Relationship between copper species in solution and leaching from alkaline copper quat (ACQ) treated wood

Abstract

Abstract The effects of pH and Cu:Mea ratio in alkaline copper quat (ACQ) solution formulation on the distribution of copper-monoethanolamine (Cu-Mea) complex species and Cu precipitation, and its influence on copper leaching from treated southern pine samples were investigated. Distribution of Cu in ACQ solution was studied by means of an equilibrium speciation model for aqueous systems (MINTEQA2). Conditions that favored a higher proportion of monovalent cationic complex, [Cu(Mea)2-H]1+, and precipitated copper as CuCO3(s), resulted in higher leach resistance in treated wood compared to conditions that favored the neutral copper complexes, [Cu(Mea)2-2H]0, and higher ligand copper complexes, [Cu(Mea)3]2+, [Cu(Mea)4]2+. Monovalent cationic Cu complex is maximized at a pH close to 9 with a Cu:Mea molar ratio of 1:4. Amounts of Cu leached at pH 9 were low compared to those at pH 12, where most of the Cu is present as neutral complex. Reduction of pH from 9 to 8 did not reduce Cu leaching, because of the lower cation exchange capacity of wood at the lower pH. Changing the Cu:Mea molar ratio to 1:3 at pH 9 significantly reduced the higher ligand complexes in the formulation and resulted in the majority of the copper being distributed as monovalent cationic complexes and some CuCO3(s) precipitation. These changes in 1:3 formulation significantly reduced copper leaching compared to Cu:Mea ratios of 1:4 and 1:10.