Valorization of Red Mud as a Catalyst in the Treatment of Olive Mill Wastewater by Fenton Reaction

Abstract

The worldwide olive oil sector is a strategic sector for olive oil producing countries due to olive oil nutrition and health benefits. With what is being produced of olive oil, it continues to produce wastewater with a significant environmental impact due to the high organic load and the biochemical composition of this wastewater, particularly, the presence of microbial growth inhibiting compounds such as phenolic compounds, which makes its biological treatment difficult. On other way, red mud, the main leaching residue resulting from the alkaline treatment of bauxite (Bayer process), can be used as catalyst in chemical processing. Bayer red mud reveals high alkalinity, strong water absorption, and a large content of iron. The presence of metals in the composition of the red mud, such as iron dioxide (hematite), titanium dioxide, etc. allows its use as a catalyst in advanced oxidation processes. In this work, red mud has been revalued as a catalyst in the treatment of olive mill wastewater (OMW) by Fenton reaction. All experiments were carried out at laboratory scale in reactor with capacity of 500 cm3. Experiments have been carried out at different concentrations of red mud 0.05, 0.10, 0.5, 1.0, 2.0, 4.0, 5.0, 20, and 30 g/L. In parallel, three control experiments were carried out using only hydrogen peroxide or hydrochloric acid or red mud without pH adjustment (adsorption experiment). Experimental results have determined that the introduction of red mud as a catalyst in the like Fenton reaction (H2O2/red mud) with concentrations higher than 0.05 g/L has allowed the increase of the degradation percentages until reaching stable values at red mud concentrations higher than 5 g/L. The removal percentages at 0.5 g/L of red mud were COD = 47.1 %, total organic carbon (TOC) = 58.1 %, total carbon (TC) = 66.8 %, total nitrogen = 44.1 %, and total phenolic compounds (TPCs) = 63.5 % versus 57.2 % for COD, 74.4 % for TOC, 79.9 % for TC, 70.7 % for TN, and 66.0 % for TPCs in Fenton like reaction with 5 g/L of red mud. The common operating conditions were initial COD of OMW = 6171.9 mg O2/L, initial TOC of OMW = 3253.7 mg/L, pH = 3, magnetic agitation speed = 460 rpm, environment temperature, and the H2O2 at 10 % (w/v) added to the OMW according to the stoichiometry of the reaction. Given these results, it can be concluded that red mud can be a promising catalyst in oxidation systems based on the Fenton reaction allowing the incorporation of wastes into new green processes leading to the achievement of circular economy in industrial processes.