Physical-chemical pretreatment methods include steam explosion, ammonia fiber explosion (AFEX), microwave pretreatment, and ultrasonic pretreatment, etc. Steam explosion is the most commonly used physical-chemical method for the pretreatment of lignocellulosic materials in particular for hardwood and agricultural biomass [12]. In this method, biomass is treated with high-pressure saturated steam and then the pressure is suddenly reduced such that the biomass undergoes an explosive decom­pression. Steam explosion is typically carried out at 160-260 °C and 0.69-4.83 MPa for several seconds to a few minutes before the materials are exposed to atmospheric pressure [13]. Like a hydrothermal pretreatment, steam explosion removes hemicel — lulose mainly by autohydrolysis and partially by chemical effects and mechanical forces. The high temperature and pressure promote the acetyl groups present in hemicellulose to be automatically hydrolyzed to acetic acid; on the other hand, the water may act as an acid under such high-temperature condition. All of these acids formed in the steam explosion process could thus hydrolyze hemicellulose. Removal of hemicellulose exposes the cellulose surface and increases enzyme accessibility to the cellulose microfibrils [14]. In the stream explosion process, lignin can also be removed to a certain extent, but is redistributed on the fiber surfaces as a re­sult of melting and depolymeriation/repolymerization reactions [15]. The removal and redistribution of hemicellulose and lignin could swell the pretreated sample and increase its accessible surface area [7]. The main drawback of this method is that many enzyme-inhibitors are produced in the pretreatment. For example, the pen­toses and hexoses formed from the hydrolyzed hemicellulose and cellulose can be further degraded to furfural, 5-hydroxymethylfurfural (HMF), levullinic acid, and formic acid, which would deactivate the enzymes used in the consecutive enzymatic hydrolysis process. In the AFEX pretreatment process, lignocellulosic materials are treated with liquid ammonia at the temperature between 60 °C and 100 °C under high pressure for a certain period of time, normally 10 min, before the pressure is released swiftly, which would cause mechanical explosion of the materials from the inside. Recycling of ammonia in the system after the pretreatment is economically feasi­ble due to the high volatility of ammonia at atmospheric pressure [16]. The AFEX process was employed for the pretreatment of a variety of lignocellulosic materials such as alfalfa, wheat straw, wheat chaff, barley straw, corn stover, rice straw, mu­nicipal solid waste, softwood newspaper, kenaf newspaper, coastal Bermuda grass, switchgrass, aspen chips, and bagasse [17, 18]. AFEX pretreatment does not remove much hemicellulose and lignin, but it can decrease the cellulose crystallinity, disrupt the lignin-carbohydrate linkages and remove the acetyl groups from hemicellulose [19]. After pretreatment, the enzymatic digestibility of lignocellulosic materials can be increased. Moniruzzaman et al. [20] achieved more than 80 % of the theoretical sugar yield from corn fiber pretreated usingAFEX at 90 °C, an ammonia-to-corn fiber mass ratio of 1:1 and 200 psi for 30 min. Superior to the steam-explosion pretreat­ment where many inhibitors are formed from the degradation of hemicellulose and cellulose, the AFEX pretreatment is likely more advantageous as no toxic byprod­ucts are formed except for some phenolic fragments from lignin [3]. However, the AFEX process was not very effective for biomass with higher lignin content such as newspaper and woody biomass [12, 13].

Microwave pretreatment may be considered a physico-chemical process since it involves both non-thermal and thermal effects. The microwave method has proved to be effective for improving enzymatic hydrolysis of many agricultural residues/biomass such as rice straw and wheat straw [21, 22]. Ma et al. [22] sys­tematically optimized the pretreatment conditions for rice straw hydrolysis, and studied the effects of microwave intensity, irradiation time, and substrate concen­tration on the hydrolytic conversion of cellulose and hemicellulose. It was believed that microwave and microwave-based pretreatment could hydrolyze hemicellulose and solubilize lignin. Moreover, the thermal and non-thermal effects arising from heating could enlarge the pore size of the lignocellulosic materials, and enhance the accessibility of cellulose to enzymes [23]. Ultrasonic pretreatment could be an­other promising method for the removal of hemicellulose and lignin, and it has been widely used in the extraction of hemicellulose and enzyme proteins from biomass and organic waste materials such as biosludge [24, 25], although there is very limited published literature with respect to the effects of ultrasonic pretreat­ment on the glucose yield and carbohydrate conversion during enzymatic hydrolysis of lignocellulosic materials. Yachmenev et al. [26] reported that saccharification of cellulose was enhanced considerably by ultrasonic pretreatment. The increased en­zymatic hydrolysis yields after ultrasonic pretreatment could be explained by the effects of ultrasonic pretreatment: cracking of the cell wall, dislocation of the sec­ondary wall of the middle layer of the cell wall, and exposure of the middle layer to enzymes [27]. Yu et al. [28] investigated the effects of ultrasonic pretreatment on enzymatic hydrolysis of rice hull using 250 W, 40 kHz at 25 °C for different period of time ranging from 10 min to 60 min. Their results showed that the yields of total sugar and glucose increased from 11.7 % to 10.9-16.3 % and 15.8 %, respectively, via ultrasonic pretreatment for 30 min. In summary, ultrasonic treatment can cause cavitation, crash the cell wall structure, and provide more accessible surfaces in the substrate. Ultrasonic pretreatment can thus be a potential method for pretreatment of lignocellulosic materials due to its lower energy consumption. Due to the limited research in this respect, it is of interest to examine the effects of ultrasonic pre­treatment on enzymatic hydrolysis of lignocellulosic materials, in particular, when combining it with other chemical pretreatment approaches such as organosolv and alkaline methods.