Effect of ammonia addition to alkaline copper quaternary wood preservative solution on the distribution of copper complexes and leaching

Abstract

Abstract The effects of ammonia (NH3) addition to monoethanolamine (Mea)-based alkaline copper quaternary (ACQ) on the predicted distribution of cupric (Cu2+) species in solution and copper leaching from treated wood were investigated. Addition of ammonia in higher proportions reduced the amount of neutral copper-Mea complex at high pH and increased the amounts of divalent copper-NH3 complexes. At lower pH, the amounts of high-ligand copper-Mea complexes were reduced. These effects should promote better fixation and diminish copper leaching. However, ammonia addition also reduced the monovalent copper-Mea complex and the amount of copper fixed by precipitation at lower pH. This effect could, potentially, reduce fixation and leach resistance. Ammonia addition to the ACQ formulation from a Cu:Mea:NH3 ratio of 1:4:0 (normal Mea formulation, pH=9) to 1:4:2 (pH 10–10.2) and 1:4:4 (pH 10.4–10.5) did not significantly affect the amount of copper leached. However, a further increase in ammonia to 1:4:6 (pH 10.6–10.7) resulted in a significant reduction in copper leaching. At pH 9, an increase in the proportional amount of ammonia increased the amount of copper leached by reducing the amount of monovalent copper-Mea complex and increasing the divalent copper-NH3 complexes. Ammonia addition with a lower amount of Mea, 1:2.5:4 formulation (pH 10.5–10.7), significantly reduced copper leaching as a result of higher amounts of divalent copper-NH3 complexes adsorbed at higher pH (higher cation exchange capacity) without compromising the amount of copper precipitated at lower pH.