CONCENTRATION OF SELECTED PLANT NUTRIENTS AND TARGET ELEMENTS FOR PHYTOREMEDIATION AND PHYTOMINING IN BIOGAS DIGESTATE

Germanium (Ge) and rare earth elements (REEs) are of increasing interest in phytoremediation and phytomining research. These elements are present in almost all soils and soil-grown plants contain considerable concentrations of these elements in their biomass. The process chain of phytomining involves i) the accumulation of target elements in harvestable plant biomass (phytoextraction), ii) production of bioenergy by burning or biogas production, and iii) the recovery of the elements from bioenergy residues. Additionally, the use of ashes and bio-digestion waste as substitutes for synthetic fertilizers offers a great chance to close the nutrient cycle. However, current research in phytomining mostly focuses on the optimization of the phytoextraction process and there is very little information available in the literature on the chemical behavior of target elements for phytomining during the process of energy production. Therefore, the aim of the present study was to assess the concentrations of selected plant nutrients (P, Fe, Mn, Zn, Cu, Ni, Mo, Co) and target elements in phytoremediation (As, Cd, Pb, Cr) and phytomining (Ge, REEs) in digestates from anaerobic fermentation and evaluate the effect of the fermentation process on the enrichment of the elements. In batch reactor experiments shoot biomass of the energy crop Phalaris arundinacea was anaerobically fermented. Major and trace element concentrations in the initial mixture of the plant material and inoculum as well as the resulting digestates were determined by ICP-MS. Concentrations of macro and micro nutrients in the digestates decreased in the order P > Fe > Zn > Mn > Cu > Ni > Mo > Co. The concentrations of potentially toxic trace elements were highest for Pb and decreased in the order Pb > Cr > As > Cd. The digestates contained considerable concentrations of REEs and Ge. Specifically, concentrations of REEs were higher than most of the toxic elements except of Pb. The fermentation process led to a significant increase of Zn, Pb, As, REEs and Ge while P, Fe, Mn, Mo and Cd were not affected. For Ni and Cr we observed a significant depletion of the digestates. Most probably the increasing concentrations result from carbon losses during microbial decomposition of the biomass and consequently an enrichment of the elements relative to the organic matrix. These results clearly show that optimization of the biogas production process offers a great opportunity to enhance the efficiency of the process chain of phytomining; however, the processes involved need to be understood and remain field for further studies.