**4.2 Chemical methods**

Aqueous acidic and alkaline solutions are known for their dissolvability of lignin. Acidic and alkaline pretreatments of lignocellulose have been so far the most effective and feasible for lignocellulosic bioethanol production technology [19, 20]:


**Figure 4a–c** shows the scanning electron microscopic (SEM) photos of pretreated rubber wood (*Hevea brasiliensis*) by acidic and alkaline solutions (Dr. Nguyen et al.). As seen from **Figure 4a**, before the pretreatment, the structure of rubber wood was well intact and recognized in order. After alkaline pretreatment (**Figure 4b**), the fibril structure was flaked due to loss of adhesive lignin and disorder. In addition, an alkaline pretreatment followed by acidic solution soaking like that at **Figure 4c** yielded an over-destruction of the material, where delignified cellulose was strongly hydrolyzed by the acid.
