Sonochemical Modification of ZrO₂ Nanoparticles with Thiamine Hydrochloride for the Development of Films with PLA for the Adsorption of Hexavalent Chromium

Abstract

Industrial wastewater can be reused in other everyday processes to help combat water scarcity worldwide. One contaminant in industrial wastewater is hexavalent chromium, which is highly toxic and can cause kidney, liver, and respiratory problems, making its removal vital. In this study, PLA-based films containing modified zirconia nanoparticles were developed via a solution-mixing process for hexavalent chromium adsorption. Obtaining the films involved two stages: the first was the chemical modification of ZrO₂ nanoparticles with thiamine hydrochloride (vitamin B1) using fixed-frequency ultrasound at an output of 750 W and 50% amplitude for 60 min. The second stage involved preparing the films by mixing them in the solution using an ultrasonic bath. The nanoparticle concentrations were 0.25, 0.5, and 1 wt%. The results obtained from characterization using Fourier-transform infrared spectroscopy (FT-IR) revealed the characteristic bands of PLA and the characteristic peak of the Zr-O bond corresponding to the ZrO₂ nanoparticles. Thermogravimetric analysis (TGA) showed that the ZrO₂ nanoparticles provided thermal stability to the PLA polymer. X-ray diffraction (XRD) showed a broad peak of amorphous PLA at 16.8° and signals corresponding to the crystalline phase of ZrO₂. The morphology of a cross-section of the films was observed using scanning electron microscopy (SEM), revealing a rough surface with pores. Finally, hexavalent chromium adsorption tests were carried out, measuring the adsorption efficiency under the parameters of pH, concentration, and contact time. The PLAZrO₂ sample achieved an adsorption efficiency of 83% at pH 2.

1. Introduction

Industrial wastewater is characterized by heavy metals such as copper (Cu), lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg), iron (Fe), and chromium (Cr). They come from industries such as chemicals, food, paper, pesticide and biocide production, petroleum and petrochemicals, pharmaceuticals, and metallurgy [1].

Chromium is a highly toxic heavy metal for animals and humans. It is generally found in industrial wastewater since its use cannot be easily replaced due to unique properties such as anti-corrosion, color, toxicity, thermal resistivity, and a high melting point [2,3]. There are different methods for removing chromium from wastewater, such as chemical precipitation, reverse osmosis, coagulation, and adsorption. Adsorption technology is simple, cheap, has the capacity to regenerate, and can be scaled to an industrial level [4,5,6].

Some adsorbent materials are montmorillonite [7], polymers [8,9], polysaccharides [10], and polyaniline [11]. In recent years, the use of nanoparticles in the adsorption of heavy metals has increased. Several works have been reported about the use of carbon-based nanoparticles [12], ceramics [13], and metallic nanoparticles [14] to efficiently remove heavy metals due to their properties, such as surface area, size, photocatalytic property which allows degrading components, and mechanical properties, among others.

The properties of nanoparticles can be enhanced by superfinely modifying these materials so that they can interact chemically or physically with other types of organic or inorganic molecules. For example, carbon black has been modified with thiamine hydrochloride to eliminate uremic toxins by achieving physical interactions and removing molecules such as urea and creatinine [15]. Recent studies have suggested that by modifying nanoparticles or developing nanocomposites, better results can be obtained for decontaminating water that contains heavy metals [16].

On the other hand, there are membranes of different types of synthetic polymers used for water filtration, but currently, the care of the environment is of great importance; because of this, the use of biodegradable polymers has had a great area of interest worldwide [17]. PLA is a thermoplastic polymer derived from lactic acid (LA) that can be used in the manufacturing of textiles. The production of 90% of LA in the global market is by lactic acid bacteria (LAB), which can stereoselectively produce L or D-LA [18].

There is a wide variety of techniques for obtaining nanocomposites, among which the most commonly used are in situ polymerization, melt mixing, and solution mixing. In the solution-mixing technique, the particles are dispersed in a liquid, generally with the help of ultrasonic waves, and subsequently mixed with a solution of dissolved polymer; subsequently, the solvent is evaporated; finally, a sample powder or film is recovered. This technique tends to disperse the nanoparticles effectively and homogenize them in the medium where they are found [19].

The combination of two adsorbent materials, such as nanoparticle-reinforced polymers, can increase the percentage of heavy metal removal. Bao et al. studied the adsorption of hexavalent chromium on PANI/ZnO; this compound was shown to be efficient and reusable, and also with photocatalytic properties [20]. Kumar et al. prepared an alginate compound with ZrO₂ for the adsorption of Cr(VI), managing to remove 73% of the heavy metal, and this adsorbent can be reused up to three cycles [21].

In this work, films based on PLA and ZrO₂ nanoparticles pre-modified with thiamine hydrochloride were prepared. The films were prepared using short sonochemical process times. The films were characterized by FT-IR, TGA, XRD, and SEM. This is the first study to evaluate hexavalent chromium adsorption on PLA-based films with modified ZrO₂ nanoparticles.

2. Materials and Methods

2.1. Materials

NatureWorks (Minneapolis, MN, USA)provided polylactic acid (PLA) resin Ingeo biopolymer 6260D with a melt index of 65 g/10 min. Zirconia ZrO₂ nanoparticles (<100 nm) from Sigma Aldrich (St. Louis, MO, USA), chloroform, thiamine hydrochloride from Faga Lab (Sinaloa, México), and distilled water with a pH of 7 were used as a solvent to obtain the aqueous solutions.

2.2. Modification of ZrO₂ Nanoparticles with Thiamine Hydrochloride by Sonochemistry

The modification of ZrO₂ nanoparticles with thiamine hydrochloride was carried out following the protocol of Sanchéz-Huerta et al., 2025 [15].

2.3. Preparation of Films PLA/ZrO₂

PLA and modified ZrO₂ films were prepared by solution mixing (Figure 1). The established concentration of ZrO₂ nanoparticles was dispersed in 10 mL of chloroform in a Branson ultrasonic bath for 5 min. The PLA pellets were then dispersed in chloroform for 10 min. The ZrO₂ nanoparticles solution was added to the polysulfone solution and subjected to 15 min of ultrasonication. After this time, the solution was allowed to dry at room temperature for 12 h.

2.4. Characterization

Fourier-transform infrared spectroscopy (FTIR) characterization was performed using a Magna Nicolet 550 spectrometer (Waltham, MA, USA) with a resolution of 16 cm⁻¹ and 100 scans, in the infrared range from 400 to 4000 cm⁻¹.

X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance eco diffractometer (Bruker, Billerica, MA, USA), with a scanning range of 20 to 80° (2θ) and a scanning speed of 0.02% per second. K-alpha copper radiation with a wavelength of 1.54 Å and a D-teX Ultra detector (Bruker, Billerica, MA, USA) were used, operating at an intensity of 35 kV and a voltage of 25 mA.

The thermogravimetric analysis (TGA) was carried out under the ASTM E-1131 standard, using a Q500 thermal analyzer from TA Instruments (New Castle, DE, USA). The TGA thermal curve of the samples was obtained. The instrumental conditions were as follows: a heating rate of 10 °C/min and a sample mass of 10 mg.

Scanning electron microscopy (SEM) analyses were conducted on a JEOL-JSM-7401 instrument (Thermo Scientific, MA, USA) with an acceleration voltage of 5 keV.

2.5. Adsorption of Hexavalent Chromium (Cr VI)

A stock solution of Cr VI at 50–100 mg/L was prepared by dissolving the appropriate amount of K₂Cr₂O₇ in distilled water. The initial pH was adjusted to 2 by adding an aqueous solution 0.1 M HCl; this pH adjustment was carried out as reported in the literature where the adsorption capacity of Cr VI aliquots of the solutions were analyzed by UV-Vis spectroscopy to determine the absorbance at 540 nm and subsequently estimate the Cr VI ions concentration using the Nesslerizations method according to the standard methods for water analysis [22]. All experiments were performed in triplicate.

$$\% adsorption efficiency={\displaystyle \frac{\left(C_i-C_e\right)}{C_i}}\times 100$$

(1)

The adsorption efficiency percentage was calculated according to Equation (1) where C_i and C_e are the initial and equilibrium Cr VI (mg) concentrations. V and m are the volume of Cr VI solution and the amount of adsorbent (g).

$$q_e={\displaystyle \frac{\left(C_i-C_e\right)V}{m}}$$

(2)

The adsorption capacity of the films PLAZrO₂ was calculated with Equation (2) at equilibrium where V is the volume in L of solution and m is the amount of mass in mg of absorbent.

3. Results and Discussions

3.1. Fourier-Transform Infrared Spectroscopy (FTIR)

In Figure 2, the spectrum of the PLA pure films is shown, where the characteristic adsorption bands were observed: 2946 cm⁻¹, 3000 cm⁻¹ CH asymmetric and CH symmetric groups, respectively, 1757 cm⁻¹ C = O carbonyl group, 1456 cm⁻¹ CH₃ bending at 1117 cm⁻¹ C–O–C asymmetric stretching ether group, 1056 cm⁻¹ corresponding to the C-O-C bond and C = O bending at 758 cm⁻¹ [23,24]. In the PLAZrO₂ 0.25% and PLAZrO₂ 0.5% films, the characteristic peaks of the PLA polymer matrix are shown, and additionally, the band is characteristic of the Zr-O bond at 485–565 cm⁻¹. However, in the PLAZrO₂ 1% sample, this signal is not observed because it is uniformly dispersed; this has already been reported by other authors [25,26].

3.2. X-Ray Diffraction (XRD)

The diffraction patterns of the films are presented in Figure 3. The PLA film presents an amorphous peak centered at 18° [27,28]. The PLAZrO₂ 0.25% film presents two sharp peaks of low intensity at 16.8°, assigned to the α crystalline phase, which confirms that PLA does not have a polymorphic crystal transition [29], and at 19°, corresponding to the PLA structure, it also presents peaks at 27, 28, and 31°, characteristic of ZrO₂ [30]. The PLAZrO₂ 0.5% and PLAZrO₂ 1% films present similar peaks at 16.8° and 19°, and a peak centered at 35°, corresponding to the ZrO₂ nanoparticles added to the film.

3.3. Thermogravimetric Analysis (TGA)

Figure 4 shows the thermogravimetric analysis of pure PLA films, where a first weight loss is observed from room temperature to 150 °C, which is attributed to chemically adsorbed water. A second part is observed starting at 265 °C and stabilizing at 334 °C; this weight loss is due to the intramolecular transesterification reaction, which agrees with what was reported [31]. The PLAZrO₂ 0.25 and 0.5% films also show a first loss from room temperature to 150 °C, which may be due to humidity or the solvent used, and a second loss from 270 °C to 333 °C corresponding to the realization of the polymeric matrix. On the other hand, the PLAZrO₂ 1% film shows the second loss from 284 °C to 350 °C. At higher concentrations of ZrO₂ nanoparticles, greater thermal stability is observed, where greater compatibility with the polymer matrix is observed; this has already been observed with TiO₂/ZnO nanoparticles and nanoclays [18,24].

3.4. Scanning Electron Microscopy (SEM)

The morphological characteristics of the films in a PLA cross-section are observed in Figure 5a where no pores are observed; it is a rough surface without the presence of particles on the surface. On the other hand, in Figure 5b, which corresponds to the PLAZrO₂ 0.25% film, small pores and a rough surface are observed; similarly, in (c) PLAZrO₂ 0.5% and (d) PLAZrO₂ 1%, a rough surface is observed in the images, and as the concentration increases, more pores are observed. The pores present in b, c, and d have a diameter of 30–60 µm. All micrographs were obtained with the same magnification of 100 µm.

Figure 6 shows the micrographs at 20 µm; it is a cross-section at higher magnification. In pure PLA, a rough surface is observed without the presence of nanoparticles. In Figure 6b, at least three irregularly shaped ZrO₂ agglomerates are observed; no smaller pores are observed, indicating that the nanoparticles are embedded in the PLA matrix [32]. Figure 6c also shows agglomerates distributed along the surface of the PLA matrix. In the case of 1% PLAZrO₂, micrometric-sized pores are observed, where nanoparticles are observed around the pore.

3.5. Adsorption of Hexavalent Chromium Cr VI onto Films

The pH of the aqueous solution is a crucial parameter that significantly influences the adsorption process. The experiments in this stage were carried out under conditions of constant temperature (25 °C), stirring speed (300 rpm), and contact time of 120 min. Figure 7b shows the effect of pH on adsorption efficiency. At an acidic pH of 2, the adsorption efficiency increases, whereas at pH 8 and 10, a drastic decrease is observed. This result is consistent with the findings of Zhang et al. [33], who investigated the adsorption of hexavalent chromium under acidic conditions. However, the effectiveness decreases from pH > 3.

Figure 7a shows the effect of contact time on adsorption. The total contact time was 120 min, and aliquots were taken every 10 min. From 30 min onward, an increase in adsorption efficiency was observed. It is also observed that at higher concentrations the adsorption efficiency increases. The PLAZrO₂ 1% presented 83% adsorption efficiency at 120 min. Despite the small quantity of nanoparticles, excellent adsorption is achieved in a short time. The synergy of the polymer with the nanoparticles represents an option to increase the adsorption of the material. In Figure 7c, the effect of the concentration of chromium VI is determined, where, despite increasing the concentration, the adsorption efficiency remains the same; this behavior has already been reported by Nameni et al. [34].

PLAZrO₂ films showed good results in the adsorption of hexavalent chromium, achieving over 80% adsorption efficiency. The pH was first evaluated to determine the optimum for maximum adsorption. Once this parameter was established, the concentration was assessed, as adsorbent materials have a concentration limit beyond which adsorption ceases because the adsorbent sites are occupied. Contact time is another important parameter, as industrial processes must optimize the time required for contaminant removal.

3.6. Adsorption Mechanisms

Figure 8 shows the possible adsorption mechanism of PLAZrO₂ films. The pH effect indicates that at acidic pH, an electrostatic interaction occurs between Cr VI and the PLAZrO₂ film, while at pH values above 8, an electrostatic repulsion is observed between the Cr VI ions and the PLAZrO₂ film. The functional groups added to the ZrO₂ nanoparticles through chemical modification with thiamine hydrochloride aid in the adsorption of Cr VI. A ph acid N- functional group becomes positively charged due to loss of two electrons, while O-functional groups are likely to deprotonate to form negative species. Due to this, N-functional groups may have electrostatic interaction with HCrO₄⁻ while some NH₂ groups still play to role of reduction in Cr(VI) to Cr(III); consequently, O-functional groups interact with Cr(III). Finally, O-functional groups interact with Cr⁺³. Furthermore, a complex can form between Cr VI and PLA functional groups such as C=O, OH, and O–C=O.

This new adsorbent material offers several advantages, including the biodegradability of the polymer used, the film preparation method, and the chemical modification process, which utilizes an alternative energy source such as ultrasound. It also employs a renewable reagent, thiamine hydrochloride, as a modifier. This methodology aligns with the principles of green chemistry. Due to its biodegradability and environmentally friendly method, this new material shows promise for contaminant removal.

4. Conclusions

In this work, a new film based on PLA and modified ZrO₂ nanoparticles was developed using sonochemistry as an alternative energy source for film modification and preparation. The films were characterized by FT-IR spectroscopy, revealing the characteristic signals of PLA and ZrO₂. X-ray diffraction patterns showed sharper peaks corresponding to PLA and characteristic peaks of ZrO₂. Thermogravimetric analysis revealed increased thermal stability in the films. The films exhibit a degree of roughness and micrometer-sized pores. The three films, including the target pure PLA, were evaluated for hexavalent chromium adsorption, achieving an 83% adsorption efficiency for the film with 1% PLAZrO₂. The adsorption mechanism is based on electrostatic interactions in an acidic medium and hydrogen bonding, which occurs due to the incorporation of amino groups on the surface of the nanoparticles embedded in the PLA matrix. It is also possible to form a complex with the functional groups of PLA and hexavalent chromium.

As a future perspective, the adsorption efficiency will be evaluated with other heavy metals and with dyes to see its viability and selectivity for use in the treatment of wastewater, which contains different contaminants, both organic and inorganic as well as biological.