Native plant colonization of brownfield soil and sludges: effects on substrate properties and pollutant mobility

Abstract

Phytostabilization with native plant species might represent an economically more realistic and cost-effective option than excavation, soil washing, and sludge disposal for rehabilitation of degraded and polluted industrial areas. This work was done to assess the changes induced by native plant revegetation in the chemical properties and mobility-bioavailability of Pb and Zn pollutants of soil and post-washing sludges from an Italian brownfield site of national interest. A 5-year native plant revegetation of polluted soil and relative post-washing sludges from a steel plant was achieved in situ and ex situ in pot and in the presence and absence of peat as organic amendment. During the experiment, the vegetation growth was monitored (Adamo et al. In Int J Environ Sci Technol 12(6):1811–1824, 2015). Before and after plant growth, the substrates were studied for pH, organic carbon, and carbonate contents. Lead, Zn, and other metal mobility and leachability were investigated by water extraction. The metal bioavailability was estimated by diethylenetriaminepentaacetic acid (DTPA) extraction at pH 7.3. Sequential extractions (BCR procedure) were used to fractionate Pb and Zn in soil main geochemical forms. Plant ability to uptake metals was evaluated on the three most representative species: Bituminaria bituminosa, Daucus carota, and Dactylis glomerata. After 5 years of revegetation with native plants, the substrate pH and organic carbon content were respectively decreased and increased by plant growth, with changes masked by peat treatments. Although metal pollutants in both substrates were characterized by low water solubility and DTPA availability, after plant growth, an increase of rhizospheric Zn, Cu, Fe, and Mn solubility in H2O was detected. According to metal speciation, Pb and Zn were largely occluded in easily reducible manganese/iron oxides and trapped in the mineral structure of silicates, with no visible changes of distribution after plants. Water extraction always underestimated plant uptake, whereas DTPA and sequential extractions better predicted Pb and Zn uptake. Despite the original extremely low mobility and bioavailability of metal pollutants in both soil and post-washing sludges, the acidification and increase of organic carbon content induced by peat amendments and plant growth enhance the solubility in water of metal-containing compounds. Therefore, attention must be paid to these effects in the long period. A continuous monitoring of the changes of pollutant mobility-bioavailability induced by native plant revegetation of brownfields is crucial to prevent risks to the surrounding environment and human health.