A further inquiry into co-pyrolysis of straws with manures for heavy metal immobilization in manure-derived biochars.

•Co-pyrolysis of CS with SM reduced total Cu, Zn, Pb, Ni, Cd and Cr of SM biochars.•Higher CS addition ratio and higher pyrolysis temperature favored Cd and Cu immobilization.•Higher temperature and lower addition ratio immobilized Zn, Ni and Pb better.•The ecological risks of SM and single pyrolysis SM biochars were dominated by Cd, Cu and Zn.•Pyrolysis at CS/SM ratio 3:1 and 700 ℃ was optimal to reduce environmental risks of SM biochars.