Heat Capacity-Manipulated Balance in Phosphorus Bioavailability Enhancement and Heavy Metal Stabilization during Sewage Sludge Copyrolysis

Abstract

Disentangling the thermochemistry of sludge copyrolysis to balance phosphorus (P) bioavailability enhancement and heavy metal (HM) stabilization is challenging due to intricate and disordering thermochemical reactions caused by sludge compositional inhomogeneity. A heat capacity assembly and dispatch strategy via antagonistic additive-paired sludge copyrolysis was conceived to quantitatively determine the balance for P reclamation from sewage sludge (SS). Calcium oxide (CaO, with additions ranging from 7.54 to 10%) and wasted rapeseed meal (RM, with additions ranging from 10 to 50%)-formulated copyrolysis experiments were designed to investigate the evolved fate of P and HMs in SS-derived biochar (SSB) production. Ryegrass plant cultivation was adopted to examine the orchestrated balance of P and HMs in soil applications. RM and CaO preferentially influenced the P distribution, including iron/aluminum-bound P and apatite/CaCO₃-associated P, respectively. In contrast, both additives consistently reached equilibrium with two HM ensembles, including soluble/reducible and oxidizable/residual fractions. SSB derived from RM-added (50% addition) and CaO-RM-paired (6% CaO and 47% RM addition) copyrolysis at 700 °C demonstrated preferential P acquisition of ryegrass (height increase by 17.4 and 4%, respectively) while maintaining a low HM ecological risk index (27.5 and 16.3, respectively). The kinetic and thermodynamic results confirmed that CaO and RM had antagonistic effects on the thermochemistry of sludge copyrolysis. The heat capacity reached its maximum at around 675 °C and was not affected by the paired additives. The outcome can rationalize P upcycling from SS-like waste resources through tapping the heat capacity precisely tuning the thermochemistry of copyrolysis, thereby boosting global P circularity.