# Removal of Antibiotics by Biochars: A Critical Review
## 4 A Critical Review of Biochar-Based Adsorption Processes
### Thermodynamic and Kinetic Considerations  
Thermodynamic and kinetic (rate and mechanism) studies are the two essential tools in adsorption studies because they answer fundamental questions such as whether an adsorption process works, how it works, how to optimize it, and how to design better adsorbents. Thermodynamics determines the feasibility of an adsorption process under various temperature and pressure conditions. The thermodynamic analysis of adsorption involves the calculation of thermodynamic parameters such as Gibbs free energy, enthalpy, and entropy (\(\Delta\)G, \(\Delta\)H, and \(\Delta\)S, respectively), which can be used to assess the thermodynamic feasibility of the adsorption process. For instance,  
* If \(\Delta\)G \(<\) 0, the process is thermodynamically favorable, and adsorption will occur spontaneously;
* If \(\Delta\)G \(>\) 0, the process is thermodynamically unfavorable, and adsorption will not occur spontaneously:
* If \(\Delta\)G \(=\) 0, the process is at thermodynamic equilibrium. Enthalpy change (\(\Delta\)H) is another thermodynamic parameter that is used to assess the feasibility of adsorption. If the adsorption is exothermic (\(\Delta\)H \(<\) 0), it releases heat and is more favorable at lower temperatures. If the adsorption is endothermic (\(\Delta\)H \(>\) 0), it absorbs heat and is more favorable at higher temperatures. Entropy change (\(\Delta\)S) is the final factor to assess the feasibility of adsorption. An increase in entropy (\(\Delta\)S \(>\) 0) favors the adsorption process. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Biochar** & **Pyrolysis** & **Proposed Mechanism** & **Modification** & **Antibiotic** & **Maximum** & **Acgorption** & **Reference** \\
**Precursor** & **Temperature (\({}^{\circ}\)C)** & **Proposed Mechanism** & **Modification** & **Used** & **(Qn)** & **(Qn)** & **(mg/g)** \\ \hline \multicolumn{8}{c}{Pristine biochars} \\ _Pinus_ & \multirow{2}{*}{600–800} & \multirow{2}{*}{n.a} & \multirow{2}{*}{n.a} & Tetracycline & \multirow{2}{*}{163} & \multirow{2}{*}{[104]} \\ _radiuta_ wood & & & & (TC) & & \\ sawdust & & & & Enrofloxacin (EF) & 45.9 (EF) & \\ Bamboo & \multirow{2}{*}{500} & \multirow{2}{*}{n.a} & \multirow{2}{*}{n.a} & Offoxacin & 45.1 & \multirow{2}{*}{[170]} \\ sawdust & & & & (OF) & & \\  & & & & Sulfadiazine & 0.12 & \\ Spent & \multirow{2}{*}{200–700} & \multirow{2}{*}{\(\pi\)–\(\pi\) EDA} & \multirow{2}{*}{n.a} & (SDZ), & (SDZ) & \\ coffee grounds & & & & sulfamethoxazole & 0.13 & \\  & & & & (SMX) & (SMX) & \\ \hline \multicolumn{8}{c}{Modified biochars} \\ \hline \multicolumn{8}{c}{Multiplier:} & \multirow{2}{*}{600} & Multiple: & Tetracycline (TC), & \multirow{2}{*}{429.3} \\ Sunflower & & chemisorption, & & ciprofloxacin & \\ seed husk & & external diffusion, & & (CIP), & \\  & & & & sulfamethoxazole & 251.3 & \\ Bamboo & \multirow{2}{*}{380} & Hydrogen bonds, & & (SMX) & \\  & & & Lewis acid–base & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \end{tabular}
\end{table} 
Table 4: Mechanisms of antibiotic adsorption onto biochars; modified from [12]. These factors are related according to the following equation:  
\[\Delta\text{G}=\Delta\text{H}-\text{T}\Delta\text{S}\]  
In order for \(\Delta\text{G}\) to be negative, either the enthalpy change (\(\Delta\text{H}\)) should be negative (exothermic process) and greater than the TAS product (typically positive), or, in the case of endothermic reactions, the entropy change (\(\Delta\text{S}\)) should be large enough to offset the positive enthalpy change (\(\Delta\text{H}\)) and temperatures (T) should be high. Exothermic adsorption (\(\Delta\text{H}<0\)) involves relatively strong adsorbate-surface interactions, such as chemical sorption or strong van der Waals forces, while endothermic adsorption involves weak adsorbate-surface interactions such as physical sorption (weak intermolecular interactions). The entropy change (\(\Delta\text{S}\)) may be positive or negative in chemical sorption, but it is usually negative in physical sorption.