Biosorption of Co 2 + Ions from Aqueous Solution by K 2 HPO 4 -Pretreated Duckweed Lemna gibba

: The wastewater of the many industries that use divalent cobalt (Co 2 + )-containing compounds has elevated levels of this metal. Thus, novel technology is needed to e ﬃ ciently remove Co 2 + ions from aqueous solutions. Biosorption is a low-cost technique capable of removing heavy metals from contaminated water. This study aims to evaluate the performance of KH 2 PO 4 -pretreated Lemna gibba ( PLEM ) as a biosorbent of Co 2 + in aqueous solutions tested under di ﬀ erent conditions of pH, particle size, and initial Co 2 + concentration. Kinetic, equilibrium, and thermodynamic studies were conducted. The capacity of biosorption increased with a greater initial Co 2 + concentration and was optimal at pH 7.0 and with small-sized biosorbent particles (0.3–0.8 mm). The pseudo-second-order sorption model best describes the experimental data on Co 2 + biosorption kinetics. The Sips and Redlich-Peterson isotherm models best predict the biosorption capacity at equilibrium. According to the thermodynamic study, biosorption of Co 2 + was endothermic and spontaneous. The e ﬀ ect of pH on the biosorption / desorption of Co 2 + suggests that electrostatic attraction is the main biosorption mechanism. SEM-EDX veriﬁed the presence of Co 2 + on the surface of the pretreated-saturated biosorbent and the absence of the metal after desorption.


Introduction
Excessive population growth, urbanization, and industrial development have increased the pollution of the planet and altered ecosystems. Of all environmental pollution, the contamination of water is the most worrisome because of affecting the primordial element on which life is based. The main source of water pollution is the discharge of industrial wastewater with diverse toxic substances, among which heavy metals are of particular concern [1].
Cobalt is a heavy metal found in the Earth's crust, being a natural component of volcanic emissions, as well as surface and subterranean water. It is released into the environment through anthropogenic activities: Burning fossil fuels, applying fertilizers, mining, electroplating, manufacturing batteries, and producing commodities with industrial processes involving cobalt-containing compounds, among others.
Although cobalt is an essential nutrient in human metabolism and the principal component of vitamin B12 [2], it is harmful to our health beyond trace levels, competing with other elements that constitute integral parts of a proper metabolic function [3]. In excess, it can give rise to skin irritation and problems in bone development, as well as respiratory, cardiac, thyroid, liver, and gastric disorders [4,5]. Due to being hazardous to humans and ecosystems [6], cobalt-contaminated wastewater should be treated prior to being released into the environment.

The Influence of Different Physicochemical Parameters on the Biosorption of Co 2+ by PLEM
Experiments to evaluate the effect of several physicochemical variables on the biosorption of Co 2+ by PLEM were carried out in 500 mL Erlenmeyer flasks. They contained 120 mL of a solution with a known concentration of Co 2+ at a predetermined pH value. Subsequently, an addition was made of 0.12 g of PLEM at a certain particle size, thus achieving a biosorbent concentration of 1 g (dry weight) L −1 . The suspensions were left at 18 • C (rt) for 2 h under constant agitation at 140 rpm in an orbital shaker (All Sheng™, Hangzhou Allsheng Instruments Co, Ltd., Hangzhou, China). The pH of the solutions was adjusted to the desired value and maintained constant throughout the assay by adding 0.1 M HCl and 0.01 M NaOH.
During the experiment, samples were taken at various exposure times and filtered to afford a solution free of biosorbent. The filtrate of each flask was diluted properly for the posterior quantification of the cobalt concentration. From the values obtained, the biosorption capacity of Co 2+ by PLEM was calculated at a series of exposure times using Equation (1): where q (mg g −1 ) is the capacity of biosorption of Co 2+ , V (L) is the total volume of the solution, M (g) is the biosorbent mass, and C ini and C (mg L −1 ) correspond to the initial concentration of Co 2+ in the solution and its concentration at time t (h), respectively. When the system reaches equilibrium, t = t eq , C = C eq and q = q eq . Based on the values of biosorption capacity found, the most suitable pH of the solution and the best particle size for the removal of Co 2+ were selected for the rest of the biosorption experiments. For each of the parameters examined, controls free of biosorbent were established and analyzed for possible changes in the concentration of cobalt.

Kinetic Modeling of the Biosorption of Co 2+ by PLEM
For the kinetic modeling of the biosorption of Co 2+ by PLEM, the equations of pseudo-first-order, pseudo-second-order, and fractional power were employed (Table 1). Table 1. Biosorption models were tested.

Isothermal models Equation Parameters
Langmuir [19,20] q eq = q mL b L C eq 1+b L C eq R L = 1 1+b L C ini q mL -maximum biosorption capacity determined by Langmuir (mg g −1 ) b L -Langmuir constant, linked to affinity for the active sites (L mg −1 ) C ini -initial concentration (mg L −1 ) R L -separation factor Freundlich [19] q eq = k F C eq 1 /nF k F -Freundlich constant, related to the biosorption capacity (mg g −1 (mg L −1 ) −1/nF ) n F -Freundlich constant, linked to the intensity of sorption Sips [19] q eq = q mSP k SP C eq n SP 1+k SP C eq n SP q mSP -maximum biosorption capacity, determined by Sips (mg g −1 ) k SP -constant of the model (mg L −1 ) −nSP n SP -exponent of the model Redlich-Peterson [19] q eq = k RP C eq 1+a RP C eq b RP k RP -constant of the model (L g −1 ) a RP -constant of the model (mg L −1 ) −bRP b RP -exponent of the model

Biosorption Isotherm Studies at Different Temperatures
In 125 mL flasks were poured 30 mL of solutions of Co 2+ at distinct concentrations (20,40,60,80,100,200, and 300 mg L −1 ), adjusting the pH to 7.0. Then 0.03 g of PLEM (particle size = 0.3-0.8 mm) was placed in each flask to ensure a concentration of 1 g L −1 of PLEM. The suspensions were left for 2 h at 18, 30, 40, 50, or 60 • C to reach biosorption equilibrium. Subsequently, the samples from each flask were filtered, and the residual concentration of Co 2+ was quantified in each filtrate. With the experimental results of the biosorption capacity found at equilibrium (q eq ) and the residual concentration of cobalt at equilibrium (C eq ) for each initial concentration of metal assayed (C ini ), the isotherm for adsorption was calculated. It was then possible to select the best mathematical model for describing the experimental behavior. With this objective in mind, models of two (Langmuir and Freundlich) and three parameters (Sips and Redlich-Peterson) were used (Table 1).

Determination of the Thermodynamic Parameters
The thermodynamic parameters examined were the changes in Gibbs free energy (∆G 0 , J mol −1 ), in standard entropy (∆S 0 , J mol −1 K −1 ), and in standard enthalpy (∆H 0 , J mol −1 ). With the data on the isotherms for biosorption at equilibrium, the coefficient of distribution (K d , L g −1 ) was obtained for each temperature and concentration assayed using Equation (2) [21]: In the graph of Ln K d vs. C eq for each temperature, the point at which the ordinate crosses the origin corresponds to Ln K 0 (K 0 being the sorption constant at equilibrium, L g −1 ). These values were substituted in Equation (3) to find the change in Gibbs free energy [22]: where R is the constant of the ideal gases (8.315 J mol −1 K −1 ), and T is the absolute temperature (K) during biosorption. The change in standard entropy (∆S 0 ) was found by Equation (4): The slope of the graph of ∆G 0 vs. T indicates the mean value of ∆S 0 . The change in the standard enthalpy was furnished by Equation (5):

Desorption of Co 2+ from the Biosorbent
To evaluate desorption, the biosorbent was first saturated by exposing PLEM (1 g L −1 , with a particle size of 0.3-0.8 mm) to a solution of Co 2+ (300 mg L −1 , pH 7.0, rt) under constant agitation at 140 rpm for 2 h. Upon completion of this time, the biosorbent was washed with deionized water several times to eliminate the excess cobalt and then dried in an oven at 60 • C for 48 h. Finally, it was stored in hermetically-sealed bottles until further use.
For the desorption of Co 2+ from PLEM, diverse solutions were tested as eluents: Water at rt (H 2 O rt, the control), water at 60 • C (H 2 O 60 • C), various acidic solutions (HCl, H 2 SO 4 , HNO 3 , C 2 H 2 O 4 , KH 2 PO 4 , and NH 4 Cl) and three alkaline compounds (NaOH, NaHCO 3 , and K 2 HPO 4 ). The concentration of all compounds was 0.1 M. Desorption was carried out by placing 120 mL of one of the distinct eluent solutions in each Erlenmeyer flask and adding the saturated biosorbent at a concentration of 1 g L −1 . The material was maintained under constant agitation at 140 rpm and 18 • C for 2 h, collecting and filtering samples from each of the flasks at different times. The concentration of desorbed metal on each filtrate was quantified. The percentage of desorption at time t was calculated with Equation (6) [23]: where C ini and C D (mg L −1 ) are the initial concentration of metal in the solution (t = 0 h) and the concentration of Co 2+ eluted from the solution at time t, respectively, and q eq (mg g −1 ) is the amount of Co 2+ retained per gram of biosorbent (determined experimentally). The results of the percentage of the desorption were compared to select the adequate solution for eluting Co 2+ from PLEM.

Biosorption-Desorption Cycles
PLEM was saturated with Co 2+ for 2 h, as described in the previous section. Upon completion of this time, samples of the solution were taken to assess the biosorption capacity of PLEM in the first stage (Equation (1)). Subsequently, the saturated biosorbent was washed, dried, and subjected to the desorption of Co 2+ (as already explained) by putting 1 g L −1 of the material in a solution with the selected eluent and leaving it under constant agitation at 140 rpm and rt for 2 h. Samples were then taken to quantify the concentration of Co 2+ in the solution and calculate the percentage of desorption for the first cycle (Equation (6)). PLEM was washed with deionized water and dried at 60 • C for 48 h to be submitted to posterior cycles. Three cycles of biosorption/desorption were carried out under the same conditions, allowing for the comparison of the capacity of biosorption and percentage of desorption from one cycle to another. The possible changes in the structure and composition of the surface of PLEM, due to the process of biosorption and the posterior desorption of Co 2+ were explored on a scanning electron microscope (SEM). The three types of samples of PLEM (unexposed to Co 2+ , saturated, and desorbed in the first cycle) were dried for 24 h at 60 • C. Subsequently, they were covered with carbon to be later observed with a JEOL high-resolution scanning electron microscopy (HR-SEM) (model JSM7800F, Jeol Ltd., Tokyo, Japan) with an acceleration voltage of 5 kV.

Analytical Methods
Co 2+ was quantified by the dimethylglyoxime (DMG) method, with which a compound is formed with an intensity of color proportional to the concentration of Co 2+ present in the solution [24]. The measurement of absorbance was conducted in a Spectronic Genesys UV/Vis 10 spectrophotometer (Thermo Electron Scientific Instruments Corp, Madison, WI, USA) at 400 nm. The concentration of Co 2+ was established by constructing metal-type curves with at least 10 distinct known concentrations.

Statistical Analysis
Each experiment was performed independently at least twice, and the determinations of residual cobalt were made at least three times, with the aim of attaining the appropriate statistical power. Data are expressed as the mean ± standard deviation (SD) of the values obtained experimentally.
Regarding the values from the kinetics of biosorption and the experimental biosorption capacity at equilibrium (q eq ), differences between groups were examined with two-way ANOVA and Tukey's test (with a confidence interval of α = 0.05) on the GraphPad Prism ® Ver 8.4 program 2020 (GraphPad Software Inc, San Diego, CA, USA). The kinetic and equilibrium parameters were scrutinized by nonlinear regression on the same software, selecting the best model in accordance with a variety of error functions: The correlation coefficient (R 2 ), the absolute sum of squares (ASE), the standard deviation of the residuals (Sy.x) and Akaike's information criterion (AICc). The data from the three cycles of biosorption/desorption were compared with one-way ANOVA and Dunnett's test (confidence interval, α = 0.05) on the GraphPad Prism ® Ver 8.4 program 2020 (GraphPad Software Inc., San Diego, CA, USA).

Results and Discussion
No change in the concentration of Co 2+ was found for the PLEM-free solutions, used as controls for the evaluation of the influence of the physicochemical conditions herein tested. Thus, the removal of Co 2+ from the aqueous solution can be fully attributed to the effect of biosorption produced by PLEM.

The Effect of pH
The level of pH is one of the physicochemical factors that most influence the biosorption of heavy metals [25]. The pH values of 2-7 were presently employed because the precipitation of cobalt was observed experimentally as of pH 8, likely due to the formation of cobalt hydroxide [26,27]. At each pH value, the biosorption capacity was evaluated with respect to time (Figure 1a). With the pH at 2 or 3, the cobalt removal capacity was near 0. The level of pH is one of the physicochemical factors that most influence the biosorption of heavy metals [25]. The pH values of 2-7 were presently employed because the precipitation of cobalt was observed experimentally as of pH 8, likely due to the formation of cobalt hydroxide [26,27]. At each pH value, the biosorption capacity was evaluated with respect to time (Figure 1a). With the pH at 2 or 3, the cobalt removal capacity was near 0. The sorption capacity was enhanced with each increment in pH from 4 to 7, which can be easily explained by considering the pH of the plant material (1.67), which results in zero point of charge (ζ 0 ) [17]. When the pH of a solution is less than that found at ζ 0 , the net charge of the surface of the biosorbent is positive. Hence, an electrostatic repulsion exists between the positive charge of both the metal ions and the surface of the biosorbent [28]. In contrast, when a solution has a pH value above that at ζ 0 , the net charge of the surface of the biosorbent is negative, and there is an attraction with the positively charged metal ion [29]. A pH value of 5-7 herein afforded the fastest biosorption of Co 2+ during the first 10 min (0.17 h) of the experiment. After this time, however, the velocity of removal of the metal decreased until reaching equilibrium, at which point the velocity of net transfer was 0. The initial rapid biosorption was due to the greater number of sites on PLEM available for the uptake of the sorbate and the higher concentration of Co 2+ in the aqueous solution. As time passed, the available sites and the concentration of free cobalt ions were both diminished, leading to a gradual decline in the velocity of the removal of Co 2+ until reaching the equilibrium dynamic. It was observed that as the pH increased, the biosorbent removed more Co 2+ , and therefore, required more time to reach equilibrium (t eq ). The same phenomenon has been reported for the effect of pH on the biosorption of other divalent metal ions [29].
A summary of the of Co 2+ removal capacity at experimental equilibrium (q eq ), the time to reach equilibrium (t eq ), and the values of the parameters and error functions for each model and at each pH value assayed are provided in Table 2. None of the kinetic models employed fit the experimental results at pH 2 or 3, probably owing to the minimal biosorption of Co 2+ under these conditions. At pH 4, a reduction in the removal capacity was only found after 0.75 h (Figure 1a), a time period not included in the process of biosorption. Hence, the corresponding data was not considered when determining the values of the parameters for the kinetic models. With a pH of 4-7, the pseudo-second-order model had the highest correlation coefficient (R 2 ) and the lowest values for ASE, Sy.x, and AICc compared to the other two models (pseudo-first-order and fractional power). The Elovich model was also evaluated, but is not listed in the tables because the R 2 was too small, and the parameters obtained had exaggerated SD values. Given that a pH of 7 produced the greatest biosorption capacity at equilibrium, this value was used for further testing.

The Effect of Particle Size
Particle size is a physical property that affects the surface area of contact between a sorbent and the liquid phase, thus playing a key role in biosorption [30,31]. When the particle size is reduced, the area of contact is amplified, and the sites of sorption are more accessible, generating a better capacity, efficiency, and velocity of biosorption and a decrease in the time to reach equilibrium (Figure 1b). The present results are in agreement with previous reports of an enhanced biosorption capacity as the particle size diminishes, considering particles from 0.3 to 2.0 mm (Table 3). Table 3. Kinetic parameters of the biosorption of Co 2+ by PLEM, using different particle sizes (C ini = 100 mg L −1 , pH = 7.0).

Parameter
Particle Size (mm) 0.3-0.5 0. The biosorption of Co 2+ was not significantly different (p > 0.05) between the size intervals of 0.3-0.5 mm and 0.5-0.8 mm. Therefore, a kinetic study was carried out to remove Co 2+ by PLEM at a particle size of 0.3-0.8 mm. The statistical analysis with two-way ANOVA and Tukey's test indicated the lack of significant difference (p > 0.05) between the equilibrium biosorption capacity q eq values of the samples with the following three particle sizes: 0.3-0.5, 0.5-0.8 mm, and 0.3-0.8 mm. The Co 2+ biosorption rate was slightly faster (as expected) at the smaller particle size range (0.3-0.5 mm), reaching equilibrium at 0.5 h. The particle size range of 0.5-0.8 mm achieved equilibrium in a longer period of time (0.75 h), probably due to the greater surface area available with a smaller particle size, leading to faster binding of Co 2+ ions to the surface of the biosorbent. With a particle size range of 0.3-0.8 mm, the time required to reach equilibrium (t eq ) of Co 2+ biosorption by PLEM was 0.5 h, similar to the time found for the smallest particles tested (0.3-0.5 mm).
One advantage of employing a particle size of 0.3-0.8 mm is that it is possible to utilize fixed-bed columns packed with the material. Volesky [32] suggested using a particle size of 0.4-0.7 mm, since smaller sizes could obstruct the bed and provoke a drop in pressure. Additionally, particles of 0.3-0.8 mm (but not smaller) allow for the application of more biosorbent material. If the particle size range is under 0.3 mm, pretreatment is more difficult. Hence, a particle size of 0.3-0.8 mm was chosen for the rest of the experiments. The experimental results of the Co 2+ removal capacity at equilibrium (q eq ) were compared to the parameters of the kinetic models assayed ( Table 3). As can be appreciated, the equation of the pseudo-second-order model shows a higher correlation coefficient (R 2 ) and lower error functions (ASE, Sy.x, and AICc) than the other two models.

The Effect of the Initial Co 2+ 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 −1 generated an 8.46-fold rise (from 5.46 to 46.17 mg g −1 ) 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 Co 2+ 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 (q eq ), the time required to reach equilibrium (t eq ), 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 Co 2+ .
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 t eq and k 2 between two initial concentrations of Co 2+ (C ini ): 10 and 300 mg L −1 . The corresponding values for t eq were 0.05 and 0.75 h, while those for k 2 were 6.847 and 1.402 g mg −1 h −1 , respectively (Table 4). Thus, an increase in the initial concentration of cobalt led to a decrease in k 2 and a longer time necessary to reach equilibrium, which is in agreement with previous reports on the biosorption of metallic ions [33,35]. Table 4. Kinetic parameters of the biosorption of Co 2+ by PLEM at various initial concentrations of the metal (particle size = 0.3-0.8 mm, pH = 7.0).

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 (q m ) 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 (q eq ) with respect to the concentration of the sorbent at equilibrium (C eq ). The relation between the experimental isotherms and those predicted by the theoretical models for the biosorption of Co 2+ by PLEM at different temperatures is shown in Figure 2.

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 Co 2+ by PLEM at different temperatures is shown in Figure 2.   The maximum experimental sorption capacity (q m 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). Regarding the isotherm models of two parameters, the Langmuir model afforded the best correlation coefficient (R 2 > 0.99) and the smallest error functions. The value of the separation factor (R L ) reflects the nature of biosorption, which is considered unfavorable with R L ≥ 1, favorable with 0 < R L < 1, an irreversible if R L = 0 [38]. The values of R L calculated presently indicate that biosorption is favorable (0.07 < R L < 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 2 > 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 (q mSP = 47.55 to 51.55 mg g −1 ) at the five temperatures herein employed were very close to the experimental data (q m exp = 46.17 to 49.35 mg g −1 ). Compared to the capacity for the biosorption of Co 2+ 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 Co 2+ . ND, no data.

Thermodynamic Parameters
Graphs were constructed to find the thermodynamic parameters, ΔG 0 (Figure 3a), ΔH 0 , and ΔS 0 (Figure 3b), and the corresponding values were determined (Table 7).    The Gibbs free energy (∆G 0 ) values are negative for the biosorption of Co 2+ by PLEM (Table 7), suggesting a spontaneous process. The biosorption has been reported to improve as the temperature rises [22]. The positive values of ∆H 0 show an endothermic biosorption, which is consistent with the enhanced biosorption capacity (q m exp ) presently found at higher temperatures ( Table 5). The change in the mean calculated standard enthalpy was ∆H 0 prom = 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 (∆S 0 ) reveals a high affinity of Co 2+ 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 Co 2+ by PLEM is spontaneous and favorable, allowing this material to be utilized for the removal of metal from polluted water.

Desorption
The elution of Co 2+ after its sorption by PLEM was tested with various acids and bases ( Figure 4). Overall, the strong acids (HCl, HNO 3 , and H 2 SO 4 ) 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 Co 2+ . The Gibbs free energy (ΔG 0 ) values are negative for the biosorption of Co 2+ by PLEM (Table 7), suggesting a spontaneous process. The biosorption has been reported to improve as the temperature rises [22]. The positive values of ΔH 0 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 ΔH 0 prom = 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 (ΔS 0 ) reveals a high affinity of Co 2+ 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 Co 2+ by PLEM is spontaneous and favorable, allowing this material to be utilized for the removal of metal from polluted water.

Desorption
The elution of Co 2+ 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 Co 2+ . 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 Co 2+ by PLEM. On the other hand, a high concentration of H + ions in the acid solutions could cause competition with Co 2+ 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 Co 2+ suggests that the main biosorption mechanism is electrostatic attraction, a physical process between negatively charged groups of the biosorbent and the positive charge of Co 2+ . The thermodynamic value of ΔH 0 prom (2.49 KJ mol −1 ) indicates a physisorption process, which reinforces the idea of electrostatic attraction being the principal mechanism of biosorption. 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 Co 2+ by PLEM. On the other hand, a high concentration of H + ions in the acid solutions could cause competition with Co 2+ 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 Co 2+ suggests that the main biosorption mechanism is electrostatic attraction, a physical process between negatively charged groups of the biosorbent and the positive charge of Co 2+ . The thermodynamic value of ∆H 0 prom (2.49 KJ mol −1 ) indicates a physisorption process, which reinforces the idea of electrostatic attraction being the principal mechanism of biosorption.

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 Processes 2020, 8, 1532 16 of 21 way to dispose of the biosorbent once it is no longer useful. Few such studies have been reported for the biosorption/desorption of Co 2+ [9,43].
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 Co 2+ removal from one cycle to the next [55]. However, PLEM maintained an elevated capacity of Co 2+ removal throughout the three cycles. During all three cycles, moreover, Co 2+ was completely desorbed (E D = 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 Co 2+ .

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 way to dispose of the biosorbent once it is no longer useful. Few such studies have been reported for the biosorption/desorption of Co 2+ [9,43].
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 Co 2+ removal from one cycle to the next [55]. However, PLEM maintained an elevated capacity of Co 2+ removal throughout the three cycles. During all three cycles, moreover, Co 2+ 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 Co 2+ .

Scanning Electron Microscopy Coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDX)
The SEM-EDX analysis of PLEM before exposure to Co 2+ (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 Co 2+ . The EDX spectra of PLEM evidences a surface free of Co 2+ .

Scanning Electron Microscopy Coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDX)
The SEM-EDX analysis of PLEM before exposure to Co 2+ (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 Co 2+ . The EDX spectra of PLEM evidences a surface free of Co 2+ .