Alkaline pretreatment of yerba mate (Ilex paraguariensis) waste for unlocking low-cost cellulosic biofuel

Abstract

Yerba mate (Ilex paraguariensis) is one of the most popular hot drinks in the world and might represent an opportunity for energetic valorization after its consumption. Alkaline pretreatment appears to be a potential strategy for unlocking cellulosic biofuels. Thereby, in this study, four alkaline agents were evaluated for improving yerba mate anaerobic digestion (AD): (1) sodium hydroxide (NaOH); (2) potassium hydroxide (KOH); (3) lime; and (4) sodium bicarbonate. Complete factorial design with center point repetition was chosen to evaluate all combinations of concentration of alkali (50, 125, and 200 galkaline agent kg−1dry yerba mate), reaction time (12.2, 30.1, and 48 h) and temperature (30, 65, and 100 °C) in carbohydrate solubilization (variable response). Pre-treatment harshness was quantified by the modified severity factor (MSF). The methane production potential was determined for the best pretreatment condition achieved for each alkali tested. Energy balance for assessing the effectiveness of the proposed treatments on net energy gain was performed. NaOH and KOH had the same effect on organic matter solubilization and produced the same optimum value for the MSF. Thermal-alkaline pretreatment of yerba mate can increase the methane yield up to seven-fold as compared to control (maximum cumulated methane yield of 700 N-LCH4.Kg-VS−1added). All of the alkalis evaluated in the yerba mate pretreatment improved the kinetics of the AD process (kh; from 1e−5 to 1e−1 d−1). NaOH pretreatment at its highest levels of alkali concentration and temperature and lowest level of reaction time was found to be the best scenario in terms of net energy gain.