Recycling of nitric acid solution in chemical pretreatment of oat hulls for biorefining

Abstract

The pretreatment of lignocellulosic bioresources is a key step in obtaining fermentable sugars. The present study explored the recycling of an HNO3 cooking solution during one-step chemical pretreatment of oat hulls and examined the enzymatic hydrolysis performance of the resultant substrates. Two series of experiments on solution recycling were performed: stabilization and experimental elevation of HNO3 concentration by adding 70 wt% HNO3. When the HNO3 concentration was stabilized at 4.4 wt%, the solution could be used five times. Meanwhile, when the HNO3 concentration was experimentally elevated from 4.4 wt% to 5.6 wt% (at the tenth pretreatment) by adding 70 wt% HNO3, the cooking solution could be used ten times. The yield of reducing sugars on a hydrolyzables content basis was 95% to 99% for ten substrates. In this case, the lignin contents of the substrates did not increase and had no adverse effect on the enzymatic hydrolysis performance.