‘active substances and preparations containing one or more active substances, put up in the form in which they are supplied to the user, intended to destroy, deter, render harmless, prevent the action of, or otherwise exert a controlling effect on any harmful organism by chemical or biological means’.

‘the construction works must be designed and built in such a way that they will, throughout their life cycle, not be a threat to the hygiene or health and safety of workers, occupants or neighbours, nor have an exceedingly high impact, over their entire life cycle, on the environmental quality or on the climate during their construction, use and demolition, in particular as a result of any of the following: … the emissions of dangerous substances,… into indoor or outdoor air; the release of dangerous substances into ground water, marine waters, surface waters or soil…’

• The wood chemistry with respect to interactions with the biocides (see Section 6.2.2)

• The exposure conditions of treated wood products (see Section 6.2.3).

6.2.1 Biocide types and their toxicological properties

Different classes of biocides were used in wood treatment according to the legislative constraints of the time, which imposed increasingly strict conditions. Thus, in the 1960s, products such as aldrin, pentachlorophenol and creosotes were usually employed. Because of their known content of carcinogenic chemicals, they were restricted or prohibited (JORF, 1992; OJEU, 2001). Then alternative products known under the name of CCA (chromated copper arsenate) were extensively used. Nowadays, CCA treatment is prohibited for residential construction (with certain exceptions) (AWPA, 2001; JORF, 2004). The alternatives to these biocides are the so-called ‘new generation biocides’ (chromium and arsenic free), such as copper boron azoles (CBA), ammoniacal copper quaternary (ACQ), copper HDO (CuHDO), etc. Recently, preservatives containing micro- or nano-sized copper particles have been introduced to the market, usually with ‘micro- nized’ or ‘micro’ designations such as MCQ or MCA.

The number of commercial preservative products is very high but the number of active substances is relatively small. The main active substances are (FCBA, 2011):

The class of toxicity of some biocides used for wood preservation treatment is given in Table 6.1. The classes of toxicity comply with the classification made by the World Health Organization (WHO, 2006).

Wood preservatives can be classified into four main categories:

The main water-borne preservatives are copper chromium arsenic (CCA), copper azoles, ammoniacal copper quaternary (ACQ), copper HDO (CuHDO) and borate-based formulations.

In CCA treatment, the copper acts primarily to protect the wood against decay fungi and bacteria, while the arsenic is the main insecticidal component of CCA. The chromium acts as a chemical fixing agent, which also provides ultraviolet (UV) light resistance and has little or no preserving properties. Its role is to help the other chemicals to fix in the wood, binding them through chemical complexes to the wood’s cellulose and lignin. It should be mentioned that even though CCA treatment is presently prohibited for residential uses, some CCA-treated woods are still in service. Moreover, CCA products are still permitted for use in various industrial and public works, such as bridges, highway safety fencing, electric power transmission and telecommunications poles (FCBA, 2011). Thus, this kind of preservative continues to be of concern to the environment.

Copper azole is a fungicide and an insecticide. There are two types of copper azole: type A (CBA-A) and type B (CA-B). Copper boron azole type A contains the following ingredients: copper (49%), boron as boric acid (49%), and azole as tebuconazole (2%). Copper azole type B is composed of copper (96.1%) and azole as tebuconazole (3.9%) (EPA, 2011). Some commercial CBA also contains glycol (US DOI, 2011). Despite research efforts, no effective means has yet been found to keep borate preservatives from leaching out of wet wood (US DOI, 2011).

Ammoniacal copper quaternary (ACQ) contains copper as fungicide and a quaternary ammonium compound as insecticide. There are currently four AWPA standardised ACQ formulations (types A to D). All ACQ types contain two active ingredients which may vary within the following limits: copper oxide (62-71%) and a quaternary ammonium compound (29-38%). ACQ is considered as an alternative to CCA treatment. Nevertheless, because of its higher level of copper, ACQ-treated wood is more corrosive to common steel and the use of double-galvanised or stainless steel fasteners is necessary for this kind of treated wood processing (US DOI, 2011).

Cu-HDO is composed of copper and N-cyclohexyl-diazeniumdioxide (HDO). Cu-HDO affects sulphydryl groups of essential amino acids of fungi and causes protein denaturation (EPA, 2011). It is classified as moderately hazardous (toxicity class II, with a LD₅₀ of 380 mg/kg) (WHO, 2006).

Borate preservatives such as boric acid, oxides and salts (borates) are supplied under numerous brand names throughout the world. Boratetreated wood is considered of low toxicity (toxicity class U – see Table 6.1). However, borate compounds do not become fixed in the wood and can be readily leached out. They are efficient rather for use class 1 and 2 (see Section 6.2.3), such as framing, sheathing, sill plates, furring strips, trusses and joists (EPA, 2011).

Oil-borne preservatives include creosote and some vegetal oils containing permethrin. Creosote is one of the oldest wood preservatives. Because of its known content of carcinogenic chemicals, it was restricted or prohibited (JORF, 1992; OJEU, 2001). However, it is still used for railroad ties and utility poles. In recent years linseed oil has been incorporated in preservative formulations as a solvent and water repellent together with preservatives of low water solubility, such as permethrin. Permethrin is a broad-spectrum non-systemic synthetic pyrethroid insecticide (Imgrund, 2003).

Light organic solvent preservatives (LOSP) are based on the use of white spirit, or light oils such as kerosene, as solvent carrier for biocides. Synthetic pyrethroids are typically used as an insecticide, such as permethrin, bifenthrin or deltamethrin. Other active compounds are propiconazole and tebuconazole as fungicides. These kinds of preservatives contain no heavymetal compounds but they are still of concern for health and environmental impacts, due to the release of volatile organic compounds (VOC) and organic biocides.

For a few years now, preservatives containing micro- or nano-sized copper particles have been used for wood treatment. There are currently two particulate copper systems on the market. One system uses a quaternary biocide system (known as MCQ) and is based on ACQ. The other uses an azole biocide (known as MCA or μCA-C) and is based on copper azole. This kind of preservative raises concerns regarding exposure to micro- and nano-sized particles, in the context of the general debate about the hazards of nanotechnology to humans and the environment (Evans et al., 2008; Matsunaga et al, 2008, 2010; ICTA, 2010).

6.2.2 Wood chemistry and its interactions with biocides

Wood is composed of mainly three biopolymers present in different proportions, namely cellulose (about 45% in weight), hemicellulose (about 30%) and lignin (20%) and a small fraction (about 5%) of low weight compounds extractible in water or other solvents, named extractives (Govin, 2004).

Cellulose is composed of linear chains of D-glucose linked by β-1,4- glycosidic bonds with a degree of polymerisation of about 10,000. Cellulose possesses hydroxyl groups and has the specific reactivity of primary and secondary alcohols (donor reactivity of hydroxyl groups).

Hemicelluloses are heteropolysaccharides (containing glucose, galactose, xylose, arabinose, mannose and uronic acids) with a lower degree of polymerisation (50–300) and amorphous structure. The chemical structure of hemicellulose varies from soft to hard wood. It contains mainly hydroxyl groups but also carboxylic groups. Lignin is a complex network of polymers (molar mass exceeding 10,000) composed of phenolpropane units and containing characteristic methoxyl groups, phenolic hydroxyl groups and some terminal aldehyde groups. Lignin is associated with hemicellulose forming complexes that are resistant to hydrolysis. There is no evidence that lignin is associated with cellulose. Other minor polysaccharides present in wood are pectins, starch and proteins.

The low weight compounds are organics belonging to many chemical classes: fats, fatty acids, fatty alcohols, phenols, terpenes, polyphenols, tannins, lignans (combinations of two phenylpropane units), steroids, resin acids, rosin, waxes, etc. Wood contains also inorganic species represented by silica, and several major elements – Ca, Mg and K (up to 80% of the ash). These metals are probably bound to carboxyl groups in pectic materials, oxalates, carbonates and sulphates. Many other metals are present in lower quantity.

Wood is a slightly acid material, having pH values in contact with water in the range of 3 to 6, depending on wood species (e.g. pH 3 for western red cedar and pH 6 for ash). The acid/base properties and general reactivity of wood materials are determined by the functional groups linked to the polymeric structure. So, carboxyl groups are attached mainly to hemicelluloses and pectins and have acidity constant pK_a values between 3 and 5. Lignin contains phenolic groups (pK_a = 7.5–10.5) and small quantities of carboxyl as a result of oxidation processes. The acid/base properties of wood extractives in water are due mainly to carboxyl groups (pK_a of 5.0–6.5) and their solubility rises with the temperature and alkali content (Balaban and Uçar, 2001; Ravat et al., 2000a; Duong et al., 2004).

Complexation and ion exchange are the two main interaction mechanisms of metal ions with wood components. The ion exchange mechanism is possible at the level of carboxyl functional groups present in hemicellulose, pectins or lignin. It manly concerns alkaline and alkaline-earth cations and is a non-specific electrostatic interaction. Heavy metals such as Cu, Cd, Ni, Zn and Pb are mainly involved through reactions of surface complexation with the functional groups phenol and carboxyl. The phenolic sites lying on lignin have significantly higher affinity for transition metals than the carboxylic sites. The conclusion of many studies is that cellulose has a lower sorption capacity for heavy metals compared to lignin. The extent of metal sorption depends on pH and aqueous ionic strength. Complexation constants for several heavy metals including Cu and Zn are reported in the literature (Ravat et al., 2000a, 2000b; Merdy et al., 2002; Guo et al, 2008).

In the wood/water system, at low pH, metals exist predominantly as free cations in solution. When the pH increases, the deprotonated carboxylic sites contribute significantly to their fixation by forming complexes. At higher pH, phenolic sites become deprotonated and participate in metal binding. These complexes are mostly monodentate, S1–O–M +, and to a lesser extent bidentate, S1–O–M–O–S2 (here S1 and S2 are organic functional groups, and M is a bivalent metal). The proportion of the aqueous metal species and fixed complexes on biopolymers depends on pH and ionic strength, and can also be influenced by the presence of extractives.

The maximum metal quantity a wood material can fix by specific interactions is given by the number of binding sites. The site density depends on the wood species and biopolymer type. For example, a lignin isolated from black liquor (a residue of the paper industry) (Guo et al., 2008) contains about 0.08 mmol carboxyl and 0.28 mmol phenolic groups per gram of dried wood. A hemicellulose extracted from cottonwood (DeGroot, 1985) contains about 0.08 mmol/g carboxyl groups.

The extractives are organic compounds able to undergo complexation reactions with free metals present in aqueous solution (by intermediation of carboxylic, phenolic groups, or other electron donor groups). Copper is particularly renowned for its capacity to complexate with dissolved organic matter, resulting in an increase of its global leachability.

In the aqueous phase chromium behaves as Cr(III) and Cr(VI). In contact with biomaterials like wood, Cr(VI) transforms into Cr(III) by an adsorption-coupled reduction mechanism involving adsorption steps of both species and reduction of Cr(VI) such that Cr(III) becomes dominant in the system (Dupont and Guillon, 2003; Park et al., 2007).

According to Bernardo et al. (2009), Cr(III) adsorption on biomaterials follows a reaction path involving phenolic or carboxylic groups leading to mono- and polydentate complexes with free and hydroxylated Cr^3 − ions. Despite the abundance of literature on the subject (Miretzky and Cirelli, 2010), no intrinsic complexation constants have been determined.

Boron leachability in treated wood is recognised to be high but its chemistry in wood is far from well understood. Most authors consider that boron does not react with wood materials and is completely available for leaching. Other authors consider that B (as boric acid) can form organic complexes via O bridges with hydroxyl groups on polysaccharides (Obanda et al., 2008; Ramos et al., 2006). No complexation constants are available in the literature.

Generally, wood preservatives are used as mixtures of several chemical substances with complementary biocidal effects. The chemical behaviour of these substances is dependent also on the additives used. Research on products containing ammonia and Cu has shown that ammoniacal copper is readily adsorbed to phenolic hydroxyls, such as are found in lignin or extractives, although, in the absence of ammonia, precipitation may occur as copper carbonates, copper oxides or copper arsenate complexes. Also, coarse deposits of almost pure copper were observed by spectroscopic investigations localised within the cell wall and on the microfibrils (Lebow, 1996).

However, there is no unanimity concerning the formation of salt precipitates, since they are reasonably soluble. Many investigations on CCA-treated wood by XAFS (X-ray absorption fine structure spectroscopy) indicated that Cu is not chemically associated with any heavy element (Bull, 2001). The results of leaching tests performed with different extractants (acetic acid, EDTA, oxalic acid, etc.) show that Cu is not dependent on Cr and As and exhibits a different release behaviour. The most plausible fixation mechanism for Cu remains the complexation on carboxyl and phenolic sites. Similar mechanisms possibly occur in the case of Zn, although the binding forces are weaker.

Little information has been published on the fixation mechanism of organic biocides on wood materials (Woo et al., 2010). It was observed that these molecules are rather leach resistant, suggesting possible interactions with the wood structure. It may be expected that the organic biocides interact with the different wood components at least by weak physical bonds (van der Waals, dipolar and charge transfer interactions), thus adsorbing on the biopolymers. Their partition between the aqueous solution and the biopolymers of the wood depends on the biocide’s hydrophobicity and aqueous solubility. So, it is expected that hydrophobic molecules have more affinity for lignin than for cellulose. Stronger interactions can occur when biocides possess functional groups able to react with specific sites on wood, such as hydroxyls (hydrogen bonds).

Investigations (Kjellow et al., 2010) on tebuconazole, propiconazole and IPBC behaviour during wood impregnation have allowed researchers to observe their partition between the carrier solvent and wood samples and have led to the conclusion that some (adsorption) interactions occur. The adsorption level decreases in the order tebuconazole > propiconazole > IPBC. It was suggested that tebuconazole forms stronger bonds with the hydrophilic biopolymers (hemicellulose, for instance) due to the presence of a hydroxyl group.

6.2.3 Exposure conditions of treated wood products

The service life and end life are the longest steps during the life cycle of treated wood products. Thus, during these two steps the environmental risk could be of more concern.

As a function of exposure conditions, European Standard EN 335 (CEN, 2006) defines five use classes which represent different service situations to which wood-based products can be exposed. The classes described in the European Standard are based on an existing classification agreed on by the European Homologation Committee (EHC), after taking into account the possibility of harmonisation with three moisture categories of Eurocode 5 and with other classes used outside Europe. It has, however, been judged that the following five classes are the most appropriate solution for European conditions:

Use classes 1 and 2 are of concern mainly for the health impact via the indoor air emissions of eventual VOC contained in the treated wood, whereas use classes 3 to 5 are of concern for the environmental impact, especially via leaching phenomena.

During the service life, the biocide amount in wood products lessens following air emission, leaching and biodegradation processes. Nevertheless, the end-of-life management (incineration, recycling, landfill) must take into account the presence of specific toxic substances.

Landfill disposal is allowed for wood wastes whose composition complies with the admission standards in different landfill categories. The biocide release in these systems will affect the landfill leachate composition and thus its management.

Biotransformation of organic compounds by using adapted leaving species (fungi, bacteria) could be envisaged before the entrance of wood residues back in the carbon cycle (Woo et al., 2010) by incineration.

Recent research (Tame et al., 2007) found that the presence of Cu-based biocides favours dioxin formation during the combustion of treated wood. The explanation resides mainly in the catalytic activity of Cu for dioxin formation, the smouldering of wood char by the metals providing a favourable temperature environment, and the presence of chlorinated organic biocides as precursors for dioxin formation.

6.3.1 Release mechanisms and parameters of influence

Release of biocides from treated wood products is the result of coupled chemical reactions, transport processes and biological activity.

In contact with water, wood material undergoes chemical and structural changes, with the release of soluble compounds such as natural mineral salts and extractives. Fixed biocides are partially desorbed following thermodynamic equilibrium. All these chemical reactions are influenced by the pH. The natural pH of wood is often acidic, so contact with neutral water could cause chemical and structural modifications of the wood material (hydrolysis, deprotonation of acidic functions, dissolution/precipitation of extractives, etc.). The pH strongly influences the mineral biocides (Cu, Cr, As, B) binding by complexation reactions on carboxyl and phenol groups, but also the interaction between hydrophilic organic compounds. The presence of oxygen is a factor in wood ageing by oxidation of, for example, phenol groups and depolymerisation of lignin.

Besides chemical processes, the organic natural compounds and biocides can be transformed by biological activity of bacteria or fungi. The decay period of organic biocides is variable as well as the secondary compounds formed, following the surrounding conditions.

Wood is a porous, hygroscopic material; all the processes mentioned above take place in the porous structure. There are gradients of composition between the core and the product surface following the conditions of water contact experienced by the product. The dynamics of the biocide release from a given piece of wood are then determined by the diffusion processes in the porous structure. Wood is an anisotropic material; the diffusion rate varies following the orientation, being maximal along the fibres, minimal in the tangential direction and intermediary in the radial direction.

Once the biocides reach the external surface of the wood, their dispersion in the environment is ensured by the water circulation in soils, surface and ground waters, potentially affecting the quality of environmental compartments (including water resources for human consumption) and the integrity of living targets. In some cases attenuation could occur through soil infiltration by chemical (retention, decay) and biological (degradation) mechanisms. Dispersion of the released pollutants into surrounding water and soils plays a diluting role for the initial leachate.

Numerous leaching studies have been realised at laboratory and field scales for determining the biocide release from wood products (Cooper, 1991; Lebow, 1996; Brooks, 1997; Hingston et al., 2001; Solo-Gabriele, 2003; etc.) and have highlighted the role of different parameters of influence, described below.

6.3.2 Methodologies of toxic effect assessment

Biocides are intrinsically toxic substances and their harmful effect will strongly depend on their release behaviour.

The extrapolation of partial field observations or laboratory assays to conditions encountered in real scenarios of service life is not obvious. The assessment of toxic effects needs comprehensive methodologies and adapted experimental and modelling tools, which is why the European authorities endeavour to develop standard assessment methods.

The potential harmful effects of biocides released in a given scenario of wood product utilisation can be assessed following classical methodologies of risk assessment on ecosystems and human health. These methodologies consist of the evaluation of three components of the cause–effect chain: the source of pollution (the wood product, for instance), the pollutant transport through environmental compartments (soils, waters) and the effects on living targets. A sound evaluation of the physico-chemical and biological processes at the level of these three terms is necessary and possible by using appropriate experiments, models and databases. A more detailed description is given in Chapter 14 of this book.

The hazardous effect of biocides as toxic molecules present in different environmental compartments is evaluated by bioassays (toxicity tests) on selected species, then the risk is evaluated by comparing the test results with toxicological parameters (lethal or effect doses and concentration).

The key point in the assessment process is the evaluation of the exposure of living targets to a biocide, i.e. knowledge of the biocide concentration and the evolution with time of the released flux from the wood product. So, knowledge of the wood product behaviour in its utilisation scenario (the source term of pollution) is fundamental for the evaluation of the exposure conditions. The most common experimental tools for studying the leaching phenomenon are the leaching tests. Some of these tests (i.e. static leaching tests) give information about different intrinsic properties of the material (e.g. acid/base neutralisation capacity, pH influence on pollutant release) and others (i.e. dynamic leaching tests) provide knowledge about the release dynamics – see also Chapter 14.

As mentioned above, the leaching of biocides from construction materials is a research subject receiving attention from the European regulation authorities. The main standards concerning the leaching behaviour that are presently under development are CEN/TC 351/WG 1 N 178 – Generic horizontal dynamic surface leaching test (DSLT) for determination of surface dependent release of substances from monolithic or plate-like or sheet-like construction products; CEN/TC 351/WG 1 N 177 – Guidance standard for CEN, Product TCs for selection of leaching tests appropriate for their product(s) – General principles; and CEN/TC 351/WG 1 N 162 – Generic horizontal up-flow percolation test for determination of the release of substances from granular construction products (CEN, 2011). Until these protocols, at the European level two standards referred to the leaching of wood preservatives: XP ENV 1250-2 (CEN, 1994) and EN 84 (CEN, 1997). XP ENV 1250-2 specified that ‘the results cannot be connected to the conditions of exposure in the service life of wood based products, as part of the building’. EN 84 was aimed at the accelerated ageing of treated wood prior to biological testing. These two standards are rather tests that aim to evaluate the effectiveness of the wood preservation treatment and not the environmental impact due to the leaching of biocides.

Much work aimed at characterising the leaching process has been carried out, at different scales and following different experimental protocols. As mentioned above, we distinguish between two types of leaching assays: (1) equilibrium tests – the wood material (generally of very small size, crushed) is put in contact with a leachant for a given duration, allowing chemical equilibria to occur (e.g. batch assays); and (2) dynamic tests – processes such as diffusion or leachant flow determine the time evolution of the released quantities (e.g. block diffusion, runoff). The results are weakly comparable and generalisation is not possible, especially when transport phenomena play a major role in the release process. For these reasons only few examples are given here; the interested reader may consult the many other journal articles.

Biocide solubilisation under different pH conditions has been investigated (Esser et al, 2001; Schiopu, 2007; Schiopu et al., 2011), showing similar leaching patterns for the inorganic biocides (Cu, Cr, As, B, Zn). In the case of wood samples treated with CCA and Cu-quat (copper oxide, didecyl-dimethylammonium chloride or DDAC) preservatives (Esser et al., 2001), it was shown that Cu, Cr and As have a maximum fixation at pH 7–8 and solubilisation is augmented by many orders of magnitude at acid or alkaline pH. So, for natural pH values of most wood species (i.e. acid), the element fixation is not optimal (see Fig. 6.1 (Tiruta-Barna and Schiopu, 2011)). The most important variation was observed for Cu, while As behaviour is slightly different following the type of preservative. When the behaviour of extractives was observed, their global concentration (determined as dissolved organic carbon) was found to be augmented with solution alkalinity.

Dynamic tests were performed at laboratory and field scales (Esser et al., 2001; Schiopu, 2007; Hingston et al., 2002; Hasan et al., 2010) and were made the subject of interlaboratory studies for the development of leaching assessment methods and leaching tests (Schoknecht et al., 2005).

A comparison was made (see Fig. 6.2) between the dynamic release in laboratory leaching tests (MBT sequential renewal of leachate, CMLT continuous renewal) and field assays on a commercial wood product treated with copper–boron–azole based preservatives (Schiopu, 2007; Schiopu et al., 2007). The field pilot simulated two scenarios of intermittent rainwater contact: immersion in rainwater (stagnation) and runoff rainwater, during one year (leachate concentrations and rainfall events were monitored). It was observed that the accumulated release increases by many orders of magnitude with the effective leaching time (the duration of dry periods was not accounted for). More or less significant differences have been observed between the tests, following the target element (Cu: similar releases; Zn: different shapes). During one year of rain exposure, the stagnation scenario accounted for more effective contact time (about 300 days) than the runoff scenario (30 days). The increase of release in the stagnation scenario after 30 days could be explained by a weathering process of the wood samples. Therefore, the release level in this scenario tends to those of an aged wood sample after about 100 days.

Despite the abundance of literature on studies dedicated to biocide leaching and wood retention mechanisms, there are few studies that structure the information and knowledge on mechanistic models. The interest in modelling of physico-chemical processes resides in the possibility of confirming hypotheses based on experimental observations, identifying and calculating related parameters, and foreseeing the system behaviour for different exposure conditions. Modelling could make interpretation bridges between different observed systems and different scales and resolve unexplained experimental results.

Diffusion transport in the wood pores was one of the earlier modelling trials (other than empirical regression equations). Recently, leaching from wood was modelled (Waldron and Cooper, 2010) by soluble species (no chemical reactions) and their three-dimensional diffusion following the wood anisotropy (along fibres, radial and tangential directions). Diffusion coefficients of 10^− 10–10^− 9 m²/s have been calculated for the longitudinal direction.

The transport mechanisms are not sufficient for representing the leaching behaviour. The chemistry of the wood/biocide system, although very complex, must be formalised and integrated on coupled chemical-transport models. Chemical models are very few in the literature. A realistic chemical model was established (Tiruta-Barna and Schiopu, 2011) for explaining the leaching behaviour of a wood product made from copper–boron–azole (CBA) treated Pinus sylvestris. The model considers the main fixation mechanisms for Cu, B and other trace species present in wood (Cr, Zn, Ca, SO₄^− 2). The influence of dissolved organic matter on Cu release was modelled by pH-dependent complexation reactions. The model can explain the leaching behaviour in a closed system (at equilibrium) for a pH range from 3 to 11 (cf. prCEN/TS 14429 (CEN, 2002)) (Fig. 6.1). This kind of model represents a step forward but more efforts are needed to integrate transport and reliable chemical models into useful predictive tools for the assessment of leaching behaviour.

6.3.3 Toxicity studies – case examples

Despite the fact that the issue of biocide release gains more and more interest, there are few available studies in the literature evaluating the harmful effect or risk for real service-life scenarios of commercial products by using standard methodologies and tools. Ecotoxicological studies have been carried out mainly on conventional preservatives using different methods, experimental assays and conditions, and expressing the results in different ways that are often not comparable.

The compilation of field observations of biocide release from wood products in their service life (Lebow, 1996; Hingston et al., 2001) highlighted the fact that the methods and experiments used for many dozens of years have varied from one study to another and a coherent analysis of the leaching process and the mechanisms involved is very difficult, due to the lack of complete information. The most monitored preservatives so far are Cu-based with different compositions: CCA (the most studied), ACA, ACZA, ACQ, DDAC, CC and CuAz. For all studies the behaviour of Cu, As and Cr was observed in terms of wood residual composition and soil or water concentration.

One of the most studied scenarios is of the ‘underground contact’ type (poles, posts, plywood, stakes), considered to be the worst example of biocide emission. The biocide emission and pollution are demonstrated by a higher soil concentration of the biocide than in the natural background. In these various studies, higher concentrations of up to 1000 times background were measured on the soil volume around the wood product within a radius of about 1 m. The biocide loss by the wood products was about 10–30% in several years, with various levels for Cu, Cr and As.

In marine environments, the immersed wood products release significant quantities of inorganic biocides, as was observed in the majority of reported studies. Biocide concentration depletion of the surface layer of wood samples of 40–85% was observed after several years of service life. It was noted that most biocide loss occurs in the first weeks of seawater contact. The contamination of living marine species was demonstrated by measuring the levels of Cu, Cr and As in green algae, oysters and crabs collected from the wood sites (including sediments); these were found to be higher than the natural concentrations.

Immersion in fresh water has been less studied. Slightly higher concentrations in CCA components were found downstream and depleted biocide concentration was measured in the wood surface layer.

‘Above-ground’ contact is considered less harmful because contact with water is rather intermittent, depending strongly on the surface area exposed to rain.

All the above-mentioned studies (based on field measurements of pollutant concentrations) concluded that there were non-impact or insignificant effects given the low concentration and the limited dispersion of the target biocides in soils and waters (effects of dilution and natural attenuation), although the toxic effect was not properly assessed.

Conversely, studies performed in controlled conditions, using leachates from treated wood with or without dilution and using various toxicity tests on living organisms, have concluded that there is a possible or effective harmful impact on aquatic compartments and, to a lesser extent, on topsoil organisms. Several case studies and their main findings are presented here.

The toxicity of commercial preservatives (propiconazole, tebuconazole, IPBC, cypermethrin) for aquatic organisms was studied (Adam et al., 2009) using bioassays (freshwater amphipods Gammarus pulex (L.)) and the results obtained for individual biocides showed that the toxicity increases from propiconazole to cypermethrin, and that there are synergistic effects enhancing the harmful effect of the commercial mixture.

The topsoil ecotoxicity from a contaminated site with wood preservatives Cu sulphate and CCA was studied (Mench and Bes, 2009) using target organisms (radish, lettuce, slug Arion rufus L., and earthworm Dendrobaena octaedra). The levels of contamination (variables on the site) were 65–2600 mg Cu kg^− 1, 0–52 mg As kg^− 1 and 0–87 mg Cr kg^− 1. The results showed a high Cu content in plants and a negative effect on plant growth for the most contaminated site.

Dubey et al. (2007) studied the aquatic toxicity of leachates obtained from blocks of wood treated with Cu-based preservatives (CCA, ACQ, CBA). A specific heavy metal toxicity assay was used, based on inhibition of β-galactosidase activity in an E. coli strain. Natural water compartments were considered (rivers, lakes, wetlands, and seawater) along with synthetic moderate hard water and deionised water. In their study, Cu was released from ACQ- and CBA-treated wood about 10–20 times more than from CCA- treated wood. The toxicity of aquatic compartments was found to correlate with the labile (non-complexated) Cu concentration in waters. The bioavailability (and hence the toxic effect) is reduced in the presence of organic and mineral complexes in natural waters.

The new generation of preservatives has replaced CCA for residential constructions; however, not enough hazard assessment has yet been performed. Furfurylated wood as an alternative to CCA-treated wood was tested for its aquatic toxicity (Pilgård et al., 2010). Aquatic organisms were used in Microtox and Daphtox, applied to leachates obtained from different treated woods, in two leaching tests. The most relevant results concern the influence of the treatment procedure on the potential toxicity: those processes which favour curing/polymerisation led to products with less impact. Hemlock stakes treated with different preservatives (e.g. CCA, creosote, ACQ, zinc naphthenate, copper naphthenate) were tested (Lalonde et al., 2011) for aqueous toxicity, acute lethality tests being performed with Daphnia magna, Vibrio fischeri, rainbow trout and three-spine stickleback. For the samples tested, the toxicity varied in the order ACQ > creosote > zinc naphthenate > copper naphthenate > CCA.

Adam, O., Badot, P.-M., Degiorgi, F., Crini, G. Mixture toxicity assessment of wood preservative pesticides in the freshwater amphipod Gammarus pulex (L.). Ecotoxicology and Environmental Safety. 2009; 72:441–449.

AWPA – American Wood Preservers’ Association. Book of Standards. Available from http://www.awpa.com, 2001. [(accessed 14 November 2011)].

Balaban, M., Uçar, G. The correlation of wood acidity to its solubility by hot water and alkali. Holz als Roh- und Werkstoff. 2001; 59:67–70.

Baysal, E., Sonmez, A., Colak, M., Toker, H. Amount of leachant and water absorption levels of wood treated with borates and water repellents. Bioresource Technology. 2006; 97:2271–2279.

Bernardo, G.R.R., Rene, R.M.J., De la Torre, A., Catalina, M. Chromium (III) uptake by agro-waste biosorbents: Chemical characterization, sorption–desorption studies, and mechanism. Journal of Hazardous Materials. 2009; 170:845–854.

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