(357g) The Wetland Biorefinery Concept

Constructed wetlands (CWs) are a passive water remediation technology, offering a low-maintenance and often low-cost alternative to conventional remediation. This technology has been shown to effectively remove a range of contaminants, including metals, acidity, nutrients, pathogens, pesticides, and pharmaceuticals. [1]–[5] Many wetland species are fast-growing, with some of the most common species estimated to yield between around 10-30 tonnes/ha/y (dry basis), comparable to those of dedicated energy crops such as miscanthus or switchgrass. [6]–[9] Although there is currently no large-scale utilisation of CW crops, production of biofuels from CWs has been shown to offer a greater reduction in greenhouse gases than current biofuel production systems (including cellulosic and algal fuels). [10]
This study aims to comprehensively study the suitability of ionic liquid (IL) pretreatment for large-scale valorisation of biomass harvested from constructed wetlands. Timecourse pretreatment at 150oC were first carried out on Phragmites Australis, using three different ILs: DiMethylButylAmmonium Hydrogen Sulfate ([DMBA][HSO4]), MethylButylAmmonium Hydrogen Sulfate ([MBA][HSO4]), and DiMethylEthanolAmmonium Formate ([DMEtA][HCOO]). Behaviour between [MBA][HSO4] and [DMBA][HSO4] was similar, with extensive delignification and hemicellulose removal producing higher-purity cellulose pulps (up to 68 and 70%) with high saccharification yields (up to 88% and 78%). [DMEtA][HCOO] displayed much slower and more selective lignin removal, with some preservation of the hemicellulose fraction and correspondingly lower pulp purities (up to 48%). Peak saccharification was similar (79%), and total sugar release significantly higher due to preserved pentoses in the hemicellulose. Lignin analysis by GPC and HSQC spectroscopy revealed that lignin recovered using [DMEtA][HCOO] appeared to be less modified, with less condensation and cleavage of β-O-4 ether linkages.
Feedstock independence of the pretreatment process is vital for large-scale valorisation of CW biomass. A total of 11 feedstocks (made up of 8 species) were pretreated using [DMBA][HSO4] at two time points, to check for inter-species and intra-species variation in pretreatment performance. Wetland species were found to have fairly similar compositions, with notably high extractives contents (typically around 20% of dry mass) and relatively low lignin (around 20%). Saccharification yields of untreated material were higher than for most lignocellulosic feedstocks (10-20%), but were still significantly enhanced by pretreatment for 30 minutes at 150oC. Out of the 11 feedstocks tested, 9 showed saccharification yields above 69%. Significant pseudo-lignin formation was observed across feedstocks even at fairly short durations, with lignin yields exceeding delignification. This was attributed to high extractives content of the biomass, which was found to be strongly correlated with lignin yield. Only Pistia Stratiotes was found to be incompatible with acidic IL pretreatment, with very high glucan losses attributed to a high content of starch or amorphous cellulose.
In order to further demonstrate feedstock independence of this process, 9 feedstocks were combined to form a mixed feedstock, which was pretreated using [DMBA][HSO4] for 45 minutes at 150oC. When using fresh ionic liquid, pretreatment performance was close to the weighted average of the individual species. However, hot water extraction was found to significantly raise pulp purity, from 70% to 80%, due to a significant non-lignocellulosic fraction present on the pulp. A series of pretreatments were then carried out in which the IL was recycled between uses (achieving over 99% recovery in each cycle). Over 4 cycles, the pretreatment performance was found to decrease consistently, as delignification decreasing substantially. This was attributed to a significant drop in the acid/base ratio of the IL, with a replacement of the consumed acid between cycles expected to regenerate performance.

[1] R. H. Kadlec and S. D. Wallace, “Treatment wetlands,” Treat. Wetl., p. 965, 2009.
[2] J. Vymazal, “Constructed wetlands for treatment of industrial wastewaters: A review,” Ecol. Eng., vol. 73, pp. 724–751, 2014.
[3] A. S. Sheoran and V. Sheoran, “Heavy metal removal mechanism of acid mine drainage in wetlands: A critical review,” Miner. Eng., vol. 19, no. 2, pp. 105–116, 2006.
[4] J. Vymazal and T. Březinová, “The use of constructed wetlands for removal of pesticides from agricultural runoff and drainage: A review,” Environ. Int., vol. 75, pp. 11–20, 2015.
[5] J. Vymazal, “The use constructed wetlands with horizontal sub-surface flow for various types of wastewater,” Ecol. Eng., vol. 35, no. 1, pp. 1–17, 2009.
[6] S. Collura, B. Azambre, G. Finqueneisel, · Thierry, Z. · Jean, and V. Weber, “Miscanthus × Giganteus straw and pellets as sustainable fuels,” Env. Chem Lett, vol. 4, pp. 75–78, 2006.
[7] M. Khanna, B. Dhungana, and J. Clifton-Brown, “Costs of producing miscanthus and switchgrass for bioenergy in Illinois,” Biomass and Bioenergy, vol. 32, no. 6, pp. 482–493, Jun. 2008.
[8] J. Vymazal, L. Kröpfelová, J. Švehla, V. Chrastný, and J. Štíchová, “Trace elements in Phragmites australis growing in constructed wetlands for treatment of municipal wastewater,” Ecol. Eng., vol. 35, no. 2, pp. 303–309, 2009.
[9] S. J. McNaughton, “Ecotype Function in the Typha Community-Type,” Ecol. Monogr., vol. 36, no. 4, pp. 297–325, Feb. 1966.
[10] D. Liu et al., “Constructed wetlands as biofuel production systems,” Nat. Clim. Chang., p. 22, 2012.