Phosphorus-Rich Biochars Can Transform Lead in an Urban Contaminated Soil

Transformation of soil Pb to pyromorphites and phosphates has the potential to be an effective strategy to immobilize this contaminant in situ. Soil treatment using monocalcium phosphate, a commercial fertilizer (NTS Soft Rock) and biochars prepared from poultry litter and from biosolids at three different temperatures (300, 400, and 500°C) and two doses (1 and 3%) were evaluated. Lead bioaccesibility, mobility, and solid speciation were measured. Leachable Pb (determined with the toxicity characterized leaching procedure) was not significantly ( > 0.05) changed after biochar addition, but a significant decrease in bioaccesible Pb was found for several treatments ( < 0.05). This was particularly notable for treatments receiving biosolids prepared at 400 and at 500°C or monocalcium phosphate at the 3% dose. The decrease in bioaccesible Pb concentration in the biochar treatments was similar to traditional phosphate amendments. Our research found transformation of Pb species to the more stable pyromorphite and Pb-phosphate to be partially responsible for the observed changes, although other mechanisms, including pH changes, might also play an important role. Overall, pyrolysis was an effective method to upgrade waste streams and facilitate Pb immobilization, although key pyrolysis parameters need to be selected carefully.