Interactions of ammoniacal copper zinc arsenate (ACZA) wood preservative with Douglas-fir

Graduation date: 1993The use of the waterborne wood preservative, ammoniacal copper zinc\ud arsenate (ACZA), is likely to increase in the near future because of environmental\ud concerns associated with oilborne preservatives. Developing an understanding of the\ud permanence of this preservative in wood will become important from both\ud environmental and performance standpoints. Conventional thinking suggests that\ud ACZA components are stabilized in the wood by the precipitation of metal/arsenate\ud complexes as the ammonia evaporates, but little work has been done to confirm this\ud theory. Experiments were conducted to clarify the mechanisms which stabilize, or\ud "fix" copper, arsenic and zinc within ACZA treated wood.\ud Studies were undertaken to determine the effect of ACZA composition on\ud fixation, the role of wood components in fixation, the role of precipitation in\ud fixation, the order of precipitation of ACZA components, the sites of copper and\ud zinc adsorption within the treated wood, the bonding environment of adsorbed copper, and the microdistribution of copper, zinc and arsenic within treated wood.\ud The results of these experiments generally support the finding that arsenic fixation\ud occurs by precipitation of metal/arsenate, complexes as ammonia evaporates from the\ud treated wood. Arsenic leaching was minimized when the metal oxide:arsenic\ud pentoxide ratio in the treating solution exceeded 2.0, and when a portion of the\ud copper was replaced with zinc. Subsequent tests revealed that zinc precipitated the\ud majority of arsenic when the ammonia was allowed to evaporate from the treating\ud solution. In contrast, the metal cations are not dependent on arsenic for fixation\ud because they have low water solubility and undergo adsorption reactions with the\ud wood. Copper, in particular was readily adsorbed by the lignin and heartwood\ud components of wood, suggesting that phenolic groups were primary reaction sites.\ud Further tests with lignin model compounds also suggested that copper reacted\ud preferentially with the hydroxyl groups of phenolic compounds. Adsorption of the\ud metals to the wood can interfere with arsenic fixation by reducing the availability of\ud metal cations in the preservative solution. The severity of arsenic leaching due to\ud metal adsorption could be affected by many treatment variables, and more work is\ud needed to determine how these variables could be manipulated to minimize arsenic\ud fixation