6 Wood Discoloration

156 7 Wood Rot
1935 from staining and from decay by fungi and insects (Prewitt et al. 2003).
The life expectancy of utility poles increased from approximately 7 years in
an untreated pole to about 35 years in a treated pole, thereby saving utility
companies millions of dollars in replacement costs. In the USA, 36 million
PCP-treated poles have been estimated to be in service in 1990. In view of the
negative impact on humans, animals, plants, and the environment, utilization
of PCP and import of PCP treated woods are however restricted in Germany.
Disposal of spent treated wood has increasingly become a major concern.
Popular methods, such as burning (incineration, combustion) and land filling,
are costly or even impractical because of increasingly strict regulatory requirements.
Recycling of the preserved wood and removal of the toxic preservatives
from the treated wood is of great importance. Research in this area (Lin and
Hse 2005) focus on direct recycling of preserved wood into composite manufacturing,
CCA removal from spent CCA-treated wood performed by lowtemperature
pyrolysis, solvent extraction, hydrogen peroxide extraction (Kim
et al. 2004), electrodialytic remediation (Christensen et al. 2005), biological
remediation, and dual treatment processes involving biological remediation
and chemical extraction. Li and Hse (2005) liquefied CCA-treated wood in
polyethylene glycol and removed more than 90% of the metals by precipitation
from aqueous solvents. Kartal and Imamura (2005) used chitin and chitosan
for remediation of CCA-treated wood. Studies on bioremediation, particularly
creosote, DDT, lindane and PCP, used several bacteria and fungi (review by
Majcherczyk and Hüttermann 1998). Fungi which excrete high amounts of oxalic
acid and are copper tolerant like Antrodia vaillantii (Collett 1992a, 1992b;
Schmidt 1995b) have been used to bio-recycle CCA and CCB treated wood
(Leithoff et al. 1995; Stephan et al. 1996; Kartal and Imamura 2003; Samuel
et al. 2003; Humar et al. 2004; Kartal et al. 2004). Clausen (1997b) enhanced
CCA removal from treated wood by Bacillus licheniformis (Weigmann) Chester.
There is a great bulk of investigations on new, alternative wood protection
procedures that deal with the chemical and/or physical modification of wood
(e.g., Militz and Krause 2003). Rapp and Müller (2005) grouped the recent wood
protection procedures that are already used or are expected to be used into
wood modification, wood hydrophobization, and supercritical fluid treatment.
Wood modification comprises various treatments that decrease the swelling
of the woody cell wall and thus its accessibility for the fungal degradation
agents.
Reactive organic compounds like acetic anhydride (“acetylation”) are introduced
in the wood (Hill et al. 1998), which react with the hydroxyl groups of
the cell wall polymers and thus increase the dimensional stability of the wood
as well as its resistance against decay and discoloring fungi. Acetylation with
acetic anhydride results in covalently bonded acetyl groups (“plugging of hydroxyl
groups”) in the wood and acetic acid as a by-product. Acetylated wood
is non-toxic and has no harmful impact on the environment, but may have an
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