Copper remains one of the key biocides for protecting timber in soil contact. Historically, copper-based preservatives relied on solubilized copper. Over the past decade, micronized copper systems have largely replaced soluble copper systems in this application. While micronized copper chemistries have been shown to perform well over time with reduced copper leaching compared to solubilized copper, data are lacking on the effects of repeated leaching cycles on resistance of the timber to fungal attack. The potential effects of repeated leaching cycles of micronized copper azole–treated southern pine (Pinus spp.) blocks on both copper losses and resistance to fungal attack were explored over 20 leaching cycles using Gloeophyllum trabeum and Rhodonia placenta as test fungi. Copper losses during leaching were elevated for the first two cycles and steadily declined with additional cycling. There were no noticeable differences in fungal-associated weight losses in blocks exposed to G. trabeum. There was only a slight increase in average block weight losses with R. placenta, although blocks exposed to this fungus experienced higher weight losses with increased leaching cycles. The results suggest that repeated leaching exposures may produce some localized increases in susceptibility to fungal attack, but the overall effects were minor.
Copper remains among the most widely used components for wood preservatives employed in ground contact. Copper reacts slightly with the wood and has broad spectrum activity against fungi and insects (Dahlgren and Hartford 1972; Pizzi 1981, 1982; Cooper 1991; Jin and Archer 1991; Ruddick 1996, 2003; Freeman and McIntyre 2008). Chromated copper arsenate (CCA) has been the most effective copper-based wood preservative, but restricting its use in residential applications led to the substitution of a host of other copper-based systems. The majority of these systems used either ammonia or ethanolamine to solubilize the copper for delivery into wood. These systems were more costly than CCA and heightened the risk of metal fastener corrosion.
The development of micronized copper systems wherein copper compounds such as copper carbonate were ground to small particles that could be suspended in a treatment solution for wood application markedly changed treating practices in large parts of the world (Freeman and McIntyre 2008). These systems were especially suited for use on more permeable species such as the southern pines (Pinus spp.), but major questions arose concerning the activity of micronized copper particles in comparison with traditional solubilized systems (Jin et al. 2008, McIntyre and Freeman 2008, Preston et al. 2008, Xue et al. 2010, Zahora 2011, Zhang and Horton 2016). Extensive studies examined distribution and efficacy of both micronized and solubilized copper systems (Cookson et al. 2008; Cooper and Ung 2008, 2009; Larkin et al. 2008; McIntyre and Freeman 2008; Stirling et al. 2008; Evans et al. 2012). Both are now standardized by the American Wood Protection Association (AWPA) for treatment of wood in both Use Categories 3 and 4 (above ground and soil contact; AWPA 2019a). However, lingering questions remain about the bioavailability of copper in micronized versus solubilized forms.
Micronized copper carbonate has very low water solubility (<0.1 g/100 mL water). Questions have arisen concerning how a compound with such low water solubility could release enough copper to inhibit growth by wood degrading fungi. One suggestion was that water in the wood cell wall solubilized low levels of copper from the particles and that this copper then reacted with wood cell walls (Stirling and Drummond 2009). Continued copper solubilization with wetting could result in a gradual increase in cell wall bound copper, provided there are sufficient reactive sites, thereby providing long-term protection against fungal attack. One advantage of micronized copper systems is that they will tend to release less copper into the surrounding environment when wood is wetted, thereby producing a smaller environmental footprint (Xue et al. 2010, Zhang and Horton 2016). Field trials have demonstrated long-term efficacy of micronized copper systems (Cookson et al. 2008, Larkin et al. 2008, McIntyre and Freeman 2008), but there are relatively few data on how exposure to multiple leaching cycles affects both copper losses and resistance to fungal attack. The objective of this research was to examine the effects of multiple leaching cycles on the decay resistance of southern pine blocks treated to the ground contact retention with micronized copper azole (MCA).
Materials and Methods
Southern pine sapwood lumber that was free of visible defects was cut into 436 blocks that were 19 by 19 by 19 mm, oven-dried at 105°C, and weighed to the nearest 0.01 g. Fifty-four blocks were set aside and used as untreated controls, while the remainder were treated with a micronized copper solution to a target retention of 2.4 kg/m3 per AWPA Standard U1 for Use Category (UC) 4 (AWPA 2019a). Briefly, the blocks were immersed in the MCA solution and subjected to a 20-minute vacuum (86 kPa) followed by a 30-minute pressure period (689 kPa). The pressure was released and the blocks were blotted dry before being weighed. Differences between initial and final weight were used to determine net uptake. Four blocks were then oven-dried and ground to pass a 20-mesh screen, and the resulting ground wood was analyzed for copper content by X-ray fluorescence spectroscopy according to AWPA Standard A9 (AWPA 2019b).
The remaining blocks were stored for 48 hours under nondrying conditions to allow any wood reactions to proceed before being air-dried for several days and finally oven-dried at 50°C for 48 hours. The blocks were then weighed.
Groups of 54 blocks each were allocated to one of seven leaching treatments. One group of 54 was set aside and not subjected to leaching, while the remaining six sets were subjected to the leaching procedures described in AWPA Standard E10 (AWPA 2019c). Briefly, blocks were submerged in an excess of distilled water, and a vacuum was drawn over the solution for 20 minutes. The vacuum was released and the blocks were left in the water. Water was changed at 6, 24, and 48 hours after immersion and then at 48-hour intervals thereafter for an additional 12 days. The leachate water was retained for analysis of copper by ion-coupled plasma spectroscopy. At the end of the leaching period, blocks were oven-dried as described above and weighed. Blocks were subjected to up to 20 leaching cycles. The effects of leaching on preservative performance were assessed after 1, 3, 7, 10, 15, or 20 leaching cycles by removing 54 blocks at each time for durability assessments.
Blocks removed after the specified leaching cycle for each group were oven-dried at 50°C for 48 hours and weighed. The blocks were then steam sterilized for 20 minutes at 100°C before being exposed in an AWPA Standard E10 soil block test (AWPA 2019c). French square jars were half-filled with a moist soil mixture and a western hemlock feeder strip was placed on the soil surface. Jars were capped and autoclaved for 45 minutes at 121°C. After cooling, a small agar disc cut from the edge of an actively growing culture of one of the test fungi was placed on the feeder strip surface. The jars were loosely capped and incubated for 10 to 14 days at 28°C to allow the fungus to colonize the feeder strip. The test fungi were Gloeophyllum trabeum (Pers ex Fr.) Murr. (Isolate No. Mad 617) and Rhodonia placenta (Fr) Niemela, Larss, and Schagel (Isolate No. Mad 698). Both fungal species cause brown rot. The sterile pine blocks were placed (cross section down) on the feeder strip surface, and the jars were loosely capped and incubated at 28°C for 12 weeks. Eighteen blocks were exposed per fungus per leaching cycle evaluated. At the end of the exposure period, blocks were removed from the decay chambers and scraped clean of adhering soil and mycelium, and a wet weight was recorded. Blocks were then oven-dried and weighed to determine mass loss during the fungal exposure period.
Results and Discussion
The test blocks were treated to an average target retention of 2.4 kg/m3 (as Cu), which corresponds to the UC 4A level listed in AWPA Standard U1 (AWPA 2019a). The actual retentions achieved were 4.0 kg/m3, which falls between the UC 4A and UC 4B levels. This level is somewhat higher than would normally be used for soil block testing, but the intent was to evaluate copper mobility in relation to changes in performance.
Copper levels in leachate were elevated after the first leaching cycle but declined from 25.21 to 5.35 μg/mL with an additional leaching cycle. Copper levels in leachate ranged from 1.56 to 3.62 μg/mL with subsequent leaching, but there was no distinct pattern during the next 18 leaching cycles, suggesting leaching losses reached a steady state (Fig. 1). Elevated metal losses are typical for the first rewetting of copper treated wood and are believed to reflect losses due to surface deposit mobility following treatment (Brooks 2011). Metal levels in leachate typically decline with continued water exposure as they did in this case (Lebow 1996, Lebow and Tipple 2001, Ye and Morrell 2015). Conceptually, micronized copper is intended to have low water solubility, but some copper will always be expected to initially leach from treated wood. This is critical for performance, since some copper needs to be soluble in the liquid within the wood cell lumen to affect the germination and growth of any invading decay fungi. Repeated leaching results were consistent with the premise that low levels of copper will always be released under wet conditions. Total copper losses in leachate over the 20 leaching cycles would represent an 8 percent loss in copper in the wood, assuming the 4.0 kg/m3 of MCA retention.
Mass losses for nontreated control blocks exposed to the test fungi averaged 40.67 percent for R. placenta and 37.92 percent for G. trabeum (Table 1). While mass losses for blocks exposed to G. trabeum were slightly below the 40 percent typically required for this test, the results indicate suitable conditions for aggressive fungal wood attack. Mass losses for blocks that were leached and exposed in chambers with no decay fungi were all near zero, indicating test conditions alone did not induce mass loss. Weight gains were sometimes observed, suggesting limited elemental uptake from the soil into test blocks.
Blocks subjected to up to 20 leaching cycles and subsequently exposed to G. trabeum experienced no measurable changes in mass loss compared to nonleached blocks. This brown rot fungus is not known for its copper tolerance but is commonly associated with the decay of wood in service (Duncan and Lombard 1965). As with control blocks not exposed to test fungi, weight gains were sometimes noted.
Rhodonia placenta is known to be copper tolerant, and this brown rot fungus is usually included in the AWPA soil block test because of this ability (AWPA 2019c). Blocks that were leached and exposed to R. placenta generally experienced minimal average mass loss over all 20 leaching cycles; however, some performance variations began to emerge as blocks were subjected to seven or more leaching cycles (Table 1). Mass losses of blocks subjected to seven leaching cycles still averaged only 2.88 percent, but three blocks experienced mass losses of 7.5, 13.0, and 37.1 percent, respectively, suggesting increased performance variation with leaching cycle. This effect was absent in blocks subjected to an additional three leaching cycles but appeared in blocks subjected to 15 or 20 cycles. Mass losses after 15 and 20 leaching cycles were still below 5 percent, but had begun to increase, and there was more variation in weight losses. Five of 18 blocks subjected to 15 leaching cycles had weight losses of at least 6 percent and ranged up to 18 percent, but most blocks still had mass losses near zero. Similar effects were noted after 20 leaching cycles; six blocks had weight losses over 6 percent, and one block experienced 23 percent weight loss.
The weight loss variations suggest that repeated leaching had begun to alter the protective effects of micronized copper in some blocks. Overall, however, results still suggest that the micronized copper system remained effective. The level tested was far above the threshold, making it impossible determine if leaching enhanced copper distribution, thereby increasing effectiveness. The results do, however, indicate that the micronized copper was resistant to leaching and retained its effectiveness against the two test fungi over prolonged leaching exposures.
MCA-treated southern pine blocks experienced relatively limited copper losses following the initial leaching cycle; repeated leaching cycles did not negatively affect overall susceptibility to decay by the two test fungi, although weight losses became more variable, suggesting some localized copper losses.
The authors are, respectively, Associate Professor, Northeast Forestry Univ., Harbin, China (firstname.lastname@example.org); Senior Faculty Research Associate and Senior Faculty Research Associate, Dept. of Wood Sci. and Engineering, Oregon State Univ., Corvallis (email@example.com, firstname.lastname@example.org); and Director, National Centre for Timber Durability and Design Life, Univ. of the Sunshine Coast, Brisbane, Australia (email@example.com [corresponding author]). This paper was received for publication in December 2019. Article no. 19-00065.