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Apr 05, 2024

Enhancement of adsorption efficiency of crystal violet and chlorpyrifos onto pectin hydrogel@Fe3O4

Scientific Reports volume 13, Article number: 10764 (2023) Cite this article 415 Accesses Metrics details The magnetic mesoporous hydrogel-based nanoadsornet was prepared by adding the ex situ

Scientific Reports volume 13, Article number: 10764 (2023) Cite this article

415 Accesses

Metrics details

The magnetic mesoporous hydrogel-based nanoadsornet was prepared by adding the ex situ prepared Fe3O4 magnetic nanoparticles (MNPs) and bentonite clay into the three-dimentional (3D) cross-linked pectin hydrogel substrate for the adsorption of organophosphorus chlorpyrifos (CPF) pesticide and crystal violet (CV) organic dye. Different analytical methods were utilized to confirm the structural features. Based on the obtained data, the zeta potential of the nanoadsorbent in deionized water with a pH of 7 was − 34.1 mV, and the surface area was measured to be 68.90 m2/g. The prepared hydrogel nanoadsorbent novelty owes to possessing a reactive functional group containing a heteroatom, a porous and cross-linked structure that aids convenient contaminants molecules diffusion and interactions between the nanoadsorbent and contaminants, viz., CPF and CV. The main driving forces in the adsorption by the Pectin hydrogel@Fe3O4-bentonite adsorbent are electrostatic and hydrogen-bond interactions, which resulted in a great adsorption capacity. To determine optimum adsorption conditions, effective factors on the adsorption capacity of the CV and CPF, including solution pH, adsorbent dosage, contact time, and initial concentration of pollutants, have been experimentally investigated. Thus, in optimum conditions, i.e., contact time (20 and 15 min), pH 7 and 8, adsorbent dosage (0.005 g), initial concentration (50 mg/L), T (298 K) for CPF and CV, respectively, the CPF and CV adsorption capacity were 833.333 mg/g and 909.091 mg/g. The prepared pectin hydrogel@Fe3O4-bentonite magnetic nanoadsorbent presented high porosity, enhanced surface area, and numerous reactive sites and was prepared using inexpensive and available materials. Moreover, the Freundlich isotherm has described the adsorption procedure, and the pseudo-second-order model explained the adsorption kinetics. The prepared novel nanoadsorbent was magnetically isolated and reused for three successive adsorption–desorption runs without a specific reduction in the adsorption efficiency. Therefore, the pectin hydrogel@Fe3O4-bentonite magnetic nanoadsorbent is a promising adsorption system for eliminating organophosphorus pesticides and organic dyes due to its remarkable adsorption capacity amounts.

Along with the rapid population increase in the world, supplying food is a significant issue that has to be considered profoundly. In this regard, it seems inevitable not to use plant pests for agriculture in order to supply food. However, this procedure releases pesticides in nature and their excessive utilization1,2. Organophosphorus pesticides are categorized as synthetic pesticides among various pesticide types, which have been employed in industrial agriculture and utilized as insecticides as well as nerve agents in many countries because of the widespread restrictions on organochloride pesticide usage since the 1970s. Near 36% of the total world market of pesticides is consisted of organophosphorus ones3. Chlorpyrifos (CPF) is a chlorinated organophosphate insecticide with high crystallinity, which has been produced and utilized worldwide since 1965 in various forms, i.e., liquid, gel, pellets, wettable powders, etc.4,5. The public’s desire to utilize the CPF pesticide is related to two main reasons, one is its inexpensiveness and the other is its facile access6. The long-lasting durability of the CPF is assigned to its physicochemical features and structural characteristics. CPF is a nonpolar and water low-soluble material with increased partition from aquatic solvents to organic solvents. Because of suppressing the acetylcholinesterase enzyme, similar to other organophosphate forms, the CPF toxic effects occur, which could cause various neurobehavioral effects7,8. Due to a novel statement, researchers have considered diverse CPF effects on target cells.

Moreover, exposure to small amounts of this pesticide causes genetic disorders in humans and genotoxicity9. Organic dyes are toxic, low-biodegradable materials that have carcinogenic impacts on the aquatic organisms according to their stable and aromatic structures10,11,12. One of the widely utilized organic dye with positively-charged nature in industrial applications, including textile, pharmaceutics, etc. is crystal violet (CV)13. Releasing CV in the water sources has become a massive issue because of its threat for mammalian cells for its mutagenic poisonous effects that cause cancer, skin irritation, and cornea complications14. In spite of CPF and CV advantages in industrial applications, namely agriculture and textile, these substances cause severe problems by polluting the environment and threaten human health. Due to the abovementioned downsides, the treatment of the contaminated water from these organic pollutants seems crucial to protect the environment and human health. Until date, various physicochemical procedures have been applied for organic contaminants’ elimination, such as precipitation15, photocatalysis16, adsorption17,18,19, membrane filtration20, degradation21, ion exchange22, etc. Although the stated procedures render highlighted profits, they have several drawbacks in large-scale performance. For instance, forming a concentrated sludge is a time and energy consuming process with low efficacy and expensive operation. Therefore, among all of the processes for contaminant removal from water, adsorption is suggested due to its economical and practicable procedure, enhanced efficacy, selectivity and retrievability, and affordability12,23.

In this line, many adsorption systems have been designed and employed for contaminant elimination such as porous substances24, bioadsorbents25, carbon nanotubes26, graphene-based composites27, polymer-based systems6. Biopolymers with their accessibility, cost fairness, low toxic nature, biodegradability and biocompatibility are well-known precursors to form an eco-friendly adsorption system28. Natural polymers tend to make three-dimensional (3D) networks via cross-linking agents. Various polypeptides29, and polysaccharides, including cellulose30, β-cyclodextrin31, agar32, arabic gum33, etc. have been used in hydrogel nanocomposites with eye-catching properties lying on their structural flexibility, facile functionalization and modifications for the desired purposes34. Apart from the 3D network of the cross-linked natural polymer-based hydrogels, which can enhance the pollutant’s entrapment, the functional groups on the hydrogel have direct interactions with contaminants to improve the adsorption efficiency35,36,37,38. In connection with this, pectin is a biopolymer with high biocompatibility and biodegradability utilized in different facets, including drug delivery, tissue engineering, and adsorption39,40. Due to the nontoxic nature of the pectin biopolymer, it is also employed as edible fibers. These pectin-containing fibers have the role of a barrier against gas and oil molecules or as active material’s carrier, like dyes, odors, and antioxidants41. Moreover, the poor mechanical strength and low adsorption efficiency and low processability of the individual polysaccharide-based hydrogels could be ameliorated by various substances, including inorganic nanomaterials, mesoporous siliceous, and carbonaceous substances14.

As preventing adsorbent loss during its isolation from the reaction mixture matters, preparing magnetic nanocomposites or applying natural substances with innate magnetic feature has been noticed42. In this regard, magnetic nanoparticles (MNPs) integration in the nanocomposites barricade the nanocomposite loss during adsorption–desorption cycles and improves the scale-up efficiency in water refinement and adsorption of contaminants43,44. The incorporation of clay-based materials, i.e., bentonite in the adsorbent system has lighten the way of adsorption capacity improvement whether in single-phase or as a composites’ component45. Bentonite renders enhanced surface reactivity, ion-exchange features, flexibility, nontoxicity, biocompatibility, and high surface area leading to an improved adsorption capacity46. Lately, many studies displayed the impressive effect of bentonite layers incorporation between natural polymer-based hydrogels and investigated their impact on the structural physicochemical characteristics in adsorption process47,48. For example, bentonite layers intercalation with polyaniline polymeric chains led to developing a product with increased surface area, high conduction, mechanical stability, and elevated adsorption capacity49,50. Furthermore, several reports have indicated the bentonite’s beneficial physicochemical features induced to the nanoadsorbent structure by integrating bentonite and many metal oxides or metal-based structures51,52.

Herein, a novel nanoadsorbent was prepared based on natural cross-linked pectin hydrogel with calcium chloride (CaCl2) cross-linking agent via an ionic cross-linking process. Then, it was magnetized with the as-prepared nanoscale Fe3O4 MNPs with a spherical morphology to be conveniently separated from the reaction mixture. Afterward, bentonite clay was introduced to the adsorbent system to elevate the surface area and porosity. The privileges of prepared pectin hydrogel@Fe3O4-bentonite nanoadsorbent are its economic production procedure from natural and inexpensive precursors, the high surface area of 68.904 m2.g−1 as a result of ion cross-linking of pectin hydrogel and the addition of bentonite to the structure with 100.011 m2.g−1 surface area, abundant reactive sites, facile magnetic recovery, as well as its reusability in three adsorption–desorption runs with no remarkable decrease in the yield of the adsorption reaction. This nanoadsorbent obeys the Freundlich isotherm and the pseudo-second-order model for the adsorption process and kinetics of the description, respectively. Also, by the acquired results from the adsorption process, the prepared nanoadsorbet exhibited a high adsorption capacity of 833.333 mg/g and 909.091 mg/g for CPF and CV adsorption from an aqueous media in an optimum condition, respectively. Based on the experimental data, the pectin hydrogel@Fe3O4-bentonite nanoadsorbent rendered a desirable performance in organophosphorus pesticide (CPF) and organic cationic dye (CV) adsorption. The current study exhibits a novel, affordable, eco-friendly, promising adsorbent with a low-cost inorganic cross-linker, advanced adsorption capacity, and high surface area for effective dye removal and dephosphorization from water. The preparation steps are depicted in Fig. 1.

The scheme of pectin hydrogel@Fe3O4-bentonite magnetic nanocomposite preparation pathway.

High-purity materials, including Pectine (Sigma-Aldrich Cas.no, (9000–69-5)), Bentonite (Sigma-Aldrich Cas.no, (1302–78-9)), Ferric chloride hexahydrate (FeCl3·6H2O) (ACS reagent, 97%), Ferrous chloride tetrahydrate (FeCl2·4H2O) (Reagent Plus®, 98%, Sigma-Aldrich), Ethanol (96%, Sigma-Aldrich), Sulfuric acid (ACS reagent, 95.0–98.0%, Sigma-Aldrich), Diethyl ether (ACS reagent, anhydrous, ≥ 99.0%, contains BHT as an inhibitor, Sigma-Aldrich), Ammonia (puriss., anhydrous, ≥ 99.9%, Sigma-Aldrich), Sodium hydroxide (reagent grade, ≥ 98%, pellets (anhydrous), Sigma-Aldrich), Calcium chloride (CaCl2)( anhydrous, powder, 99.99% trace metals basis).

The chemical solvents and reagents applied in this work, viz., FeCl3.6H2O, FeCl2.4H2O, distilled water, ammonia solution (25%), acetone, pectin, calcium chloride, methanol, sodium hydroxide, bentonite, and ethanol were provided by Sigma-Aldrich Company, USA. Based on the preparation process of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent, some analytical and spectroscopic experiments have been taken to authenticate the structural features in every preparation stage as follows. Fourier-transform infrared (FT-IR) spectra by Shimadzu FTIR-8400S model, Japan, apparatus using KBr pellet. The nanoadsorbent’s elemental detection was executed by energy-dispersive x-ray analysis (EDX) (VEGA-TESCAN-XMU model, Czech Republic). The X-ray diffractometer (DRON-8, Saint-Petersburg, Russia) was utilized to record the prepared samples’ X-ray diffraction (XRD) pattern. The morphology of the samples was perused by the Hitachi S-5200 apparatus, Japan, and ZEISS SIGMA, Germany, for the field emission scanning electron microscope (FESEM) investigation. The study of the sample’s magnetic behavior was accomplished by VSM analysis (Meghnatis, daneshpajooh Kashan, Iran). Also, the BAHR-STA 504 apparatus, Germany, was employed to investigate the change in the weight of the samples with temperature enhancement by thermogravimetric analysis (TGA) under an argon atmosphere. The samples’ specific surface area, pore width, and pore volume were determined by the Brunauer–Emmett–Teller (BET, Micrometics ASAP2020, USA). The samples’ surface charge was determined by zeta potential (Bruker, D8 advance, USA). Moreover, the analyses were executed to specify the properties and considerable adsorption performance of the nanoadsorbent.

Co-deposition approach was applied to prepare Fe3O4 MNPs. Initially, 2.35 g of FeCl3.6H2O and 0.86 g FeCl2.4H2O, along with 40.0 mL of distilled water, were magnetically stirred under nitrogen gas flow at 80 °C for 30 min. Then, 15.0 mL of ammonia was added to the suspension to form Fe3O4 MNPs. Ultimately, the black precipitate was magnetically collected and rinsed with distilled water several times; then, it was dried at 70–80 °C for 12 h.

Pectin (0.75 g) was dissolved in 75.0 mL of distilled water at 50 °C for 30 min. Then, 1.5 g of CaCl2 with 30.0 mL of distilled water was injected dropwise and stirred for 40 min. Afterward, 30.0 mL ethanol was added. 0.12 g of sodium hydroxide with 15.0 mL of distilled water was poured dropwise and mixed for 30 min, and the pectin hydrogel was formed. It was taken into the ice bath to fix its gelly nature. After forming and hardening the gel, the reaction solution was washed with 50.0 mL of ethanol and 50.0 mL of distilled water.

First, 0.25 g of pectin hydrogel was dissolved in 50.0 mL of distilled water for 30 min at 50 °C. Afterward, 0.15 g of CaCl2 was slowly poured into the reaction mixture and stirred for 30 min at room temperature. 0.35 g of the as-prepared Fe3O4 MNPs were added into 20.0 mL of distilled water and dispersed via sonication for 40 min53. Next, it was added gently to the reaction solution in 2–3 steps and stirred for 30 min. Eventually, 10.0 mL of methanol, 0.04 g of sodium hydroxide, and 5.0 mL of distilled water were added to the mixture and stirred for 1 h. After performing the mentioned steps, the reaction solution was washed many times with distilled water and ethanol, and a magnetic hydrogel was obtained. The obtained magnetic hydrogel was freeze-dried for 24 h. The pectin hydrogel@Fe3O4 magnetic hydrogel was prepared by an ionic cross-linking procedure. The utilization of Ca2+ divalent cation as a cross-linking agent in this preparation helped to create a 3D structure. The carboxylic acid functional groups on the pectin polymeric chain changed into COO- in an alkaline peripheral caused by NaOH. Also, the hydroxyl groups became O-. These anions have electrostatic interactions with Ca2+. Finally, the precipitate was formed by applying ethanol and diluted NaOH solution.

For the preparation of pectin hydrogel@Fe3O4-bentonite magnetic nanoadsorbent, initially, 1.0 g bentonite was added in 100.0 mL of distilled water and remained for 2 h at an ambient temperature. Then, pectin hydrogel@Fe3O4 (0.25 g) was dispersed in 25.0 mL of distilled water for 20 min without temperature via ultrasonication. Bentonite suspension was added to the pectin hydrogel@Fe3O4 solution and dispersed ultrasonically for another 20 min. In the last step, the obtained solution was stirred at 25 °C for 16 h. Finally, the obtained particles were magnetically separated, rinsed with distilled water and ethanol, and dried in a vacuum oven for 24 h. In this preparation stage, bentonite attaches to the surface of pectin hydrogel@Fe3O4 magnetic nanocomposite from the silicon head, and the pectin hydrogel@Fe3O4-bentonite magnetic nanoadsorbent was formed. Ultimately, ethanol and diluted NaOH solution were utilized to form the precipitate, as shown in Fig. 1.

Experimental investigations were performed to peruse pectin hydrogel@Fe3O4-bentonite magnetic nanocomposite capability for CPF and CV adsorption from aqueous media.

The variable parameters were perused to calculate the adsorption capacity, like pH, adsorbent amount, contact time, and initial concentration of CPF and CV. To investigate the optimum adsorption conditions, various parameters were examined. For pH adjustment in the range of 4 to 9, 0.1 M of HCl and 0.1 M of NaOH were utilized, and 5–25 mg of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent was applied in the 5–25 min contact time, and the initial concentrations of the CPF and CV were 50–400 ppm. Besides, the adsorption isotherms were perused by comparing the experimental results and Freundlich, Langmuir, Temkin, and Dubinin-Radushkevich (D-R) models. Furthermore, the adsorption kinetics were investigated via pseudo-first-order, pseudo-second-order, and Elovich adsorption kinetic models. The concentrations of CPF and CV were determined by a UV–Vis spectrophotometer. In addition, the CPF and CV’s adsorption capacity by the pectin hydrogel@Fe3O4-bentonite nanoadsorbent was measured via Eq. (1) and Eq. (2), respectively6.

where Ci (mg/L) is the initial concentration and Ce (mg/L) is an equilibrium concentration of CPF and CV in an aqueous solution. V (L) stands for the solution volume. And m (g) represents the pectin hydrogel@Fe3O4-bentonite nanoadsorbent’s weight.

The retrievability assessment was executed to give a view of the regeneration potential of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent and the water treatment procedure expenses. In this study, three consecutive reuse cycles in optimum conditions were carried out to observe the pectin hydrogel@Fe3O4-bentonite nanoadsorbent’s regeneration after CPF and CV adsorption. The desorption experiment of CPF was executed as follows. After CPF adsorption by pectin hydrogel@Fe3O4-bentonite nanoadsorbent in optimum conditions, i.e., 10.0 mL of solution volume, 0.005 g of adsorbent dosage, 20 min contact time, solution pH of 7, 300.0 mg/L initial concentration at an ambient temperature, the nanoadsorbent was poured into 10.0 mL ethanol and stirred at 25 °C for 3 h. Moreover, for the CV desorption experiment, after CV adsorption by pectin hydrogel@Fe3O4-bentonite nanoadsorbent in optimum conditions, viz., 10.0 mL of solution volume, 0.005 g of adsorbent dosage, 15 min contact time, solution pH of 9, 300.0 mg/L initial concentration at an ambient temperature, the nanoadsorbent was added into a HCl solution (0.1 M) and stirred at 25 °C. After desorption, the nanoadsorbent was magnetically isolated from the mixture. The released CPF and CV amount were then investigated through a UV–Vis spectrophotometer. The desorption percentage (D%) was calculated using Eq. (3).

where A (mg) belongs to the desorbed contaminant quantity in the elution medium, and B (mg) stands for the adsorbed contaminant quantity by pectin hydrogel@Fe3O4-bentonite nanoadsorbent.

Regarding the preparation steps of the magnetic nanoadsorbent, such as Fe3O4 MNPs preparation, pectin hydrogel formation, ex-situ magnetization of pectin hydrogel by adding the as-prepared Fe3O4 MNPs (pectin hydrogel@Fe3O4), and bentonite incorporation into the composite (pectin hydrogel@Fe3O4-bentonite), different characterizations were employed to investigate the prepared nanoadsorbent’s features as follows.

Pectin hydrogel@Fe3O4-bentonite magnetic nanocomposite was prepared and applied as an efficient nanoadsorbent for CPF and CV removal from aqueous media. As demonstrated in Fig. 1, preparing this nanoadsorbent is comprised of Fe3O4 MNPs preparation via a co-sedimentation approach, pectin hydrogel preparation via employing the Ca2+ divalent cation as a cross-linker, and ultimately, bentonite as a clay material was added to the composite to enhance the surface area. As stated, due to their cross-linked 3D structure, hydrogels render high porosity leading to higher CPF and CV adsorption via the physical capturing of contaminants into the pores. On the other hand, diverse functional groups on the hydrogel, i.e., hydroxyl and carboxyl groups, result in hydrogen bonding and electrostatic interactions due to the CPF and pectin heteroatom structures.

However, the main interaction between the pectin hydrogel@Fe3O4-bentonite nanoadsorbet with negative surface charge (based on the data reported in Table 2) and CV cationic dye is electrostatic interactions. Therefore, the magnetic pectin hydrogel@Fe3O4-bentonite nanoadsorbet with heteroatom structures and clay substances led to an improved organic dye and organophosphorus pesticide adsorption capacity.

Fourier transform infrared spectroscopy (FTIR) is one of the most suitable and conventional techniques for the qualitative identification of the structural functional groups of species and for determining the structure of different molecules, especially organic species. Figure 2 shows the FTIR spectra of pectin hydrogel, Fe3O4 MNPs, pectin hydrogel@Fe3O4, bentonite, and pectin hydrogel@Fe3O4-bentonite, respectively. As depicted in the FTIR spectrum of the pectin hydrogel (Fig. 2a), the formation of the pectin hydrogel is confirmed by the presence of five distinctive absorption peaks. The broad absorption peak that arose at 3425 cm−1 is correlated with the –OH groups’ stretching vibration in hydrogen bonding54. Two sharp peaks at 2917 cm-1 and 1425 cm-1 are due to the C–H stretching and bending vibrations, respectively. Furthermore, the stretching vibrations of –C–O–C in the glycosidic bond and carbonyl group in –COOCH3 emerge at 1049 cm-1 and 1736 cm-1, respectively42,55. As observed in Fig. 2b, the spectrum of Fe3O4 has three characteristic absorption peaks. The vibration peak at 570 cm−1 is related to the bonds between iron and oxygen (Fe–O)6,56. The H–O–H vibration has an absorption peak in the region of 1624 cm−157. Besides, The band at 3403 cm−1 corresponds to the hydroxyl groups’ stretching vibration providing a hydrophilic component for Fe3O4 MNPs and increasing their dispersity58. In the pectin hydrogel@Fe3O4 spectrum (Fig. 2c), all peaks are associated with the pure Fe3O4 NPs and pectin hydrogel. As debated above, the broad absorption peak at 3410 cm−1 corroborates the stretching vibration of the –OH hydrogen bond observed in both pure substances. The peaks at 2920 cm-1 and 1425 cm−1 confirm the pectin hydrogel’s C–H stretching and bending vibrations, respectively. The peak at 1626 cm−1 is attributed to the H–O–H bond of Fe3O4 MNPs. The pectin hydrogel’s stretching vibrations of the –C–O–C group in the glycosidic bond and C = O in –COOCH3 arose at 1049 cm−1 and 1736 cm−1, respectively. Also, a distinguished peak at 560 cm−1 is allocated to the vibrations of the Fe–O bond in Fe3O4 MNPs31,59,60. The presence of all peaks indicates the formation of pectin hydrogel@Fe3O4 nanocomposite.

The FT-IR spectra of (a) Pectin hydrogel, (b) Fe3O4 MNPs, (c) Pectin hydrogel@Fe3O4, (d) bentonite (e) Pectin hydrogel@Fe3O4-bentonite.

Figure 2d represents the bentonite spectrum which resembles amorphous silica, with absorption peaks at 1040 cm−1, 794 cm−1, 524 cm−1, and 473 cm−1, caused by stretching and bending vibrations of SiO42. The Si–O bonds are evident in the silicate structure and are easily identified in the FTIR spectrum by strong absorption peaks in the 1000–1100 cm−1 region. In contrast, 473 cm−1 and 524 cm−1 peaks are caused by Si–O-Al and Si–O-Si bending vibrations, respectively. In addition, the broad absorption peak at 3440 cm−1 is due to the stretching of the –OH hydrogen bond, which corresponds to the frequencies of –OH (silanol group (Si–O–H))61,62.

Figure 2e demonstrates the FTIR spectrum of the prepared pectin hydrogel@Fe3O4-bentonite nanoadsorbent. When the pectin hydrogel@Fe3O4 was functionalized with bentonite binder, the pectin hydrogel@Fe3O4 became more reactive, some changes were caused in the peaks of pectin hydrogel@Fe3O4, and new absorption peaks appeared. The peak that arose at 3420 cm−1 is assigned to the stretching vibration of the –OH, which was observed in all three pure substances. The adsorption peak at 1662 cm−1 is allocated to the vibrations of H–O–H in the Fe3O4 MNPs. The bands observed at 2922 cm−1 and 1420 cm−1 are due to C–H stretching and bending vibrations63. The peak at 1043 cm−1 corresponds to the stretching vibrations of –C–O–C (glycosidic bond) of the –COOCH3 group; all these peaks are related to pectin hydrogel. As mentioned above, the absorption bonds at 1038 cm−1, 794 cm−1, and 470 cm−1 are due to stretching and bending vibrations (Si–O-Si), belonging to four-dimensional SiO42, which is very evident in bentonite structure. Furthermore, the 524 cm−1 peak is caused by Al–O–Si bending vibrations, which are all specific to bentonite material. Ultimately, by combining all the distinctive absorption peaks and the bonding of materials with unique functional groups with a new chemical structure, the prepared magnetic nanoadsorbent was formed.

For the investigation of the sample’s elements and elemental distribution in the sample, the EDX analysis was executed (Fig. 3). The O and Fe elements are assigned to the Fe3O4 superparamagnetic MNPs with 38.22 W% and 61.78 W%, respectively (Fig. 3a). Based on the EDX spectrum in Fig. 3b, two distinctive C and O elements represent pectin hydrogel, showing 49.84 W% and 50.14 W% due to the quantitative results. The integration of Fe3O4 MNPs and pectin hydrogel was successfully confirmed via the presence of the Fe element (8.78 W%) in the pectin hydrogel spectrum (Fig. 3c). According to adding Al (2.49 W%), Si (14.41 W%), and O (57.19 W%), indicating the bentonite structural elements, the formation of the final pectin hydrogel@Fe3O4-bentonite is authenticated (Fig. 3d). Furthermore, Fig. 3e depicts the well-distribution of the elements in the magnetic pectin hydrogel@Fe3O4-bentonite nanoadsorbent.

EDX analysis of (a) Fe3O4 MNPs, (b) Pectin hydrogel, (c) Pectin hydrogel@Fe3O4, (d) Pectin hydrogel@Fe3O4-bentonite nanoadsorbent, and (e) EDX mapping of Pectin hydrogel@Fe3O4-bentonite nanoadsorbent.

The FESEM images were provided to clarify and investigate the samples’ morphology, size, shape uniformity, distribution of particle size, and possible agglomerations. Figure 4a displays the FESEM image of the prepared Fe3O4 MNPs. Although the nanoparticles have a generally uniform size and spherical morphology, they are aggregated in some areas. Besides, the prepared nanoparticles rendered a narrow size distribution of ca. 40–70 nm, participating in the final adsorbent to form the Pectin hydrogel@Fe3O4-bentonite nanoadsorbent. Based on the FESEM images in Fig. 4b,c, the bentonite components demonstrated bullet-like morphology and porous structure. The size and molecular mass of bentonite’s structural phases were not alike; the material’s size was different. The FESEM images of the Pectin hydrogel@Fe3O4 have been taken in 700 nm and 1 μm magnifications, as shown in Fig. 4d,e, respectively. The spherical Fe3O4 MNPs were well placed on the pectin hydrogel substrate, functionalized, and a magnetic hydrogel was formed. However, since the ion cross-linking of the pectin polymer strings was performed in the presence of the as-prepared Fe3O4 MNPs, these MNPs are not simply visible in the SEM images of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent, and the hydrogel network covers them. From the comparison of the FESEM images of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent and pectin hydrogel@Fe3O4 nanocomposite, the modifications made on the surface in order to apply the nanocomposite with better efficiency in water contaminant and chlorpyrifos poison absorption. In Fig. 4f–i, adding bentonite exhibited significant alterations on the surface of the magnetic hydrogel, including smoothing the pores’ surface and forming a layered structure.

FESEM images of (a) Fe3O4 magnetic nanoparticles, (b,c) Bentonite, (d,e) Pectin hydrogel@Fe3O4, (f–i) Pectin hydrogel@Fe3O4-bentonite nanoadsorbent.

The magnetic characteristics of the samples, i.e., Fe3O4 MNPs, pectin hydrogel@Fe3O4, pectin hydrogel@Fe3O4-bentonite nanoadsorbent were conducted via the VSM analysis in an applied magnetic field from − 10,000 to + 10,000 Oe at 25 °C, as displayed in Fig. 5a–c. The magnetic coercivity and magnetic retentivity properties of the Fe3O4 MNPs, pectin hydrogel@Fe3O4, and pectin hydrogel@Fe3O4-bentonite nanoadsorbent are zero. Hence, the prepared samples have superparamagnetic behavior. According to the VSM curve shown in Fig. 5, the magnetic saturation of the bare Fe3O4 MNPs is 56.40 emu/g. Then, the magnetic saturation reduced from 33.76 emu/g for pectin hydrogel@Fe3O4 to 20.53 emu/g for pectin hydrogel@Fe3O4-bentonite nanoadsorbent. The observed decrease in magnetic saturation is related to adding pectin as a natural polysaccharide and bentonite to increase the surface’s active sites. Besides, these materials are non-magnetic, and reducing the magnetic saturation due to the chemical modifications and incorporation of the organic substances on the nanoadsorbent’s surface seems logical. Yet, the magnetic saturation of both pectin hydrogel@Fe3O4 and pectin hydrogel@Fe3O4-bentonite nanoadsorbents is sufficient to be isolated from the mixture by a magnet.

VSM curves of (a) Fe3O4 MNPs, (b) Pectin hydrogel@Fe3O4, (c) Pectin hydrogel@Fe3O4-bentonite nanoadsorbent.

Figure 6 demonstrates the thermogravimetric behavior of the pectin hydrogel, pectin hydrogel@Fe3O4, bentonite, and pectin hydrogel@Fe3O4-bentonite nanoadsorbent, and their stability over temperature rise to 800 °C with 10 °C/min stable heating rate under an air atmosphere by the thermogravimetric analysis (TGA). The thermogravimetric behavior of the pectin hydrogel (Fig. 6a) shows a 2% weight loss at 50–223 °C, indicating the evaporation and removal of the entrapped and absorbed water molecules in the pectin hydrogel’s pores and surface. Further, increasing the temperature to 350 °C shows a 25% weight loss due to organic moieties’ separation and thermal dissociation. The thermal behavior of the pectin hydrogel shows that the degradation of pectin hydrogel starts at ca. 250 °C and continues to 350 °C64,65. Approximately 70% of pectin hydrogel weight decreases between 200 and 550 °C. The residual weight of pectin hydrogel at 800 °C is reported to be about 20%. As can be observed in the pectin hydrogel@Fe3O4 thermogram curve (Fig. 6b), with the temperature increase to 800 °C, ca. 67% of the weight of this sample remains, and the increase in thermal resistance compared to pectin hydrogel is only due to the formation of Fe3O4 inorganic MNPs in its substrate66. The observed gradual weight loss can be assigned to water evaporation from the holes in this structure as well as dehydrogenation or dehydroxylation of its surface.

TGA curve of (a) pectin hydrogel, (b) Pectin hydrogel@Fe3O4, (c) bentonite, (d) Pectin hydrogel@Fe3O4-bentonite nanoadsorbent.

According to the previously reported results, bentonite represents high thermal stability so that more than 91–95% of its weight has retained up to 700 °C67. Bentonite thermogram has demonstrated a continuous but partial weight loss over enhancing the temperature due to the surface’s dehydrogenation or dehydroxylation (Fig. 6c). Moreover, with a quick look at the thermogram of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent in Fig. 6d, a partial weight loss (2%) in the temperature range of 150–250 °C is attributed to evaporating and removal of the absorbed water inside and on the surface of the mesoporous structure. In addition, with the increase of temperature up to 551 °C, a weight loss of about 12% occurs because of the separating and thermal decomposition of the organic moieties of the alkyl chain that covalently bonded to bentonite68. Also, from the weight loss difference of the three samples, ca. 21% of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent’s weight consists of the organic part. As a result, it can be concluded that pectin hydrogel@Fe3O4-bentonite nanoadsorbent has high-temperature resistance.

The crystallinity investigation of the Fe3O4 MNPs, pectin hydrogel@Fe3O4, and pectin hydrogel@Fe3O4-bentonite nanoadsorbent was carried out by X-ray diffraction (XRD) analysis from 10° to 80° (Fig. 7). The Bragg diffraction peaks originate from (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 2 2), and (5 1 1) planes in Fe3O4 MNPs, indicating cubic (fcc) structures of Fe3O4 magnetite NPs, as displayed in Fig. 7a69. Due to Fig. 7b, the pectin XRD pattern have an amorphous structure which is characterized by a peak emerged at 2θ = 20°70. According to the data reported in the articles, the XRD patterns of Fe3O4 MNPs and pectin hydrogel have strong, distinctive peaks at 2θ = 20° and 2θ = 35°, respectively71,72,73,74. Pectin hydrogel@Fe3O4 composite demonstrated both of these peaks at 2θ = 20° and 2θ = 35° as well, which correspond to 2θ of pectin hydrogel and Fe3O4 MNPs (Fig. 7c). A reduction in the peak intensity indicates the chemical interactions between pectin hydrogel and Fe3O4 MNPs. Moreover, based on the pattern of Fig. 7d, the crystalline peaks at 2θ = 19.76º, 21.48º, 28.52º, 35.68º, 62.16º have complied with the bentonite presence in the pectin hydrogel@Fe3O475. In the XRD pattern of pectin hydrogel@Fe3O4-bentonite nanoadsorbent, a decrease in the peak intensity at 2θ = 20° and 2θ = 36° is because of the composition and related to the chemical modifications of the magnetic pectin hydrogel@Fe3O4 surface (Fig. 7e).

XRD pattern of (a) Fe3O4 MNPs, (b) Pectin, (c) Pectin hydrogel@Fe3O4, (d) bentonite, (e) Pectin hydrogel@Fe3O4-bentonite nanoadsorbent.

The BET analysis was carried out to explain the surface behavior and porosity of the bentonite and pectin hydrogel@Fe3O4-bentonite nanoadsorbent using N2 adsorption–desorption. As demonstrated in Fig. 8, the isotherm profiles are type-IV, representing the H4 hysteresis loop at 0.42–0.1 p/p0 pressure. Mesoporous materials are in the type-IV isotherm profile category based on the IUPAC categorization. According to the results, the BET surface area of the bentonite was calculated 100.011 m2/g, demonstrating an advanced surface area compared to the reported studies (Table 1). Furthermore, the pectin hydrogel@Fe3O4-bentonite nanoadsorbent has provided a BET surface area of 68.904 m2/g, which is sufficiently good. Besides, the functionalization of the pectin hydrogel, i.e., magnetization by Fe3O4 MNPs, and pectin hydrogel@Fe3O4 composition with bentonite, a reduction in the pore volume and surface area were observed. Considerably, the as-prepared pectin hydrogel@Fe3O4-bentonite nanoadsorbent demonstrating satisfactory textural features, enhanced porosity, and specific surface area could be contemplated as a potent nanoadsorbent in different water contaminant removal.

The N2 adsorption–desorption isotherms of bentonite and pectin hydrogel@Fe3O4-bentonite nanoadsorbent.

The superficial charges of the as-prepared pectin hydrogel@Fe3O4-bentonite nanoadsorbent were measured by zeta potential test. Zeta potential measurements were performed once before adsorption and once after CV dye adsorption at room temperature. Due to the reported results, at pH of 3, 5, 7, and 9, the measured zeta potential of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent in deionized (DI) water were − 32.2, − 33.1, − 34.1, and − 42.8, respectively (Table 2). The increase in the absolute value of the zeta potential value is related to the pH value increase, as the structural hydroxyl and carboxylic acid groups are deprotonated. More importantly, due to the noticeable negative surface charge of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent, the nanoadsorbent particles do not aggregate at various pH values. By investigating the zeta potential measurements after CV dye adsorption, it is deduced that at pH of 3, 5, 7, and 9, the pectin hydrogel@Fe3O4-bentonite nanoadsorbent’s zeta potential in the CV solution was − 32.8, − 34.3, − 35.9, and − 36.2, respectively. Since the CV dye has a cationic structure, it has robust electrostatic interactions with negatively charged pectin hydrogel@Fe3O4-bentonite nanoadsorbent.

Before optimizing various parameters, the CPF pesticide and CV dye calibration curves are provided in the supporting information file as Figs. S1 and S2, respectively. Different factors, such as the solution pH, employed adsorbent dosage, contact time, and initial concentration of the contaminant, affect the adsorption process of organic dyes and pesticides76,77. Further, the optimization of these effective parameters upgrades the contaminant removal. The pH effect on the CPF and CV adsorption on the pectin hydrogel@Fe3O4-bentonite nanoadsorbent is presented in Fig. 9a. Through the solution pH increase from 4 to 7, the adsorption capacity incremented from ca. 78.6 and 80.2 mg/g to 91.9 and 88.3 mg/g for CV and CPF, respectively. However, by elevating the pH to 8, the adsorption capacity of CPF was reduced to 82.6 mg/g, and the adsorption capacity of CV climbed to 96.5 mg/g. At acidic pH (i.e., pH 5), the CPF adsorption capacity declined and reached 80.6 mg/g because of the competition between the hydroxyl groups of the nanoadsorbent and excess H+ around the adsorbent for unoccupied adsorption sites. On the other hand, the lower CPF adsorption capacity at high pH amounts (i.e., 9) can be assigned to the electrostatic repulsion between extra OH anions and the nanoadsorbent’s OH groups78,79. Also, the leading bondings are the hydrogen bonding between the pectin hydrogel@Fe3O4-bentonite nanoadsorbent and CPF molecules with a non-ionic structure as an organophosphorus pesticide. Thus, the solution pH of 7 provides the minimum amount of cationic and anionic substances, diminishing their impact on the pollutant and nanoadsorbent active sites’ interactions. In this regard, for the CV with a positively-charged structure, the driving forces for its adsorption on the pectin hydrogel@Fe3O4-bentonite nanoadsorbent with negative surface charge are the electrostatic attraction, which led to an enhanced adsorption capacity at higher solution pH. This redult was consistent with Eltaweil et al. study which evinced the proper effect of the neutral and alkaline environment for adsorptive elimination of detrimental cationic dyes80. According to the experimental results, an ideal solution pH for CPF and CV was 7 and 8, respectively, representing the largest adsorption capacity.

(a) Effect of solution pH (4–9), adsorbent dosage (0.01 g), initial concentration (100 mg/L), V (10 mL), Time (15 min), T (298 K), (b) adsorbent dosage (0.005–0.025 g), pH 7 and 8 for CPF and CV respectively, initial concentration (100 mg/L), V (10 mL), Time (15 min), T (298 K), (c) contact time (5–25 min), pH 7 and 8 for CPF and CV respectively, adsorbent dosage (0.005 g), initial concentration (100 mg/L), V (10 mL), T (298 K), (d) initial concentration (50–400 mg/L), pH 7 and 8 for CPF and CV respectively, adsorbent dosage (0.005 g), V (10 mL), Time (20 and 15 min for CPF and CV respectively), T (298 K).

The relation between the nanoadsorbents amount and the related adsorption capacity for CPF and CV was investigated using various pectin hydrogel@Fe3O4-bentonite nanoadsorbent amounts at optimum pH. Based on the results in Fig. 9b, with an increase in the adsorbent dosage from 0.005 g to 0.025 g, the adsorption capacity of the magnetic nanoadsorbent for CPF and CV decreased from ca. 170.2 mg/g and 192.3 mg/g to 37.5 mg/g and 38.1 mg/g, respectively. The availability of CPF and CV with a large amount to be adsorbed by pectin hydrogel@Fe3O4-bentonite nanoadsorbent corresponds to the enhanced adsorption capability at lower adsorbent dosages, which is associated with the increased contaminant amount available for the nanoadsorbent. Therefore, 0.005 g of pectin hydrogel@Fe3O4-bentonite nanoadsorbent was demonstrated to have the most efficient adsorbent dosage for further studies.

The contact time impact on the pectin hydrogel@Fe3O4-bentonite nanoadsorbent to adsorb CPF and CV was investigated from 5 to 25 min at optimum pH and adsorbent dosage. In this regard, the CPF adsorption capacity demonstrates an increasing trend of ca. 200.0 mg/g by the progress of the time to 15 min (Fig. 9c). However, the adsorption capacity decreases slightly to 20 min and has an approximately steady trend to 25 min. The adsorption capacity of CV shows improvement to ca. 178.8 mg/g after 20 min contact time and gradually reduces to 171.6 mg/g after 25 min. Thus, an optimum contact time for CPF and CV was ascertained as 15 min and 20 min, respectively. The increasing trend at the initiation of the adsorption reaction is related to the numerous unoccupied active sites of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent available for interaction with CPF organophosphorus pesticide and CV organic dye, resulting in a quick mass transfer. Efficient interactions among the nanoadsorbent’s functional groups and CPF pesticide and CV dye are progressive until it reaches the maximum equilibrium adsorption capacity after passing 15 min and 20 min of the reaction process. In contrast, no progress in adsorption capacity was observed after the optimum contact time, which is assigned to occupying the nanoadsorbent’s active sites after reaching the maximum equilibrium adsorption capacity.

The employed contaminant concentration was the last parameter debated in this section to determine its effect on the adsorption capacity of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent through regulating the CPD and CV initial concentration from 50 to 400 mg/L at optimum pH, adsorbent dosage, and contact time. Based on Fig. 9d, increasing the CPF and CV initial concentrations from 50 to 400 mg/L enhanced the adsorption capacity to 703.8 mg/g and 665.4 mg/g, respectively. Certainly, by increasing the CPF and CV initial concentration at a constant adsorbent dosage, the CPF organophosphorus pesticide and CV dye adsorbate amount to pectin hydrogel@Fe3O4-bentonite nanoadsorbent ratio was enhanced, leading to an improved adsorption capacity.

The adsorption isotherms were perused to investigate the interactions between the pectin hydrogel@Fe3O4-bentonite nanoadsorbent and CPF pesticide and CV dye. Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R)’s isotherms have been investigated to determine equilibrium adsorption isotherms and compute the highest amount of the adsorption capacity81. Langmuir isotherm (Eq. (4)) is a model to describe the one-layer contaminant adsorption on the adsorbent’s surface. In this case, all of the adsorbent’s superficial sites have similar energy and affinity to interact effectively with contaminants, leading to homogeneous adsorption17. Indeed, the maximum adsorption capacity is considered when a complete monolayer of the contaminants is formed on the adsorbent’s superficial. On the opposite, the Freundlich isotherm (Eq. (5)) defines as a model to describe multilayer adsorption of pollutants on the heterogeneous surface of the adsorbent. Temkin isotherm (Eq. (6)) defines how the adsorbate substances and the adsorbent’s system have interacted and the adsorption procedure’s bonding energies. In this isotherm, the adsorption energy between the adsorbent’s covered surface and adsorbate diminished based on the descending linear pattern. The Temkin model represents the adsorption procedure’s free energy as an adsorbent’s surface coating function82. The Dubinin-Radushkevich (D-R) model (Eqs. (7), (8)) is an empirical isotherm utilized to describe the adsorption procedure based on the pore-filling mechanism. The wide applicability of this isotherm is to represent the adsorption mechanism executed onto homogeneous and heterogeneous surfaces83. The equations of the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherms are defined in Eqs. (4), (5), (6), (7) and (8) , respectively58.

where Ce (mg/L) belongs to the equilibrium concentration of CPF and CV, Qe (mg/g) is the equilibrium adsorption capacity, and Qmax (mg/g) represents the highest adsorption capacity. KL (L/mg) and KF (L/mg) are constants in Langmuir and Freundlich isotherms computed from the plot between Ce/Qe and Ce, and between log Qe and log Ce, respectively. In the case of n > 1, the CPF and CV’s adsorption at high concentrations on the adsorption surface is favorable17. For the Temkin isotherm (Eq. (6)), R represents the universal gas constant, T (K) stands for the temperature, bT is ascribed to the adsorption heat, and KT (L.mg−1) is the constant of the Temkin model. In Eqs. (7), (8) for Dubinin-Radushkevich (D-R) isotherm model, qs (mg P/g) is adsorption capacity-related Dubinin-Radushkevich (D-R)’s constant, KDR (mol2/kJ2) stands for adsorption’s average free energy, R (J/mol K) stands for the gas constant, and T (K) is the temperature. According to the charts of the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherms in Fig. 10a–d, respectively, and based on the data presented in informative Table 3, obtained from Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherms, Freundlich isotherm well matches the experimental information compared to Langmuir, Temkin, and Dubinin-Radushkevich (D-R) isotherms for CPF and CV. The adsorption kinetics linear plots and the computed CPF and CV contaminants’ parameters were exhibited in Fig. 10e–g. Regarding the R2 term, which is corresponded to the correlation coefficient and various amounts of Qe,experimental, and Qe,calculated for CPF and CV pollutants, pseudo-second-order models described the adsorption kinetics. Besides, the R2 term was reported to be 0.9972 and 0.9987 for CPF and CV, respectively, indicating a close value to the unit that matched the pseudo-second-order kinetics model compared to the R2 term calculated from the pseudo-second-order (CPF: 0.9867, CV: 0.9103) and Elovich (CPF: 0.9155, CV: 0.9031) model. Moreover, Table 4 compares CPF and CV’s adsorption capacity on the prepared pectin hydrogel@Fe3O4-bentonite nanoadsorbent with other adsorbents reported in prior studies. Among all of the described adsorbents investigated previously, the prepared pectin hydrogel@Fe3O4-bentonite nanoadsorbent demonstrated a desirable Qmax. Various physicochemical characteristics of the prepared nanoadsorbent, such as facile contaminant diffusion into the 3D cross-linked network and mesoporous structure of the magnetic hydrogel-based nanoadsorbent, a large surface area mainly owed to the bentonite addition, numerous adsorption reactive sites, such as hydroxyl and carboxylate groups, and nanosized Fe3O4 NPs are some of the reasons that cause a great Qmax amount. Therefore, the prepared pectin hydrogel@Fe3O4-bentonite nanoadsorbent is recommended for adsorbing the organophosphorus pesticide and toxic dye pollutants from wastewater.

(a) Langmuir, (b) Freundlich, (c) Temkin, and (d) Dubinin-Raduskevich (D-R) isotherms (condition: initial concentration (50–400 mg/L), pH 7 and 8, adsorbent dosage (0.005 g), contact time (20 and 15 min), T (298 K)). (e) Pseudo-first-order, (f) Pseudo-second-order, and (g) Elovich models (conditions: contact time (5–25 min), pH 7 and 8, adsorbent dosage (0.005 g), initial concentration (50 mg/L), T (298 K)) for CPF and CV respectively.

The retrievability of nanoadsorbent is highly significant for application in industrial facets17. According to the attained results, as shown in Fig. 11, the recyclability of pectin hydrogel@Fe3O4-bentonite magnetic mesoporous nanoadsorbent was accomplished for three consecutive adsorption–desorption cycles. To peruse the desorption, the magnetic nanoadsorbents were added into the HCl solution with 0.1 M concentration and shaken for desorption at room temperature separately to dissociate the nanoadsorbent-CV and nanoadsorbent-CPF bonds. Then, the magnetic nanoadsorbents were isolated by a magnet, rinsed with distilled water, and dried in an oven at 60 °C. Due to the consecutive reusability cycles in three runs, no substantial decrease was detected in the CPF and CV adsorption capacity. In this line, the pectin hydrogel@Fe3O4-bentonite magnetic mesoporous nanoadsorbent is a great system for removing CPF and CV, and it can be conveniently collected without any considerable diminish in adsorption efficiency.

The recyclability diagram of the pectin hydrogel@Fe3O4-bentonite magnetic nanoadsorbent for eliminating (a) CPF and (b) CV for three successive runs.

After the CPF and CV adsorption at optimum conditions, the pectin hydrogel@Fe3O4-bentonite nanoadsorbent was filtered, and the ICP test was taken from the filtrate. Based on the ICP results, the Fe3+ and Ca2+ ions concentration released into the filtrate solution was measured to be 3.21 ppm and 2.09 ppm, respectively. This meager leaching amount is ascribed to the incomplete magnetic nanoadsorbent separation from the reaction mixture after completing the adsorption with a magnet. Hence, the released ions to the filtrate solution were so few, indicating the nanoadsorbent's stability and applicability to be recycled for three successive cycles without any loss in adsorption efficiency.

As previously stated, the pectin hydrogel@Fe3O4-bentonite magnetic nanoadsorbent has a prominent role in organic dye and pesticide elimination according to its enhanced surface area and mesoporosity. Additionally, abundant hydroxyl and carboxyl functional groups of the pectin hydrogel tend to provide a hydrogen bond network with CV and CPF. The electrostatic interaction between CPF and the magnetic nanoadsorbent occurs due to their heteroatom structures. It should be considered that based on the anionic nature of the prepared nanoadsorbent in different acidic, neutral, and alkaline solution pH, as shown in Table 2, and the cationic charge of the CV, the leading interaction between CV and the prepared magnetic nanoadsorbet would be an electrostatic attraction (Fig. 12). Notably, the highest adsorption efficiency related to the most effective electrostatic interaction between the nanoadsorbent and CV occurred at pH = 8 because, in the acidic pH, the produced H+ in the solution acts as a competing species with positively-charged CV for proper binding sites on the surface of the prepared adsorbent; therefore, in acidic pH, the adsorption efficiency reduces. Furthermore, with a quick glance at the remarkable porous structure of the cross-linked pectin hydrogel, plausible physicochemical adsorption, and BET surface area of the nanoadsorbent (68.904 m2/g), which mainly arose from the bentonite addition to the nanocomposite, it can be deduced that CPF and CV can be physically trapped in the nanoadsorbent structure. Overall, numerous pores, OH and COOH groups, and also the heteroatom structure of the nanoadsorbent have promoted the affinity and adsorption capacity toward CPF and CV contaminants.

Possible adsorption mechanism of CV and CPF onto the pectin hydrogel@Fe3O4-bentonite magnetic nanoadsorbent.

In this work, the prepared pectin hydrogel@Fe3O4-bentonite magnetic nanoadsorbent demonstrated an excellent magnetism, due to the presence of the Fe3O4 MNPs in its structure. The magnetic nanoadsorbent was characterized via different analatical approaches, such as FTIR, EDX, FESEM, XRD, TGA, BET, and VSM to find out the precise structural properties of the prepared nanoadsorbent. The adsorption efficiency of the organophosphorus CPF and CV organic dye in an aqueous medium was calculated. The CPF and CV adsorption capacity was 833.333 mg/g and 909.091 mg/g in optimum conditions, respectively. TGA curve displayed ca. 15% weight loss to 800 °C, indicating the enhanced thermal stability of the pectin hydrogel@Fe3O4-bentonite nanoadsorbent. Besides, XRD patterns clearly show the immobilization and uniformity in the distribution of Fe3O4 MNPs in the nanoadsorbents, enhancing the crystallinity of the structure even after adding bentonite. Due to the VSM results, the magnetic saturation of the nanoadsorbent was 20.53 emu/g without magnetic Remanence and coercivity, and it represented the superparamagnetic properties. The FESEM images showed the uniformity in size and shape of the prepared Fe3O4 MNPs throughout the cross-linked structure of pectin hydrogel. Also, after bentonite addition, smoothing the pore’s surface and the formation of the layered structure in FESEM images are evident. The pectin hydrogel@Fe3O4-bentonite nanoadsorbent was introduced as an effective adsorption system for eliminating CPF and CV from aqueous media based on the enhanced surface area, numerous active interaction sites, and effective interactions with contaminants. Generally, the prepared nanoadsorbent was an appropriate material to scale up and industrialization regarding the convenient isolation from the reaction, preparation from affordable substances, efficient binding to the pollutants, and maintenance the structural stability after three consecutive retrievability cycles with mo remarkable decrease in the adsorption yield.

All data generated or analysed during this study are included in this submitted article (and its Supplementary Information files).

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The authors gratefully acknowledge the partial support from the Research Council of the Iran University of Science and Technology (IUST).

These authors contributed equally: Paria Beigi, Fatemeh Ganjali and Fereshte Hassanzadeh-Afruzi.

Department of Physics, Iran University of Science and Technology, Tehran, 16846–13114, Iran

Paria Beigi

Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, Tehran, 16846-13114, Iran

Fatemeh Ganjali, Fereshte Hassanzadeh-Afruzi, Mohammad Mehdi Salehi & Ali Maleki

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P.B.: Data curation-Equal, Formal analysis-Equal, Writing—original draft-Equal, Writing—review & editing-Equal; F.G.: Formal analysis-Equal, Investigation-Equal, Methodology-Equal, Writing—original draft-Equal; F.H.-A.: Data curation-Equal, Formal analysis-Equal, Writing—original draft-Equal, Writing—review & editing-Equal; M.M.S.: Methodology-Equal, Writing—original draft-Equal; A.M.: Conceptualization-Equal, Funding Acquisition-Equal, Supervision-Equal.

Correspondence to Ali Maleki.

The authors declare no competing interests.

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Beigi, P., Ganjali, F., Hassanzadeh-Afruzi, F. et al. Enhancement of adsorption efficiency of crystal violet and chlorpyrifos onto pectin hydrogel@Fe3O4-bentonite as a versatile nanoadsorbent. Sci Rep 13, 10764 (2023). https://doi.org/10.1038/s41598-023-38005-z

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Received: 31 March 2023

Accepted: 30 June 2023

Published: 04 July 2023

DOI: https://doi.org/10.1038/s41598-023-38005-z

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