Table 1 Various physical and chemical pretreatments with associated advantages and disadvantage

Physical pretreatment

 Mechanical process

Mechanical force is required to reduce particle size as well as disrupt their surface configuration

In this process, there has been durability and robustness of the equipment used that’s why most suitable for industrial applications

Highly energy-devouring process along with slight cellulose digestibility. To enhance substrate durability there is a need for design equipment

[32,33,34]

 Extrusion process

Extrusion process is the processing technology that includes mixing, heating, stirring, crushing and shearing of the biomass

Simple process, low energy consumption, short residence time, moderate temperature and pressure and ease of commercial usability

The amount of compressive force is very high due to which regular checking to ensure its setup, a highly time-consuming process, setup cost is very costly

[36, 37]

 Irradiation

Frequency required in microwave radiation is around 0.3-300 GHz. It induces the breakdown of LCBs through molecular collision and causes dielectric polarization in a chemical bond

Shorter heating period,

Moderate temperature, less formation of inhibitors

Zone with higher temperature led to the formation of furfural and HMF compounds as well as carbohydrate degradation

[29, 38, 39]

 Sonification

The sonification wave is higher than 2000 Hz with the best directive and penetrating power

Eco-friendly and chemical-free pretreatment methodology

Difficulty in handling the process and parameters required for the process

[40,41,42]

Chemical pretreatment

 Alkaline pretreatment

In this, an alkaline solution is used to remove the uronic acid substituted on hemicellulose exterior portion that reduces the enzymes accessibility for the hydrolysis process

This process has removed the lignin fraction of mass without much sugar degradation. High efficiency and milder condition are utilized during the process

High reaction volumes and longer reaction times along with a decrease in the crystallinity, possibility of alkaline being converted to their respective salt

[44, 48]

 Organosolvent

The solvents used in this process are amalgamated in water and thus operated in a homogenous system

Non-toxic, low cost, environmentally friendly as mostly are biomass derivatives and hence can easily be recycled

It can easily be evaporated due to its low boiling point

[57]

 Acidic pretreatment

Using acid to solubilize hemicellulose fraction which increases the accessibility of enzyme towards cellulosic fraction and to modify lignin structure

Help in lignocellulosic matrix disruption and amorphous cellulose conversion

Acid corrosion

The high reaction temperature and lower reaction time,

A neutralization step is required after acidic pretreatment

[62]

 Ionic liquid pretreatment

It is termed a green solvent and can disintegrate cellulose under natural conditions

High thermal stability, non-volatile and recyclable, reinforce the hydrogen bond acceptor and huge polarity, recycling and reuse facilitate it for reducing cost

High cost, sufficient towards removing lignin and hemicellulose avoids its use in a broad variety of applications

[68, 71,72,73]