FROM THE ACS MEETING

A. MAUREEN ROUHI, C&EN WASHINGTON

As concerns about the safety of chemically treated wood increase, researchers are seeking alternative ways to preserve wood.

About 80% of wood used in the U.S. is treated with chromated copper arsenate (CCA). This mixture of the oxides of chromium, copper, and arsenic imparts a familiar green color to wood, which is obvious in newly installed decks or telephone poles. The American Council on Science & Health, a consumer education consortium, says CCA-treated wood poses no known health hazard. Studies by the Environmental Protection Agency have concluded that the CCA-treated wood does not pose unreasonable risks.

SOCIAL WORK The behavior of termites, such as this communal effort to repair damage to a nest, could point to new control methods for protecting wood.

PHOTO BY SCOTT BAUER

But recent developments have placed CCA-treated wood under siege. Last month, for example, a lawsuit filed in U.S. District Court in Miami against producers and distributors of CCA-treated wood claims the product is unsafe and seeks, among other demands, medical monitoring of people who come in contact with it.

The complaint cites a study by the University of Florida Center for Solid & Hazardous Waste Management, which found that arsenic levels in soils below decks made with CCA-treated wood routinely exceed EPA limits. The same study has spurred Florida regulators to review the state's policy of allowing CCA-treated wood to be dumped in unlined landfills. That policy likely will be made more restrictive, William Hinkley, bureau chief for solid and hazardous waste management at Florida's Department of Environmental Protection, told C&EN.

  THE GROWING PERCEPTION that CCA-treated wood is hazardous to human health and the environment has become an impetus for change. Researchers previewed the direction that change is taking at a symposium sponsored by the Cellulose, Paper & Textile Division and organized by Barry Goodell, a professor of wood science and technology at the University of Maine, and forest products professors Darrel D. Nicholas and Tor P. Schultz of Mississippi State University. The consensus is the field should move away from general poisons to targeted treatments. That evolution requires a better understanding of the mechanisms of decay and the biology of wood-attacking organisms.

Wood decay is initiated primarily by fungi. Brown-rot fungi preferentially degrade cellulosic components of wood over lignin, whereas white-rot fungi attack both lignin and cellulose. Goodell and Steven D. Aust, a biotechnologist at Utah State University, described efforts to understand how these fungi work.

About 10% of the annual timber cut in the U.S. is used to replace damage due to brown-rot fungi, Goodell told C&EN. Damage caused by these fungi can cause injury and even death because they rapidly weaken structures even before decay is visible, he said.

Goodell's work has helped clarify how brown-rot fungi degrade wood. Because the extracellular enzymes these fungi secrete are too big to penetrate deep within the wood, low-molecular-weight compounds must be involved, he told C&EN.

  THE INITIAL STAGES of decay involve reactions of ferrous ions and hydrogen peroxide. Among the products are oxygen radicals, which attack lignocellulose. The fungi produce the hydrogen peroxide, but where the ferrous ions come from has not been clear, Goodell said, because almost all iron in aerobic environments where wood and fungi exist occurs in the ferric form.

"Our work has shown that oxalate and phenolic compounds produced by brown-rot fungi function as ferric iron chelators and reduce the iron in a pseudocatalytic fashion," Goodell said. Unlike enzymes, the low-molecular-weight reactants are small enough to penetrate intact wood. Because iron binds to cellulose more than to lignin, brown-rot fungi preferentially depolymerize cellulose.

White-rot fungi have developed nonspecific mechanisms for degrading lignin, according to Aust. Most of the mechanisms depend on heme-containing peroxidases secreted by the fungi. In the presence of hydrogen peroxide, which the fungi also produce in various ways, peroxidases promote the one-electron oxidation of substrates to free radicals. The nonspecificity and the availability of multiple mechanisms are apt for lignins, which are believed to be structurally highly irregular.

The involvement of free radicals in decay has spurred interest in preservatives with free-radical-scavenging antioxidants. Also supporting this approach is the nature of the protective effect of components of various woods.

Some woods are durable because of compounds called extractives. These compounds occur naturally in the heartwood--the dark center of the trunk--of old trees and have been thought to deter fungi. But Schultz and Nicholas have found that they are less effective fungicides than commercial biocides. However, they are excellent free-radical scavengers.

Schultz and Nicholas have suggested that extractives must have both antifungal and antioxidant activities to impart durability. To test the hypothesis, they have studied the effects of butylated hydroxytoluene (BHT), an antioxidant with no antifungal property, on the protective effect of potential wood preservatives with no antioxidant properties.

BHT alone offers little protection against fungi, they have found. But BHT combined with a biocide affords more protection than the biocide alone. Such synergism could be the basis of more environmentally benign preservatives. "By adding antioxidants, less of the expensive organic biocide is necessary," Schultz said. "The cost and possible environmental concerns are reduced."

Early detection of decay is critical to preservation, according to Timothy R. Filley, an assistant professor of earth and atmospheric sciences at Purdue University. By the time wood looks chewed, discolored, or frayed, the damage may be too far gone to repair. In the lab, for example, despite many weeks of exposure to fungi, wood appears normal. "Yet when I look at the sample chemically, it is highly degraded," Filley said. "If you can determine decay early, you can apply the needed treatments or, with historical artifacts, put them in the proper environment before it is too late."

  RECOGNIZING EARLY STAGES of decay also is key in testing new preservatives, Nicholas said. At present, potential preservatives are evaluated by mass loss and visual inspection of wood. But those are "poor predictors of the early stages of decay because they can't detect chemical reactions that fungi use to open up the wood structure," he said. His research has shown that mechanical testing of wood provides a good indication of the extent of decay in wood attacked by fungi. Using mechanical tests can reduce the time to test candidate preservatives from 14 weeks to six weeks.

Methods to detect early decay are not yet available in a form useful to homeowners. However, homeowners already are gaining from what is being learned about termites. These social insects are serious pests all over the world. In New Orleans alone they cause about $350 million worth of damage annually, chewing through the historic buildings in the French Quarter.

According to J. Kenneth Grace, an entomology professor and director of the University of Hawaii Termite Project, one of the most innovative products for termite control is a bait containing a compound that interferes with molting. The bait does not kill the termite directly. Instead it perturbs a key process that occurs at an unpredictable time after the termite has ingested the bait.

"The bait can be distributed through the colony and eaten because it does not make the termites immediately ill," Grace told C&EN. "Its success indicates that other physiological systems merit scrutiny for development of future bait materials."

	HOLEY WOOD Shipworms drill holes that are up to 4 mm wide, leaving wood that easily crumbles.
	PHOTO BY BARRY GOODELL

How termites communicate is another facet of termite biology that may lead to novel control methods. "Termites are social insects with a rigid caste system that is maintained by pheromones," Grace said. But except for so-called juvenile hormone, which termites use to regulate the proportion of reproductive and soldier classes in a colony, "we still know next to nothing of the identity of these messengers." If the chemicals that termites use to maintain social homeostasis were better known and understood, controls based on disrupting the social organization might be developed.

Observing termites is difficult, however, because termites lead secretive lives. Grace and his students use computerized spatial analysis to study the patterns that termite tunnels form beneath the soil. But Claudia Husseneder, an entomologist and a collaborator of Grace's, uses molecular biology as the gateway to the termites' subterranean world.

To contain termites, researchers need to know where they come from, where they go, and how they spread. For cryptic organisms such as termites, genetics can be used to ferret out that information, Hussender told C&EN. Similarly, genomic profiles can reveal the organization of colonies, which can be complicated by interconnected nests and the occurrence of multiple kings and queens. The complexity can affect the effectiveness of baits. "We are working on a comprehensive database of population and colony genetic structures of termites," Hussender said. "With that we will lay the base for improving control measures."

The kind of damage wreaked by termites on land is brought about at sea by bivalves called shipworms. According to Daniel L. Distel, an associate professor of biochemistry, microbiology, and molecular biology at the University of Maine, the incidence of damage due to shipworms has been increasing, reaching northern areas where it had never been seen before.

Shipworms burrow through wood, and under good conditions they can go through 1.5 cm of wood in a month. When they're done, what was once solid wood looks like a sponge. Piers and wooden trestles have collapsed because of shipworm damage. In New York City, devastation of timber-pile-supported structures in waterfront properties is currently a big problem.

Distel's research aims to help understand how shipworms metabolize wood. It appears that the bivalves have symbiotic associations with multiple species of bacteria, which they harbor in their gills. The symbiotic relationship is complex, with different partners likely contributing different enzymatic machineries for the digestion of wood. "We really know very little about the biology of this organism," Distel said. "A better understanding of this symbiotic interaction may reveal vulnerabilities that can be used to control shipworm damage."