments, the organic compounds were removed according to the standard test method D-1103-96[51] to avoid secondary reactions with metal ions. Then, AB was dried, milled, and sieved in a vibratory sieve shaker Retsch AS-200 (Germany) for 5 min to select the particle size.

Adsorption Behavior

A batch system was performed thrice with 0.4 ± 0.01 g of AB and 45 mL of a solution containing metal ions separately in 100 mg L–1 with magnetic stirring of 300 rpm at 25 °C for 30 min. The pH influence on adsorption capacity was studied in the range of 3–6.5. The pH was measured during all the experiments using an HI2550 equipment (Hanna Instruments, Germany) with a combined electrode, and it was adjusted by the addition of 0.1 mol L–1 NaOH or 0.1 mol L–1 HCl, as required. Four particle size ranges were used: >250, 250–180, 180–150, and <150 μm. Experiments were conducted at different AB dosages in a range of 0.1–0.6 g (<250 μm) at pH 5.5.

For kinetic studies, the adsorption of each metal ion was performed between 5 and 120 min, at pH 5.5, and with 0.4 g of AB (<250 μm). The pseudo-first- and pseudo-second-order models were discussed for experimental data.

The adsorption isotherm for each metal ion in a wide interval of concentration was obtained using 0.4 g AB, for 15 min, and at pH 5.5. For these experiments, solutions at different initial concentrations from 10 to 1000 mg L–1 were used. Langmuir and Freundlich equations were considered to predict the adsorption behavior.

The influence of metal ions on adsorption from binary mixtures was studied using 100 mg L–1 for [Pb(II)–Cd(II), Pb(II)–Zn(II), and Zn(II)–Cd(II)], maintaining the pH at 5.5, using 0.4 g of biosorbent (<250 μm), and holding for 15 min.

After all adsorption experiments, AB was separated, dried for 2 h in a furnace, and reserved for further characterization. The initial and final concentrations of metal ions were quantified using a 3100 (PerkinElmer, USA) atomic absorption spectrometer.

In adsorption systems, the adsorption capacity (qM(II), mg g–1) and the removal efficiency for metal ions [M(II) = Pb(II), Cd(II), or Zn(II)] were calculated at different conditions by eqs and 8, respectivelywhere, V0 and Vf are the initial and final volumes, in liters. C0 and Cf are the initial and final concentrations of metal ions M(II) expressed in mg L–1, whereas wAB is the AB dosage in grams.

Characterization of Biosorbent

The solid addition method was used to determine the point of zero charge (pHPZC) of AB.[52] Several 0.1 mol/L KNO3 solutions were adjusted at pH values between 2 and 8 by adding 0.1 HNO3 or NaOH. Each solution with 0.1 g of AB was placed in a sealed flask under constant stirring (150 rpm). After 48 h, the final pH was measured. The plot of ΔpH (pHf – pH0) against the initial pH (pH0) was made to obtain pHPZC.

AB characterization was carried out before and after the adsorption experiments by SEM, XRD, and solid-state 13C CP/MAS NMR. The morphology of the biosorbent was evaluated by using a scanning electron microscope, JEOL JSM-6400 (US). EDS microanalysis was conducted to identify the elements in AB. The samples were weighed (0.2 g) and characterized with an X-ray diffractometer, Bruker D8 Advance Da Vinci (USA), using Cu Kα radiation at a step size of 0.03°. Nuclear magnetic resonance analysis was performed using CP/MAS for 13C nuclei in a 500 MHz spectrometer (Bruker, USA).