Geochemical assessment of platinum group metals for phytomining

Evaluation of biogas production potential of trace element-contaminated plants via anaerobic digestion

Within the domain of phytoremediation research, the proper disposal of harvestable plant parts, that remove pollutants from contaminated soil, has been attracted extensive attention. Here, the bioenergy generation capability of trace metals (Cu, Pb, Zn, Cd, Mn, and As) polluted plants was assessed. The biogas production potential of accumulators or hyperaccumulator plants, Elsholtzia haichowensis, Sedum alfredii, Solanum nigrum, Phytolacca americana and Pteris vittata were 259.2 ± 1.9, 238.7 ± 4.2, 135.9 ± 0.9, 129.5 ± 2.9 and 106.8 ± 2.1 mL/g, respectively. The presence of Cu (at approximately 1000 mg/kg) increased the cumulative biogas production, the daily methane production and the methane yield of E. haichowensis. For S. alfredii, the presence of Zn (≥500 mg/kg) showed a significant negative impact on the methane content in biogas, and the daily methane production, which decreased the biogas and methane yield. The biogas production potential increased when the content of Mn was at 5 000-10,000 mg/kg, subsequently, decreased when the value of Mn at 20,000 mg/kg. However, Cd (1-200 mg/kg), Pb (125-2000 mg/kg) and As (1250-10,000 mg/kg) showed no distinctive change in the cumulative biogas production of S. nigrum, S. alfredii and P. vittata, respectively. The methane yield showed a strong positive correlation (R² =0.9704) with cumulative biogas production, and the energy potential of the plant residues were at 415-985 kWh/ton. Thus, the anaerobic digestion has bright potential for the disposal of trace metal contaminated plants, and has promising prospects for the use in energy production.