Concurrent calcium peroxide pretreatment and wet storage of water hyacinth for fermentable sugar production
Introduction
Fermentable sugar generated from plant biomass is the key issue to sustain the production of bio-products such as bioethanol and lactic acid. Presently, fermentable sugars used for bio-productions are majorly generated from starch crops. However, using starch crops as feedstock will cause the land competition between food production and bioproductions (Valentine et al., 2012). Therefore, there is a growing demand of cheap and sustainable source of feedstock for production of fermentable sugars. Many research efforts have been devoted to utilizing lignocellulosic biomass as feedstock for fermentable sugar production (Mood et al., 2013). In order to liberate the monosaccharides from the cell wall complex, pretreatments are usually applied to increase enzymatic digestibility of lignocellulosic biomass. Among several pretreatment methods, alkali pretreatment possesses many favorable advantages including low operation cost, less degradation of cellulose and low formation of inhibitors for enzymatic hydrolysis and fermentation (Galbe and Zacchi, 2012). The main mechanisms of alkali pretreatment are degradation of ester bonds and cleavage of glycosidic linkages in the cell wall matrix, which causes the depolymerization of lignin structure, and therefore reduces the interaction between lignin and hemicellulose and increases the accessibility of cellulose to enzymatic attack (Cheng et al., 2010). Additionally, alkali pretreatment can also cause partial de-crystallization of cellulose fibers and therefore increases the efficiency of cellulase hydrolysis.
Aquatic plants have been proposed as future feedstock for production of third generation biofuels and could also be a potential feedstock for other bioproductions. In comparison with terrestrial plants, aquatic plants have many advantages to be utilized as feedstock for fermentable sugar production including fast growth rate, less lignin content and no competition for food production (Wilkie and Evans, 2010). Moreover, the cultivation of aquatic plants can be integrated into wastewater treatment which offers an extra benefit for process development. Water hyacinth is a floating aquatic plant which can be found globally in subtropical and tropical regions because of their abilities of rapid nutrient uptake and promote biomass growth. The annual productivity of water hyacinth could reach 200 ton dry matter in eutrophic waters in the tropical areas (Hasan and Chakrabarti, 2009). However, owing to its prolific growth characteristic, the over growing water hyacinth can block the water surface and cause the degradation of water quality especially in a eutrophic water system; thus, water hyacinth is recognized as an invasive species by many countries. Regardless of its infamous reputation as an invasive aquatic plant, nonetheless, water hyacinth has also been utilized for bioremediation of pollutants in fresh water system including heavy metals and organic matters (Ismail et al., 2014).
However, the high moisture content of fresh aquatic biomass makes the storage a challenge. Fresh harvested aquatic biomass contains more than 90% water. If the fresh harvested aquatic biomass is not stored appropriately, it will spoil quickly. Additionally, storage is a key issue to ensure year round supply of biomass for bioproductions (Kenney et al., 2012, Tian, 2013). Biomass storage can be categorized into dry storage and wet storage based on the moisture content of biomass. For dry storage, the moisture content of biomass is generally lower than 25%. Therefore, biomass drying usually takes extensive processing period depending on the surrounding environment conditions (e.g., humidity and temperature). Wet storage of biomass typically holds more than 50% moisture content during the storage period. Wet storage has several merits over dry storage including lower loss of energy content during storage period, reduced fire risk and better enzymatic digestibility (Liu et al., 2013). Wet storage can be achieved with the aid of microorganisms or chemicals to drop or increase the environmental pH to inhibit the growth of unfavorable microorganism.
In this study, a novel process of concurrent pretreatment and wet storage of water hyacinth with calcium peroxide was proposed and invested. Calcium peroxide is a white to yellowish powder which has been extensively applied in water treatment, seed disinfection and food processing (Pohanish, 2008, Zhai and Jiang, 2014). When reacted with water at pH lower than 12, calcium peroxide decomposed into hydrogen peroxide, hydroxide ions and carbonate, and the generated hydrogen peroxide will further decompose into highly reactive superoxide and hydroxyl radical. The mechanism is briefly described by following equations (Novotortsev et al., 2012):
Decomposition of calcium peroxide
Generation of superoxide and hydroxyl radical
Calcium peroxide possesses many distinctive properties in comparison with other peroxides when it is used as disinfectant and oxygen generator, including better thermal stability, environmental harmless end products, extended release period of hydrogen peroxide and reasonable cost (Chevalier and McCann, 2008, Olyaie et al., 2012). Moreover, the presentation of calcium hydroxide together with hydrogen peroxide during the decomposition of calcium peroxide might be able to increase the efficiency of delignification and enzymatic saccharification of biomass (Cabrera et al., 2014, Yu et al., 2013). Based on the described mechanism and properties, applying calcium peroxide to wet storage of aquatic biomass might be able to simultaneously disinfect the biomass to reduce the loss of energy content and depolymerize the lignin structure to increase the enzymatic digestibility. Moreover, the alkaline in the process effluent could be partially recovered through carbonation to form precipitated calcium carbonate. The calcium carbonate is then thermally converted to calcium oxide which can be further converted to calcium peroxide by reacting with hydrogen peroxide. Because the decomposition of calcium peroxide is related to moisture content in its surrounding environment, the aims of this study is to develop the process and evaluate the effects of calcium peroxide loading concentration and biomass moisture content on the carbohydrate preservation, lignin removal and enzymatic convertibility of water hyacinth after the concurrent pretreatment and storage period.
Section snippets
Chemicals and enzymes
All chemicals used in this study were either analytical grade or HPLC grade purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Alpha-amylase (product# A0447) and glucoamylase (product# M0035) were also obtained from Tokyo Chemical Industry Co., Ltd. Cellulase enzyme complex ACCELLERASE® 1500 and beta-glucosidase ACCELLERASE® BG were provided by DuPont™ Genencor® Science (Rochester, NY, USA) as gifts. All enzymes were stored at 4 °C to maintain the activities.
Biomass preparation
The whole plants of
Variation of sample pH during the storage period
Since the pH value is critical for low temperature alkali pretreatment and microorganism depression (Novotortsev et al., 2012, Pedersen et al., 2011), the variation of sample pH was monitored during the storage period and presented in Fig. 1. The experimental combinations I–III which had relative less CaO2 loading in comparison with other experimental combinations had pH dropped quickly from alkaline to around neutral during the first month of storage because of the consumption of hydroxide
Conclusion
This study discovered that the concurrent pretreatment and wet storage of fresh aquatic biomass could be achieved simply by using CaO2. It suggested that at least 10% CaO2 loading and 80% moisture content are required to maintain the alkali pH and obtain good solid recovery over a 90 day storage. The best results of lignin removal, carbohydrate preservation and enzymatic saccharification obtained in this work were comparable to that in other reports. The concurrent pretreatment and wet storage
Acknowledgement
The funding for this work was provided by the Ministry of Science and Technology of Taiwan (contract # MOST101-2218-E-224-003-MY2).
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