Synthesis of a Chemically Modified Biosorbent Based on the Invasive Plant Leucaena leucocephala and Its Application in Metformin Removal

Abstract

The present study investigated the use of a biosorbent produced from Leucaena leucocephala pods for the removal of metformin from aqueous solutions. The pods were subjected to chemical and thermal treatments and were referred to as L. leucocephala modified, which was characterized using scanning electron microscopy (SEM). The parameters investigated in the sorption process were temperature, contact time, adsorbent dosage, pH, and initial metformin concentration. The experimental data were in accordance with the Langmuir isothermal model. The maximum adsorption capacity reached was 56.18 mg g⁻¹ at 313 K. In the kinetic study, stability was achieved in 300 min, with 53.24% removal, and the pseudo-first-order model agreed well with the experimental data. The thermodynamic parameters indicated a spontaneous, favorable, and exothermic reaction. The presence of NaCl, CaCl₂, and MgCl₂ negatively affected metformin adsorption. Thus, the importance of the study was that a developed material showed promising results in the removal of metformin, particularly because it is an innovative material, and there are no studies in the literature on drug removal using L. leucocephala.

1. Introduction

The technological development of contemporary society has advanced in different areas of everyday life, such as communication, locomotion, and the use of increasingly specialized therapeutic treatments for various pathologies. Paradoxically, the modern lifestyle is a great driver of scientific progress while also making society increasingly dependent on the indiscriminate use of various chemical substances. In this sense, the detection of synthetic compounds originating from anthropogenic activities in surface and groundwater is already a reality in several locations around the world [1].

The scientific community has signaled its constant concern about the subject since the so-called emerging compounds (EC) are capable of compromising the quality of the aquatic ecosystem, even at low concentrations [2]. Such compounds have diverse natures and include pesticides, dyes, sweetening substances, plasticizing agents, hormones, personal care products, and drugs. Pharmaceutical compounds, in particular, are a class of substances considered environmentally recalcitrant in aqueous matrices, given their high degree of stability, and, therefore, their toxic effects must be strongly taken into account [3].

The access routes of pharmaceutical metabolites to the environment are diverse and include wastewater from livestock properties as well as urban, hospital, and industrial wastewater [4]. It is estimated that, ultimately, the pharmaceutical sector has a consumption rate of about 22% of fresh water in its processes compared to other sources, thus being a very extensive source of release of such compounds into the environment through effluents not treated or treated inefficiently [5].

Among many commercialized compounds, metformin (MET) is the most prescribed oral hypoglycemic factor in the world for the treatment of type 2 diabetes mellitus (DM2) [6]. According to the International Diabetes Federation (2013), the number of people diagnosed with DM2 in 2011 was 382 million worldwide, and an estimated 592 million will be carriers of the disease by the year 2035. In 2019, metformin was ranked fourth among the most prescribed drugs in the United States (Agency for Healthcare Research and Quality 2022).

As MET is not metabolized by the human body and is excreted almost completely unchanged in the urine and feces [7], the high level of consumption of the drug by the population has led to its frequent detection in the wastewater pathway system, contaminating the hydric resources that are intended for potable water supplies [8]. Moreover, its discharge into water can expose aquatic organisms to unwanted genetic and physiologic disturbances, such as malformation, feminization, and infertility [9]. This is a relevant issue, as it is an emerging contaminant with high levels in the environment [10,11].

As already reported in many papers, conventional wastewater treatment plants are still not prepared to remove EC from water [12]. For such purposes, several methodologies have been used to ensure drug removal from contaminated water. Adsorption is a technique that has stood out because it is simple, economical, and easy to perform [13]. A great advantage of this process is the wide variety of materials that can be employed as adsorbents, including agricultural wastes from different origins, geomaterials, and minerals. In order to improve the contaminants’ adsorption capacity, different approaches to physical or chemical modifications have been applied to these materials, such as acidic, basic, thermal, or steam activation [14]. Regarding MET wastewater removal through plant-based adsorbents, there are reports in the literature of the use of Moringa oleifera seed husks functionalized with iron oxide nanoparticles [15], activated carbon from Zea mays tassels [16], orange peel [17], and chichá-do-cerrado (Sterculia striata St. Hil. et Naud) fruit shells [18].

In this context, Leucaena leucocephala is a leguminous species tree originating from Central America that has been explored for many uses, such as a cellulose source for the paper industry, firewood, and protein biomass for animal feed enrichment [19]. However, in some locations, as in Brazil, it is considered an invasive plant due to toxins that inhibit the development of endemic species [20].

In the last decade, L. leucocephala has received considerable attention from researchers. The total number of articles published using different parts of the shrub is approximately 928, and most of them are published mainly in agricultural, energy, environmental, medicinal, and pharmacology journals [21]. Focusing on the environmental field, L. leucocephala has been widely investigated as a coagulant agent [22] and biosorbent for contaminant removal from wastewater, such as heavy metals (Pb²⁺, Cd²⁺ and Ni²⁺) [23,24] and malachite green dye [25]. The L. leucocephala pod has already been used by the adsorption process to remove contaminants such as hexavalent chromium and malachite green dye [25,26].

Considering the circular economy perspective, the use of L. leucocephala as a biosorbent for pollutant removal from water can be a promising application for agricultural wastes or an option in places where the plant has become invasive [21]. Therefore, the investigation of the potential of L. leucocephala bark for pharmaceutical removal from an aqueous solution such as MET is extremely pertinent and urgent, given the actual scenario. With this objective, L. leucocephala bark was submitted to chemical and thermal treatment. The modifications were characterized through MEV and zeta potential, Fourier-transform infrared spectroscopy (FTIR), structural analysis (BET), and N₂ fission process analysis. The MET adsorption performance was evaluated by the analysis of variations in solution pH, adsorbate dosage, adsorbent mass, and stirring speed. The obtained experimental data was submitted to the adjustment of mathematical models. Thus, the objective of the present work was to develop a new bioadsorbant for metformin removal.

2. Materials and Methods

2.1. L. leucocephala Seed Preparation

L. leucocephala material was prepared using the modified method of Akhtar et al. (2007). The in natura pods were washed with tap water and deionized water for the removal of debris. Then, they were oven-dried at 105 °C for 24 h.

The L. leucocephala seeds were subjected to chemical treatment by being kept in contact with 1 M methanol (CH₃OH) and 1 M nitric acid (HNO₃). Subsequently, heat treatment of the material was performed using a muffle oven (Oven Jung 10.012) at 300 °C for 1 h. The objective of chemical and thermal treatment is to increase the surface area, pore volume, and porosity of the pods. After chemical and thermal treatments, the material was called L. leucocephala modified (LLMO).

2.2. LLMO Characterization

2.2.1. SEM

For scanning electron microscopy (SEM), LLMO were fixed on a support that was coated with gold metallization and observed with a high-resolution double-beam electron microscope. FEI SCIOS (Quanta 250 FEI, Eindhoven, The Netherlands) was used at a voltage of 15 kV.

2.2.2. Zeta Potential Analyzer

A particle analyzer, Delsa^TMNanoC (Beckman Coulter, Brea, CA, USA), was used to analyze the surface load of the material, and a pH between 2 and 12 was used to verify which would be the isoelectric point and to observe the cationic and anionic charges.

2.2.3. FTIR and BET

Fourier-transform infrared spectroscopy (FTIR) was performed using a Vertex 70 v (Bruker, Billerica, MA, USA) to analyze the functional groups of the studied adsorbent, and the Brunauer–Emmett–Teller (BET) method was employed using adsorption/desorption isotherms.

2.3. Metformin Samples

The metformin solutions were prepared with distilled water and different concentrations of metformin (MET, PA > 99%, Sigma-Aldrich, St. Louis, MO, USA) (5–200 mg L⁻¹). The solution was diluted with the aid of a magnetic stirrer, in which the masses of the metformin standard were weighed and consequently diluted in a 1 L volumetric flask using ultra-pure water.

2.3.1. Adsorption Experiments

Metformin was used in this study (MET, PA > 99%, Sigma-Aldrich) and adsorption tests were carried out in a shaker (Tecnal TE-4200) at 150 rpm and 25 °C. For that, different masses of adsorbent (0.01–0.05 g) were stirred in contact with 30 mL of MET solutions (w/v) at different concentrations (5–200 mg L⁻¹) [18,27]. The parameters that were considered for the adsorption experiments are listed in

Table 1. Adsorption experiments evaluated parameters.

pH	Temperature (°C)	Metformin Concentration (mg L−1)	Adsorbent Mass (g)	Stirring Speed (rpm)
7	25	10	0.01, 0.02, 0.03, 0.04 and 0.05	150
4, 7, and 10	25	10	0.03	150
7	25	10	0.03	150
7	25, 35, and 45	5–200	0.03	150

.

2.3.2. Effect of Biosorbent Concentration and pH

Initially, preliminary tests were controlled using 0.03 g LLMO in 30 mL of 10 mg L⁻¹ metformin solution in constant rotation of 150 rpm at 25 °C for 24 h. After this test, the effect of the biosorbent mass on 0.01, 0.02, 0.03, 0.04, and 0.05 g was determined. In addition, the effects of pH were determined using acid, neutral, and alkaline pH (4, 7, and 10).

2.3.3. Kinetic and Equilibrium Study

The procedure was performed with 0.01 g LLMO in 30 mL of 10 mg g⁻¹ metformin solution maintained at an inspiration rate of 150 rpm, pH 7, and temperature of 25 °C. Aliquots were taken at specific intervals from 1 to 1440 min, and transferred to airtight jars. After withdrawing each sample, the solutions were filtered through a 0.45 µm cellulose acetate membrane. Final readings were submitted using a spectrophotometer at 229 nm. All tests were performed in duplicate. With the final concentration of each sample, the adsorption capacity was shown as shown in Equation (1),

$${\mathrm{q}}_{\mathrm{e}}=\frac{{(\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{f}}).\mathrm{V}}{\mathrm{m}}$$

(1)

where q_e is the capacity of adsorption (mg g⁻¹). C_i (mg g⁻¹) and C_f (mg g⁻¹) are the initial and final concentrations of the pollutant, respectively; V is the volume of the pollutant solution (L); and m is the mass of the biosorbent (g). To explain the kinetic models, classic models such as pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion were applied to the experimental data. The model PFO was described by Lagergren [28] (1898) as a kinetic operation that represents the behavior of a reactance, like the decomposition of a substance, as if it were a first order. It is based on the hypothesis that the speed of the reaction is directly proportional to the concentration of the species reacting at each moment. The PSO model was described by Ho (1937) and represents reactions that depend not only on the concentration of the reacting substance, but also on the resulting concentration of the reaction product. These models aim to represent the behavior of a reactance as if it were second order. Finally, the intraparticle diffusion model refers to mass transfer carried out by diffusion between interior components and is presented in Equation (2).

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}[1-{\mathrm{e}}^{-\mathrm{k}1\mathrm{t}}]$$

(2)

Since qt and qe are the adsorption capacities with respect to time (t) at their equilibrium (mg g⁻¹), k1 is the constant (min⁻¹), and t (min). The PSO model was developed by Ho and McKay [29], as in Equation (3).

$${\mathrm{q}}_{\mathrm{t}}=\frac{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2\mathrm{t}}{1+{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}\mathrm{t}}$$

(3)

Let k₂ be a constant PSO adsorption rate and the intraparticle difference to those proposed by Weber and Morris in which it was performed in the present study. With the help of Equation (4), the effective transmission rates for the liveries were determined [30],

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{K}}_{\mathrm{D}\mathrm{i}\mathrm{f}}\sqrt{\mathrm{t}}+\mathrm{C}$$

(4)

where K_Dif is the constant intraparticle diffusion rate (mg g⁻¹ min^1/2), and C is a constant that is related to the thickness of the boundary layer (mg g⁻¹). Thus, considering that the equation is valid for all solutes that tend to move by intraparticle diffusion, we can state that this equation describes the dependence of the soil solute transport rate on the thickness of the boundary layer.

2.4. Adsorption Isotherms

The Freundlich experiment was maintained at three different temperatures: 25, 35, and 45 °C, using 30 mL of metformin solution with concentrations distributed from 5 to 200 mg L⁻¹ in contact with 0.01 g of LLMO at pH 7 under 150 rpm for 480 min. From the adsorption capacity calculations, the most classic models of adsorption isotherms were evaluated, namely, Langmuir and Freundlich.

The Langmuir isotherm is based on the idea that catalyst molecules have individual places where they chemically bond to the reactive material. It describes the relationship between the amount of reactive material bound to the catalyst per unit surface area (or surface occupation) through Equation (5) [31]:

$${\mathrm{q}}_{\mathrm{e}\mathrm{q}}=\frac{{\mathrm{q}}_{\mathrm{m}}{\mathrm{b}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{b}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}},$$

(5)

where b_L is the Langmuir isotherm (L mg⁻¹) constant. The Langmuir isotherm describes how a particle adsorbs onto a specific surface or interface. It is used in separation and decanting processes, as well as to describe the adsorption of gases onto solid surfaces. The Freundlich model becomes useful in modeling adsorption/desorption phenomena, especially when the presence of two or more components favors their absorption. The Freundlich model can be applied in several systems, such as liquid mixtures of surfactants, in which a fraction of the species is adsorbed on a solid surface material, in addition to other applications in gas–solid, liquid–solid, and others. Furthermore, the Freundlich model can also be applied to electrical processing problems, when a circuit is formed by a conductor and 2 or more components. In this case, the Freundlich model must be adjusted according to the specific conditions of the circuit, given that the temperature and density conditions directly influence the response of the circuit components [32], as presented in Equation (6):

$${\mathrm{q}}_{\mathrm{e}\mathrm{q}}={\mathrm{k}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{1/\mathrm{n}},$$

(6)

where k_F is the Freundlich isotherm constant [(mg/g)/(mg/L)^1/n]. After applying the adsorption isotherm models, the equilibrium data were used to calculate thermodynamic parameters such as enthalpy (ΔH), entropy (ΔS), equilibrium constant Kc, and Gibbs free energy (ΔG). The Freundlich isotherm constant (kF), expressed in mg L⁻¹ (L g⁻¹)1/n, was also considered in the calculations.

2.5. Effect of Ionic Strength

An analysis of the effect of ionic strength on the adsorption of metformin in LLMO was carried out, considering the presence of common salt in surface water and its dissociated ions. For this, different salts (NaCl, CaCl₂, and MgCl₂) were added with concentrations of 0.1 and 0.3 M of free ions and a contact time of 24 h at 25 °C. The objective was to determine the influence of this salt and its ions on the adsorption of metformin.

3. Results and Discussion

3.1. Biosorbent Characterization

SEM images of LLMO are presented in Figure 1, revealing the formation of pores in the form of small cavities on the surface of the sample, which indicates a good possibility of metformin being adsorbed. The appearance of pores can be explained by the evaporation of volatile components during the chemical and thermal treatment processes [33,34].

The influences of particle diameter, mass, and pH were evaluated via adsorption experiments (Figure 2). The highest sieve yield and adsorption capacity of LLMO were obtained with the 600 μm sieve. It was then observed that the granulometry of 600 μm, in this particular case due to the higher amount of coal of heterogeneous size, presented a better removal rate. The adsorption capacities for adsorbent masses of 0.01, 0.02, 0.03, 0.04, and 0.05 g L⁻¹ were 16.07, 9.25, 8.69, 7.64, and 7.19 mg g⁻¹, respectively. Therefore, an adsorbent mass of 0.05 g L⁻¹ was selected in subsequent experiments.

At pH 4, 7, and 10, the adsorption capacities were 5.84, 7.21, and 3.82 mg g⁻¹, respectively. The adsorption capacity of metformin is higher at pH 7, which is in line with the results obtained for other adsorbents. Similar results were found by [35]. In that study, metformin adsorption from aqueous solutions was performed using Moringa oleifera Lam. seed husks and it was verified that pH 7 favored the interaction between the bioadsorbant and metformin. Metformin has a positive charge at pH 7, which is lower than its pKa value, while LLMO has a slightly negative surface charge at pH 7 owing to its low isoelectric point (6.2) [36]. This indicates favorable electrostatic interactions between metformin and LLMO at pH 7.

FTIR analysis was used to determine the functional groups present, as shown in Figure 3.

Cellulose nitrate is a modified cellulose polymer. It is characterized by wide bands in its FTIR spectrum, notably at 3707–3633 cm⁻¹, indicating hydrogen bonding. If methylation has occurred, a peak at 2920 cm⁻¹ indicates the asymmetric stretching of the C–H bond of the CH₂ group. A peak at 2831 cm⁻¹ refers to the symmetrical stretching of the C–H bond of the CH₃ in a β linkage [37]. The band at 1697 cm⁻¹ corresponds to the conformational release vibration of the bond between carbon and nitrogen (C–N). The increase in relative area and the displacement of the peak suggest that this bond was distorted, which happened to the carbon-to-carbon (C–C) bond as well, as it shifted to 1680 cm⁻¹. These changes are related to the breakdown of the angular structure of the molecule, which occurs when it is heated. Thus, this indicates that the temperature increased and there was a change in the molecular structure.

This alteration is probably related to the increase in the length of the COO⁻ bonds and the partial distortion of the N–H secondary amine groups [38]. The peak observed at 927 cm⁻¹ is typical of the C–O stretching vibration, which indicates the potential presence of lignin, cellulose, and hemicellulose. Possibly, the adsorption mechanism of metformin in LLMO occurred due to the assumption of electrostatic interactions between the carboxylate groups of the biosorbent particle surface along with metformin’s protonated amine and hydrogen bonds. After carrying out the adsorption, a BET analysis was performed to evaluate the porosity of the investigated material. The large specific area contributes to the adsorption of metformin in the available pores. The results obtained are shown in

Table 2. Texture properties of LLMO.

Parameters	LLMO
BET Specific surface area (m² g−1)	5.32
Average pore diameter (Å)	14.79
Total pore volume (cm³ g−1)	0.0892
Micropore volume (cm³ g−1)	0.0874
Mesopore volume (cm³ g−1)	0.0018

.

According to IUPAC, there are three categories of pore diameters: micropores, mesopores, and macropores. The developed material presents a predominance of micropores (20 Å), which indicates good interaction between the material and the contaminant. The more micropores present, the better the adsorption process [39]. This fact can be observed in [40], in which a study was conducted to evaluate the use of biosorbents in metformin removal, but it was observed that the pore volume available in the biosorbents was relatively small.

3.2. Adsorption Study

The pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models are shown in Figure 4, and the parameters are listed in

Table 3. Kinetic parameters for metformin biosorption by Leucaena leucocephala modified (LLMO).

Models	Parameters	LLMO
Pseudo-first-order (PFO)	qe (mg g−1)	7.825
	k1 (min−1)	0.008
	R2	0.980
	χ2	0.097
Pseudo-second-order (PSO)	qe (mg g−1)	11.422
	k2 (g mg−1 min−1)	0.003
	R2	0.974
	χ2	0.173

.

The initial adsorption process was rapid, and saturation was reached approximately 300 min after the onset of agitation. After stabilization, the kinetic studies revealed a maximum adsorption capacity of 6.70 mg g⁻¹ and a removal percentage of 53.24%. The PFO model calculated a correlation coefficient value (R²) of 0.980 and q_and 7.825 mg g⁻¹, while the PSO model calculated an R² value of 0.974 and q_and of 11.422 mg g⁻¹. The chi-squared value (χ² = 0.097) was also lower for the PSO model than that of the PFO model. Thus, the PSO model fit the data the best, and the calculated q_and was close to the experimental value (6.70 mg g⁻¹).

Adsorption data were analyzed according to the Langmuir and Freundlich models; the calculated parameters are presented in

Table 4. Model of Langmuir and Freundlich at 293, 303 and 313 K.

Models	Parameters	293 K	303 K	313 K
Langmuir	qmáx (mg g−1)	14.64	39.79	56.18
	KL (L mg−1)	0.024	0.057	0.073
	R2	0.920	0.964	0.975
Freundlich	KF [(mg/g)/(mg/L)1/n]	19.60	14.82	11.67
	nF	1.160	1.012	0.945
	R2	0.905	0.926	0.913

and the adsorption isotherms are shown in Figure 5.

The amount of metformin adsorbed decreased with increasing temperature, suggesting an exothermic process. The Langmuir model satisfactorily fitted the experimental data. This means that adsorption occurs in a monolayer without interactions between contaminant molecules, and that each active site adsorbs only a single molecule [41]. The correlation coefficients (R²) obtained using the Langmuir model (0.920, 0.964, and 0.975 at 25, 35, and 45 °C, respectively) were higher than those obtained using the Freundlich model (0.905, 0926, and 0.913). A higher value of K_L was obtained at 25 °C, indicating that interactions were stronger at this temperature, resulting in a higher adsorption capacity. The maximum adsorption capacity (q_max) (56.18 mg g⁻¹) was obtained at a temperature of 313 K. Another biosorbent, containing chitosan and activated charcoal, reached the maximum adsorption capacity of 60.9 mg g⁻¹ at 303 K for the removal of thionine, and thermodynamic results indicated a spontaneous and exothermic adsorption process [42].

In

Table 5. Thermodynamic parameters obtained from the adsorption of metformin at temperatures of 25 °C, 35 °C, and 45 °C.

T (°C)	T (K)	(ΔG) (KJ mol−1)	(ΔH) (KJ mol−1)	(ΔS) (KJ mol−1)
25	298	−22.36	−30.39	0.027
35	308	−21.31
45	318	−20.79

, the negative variation in free energy (ΔG) indicates a favorable and spontaneous process for metformin removal. The negative adsorption enthalpy (−30.39 kJ mol⁻¹) indicates a chemical process and confirms the exothermic nature of the adsorption process [43]. The positive entropy (ΔS) value (0.027 J K⁻¹ mol⁻¹) indicates a favorable process at the solid–liquid interface [44].

The results indicate that, for the three different temperatures (293 K, 303 K, and 313 K), there was a favorable and spontaneous adsorption since the ΔG values were negative (−22.36, −21.31, and −20.79 KJ mol⁻¹). This means that the system does not require external energy input during the adsorption process and that the process is favorable due to negative Gibbs free energy values [45]. The confirmation of the negative value of ΔH indicates that the process is exothermic, that is, it releases energy in the form of heat. This means that as the process takes place, the temperature of the system increases. The value of −20.39 KJ mol⁻¹ suggests that the process occurs through a chemisorption mechanism, which means that a chemical reaction occurs between the adsorbent and the adsorbate during adsorption. This value is a measure of the amount of energy released per mole of adsorbate and can help to understand the adsorption mechanism involved in the process [43]. The results indicate that the interaction between metformin and LLMO occurred randomly at the solid–liquid interface, since the values obtained for ΔS are low and positive (0.027 KJ mol⁻¹) [46]. A similar behavior was observed by [47], who confirmed that the adsorption of solutions containing metformin on activated carbon occurred spontaneously and exothermically.

In general, lower metformin adsorption was observed in the presence of different salts (NaCl, CaCl₂, and MgCl₂), as shown in Figure 6. The salts influenced contaminant adsorption, resulting in the final concentration of metformin in the solution. In their metformin studies, ref. [10] verified the metformin adsorption using graphene oxide and presented similar results, indicating significant inhibiting effects on metformin adsorption in the presence of salts.

The optimum parameters of metformin adsorption to LLMO are presented in

Table 6. Studies of metformin removal with biosorbents.

Adsorbent	qe (mg g−1)	Mass (g)	pH	Temperature (K)	Reference
Acid modification of orange peels (OPAC)	50.99	0.1	7	323	[17]
Byrsonima crassifolia activated hydrochar	113.6	2.0	7	293	[48]
Graphene oxide microcrystalline cellulose (GOMCC)	132.10	0.5	8.5	31.8	[49]
Activated Carbon chemical activation	122.47	1.0	8.5	318	[50]
Peach pit chemically treated biomass	82.54	0.5	8	323	[51]
Moringa seed husks are functionalized with nanoparticles	65.01	0.03	7	298	[15]
Microalgae	45.67	1	6.3	298	[52]
Artichoke Leaves	17.1	0.01	2–12	333	[53]
Banana peel	43.28	1.5	4	298	[54]
Peach (Prúnus pérsica)	38.97	0.5	8	303	[55]
LLMO	56.18	0.03	7	298	This study

, in which we present others’ studies in which metformin was removed with biosorbents.

One can see that the adsorption capacity of LLMO is similar to that of other bioadsorbants; thus, we can consider L. leucocephala pods a potential material for the adsorption of metformin from aqueous solutions.

4. Conclusions

Although the use of L. leucocephala pods is an innovative application, there are no studies in the literature that use L. leucocephala for the removal of contaminants from aqueous solutions. In addition, the material is a sustainable and economically viable solution because it is an invasive plant that is present in abundance in the environment, and its application as an adsorbent has the advantage of low cost and reduces its harmful presence in the environment.

In this study, the optimal parameters for the maximum removal percentage were pH 7, an adsorbent dose of 0.05 g L⁻¹, a contact time of 300 min, and 25 °C. Surface characterization was performed using SEM, which verified the formation of pores. For adsorption and kinetic isothermal analysis, the Langmuir and PSO models satisfactorily fit the experimental data. The thermodynamic parameters confirmed a spontaneous, favorable, and exothermic process. The adsorption capacity decreased in the presence of salts. It is concluded that the new adsorbent presents promising results in the removal of metformin, demonstrating the potential of using an invasive plant as an adsorbent following chemical and thermal treatments, and more studies are being developed aimed at increasing the removal performance of this and other drugs and for the development of a biocomposite. Future studies may report the use of this discovered material on an industrial scale, which will greatly contribute to the environment.