Evaluating the potential for environmental pollution from chromated copper arsenate (CCA)-treated wood waste: A new mass balance approach
Introduction
Chromated copper arsenate (CCA) is a wood preservative containing arsenic, chromium and copper. It has been extensively used worldwide since the 1970s [1]. Recent studies have highlighted the leaching potential of this preservative, particularly from freshly treated wood and in-service structures [2], [3], [4]. As a result, many countries such as Canada, the US, the UK, Australia and Norway have limited the use of CCA as a wood preservative [5]. Despite this, the extended service life of the wood will result in high volumes entering the wood waste stream over the next 50 years. Recently, several scientific studies have looked at the leaching properties of the wood once it is taken out of service to assess its environmental impact. Jambeck [6], Jambeck et al. [7] and Khan et al. [8] evaluated leaching of the waste wood using enclosed lysimeters that simulated various landfill scenarios. Arsenic was preferentially leached, with As(V) present in the monofill and As(III) in the municipal solid waste (MSW) lysimeters. Hasan et al. [9] used lysimeters to quantify leaching from CCA weathered wood exposed to natural conditions. Their findings support those of Jambeck [6] with regards to the preferential leaching of As. Another study by Shibata et al. [10] assessed the leaching potential of arsenic from mulch made from recycled construction and demolition wood including the impact of iron-oxide colorants. Arsenic was found to leach out up to 15% of the initial mass of arsenic. These results demonstrated raised concentrations of leached metal(loid)s and were attributed to rainfall and weathering. Mercer et al. [11] and Mercer and Frostick [12] also observed increased concentrations of As, Cr and Cu in leachate taken from lysimeters containing CCA-treated wood waste and soil. Furthermore, the conditions during the study promoted the presence of the more toxic and mobile forms of As and Cr.
Leaching from weathered CCA wood is known to occur in three stages, with initial surface wash-off followed by diffusion leaching and final release as the wood weathers [13]. The weathering wood releases the metal(loid)s due to breakdown caused by radical-induced depolymerisation (UV exposure) [14]. As the lignin breaks down, the metal(loid)s which are bound to it are released (washed off and dissolved). Release mechanisms resulting from weathering include loss of wood fibres [15] and associated warping, cracking and splitting [16]. During the weathering process the metal(loid)s will therefore be leached by a combination of surface wash-off, diffusion leaching and loss of metal(loid)s still bound to the wood (due to loss of wood fibres).
The above studies have demonstrated that CCA-treated wood waste can act as a source of heavy metal contamination following the end of its service life. However little is known about the behaviour of heavy metal pollutants in this waste when applied to the soil surface. This is particularly important since much of the wood that enters the waste stream is chipped and applied to the soil surface as mulch. The objective of the research reported here is to evaluate the potential for serious contamination of soil and the wider environment through this route. In particular, new mass balance models are used to identify mobility and pathways of leached metal(loid)s. This approach has been used in other areas of research but not for heavy metals in the soil environment. One study that evaluated leaching of CCA-treated wood using a mass balance approach was conducted by Shibata et al. [17] however they studied leachable arsenic and chromium in situ from a CCA-treated structure. Indeed this study led them to argue for the need to evaluate releases to air, water and soil together rather than separately. They highlighted the importance of this approach to quantify the release of elements from this wood and ascertain movement within different environmental media. To date, no such approach has been applied to assess the leaching characteristics of CCA-treated wood waste mulch applied to soil. This paper fills this gap.
Section snippets
Lysimeter study with natural exposure
The leaching results of a five month lysimeter study are reported in Mercer et al. [11] and Mercer and Frostick [12]. The current study is based on this work. In summary, the experimental design consisted of 14 lysimeters (0.65 m × 0.35 m × 0.165 m) each containing 20 kg of commercially purchased topsoil. The CCA-treated wood used in the study was sourced from a local industry where it had been used in cooling towers for 15 years. The wood species was identified as Picea excels which is commonly
Temporal changes in mass balances under natural weather conditions
Mass balances for arsenic, chromium and copper in the study exposed to natural weather conditions (Fig. 2) show similar trends for all the metal(loid)s within each of the media studied (wood, soil and leachate). In the wood, levels of chromium were highest in the wood followed by arsenic then copper (Cu < As < Cr: F(2,15) = 839.05, p < 0.01). This is a reflection of the original preservative formulation. There was an initial decrease in the total mass of metal(loid)s in the wood over time before levels
Loss of metal(loid)s from the system
The mass balances were based on the total metal(loid)s in the wood, soil and leachate. There were significant and systematic losses of all three metal(loid)s from the wood in all studies suggesting that CCA-treated wood continues to leach after a long service life, particularly when chipped down and applied to soil as mulch. The wood lost the largest proportion of the metal(loid)s within the initial stages of both studies. Releases from the wood was reflected in the increased levels observed
Acknowledgements
The authors would like to thank the following for their assistance:
The two anonymous reviewers whose comments greatly improved the manuscript; Gervais Sawyer for his help with wood species identification; Leeds University for the use of their cutting mill; Bob Knight for his assistance with metal(loid) analysis; the staff at the University of Hull Facilities for assistance with experimental setup; and the field assistants (Dr Andrew Kythreotis and Edwina Mercer). Funding for this project was
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