Introduction

Nanomaterials have unique characteristics compared to analog bulk particles (Wu et al. 2022). Their colossal surface area to volume proportion was used to drive new applications or in different technologies for a wide range of functions; in the last decade, they have been commercialized in more than a thousand products in different branches of the global market (Fajardo et al. 2022). Metal oxide nanoparticles (MONPs) became relevant constituents in consumer products such as cosmetics, pharmaceuticals, textiles, solar cells, and paint (McIntyre 2012; Nagarajan 2008; Mbonyiryivuze et al. 2015; Malhotra and Mandal 2019; Sungur 2021). The environment represents the ultimate sink where these nano-consumer products converge through different discharging pathways such as landfill treatment, wastewater plants, or even air emissions (Hurley and Nizzetto 2018; Pacitto et al. 2021). A key concern is the effect of disposed of MONPs on certain bacteria, such as soil microorganisms that play essential roles in a healthy ecosystem food web, including organic waste decomposition and nutrient cycling processes. According to some research in this field, such contaminants could heavily affect the soil microbial community (Liu et al. 2018; Xie et al. 2016; Mayor et al. 2013; Zhao et al. 2021).

It was well established that MONPs show a proven antimicrobial activity against pure bacterial cultures such as Escherichia coli, Bacillus subtilis, Streptococcus aureus, and Pseudomonas aeruginosa (Kumar et al. 2013; Hajipour et al. 2012; Madkhali et al. 2021) at a wide range of nanoparticle concentrations starting from 10 up to 5000 mg L−1 (Aruoja et al. 2009; Heinlaan et al. 2008; Santos-Rasera et al. 2022; Gelover et al. 2006). Some previous studies suggest that the toxicity of MONPs to microorganisms is assigned to the disruption of cell membrane activity (Neal 2008; Kubo et al. 2005; Lovrić et al. 2005). Some MONPs, such as TiO2 NPs, could penetrate the cell membrane through the cytoplasm, destroying the lipid bilayer due to the generation of reactive oxygen species (ROS) in light (Mileyeva-Biebesheimer et al. 2010; Duan et al. 2021). However, even in the absence of light, the toxicity to microorganisms exerted by ROS formation after interaction with MONPs was also identified (Adams et al. 2006; Niazi and Gu 2009).

NPs dissociation behavior producing metal ions has also been reported as a significant element of toxicity to microorganisms (Aruoja et al. 2009). Moreover, the NPs interaction with soil organic matter (SOM) may pose a significant risk to certain soil microorganisms (Neal 2008). For instance, the common NPs tendency to aggregate to themselves and other compounds and their propension to adhere to the microbial cells' surface might also play a significant role in their toxicity (Jiang et al. 2009). However, other studies demonstrated that SOM could promote NPs stability, dispersion, and separation from the surface of bacteria (Li et al. 2010; Neal 2008). Different NPs properties, such as chemical composition, particle size, shape, and surface charge, showed conflicting results concerning the toxicity to microorganisms (Aruoja et al. 2009; Heinlaan et al. 2008; Adams et al. 2006).

This research focused on zinc oxide (ZnO) nanoparticles due to their growing presence in consumer products such as sunblocks. ZnO NPs are employed in these cosmetic products since they behave as UVA (320–400 nm) and UVB (280–320 nm) reflectors because of their high ability to grasp sun radiation. They have also been proven safe for human use, photostable, non-allergenic, and non-irritating when treating human skin (Smijs and Pavel 2011). For these reasons, ZnO NPs are primarily used as an additive in sunblock cream preparations by prestigious international dermo-cosmetic brands (Kołodziejczak-Radzimska and Jesionowski 2014). The main objective of this research was to determine if metal oxide nanoparticles derived from sunblock-containing ZnO NPs could impact soil microorganisms. The objectives of the current work were to:

  • Characterize ZnO nanoparticles obtained from commercial sunblock products.

  • Simulate the in-house disposal pathway of ZnO NPs-derived sunblock removed from human skin by a water bath.

  • Determine the solubility and dissolution behaviors of ZnO NPs-derived sunblock.

  • Evaluate the toxicity of disposed of water containing different residual sunblocks prepared with bulk ZnO or ZnO NPs to soil microorganisms.

Materials and methods

Reagents and commercial sunblock

All chemical substances used were of Trace Analysis Grade (TAG); ultrapure water (18.2 MΩ cm−1 at 25 °C) was used during the dissolution experiment. Zinc oxide nanoparticles (ZnO NPs) were purchased from 'Micronisers Pty Ltd, Australia (line W45/30). According to the manufacturer specification, the ZnO NPs water suspension contained a ZnO concentration of 99.5% w/v, and the average particle size was 40 nm with a specific surface area of 25 m2 g−1. Two commercial sunblock creams were purchased from the local market: the first one containing 18% w/v bulk ZnO concentration (Caribbean Sol Sunblock SPF 30 from the USA) and the second containing (18% w/v) nano-size ZnO particles (Bioderma Photoderm Nude Touch Solaire Perfecteur SPF50, France).

ZnO NP-containing sunblock disposal route simulation

The specific behavior of ZnO NPs sunblocks in municipal greywater disposal has not yet been well addressed. The proposed disposal route of ZnO NP-derived sunblock through a wastewater treatment plant (WWTP) is presented in Fig. 1. A disposal route simulation was performed using four volunteers recruited among postgraduate students and employees of Arish University. A proportion of sunblock cream was spread randomly on each volunteer's hand skin and left for 30 min under laboratory conditions. Each volunteer washed his/her hand continuously in a warm water bath (30 °C) by immersing their hands in the water for 15 min. The supernatant from each test was collected and kept in the dark at 5 °C for future use. The collected supernatant was used to simulate the sunblock released by sunbathers/swimmers after washing/showering in realistic environmental conditions. Aliquots of 10 mL from each sample were analyzed for Zn concentrations.

Fig. 1
figure 1

Graphical mechanism for the proposed disposal route of ZnO NPs-derived sunblock

Full size image

Characterization of tested samples

The samples were sonicated and then prepared to be imaged using a Hitachi-S 3400N SEM (scanning electron microscope with high-energy electrons beam configurable up to 130 kV) (Ibrahim et al. 2022). The presence of elemental Zn was confirmed through an energy dispersive analysis X-ray spectrometer (EDX) coupled with SEM. The samples were also examined using a transmission electron microscope (TEM) operated at an accelerated voltage of 100 kV (JEOL JEM-1000 cx, Japan) (Liu et al. 2009a). The image processing for the particle size analysis was performed by ImageJ software (Ferreira and Rasband 2012).

Zinc oxide content was estimated by a UV–vis spectrophotometer (Hitachi U-3900). For phases identification of all tested samples, it was used a Rigaku Ultima IV X-ray diffractometer (XRD) adopted a Cu K-α radiation at 40 kV and 30 mA, with a 2θ angles range of 5°–90°, a step size of 0.01, and a scan speed of 2° min−1. Characterization using Fourier transform infrared spectroscopy (FTIR) was undertaken to identify the unknown specimens and provide information on molecules' organic or inorganic nature. FTIR spectra were acquired on a PerkinElmer Spectrum GX FT-IR spectrometer. Initial Zn concentrations in ZnO NPs aqueous suspension were measured using an inductively coupled plasma atomic emission spectrometer (Agilent 5800 ICP-AES). The samples were digested using concentrated HNO3 and then diluted to 5% HNO3 before the analysis.

Stability and Zn release studies

A dissolution study was performed as described by Misra et al. (2011) by placing a portion of the nano form of Zn-containing sunblock dispersed in water inside the interior of a dialysis tube (MWCO = 13.5 kDa) and transferring it to 250 mL plastic bottles (Nalgene) containing 200 mL of 1 mM Ca(NO3)2 (pH 7). In the current study, a relatively higher MW cut-off tube was used than Misra's (12.4 kDa), which was roughly estimated to be equivalent to a 1.2 nm pore size diameter. The starting ZnO content inside the dialysis tube was kept at 500 mgL−1. The samples were incubated at 25 °C in an end-over-end shaker. A proportion of 1 mL was taken from the medium outside the dialysis tube at regular intervals (from 0.5 to 98 h), acidified with 5% HNO3, and the Zn concentration was then measured using a triple quad inductively coupled plasma mass spectrometer (QQQ-ICP-MS, Agilent 8800, Tokyo, Japan).

Zn antimicrobial bioassay

The disc diffusion bioassay method (Akujobi and Njoku 2010; Hossain et al. 2022) was used to test the antibacterial activity of bulk ZnO and NPs at different concentrations. The soil sample used in the test was obtained from agricultural land in Rashid City, Behira Governorate, Egypt (location: 36.96° 31′ 23″ N, 20.80° 30′ 24″ W). The soil was classified as clay according to the FAO/USDA system, and it was characterized by sand, silt, and clay fractions of 30, 25, and 45%, respectively, and by pH moderately alkaline (pH 8.20).

Around 10 g of air-dried soil was added to 90 mL of distilled sterile water and shaken for 30 min. Another decimal dilution (10–2) was prepared from the initial soil suspension, and further serial dilutions were undertaken till required (4th, 5th and 6th dilution). Soil water suspensions prepared from the fourth, fifth, and sixth dilution steps were plated into a Petri dish. Nutrient agar was used as a growing medium; aliquots of 1 mL of the soil suspensions at selected dilutions were injected into each sterile Petri dish. Then, 20 mL of pre-prepared molten nutrient agar was added. The plates were left to dry for about 3 min, and sterile paper discs (5 mm in diameter) were placed on their surface with different concentrations of different ZnO forms. Discs were soaked in a solution containing different concentrations of ZnO in different forms. Discs soaked in 20 μL of 70% ethanol served as a negative control. Augmentin®, a commercial large-spectrum antibiotic containing amoxicillin and clavulanate as active ingredients, was used as a positive control. The inoculated plates were incubated at 30 °C for 4 days. Inhibition zone areas were measured around each disc with a scale as demonstrated in Fig. 2 using the formula:

$$A_{{{\text{in}}}} = \pi R^{2} - \pi r^{2} ,$$
(1)

where Ain = zone of inhibition π = 3.1416, R = inhibition zone radius, and r = original radius of the paper disc.

Fig. 2
figure 2

Measurement method of the area of inhibition zone using disc diffusion methods according to Eq. 1

Full size image

All tests were undertaken in triplicates. In detail, all forms and concentrations of samples tested in the bioassay study included: (1) ZnO in both forms of nanoparticles and pure bulk compounds at different concentrations (0, 0.5, 1.0, 5.0, and 10.0 mg L−1); (2) water samples collected from the handwashing simulation experiment involving the four volunteers as described in "ZnO NP-containing sunblock disposal route simulation" (namely V1, V2, V3 and V4); (3) nano and non-nano-sized ZnO sunblocks (respectively, SunNP and SunBK for ZnO NPs and bulk ZnO containing sunblock) taken directly from the original containers and used at two different concentrations (50% and 100% which represent 9.0% and 18% of ZnO for both forms).

Statistical analysis

Each trial was conducted with three replicates except for the dissolution experiment, where two replicates were used to minimize the costs of ICP-MS analysis. The results were presented as arithmetic mean ± standard deviation (SD). Paired t-test and Pearson correlation were undertaken using Minitab® 17.1.0 software.

Results and discussion

Characterizations of different forms of zinc oxide

SEM, TEM, and XRD characteristics

For their diameter and morphology, different techniques examined zinc oxide in different forms (ZnO NPs, ZnO bulk, SunBK, and SunNP). SEM presented a clear vision of the shape and size details of the ZnO NPs. The SEM demonstrated that the bulk ZnO particle size diameter ranged between 200 and 500 nm and that ZnO bulk exhibited a rocky surface and hexagonal shape, as clearly shown in Fig. 3. The particle diameter of ZnO NPs has estimated about 20–30 nm, as shown in Fig. 4. They displayed a uniform spherical shape instead.

Fig. 3
figure 3

SEM image of pure non-nano-form of zinc oxide (ZnO-Bulk)

Full size image
Fig. 4
figure 4

SEM image of pure zinc oxide nanoparticles (ZnO NPs)

Full size image

The SEM image of SunBk and SunNP (Figs. 5, 6) confirmed that both ZnO forms were coated by sunblock's organic ingredients, which hindered the ZnO primary particle size distribution estimation.

Fig. 5
figure 5

SEM image of zinc oxide sunblock (SunBK)

Full size image
Fig. 6
figure 6

SEM image of zinc oxide nanoparticles sunblock (SunNP)

Full size image

The TEM imaging provided a clearer and more detailed insight into all the particles under scrutiny (Figs. 7, 8, 9). The nanoparticle size estimated from the TEM micrograph showed a size of less than 30 nm for ZnO NPs and SunNP. The TEM graph also validated that the zinc oxide nanoparticles were semi-spherical in shape and nearly uniform in size. The current results align with what was reported by Awasthi et al. (2017), which exhibited their ZnO NPs in an agglomerated state with almost spherical primary shapes. From the TEM image of non-nano sunblock (Fig. 9), it was also clear that ZnO size showed particle distribution with a primary size of more than 100 nm.

Fig. 7
figure 7

TEM image of pure zinc oxide nanoparticles (ZnO NPs)

Full size image
Fig. 8
figure 8

TEM image of zinc oxide nanoparticles in sunblock (SunNPs)

Full size image
Fig. 9
figure 9

TEM image of bulk zinc oxide in sunblock (SunBK)

Full size image

X-ray diffraction (XRD) patterns of ZnO NPs, SunBk, and SunNPs are shown in Fig. 10. All the powders have a similar XRD pattern, but the intensity levels are different. This is because of the random orientation of crystallographic planes (Tsutsumi et al. 2021). At 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, 62.8°, 66.4°, 68.0°, and 77.0°, the diffraction peaks are obtained, which correspond to hexagonal ZnO structure with crystallographic (100), (002), (101), (102), (110), (103) (200), (112) and (202) directions as shown in Fig. 10 (Ladanov et al. 2011; Huang et al. 2001). Furthermore, the diffraction peak at 25.3° and 53.6° show the presence of TiO2 in ZnO nanostructures-containing sunblock (Fig. 13, SunNP; matches with ICDD DB card no. 01-076-1939); in fact, TiO2 is commonly used as an additive to improve sunblock efficacy in combination with ZnO nanostructures. The results confirmed the presence of hexagonal phase ZnO with the ICDD card of ICSD 082028, which corresponded with both SEM and TEM findings.

Fig. 10
figure 10

The XRD pattern of ZnO nanoparticle (ZnO NPs), ZnO NPs-containing sunblock (SunNP), and ZnO bulk-containing sunblock (SunBK)

Full size image

To estimate the crystalline sizes of the ZnO NPs, the famous well-defined Scherrer equation (D = (kλ/βhklcosθ)) was used. The crystalline sizes of the ZnO NPs, SunNPs, and SunBK were observed to be 149 nm, 2.72 nm, and 192 nm, respectively. This result demonstrated aggregation of ZnO NPs. Aggregation behavior of ZnO NPs could exist because the tiny particle has notably higher surface energy, and the development of particle attachment can minimize the free enthalpy of the system by lowering the surface area at a constant quantity. It has been reported by other studies that the bigger size of nanoparticles determined by different technology compared with TEM and XRD results is more likely due to the development of nano-metal oxide aggregates (Zhang 2003; Gericke and Pinches 2006). The actual environmental fate experiments and behavior of NPs generally focus on aggregation or agglomeration state and dispersion resulting from biological or environmental processes (Sekine et al. 2017). However, the size of ZnO in sunblock appears reasonably and relatively consistent as the sunblock cream behaves as a stabilizer agent to maintain ZnO in its original size.

FTIR and UV–Vis

FTIR spectroscopy was performed to determine the functional groups in ZnO powders. Figure 11 shows the FTIR spectrum of different ZnO forms with wave range from 4000 to 500 cm−1. As shown in Fig. 11, all spectrums showed the same trend; the same component is dominant with some exceptions. The broad typical band of IR spectroscopy at 2900–3000 is due to absorbed H2O (OH bond stretching). Moreover, the peaks in this region (2900–3000) may be due to C–H stretching of organic impurities in the Sun-NP and Sun-BK. The peak at 3450 cm−1 correlates to OH presence due to atmospheric moisture absorbed onto the surface of ZnO nanostructures (Anžlovar et al. 2012). The stretch peak around 400 to 500 for SunNP could represent the impurity of ZnO NPs due to the existence of TiO2 in the sunblock cream used in the current study, as shown in the XRD results in Fig. 10. The peaks between 1600.00 and 600.93 cm−1 correspond to ZnO stretching and deformation vibration, respectively. These results were similar to FTIR spectra observed by Kumar and Rani (2013).

Fig. 11
figure 11

FTIR Spectra of the different ZnO forms under scrutiny

Full size image

UV–Visible absorption spectrum of synthesized nanoparticles for SunBK and SunNP is shown in Fig. 12. The distinct peak centered around 300 nm is specific for ZnO NPs immersed in sunblock (SunNP) due to their multiple excitation binding energy at room temperature (Bhuyan et al. 2015). The disappearance of this peak in the case of SunBK can be due to a high increase in particle size (Fakhari et al. 2019).

Fig. 12
figure 12

UV–Visible spectrum of SunNP and SunBK

Full size image

ICP-AES and EDX

The ICP-AES results showed that the total Zn concentrations in the water disposed of during the hand washing test involving the four volunteers were 0.20, 0.23, 0.21, and 0.34 mg L−1 for V1, V2, V3, and V4, respectively. The EDX confirmed that the significant element identified in all measured materials was ZnO but that also TiO2 was identified in the SunNPs product. The ZnO percentage obtained from the EDX results was 98.2 ± 1.74% for the bulk ZnO, 82.3 ± 1.45% for the ZnO NPs, and 45.7 ± 0.96% for the SunBK. For the SunNPs sunblock, the ZnO and the TiO2 amounts were, respectively, 3.80 ± 0.29% and 1.81 ± 0.10%.

Zinc oxide nanoparticle dissolution study

This experiment aimed to study the chemical stability of engineered ZnO nanoparticles in soil water. Since the release of an element from its analog NPs could be primarily influenced by water chemistry composition (Van der Vliet and Ritz 2013; Sharon et al. 2010), the ZnO NPs stability/dissolution study was undertaken in 0.01 M Ca(NO3)2 that is considered as a good proxy standard for soil solution (Degryse et al. 2011; Marzouk et al. 2013). Table 1 shows Zn concentrations dissolved from the ZnO NPs as a function of time. The results showed that the Zn dissociated from NPs increased by increasing the incubation time, although the estimated dissolution was very small. In this regard, the amount of Zn ions dissociated from NPs, as a proportion of the total ZnO NPs addition, increased from 0.017% after 30 min to 0.47% at 98 h.

Table 1 Zinc concentration (µg L−1) measured in the Ca-nitrate electrolyte as a result of ZnO NPs dissolution outside the dialysis tube as a function of time (hours)
Full size table

The dissolution of ZnO NPs is a complicated process that a simple mathematical model cannot express. The modified first-order kinetics model (Eq. 2), as described by van de Venter (2001), Halliwell and Gutteridge (1984), has been used to describe these data.

$$Y_{t} = (Y_{{{\text{final}}}} \left( {1 - {\text{exp}}^{ - kt} } \right),$$
(2)

where Yt is the dissolved ZnO NPs (µg L−1 Zn) at the time t (h), Yfinal is the final solubility, and k is the dissolution rate coefficient (h−1). This equation has been selected as it describes the growth rate of Zn outside the dialysis tube. It was primarily used to describe organism growth or mortality as a function of time. However, it will describe the kinetic reaction of the current study as demonstrated by Halliwell and Gutteridge (1984).

The estimated k and Yfinal values for Zn were 0.176 µg L−1 h−1 and 2478 µg L−1, respectively, indicating low dissolution impacts on the kinetics and steady-state solubility of ZnO NPs under the Ca-nitrate condition. This test estimated a high R2 value (0.97), which suggests that the proposed model could well describe the kinetics of zinc dissolution, as shown in both Figs. 13 and 14. Nevertheless, Eq. 2 slightly overestimates the Zn solubility throughout equilibration times, and the general average predicted from the proposed model (1744 µg L−1) was relatively higher than the solubility determined after 98 h (1718 µg L−1) although still in a reasonable range (Figs. 13, 14). It should be remarked that this kind of mathematical model is an algebraic simplification of the occurring phenomenon like any other similar model, based on a few variables that do not consider all the specific mechanistic processes and the related consequences. To try to achieve better performances, other variables, such as the exact shape, size, and chemical composition of nanoparticles and interactions with their released free ions and physicochemical characteristics of the organics added, should/could be included in the proposed model (Assche and Clijsters 1990). However, it should also be stressed that the increase in model complexity does not often produce the expected improvements in performance and accuracy.

Fig. 13
figure 13

Dissolution kinetics of ZnO NPs in 0.01 M Ca(NO3)2. The data were fitted to a modified first-order reaction equation. Error bars represent standard deviations (n = 2)

Full size image
Fig. 14
figure 14

Predicted against measured values of Zn dissociated from ZnO NPs as described by Eq. 2. The solid line represents a 1: 1 relationship while broken lines represent 1 RSD value [residual standard deviation, the model error (Meharg 1994)]. The plotted points represent all the individual replicates instead of the average values

Full size image

Because the dissolution rate of the nanomaterial needs to be considered to explain the results and, more importantly, design experiments, the benefit of the dissolution experiment is very important in monitoring the toxicity and bioaccumulation of nanoparticulated systems. As a result, this work could not have been completed without first studying the rate at which ZnO NPs dissolve.

Disc diffusion bioassay

Effect of pure materials

In the present investigation, soil bacteria were used to investigate the antimicrobial properties of different forms of Zn using disc diffusion bioassay (DDB). Different forms of ZnO demonstrated high antimicrobial activity and displayed a large inhibition zone area. The 10 mg L−1 Zn concentration showed the highest inhibition zone compared with other treatments. Figures 15 and 16 show that the area of the inhibition zone increased with the increasing Zn concentrations for both treatments (pure ZnO nanoparticles and pure bulk forms).

Fig. 15
figure 15

Effect of different ZnO NPs concentrations on the inhibition zone area for soil bacteria using the disc diffusion bioassay. Error bars represent standard deviations (n = 9)

Full size image
Fig. 16
figure 16

Effect of different bulk ZnO concentrations on inhibition zone area for soil bacteria using disc diffusion bioassay. Error bars represent standard deviations (n = 9)

Full size image

The highest inhibition zone was detected at the highest Zn concentrations (10 mg L−1), with 82.9 and 67.6 mm2 for pure ZnO NPs and bulk, respectively. Likewise, the lowest inhibition was paralleled with the lowest Zn concentrations (0.5 mg L−1) with values of 25.9 and 35.4 mm2 for pure ZnO NPs and bulk, respectively. Figure 17 shows that with all ZnO concentrations, the inhibition zone area showed one order of magnitude smaller compared with the effect of the positive control (Augmentin antibiotic). The effects of bulk ZnO are dominant at lower concentrations, up to 1 mg L−1. Compared with a concentration higher than 1 mg L−1 of ZnO, the nanosized ZnO showed more inhibition zone area than bulk size (Fig. 17). This result indicates a larger antimicrobial activity influence due to the nano-specificity at higher concentrations only. Raghupathi et al. (2011) used different particle sizes of ZnO and studied their impact on bacteria. They found that antibacterial activity increased with decreasing particle size from approximately 200 to 10 nm. The authors found that with a particle size of ZnO of more than 100 nm, the toxicity of methicillin-sensitive S. aureus was inadequate. It seemed that the bacteriostatic function of ZnO NPs was a phenomenon occurring where the NPs size is smaller than the diameter of the bulk form (Figs. 3, 4).

Fig. 17
figure 17

Inhibition zone area (mm2) as affected by pure ZnO (both nano and bulk size) at different concentrations; 0.0, 0.5, 1, 5, and 10 mg L−1. Augmentin, a commercial antibiotic, was included as a reference line (Aug). Error bars represent standard deviations (n = 9 for each ZnO concentration and n = 3 for Aug)

Full size image

A correlation test was run between the inhibition zone area and the Zn concentration. A positive correlation was observed only in the case of the nanoparticle treatment (R2 = 0.88, p < 0.05 for ZnO NPs while R2 = 0.69, p > 0.05 for bulk ZnO). Furthermore, a t-paired test showed that although there was a difference between the effects of ZnO NP and bulk forms on the inhibition zone area, it was not a significant difference (t = 015, p = 0.89). It has been reported that the toxicity effect of ZnO NPs is comparable to the concentration level of NPs (Liu et al. 2009b; Janaki et al. 2015; da Silva et al. 2019).

In an attempt to show the direct effects of sunblock cream on the inhibition zone area, Fig. 18 reported the effect of SunNP and SunBK at 50% and 100% of the initial concentration in the cream. The results showed that the effect of pure sunblock containing nano-size ZnO was more toxic than the bulk-size ZnO form. The inhibition zone area increased with the sunblock ZnO concentration in both forms (ZnO NP; R2 = 0.99, p < 0.01, ZnOBK; R2 = 0.95, p < 0.01).

Fig. 18
figure 18

Inhibition zone area (mm2) of both nano and bulk sunblock types at different concentrations (0, 50, and 100% of the commercial product, i.e., 0.0, 9.0, and 18% of ZnO). Error bars represent standard deviations (n = 9)

Full size image

According to previous studies, different pathogenic bacteria (such as Salmonella spp., Staphylococcus aureus, Listeria monocytogenes, and Escherichia coli) have been monitored under ZnO NPs applications (Applerot et al. 2009; Jin et al. 2009; Liu et al. 2009b; Menazea et al. 2021) with variable toxicity outcome. According to these published results, it might be hypothesized that particle size and specific bacterial sensitivity could be relevant among the various factors controlling toxicity. Three criteria could be used to manage the toxicity level of ZnO NPs to microorganisms: (1) dissolution kinetic of ZnO NPs, (2) promotion of ROS generation, (3) immediate contact with biologic targets, and (4) environmental conditions (Book and Backhaus 2022; Liu et al. 2022; Li et al. 2011).

Effect of disposed water from handwashing trial

The handwashing trial performed in the current study was aimed to simulate and model the potential effects on the ZnO NPs during the route into the WWTP. Figure 19 showed that the effects recorded in these tests were similar to those in the pure materials. There was a significant increase in inhibition zone area (IZA) with increased Zn concentrations in the disposed washing solution (DWS). Pearson correlation coefficient between Zn content in DWS and IZA was 0.86 with a p-value < 0.01. The highest IZA was observed with the highest Zn content in DWS, 8.45 mm2 with 0.34 mg L−1 (Fig. 20). The interaction between ZnO NP-DWS and soil microorganisms was significantly negative. In addition, it should be considered that other factors apart from the nanoparticles may as well contribute to the toxicity. Sunblocks usually contain other ingredients and different coating materials to improve the ZnO NPs stability. For example, it has been reported that the organosilanes coating process could change the surface of ZnO NPs to increase the sunblock formula's stability (Chen and Yakovlev 2010). Therefore, the higher toxicity of soil microorganisms from the sunblock-derived ZnO NPs might be explained by the additional effects of the coating materials and other additives in the preparation.

Fig. 19
figure 19

Relationship between inhibition zone area (mm2) and Zn concentration in the disposed of washing solution obtained from the SunNP applied to the human skin. Error bars represent standard deviations (n = 9)

Full size image
Fig. 20
figure 20

Effect on Inhibition zone area (mm2) of different concentrations of SunNP from the disposed of washing solution. Error bars represent standard deviations (n = 9)

Full size image

Conclusion

This work aimed to investigate the impact of zinc oxide nanoparticles (ZnO NPs) present in sunblock cosmetics on soil microorganisms. We found that the toxic effect of pure sunblock containing nano ZnO on soil microbes is higher than bulk ZnO. The toxicity effect of sunblock ZnO NPs might be driven by some coating materials added to the nanoparticles and other co-formulants. Our work provides valuable information on the impact of ZnO NPs on soil microorganisms. This is mainly because previous studies have mainly focused on the effects of bulk ZnO on isolated organisms rather than investigating the impact on real environmental studies. However, this study examines the influence of ZnO NPs, specifically in sunblock products, on soil microorganisms. Future research should further elucidate the mechanisms by which ZnO NPs impact soil microorganisms and the larger soil ecosystem. Additionally, studies should aim to compare the toxicity of different ZnO NPs under controlled conditions to understand better the impact of particle size, shape, coating materials, and co-formulants on toxicity. Further research on the transport and fate of ZnO NPs in the environment would also be valuable in understanding their potential ecological impacts.