Effects of post-washing on pretreated biomass and hydrolysis of the mixture of acetic acid and sodium hydroxide pretreated biomass and their mixed filtrate
Introduction
Physicochemical pretreatment is a crucial stage for biomass-to-bioenergy production to render cellulose and hemicellulose more accessible to enzymatic and microbial digestion. Dilute acid and alkali pretreatments have been highly preferred due to the low cost of chemical reagents and simple operating procedures (Kim et al., 2016, Solarte-Toro et al., 2019). However, extreme post-washing is typically applied to alleviate the pH and toxicity of pretreated biomass with the resulting filtrate being discarded finally (Li et al., 2016, Skiba et al., 2017, Sritrakul et al., 2017, Wen et al., 2019), which inevitably results in massive water consumption and chemical (such as acid, alkali, sugars, and lignin) loss. Therefore, it will increase the cost of production and the problem of environmental pollution.
For acid pretreatment, physical and chemical treatments such as membrane filtration, ion exchange, and biochar adsorption were used to make the pretreated filtrate or pre-hydrolysate amenable to microbial digestion (Fayet et al., 2018, Kumar et al., 2020, Lee and Park, 2016, Pan et al., 2019, Roque et al., 2019, Sarawan et al., 2019, Zhu et al., 2015). However, there are several huge challenges for the commercialization: (1) selectively separating low-molecular-weight derivatives [such as acetic acid (HOAc), furfural, and 5-hydroxymethylfurfural (HMF)] from high-molecular-weight sugars via membrane filtration is inefficient to conduct (Pan et al., 2019); (2) extra chemicals input is unavoidable for ion exchange and biochar adsorption (Bhatia et al., 2020, Kim, 2018, Kumar et al., 2020); and (3) concentrated alkali is required to neutralize the residual acids in the filtrate regardless of which strategies are applied. For alkali pretreatment, sequential recycling of black liquor for biomass pretreatment has been encouraged (Cha et al., 2016, Goshadrou, 2019, Li et al., 2015, Wang et al., 2016, Zhou et al., 2019). Seemingly, it has a great potential to reduce water consumption and chemical discharge, but the accumulation of inhibitors and dilution of alkali concentration in recycled black liquor can increase the difficulty of detoxification and reduce the pretreatment effectiveness. This has been evidenced by extreme post-washing by water and a progressive reduction in sugar yield (Rocha et al., 2014, Zhou et al., 2019). Also, the resulting black liquor was commonly abandoned (Alencar et al., 2017, Cha et al., 2016). In this regard, our previous study has demonstrated that directly hydrolyzing the mixture of HOAc and NaOH pretreated filtrate and biomass without post-washing of the pretreated biomass can significantly reduce water consumption. Additionally, the NaOH soluble lignin exhibiting the nearly identical features with the commercial alkali lignin can be recovered (Zhao et al., 2021).
Solid loading is a critical factor affecting sugar conversion efficiency of enzymatic hydrolysis of pretreated biomass (Modenbach and Nokes, 2013). Low solid loading has been generally applied to promote pretreatment technology with higher sugar yield. However, maintaining high-solid enzymatic hydrolysis is highly preferred to reduce water and enzyme consumption (Chen and Liu, 2017). Whether the higher solid loading used for enzymatic hydrolysis is better? A previous study reported that glucose yield had a negatively linear correlation with solid loading (Kristensen et al., 2009). Therefore, the decrease in glucose yield can offset the advantages of enzymatic hydrolysis conducted at high solid loading. Exploring the turning point of solid loading can not only avoid a random choice of solid loading used for enzymatic hydrolysis but also reduce extra enzymes and chemicals input.
In this study, hemp and poplar biomass were first pretreated by HOAc and sodium hydroxide (NaOH) parallelly and washed by one-volume water (I-VW) or two-volume water (II-VW). After solid and liquid separation, their pretreated biomass and filtrate were combined, respectively. The mixing process is able to promote the neutralization reaction of residual HOAc and NaOH in the pretreated biomass and filtrate to form sodium acetate (Zhao et al., 2021). The mixed filtrate was filtered to recover the NaOH soluble lignin partially and then used as a buffer for enzymatic hydrolysis of the mixed biomass. Effects of post-washing on the pretreated biomass were illuminated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) in terms of microstructural characteristics, crystallinity, and chemical linkage, respectively. Compositional analysis of solid and filtrate as well as solid and sugar recoveries and lignin removal with different post-washing were offered. The recovered lignin was compared with the commercial alkali lignin. Moreover, the relationships between sugar concentration/conversion efficiency and solid loading were determined and can be used to boost sugar concentration.
Section snippets
Materials
Industrial hemp and poplar biomass were initially ground by an SM 2000 cutting mill (Restsch Inc. Newton, PA) and then pulverized to achieve particle sizes approximately lower than 2 mm (Zhao et al., 2021). Cellulase (Cellic® CTec3, 516 mg protein/mL) and hemicellulose (NS22244, 266 mg protein/mL) were supplied by Novozymes (Franklinton, NC). The commercial alkali lignin (370959-100G) was bought from Sigma-Aldrich chemicals company (St. Louis, MO).
Biomass pretreatment
The pretreatment procedures and conditions can
Effect of post-washing on the composition of pretreated biomass
Principal components, including glucan, xylan, and lignin in raw and pretreated biomass, are shown in Fig. 1. It was observed that post-washing inhibited the recondensation of the solubilized lignin to the surface of cellulose and hemicellulose, which was evidenced by that II-VW washed biomass had lower lignin contents than I-VW washed biomass [hemp: 31.63 vs. 33.13% for HOAc and 7.51 vs. 8.94% for NaOH (Fig. 1A); poplar: 31.63 vs. 33.13% for HOAc and 7.51 vs. 8.94% for NaOH (Fig. 1B)].
Conclusions
Effects of post-washing pretreated biomass on pretreatment effectiveness were illuminated. Post-washing was able to alter the composition of pretreated biomass through removing extractives and lignin units, increasing glucan content, lignin removal, and CrI. Mixing HOAc and NaOH pretreated filtrates led to the precipitation of solubilized lignin. Lignin recovery decreased with post-washing increased from I-VW to II-VW, but their FTIR characteristics were still comparable with the commercial
CRediT authorship contribution statement
Jikai Zhao: Conceptualization, Data curation, Formal analysis, Writing - review & editing. Yang Yang: Investigation, Methodology. Meng Zhang: Supervision. Donghai Wang: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This publication is contribution no. 21-270-J from the Kansas Agricultural Experiment Station, Manhattan, KS, USA.
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