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Accumulation of heavy metals in unconsolidated soils can prove toxic to proximal environments, if measures are not taken to stabilize soils. One way to minimize the toxicity of metals in soils is the use of materials capable of immobilizing these contaminants by sorption. Biochar (BC) can retain large amounts of heavy metals due to, among other characteristics, its large surface area. In the current experiment, sugarcane-straw-derived biochar, produced at 700 °C, was applied to a heavy-metal-contaminated mine soil at 1.5, 3.0, and 5.0% (w/w). Jack bean and Mucuna aterrima were grown in pots containing a mine contaminated soil and soil mixed with BC. Pore water was sampled to assess the effects of biochar on zinc solubility, while soils were analyzed by DTPA extraction to confirm available metal concentrations. The application of BC decreased the available concentrations of Cd, Pb, and Zn in the mine contaminated soil leading to a consistent reduction in the concentration of Zn in the pore water. Amendment with BC reduced plant uptake of Cd, Pb, and Zn with the jack bean uptaking higher amounts of Cd and Pb than M. aterrima. This study indicates that biochar application during mine soil remediation could reduce plant concentrations of heavy metals. Coupled with this, symptoms of heavy metal toxicity were absent only in plants growing in pots amended with biochar. The reduction in metal bioavailability and other modifications to the substrate induced by the application of biochar may be beneficial to the establishment of a green cover on top of mine soil to aid remediation and reduce risks.

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Purpose Biochars may enhance the retention capacity of metals in soils, especially in highly weathered tropical soil whose low cation exchange capacity renders heavy metals mobile, and thus be able to leach from soils. We evaluated the effect of sugarcane-straw-derived biochar on sorption and desorption of Cd(II) and Zn(II) in two tropical soils in particular to distinguish primary and secondary mechanisms of metal retention. Material and methods To test the efficiency of biochar to retain heavy metals, sugarcane-straw-derived biochar was mixed with a clayey Oxisol and an Entisol both from the state of Sao Paulo, Brazil, in batch testing to obtain sorptiondesorption isotherms of Cd(II) and Zn(II) and measure the release/displacement of cations (Ca 2+ and Mg 2+ ) or precipitation with phosphate during the sorption process. Results and discussion Biochar increased the sorption (including adsorption and precipitation) of both metals in both soils but that most sorption reactions were reversible under buffer acidic conditions, due to dissolution of precipitates in low pH values (<4.9). Exchange of Cd or Zn with Ca and Mg from the biochar was found to play a minor role on the retention mechanism, whereas surface precipitation (mainly in the Entisol) of the metals (e.g. with phosphate) was likely to be the main sorption mechanism. Conclusions Application of sugarcane-straw-derived biochar to heavy-metal-contaminated tropical soils seems justified because of its sorptive capacity for Cd and Zn. However, binding reactions on surfaces were reversible, mainly for Cd(II), resulting in the likelihood that repeat applications of biochar would be required to maintain reduced soil solution concentrations of Cd and Zn over time, thus avoiding phytotoxicity and associated environmental risks.

Submetido em 30/11/2010 Aceito para publicação em 12/04/2011 ResumoA alface é a mais popular das hortaliças folhosas, sendo conhecida mundialmente e o seu consumo ocorre principalmente na forma "in natura". Objetivou-se avaliar o efeito da omissão de macronutrientes no crescimento H QR HVWDGR QXWULFLRQDO GD DOIDFH FY 9HU{QLFD EHP FRPR GHVFUHYHU VLQWRPDV YLVXDLV GH GH¿FLrQFLD QXWULFLRQDO 2V tratamentos foram: solução completa e omissão individual de N, P, K, Ca, Mg e S, sob delineamento inteiramente casualizado, com três repetições. As plantas foram cultivadas em solução nutritiva de Hoagland & Arnon, em vasos (8L). Aos 56 dias após o transplantio, foram avaliados: a altura das plantas, o número de folhas, a área foliar, o índice SPAD, a matéria seca da parte aérea, das raízes e planta inteira, os teores dos macronutrientes da parte aérea e raízes e descrição das desordens nutricionais. As omissões dos nutrientes afetaram as variáveis de crescimento. Os teores de nutrientes observados nas plantas de alface do tratamento com solução nutritiva completa e da omissão, na parte aérea foram, respectivamente, N= 23,2 -9,5; P= 5,4 -1,3; K= 58,9 -3,2; Ca= 12,1 -3,6; Mg= 5,5 -0,7 e S= 3,2 -1,5g.kg