# Unravelling the Environmental Application of Biochar as Low-Cost Biosorbent: A Review
## 3 Biochar Adsorption of Heavy Metals
### Lead  
In the literature, various kinds of biochar have been used for the removal of lead. Thus, [PERSON] et al. [63] determined the high adsorption capability of three unmodified biochars produced from peanut shell, ginkgo and metasequoa leaf. However, better results were reached by rice straw biochar prepared by pyrolysis at 700 \({}^{\circ}\)C (Pb uptake 171.34 mg/g) [64].  
Likewise, Pb removal using the biochar produced from different feedstocks (peanut shells, corn cobs, poultry manure, buckwheat husk, and white mulberry wood) was considerably high above 97.5% [65]. This fact could be attributed to four adsorption mechanisms driven by the main biochar properties, such as organic functional groups, mineral content, ionic content and \(\pi\)-electrons (Figure 3). These sorption mechanisms may take place simultaneously or supplementing each other.  
[PERSON] et al. [32] studied the Pb adsorption by jujube (_Ziziphus jujuba_) pit biochar, determining the positive effect by increasing the pH of the solution and the influence of coexisting ions, such as K\({}^{+}\) and Na\({}^{+}\). In addition, the reusability of the biochar was measured for five cycles. The regeneration process was done by solvent extraction (Figure 3) using 0.2 M hydrochloric acid and washed using distilled water followed by drying. It is detected that the removal level was around 70% of the initial adsorption capacity after the last round; this procedure can reduce the operational cost of the adsorption process.  
To improve the Pb adsorption capability of biochar, several treatments of biochar have been proposed, such as alkali [66] or acid treatment, [67] chemical modification with ammonia, hydrogen peroxide and nitric acid [68] and chemical graft [69]. Recently, [PERSON] et al. [30] used the redox precipitation method to load manganese dioxide (MnO\({}_{2}\)) nanoparticles on invasive water hyacinth biochar produced by pyrolysis at 450 \({}^{\circ}\)C at 5 \({}^{\circ}\)C/min. They determined higher calculated maximum uptakes (216.22-351.37 mg/g) concerning other biochars. Based on the kinetic study, it was determined that the rate-limiting step is chemisorption, Langmuir model represents the isotherm model, and Pb was adsorbed onto biochar via carbonyl and hydroxyl groups. Thus, the high Pb uptakes decreased when the concentration of coexistent natural cations common present in wastewater or natural water, such as K\({}^{+}\), Na\({}^{+}\), Ca\({}^{+}\), and Mg\({}^{2+}\), was increased.  
In this line, [PERSON] et al. [70] reported a facile one-pot synthesis technique to obtain a carboxymethyl chitosan coating on sewage sludge biochar with Pb and Hg removal capacity and good reusability. The modified biochar showed shorter equilibrium time for Pb and Hg adsorption with a capacity of 210 and 594.17 mg/g, respectively, a fact that could be explained by the strong chemical interactions between the amino, hydroxyl and carboxyl functional groups present in the modified biochar and heavy metals. Likewise, the adsorbent exhibited good stability, its regeneration being made possible by the use of Na\({}_{2}\)-EDTA solution (0.05M) as a regenerative agent without significant biochar alteration after five regeneration cycles, keeping a similar adsorption capacity.