Transformation and Release of Micronized Cu used as a Wood Preservative in Treated-Wood in Wetland Soil

Micronized Cu (µ-Cu) is used as a wood preservative, replacing toxic chromated copper arsenate (CCA). Micronized Cu is malachite [Cu2CO3(OH)2] that has been milled to micron/submicron particles, many with diameters less than 100 nm, and then mixed with quat or azole biocides and used to treat wood. In addition to concerns about the fate of the Cu from µ-Cu, there is interest in the fate of the nano-Cu (n-Cu) constituents. We examined movement of Cu from µ-Cu treated-wood after placing treated-wood stakes into model wetland ecosystems. Release of Cu into surface and subsurface water was monitored. Surface water Cu reached maximum levels 3 days after stake installation and remained elevated if the systems were inundated. Subsurface water Cu levels were 10% of surface water levels at day 3 and increased gradually thereafter. Sequential filtering indicated that a large portion of the Cu in solution was associating with soluble organics, but there was no evidence for n-Cu in solution. After 4 months, Cu in thin-sections of treated-wood and adjacent soil was characterized with micro X-ray absorption fine structure spectroscopy (µ-XAFS). Localization and speciation of Cu in the wood and adjacent soil using µ-XAFS clearly indicated that Cu concentrations decreased over time in the treated-wood and increased in the adjacent soil. However, n-Cu from the treated-wood was not found in the adjacent soil or plant roots. A reduced form of Cu (Cu2S) was identified in deep soil proximal to the treated-wood, indicating strong reducing conditions. These results indicate that Cu in the µ-Cu treated-wood dissolves and migrates into adjacent soil and waters primarily in ionic form (e.g., Cu2+) and not as nano-sized Cu particles.

Impact/Purpose

Engineered nanoparticles (ENPs) have been recognized as valuable components of novel technologies and are currently being used in a variety of consumer products due to their unique physical, chemical, and electrical properties. The properties that make these particles functionally unique also influence their behavior and interactions with biotic and abiotic components of the environment. One of the challenges scientists face when examining the potential environmental health and safety aspects of ENPs is to understand the underlying chemistry of these interactions in order to better predict and model potential adverse environmental effects. EPA scientists are investigating micronized Cu (µ-Cu), which is used as a wood preservative, replacing toxic Chromated Copper Arsenates. One concern is that nano-sized Cu (n-Cu) may move out of the wood and into the soil, where it might adversely affect beneficial soil fauna and flora. EPA scientists monitored the amount and form of Cu released into surface water and deep soil leachate and found that surface water Cu released from the treated wood reached maximum levels 3 days after stake installation and remained elevated. Sequential filtering of these solutions indicated that some of the Cu in solution was associating with soluble organics, but there was no evidence for free n-Cu in solution. After 5 months of exposure, XAS analysis indicated that Cu concentrations in the treated wood decreased and increased in the adjacent soil. However, n-Cu from the treated wood was not found in the adjacent soil. These results indicate that Cu in the µ-Cu treated wood dissolves and leaches into adjacent wetland soil and waters primarily in ionic form (e.g., Cu2+) and not as nano-sized Cu particles. This research provides evidence that µ-Cu treated wood does not appear to be a source of n-Cu to the environment, and therefore may not present a unique exposure risk specific to nano forms of copper.

Citation

Johnson, Mark G, Todd P Luxton, P. Rygiewicz, Jay R Reichman, Mike A Bollman, G. King, M. Storm, M. Nash, AND C. Andersen. Transformation and Release of Micronized Cu used as a Wood Preservative in Treated-Wood in Wetland Soil. Elsevier Science Ltd, New York, NY, 287:117189, (2021). [DOI: 10.1016/j.envpol.2021.117189]

DOI: Transformation and Release of Micronized Cu used as a Wood Preservative in Treated-Wood in Wetland Soil