Applicability of alfalfa and goldenrod residues after supercritical CO2 extraction to plant micronutrient biosorption and renewable energy production

Abstract

The residues of alfalfa and goldenrod, after supercritical CO2 extraction, were examined against raw plant materials to determine the capacity to carry micronutrients and energy production.Materials were enriched with Cu(II), Mn(II) and Zn(II), via biosorption and subjected to multielemental analysis and determination of nitrogen. The materials showed a high capacity for Cu(II)>Zn(II) > Mn(II), whose content increased by 104–1262 and 54–2460 times in post- and prior-extraction samples, respectively. The goldenrod residues provided the best results for Mn(II) (14,000 mg/kg) and Zn(II) (18,500 mg/kg), while alfalfa residues bound more Cu(II) (13,800 mg/kg). Materials meet the EU regulation requirements for organo-mineral fertilisers.During thermal breakdown, two combustion steps lead to 83–85% weight loss. The thermal response included two exothermic effects at 120–400 °C and 400–620 °C releasing the energy of 3.4–4.5 μV/mg and 6.1–7.0 μV/mg, respectively. Alfalfa matrices provided stronger thermal effects. The materials emit H2O, CO2 and SO2. Extraction caused an increase and a slight decrease of the energy released by alfalfa during the first and second combustion steps, respectively.The residues contained less water than the raw materials and could be applied as solid biofuel or as combustion promoters. Laboratory-scale experiments proved that the residues after extraction were suitable in both applications.