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. Author manuscript; available in PMC: 2020 Dec 1.
Published in final edited form as: Sci Total Environ. 2019 Jul 30;694:133669. doi: 10.1016/j.scitotenv.2019.133669

Transformation and release of nanoparticle additives & byproducts from commercially available surface coatings on pressure treated lumber via dermal contact

Justin G Clar 1, William E Platten 3rd 2, Eric Baumann 2, Andrew Remsen 2, Steve Harmon 3, Kim Rodgers 4, Treye Thomas 5, Joanna Matheson 5, Todd P Luxton 6
PMCID: PMC7440215  NIHMSID: NIHMS1615253  PMID: 31382174

Abstract

Production and marketing of “nano-enabled” products for consumer purchase has continued to expand. However, many questions remain about the potential release and transformation of these nanoparticle (NP) additives from products throughout their lifecycle. In this work, two surface coating products advertised as containing ZnO NPs as active ingredients, were applied to micronized copper azol (MCA) and aqueous copper azol (ACA) pressure treated lumber. Coated lumber was weathered outdoors for a period of six months and the surface was sampled using a method developed by the Consumer Product Safety Commission (CPSC) to track potential human exposure to ZnO NPs and byproducts through simulated dermal contact. Using this method, the total amount of zinc extracted during a single sampling event was <1 mg/m2 and no evidence of free ZnO NPs was found. Approximately 0.5% of applied zinc was removed via simulated dermal contact over 6-months, with increased weathering periods resulting in increased zinc release. XAFS analysis found that only 27% of the zinc in the as received coating could be described as crystalline ZnO and highlights the transformation of these mineral phases to organically bound zinc complexes during the six-month weathering period. Additionally, SEM images collected after sampling found no evidence of free NP ZnO release during simulated dermal contact. Both simulated dermal contact experiments, and separate leaching studies demonstrate the application of surface coating solutions to either MCA and ACA lumber will reduce the release of copper from the pressure treated lumber. This work provides clear evidence of the transformation of NP additives in consumer products during their use stage.

Keywords: Zinc exposure, Copper exposure, Nanomaterial, Pressure-treated wood, Nano-enabled, Surface coating

Graphical Abstract

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1. Introduction

Fundamental research in nanoscale chemistry and engineering has paved the way for increasing amounts of “nano-enabled” products available for consumer purchase and use. (Foss Hansen et al., 2016; Mitrano et al., 2015; Sun et al., 2017) As a result, researchers have begun to recognize the need for studies focusing on the transformation and release of nanoparticles (NPs) and NP daughter products from these newly available consumer goods. These studies have ranged from tracking Ag NPs from textiles (Quadros et al., 2013), TiO2 from floor panels (Bi et al., 2017; Bressot et al., 2018), Cd/Se quantum dots from polymer composites (Gallagher et al., 2018), micronized copper from pressure treated lumber (Lankone et al., 2019; Platten III et al., 2016), and a variety of NP additives in spray and aerosol products (Nazarenko et al., 2014; Park et al., 2017). Interestingly some researchers have even attempted to distinguish changes in release and transformation patterns based on passive factors (dissolution) and active matrix degradation (Duncan, 2015; Duncan and Pillai, 2015). One area of special interest is “nano-enabled” surface coatings as they represent an area of direct contact between the product and numerous end users after application. Additionally, these products are applied to exposed surfaces and therefore may be more vulnerable to environmental degradation, which may accelerate their transformation and release.

Previous research has attempted to understand the release of NPs from surface coating products (Al-Kattan et al., 2015; Clar et al., 2018; Mackevica et al., 2018; Platten III et al., 2016; Shandilya et al., 2015a; Zuin et al., 2013). Most of these studies have tracked NP transformation in terms of leaching or diffuse release into environmental systems. These studies help inform questions on the long-term release of NPs to the environment, but lack detail on specific exposure doses to product users. Recently, Platten et al. utilized a wiping methodology previously developed by the Consumer Product Safety Commission (Cobb, 2003; Thomas et al., 2005) (CPSC) to quantity potential exposure quantities of copper particles and other daughter products released from micronized copper azole (MCA) pressure treated lumber during dermal contact (Platten III et al., 2016). This methodology has since been utilized to study the release of both ZnO (Clar et al., 2019) and CeO2 (Clar et al., 2018) NPs applied to two different surfaces, MCA lumber and composite decking. More recently, Mackevica and coworkers used a hand wiping technique developed by the National Institute for Occupational Safety and Health to study the release of NPs from coated surfaces (Mackevica et al., 2018). Other studies have used more aggressive approached to simulate NP release from surface coatings including taber abrasion (Morgeneyer et al., 2015) and sanding. (Göhler et al., 2010) In each of these studies, the extent of release was highly dependent on both the nature of the application and the level of surface degradation during sampling. Other studies have also concluded that the release of NPs from consumer products is primarily driven by the nature of the matrix they are embedded in, and not the intrinsic properties of the NP itself (Wohlleben and Neubauer, 2016). Even with these initial studies, there is an increasing need for research on NP release under realistic use conditions to better inform occupational and environmental risk models (Kuhlbusch et al., 2018; Shandilya et al., 2015b; Shandilya et al., 2015c). Several researchers have called for development of universal standards for testing NP release form consumer products in order to enable comparisons between data sets and thereby placing the results in better context (Koivisto et al., 2017).

In the present study, we track the transformation and release of ZnO NPs and daughter products from a commercially available “nano-enabled” wood stain and sealant via simulated dermal contact and leaching after application to pressure treated lumber. Critically, experiments are completed under realistic use scenarios to understand how the extent of material release may change during the six months of outdoor weathering after product application. Emphasis is placed on determining both the form (particulate vs. ion) and speciation of material dislodged from the surface over the six-month sampling period. Collecting both concentration and speciation data is key for determining potential risk for end users of “nano-enabled” consumer products.

2. Materials and methods

2.1. Surface specimens

Two different surfaces were investigated throughout this study. Micronized copper azole-treated lumber (MCA) and aqueous copper azole-treated lumber (ACA). The major difference between these two samples is the nature of the copper azole used in pressure treatment. MCA utilized particulate copper in the form of micro and nano sized copper carbonate/hydroxide while ACA utilized a water-based aqueous copper formulation. Acidic digestion and elemental analysis determined copper concentration in the MCA and ACA lumber to be approximately 1600 and 3800 mg/kg respectively. Detailed analysis of these wood samples is presented in previous publications (Platten et al., 2014; Platten III et al., 2016). All lumber was purchased within 50 miles of Cincinnati, OH in bulk to minimize heterogeneity between production runs.

2.2. Surface coating selection and characterization

The three products used in the study were selected from a family of coatings manufactured by the same company. The first was a commercially available wood stain (Stain-1) that was listed as Light Walnut in color that was not advertised to contain ZnO NP additives. The second coating was the same color but advertised the inclusion of ZnO NPs for enhanced UV protection (Stain-NP). The final coating was a transparent UV protective sealant that also contained ZnO NP additives (Clear-NP). The manufacture advertises that both the Stain-NP and Clear-NP products contain enough added ZnO NPs to result in “over 30 trillion NP per square inch” of coated surface to increase the UV protective properties of the product. The pH of all solutions was slightly basic ranging from 8 to 8.5.

Concentrations of quantifiable metals in all products were determined by acidic digestion using EPA method 3051a followed by ICP-OES analysis. (Thermo Fisher iCAP 6000 Series). The nature of the organic contents of each surface coatings were investigated by FT-IR using a Bruker Vertex 80 equipped with a Diamond Tip ATR (Bruker Corporation Billerica, Massachusetts). The morphology and size of the ZnO NP additives in Stain-NP and Clear-NP formulations were investigated using a Scanning Transmission Electron Microscopy (STEM) at North Carolina State University (Raleigh, NC). Samples for STEM analysis were prepared by dropping 10 μL of the product onto a formvar coated Cu TEM grid and air drying prior to analysis. STEM images were collected on an aberration corrected FEI Titan 80–300 probe with a monochromator operating at 200 kV. The instrument was equipped with Bruker 4 SDD Energy Dispersive Spectroscopy (EDS) probe for elemental analysis. Finally, the speciation of zinc in both the Stain-NP and Clear-NP formulations were determined by X-Ray Absorbance Fine Structure (XAFS) Spectroscopy.

2.3. Coating procedure and experimental design

All test solutions were applied to MCA lumber, while only the Stain-1 (no NPs) was applied to the ACA lumber. Surface coating applications were completed identically to our previous publications and per manufactures instructions (Clar et al., 2019; Clar et al., 2018). Coatings were vigorously mixed using a reciprocating shaker before application to ensure a homogeneous solution. After mixing, coatings were applied in two coats using a 2-inch roller making sure that the initial coat was still damp before application of the second coat. Final quantities of zinc applied were estimated by mass. After application, all surfaces were air dried for between 48 and 72 h before use in wiping and leaching experiments. Samples of coated material were split into two treatment conditions. A subset of coated materials was placed outdoors at the U.S Environmental Protection Agency (EPA) Center Hill Research Facility in Cincinnati OH. These boards were left exposed to natural weathering including UV degradation, precipitation, and temperature fluctuation through the duration of the study. Weather conditions during this period are summarized in the Supporting Information Table S1. The remaining boards were placed indoors and remained in constant darkness and temperature controlled to act as experimental controls.

2.4. Wiping procedure

Details of the CPSC wiping procedure can be found elsewhere (Cobb, 2003; Platten III et al., 2016; Thomas et al., 2005). The CPSC methodology has been previously used to track NP release from both lumber and UV protective surface coatings (Clar et al., 2019; Clar et al., 2018; Platten III et al., 2016). The wiping procedure was completed identically to those previously reported studies that demonstrated a high correlation with wiping by human hands (Cobb, 2003; Platten III et al., 2016; Thomas et al., 2005). Polyester fabric cloths were soaked in a saline solution for 24 h before wipe sampling. The CPSC wiping apparatus was then attached to the desired sampling area with the cloth attached and an effective surface area of 450 cm2 was sampled as shown in Fig. S1. Post sampling, polyester cloths were placed in individual 50 mL HDPE tubes in preparation for extraction. Metals extracted from the boards were solubilized through the addition of 20 mL 10% Nitric Acid and 20% hydrogen peroxide to HDPE tubes. Tubes were placed in a shaking water bath held at 60°C for 24 h to assist in metals extraction. The resulting solutions were diluted when appropriate and analyzed via ICP-OES.

Identically to our previous studies, a single wiping cloth was extracted with 20 mL of Milli-Q water to track the size and form of particulate matter dislodged during wipe events. The resulting solution was sequentially filtered through both 0.45 μm (Track-Etched Membrane, Whatman) and a 10 kDa (~3–5 nm) Amicon Ultrafiltration filters (Millipore, Billerica MA). Subsamples of fractionated filtrates were acidified and analyzed via ICP-OES while filters were reserved for imaging and speciation analysis where appropriate. Both indoor and outdoor treatments were sampled periodically over the course of 6 months (2, 4, 8, 12, 16, and 24 weeks).

2.5. Leaching procedure

All leaching experiments were conducted identically to those previously described (Clar et al., 2019; Clar et al., 2018). Subsamples of coated surfaces (4 cm by 4 cm “coupons”) were removed from coated boards after the drying period and placed faced down in 250 mL beakers filled with 100 mL of synthetic precipitation leaching procedure (SPLP) solution with a pH of 4.2. Samples were covered and mixed with a magnetic stir bar at 200 RPM for 72 h. At no time during the mixing period did the pH increase above 5.5. The leachate was then sequentially filtered through 0.45 μm and 10 kDa (~3–5 nm) membrane filters. Aqueous subsamples were collected and acidified at each step, in preparation for ICP-OES analysis. 0.45 μm and 10 kDa filters were reserved for additional analysis.

2.6. Metal speciation by X-Ray Absorption Fine Structure Spectroscopy (XAFS)

The initial speciation of zinc in all surface coatings, as well as changes to zinc speciation during this study were tracked using X-Ray Absorption Fine Structure Spectroscopy (XAFS). XAFS spectra was collected on the pristine surface coatings, as well as filters produced from extracting particles retained in the cloths using Milli-Q Water. Analyses were conducted at Sector 10BM (MRCAT) (Kropf et al., 2010) at the Advanced Photon Source at Argonne National Laboratory, U.S. Department of Energy, Argonne, IL identically to our previous publication (Clar et al., 2019). Zinc data collection was done at the Zn— K edge (9659 eV) after calibration using a Zn foil. Details of standard preparation can be found in our previous work (Clar et al., 2019). Data reduction and analysis, including linear combination fitting (LCF) was completed using the IFEFFIT software package (Ravel and Newville, 2005). The fitting range was −20 to 60 eV relative to the Zn— K edge.

3. Results and discussion

3.1. Stain characterization

FT-IR analysis of the Stain-1, Stain-NP, and Clear-NP showed similar spectral signatures (Fig. S2). This result is expected as all three products are predicted to have similar base constituents (i.e., polymers/resins) to their formulations commonly found in outdoor commercial coatings. As the major focus of this study was on the inorganic aspects of the materials, additional IR analysis was not completed.

A summary of the major elements present in each of the surface coatings is summarized in Table 1. The concentration of most inorganic constituents of all three products is similar. The concentrations of silicon, sulfur, and phosphorous indicates the manufacturer likely uses similar active components in all products. Notably, the concentration of iron is substantially higher in both the stain-based products. This is not unreasonable as the inclusion of it is potentially needed for the desired “light walnut” color. Most critical to this work is the large difference in zinc concentration between the three products. Results of the elemental analysis indicated that the zinc concentration in the Stain-NP is three orders of magnitude larger than the concentration found in the Clear-NP and two orders of magnitude larger than the Stain-1 (Table 1). Lower concentrations of zinc in the Stain-1 are expected, as the product does not list ZnO NPs as an additive. The difference in zinc concentrations between the Clear-NP and Stain-NP coatings is especially surprising considering the manufacturer advertises that both NP-enabled products contain enough ZnO nanoparticles additives to result in “over 30 trillion NPs per square inch,” after application.

Table 1.

Elements of interest found in both Stain-1, Stain-NP and Clear-NP coatings. All concentrations determined in at least triplicate. B.D.L: Below Detection Limit.

Element Concentration (mg/L)
Stain-1 Stain-NP Clear-NP
Cu 0.78 ± 0.07 9.6 ± 0.8 0.07 ± 0.06
Fe 133 ± 13 85 ± 6 <0.030
P 821 ± 130 795 ± 85 807 ± 66
S 355 ± 33 295 ± 24 226 ± 5
Si 53 ± 8 79 ± 14 97 ± 15
Zn 17 ± 2 2619 ± 285 3.2 ± 0.1
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To better characterize the size and morphology of the ZnO found in both products, the Stain-NP, and Clear-NP coatings where characterized via STEM. Representative images of the identified particles are shown in Fig. 1, Fig. 2. Spherical ZnO particles with a diameter of roughly 20 nm were identified in the Stain-NP. During this analysis, iron oxyhydroxides NP rods were also identified in Stain-NP samples. This highlights the complications with labeling and marketing of “nano-enabled” products. While the ZnO was advertised as being the NP additive in this system, other materials in the stain formulation are clearly in the nm domain. ZnO NP found in the Clear-NP have a drastically different shape and size compared to those observed in the Stain-NP. Not only are the particles larger, but they appear to have an amorphous morphology between traditional rods, and hexagonal plates. Both the elemental analysis, and collected images seem to suggest that both the Stain-NP, and Clear-NP products utilized different sources of ZnO in product formulation.

Fig. 1.

Fig. 1.

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STEM image of ZnO and iron oxide/iron hydroxide particles found in Stain-NP coating.

Fig. 2.

Fig. 2.

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STEM image of ZnO particles found in Clear-NP coating.

To elucidate and confirm the speciation of zinc in each of the products, subsamples of coating material were dried on silicon wafers before XAFS analysis. A variety of zinc compounds were evaluated as potential models for speciation analysis, and five were found to be enough to fit all zinc samples generated in this work. Zinc absorbed to Suwannee River Humic Acid (SRHA) and zinc citric acid were used as a proxy for zinc bound to organic matter. Zinc phosphate was used based on the high concentrations of phosphate identified in ICP-OES analysis, and zinc chloride was used as a proxy for ionic zinc. Finally, an external ZnO standard was included based on the advertised inclusion of the NP additive. Results of linear combination fitting (LCF) were not significantly different between normalized and first derivate of normalized spectra. Therefore, only results from first derivate fits are presented. Plots of both XAFS spectra and LCF results for collected samples are shown in the Supporting Information (Figs. S3–S6). For context, it is generally accepted throughout the literature that the error in any LCF results ranges between 5 and 10% (Gräfe et al., 2014).

Analysis of the Stain-1 (no added NPs) suggest a combination of zinc species distributed between organic complexes (70%), phosphates (30%). The presence of zinc oxide material was not anticipated in this product, as it is not explicitly noted as an ingredient by the manufacturer. Alternatively, XAFS data collected on the Stain-NP formulation did indicate the presence of a zinc oxide mineral phase. (Fig. S5) LCF data suggested zinc speciation in the Stain-NP product is distributed between crystalline zinc oxide (27%), phosphates (19%), and zinc organic complexes (43%) and ionic zinc (11%). These XAFS results suggest that transformation of the ZnO NP additives likely take place immediately after the manufacturing process and continue until end use by the consumer based on the addition of ionic zinc in XAFS modeling. While the STEM images of the Stain-NP solution show a potential collocation of some NP ZnO with iron oxyhydroxide minerals their abundance is much lower than the threshold necessary for including in LCF results. Finally, as the concentrations of zinc were very low in the Clear-NP coating, no reliable XAFS data was obtained.

3.2. Estimates of zinc release through simulated dermal transfer using CPSC methods

Studies on zinc release were focused on both the Stain-NP, and the Clear-NP products applied to the MCA lumber. A summary of the total zinc extracted during the wipe procedure is presented in Fig. 3. The amount of zinc released has been normalized to the area contacted during each sampling event (450 cm2). The zinc release from both products is extremely low. Not a single wipe event resulted in zinc release above 1 mg/m2 (0.03 mg Zn) during the six-month weathering period. This observed release pattern is lower than the releases observed in our previous work. In that study, ZnO NPs were added to commercial stains and surfaces wiped using the identical CPSC methodology with zinc release ranging from 5 to 20 mg/m2.(Clar et al., 2019) This limited release of zinc from the commercial products may be driven by the physical characteristics of the zinc additive or when it is added to the stain formulation in the manufacturing process.

Fig. 3.

Fig. 3.

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Release of zinc from coated MCA lumber coated with a) Clear-NP and b) Stain-NP solutions during simulated dermal contact. Data for both weathered boards (Outdoor) and controls (Indoor) are an average of at least three 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 and represents the average and standard deviation of four replicate measurements.

The collected data also indicates that the effects of outdoor weathering had limited impact on the total zinc release during the initial sampling events. Specifically, for the Clear-NP samples there was minimal difference in zinc release between samples left to weather outdoors, and control samples left indoors. A similar trend is also evident in the Stain-NP samples for the first four months of weathering. After 16 weeks samples collected from boards weathered outdoors show an increase in the total zinc released. This is consistent with our previous study which hypothesized degradation of the polymers and binders in the stain during weathering may serve to increase zinc release over long time scales.(Clar et al., 2019) Even with the increase in zinc release relative to the indoor samples, the total release from a single simulated dermal contact event is low (<1 mg/m2).

To estimate the relative loss of zinc from coated surfaces four samples from unwiped sections of control boards were removed, digested and analyzed for total zinc content. These results match the zinc application estimates for Stain-NP solutions determined by ICP-OES (Table 1), and the volumes of applied solution. Therefore, roughly 0.5% of the applied zinc was removed during the 6-month study during simulated dermal contact, with limited disuse environmental release.

3.3. Characterization of zinc released through simulated dermal contact

Although the total concentration of zinc released from these systems through simulated dermal contact is low, it is critical to identify both the nature and speciation of the material released for an understanding of the mechanisms responsible for release. Similar to our previous work, a single replicate from the wipe procedure was extracted with Milli-Q water to collect a sample of the particulate materials retained on the cloth. Visual analysis and preliminary chemical testing indicated that substantial material from the wipe cloths was retained on the 0.45 μm filters. Analysis of these filters was completed using an FESEM with EDS capabilities to understand particle morphology associated with zinc release. Fig. 4 is a representative image of the material collected from wipe cloths of the Stain-NP on MCA lumber after 3 months of outdoor weathering. The goal of this analysis was to determine the speciation and nature of zinc in the samples, EDS did not find large amounts of zinc retained on the filters. The aggregates collected on the filters consist of what appear to be silicon dioxide particulate. This is reasonable as silicon was found to be a component of the product in elemental analysis (Table 1). The collected images help support the results of the total zinc analysis, showing that very little is removed from the surface during a single simulated dermal contact event. Furthermore, there is no evidence of the release of free ZnO NPs during simulated dermal contact using the CPSC methodology.

Fig. 4.

Fig. 4.

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BSE FESEM image of particulate matter retained on a 0.45 μm filter after release and extraction using the CPSC estimate of dermal transfer test method. The sample is from MCA lumber coated with Stain-NP solution after three months of outdoor weathering. Strong evidence of the release of large aggregates of silicon oxides are shown in the EDS Micrographs.

Filters collected from wipe cloth extraction were also subjected to zinc speciation analysis using XAFS. The same zinc standards used in the analysis of pure coatings were used to track changes in zinc speciation during dermal contact (Fig. S3). The values presented in Table 2 highlight the changes to the zinc species removed during simulated dermal contact throughout the 6-month study. In all cases, there was insufficient material on the samples coated with the Clear-NP to obtain an interpretable XAFS spectra even after repeated scans and were therefore not included in the analysis. An example of an LCF model fit is outlined in Fig. S6.

Table 2.

Summary of zinc speciation for as received Stain-NP coating and debris extracted form dermal transfer wipe clothes using CPSC methods. Speciation determined by LCF analysis of first derivate normalized XAFS Spectra. Time Zero represents the first wipe event directly after coating application before outdoor weathering. Samples marked N/A had low zinc concentrations resulting in insufficient signal to noise ratios during XAFS data collection and processing and therefore were not used in LCF analysis. The Zn-Organic species is a sum of the contribution of zinc citric acid and zinc absorbed Suwannee River humic acid. It is generally accepted that the inherent error in any LCF analysis is between 5 and 10%.

Species Stain-NP Time zero 2 weeks 4 weeks 8 weeks 12 weeks 16 weeks 20 weeks 24 weeks
Zn-organic 43 N/A N/A N/A 69 58 76 79 N/A
Zn-phosphate 19 6 19 14 13
Ionic zinc 11 19 13 5 4
ZnO 27 6 9 5 4
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After the eight weeks of outdoor weathering, the majority of zinc can best be described as absorbed to organic material with ionic zinc serving as the next major component as shown in Table 2. After sixteen weeks, the LCF analysis indicates the Zn-Organic complexes increase in abundance compared to ionic zinc. The transition from ionic species to Zn-Organic species correlates with the increase in total zinc released during simulated dermal contact (see Fig. 3). The reason for this correlation are unknown at this time. Additional research using longer weather periods would help to determine if this trend continued for periods longer than six months. The collected XAFS data demonstrates that after application, the zinc additive in the Stain-NP undergoes a transformation during outdoor weathering. While spectra obtained here were only able to determine these transformations after eight weeks of outdoor exposure, it is likely that transformation began much earlier.

3.4. Estimates of copper release through simulated dermal contact

Previously, the U.S. EPA has demonstrated that the application of surface coating to Chromated Copper Arsenate (CCA) pressure treated lumber reduced the amount of dislodgeable arsenic for a period of time (Arcadis, 2005). The length of time the coating was effective at reducing the release of arsenic was related to the integrity of the surface coating. As part of the current study, the effectiveness of each surface coating at reducing Cu release from MCA and ACA treated lumber was evaluated. Platten et al. used the CPSC wiping methodology to simulate exposure to both micronized and nanoscale copper particulates during dermal contact with uncoated MCA and ACA lumber exposed to the environment and the data generated during that studies is reproduced with permission in Fig. 5 (Platten III et al., 2016). As shown in Fig. 5a, during initial contact events on MCA lumber, total copper released ranged from 2 to 6 mg/m2. Alternatively, the copper release from ACA lumber was less variable, between 3.25 and 5 mg/m2 during the initial sampling events (Fig. 5b). After approximately one month of outdoor weathering, copper release from both MCA and ACA samples reached a steady state of 1.5 mg/m2 during each subsequent sampling event.

Fig. 5.

Fig. 5.

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Release of total copper from MCA and ACA lumber uncoated and coated treatments during simulated dermal contact. All data is from boards left outdoors to undergo natural weathering throughout sampling. Data for uncoated systems is reproduced with permission from Platten III et al. (2016). 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 and represents the average and standard deviation of four replicate measurements.

To examine how copper release from both MCA and ACA lumber during simulated dermal contact may change based on the presence of a coating, copper release was tracked simultaneously to zinc released via the CPCS wipe methodology throughout the 6-month sampling period. While the timescale of the wiping events is slightly different between uncoated and coated samples, general trends are observed. Regardless of the coating formulation, the copper release is diminished with initial sampling events only removing between 0.1 and 0.3 mg Cu/m2. After 2-months of outdoor weathering, the copper release for MCA lumber coated with both Stain-1 and Stain-NP formulations increases slightly. However, it does not cross the 1.5 mg/m2 average of uncoated MCA lumber observed by Platten et al. For ACA lumber, the reduction in copper release is more obvious. Although both the Stain-1 and Stain-NP coatings contain additional copper (Table 1) the total release of copper in these systems is still lower than coated MCA or ACA lumber highlighting the effectiveness of the coatings on reducing copper release. Although not completed here, it is likely that continued sampling past the 6-month mark would result in the similar steady state release for each contact event. As the coating continues to break down, the surface will behave more like uncoated lumber.

The trends seen in Fig. 5 highlight the reduction in copper release after application of surface coatings to MCA and ACA lumber. However, it is also important to understand the degree to which outdoor weathering helps promote copper release during simulated dermal contact. To track the effects of outdoor weathering on copper release, an identical set of boards was coated with Stain-1 and placed indoors as experimental controls. A summary of total copper release from these samples is presented in Fig. 6. For ACA samples coated in Stain-1, there is an increase in the amount of copper release from samples left to weather outdoors compared to indoor controls. While the copper release is only slightly higher during each sampling period, the total Cu release from ACA outdoor samples totaled ~ 2 mg, while indoor controls released only 0.9 mg of copper. It is important to note that in previous work release of copper from uncoated ACA lumber during simulated dermal contact also noted an increase in release from samples left to weather outdoor relative to controls (Platten III et al., 2016). Increased release from outdoor MCA samples is much more pronounced. After the initial sampling event, the outdoor and indoor treatments begin to diverge. Copper release from MCA-outdoor samples increases throughout sampling reaching roughly 0.9 mg/m2 after six months of outdoor exposure, while indoor samples stay below 0.2 mg/m2. Throughout the sampling period MCA samples coated in Stain-1 released more copper than indoor control at 5.4 and 1.1 mg of Cu respectively.

Fig. 6.

Fig. 6.

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Release of total copper from MCA and ACA lumber coated with Stain-1 during simulated dermal contact. 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 and represents the average and standard deviation of four replicate measurements.

We hypothesize that the difference in copper release for the outdoor samples is based on the breakdown of polymers and binders in the stain during continued UV exposure and precipitation events. As the stain degrades with continued outdoor exposure, subsequent contact events are more likely to make direct contact with the lumber itself, thereby increasing the release. Indeed, a similar release pattern for other surface coating formulations has been previously documented (Clar et al., 2019). Even with the differences between outdoor and indoor samples, during this study no single wipe event from MCA or ACA boards meet or exceeded the 1.5 mg/m2 threshold of uncoated boards observed by Platten and coworkers (Fig. 5) (Platten III et al., 2016). After six months of outdoor weathering, the MCA coated with stain approach 1.0 mg/m2, just below the previously established steady state release from uncoated lumber.

Overall, the data presented here demonstrate that under environmentally relevant conditions, the copper release from MCA and ACA lumber is diminished upon application of a surface coatings, at least for the first six months. Harsh outdoor conditions (extended heat, large temperature swings, etc) may serve to reduce the properties of the coating and shorten the protective period. Indeed, previous research on the passive release of copper particulate from MCA lumber found release to be climate dependent over 18 months. (Lankone et al., 2019) Although not tested here, it is likely that a similar climate dependence would be evident using coated MCA lumber samples. In general, if copper release is determined to be an environmental risk from MCA or ACA lumber depending on its determined use, the results here indicate that coating the surface will reduce the release of copper during dermal contact.

3.5. Zinc and copper release from simulated leaching

Passive release of ZnO or other products during extended weathering by precipitation was evaluated by subjecting the MCA coated lumber (Stain-NP and Clear NP) to a 72-h leaching period in SPLP solution (pH 4.2). A summary of zinc release from these products is presented in Fig. 7. The drastic differences in zinc released from MCA surfaces coated the Stain-NP and Clear-NP coatings are likely caused by the different zinc concentrations in the two products (Table 1). However, the overall zinc release from both products is low. For context, our previous work demonstrated that zinc release from systems coated with ZnO dispersed in water can range as high as 30 mg/L, two orders of magnitude above the concentrations observed in this study (Clar et al., 2019). Moreover, the results of the leaching study suggest that any zinc leached from the coatings is released in the ionic form. For both coatings, the concentration of zinc in the unfiltered subsamples correspond with the concentration in the 10 kda (~ 3 nm) filtrates. This is consistent with our previous work, and reasonable based on the pH of SPLP solution. Additional testing using different leaching solutions may find the presence of zinc based particulate matter retained on the filters.

Fig. 7.

Fig. 7.

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Results of Leaching test performed on MCA lumber coupons that had been coated with either Stain-NP or Clear-NP formulations. Samples were mixed in SPLP (pH 4.2) solution for 72 h and then sequentially filtered to determine size fractions of released material. Data presented is the average and standard deviation of four replicate measurements.

As previously discussed, the application of a coating to both MCA and ACA lumber reduced the release of copper during simulated dermal contact. Identical leaching protocols were conducted on uncoated MCA and ACA lumber, as well as those coated in Stain-1 to determine any changes in copper release. The copper release data is summarized in Fig. 8. Using uncoated MCA lumber, the total concentration of copper released is approximately 10 mg/L after 72-h soak in SPLP. Importantly, data from the sequential filtrations indicated that most of the copper released in these systems in in the ionic form as ~90% of the copper passed through the 10 kDa filter. While there appears to be a slight decrease in the copper concentration with each filtration step, there is variability in replicate measurements. Corresponding with the results of similar dermal contact studies, the presence of the coating reduces copper release. In the case of MCA lumber coated with Stain-1, the release is below 3 mg/L for all filtration steps indicating a 75% reduction in copper release. The same trends are observed in coated and uncoated samples of ACA lumber. The overall copper release from the uncoated ACA and stain coated ACA is slightly lower than the corresponding MCA samples, roughly 8.8 mg/L. The release can again best be characterized on ionic based on the results of sequential filtration. This result indicates that copper release from lumber is deemed a potential environmental risk, and surface coating application will mitigate release.

Fig. 8.

Fig. 8.

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Results of Leaching test performed on both uncoated MCA and ACA lumber samples, and samples coated with Stain-1. Samples were mixed in SPLP (pH 4.2) solution for 72 h and then sequentially filtered to determine size fractions of released material. Data presented is the average and standard deviation of four replicate measurements.

4. Summary and outlook

“Nano-enabled” or “nano enhanced products” have continued to expand on the consumer market. In many cases, products have begun widely advertising the benefits of included NPs in products to peak consumer interest. In fact, it has been suggested that some products marketed as “nano-enabled” may in fact not utilize nanoscale particles at all (Gruère, 2011). While the advanced properties of these materials are being harnessed for a potential benefit, it is critical to understand the potential environmental and human health consequences of widespread use of these products. Not only is there a need to understand the total release of specific materials (i.e., ZnO) from nano-enabled products, efforts must be made to understand how these NPs transform during the use phase, when most of exposure is likely to occur. In the present study, results of simulated dermal contact demonstrate that potential exposure is linked to product degradation and temporally variability (Fig. 3). Early contact events showed low total zinc releases, but increased outdoor exposure correlated with increases in zinc removal during simulated dermal contact, specifically from the stain-based product. While this study only focused on the first 6 months after application and weathering, much longer incubation periods (i.e., years) could result in even higher dermal transfer rates. More importantly, this increase in release corresponds to a change in zinc speciation. These transformations highlight how consumers may be exposed to daughter products of nano-additives, which alter their potential toxicity. Although the release of nano-size particles was not observed in either simulated dermal contact or leaching studies herein, different product formulations or sampling techniques may result in different observations. Additionally, this study also demonstrates how interactions between nano-enabled and traditional products alter the release behavior. Specially, coating MCA or ACA lumber with a traditional deck stain drastically reduced the amount of copper released in both simulated dermal contact and leaching. Overall, the results of this study highlight the importance of examining both near field and long-term exposure scenarios. For example, data collected after five months of outdoor weathering found increases in zinc release and changes in zinc speciation. Future studies on similar products may gain valuable information through simulating years of outdoor exposure.

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Acknowledgements

Any opinions expressed in this paper are those of the authors(s) and do not, necessarily, reflect the official positions and policies of the U.S. EPA or the U.S. CPSC. Any mention of products or trade names does not constitute recommendation for use by the U.S. EPA or U.S. CPSC. Use of the Advanced Photon Source (APS), a U.S Department of Energy (DOE) Office of Science User Facility operated for the DOC Office of Science by Argonne National Laboratory under contract No. DE-AC0206CH1357. MRCAT operations (Sector 10) are supported by the Department of Energy and the MRCAT member institutions. This project was supported in part by an appointment in the Research Participation Program at the Office of the Research and Development (ORD), EPA administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S Department of Energy and EPA.

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