# Unravelling the Environmental Application of Biochar as Low-Cost Biosorbent: A Review
## 6 Regeneration of Biochar  
The desorption process for the recovery of the biochar is of extreme importance in assessing their potential to be reused. The metal recovery of a solution may be based in different mechanisms (precipitation, complexation or ion exchange) [122]. Solvent regeneration achieves the pollutant desorption from biochar, taking into account the equilibrium relationship between biochar, the solvent and pollutant to break the adsorption equilibrium by changing the temperature and the pH value of the solvent [123]. Different acids and bases (NaOH, HCl, H\({}_{2}\)SO\({}_{4}\), EDTA, HNO\({}_{3}\), Ca(NO\({}_{3}\))\({}_{2}\), NaNO\({}_{3}\)) are used as reagents for desorbing [124]. Thus, alkaline regeneration agents, such as NaOH, NaHCO\({}_{3}\), and NaH\({}_{2}\)PO\({}_{4}\), have been found to efficiently desorb As from Fe-impregnated biochar [125]. Additionally, 0.1 M NaOH is able to desorb 72% of initially sorbed As(V) on biochar, showing that desorption and regeneration rates decreased gradually with the number of cycles, from 72% (the first cycle) to 60% (the third cycle) [125]. Similarly, Fe\({}_{3}\)O\({}_{4}\)@-orange peel can be recycled using 0.1 M HNO\({}_{3}\) with a desorption capacity of 98.5% after five recycling cycles [126]. The efficacy of recycling biochar using different organic solvents was examined in several studies. [PERSON] et al. [95] revealed that methanol, ethanol, and acetone have a higher regeneration capacity by separation of organic contaminants from the biochar. Another alternative to achieve the pollutant desorption from biochar is the thermal method or microwave irradiation in which the adsorbed pollutant is carbonized and decomposed and eventually the molecule becomes smaller than the pore size of biochar and escapes [123]. In thermal regeneration, there is a relationship between the temperature and the regeneration efficiency--for example, when _Enteromorpha prolifera_ biochar polluted by pyrene was regenerated at 80, 150, and 200 \({}^{\circ}\)C, the regeneration efficiency was 35.00%, 45.00% and 48.00%, respectively [127]. In the microwave irradiation method, the trapped polar substance molecules in biochar are used to produce dipole to polarization and the electromagnetic field can be transformed into heat energy, generating the heating and volatilization of the organic substances present in the biochar. The main advantage of this method over thermal method is the uniform heating of the biochar, preserving its porous structure [128]. Latterly, the combination of treatment processes has led to improved treatment techniques due to the synergistic effect between them. Among the different alternatives, advanced oxidation processes (AOPs) have been demonstrated to be effective for removing organic pollutants from wastewater [129; 130]. Recently, in the literature several processes have been reported in which the biochar is used as a catalyst to AOPs or the AOPs are applied to the exhausted biochar regeneration. Thus, [PERSON] et al. [131] synthesized a Fe, N co-doped wheat straw biochar using urea, and ferrous sulfate as precursors and apply to activate persulfate for organic contaminant degradation (such as acid orange, methyl orange, phenol, bisphenol A and tetracycline hydrochloride). In this line, [PERSON] et al. [132] suggested a dual application of biochar as an adsorbent of nickel and lead and subsequent degradation of phenol through in situ generation of hydroxyl radicals. [PERSON] et al. [133] concluded that the defective structures and functional groups (C-OH) of mesoporous biochar (by bagasse calcination with KOH and CaCl\({}_{2}\) modified activation) to act as reaction active sites which played important roles in the oxidative degradation of phenol. Thus, in the degradation of sulfamethoxazole by persulfate activation, [PERSON] et al. [134] figured out a clear linear relationship between the spent-coffee biochar activity and the sodium persulfate concentration, and the process is favored by the adsorption of sulfamethoxazole on the biochar surface.