EFFECT OF DE-ICING SALTS ON THE MOBILIZATION OF ZINC FROM TIRE PARTICLES

Tire tread material contains about 1% Zn. Previous work from our group (Councell et al. 2004) has shown tire particles are a significant source of Zn to the environment. These particles may be in contact with deicing salts in a variety of environments; for example, on paved surfaces (roadways, parking lots) subject to salt application, and in storm water retention ponds that collect runoff from such surfaces. Our goal is to study the possible effect of de-icing salts on the mobilization of Zn from tire particles. We have included alternative road salts (MgCl₂ and CaCl₂) as well as the commonly applied NaCl in this investigation.

Tire particles consisted of ground tire rubber (≤ 590 µm; minus 30 mesh), from a commercial scrap tire facility. The range of chloride concentrations tested [4700, 2000, 860, 230, 0 mg/L (deionized water (DI)] was chosen to include the maximal value observed in streams in the Baltimore, Maryland region (Kaushal et al. 2005) and the USEPA water quality criteria for protection of aquatic life (acute and chronic exposure). Three sets of room-temperature, laboratory-shaker, batch experiments were conducted at various solid-to-solution (g/mL) ratios (1:20, 1:50 and 1:500), and samples were collected at extraction times that ranged from 30 minutes to 8 weeks. Filtered (0.45µm) solutions were analyzed for Zn by ICP-ES.

For the 1:20 series, initial wetting of the tire material with either DI water or the saline solutions resulted in Zn concentrations of about 20 mg/L in the filtrate. This increased at 8 weeks to about 80-100 mg/L for the highest chloride concentrations of each salt, as compared to about 70 mg/L for the DI control. For the 1:20 series, the quantity of Zn released into solution in 8 weeks amounted to about 10% of that in the tire material present, increasing to greater than 25% at solid-to-solution ratios of 1:500.