Fate of Iron from Tire Derived Aggregate: An Experimental Study
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Tire derived aggregate (TDA) has been shown to be useful in a variety of civil and environmental engineering applications. It is often used as backfill for construction projects or as media for leachfields. While research has shown that the leaching of metals from tires does not result in significant accumulation in test beds, the fate of metals in the soil sub-layers is unknown. The purpose of this study was to understand the fate of metals leached from TDA, as well as to investigate the sorptive capacity of iron oxides formed from leached iron. The study approach was four-fold: equilibrium experiments of iron, both soluble and particulate, at varying pH levels; kinetic experiments for metal leaching from TDA under constant pH; column experiments to evaluate transport and fate of TDA-derived metals; and sorption experiments to determine the sorptive capacity of iron floc. The results of the experiments helped to form a conceptual model of the fate of iron after leaching from TDA. The conceptual model is as follows: The zero-valent metal in the wire oxidizes to form Fe floc. This floc is transported to underlying soil. The presence of organic ligands in the TDA rubber increases the solubility of Fe. However, the majority of Fe in suspension is in the particulate form. Intermittent saturation of TDA may result in a faster rate of Fe leaching than that of a constant saturation. In laboratory columns receiving Fe-rich suspensions, a high percentage of Fe was retained for both sandy and clay soil types. At higher pH levels, more Fe will be leached from TDA. This affects the total mass of Fe loading to soil, but suspension pH does not seem to affect behaviors in the soil. Fe floc generated by TDA exhibits sorptive properties for phosphate and zinc, and likely for other cations and anions.