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

Co-pyrolysis of straws with manures has been found effective to mitigate heavy metal risks in manure-derived biochars. This study further investigated co-pyrolysis strategy on the levels, species and risks of metals (Cu, Zn, Cr, Ni, Pb, and Cd) carried by manure-based biochars through co-pyrolyzing swine manure (SM) and corn straw (CS) with different mixture ratios (1:0, 0:1, 3:1, 1:1, and 1:3, w/w) at 300 ℃, 500 ℃ and 700 ℃. The total heavy metals in SM biochars were significantly reduced by CS addition except when SM/CS ratio was 3:1 at 300 ℃. Notably, CS addition increased stable Ni, Zn, Cu, Pb and Cd, but simultaneously mobilized part of Ni, Zn, Cu and Pb in SM biochars, especially at higher CS ratio and higher temperature. Co-pyrolysis converted less stable Cd to more stable Cd at all pyrolysis conditions, with higher CS ratio and higher temperature more effective. Overall, higher temperature (700 ℃) and higher addition ratio of CS (SM/CS 1:3) were more favorable for mitigating the potential ecological index of biochar-bearing Cd, Cu and Zn, the dominating risky contributor to SM biochars, hence more effective to mitigate the overall environmental risks of heavy metals in the derived SM biochars.