Combining phytoremediation with bioenergy production; exploring options for sustainable remediation

Uncontrolled metal deposition in soil constitutes a serious concern for the environment due to associated risks of metal toxicity to the biota and consequently to humans. Since metals are non-biodegradable, mitigating exposure risks is reliant on either removing metals from soil, or altering their speciation in ways where their bioavailability and mobility is reduced to safer levels. The use of plants to extract metal contaminants from soil has been proposed as a cost-effective means of remediation, especially when augmented with plant growth promoting bacteria and utilizing energy crops for this process is a useful way of attaining sustainable added value from the process. The focus of this research was to examine phytoremediation as a sustainable biotechnology to remediate metal-contaminated soil, generate bioenergy and to explore the potential of using its by-products for contaminant stabilization and as adsorbents.
A multicriteria decision analysis, based on relevant criteria and key performance indicators was used to uniquely develop a mechanism for selecting plant species that satisfies the suitability criteria for both phytoremediation and biomass valorisation and silvergrass and sunflower emerged as the top performers as they incorporate important features beneficial for phytoremediation and bioenergy production. Greenhouse phytoextraction studies were carried out using sunflower plants in pots and the effect of plant growth promoting bacteria, Bacillus aryabhattai on growth and phytoextraction effectiveness was investigated. Sunflower plants were found to be largely effective in accumulating metal contaminants into its aboveground tissues (with bioconcentration factor and translocation factor ranging from 0.81 – 0.94), and this was enhanced significantly by the application of plant growth promoting bacteria, Bacillus aryabhattai (with bioconcetration and translocation factor > 1 for all metals, thus attaining hyperaccumulator status). Metal-rich post-remediation sunflower residues had calorific values ranging from 17.01 to 18.04 MJ/kg and these were converted thermochemically via pyrolysis producing an estimated 22.3 % bio-oil yield free of metal contaminants and biochars (51.6% yield) and the speciation of metals in biochar matrix was analysed. Speciation studies showed that about 73.69% - 86.04% of the metals were stably stored in the non-bioavailable F3 and F4 fractions of the biochar matrix following pyrolysis, thus significantly reducing their bioavailability and mobility. Metal-rich sunflower-derived biochar were further utilized to perform column and batch experiments to ascertain the feasibility of further attaining practical adsorbent-based remediation from metal contaminated aqueous solution using post-remediation sunflower biochar. The metal-enriched biochar was demonstrated to be highly effective adsorbent for the removal of metal contaminants in aqueous solution (91.66 – 93.67% removal in mono-metal conditions and 81 – 88.1% removal in multi-metals condition) in the order Pb > Cd > Zn. Phytoremediation offers a less intrusive, environmentally sustainable technology option for contaminant control and when combined with energy production, it opens opportunities to attain society’s economic and environmental goals in a sustainable manner. By-products attained from the process like biochars can potentially offer practical application in contaminant risk management schemes and soil improvement technologies.