The mechanism of leaching of copper-chrome-arsenic preservatives from treated timber in saline waters
J Irvine, S-E Dahlgren
A mechanism for the leaching of copper-chrome-arsenic preservatives is proposed on the basis of theoretical calculations and experimental studies using timber subjected to a range of water types of different ionic compositions. Complex formation by copper (II) and chromium (III) with chloride and hydroxide ions is shown to affect the leaching rate. Of even greater importance is the salt effect on the activity coefficients. The various complexes formed at different pH values are discussed. Two minima are observed in the plot of copper leaching versus water salinity; these are attributed to the presence of the copper fixation compounds Cu2(OH)AsO4 and Cu3(AsO4)2. At low salinities NaCl is shown to have a coagulating effect on the copper fixation compounds, reducing their solubility, whereas at higher salinities complex formation dominates. With chromium the coagulation effect is not observed and leaching increases with increasing salinity. Loss of arsenic is shown to lag behind that of copper and chromium and is related to the chromium excess in the preservative formula: the greater the excess, the longer the delay in arsenic loss. The suggested mechanism adequately explains the experimental results.