# Removal of Antibiotics by Biochars: A Critical Review
## 4 A Critical Review of Biochar-Based Adsorption Processes
### Modification of Biochars
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Treatment No.** & **Treatment** & **Advantages** & **Disadvantages** \\ \hline
1 & Acid treatment & Removal of metals; & Lower biochar yield due to acid hydrolysis; \\  & & increased surface area & inefficient removal of silica; \\  & & & high cost \\
2 & Alkali treatment & Removal of silica; & Lower biochar yield due to alkaline hydrolysis; \\  & & increased surface area & inefficient removal of metals; \\
3 & Demineralization with hot water & Efficient removal of metals and & Energy- and time-consuming; \\  & & silica & additional drying step required \\
4 & Ball milling & Increased surface area & High cost; \\
5 & Steam activation & Increased surface area & less effective than chemical methods \\
6 & Doping with organic compounds & Addition of surface functional & Reduced biochar yield \\
7 & Surfactant modification & Addition of acidic or basic surface & High cost \\  & & functional groups & High cost \\
8 & Mineral impregnation & Addition of metal oxides on the & Secondary contamination by leaching of \\  & & biochar surface & minimal \\  & & Addition of iron atoms on the & \\
9 & Mineral impregnation: iron & biochar surface; & Secondary contamination by leaching of iron \\  & & easy removal of magnetic & \\  & & particles from water & \\
10 & Composite-forming clays & Enhanced ion-exchange mechanism & Environmental impact of clay processing \\
11 & Composite-forming by carbon & Addition of adsorption sites & High cost \\
12 & Composite-forming by & Addition of surface & heteroatom leaching; \\  & heteroatom doping & functional groups & specialized process \\  & & Production of a specialized type & \\  & & of biochar selective to target & High cost; \\  & & (imprint) molecules; & specialized process \\  & & reusable & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Critical comparison of the advantages and disadvantages of the main biochar modification methods. Acid or alkali activation is the most widely used and effective way to enhance biochars' surface area and porosity. Both acid and alkali treatments increase the porosity of biochars by altering the biochar's structure and surface functional groups via depolymerization, dehydration, and dehydrogenation reactions (i), creating micro- and mesopores inside the biochar's structure (ii), and removing the inorganic compounds (iii) [49; 143; 144; 145]. Acid-base combined treatments can be considered for low-porosity biochars bearing limited surface functional groups, such as municipal sewage sludge biochars [49; 146; 147]. These treatments seem to be superior to the single acid or alkali treatments [49]. However, the available data are still scarce. More experimental results with a broader range of biochars are required to better understand the effects of the combined acid-base treatments. Physical treatment methods, such as coating with carbonaceous materials, ball milling, and template formation, can also result in surface enhancement. Ball milling seems to be a feasible method to produce biochar nanoparticles [148]. Future research should focus on developing technologies to simultaneously achieve enhanced functionality and porous structure of biochars. Cationic or anionic surfactants such as cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) are used to alter the adsorbent's surface and, in particular, to change the surface charge [149]. Certain organic compounds, such as humic acid (HA) [150], methanol [151], and chitosan [152; 153; 154], have been used in the modification of biochars because they introduce supplementary functional groups (e.g., carbonyl (-C=O-), amino (\(-\)NH\({}_{2}\)) and hydroxyl (-OH)) to the surface of biochar [49]. However, organic compound modification has cost disadvantages, which limit its development [49]. Metal or metal oxide modification provides a higher number of adsorption sites and creates a larger surface area in biochars [49; 146; 155; 156; 157]. The metal modification is particularly effective in the recycling of biochars after adsorption.