2.1 Extraction and drying of nopal mucilages

For the experimental work, cactus cladodes of the species Opuntia Ficus-indica (OFI) were collected in the town of San Juan Tlacotenco (altitude: 2378 m.a.s.L.; latitude: 19°01′73″ W and longitude: 99°09′28″ N) in the municipality of Tepoztlan, Morelos, Central Mexico. The conditions of the plots were: igneous-sandy soil of medium depth, pH 6.5 to 8.5, mean annual temperature of 18 °C, and mean annual rainfall of 750 mm. The nopals were harvested according to their age of maturation proposed by Rodríguez-González et al. (2021) to obtain different concentrations of uronic acids (UA) in the extracted mucilage. The choice and harvesting of the nopals was carried out randomly from different plants, after which they were taken to the laboratory. The prickles were removed from all the nopals, and these were washed with running water and neutral liquid soap (Hycel, Mexico) to remove dust and impurities, and then washed with distilled water. Subsequently, the nopals were cut into strips with a polyethylene grater with 3 mm openings to obtain a fine grating.

On the other hand, the mucilage extraction process of the nopals was carried out according to the methodology proposed by Rodríguez-González et al. (2021), it is important to mention that OFI mucilages were not purified; because we wanted to use and work with an inexpensive and quick-to-use coagulant materials for people. For this, grated nopals and distilled water in a 1:1 ratio (wt./v) were added in beakers and placed on a stirrer hot plate at a temperature of 40 ± 2 °C and left for agitation at 300 rpm for 4 h. Then, the cactus mucilage from each of the vessels was filtered separately through a 250 μm aperture mesh and stored at 4 °C for 18 h. Finally, the filtered mucilages were freeze-dried (Genesis 12 SE, model VirTis, U.S.A) under high vacuum (0.04 mbar) for 6 days to obtain the dry powdered OFI mucilages, namely M1, M2 and M3. These were classified according to the age of maturation: M1 for mucilage extracted from prickly pear cactus with 20–35 days of maturation, M2 for mucilage extracted from prickly pear cactus with 80–100 days of maturation, and M3 for mucilage extracted from prickly pear cactus with 550–600 days of maturation.

2.2 Quantification of uronic acids

Quantification of UA of OFI mucilages (M1, M2 and M3) was performed by the carbazole colorimetric method (Dische, 1947). For this purpose, 20 mg of mucilage were dispersed in 10 mL of distilled water at 40 °C for 1 h; then a 100 μL aliquot of the mucilage solutions was mixed with 6 mL of concentrated H₂SO₄ immersed in a stainless-steel vessel with 3 L of chilly water, and then the mixture was placed in water bath for 10 min. Subsequently, the mixture was cooled at room temperature and 0.4 mL of an alcoholic solution of carbazole (0.1 v/v%) was added. The absorbance of this mixture was measured in a UV–vis spectrophotometer (Shimadzu, model 160A, Japan) at λ = 530 nm after 30 min; and the content of UA was determined by a calibration curve using a galacturonic acid as standard (Sigma-Aldrich, Germany) (0.25–1 mg/mL solutions in distilled water were used). The above procedure was performed for each of the mucilages (M1, M2 and M3) separately.

2.3 Study area and sampling

The Yautepec River is in the state of Morelos, central Mexico (latitude 18°57′3″ to 18°34′10″ N and longitude 98°52′14″ to 99°11′14″ W), near the corridor the Chichinautzin biological park, the Iztaccíhuatl-Popocatepetl National Park (Popocatepetl volcano has been active since 1994), the Apatlaco, Cuautla, and Amacuzac river basins. The Yautepec River is 95 km long and has an average annual temperature of 21 °C; approximately 410,000 people live along the river (INEGI, 2018). This water is used for agricultural, livestock, recreational, public, and industrial activities (CONAGUA, 2018). The industries near the river belong to the pharmaceutical, textile, agrochemical, metallurgical, automotive, chemical, food, and paint areas (INEGI, 2018). Also, the Yautepec River water experiences pollutant deposition (cadmium, iron, manganese, lead, sulfates) due to ash and gases spewed by the Popocatepetl volcano (D’Addabbo et al., 2015; Shruti et al., 2018).

The water samples were collected at 10 different points (S1–S10) along the Yautepec River (Fig. 1) in December 2016. These points were selected based on the results obtained by Vargas-Solano et al. (2019). Water samples were stored in high-density polyethylene containers, that were previously washed with neutral detergent and deionized water, and then immersed in a 2 % ultrapure nitric acid solution (HNO₃, Fermont, Mexico). Finally, the containers with water were kept at a temperature of 4 °C until processing in the laboratory (NMX-AA-14, 1980).

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Fig. 1

Map of the study area indicating the sampling sites of the Yautepec River, Morelos central Mexico.

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2.4 Field and laboratory analysis

Turbidity, pH, and EC of the water samples were determined in situ using portable equipment, namely, turbidity meter (HANNAH instrument, model HI 93703, Romania), pH meter (Oakton, model pH Tester 30, Singapore), and conductivity meter (HM Digital, model aqua, China), respectively. The portable equipment was previously calibrated. Subsequently, the pH meter and conductivity meter were introduced into the water river for the determination of pH and EC, respectively; while, for the determination of turbidity, the water samples were placed in a glass cell of the equipment that allowed it to be read. On the other hand, the determination of the total initial concentration of HM in the water samples was performed in the same month of the collected of water as follows; the water samples (500 mL) were acidified with 2 mL of ultrex HNO₃ (Fermont, Mexico), digested and analyzed using flame atomic absorption spectrometry (FAAS) (Perkin Elmer, model Analyst 100, U.S.A) for cooper, cadmium, chromium, nickel, lead, iron, zinc and manganese; and hydride generation for arsenic. Milli Q water was used as an internal control, and a standard reference material (high purity 1.605.428) was used to maintain the precision of analysis. Each of the measures was carried out in triplicate.

2.5 Heavy metal removal experiments

The heavy metal removal experiments in the 10 water samples of the Yautepec River using OFI mucilages (M1, M2 and M3) were carried out using the jar method (ASTM D2035-19, 2019). During the experiments, the water samples presented a pH between 5.9 and 8.5 and a temperature of 25 ± 1 °C. For these experiments, the water samples (500 mL) were shaken at 300 rpm for 1 min, and at that moment OFI mucilage in solution (3.5 mg/mL distilled water) was added; this concentration was used according to reported results (Vargas-Solano et al., 2022b). Afterwards, the water samples were shaken at 80 rpm for 30 min and rested for 60 min; then they were centrifuged at 10,000 rpm for 10 min (Hermle, model 323-K, Germany), and immediately the measurement of turbidity, pH and EC of the recovered water (supernatants) was carried out in the laboratory; also, the concentration of copper, cadmium, chromium, nickel, lead, iron, zinc, manganese and arsenic was determined using the equipment and methodology described above. In this part of the research, the three mucilages studied (M1, M2 and M3) were used in triplicate for each of the 10 river water samples. On the other hand, the initial total heavy metal concentration (IC) in the different water samples was calculated using Eq. (1) (Vargas-Solano et al., 2022b):

Finally, the total percentage of HM removed (%TCHR) from the water samples was calculated using Eq. (4):

2.6 Identification of functional groups in the different OFI mucilages

The identification of the functional groups in the different OFI mucilages (M1, M2, and M3) before and after the removal process was carried out by Fourier transform infrared spectroscopy (FTIR) using a spectrophotometer (Shimadzu, model IR-Affinity 1, Japan) operated in a frequency range of 4000–600 cm⁻¹ and a resolution of 4 cm⁻¹, using an Attenuated Total Reflection (ATR) plate. For this purpose, a dehydrated sample (1 mg) of each of the OFI mucilage powder was placed on the ATR plate; the experiments were performed in triplicate at room temperature.

2.7 Statistical analysis

Statistical analyzes of the UA content and the physicochemical characteristics of the studied water samples were carried out to calculate the mean values, standard deviation and statistical differences using one-way ANOVA and Tukey's test with a significance level of p < 0.05; as well as the correlation matrix between the physicochemical characteristics, the IC, as well as TCHR had a significance level of p < 0.05, 0.01 and 0.001. All tests were performed using Statistica software version 12.0 (2013) (StatSoft Inc.).

3.1 Uronic acid content in OFI mucilages

The content of UA in OFI mucilages (M1, M2 and M3) obtained in the present work and in previous research is presented in Table 1; where it is reported that mucilage M3 had the highest content of UA with 2.22 ± 0.04 mg/mL; this value is statistically different (p < 0.05) to the content of these compounds quantified in mucilages M1 and M2, which presented 1.87 and 1.99 mg/mL, respectively. The content of UA quantified in mucilages M1 and M2 was similar to that reported by Rodríguez-González et al. (2021), who extracted mucilages from OFI with maturation ages of 20 and 80 days, with contents of 1.92 and 2.14 mg/mL, respectively; this result may be related to the same plant growth factor (variety and age of the cladode, type of soil and crop climate). Also, Table 1 shows that the content of UA increased with the age of maturation of the nopals of 20–600 days; this is consistent with reports of purified mucilage extracted from nopals of 80 and 600 days or more of maturation, and with different varieties of the OFI species (Ribeiro et al., 2010). The increase in UA content in the different mucilages has been related to an increased biosynthesis of carbohydrate precursors of homogalacturonan compounds in mucilaginous cells, which have the function of regulating water content in the plant (Baba, 2006; Ribeiro et al., 2010). Furthermore, the difference between the UA content in the OFI mucilage of the present work and the results obtained by other researchers (Ribeiro et al., 2010), can be attributed to the different varieties of Opuntia studied and the purification of the mucilage. Also, it has been mentioned that the UA content in OFI mucilage can differ due to the modifications made in the techniques used to determine the UA content, as well as in the maximum wavelength (λ_max) used in the spectrometer (Li et al., 2007). The results of this section indicate that the change in UA content quantified in OFI mucilages is related to the age of maturation of nopals.

3.2 Physicochemical characteristics of water samples

The values obtained for pH, EC, and turbidity of the river water samples before and after the HM removal process using OFI mucilages are presented in Table 2, where a decrease in pH with respect to its initial value is reported, except in water sample S1. In addition, water samples S2, S3, and S4 presented initial pH values < 8, and after the removal process, the pH decreased between 1.3 and 5.2 %. The other six water samples had initial pH values ≥ 8, and a pH decrease between 3.7 and 15.3 % after the removal process; water sample S10 showed the largest pH decrease. The decrease in pH in the water samples after the removal process did not show a linear trend with the initial pH values; however, it was observed that the water samples with initial pH values ≥ 8.1 showed the largest decreases. These results may be related to a higher or lower proportion of deprotonation of the functional groups present in the molecules that constitute OFI mucilages. That is, in water samples with pH ≈ 8, a deprotonation of the functional groups of the mucilage occurred when incorporated and solubilized in water; in this case, the H⁺ ions of the functional groups were released to water to produce a charge equilibrium. On the other hand, water samples with a pH < 7 presented a lower or slight deprotonation of the carboxyl, carbonyl, and hydroxyl functional groups than waters with pH ≈ 8. This could be due to the lower amount of H⁺ ions and the little deprotonation of the functional groups of the OFI mucilage, which slightly affected the pH of the waters (Choudhary et al., 2019; Nharingo et al., 2015; Onditi et al., 2016; Vargas-Solano et al., 2022b). Finally, Table 2 shows that the river water samples, except for water sample S1, are within the permissible pH limits, according to the international standard (WHO, 2022); the sample pH values are between 6.2 and 8.0. On the other hand, in water bodies, pH conditions can be modified by several factors, including pollution from recreational, domestic, and industrial activities (Nobel et al., 1992), as well as natural substances generated during the rainy season, CO₂ emissions, and climate change (Rahmanian et al., 2015; Sarker et al., 2021).

For EC values, Table 2 shows that this parameter exhibited a decrease in all water samples after the HM removal process with respect to its initial value, except for water sample S1. This decrease is between 4 and 528 μS/cm; however, water sample S1 presented an average increase of 99 μS/cm. The decrease in EC in the water samples may be related to a reduction in the concentration of HM present in them; this is attributed to the removal of HM from the water samples by the application of OFI mucilages as adsorbent material. In the case of water sample S1, the increase in EC may be related to the release of mineral salts such as Na²⁺, K⁺, Ca²⁺ from the mucilage into the water sample during the removal process (Choudhary et al., 2019; Rubio-Arias, et al., 2016; Vargas-Solano et al., 2022b). As seen in Table 2, the water samples collected in the Yautepec River present different values of electrical conductivity. The differences in electrical conductivity after the removal process would be given by the pH and the concentration of the total presence of ions in solution, which affect, to a greater or lesser degree, the hydrodynamics of the mucilage molecule. Also, columns 2 and 3 of Table 2 show the pH and EC of the water samples of the Yautepec River; as can be seen, these do not present a proportional relationship. In the measurement of EC, all the ions present in the water samples contribute to the magnitude of this physicochemical characteristic (Marandi et al., 2013); therefore, the EC values could be due to the composition of the water itself, as well as the presence of contaminants and chemical substances, which were able to ionize and modifying the electrical conductivity of the water samples.

For turbidity, Table 2 shows that after the HM removal process the values of this physicochemical characteristic changed, which are discussed in two scenarios: those related to an increase and a decrease in the turbidity of the water samples. Also, these values showed a dependence on the initial turbidity values. The first scenario was presented in the water samples with initial turbidity values <1 NTU, which after the removal process, had an increase in their final value, which was between 0.46 and 1.9 NTU. The second scenario emerged in the water samples with initial turbidity values between 1.0 and 3.4 NTU, which decreased turbidity between 0.74 and 1.27 NTU. The decrease in turbidity after the removal process was observed in all the water samples that presented turbidity ≥ 1 NTU. Furthermore, these coincide with initial values of pH ≥ 8.1 and pH = 7.8 (sample S4). The increase in this characteristic may be due to an excess of fine particles suspended or dispersed from the added mucilage (organic matter), which had no interaction with the heavy metals or other particles that generate turbidity, due to a low concentration of ions or absence of particles in the water (Belbahloul et al., 2015). Moreover, these findings establish an influence of the initial turbidity and pH values of the water samples; also, the results of the present work are consistent with those obtained by Zhang et al. (2006), who used nopal flour as a coagulant material for turbidity reduction of synthetic water samples with kaolinite; these had initial turbidity values between 56 and 104 NTU. The former authors found a higher coagulation activity of nopal flour in water samples with pH = 10 and nopal flour dosage between 30 and 60 mg/L; under these conditions they obtained removal efficiencies higher than 90 %. Miller et al. (2008) observed the same behavior using nopal flour to remove turbidity from water samples with kaolinite (0 to 125 NTU). They noted a higher coagulation activity of nopal flour in water samples with values of 8 ≤ pH ≤ 10. Finally, the results presented in Table 2 indicate that, after the HM removal process using OFI mucilages, all water samples had turbidity values below 5 NTU, which are within the values established by international and national standards (WHO, 2022; Zhang et al., 2006). The results of this section indicate that the water samples from the Yautepec River, except for water samples S1, after the removal process of HM present pH and turbidity values permitted for human use and consumption, according to the international standard.

3.3 Presence of heavy metals

The concentrations of cooper, cadmium, chromium, nickel, lead, zinc, iron, manganese, and arsenic, quantified in each of the river water samples before the removal process are presented in Table 3, as well as the permissible limits of the HM established in international and national standards (WHO, 2022; Zhang et al., 2006). The results reported in Table 3 show that each water sample presented differences in the concentrations of each of the HM studied, as well as in the initial total HM concentration (IC). Water sample S1 presented the lowest IC with 55.1 μg/L; while samples S7, S8 and S9 exhibited the highest ICs. It is important to mention that sample S1 was collected in a spring located 32 km from the active Popocatepetl volcano; while water samples S7, S8, and S9 were collected in places of the Yautepec River near agricultural, industrial, and urban areas, respectively (see Fig. 1). Sample S1 presented different concentrations of the different HM studied (Table 3); this can be attributed to the presence of volcanic ash with a surface coating of soluble salts, which could have caused an increase in acidity in the water sample (Stewart et al., 2006). Likewise, the high iron and lead values may be related to the presence of dissolved iron oxide from andesite rocks and geochemical elements in basaltic andesites, which dissolve in surface water (Chen et al., 2007). The presence of arsenic in this water sample can be attributed to volcanic emissions and hydrothermal fluids from the active volcano (Jiao et al., 2012; Vargas-Solano et al., 2019).

On the other hand, the high concentration of HM presented in water samples S7, S8, and S9 may be related to the agricultural, industrial, and domestic activities that take place in the sites near the collection areas (Al-Asadi et al., 2020; Chen et al., 2007). Vargas-Solano et al. (2019) reported that some sugarcane crops near the sampling sites are treated with agrochemicals to eliminate pests, so chemical residues are carried to the Yautepec River by runoff during crop irrigation and rainfall. In turn, Nharingo et al. (2015) reported a high concentration of lead, cadmium, and copper in water samples from the Mukuvisi River, which was attributed to contaminated water discharged into the river by industries and residential houses. Table 3 shows that in some water samples the concentration of cadmium, lead, iron, and manganese exceeded the permissible limits established for human use and consumption; while other HM such as copper, chromium, nickel, zinc, and arsenic were present in low concentrations; these did not exceed the permissible limits for human use and consumption, as established in international and national standards (WHO, 2022; Zhang et al., 2006). The low concentrations of such HM in the river water samples may be due to the low solubility of such metals in water and the presence of metal ions that are associated with colloids suspended (sand, silt, and clay) by the adsorption process (Al-Asadi et al., 2020; Campillo-Cora et al., 2020; Mimba et al., 2017).

3.4 Removal of heavy metals

The IC quantified in the ten water samples studied before the removal process and the TCHR by OFI mucilages (M1, M2 and M3) are displayed in Fig. 2, which shows that the TCHR presented different HM removal values. The different letters at the top of the bars for each water sample establish significant differences at p < 0.05. Water samples S1, S3, and S5 (with pH ≤ 8.0) showed the lowest TCHR values, between 4.33 and 5.22 μg/L, 95.1 and 114.6 μg/L, and 98.1 and 121.9 μg/L, respectively. Whereas, the highest TCHR values were exhibited in samples S4, S7, S8, and S9 (with pH > 8.0, except sample 4), ranging between 590.8 and 639.8 μg/L, 586.1 and 625.8 μg/L, 1593.3 and 1621.3 μg/L, and 871.7 and 932.2 μg/L, respectively. The minimum and maximum values of all the above intervals of TCHR correspond to HM removal by mucilages M1 and M3, respectively; while intermediate values of the intervals are related to HM removal by mucilage M2 (see Fig. 3).

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Fig. 2

Concentration of total heavy metals determined in water samples from the Yautepec River before and after the removal process using OFI mucilages M1, M2 and M3. The results in the bars labeled with different letters indicate significant differences for Tukey (p < 0.05).

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Fig. 3

Percentage of heavy metals removed from the Yautepec River water samples with a pH of 8.1 ± 0.1 using OFI mucilages (M1, M2 and M3). The results in the bars labeled with different letters indicate significant differences for Tukey (p < 0.05).

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The differences in TCHR values show a dependence on the mucilage used for removal; these differences can be attributed to the UA content of OFI mucilages due to the presence of the functional groups in the mucilage molecules (Barka et al., 2013; Fox et al., 2012; Nharingo et al., 2015; Vargas-Solano et al., 2022b). Fox et al. (2012) studied the removal of arsenic ions by GE and NE fractions of Opuntia mucilage; and showed that GE removed a higher percentage of arsenic, for a given pH and mucilage dose with respect to NE. This result was attributed to the UA content of pectic polysaccharides present in both fractions. On the other hand, the changes in TCHR values also presented a dependence on the initial pH and IC in each water sample (Barka et al., 2013; Nharingo et al., 2015; Onditi et al., 2016). The dependence of TCHR values on these parameters is analyzed in Sections 3.4.1 and 3.4.2, respectively.

On the other hand, the removal percentages of total heavy metals (%TCHR) in the 10 water samples of the Yautepec River by mucilages M1, M2 and M3 are presented in Table 4; %TCHR values were calculated using Eq. (4). In this Table, it can be seen that the lowest %TCHR values were obtained in all water samples evaluated with mucilage M1, with removal percentages from 7.88 % to 88.6 %, referring to samples S1 and S8, respectively. Meanwhile, when evaluating the water samples with the mucilages M2 and M3, an increase in the %TCHR was observed with respect to the increase in the UA content of the mucilages. In the case of mucilage M2, heavy metal removal percentages between 8.94 % and 89.1 % were obtained, and for mucilage M3, removal percentages between 9.47 % and 90.26 % were obtained, for water samples S1 and S8, respectively. The increases in %TCHR as the UA content of the mucilages used for the removal of HM present in the water samples increased, may be related to the increase in adsorption sites by the functional groups of the uronic acids of the OFI mucilages (Fox et al., 2012). On the other hand, considering the physicochemical characteristics, the removal process can be attributed to the adsorption mechanism due to opposite charges between the functional groups and the heavy metals, as well as the electrostatic interactions of these components and/or adsorption and bridging among the molecules of mucilage and solid particles present in water (Bouaouine et al., 2018; Choudhary et al., 2019; Chamani, 2006; Kalhori et al., 2022; Moosavi-Movahedi et al., 2003; Taheri et al., 2022). This result is consistent with what was reported by Qi et al. (2024), who worked with a carbon/iron compound derived from biomass (FexOy-BC (RM)) and carried out the adsorption of cadmium(II) present in wastewater, they observed that by increasing the dose of the adsorbent compound, an increase in the percentage of metal removal was obtained.

3.5 Study of mucilage with heavy metals by FTIR

The Fig. 4a, b, and c present the normalized FTIR spectra of OFI mucilages (M1, M2, and M3, respectively) before and after the HM removal process for water samples S1 and S10. In general, the spectra show the characteristic bands of the specific functional groups of pectic polysaccharides, i.e., carboxylic acid, carbonyl, and hydroxyl groups (Barka et al., 2013; Bayar et al., 2016; Choudhary et al., 2019; Fox et al., 2012; Nharingo et al., 2015; Vargas-Solano et al., 2022b). Also, the spectra exhibit some major features; the first is a broad band at 3290 cm⁻¹ corresponding to the –OH stretching vibrational mode with hydrogen bonds of the hydroxyl groups. The second is a band located at 2930 cm⁻¹ that concerns the C–H stretching vibration of the methyl ester group of galacturonic acid; this band is more pronounced in mucilage M3 with respect to mucilages M2 and M1 (see the FTIR spectra of the graphical abstract). This can be attributed to the higher concentration of UA in mucilage M3 (see Table 1). The third, is a region between 1716 and 1330 cm⁻¹ that is related to the presence of carboxylic groups in OFI mucilages. The band corresponding to 1716 cm⁻¹ is attributed to the stretching of the C=O bond, a non-ionized form of the carboxyl group. Two other bands were identified at 1590 cm⁻¹ and 1395 cm⁻¹ corresponding to symmetric and antisymmetric COO– stretching vibration, respectively.

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Fig. 4

Normalized FTIR spectra of OFI mucilage before and after the heavy metal removal process from Yautepec River water samples from sites S1 and S10. (a) M1, (b) M2 and (c) M3.

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The fourth feature is a band at 1245 cm⁻¹, referred to the stretching mode of the ether group (C–O–C). Finally, the fingerprint region, the bands at 1200 cm⁻¹ and 800 cm⁻¹ are related to the presence of functional groups of the polysaccharides, namely bending of –OH, stretching and deflection of C–O–C, and CH₃ groups, respectively. In addition, the presence of bands between 1120 cm⁻¹ and 1040 cm⁻¹, which are related to UA (Fox et al., 2012). The intensity of these bands can be correlated with the UA content; such effect is observed in the spectra of the studied mucilages, where the intensities of the bands are higher in the spectrum of mucilage M3 than the bands of M2 and M1 mucilages (see the FTIR spectra of the graphical abstract); which confirms the result shown in Table 1. On the other hand, when comparing the FTIR spectra of OFI mucilages before and after the HM removal process, some differences between them can be observed. The first one corresponds to the decrease in the intensity or disappearance of some bands of the spectrum after the removal process; particularly, a decrease was recorded in the bands corresponding to the frequencies of 3290 cm⁻¹, 2930 cm⁻¹, as well as in the region between 1716 and 1330 cm⁻¹ and in the fingerprint region. Also, the disappearance of the bands referring to the frequencies of 1716 cm⁻¹ and 1245 cm⁻¹ is observed. This may be related to the interaction of HM and functional groups present in OFI mucilage molecules (Fox et al., 2012; Nharingo et al., 2015; Vargas-Solano et al., 2022b). The second difference concerns a phase shift of the FTIR spectra before and after the removal process in the bands corresponding to the frequencies of 1590 cm⁻¹, 1512 cm⁻¹, 1395 cm⁻¹ and 1040 cm⁻¹; these frequencies correspond to the participation of carboxylic acids and polysaccharides in the removal of HM (Barka et al., 2013; Bayar et al., 2016; Choudhary et al., 2019; Fox et al., 2012; Nharingo et al., 2015; Vargas-Solano et al., 2022b).

The results obtained are consistent with those reported by other researchers, who evaluated nopal mucilage in the removal of metal ions (Fox et al., 2012; Nharingo et al., 2015; Onditi et al., 2016). Nharingo et al. (2015) used OFI mucilage for Pb²⁺ removal and reported changes in the FTIR spectra of mucilage before and after removal; these changes were attributed to the electrostatic interactions between the functional groups present in the pectic molecules and compounds of the OFI mucilage with the Pb²⁺ ions. In the present study, the FTIR spectra of OFI mucilage before and after the HM removal process underwent changes, namely, dephasing, decrease in the intensity and disappearance of some bands; these changes result from the electrostatic interactions between the functional groups present in the mucilages and the HM.