European plant researchers developed and tested under field conditions a method of biomass production on heavy metal contaminated (HMC) sites that simultaneously improves soil quality and function.
Studies show that plants grown as energy crops to produce biomass can also be used for removal of heavy metals from contaminated soil or securing further spread of these pollutants to soil, air and water. The EU-funded Phyto2Energy initiative developed an innovative concept for the phytoremediation of contaminated sites, combining and testing beneficial properties shown by energy crop species. Besides biomass production some can accumulate heavy metals in their above-ground parts, while others are able to grow on contaminated soils without absorbing heavy metals.
Although being a long-term process, phytoremediation allows the restoration of contaminated arable soil to even normal agricultural use, including food and feed crops. Phytostabilisation, meanwhile, reduces the mobility of substances in the environment, focusing on the long-term stabilisation and containment of the pollutant and is applied to heavily contaminated marginal land that would never be used for agricultural purposes. “However, thanks to the properties of some energy crop species resistant to the uptake of heavy metals this land could be used for ‘clean’ biomass production,” explains project coordinator Izabela Ratman-Klosinska, from the Institute for Ecology of Industrial Areas (IETU) in Katowice, Poland.
Heavy metal uptake
Researchers studied pre-selected energy crop species most suitable for phytoremediation-driven energy crops production: Miscanthus x giganteus, Sida hermaphrodita, Spartina pectinata and Panicum virgatum – at two field sites. The first field comprised arable land contaminated with heavy metals, while the second was a former sewage sludge dewatering area.
Results showed that organic matter content determined the bioavailability level of heavy metals and therefore the level of heavy metal uptake by energy species. “Irrespective of the site the most promising results in terms of the lowest metal uptake and the highest biomass production were found in Spartina pectinata,” claims Dr Marta Pogrzeba of IETU, who coordinated the plant investigations and field trials. “This makes it a candidate species for safe biomass production, even at heavily contaminated sites with high heavy metal bioavailability level,” she adds.
The highest lead uptake was determined to be for Miscanthus x giganteus, while Sida hermaphrodita had the highest cadmium and zinc uptake. These species also produced a satisfactory biomass yield, making them suitable candidates for phytoremediation-driven energy crop production.
Soil microbes increase biomass
Scientists also successfully demonstrated the potential of soil microbes to increase biomass yield, investigating over 140 bacterial strains isolated from the root system of the tested energy crops species. “Out of 140, researchers identified 3 bacterial strains from the Pseudomonas putida family, which after further genome sequencing proved to be unique in terms of facilitating plant growth and increasing resistance to heavy metals. These features made them promising candidates for a prototype biostimulant formula dedicated to energy crops,” states Professor Grażyna Płaza of IETU, responsible for coordinating the microbiological investigations.
In addition, the team investigated the safe conversion of biomass into energy using the gasification process. “We achieved an in-depth knowledge of the fuel characteristics, its gasification parameters, the fate of the contaminants and the properties of the produced end-products,” claims Dr Sebastian Werle from the Silesian University of Technology who led the gasification experiments. “The combined new knowledge creates a solid foundation for designing future installations appropriate for treating heavy metal contaminated biomass.”
Phyto2Energy results may benefit engineers working on large-scale phytoremediation projects, as well as projects related to land management. “The three selected P. putida bacterial strains may also be used as components of microbiological composite inoculants dedicated to stimulating the growth of plants, potentially finding wider application in industrially orientated metabolic engineering,” Prof. Plaza concludes.