Strategies for the use of plant biomass obtained in the phytostabilisation of trace-element-contaminated soils

Abstract

The recycling options of the plant biomass produced during the phytoremediation of contaminated soils include the production of renewable energy (such as biogas or thermal energy) and also of useful products such as compost. The presence of trace elements (TEs) and the nature of the organic matter can affect the biodegradability of the biomass. The biomass of four plant species, Silybum marianum, Piptatherum miliaceum, Nicotiana glauca and Helianthus annuus, grown in a pot experiment for the phytostabilisation of a TEs contaminated soil was studied for biodegradability by biological aerobic and anaerobic transformation and for thermal energy production by thermal analysis and differential scanning calorimetry. According to the aerobic degradation results, all these plant materials are adequate for recycling by composting, with S. marianum being the least recommended due to the lowest potentially-mineralisable carbon. However, S. marianum was suitable for biogas production through anaerobic digestion, as together with P. miliaceum it showed the best results for biogas production potential, whereas, N. glauca gave low biogas production, related to its high Pb, lignin and VS concentrations and C/N ratio. The species most suitable for thermal energy purposes were again S. marianum and P. miliaceum, since both exhibited high energy release in the high-temperature regions of the thermal profile. Contrastingly, H. annuus and N. glauca produced high losses of mass at temperatures below 200 °C, which may indicate flammability risks during handling and storage.