# Biochars as a Solution for Silver Removal and Antimicrobial Activity in Aqueous Systems
## 3 Results and Discussion
### FTIR Analysis  
Biochars derived from rice husks and cotton exhibit FTIR bands that reflect the presence of typical organic and inorganic components found in pyrolyzed biomass. During pyrolysis, emitted gases partially condense onto the solid residue, contributing to surface functionalities. All three biochars (RHB, ARHB, and CB) show common bands at approximately 3450 cm\({}^{-1}\), corresponding to O-H stretching of hydrogen-bonded hydroxyl groups, and a doublet between 2930 and 2850 cm\({}^{-1}\), attributed to aliphatic C-H stretching (see Figure 3).  
The FTIR spectra of RHB and ARHB (Figure 3a,c) are similar, with peaks at 1617 cm\({}^{-1}\) (aromatic C=C or C=O stretching, possibly indicative of carbonyl groups in amides), 1453 cm\({}^{-1}\) (C-H bending in aliphatic chains), 1112 cm\({}^{-1}\) (C-O stretching or Si-O-Si bonds), 850 cm\({}^{-1}\) (aromatic C-H bending or Si-O/Si-O-C bonds), and 658 cm\({}^{-1}\) (Si-O-Si symmetric stretching). However, ARHB, exhibits additional oxygen-containing groups due to acti  
Figure 3: FTIR spectra of the biochars before and after Ag adsorption: (**a**) ARHB; (**b**) ARHB/Ag; (c) RHB; (**d**) RHB/Ag; (**e**) CB; and (**f**) CB/Ag.  
Figure 2: XRD diffractograms of the samples before (**a**) and after Ag adsorption (**b**).  
vation, as evidenced by stronger bands at 1100 cm\({}^{-1}\) and 1005 cm\({}^{-1}\), corresponding to C-O and Si-O stretching. This activation increases the availability of functional groups, enhancing ARHB's reactivity compared to RHB. Residual SiO\({}_{2}\) remains in both RHB and ARHB, indicating incomplete extraction during preparation.  
The FTIR spectrum of CB (Figure 3e) shows distinct bands at 1730 cm\({}^{-1}\) (carbonyl group stretching), 1654 cm\({}^{-1}\) (C=C stretching in aromatic rings, or possibly C=O stretching in conjugated carbonyl groups, such as amides), 1370 cm\({}^{-1}\) (aliphatic structures or oxygen-containing groups), 1154 cm\({}^{-1}\) (C-O stretching), and 1030 cm\({}^{-1}\) (cellulose derivatives and oxygen-containing groups).  
After silver ion adsorption, all three biochars exhibit notable changes in their FTIR spectra, indicating interactions between silver ions and oxygen-containing functional groups on their surfaces. These changes include shifts in peak positions, the appearance of new peaks, and variations in peak intensities. Specifically, shifts are observed in peaks associated with O-H stretching, aliphatic C-H stretching, aromatic C=C and C=O groups, and C-O stretching (Figure 3b,d,f). For RHB and ARHB, the disappearance of peaks at 850 cm\({}^{-1}\) and 658 cm\({}^{-1}\) suggests strong interactions between Ag\({}^{+}\) and Si-O/Si-O-Si groups, leading to structural rearrangements and bond modifications. These interactions may involve chemical bonding or electrostatic attraction, disrupting the vibrational modes associated with Si-O-Si symmetric stretching and Si-O/Si-O-C bonds, causing these peaks to vanish from the FTIR spectrum.  
The observed spectral changes after silver ion adsorption suggest that the silver ions primarily interact with oxygen-containing functional groups, such as hydroxyl, carbonyl, and silicate sites, on the biochar surface. These interactions likely involve coordination bonds with oxygen-containing functional groups, electrostatic attractions, surface complexation, and possible chemical bonding, resulting in alterations to the biochar's surface structure and functional group.