Background. Recent studies have explored the potential for using biochar as a soil amendment in agriculture. However, it can also be used as a gentle remediation option for contaminant reduction. Biochar is a by-product obtained from the pyrolysis of biomass (organic matter). It is known for its long-lasting chemical properties, wide surface area values, and carbon-richness, which make it an efficient method for the immobilization of organic and inorganic contaminants such as heavy metals. Objective. The aim of the present study was to analyze the efficiency of biochar, obtained from the gasification of corncob, for the immobilization of lead in contaminated soils. Methods. In the present study, biochar from corncob was used as an amendment for soil contaminated with lead (extracted from the municipality of Malambo, Colombia) in order to estimate its ability to immobilize leaching lead. A comparison laboratory test applied a modified biochar produced with a 10% hydrogen peroxide chemical treatment. In addition, a pot experiment was done with both biochar by sowing seeds of Pennisetum clandestinum for 33 days. During this period, plant growth was measured for the different amendments of biochar concentrations. Results. Laboratory tests indicated that unmodified biochar obtained a maximum retention of 61.46% of lead, while the modified biochar obtained only 44.53% retention. In the pot experiments, the modified biochar indicated high germination and growth of seeds (up to 89.8%). Conclusions. Although the lead immobilization in soil was positive for both cases, the use of soil with high concentrations of lead (167.62 g/kg) does not indicate biochar's effectiveness for purposes of comparison with the current United States Environmental Protection Agency (USEPA) limit value (400 ppm for bare soil in urban play areas). Therefore, further studies are recommended using soil with lower lead concentration levels. Competing Interests. The authors declare no competing financial interests. One author is an employee of Pure Earth.

The following text is now added to the beginning of the fourth paragraph of the 'Statistical analysis' subsection of the Methods section: "A normalization factor can be added to account for the uncertainty in the symptom onset dates of the index cases. Assuming a uniform distribution, the likelihood would only differ by a multiplicative constant and give the same estimates. " Also, the time frame in the final sentence of that paragraph ("from day 1 to 7") was incorrect. The correct time frame is "from days 5, 8 and 11. " Finally, Fig. 1c and Extended Data Fig. 1 have been replaced accordingly, and the legend to Extended Data Fig. 1 ("to start from 1 days (top left) to 7 days (bottom right) before symptom onset") is now rephrased ("to start from 5 days (top), 8 days (middle) and 11 days (bottom) before symptom onset") to match the revised figure. The errors have been corrected in the HTML and PDF versions of the article. The original code has also been replaced with updated code in Supplementary Software 1.

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