Characterization of copper speciation on waste biomass of phytofiltration systems using energy dispersive Inelastic X-ray scattering

Background: Remediation of heavy metal-contaminated water using phytoremediation with accumulator aquatic plants is a promising low-cost emerging technology that adapts very well to the surrounding ecosystem. For the system to work efficiently, metal-saturated plants must be replaced, producing a potentially toxic amount of biomass that is usually stored dry to reduce its volume. The speciation of the high metal content in this biomass is crucial to define its final destination. This work explores the application of synchrotron-based EDIXS (Energy Dispersive Inelastic X-ray Scattering) to monitor the speciation of copper in regional aquatic plants from a laboratory-scale phytoremediation system.

Results: The phytofiltration system utilized Lemna minor L. and Salvinia biloba Raddi species grown under controlled conditions of light and nutrient availability. Both species are known hyperaccumulators of copper and are prevalent in lakes and rivers across South America. The validation of EDIXS was previously carried out by comparing the results of copper standard samples with those obtained by XANES. The findings revealed that both plant species retained copper in chemical complexes exhibiting octahedral coordination with a Cu valence of 2. Notably, differences emerged between the leaves and roots of Lemna minor L., suggesting a more pronounced adsorption of copper in its leaves, a trend that intensified with exposure. In opposite, for Salvinia the differences between leaves and roots suggests the presence of specific protective mechanisms to cope the copper exposure. Surprisingly, no significant dependence on copper concentration of the aqueous media was observed for either species.

Significance and novelty: These promising results endorse the viability of the proposed methodology in identifying the most effective fate of biomass generated in phytoremediation systems. EDIXS provides a valid tool for performing local copper speciation in aquatic plants with sufficient selectivity to identify subtle differences in various biological tissues. The simplicity of this methodology renders it a valuable tool for advancing our comprehension of metal speciation within waste biomass, thereby holding significant implications for the development of environmental remediation strategies.