From Agricultural Waste to Sustainable Adsorbent: Characterization of Hazelnut Shell Biochar and Its Performance in Waste Frying Oil Purification

Agricultural hazelnut shell waste was converted into biochar by slow pyrolysis at 500, 700, and 900 °C and evaluated as a sustainable adsorbent for waste frying oil purification. The biochars were characterized by proximate, thermal, spectroscopic, morphological, and elemental analyses, including Brunauer–Emmett–Teller (BET) surface-area measurement, and benchmarked against activated carbon and Magnesol. Pyrolysis markedly developed the pore structure, increasing the BET surface area from 1.68 m2/g in the raw shell to 666.05 m2/g at 900 °C, and an exploratory principal component analysis summarized 90.85% of the total variance. Acid-extractable element concentrations were screened against feed-grade biochar and oenological bentonite reference values, without establishing regulatory compliance or food-contact suitability. All three biochars significantly reduced free fatty acidity (27.6–37.8%) and p-anisidine value (23.3–27.5%), with no significant difference relative to activated carbon (Tukey’s test, p > 0.05); however, total polar material was not significantly reduced under these mild, short-contact conditions. Gas chromatography showed no significant changes in the major unsaturated fatty acids of the glyceride-bound fraction, and total color differences remained small (ΔE* = 0.71–1.82). Hazelnut-shell biochar therefore shows potential as a waste-derived, circular adsorbent for selected oil-degradation products.