Beneficial and detrimental effects of choline chloride-oxalic acid deep eutectic solvent on biogas production

Deep eutectic solvents (DES), a new class of alternative solvents, have recently been used in the pre-treatment of lignocellulosic biomass. Due to the ability to dissolve phenolic compounds, they have been efficiently applied as delignification agents. However, to extend DES application to bioprocesses, such as Anaerobic Digestion (AD), their toxicity to microbial consortia must be evaluated. In this work, an effective delignifying DES, composed of choline chloride (ChCl) and oxalic acid (OA) (1:1) was prepared and its effect evaluated, for the first time, in biogas production. Results show that the presence of DES had both beneficial and detrimental effects on the anaerobic consortium, depending on its concentration. In the concentration range of 0.3-12.5 g/L, the presence of DES led to a lag-phase of 1 to 8 d as the DES concentration increased. However, after the lag-phase has been surpassed, DES up to a concentration of 12.5 g/L improved the biogas production, reaching an accumulated biogas volume three times higher than the control assay for the concentration of 12.5 g/L. For the highest DES concentrations (19.8-78.1 g/L), the biogas production was inhibited. The assays performed with DES components alone have indicated that OA at 3.2 g/L was the main responsible for the inhibition of biogas production (50% less biogas produced than the control). ChCl at 4.9 g/L has not presented a lag-phase and produced an accumulated biogas volume like the control assay (1200 mL for 30 d incubation). This work points out that ChCl:OA DES may be used in the delignification of biomass further submitted to AD, provided the inhibitory concentrations of OA are not achieved.

The effects of solid lignin on the anaerobic digestion of microcrystalline cellulose and application of smoothing splines for extended data analysis of its inhibitory effects

Lignocellulose is an abundant substrate for biogas production; however, for efficient utilization, proper pre-treatment is required to enhance the biomethane yield and hydrolysis rate significantly. Phenolic compounds from dissolved lignin, produced during alkali pre-treatment, have inhibitory effects on the anaerobic digestion; however, the possible inhibitory effects of solid lignin have not gathered enough interest. Especially, the effect of solid lignin on methanogenesis remains a knowledge gap. In this study, kraft lignin was used as a model solid lignin substrate for its co-digestion with microcrystalline cellulose. A new approach of modelling biomethane production curves using smoothing splines was developed to describe the long-term inhibitory effects of solid lignin on hydrolysis and methanogenesis. The method gives possibility to describe long-term inhibitory effects by using batch instead of continuous test data. Results revealed that kraft lignin showed mild inhibitory effects on methanogens. However lignin impact combined with volatile fatty accumulation can prolong hydrolysis and reactor recovery start-up by 47.3% and 75.3%, respectively. For small dosages of solid lignin adaptation of methanogens is possible.

Water-Based Synthesis of Hydrophobic Ionic Liquids [N8888][oleate] and [P666,14][oleate] and their Bioprocess Compatibility

The conversion of organic waste streams into carboxylic acids as renewable feedstocks results in relatively dilute aqueous streams. Carboxylic acids can be recovered from such streams by using liquid-liquid extraction. Hydrophobic ionic liquids (ILs) are novel extractants that can be used for carboxylic acid recovery. To integrate these ILs as in situ extractants in several biotechnological applications, the IL must be compatible with the bioprocesses. Herein the ILs [P666,14][oleate] and [N8888][oleate] were synthesized in water and their bioprocess compatibility was assessed by temporary exposure to an aqueous phase that contained methanogenic granular sludge. After transfer of the sludge into fresh medium, [P666,14][oleate]-exposed granules were completely inhibited. Granules exposed to [N8888][oleate] sustained anaerobic digestion activity, albeit moderately reduced. The IL contaminants, bromide (5-500 ppm) and oleate (10-4000 ppm), were shown not to inhibit the methanogenic conversion of acetate. [P666,14] was identified as a bioprocess-incompatible component. However, our results showed that [N8888][oleate] was bioprocess compatible and, therefore, has potential applications in bioprocesses.

Enhancement of anaerobic digestion of grass by pretreatment with imidazolium-based ionic liquids

In this work, the toxicity of imidazolium-based ionic liquids (ILs) and the enhancement of high-solid anaerobic digestion by pretreatment were studied. Compared with [Bmim]Cl, [Bmim]OAc and [Bmim]BF₄, [Bmim]PF₆ had the highest toxicity. When the mass ratio of [Bmim]PF₆ to grass was higher than 1:10, biogas was not produced within 30 days. The ability to remove lignin and hemicellulose followed the sequence of [Bmim]OAc, [Bmim]Cl, [Bmim]BF₄ and [Bmim]PF₆. The crystallinity index of grass pretreated with [Bmim]OAc, [Bmim]Cl, [Bmim]BF₄ and [Bmim]PF₆ reduced by 73.83%, 54.44%, 17.52% and 7.47%, respectively. The pretreatment with ILs enhanced the methane yield of grass by reducing crystallinity and particle size. The grass pretreated with [Bmim]OAc had the highest methane yield, about 221 mL/g volatile solids, due to its good lignin removal ability and relative low toxicity. After 10 times recycling, the cumulative methane yield of grass pretreated by recycled [Bmim]OAc decreased by 11.95%.