# Application of Biochar for the Restoration of Metal(loid)s Contaminated Soils
## 4 Discussion
### Effect of Biochar on Soil Properties and Metal(oid)s Concentration
#### 4.1.3 Soluble and Bioavailable Forms  
The increase in soluble forms of Cu and Zn is strongly related to soil properties, the acid pH being mainly responsible [43]. The opposite is observed for As, which tends to be more mobile at alkaline pH [48]. Indeed, unlike cationic metals, As is in the form of an oxyanion in solution, which make the remediation more difficult because its mobility in soil may increase with increasing pH when anion release occurs [49]. In our study, the addition of biochar significantly reduced the concentration of the mobile fractions of metals, especially for Zn and Cu. Thus, concentration of the mobile forms of Zn and Cu were inversely correlated with pH (r = 0.735-0.942, \(p>0.01\)) and directly with EC (r = 0.507-0.834, \(p<0.01\)) (Table 18). Previous studies have also reported a reduction of Zn mobility due to the application of biochar. [PERSON] and [PERSON] [41] observed that biochar reduced Zn concentration in the leachate through surface sorption. [PERSON] et al. [50] also reported a decrease in Zn concentrations in the leachate after the addition of biochar, but it was related to an increase in pH. Zn bioavailability is drastically reduced in RA8B in response to the sharp increase in pH. This behavior was also observed by [PERSON] et al. [26], who found that the application of biochar to a mining-contaminated soil, increase pH and CEC, and reduced the bioavailable concentrations of metals. In the case of Cu, the bioavailability is reduced with the biochar addition, but this reduction was of a similar magnitude in the soil amended with biochar, regardless the biochar dosage applied. This result differs from those described by [PERSON] et al. [51], who observed a decrease in bioavailable forms related to the amount of biochar used, although these authors worked with much higher doses (15-25%) of biochar. The main mechanism related to the reduction of Cu mobility after biochar addition is cation exchange capacity [52], so a certain amount of Cu retained is in bioavailable forms. For the mobile forms of Pb, a low correlation with soil properties was found, positive with pH (r = 0.539, \(p<0.01\) and r = 0.852, \(p<0.01\), for soluble and bioavailable forms, respectively) and negative with EC (r = \(-0.557\), \(p<0.01\) and r = \(-0.732\), \(p<0.01\), for soluble and bioavailable forms, respectively) (Table 18). In fact, Pb presented the highest solubility ratio in NC, with negligible fractions in RA. Lead is a very immobile element in soils, being clay minerals, pH and organic matter, the most important factors determining its fixation [53,54]. Probably in contaminated soils, Pb may have been immobilized by Fe hydroxysulphates [38,55], and Pb accumulation is also described in the most superficial part of soils, due to sorption by organic matter, by the formation of stable complexes, thus reducing phytotoxicity when the soil has an acidic pH [56]. However, in NC the presence of vegetation may have promoted the formation of complexes between Pb and organic matter contributing to the high solubility of Pb and consequently to the increase of its availability when the pH was raised above 6.5 [57]. Arsenic also showed a low mobility in the polluted soils. The anionic behavior of As favors adsorption under acidic conditions due to its binding to Fe/Mn oxides present in the soil [43,58]. However, when the pH approaches neutrality, an increase in bioaccessibility may also occur. According to correlation analyses, the variation of the soluble fraction of As with respect to pH value is not significant, while the bioavailable fraction is positively related to pH (r = 0.672, \(p<0.01\)) and negatively related to EC (r = \(-0.311\), \(p\) = 0.032) (Table A2). The addition of biochar did not lead to immobilization of this metal(loid), coinciding with the results obtained by [PERSON] and [PERSON] [41], who found that biochar had a negligible influence on mobile As concentrations in soil contaminated with multiple metals.