###### Abstract
However, many treatment methods--except the adsorption process--are characterized by many economic problems such as installation, operation, and management cost. Moreover, the generation of secondary sludge represents another environmental pollutant problem and needs careful attention in the sludge disposal process [19]. The adsorption techniques have been considered as the most appropriate technology in the decrease and removing of the toxic compounds due to their economic benefit, high effectiveness, simple operation and process control, technological flexibility, lower and affordable energy requirements, and environmental friendliness [20; 21; 22; 23; 24]. The adsorbents are porous materials with high specific surface area, which may result in excellent adsorption capacity. The adsorption capacity strongly depends on a series of variables: dosage of the adsorbent, concentration of pollutants in the contaminated water, temperature, contact time and, above all, pH. The efficiency of the adsorption process is also determined by the porosity of the adsorbent, the surface area, the polarity, and the functional groups on the adsorbent surfaces [25]. It is possible to identify two sorption mechanisms: (i) the physisorption, in which the pollutants are adsorbed on the surfaces of the adsorbents by weak van der Waals forces or electrostatic forces, and (ii) chemisorption, in which the pollutants can be immobilized on the active matrix thanks to the presence of functional groups on its surfaces, creating chemical bonds [25]. Regarding adsorption of rice waste-based materials, the interaction mechanisms with As and fluorides are mainly explained by comparing the FTIR spectra before and after adsorption, by porosimetric measurements and surface observation by SEM on the same samples. Moreover, artificial neural network modeling is frequently used as a theoretical tool [26]. In particular, concerning As adsorption, interesting results were obtained at pH between 6 and 9, and the adsorption efficiency of the rice husk biochar as prepared was a little bit lower or similar to the performance obtained for functionalized biochar (surface modifications, for example, by treatments with FeCl\({}_{3}\) salt), even if the specific surface area was much higher (25.161 m\({}^{2}\) g\({}^{-1}\) vs. 1.890 m\({}^{2}\) g\({}^{-1}\)) [27]. This points out to the fact that chemical interactions among functional groups present on the adsorbent surface and detected by FTIR (O- and N-containing groups) and the As containing species and polarity indices [(O + N)/C] of the same external groups are more important than porosity in determining adsorption. Really, the formation of complexes between As(III) and As(V) and the detected functional groups is evident by the shift or the disappearance of some IR bands of the biochar after contaminants' adsorption. The functionalization with Fe(III) leads again to adsorption by chemical interactions, with formation of composites like Biochars-FeH\({}_{2}\)AsO\({}^{3-}\), Biochars-FeHAsO\({}_{3}\)\({}^{2-}\) or Biochars-FeOH\({}_{2}\)-H\({}_{2}\)AsO\({}_{3}\) for As(III), and Biochars-FeH\({}_{2}\)AsO\({}_{4}\), Biochars-FeHAsO\({}_{4}\)\({}^{2-}\) or Biochars-FeOH\({}_{2}\)-H\({}_{2}\)AsO\({}_{4}\) for As(V). Concerning fluoride adsorption, its removal efficiency by rice husk biochar seems to be maximum at acidic pH (values = 4 being the best option); physisorption is the most important mechanism [28], with the adsorption efficiency increasing by increasing the surface area and the pores' number and volumes in the matrix. After surface functionalization with metals (Zn, Fe), adsorption of fluorides is favored also by an electrostatic force of attraction between positively-charged surface of biochar and negatively-charged F\({}^{-}\), leading to the obtainment of--OH\({}_{2}\)--F pendant groups on the biochar structure [26]. Currently, there is an uptrend on using biochar derived from different materials and various waste streams as an adsorbent in different adsorption processes [29; 30; 31; 32; 33].