This study investigated the effects of iron-based materials—microscale zero-valent iron (mZVI), nanoscale zero-valent iron (nZVI), and nanoscale magnetite (nano-Fe3O4, in two size ranges: 50 \(\:-\) 100 nm and 14 \(\:-\) 29 nm)—on the anaerobic digestion (AD) of thermally hydrolyzed sewage sludge (THSS). Batch experiments were conducted under mesophilic conditions with three dosages (5, 15, and 30 mg/g-TS) of each material. Methane production kinetics were analyzed using the modified Gompertz model. A sequential extraction procedure was used to assess the speciation of potentially toxic elements (PTEs), namely, Zn, Cu, Pb, Ni, and Cr, in the digestates. The results showed that both mZVI and nZVI enhanced cumulative CH4 production more than either size of nano-Fe3O4. The highest cumulative CH4 yields (223 mL/g-VSadded), approximately 9% higher than the control, were achieved at nZVI dosages of 5 and 15 mg/g-TS. Among iron-based materials, nZVI most effectively shortened the lag phase (1.6-fold decrease at 15 mg/g-TS), whereas both sizes of nano-Fe3O4 had minimal effect (maximum 1.06-fold decrease for the 50–100 nm Fe3O4 at 30 mg/g-TS). The addition of mZVI and nZVI increased the mobility of Zn, Cu, and Ni in the digested THSS samples, while both nano-Fe3O4 materials reduced mobility of all studied PTEs. Overall, the results indicate a trade-off between enhanced methane production and environmental risk; mZVI and nZVI improve AD but increase PTE mobility, whereas nano-Fe3O4 mitigates PTE mobility with little or no effect on CH4 production.
