# Removal of Antibiotics by Biochars: A Critical Review
## 2 Biochars and Adsorption: A Bibliometric Survey  
Biochars are carbon-based materials that are produced by pyrolysis processes of organic biomass (wood, leaves, manure, etc.) and used for non-fuel purposes such as adsorption [8; 43], soil amendment [44; 45], and as catalysts for chemical reactions [46]. The production of biochar is a carbon-negative process that contributes to mitigating the greenhouse effect [47; 48]. Biochars typically have larger specific surface areas than raw biomass (generally \(<\)1 m\({}^{2}\) g\({}^{-1}\) for lignocellulosic biomass), but their specific surface areas are generally much smaller than those of activated carbons (500-3000 m\({}^{2}\) g\({}^{-1}\)). The range of surface area and pore structure of different biochars is not well known. Biochars usually contain surface chemical groups, while in activated carbons these groups are modified or lost during the thermal or chemical activation processes. Biochars have a significant cost advantage over activated carbons. The production of biochars costs between 0.35 USD/kg and 1.2 USD/kg [8], which is about one-sixth the cost of activated carbons [49]. Another interesting property of biochars is their high affinity for adsorbing organic pollutants [50]. This set of properties of biochars suggests their use for the removal of antibiotics from wastewater. The favorable cost of biochars, along with their promising adsorption properties, has led to an increased usage of biochars for the removal of antibiotics [12]. Biochars show selectivity in the adsorption of antibiotics; for instance, they showed higher sulfamethoxazole removal capacities than multiwalled carbon nanotubes, graphite, and clay minerals [4]. The adsorption properties were compared based on adsorption coefficient values, which showed partitioning of antibiotics between the adsorbent and water [4]. This result is interesting because biochars adsorbed higher amounts of sulfamethoxazole antibiotics than carbonaceous adsorbents with low specific surface areas such as graphite (1-10 m\({}^{2}\)/g) and multiwalled carbon nanotubes (100-1000 m\({}^{2}\)/g). Tetracycline antibiotics showed stronger adsorption to graphite than to activated carbon and multiwalled carbon nanotubes, in spite of the latter's higher specific surface areas [4]. Therefore, specific surface area is not the only determining factor in the adsorption of antibiotics onto biochars. The pore structure and surface chemical groups of the carbonaceous materials also play an important role in the adsorption of antibiotics [41]. The pore structure is particularly important in the adsorption of bulky antibiotics such as tetracycline, where high-surface-area-bearing microporous activated carbons and carbon nanotubes demonstrated low affinity due to size exclusion and slow adsorption kinetics [51]. The size exclusion effect, on the other hand, may be beneficial in biochars containing macro- and mesopores [51], similar to reverse osmosis or nanofiltration membranes [27; 52]. Surface chemical groupssuch as hydroxyl, carbonyl, carboxyl, and amine groups also contribute to the adsorption through intermolecular interactions and covalent bonding. Metal components of the inorganic biomass fraction also contribute to adsorption through surface complexation mechanisms [53]. The effect of the inorganic composition of the biochar is often ignored in adsorption studies. The adsorption capacity of biochars may be further increased by physical or chemical treatments, as well as by metal oxide or heteroatom doping. Physical treatments are principally used to increase the surface area, while chemical treatments are applied to modify the surface chemistry and pore structure of biochars [8]. Metal oxides and hydroxides are frequently used to prepare biochar-metal composites. This application is essentially applied to convert the negative surface charge of biochars to positive [8], to produce magnetic biochars by using Fe\({}_{3}\)O\({}_{4}\)[54], and to increase the porosity [8].