Enhancing the photo-Fenton treatment of contaminated water by use of ultrasound and iron-complexing agents

The photo-Fenton process is a promising emerging technology for the treatment of contaminated water streams. It is based on the generation of reactive oxygen species that are capable of destroying recalcitrant contaminants otherwise unaffected by conventional municipal wastewater treatment method. Among its innate advantages are that it can be catalyzed naturally by solar radiation and the main reagents required are iron and hydrogen peroxide, both abundant and relatively cheap. There are however some important limitations. The process functions best at acidic pH, is negatively affected by some inorganic ions present in natural water and the water requires pH neutralization and iron recuperation following the treatment. These limitations inhibit the application of the process at larger-scale. This work is an attempt to develop new strategies for enhancing the photo-Fenton process for making it a more attractive option for the treatment of municipal or industrial wastewaters. Two main pathways are investigated, the use of ultrasound and the use of iron-complexing agents. Ultrasonic processes have been shown to improve degradation kinetics when applied in combination with photo-Fenton, while at the same time are not as negatively affected by some of the inorganic ions present in water. Iron-complexing agents can maintain iron soluble at a wider operational pH and can also improve process efficiency via pathways discussed at length in the text. Chapter 2 is focused on the application of ultrasound in combination with photo-Fenton for two very distinct applications: 1) A pilot-scale ultrasound/photo-Fenton system was set-up and evaluated for the treatment of three different contaminants with distinct physicochemical properties and at different concentrations. Synergy was evaluated in terms of degradation kinetics and H2O2 consumption efficiency. 2) Ultrasound was applied alone and in combination with photo-Fenton for the treatment of the iodinated contrast agent Iohexol. These agents are injected in patients during radiographic procedures and excreted via urine. This provided an opportunity to examine if ultrasound could improve their treatment in such a complex aquatic medium. Treatment options both in urine (at the source) and in the context of municipal wastewater are discussed. Chapter 3 is focused on the use of EDDS as an iron-complexing agent for degrading contaminants at near neutral pH across a concentration range usually found in different wastewaters (from IŒg L-1 to mg L-1 ). EDDS was used both for the treatment of microcontaminants (IŒg L-1) in municipal wastewater treatment plant effluents as well as for highly contaminated ( several mg L-1) water. The importance of several factors and operational conditions is discussed. Chapter 4 discusses the possibility of exploiting natural iron-complexing agents (mainly phenolic/polyphenolic compounds) that can be found in some types of wastewater (e.g from natural products processing industries). The concept of reusing polyphenol-rich wastewater in small quantities as an alternative to artificial complexing agents is introduced. As a case study, water containing highly concentrated cork bark extracts (Cork Boiling Wastewater) has been added in natural water spiked with one or several contaminants. Its capacity to maintain Fe3+ soluble at near-neutral pH as well as the effect on Fenton/photo-Fenton degradation were observed.