Constructed wetland remediation has significant potential as a low-cost and low-maintenance water treatment technology. The high growth rates and yields of wetland species mean that constructed wetlands can also serve as a source of lignocellulosic biomass, which can be fractionated and processed into a wide range of useful fuels, chemicals, and materials. The work presented in this thesis investigates the potential for the widespread adoption of constructed wetlands for wastewater remediation, and for the valorisation of the generated biomass through an ionic liquid biorefinery.
Firstly, this work investigates the effect of varying the degree of substitution of the cation in aqueous alkylammonium hydrogen sulfate ionic liquids. A decreasing degree of substitution was found to significantly enhance solution acidity in a way that differed from addition of excess sulfuric acid. The effects of this acidity were tested for the ionic liquid pretreatment of Miscanthus, through which a promising new pretreatment solvent was identified. Methylbutylammonium hydrogen sulfate produced pulps with purities of up to 94%, in two-thirds of the time of current benchmark ionic liquids.
Secondly, a desk study was carried out to quantify the economic potential for wetland remediation, for the removal of organic and nutrient contamination, and for the remediation of Acid Mine Drainage, relative to conventional technologies. Constructed wetlands were found to be comparatively economically favourable at a wide range of conditions, particularly those with lower flow rates and/or contamination. Valorisation of the wetland biomass was found to be able to offset a significant proportion of total costs, but may require mixing with other feedstocks due to areal requirements.
Valorisation of wetland feedstocks through ionic liquid pretreatment was then investigated experimentally. Feedstock independence was validated by pretreating 7 common wetland species. Large improvements in digestibility and pulp purity were observed (to around 80% and 70% respectively), with low variability between species. These feedstocks’ high extractives content was found to increase the rate of pseudolignin formation, but this could be avoided through pre-extraction with ethanol and water. Recycling performance was found to decrease with each cycle, although this was offset to a certain degree through biomass pre-extraction and through addition of acid between cycles.
Finally, a novel fractionation process was suggested and developed using duckweed as a feedstock, allowing the full range of its promising features to be utilised. Ionic liquid interactions with pure starch were probed, allowing conditions amenable to starch processing to be determined. Pretreatment of duckweed with ionic liquids at mild conditions allowed near-quantitative sugar release to be achieved in 8 hours of hydrolysis, while a protein-enriched post-saccharification residue was also generated.