*5.3.1 Physical activation*

The physical activation method involves heating carbonized biomass material to a predetermined activation temperature, typically ranging from 700 to 1100°C [40]. The activation agents used in this method include air, nitrogen, steam, or CO2 [40]. The use of activation gas can prevent CO2 formation and generate a wide range of pore sizes in the activated carbon during the activation process [43]. Carbon dioxide is the most commonly used activation agent in the physical activation process due to its ease of handling [44], which leads to higher carbon yield and the development of uniform pores [45]. However, the steam activation process results in lower carbon yield, which could be attributed to the rapid degradation of biomass materials at high temperatures [46]. Furthermore, physical activation with CO2 produces microporous carbons, whereas activation with steam at identical conditions results in mesoporous and macroporous carbons [15]. Activation reactions of O2, CO2, and steam are given below [15]:


The main parameters that affect the physical activation process are activation temperature, activation time, heating rate, and activator flow rate.

#### *5.3.2 Chemical activation*

The chemical activation process involves the use of a chemical activator to enhance the surface properties of biochar such as porosity, pore size distribution, and surface area. Various chemicals can be used as activation agents, including H3PO4, H2SO4, KOH, K2CO3, and ZnCl2 [47]. It is crucial to ensure complete mixing between the biomass material and the activation agent before the activation step to achieve the desired quality of the produced activated carbon. Chemical activators work by degrading and dehydrating organic carbon molecules of the starting biomass materials to increase the porosity and surface area of the produced BAC [40]. Chemical activation can be carried out in a single step or in two steps. In the single-step method, the biomass material is impregnated with the activator and then carbonized, while in the two-step method, the biomass material is initially carbonized in the first step and then impregnated with the activation agent and subjected to thermal activation in the second step. **Figure 4** shows the steps of the main methods for the preparation of biomass-based activated carbons.

#### *5.3.3 Activation agents*

Chemical activation has proven to be a successful method for producing activated carbons using a range of chemical reagents such as KOH, NaOH, ZnCl2, H2SO4, and H3PO4.
