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Abstract:

Phytomining, a plant based technology which uses the capacity of plants and their associated microorganisms to accumulate commercially valuable elements in their biomass, is an alternative for winning commercially valuable elements when conventional mining is economically not viable e.g. due to low concentrations of metals in ore bodies. These elements will be accumulated in above-ground compartments of the plants according to the plant’s capacity for uptake and distribution patterns. After harvesting the plants, the biomass can be utilized for energetic purposes, either by burning the phytomass or generating biogas via microbial fermentation. The classical elements used for phytomining usually occur at a few sites with high concentration, e.g. ultramafic soils, thus allowing high rates of accumulation in plants. An alternative approach is to use elements occurring ubiquitously in soils such as germanium (Ge) and rare earth elements (REEs), which are not essential for plant nutrition, but will be taken up by plants due to their chemical similarity with (essential) nutrients. Experiments in the greenhouse and in the field with several species of energy crops have shown high yields of Ge (up to 200 μg m^-2) in grass species (Hordeum vulgare, Panicum miliaceum, Phalaris arundinacea, Avena sativa, Zea mays), whereas REEs preferentially accumulated (up to 400 μg m^-2) in forbs (Lupinus albus, L. angustifolius, Fagopyrum esculentum, Brassica napus). However, a major limitation for the economic feasibility of phytomining in areas even with elevated soil concentrations of valuable elements is the low concentration of these elements which is usually found in the soil solution. Bioavailability of elements such as (heavy) metals and metalloids can be enhanced by processes in the rhizosphere, i.e. the root–soil interface with physicochemical properties of the soil being heavily modified by plant roots and their associated microorganisms. These processes include the exudation of metabolites from roots, causing many chemical reactions in the rhizosphere such as changes in pH and redox potential, sorption/desorption of cations and anions on/from soil colloids. Field experiments with mixed cultures of oat (Avena sativa) and white lupine (Lupinus albus) have shown elevated concentrations of, e.g., iron, lanthanium and neodyme both in the soil solution and in the above-ground biomass of oat. The elements accumulated in the plant biomass can be recovered e.g. by digesting the plant material via microbial fermentation, chemical agents or burning. Burning of biomass yields ashes, from which elements like Ge can be recovered via acidification and distillation with HCl. In the residues from the microbial fermentation concentrations of Ge and REEs were elevated by a factor of 100 compared to the original plant material. Furthermore, nutrients in the digestate can be used as fertilizer back in the field. In conclusion, the principal components of phytomining are (i) accumulation of target elements in plants, (ii) biomass yield for high rates of element and biogas harvesting, (iii) chelants and other root and microbial exudates to increase bioavailability of target elements, (iv) digestion of plant material, (v) extraction of target elements from digestates, (vi) fertilizer from fermentation residues or ashes used back in the field thereby closing material cycles.