*3.3. The E*ff*ect of the Initial Co2*<sup>+</sup> *Concentration*

The initial concentration of metallic ions is an important variable because it significantly affects the biosorption capacity and the time to reach equilibrium [33]. A boost in the initial concentration of the metal from 10 to 300 mg L−<sup>1</sup> generated an 8.46-fold rise (from 5.46 to 46.17 mg g−<sup>1</sup> ) in the biosorption capacity at equilibrium (Figure 1c). Increasing the initial concentration of the sorbate, while maintaining the concentration of the biosorbent constant likely amplified the driving force behind sorption (the transfer of the cobalt ions from the aqueous solution to the surface of the biosorbent), a consequence of the higher gradient of concentration. Moreover, there is a greater probability of Co2<sup>+</sup> binding to the active sites available in the sorbent, which would bring about a better biosorption capacity [34]. The experimental data on biosorption capacity at equilibrium (*qeq*), the time required to reach equilibrium (*teq*), and the values of the parameters of the kinetic models and their corresponding error functions are listed in Table 4. Of the theoretical models applied to the data, the pseudo-second-order model gave the values closest to those found experimentally (as occurred with the other environmental variables) for the distinct initial concentrations of Co2+.

The sorption velocity (*k*2) is a kinetic parameter known to be related to the time to reach equilibrium, and therefore, depends on the initial concentration of the metal. The analysis of the kinetic parameters with two-way ANOVA and multiple comparisons by Tukey's test revealed a significant difference in relation to *teq* and *k*<sup>2</sup> between two initial concentrations of Co2<sup>+</sup> (*Cini*): 10 and 300 mg L−<sup>1</sup> . The corresponding values for *teq* were 0.05 and 0.75 h, while those for *k*<sup>2</sup> were 6.847 and 1.402 g mg−<sup>1</sup> h −1 , respectively (Table 4). Thus, an increase in the initial concentration of cobalt led to a decrease in *k*<sup>2</sup> and a longer time necessary to reach equilibrium, which is in agreement with previous reports on the biosorption of metallic ions [33,35].



*kFP* (mg g−1

*v* (h−1 )

R

2

)

*AICc* −98.35 −81.02 −70.50 −30.34 37.94 −81.09 30.34 18.74 **Fractional power** 5.89 ± 0.16 12.92 ± 0.26 20.84 ± 0.32 30.05 ± 0.42 36.85 ± 0.36 40.78 ± 0.53 44.33 ± 0.41 46.49 ± 0.46

0.099 ± 0.02 0.099 ± 0.01 0.083 ± 0.009 0.083 ± 0.008 0.095 ± 0.005 0.085 ± 0.007 0.098 ± 0.005 0.088 ± 0.006

 0.4581 0.5935 0.6402 0.6845 0.8463 0.7226 0.8675 0.8184 *ASE* 31.46 84.08 128.5 218.8 157.8 348.1 206.0 269.9 *Sy.x* 0.7932 1.297 1.603 2.092 1.777 2.638 2.030 2.323 *AICc* −19.63 31.48 53.53 81.21 64.23 105.4 78.08 92.13

#### *3.4. Biosorption Isotherm Studies at Various Temperatures 3.4. Biosorption Isotherm Studies at Various Temperatures*

To understand the sorbate-sorbent interaction, it is crucial to assess the isotherm of biosorption and model it at several temperatures. This approach also allows for the prediction of the maximum biosorption capacity of the sorbent (*qm*) and consequently a comparison of distinct sorbents (a prerequisite for the design of an adsorption system) [36,37]. Biosorption at equilibrium was established by examining the variation of the biosorption capacity at equilibrium (*qeq*) with respect to the concentration of the sorbent at equilibrium (*Ceq*). The relation between the experimental isotherms and those predicted by the theoretical models for the biosorption of Co2<sup>+</sup> by *PLEM* at different temperatures is shown in Figure 2. To understand the sorbate-sorbent interaction, it is crucial to assess the isotherm of biosorption and model it at several temperatures. This approach also allows for the prediction of the maximum biosorption capacity of the sorbent (*qm*) and consequently a comparison of distinct sorbents (a prerequisite for the design of an adsorption system) [36,37]. Biosorption at equilibrium was established by examining the variation of the biosorption capacity at equilibrium (*qeq*) with respect to the concentration of the sorbent at equilibrium (*Ceq*). The relation between the experimental isotherms and those predicted by the theoretical models for the biosorption of Co2+ by *PLEM* at different temperatures is shown in Figure 2.

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**Figure 2.** Isotherms for the adsorption of Co2+ by *PLEM* at the following temperatures: (**a**) 18 °C, (**b**) 30 °C, (**c**) 40 °C, (**d**) 50 °C, and (**e**) 60 °C (pH = 7.0, particle size = 0.3–0.8 mm). **Figure 2.** Isotherms for the adsorption of Co2<sup>+</sup> by *PLEM* at the following temperatures: (**a**) 18 ◦C, (**b**) 30 ◦C, (**c**) 40 ◦C, (**d**) 50 ◦C, and (**e**) 60 ◦C (pH = 7.0, particle size = 0.3–0.8 mm).

The maximum experimental sorption capacity (*qm exp*) was determined at each temperature, as were the values of the other parameters and the error functions (*R* 2 , *ASE*, *Sy.x*, and *AICc*) for the models of isotherms (Table 5).


**Table 5.** Isotherms for the biosorption of Co2<sup>+</sup> by *PLEM*.

Regarding the isotherm models of two parameters, the Langmuir model afforded the best correlation coefficient (*R* <sup>2</sup> > 0.99) and the smallest error functions. The value of the separation factor (*RL*) reflects the nature of biosorption, which is considered unfavorable with *R<sup>L</sup>* ≥ 1, favorable with 0 < *R<sup>L</sup>* < 1, an irreversible if *R<sup>L</sup>* = 0 [38]. The values of *R<sup>L</sup>* calculated presently indicate that biosorption is favorable (0.07 < *R<sup>L</sup>* < 0.5) at all temperatures assayed.

On the other hand, each of the models of three parameters (Sips and Redlich-Peterson) provided a higher correlation coefficient (*R* <sup>2</sup> > 0.996) and lower error functions than the models of two parameters. Overall, the Redlich-Peterson model gave the lowest error functions. The values of maximum biosorption capacity predicted by the isotherm of Sips (*qmSP* = 47.55 to 51.55 mg g−<sup>1</sup> ) at the five temperatures herein employed were very close to the experimental data (*qm exp* = 46.17 to 49.35 mg g−<sup>1</sup> ). Compared to the capacity for the biosorption of Co2<sup>+</sup> previously reported for diverse biosorbents, the value found in the current study reveals an excellent capacity for *PLEM* (Table 6). Thus, it is an attractive biosorbent for the detoxification of water contaminated with Co2+.


**Table 6.** Capacity for the biosorption of Co2<sup>+</sup> by different materials. Spent coffee 5.37 6 [40]

**Material Biosorption Capacity (mg g<sup>−</sup>1) pH Reference**  *Cocos nucifera* leaf 3.69 5 [39]

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**Table 6.** Capacity for the biosorption of Co2+ by different materials.

ND, no data.

#### *3.5. Thermodynamic Parameters 3.5. Thermodynamic Parameters*

Graphs were constructed to find the thermodynamic parameters, ∆*G* 0 (Figure 3a), ∆*H*<sup>0</sup> **,** and ∆*S* 0 (Figure 3b), and the corresponding values were determined (Table 7). Graphs were constructed to find the thermodynamic parameters, Δ*G*0 (Figure 3a), Δ*H*0**,** and Δ*S*<sup>0</sup> (Figure 3b), and the corresponding values were determined (Table 7).

**Figure 3.** Graphs based on the values of (**a**) Ln *Kd* vs. *Ceq* and (**b**) Δ*G*0 vs. *T*, which were used to calculate the thermodynamic parameters of biosorption of Co2+ by *PLEM*. **Figure 3.** Graphs based on the values of (**a**) Ln *K<sup>d</sup>* vs. *Ceq* and (**b**) ∆*G* <sup>0</sup> vs. *T*, which were used to calculate the thermodynamic parameters of biosorption of Co2<sup>+</sup> by *PLEM*.

**Table 7.** Thermodynamic parameters of biosorption of Co2<sup>+</sup> by *PLEM*.


The Gibbs free energy (∆*G* 0 ) values are negative for the biosorption of Co2<sup>+</sup> by *PLEM* (Table 7), suggesting a spontaneous process. The biosorption has been reported to improve as the temperature

60 −1172.1 2427.9

rises [22]. The positive values of ∆*H*<sup>0</sup> show an endothermic biosorption, which is consistent with the enhanced biosorption capacity (*qm exp*) presently found at higher temperatures (Table 5). The change in the mean calculated standard enthalpy was ∆*H*<sup>0</sup> prom = 2.49 KJ mol−<sup>1</sup> . A value below 40 kJ mol−<sup>1</sup> is indicative of a process of physisorption [21]. The positive value of standard entropy (∆*S* 0 ) reveals a high affinity of Co2<sup>+</sup> for *PLEM* [22], and thus, the probability that the metal promotes structural changes in the biosorbent. Hence, the process of biosorption likely increases the degree of disorder of the whole system [25,52]. According to the values of the thermodynamic parameters, adsorption of Co2<sup>+</sup> by *PLEM* is spontaneous and favorable, allowing this material to be utilized for the removal of metal from polluted water. enhanced biosorption capacity (*qm exp*) presently found at higher temperatures (Table 5). The change in the mean calculated standard enthalpy was Δ*H*0prom = 2.49 KJ mol−1. A value below 40 kJ mol−1 is indicative of a process of physisorption [21]. The positive value of standard entropy (Δ*S*0) reveals a high affinity of Co2+ for *PLEM* [22], and thus, the probability that the metal promotes structural changes in the biosorbent. Hence, the process of biosorption likely increases the degree of disorder of the whole system [25,52]. According to the values of the thermodynamic parameters, adsorption of Co2+ by *PLEM* is spontaneous and favorable, allowing this material to be utilized for the removal of metal from polluted water.

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rises [22]. The positive values of Δ*H*<sup>0</sup> show an endothermic biosorption, which is consistent with the

#### *3.6. Desorption 3.6. Desorption*  The elution of Co2+ after its sorption by *PLEM* was tested with various acids and bases (Figure

The elution of Co2<sup>+</sup> after its sorption by *PLEM* was tested with various acids and bases (Figure 4). Overall, the strong acids (HCl, HNO3, and H2SO4) were the best eluent solutions, giving superior desorption percentages (>94%) compared to the weak acids (<65%) or alkaline compounds (<20%). Water, whether at rt or 60 ◦C, was not capable of eluting more than 10% of Co2+. 4). Overall, the strong acids (HCl, HNO3, and H2SO4) were the best eluent solutions, giving superior desorption percentages (>94%) compared to the weak acids (<65%) or alkaline compounds (<20%). Water, whether at rt or 60 °C, was not capable of eluting more than 10% of Co2+.

**Figure 4.** Kinetics of desorption of Co2+ from *PLEM* with distinct eluent solutions. **Figure 4.** Kinetics of desorption of Co2<sup>+</sup> from *PLEM* with distinct eluent solutions.

Thus, the biosorbent was positively charged at the pH of acid solutions, resulting in an electrostatic repulsion with the sorbate [53]. Accordingly, physisorption seems to play a key role in the process of biosorption of Co2+ by *PLEM*. On the other hand, a high concentration of H+ ions in the acid solutions could cause competition with Co2+ for these sorption sites, favoring ionic interchange, and consequently, the desorption process [54]. Since 0.1 M HCl was the eluent with the greatest percentage of desorption (100%), the biosorbent was eluted with this solution in posterior assays. Thus, the biosorbent was positively charged at the pH of acid solutions, resulting in an electrostatic repulsion with the sorbate [53]. Accordingly, physisorption seems to play a key role in the process of biosorption of Co2<sup>+</sup> by *PLEM*. On the other hand, a high concentration of H<sup>+</sup> ions in the acid solutions could cause competition with Co2<sup>+</sup> for these sorption sites, favoring ionic interchange, and consequently, the desorption process [54]. Since 0.1 M HCl was the eluent with the greatest percentage of desorption (100%), the biosorbent was eluted with this solution in posterior assays.

The effect of pH on the biosorption/desorption of Co2+ suggests that the main biosorption mechanism is electrostatic attraction, a physical process between negatively charged groups of the biosorbent and the positive charge of Co2+. The thermodynamic value of Δ*H*0prom (2.49 KJ mol−1) indicates a physisorption process, which reinforces the idea of electrostatic attraction being the principal mechanism of biosorption. The effect of pH on the biosorption/desorption of Co2<sup>+</sup> suggests that the main biosorption mechanism is electrostatic attraction, a physical process between negatively charged groups of the biosorbent and the positive charge of Co2+. The thermodynamic value of ∆*H*<sup>0</sup> prom (2.49 KJ mol−<sup>1</sup> ) indicates a physisorption process, which reinforces the idea of electrostatic attraction being the principal mechanism of biosorption.

#### *3.7. Biosorption-Desorption Cycles*

Considering the indispensable requirement of recyclability for the practical application of a biosorbent, an evaluation of the cycles of biosorption/desorption is necessary to assure that the material can be regenerated in a cost-effective manner [23]. Additionally, insights are provided as to the best

of Co2+.

*3.7. Biosorption-Desorption Cycles* 

way to dispose of the biosorbent once it is no longer useful. Few such studies have been reported for the biosorption/desorption of Co2<sup>+</sup> [9,43]. the best way to dispose of the biosorbent once it is no longer useful. Few such studies have been reported for the biosorption/desorption of Co2+ [9,43].

material can be regenerated in a cost-effective manner [23]. Additionally, insights are provided as to

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The biosorption capacity of *PLEM* in the first cycle (46.17 <sup>±</sup> 0.41 mg g−<sup>1</sup> ) was diminished 8.53% in the second cycle and a cumulative 17.89% by the end of the third cycle (Figure 5a), representing significant differences. Hence, the eluent herein employed (0.1 M HCl) could have damaged the composition and structure of the biosorbent, affecting the sorption sites and reducing the capacity of Co2<sup>+</sup> removal from one cycle to the next [55]. However, *PLEM* maintained an elevated capacity of Co2<sup>+</sup> removal throughout the three cycles. During all three cycles, moreover, Co2<sup>+</sup> was completely desorbed (*E<sup>D</sup>* = 100%) from the biosorbent (Figure 5b), evidencing its recyclability. After the end of its useful life, *PLEM* can be integrated into compost with null impact on the environment because of not containing any Co2+. The biosorption capacity of *PLEM* in the first cycle (46.17 ± 0.41 mg g−1) was diminished 8.53% in the second cycle and a cumulative 17.89% by the end of the third cycle (Figure 5a), representing significant differences. Hence, the eluent herein employed (0.1 M HCl) could have damaged the composition and structure of the biosorbent, affecting the sorption sites and reducing the capacity of Co2+ removal from one cycle to the next [55]. However, *PLEM* maintained an elevated capacity of Co2+ removal throughout the three cycles. During all three cycles, moreover, Co2+ was completely desorbed (*ED* = 100%) from the biosorbent (Figure 5b), evidencing its recyclability. After the end of its useful life, *PLEM* can be integrated into compost with null impact on the environment because of not containing any Co2+.

**Figure 5.** (**a**) Capacity of biosorption and (**b**) the percentage of desorption of Co2+ by *PLEM* during three cycles of biosorption/desorption. **Figure 5.** (**a**) Capacity of biosorption and (**b**) the percentage of desorption of Co2<sup>+</sup> by *PLEM* during three cycles of biosorption/desorption.

#### *3.8. Scanning Electron Microscopy Coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDX) 3.8. Scanning Electron Microscopy Coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDX)*

The SEM-EDX analysis of *PLEM* before exposure to Co2+ (Figure 6a) reveals a course and porous surface with agglomerations of the biosorbent. Hence, the surface is characterized by an ample exposure of the active sites for the capture of Co2+. The EDX spectra of *PLEM* evidences a surface free The SEM-EDX analysis of *PLEM* before exposure to Co2<sup>+</sup> (Figure 6a) reveals a course and porous surface with agglomerations of the biosorbent. Hence, the surface is characterized by an ample exposure of the active sites for the capture of Co2+. The EDX spectra of *PLEM* evidences a surface free of Co2+.

**Figure 6.** SEM and EDX micrograph of *PLEM* during the first biosorption/desorption cycle*:* (**a**) Before exposure to Co2+, (**b**) saturated with Co2+, and (**c**) subsequent to desorption of the biosorbent with 0.1 M HCl.

The micrograph of *PLEM* saturated with Co2<sup>+</sup> (Figure 6b) shows a more homogenous surface (on which rectangular particles are dispersed) than *PLEM* prior to contact with Co2+. The following desorption with 0.1 M HCl (Figure 6c), the appearance of the surface of *PLEM* is similar to that observed before exposure to Co2+. In the EDX spectrum, two peaks corresponding to Co2<sup>+</sup> indicate its presence after the biosorption step (Figure 6b). The absence of such peaks after desorption (Figure 6c) evidenced the adequacy of the eluent solution for the total recovery of the metallic ion. Consequently, HCl was able to regenerate the biosorbent for posterior cycles of biosorption/desorption.

#### **4. Conclusions**

The results demonstrate that *PLEM* is an attractive, economical, sustainable, and environmentally friendly material for the removal of Co2<sup>+</sup> from aqueous solutions. The capacity of biosorption of Co2<sup>+</sup> by *PLEM* was enhanced by smaller particle size, a greater pH of the solution, and a higher initial concentration of the metal. The main mechanism of removal of Co2<sup>+</sup> from the aqueous solution is physisorption based on electrostatic attraction. While the kinetics of the experimental biosorption data were adequately described by the pseudo-second-order model, the isotherms of biosorption at equilibrium at different temperatures were properly predicted by the Sips and Redlich-Peterson models. According to the thermodynamic study, the biosorption of Co2<sup>+</sup> by *PLEM* is an endothermic and spontaneous process. The best eluent solution for the recovery of both the metal and the biosorbent material was 0.1 M HCl. *PLEM* can be used for at least three cycles of biosorption/desorption, with a high capacity of biosorption and complete desorption in each cycle, revealing the recyclability of the material, and therefore, the possibility of its economical use. The SEM-EDX analysis confirmed the biosorption of Co2<sup>+</sup> by *PLEM* and the posterior desorption of the plant material by means of its exposure to 0.1 M HCl.

**Author Contributions:** Conceptualization, L.M.-B. and E.C.-U.; methodology, J.L.R.-L., L.M.-B. and E.C.-U.; software, J.L.R.-L.; validation, J.L.R.-L. and L.M.-B.; formal analysis, L.M.-B. and E.C.-U.; investigation, J.L.R.-L.; resources, L.M.-B. and E.C.-U.; writing—original draft preparation, review and editing, L.M.-B. and E.C.-U.; visualization, L.M.-B.; supervision, L.M.-B. and E.C.-U.; project administration, L.M.-B. and E.C.-U.; funding acquisition, L.M.-B. and E.C.-U. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Instituto Politécnico Nacional, Secretaría de Investigación y Posgrado, project number: 20201814.

**Acknowledgments:** The authors are grateful for the technical support provided by the Centro de Nanociencias y Micro y Nanotecnologías, IPN. The CONACyT awarded a graduate scholarship to the coauthor J.L.R.-L., L.M.-B. and E.C.-U. hold grants from EDI-IPN, COFAA-IPN, and SNI-CONACYT. The authors thank Bruce Allan Larsen for proofreading the manuscript.

**Conflicts of Interest:** The authors declare that they have no conflict of interest.

### **References**


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