Uptake, Distribution and Speciation of Heavy Metals within Miscanthus Grown in Contaminated Soils

This project assessed uptake of Zn and Pb by a novel Miscanthus hybrid grown in contaminated soils. Data from a young plants grown as part of a pot-trial under controlled conditions were compared with those from biomass derived from a long-term field trial. Sequential extractions combined with analysis of X-ray Absorption Fine Structures (XAFS) were used to characterise the bioavailable Zn and Pb within the soil at the field trail site. These data were used to engineer a suite of different soil conditions for the pot-trial. The pot-trial soils encompassed bioavailable Zn and Pb comparable to that at the field-trial location and a range of incrementally increasing contamination levels. Both Zn and Pb uptake by the plant increased in proportion to their concentrations within the soil. Retention of both Zn and Pb differed between leaf and stem structures, and within mature biomass compared to juvenile plants. XAFS analysis of the biomass revealed different Zn species within stems and leaves, and differences between growth phase and the mature biomass at time of harvest. Sulfur complexes were the dominant Zn species in juvenile plant leaves, with a minority contribution from octahedral O/N complexes. Sulfur complexes were also prevalent in stems from juvenile plants, but predominant O/N speciation shifted towards more tetrahedral O/N coordination. In contrast, the biomass crop was typified by tetrahedral O/N complexes. X-ray Fluorescence (XRF) imaging identified that Zn retention in the plants displayed greater abundance within vascular bundles, in particular around phloem tissues. Despite S complexes being the predominant form of Zn sequestration, signal intensity within XRF maps demonstrated that location of peak S and Zn concentration were not spatially correlated proving a degree of mutual independence. Microprobe analysis (µXAFS) of Zn speciation revealed that tetrahedral O/N complexes significantly increased at locations of highest Zn concentration.