Dislodgeable copper, chromium and arsenic from CCA-treated wood surfaces

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Abstract

Chromated copper arsenate (CCA) is commonly used to preserve wood, but its use poses risk of arsenic exposure. In order to evaluate the extent of exposure to As from physical contact with CCA-treated wood, dislodgeable As from treated wood surfaces (as well as Cu and Cr) was determined as a function of weathering time using dampened polyester wipe materials. Six sets of 2.5-m-long CCA-treated boards, three–four boards per set, were purchased from lumber yards and cut into 30- or 60-cm coupons. A total of 44 such coupons were placed outdoors and the dislodgeable CCA components from the surfaces of the wooden coupons were periodically determined over a 1- or 2-year period by a systematic wipe method followed by nitric acid extraction of the CCA components from the cloth. In all 316 samples, appreciable amounts of the three elements, Cu, Cr and As, were detected. The amounts of surface-dislodgeable As, the most potentially hazardous element and the one of major concern in this study, varied from 5 to 122 μg/100 cm2 with an average value of 37±22 μg/100 cm2. There was considerable variation in As dislodged among coupons, boards, sets and time. Test coupons that tended to release relatively higher (or lower amounts) over time initially, continued to do so over time. However, the amounts of arsenic dislodged over time did not follow a simple pattern. While the As dislodged tended to decrease with time during the first year, it approached the initial value or increased somewhat during the second year, presumably due to surface rejuvenation effects caused by erosion and weathering. When all the data were normalized to the initial values, no trend emerged, as indicated by the average normalized value of 1.0±0.4 for As dislodged over time. Apparently, on installations constructed with CCA-treated wood, arsenic may remain available for a number of years.

Introduction

Due to its excellent fungicidal and insecticidal properties, chromated copper arsenate (CCA) is currently the most widely used wood preservative. Treated wood is used outdoors for both residential and commercial applications. Such uses include decks, picnic tables, landscape timbers, highway sound barriers, telephone poles and docks. Recent concerns that dispersal of these additives from the wood could impact the environment (Weis and Weis, 1996, Weis and Weis, 2002, Stilwell and Gorny, 1997, Stilwell and Graetz, 2001, Lebow et al., 2000, Hingston et al., 2001, Townsend et al., 2001) have resulted in a phase-out of all new uses of this material for residential purposes in the US, effective January 2004 (Federal Register, 2002). Nonetheless, potential environmental impacts, especially with regard to arsenic, still exist since the wood is expected to remain in service for many years (Solo-Gabriele and Townsend, 1999). Various aspects of the environmental and health hazards of arsenic exposure have been extensively covered (Nriagu, 1994, NRC, 1999).

The extent of arsenic exposure to individuals due to physical contact with CCA wood surfaces, including playground equipment, decks and picnic tables built using CCA-treated wood, is of major concern. Because of children's frequent hand-to-mouth activity, they are considered the most vulnerable to this potential risk. In this study, we provide data that will assist in the assessment of this potential risk by analyzing the Cu, Cr and As in wipe samples taken from CCA-treated wood surfaces. These surfaces consisted of test coupons, cut from wood bought at lumber yards, that were placed outside to weather and periodically sampled for up to 2 years. While surface-dislodgeable As in treated wood using wipe samples has been determined in some instances, primarily limited to surveys, there is no information on how its availability varies over time.

Section snippets

Test samples

Six sets of 2.5-m-long CCA pressure-treated pine boards, treated nominally with 6.4 kg/m3 CCA preservative, were purchased from three lumber yards. Each set consisted of three–four boards, and the total number of boards used was 19. Each board was cut into 30- or 60-cm test coupons. Either two or four coupons from the interior portions of each board were tested. The coupons were placed on racks outside, and were periodically tested over 1 (sets 3–6) or 2 (sets 1–2) years. Coupons from set 1

Extraction efficiency of nitric acid solutions

Shown in Fig. 1a are the extraction efficiencies of the various components of CCA in wood powder over time using 20% HNO3 solution at room temperature (approx. 25 °C). These results show that Cu and As are readily extracted within 2 h. The average extraction efficiency for these elements for periods greater than 1 h was 98±3% for Cu and 95±4% for As. The Cr in the CCA wood powder was not completely extracted until the extraction time approached 48 h. However, by raising the extraction

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