Evaluation of alkali black liquor recycling for rice straw delignification and its effect on enzymatic saccharification

Abstract

Preceding bioethanol generation, the pretreatment of lignocellulosic biomass with alkali leads to the release of toxic effluents. In the light of growing environmental concerns, there is a strong need to minimize the pollution of water bodies and agricultural land through alternative strategies. Successive recycling of alkali black liquor (ABL), obtained from earlier pretreatment batch, may cut down the toxic effluents by many folds. The present study was conducted to evaluate the delignification performance of ABL for pretreatment of rice straw through recycling. Results indicated that instead of disposing ABL, it can be reused for at least up to five consecutive cycles. Furthermore, saccharification of ABL pretreated biomass using Cellic CTec2 was evaluated and morphological and chemical alterations were analyzed. Over the seven cycles, efficacy of delignification remained up to 60.36% and the enzymatic conversion of cellulose ranged between 99% (1–2 cycles) to 74% (7th cycle). Scanning electron micrographs indicated delignification of ABL pretreated rice straw samples. Crystallinity indices (CrI) of the ABL pretreated and enzyme treated biomass were found to change from 53.70% to 27.96% as compared to untreated rice straw (45.07%) and enzyme treated raw rice straw (42.16%). The present study suggests that the ABL could be used for pretreatment of rice straw, since no adverse effects were noticed on the delignification and saccharification due to ABL recycling.