Integration of olive stones in the production of Fe/AC-catalysts for the CWPO treatment of synthetic and real olive mill wastewater

Abstract

A by-product of olive mill’s operation (olive stone, OS) was transformed into activated carbon (AC) and used as support to prepare Fe-based catalysts, which were employed on the catalytic wet peroxide oxidation (CWPO) of olive mill wastewater (OMW). Three Fe-impregnation routes were tested: incipient wetness impregnation (IWI), adsorption (Ads), and hydrothermal (HT), resulting in catalysts with distinct iron loadings, particle sizes and surface dispersion. OSAC-Fe catalysts were characterized by N2 and CO2 physisorption, XRD, XPS, FTIR, HRTEM, EDX, and HRSEM techniques. Catalysts’ activity and stability was first compared in the degradation of synthetic polyphenolic solutions. After one cycle (240 min), catalysts’ sorption capacity was considerably exhausted and OH radicals were found to be the main oxidative species responsible for total phenolic content (TPh) removal. OSAC-Fe-IWI and OSAC-Fe-Ads performed better than OSAC-Fe-HT after four consecutive cycles (53 and 48 vs. 38% TPh removals, respectively), also showing considerably lower cumulative Fe leaching values (2.2 and 2.8 vs. 10 wt%). The most promising materials were used for depuration of real OMW samples. Under smooth operational conditions ([OSAC-Fe-IWI] = 2.0 g/L, [H2O2] = 0.5 g/L, pH0 ~ 4.9, T0 = 25 °C), 55% TPh removal was attained after 240 min, resulting in a significant reduction of the effluent’s toxicity (from 100% Vibrio fischeri bioluminescence inhibition to 36%), 37% chemical oxygen demand degradation, and 21% total organic carbon mineralization. Promising catalytic performances were also achieved by OSAC-Fe-Ads, despite its considerably lower iron loading, highlighting the importance of Fe surface dispersion.