Biochar (via slash-and-char) could be an effective remediation strategy in agricultural soils contaminated with cadmium, lead, and zinc, according to a recent publication in Environmental Pollution by Niu et al. (2015). Slash-and-char is an ancient agricultural alternative to slash-and-burn, in which vegetation is cut, allowed to dry, converted to biochar by smoldering in simple earthen mounds or pits, and mixed into surrounding soil. The study found that biochar produced in this way could reduce metal concentration to a safe level in vegetable crops.
Contaminated soils could be remediated in a different way. See what ISTC researchers have done with the Mud-to-Parks program.