As the wood chips are primarily applied to the sidewalks bordering roadways, we wanted to know if wood could absorb some of the heavy metals generated by traffic and, if so, in what proportion. This is an important element when the goal is to reclaim the materials at the end of the winter, notably by composting.
The study consisted of collecting samples of wood chips at a few sites in [downtown Chaux-de-Fonds, Switzerland] at the end of the winter and measuring the heavy metal concentrations. For comparison purposes, we also determined the heavy metals in samples of unused treated chips. The selected analytic method is based on the Federal Ordinance on Chemical Risk Reduction (Switzerland).
In the untreated wood chips, any heavy metal concentrations are derived from the wood proper, as well as from some impurities in the salt used to treat them.
The analytic results of the chips recovered at winter’s end at various locations in Chaux-de-Fonds were well below the applicable limit values, with the exception of cadmium (09.0977). The increase in the zinc, copper and lead content in the recovered chips (as compared to the unused treated chips) remained low. The most significant increase related to lead. We found that, contrary to our fears, the wood chips applied to pedestrian walkways along high-traffic roads do not act as sinks for heavy metals.
The wood chips should therefore be able to be recovered by composting, provided that the contents of certain inert foreign substances be considered (e.g. metal, glass, plastic pieces, bags, strings). By mixing the recovered wood chips with the organic waste most common in summer (grass, in particular), it is possible to improve the carbon/nitrogen ratio of the compost.
See the ARPEA article in its entirety (in French only).
See the MSDS for magnesium chloride.
Table 1: Concentrations of metals in wood chips