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Reuse of Homogeneous Fenton’s Sludge from Detergent Industry as Fenton’s Catalyst

Abstract Fenton’s reaction is an advanced oxidation process where, classically, hydrogen peroxide is the oxidizing agent and an iron catalyst promotes the formation of hydroxyl radicals (•OH). Among the studies that evaluated different metals as Fenton-like catalysts, our group of investigation has recently used cerium-based solids as heterogeneous catalysts in slurry reaction and, in this work, iron sludge coming from an industrial Fenton’s reactor used for the wastewater depuration of a detergent production factory is being appraised while treating a synthetic effluent containing 0.1 g.L-1 of seven phenolic acids commonly found in olive mill wastewaters - one of our group’s major focus. The treatment facility uses a homogeneous Fenton’s process and the dried sludge contains approximately 37% (w/w) iron content. As it comes directly from the plant and it is a pretty raw mud, it is expected to find high amounts of organic matter in the catalyst, because there are no further steps besides baking to enhance its efficiency. This mud was dried, baked and milled to become the catalyst. Calcination temperatures of 300, 400, 500 and 1000 °C were employed and a comparison with the non-baked catalyst was performed. Chemical oxygen demand (COD), total phenolic content (TPh) and total organic carbon (TOC) removals were assessed. Higher calcination temperatures were most-likely to show better organic depletion, although higher levels of iron leaching were detected. Biological oxygen demand in five days (BOD5) was increased in some cases, what could enable the application of a subsequent biological treatment step. Adsorption tests were carried out in order not to erroneously associate the abatements to oxidation and have proved that TPh, COD, TOC and BOD5 values did change by chemical reaction rather than single adsorption.