Biological soil remediation: phytoremediation with plants and their associated microbes
Mineral-dissolving bacteria can be used alongside plants as natural soil conditioners. For example, they can be used to remove cadmium. The principle is based on the ability of bacteria to break up cadmium-containing soil particles; the released cadmium is then taken up by the plants and removed as the plants are pruned and disposed of.
Geology meets biology meets ecology: scientists led by Prof. Dr. Andreas Kappler at the University of Tübingen are investigating how cadmium and other metal compounds that are toxic to humans and animals can be removed from soil. Rather than using complex chemical and mechanical methods for removing the metal contaminations, the researchers use natural helpers, i.e. bacteria and plants. Amongst other things, phytoremediation has the advantage that the soil structure remains intact and the natural soil fauna is not destroyed. The German Environmental Foundation (DBU) is funding a research project at the University of Tübingen specifically focussed on the removal of cadmium from soil.
Biologist Eva Marie Mühe, who is doing her doctoral thesis in Kappler’s laboratory, explains: “Cadmium has become a problem in Central Europe. Once introduced into the soil, through fertiliser for example, cadmium cannot be degraded naturally and can pass through the food chain into the human organism where it is integrated into bone in place of calcium, resulting in brittle bones. In addition, cadmium is carcinogenic.” So the question is why phosphate fertiliser contains ever increasing quantities of cadmium. Kappler explains that the worrying development is somehow related to phosphate, which is important for plants and people and hence an important part of fertilisers. "The sale of phosphate fertilisers is decreasing around the world. The quality is also decreasing due to the presence of cadmium and other metal contaminants. Metals bind to phosphate very well, in the case of both iron and cadmium. Moreover, cadmium is an impurity in the phosphate rock used to manufacture fertilisers."
Plants can withdraw metal ions from soil
This is why researchers are looking for effective, environmentally friendly and inexpensive methods that can be used to remove cadmium from soils once it is in there. Cadmium is not present as pure metal, but as a charged ion, for example bound to iron(III)-containing oxidic minerals which we all know as rust.
And that's what provides the Tübingen researchers with a starting point for their idea of removing cadmium from the soil: there are bacteria with a particularly high tolerance to cadmium, and these bacteria can solubilise cadmium. Geobacter metallireducens is one such bacterium, and its name speaks volumes. “These organisms dissolve the rusty iron(III) minerals, resulting in the mobilisation of cadmium,” explains Kappler.
Phytoremediation involves the use of plants and their associated microbes for the treatment of environmental pollution. “Studies have shown that plants that are true hyperaccumulators are extremely tolerant to cadmium and efficient in taking up cadmium and storing it in large quantities in their leaves. The plant Arabidopsis halleri, which is a relative of Arabidopsis thaliana, is one such hyperaccumulator,” said Kappler. Researchers have found out that the small plant is able to actively mobilise cadmium from the soil. “The plant releases organic molecules that are able to attract microorganisms such as Geobacter,” said Kappler.
Cadmium has the potential to become a valuable material
The cadmium-containing plants are harvested and instead of being thrown away, they are increasingly fed into metal-waste recycling streams. Cadmium is a precious metal that is used by the semiconductor industry for the manufacture of thin-film solar cells as well as in batteries and accumulators. Basically, it is an economic issue. If the need for such applications continues to increase, it is worthwhile recycling the cadmium from the plants or from their combustion residues. Similar ideas already exist for other metals. Researchers have shown that plants can be used to extract and recycle zinc from soil. Since the phytoremediation of cadmium is still quite time-consuming and not always efficient, researchers are considering other alternatives. “The second alternative is to immobilise cadmium in soil, transfer it into stable compounds that can no longer do any damage,” said Kappler. Cadmium ions can react with carbonate ions and are precipitated as cadmium carbonate. “This iron-calcium carbonate mixture is solid and cannot be taken up by plants,” the researcher said.
However, the whole endeavour has the specific drawback that carbonates easily dissolve in an acidic environment. However, Kappler also has a solution to this. How about adding nucleation centres? Magnetites, i.e. strongly magnetic iron oxides, would then form and bind to cadmium. “They are up to ten to fifteen times less soluble than carbonates. You do not need a strong acid, even lemon juice is acidic enough to dissolve carbonates; six molar hydrochloric acid and a temperature of 70°C is required for dissolving magnetite.”
The use of bacteria is another possibility for producing magnetites, and a very smart one at that. Iron-reducing and iron-oxidising bacteria that produce magnetite could be used for this purpose. This “bioproduction” has the huge advantage that the magnetite crystals do not aggregate. “The surface of biomagnetite is negatively charged and this prevents the molecules from aggregating. The negative charge comes from bacterial surface proteins such as phospholipids and lipopolysaccharides,” explains Kappler. Working with researchers from other disciplines opens up possibilities for developing useful concepts. However, not all are suitable for practical application, which heavily depends on the composition and micro- and macroclimate of the soil.
University of Tübingen
Centre for Applied Biosciences
Prof. Dr. Andreas Kappler
Tel.: +49 (7)071 29-74992