Effect of particle size on phosphorus and heavy metals during the preparation of biochar from food waste biogas residue

In this paper, biochar (BRC) produced from biogas residue (BR) with three particle sizes (d≤0.15 mm, 0.15 mm <d≤0.25 mm, d>0.25 mm) were obtained by pyrolysis at 600℃. The effects of different particle sizes on the migration and transformation of phosphorus and heavy metals between BR and BRC were systematically studied. And toxicity characteristic leaching procedure (TCLP) leaching toxicity test and potential ecological risk assessment were used to evaluate the safety risk of BR and BRC. The results showed that the phosphorus in BR and BRC were mainly acid-soluble phosphorus (HCl-P), followed by residual phosphorus (Res-P), and the contents of other phosphorus forms were low. TP content showed a decreasing trend with increase of particle size in BR and BRC. Pyrolysis can promote the conversion of H2O-P, NaHCO3-P and NaOH-P to HCl-P and Res-P. As the particle size increases, the total amount of Cu and Zn in BR increases, while the amount of Cr decreases, the total amount of Cr and As in BRC increases, and the total amount of Zn and Pb decreases. Meanwhile, the F3+F4 fractions (oxidizable and residue fraction) of Cr, Zn, Pb, As in BR and that of Cr, Pb, As in BRC decreased with the increase of particle size, while Cu, Zn and Cd of BRC showed an opposite trend. The assessment results of TCLP leaching toxicity and potential ecological risk of heavy metals indicate that the heavy metals in BR and BRC are at low risk levels. © 2021, Editorial Board of CIESC Journal. All right reserved.