Increasing Photo-Fenton process Efficiency: The effect of high temperatures

Abstract

The possibility to intensify the homogeneous photo-Fenton by increasing temperature (up to 90 °C) has been deeply investigated using phenol as model pollutant and treating a real landfill leachate effluent.

TOC removal rate was significantly improved as temperature increases. The required irradiation time to mineralize 1 g/L of phenol using the stoichiometric amount of H2O2 and 10 mg/L Fe2+ decreases from 120 min at 50 °C to less than 30 min when the experiments were carried out at 90 °C. Besides, within this range, the value of H2O2 consumption efficiency (ηH2O2), defined here as g of TOC converted per g of H2O2 consumed, was maintained around 0.15, which confirm the scarce effect of thermal H2O2 decomposition into O2 and H2O. More interestingly, the Irradiation Energetic Efficiency, defined as the TOC converted per W-h of energy emitted to the solution, dramatically increased with temperature, with a maximum value of 1.04 g of TOC per W-h reached at 90 °C. These results significantly improve those reached using dark Fenton under comparable experimental conditions.

Similar trend was observed when high temperature photo-Fenton was applied to landfill leachates, using the stoichiometric amount of H2O2 (2.12 g/g COD) and 10 mg/L Fe2+. The required irradiation time to achieve maximum TOC and COD removals (around 80%) was reduced from 180 to 45 min by increasing temperature from 50 to 90 °C and the irradiation efficiency increased 4-fold withing this range of temperature.

In addition, a kinetic model has been proposed to fit TOC evolution in both phenol and the leachate treatment. According to this, firstly temperature promotes the rapid degradation of TOC by the hydroxyl radicals produced by Fenton reaction decreasing the wastewater turbidity and colour. Subsequently, photo-Fenton reaction plays a key role, promoting organic acids and other by-products mineralization.

The results of this paper reveal that, far from being a drawback due to undesirable H2O2 thermal decomposition into O2 and H2O, high temperatures could be efficiently used to enhance the cost-efficiency of photoassisted processes. To the best of our knowledge, this is the first time that such a wide range of temperature up to 90 °C is explored. Thereby, the outcomes of this research confirm temperature as a promising way to intensify photoassisted treatments, reducing both the reaction time and the required energy (W-h) and operational costs to partially oxidized and/or mineralize organic matter in highly organic loaded wastewaters.