Valorization of olive stone by-product as Fenton-like catalysts for olive mill wastewater treatment

The olive oil industry is responsible for the generation of highly-loaded and complex effluents (olive mill wastewaters, OMW) that pose several environmental threats. In particular, OMW’s polyphenolic fraction is known for its recalcitrant behavior and phytotoxic properties (McNamara et al., 2008). In a perspective of by-products’ valorization within the same agro-industry, olive stones (OS) were transformed into porous activated carbon (AC) materials for Fe-anchoring. In line with the circular economy trend, the resulting Fe-catalysts were then used in the heterogeneous Fenton-like oxidation of OMW. OSAC supports were synthetized by sequential carbonization and CO2-activation of OS in a horizontal tube furnace (800 °C). Three distinct Fe-impregnation routes were employed to synthetize OSAC-Fe catalysts: incipient wetness impregnation (IWI), adsorption (Ads), and hydrothermal (HT). All materials were extensively characterized by several techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission (TEM) and scanning (SEM) electron microscopy, as well as N2- and CO2-physisorption. Screening of the resulting catalysts was performed with a synthetic solution simulating the toxic polyphenolic fraction of OMW; then a real OMW solution was used with the most promising materials. All catalysts presented well-developed microporosity with a slight contribution of mesopores in their structure. XRD confirmed the formation of mixed iron-oxides (hematite, magnetite) as the active-phases. Key differences in the catalyst’s Fe-loadings, metallic particle sizes and surface dispersion were observed, highlighting the importance of the impregnation procedure step in the production of catalysts (Esteves et al., 2021). Despite their developed porosity, the catalysts’ adsorptive capacity was substantially exhausted after one 240-min cycle. Thereafter, hydroxyl radicals (OH•) were identified by radical scavenging tests as the main oxidizing agents of the phenolic compounds. After four consecutive cycles, catalysts prepared by IWI and Ads performed better than the one synthetized by HT (53 and 48% vs. 38% phenolic content removal, TPh, respectively), also showing higher stability (4-5 times lower Fe-leaching from the support). Using the most promising catalyst ([OSAC-Fe-IWI] = 2.0 g/L) under smooth operational conditions (25 °C, no pH adjustments, [H2O2] = 0.5 g/L), 61% TPh, 38% chemical oxygen demand (COD), and 22% total organic carbon (TOC) removals were achieved with real OMW samples. The produced effluent also showed a toxicity reduction towards V. fischeri from initial 100% to 31% after only 240 min (as inferred by the decrease of bioluminescence inhibition after 30 min of contact with the bacteria). Under the same experimental conditions, promising catalytic performances were also attained by OSAC-Fe-Ads, emphasizing the importance of the superior Fe-surface dispersion in this sample, despite the lower Fe-load obtained as a result of the impregnation procedure. Though legislated discharge values were not met, the significant toxicity reductions of the OMW combined with an improvement of biodegradability indices, suggests that the stress of a downstream biological step would be greatly reduced by a preliminary oxidation step. Moreover, the heterogeneous Fenton-like process is able to tackle some constraints associated to analogous homogeneous processes, such as the formation and downstream management of Fe-sludges and strict pH range of operation.