# Biochars as a Solution for Silver Removal and Antimicrobial Activity in Aqueous Systems
## 3 Results and Discussion
### SEM Analysis and Textural Characterization  
Scanning electron microscopy was employed to analyze the structural characteristics of the biochar samples (Figure 4).  
After the removal of silica and pyrolysis, the inner surface of rice husk biochar (RHB) reveals a well-organized, dense, and compact cellular structure with a homogeneous porous texture. Some areas are smooth, while others display a honeycomb-like pattern. The visible macropores may facilitate the effective transport of adsorbates. Despite the chemical and thermal treatment of the feedstock, the laminar structure of the raw rice husk is preserved, as shown in Figure 4a1,a2. Chemical activation of the RHB sample does not alter its external surface; however, it induces the development of a three-dimensional network of pores between the external and internal surfaces of the husk, as illustrated in Figure 4b1,b2. In contrast, the CB biochar consists of distinct, well-formed fragments with irregular shapes. This biochar exhibits a more compact and solid structure compared to the other samples, as depicted in Figure 4c1,c2.  
The N\({}_{2}\) adsorption isotherms of the three biochars exhibit behaviour without a clear plateau of maximum adsorption (see Figure 5). According to the International Union of Pure and Applied Chemistry (IUPAC) classification, these are Type IV isotherms. Adsorption and desorption curves do not coincide, describing isotherms with hysteresis regions that are characteristic of adsorbents with a wide range of pore size distributions.  
For the biochar CB, a Type H3 hysteresis loop, as per the IUPAC classification, is observed during the desorption of condensed gas from the mesopores of the adsorbent. The Type H3 loop is characteristic of materials with non-rigid aggregates of plate-like particles or slit-shaped pores. This loop appears in the hysteresis region of the isotherm,and is typically associated with mesoporous materials that have non-uniform sizes and shapes. Biochars RHB and ARHB exhibit H4 hysteresis in their nitrogen adsorption-desorption isotherms. H4 hysteresis loops are indicative of materials with narrow slit-like pores, which can be found in some mesoporous materials but are particularly characteristic of microporous materials (pores less than 2 nm in width). The microporous materials often display H4 hysteresis due to capillary condensation and adsorption-desorption mechanisms occurring within these very small pores [40].  
Figure 4: SEM micrographs of (**a1**,**a2**) RHB; (**b1**,**b2**) ARHB; and (**c1**,**c2**) CB.  
Figure 5: Nitrogen adsorption–desorption isotherms and pore size distribution of the biochars: CB, RHB, and ARHB.  
The pore size distribution was calculated using the BJH method. It was observed that biochar CB exhibits a multimodal distribution in both the micropore and mesopore domains, whereas micropores are dominant in biochars RHB and ARHB.  
The surface characteristics of the analyzed materials are presented in Table 1.  
where \(S_{\text{BET}}\) is BET surface area; \(S_{\text{mi}}\) is micropore surface area; V is total pore volume; D\({}_{\text{av}}\) is average pore diameter; and V\({}_{\text{mi}}\) is micropore volume.  
As shown in the table, the ARHB biochar demonstrates the best textural parameters, including the highest specific surface area and the largest total pore volume, predominantly comprising micropores. These characteristics are expected to enhance the adsorption of silver ions through physisorption.