Co-pyrolysis of sewage sludge and food waste digestate to synergistically improve biochar characteristics and heavy metals immobilization

Food waste digestate (FWD) is a desirable additive in sewage sludge (SS)-based biochar preparation owing to its high contents of intrinsic inorganic minerals and lignocellulosic compounds. In this study, we investigated the co-pyrolysis of SS with FWD at different mixing ratios (4:0, 3:1, 2:2, 1:3, and 0:4; SS:FWD w/w) at 550 °C to synergistically improve the biochar characteristics and immobilize the heavy metals in the SS. The results showed that co-pyrolysis of SS with FWD greatly increased the aromaticity and pH (by 13.22–26.56%) of the blended biochar, and significantly reduced the contents of total and bioavailable heavy metals. The addition of FWD effectively enhanced the conversion of heavy metals from less stable fractions to more stable forms, but led to the transformation of Cr from the residual fraction (F4) to the oxidizable fraction (F3) when the FWD:SS ratio was ≥ 3:1. Overall, the formation of co-crystal compounds, stable kaolinite, and metal oxides together with the enhancement of biochar characteristics during co-pyrolysis significantly reduced the heavy metal-associated ecological risk (potential ecological risk index lower than 15.51) and phytotoxicity (germination index higher than 139.41%) of the blended biochar. These findings suggest that high levels of mineral components in FWD greatly immobilize more heavy metals in biochar.