For further resource utilization of solid waste steel slag and the reduction in biodiesel production costs, this study used steel slag as a carrier to synthesize a CaO-CeO₂/slag solid base catalyst for the effective transesterification of palm oil into fatty acid methyl esters (FAMEs). The synthesis involved a two-step impregnation of steel slag with nitrate of calcium and cerium and thermal activation at 800 °C for 180 min. Then, the catalysts’ textural, chemical, and CO₂ temperature-programmed desorption properties were characterized. The catalytic activity depended highly on the ratio of Ca-Ce to steel slag mass; the CaO-CeO₂/slag-0.8 catalyst showed outstanding performance. Characterization showed that the surface area and total basicity of the Ca-Ce/slag-0.8 catalyst were 3.66 m²/g and 1.289 mmol/g, respectively. The reactivity results showed that FAMEs obtained using 7 wt.% catalyst, 9:1 of methanol-to-palm-oil molar ratio, 180 min reaction duration, and 70 °C reaction temperature was optimum (i.e., 95.3% yield). In addition, the CaO-CeO₂/slag-0.8 catalyst could be reused for at least three cycles, retaining 91.2% of FAMEs yield after n-hexane washing. Hence, the catalyst exhibits an excellent potential for cost-effective and environmentally friendly biodiesel production.

Hydrothermal carbonization (HTC) of corn straw causes hydrolysis and pyrolytic reorganization of the carbon skeleton, leading to the formation of polycyclic aromatic hydrocarbons (PAHs). When used as a soil amendment, hydrothermal carbon can lead to soil contamination, increased biotoxicity, and potential harm to ecosystem health. To systematically evaluate PAHs formation mechanisms, single-factor experiments were carried out by treating corn straw under varying temperatures (180 to 300 °C) and durations (2 to 6 h) in a closed batch reactor. PAHs were quantified via gas chromatography-mass spectrometry (GC-MS) with deuterated internal standards. Results revealed that total PAHs concentrations increased by 409%, 66.5%, and 68.3% at 180 °C, 210 °C, and 240 °C (4 h and 2 h), respectively, attributed to intensified dehydration and aromatization reactions under subcritical conditions. Conversely, PAHs levels decreased by 80.4% and 78.1% at 270 °C and 300 °C (4 h and 2 h), likely due to thermal cracking of PAHs macromolecules into low-molecular-weight fragments. Prolonged treatment (6 h and 4 h) reduced PAHs by 62.9 to 70.8% at ≤240 °C, suggesting oxidative degradation pathways dominate over pyrolysis under extended residence time. Mechanistic analysis indicated that optimizing HTC at 270 °C for 4 h achieves a critical balance between carbonization efficiency and PAHs suppression, providing a feasible strategy to mitigate ecotoxicological risks of hydrothermal carbon in soil remediation.