Numerous pretreatment technologies such as acid-based pretreatments7 alkaline-based pretreatments8 organosolv pretreatments9 ionic liquid pretreatments10 and physically assisted chemical pretreatments11 have been explored to remove lignin, break resistance and depolymerize cellulose for fermentable sugar production.

Sulphite pretreatment is a traditional route in the pulp industry for papermaking; it can be performed over a wide pH and temperature range, as described in a textbook.12 The goal of pulping is to remove lignin as much as possible without simultaneous loss and degradation of hemicellulose and cellulose; this would result in a pulp with high yield and strength.13 The sulfite process was also used to pretreat wood chips,12 and it was first used for softwoods (spruce and red pine) by enzymatic saccharification. The result of the study showed that the sulfite-treated softwood chips could be significantly softened and the enzymatic cellulose conversion yield of over 90% was achieved.13 Then, alkaline sulfite or acid sulfite was used for the pretreatment of other lignocellulosic biomass, such as bamboo14 and switchgrass.15 Im Pretreatment process can be the active reagents sulfite (SO32−), bisulfite (HSO3−) or a combination of two of the three reagents sulfite (SO32−), bisulfite (HSO3−) and sulfur dioxide (SO2 or H2SO3) depending on the pH of the pretreatment liquor at pretreatment temperature.16

Ammonium sulfite as a kind of neutral sulfite has long been used for papermaking17 and can be easily decomposed into ammonia and sulfite at around 70 °C. Therefore, the effects of pretreatment of ammonium sulfite and sodium sulfite can be significantly different. Ammonium sulfite can show the effects of both ammonia and sulfite on the pretreated bamboo. However, few studies have been reported on the pretreatment of lignocellulosic biomass. It was first used for wheat straw, and the result showed that ammonium sulfite could improve enzymatic hydrolysis significantly, but under more severe pretreatment conditions; Since bamboo is stiffer and more compact than wheat straw, stringent pretreatment conditions would be required.18 Considering the pretreatment cost and the final value of glucose produced, the ammonia sulfite pretreatment conditions of bamboo need to be optimized. The aim of ammonium sulfite this study was to find low intensity pretreatment parameters (pretreatment temperature, time and ammonia sulfite concentration) to achieve higher cellulose recovery yield. Ground bamboo was chosen as the starting material and a series of pretreatments were performed based on the Box-Behnken design with three variables: pretreatment temperature, time, and ammonia sulfite concentration. The total cellulose recovery yield (TCRY) was calculated by the enzymatic hydrolysis efficiency obtained after 48 h of enzymatic hydrolysis multiplied by the solids recovery rate after the ammonia sulfite pretreatment. Environmental scanning electron microscopy was performed to compare the structural changes of raw bamboo and the preferred pre-treated substrate.