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.