Biosorption of Co²⁺ Ions from Aqueous Solution by K₂HPO₄-Pretreated Duckweed Lemna gibba

Abstract

The wastewater of the many industries that use divalent cobalt (Co²⁺)-containing compounds has elevated levels of this metal. Thus, novel technology is needed to efficiently remove Co²⁺ 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 KH₂PO₄-pretreated Lemna gibba (PLEM) as a biosorbent of Co²⁺ in aqueous solutions tested under different conditions of pH, particle size, and initial Co²⁺ concentration. Kinetic, equilibrium, and thermodynamic studies were conducted. The capacity of biosorption increased with a greater initial Co²⁺ 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 Co²⁺ biosorption kinetics. The Sips and Redlich-Peterson isotherm models best predict the biosorption capacity at equilibrium. According to the thermodynamic study, biosorption of Co²⁺ was endothermic and spontaneous. The effect of pH on the biosorption/desorption of Co²⁺ suggests that electrostatic attraction is the main biosorption mechanism. SEM-EDX verified the presence of Co²⁺ on the surface of the pretreated-saturated biosorbent and the absence of the metal after desorption.

1. 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.

Since low concentrations of cobalt are difficult to remove from water by conventional physicochemical treatments, it is necessary to apply innovative technology characterized by safety, efficiency, and versatility. One alternative is biosorption, a process independent of cell metabolism [7]. This technique, which has been little studied as a remedy for Co²⁺ pollution, can be carried out by living, dead, or inactive biological material [8,9].

Biosorption is a process of capturing heavy metals by physical adsorption (physisorption), ionic interchange, chemisorption (e.g., complexation, coordination, and chelation), and microprecipitation [10]. Diverse biological materials are capable of biosorption, including agroindustrial waste, microbial biomass, and biopolymers. These economical materials are available in great quantities, and the respective processes are environmentally friendly [11,12]. Unlike physicochemical methods, biosorption techniques can efficiently remove low concentrations of metals from aqueous solutions. If biosorption is followed by desorption, the metals can be recovered and the biosorbents regenerated for later use [13].

The current contribution focuses on the biosorbent potential of Lemna gibba, a macrophyte of universal distribution commonly known as duckweed. This plant, which quickly proliferates to double its biomass in about two days, lends itself to the bioremediation of aquatic systems, due to its small size (2–4 mm) and ability to bioaccumulate toxic compounds (e.g., heavy metals) [14]. Because eutrophication has provoked an excessive spread of Lemna gibba, it is now a plague in many places. Its excessive growth in the form of a thick mat on the aquatic body leads to navigation problems, harbors harmful fauna, and prevents sunlight from reaching photosynthetic species in the water below, thus interrupting the correct oxygenation of its environment [15,16]. Apart from being abundantly available, the plant material holds promise as a sustainable biosorbent for treating wastewater contaminated with cobalt and other heavy metals.

According to a previous report, pretreatment of L. gibba with K₂HPO₄ substantially improves the availability of sorption sites on the surface of plant cells, and therefore, their capacity for Co²⁺ biosorption, which is achieved by removing salts and producing a higher negative charge (−35 vs. −26 mV). The zero point of charge (ζ₀) was 2.37 for unpretreated and 1.62 for K₂HPO₄-pretreated Lemna gibba, thus creating a greater attraction in the latter for positively charged Co²⁺. The ATR-FTIR analysis of K₂HPO₄-pretreated Lemna gibba revealed an important role of its hydroxyl and carboxyl groups in the removal of Co²⁺ [17]. The aim of the present study was to analyze the performance of K₂HPO₄-pretreated L. gibba as a biosorbent under distinct conditions of pH, particle size, and the initial concentration of Co²⁺. Various theoretical models were tested to find the best one for describing the experimental data on biosorption. To determine the best eluent solution for desorption, saturated L. gibba was processed with strong and weak acids, as well as some alkaline compounds. Considering that recyclability is a prerequisite for the practical application of biosorption technology, three biosorption/desorption cycles were herein evaluated.

2. Materials and Methods

2.1. Reagents

The reagents employed in the experiments were all of analytical grade (JT Baker^®, Monterrey, Mexico). During the biosorption experiments, the pH of the solutions was maintained constant by adding HCl and NaOH in the solution at a concentration of 0.1 M and 0.01 M, respectively. The different concentrations of Co²⁺ were prepared by making dilutions of a stock solution of CoCl₂·6H₂O (>98% purity) containing 1 g L⁻¹ of Co²⁺.

2.2. Preparation of the Biosorbent

Lemna gibba was collected from the Xochimilco canals in Mexico City (19°15′31.8″ N 99°05′05.3″ W). It was cleaned with running tap water and then deionized water before being dried in a Luzeren^® oven (Proveedor de Laboratorios, Mexico) at 60 °C for 48 h. Afterward, the material was ground in a hammer mill (Glen Creston, Ltd., London, UK) and sieved (U.S. ASTM) to obtain fractions of the biosorbent, each with a particular particle size between 0.3 and 2.0 mm (0.3–0.5, 0.5–0.8, 0.8–1.4, and 1.4–2.0 mm). The fractions were all pretreated with K₂HPO₄. Briefly, 5 g (dry weight) of Lemna gibba per liter were exposed to K₂HPO₄ (0.3 M) at 18 °C for 30 min. During the pretreatment, the material was agitated at 140 rpm in an orbital shaker (All Sheng™, Hangzhou Allsheng Instruments Co, Ltd., Hangzhou, China). Upon completion of the exposure time, the biosorbent was washed with deionized water. When the resulting wash water had a pH near the deionized water being used, the material was dried in an oven at 60 °C for 48 h [17]. Each fraction of dried, pretreated Lemna gibba (PLEM) was stored in a separate, well-labeled, hermetically-sealed bottle at room temperature (rt).

2.3. The Influence of Different Physicochemical Parameters on the Biosorption of Co²⁺ by PLEM

Experiments to evaluate the effect of several physicochemical variables on the biosorption of Co²⁺ by PLEM were carried out in 500 mL Erlenmeyer flasks. They contained 120 mL of a solution with a known concentration of Co²⁺ 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.

Firstly, the pH varied (2, 3, 4, 5, 6, and 7), while maintaining the initial concentration of Co²⁺ (C_ini) at 100 mg L⁻¹ and the particle size of PLEM at 0.3–0.5 mm. Later, distinct particle sizes (0.3–0.5, 0.5–0.8, 0.8–1.4, 1.4–2.0, and 0.3–0.8 mm) were utilized, while maintaining C_ini at 100 mg L⁻¹ and the pH at 7.0. Finally, different initial values of C_ini (10, 20, 40, 60, 80, 100, 200, and 300 mg L⁻¹) were used, while maintaining the pH at 7.0 and the particle size at 0.3–0.8 mm.

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 Co²⁺ by PLEM was calculated at a series of exposure times using Equation (1):

$$q=\frac{V}{M}\left(C_{ini}-C\right)$$

(1)

where q (mg g⁻¹) is the capacity of biosorption of Co²⁺, 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 Co²⁺ 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 Co²⁺ 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.

2.4. Kinetic Modeling of the Biosorption of Co²⁺ by PLEM

For the kinetic modeling of the biosorption of Co²⁺ by PLEM, the equations of pseudo-first-order, pseudo-second-order, and fractional power were employed (

Table 1. Biosorption models were tested.

Kinetic Models	Equation	Parameters
Pseudo-first-order [18]	$q=q_{eq1}\left[1-e^{\left(-k_{1}t\right)}\right]$	k1—pseudo-first-order sorption velocity constant (min−1) qeq1—equilibrium biosorption capacity predicted by the model (mg g−1)
Pseudo-second-order [18]	$q=\frac{t}{\left(\frac{1}{k_2\ast {q_{eq2}}^2}\right)+\left(\frac{t}{q_{eq}}\right)}$	k2—pseudo-second-order sorption velocity constant (g mg−1 min−1) qeq2—equilibrium biosorption capacity predicted by the model (mg g−1)
Fractional power [18]	$q=k_{FP}{t}^v$	kFP—constant of the model (mg g−1) ν—velocity constant (h−1)
Isothermal models	Equation	Parameters
Langmuir [19,20]	$q_{eq}=\frac{q_{mL}{b}_L{C}_{eq}}{1+b_L{C}_{eq}}$ $R_L=\frac{1}{1+b_L{C}_{ini}}$	qmL—maximum biosorption capacity determined by Langmuir (mg g−1) bL—Langmuir constant, linked to affinity for the active sites (L mg−1) Cini—initial concentration (mg L−1) RL—separation factor
Freundlich [19]	$q_{eq}=k_F{C}_{eq}{}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$nF$}\right.}$	kF—Freundlich constant, related to the biosorption capacity (mg g−1 (mg L−1)−1/nF) nF—Freundlich constant, linked to the intensity of sorption
Sips [19]	$q_{eq}=\frac{q_{mSP}{k}_{SP}{C}_{eq}{}^{n_{SP}}}{1+k_{SP}{C}_{eq}{}^{n_{SP}}}$	qmSP—maximum biosorption capacity, determined by Sips (mg g−1) kSP—constant of the model (mg L−1)−nSP nSP—exponent of the model
Redlich-Peterson [19]	$q_{eq}=\frac{k_{RP}{C}_{eq}}{1+a_{RP}{C}_{eq}{}^{b_{RP}}}$	kRP—constant of the model (L g−1) aRP—constant of the model (mg L−1) −bRP bRP—exponent of the model

).

2.5. Biosorption Isotherm Studies at Different Temperatures

In 125 mL flasks were poured 30 mL of solutions of Co²⁺ 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 Co²⁺ 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).

2.6. Determination of the Thermodynamic Parameters

The thermodynamic parameters examined were the changes in Gibbs free energy (ΔG⁰, J mol⁻¹), in standard entropy (ΔS⁰, J mol⁻¹ K⁻¹), and in standard enthalpy (ΔH⁰, 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]:

$$K_d=\frac{q_{eq}}{C_{eq}}$$

(2)

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 K₀ (K₀ 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]:

$$\Delta G^0=-RTLn{K}_0$$

(3)

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 (ΔS⁰) was found by Equation (4):

$$\Delta S^0=\frac{\partial \Delta G^0}{\partial T}$$

(4)

The slope of the graph of ΔG⁰ vs. T indicates the mean value of ΔS⁰. The change in the standard enthalpy was furnished by Equation (5):

$$\Delta G^0=\Delta H^0+T\Delta S^0$$

(5)

2.7. Desorption of Co²⁺ 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 Co²⁺ (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 Co²⁺ from PLEM, diverse solutions were tested as eluents: Water at rt (H₂O rt, the control), water at 60 °C (H₂O 60 °C), various acidic solutions (HCl, H₂SO₄, HNO₃, C₂H₂O₄, KH₂PO₄, and NH₄Cl) and three alkaline compounds (NaOH, NaHCO₃, and K₂HPO₄). 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]:

$$E_D(\%)=\frac{V(C_D-C_{ini})}{M{q}_{eq}}\times 100$$

(6)

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 Co²⁺ eluted from the solution at time t, respectively, and q_eq (mg g⁻¹) is the amount of Co²⁺ retained per gram of biosorbent (determined experimentally). The results of the percentage of the desorption were compared to select the adequate solution for eluting Co²⁺ from PLEM.

2.8. Biosorption-Desorption Cycles

PLEM was saturated with Co²⁺ 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 Co²⁺ (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 Co²⁺ 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.

2.9. Scanning Electron Microscope Coupled to Energy-Dispersive X-ray Spectroscopy (SEM-EDX)

The possible changes in the structure and composition of the surface of PLEM, due to the process of biosorption and the posterior desorption of Co²⁺ were explored on a scanning electron microscope (SEM). The three types of samples of PLEM (unexposed to Co²⁺, 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.

2.10. Analytical Methods

Co²⁺ was quantified by the dimethylglyoxime (DMG) method, with which a compound is formed with an intensity of color proportional to the concentration of Co²⁺ 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 Co²⁺ was established by constructing metal-type curves with at least 10 distinct known concentrations.

2.11. 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 (R²), 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).

3. Results and Discussion

No change in the concentration of Co²⁺ 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 Co²⁺ from the aqueous solution can be fully attributed to the effect of biosorption produced by PLEM.

3.1. 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 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 (ζ₀) [17]. When the pH of a solution is less than that found at ζ₀, 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 ζ₀, 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 Co²⁺ 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 Co²⁺ 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 Co²⁺ until reaching the equilibrium dynamic. It was observed that as the pH increased, the biosorbent removed more Co²⁺, 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 Co²⁺ 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. Kinetic parameters of the biosorption of Co²⁺ by PLEM at various pH values of the solution (C_ini = 100 mg L⁻¹, particle size = 0.3–0.5 mm).

Parameter	pH
	4.0	5.0	6.0	7.0
qeq (mg g−1)	12.35 ± 0.09	17.85 ± 0.08	29.78 ± 0.18	40.13 ± 0.18
teq (h)	0.25	0.25	0.25	0.5
Pseudo-first-order
qeq1 (mg g−1)	12.19 ± 0.15	17.51 ± 0.13	29.20 ± 0.22	38.80 ± 0.26
k1 (h−1)	18.81 ± 0.98	25.03 ± 1.11	41.79 ± 2.32	34.96 ± 1.59
R2	0.9740	0.9757	0.9699	0.9777
ASE	17.570	33.360	104.200	139.400
Sy.x	0.6468	0.7860	1.389	1.607
AICc	−33.79	−22.54	41.25	57.55
Pseudo-second-order
qeq2 (mg g−1)	13.69 ± 0.22	18.61 ± 0.11	30.57 ± 0.16	40.86 ± 0.09
k2 (g mg−1 h−1)	1.88 ± 0.17	2.28 ± 0.10	2.51 ± 0.13	1.50 ± 0.03
R2	0.975	0.990	0.989	0.998
ASE	16.81	13.75	35.23	12.31
Sy.x	0.6327	0.5046	0.8077	0.4775
AICc	−35.73	−72.18	−19.50	−78.37
Fractional power
kFP (mg g−1)	15.03 ± 0.56	18.37 ± 0.33	30.61 ± 0.39	40.85 ± 0.52
v (h−1)	0.211 ± 0.02	0.107 ± 0.01	0.073 ± 0.007	0.086 ± 0.008
R2	0.758	0.676	0.667	0.7293
ASE	68.31	133.90	190.00	342.90
Sy.x	1.341	1.637	1.949	2.619
AICc	55.70	80.49	73.88	104.6

. None of the kinetic models employed fit the experimental results at pH 2 or 3, probably owing to the minimal biosorption of Co²⁺ 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 (R²) 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 R² 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.

3.2. 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. Kinetic parameters of the biosorption of Co²⁺ by PLEM, using different particle sizes (C_ini = 100 mg L⁻¹, pH = 7.0).

Parameter	Particle Size (mm)
	0.3–0.5	0.5–0.8	0.8–1.4	1.4–2	0.3–0.8
qeq (mg g−1)	40.13 ± 0.18	39.77 ± 0.25	33.38 ± 0.17	32.25 ± 0.07	40.05 ± 0.16
teq (h)	0.5	0.75	1.0	1.0	0.5
Pseudo-first-order
qeq1 (mg g−1)	38.80 ± 0.26	38.01 ± 0.29	31.74 ± 0.32	30.91 ± 0.31	38.76 ± 0.25
k1 (h−1)	34.96 ± 1.59	36.02 ± 1.98	13.66 ± 0.62	12.41 ± 0.52	35.08 ± 1.56
R2	0.978	0.968	0.969	0.973	0.979
ASE	139.4	191.9	154.2	130.8	134.0
Sy.x	1.607	1.885	1.690	1.556	1.575
AICc	57.55	75.43	63.20	53.97	55.32
Pseudo-second-order
qeq2 (mg g−1)	40.86 ± 0.09	40.07 ± 0.10	34.45 ± 0.23	33.64 ± 0.19	40.80 ±0.09
k2 (g mg−1 h−1)	1.50 ± 0.03	1.55 ± 0.04	0.61 ± 0.02	0.56 ± 0.02	1.52 ± 0.09
R2	0.998	0.998	0.991	0.994	0.998
ASE	12.31	14.66	46.04	27.98	11.73
Sy.x	0.478	0.521	0.923	0.720	0.466
AICc	−78.37	−68.58	−4.504	−32.39	−81.09
Fractional power
kFP (mg g−1)	40.85 ± 0.52	40.18 ± 0.45	33.06 ± 0.58	32.02 ± 0.59	40.78 ± 0.52
v (h−1)	0.09 ± 0.01	0.09 ± 0.01	0.17 ± 0.01	0.18 ± 0.01	0.08 ± 0.01
R2	0.729	0.781	0.819	0.814	0.723
ASE	342.9	256.4	422.2	453.9	348.1
Sy.x	2.619	2.265	2.906	3.013	2.638
AICc	104.6	89.47	115.4	119.2	105.4

).

The biosorption of Co²⁺ 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 Co²⁺ 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 Co²⁺ 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 Co²⁺ 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 Co²⁺ 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 Co²⁺ 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 (R²) and lower error functions (ASE, Sy.x, and AICc) than the other two models.

3.3. The Effect of the Initial Co²⁺ 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 Co²⁺ 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. Kinetic parameters of the biosorption of Co²⁺ by PLEM at various initial concentrations of the metal (particle size = 0.3–0.8 mm, pH = 7.0).

Parameter	Cini (mg L−1)
	10	20	40	60	80	100	200	300
qeq (mg g−1)	5.46 ± 0.16	12.18 ± 0.20	20.20 ± 0.13	29.22 ± 0.19	36.44 ± 0.48	40.05 ± 0.16	44.22 ± 0.31	46.17 ± 0.41
teq (h)	0.05	0.08	0.16	0.2	0.5	0.5	0.75	0.75
Pseudo-first-order
qeq1 (mg g−1)	5.71 ± 0.03	12.44 ± 0.07	19.95 ± 0.14	28.64 ± 0.23	34.53 ± 0.41	38.76 ± 0.25	41.56 ± 0.46	43.81 ± 0.42
k1 (h−1)	24.01 ± 0.85	25.00 ± 0.81	32.93 ± 1.58	34.58 ± 1.91	35.81 ± 2.94	35.08 ± 1.56	33.50 ± 2.53	37.68 ± 2.58
R2	0.9853	0.9871	0.9741	0.9666	0.9288	0.9785	0.9371	0.9514
ASE	2.324	8.894	43.08	113.9	356.2	134.0	456.9	384.9
Sy.x	0.2075	0.4058	0.8932	1.452	2.568	1.575	2.909	2.670
AICc	−171.7	−96.58	−8.221	46.20	110.1	55.32	124.0	114.4
Pseudo-second-order
qeq2 (mg g−1)	6.05 ± 0.09	13.15 ± 0.09	20.97 ± 0.10	30.13 ± 0.15	36.52 ± 0.27	40.80 ± 0.09	44.03 ± 0.26	46.19 ± 0.23
k2 (g mg−1 h−1)	6.85 ± 0.76	3.37 ± 0.20	2.85 ± 0.13	2.06 ± 0.09	1.65 ± 0.11	1.52 ± 0.03	1.27 ± 0.07	1.40 ± 0.07
R2	0.9456	0.9830	0.9915	0.9915	0.9804	0.9981	0.9882	0.9912
ASE	8.617	11.74	14.17	29.02	98.25	11.73	85.78	69.72
Sy.x	0.3995	0.4663	0.5122	0.7331	1.349	0.4660	1.260	1.136
AICc	−98.35	−81.02	−70.50	−30.34	37.94	−81.09	30.34	18.74
Fractional power
kFP (mg g−1)	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
v (h−1)	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
R2	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

. 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 (k₂) 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 k₂ between two initial concentrations of Co²⁺ (C_ini): 10 and 300 mg L⁻¹. The corresponding values for t_eq were 0.05 and 0.75 h, while those for k₂ 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 k₂ and a longer time necessary to reach equilibrium, which is in agreement with previous reports on the biosorption of metallic ions [33,35].

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 (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 Co²⁺ 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 (R², ASE, Sy.x, and AICc) for the models of isotherms (

Table 5. Isotherms for the biosorption of Co²⁺ by PLEM.

Parameter	Temperature (°C)
	18	30	40	50	60
qm exp (mg g−1)	46.17 ± 0.41	46.33 ± 0.19	47.73 ± 0.12	48.49 ± 0.21	49.35 ± 0.22
Langmuir
bL (L mg−1)	0.047 ± 0.005	0.046 ± 0.005	0.046 ± 0.004	0.045 ± 0.003	0.048 ± 0.006
qmL (mg g−1)	51.12 ± 1.49	52.66 ± 1.82	52.78 ± 1.48	54.97 ± 1.27	55.57 ± 1.91
RL	0.508 − 0.068	0.509 − 0.073	0.508 − 0.075	0.513 − 0.076	0.498 − 0.071
R2	0.993	0.990	0.993	0.996	0.990
ASE	13.65	19.18	12.9	8.856	21.46
Sy.x	1.508	1.788	1.466	1.215	1.891
AICc	16.28	18.99	15.82	12.81	19.89
Freundlich
kF (mg g−1 (L g−1)1/nF)	11.05 ± 2.65	10.82 ± 2.66	10.6 ± 2.39	10.94 ± 2.43	11.71 ± 2.80
nF	3.64 ± 0.70	3.504 ± 0.67	3.433 ± 0.59	3.422 ± 0.58	3.53 ± 0.67
R2	0.924	0.922	0.938	0.939	0.926
ASE	140.7	150.4	120.7	126	161
Sy.x	4.842	5.006	4.484	4.582	5.179
AICc	34.94	35.47	33.71	34.05	36.01
Sips
kSP (L g−1)	0.022 ± 0.005	0.018 ± 0.005	0.022 ± 0.005	0.026 ± 0.004	0.023 ± 0.008
qmSP (mg g−1)	47.55 ± 1.026	48.25 ± 1.065	48.79 ± 1.122	51.57 ± 0.975	51.55 ± 1.677
nSP	1.295 ± 0.086	1.367 ± 0.095	1.294 ± 0.092	1.224 ± 0.066	1.304 ± 0.131
R2	0.998	0.998	0.998	0.999	0.996
ASE	3.713	4.38	4.055	2.504	9.674
Sy.x	0.8617	0.936	0.900	0.7077	1.391
AICc	15.19	16.51	15.9	12.04	22.85
Redlich-Peterson
kRP (L g−1)	1.822 ± 0.096	1.789 ± 0.126	2.006 ± 0.222	1.964 ± 0.099	1.996 ± 0.178
aRP (L mg−1)bRP	0.017 ± 0.003	0.014 ± 0.004	0.023 ± 0.009	0.019 ± 0.004	0.015 ± 0.006
bRP	1.142 ± 0.029	1.171 ± 0.044	1.095 ± 0.057	1.116 ± 0.028	1.162 ± 0.054
R2	0.999	0.998	0.996	0.999	0.997
ASE	2.111	4.197	7.774	1.824	7.069
Sy.x	0.6498	0.9162	1.247	0.604	1.189
AICc	10.68	16.17	21.1	9.507	20.34

).

Regarding the isotherm models of two parameters, the Langmuir model afforded the best correlation coefficient (R² > 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 (R² > 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 Co²⁺ previously reported for diverse biosorbents, the value found in the current study reveals an excellent capacity for PLEM (

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

Material	Biosorption Capacity (mg g−1)	pH	Reference
Cocos nucifera leaf	3.69	5	[39]
Spent coffee	5.37	6	[40]
Rhytidiadelphus squarrosus	7.38	6	[41]
Natural hemp fibers	13.58	4.5–5	[26]
Activated carbon from hazelnut shells	13.88	6	[42]
Alginate from Callithamnion corymbosum sp.	18.79	4.4	[43]
Sargassum wightii	20.63	4.5	[44]
Carbonized lemon peel	22.00	6	[45]
Watermelon rind	23.30	5	[46]
Prunus dulcis bio-char	27.86	7	[36]
Teak leaves	29.48	5	[47]
MgCl2-pretreated Ficus carica leaves	33.9	6	[48]
Acacia nilotica	35.45	5	[37]
NaOH-treated lemon peels	35.71	6	[49]
Almond green hull	45.50	ND	[50]
PLEM	46.47	7	The present study
Corn silk	82.04	6	[25]
NaOH-pretreated Mangifera indica leaves	114	5	[51]

ND, no data.

). Thus, it is an attractive biosorbent for the detoxification of water contaminated with Co²⁺.

3.5. Thermodynamic Parameters

Graphs were constructed to find the thermodynamic parameters, ΔG⁰ (Figure 3a), ΔH⁰, and ΔS⁰ (Figure 3b), and the corresponding values were determined (

Table 7. Thermodynamic parameters of biosorption of Co²⁺ by PLEM.

Temperature °C	ΔG0 (J mol−1)	ΔH0 (J mol−1)	ΔS0 (J mol−1 K−1)
18	−684.8	2461.3	10.81 ± 1.89
30	−804.3	2471.6
40	−825.0	2558.8
50	−959.4	2532.6
60	−1172.1	2427.9

).

The Gibbs free energy (ΔG⁰) values are negative for the biosorption of Co²⁺ by PLEM (Table 7), suggesting a spontaneous process. The biosorption has been reported to improve as the temperature rises [22]. The positive values of ΔH⁰ 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 ΔH⁰_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 (ΔS⁰) 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.

3.6. Desorption

The elution of Co²⁺ 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⁺.

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.

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 ΔH⁰_prom (2.49 KJ mol⁻¹) 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 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].

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 (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 Co²⁺.

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 of Co²⁺.

The micrograph of PLEM saturated with Co²⁺ (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⁺ 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⁺ from aqueous solutions. The capacity of biosorption of Co²⁺ 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⁺ 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⁺ 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⁺ by PLEM and the posterior desorption of the plant material by means of its exposure to 0.1 M HCl.