# Unravelling the Environmental Application of Biochar as Low-Cost Biosorbent: A Review
## 3 Biochar Adsorption of Heavy Metals  
It is known that some heavy metals are essential to organisms in small amounts, such as Fe, Mn, Zn, B, V, Cu, Ni or Mo, but become harmful when they are present in high concentrations, while others, such as Cd, Cr, Hg or Pb, have no biological function and are highly toxic. Currently, mining operations, metal plating facilities, industrial development and other human activities are the main contributions to the discharge of a high level of heavy metals in water. Thus, their presence in drinking water constitutes a threat to the environment and living organisms' health [23; 24]. The affinity for biochar by heavy metals is limited, as a result of the active surface groups generated after the manufacturing process. Therefore, at the present time, the use of physical-chemical pre-treatments of the raw material and post-treatments of the biochar constitutes the main strategy to obtain efficient adsorbents for the treatment of streams polluted with heavy metals. In Table 1 are summarized recent examples of the biochar manufacturing in order to increase the metal adsorption, and in the following subsections, the particularities of the more toxic heavy metals are studied in depth.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Biochar** & **Temperature (CO)** & **Procedure** & **Pollutat** & **Uptake (mg/g)** & **Sorption Mechanism** & **Reference** \\ \hline Montmorillonite modified & 350 & Sodium-mortmorillonite and corn straw 15, 30 min of ultrasonic-dispection. Neutralization and dried at 80° C. & Zn & 8.16 & Chemical interaction & [25] \\ \hline Iron-impregnated food waste biochar & 400 & Ratio food wasteFciD1; 13, stirring at Offspring for 24 h. & Se & 11.73 & Electrostatic interactions & [26] \\ \hline \multirow{3}{*}{Corn stalk} & \multirow{3}{*}{600} & Mass ratio com stalk: KOH 4.81. Mixture poured in 500 & \multirow{3}{*}{Hg} & \multirow{3}{*}{38.30} & Chemical interactions & [27] \\  & & In: Identified stainless steel autoclive indoor 20° C for 24 h. & & & (Hg-O coordination) & [27] \\  & & After cool and dried, pyrolysis at 800° C for 2 h. & & & \\ \hline Pomele pets & 500 & Donald peke: NaOH 51.8 (51.5) (at 30° C for 24 h. & Mn & 163.19 & Chemical adsorption & [28] \\  & & dried at 60° C. Pyrolysis were 3m; atmosphere for 2 h. & Mn & & \\ \hline \multirow{3}{*}{Egaria nips (EN)-derived biochar supported a2 VI composite} & \multirow{3}{*}{200} & EN powder dispersion in SDS alkaline solution at 95° C for 24 h. & \multirow{3}{*}{Cr(VI)} & Surface complex formation, reduction and an exchange reaction & [29] \\  & & & & & and in exchange reaction & \\ \hline \multirow{3}{*}{MoP\({}_{\text{3}}\) loaded water bismuth plants biochar} & \multirow{3}{*}{-700} & Naminated MnO-loaded biochar was prepared using & Pb & 268.9 & \multirow{3}{*}{Electrostatic interactions} & [30] \\  & & Nd/Pd/Al & at 25° C for 30 min, ultra-fluorizing addition of & Ca & 79.2 & \multirow{3}{*}{Electrostatic interactions} & [30] \\  & & 40 m/L of 30°, HQ\({}_{\text{3}}\) and pH adjusted to 7.0 & Zn & 68.6 & & \\ \hline \multirow{3}{*}{30 MnO\({}_{\text{2}}\) modified rice hanks biochar} & \multirow{3}{*}{500} & Rice hanks biochar mobility in rapid redox reaction & Ca & 84.76 & \multirow{3}{*}{Chemical adsorption} & [31] \\  & & between Nd/Pd/Al & Nd/Pd/Al & at, embedded in the & & \\  & & polyacrylamide gold to polymerization & Pb & 70.90 & & \\ \hline \multirow{3}{*}{Jusube pit} & \multirow{3}{*}{800} & Pyrolysis under x-atmosphere for 2 h. & \multirow{3}{*}{Pb} & \multirow{3}{*}{137.1} & \multirow{3}{*}{Ion exchange} & [23] \\  & & & & & \\ \hline \multirow{3}{*}{Grape pomace lignin} & \multirow{3}{*}{700} & Feedback particle-a058 nm. From room temperature & \multirow{3}{*}{Pb} & \multirow{3}{*}{150} & Physical and chemical segregation & [33] \\  & & & & & \\  & & & & & \\ \hline \multirow{3}{*}{Corn Stinks (L-cy/FOOT100 ptons 102)} & Mass ratio Fed/F