Chemical speciation and risk assessment of Cu and Zn in biochars derived from co-pyrolysis of pig manure with rice straw

Pig manure has been utilized as a good feedstock to produce biochar. However, the pig manure-derived biochar from intensive pig cultivation contains high levels of total and bioavailable heavy metals. In this study, the co-pyrolysis of pig manure with other biomass (e.g. rice straw) at 300–700 °C was investigated to solve the above-mentioned topic. The ammonium acetate (CH3COONH4), Tessier sequential extraction procedure and hydrogen peroxide were adopted to evaluate the bioavailability, chemical speciation, and potential risk of Cu and Zn in the biochars. Results showed that the addition of rice straw significantly reduced the concentrations of total, exchangeable and carbonate-associated Cu and Zn in the biochars compared to the single pig manure biochars. Co-pyrolysis of pig manure with rice straw at a mass ratio of 1:3 and at 600 °C could be most effective to reduce the concentrations of CH3COONH4-extractable and potential released Cu and Zn in the biochars. In conclusion, the co-pyrolysis process is a feasible management for the safe disposal of metal-polluted pig manure in an attempt to reduce the bioavailability and potential risk of heavy metals at relatively high pyrolysis temperatures. •Co-pyrolysis of pig manure and rice straw for biochars was conducted.•Hydrogen peroxide was used to evaluate the potential risk of Cu and Zn in biochars.•Reduction of total Cu and Zn contents in blended biochar was due to dilution effect.•Exchangeable metals were transformed to more stable forms due to the co-pyrolysis.•Pyrolysis of pig manure with rice straw at a ratio of 3:1 and at 600 °C were optimum.