Solid waste management is a major public health and environmental concern especially in developing countries like Egypt. Solid waste is increasingly accumulating in open areas and along water canals’ banks causing considerable air, soil and ground water contamination, and thus indirect detrimental health impacts. Its accumulation contributes to the global greenhouse gases (GHG) emissions for up to 0.57% based on emission increase rate of 5.1% annually and population increase rate of 1.7% − 2.3%. The trend of GHG emissions in Egypt for the agricultural solid waste within years 2000–2010 shows dramatic increasing trends from 31.7- 52.18 million tons/year (1). Solid waste is categorized according to its origin namely domestic, industrial, and agricultural, etc. These categories accord to the solid waste contents like organic materials, glass, metal, plastic and paper, or their hazardous potentials such as toxic, flammable, radioactive, infectious substances, etc. In addition, the density of solid waste varies based on the point of measurement (at source, during transportation or at disposal) (2).
The Egyptian agricultural waste amount ranges from 30–35 million tons per year. About 7 million tons of them are utilized as animal feed and other 4 million as organic manure (3). The remainder is burnt directly on the fields or used for heating in small villages (4). They represent however important source of bioenergy and valuable products. Sugarcane bagasse (SCB) is one of the most important parts of agricultural waste in Egypt and all over the world. It is resulted from sugarcane after extraction of juice, amounts up to 4.7 million tons per year across the country. Sugar mills generate bagasse about 500 g/kg water content at a rate of 270 kg/t of harvested cane (5). In south Egypt, for instance, Quena has 3 mills: in (Naga Hammadi, Deshna and Qous) with a total production capacity of 4.3 million tons of SCB per season. Despite being good for business, this huge amount of sugarcane production brings some environmental inconvenience. Most of the SCB is used for heat generation in the sugar mills with very poor efficiency and emits black smoke (1).
SCB is a recalcitrant lignocellulosic waste against biodegradation and bioconversion. Lignocelluloses are composed of cellulose, hemicelluloses and lignin in an intricate structure, which is hard to be decomposed (6). It must be pretreated to increase their biodigestibility and make cellulose more accessible to the cellulolytic enzymes, affording a wealthy bioethanol as a clean energy source. Furthermore, the polymers contained in lignocelluloses are themselves relatively difficult to be hydrolyzed to their sugar monomers. Lignin has very complicated structure which covers both cellulose and hemicellulose and plays the role of cement between them, forming a rigid three-dimensional structure of the cell wall. Lignin can be used though the production of chemicals, as a source of combined heat and power, pharmaceutical industry, etc. (7).
There are several pretreatment methods of SCB which can be classified into four categories: physical, chemical, biological and physicochemical pretreatment (4) and (6). The main objective of the pretreatment is to increase the cellulose digestibility and open its recalcitrant crystal structure. This could be done by disrupting hydrogen bonds in crystalline cellulose. Furthermore, hemicellulose and lignin would be disrupted and solubilized. This facilitates rapid and efficient hydrolysis of carbohydrates (cellulose and hemicellulose) to fermentable sugars via enzymatic hydrolysis.
The choice of the optimum pretreatment process depends mainly on the objective of the solid waste pretreatment, composition, economic assessment and environmental impact. According to the literature, application of freezing for SCB fractionation possesses important advantages. Significant lower environmental impact due to less discharge of hazardous derivatives and null used chemicals are examples of these advantages (8). Therefore, inhibition of the subsequent hydrolysis and fermentation steps due to those derivatives would be avoided, and thus, less costs and clean environment can be gained. The mechanism of freezing pretreatment is based on the expanding of liquid volume as it freezes related to its crystal structure (9). When water freezes, it stacks on the crystalline lattice configuration and in turn stretches the rotational and vibrational components of the bond. Consequently as observed by Pinsky et al. (10), the freeze-thaw process could disrupt bulk hydration layer; making the crystal lattice of ice and protein inactive. Franceschini et al. (11) have shown that the best results of fractionation were obtained after one cycle of freeze-thaw below − 10°C. However, Vahur Rooni (13) found that the optimum results were obtained after 4 times of the freeze-thaw. Ken-Lin Chang et al. (9) observed a significant increase in the enzyme digestibility of rice straw from 48–84% after freeze-thaw. Their obtained reducing sugar yield of native and pretreated rice straw after 48 h were 93.84 g/kg and 226.77 g/kg substrate, respectively. However, there is no common conclusion about the required total time and the resulted biomass characteristics after fractionation.
On the other hand, alkaline hydrogen peroxide (AHP) proved promising results in SCB pretreatment especially for lignin dissolution. In such a process, the ester and ether bonds in lignin-carbohydrate complexes are broken, while the internal surface area of biomass increased (14). Besides, it is a typical and relatively safe eco-friendly agent used for delignification during wood pulping processes. Previous researches (15) and (16) found that dilute alkaline solutions of (1–5% (v/v)) H2O2 removed about 50% of the lignin present in lignocelluloses; yielding a cellulose-rich insoluble residue that can be enzymatically converted to glucose with up to 90% overall efficiency. Irfan et al. (17) experienced that SCB pretreatment using 3% (v/v) H2O2 yielded about 1.55 mg/mL of total and reducing sugars with maximum delignification ratio was 36%.
Therefore, the objective of this research is to check a promising eco-friendly strategy for SCB fractionation and further pretreatment. This was achieved in two successive sets: 1. to fractionate the physical composition of SCB by means of freeze-thaw, followed by 2. dissolution of the lignin by dilute H2O2 to release cellulose-rich insoluble residue for further enzymatic hydrolysis. To more come up with a reliable approach, different key operating conditions were investigated taking the interaction between them into consideration. This target was handled in this study by using response surface methodology (RSM) investigating duration, temperature and H2O2 concentrations for the best fractionated sample during freezing. In addition, enzymatic hydrolysis and compositional analysis with (XRD, FTIR and SEM) have been conducted for assuring the efficiency of the studied pretreatment approach.