Dermal transfer and environmental release of CeO2 nanoparticles used as UV inhibitors on outdoor surfaces: Implications for human and environmental health

Abstract

A major area of growth for "nano-enabled" consumer products have been surface coatings, including paints stains and sealants. Ceria (CeO₂) nanoparticles (NPs) are of interest as they have been used as additives in these these products to increase UV resistance. Currently, there is a lack of detailed information on the potential release, and speciation (i.e., ion vs. particle) of CeO₂ NPs used in consumer-available surface coatings during intended use scenarios. In this study, both Micronized-Copper Azole pressure-treated lumber (MCA), and a commercially available composite decking were coated with CeO₂ NPs dispersed in Milli-Q water or wood stain. Coated surfaces were divided into two groups. The first was placed outdoors to undergo environmental weathering, while the second was placed indoors to act as experimental controls. Both weathered surfaces and controls were sampled over a period of 6months via simulated dermal contact using methods developed by the Consumer Product Safety Commission (CPSC). The size and speciation of material released was determined through sequential filtration, total metals analysis, X-Ray Absorption Fine Structure Spectroscopy, and electron microscopy. The total ceria release from MCA coated surfaces was found to be dependent on dispersion matrix with aqueous applications releasing greater quantities of CeO₂ than stain based applications, 66±12mg/m² and 36±7mg/m², respectively. Additionally, a substantial quantity of CeO₂ was reduced to Ce(III), present as Ce(III)-organic complexes, over the 6-month experimental period in aqueous based applications.

Keywords: Ceria; Exposure; Nano-enabled; Nanomaterial; Surface coating; XAFS.

Figure 1:

STEM images of CeO₂ NPs used in this study. Particle size was calculated as 6.4±1.3 nm by manual analysis of 53 particles.

Figure 2:

Release of cerium from coated boards. Data for both weathered boards (Outdoor) and controls (Indoor) are an average of at least three separate replicates. Time zero corresponds with the first wipe event after application of surface coatings and dry time of at least 48 h. Data has been normalized based on the area wiped per sampling event.

Figure 3:

Total cerium wiped from the surface of the wood and composite surfaces for the outdoor and indoor treatments. The Out and In designations refer to whether the surfaces coating were aged outdoors (Out) or indoors (In). Indoor samples function as experimental controls for outdoor samples. The lower-case letters indicate statistical differences in the mean concentration of cerium wiped from the surface over the 6-month experiment as determined by a Fisher LSD means test (n= 4; α = 0.05). Identical letters indicate no statistical difference in the mean.

Figure 4:

Release of cerium from MCA-coated boards that have undergone 1260 h of UV degradation before application of surface coatings. Data were collected in triplicate and have been normalized by the total area wiped during a sampling event. The data at Time zero correspond with the first wipe event after application of surface coatings and dry time of at least 48 h.

Figure 5:

Release of cerium from pristine MCA boards (controls) and UV degraded boards per wipe event. Data were collected in triplicate and have been normalized by the total area wiped during a sampling event.

Figure 6:

SEM image of particulate matter retained on a 0.45μm filter after release and extraction using the CPSC estimate of dermal transfer test method. Sample is from a micronized copper pressure-treated lumber coated with CeO₂ NPs dispersed in water after 1 month of outdoor weathering. Cerium particles are heavily co-located with copper particles, as shown in the EDS micrograph.

Figure 7:

Results of leaching test performed on surface coupons that had been painted with different experimental treatments. Samples were allowed to mix in SPLP solution for 72 h and then sequential filtered to determine size fractions of released material. Composite samples are coated with CeO2 dispersed in water as wood stain is not intended for application on composite surfaces. Detection limit for this analysis is 0.230 mg/L Ce.

Figure 8:

XAFS data collected on a 10 kDa filter produced after leaching a CeO2/Water/Composite sample in SPLP solution for 72 h. Resulting data clearly demonstrate the presence of both Ce(III) and Ce(IV) oxidation states.