Effects of inorganic ions on leachability of wood preserving N'N-hydroxynapthalimide (NHA)

Abstract

Southern yellow pine sapwood stakes and blocks were treated with the sodium salt of the calcium-precipitating compound N'N-hydroxynapthalimide (NHA) and leach tested for 2 weeks using the American Wood-Preservers' Association (AWPA) standard. Leacheates were measured for NHA using a microplate optical density ultraviolet reader, and leach rates were estimated for tap water, distilled water, and seawater. The percentage of NHA lost from blocks ranged from a low of 5.0 percent for seawater to 26.4 percent for distilled water. Clearly, higher inorganic ion concentrations in the leaching solutions resulted in less chemical leaching of NHA. To further reduce NHA leachability, additional groups of NHA-treated blocks were post-fixed with 2 percent CaCl2. With CaCl2 post-fixation prior to leaching, the percentage of leached NHA was reduced to 4.3 percent for seawater and 12.7 percent for distilled water. Additional stakes were pressure-treated with aqueous NHA concentrations ranging from 0.1 to 2.0 percent and then leached in distilled water. The percentage of release of NHA in 2 percent treated stakes was 9.1 percent in distilled water; in 0.1 percent treated stakes, 31.5 percent NHA was released. We conclude that NHA leach rates can be decreased by precipitation with inorganic ions (such as calcium) and increased concentration levels of NHA during pressure treatment.