Leaching and fractionation of heavy metals in mining soils amended with biochar

Contaminated mining soils might pose risk to the environment due to leaching of heavy metals into ground water, especially under acid conditions. Biochar might be an option for remediation of contaminated mining soils. The aim of this study was to evaluate the effect of soil acidification and biochar (BC) application on Cd, Pb and Zn mobility and chemical fractionation in two mining soils (A and B). Fifteen leaching columns per soil were packed, applying the following treatments: soil + 3% BC; acidified soil; acidified soil + 3% BC and control soil + CaCO3. The control treatment was constituted by the original polluted soil (soil A or soil B). BC was produced by slow pyrolysis at 700 °C from sugarcane straw. Ten leaching events were performed and Cd, Pb, Zn and pH were determined for each leaching. A sequential extraction procedure was performed after leaching was finished. The following fractions were evaluated: mobile, exchangeable, bound to Mn oxide, bound to OM, bound to Fe oxide and residual. In the polluted control soil metal mobility was very low. In acidified soils metal mobility increased drastically and BC application reduced the Cd (57-73%), Pb (45-55%) and Zn (46%) concentrations in the leachate. With the sequential extraction it was observed that Cd was mostly bound to the mobile fraction (15-35%) and exchangeable fraction (7-38%), Pb was mainly bound to the Mn oxide fraction (32-70%) and OM (31-43%) and Zn was principally bound to the residual fraction (31-68%). Cadmium presented the highest mobility index (33-53%) and Pb the lowest (1-3%), which is related to the fractions they are bound in the soil. Field studies would be necessary to confirm the effectiveness of BC application on heavy metals' retention, prior to large scale application.