Natural cellulose fibers from Agave Americana L. ASPARAGACEAE as an effective adsorbent for mercury in aqueous solutions

Adsorption (2025) 31:40 https://doi.org/10.1007/s10450-024-00590-4 copper (Cu), zinc (Zn), lead (Pb), chromium (Cr), nickel (Ni), and mercury (Hg) are harmful by-products of these industrial processes [1, 2]. Once introduced into the environment, these metals disperse through air, soil, and water, accumulating in living organisms due to their non-biodegradable nature [3]. Their toxicity is especially alarming for aquatic life and can have detrimental effects on human health, even at very low concentrations [4, 5].The impact of heavy metal pollution extends beyond health issues, significantly affecting the economic stability and quality of life of the affected communities [6]. Among heavy metals, mercury is considered one of the most hazardous due to its high toxicity and capacity for bioaccumulation [7]. Excessive mercury intake poses significant risks to the central nervous, digestive, and renal systems in humans [8]. The World Health Organization (WHO) recommends a maximum allowable limit of 0.006 mg/L for mineral mercury in drinking water, with a total daily intake not exceeding 2 µg/ 1 Introduction Heavy metal pollution, particularly that which arises from activities such as mining, battery production, and the pharmaceutical industry, poses a serious risk to both the environment and public health. Metals such as cadmium (Cd), Lourdes García-Rodríguez [email protected] Hugo Sánchez-Moreno [email protected] 1 Grupo de Investigación de Energías Alternativas y Ambiente, Facultad de Ciencias, Escuela Superior Politécnica de Chimborazo (ESPOCH), Panamericana Sur Km 1 ½, EC060155 Chimborazo, Ecuador 2 Departamento de Ingeniería Energética, Escuela Técnica Superior de Ingeniería (ETSI), Universidad de Sevilla. ETSI, Camino de Los Descubrimientos, s/n, Sevilla 41092, Spain Abstract This study investigated the use of functionalized cabuya fibers (FCF) as an effective adsorbent for Hg (II) removal from aqueous solutions. The composition, surface properties, and morphology of the FCF were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and Fourier transform infrared spectroscopy (FTIR). The effects of the pH, contact time, temperature, adsorbent dosage, and initial Hg (II) concentration on the adsorption process were studied. Under optimized experimental conditions, FCF achieved a removal efficiency exceeding 92%, with a maximum adsorption capacity of 8.29 mg/g. The experimental data for the FCF isotherm were analyzed using the Langmuir, Freundlich, DR, and Temkin adsorption models. Notably, the Langmuir isotherm exhibited the highest R² value of 0.99, indicating the model’s strong applicability. The pseudo-second-order kinetic model k2 = 0.42 mg/g.min was employed to elucidate the adsorption mechanism. Thermodynamic studies of the adsorbent FCF were conducted, and ΔG° (-6.16 kJ/mol), ΔH° (36.29 kJ/mol), and ΔS° (141.98 kJ/mol·K) were calculated, assessing the feasibility of the process. Additionally, the desorption results of FCF were evaluated, demonstrating that it can be reused for up to three cycles, achieving adsorption rates of 74% and 62% in the third cycle. This indicates its stability and recycling capacity. Finally, the effectiveness of the FCF was demonstrated by eliminating approximately 91% of Hg (II) from real mineral water samples in Ecuador. These results highlight the p of FCF as promising, eco-friendly, and sustainable adsorbents for the remediation of Hg (II) contamination in aquatic systems. Keywords Hg (II) adsorption · Mining wastewater treatment · Adsorption isotherm · Kinetic models · Adsorption isotherms · Natural fibers Received: 6 November 2024 / Revised: 16 December 2024 / Accepted: 26 December 2024 / Published online: 31 January 2025 © The Author(s) 2025 Natural cellulose fibers from Agave Americana L. ASPARAGACEAE as an effective adsorbent for mercury in aqueous solutions HugoSánchez-Moreno1· LourdesGarcía-Rodríguez2· CelsoRecalde-Moreno1 1 3 Adsorption (2025) 31:40 kg of body weight [9]. Therefore, the removal of Hg (II) from aqueous solutions is essential, highlighting the need for effective methods to eliminate Hg (II) [10]. Common remediation techniques for the elimination of contaminants from wastewater, such as chemical precipitation [11], ion exchange [12], reverse osmosis [13], coagulation [14], electrochemical treatments [15], and membrane processes, have been reported in the literature [16]. However, most of these methods have some disadvantages such as high cost, toxic waste production, reduced efficiency, and high energy consumption [17]. Adsorption is considered one of the best wastewater treatment techniques and has received considerable attention due to its operational feasibility, simple design, high efficiency in contaminant removal, as well as its recyclability and reusability [18, 19]. In recent years, numerous studies have been conducted to identify sustainable and cost-effective adsorbents derived from natural biomass [20]. Among these materials, cellulosic biomass has garnered significant interest due to its low cost and abundance [21]. This type of biomass is characterized by its high number of hydroxyl groups on the surface, which facilitates modification, as well as its excellent specific resistance, large surface area, and remarkable thermal and chemical stabilities [22, 23]. Examples of cellulosic biomass include American agave [24],flax fibers [25], Cissus quadrangularis Linn plant [26], and palm oil fibers [27]. Plant fibers are known as the most abundant renewable resource in nature, composed mainly of cellulose, hemicellulose, and lignin, with a hierarchical structure that spans from the metric scale to the nanometric scale [28]. In this context, agave is presented as a group of desert plants belonging to the monocot family Agavaceae [29]. It is characterized by its spiny leaves, which produce a variety of fibers [30]. Despite their abundant availability and environmental sustainability, agave fibers have yet to achieve significant economic value [31]. It has been demonstrated that agave fibers are highly effective adsorbents for the removal of dyes and chromium from [29]. However, despite their remarkable adsorption capacity, the properties of these fibers in capturing heavy metal ions have yet to be explored in depth. In this study, the main objectives were to explore the removal efficacy and adsorption mechanism of Hg (II) on cabuya (Agave Americana L. ASPARAGACEAE) fibers functionalized with NaOH (FCF). A series of characterizations, such as FTIR, SEM, and EDS, were conducted to investigate the physicochemical properties. The resistance to environmental exposure was examined against pH levels and temperature, as well as the reusability of FCF. Subsequently, adsorption kinetics, isotherms, and thermodynamics were studied to investigate the adsorption properties of Hg (II) on FCF. The practical application of FCF in real water bodies was also explored, specifically in wastewater from artisanal mining at the Virgen de las Nieves Beneficiation Plant in Zaruma. Through this comprehensive approach, it is expected not only to contribute to the understanding of the adsorption capacity of Hg (II) on FCF but also to provide a viable solution for the remediation of contaminated waters in Ecuador. This addresses a significant environmental issue and promotes public health, marking a step forward in strategies for mitigating heavy metal contamination in a country with abundant vegetation of Agave. 2 Materials and methods 2.1 Reagents and materials American Agave L. ASPARAGACEAE was collected in the center of the inter-Andean region of Ecuador, in the Riobamba sector. The biosorbent was treated with aqueous solutions of appropriate concentrations prepared using NaOH, which was acquired from ISOLAB Laborgerate GmbH, Wertheim, Germany. The Hg (II) standard solution of 1000 mg/L was acquired from AccuStandard (New Haven, Connecticut, USA). HCl and HNO3 were purchased from LOBA CHEMIE PVT. Ltd. All chemicals and solvents used in this study were of analytical grade, and no additional purification systems were required. Milli-Q water was used to prepare the solutions (resistivity: 18.2 MΩ.cm at 298 K). 2.2 Methods Cabuya leaves were collected and functionalized by salt hydrolysis at 40 °C for 4 h. The resulting FCF were washed to remove non-cellulosic compounds and were characterized using SEM, EDX, and FTIR. The FCF were then tested as Hg (II) absorbents with mercury solutions of known concentrations placed in contact with the FCF. The effects of various factors, including the contact time, sample dose, pH, temperature, and concentration, were recorded to determine the amount of Hg (II) retained. Mercury adsorption was analyzed using atomic absorption spectroscopy. To determine the type of adsorption between the FCF and analyte, an adsorption isotherm test was performed, which evaluated the equilibrium data based on the Langmuir and Freundlich isotherm models under experimental conditions. Additionally, the adsorption kinetics of mercury in the FCF were studied by fitting the experimental data to pseudo-firstorder and pseudo-second-order kinetic models. 1 3 40 Page 2 of 27 Adsorption (2025) 31:40 3 Experimentation 3.1 Preparation of FCF The extraction of natural fibers poses a considerable challenge in the processing of plant fibers. Leaves of black cabuya (Agave americana L. ASPARAGACEAE) were collected, which initially exhibited a gray-green color. The fibers from the plant were manually extracted, and the thorns from the edges were trimmed (Kozłowski et al., 2020). The collected fibers underwent a purification and functionalization process using a 5% (w/v) sodium hydroxide (NaOH) solution, which is generally optimal for most natural fibers naturales [32].This process was carried out at a temperature of around 40 °C for a period of 4 h [33]. Subsequently, the treated fibers were repeatedly washed with deionized water using ultrasonic cleaning equipment until a pH of 6 was reached. Finally, the functionalized cabuya fibers (FCF) were dried in an oven at 75 °C for 24 h to remove excess water [34]. 3.2 Characterization studies The microstructure of the FCF was analyzed using a Nikon Eclipse E200 data acquisition optical microscope with a parafocal length of 60 mm and a 10X field of view scanning electron microscope (SEM, JSM-IT100LA). The samples were mounted on stubs with double-sided adhesive tape and coated with a thin layer of evaporated gold (Au) [35]. The FTIR spectrum of the FCF was obtained using a Fourier transform infrared spectrometer (JASCO FT/IR-4100) from 4000 to 560 cm−1. The corresponding graph was transformed into a peak absorbance graph with a resolution of 16 cm−1. 3.3 Feasibility of FCF as hg (II) retainers in water To establish the capacity of the FCF to retain mercury, a certified mercury (II) standard of 1000 mg/L was used, and solution of 1.0 mg/L was preparedfor various assays (effect of contact time, adsorbent quantity, agitation, initial Hg (II) concentration, pH, and temperature). These solutions were placed in contact with the FCF and once equilibrium was reached, the percentage of mercury adsorbed for each test was determined using atomic absorption spectrometry. 3.3.1 Effect of contact time The adsorption of Hg (II) metal ions was measured by placing 0.5 and 1.0 g of FCF in a flask containing 100 mL of a solution of mercury (1.0 mg/L) without pH adjustment. The contents of the flask were agitated under ultrasound for 60 s and mechanical agitation was continued at 150 rpm for 24 h at a constant temperature of 25 °C. Samples of the liquid phase (1 mL) were collected at time intervals of 5, 10, 15, 30, and 60 min, and then at 2 h and 4 h. After each aliquot was taken, it was titrated to 10 mL with deionized water and mercury retention was determined by atomic absorption spectroscopy. 3.3.2 Effect of adsorbent quantity (FCF) The effect of the FCF quantity on mercury retention was determined by varying the adsorbent dose for each assay (0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 g) in a fixed-volume flask containing 100 mL of a solution with a 1.0 mg/L mercury ion concentration at a temperature of 25 °C without pH adjustment. The contents of the flask were shaken under ultrasound for 60 s and mechanically agitated at 150 rpm until equilibrium was reached. An aliquot of 1 mL was then taken and diluted to 10.0 mL with deionized water, and the concentration of the retained mercury was determined using atomic absorption spectroscopy equipment. 3.3.3 Effect of temperature The effect of temperature was evaluated with values between 25, 35, and 45 °C, for which the optimal conditions for FCF identified in Sect. 3.3.1 and 3.3.2, which were the equilibrium time and dosage of the sample, preserving a concentration of 1.0 mg/L of mercury, at a volume of 100 mL, without pH adjustment. Once equilibrium was reached, aliquots of 1 mL were taken, filled to the 10.0 mL calibration mark, and the mercury concentration was determined by atomic absorption spectroscopy. 3.3.4 Effect of pH on retention The point of zero charge (PZC) for FCF biomass was calculated by mixing 50 mg of FCF with 10 ml of a 0.01 M NaCl electrolyte solution. The initial pH was adjusted using 0.1 M NaOH and 0.1 M HCl, and the mixture was agitated for 24 h at room temperature. The pH of the filtrate was measured after filtering, and the PZC was determined as the intersection of the curve with the y-axis = 0. To study the effect of pH on Hg (II) adsorption, different solutions were prepared at pH values of 3.0, 5.0, 7.0, and 9.0, with mercury concentration of 1.0 mg/L of mercury. The adsorbent samples were added to a stable volume of 100 mL of each prepared solution, and the contact time was reached. The mercury concentration was determined using atomic absorption spectroscopy and the pH was adjusted using analytical-grade HCl and NaOH. 1 3 Page 3 of 27 40 Adsorption (2025) 31:40 FCF were placed in 100 mL of a solution of HCl and HNO3 with concentrations of 0.1 mol L−1 in different assays, with 150 rpm agitation at 25 °C for 8 h. The desorption process was performed in duplicate during one desorption cycle. Once the test was completed, the retained mercury concentration was determined using atomic absorption spectroscopy. 3.7 Retention Capacity in hg (II)-Fortified Real Water samples To evaluate the mercury retention capacity of the FCF, samples of mercury-contaminated wastewater from the gold extraction systems of the Virgen de las Nieves Beneficiation Plant in Zaruma, Ecuador (South America) were used. Mine water (100 mL) was collected, and Hg (II) concentrations of 10 mg/L, 20 mg/L, and 30 mg/L were added to obtain a fortified sample. The mine water used was obtained from four sampling points: the entrance, exit, gold extraction points, and wastewater discharge pool. The fortified mine water was brought into contact with 1.0 g of FCF and agitated until an equilibrium was reached. After equilibrium, aliquots of 1.0 mL were taken, diluted to 10 mL, digested, and the mercury concentration was determined by atomic absorption spectroscopy. 4 Results and discussion 4.1 Characterization of FCF 4.1.1 Scanning electron microscopy (SEM) and energydispersed spectroscopy (EDS) Figure 1 shows the morphology of the cellulose surface with increase of 800X (Fig. 1a) and 370X (Fig. 1b). The SEM micrographs show the structure of long, thick, and FCF with an average diameter of 65 μm. A study by [38] the University of São Paulo reported a diameter of approximately 50 μm. On the other hand, Fig. 1b depicts a smooth surface structure of fibrils with uniform sizes, due to treatment with sodium hydroxide, which successfully removed impurities. In Fig. 1a, holes with an average size of 3 μm were observed. The EDS analysis of the FCF is presented in Fig. 2, namely, the area analyzed (Fig. 2a) and its spectrum (Fig. 2b). The EDS mapping of the fiber revealed the presence of carbon, oxygen, and gold. Specifically, the presence of carbon and oxygen indicates that there were no impurities in the other element. In addition, the appearance of gold was because the fibers were coated with a thin layer of gold to make them conductive. Finally, the spectrum indicates an 3.3.5 Effect of initial hg (II) concentration The effect of the initial Hg (II) concentration on adsorption by the FCF obtained was carried out by placing 1.0 g of dry FCF fibers in a series of flasks containing 100 mL of metal ions at defined concentrations (10–100 mg/L), without pH adjustment. The contents of the flasks were agitated at 150 rpm and 25 °C for 120 min. After adsorption, the residual concentration of the metal ions was determined using atomic absorption spectroscopy. 3.3.6 Effect of agitation The effect of agitation on Hg (II) adsorption was determined to analyze the influence of this factor. For this purpose, an initial mercury concentration of 10 mg/L was prepared, maintaining parameters such as an adsorbent dose of 1.0 g, contact time of 120 min, pH of 5, and a constant temperature of 25 °C. Tests were performed with and without agitation. After adsorption, the final mercury concentration was determined by atomic absorption spectroscopy. 3.4 Adsorption isotherm and adsorption kinetics The adsorption isotherm describes the equilibrium of a material on a surface based on the relationship between adsorbate concentration and adsorbed [36]. In this study, Langmuir, Freundlich, Temkin and Dubinin-RudushKevich adsorption isotherms were investigated to identify the best isothermal model for describing mercury adsorption in FCF. Additionally, a fundamental aspect was addressed using kinetic analysis, which provided a description of the rate and mechanism of Hg (II) adsorption. To this end, the adsorption kinetics of Hg (II) were studied by fitting the experimental data to the pseudo-first-order, pseudo-secondorder, and Elovich kinetic models. 3.5 Thermodynamic study The thermodynamic adsorption model is mainly used to characterize the thermodynamic properties of the adsorption process by providing information on the mechanism of the temperature effects on the water-mercury interface adsorption [37]. To this end, the thermodynamic parameters for the adsorption of Hg (II) ions in FCF were studied through the changes of Gibbs free energy, entropy and enthalpy modifying the temperature of the system between 25 °C and 40 °C. 3.6 Desorption analysis Once the adsorption process was complete, two leaching agents were used to assess Hg (II) retention. Hg (II)-loaded 1 3 40 Page 4 of 27 Adsorption (2025) 31:40 a significant increase in the relative intensity of the OH band from 3332 cmto 1to 3370 cm−1, a decrease in the intensity of the C = O band from 1666 cmto 1 to 1627 cm−1, and the appearance of the relative intensity of the C-H band at 2360 cm−1 (Fig. 3b). 4.2 Influence of different experimental parameters on hg (II) retention by FCF To test the viability of the FCF as a Hg (II) adsorbent, a certified Hg (II) standard of 1000 mg/L was used, from which a standard solution (1.0 mg/L) was prepared to evaluate the influence of different operating parameters on the FCF (contact time, adsorbent dose, agitation, temperature and pH). Moreover, variations in the initial Hg (II) concentration (10, 20, 30, 40, 60, 80, and 100 mg/L) were studied to establish the optimal working conditions. The adsorption percentage was calculated using Eq. (1). atomic ratio between O: C equal to 46:54 and a strong signal around 0.4 eV corresponding to C. 4.1.2 Infrared spectroscopy (FTIR) The untreated cabuya fiber (CF) displayed strong FTIR bands at 1029 cm−1, 1322 cm−1, 1666 cm−1, 2920 cm−1, and 3332 cm−1 (Fig. 3a). The 1029 cm−1 bands are characteristic of natural cellulosic fibers and are attributed to the stretching of the primary alcohol (-C-O-H) [39]. The bands at 1322 cm−1 are attributed to the -C-O stretching band and the presence of an isopropyl group on the surface. The bands around 2920 cm−1 are associated with asymmetrical and symmetrical methyl and methylene stretching groups. The band at 1666 cm−1 corresponds to the C = O ester, while the bands around 3332 cm−1 are attributed to the different stretching modes of the O-H group [38]. After exposure to NaOH, the FTIR spectrum of the fiber showed some changes, including Fig. 2 (a) SEM– EDS FCF analyzed area, (b) SEM– EDS FCF spectrum Fig. 1 Morphology of the cellulose surface (FCF) (a) Increase of 800 × (20 μm) and (b) increase of 370 × (50 μm) 1 3 Page 5 of 27 40 Adsorption (2025) 31:40 of Hg (II) under these conditions were 86.61% and 92.32%, respectively. It is also evident that within the first 5 min, the adsorption exceeded 65%, confirming that FCF has several active sites with adequate affinity between the OH groups for Hg (II), allowing immediate interaction after contact, which results in effective adsorption [40]. 4.2.2 Effect of adsorbent dose This study investigated the effect of adsorbent dose on the removal of Hg (II) ions from aqueous solutions (Fig. 5), revealing that the adsorption process is closely related to the availability of active sites for contaminant interaction [41]. Using different amounts of adsorbent (0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 g) at a concentration of 1.0 mg/L, under agitation for 120 min at 150 rpm and a constant temperature Adsorption (%) = Ci −C Ci ×100% (1) Where Ci (µg/L) and C (µg/L) represent the metal concentrations in the prepared Hg (II) solution before and after adsorption, respectively. 4.2.1 Effect of contact time The effect of contact time on the FCF was investigated with 0.5 and 1.0 g of FCF samples in 100 mL of a solution of 1.0 mg/L of Hg (II), agitation (150 rpm), time range (5–240 min), and a constant temperature of 25 °C. Figure 4 shows that the adsorption efficiency of Hg (II) increased over time from 5 to 120 min, followed by slight desorption at 240 min. Therefore, 120 min was chosen as the optimal contact time for the subsequent experiments. The maximum adsorptions Fig. 3 FTIR spectra of cabuya fiber spectra (a) dried before exposure to sodium hydroxide (NaOH), (b) after exposure to sodium hydroxide (NaOH) (time 4 h and temperature 40 °C 1 3 40 Page 6 of 27 Adsorption (2025) 31:40 4.2.4 Effect of solution pH The pH of a solution can significantly affects the adsorption process of metal ions on adsorbents. This is because the pH can affect both the metal species in the solution and the surface characteristics of the adsorbents. For instance, it can influence the dissociation of functional groups and generation of charges on the surface [44]. The point of zero charge (PZC) of FCF (Fig. 7) is a crucial parameter in the adsorption process and was determined to be 2.90. This value has significant implications for Hg (II) removal. When the solution pH is greater than the pH PZC, the FCF surface acquires a net negative charge, which increases the adsorption capacity by promoting electrostatic interactions between cationic Hg (II) and electron-rich surface sites. Conversely, when the pH is lower than the pH PZC, the adsorbent surface exhibits a net positive charge, generating repulsive forces that, in theory, should reduce Hg (II) adsorption [4, 45]. This effect is reflected in the experimental results (Fig. 8), where adsorption increases significantly at pH conditions above 2.9, reaching a maximum in the pH range of 5. of 25 °C, it was observed that the percentage of Hg (II) ion removal increased significantly from 48.96 to 92.32% as the adsorbent dose rose from 0.2 g to 1.0 g. Beyond this point, the removal efficiency remained nearly constant, identifying 1.0 g as the optimal dose. This stabilization could be attributed to system saturation or adsorbent aggregation, phenomena that reduce the specific adsorption efficiency despite the increased amount of adsorbent [42]. 4.2.3 Influence of temperature Temperature is an additional factor that affects the sorption process. In this study, equilibrium data in the temperature ranges of 25, 35, and 40 °C were used, using a dose of 1.0 g of adsorbent and an analyte concentration of 1.0 mg/L. Figure 6 shows how the adsorption capacity for Hg ions increased progressively with increasing temperature, with a maximum adsorption of 96.81%. This could be explained by the fact that at higher temperatures, there is better interaction between the Hg ions and the active groups of the FCF [43]. Fig. 4 Influence of contact time on Hg (II) adsorption. Test conditions: 1.0 mg/L; 150 rpm; T=25 °C; V = 100 mL, mAdsorbent = 0.5 g and 1.0 g 1 3 Page 7 of 27 40 Adsorption (2025) 31:40 4.2.5 Effect of initial hg (II) concentration The impact of the initial concentration of Hg (II) on the percentage of Hg (II) adsorption by FCF in the aqueous solution was investigated to identify the possible saturation of FCF. The initial concentration of Hg (II) (10, 20, 30, 40, 60, 80, and 100 mg/L) was varied while maintaining other parameters such as an adsorbent dose of 1.00 g, agitation speed of 150 rpm, contact time of 120 min, no pH adjustment, and a constant temperature of 25 °C. The results obtained for the effect of the initial concentration of Hg (II) on the adsorption of Hg (II) ions is shown in Fig. 9. 4.2.6 Effect of agitation Figure 10 shows the Hg (II) ion adsorption capacity of FCF when agitated at 150 rpm without agitation. The assay was performed at an initial concentration of 10 mg/L, maintaining parameters such as an adsorbent dose of 1.0 g, contact The study revealed that the maximum adsorption of Hg (II) (Fig. 8) was achieved at a pH of 5, while the best adsorption results with FCF were observed at pH levels of 3, 5, and 7, with adsorption efficiencies of 90%, 92%, and 89%, respectively. These results indicate that FCF can effectively absorb Hg (II) at pH values close to neutral, which is an advantage when dealing with real effluents in mining areas [46]. On the other hand, the lowest recorded adsorption, 67%, occurred at a pH of 1, which is attributed to the acidic nature of the medium. Under these conditions, the active groups of the adsorbent may undergo partial protonation, limiting its adsorption capacity, in addition to competition between H(I) and Hg (II) ions for the active sites of FCF [47]. At a pH of 9, a decrease in Hg adsorption was observed, likely due to the presence of various Hg species in water, such as Hg (OH), Hg (OH)2, and Hg (II), which promote the formation of Hg (II) hydroxide complexes and reduce adsorption efficiency [48]. Moreover, the presence of high concentrations of Na+ ions could compete with Hg (II) ions for the adsorption sites on the FCF surface [49]. Fig. 5 Dosing of adsorbent mass in the Hg (II) adsorption experiments. Test conditions: 1.0 mg/L; 150 rpm; T=25 °C; V= 100 mL, t=120 min 1 3 40 Page 8 of 27 Adsorption (2025) 31:40 the surface becomes rougher, leading to a partial blockage of some pores and reducing the available surface area, thereby decreasing the adsorption capacity [51]. Figure 11b and d present the results of the EDS analysis following the adsorption of Hg at concentrations of 1 mg/L and 10 mg/L, respectively. In the two comparative analyses, significant differences were observed in the elemental composition. In the first case, Hg II was not detected, likely due to working with an initial concentration of only 1 mg/L of Hg (II) for the adsorption process with FCF. This resulted in traces that were too small to be detected by the limited sensitivity of the analysis. In contrast, in the second analysis, minimal traces of Hg (II) were identified (0.01% by mass and 0.02% by atoms), as a higher initial concentration of 10 mg/L of Hg (II) was used for the adsorption process. This increase may have improved the sensitivity of the analysis. Regarding carbon (C) and oxygen (O), both elements predominated at the atomic level in the analyses; however, their mass contribution was greater in the first case (8.48% and 10.45%, respectively, compared to 2.50% and 2.21% in the second). These differences could be attributed to a larger surface time of 120 min, pH of 5, and a constant temperature of 25 °C. It can be found that the efficiency in the removal of Hg ions is higher when working with agitation reaching an adsorption of 91.84%, while adsorption without agitation reaches 87.68%. From the results obtained, it is evident that agitation is a critical aspect to be considered for improving adsorption, because it allows the adsorbate molecules to have greater contact with the adsorbent, guaranteeing greater homogeneity in all empty sites on the surface of the adsorbent [50]. 4.2.7 SEM and EDS after the adsorption process Figure 11 shows the surface morphology FCF after the adsorption of Hg (II), observed at a magnification of 300X (Fig. 11a and c). Comparing this morphology with that of the initial adsorbent, presented in Fig. 1a and b of Sect. 4.1.1, a significant change can be observed. Prior to adsorption, the surface of the adsorbent is clean and homogeneous, providing numerous active sites for interaction with contaminants. However, after the adsorption process, Fig. 6 Adsorption of Hg (II) on FCF at different temperatures. Test conditions: 1.0 mg/L; 150 rpm; mAdsorbent = 1.0 g; V= 100 mL, t=120 min 1 3 Page 9 of 27 40 Adsorption (2025) 31:40 Ce is the concentration of the metal at equilibrium (mg/L) and Co is the initial concentration of Hg (II) (mg/L). The values of the standard changes in enthalpy (ΔH0) and entropy (ΔS0) were calculated from the slope and intercept of the ln Kc versus 1/T plot. Table 4 shows the thermodynamic parameters for the adsorption of Hg (II) ions on the FCF. Other authors have suggested studying the standard change in the Gibbs free energy using the following equations [77]: K0 e= (1000 •K L •M)•1mol/L γ (14) ∆G 0=−RTlnK0 e (15) where KL (L/mg) is the Langmuir adsorption constant, which is related to the properties of the adsorbent, the adsorbate, and the temperature. Where M represents the 4.5 Thermodynamic study Several researchers have analyzed the thermodynamic properties of cellulose-based adsorbents with respect to Hg (II). Some studies have described the endothermic nature of Hg (II) adsorption by cellulose-based adsorbent materials [75]. The thermodynamic parameters can be studied with the Gibbs Free Energy standard change using the following equation: ∆G 0=−RTlnKc (12) where R is the universal gas constant, T is the absolute temperature (K), and Kc is the adsorption equilibrium constant [76]: Kc = Co −Ce Ce (13) Fig. 13 Langmuir isotherm plots for the adsorption of Hg (II) on FCF 1 3 40 Page 16 of 27 Adsorption (2025) 31:40 Fig. 15 Dubinin-Radushkevich isotherm plots for the adsorption of Hg (II) on FCF Fig. 14 Hg (II) adsorption isotherms on FCF according to the Temkin model 1 3 Page 17 of 27 40 Adsorption (2025) 31:40 concentration during desorption. The desorption percentages, denoted as D, were calculated from the adsorption and desorption data using the following Equation [81]: %D= C des Cads (17) where Cdes (mg/g) is the amount of metal desorbed by (HCl and HNO3) and Cads (mg/g) is the amount of metal adsorbed by the solid. Figure 19 shows that the desorption processes were incomplete when using 0.1 mol/L HCl and 0.1 mol/L HNO3 over 8 h. The percentage of Hg desorbed using HNO3 (31%) was higher than that desorbed using HCl (21%). The mostly irreversible nature of adsorption can be attributed to specific interactions at high-energy sites in FCF [82]. This observation is consistent with the pseudo-second-order model, which proved to be the most suitable for representing the experimental Hg (II) adsorption values in the FCF. This model is typically associated with chemical adsorption processes. 4.7 Reusability Reusability is a crucial component of good adsorbents, in terms of stability and cost-effectiveness for practical applications. Desorption tests were performed using 0.1 mol/L HCl and 0.1 mol/L HNO3 as an eluent to evaluate the regeneration potential of FCF and its reusability as adsorbents for the removal of Hg (II) ions. The results are shown in Table 6. The results of the reusability study showed that desorption resulted in minimal loss in the adsorption capacity of the cellulosic fiber. The percentage of Hg (II) ion removed decreased as the number of reusability cycles increased. The percentage of Hg (II) ion removal using HNO3 decreased from 92 to 74% (18% loss), whereas when HCl was used, it decreased from 90 to 62% in both cases after three adsorption/desorption cycles. This decrease in the percentage of adsorption can be explained by repetitive adsorption/desorption processes that cause depletion of active sites [83]. However, these findings indicate that the functionalized fiber can regenerate even after being repeatedly used for Hg (II) ions, molecular weight of the adsorbate, γ is the coefficient of activity (dimensionless), assumed to be 1.0, and Ke 0 is the dimensionless thermodynamic equilibrium constant. ∆G (kJ/mol) is the Gibbs free energy change and R is the gas constant (8.314 J/mol/K). Table 5 summarizes this method. This can be seen in Tables 4 and 5 all ΔG° values were negative, demonstrating that the adsorption of Hg (II) in FCF is thermodynamically viable and spontaneous in nature. In addition, ΔG° values below 40 kJmol−1 indicate that the process of Hg (II) adsorption in FCF should not be considered a chemically pure mechanism, but also that it occurs by a physical mechanism [78]. Finally, the positive values of ΔH° and ΔS° for Hg (II) adsorption indicate the endothermic nature of this process and a trend of increasing randomness in the equilibrium between the adsorbent and the adsorbed metal [79]. 4.6 Desorption Analysis To fully characterize the system in this study, the desorption process must also be analyzed. The amount of retained metal (mg Hg (II) per g FCF) was calculated from the difference between the amount adsorbed and desorbed, according to the following Equation [80]: Cr=Cads −Cdes (16) where Cr (mg/g) and Cads (mg/g) are the amounts retained and adsorbed, respectively, and Cdes (mg/g) is the equilibrium Table 1 Isothermal parameters of the Langmuir and Freundlich models for hg (II) adsorption on FCF at 25 °C Langmuir Model Freundlich model qm (mg/g) KL (L/mg) R2Kf (mg/g) 1/n (L/mg) R2 8.29 0.146 0.99 0.87 0.72 0.95 Temkin Model Dubinin–Radushkevich Model AT(L/g) bT(kJ/mol) R2qm (mg/g) B E(kJ/mol) R2 5.07 0.21 0.88 3.40 9,00E-05 105.41 0,82 Separation Factor Concentration (mg/L) 10 20 30 40 60 80 100 RL 0.40 0.25 0.18 0.14 0.10 0.07 0.06 Table 2 Comparison of the maximum sorption capacity of FCF (qm) with those reported in the literature for another sorbent cellulose Metal Adsorbent T (°C) qm (mg/g) Reference Hg (II) Diaminoguanidine functionalized cellulose 25 °C 55 mg/g [64] Hg (II) Thiophene functionalized cellulose 25 °C 109.7 mg/g [65] Hg (II) Guanyl-modified cellulose 25 °C 49 mg/g [66] Hg (II) Modified hemp fibers 30 °C 57 mg/g [45] Hg (II) Flax fibers 25 °C 5.2 mg/g [25] Hg (II) FCF (Agave Americana ASPARAGACEAE) 25 °C 8.29 mg/g This study 1 3 40 Page 18 of 27 Adsorption (2025) 31:40 Fig. 16 Pseudo first-order kinetic model for Hg (II) removal by FCF 1 3 Page 19 of 27 40 Adsorption (2025) 31:40 las Nieves Beneficiation Plant was found. The collected samples had pH values close to neutral, ranging from 6.3 to 6.9. The conductivity values varied from 1157 µS/cm to 1463 µS/cm, exceeding the maximum acceptable level of 1000 µS/cm, indicating an increased salt concentration. Alkalinity values were between 2.8 mg/L CaCO3 and 3.8 mg/L CaCO3, suggesting dissolution of rocky material, and were classified as low at below 75 mg/L CaCO3 [84]. Pre-analysis of wastewater samples from selected stations revealed Hg (II) concentrations ranging from 1.8 µg/L to 26.27µg/L. Physicochemical parameters of the water samples are listed in Table 7. To assess the efficiency of FCF in field applications, real samples from four points were mixed, and 10, 20, and 30 mg/L of Hg (II) were added to fortify the mining water with Hg (II) and to evaluate the critical adsorption conditions of FCF. The results in Fig. 21 demonstrate the high percentages of Hg (II) elimination under critical conditions. At 10 mg/L, the expected adsorption of 91% was achieved; however, when 20 mg/L and 30 mg/L were added, the FCF began to saturate with other ions, such indicating the reusability and stability potential of the FCF for Hg ion removal by adsorption. Additionally, these findings showed that HNO3-treated FCF were more suitable than HCl-treated FCF for repeated use. The difference between the fibers after acid treatment in terms of reusability was possibly due to the stretching band of the hydroxyl (OH) group. The FTIR spectrum (Fig. 20c) shows that after treatment with 0.10 mol/L HCl, there is almost no presence of (OH) groups, producing losses of many FCF functionalities. On the other hand, when observing the stretch band after treatment with 0.10 mol/L HNO3 (Fig. 20b), it was observed that the (OH) groups were not significantly reduced, resulting in better reusability due to the greater presence of active sites, so that they interact in the adsorption of Hg (II). 4.8 Holding capacity in real hg (II)-Fortified Water samples Samples were collected from real water sources where artisanal mining waste from the Zaruma area of the Virgen de Fig. 17 Pseudosecond-order kinetic model for Hg (II) removal by FCF 1 3 40 Page 20 of 27 Adsorption (2025) 31:40 as Cu and Pb, decreasing the capacity to remove Hg (II), resulting in adsorption rates of 83% and 57%, respectively. 5 Summary results This study analyzes the adsorption of Hg (II) at concentrations ranging from 1.0 mg/L to 100.0 mg/L, under temperatures from 25 to 45 °C and pH varying from 1 to 9. Adsorbent doses between 0.2 and 1.2 g were employed, and tests were conducted over time intervals from 5 to 120 min, all at a stirring speed of 150 rpm. The functionalization of fique fiber was performed through NaOH treatment to enhance its adsorption capacity. The functionalized fiber (FCF) was characterized using SEM, EDS, and FTIR. FTIR analysis revealed bands at 1029 cm⁻¹, 1322 cm⁻¹, 1666 cm⁻¹, 2920 Table 3 Kinetic Model Parameters Pseudo 1st order model 10 mg/L 20 mg/L 30 mg/L 40 mg/L qe (mg/g) 0.124 0.092 0.187 0.142 k1(min−1) 0.0071 0.0093 0.0204 0.0242 R20.83 0.96 0.84 0.90 Pseudo 2nd order model 10 mg/L 20 mg/L 30 mg/L 40 mg/L qe (mg/g) 0.91 1.82 2.69 5.56 K2mg/(g.min) 0.42 1.01 0.244 0.249 R20.99 0.99 0.99 0.99 Elovich model 10 mg/L 20 mg/L 30 mg/L 40 mg/L β(mg/g) 28.25 36.23 21.88 21.79 αmg/(g.min) 1.82E + 07 3.64E + 24 2.85E + 21 9.53E + 29 R20.97 0.93 0.82 0.92 Table 4 Thermodynamic Parameters for Hg (II) Ion Adsorption in FCF T (K) Ln Kc ∆G0 (kJ/mol) ΔH0 (kJ/mol) ΔS0 (J/mol·K) 298 2.487 −6.16 36.29 141.98 308 2.784 −7.12 318 3.412 −9.02 Table 5 Gibbs free energy for adsorption of Hg (II) ions in FCF in relation to Langmuir adsorption constant T (K) Ln K e 0∆G0 (kJ/mol) 298 5.11 − 12.66 Fig. 18 Elovich kinetic model for Hg (II) removal by FCF 1 3 Page 21 of 27 40 Adsorption (2025) 31:40 FCF was favored when pH approached 5. FCF’s adsorption capacity increased as temperature rose to 25, 35, and 45 °C, reaching values of 92%, 94%, and 97%, respectively. Agitation proved not to be a highly relevant factor, achieving 92% adsorption with agitation and 89% without it. For Hg (II) concentrations of 10, 20, 30, and 40 mg/L, FCF adsorption remained stable at around 90%, indicating that FCF maintains its adsorption without saturation up to 40 mg/L. The highest mercury adsorption value was 97% with 1.0 mg/L of mercury. This result was achieved using FCF at pH 5, 240 min contact time, 150 rpm, and 45 °C. Langmuir, Freundlich, Temkin, and D-R isotherms were used to describe Hg adsorption behavior on FCF. Equilibrium data results showed better fitting to the Langmuir isotherm, reaching a maximum adsorption capacity of 8.29 mg/g. This behavior suggests a homogeneous distribution of active sites on the adsorbent surface and the predominance of chemical interactions during the adsorption process. Adsorption kinetics were evaluated using pseudo-first-order, pseudo-secondorder, and Elovich models. Experimental data showed better fitting to the pseudo-second-order model, characterized by cm⁻¹, 2360 cm⁻¹, and 3370 cm⁻¹. Notably significant is the band at 3370 cm⁻¹, attributed to different OH group stretching modes. The abundance of hydroxyl groups (OH) in the fiber structure suggests enhanced adsorption capacity. To analyze the viability of Hg (II) adsorption with FCF, parameters such as time, concentration, pH, adsorbent amount, and temperature were evaluated. The equilibrium time for mercury retention was reached within the first 2 h, achieving a remarkable adsorption percentage of 92%. Regarding the adsorbent amount effect, 1.0 g of FCF was determined as the optimal quantity to maximize adsorption. Increasing the adsorbent mass above this value did not result in significant improvement in adsorption capacity. Mercury adsorption on Table 6 Reutilization of functionalized (FCF) for Hg (II) Reusability Cycles Hg (II) Recovery (%) Treatment HNO3 Treatment HCl 1 92 90 2 81 76 3 74 62 Fig. 19 Percentage of Hg (II) desorption using HCl and HNO3. Test conditions: 150 rpm; m FCF(Hg (II)) = 1.0 g; V = 100 mL, t = 8 h, t = 25 °C 1 3 40 Page 22 of 27 Adsorption (2025) 31:40 achieving three effective reuse cycles with HNO₃ and HCl, with efficiencies of 74% and 62%, respectively. The obtained results demonstrate that FCF is effective for Hg (II) removal in both synthetic aqueous solutions and real waters, positioning it as a viable and economical alternative for treating mercury contaminated waters. 6 Conclusions Experimentally, the synthetic samples showed that the zerocharge point of FCF was relatively low, indicating a good ability to adsorb cations. The FCF with Hg (II) had an equilibrium time of approximately 120 min, with a maximum adsorption percentage of 92%, and 1.0 g of adsorbent and agitation at pH 5.0, under standard conditions. The maximum adsorption obtained (qm) was 8.29 mg/g, which fitted the Langmuir isotherm. Moreover, an increase in temperature to 45 °C increased the adsorption percentage of FCF, reaching 97% of Hg (II). The thermodynamic parameters a rapid initial adsorption rate. Thermodynamic parameters confirmed that Hg (II) adsorption on FCF is a spontaneous and endothermic process. The established optimal conditions were applied to real water samples from the Virgen de las Nieves Processing Plant, achieving an adsorption efficiency of 91% for Hg (II). The adsorbent’s regeneration capacity was also evaluated through desorption cycles, Table 7 Physicochemical parameters and heavy metals present in real water samples Virgen de las Nieves Beneficiation Plant Physicochemical Parameters Unit M1 M2 M3 M4 Alkalinity mg/L 3.4 3.8 3.1 2.8 TDS mg/L 733.5 768.6 856.3 875.6 Conductivity µS/cm 1203 1157 1463 1225 pH 6.47 6.78 6.89 6.58 Turbidity NTU 1314 1236 1456 1025 Heavy metals Unit M1 M2 M3 M4 Mercury (Hg (II)) µg/L 3.35 1.19 26.27 1.82 Lead (Pb) µg/L 143 3.50 - 473.3 Copper (Cu) µg/L 2166.7 172.4 2083.26 845.6 Fig. 20 FTIR spectra of FCF (a) dried before adsorption (b) after adsorption and treated with nitric acid (HNO3) (c) after adsorption and treated with hydrochloric acid (HCl) 1 3 Page 23 of 27 40 Adsorption (2025) 31:40 Declarations Competing interests The authors declare no competing interests. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit h t t p : / / c r e a t i v e c o m m o n s . o r g / l i c e n s e s / b y / 4 . 0 / . References 1. 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