Analysis of the Solid Contents of Toothpastes Available in UAE (United Arab Emirates) Markets

In order to find out whether any toothpastes commercially available in the United Arab Emirates (UAE) carry microplastic content in form of plastic microbeads, the filterable solid contents of 31 toothpastes from UAE markets and 2 toothpastes imported from Syria were analyzed. FT-IR studies of the solids revealed that the major solid components were hydrated silica and calcium carbonate, where the individual toothpaste product exhibited either one or the other as the dominant constituent. Titrimetric analysis of the alkalinity of the ash of the toothpastes was carried out. The solids, ashed at 600˚C were subjected to FT-IR and EDS (energy dispersive X-ray spectroscopic) analysis. The ash of some of the products was shown to have TiO 2 and Ca 3 (PO 4 ) 2 as minor components. Mostly organic dyes were used as colorants; however, iron oxide (Fe 2 O 3 ) was also found. Importantly, none of the toothpastes carried any solid microplastic particles. Only 3 toothpastes carried microbeads at all, which were made of either silica or microcrystalline cellulose. This finding indicates that toothpastes, at least in the UAE, are no longer a significant source of microplastic in the environment. The results were compared to a toothpaste bought through the internet with a formulation from 2014, which exhibited polythene microplastic at 1.31 ± 0.39 w% of the filterable solid content.


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and calcium hydrogen phosphates, among others, constitute the abrasives that help remove the plaque from the tooth enamel [4]. It is this solid content of toothpastes that is the topic of this contribution.
In the last decades, microplastics could be found within the solid contents of certain toothpaste brands. Microplastics are plastic particles of less than 5 mm in length [5]. They have been labeled emerging environmental pollutants, reaching the aquatic environment through effluent water from wastewater treatment plants [6] [7] or directly through run-offs [8] [9], where it has been estimated that 485 × 10 10 plastic particles of a size less than 5 mm are floating on the world's oceans [10]. Equally, Microplastics can be found on land, where partly they are entered into the soil through sewage sludge [11] [12]. While many microplastics, especially when made of polythene, polypropylene and polystyrene, are chemically inert and not toxic per se, additives, monomers remaining from the preparation, albeit at very low concentrations, as well as adsorbed chemicals (however also see [13]) and biological material can pose a risk to organisms, however small. While a high percentage of microplastics reaching the environment is secondary microplastic, derived from the fragmentation of meso-and microplastics, a certain fraction reaches nature as constituents of products, specifically fabricated at that small size.
Over the last 40 years, personal care products have been found to be one of the sources of such primary microplastics [14] [15]. Historically, these personal care products include rinse-off cosmetics [16] [17] [18] [19] [20] and toothpastes [21]. Microplastics have been used as abrasives as well as bulking agents in toothpastes. Sometimes, they are dyed and thus add a color pattern against a differently colored background. Nevertheless, the presence of microplastics in personal care products (PCPs) has led to many cautionary voices coming from scientists and policymakers alike [22]

General
For weighing, either a Radwag balance AS 220.R2 (readability limit: 0.1 mg) or a Kern balance ABT 220-5DM (readability limit: 0.1 mg/0.01mg) was used. FT-IR spectra of the toothpastes' solid contents and their ash were measured as KBr pellets with ThermoNicolet Nexus 670 and Perkin Elmer Spectrum Two FT-IR spectrometers.  analysis. Thereafter, the ash contents of selected toothpastes were examined with energy-dispersive-X-ray spectroscopy coupled with scanning electron microscopy (SEM-EDS) as well as with X-ray fluorescence spectroscopy (XRF). The alkalinity content of the ash was measured (as w% CaCO 3 ) by reacting a precisely determined amount of the ash (50 -100 mg) with aq. HCl (0.1 M, 20 mL) at 60˚C for 15 min. and subsequently back-titrating the reaction mixture with aq.

Thermolysis of the Toothpastes' Solid Contents and Analysis of the Ash
NaOH (0.1 M) using phenolphthalein as indicator [37]. The titrations were carried out in triplicate.

Wavelength Dispersive XRF (WD XRF) Analysis of the Ash
Sample preparation-The ash sample obtained after thermolysis was ground to a fine powder. The resulting fine powder was pressed in a 13 mm bore steel die in a manually operated hydraulic press (Specac). The pressure was applied until the reading was stable at 10 tons and left for the 40s. This produced mechanically stable round pellets of 13 mm diameter. The pellets were generally analyzed within an hour and great care was taken that the two flat surfaces intended for XRF analysis were not touched. The weight and exact diameter of the pellet were measured and used for the semi-quantitative X-Ray analysis.
XRF Analysis-The XRF analysis was done on a wavelength dispersive (WD) XRF spectrometer (Rigaku ZSX Primus IV) equipped with an Rh X-ray tube. The instrument is controlled by ZSX Guidance software intended for the analysis of approximately 70 elements from F to U. The resulting pellet was placed in a sample holder cup with the aid of 10 µm polypropylene film which had a high X-ray transmission rate and low level of impurities. All samples were arranged on a sequential basis controlled by an automated autosampler system. The spectra were processed with a semi-quantitative SQX software package, capable of automatically correcting all matrix effects, including line overlaps. SQX also corrected for secondary excitation effect by photoelectrons (light and ultra-light elements), varying atmospheres, impurities, and different sample sizes. Finally, the spectra of each sample were matched with a library and Perfect Scan Analysis Programs [38].

Scanning Electron Microscopy (SEM)/Energy-Dispersive X-Ray Spectroscopy (EDS) Analysis
The microstructural features of the toothpaste samples were obtained using a G-100DB, Miyazaki, Japan) and micrographs of the sample were recorded using InTouch Scope JSM software using a power of 20 kV. The elemental composition was examined by JEOL-SEM equipped with an energy dispersive X-ray detector (EDS).

Floating Experiments with the Solid Contents of the Toothpastes-Analysis of Floating Microplastics (MPs)
Most microplastics, especially those consisting of polythene, have a lower density than water, i.e., less than 1.0 g/mL. Following the method of Ustabasi and Baysal [30], the filtered and dried solids of the respective toothpastes were In case of an observed presence of microplastic, the material was subjected to DSC (differential scanning calorimetry) analysis (see below).

Differential Scanning Calorimeter (DSC)
The thermal response of the isolated microplastic from toothpaste (H-1) was measured using a differential scanning calorimeter (Shimadzu DSC-60 Plus, Japan). About 7 mg of powdered sample was precisely weighed into a DSC sample pan, which was sealed with a top lid utilizing a pellet press. The sample was measured in the temperature range of 25˚C -300˚C at a heating rate of 10˚C/min under a constant flow of N 2 (50mL/min). Data analysis was performed using the LabSolutions TA software [39].

Ash Content of the Toothpastes
The examined toothpastes exhibited a solid content of 11.18 ± 0.12 w% to 48.55 ± 0.28 w%, as shown in Table 1 major abrasive components, the toothpastes for the most part had either silica or calcium carbonate, whereas toothpaste brands typically offered at least one toothpaste of each version. Those toothpastes with silica as the major ingredient have little alkalinity. Table 1 shows the alkalinity values obtained through titration expressed in w% CaCO 3 . Numbers of CaCO 3 exceeding 100 w% of the ash, as found in T3, T10, T13-T14, and T18 indicate that these toothpastes have in addition to CaCO 3 other basic salts. Also, the ash content of selected toothpastes was used the determination of the elemental composition of the solids using the WD XRF analysis method. All the results are expressed in mass% and calculated based on the weight of ash obtained after pyrolysis. The multi-element analysis report for each sample is shown in Table 2. As mentioned above, most of the samples contain silica and calcium carbonate/calcium hydrogen phosphate as major components, even though samples T16 and T27 carry alumina/aluminum hydroxide as the major component. All of these compounds mainly act as abrasives and are usually used as major components in formulations of toothpastes [42]. The presence of calcium, silica and alumina is identified and quantified by characteristic elemental spectra of each element (calcium, silicon, and aluminum) as presented in Figure 1. The spectrum for each element is the same for each toothpaste. Solely, the intensity varies (expressed in Kcps as shown in Table 2) based on the concentration of the element. A small peak of the Kα satellite is characteristic of silica and alumina.
The other minor elements in the ash observed were titanium, strontium, iron, zinc, magnesium, potassium, phosphorus, and sulfur. The X-fluorescence spectra for magnesium, sulfur, phosphorous, potassium and sodium are presented in Figure 2 (taken from sample T26), the spectra for strontium and iron are presented in Figure 3 (for sample T26) and the spectra for zinc and titanium are presented in Figure 4 (for sample T28). Titanium dioxide (TiO 2 ) is used as an opacifying agent in toothpastes and was found in T10, T27 and T28 in quantities of <5.0 w%. The European Chemicals Agency (Echa) considers that TiO 2 may cause cancer if inhaled. In 2022, the EU classified the substance as a suspected carcinogen by inhalation in certain powder forms. Thus, titanium dioxide (as E171) is no longer considered safe, when used as a food additive [43].
Also, ash samples of selected toothpastes were analyzed with energy-dispersive-X-ray spectroscopy coupled with scanning electron microscopy (SEM-EDS) ( Figure 5 and Figure 6). As expected silicon and calcium were found to be the most abundant elements, attributable to SiO 2 and CaCO 3 . Mg is attributed to CaMg(CO 3 ) 2 or MgCO 3 , Ti to TiO 2 , Fe mostly to Fe 2 O 3 . P we believe to be attributed to Ca 3 (PO 4 ) 2 as sodium monofluorophosphate (Na 2 PO 3 F) is water-soluble.

Microbeads, Floating Experiments and the Quest to Find Microplastics as Constituents
Floating experiments were conducted with all 34 toothpastes (Table 1). Only         The indication that microplastic containing toothpastes are no longer available directly from UAE markets contrasts with recent studies from Turkey [30] and India [33] that seem to suggest that microplastic containing toothpastes can still be bought in these countries. Also in China microplastics have still been found in toothpastes [46]. Nevertheless, the current study is an indicator that the ban on microplastic content in rinse-off cosmetics, including toothpastes, in an ever increasing number of countries has a beneficial effect also on products that are sold in regions where bans have not yet taken hold.

Colorants
21 out of the 31 toothpastes (67.7%) bought in UAE markets were significantly colored ( Figure 9). 10 out of the 31 toothpastes (32.3%) contained titanium dioxide (TiO 2 ) as a white pigment. As colors green (11 toothpastes, 35.5%) and blue Quinoline is deemed not to present any health risk, and is even permitted as a colorant in beverages in the European Community and in Australia. The toothpaste also included iron oxide.

Conclusion
The solid contents of 31 toothpastes from across the main brands, bought in UAE markets, and of 2 toothpastes synthesized and acquired in Syria were studied. None of the toothpastes showed microplastic content. This is a good indication that microplastic content in toothpastes in these countries is being/has been phased out. Therefore, it seems that restrictive regulations in countries of the European Community, the United States and other countries in regard to microplastic in rinse-off cosmetics have a beneficial effect of reducing microplastic containing personal care products also in other regions. By chance, for this study, toothpaste could be acquired online that, long past its expiry date, included polythene based microplastic in its formulation. It is important to note that when toothpastes are formulated with microplastic content, then microplastic is an abundant constituent in the products that cannot be overlooked and should not be seen and analyzed as a sparse contaminant.

Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.