Energy production from heavy-metal-contaminated biomass obtained from phytoremediation

Phytoremediation is an eco-friendly and cost-effective approach to remediate the heavy metal(loid) (HMs) polluted lands and water bodies. The contaminants can be absorbed by vegetation on site, and be removed by harvesting the plants. While extensive efforts have been made to improve the phytoremediation efficiency, the sustainability of this technology is questioned, due to the problematic disposal of the heavy metal enriched biomass (HMEB). This work uses Avicennia marina biomass collected after remediation of lead and zinc contaminated soil as an energy resource, investing its potential to produce clean and value-added bio-products while preventing emissions of secondary contaminants.

To develop the fundamentals of the phytoremediation-pyrolysis scheme, five basic aspects during pyrolysis process were investigated in this work: i) role of HMs during the pyrolysis process; ii) transformation of HMs during pyrolysis; iii) recovery of HMs in the end-products at the end of the pyrolysis process; iv) chemical speciation of HMs in biochars; v) properties of pyrolytic products and their potential application/upgrading techniques. Furthermore, certain biomass engineering methods were applied to modify the HM deportment (i.e. distribution and mobility) and pyrolysis product properties, in order to sequestrate the HMs in biochars, while improve the product qualities.

The main contributions of this thesis are: a state-of-the-art of the pyrolysis of heavy-metal enriched biomass obtained from a phytoextraction process (Chapter 2); a preliminary study on the pyrolysis of HMEB to characterise the pyrolysis products (Chapter 3); a characterisation study on the biomass and pyrolytic products from pyrolysis of heavy metal contaminated and un-contaminated biomass at varying temperatures (Chapter 4); a study on the impact of pyrolysis temperatures on HM deportment during pyrolysis of HMCB (Chapter 5); a study on the impact of co-pyrolysis of HMCB with magnesium carbonate on HM deportment and pyrolytic product properties (Chapter 6); a characteristic study on the biomass and pyrolytic products from phosphate pre-treated HMCB (Chapter 7); a study on HM deportment and pyrolytic product properties from ferric salt pre-treated HMCB (Chapter 8). The results show that the pyrolysis temperature drastically affects the HM recovery and mobility in pyrolytic products. Higher temperatures promote volatilisation of HMs into gaseous products, while reducing the mobility of HMs in biochars. The volatile pattern of each HM varies with temperature and pyrolysis conditions. For example, Cd in biomass is highly volatile, while Fe and Cu are non-volatile elements. The presence of HMs in biomass can catalyse the pyrolysis process, producing more hydrocarbons in bio-oils compared to un-contaminated biomass. The engineering of HMCB makes noticeable changes on pyrolytic product properties and HM deportment. For example, co-pyrolysis of HMCB with magnesium carbonate enhances the HM sequestration in solid phase significantly. The impregnation of ferric salts into HMCB can catalyse the pyrolysis process, as well as reduce the mobility and bioavailability of HMs in biochars.