# Removal of Antibiotics by Biochars: A Critical Review
## 5 Knowledge Gaps, Critical Evaluation, and Future Directions  
The application of biochars in the adsorption of emerging pollutants such as antibiotics is still in the development stage. The number of scientific studies on the adsorption of antibiotics is increasing, but only a few studies have used pristine biochars for adsorbents. On the other hand, biochar modification seems to be a promising approach to increase the adsorption capacity of biochars. In order to maximize the potential of biochars in adsorption, the first step is to determine the knowledge gaps. This review allowed us to identify the following knowledge gaps:  
1. Currently, there is deficient monitoring of specific antibiotic compounds in water sources. 2. Biochars' surface structure and chemical properties are highly variable among different biochars. Waste biomass materials are usually applied for biochar production, but they have limited potential compared to porous wood biochars. 3. The effect of ash contents on adsorption is usually ignored, although inorganics are involved in the surface complexation. More studies are required to screen biochars for ash composition. Most antibiotic adsorption studies were performed at the batch scale, which is not representative of real water conditions. Also, studies on the simultaneous removal of antibiotics and other pollutants are still scarce. 5. Thermodynamic and kinetic studies on the adsorption of antibiotics on biochars are scarce. The adsorption mechanisms of different antibiotics on pristine chars are not well known. 6. Heteroatom doping and iron doping of biochars are promising approaches to increase the adsorption capacity of biochars, but they are costly. 7. Molecular imprinting of biochars is another possible way to improve the adsorption capacity of biochars, but this process is also costly. 8. The combination of biochars with other antibiotic removal methods, such as the activation of AOPs, seems to be a promising approach. Previous studies have demonstrated that biochars are efficient adsorbents for the removal of antibiotics, while it is important to screen target antibiotics for adsorption. Among the different antibiotics, \(\beta\)-lactams are the most frequently used, but they are not stable in water because of the instability of the lactam ring; therefore, they are usually not detected or are detected at low concentrations in the wastewater [33]. The high-molecular-weight tetracyclines are the sorbents most readily adsorbed onto biochars [12], suggesting the size exclusion effect and the role of surface chemistry in adsorption. Another implication of this result is that biochars are low-cost alternatives to nanofiltration and reverse-osmosis membranes [27], as well as activated carbons [181], for tetracycline removal. Hydrothermally produced chars (hydrochars) may be preferentially used for the removal of tetracyclines because they usually retain surface chemical groups better than biochars. The adsorption process can be integrated into the activated sludge process as a tertiary process where antibiotics that are not retained by the activated sludge can be targeted. For this aim, the octanol-water partition parameter criterion (logKow) can be applied. According to the classification of [PERSON] (1996) [182], the antibiotics can be grouped as low-sorption (i.e., tetracyclines, sulfonamides, aminoglycosoides; logKow < 2.5), medium-sorption (i.e., \(\beta\)-lactams, macrolides; <2.5 logKow < 4.0), and high-sorption (i.e., glycoptides; logKow > 4.0) [60]. However, this result should be interpreted with caution, because environmental conditions such as the pH of the solution can affect the sorption of antibiotics [33]. However, in all cases, the principal adsorption targets of biochars should be low-sorption and medium-sorption antibiotics that cannot be removed by activated sludge.