Phenolic Fingerprints of Spanish Olive Mill Wastewaters (Alpechin): A Step Toward Regional Valorization Through Antioxidant Recovery.

Olive mill wastewater (OMW), a by-product of olive oil extraction, poses significant environmental challenges due to its toxicity and heterogeneity. This study evaluates the phenolic and mineral composition of OMW and alpechin sludges from abandoned ponds in Spain, and establishes a standardized conventional method to recover phenolic fractions and promote their safe valorization as bioactive ingredients. Phenolic compounds were identified by triple-TOF-LC-MS/MS, and minerals and heavy metals were quantified by ICP-MS. Across thirteen ponds analyzed, samples from Cordoba, Tarragona, Alicante and Toledo showed higher phenolic levels, ranging from 7.2 g GAE/kg to 18.9 g GAE/kg, with methanolic extracts reaching 10.98-15.67 mg GAE/mL. Thirty-one phenolic compounds were identified, predominantly luteolin, apigenin, quercetin, and secoiridoid derivatives, notably hydroxytyrosol and tyrosol, supporting their functional potential as natural antioxidants. The mineral profile was dominated by K and Ca and showed negligible carryover to the phenolic organic fraction (<1%). Heavy metal concentrations in fresh OMW were 0.32-1.06 µg/kg for Cd and Hg and 9-43.9 µg/kg for As and Pb. In OMW sludge, they ranged between 0.033 and 0.19 mg/kg for Cd, 0.01 and 0.12 mg/kg for Hg, 5.45 and 8.06 mg/kg for As, and 4.45 and 23.70 mg/kg for Pb, whereas phenolic extracts contained only 0.15-21.50 µg/kg, remaining below EU food safety limits. This work presents one of the first integrated approaches to risk-benefit mapping of abandoned ponds in Spanish soils and advances extraction standardization by jointly considering functional potential, contaminant profiles, and matrix location.