Advanced oxidation processes (AOPs) for the treatment of liquid wastes.

Advanced oxidation processes (AOPs) are very efficient technologies for wastewater treatment. The main objective of this study is to develop new, sustainable and cost effective AOPs for the treatment of liquid wastes. Heterogeneous Fenton like oxidation has been chosen as the model AOP for the treatment of liquid wastes due to ease of operation, recyclability of the catalyst, cost effectiveness and above all high organic removals. The development of a sustainable heterogeneous catalyst is a challenging task and requires a perfect combination of a metal over an appropriate support. First, a range of metallic catalysts supported over ZrO2, CeO2 and ZrO2-CeO2 are prepared by wet impregnation method, characterized by standard techniques and tested for their activities in oxidation process, using aqueous solution of ibuprofen (IBP) as model pollutant. Iron and copper-based catalysts supported over ZrO2 achieved maximum degradation and mineralization of IBP. Moreover, the oxidation pathway confirmed that they do not form more toxic metabolites than that of the parent molecule. Second, the catalyst Cu/ZrO2 is used for the oxidation of model pollutant IBP and heterogeneous Fenton process is optimized by investigating the favorable conditions of pH, catalyst dose, H2O2 dose and temperature. The optimized process was employed for the treatment of landfill leachate and analogous results with similar process efficacy were obtained. The Fenton process removed 65 and 92% of the total dissolved organic carbon (TDOC) from landfill leachate in a single-step and two-step processes, respectively. The developed catalyst is efficient for the removal of adsorbable organic halogens (AOX) and is also recyclable without losing activity. The qualitative abatement of organics is determined through proton nuclear magnetic resonance (1H NMR) analysis which revealed that the oxidation of leachate significantly degraded the organics. Third, to further enhance the efficacy of the Fenton process, a series of Fe/ZrO2 and Cu/ZrO2 catalysts with varying metal loadings are prepared and their activities are investigated in terms of IBP mineralization. The optimum metal loading for both type of catalysts is 7.5% but Fe/ZrO2 achieves maximum activity (80%) at pH 3 while Cu/ZrO2 affords maximum activity (70%) at pH 5. Moreover, both type of catalysts with 7.5% metal loading undergo minimal leaching at optimum conditions which are lower than the European Union directives for release in wastewater. However, Fe/ZrO2 requires higher catalyst doses and loses its activity when recycled due to complex formation with the degradation products. On the other hand, lower doses of Cu/ ZrO2 achieve higher and stable IBP mineralization upon recycling. Fourth, a series of Fe and Cu bimetallic catalysts are prepared to induce better redox cycle in the catalyst composites. These catalysts are very active within the pH range of 3 and 4. The extent of bimetallic activity is governed by the limiting metal while the excess metal component also exhibits monometallic behavior. Among the developed catalysts 5Cu/5Fe not only displays the highest TOC abatement (82%) under optimum conditions but also sustains minimum metal loses. Moreover, the bimetallic catalysts with balanced metal loadings are better able to yield stable activities upon recycling. Finally, iron slag is employed for the treatment of leachate because it contains iron species that can catalyze the heterogeneous Fenton process. Characterization of iron slag revealed that it contains iron in the form of metal oxides which are suitable as heterogeneous catalyst. Iron slag mineralized 75% of organics present in leachate under optimum conditions; pH 3, catalyst dose – 1g/L and reaction temperature 70 °C. This is very important from a circular economy perspective because a solid waste (iron slag) that is produced in huge quantities can be effectively used as a catalyst for the treatment of highly polluted liquid wastes.