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Article

Composite Copolymer Beads Incorporating Red Mud for Water Amendment by Adsorption—Oxidation Processes

by
Teodor Sandu
1,
Elena Alina Olaru
2,
Raul-Augustin Mitran
3,
Andreea Miron
1,
Sorin-Viorel Dolana
1,
Anamaria Zaharia
1,
Ana-Mihaela Gavrilă
1,
Marinela-Victoria Dumitru
1,
Anita-Laura Chiriac
1,
Andrei Sârbu
1,* and
Tanța-Verona Iordache
1,*
1
National Institute for Research and Development in Chemistry and Petrochemistry—ICECHIM, Splaiul Independenței 202, 6th District, 060021 Bucharest, Romania
2
PROTMED Research Centre, University of Bucharest, Splaiul Independenței 91-95, 5th District, 050107 Bucharest, Romania
3
Ilie Murgulescu Institute of Physical Chemistry, Splaiul Independenței 202, 6th District, 060021 Bucharest, Romania
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2024, 14(14), 6386; https://doi.org/10.3390/app14146386
Submission received: 14 June 2024 / Revised: 8 July 2024 / Accepted: 19 July 2024 / Published: 22 July 2024
(This article belongs to the Special Issue Pollution Control Chemistry II)
Browse Figures
Versions Notes

Abstract

:
We face significant environmental pollution problems due to various industries, such as the aluminum industry, which generates large amounts of red mud (RM) waste, or agriculture, in which case the use of pesticides creates huge water pollution problems. In this context, the present study offers a better perspective to originally solve both environmental issues. Thus, the main target of the study referred to using RM waste as a filler for preparing composite copolymer beads. Thereafter, this can achieve significant removal of water pollutants due to their adsorption/oxidation characteristics. As evidenced by the changes in chemical structure and composition, thermal stability, morphology, and porosity, RM was homogenously incorporated in poly(acrylonitrile-co-acrylic acid) beads prepared by wet phase inversion. The final assessment for the removal of pesticides by adsorption and oxidation processes was proven successful. In this regard, 2,4-dichlorophenoxyacetic acid was chosen as a model pollutant, for which an adsorption capacity of 16.08 mg/g composite beads was achieved.

1. Introduction

Aluminum is a structural metal found in many domains, being the third most abundant element in the earth’s crust. The primary source of aluminum is bauxite, from which it is extracted through a two-stage sequence: bauxite refining to alumina (Bayer approach), followed by the actual obtainment of aluminum by alumina smelting (according to Hall-Heroult) [1]. However, aside from aluminum, red mud (RM) by-product is yielded in large amounts and needs large land surfaces for disposal. The situation worsens as climatic conditions (rain or wind) determine the spread on larger land surfaces or, even worse, in the water table. RM is called as such because of its high iron content, which gives it a red color. RM is highly alkaline due to the bauxite digestion with caustic soda, its pH attaining values of 10.5–12.5. Therefore, RM negatively impacts the environment, requiring recycling measures, which is highly desirable given its content in metal oxides, mainly iron oxides [2,3,4,5,6,7].
Although RM also finds use in the construction industry [8,9,10], the main ways of recycling RM refer to its use as a coagulant, adsorbent (anions, heavy metals), and catalyst (hydrogenation; dechlorination and hydrodechlorination; a catalyst for the pyrolysis of municipal wastes) [11,12,13]. Also, it may be regarded as an alternative source of metal oxides (Al2O3, Fe2O3, TiO2) [14]. Tandekar describes using RM as a source of mixed oxides to develop composites with water decontamination features [15]. Hence, one research direction uses RM for water amendment, relying on its content in adsorbents and flocculants [16,17]. RM and materials thereof developed in different studies were able to remove several classes of compounds: hydrogen sulfide [18], hexavalent chromium [19], phosphate [20,21,22], phosphorous [23], dyes [24,25,26,27], leather industry wastewaters [28], nickel [29,30,31], ferricyanide [12], and lead [13]. However, as taken from the landfill, RM is difficult to use as such for water reconditioning. Thus, it must be modified or pre-treated with acid, seawater, or carbon dioxide before use [32].
Recently, it has been observed that compounding the RM with polar polymers to yield composite materials can represent a more efficient way to recycle RM and restore water quality [33,34]. In this sense, it should be mentioned that several studies have tested different polymers for this purpose. These include polyvinyl alcohol [2,35,36,37,38], alginate [39], polypropylene [40], polyaniline [41], poly(2-chloroaniline) [42], polyvinyl chloride [43], epoxy resin [11], acrylonitrile-butadiene-styrene [44], polyethylene [45], and poly(hydroxyl ether) of bisphenol A (PHEBA) [46]. For instance, Babu et al. [39] developed zinc alginate beads and impregnated them with zirconium-treated RM, which was further used to remove lead from water. Another polymer of interest for RM compounding into composite materials is polyacrylonitrile (PAN). This is a well-known precursor for materials with adjustable porosity, such as beads, membranes, and fibers [47,48,49,50,51]. PAN exhibits chemical, mechanical, and thermal stability [49,50,51,52,53,54,55], availability, and low cost. Nonetheless, in the context of water treatment, the use of PAN alone is drawbacked by its brittleness and hydrophobicity [56]. Moreover, using PAN by itself is unsatisfactory because of fouling issues. However, these shortcomings can be overcome by copolymerization of acrylonitrile (AN) with other acrylic monomers, such as methacrylic acid, acrylic acid, vinyl acetate, glycidyl methacrylate, N-vinylimidazol, hydroxy ethyl methacrylate, vinyl pyrrolidone, acrylamide, or vinyl chloride [49,57,58,59,60,61].
Another advantage of PAN copolymers for preparing composite materials is their ability to coagulate in non-solvents. This makes them suitable for developing materials of various shapes and sizes using a less complex method known as wet phase inversion (WPI). In this approach, the copolymer solution is poured dropwise in the coagulation bath (containing the non-solvent) to form beads [62], or it can be poured onto suitable surfaces, followed by immersion in the coagulation bath, to form films and membranes [63]. Abbasi et al. [64] used this technique to prepare zeolitic imidazolate framework ZIF-8/polymer composite beads, and Zeng et al. [65] prepared hollow carbon beads, followed by carbonization, for oil sorption. Sattar et al. also used WPI to develop poly(lactic acid)/activated carbon beads to retain Rhodamine B [66]. In contrast, Tandekar et al. developed RM-chitosan beads with adsorptive features toward three types of ions, i.e., fluoride, chromate, and phosphate [15].
Therefore, starting from the presented literature, this work aimed to develop copolymer composite beads based on PAN copolymers and RM, capable of removing pesticides efficiently by an adsorption/oxidation mechanism. For this purpose, the study considered a model pesticide, 2,4-dichlorophenoxyacetic acid (abbreviated 2,4-DCFA), a hard-biodegradable pesticide commonly used in plant protection. Its effect is very harmful if it reaches water sources, the half-life being several weeks under aerobic conditions and over 120 days under anaerobic conditions [67,68,69,70,71]. Among the procedures for removing this pesticide, photocatalytic degradation, ultrasounds, electrocoagulation, adsorption on activated carbon [71], and removal by iron nanoparticles or magnetic materials (magnetite) [33] have been reported so far.

2. Experimental

2.1. Materials

The first component of the composites is a copolymer of acrylonitrile (AN) and acrylic acid (AA), poly(AN-co-AA), notated PAN-co-PAA for convenience. Both monomers, AN and AA, over 99% purity and supplied by Merck (Darmstadt, Germany), were distilled for the inhibitor removal. Copolymerization was promoted using a redox initiation system consisting of potassium peroxydisulfate (KPS, p.a.) and sodium metabisulfite (MS, p.a) (both acquired from Reactivul, Bucharest, Romania) under acidic conditions accomplished with the use of concentrated sulfuric acid (technical-grade, provided by Reactivul, Bucharest, Romania). The preparation of the targeted composite beads also involved the use of specific additional reagents: dimethyl sulfoxide (p.a., Merck, Darmstadt, Germany) for dissolution of copolymers and isopropyl alcohol (p.a., Merck, Darmstadt, Germany) for bead formation by phase inversion. The second component of the composite beads was the RM inorganic filler, which was kindly provided by Alumol Plant (Tulcea, Romania) as a slurry, with the following chemical composition: 41.1% Fe2O3, 1.6% FeO, 17.5% Al2O3, 7.3% SiO2, 5.7% CO2, 3.6%Na2O; 7.5% CaO; 5.2% TiO2, 9.5% H2O; 15.1% residue of calcinations at 1000 °C. Prior to its use, RM was dried and neutralized with CO2 up to pH7. The pesticide, 2,4-dichlorophenoxyacetic acid (abbreviated 2,4-DCFA, chemical structure given in Figure 1A), chosen as a model pollutant to verify the decontamination efficiency of developed composite beads, was provided by Merck (Darmstadt, Germany).

2.2. Synthesis of Composite Copolymer Beads

2.2.1. Preparation of PAN-co-PAA

For a reliable evaluation of composites’ performance, the study employs three copolymer compositions for the PAN-co-PAA, prepared by changing the weight ratio between the two monomers in the initial mixture submitted to copolymerization, AN and AA, as follows: 95% AN- 5% AA (copolymer index C1), 90% AN- 10% AA (copolymer index C2), and 80% AN- 20% AA (copolymer index C3), respectively. The three copolymers were prepared identically (Figure 1B), according to the previously described procedure [72]. Briefly, in the 4-necked flask, where the copolymerization process takes place, 325 mL of distilled water out of the total of 425 mL is introduced (the rest being kept for dissolving the components of the redox initiation system, 50 mL for KPS and 50 mL for MS), after which sulfuric acid is added drop by drop until the pH reaches 2.5. The medium is heated to 45 °C under nitrogen gas, after which AN, AA, MS solution, and KPS solution are added in this sequence. Both the concentration of MS and that of KPS represent 1 wt.% relative to the monomer mixture.

2.2.2. Preparation of Copolymer Composite Beads

In the following step, the copolymer beads with RM content were prepared in the following sequence (as presented in Figure 1B): (i) preparation of an 8 wt.% copolymer solution by the dissolution of the copolymer (2 g) in DMSO (21 mL) at 70 °C and stirring 200 rpm (approximately 2 h until homogenization); (ii) integration of the RM powder (0.16 g, representing 8 wt.% relative to the copolymer) in the previously prepared copolymer solution to obtain a precursor composite solution and maintaining the temperature and stirring up to 2 h; (iii) formation of composite beads following a phase inversion process, which involved dripping the precursor solution, with the help of a syringe, over a coagulation bath (containing 75 mL isopropyl alcohol and 75 mL water). For each copolymer composition (C1, C2, and C3), two sets of samples were prepared (as shown in Figure 1C), namely, the test beads with RM (noted accordingly as 1-RM, 2-RM, and 3-RM) and the control beads without RM (noted accordingly as 1-Ref, 2-Ref, and 3-Ref).

2.3. Characterization Techniques for Composite Beads and Instruments

2.3.1. Characterization of Composite Beads

To evaluate the properties of composite beads vs. their control, samples from each series of beads were previously dried by lyophilization as follows: beads freshly taken from the liquid medium were placed in Falcon tubes, kept in the refrigerator (at −20 °C) for 24 h, and then lyophilized (at −55 °C) for 48 h.
Structure and composition: Fourier-Transformed Infrared (FTIR) spectra were recorded in Attenuated Total Reflectance (ATR) mode on a Thermo Fisher Summit Proequipment (Madison, WI, USA) running 16 scans at 4 cm−1 resolution in the 4000–400 cm−1 range. The lyophilized beads were ground in a ball mill before analysis.
Thermal stability: Thermogravimetric analyses (TGA) coupled with differential thermal analysis (DTA) were performed using a Mettler Toledo TGA/SDTA851e thermogravimeter (New Castle, DE, USA) at a heating rate of 10 °C/min, under 80 mL/min synthetic air flow in the 25–1000 °C temperature range.
Morphology: The morphology of beads was investigated using Scanning Electron Microscopy (SEM). SEM images for the reference and hybrid samples were recorded on a Hitachi TM4000 plus II benchtop (Tokyo, Japan) at an accelerating voltage of 15 kV with a cooling stage. Before SEM analysis, samples were uniformly coated with gold by cold sputtering using a Sputter Coater Q150R ES Plus (Quorum; Cambridge, UK) to create an electrically conductive thin film (~5 nm film thickness), thus inhibiting “charge-up”, reducing thermal damage and increasing the emission of secondary electrons. The lyophilized beads were cut in two for analyzing the inner morphology.
Porosity and specific surface: Using nitrogen porosimetry, the specific surface area of beads, the pore surface area, pore size, and volume were evaluated using a NOVA 2200 e (Quantachrome Instruments, Boynton Beach, FL, USA) under a nitrogen atmosphere at −196 °C (liquid nitrogen temperature). The lyophilized beads were ground in a ball mill before analysis and degassed at 40 °C for 4 h under vacuum.

2.3.2. Evaluation of Removal Efficiency for 2.4- DCFA Pesticide

A pesticide solution containing 100 mg 2.4- DCFA/L, with a theoretical content of 43.43 mg of carbon/L, was used for this assay. For reliability, the 100 mg 2,4DCFA/L solution was also analyzed at pH = 3.46 to determine the real concentration of carbon, i.e., 46.35 mg carbon/L. The removal of 2.4- DCFA was assayed by both adsorption and oxidation processes. In both cases, a preliminary stage of sample preparation was required. In this respect, the beads were washed with ultrapure water (510 mL) on a glass frit in portions of 15 mL and kept afterward in ultrapure water under orbital agitation for 24 h.
Two samples (1-Ref and 1-RM) that showed the highest TOC values after the first washing step underwent a second washing step, being exposed for another 24 h to orbital agitation. The aim was to remove alcohol traces from the internal pores of the material. The total organic carbon (TOC, centralized in Table 1) in the washing waters was determined by approaching the UV-persulfate method and using a HiPerTOC device (Thermo Electron, Waltham, MA, USA).
After proper purification, the bead samples were placed in glass ampoules, dried at 50 °C for approximately 15 h in an oven with ventilation, and then kept in the desiccator until testing as an adsorbent and oxidizing agent for 2,4-DCFA. The adsorption tests were carried out on volumes of 15 mL of 100 mg/L 2,4-DCFA solution in glass vials with stoppers using a vertical stirrer (GFL 3025) at a constant temperature of 20 °C for 24 h. The tests were carried out without pH correction. The following oxidation tests were carried out in a closed system on 50 mL of 2,4-DCFA for 4 h with orbital stirring of 200 rpm in the presence of 1 mL of hydrogen peroxide (30%). For the maximum dosage of released oxygen, 1 mL of oxygenated water diluted with 25 mL of ultrapure water (to which 5 mL of H2SO4 was added) was titrated with a solution of KMnO4, 0.02 M, F = 1.000 (1 mL resulting in 6.48 mmole oxygen). Some of the composite beads recovered after adsorption/oxidation (i.e., 3-RM) were analyzed by FTIR, and the other composite beads were washed with 3 portions of distilled water, dried in the oven at 50 °C until constant weight, and analyzed for structure modifications by FTIR vs. initial composite beads.

3. Results and Discussion

3.1. Investigation of Structure and Composition for Composite Beads

The spectra were recorded comparatively for the composite beads with RM content (1-RM ÷ 3-RM) vs. the reference counterparts (1-Ref ÷ 3-Ref) and the precursor materials (copolymers and RM alone) to determine the effect of the addition of RM on the chemical structure and composition and the importance of the copolymer/RM ration in the composite beads (Figure 2A–C). In the area of low wavelengths, the signals corresponded to the sulfate ions from the redox initiation system, with symmetric stretch ~950 cm−1 [73] (for beads and copolymers) and stretching of S=O from DMSO around 1030 cm−1 (beads) [74], and overlapped with the signals in RM corresponding to the Si-O-Al bonds (~1000 cm−1) in all the spectra of composite beads (1-RM, 2-RM, and 3-RM).
Figure 2A shows the FTIR spectra for the series 1-RM and 1-Ref, which reveals the characteristic bands of the copolymer at 3386 cm−1 and 1730 cm−1 (stretching O-H and C=O, respectively, from the –COOH group of AA), 2243 cm−1 (stretching C≡N bond of AN), and 2926 cm−1 (stretching -CH2 copolymer backbone) [72]. In this series (1-Ref vs. 1-RM), no significant differences can be observed between the analyzed samples. However, the band’s intensity at 1450 cm−1 is visibly lower after RM integration. This latter band is attributed to the in-plane deformation of the aliphatic CH2 groups in the copolymer [75]. The band’s intensity reduction can be attributed to physical interactions of the backbone with RM, which reduces the intensity of vibrations. Figure 2B (2-RM and 2-Ref spectra) indicates the appearance of characteristic bands of the copolymer at 3428 cm−1 (O-H), 1735 (C=O), 2241 cm−1 (C≡N), and 2929 cm−1 (-CH2), but also the decrease in the band intensity at 1456 cm−1 in the spectrum of 2-RM. For the last series of samples, 3-RM and 3-Ref, the spectra (Figure 2C) are somewhat different from the previous series, presenting (i) a large hump in the region of 2300–3600 cm−1, which can be attributed to the higher capacity of the C3 copolymer to retain intramolecular water (C3 contains the highest amount of AA 20%) [72], (ii) lower intensities of characteristic bands from the copolymer (3447 cm−1, 1741 cm−1, 2238 cm−1, and 2926 cm−1) as a result of increased copolymer–RM interactions, and (iii) a significant decrease in the band intensity characteristic for -CH2 deformation that shifts to 1436 cm−1 after the addition of RM (3-RM spectrum).
The following TGA results (Figure 3) were used to characterize the thermal stability of beads and compute the RM content after complete combustion of the copolymeric phase.
For all copolymers and composite beads, three distinct mass loss ranges can be noticed between 25 and 200 °C, 220 and 500 °C, and 500 and 750 °C (Figure 3A–C). The first mass loss is accompanied by endothermic effects (Figure 3D–F) caused by drying and loss of residual solvent molecules. The following two decomposition steps above 200 °C are accompanied by exothermic events (Figure 3B,D,F), indicating oxidation thermal decomposition reactions. The 220–500 °C mass loss events correspond to the thermal degradation of PAN-co-PAA, and the 500–750 °C mass loss ranges correspond to the complete combustion of the residual carbon and organic moieties [76,77].
RM alone exhibits a 4% wt. mass loss up to 200 °C due to dehydration and a 7% wt. mass loss event between 200 and 375 °C, whereas all the reference beads without RM show complete oxidation at 1000 °C and no significant residue. However, variable residue is obtained at 1000 °C for the composite beads, which is attributed to the RM content (Table 2). The presence of RM also downshifts the maximum degradation temperatures in the final decomposition stage of composite beads by 30–50 °C compared to the reference beads. Yet, this same degradation stage also indicates a more homogenous decomposition of the remaining copolymer backbone (strong interactions between the -CH2 groups of the copolymer backbone and RM were noticed in FTIR spectra). Considering the copolymer is wholly decomposed, the remaining residues from the composite beads were attributed to RM. Using this hypothesis, the real RM content in the composite beads was calculated as 3.42% wt. for 1-RM, 3.44% wt. for 2-RM, and 5.63% wt. for 3-RM (Table 2). These results indicate that 3-RM is the most homogenous sample according to the initial RM content, with a good incorporation of RM in the copolymer matrix (8% wt. relative to the copolymer). The results are also consistent with the observations from the FTIR analysis, in which case the characteristic band for CH2 deformation was more intense in 1-RM and 2-RM spectra, suggesting the existence of lower amounts of RM in these samples as compared to 3-RM.

3.2. Porosity and Morphology Investigation of Composite Copolymer Beads

To obtain information regarding the effect of RM on the morphology of the samples, SEM images were recorded at a 50 µm scale in section and surface views for both reference and composite beads (Figure 4). The micrographs in Figure 4A show that using RM changes the composite beads’ surface and inner pore structure. Samples with lower amounts of AA (5% for 1-RM and 10% for 2-RM) in the copolymer matrix present heterogeneous pore structures and large voids (section view-1-RM) and alternating porous and smooth zones (section view 2-RM), probably due to uneven RM distribution in the copolymer matrix (as indicated by FTIR and TGA determinations). The section view of 3-RM reveals an interesting porous structure of the composite, with smaller smooth zones than 2-RM.
A different morphology is also noticed between reference samples in the section view (Figure 4A). Therefore, apart from the uneven RM content, the AA content in the beads’ formulation also influences the porosity and homogeneity of samples. In this case, the increase in AA content leads to more homogenous pore networks and surfaces as a result of the higher hydrophilicity of the copolymer [72].
When analyzing the surface of beads (Figure 4B), it looks like adding RM negatively contributes to the surface porosity, especially for 2-RM and 3-RM. Thus, the overall morphology assessment indicates that 3-RM presents the most homogenous porous network, but with reduced access to the surface of the bead compared to 1-RM. It can be mentioned that the presence of RM in the pores or on the surface cannot be distinguished due to the presence of small white particles/dots appearing in both types of beads, reference and RM-based beads, which can also be attributed to insolubilized copolymer particles. Nevertheless, it is essential to point out that for the 3-RM/3-Ref series, the presence of such white dots is very low, indicating a more homogenous dissolution of the copolymer in this case.
Further on, SEM results were backed up by nitrogen porosimetry determinations (results summarized in Table 3). Comparing 1-Ref and 1-RM and the following C2-Ref and C2-RM pair, it was noticed that the integration of RM resulted in a significant decrease in both BET surface area and pore surface area, which is in good agreement with SEM results (Figure 4) that revealed that the inner pore structure is affected by the addition of RM. As for pore diameter and volume, the decrease is not significant, but the downward trend is maintained.
For the third series of samples (3-Ref and 3-RM), the effect of RM is rather the opposite from the first two series, indicating an increased BET surface area, pore surface area, and pore volume upon the addition of RM. However, a significant difference was noticed between the first two series and the third series of copolymer beads in terms of BET surface area, pore surface, and pore volume, especially for the Ref samples, in which case the values are higher by one order of magnitude (1-Ref and 2-Ref). Hence, this observation may only be explained by the fact that 3-Ref and 3-RM present lower surface porosity (consistent with the SEM images) and may be due to the copolymer composition [72]. As observed previously in the FTIR spectra, this copolymer matrix is more hydrophilic. It seems to affect the pore formation in the phase inversion process by the fact that the solvent is squeezed out of the forming solid beads more slowly. This leads to smaller pore volumes and pore surface areas, as also described by Hamta et al. for acrylonitrile/styrene copolymer membranes [78]. Most authors link the increase in adsorption capacity to the high surface area of adsorbents (some carbon-based adsorbents have a 1200 m2/g surface area [79]). Thus, the relatively low BET surface area of composite beads obtained herein can ultimately limit the adsorbed 2,4-DCFA.

3.3. Retention of 2,4-DCFA by Composite Beads in Adsorption/Oxidation Processes

The adsorption tests were performed using the whole bead, which led to a difference between the amount of sample of 40.0 ± 1.0 mg. Even though the beads were thoroughly washed to remove the solvent (Table 1), some residual alcohol remained in the more confined pore structures of beads and leached out during the 24 h adsorption interval, interfering with the TOC measurement for 2,4-DCFA quantification. As observed from the results centralized in Table 4, the supernatants of 2,4-DCFA solutions after adsorption show higher values of TOC compared to the initial value of 46.35 mg C/L (corresponding to the solution of 100 mg 2,4-DCFA/L), except for sample 3-RM. Another interference in TOC measurement for RM-based based composite beads may be associated with the small (5.7%) but existing amount of CO2 in the RM, which may be released at low pH values. Nevertheless, the results may be interpreted qualitatively, and aside from the potential CO2 release, 3-RM seems to be the only system capable of absorbing at least 9.7% 2,4-DCFA from the initial solution. This is why the following oxidation trials were performed only for this system and compared to its reference, 3-Ref, as well as to the values obtained for RM alone. The results of the oxidation trial are summarized in Table 5, in which case a similar amount of adsorbent (40.0 ± 0.2 mg) was used.
In support of the selection of the 3-RM system, it can be mentioned that the increase in carboxylic groups in the precursor copolymer (resulting from the use of higher AA ration) contributes to the particular performance of 3-RM. If we evaluate the chemical structure of AA and AN monomer units and that of 2,4-DCFA, it is more likely that the cyano groups in AN interact with the carboxylic groups in 2,4-DCFA through hydrogen bonds. Therefore, to facilitate the access of 2,4-DCFA to the cyano groups, a more hydrophilic environment is needed to favor the diffusion processes. The increase in AA contributes to the creation of this environment [80,81], especially when using 20% (the case of 3-RM and 3-Ref). On the other hand, if the content of AA is increased over 20%, the copolymer becomes too hydrophilic to be used with success for the wet phase inversion process [80]. Another relevant study [81] highlighted that even methacrylic acid, which is less water-soluble than AA, should be used in ratios of maximum 20% relative to AN for achieving a good adsorption of analytes.
Following the oxidation assays, a clear decrease in the TOC value is observed for both 3-RM and the reference, indicating that 2,4-DCFA is retained quantitatively in the copolymer beads (Table 5). Another exciting value of TOC is attained for RM alone, which now suggests that RM may release CO2 upon adsorption/oxidation. The TOC value for 3-RM calculated against the initial TOC value after oxidation of 2,4-DFCA is close to that obtained in the previous adsorption test (12.8% vs. 9.7%), confirming the potential adsorbent character of 3-RM. In addition, comparing the TOC value of 3-RM with that of the reference, it looks like the contribution of RM to the overall adsorption/oxidation of 2,4-DCFA is significant (up to 2.92%), corresponding to a 22.81% increase in the specific activity of the composite for retaining 2,4-DCFA. In this case, the increased specificity can be due to the oxidant character of RM, as it contains in its structure 1.6% FeO and 5.2% TiO2, the latter being used as a catalyst in some oxidation processes [82].
Analyzing the results in Table 5, the uptake for 3-RM by 5.97 mg C/L, corresponding to an adsorption capacity of 7.50 mg C/g of adsorbent (equivalent to a value of 16.08 mg 2,4-DCFA/g composite), is a good value considering that other studies have reported lower performances for other composites. For instance, in the work of Andrunik et al. [83], the maximum adsorption of 2,4-DCFA was around 8.50 mg/g at pH 7 when using a zeolite–carbon composite.

3.4. Investigation of Structure Modification after Composite Beads’ Recovery

Finally, to evaluate the integrity of the material for potential reuse, the 3-RM system was analyzed after each procedure. In this respect, the structure and composition of the material upon contact with the contaminant (2,4-DCFA) were verified by FTIR (spectra given in Figure 5). Although no critical changes in the copolymer structures are observed either in the case of oxidation or adsorption processes, some essential bands appear due to the presence of 2,4-DCFA in the beads.
Figure 5A shows that -OH groups appear following the adsorption/oxidation processes (3550–3650 cm−1). These can be assigned to the carboxyl group in 2,4-DCFA. The sharp band appearing at 600 cm−1 is associated with the presence of -C-Cl bonds in 2,4-DCFA. In addition, bands previously associated with sulfated ions from the redox system and S=O in DMSO (950 cm−1 and 1030 cm−1, respectively) disappear due to the intense purification procedure performed before adsorption and oxidation tests. Last but not least, the characteristic bands of the copolymer at 3380 cm−1 (O-H), 1732 cm−1 (C=O), 2244 cm−1 (C≡N), and 2940 cm−1 (stretching -CH2) are more intense but with no specific modifications. The band at 1455 cm−1 (in plane deformation of the aliphatic CH2 groups in the backbone) that was more affected by the presence of RM is more intense after adsorption and oxidation. This could mean that the interactions with the copolymer backbone have diminished due to RM implication in the adsorption/oxidation processes of 2,4-DCFA. The spectra of reconditioned 3-RM (after adsorption and oxidation, in Figure 5B) show more similarities compared to the spectrum of initial 3-RM (in Figure 5A), with the difference that that all the characteristic bands of the copolymer present lower intensities, except for the large hump in the region 3150–3600 cm−1, which may be an indicator that larger amounts of water were retained after reconditioning. High water contents can also affect the intensity of other characteristic bands in the analyzed sample, as observed in this case. Considering these results, 3-RM composite beads seem to be strong candidates for future reuse trials.

4. Conclusions

Composite materials based on poly(acrylonitrile-co-acrylic acid) and red mud were prepared in the form of beads and used for pesticide removal in synthetic wastewater using adsorption/oxidation processes. The composite beads were primarily investigated for structure, composition, thermal stability, morphology, and porosity to identify at least one viable system of beads with potential application in wastewater treatment. In this respect, FTIR spectra indicated an exciting interaction of RM with the copolymer backbone as a function of copolymer composition. At the same time, the thermogravimetry assays allowed for the evaluation of the influence of RM upon the stability and homogeneity of the composite beads, by comparing the initial RM content with the calculated amount of RM from TGA in the analyzed samples. The morphology and porosity of beads depended on the composition of the copolymer, but also on the homogenous distribution of RM. Thus, the composite beads based on copolymers with higher acrylic acid content (20%) exhibited less extended pore structures with poor surface communication and smaller pore surface areas due to their higher hydrophilicity. Nevertheless, this same system was the only one prone to absorb the target pesticide, 2,4-DCFA, as revealed by the TOC measurements in the adsorption trial. The adsorbent/oxidizing character was better highlighted by the following oxidation processes, which pointed out that RM contributed to a specific retention of 2,4-DCFA relative to the TOC values registered for the reference beads, up to 22.81%. Although the study may need further optimization and evaluation, the results obtained in this study show that RM can be repurposed for developing adsorbents for wastewater purification using low-cost precursors and methods, which for the industry may represent a fair trade-off vs. other performant carbon-based adsorbents that require complex preparation methods and pose a risk for CO2 pollution due to preliminary carbonization procedures [78,84,85]. Future studies regarding the synthesis optimization of the composite beads in terms of the maximum amount of RM that can be integrated in the precursor solution, as well as reuse studies for multiple adsorption/oxidation cycles, will bring this potential option closer to the industry.

Author Contributions

Conceptualization, T.S. and A.S.; Formal analysis, T.S., A.M., S.-V.D., A.Z. and A.-M.G.; Investigation, E.A.O., R.-A.M., A.M., S.-V.D., A.Z., A.-M.G. and M.-V.D.; Methodology, T.S., E.A.O., R.-A.M., M.-V.D., A.-L.C. and T.-V.I.; Supervision, A.S. and T.-V.I.; Validation, A.Z., A.-L.C., A.S. and T.-V.I.; Writing—original draft, T.S. and T.-V.I. All authors have read and agreed to the published version of the manuscript.

Funding

The authors from ICECHIM acknowledge the funding from UEFISCDI and the European Commission for ctr. 57/2024 “WATER-BIOFIL” within the WATER4ALL 2022 JOINT TRANSNATIONAL CALL and the support of the Ministry of Research, Innovation, and Digitalization for the study through the institutional project 2N/03.01.2023 (PN 23.06.01.01. AQUAMAT).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the data are available from the corresponding author, Tanta-Verona Iordache, [email protected].

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Applsci 14 06386 g001
Figure 1. (A). Chemical structure of 2,4-DCFA, with formula C8H6Cl2O3 and a molar mass of 221.04 g/mol; (B). schematic representation of the preparation process for composite beads with RM; (C). pictures of lyophilized series of beads with RM (1-RM, 2-RM, and 3-RM) and the control beads (1-Ref, 2-Ref, and 3-Ref) with diameters of 2–4 mm.
Figure 1. (A). Chemical structure of 2,4-DCFA, with formula C8H6Cl2O3 and a molar mass of 221.04 g/mol; (B). schematic representation of the preparation process for composite beads with RM; (C). pictures of lyophilized series of beads with RM (1-RM, 2-RM, and 3-RM) and the control beads (1-Ref, 2-Ref, and 3-Ref) with diameters of 2–4 mm.
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Figure 2. FTIR spectra of analyzed composite copolymer beads compared to RM alone and their corresponding precursor copolymer: (A) C1-based series; (B) C2-based series; (C) C3-based series.
Figure 2. FTIR spectra of analyzed composite copolymer beads compared to RM alone and their corresponding precursor copolymer: (A) C1-based series; (B) C2-based series; (C) C3-based series.
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Figure 3. Thermogravimetric analyses (A–C) and differential thermal analysis (D–F) of the composite copolymer beads and their corresponding references, compared to the precursor materials of RM alone and corresponding copolymers.
Figure 3. Thermogravimetric analyses (A–C) and differential thermal analysis (D–F) of the composite copolymer beads and their corresponding references, compared to the precursor materials of RM alone and corresponding copolymers.
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Figure 4. Micrographs of reference (top) and composite beads (bottom) taken at 50 µm scale: (A). section view and (B). surface view.
Figure 4. Micrographs of reference (top) and composite beads (bottom) taken at 50 µm scale: (A). section view and (B). surface view.
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Figure 5. FTIR spectra of (A) 3-RM, 3-RM before and after adsorption (3-RM-Ads)/oxidation (3-RM-Ox) processes; (B) 3-RM after reconditioning from the two procedures (3-RM-R-Ads after adsorption and 3-RM-R-Ox after oxidation).
Figure 5. FTIR spectra of (A) 3-RM, 3-RM before and after adsorption (3-RM-Ads)/oxidation (3-RM-Ox) processes; (B) 3-RM after reconditioning from the two procedures (3-RM-R-Ads after adsorption and 3-RM-R-Ox after oxidation).
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Table 1. Total organic carbon (TOC) in the supernatants resulting from the washing procedure of copolymer beads.
Table 1. Total organic carbon (TOC) in the supernatants resulting from the washing procedure of copolymer beads.
Sample
Code		TOC (mg/L)
1st Step		TOC (mg/L)
2nd Step
1-REF8.991.76
1-RM14.551.28
2-REF1.21-
2-RM1.84-
3-REF0.71-
3-RM1.78-
Table 2. TGA calculation of RM content in composite beads as a function of the residue at 1000 °C.
Table 2. TGA calculation of RM content in composite beads as a function of the residue at 1000 °C.
Sample CodeResidue at 250 °CResidue
at 500 °C		Residue
at 1000 °C		Residue at 1000 °C of Dried Sample,
RD (% wt) *		RMCalc
(% wt) **
1 REF55.4939.580.050.100.00
1 RM53.6939.791.733.233.42
2 REF46.7129.710.040.090.00
2 RM47.4432.821.543.243.44
3 REF73.9446.920.030.040.00
3 RM64.1340.063.335.195.63
RM96.2789.0488.1191.53-
* RD, (%) = (Residue at 1000 °C/Residue at 250 °C)·100; ** RMcalc, (%) = (RD, x-RM − RD, x-Ref)/RD, RM·100, where x is the sample code, x = 1, 2, 3.
Table 3. Parameters of samples determined by N2 porosimetry.
Table 3. Parameters of samples determined by N2 porosimetry.
SampleBET Surface Area
(m2g−1)		Pore Surface Area
(BJH) (m2g−1)		Mean Pore Diameter
(BJH) (nm)		Mean Pore Volume (BJH)
(cm3g−1)
1-Ref56.7265.513.470.0277
1-RM7.075.993.170.0113
2-Ref41.7966.594.540.0545
2-RM2.7418.604.540.0150
3-Ref1.153.094.540.0037
3-RM2.183.474.540.0070
Table 4. Adsorption test results as function of amount and type of beads.
Table 4. Adsorption test results as function of amount and type of beads.
Sample CodeAdsorbent Amount (mg)pHTOCfinal
(mg C/L)		Adsorption Capacity *
(mg C/g Adsorbent)		Adsorbed Amount ** (%)
1-Ref40.53.7048.54//
1-RM40.73.6747.23//
2-Ref39.63.6447.99//
2-RM40.43.7049.07//
3-Ref40.13.8047.83//
3-RM41.03.7541.851.659.71
* Calculated as (TOCinitial-TOCfinal)·Vsolution/madsorbent, where TOCinitial is 46.35 mg C/L and Vsolution is 0.015 L; ** calculated as (TOCinitial-TOCfinal)·100/TOCinitial, where TOCinitial is 46.35 mg C/L.
Table 5. Results of oxidation tests for the 3-RM/3-Ref series compared to RM alone.
Table 5. Results of oxidation tests for the 3-RM/3-Ref series compared to RM alone.
SampleAdsorbent Amount (mg)TOCinitial
(mg C/L)		Adsorption/
Oxidation Capacity *
(mg C/g Adsorbent)		Adsorbed/
Oxidized Amount vs. TOCinitial (%) **
2,4 DCFA+H2O2-46.64--
RM+H2O240.41.34--
3-Ref +
2,4 DCFA		+H2O240.242.035.739.88
3-RM +
2,4 DCFA		+H2O239.840.677.5012.80
* Calculated as (TOCinitial-TOCfinal)·Vsolution/madsorbent, where TOCinitial is 46.64 mg C/L and Vsolution is 0.050 L; ** calculated as (TOCinitial-TOCfinal)·100/TOCinitial, where TOCinitial is 46.64 mg C/L.
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Sandu, T.; Olaru, E.A.; Mitran, R.-A.; Miron, A.; Dolana, S.-V.; Zaharia, A.; Gavrilă, A.-M.; Dumitru, M.-V.; Chiriac, A.-L.; Sârbu, A.; et al. Composite Copolymer Beads Incorporating Red Mud for Water Amendment by Adsorption—Oxidation Processes. Appl. Sci. 2024, 14, 6386. https://doi.org/10.3390/app14146386

AMA Style

Sandu T, Olaru EA, Mitran R-A, Miron A, Dolana S-V, Zaharia A, Gavrilă A-M, Dumitru M-V, Chiriac A-L, Sârbu A, et al. Composite Copolymer Beads Incorporating Red Mud for Water Amendment by Adsorption—Oxidation Processes. Applied Sciences. 2024; 14(14):6386. https://doi.org/10.3390/app14146386

Chicago/Turabian Style

Sandu, Teodor, Elena Alina Olaru, Raul-Augustin Mitran, Andreea Miron, Sorin-Viorel Dolana, Anamaria Zaharia, Ana-Mihaela Gavrilă, Marinela-Victoria Dumitru, Anita-Laura Chiriac, Andrei Sârbu, and et al. 2024. "Composite Copolymer Beads Incorporating Red Mud for Water Amendment by Adsorption—Oxidation Processes" Applied Sciences 14, no. 14: 6386. https://doi.org/10.3390/app14146386

APA Style

Sandu, T., Olaru, E. A., Mitran, R. -A., Miron, A., Dolana, S. -V., Zaharia, A., Gavrilă, A. -M., Dumitru, M. -V., Chiriac, A. -L., Sârbu, A., & Iordache, T. -V. (2024). Composite Copolymer Beads Incorporating Red Mud for Water Amendment by Adsorption—Oxidation Processes. Applied Sciences, 14(14), 6386. https://doi.org/10.3390/app14146386

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