Abstract
Pretreatments are often needed for lignocellulosic biomass feedstocks before either thermochemical or biochemical conversion processes. Our previous research has demonstrated the potential of bioleaching, with its superior capability of removing certain inorganic compounds compared to water leaching, to improve biomass quality for thermochemical conversion in biofuel production. In this study, the bioleaching process was scaled up from 250 mL beakers to be carried out in custom-designed 2.5 L bioreactors. The fungus Aspergillus niger was used in the bioreactors for leaching sorghum straw biomass with an initial ash content of 6.0%. The effects of three operating parameters on leaching efficiency (i.e., residual ash content) were extensively studied, including the fungal mass added to each reactor, leaching time, and glucose concentration in the starting liquid phase. Response surface methodology (RSM) was used for the experiment design. The results showed that the average residual ash content of the sorghum feedstock after bioleaching was significantly lower (3.63 ± 0.19%) than that of the ash content (4.72 ± 0.13%) after water leaching (p < 0.00001). Among the three parameters, glucose concentration in the starting liquid phase had the most significant effect on leaching effectiveness (p = 0.0079). Based on this outcome, subsequent bioleaching experiments yielded reductions in residual ash content to as low as 2.73%.
Highlights
The same settings were used as in bioleaching, except: no fungal biomass was added; de-ionized water (DI) water was added as a starting liquid phase; and the leaching time was 48 h, as we found that longer leaching time with water did not yield further ash removal
This study focused on the bioleaching pretreatment of sorghum straws in self-designed bioreactors
Our bioleaching process was more effective in reducing the ash content of the sorghum straw (3.63%) compared to the water leaching process (4.72%)
Summary
To improve the quality of lignocellulosic biomass as a potential fuel, a pretreatment known as leaching has been extensively studied and proven effective in removing inorganic ingredients in the biomass [4]. The most common pretreatment is water leaching, which washes the biomass with an adequate amount of water [5]. While this is effective in removing certain inorganic ingredients that are water-soluble, more effective methods are necessary to reduce elements such as Ca, Mg, Si, and S that tend to form insoluble compounds in the plant [6].
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