Collaborative Research: Catalyst Free Activation of Peroxydisulfate under Visible Light to Degrade Contaminants in Water: Elucidation of Kinetics and Mechanism

Organic micropollutants (OMPs) have become major threats to human and ecosystem health in the United States and worldwide. Many OMPs are not efficiently removed by conventional water treatment processes. Advanced oxidation processes (AOPs) such as the commercial UV/AOP process are increasingly being utilized as a final treatment barrier to remove OMPs in advanced water reclamation and reuse plants in the United States and worldwide.

In a typical UV/AOP process, UV-C light (254 nm in wavelength) is combined with an oxidant (e.g., hydrogen peroxide) to generate OH● free radicals that can destroy and mineralize OMPs including personal care products, pharmaceuticals, pesticides, herbicides, etc. Recently, AOPs based on sulfate radicals (SO4●-) have gained worldwide popularity due to the higher redox potential and longer lifetime of SO4●- radicals compared to those of OH● radicals.

Sulfate radicals are typically generated on site by activating one of two common precursors: peroxymonosulfate (PMS) and peroxydisulfate (PDS) using a catalyst or photolysis. Recent studies have shown that PDS can be activated by visible light without using catalyst thereby providing new opportunities to develop more cost-effective AOPs for large-scale water treatment and wastewater reclamation.

The overarching goal of this project is to advance the fundamental understanding of the mechanisms of sulfate radical generation from PDS by visible light activation and determine the efficacy of this sulfate radical-based AOP to degrade and mineralize different classes of OMPs. The successful completion of this project will generate new fundamental knowledge to guide the design and implementation of sulfate radical-based AOPs for the removal and destruction of OMPs from wastewater and contaminated drinking water sources.

Additional benefits to society will be achieved through student education and training including the mentoring of one undergraduate and two graduate students at Texas A&M University and one graduate student at the University of Cincinnati.

Sulfate radical (SO4●)-based advanced oxidation processes (AOPs) are particularly attractive due to their larger redox potential and much longer half-life (30-40 microseconds) compared to those of hydroxyl radicals (OH●, 20 nanoseconds). In addition, the low bond dissociation energy of the O-O bond in peroxydisulfate (PDS), a primary precursor of SO4●-, suggests that activation of PDS is feasible by visible light without a catalyst.

However, the practicality of a catalyst-free and visible light activated PDS AOP depends on the quantum yield of SO4●- and the properties of OMPs, which have been shown to promote the formation of other radical species including OH●, superoxide (O2●), and singlet oxygen (1O2) in aqueous solutions containing anions (chloride, carbonate, and phosphate) and natural organic matter. To address these challenges, the Principal Investigators (PIs) of this project propose to carry out a fundamental study of the kinetics and mechanisms of degradation of six (6) target OMPs with distinctively different molecular structures using a catalyst free and visible light activated PDS advanced oxidation process.

The specific objectives of this research are to 1) measure the quantum yields of activated PDS by visible light at three monochromatic wavelengths; 2) identify and quantify reactive species in a broad spectrum of light to establish the potential advantage of visible light activation of PDS over UV light activation for the generation of SO4●- radicals ; 3) determine the degradation kinetics of six (6) target OMPs under different environmental conditions; and 4) combine and integrate multiple experimental assays/tools (e.g., colorimetry and electron paramagnetic resonance spectroscopy) to elucidate and confirm the primary and secondary reactive species responsible for contaminant degradation in sulfate radical-based AOPs. The successful completion of this project has the potential for transformative impact through the generation of new fundamental knowledge to guide the design of more cost-effective and sustainable AOPs for water treatment and wastewater reclamation.

To implement the education and training goals of the project, the PIs propose to leverage existing programs at Texas A&M University (TAMU) and the University of Cincinnati (UC) to recruit and mentor undergraduate students from underrepresented groups to work on the project. In addition, the PIs plan to integrate the findings from this research into existing environmental engineering graduate/undergraduate courses at TAMU and UC.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.