6. Conclusions
Leaching from CBA treated wood was investigated with the aim to identify chemical mechanisms of fixation and mobilisation of biocides based on a pH dependency leaching test (ANC test) that has been already widely applied for waste and concrete construction materials. Despite the fact that it is an aggressive test (test conditions are unlikely to occur under service conditions), ANC results, in combination with a suitable set of analytical methods, provide manifold information about the studied system and chemical reactions that occur in this system, information which is indispensable for the interpretation of leaching processes. However, in the case of CBA treated wood it proved to be difficult to describe the entire composition of the eluates without INVESTING great effort for molecule identification and set up of methods for quantification.
The main conclusions from this study can be summarised as follows:
-
Inorganic endogenous elements like Ca and K, and ions like sulphate and phosphate are released from untreated and treated wood in contact with water at different pH. They are linked to the solid phase (e.g. Ca in treated wood) or solubilised in wood pore water (e.g. K) and are the main electrolytes that regulate the ionic strength.
-
Organic compounds are released from natural wood. The quantity of released TOC as well as substances with carboxyl and phenolic groups increases with pH. This observation is mainly explained by hydrolysis reactions on wood polymers.
-
The chemical composition of wood changes during storage, at least for sapwood from P. sylvestris. This affects not only binding and release of substances in the ANC test, but also the amount and composition of extractives, including the availability of binding sites in the solid as well as the liquid phase.
-
Wood is modified by the treatment with preservatives. In the case of products containing Cu and amines, this does not only involve binding reactions of Cu–amine complexes, but also modifications of wood components due to reactions with amines. A number of compounds containing nitrogen are released from wood treated with a CBA solution, probably due to an excess of amines and reactions of amines with wood components. It can be expected that these reactions depend on the identity of the amines, the ratio between Cu and amines and the pH conditions in the preservative solution and in wood.
-
Cu release is relatively high with respect to its initial content. Cu is bound on carboxyl and phenolic sites and forms soluble complexes with extractives. The balance between binding to solid and extractable components, as well as the acid/base properties of the binding sites, result in the lower release at neutral pH.
-
B is released at a high level. Even if some fixation phenomena are possible, their effectiveness is weak (demonstrated by the non-dependency on pH) and cannot effectively hinder the leaching.
-
Tebuconazole is released in lesser extent if compared to its initial content in wood samples. Its fixation on solid wood structure seems to be influenced by pH, suggesting hydrogen bond interactions with OH groups on wood.
6. Conclusions
Leaching from CBA treated wood was investigated with the aim to identify chemical mechanisms of fixation and mobilisation of biocides based on a pH dependency leaching test (ANC test) that has been already widely applied for waste and concrete construction materials. Despite the fact that it is an aggressive test (test conditions are unlikely to occur under service conditions), ANC results, in combination with a suitable set of analytical methods, provide manifold information about the studied system and chemical reactions that occur in this system, information which is indispensable for the interpretation of leaching processes. However, in the case of CBA treated wood it proved to be difficult to describe the entire composition of the eluates without INVESTING great effort for molecule identification and set up of methods for quantification.
The main conclusions from this study can be summarised as follows:
-
Inorganic endogenous elements like Ca and K, and ions like sulphate and phosphate are released from untreated and treated wood in contact with water at different pH. They are linked to the solid phase (e.g. Ca in treated wood) or solubilised in wood pore water (e.g. K) and are the main electrolytes that regulate the ionic strength.
-
Organic compounds are released from natural wood. The quantity of released TOC as well as substances with carboxyl and phenolic groups increases with pH. This observation is mainly explained by hydrolysis reactions on wood polymers.
-
The chemical composition of wood changes during storage, at least for sapwood from P. sylvestris. This affects not only binding and release of substances in the ANC test, but also the amount and composition of extractives, including the availability of binding sites in the solid as well as the liquid phase.
-
Wood is modified by the treatment with preservatives. In the case of products containing Cu and amines, this does not only involve binding reactions of Cu–amine complexes, but also modifications of wood components due to reactions with amines. A number of compounds containing nitrogen are released from wood treated with a CBA solution, probably due to an excess of amines and reactions of amines with wood components. It can be expected that these reactions depend on the identity of the amines, the ratio between Cu and amines and the pH conditions in the preservative solution and in wood.
-
Cu release is relatively high with respect to its initial content. Cu is bound on carboxyl and phenolic sites and forms soluble complexes with extractives. The balance between binding to solid and extractable components, as well as the acid/base properties of the binding sites, result in the lower release at neutral pH.
-
B is released at a high level. Even if some fixation phenomena are possible, their effectiveness is weak (demonstrated by the non-dependency on pH) and cannot effectively hinder the leaching.
-
Tebuconazole is released in lesser extent if compared to its initial content in wood samples. Its fixation on solid wood structure seems to be influenced by pH, suggesting hydrogen bond interactions with OH groups on wood.
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