# Unravelling the Environmental Application of Biochar as Low-Cost Biosorbent: A Review
## 4 Biochar Adsorption of Emerging Contaminants
### Dyes  
In the literature, a multitude of studies have been reported in which the biochar has been manufactured from different feedstocks, evaluating the effect of the pyrolysis parameter in its dye adsorption capacity [97; 98]. Thus, recent improvements in different biochars to enhance the adsorptive properties of the biochar will be presented. One alternative is the use of ball-milling technology, which offers advantages such as consuming less energy, not requiring chemicals during the ball-milling process and increasing the specific surface area of biochar by 65 times [99]. [PERSON] et al. [100] applied this technology in a planetary ball mill to sugarcane bagasse biochar to increase its sorption ability to a soluble cationic dye, methylene blue. The reported results reveal that this technology increases the external and internal surface area, oxygen-containing functional groups and reduces the point of zero charges from 4.2 to 2.7, thereby enhancing methylene blue adsorption via \(\pi\)-\(\pi\) interaction and electrostatic attraction (Figure 3). As was reported by [PERSON] et al. [101], the increase in oxygen species on the surface of biochar by incorporation of carboxylic, carbonyl, lactonic, and phenolic groups affects the dye removal. In this sense, several strong acids (HNO\({}_{3}\), KMnO\({}_{4}\), H\({}_{2}\)O\({}_{2}\), H\({}_{3}\)PO\({}_{4}\) or HCl) have been tested, with a positive effect on biochar adsorptive properties [102]. The biochar modification process could be improved by combination with other agents. Thus, methylene blue was also removed by bamboo hydrochar modified with HCl and NaOH [103]. By a sequential acid/NaOH treatment, it is possible to enlarge the surface area and pore volume of hydrochar in the first stage, followed by the enhancement of the oxygen-containing functional groups by the activation process with NaOH. In addition, it was concluded that by activation, the \(\pi\)-\(\pi\) interactions could be considered as the primary mechanism, and electrostatic attraction and ion-exchange mechanism interactions between cationic dye and hydrochars are also involved. Another alternative is the use of activating agents (such as ammonium chloride or ammonium acetate) to produce nitrogen-doped biochar due to the binding with carbon and generating nitrogen-containing functional groups during the heating process (such as nitrogen-pyrrole, nitrogen-pyridine, or nitrogen-oxidized). In a comparative study of nitrogen-doped biochar with one derived from phosphoric acid activation for the removal of an azo dye, acid red 18, the authors concluded that maximum adsorption capacity increased by 40% due to the beneficial effect of N- functional [104]. To obtain similar nitrogen-doped biochar, [PERSON] et al. [105] applied a simple method by ball-milling bagasse biochar with ammonium hydroxide. In this case, it was detected that the nitrogen species were amine and nitrile, whose presence enhanced the sorption of the dye reactive red to 16 times the level of unmodified biochar. New composites have been produced satisfactorily by the co-precipitation method, such as MgAl-layered double hydroxide (LDH) bovine bone biochar composite. [PERSON] et al. [106] tested this composite in the adsorption of methylene blue, achieving uptake around 406 mg/g, with the capacity for solvent regeneration with methanol and sodium chloride (Figure 3). However, the efficiency of adsorbent decreases until uptake around 70 mg/g after six cycles due to the disintegration of LDH. Better results were obtained when a green banana peel biochar/iron oxide composite was manufactured, enhancing its adsorb dye capacity in a wide range of pH, keeping the uptake level after five regeneration cycles [107]. Similarly, a novel Fe\({}_{3}\)O\({}_{4}\)-graphene-biochar composite (GBC-Fe\({}_{3}\)O\({}_{4}\)) was prepared for the removal of crystal violet dye.