Biocide leaching from CBA treated wood — A mechanistic interpretation

Abstract

Treated wood is frequently used for construction. However, there is a need to ensure that biocides used for the treatment are not a threat for people or environment. The paper focused on Pinus sylvestris treated with copper–boron–azole (CBA), containing tebuconazole as organic biocide and monoethanolamine (Mea).

This study investigates chemical mechanisms of fixation and mobilisation involved in the leaching process of the used inorganic and organic biocides in CBA.

A pH dependent leaching test was performed, followed by a set of complementary analysis methods in order to identify and quantify the species released from wood. The main findings of this study are:

• Organic compounds are released from untreated and treated wood; the quantity of released total organic carbon, carboxylic and phenolic functions increasing with the pH.

• Nitrogen containing compounds, i.e. mainly Mea and its reaction products with extractives, are released in important quantities from CBA treated wood, especially at low pH.

• The release of copper is the result of competitive reactions: fixation via complexation reactions and complexation with extractives in the liquid phase. The specific pH dependency of Cu leaching is explained by the competition of ligands for protonation and complexation.

• Tebuconazole is released to a lesser extent relative to its initial content. Its fixation on solid wood structure seems to be influenced by pH, suggesting interactions with OH groups on wood. Boron release appears to be pH independent and very high. This confirms its weak fixation on wood and also no or weak interaction with the extractives.

Introduction

Pollutant release from construction products in contact with water is currently a research subject receiving attention from the European regulation and standardisation authorities (EC, 2005). Being an economical, durable and aesthetically pleasing building material, treated wood is a frequent choice for construction projects. However, we need to ensure that biocides used for the treatment do not pose a threat to people or environment. Previous studies generally report high leaching rates from treated wood, depending on the type of preservative, exposure conditions and wood specimen (Lebow, 1996, Ahn et al., 2010, van Zomeren et al., 2005, Schiopu et al., 2012).

Understanding the leaching process is crucial for the management of wood products over their whole life cycle, especially during their use stage and end of life (wastes), but also for developing new treatment methods and products. So far, leaching from wood has been investigated for inorganic and a few selected organic preservatives, and factors that influence their release have already been summarised by several authors (Schoknecht et al., 2004, Schoknecht et al., 2005). However, most of these studies only report on results of experimental leaching tests, but do not investigate the chemical mechanisms involved.

As discussed in a recent paper (Tiruta–Barna and Schiopu, 2011), a leaching process implies complex chemical and physical phenomena: biocide interactions with the solid wood structure (fixation), distribution between the solid and liquid phase (mobilisation), and transport processes (mainly diffusion inside the wood porous system). The fixation mechanism of copper on the wood structure has been intensively studied (Jiang, 2000, Lee, 2011, Mettlemary, 2011). This is not the case for other compounds, especially organic biocides, for which the fixation mechanisms have not been elucidated so far. Irrespective of the biocides, very few reaction constants exist in literature allowing a quantitative estimation of the fixation level. Also the mobilisation mechanisms are hardly investigated. What is the role of the wood/water chemistry in terms of pH and extractives on the biocide leaching? No studies have been reported on the composition of the organic matrix in eluates, and no leaching mechanism that includes the role of these extractives has been proposed so far.

The intention of this study was to investigate the leaching behaviour of treated wood and to propose leaching mechanisms for inorganic and organic compounds. The product studied here is sapwood of Pinus sylvestris treated with copper–boron–azole (further called CBA), containing tebuconazole as organic biocide, and monoethanolamine as major constituent. Copper is an essential micronutrient for most living cells, but in larger doses it acts as an algicide, bactericide, fungicide, insecticide, and moldicide (Freeman and McIntyre, 2008); boron is used as insecticide and fungicide (Lebow, 1996); tebuconazole is a fungicide that is added to copper containing wood preservatives to act against copper-tolerant fungi (Grundlinger and Exner, 1990). These properties are hazardous when the biocides are controlees released in surrounding environment.

Experiments based on the leaching procedure of the European standard XP CEN/TS 14429 (‘pH dependence leaching test’ or ‘Acid Neutralisation Capacity test’ — further called ‘ANC’) (CEN, 2006) were performed to determine the acid/base properties of samples from untreated and treated wood and investigate the pH influence on the release of elements, ions and organic compounds. The ANC test is in line with the Construction Products Regulation (EC, 2011). In order to comply with the Basic Requirement BR 3 ‘Hygiene, health and the environment’ of the CPR, the European Commission mandated CEN/TC 351 to develop methods for the assessment of the leaching behaviour of various construction products (based on wood, concrete, etc.) with the same methods, i.e. horizontal approach (CEN, 2012). Other leaching procedures were developed specifically for wood products (e.g. CEN/TS 15119 (CEN, 2007)). Nevertheless, these specific methods are suitable for some regulatory purposes but not for chemical modelling. The method applied in our research was chosen because it is more suitable for a mechanistic model development (equilibrium conditions are reached at different pH values and therefore chemical mechanisms can be identified).