Direct sulfonation of defatted sludge to prepare sulfonic acid solid catalysts for biodiesel production from sludge lipids

Sewage sludge lipids have been confirmed as ideal oil feedstock for the production of green fuel biodiesel. However, the resource utilization of the residual residue (defatted sludge) produced during lipid extraction has not yet been explored. This study found that the defatted sludge can be used to prepare sulfonic acid solid catalysts for the esterification or transesterification of lipids extracted from sewage sludge to produce biodiesel, thereby achieving the full high-value utilization of sludge components. Lipids were extracted by Soxhlet extraction, and sulfonic acid solid catalysts were prepared by direct sulfonation using defatted sludge as the carrier. The study showed that the catalyst (D-SS-160) prepared at a sulfonation temperature of 160 °C had the best catalytic performance, which was attributed to its surface-rich -SO3H active sites, as verified by characterization techniques such as Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). After optimizing the reaction parameters by Response Surface Methodology (RSM-BBD) method, the biodiesel yield reached 42.3%. Gas chromatography mass spectrophotometer (GC-MS) analysis confirmed that the chromatographic peaks of the biodiesel product were composed of fatty acid methyl esters such as methyl palmitate and methyl stearate, which met the diesel standard specified in EN 14212. In addition, the catalyst showed good stability and could still maintain a yield of 34.5% after four cycles. The preparation of sulfonic acid solid catalysts from defatted sewage sludge for the production of biodiesel from lipids extracted from sewage sludge is feasible, which can achieve the complete reuse of all components in sewage sludge and can be proposed as a feasible recycling strategy.