# Removal of Antibiotics by Biochars: A Critical Review
###### Abstract
Another difficulty in estimating the total quantity of antibiotics in the environment arises due to their highly variable metabolism, stability, and solubility [26]. Antibiotics are classified as emerging pollutants in aquatic ecosystems, owing to their continuous input and persistence at low concentrations [3]. The traditional method of removing antibiotics from waters is sewage treatment by screening and sedimentation, followed by secondary biological treatment. A number of novel methods, such as advanced oxidation processes (AOPs) (including photocatalysis, Fenton, ozonation, and UV irradiation AOPs), application of nanofiltration and reverse osmosis membrane filtration, and membrane bioreactors, as well as chlorination [3; 27; 28; 29; 30], have been shown to be effective in removing antibiotics from wastewater at a small scale, but they have high capital and operation costs [31] or slow kinetics [32] and, therefore, their applicability in large-scale sewage treatment plants is unclear. Conventional sewage treatment in wastewater treatment plants (WWTPs) is the most important process in removing antibiotics from wastewater. The WWTPs are the principal sources of the antibiotics that are released into the environment [31]. Conventional sewage treatment plants cannot effectively remove antibiotics from wastewater [33] because they include a large number of chemicals with different physicochemical properties that result in highly variable (17-90%) removal percentages; for instance, the removal percentage of tetracyclines may be as high as 90%, while it can be as low as 17% for \(\beta\)-lactams [34; 35; 36]. Certain antibiotics can be considered to be non-biodegradable or poorly biodegradable and present at ng/L or \(\upmu\)g/L concentrations [37]. The low concentrations usually range from nanograms to micrograms per liter, and the half-lives are highly variable in wastewaters [4; 22; 38]. are usually widely available, require little or no processing, and have good adsorption properties [39; 40]. Different materials are currently being used as adsorbents, and their adsorption properties may be tuned by applying a thermal or chemical process. The activated carbons are possibly the most promising materials for the removal of organic pollutants from water, but they are costly. Alternative low-cost adsorbents such as microalgae [31] and carbon-based materials such as graphene-based nanomaterials, carbon nanotubes, hydrocarbons, and biochars [4; 22; 41] are currently being investigated for antibiotic removal. Biochars are promising adsorbents because they can be produced by using waste products as feedstocks, ranging from agricultural and forest residues to sewage sludges and manures. Their adsorption properties can be customized by changing the biomass feedstock and pyrolysis conditions to obtain distinct surface physicochemical properties for the adsorption of specific contaminants, such as antibiotics [42]. The objective of this review is to critically analyze the previous antibiotic adsorption studies, with particular attention to biochar sorbents and their properties (e.g., specific surface areas, pore structures, and surface functional groups), modification, and interactions with the different antibiotic classes to provide a state-of-the-art reference to screen biochars for different antibiotic adsorption processes, as well as to optimize their performance.