Binding characteristics of humic substances with Cu and Zn in response to inorganic mineral additives during swine manure composting

Composting is suitable for recycling livestock manure into valuable organic fertilizer, which can improve soil quality while mitigating potential risk of heavy metal pollution. Humic substances (HS) in compost have been demonstrated to play a key role in regulating the redistribution of heavy metal fractions. However, limited direct information have been reported on how different components of HS complexes with heavy metals to affect their bioavailability during composting. In this study, sequential extraction procedures (H2O, KCl, Na4P2O7, NaOH and HNO3) were used to assess the characteristics that HS bound with Cu and Zn during composting of swine manure and straw added either 5% boron waste (BW) or 5% phosphate rock (PR). Organically complexed fraction extracted by Na4P2O7 contained only 33–41% of the Cu but most of the Zn (81–87%). During composting, initially mobile fractions of Cu and Zn (extracted by H2O or KCl) changed into more stable fractions (extracted by NaOH and HNO3), and both organic matter and fulvic acids (FA) were identified as critical factors to explain this redistribution based on redundancy analysis. Over 80% of Cu and Zn were complexed with FA of HS. However, exogenous additives (phosphate rock and boron waste) enhanced Cu conversion by promoting humification (Humic acid/Fulvic acids, HA/FA) whereas they had limited influence on Zn, due to the relatively weak binding relationship between Zn and HA. •Cu and Zn were mainly concentrated in organically complexed fraction.•Cu and Zn were mainly complexed with fulvic acid rather than humic acid during composting.•Inorganic additives improved Cu stability by promoting complexation with humic acid.