# Removal of Antibiotics by Biochars: A Critical Review
## 4 A Critical Review of Biochar-Based Adsorption Processes
### Biochar Properties  
The most relevant properties of biochars in relation to their adsorption ability are the specific surface area (i), the pore size distribution (ii), and surface functional groups (iii) such as hydroxyl, carboxyl, carbonyl, etc. [68]. The mineral content and composition (iv) of biochars are also important in the adsorption of bulky antibiotics such as tetracyclines through surface complexation [53; 69]. The biochar's properties contribute to the available active sites for adsorption, improving the adsorption capacity [70]. The pore structure is formed due to the release of volatile compounds and water loss in the dehydration process during pyrolysis. Thus, the feedstock and the pyrolysis conditions, especially the temperature, significantly affect the biochar's pore structure and, consequently, the adsorption capacity of the biochar [49; 71; 72]. A high pyrolysis temperature has been linked to a larger surface area, higher microgravity, and graphitic structures, due to the increase in volatilization at higher temperatures [73; 74; 75; 76; 77]. On the other hand, at low pyrolysis temperatures, the functional groups are retained and they contribute to adsorption [69; 78]. Therefore, generally, moderate temperatures (400-700 \({}^{\circ}\)C) are more suitable for the development of favorable pore structures [74]. The aromatic carbon groups (C=C), carbonyl groups (C=O), and aliphatic groups (CH\({}_{2}\) + CH\({}_{3}\)) were determined for four biochars produced by carbonizing corn crop residue (_Zea mays_ L) and wood shavings of oak (_Quercus_ ssp.) at 350 \({}^{\circ}\)C and 600 \({}^{\circ}\)C using slow pyrolysis. The results showed that the aromatic carbon content increased with temperature for both biochars, while the carbonyl and aliphatic groups decreased [79; 80], which is in agreement with the results of [PERSON] et al. [81]. Thus, a high pyrolysis temperature is almost always advantageous for the adsorption process, although the biochar yield decreases with increasing pyrolysis temperature and the economic viability of the process is reduced. Several pore measurements have been reported for biochars. The most common measure is the total pore volume, which includes all pores. According to the International Union of Pure and Applied Chemistry (IUPAC), pores can be divided into three main groups: micropores (<2 nm), mesopores (2-50 nm), and macropores (>50 nm) [71]. However, some authors also use the term \"nanopores\" to indicate micropores, probably because they are in the nanometer range. The role of each type of pore in adsorption is different. Macropores are primarily linked to the diffusion of substances, mesopores serve as channels for mass transfer, and micropores provide space for trapping [71; 82]. The high temperatures in pyrolysis have been stated to be responsible for the presence of pores with sizes around 1.2 and 1.0 nm--the so-called micropores--leading to an increased surface area [83]. Nevertheless, the surface area only increases with the pyrolysis temperature up to a maximum, after which the surface area decreases [68]. For instance, some authors state that there are two competing phenomena: the first increases the volatile release and, consequently, the surface area; and the other is thermal deactivation that leads to char melting, pore fusion, and structure ordering, which decrease the surface area and pore volume [84; 85; 86]. The heating rate is also important in the formation of the pore structure. For example, tests conducted at two different heating rates (10-30 \({}^{\circ}\)C/min and 50 \({}^{\circ}\)C/min) showed that at the lower heating rate the volatiles formed were released from the surface, leading to an open fiber structure with the formation of cavities and, therefore, increasing the surface area [87]. On the other hand, a higher heating rate led to a decrease in surface area and pore volume, which was believed to be due to some of the pore walls becoming too thin and breaking [87]. The same effect was also observed with pyrolysis residence time.