Zinc mineral weathering as affected by plant roots

The revegetation of soils contaminated by heavy metals is generally considered a suitable option to reduce the metal dispersion in surrounding environments. A continuous flow experiment was carried out to quantitatively assess the effect of Italian ryegrass (Lolium multiflorum Lam.) root activity on the weathering of smithsonite (ZnCO3), a common zinc mineral. Results showed that the rate of Zn release from smithsonite to leachates was 1.06 µg g-1 h-1 and 0.54 µg g-1 h-1 in the presence and the absence of plants, respectively. The strong correlation (r = 0.95; p < 0.001) between concentrations of Zn and dissolved organic carbon (DOC) exuded by the roots in leachates suggests that organic root exudates were closely involved in the smithsonite weathering. After 10 days, the total Zn amount released by smithsonite (i.e. the sum of Zn amounts lost by leaching and taken up by plant) was multiplied by a factor of 2.25 in the presence of plants. Although this was a laboratory study and further in situ long-term investigations are required, our results clearly highlighted that plants can enhance metal release into the environment by accelerating mineral weathering. Thus, the potential role of plants on metal phases weathering should be further considered when revegetation strategies are proposed for the reclamation of heavy metal polluted soil