Waste refractory brick material added chitosan/oxidized pullulan complex gel production and removal of heavy metals from waste water | Scientific Reports

Scientific Reports volume 14, Article number: 26229 (2024) Cite this article

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Wastewater is a by-product of numerous industrial processes that have been demonstrated to have adverse effects on human and natural health due to the pollutants it contains. The pollutants in these substances are organic or inorganic molecules and heavy metal ions that significantly harm the environment and human health. A variety of techniques have been devised for the removal of heavy metal ions from wastewater. The adsorption process has attracted significant attention due to its straightforward implementation, cost-effectiveness, and the environmentally friendly production of adsorbent materials using biocompatible substances. In this study, the removal of Cu2+ ions from wastewater was conducted using chitosan pullulan, a biocompatible and biodegradable polymer. In addition to chitosan and pullulan, waste refractory materials from a furnace used in iron and steel production were added to these polymer materials to increase the adsorption capacity. The initial step involved grinding the waste refractory brick material. Subsequently, chitosan was dissolved in acetic acid. After that, the refractory material was suspended in this solution, facilitating the formation of hydrogel beads using a NaOH solution. The obtained hydrogels were coated with pullulan to produce polyelectrolyte gel. Pullulan was oxidized to 6-carboxypullulan by the TEMPO (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) oxidation method and the negatively charged groups in its structure interacted with the positively charged groups in the chitosan structure to produce a complex gel. The chemical structure, morphological analysis, thermal analysis, and water release analysis of the produced waste refractory brick material added chitosan/oxidized pullulan complex gels were examined. The impact of the 6-carboxypullulan coating on the gels’ properties was elucidated. Furthermore, the adsorption of Cu2⁺ was conducted using solutions containing 100, 500, and 1000 ppm Cu2⁺ ions. It has been observed that the material can clean water with over 98% efficiency, even in solutions that exceed the standards set for wastewater. The material’s efficacy in cleaning solutions with concentrations above the standard for wastewater cleaning is evidence of its high performance. Furthermore, the kinetics and isotherm of the adsorption reaction were examined. The kinetics were determined to be consistent with the Pseudo Second Order (chemical reaction controlled) and aligned with the Langmuir and Freundlich Isotherm (mixed adsorption occurred on homogeneous and heterogeneous surfaces).

Water resources worldwide are contaminated due to the extensive release of wastewater from agricultural, industrial, and urban sources into water streams containing harmful chemicals such as dyes and heavy metals1. The issue of water contamination and its treatment has become increasingly prevalent globally. The presence of metals in wastewater, whether in minute levels or large concentrations, poses a significant threat to the health of living organisms, particularly in industrial settings where metals are widely utilized. Several methodologies for wastewater treatment have been documented in the literature2. These contain physical, chemical, and biological methods widely regarded as successful in water purification. Standard techniques employed to decrease the concentration of heavy metal ions effectively encompass redox treatment, reverse osmosis, electrochemical treatment, bioremediation, coagulation, smoothing of the limestone, precipitation, ion exchange, and membrane filtration and adsorption3. The adsorption technique is well recognized as a cost-effective and efficient approach for purifying water4. Over the past few years, numerous efficient adsorbents have been created and employed for wastewater treatment5. Adsorption is a mechanism of mass transfer where soluble or extractable substances are moved from a fluid phase to the surface of a solid phase. Species that are adsorbed by physicochemical interactions attach to the solid surface6. The adsorption process can achieve a removal effectiveness of up to 99.9%7. Using adsorbent materials, including carbon nanotubes8, Mg–Co9, and chitosan-based materials10, has achieved adsorption efficiency over 99%. The United States Environmental Protection Agency (USEPA) has officially recognized the adsorption process as a very effective and superior methodology for treating wastewater, surpassing other methods11.

Various adsorbents, including hydrogels, have been used in water treatment techniques made by the adsorption method12. Hydrogels are polymeric networks with covalent or physical crosslinks and are generally not very strong in terms of mechanical strength, elastic modulus, or tensile strength; however, these properties can be eliminated by creating nanocomposites obtained by incorporating nanofillers into the hydrogel matrix13. Hydrogels can be formed using a variety of nanofillers, including metallic, polymeric, and carbon-based14. Among these, using polymeric hydrogels in adsorption is essential due to their high adsorption capacity, tunability, biocompatibility, environmental friendliness, and reliability in water treatment applications14. For example, chitosan is a bio-adsorbent material, and chitosan can be converted into a gel matrix and modified using various techniques. It can also adsorb heavy metal ions from a solution15,16.

The study conducted by Duceac et al. demonstrated that pullulan-based hydrogels exhibited biodegradability and biocompatibility, high modification capacity, low toxicity, high water solubility, chemical stability, and strong adsorption capacity17. Aureobasidium pullulans, a polymorphic fungus, synthesizes pullulan, and its structure is made up of maltotriose units linked together by α-1,6-glycosidic bonds; the formation of different coexisting glycosidic bonds gives it unique physical and chemical properties18. The hydroxyl groups in the pullulan structure facilitate its participation in chemical reactions, allowing new derivatives with distinct properties to form. Pullulan oxidation occurs via the replacement of hydroxyl groups with functional groups. The formation of new bonds during the oxidation reaction enhances the mechanical properties of the resulting pullulan19. In the wastewater treatment method, pullulan is initially functionalized by oxidation, after which an ionic crosslinking process creates a pullulan-based hydrogel.

Two distinct oxidation mechanisms, TEMPO (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) and periodate, can be applied in conjunction with pullulan to achieve functionalization. In the TEMPO oxidation mechanism, which was first published by Nooy et al. in 1996, 6-carboxypullulan was obtained by using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) as a catalyst20. In the period oxidation mechanism, first published by Bruneel et al. in 1993, 2,3-dialdehyde pullulan is obtained using sodium periodate21. Then, the functional pullulan is crosslinked with the appropriate polymer to produce pullulan-based hydrogel. This study preferred chitosan as a hydrogel matrix due to its superior properties. Chitosan-oxidized pullulan complex gels are formed by physical bonds between TEMPO-oxidized pullulan and chitosan.

Chitosan, which follows cellulose as the second most prevalent polysaccharide, is composed of N-acetyl-β-d-glucosamine and 1,4-linked β-d-glucosamine residues, and unique components in biomedical, wastewater treatment, food, and other fields are chitosan and its derivatives due to their antibacterial activity, biodegradability, and low toxicity17. Chitosan is produced through the process of deacetylation of chitin. Both deacetylated and undeacetylated groups are present in chitosan. The deacetylated group undergoes protonation and acquires solubility in water at low pH22. The biopolymer chitosan derives its initial properties from its amino groups, displaying unparalleled cationic behavior among polysaccharides in acidic solutions, where protonation of amine groups causes electrostatic attraction and ion exchange, and the free electron pair of nitrogen potentially accounts for its ability to chelate metal cations in nearly neutral solutions23.

The incorporation of diverse agents into the composition of gels has the potential to enhance a multitude of physical and chemical properties, particularly concerning adsorption capacities. Magnesium oxide (MgO) particles are an affordable, secure, and environmentally friendly adsorbent with strong sorption properties, high surface reactivity, and the ability to adsorb large amounts of substances, making them highly promising for wastewater and heavy metal treatment, and they are easy to produce from readily available natural minerals24. The study by Ghanavati et al. is on hydrogel with MgO additive deposited on chitosan, which was investigated for removing heavy metal cobalt II using atomic absorption spectrometry at various concentrations25.

In this study, the synthesis and characterization of oxidized pullulan-coated chitosan hydrogel beads containing varying concentrations of MgO particles were performed. Furthermore, a proposed process will enable cleaning a solution with a very high adsorption capacity and a very high level of contaminants, such as 10,000 ppm. These materials are designed for the adsorption of Cu2+ ions from aqueous solutions. An essential aspect of this research is waste refractory brick material (WRBM) obtained from the furnace of an iron and steel production facility in Turkey. The use of WRBM is crucial in terms of cost-effectiveness and environmental benefits because industrial waste that would otherwise contribute to ecological degradation is reused. By integrating WRBM into the gel matrix, we improve hydrogels’ adsorption capacity and structural properties while promoting a sustainable approach to wastewater treatment. The materials and methods used in the synthesis, characterization, and adsorption experiments are explained in detail in the following sections. The description, contents, and abbreviations of the composites that were produced are given in Table 1.

The chemical structure of the additive material directly affects its adsorption properties. It is, therefore, essential that a detailed characterization of the additive material is carried out. At the end of the life cycle of the ladle furnace refractory material, t was characterized by a process of crushing, grinding, and characterization. The XRD pattern of the ground refractory material is shown in Fig. S1. It was established that the peaks in the pattern correspond to those of MgO (#96-900-6785) and graphite (#96-900-8570). No other phase in the structure was observed. The inclusion of graphite in refractories is because the carbon cannot be readily wetted by the slag, thus preventing the ingress of the slag into the brick and the subsequent corrosion of the MgO26. Moreover, in MgO-graphite refractory materials, the content of MgO ranges from 80 to 93% by weight, while the remainder, approximately 7–20%, is graphite27. A carbon–sulfur analysis was conducted to ascertain the quantity of graphite in the waste material. The results indicated that the structure contained 11.1845% carbon and 0% sulphur by weight. Figure 1 shows the SEM and EDS analysis results of the ground WRBM particles. It was observed that the particles exhibited a near-spherical but heterogeneous morphology (see Fig. 1a,c,d). Furthermore, the EDS map of the particles is presented in Fig. 1b. It was observed that carbon and MgO particles do not bond chemically or physically to each other. Graphite and MgO are not bonded chemically or physically and are mixed with binders to produce refractory materials26. The average particle diameter for MgO particles is about 35 µm, while for graphite, it is about 8 µm. Moreover, the elemental distribution results are presented in Fig. 1e,f. No other elements were observed in the structure except Mg, C, and oxygen. The observed excess weight of C and O can be attributed to the preparation of the sample with carbon tape and the substrate material. Consequently, it was established that MgO and graphite particles were not bonded to each other within the structure. Incorporating MgO particles and graphite particles into the gels was anticipated to enhance the adsorption efficiency28.

SEM images of MgO particles; ×1000 (a), ×2000 (b), ×4000 (c), EDS map of particles (b), elemental composition of particles (e), and EDS energy intensity graph (f).

The FTIR spectrum of pullulan is used as a precursor, and oxidized pullulan is given in Fig. 2a. The broad peak in the spectrum of pullulan in the wavelength range of 3000–3600 cm−1, with a peak at 3319 cm−1, is attributed to the presence of –OH groups in the structure29. A similar peak structure was observed in the FTIR spectrum of oxidized pullulan, but the peak was observed at a different wavelength, 3245 cm−1. It was observed that the peak at 2924 cm−1 was caused by C–H bond vibration30. However, the intensity of this peak decreased in the FTIR spectrum of oxidized pullulan, which was thought to be due to the intensity of the new peaks formed. The peak at 1652 cm−1 was attributed to the O–C–O bond, while the peak at 1345 cm−1 was associated with the C–O–H bond31. The observed peak at 987 cm−1 was attributed to the C–O–C stretching of α-(1 → 4)-glycosidic type linkages29,32. In addition to the bonds, as mentioned earlier in the oxidized pullulan structure, the formation of these bonds can be attributed to the vibration of the C=O bond in the carboxylate group at 1661 cm−1 and the C–O symmetric stretching of dissociated carboxyl groups32,33 at 1427 cm−1. Figure 2b gives FTIR spectra of chitosan beads (MgO_0) and chitosan beads coated with oxidized pullulan (PMgO_0). The infrared spectrum of MgO_0 exhibited a broad peak at a wavelength of 3293 cm−1, which was attributed to the vibration of O–H and N–H bonds. The 859 cm−1 and 1029 cm−1 peaks correspond to the C–O and C–C stretching in the ester bond and the saccharide structure, respectively17. The peak at 1442 cm−1 was attributed to O–H, while the peak at 1640 cm−1 was attributed to N–H17. Coating of chitosan beads with pullulan caused some changes in the FTIR spectrum. The peak observed at 1406 cm−1 in the MgO_0 spectrum is observed to be much more intense at 1373 cm−1 in the PMgO_0 spectrum due to the C–O symmetric stretching of dissociated carboxyl groups in the structure of the oxidized pullulan. In addition, the broadening of the peak around 1640 cm−1 is due to the vibration of the C=O bond in the carboxylate group in oxidized pullulan. Furthermore, the 1427 and 1661 cm−1 peaks in the FTIR spectrum of oxidized pullulan shifted to 1373 and 1638 cm−1, respectively. This indicates that the carboxyl groups in the structure of pullulan and the amine groups in the structure of chitosan ionically interacted, thereby confirming the successful coating of the chitosan beads. Figure 2c presents the FTIR spectra of pullulan-coated gels doped with varying quantities of MgO particles. It was observed that the peak at 400–700 cm−1, caused by the vibration of the Mg-O bond, increased in broadness and intensity as the amount of MgO increased. Moreover, it was observed that the intensity of the peak at around 1400 cm−1 decreased with the amount of MgO and exhibited two peaks. The results demonstrated that MgO addition had a detrimental impact on ionic interaction between chitosan and oxidized pullulan, indicating that MgO addition should be maintained within the optimal amount.

FTIR spectrum of pullulan-oxidized pullulan (a), chitosan bead- pullulan coated bead (b), and MgO particle doped pullulan coated gels (c).

The amount of water trapped by the gels in the structure during the production stage and the release behavior of the gels give an idea about the structure of the gels. The change in weight of the gels depending on time at 60 °C is presented in Fig. S2a. It was established that the gels did not release the water within their structure relatively rapidly. The study conducted by Bercea et al. reported that the gels produced with poly(vinyl alcohol) and carboxyl oxidized pullulan almost entirely released the water within them and reached equilibrium in approximately 60 min at 25 °C29. Furthermore, Duceac et al. observed that chitosan and carboxyl oxide pullulan hydrogels released the water they contained within 20–30 min in PBS solution at 37 °C, pH = 7.4, reaching equilibrium17. The slower release of the gels in this study may be due to the higher amount of pullulan interacting with ionic, and a denser structure may have been obtained due to waiting 7 days for the ionic interaction to be completed. Figure S2b demonstrates the extent to which the gels retained initial weight at the end of the 6th hour. It shows that pullulan coating gels trap more water. The hydroxyl groups in pullulan have a positive effect on the water-trapping behavior. Adding MgO enhanced the water-trapping capacity of pullulan-coated gels, although no discernible change was observed in gels devoid of such a coating. The presence of hydroxyl groups can be attributed to the increasing pullulan interaction, further by the presence of MgO particles, which promote ionic interaction.

To understand gels’ releasing behavior, the gels’ water-releasing kinetics were examined. Figure 3 shows the water release data of different gels during drying at 60 °C, along with their fit to Pseudo-Zero-Order and Pseudo-First-Order models. It was understood that the release kinetics in both types of gels were compatible with Psedou-First-Order, and it was understood that the release mechanism depended on the concentration of water in the gels34,35. It was observed that pullulan-coated gels exhibited a greater fit with the First-Order model. The water concentration was more effective in releasing water from their chemical structure as the number of -OH groups increased.

Releasing kinetics of gels; compatibility of pullulan coated gels with Pseudo-Zero-Order (a), and Pseudo-First-Order (c), compatibility of not coated gels with Pseudo-Zero-Order (b), and Pseudo-First-Order (d).

DSC analysis of gels is given in Fig. 4. MgO_0 gel showed a broad peak at 144.4 °C due to water removal in the structure. At 262.3 °C, an exothermic peak occurred due to the deterioration of the amine groups in the chitosan structure36. The chitosan-pullulan complex gel (PMgO_0) exhibited a broad dehydration peak similar in temperature to that observed for chitosan, at 141.2 °C. However, the thermal decomposition peak decreased to 211.3 °C. The fact that thermal decomposition decreases at lower temperatures indicates that the carboxyl group in the structure of oxidized pullulan and the amine groups in the structure of chitosan interact ionically. It was observed that the thermal decomposition peak exhibited a further increase with adding MgO. Furthermore, the addition of MgO led to the formation of a sharp, endothermic peak between 170 and 185 °C. This peak proves that MgO particles have a secondary interaction with the amine groups in chitosan and the carboxyl groups in pullulan and that adding MgO, as seen in the FTIR results, negatively affects the ionic interaction. It can be thought that the amine and carboxyl groups interact with MgO particles instead of interacting with each other, weakening the ionic interaction. Furthermore, it was observed that the peak intensity and width increased in proportion to the quantity of MgO particles added.

DSC analysis of gels.

The morphology of the adsorbent plays a pivotal role in influencing the performance of adsorption studies37. SEM analysis was conducted to elucidate the relationship between the adsorption performance of the gels and their surface morphologies. SEM images of the gels at different magnifications are presented in Fig. 5; also, surface plots of gels are presented in Fig. S3. It was observed that the surface roughness was significantly elevated in the gels containing particle additives at all three different amounts. Previous studies have indicated that gels produced by freeze drying, primarily using chitosan-pullulan, exhibit high surface roughness and a highly porous structure, with pore sizes ranging from several µm to 380 µm38,39. In contrast to the findings reported in the literature, pores were observed in the structure of the gels. However, the number and size of these pores were found to be smaller and smaller, respectively. This discrepancy can be attributed to the drying of the gels at 60 °C under normal atmospheric conditions before SEM analysis. Furthermore, it has been previously demonstrated that the shrinkage of lyophilized hydrogel structures can form closed pores40,41. It is thought that during SEM sample preparation, the number of pores in the gels decreases, and the size of the existing pores increases due to water evaporation. Moreover, it was observed that the surface roughness increased slightly as the amount of MgO particles increased. The study by Gasti et al. reported that while chitosan/pullulan films exhibited homogeneous surface distribution, surface roughness increased when chitosan-ZnO hybrid nanoparticles were added42. The researchers interpreted this phenomenon as the dispersion of nanoparticles. This study observed an increase in surface roughness, which was attributed to the combined effects of particle dispersion and ionic interaction. Furthermore, pullulan was observed to accumulate in a needle-like manner on the chitosan surface (see Fig. 5g,h). Pullulan oxidized by TEMPO is attached to the physical forces occurring on the outer surface of chitosan beads17. It has been established that the pullulan, via physical forces, accumulates in a needle-like form. In their study, Elangwe et al. observed increased pullulan content in their physically crosslinked hydrogels based on chitosan and pullulan, resulting in a corresponding enhancement in the gels’ porous structure41. The interaction of pullulan with chitosan results in an increase in the porous structure of the material. This study demonstrates that the needle-like form of pullulan promotes porosity.

SEM images of PMgO_0.1 with ×500 (a) and ×1000 (b) magnification; PMgO_0.25 with ×500 (c), and ×1000 (d) magnification; PMgO_0.5 with ×500 (e), ×1000 (f), ×5000 (e), and ×10,000 (f) magnification.

Adsorption reactions involve the adhesion of the target structure (adsorbate) to the solid surface (adsorbent)43. Furthermore, metal ions in the solution can be adsorbed onto the surface of the adsorbent material44. The parameters that are effective in the adhesion of metal ions to the adsorbent surface can be listed as time, temperature, concentration, etc.45. These parameters exert a significant influence on the efficiency of the adsorption reaction and thus have the potential to alter the outcome. Investigating these parameters’ effects on adsorption efficiency is essential to better understand the underlying chemical reaction. In addition, this adhesion can occur on a single surface or multiple surfaces as chemisorption, physisorption, or physicochemical sorption46. The adsorption reaction’s kinetics and isotherms are fundamental to determining the reaction’s nature and limiting factors. This study investigates the influence of diverse parameters on the adsorption reaction, kinetics, and isotherm to provide a comprehensive understanding of the reaction.

One of the critical parameters of the adsorption reaction is time. The change in adsorption efficiency over time is given in Fig. 6a. The adsorption reactions were carried out in 20 mL solutions containing 100 ppm Cu at 25 °C, pH 5.5, and 2 g of each gel. PMgO_0.1, PMgO_0.25, and PMgO_0.50 gels reached equilibrium at 30, 45, and 60 min of the reaction. In addition, the adsorption efficiency was calculated as 96.01%, 93.48%, and 98.47%, respectively. The adsorption process includes three steps: 1. Transfer of heavy metal ions to the adsorbent surface; 2. Diffusion of heavy metal ion molecules into the adsorbent interior; 3. Heavy metal ion molecules interact with reactive sites and the polymer structure47,48. As the quantity of MgO particles increases, the time required to achieve equilibrium becomes longer, and it is interpreted as the particles making it more difficult for the ionic interactions between chitosan and pullulan to occur. Hence, a reduction occurs in the adsorption surface due to decreasing reactive sites of polymers. Despite reaching equilibrium at different times in three distinct gels, the adsorption efficiency of the materials is high, at approximately 95%. While PMgO_0.50 gel provided the highest adsorption efficiency, PMgO_0.25 showed lower efficiency. Even though it has a negative effect on the ionic interaction, the increased amount of MgO particles provides active surface area for adsorption and increases metal ion adsorption49. In addition, the time-dependent adsorbent capacity of the gels was calculated and given in Fig. 6b. It was calculated as 1.92, 1.86, and 1.96 mg/g for PMgO_0.1, PMgO_0.25, and PMgO_0.50 gels, respectively.

Time-dependent adsorption efficiency (a) and adsorbent capacity (b) (20 mL, 100 ppm Cu, pH 5.5, 2 g gels).

Adsorption kinetics provide information about the adsorption time and rate50, determine the rate-limiting step in the adsorption process51, and determine the adsorption mechanism52. The adsorption reaction data were examined for fitting with the Pseudo-First-Order, Pseudo-Second-Order, Elovich, and Weber–Morris models (see Fig. 7). These reactions were done using 20 mL solutions containing 100 ppm Cu2+ ions at 25 °C, pH 5.5, and 2 g of PMgO_0.1, PMgO_0.25, and PMgO_0.50 gels. Equation constants for all 4 models are given in Table 2. Analysis of the R2 values revealed that the adsorption kinetics for all three gels followed the Pseudo-Second-Order model. It was established that the adsorption reaction was chemisorption-controlled22,44. Same as this study, in a study involving pullulan-graft-poly(acrylic acid) and pullulan-graft-poly(acrylic acid-co-acrylamide) gels, it was reported that the swelling kinetics of the gels aligned with the Pseudo-Second-Order model53. The theoretical maximum adsorbent capacities calculated by pseudo-second-order were in close agreement with the maximum adsorbent capacities obtained experimentally. Moreover, “k2” values were calculated as 1.1857, 0.4, 0.197 g/mg·min for PMgO_0.1, PMgO_0.25, and PMgO_0.5 gels, respectively. The rates of reaching reaction equilibrium are listed as PMgO_0.1 > PMgO_0.25 > PMgO_0.5.

The absorption data were compared with the predictions of four different models: (a) pseudo-first-order, (b) pseudo-second-order, (c) Elovich, and (d) Weber Morris.

The structure of chitosan contains both –NH2 and –OH, whereas 6-carboxypullulan contains –COOH and –OH. Chitosan has a high density of these groups that are effective in the adsorption of metal ions, and in this process, the complexation of metal ions on the chitosan surface and electrostatic interactions are the primary mechanisms; free electrons in –NH2 and –OH groups donate their electrons to the D-orbital of electron-free metal cations, forming a complex/chelate54. Similarly, the functional groups in 6-carboxypullulan enable metal ions to adsorb to the surface of the adsorbent material. The kinetic study indicated that the adsorption reaction was of the chemisorption. The chemisorption phenomenon is predicated on the assumption that the rate of adsorption is directly proportional to the concentration of specific adsorption sites on the surface of the sorbents; furthermore, the formation of a chemical bond between the adsorbate and the adsorbent surface is postulated to be the mechanism by which the adsorbate is retained on the adsorbent surface45. It is observed that chemical reactions occur between these groups and Cu2⁺, with the potential for ion exchange to occur as a result of these reactions.

The initial concentration of metal ions is a pivotal parameter that significantly affects the adsorption reaction55. As the initial amount of metal ions increases, it provides a greater driving force for mass transfer. With the increasing driving force, the adsorbate and the adsorbent interaction increases. As a result of this interaction, the adsorption capacity of the adsorbent also increases. In addition, changes in the adsorption rate may occur due to higher collisions at higher concentrations. However, the increased initial metal ion may have a negative effect on the adsorption efficiency. When the saturation point is reached, a decrease in the adsorption efficiency is likely to be observed. The critical point here, i.e., the adsorption capacity, efficiency, and initial metal ion concentration, is the solution volume and the amount of adsorbent material used. Using low-volume wastewater with a high adsorbent amount will cause high adsorption efficiency and low capacity, even at very high concentrations. Adsorption experiments were conducted with solutions containing 100 ppm, 500 ppm, 1000 ppm, 3000 ppm, 5000 ppm, and 10,000 ppm Cu2+ ions using PMgO_0.1 gel (at 25 °C, 45 min., pH 5.5, with 2 g of gel and 20 mL solution). The adsorption efficiencies are presented in Fig. 8a. The solution containing 100 ppm pollutant was cleaned to 98.474%, the solution containing 500 ppm pollutant to 99.34%, and the solution containing 1000 ppm pollutant to 98.955%. Furthermore, the efficacy of cleansing solutions containing 3000, 5000, and 10,000 ppm of pollutants was evaluated, with the results demonstrating a 98.45%, 98.52%, and 97.67% removal efficiency, respectively. PMgO_0.1 gels were highly effective in removing pollutants from a solution containing 10,000 ppm, with a success rate greater than 97%. Moreover, qe values were calculated for different pollutant concentrations and are presented in Fig. 8b. It was calculated as 1.9684, 8.1595, 19.7909, 29.65, 49.26, and 97.67 mg/g for initial concentrations of 100, 500, 1000, 3000, 5000 and 10,000 ppm, respectively. It has been established that as the concentration of the initial metal increases, so does the capacity of the adsorbent. This is due to increased collisions between metal ions and the active surfaces of the adsorbent in the solution56. Consequently, the adsorbent’s capacity increases in proportion to the initial concentration.

Adsorption efficiencies of different initial metal ion concentrations (a) and adsorption capacities (b).

An isotherm study was carried out to understand the adsorption behavior of Cu2+ ions. The compatibility of Langmuir and Freundlich isotherm models with experimental data at different concentrations was examined and given in Fig. 9a,b. Additionally, model constants are presented in Table 3. R2 values for both models were calculated as 0.8995 and 0.9793 for the Langmuir and Freundlich models, respectively. The adsorption isotherm is compatible with Freundlich, indicating that the adsorption reaction occurs multilayer and heterogeneously57. For Langmuir isotherm, the RL values in the Langmuir model were calculated; it was found that the system had a favorable adsorption process in 6 different C0, and the theoretical qmax value was calculated as 117.64 mg/g. The discrepancy between the qmax experimental data and the qmax theoretical value can be attributed to the fact that the adsorption reaction described by the Langmuir isotherm model is not aligned with the observed experimental data. Furthermore, the 1/nf value calculated in the Freundlich model is less than 1, so the reaction is normal adsorption.

Fitting experimental data to Langmuir (a) and Freundlich (b) models.

To investigate the effect of gel quantity on the adsorption reaction, 2, 1, 0.5, and 0.02 g of PMgO_0.1 gel at 25 °C, 45 min, pH 5.5, and 20 mL solution were reacted with solutions containing 100 ppm Cu2⁺ ion. The adsorption efficiencies and adsorbent capacities depend on the ratio of solids to liquids, as illustrated in Fig. 10. The adsorption efficiencies were above 96% in adsorption reactions utilizing 2 and 1 g of gel. However, a decrease in efficiency was demonstrated in reactions using 0.5 and 0.02 g of gel, with the adsorption efficiencies reaching approximately 93%. As the amount of material used increases, the increased surface area provides an increase in adsorption efficiency22. Furthermore, it was observed that the calculated adsorption capacity increased with decreasing gel amount. An increase in the quantity of adsorbent results in an augmentation of the number of unsaturated sites58. Conversely, a decrease in the amount of adsorbent leads to a reduction in the number of these sites and an enhancement in the adsorbent capacity. The gel demonstrated high adsorbent capacity for the adsorption of low-concentration metal ions, as well as for the adsorption of high-concentration metal ions.

Adsorption efficiencies (a) and adsorbent capacities (b) with the varying solid-to-liquid ratio.

Adsorption capacities, efficiencies, initial concentrations, kinetics, and isotherms of different studies are given in Table 4. This study evaluated waste refractory material from our perspective for the first time and used in wastewater treatment. The produced material passed wastewater standards and cleaned wastewater containing 10,000 ppm pollutants with over 97% efficiency. This study also examined the material’s properties, and the adsorption reaction’s kinetics, mechanism, and isotherm were explained.

Waste refractory brick material (WRBM) was supplied from the furnace of a company actively producing iron and steel in Türkiye. Pullulan was provided by a local company in Türkiye. TEMPO (Sigma-Aldrich, 98%), NaBr (Merck, ≥ 99%), and NaClO (Merck, 6–14% active chlorine) were used in the oxidation of pullulan. NaOH (TEKKİM) increased the pH during the oxidation reaction and produced chitosan-based gels. Chitosan (Sigma Aldrich) was used for the gel matrix, and acetic acid (Merck) was used to prepare the chitosan solution. Ethanol (Merck), acetone (TEKKİM, 99.5%), HNO3 (Tekkim, 65%), and de-ionized (produced from Milli-q, ITU) water were used as precipitation, sample preparation agents, etc., in different processes. CuSO4 (TEKKİM) was used for the preparation of wastewater.

The oxidation of pullulan with TEMPO was similar to that in the literature, with some modifications29,32,63. Briefly, the pullulan solution was prepared with 10 g pullulan in 300 mL de-ionized water. Then, TEMPO (0.24 g) and NaBr (1.64 g) were added to the polymer solution under vigorous magnetic stirring (model: MS300HS-MTOPS, 500 rpm, 1 h). After a homogenous solution was obtained, 200 mL of 8% NaClO solution was added. The reaction was continuously monitored with a pH meter to ensure the optimal pH value of 10 for the oxidation reaction. The pH was constantly adjusted to 10 throughout the reaction by using the solution created by adding 2 M of NaOH to 150 mL of de-ionized water. The reaction is considered complete once the pH has reached a stable value. The reaction was stopped by adding a few drops of ethylene glycol and then precipitated with acetone. The precipitated particles were separated from the liquid phase by filtration and freeze-drying (Freezone 4.5, model: Labconco) (24 h, − 80 °C). The experimental scheme is presented in Fig. S1a.

A chitosan solution was prepared by adding 2 g of chitosan powder to 100 mL of 1% acetic acid solution and mixing it in a magnetic stirrer (500 rpm, 30 min) until a homogeneous solution was achieved. The resulting chitosan solution was subsequently divided into four separate 25 mL beakers. 0.1% w/w MgO (0.025 g) was added to the first beaker, 0.25% w/w MgO (0.625 g) was added to the second beaker, and 0.5% w/w MgO (0.125 g) was added to the third beaker, which was then the solution is to get homogenous by ultrasonic homogenizer (model: UP200HT Hielscher) for 30 min. The fourth beaker was not subjected to the addition of MgO. These homogenized solutions were added separately to 50 mL of 20% NaOH solution as 100 µL droplets. They were kept in solution at room temperature for one day to complete gelation and then washed 5 times with de-ionized water. To coat the obtained gels with oxidized pullulan, 5 g of oxidized pullulan was dissolved in 100 mL of de-ionized water. The gels were stirred separately in 25 mL of oxidized pullulan solution for 1 h and then kept in the solution for 7 days to ensure complete ionic interaction. Pullulan-coated chitosan gels containing 0.1% w/w MgO (PMgO_0.1), 0.25% w/w MgO (PMgO_0.25), 0.5% w/w MgO (PMgO_0.5) were produced and the production scheme is given in Fig. 11.

Experimental scheme of oxidation of pullulan (a), production of MgO added pullulan coated chitosan hydrogel beads (b), and coating chitosan beads with pullulan (c).

In adsorption experiments, wastewater was prepared by dissolving the desired CuSO4 in de-ionized water. Adsorption studies were carried out with 20 mL solutions of different Cu2+ ion concentrations using a magnetic heater stirrer. Adsorption reactions were carried out at pH 5.5 and using 2 g of various gels.

Adsorption efficiency was calculated by Eq. (1)22;

C0 is the initial Cu2+ ion concentration, and Ce is the Cu2+ ion concentration at equilibrium. Equation (2) was used to calculate the adsorbent capacity22;

Here, qt is adsorbent capacity (mg/g). Also, the variables in this equation are V, which is the volume of solution (L); M, which is the quantity of adsorbent (g); C0, which is the initial concentration (mg L−1); and Ct, which is the concentration of the solution at a particular time (mg L−1).

Adsorption kinetics were determined by taking samples at certain time intervals from the adsorption reactions carried out with 20 mL of 100 ppm Cu2+ ions containing solution and 2 g gels at 25 °C, pH 5.5. The compatibility of pseudo-first-order, pseudo-second-order, Elovich, and Weber–Morris equations with experimental data has been investigated. Equation (3) gives the adsorption kinetics with linearized Psedou-first-order44;

This equation represents the adsorption capacity at equilibrium and time “t” (mg·L−1) by qe and qt, respectively. K1 (min−1) is the equation constant, and t is the time (min). Based on the concept that the difference in saturation concentration and the amount of adsorbent with time is directly proportional to the rate of change of solute concentration, the pseudo-first-order model is developed64. This model is commonly used to represent physisorption, where the adsorption rate is constrained by the adsorbate’s diffusion over the interface. The linearized Pseudo-Second-Order equation is given in Eq. (4)44;

Here, the variables are the same as in Pseudo-First-Order, and K2 (g/mg·min) is the equation constant. It is acknowledged that chemisorption is the rate limiter in adsorption investigations that adhere to the pseudo-seconder-order equation22. The equation of the Weber–Morris model is also given in Eq. (5)22;

Kd is the model constant (g/mg·min1/2), and t is the time (min.). According to this model, the adsorbate diffuses into the adsorbent’s pores during adsorption65. Moreover, the Elovich equation is represented by Eq. (6)22;

In this case, the adsorption constant is α (mmol/t·g), while the deadsorption is β (mmol/g). When describing heterogeneous and chemisorption processes, the Elovich model is utilized22.

Isotherm models are used to detail the interaction between adsorbent and adsorbate. The Langmuir isotherm model is given in Eq. (7)66;

The variables Ce, qm, and KL represent the equilibrium concentration (mg·L−1), maximum adsorbent capacity (mg·g−1), and Langmuir constant, respectively. It is possible to assume that homogenous adsorption occurs in the monolayer and on the surface of the adsorbent for adsorption investigations that follow the Langmuir model. Moreover, the separation factor (RL) in the Langmuir model is represented by Eq. (8);

The RL value indicates the type of adsorption process: RL > 1 (unfavorable adsorption), RL = 1 (Linear adsorption), 0 < RL < 1 (favorable adsorption), RL = 0 (irreversible adsorption)22. Furthermore, the Freundlich isotherm model is presented in Eq. (9)66;

Here, Kf is the model constant, and the Freundlich exponent, correlated with the sorption intensity, is represented by the parameter nf in the Freundlich equation67. If the 1/nf value is higher than 1, it indicates favorable adsorption, meaning that adsorption is substantial at low concentrations and less significant at higher concentrations. On the other hand, a lower value of 1/nf indicates that adsorption becomes more favorable at higher concentrations. If the adsorption reaction fits the Freundlich model, adsorption occurs heterogeneously in more than one plane68.

The amount of water trapped by the gels during production and the time-dependent release of this water provide information about the physical properties of the gels. Gels were dried at 60 °C under normal atmospheric conditions. The weight change of the gels was calculated using Eq. (10)69;

Here, whydrogel refers to the initial weight, and wt refers to the weight of the gel at time t. The kinetics of time-dependent release are important in interpreting the behavior of gels. Pseudo-Zero-Order and Pseudo-First-Order were used to understand the release kinetics of the gels. Pseudo-zero-order is given in Eq. (11)34;

where k0 is the zero-order release rate constant, mt is the quantity of water released during time t, and mb is the amount of water in solution before release. Pseudo-zero-order indicates that release occurs independently of the gel’s water concentration. Also, Pseudo-First-Order is given in Eq. (12)34;

Here, the variables are the same, and k1 is the model constant. Releasing consistent with Pseudo-First-Order indicates that water release depends on the concentration of water in the gels.

X-ray diffraction analysis (XRD) (Rigaku Miniflex, Cu Kα, 10° ≤ 2Θ ≤ 90) was used for phase analysis of WRBM. Scanning Electron Microscopy Energy Dispersive Spectroscopy analysis (SEM/EDS) (model: FEI Quattro Analytical Scanning Electron Microscope, Thermo Fisher) was used for morphology, elemental distribution, and particle size analysis of WRBM. Carbon–sulphur (Eltra) analysis was carried out to determine the carbon content of WRBM. Fourier Transform Infrared Spectroscopy analysis (FTIR) (Bruker) determined the chemical structure of the precursor materials and gels after completely removing water from their structures. Thermal properties of the gels were determined by Differential Scanning Calorimetry (DSC) analysis (N2 atmosphere, 10 K/min) (204f1, Netzch). The morphological features of the gels were investigated by SEM analysis (model: FEI Quattro Analytical Scanning Electron Microscope, Thermo Fisher) using completely dried gels. The concentration of Cu2+ ions contained in the solutions in the adsorption experiments was determined by an Atomic Absorption Spectrometer (AAS) (Agilent).

In this study, oxidized pullulan-coated chitosan hydrogel beads encapsulating MgO particles at different concentrations were synthesized and evaluated for their effectiveness as adsorbents for Cu2+ ions in wastewater treatment. The synthesis involved the oxidation of pullulan via TEMPO and subsequent coating of oxidized pullulan on chitosan gels. Structural and morphological properties of the synthesized materials were analyzed using XRD, SEM/EDS, FTIR, and DSC techniques. Our results showed that the oxidation of pullulan significantly changed its chemical structure and increased its ability to interact with chitosan through ionic interactions. Furthermore, the MgO particles utilized in this study were sourced from waste refractory bricks. Incorporating MgO particles into the chitosan matrix increased the adsorption capacity and efficiency of Cu2+ ions. Adsorption experiments revealed that the optimized gel composition exhibited high adsorption efficiency and capacity, and the best performance was observed at a specific MgO concentration. It was observed that PMgO_0.1 gel (at 25 °C, 45 min., pH 5.5, with 2 g of gel and 20 mL solution) provided 98.474%, 99.34%, and 98.955% adsorption efficiency from solutions containing 100, 500, and 1000 ppm Cu2+, respectively. Adsorption kinetics followed pseudo-second-order models, indicating the involvement of chemical adsorption processes. Adsorption isotherms are best described by the Langmuir model, which posits monolayer adsorption on a homogeneous surface. In addition, the water release behavior showed that the pullulan coating increased water retention and conformed to the pseudo-first-order release kinetics model. Thermal analysis revealed that the interaction between chitosan and oxidized pullulan and the presence of MgO particles affected the thermal stability of the gels. Morphological analysis showed an increase in surface roughness upon MgO addition; this may be useful for adsorption applications. In conclusion, oxidized pullulan-coated chitosan hydrogel beads encapsulated with MgO particles offer a promising approach for effectively removing heavy metal ions from wastewater. The findings of this study provide valuable information regarding the design and application of biopolymer-based adsorbents for environmental remediation. In this study, evaluating waste raw materials has enabled the production of an innovative material from a biocompatible and biodegradable polymer, providing high adsorption efficiency. Future research can focus on optimizing gel compositions and investigating the adsorption performance of these materials for other pollutants, thereby expanding their scope of application.

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

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The authors thank Prof. Dr. Huseyin Kizil for DSC analysis, Assoc. Prof. Dr. Duygu Agaogulları for FTIR analysis. Also, thanks to Dr. Nuri Solak for the SEM/EDS analysis.

Department of Metallurgical and Materials Engineering, Istanbul Technical University, 34469, Istanbul, Turkey

Emircan Uysal, Halide Nur Dursun, Rasim Güler, Uğur Takmaz, Ayşegül Küt, Mehmet Çeri & Sebahattin Gürmen

Department of Mineral Processing Engineering, Istanbul Technical University, 34469, Istanbul, Turkey

Halide Nur Dursun

Department of Chemistry, Istanbul Technical University, 34469, Istanbul, Turkey

Emre Can Uysal

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Emircan Uysal: Methodology, Validation, Investigation, Formal analysis, Writing—original draft, Writing—review and editing, Visualization, Conceptualization. Halide Nur Dursun: Validation, Formal analysis, Writing—original draft, Writing—review and editing, Visualization. Rasim Güler: Methodology, investigation. Uğur Takmaz: Methodology, investigation. Ayşegül Küt: Methodology, investigation. Mehmet Çeri: Methodology, investigation. Emre Can Uysal: Validation, Formal analysis. Sebahattin Gürmen: Validation, Resources, Supervision, Project administration.

Correspondence to Emircan Uysal or Sebahattin Gürmen.

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Uysal, E., Dursun, H.N., Güler, R. et al. Waste refractory brick material added chitosan/oxidized pullulan complex gel production and removal of heavy metals from waste water. Sci Rep 14, 26229 (2024). https://doi.org/10.1038/s41598-024-72187-4

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