How additives steer sewage sludge hydrochar properties: Divergent pathways of MgO/CaO versus sawdust in carbon/nutrient fate and magnetism

How contrasting additives mediate nutrient/carbon (C) fate and hydrochar magnetism in sewage sludge co-hydrothermal carbonization remains unclear. We compared a lignocellulosic biomass (sawdust) with alkaline earth metal oxides (MgO, CaO). MgO/CaO raised hydrochar pH from 5.0 (sludge-alone, SC) to 5.8–7.8 and phosphorus (P) partitioning from 89.5% to 90.3%-96.0%, but lowered C and nitrogen (N) distribution (C: 64.5% to 59.6%-62.1%; N: 47.1% to 41.3%-46.1%) and C content (202.8 to 148.7–178.9 mg/g). Sawdust decreased pH to 4.3–4.7, increased C and N distribution (C: 64.5% to 70.5%-83.1%; N: 47.1% to 57.2%-84.9%) and C content (202.8 to 279.7–444.2 mg/g), but reduced P partitioning (89.5% to 60.1%-72.0%). Both additives (MgO/CaO and sawdust, respectively) decreased N and P content (N: 26.3 to 18.4–22.9 and 17.6–24.7; P: 29.4 to 24.6–27.4 and 7.1–18.1 mg/g), increased C/N ratio (4.5 to 4.9–5.8 and 6.7–16.5), and raised potassium (K) distribution (19.5% to 25.9%-37.5% and 21.5%-47.2%). Both reduced available N (3.6 to 1.1–2.4 and 1.5–2.9 mg/g), but increased available P (193.4 to 326.9–587.4 and 212.6–243.0 mg/kg) and available K (284.7 to 294.3–333.3 and 305.0–357.7 mg/kg). Dissolved organic carbon rose with MgO/CaO (17.2 to 21.3–29.6 mg/g) but fell with sawdust (17.2 to 11.6–14.9 mg/g). MgO/CaO increased pH, converting augelite to brushite to enhance P availability, while forming magnetic MgFe₂O₄. Despite opposite pH effects and distinct pathways (Mg2+/Ca2+ vs. carbon skeleton supply), both additives enriched heterocyclic-/quaternary-N and carboxyl groups, modulating nutrient/C distribution and availability. Thus, additives enable targeted hydrochar design (fertilizer, soil amendment, magnetic adsorbent), with MgO/CaO offering a novel route to magnetic products.