# Removal of Antibiotics by Biochars: A Critical Review
## 4 A Critical Review of Biochar-Based Adsorption Processes
### Thermodynamic and Kinetic Considerations
Pressure can also affect the adsorption process, but its impact in liquid adsorption is less pronounced than temperature. High pressures increase the entropy and favor the adsorption process, but they may also lead to degradation of the adsorbent. Thus, in order to optimize the adsorption of antibiotics onto biochars, it is necessary to calculate the thermodynamic properties. If the adsorption is exothermic, it should be performed at low temperatures, and if the adsorption is endothermic it should be performed at high temperatures. If the adsorption occurs due to surface chemical reactions, the adsorbent's surface should be modified with metal oxides or heteroatoms to increase the number of available complexation sites and drive the adsorption process in a thermodynamically favorable manner. If the adsorption occurs through physical sorption, oxygenated surface functional groups should be introduced to the biochars to increase the intermolecular interactions, such as hydrogen bonds. It should be noted that the adsorption of antibiotics is a complex reaction and involves both surface chemical reactions and physical sorption [172]. Therefore, different experimental conditions should be tested to optimize the adsorption process. A particular case of thermodynamic studies is the study of adsorption isotherms that describe the equilibrium relationship between the concentration of adsorbate molecules and the amount of adsorbate adsorbed onto the surface of the adsorbent. Thus, they provide information about the adsorption capacity and the adsorbent-adsorbate surface interactions. The Langmuir isotherm and Freundlich isotherm are the most frequently used adsorption isotherms. According to the Langmuir isotherm, the adsorbent's surface is homogeneous, and adsorption occurs as a monolayer until all available sites are occupied by adsorbate molecules and there are no interactions among the adsorbed molecules. The Freundlich isotherm assumes a heterogeneous adsorbent surface, multilayer adsorption, and interactions among the adsorbed molecules. Kinetic models of adsorption describe how adsorbate molecules are adsorbed onto the surface of an adsorbent material as a function of time. The kinetic models explain the reaction rate and the mechanisms, and they provide insights into the dynamic aspects of adsorption. The most frequently used kinetic models are the pseudo-first-order model, pseudo-second-order model, Elovich model, and intraparticle diffusion model. The pseudo-first-order and pseudo-second-order models consider the surface reaction as the rate-limiting step, while the intraparticle diffusion model considers the intraparticle diffusion as the rate limiting step. According to the pseudo-first-order kinetic model, the adsorption rate (\(\text{dq}/\text{dt}\)) is proportional to the difference between the equilibrium concentration (qe) and the concentration at a given time (q). while according to the pseudo-second-order kinetic model the adsorption rate is proportional to the square difference between the equilibrium concentration and the concentration at a given time. The Elovich model assumes that rate of the adsorption is not constant over time and that there are interactions between the adsorbate molecules. The intraparticle diffusion model describes the rate of intraparticle diffusion. The adsorption of tetracycline antibiotics onto zinc chloride activated biochar was described by the pseudo-second-order kinetic model and the Langmuir isotherm, with a maximum (monolayer) adsorption capacity of 200 mg/g tetracycline. Hydrogen bonding and electrostatic interactions were the main proposed mechanisms [176]. The adsorption of quinolone antibiotics onto magnetic biochar also resulted in a similar trend. The adsorption was described by the pseudo-second-order kinetic model and the Langmuir isotherm, with a maximum adsorption capacity of 68.9 mg/g [177].