1. Introduction
Today, environmental conservation has emerged as a crucial concern for the sustainable progress of our society, driven by the increasing recognition of the detrimental consequences posed by our industrial, economic, and societal practices on the environment and the overall quality of life [
1]. One of the most critical challenges we face is properly managing wastewater since its untreated discharge can have significant negative consequences for aquatic ecosystems and human health [
2,
3,
4,
5]. One of the main parties responsible for the emission of a large number of pollutants into water is the petrochemical industry since they unintentionally or deliberately release chemical substances in the production of polymers [
6,
7,
8]. Within the broad spectrum of pollutants present in wastewater, chemical additives play a prominent role due to their wide use in various industries [
7]. The additive 1,2,3-trideoxy-4,6:5,7-bis-
O-[(4-propylphenyl)methylene]nonitol is one of the substances present in wastewater, known commercially as Millad NX 8000 (
Figure 1), a substance widely used in the manufacture of plastic products, especially in the production of bottles and containers, which are present in many aspects of daily life and their production has increased dramatically in the last 50 years [
9]. Millad NX 8000 belongs to a class of additives known as nucleants, which are used to improve polymers’ mechanical and optical properties. Millad NX 8000 is used from 0.01% to 1% during polypropylene (PP) manufacturing to improve its transparency and resistance to high temperatures [
10]. Since polypropylene crystallizes very slowly by forming relatively large spherulites [
11], this causes considerable light scattering and opacity in the polymer [
11]. During the cooling process, the segregation and crystallization of the sorbitol derivatives result in the formation of spherulites, which are observed as fibers with a diameter of 10 nm [
12,
13,
14,
15,
16]. Millad acts as a nucleation and clarification agent, allowing the spherulites formed to be smaller than the wavelength of light, and therefore a polymer with less opacity.
In the last 39 years, various types of nucleating agents with a clarifying effect on PP have been developed. One main disadvantage of PP clarifiers has been their organoleptic problems (taste and odor) since most of these clarifiers are based on sorbitol acetal compounds, which hydrolyze during the PP processing stage, leading to the release of free aldehydes, which then cause unpleasant flavors and odors in PP [
17]. Among its most recent products and with innovative chemistry is the Millad NX 8000, which is characterized by providing a decrease in turbidity in the PP greater than 50% concerning previous clarifying agents; it is also free of the odors and flavors that other nucleating agents used in the packaging products made of PP transmits [
17]. The national and international industries that produce PP use billions of tons of these additives per year, and it is known that their production processes are not entirely efficient. Significant amounts of these additives can end up in wastewater. This can have negative consequences for the environment and human health since it is known that many of these chemical additives can be toxic and persistent in the environment [
17], for which Millad NX 8000 can be considered a persistent organic pollutant (POP). Considering that this additive is frequently used in the production of plastic products and different studies have been carried out for its detection in macro and microplastic waste, it is to be expected that they are found in environmental matrices (water, sediments, and biota) and may represent an ecological concern [
18]. The existence of additives, such as Millad NX 8000, in polymer matrices is potentially a severe factor in the physical recycling of plastics, which delays progress toward the circular economy, which is where the plastics industry intends to go [
19].
In a study carried out in rats for 90 days to observe the acute toxicity, sensitization, irritation, oral toxicity, and possible effects on the public and environmental health of the additive studied in this article, it was found that Millad NX 8000, not being chemically bound to the polymeric matrix, has a high potential to migrate from the articles in which it is incorporated, which allows the public to be exposed to this chemical [
20]. Some histopathological changes were observed in rats exposed to high doses (1682 mg/kg) [
20]. The toxicity data for this additive report a low potential to cause adverse effects after dermal or oral exposure. However, not enough studies have been carried out to quantitatively evaluate the risks of its use in materials in contact with food [
20]. Millad NX 8000 is slightly irritating to the eyes and may cause respiratory tract irritation, but acute and long-term effects on the lungs/respiratory system have not been thoroughly studied. The presence of Millad NX 8000 in wastewater poses environmental and health problems due to its chemical properties and potential impact on aquatic ecosystems. Therefore, it is necessary and urgent to recover this additive from wastewater to minimize the associated risks and promote sustainable practices in waste management. The scarce information related to the environmental impact of the Millad in different bodies of water is also associated with the limitations presented in the identification and quantification processes in the said bodies of water [
21]. This is most likely due to the poor solubility of these substances in various solvents, which complicates the preparation of stock solutions and the choice of a suitable extraction solvent; unfortunately, there are minimal data available on the solubility and thermal properties of such a substance [
22,
23,
24].
Among the various instrumental techniques employed to investigate Millad NX 8000 and its role as a nucleation/crystallization agent in polymers, Fourier transform infrared spectroscopy (FTIR) [
25,
26] and scanning electron microscopy (SEM) [
26] have been utilized. Noteworthy studies, such as the comprehensive research conducted by Smith et al. on the binary system of polypropylene and 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS), have successfully determined the solubility of the additive using optical microscopy (OM) and differential scanning calorimetry (DSC). Additionally, they have developed a non-equilibrium phase diagram that intricately describes the various phases formed during cooling or heating, and their structures across the entire composition and within a broad temperature range [
27]. In another study, gas chromatography-mass spectrometry (GC/MS) techniques coupled with microwave-assisted extraction were employed to identify the nucleating agents in plastic materials [
28].
Presently, it is evident that a comprehensive and reliable analytical methodology for the extraction and quantification of Millad NX 8000, a common component found in industrial waste, has not been established. Consequently, the production process of polypropylene (PP) is not environmentally sustainable. Therefore, there is a critical necessity to develop robust analytical methods that can accurately identify, quantify, and recover Millad NX 8000, addressing the urgent need for environmentally friendly practices in the PP manufacturing industry.
The objective of this research is to propose a suitable analytical methodology for the extraction and recovery of Millad NX 8000 present in wastewater using solid-phase extraction with an adsorbent phase of glass fiber and activated carbon. GC-MS technique was used for quantification and FTIR, DSC, TGA and UV-VIS were used to characterize the recovered material. The recovered additive was added to a PP matrix to evaluate its performance via FTIR, DSC and TGA.
In doing so, this study makes a contribution to the circular economy of petrochemical plants producing PP worldwide, providing new knowledge that contributes to better monitoring and optimization of all industrial processes in which this additive is used.