Leaching of heavy metals from chromated copper arsenate (CCA) treated wood after disposal

Abstract

Wood treated by preservatives is commonly found in solid waste. Among the different types of preserved wood, chromated copper arsenate (CCA) treated wood recently has received much attention due to the scale of usage and its significant role in soil and water contamination. As the ash of CCA treated wood would be hazardous if the wood were to be incinerated, this is not a good alternative, and the best available disposal method is thus landfilling in the US, Canada and Australia. Leaching of the metals from preserved wood that is disposed in unlined landfills for construction debris pollutes the soil and water environments. Several factors affecting leaching of the metals from wood, including pH of the leachant, temperature, the duration of leaching and the type of leachant, were investigated. These factors affect each of the metals, chromium, copper and arsenic, differently. A comparison of these effects on each metal was performed. The results of the experiments showed that the pH of the leachants has a significant effect on the leaching process, and sulfuric acid (pH 3) is the most effective leachant compared to nitric and acetic acid (pH 3-4-5). The amounts of leached chromium, copper and arsenic by sulfuric acid (pH 3) during 15 days were, respectively, 0.2, 0.14 and 0.15 mg more than leachates by nitric acid (pH 5) on the basis of 1 g of wood (initial contents of 1.03 mg, 0.42 g and 0.8 mg per g of wood). Most of the leaching occurs in the first 5 days, and the rate of leaching decreases significantly after 5 days. Increasing temperature increases the amount of leached metals, and arsenic is the least resistant metal to the leaching when the temperature increases. Increasing the temperature from 15 °C to 35 °C during 15 days increases the amount of leached chromium, copper and arsenic by acetic acid at pH 5 by about 0.1, 0.4 and 1.2 mg per g of wood, respectively.

Introduction

Today, municipal solid waste management has become an important part of environmental protection activities. In 2003, the US produced more than 236,000 million kg of municipal solid waste, of which 30% was recycled and the rest was disposed of by landfilling or composting (US EPA, 2006). In Canada, even though the population is less, the same problem exists. As there are some limitations for the types of wastes that can be recycled or combusted, landfilling is an important method of municipal and construction debris waste management (Ress et al., 1998).

Following wood removal from the forests, several types of deterioration by fungi and insects threaten the untreated wood and reduce the lifetime of the wood and the wooden building materials. Chemicals are utilized to treat the wood to protect wood against bacterial, fungal, and insect attack. Chemical treatment has been practiced for centuries and is intended to enhance wood durability, and thereby increase the life expectancy of wood in service. As an example, railroad cross ties that are used in North America would have an average lifetime of 5 years without treatment (Konasewich and Henning, 1998), whereas the lifetime of most creosote preserved wood is estimated to be 30 years (Webb, 1990). In Florida, it has been found that up to 30% of the construction and demolition (C&D) wood can be CCA (Chromated Copper Arsenate) treated wood (Solo-Gabriele et al., 2003). The method of disposal most used for treated wood is landfilling in US, Canada and Australia without any pretreatment. Countries such as Germany ban landfill disposal, but the ash of treated incinerated wood can be hazardous since CCA concentrates in the ash (Solo-Gabriele et al., 2002) and, in addition, proper air pollution equipment is required (Iida et al., 2004). Some methods for the treatment of CCA treated wood are being developed, such as thermochemical means as reviewed by Helsen and Van den Bulck (2005), and biological treatment.

One of the most problematic preserved wood in United States as well as in Canada is CCA treated wood, which is used in outdoor decks, playgrounds, and fences. CCA has been favored for lumber treatment because it is inexpensive, leaves a dry, paintable surface, and binds to become relatively leach-resistant. However, there is increasing concern about potential environmental contamination from the leaching of Cu, Cr and As from treated wood in service and from wood removed from service and placed in landfills in North America (Shalat et al., 2006). The life cycle of treated wood is estimated to be about 25 years and then the wood is discarded as waste (Illman et al., 1996). By 1995, more than 90% of 67 million kg of utilized waterborne preservatives was CCA (Solo-Gabriele et al., 1999). Despite this, the quantity of removed treated wood from service is estimated to increase to 12 million m³ by the year 2004 (McQueen and Stevens, 1998) and to 16 million m³ by the year 2020 in the US (Cooper, 1993).

During rain, water penetrates into C&D landfills and causes leaching of wood preservatives (copper, chromium and arsenic) from the disposed Chromated Copper Arsenate (CCA) treated wood. Precipitation is the only cause of leachate formation in these types of landfills for construction material. This was demonstrated by Weber et al. (2002). The leached metals can pollute ground and surface water used for drinking purposes as C&D landfills are unlined. Concentrations of arsenic in the leachate exceeded the US water quality standards. As the leachate of treated wood in landfills is genotoxic and carcinogenic, the contaminated water is dangerous for human beings and animals and the investigation of leaching and biodegradation of treated wood is important. Leaching of CCA-treated wood in a simulated monofill was also demonstrated by Jambeck et al. (2006). Arsenic leached to the greatest level and was found in concentrations of 42 mg/L. A monofill is a landfill for disposal of a specific single type of waste.

Recently, a significant amount of arsenic has been found in children’s playgrounds due to the use of CCA treated wood in Canada and US, and it has received significant attention (Hauserman, 2002). Shalat et al. (2006) recently performed a study on children’s exposure to CCA-treated wood in playground material. They determined that dislodegeable arsenic was found on children’s hands after brief periods of play exposure. Levels of 0.6 μg of arsenic were found on their hands after CCA-treated wood exposure compared to less than 0.2 μg without the exposure. The US Consumer Product Safety Commission (CPSC) stated that it might even cause cancer in children (Green Building News, Feb. 2003 issue (oikos.com/news/2003/02.html)). Since December 31, 2003, the sale of CCA-treated wood is no longer allowed in Canada for non-industrial uses (Health Canada, 2005). Although replacing the CCA-treated wood with other preservatives alleviates one problem, it does not decrease the demand for disposal of existing CCA-treated wood. Landfilling of existing CCA-treated wood and its related environmental problems will continue to be problematic due to leaching (Jambeck et al., 2006). Thus it is necessary to know how much of the metals leach and which factors affect the leaching process, and to find solutions to decrease its damage to the environment. According to the toxicity characteristics leaching procedure (TCLP) method, the acid used for leaching is acetic acid (0.1 N) to simulate the organic acids produced in landfills. However, in landfills the disposed materials would also be exposed to nitric acid and sulfuric acid during acid rain, In a municipal landfill, exposure of the CCA-treated wood would be to many types of chemicals and thus is highly difficult to evaluate. In a C&D landfill, precipitation would be the only leaching agent. Another scenario is to dispose of the wood in a lined landfill with only CCA-treated wood, which makes it easier to manage the leachate. Therefore, in these experiments, nitric, sulfuric and acetic acids were used. The objective of this study was to evaluate the factors affecting the leaching of the metals from wood from a disposal point of view, including pH of the leachant, temperature, the duration of leaching and the type of leachant.