Surface soil-dust contamination of Phalaris arundinacea grown on former lead mine sites: implications for biomass use, phytoremediation and phytomanagement

This study evaluated the contribution of soil dust deposited on the surface of reed canary grass (Phalaris arundinacea) grown on historic lead (Pb) mine sites to the overall contamination of the biomass, with implications for phytoremediation, valorization and utilization. By applying a novel combination of imaging of plant material using X-ray computed tomography (XCT) and scanning electron microscopy (SEM), with washing experiments and bulk analysis, the research aimed to distinguish between (a) Pb uptake through biological processes (phyto-extraction), and (b) surficial dust and physical entrapment of Pb-rich dust on plants cultivated in contaminated soils (surface-contamination). The study established the presence and distribution of Pb-rich particles, which were difficult to remove even by means of sequential washing in 1 M hydrochloric acid and surfactant. Analysis confirmed that the majority of Pb contamination was due to dust, but with significant levels remaining even after intense washing. This questions the effectiveness of phytoremediation in reducing bioavailable soil Pb levels through phyto-extraction, compared to achieving mechanical stabilization or reducing dispersion during phyto-management, and may represent a challenge to the viability of subsequent processing and use of the biomass product. Site-specific variations in contamination levels were observed, underscoring the influence of both local environmental conditions and plant morphology on dust accumulation. These observations highlight the necessity for standardized washing protocols to be adopted and for better reporting of the actual washing methods used in phytoremediation research, so as to correctly assess levels of contaminant uptake and actual remediation. The conclusion is that residual surficial dust contamination of biomass may have been overlooked in many previous phytoremediation studies and as a consequence the reported phyto-extraction potential has been over-estimated.